section overview of biology, development,...

CHAPTER 7

■

DEVELOPMENT AND STRUCTURE OF SKIN

57

S E C T I O N

3

OVERVIEW OF BIOLOGY, DEVELOPMENT, AND STRUCTURE OF SKIN

C H A P T E R 7

Development and Structure of Skin

David H. Chu

SKIN: AN OVERVIEW

Skin is a complex organ that protects itshost from its environment, at the sametime allowing interaction with the envi-ronment. It is much more than a static,impenetrable shield against external in-sults. Rather, the skin is a dynamic,complex, integrated arrangement ofcells, tissues, and matrix elements thatmediates a diverse array of functions:skin provides a physical permeabilitybarrier, protection from infectious agents,thermoregulation, sensation, ultravio-let (UV) protection, wound repair andregeneration, and outward physical ap-pearance (Table 7-1). These variousfunctions of skin are mediated by oneor more of its major regions—the epi-dermis, dermis, and hypodermis (Fig.7-1; see also Fig. 6-1, Chap. 6). Thesedivisions are interdependent, functionalunits; each region of skin relies uponand is connected with its surroundingtissue for regulation and modulation ofnormal structure and function at molec-ular, cellular, and tissue levels of organi-zation (see Chap. 6).

Whereas the epidermis and its outerstratum corneum provide a large partof the physical barrier provided byskin, the structural integrity of the skinas a whole is provided primarily by thedermis and hypodermis. Antimicrobialactivities are provided by the innateimmune system and antigen-presentingdendritic cells of the epidermis, circu-lating immune cells that migrate from

the dermis, and antigen-presentingcells of the dermis (see Chap. 10). Pro-tection from UV irradiation is providedin great measure by the most superfi-cial cells of the epidermis. Inflamma-tion begins with the keratinocytes ofthe epidermis or immune cells of thedermis, and sensory apparatus ema-nates from nerves that initially traversethe hypodermis to the dermis and epi-dermis, ending in specialized receptiveorgans or free nerve endings. The larg-est blood vessels of the skin are foundin the hypodermis, which serve totransport nutrients and immigrant cells(see Fig. 6-1, Chap. 6). The cutaneouslymphatics course through the dermisand hypodermis, serving to filter debrisand regulate tissue hydration. Epidermalappendages provide special protectiveor sensory functions. Skin also deter-mines a person’s physical appearance,influenced by pigmentation providedby melanocytes, with body contours,appearance of age, and actinic damageinfluenced by the epidermis, dermis,and hypodermis. The skin begins to beorganized during embryogenesis, whereintercellular and intracellular signals,as well as reciprocal crosstalk betweendifferent tissue layers, are instrumen-tal in regulating the eventual matura-tion of the different components ofskin.

What follows is an integrated de-scription of the major structural fea-tures of the skin and how these struc-tures allow the skin to achieve itsmajor functions, followed by a reviewof their embryologic origins. Also high-lighted are illustrative cutaneous dis-eases that manifest when these func-tions are defective. Understanding thegenetic and molecular basis of skin dis-ease has confirmed, and in some casesrevealed, the many factors and regula-tory elements that play critical roles inskin function.

EPIDERMIS

One of the most fundamental and visi-ble features of skin is the stratified,cornified epidermis (Fig. 7-2). The epi-dermis is a continually renewing struc-ture that gives rise to derivative struc-tures called appendages (pilosebaceousunits, nails, and sweat glands). The epi-dermis ranges in thickness from 0.4 to1.5 mm, as compared with the 1.5- to4.0-mm full-thickness skin. The major-ity of cells in the epidermis are kerati-nocytes that are organized into four lay-ers, named for either their position or astructural property of the cells. Thesecells progressively differentiate from

STRUCTURE AND FUNCTION

OF SKIN

AT A GLANCE

■

Three Major Layers—Epidermis, Dermis, Hypodermis:

■

Epidermis: major permeability barrier, innate immune function, adhesion, ultraviolet protection.

■

Dermis: major structural element, three types of components—cellular, fibrous matrix, diffuse and filamentous matrix. Also site of vascular, lymphatic, and nerve networks.

■

Hypodermis (subcutis): insulation, mechanical integrity, containing the larger source vessels and nerves.

SECTION 3

■

OVERVIEW OF BIOLOGY, DEVELOPM

ENT, AND STRUCTURE OF SKIN

58

proliferative basal cells, attached to theepidermal basement membrane, to theterminally differentiated, keratinized stra-

tum corneum, the outermost layer andbarrier of skin (see Chap. 45). Interca-lated among the keratinocytes at vari-ous levels are the immigrant residentcells—melanocytes, Langerhans cells,and Merkel cells. Other cells, such aslymphocytes, are transient inhabitantsof the epidermis and are extremelysparse in normal skin. There are many

regional differences in the epidermisand its appendages. Some of these dif-ferences are apparent grossly, such asthickness (e.g., palmoplantar skin vs.truncal skin, Fig. 7-3); other differencesare microscopic.

Pathologic changes in the epidermiscan occur as a result of a number of dif-ferent stimuli: repetitive mechanicaltrauma (as in lichen simplex chronicus),inflammation (as in atopic dermatitisand lichen planus), infection (as in ver-ruca vulgaris), immune system activityand cytokine abnormalities (as in psori-asis, Fig. 7-4), autoantibodies (as inpemphigus vulgaris and bullous pem-phigoid), or genetic defects that influ-ence differentiation or structural pro-teins [as in epidermolysis bullosa (EB)simplex, epidermolytic hyperkeratosis,the ichthyoses, and Darier disease].

Layers of the Epidermis

BASAL LAYER

The keratinocyte is an ec-todermally derived cell and is the pri-mary cell type in the epidermis, ac-counting for at least 80 percent of thetotal cells. The ultimate fate of thesecells is to contribute the components forthe epidermal barrier as the stratum cor-neum. Thus, much of the function ofthe epidermis can be gleaned from thestudy of the structure and developmentof the keratinocyte.

Keratinocyte differentiation (keratini-zation) is a genetically programmed,carefully regulated, complex series ofmorphologic changes and metabolicevents whose endpoint is a terminallydifferentiated, dead keratinocyte (cor-neocyte) that contains keratin filaments,matrix protein, and a protein-reinforced

�

FIGURE 7-1

The major regions of skin. Skin iscomposed of three layers: (1) epidermis, (2) dermis,and (3) hypodermis. The outermost epidermis is sep-arated from the dermis by a basement membranezone, the dermal-epidermal junction. Below the der-mis lies the subcutaneous fat (hypodermis). Epider-mal appendages, such as hair follicles and eccrineand apocrine sweat glands, begin in the epidermisbut course through the dermis and/or the epidermis.Blood vessels, lymphatics, and nerves coursethrough the subcutaneous fat and emerge into thedermis. (Used with permission from O. Kovich, MD.)

TABLE 7-1

Functions of Skin

F

UNCTION

T

ISSUE

L

AYER

S

OME

A

SSOCIATED

D

ISEASES

Permeability barrier Epidermis Atopic dermatitisEctodermal dysplasiasIchthyosesKeratodermasExfoliative dermatitisBullous diseases

Protection from pathogens Epidermis Verruca vulgarisDermis Ecthyma

CellulitisLeishmaniasisHuman immunodeficiency virusTinea pedis/corporis

Thermoregulation Epidermis Ectodermal dysplasiasDermis RaynaudHypodermis Hyperthermia

Sensation Epidermis Diabetic neuropathyDermis LeprosyHypodermis Pruritus

Postherpetic neuralgiaUltraviolet protection Epidermis Xeroderma pigmentosum

Oculocutaneous albinismWound repair/regeneration Epidermis Keloid

Dermis Venous stasis ulcerPyoderma gangrenosum

Physical appearance Epidermis MelasmaDermis VitiligoHypodermis Scleroderma

Lipodystrophy

�

FIGURE 7-2

Schematic of epidermis. The epidermis is a stratified, cornified epithelium. The deepestlayer consists of basal cells (BL) that rest upon the basement membrane of the dermal-epidermal junc-tion (DEJ). These cells differentiate into the cells of the spinous layer (SL), characterized by abundantdesmosomal spines. Spinous cells eventually become granular layer cells (GL), producing many of thecomponents of the cornified envelope. Ultimately, the terminally differentiated keratinocytes shed theirnuclei and become the stratum corneum (SC), a cross-linked network of protein and glycolipids.

CHAPTER 7

■


59

plasma membrane with surface-associ-ated lipids (see Chap. 44).

Keratins are a family of intermediatefilaments and are the hallmark of all epi-thelial cells, including keratinocytes (re-viewed in refs. 1 and 2). They serve apredominantly structural role in thecells. Fifty-four different functional ker-atin genes have been identified in hu-mans—34 epithelial keratins and 17 hairkeratins.

3

The co-expression ofspecific keratin pairs is dependent oncell type, tissue type, developmentalstage, differentiation stage, and diseasecondition (Table 7-2). Furthermore, thecritical role of these molecules is under-scored by the numerous manifestationsof disease that arise because of muta-tions in these genes (see Table 7-2).Thus, knowledge of keratin expression,

regulation, and structure provides in-sight into epidermal differentiation andstructure.

The basal layer (stratum germinati-vum) contains mitotically active, colum-nar-shaped keratinocytes that attach viakeratin filaments (K5 and K14) to thebasement membrane zone at hemides-mosomes (see Chap. 51), attach to othersurrounding cells through desmosomes,and that give rise to cells of the more su-perficial, differentiated epidermal layers.Ultrastructural analysis reveals the pres-ence of membrane-bound vacuoles thatcontain pigmented melanosomes trans-ferred from melanocytes by phagocyto-sis.

4

The pigment within melanosomescontributes to the overall skin pigmenta-tion perceived macroscopically.

5

Thebasal layer is the primary location of mi-

totically active cells of the epidermis.Cell kinetic studies suggest that the basallayer cells exhibit different proliferativepotentials (stem cells, transit amplifyingcells, and postmitotic cells), and in vivoand in vitro studies suggest that there ex-ist long-lived epidermal stem cells

6

(seeChap. 44). Because basal cells can be ex-panded in tissue culture and used to re-constitute sufficient epidermis to coverthe entire skin surface of burn patients,

7,8

such a starting population is presumed tocontain long-lived stem cells with exten-sive proliferative potential.

A large amount of data supports theexistence of multipotent epidermal stemcells within the bulge region of the hairfollicle based on these traits.

9–17

Cellsfrom this region are able to contributeto the formation not only of the entirepilosebaceous unit, but to the interfol-licular epidermis as well.

14–17

The existence of an additional progen-itor population of cells, within the sur-face epidermal basal layer, is also sup-ported by a number of lines of evidence,both in vitro and in vivo. These putativebasal stem cells appear to be clonogenic,progress rapidly through S-phase of thecell cycle, and divide infrequently duringstable self-renewal (retaining radiola-beled nucleotide label over long peri-ods). Additionally, they are capable ofcell division in response to exogenousand endogenous agents. Early lineagetracing experiments in the epidermisidentified that keratinocytes are orga-nized into vertical columns of progres-sively differentiating cells, termed

epider-mal proliferating units

.

18–21

The second type of cell, the transit

amplifying cells of the basal layer, arisesas a subset of daughter cells producedby the infrequent division of stem cells.These cells provide the bulk of the celldivisions needed for stable self-renewaland are the most common cells in thebasal compartment. After undergoingseveral cell divisions, these cells giverise to the third class of epidermal basalcells, the postmitotic cells that undergoterminal differentiation. Although longbelieved to detach from the basal laminato migrate to a more superficial (supra-basal) position in the epidermis, recentevidence has suggested that asymmetricdivision of basal cells relative to the base-ment membrane can directly give riseto a suprabasal differentiating daughtercell.

30

In humans, the normal transittime for a basal cell, from the time itloses contact with the basal layer to thetime it enters the stratum corneum, is atleast 14 days. Transit through the stra-

�

FIGURE 7-3

Anatomic vari-ation in epidermal thickness.

A.

Acral skin.

B.

Eyelid skin.Note that the epidermis is con-siderably thicker in (

A

) than (

B

),including the compact layers ofthe stratum corneum, as well asthe deeper epidermal layers.(Used with permission from O.Kovich, MD.)

�

FIGURE 7-4

Epidermal hy-perplasia. Hyperproliferation ofthe epidermis can occur due toa number of causes, as mani-fested in diseases such as pso-riasis (pictured), as well as li-chen simplex chronicus, atopicdermatitis, lichen planus, andverruca vulgaris. (Used with per-mission from O. Kovich, MD.)

A

B

SECTION 3

■



60

tum corneum and subsequent desqua-mation require another 14 days. Theseperiods of time can be altered in hyper-proliferative or growth-arrested states.

SPINOUS LAYER

The shape, structure,and subcellular properties of spinouscells correlate with their position withinthe mid-epidermis. They are named forthe spine-like appearance of the cellmargins in histologic sections. Supra-basal spinous cells are polyhedral inshape with a rounded nucleus. As thesecells differentiate and move upwardthrough the epidermis, they becomeprogressively flatter and develop or-ganelles known as

lamellar granules

(seeGranular Layer). Spinous cells also con-tain large bundles of keratin filaments,organized around the nucleus and in-serted into desmosomes peripherally.

Spinous cells retain the stable K5/K14keratins that are produced in the basallayer but do not synthesize new mes-senger RNA (mRNA) for these proteins,except in hyperproliferative disorders.Instead, new synthesis of the K1/K10keratin pair occurs in this epidermallayer. These keratins are characteristic ofan epidermal pattern of differentiationand thus are referred to as the

differentia-tion-specific

or

keratinization-specific ker-atins

. This normal pattern of differenti-ation is switched to an alternativepathway in hyperproliferative states,however. In conditions such as psoriasis,actinic keratoses, and wound healing,synthesis of K1 and K10 mRNA and pro-

tein is downregulated, and the synthesisand translation of messages for K6 andK16 are favored. Correlated with thischange in keratin expression is a disrup-tion of normal differentiation in the sub-sequent granular and cornified epidermallayers (see Granular Layer and StratumCorneum). mRNA for K6 and K16 arepresent throughout the epidermis nor-mally, but the message is only translatedon stimulation of proliferation.

The “spines” of spinous cells are abun-dant desmosomes, calcium-dependentcell surface modifications that promote

adhesion of epidermal cells and resis-tance to mechanical stress (reviewed inref. 31; see also Chaps. 44 and 51).Within each cell is a desmosomal plaque,which contains the polypeptides plako-globin, desmoplakins I and II, kera-tocalmin, desmoyokin, and plakophilin.Transmembrane glycoproteins—desmo-gleins 1 and 3 and desmocollins I and II,members of the cadherin family—pro-vide the adhesive properties of the extra-cellular portion of the desmosomes,known as the

core

. Whereas the extracel-lular domains of the cadherins form partof the core, the intracellular domains in-sert into the plaque, linking them to theintermediate filament (keratin) cytoskel-eton. Although desmosomes are relatedto adherens junctions, the latter associ-ate with actin microfilaments at cell–cellinterfaces, via a distinct set of cadherins(e.g., E-cadherin) and intracellular cate-nin adapter molecules.

That the desmosomes are integralmediators of intercellular adhesion isclearly demonstrated in diseases inwhich these structures are disrupted(Table 7-3).

32

The autoimmune bullousdiseases pemphigus vulgaris and pem-phigus foliaceus (see Chap. 52) arecaused by antibodies that target the des-moglein proteins within the desmo-somes. Loss of desmosomal adhesionresults in the characteristic roundingand separation of keratinocytes (acan-tholysis), ultimately forming a blisterwithin the epidermis. Strikingly, theclinical presentation of these diseasesreflects the relative expression in the tis-sue of the desmoglein 1 and 3 proteins

TABLE 7-2

Expression Patterns of Keratin Genes and Keratin-Associated Diseases

B

ASIC

A

CIDIC

T

ISSUE

E

XPRESSION

D

ISEASE

A

SSOCIATION

1 10 Suprabasal keratinocytes Bullous congenital ichthyosiform erythro-derma; diffuse nonepidermolytic PPK (keratin 1)

1 9 Suprabasal keratinocytes (palmoplantar skin)

Epidermolytic PPK (epidermolytic hyper-keratosis)

2 10 Upper spinous and granular layers Ichthyosis bullosa of Siemens

3 12 Cornea Meesmann’s corneal dystrophy

4 13 Mucosal epithelium White sponge nevus

5 14 Basal keratinocytes Epidermolysis bullosa simplex

6a 16 Outer root sheath, hyperprolifera-tive keratinocytes, palmoplantar keratinocytes

Pachyonychia congenita type I; focal non-epidermolytic PPK

6b 17 Nail bed, epidermal appendages Pachyonychia congenita type II; steatocys-toma multiplex

8 18 Simple epithelium Cryptogenic cirrhosis

PPK = palmoplantar keratoderma.

TABLE 7-3

Disease Resulting from Disruption of Desmosomal Proteins

P

ROTEIN

D

ISEASES

Desmoglein 1 Pemphigus foliaceusStriate palmoplantar keratodermaStaphylococcal scalded-skin syndromeBullous impetigo

Desmoglein 3 Pemphigus vulgaris

Desmoglein 4 Autosomal recessive hypotrichosis

Plakoglobin Palmoplantar keratoderma with wooly hair and arrhythmogenic right ventric-ular cardiomyopathy (Naxos disease)

Plakophilin 1 Ectodermal dysplasia/skin fragility syndrome (skin erosions, dystrophic nails, sparse hair, and painful palmoplantar keratoderma)

Plakophilin 2 Arrhythmogenic right ventricular cardiomyopathy

Desmoplakin Lethal acantholytic epidermolysis bullosaStriate palmoplantar keratoderma, type IPalmoplantar keratoderma with left ventricular cardiomyopathy and wooly hairAutosomal dominant arrhythmogenic right ventricular cardiomyopathy

CHAPTER 7

■


61

(see Chaps. 51 and 52). Pemphigus vul-garis results from autoantibodies di-rected against desmoglein 3 and resultsin disruption of the epidermis betweenthe basal and suprabasal layers (re-viewed in ref. 33). On the other hand,desmoglein 1 is expressed in the upperepidermal layers, and antibodies to thisprotein in patients with pemphigus foli-aceus result in blisters in the more su-perficial granular layer. Other diseasesthat target the same desmoglein 1 pro-tein but by a different mechanism arestaphylococcal scalded skin syndrome(see Chap. 178)

and bullous impetigo, inwhich a bacterial protease cleaves andinactivates desmoglein 1, resulting inthe same superficial blistering seen inpemphigus foliaceus.

34

Genetic muta-tions in other desmosomal componentsalso reveal a role for these proteins inadhesion as well as cell signaling (seeTable 7-3).

32

The importance of calcium as a media-tor of adhesion is well illustrated in thecases of two conditions that exhibit char-acteristic epidermal dyscohesion, Darierdisease (keratosis follicularis) and Hailey-Hailey disease (benign chronic pemphi-gus) (see Chap. 49).

35,36

Both of these dis-eases are caused by mutations in genesthat regulate calcium transport, SERCA2(sarco/endoplasmic reticulum Ca

2+

-ATPasetype 2 isoform) in the case of Darier dis-ease, and ATP2C1 (ATPase, Ca

2+

trans-porting, type 2C, member 1, a regulator ofcytoplasmic calcium concentration) in thecase of Hailey-Hailey disease.

Lamellar granules are also formed inthis layer of epidermal cells (Fig. 7-5).These secretory organelles deliver pre-cursors of stratum corneum lipids intothe intercellular space. Lamellar gran-ules contain glycoproteins, glycolipids,phospholipids, free sterols, and a num-ber of acid hydrolases, including lipases,proteases, acid phosphatases, and gly-cosidases. Glucosylceramides, the pre-cursors to ceramides and the dominantcomponent of the stratum corneum lip-ids, are also found within these struc-tures (see Chap. 45). Genetic diseasesdemonstrate the importance of steroidand lipid metabolism for sloughing ofcornified cells—in recessive X-linkedichthyosis, for example, mutation ofsteroid sulfatase results in a retentionhyperkeratosis (see Chap. 47).

37,38

GRANULAR LAYER

Named for the baso-philic keratohyalin granules that areprominent within cells at this level ofthe epidermis, the granular layer is thesite of generation of a number of the

structural components that will form theepidermal barrier, as well as a number ofproteins that process these components(see Fig. 7-2) (reviewed in refs. 39 and40). Keratohyalin granules (see Fig. 7-5)are composed primarily of profilaggrin,keratin filaments, and loricrin. It is in thislayer that the cornified cell envelope be-gins to form. Release of profilaggrinfrom keratohyalin granules results in thecalcium-dependent cleavage of theprofilaggrin polymeric protein into filag-grin monomers. These filaggrin mono-mers aggregate with keratin to formmacrofilaments. Eventually, filaggrin isdegraded into molecules, including uro-canic acid and pyrrolidone carboxylicacid, which contribute to hydration ofthe stratum corneum and help filter UVradiation. Loricrin is a cysteine-rich pro-tein that forms the major protein com-ponent of the cornified envelope, ac-counting for more than 70 percent of itsmass. Upon its release from keratohyalingranules, loricrin binds to desmosomalstructures. Loricrin, along with involu-crin, cystatin A, small proline-rich pro-teins (SPR1, SPR2, and cornifin), elafin,and envoplakin are all subsequentlycross-linked to the plasma membrane bytissue transglutaminases (TGMs, primar-ily TGMs 3 and 1), forming the cornifiedcell envelope.

Mutations in the TGM1 gene havebeen shown to be the basis of somecases of lamellar ichthyosis, an autoso-mal recessive condition characterizedby large scales and a disruption in theuppermost differentiating layers of theepidermis.

41,42

Another form of ichthyo-sis, ichthyosis vulgaris, is caused by mu-tations in the gene encoding filag-

grin.

43,44

Loricrin abnormalities result ina form of Vohwinkel syndrome withichthyosis and pseudoainhum, as wellas the disease progressive symmetrickeratodermia.

45–47

These findings em-phasize the importance of proper for-mation of the cornified envelope in nor-mal epidermal keratinization.

The final stage of granular cell differ-entiation involves the cell’s own pro-grammed destruction. During this pro-cess, in which the granular cell becomesa terminally differentiated corneocyte, anapoptotic mechanism results in the de-struction of the nucleus and almost allcellular contents, with the exception ofthe keratin filaments and filaggrin matrix.

STRATUM CORNEUM

(See Chap. 45) Com-plete differentiation of granular cells re-sults in stacked layers of anucleate, flat-tened cornified cells that form thestratum corneum. It is this layer thatprovides mechanical protection to theskin and a barrier to water loss and per-meation of soluble substances from theenvironment (reviewed in refs. 39 and48). The stratum corneum barrier isformed by a two-compartment systemof lipid-depleted, protein-enriched cor-neocytes surrounded by a continuousextracellular lipid matrix. These twocompartments provide somewhat seg-regated but complementary functionsthat together account for the “barrier ac-tivity” of the epidermis. Regulation ofpermeability, desquamation, antimicro-bial peptide activity, toxin exclusion,and selective chemical absorption are allprimarily functions of the extracellularlipid matrix. On the other hand, me-chanical reinforcement, hydration, cyto-

�

FIGURE 7-5

Junction of the stratum granulosum (SG) and stratum corneum (SC). Lamellar granules(LG) are in the intercellular space and cytoplasm of the granular cell. Keratohyalin granules (KHG) arealso evident.

Inset:

Lamellar granule,

×

28,750. (From Holbrook K: Structure and development of theskin, in

Pathophysiology of Dermatologic Disease

, 2nd ed., edited by Soter NA, Baden HP. New York,McGraw-Hill, 1991, p 7, with permission. Inset used with permission from EC Wolff-Schreiner, MD.)

SECTION 3

■



62

kine-mediated initiation of inflamma-tion, and protection from UV damageare all provided by the corneocytes.

Nonkeratinocytes of the Epidermis

Melanocytes are neural crest-derived,pigment-synthesizing dendritic cells thatreside primarily in the basal layer (seeChap. 70).

49

By light microscopy, thesecells are recognized by their pale-stain-ing cytoplasm, ovoid nucleus, and colorof the pigment-containing melano-somes, the distinctive organelle of themelanocyte. The function of melano-cytes has been highlighted by disordersin melanocyte number or function. Theclassic dermatologic disease, vitiligo, iscaused by the autoimmune depletion ofmelanocytes.

50

Causes of other disor-ders of pigmentation are found in vari-ous defects in melanogenesis, includingmelanin synthesis, melanosome produc-tion, and melanosome transport andtransfer to keratinocytes (see Chaps. 70and 73). Regulation of melanocyte pro-liferation and homeostasis is under in-tensive study as well as a means to un-derstanding melanoma (see Chap. 124).

4

Keratinocyte–melanocyte interactions arecritical for melanocyte homeostasis anddifferentiation, influencing proliferation,dentricity, and melanization.

Merkel cells are slow-adapting type Imechanoreceptors located in sites ofhigh-tactile sensitivity (see Chap. 120).

51

They are present among basal kerati-nocytes in particular regions of the body,including hairy skin and in the glabrousskin of the digits, lips, regions of the oralcavity, and the outer root sheath of thehair follicle. Like other nonkeratinocytes,Merkel cells have a pale-staining cyto-plasm. Immunohistochemical markers ofthe Merkel cell include K8, K18, K19, andK20 keratin peptides. K20 is restricted toMerkel cells in the skin and thus may bethe most reliable marker. Ultrastruc-turally, Merkel cells are easily identifiedby the membrane-bounded, dense-coregranules that collect opposite the Golgiand proximal to an unmyelinated neurite(Fig. 7-6). These granules are similar toneurosecretory granules in neurons andcontain neurotransmitter-like substancesand markers of neuroendocrine cells, in-cluding metenkephalin, vasoactive in-testinal peptide, neuron-specific enolase,and synaptophysin. Although increasinglymore is being learned about the normalfunction of Merkel cells, they are of par-ticular clinical note because Merkel cell-derived neoplasms are particularly aggres-sive and difficult to treat (see Chap. 120).

Langerhans cells are dendritic antigen-processing and -presenting cells in the epi-dermis (see Chap. 10).

52

Although theyare not unique to the epidermis, theyform 2 percent to 8 percent of the totalepidermal cell population. They aremostly found in a suprabasal position butare distributed throughout the basal, spi-nous, and granular layers. In histologicpreparations, Langerhans cells are pale-staining and have convoluted nuclei. Thecytoplasm of the Langerhans cells con-tains characteristic small rod- or racket-shaped structures called

Langerhans cellgranules

or

Birbeck granules

(Fig. 7-7). Theyprincipally function to sample and presentantigens to T cells of the epidermis. Be-cause of these functions, they are im-plicated in the pathologic mechanismsunderlying allergic contact dermatitis,cutaneous leishmaniasis, and human im-munodeficiency virus infection. Langer-hans cells are reduced in the epidermis ofpatients with certain conditions, such aspsoriasis, sarcoidosis, and contact derma-titis; they are functionally impaired byUV radiation, especially UVB.

Because of their effectiveness in anti-gen presentation and lymphocyte stim-ulation, dendritic cells and Langerhanscells have become prospective vehiclesfor tumor therapy and tumor vaccines.These cells are loaded with tumor-spe-cific antigens, which will then stimulatethe host immune response to mount anantigen-specific, and therefore tumor-specific, response.

DERMAL-EPIDERMAL JUNCTION

The dermal-epidermal junction (DEJ) isa basement membrane zone that formsthe interface between the epidermis and

dermis (see Chap. 51).

53,54

The majorfunctions of the DEJ are to attach theepidermis and dermis to each other andto provide resistance against externalshearing forces. It serves as a support forthe epidermis, determines the polarityof growth, directs the organization ofthe cytoskeleton in basal cells, providesdevelopmental signals, and serves as asemipermeable barrier.

The DEJ can be subdivided into threesupramolecular networks: the hemides-mosome-anchoring filament complex,the basement membrane itself, and theanchoring fibrils. The critical role of thisregion in maintaining skin structural in-tegrity is revealed by the large numberof mutations in DEJ components thatcause blistering diseases of varying se-verity, covered in detail in Chap. 60.These bullous diseases are grouped ac-cording to the level of the cleavagewithin the DEJ—the most superficial,EB simplex, involves basal keratinocytecleavage. Junctional EB occurs withinthe lamina lucida and lamina densa re-gions. Dystrophic EB is the deepest levelof blistering, within the sublaminadensa/anchoring filaments. Chap. 51provides a detailed discussion of theDEJ networks.

DERMIS

The dermis is an integrated system of fi-brous, filamentous, diffuse, and cellularconnective tissue elements that accom-modates nerve and vascular networks,epidermally derived appendages, andcontains many resident cell types, in-

�

FIGURE 7-6

Merkel cells from the finger of a130-mm CR (crown-rump) 21-week human fe-tus. Note nerve (N) in direct contact with the lat-eral and basal surfaces of the cell and densecore cytoplasmic granules (G).

×

13,925.

Inset:

Merkel cell granules,

×

61,450.

�

FIGURE 7-7

Langerhans cell. Note indentednucleus, lysosomes, as well as rod- and racket-shaped cytoplasmic granules (Birbeck granules),and the absence of keratin filaments.

×

13,200.

Inset:

Birbeck granules

×

88,000. (Used with per-mission from N. Romani, MD.)

CHAPTER 7

■


63

cluding fibroblasts, macrophages, mastcells, and transient circulating cells ofthe immune system (see Figs. 6-9 and6-14). The dermis makes up the major-ity of the skin and provides its pliability,elasticity, and tensile strength. It pro-tects the body from mechanical injury,binds water, aids in thermal regulation,and includes receptors of sensory stim-uli. The dermis interacts with the epi-dermis in maintaining the properties ofboth tissues, collaborates during devel-opment in the morphogenesis of theDEJ and epidermal appendages (see De-velopment of Skin Appendages), and in-teracts in repairing and remodeling skinafter wounding.

The dermis is arranged into two ma-jor regions, the upper papillary dermisand the deeper reticular dermis. Thesetwo regions are readily identifiable onhistologic section, and they differ intheir connective tissue organization, celldensity, and nerve and vascular pat-terns. The papillary dermis abuts theepidermis, molds to its contours, and isusually no more that twice its thickness(see Fig. 6-9). The reticular dermis formsthe bulk of the dermal tissue. It is com-posed primarily of large-diameter col-lagen fibrils, organized into large, inter-woven fiber bundles, with branchingelastic fibers surrounding the bundles(see Fig. 6-14). In normal individuals,the elastic fibers and collagen bundlesincrease in size progressively towardthe hypodermis. The subpapillary plexus,a horizontal plane of vessels, marksthe boundary between the papillaryand reticular dermis. The lowest bound-ary of the reticular dermis is definedby the transition of fibrous connectivetissue to adipose connective tissue ofthe hypodermis.

Fibrous Matrix of the Dermis

The connective tissue matrix of the der-mis is comprised primarily of collage-nous and elastic fibrous tissue.

55,56

These are combined with other, non-fibrous connective tissue molecules,including finely filamentous glycopro-teins, proteoglycans (PGs), and gly-cosaminoglycans (GAGs) of the “groundsubstance.”

57

In terms of acellular components, col-lagen forms the bulk of the dermis, ac-counting for approximately 75 percentof the dry weight of skin, and providingboth tensile strength and elasticity. (Fordetails regarding the polypeptide struc-ture and distribution of collagens, seeChap. 61.) The periodically banded, in-

terstitial collagens account for the great-est proportion of collagen in adult der-mis (type I, 80 percent to 90 percent; III,8 percent to 12 percent; and V, < 5 per-cent). Although type V collagen ac-counts for a relatively small proportionof total collagen, it codistributes withboth types I and III collagen to assist inregulating fibril diameter. It is locatedprimarily in the papillary dermis and thematrix surrounding the basement mem-branes of vessels, nerves, and epidermalappendages, and at the DEJ. Type VIcollagen is associated with fibril and inthe interfibrillar spaces. Type IV col-lagen is confined to the basal lamina ofthe DEJ, vessels, and epidermal appen-dages. Type VII collagen forms anchor-ing fibrils at the DEJ.

Elastic connective tissue (see Chap.61) is complex molecular mesh, assem-bled into a continuous network that ex-tends from the lamina densa of the DEJthroughout the dermis and into the con-nective tissue of the hypodermis.

56

Elas-tic fibers return the skin to its normalconfiguration after being stretched ordeformed. Elastic fibers are also presentin the walls of cutaneous blood vesselsand lymphatics and in the sheaths ofhair follicles. By dry weight, elastic con-nective tissue accounts for approxi-mately four percent of the dermal ma-trix protein. The components of elasticfibers include fibrillin-1, a 350-kd mole-cule, mutations in which cause Marfansyndrome (see Chap. 139). Elastin is theelastic fiber matrix component, and mu-tations in this protein cause the diseasecutis laxa (see Chap. 139). Oxytalan fi-bers extend from the DEJ to the papil-lary/reticular dermal junction, wherethey merge with elaunin fibers. Elauninfibers, in turn, evolve into mature elas-tic fibers of the reticular dermis. Thesefibers are normally located betweenbundles of collagen fibers, although incertain pathologic conditions, suchas Buschke-Ollendorff syndrome, bothelastic and collagen fibers become as-sembled within the same bundle. Theimportance of this elastic fiber networkis clearly seen in the number of multi-system diseases that arise because ofmutations in components of this net-work. Recently, the defect underlyingpseudoxanthoma elasticum (PXE) wasfound to be mutation in ABCC6, amember of the large adenosine triphos-phate-dependent transmembrane trans-porter family. Thus, this disease that ischaracterized by loss of skin elasticityand calcified elastic fibers is unlikely aprimary defect in elastic tissue, but

rather a metabolic disorder with sec-ondary involvement of elastic fibers.

58–

60

In addition to genetic mutations, solarradiation and aging also contribute toelastic fiber damage.

61

Filamentous and Diffuse Matrix Components of the Dermis

(SeeChap. 61)

The fibrous and cellular matrix elementsare embedded within more amorphousmatrix components, which also arefound in basement membranes.

62–64

PGsand GAGs surround and embed the fi-brous components. PGs are large mole-cules consisting of a core protein thatdetermines which GAGs will be incor-porated into the molecule. The PG/GAG complex can bind water up to1000 times its own volume and haveroles in regulation of water-binding andcompressibility of the dermis, as wellas increasing local concentrations ofgrowth factors through binding (e.g.,basic fibroblast growth factor). Theyalso link cells with the fibrillar and fila-mentous matrix, influencing prolifera-tion, differentiation, tissue repair, andmorphogenesis.

The major PGs in the adult dermis arechondroitin sulfates/dermatan sulfate,including biglycan, decorin, and versi-can; heparan/heparan sulfate PGs, in-cluding perlecan and syndecan; andchondroitin-6 sulfate PGs, which arecomponents of the DEJ (see Chap. 61).The relative amounts of these differentPGs change with age, as adult dermisafter age 40 years generally has in-creases in dermatan sulfate, but de-creases in chondroitin-6 sulfate and chon-droitin sulfate.

Glycoproteins that are found in thedermis include fibronectin, thrombo-spondin, laminin, vitronectin, and tenas-cin. Like the PGs/GAGs, they interactwith other matrix components via inte-grin receptors. These molecules facilitateprocesses of migration, cell adhesion,morphogenesis, and differentiation. Fi-bronectin is synthesized by both epithe-lial and mesenchymal cells, and it coverscollagen bundles and the elastic network.Vitronectin is present on all elastic fibersexcept for oxytalan. Tenascin expressionis found around the smooth muscle ofblood vessels, arrector pili muscles, andappendages such as sweat glands.

Cellular Components of the Dermis

Fibroblasts, macrophages, and mast cellsare the regular residents of the dermis,

SECTION 3

■



64

mostly found around the papillary re-gion and surrounding vessels of the sub-papillary plexus (see Fig. 6-20). Theyalso occur in the reticular dermis in theinterstices between collagen fiber bun-dles. The fibroblast is a mesenchymallyderived cell that migrates through thetissue and is responsible for the synthe-sis and degradation of fibrous and nonfi-brous connective tissue matrix proteinsand a number of soluble factors. Fibro-blasts provide a structural extracellularmatrix framework as well as promoteinteraction between epidermis and der-mis by synthesis of soluble mediators.Studies of human fibroblasts indicatethat even within a single tissue, pheno-typically distinct populations exist, someof which relate to regional anatomicaldifferences.

65,66

They are also instru-mental in wound healing and scarring,increasing their proliferative and syn-thetic activity during these processes.

The monocytes, macrophages, anddermal dendrocytes constitute the mono-nuclear phagocytic system of cells in theskin. Macrophages are derived fromprecursors in the bone marrow, differ-entiate into circulating monocytes, thenmigrate into the dermis to differentiate.These cells are phagocytic; process andpresent antigen to immunocompetentlymphoid cells; are microbicidal (pro-ducing lysozyme, peroxide, and super-oxide), tumoricidal, secretory (growthfactors, cytokines, and other immuno-modulatory molecules), and hematopoi-etic (see Chap. 10); and are involved incoagulation, atherogenesis, wound heal-ing, and tissue remodeling.

Mast cells (see Chap. 150) are special-ized secretory cells that, in skin, arepresent in greatest density in the papil-lary dermis, near the DEJ, in sheaths ofepidermal appendages, and aroundblood vessels and nerves of the subpap-illary plexus. They are also common inthe subcutaneous fat. Mast cells areidentified histologically by a round oroval nucleus and abundant, darkly stain-ing cytoplasmic granules. The surface ofdermal mast cells is coated with fi-bronectin, which probably assists in se-curing cells within the connective tissuematrix. Mast cells can become hyper-plastic and hyperproliferative in masto-cytosis (see Chap. 150). Mast cells aresecretory cells that are responsible forimmediate-type hypersensitivity reac-tion in the skin and are involved in theproduction of subacute and chronic in-flammatory disease. They synthesizesecretory granules composed of hista-mine, heparin, tryptase, chymase, car-

boxypeptidase, neutrophil chemotacticfactor, and eosinophilic chemotactic fac-tor of anaphylaxis, which are mediatorsin these processes.

The dermal dendrocyte is a stellate,dendritic, or sometimes spindle-shaped,highly phagocytic fixed connective tis-sue cell in the dermis of normal skin.They are a subset of antigen-presentingmacrophages or a distinct lineage thatoriginates in the bone marrow. Similarto many other bone marrow-derivedcells, dermal dendrocytes express factorXIIIa and CD45, and they lack typicalmarkers of fibroblasts (e.g., Te-7). Thesecells are particularly abundant in thepapillary dermis and upper reticular der-mis, frequently in the proximity of ves-sels of the subpapillary plexus. Dermaldendrocytes function in the afferentlimb of an immune response (see Chap.10). They are also likely the cell of ori-gin of a number of benign fibrotic prolif-erative conditions in the skin, such asdermatofibromas and fibroxanthomas(see Chap. 64).

CUTANEOUS VASCULATURE

Blood Vessels

(See Chap. 163)

The blood vessels of the skin providenutrition for the tissue, but in addition,they are involved in temperature andblood pressure regulation, wound repair,and numerous immunologic events.

67

The microcirculatory beds in skinprogress from arterioles to precapillarysphincters. Extending from the sphinc-ters are arterial and venous capillaries,which become postcapillary venules,and finally, collecting venules. Whencompared with vasculature of otherorgans, the vessels of skin are adaptedto shearing forces, as they have thickwalls supported by connective tissueand smooth muscle cells. Special cells,known as

veil cells

, surround the cutane-ous microcirculation, defining a domainfor the vessels within the dermis whileremaining separate from the vesselwalls.

The rich vascular network of the skinis located at boundaries within the der-mis and supplies the epidermal append-ages (see Fig. 163-2). The vessels thatsupply the dermis branch from muscu-locutaneous arteries that penetrate thesubcutaneous fat and enter the deep re-ticular dermis. At this point, they are or-ganized into a horizontal arteriolarplexus. From this plexus, ascending arte-rioles extend toward the epidermis.These arterioles contain two layers of

smooth muscle cells, as well as peri-cytes, a second type of contractile cell ofthe vessel wall. At the junction betweenthe papillary and reticular dermis, termi-nal arterioles form the subpapillaryplexus. The arterioles at this level haveonly a single layer of smooth musclecells, organized in a manner to suggestthat they function as precapillarysphincters. Capillary loops then extendfrom the terminal arterioles of theplexus into the papillary dermis. At theapex of each capillary loop is the thin-nest portion, in which both the endo-thelium and basal lamina of the vesselsare attenuated, allowing for transport ofmaterial out of the capillary. The extra-papillary descending limbs of capillaryloops are venous capillaries that draininto venous channels of the subpapillaryplexus that lies above and below the ar-teriolar vascular plexus. The postcapil-lary venules of the subpapillary plexusare physiologically important compo-nents of the microcirculation. They areresponsive to mediators such as hista-mine, developing gaps between adja-cent endothelial cells that allow for theextravasation of fluid and cells, and aretherefore often the sites of inflamma-tory cells during these responses.

Certain regions of the skin, such asthe palms and soles, contain direct con-nections between arterial and venouscirculation as potential shunts aroundcongested capillary beds. These sitesconsist of an ascending arteriole (a glo-mus body), which is modified by threeto six layers of smooth muscle cells andhas associated sympathetic nerve fibers.The glomus can close completely whenthe blood pressure is below a criticallevel.

In the adult, the cutaneous vascula-ture normally remains quiescent, withthe exception of certain hair cycle–de-pendent angiogenesis during anagen.Quiescence of vessels is in part due toinhibition of angiogenesis in the dermalmatrix by factors such as thrombospon-din. Pathogenic stimuli sometimes re-sult in secondary angiogenesis, from tu-mors or during wounding. One of thekey mediators of such angiogenesis isvascular endothelial growth factor(VEGF), often secreted by tumors or bykeratinocytes (see Chap. 163).

68,69

Numerous disorders can manifestthemselves within the cutaneous vascu-lature. Leukocytoclastic vasculitis (cuta-neous necrotizing venulitis) occurswithin the venules in response to anumber of potential pathogenic mecha-nisms (see Chap. 164). Stasis dermatitis,

CHAPTER 7

■


65

urticaria, polyarteritis nodosa, thrombo-sis, and thrombophlebitis all affect ves-sels in the skin, of different sizes, someby occlusion of vessels (vasculopathy)and others by inflammation of the ves-sels (vasculitis).

Lymphatics

The lymph channels of the skin are im-portant in regulating pressure of the in-terstitial fluid by resorption of fluid re-leased from vessels and in clearing thetissues of cells, proteins, lipids, bacteria,and degraded substances.

70,71

The ves-sels begin in blind-ending initial lym-phatics (also known as

lymphatic capillar-ies

, prelymphatic tubules, and terminal orperipheral lymphatics) in the papillary der-mis. They drain into a horizontal plexusof larger lymph vessels located deep tothe subpapillary venous plexus. A verti-cal system of lymphatics then carriesfluid and debris through the reticulardermis to another deeper collectingplexus at the reticular dermis–hypoder-mis border.

Lymph flow within the skin dependson movements of the tissue caused byarterial pulsations and larger-scale mus-cle contractions and movement of thebody, with backflow prevented by bi-cuspid-like valves within the vessels.

Although lymphatic vessels are onlyseen with difficulty on histologic sec-tion, because they are often collapsed inskin, they are composed of a large lu-men and a thinner wall than blood ves-sels, with endothelium, discontinuousbasal lamina, and elastic fibers. Molecu-lar characterization of these vessels hasidentified Prox1, VEGFR-3, and LYVE-1as specific markers of lymphatic charac-ter, and one of the most heavily-studiedlymphangiogenic molecules is VEGF-C(see Chap. 163).

Certain pathologic conditions in-volve or highlight the lymphatic vessels,such as lymphedema, lymphangiomacircumscriptum, and stasis dermatitis.The importance of lymphatics in theprogression and spread of cancer is alsobecoming more clear, as melanoma cellsdestroy endothelial cells of the initiallymphatics to gain entry to the lymphcirculation, and recent studies haveshown that tumors themselves can pro-mote lymphangiogenesis as part of theirearly program on the way to metasta-sis.68 The discovery of the molecular de-fects in hereditary lymphedemas [type IMilroy disease VEGFR-3,72–74 type IIlymphedema praecox, lymphedema dis-tichiasis, lymphedema and yellow nails,

and lymphedema and ptosis MFH1(forkhead transcription factor, FoxC2)75]has implicated the VEGFR-3 and FoxC2in lymphatic development.

CUTANEOUS NERVES AND RECEPTORS

(See Chaps. 101 and 102) The nerve net-works of the skin contain somatic sen-sory and sympathetic autonomic fi-bers.76 The sensory fibers alone (freenerve endings) or in conjunction withspecialized structures (corpuscular re-ceptors) function as receptors of touch,pain, temperature, itch, and mechanicalstimuli. The density and types of recep-tors are regionally variable, accountingfor the variation in acuity at differentsites of the body. Receptors are particu-larly dense in hairless areas such as theareola, labia, and glans penis. Sympa-thetic motor fibers are codistributedwith the sensory nerves in the dermisuntil they branch to innervate the sweatglands, vascular smooth muscle, the ar-rector pili muscle of hair follicles, andsebaceous glands.

The nerves of skin branch from mus-culocutaneous nerves that arise segmen-tally from spinal nerves. The pattern ofnerve fibers in skin is similar to the vas-cular patterns. That is, nerve fibers forma deep plexus, then ascend to a superfi-cial, subpapillary plexus.

Free nerve endings include the peni-cillate and papillary nerve fibers and arethe most widespread and importantsensory receptors of the body. In hu-mans, they are ensheathed by Schwanncells and a basal lamina. Free nerve end-ings are particularly common in thepapillary dermis; the basal lamina of thefiber may merge with the lamina densaof the basement membrane zone.

The penicillate fibers are the primarynerve fibers found sub-epidermally inhaired skin. These are rapidly adaptingreceptors that function in the perceptionof touch, temperature, pain, and itch. Be-cause of overlapping innervation, dis-crimination tends to be generalized inthese regions. On the other hand, freenerve endings present in non-haired,ridged skin, such as the palms and soles,project individually without overlappingdistribution. These receptors are thoughtto function in fine discrimination.

Papillary nerve endings are found atthe orifice of a follicle. These branchesfrom nerves that innervate the deeperlevels of the follicle are thought to beparticularly receptive to cold sensation.Hair follicles also contain other recep-

tors, from myelinated stem axons in thedeep dermal plexus, thought to be slow-adapting receptors that respond to thebending or movement of hairs. Cholin-ergic sympathetic fibers en route to theeccrine sweat gland and adrenergic andcholinergic fibers en route to the arrec-tor pili muscle are carried along with thesensory fibers in the hair basket.

Free nerve endings are also associatedwith individual Merkel cells. Merkelcell–nerve complexes are described by avariety of names (touch domes, hederiformendings, Iggo’s capsule, Pinkus corpuscles,Haarscheibe), depending on their compo-sition and location. In haired skin, touchdomes are associated with hair follicles.In palmoplantar skin, these complexesare found at the site where the eccrinesweat duct penetrates a glandular epi-dermal papilla.

Corpuscular receptors, both Meiss-ner’s and Pacinian, contain a capsule andinner core and are composed of bothneural and non-neural components. Thecapsule is a continuation of the perineu-rium, and the core includes the nerve fi-ber surrounded by lamellated wrap-pings of Schwann cells. Meissner’scorpuscles are elongated or ovoid mech-anoreceptors located in the dermal pa-pillae of digital skin and oriented verti-cally toward the epidermal surface (Fig.7-8). One to six axons enter the corpus-cle, ramify extensively, and terminate inbulboid endings that are surrounded bylamellae.

The Pacinian corpuscle lies in thedeep dermis and subcutaneous tissue of

� FIGURE 7-8 Meissner’s corpuscle. Note thecapsule and inner core located in the dermal papil-lae. These collections of cells serve as mechanore-ceptors. (Used with permission from O. Kovich, MD.)

SECTION 3■



66

skin that covers weight-bearing surfacesof the body. It has a characteristic cap-sule and lamellar wrappings (Fig. 7-9).The perineural capsule is organized into30 or more concentric layers of cells andfibrous connective tissue. The middlesubcapsular zone is composed of col-lagen and fibroblasts, and the inner coreconsists of Schwann cell-derived hemi-lamellae: flattened semicircles that alter-nate with those of the opposite side. Pa-cinian corpuscles serve as rapidlyadapting mechanoreceptors that re-spond to vibrational stimuli.

HYPODERMIS (SUBCUTIS)The tissue of the hypodermis insulatesthe body, serves as a reserve energy sup-ply, cushions and protects the skin, andallows for its mobility over underlyingstructures. It has a cosmetic effect inmolding body contours. The boundarybetween the deep reticular dermis andthe hypodermis is an abrupt transitionfrom a predominantly fibrous dermalconnective tissue to a primarily adiposesubcutaneous one (see Fig. 6-1, Chap.6). Despite this clear distinction ana-tomically, the two regions are stillstructurally and functionally integratedthrough networks of nerves and vesselsand through the continuity of epidermalappendages. Actively growing hair folli-cles span the dermis and extend into thesubcutaneous fat, and the apocrine andeccrine sweat glands are normally con-fined to this depth of the skin.

Adipocytes form the bulk of the cellsin the hypodermis.77,78 They are orga-nized into lobules defined by septa of fi-brous connective tissue. Nerves, vessels,and lymphatics are located within thesepta and supply the region. The syn-thesis and storage of fat continuesthroughout life by enhanced accumula-tion of lipid within fat cells, proliferationof existing adipocytes, or by recruit-ment of new cells from undifferentiatedmesenchyme. The hormone leptin, se-creted by adipocytes, provides a long-term feedback signal regulating fatmass. Leptin levels are higher in subcu-taneous than omental adipose, suggest-ing a role for leptin in control of adiposedistribution as well.

The importance of the subcutaneoustissue is apparent in patients withWerner syndrome (see Chap. 139), inwhich subcutaneous fat is absent in le-sion areas over bone, or with sclero-derma (see Chap. 158), where the sub-cutaneous fat is replaced with densefibrous connective tissue. Such regions

in Werner patients ulcerate and healpoorly. The skin of patients with sclero-derma is taut and painful. In the heredi-tary and acquired lipodystrophies, lossof subcutaneous fat disrupts glucose, tri-glyceride, and cholesterol regulation,and causes significant cosmetic alter-ation, increasing the interest in possiblehormonal therapy for these disorders(see Chap. 69).79 The subcutaneous tis-sue is involved in different inflamma-tory conditions, and these are discussedin Chap. 68.

DEVELOPMENT OF SKINSignificant advances in the understand-ing of the molecular processes responsi-ble for the development of the skin havebeen made over the last several years.Such advances increase the understandingof clinicopathologic correlation amongsome inherited disorders of the skin andallow for the early diagnosis of such dis-eases.80,81 The developmental progres-sion of various components of the skinis well documented, and a time line in-dicating the events that occur duringembryonic and fetal development isprovided (Table 7-4).82,83 Of note, theestimated gestational age (EGA) is used

throughout this chapter; this system re-fers to the age of the fetus, with fertili-zation occurring on day 1. To avoid con-fusion, it should be pointed out thatobstetricians and most clinicians defineday 1 as the first day of the last men-strual period (menstrual age), in whichfertilization occurs on approximatelyday 14. Thus, the two dating systemsdiffer by approximately 2 weeks, suchthat a woman who is 14 weeks preg-nant (menstrual age) is carrying a 12-week-old fetus (EGA).

Conceptually, fetal skin developmentcan be divided into three distinct buttemporally overlapping stages, those ofspecification, morphogenesis, and dif-ferentiation. These stages roughly corre-spond to the embryonic period (0 to 60days), the early fetal period (2 to 5months), and the late fetal period (5 to 9months) of development. The earlieststage, specification, refers to the processby which the ectoderm lateral to theneural plate is committed to becomeepidermis, and subsets of mesenchymaland neural crest cells are committed toform the dermis. It is at this time thatpatterning of the future layers and spe-cialized structures of the skin occurs, of-ten via a combination of gradients of

� FIGURE 7-9 Pacinian corpus-cle. Note the characteristic peri-neural capsule, likened to theappearance of an “onion-skin.”Pacinian corpuscles serve as rap-idly adapting mechanoreceptorsthat respond to vibrational stimuli.(Used with permission from O.Kovich, MD.)

TABLE 7-4Timing of the Major Events in the Embryogenesis of Human Skina

FIRST TRIMESTER

SECOND TRIMESTER

THIRD TRIMESTER

1 2 3 4 5 6 7 8 9Epidermis

Appearance of epidermal cell layersStratum basale XPeriderm XStratum intermedium XStratum granulosum XStratum corneum XPeriderm disappearance X

(continued)

CHAPTER 7■


67

proteins and cell–cell signals. The sec-ond stage, morphogenesis, is the pro-cess by which these committed tissuesbegin to form their specialized struc-tures, including epidermal stratification,epidermal appendage formation, subdi-vision between the dermis and subcutis,and vascular formation. The last stage,differentiation, denotes the process bywhich these newly specialized tissuesfurther develop and assume their ma-ture forms. Table 7-5 integrates specifi-cation, morphogenesis, and differentia-tion with skin morphology and geneticdiseases.

For simplification and greater clarity,the stages of development of the epider-mis—dermis and hypodermis, dermal–epidermal junction, and epidermal ap-pendages—are presented sequentially.

Epidermis

EMBRYONIC DEVELOPMENT During thethird week after fertilization, the humanembryo undergoes gastrulation, a com-plex process of involution and cell redis-tribution that results in the formation ofthe three primary embryonic germ layers:ectoderm, mesoderm, and endoderm.Shortly after gastrulation, ectoderm fur-ther subdivides into neuroectoderm andpresumptive epidermis. The specifica-tion of the presumptive epidermis isbelieved to be mediated by the bonemorphogenetic proteins (BMPs). Laterduring this period, BMPs again appearto play a critical role, along with En-grailed-1 (En1), in specifying the volarversus interfollicular skin.84–86 By 6weeks EGA, the ectoderm that coversthe body consists of basal cells and su-perficial periderm cells.

The basal cells of the embryonic epi-dermis differ from those of later devel-opmental stages. Embryonic basal cellsare more columnar than fetal basal cells,and they have not yet formed hemides-mosomes. Although certain integrins(e.g., α6β4) are expressed in these cells,they are not yet localized to the basalpole of the cells. Before the formation ofhemidesmosomes and desmosomes, in-tercellular attachment between individ-ual basal cells appears to be mediatedby adhesion molecules such as E- and P-cadherin, which have been detected onbasal cells as early as 6 weeks EGA. Ker-atins K5 and K14, proteins restricted todefinitive stratified epithelia, are ex-pressed even at these early stages of epi-dermal formation.

At this stage, periderm cells form a“pavement epithelium.” These cells are

TABLE 7-4Timing of the Major Events in the Embryogenesis of Human Skina (Continued)

FIRST TRIMESTER

SECOND TRIMESTER

THIRD TRIMESTER

1 2 3 4 5 6 7 8 9Epidermal cell junctions

Desmosomes without associated keratin filaments XDesmosomes with associated keratin filaments XTight junctions XHemidesmosomes X

AntigensPemphigus and pemphigoid antigen XA, B, H blood group antigens X

Immigrant cellsPresent, but type uncertain XMelanocyte

With premelanosomes XWith melanosomes that synthesize melanin XTransfer of melanosomes to keratinocytes XLangerhans cells X

Merkel cells XEpidermal appendages

Pilosebaceous apparatusHair follicle development begins XHair exposed on skin surface and patterns estab-lished on the scalp

X

Sebaceous gland primordium XSebaceous gland function XApocrine gland primordium XApocrine gland function X

Eccrine sweat glands (trunk)Duct and gland patent and functioning X

NailsNail fold and establishment of matrix primordium XNail plate forms X

Keratinization of epidermis and appendagesDorsal ridge of presumptive nail XNail plate XPalmar/plantar surface of digits XHair cone XHair tract XHair shaft XSebaceous duct XEccrine sweat gland duct (intraepidermal) XApocrine duct X

DermisStructural organization

Papillary and reticular regions established XDermal papillae established XDermal-subcutaneous boundary XPanniculus adiposus established X

Connective tissue matrix proteinsCollagen present by ultrastructural observation XCollagen present by biochemical analysis

Type I ? XType III ? X

Elastic microfibrils ? XElastic matrix XElastic fibrous networks X

aData are representative of the trunk unless stated otherwise.

SECTION 3■



68

TABLE 7-5Proteins Involved in Cutaneous Development and Differentiation

EPIDERMIS DERMIS/SQ DEJ APPENDAGES

Specification • BMPs • Lmx-1B (Nail-patella syndrome)

• Not known • Lmx-1B• Engrailed-1 • Wnt7a• (Aplasia cutis) • Engrailed-1 • NGFR

• Wnt7a

Morphogenesis • p63 • Lamin A/C, ZMPE STE24 (restrictive der-mopathy)

• Laminin 1 • Ectodysplasin A (EDA) (X-linked hypohidrotic ectodermal dysplasia)• Dlx-3 (Tricho-dento-osseous syn-

drome)• Collagen IV• Heparin sulfate • Connexin 30 (Autosomal hypo-

hidrotic ectodermal dysplasia, type 2)• PTEN (Proteus syn-drome)

• Proteoglycans• EDA receptor (Autosomal hypo-

hidrotic ectodermal dysplasia, type 3)• (Focal dermal hypopla-sia/Goltz syndrome) • MSX1 (Witkop syndrome/tooth and

nail syndrome)• c-kit (Piebaldism)• PAX-3 (Waardenburg type 1,3)• p63 (Hay-Wells/AEC, EEC)• Beta-catenin (pilomatricomas)• Shh• Wnt• BMPs• FGF5• LEF1• Dlx-3

Differentiation • Structural proteins • Capillary morphogen-esis protein-2 (juve-nile hyaline fibromato-sis, infantile systemic hyalinosis)

• BPAG2 • Hair• K5, K14 (EB simplex) • Collagen VII • BMPs• Plectin (EB with MD) • α6β4 integrin • Hoxc13• BPAG2 (GABEB) • Laminin 5

(junctional EB)• Foxn1

• α6β4 integrin (EB with PA) • Plakoglobin (Naxos disease)• K1, K10 (BCIE) • Collagen I, a1, or a2

(osteogenesis imper-fecta)

• Plakophilin/desmosomal band 6 (ectodermal dysplasia, skin fragility syndrome)

• K1, K9 (Vorner, Unna-Thost, Greither)• Loricrin (NCIE, Vohwinkel, progres-

sive symmetric erythrokeratodermia) • Collagen V, a1, or a2 (Ehlers-Danlos syn-drome)

• Hairless (papular atrichia)• Filaggrin (ichthyosis vulgaris) • Nail

• Post-translational modifiers • K6a, K16 (pachyonychia con-genita type I)• LEKTI (Netherton) • Collagen VII (dystro-

phic EB)• Transglutaminase 1 (lamellar ichthyosis; NCIE)

• K6b, K17 (pachyonychia con-genita type II, steatocystoma multiplex)

• Fibrillin (Marfan syn-drome)• Phytanoyl CoA hydroxylase

(Refsum) • Elastin (cutis laxa) • Plakophilin• Fatty aldehyde dehydrogenase

(Sjögren-Larsson)• MRP6 (PXE) • Sebaceous gland• Tie-2 (inherited venous

malformations)• Blimp-1

• Steroid sulfatase/arylsulfatase C (X-linked ichthyosis)

• K6b, K17• Endoglin, activin

receptor-like kinase 1 (HHT/Osler-Weber-Rendu)

• Transporter/channel proteins• ABCA12 (harlequin fetus)• Connexin 26 (KID syndrome, palmo-

plantar keratoderma with deafness) • VEGF receptor-3 (hereditary lymphe-dema type I)

• Connexin 30.3 or 31 (erythrokera-toderma variabilis, progressive sym-metric erythrokeratodermia) • MFH1 (hereditary

lymphedema type II)• SERCA2 (keratosis follicularis)• ATP2C1 (Hailey-Hailey disease) • Prox-1

• Signal transduction proteins • LYVE-1• Patched (basal cell nevus syndrome)

AEC = ankyloblepharon-ectodermal dysplasia-clefting; BCIE = bullous congenital ichthyosiform erythroderma; BMPs = bone morphogenetic proteins; BPAG = bullous pemphigoid antigen; EB = epidermolysis bullosa; EEC = ectrodactyly-ectodermal dysplasia-clefting; GABEB = generalized atrophic benign epidermolysis bullosa; HHT = hereditary hemorrhagic telangiectasia; K = keratin; KID = keratitis-ichthyosis-deafness; MD = multiple dystrophy; NCIE = non-bullous congenital ichthyosiform erythroderma; NGFR = nerve growth factor receptor; PA = pyloric atresia; PXE = pseudoxanthoma elasticum.Protein names are indicated in boldface. Associated diseases/genodermatoses are listed in parentheses. Multiple names for the same protein or syndrome are sepa-rated by /. Genes and associated diseases can be found in Online Mendelian Inheritance in Man (OMIM) at http://www.ncbi.nlm.nih.gov/omim.

CHAPTER 7■


69

embryonic epidermal cells that arelarger and flatter than the underlyingbasal cells. Apical surfaces contact theamniotic fluid and are studded with mi-crovilli. Connections between peridermcells are sealed with tight junctionsrather than desmosomes. By the end ofthe second trimester, these cells aresloughed and eventually form part ofthe vernix caseosa. Like stratified epi-thelial cells, periderm cells express K5and K14, but they also express simpleepithelial keratins K8, K18, and K19.

Aplasia cutis (see Chap. 106) may re-flect focal defects in either epidermalspecification or development caused bysomatic mosaicism, or mutations thatoccur postzygotically. The moleculardefect for this disorder is not known,however. The fact that few genetic dis-eases have been described in which epi-dermal specification or morphogenesisare defective likely reflects the fact thatsuch defects would be incompatiblewith survival.

EARLY FETAL DEVELOPMENT (MORPHOGEN-ESIS) By the end of 8 weeks of gestation,hematopoiesis has switched from the ex-traembryonic yolk sac to the bone mar-row, the classical division between em-bryonic and fetal development. By thistime, the epidermis begins its stratifica-tion and formation of an intermediatelayer between the two pre-existing celllayers. The cells in this new layer are sim-ilar to the cells of the spinous layer in ma-ture epidermis. Like spinous cells, theyexpress keratins K1/K10 and the desmo-somal protein desmoglein-3. The cells arestill highly proliferative and, during thisperiod of development, they evolve into amultilayer structure that will eventuallyreplace the degenerating periderm.

Expression of the p63 gene plays a crit-ical role in the proliferation and mainte-nance of the basal layer cells. Epidermalstratification does not occur in mice defi-cient for p63.87–89 In humans, althoughno null mutations have been isolated,partial loss of p63 function mutationshave been identified in ankyloblepharon,ectodermal dysplasia, and cleft lip/palatesyndrome (Hay-Wells syndrome) as wellas ectrodactyly, ectodermal dysplasia,and cleft lip/palate syndrome (see Chap.143).90–92 The pre-existing basal cell layeralso undergoes morphologic changes atthis time, becoming more cuboidal andexpressing new keratin genes, K6, K8,K19, and K6/K16, that are usually ex-pressed in hyperproliferative tissues. Thebasal layer also begins to elaborate pro-teins that will ultimately anchor them to

the developing basal lamina (see Dermal-Epidermal Junction), including hemides-mosomal proteins BPAG1, BPAG2, andcollagens V and VII (see Chaps. 51, 54,and 60).

Embryonic lines of ectodermal forma-tion are revealed in mosaic disordersthat follow the lines of Blaschko, includ-ing congenital, nevoid, and acquiredconditions.93–95 Molecular demonstrationof genetic mosaicism has been reportedfor a number of X-linked disorders (re-viewed in ref. 94), as well as epidermalnevi in epidermolytic hyperkeratosis.96

LATE FETAL DEVELOPMENT (DIFFERENTIA-TION) Late fetal development reveals thefurther specialization and differentia-tion of keratinocytes in the epidermis. Itis at this time that the granular and stra-tum corneal layers are formed, andthe rudimentary periderm is sloughed.Keratinization of the surface epidermisis a process of keratinocyte terminal dif-ferentiation which begins at 15 weeksEGA. The granular layer becomes prom-inent, and important structural proteinsare elaborated in the basal layer cells.The hemidesmosomal proteins plectinand α6β4 integrin are expressed andcorrectly localized at this time. Muta-tions in these genes result in variousbullous genodermatoses (reviewed inChap. 60). The more superficial cells un-dergo further terminal differentiation,and the keratin-aggregating protein fil-aggrin is expressed at this time.

The formation of the cornified enve-lope is a late feature of differentiatingkeratinocytes, and it relies on a numberof different modifications to create animpermeable barrier. Enzymes such astransglutaminase, LEKTI (encoded bythe gene SPINK-5), phytanoyl coen-zyme A reductase, fatty aldehyde dehy-drogenase, and steroid sulfatase are allimportant in the elaboration of thecornified envelope and mature lipid bar-rier, and defects in these enzymes canlead to abnormal epidermal barrier for-mation (see Chap. 47).

SPECIALIZED CELLS WITHIN THE EPIDER-MIS The three major nonepidermal celltypes—melanocytes, Langerhans cells,and Merkel cells—can be detectedwithin the epidermis by the end of theembryonic period. Melanocytes are de-rived from the neural crest, a subset ofneuroectoderm cells. Pigment mosa-icism (formerly called hypomelanosis of Itoand linear and whorled hypermelanosis) (seeChap. 73) following the lines of Blaschkomay reflect the migratory paths of mel-

anoblasts, or alternatively, mosaic defectsin pigment transfer from melanocytes tokeratinocytes. The founders of each mel-anoblast clone originate at distinct pointsalong the dorsal midline, traversing ven-trally and distally to take up residence inthe epidermis.

Melanocytes are first seen within theepidermis at 50 days EGA. Melanocytesexpress integrin receptors in vivo and invitro and may use these to migrate to theepidermis during embryonic develop-ment. Migration, colonization, prolifera-tion, and survival of melanocytes in devel-oping skin depend on the cell surfacetyrosine kinase receptor, c-kit, and its li-gand, stem cell factor.97,98 Melanin be-comes detectable between 3 and 4 monthsEGA, and by 5 months, melanosomes be-gin to transfer pigment to keratinocytes.Many genetic disorders of pigmentationhave been characterized and are presentedin detail in Chaps. 71, 73, and 144. In theadult, a pool of melanocyte precursor cellsresides in the upper permanent portion ofthe hair follicle, capable of producing ma-ture melanocytes.97,99,100

Langerhans cells, another immigrantpopulation, are detectable by 40 daysEGA. They begin to express CD1 on theirsurface and to produce their characteristicBirbeck granules by the embryonic–fetaltransition. By the third trimester, most ofthe adult numbers of Langerhans cellswill have been produced.

Merkel cells, as described earlier inthe chapter (see Nonkeratinocytes ofthe Epidermis), reside in the epidermis.They are first detectable in the volar epi-dermis of the 11- to 12-week EGA hu-man fetus. The embryonic derivation ofthis population of cells is controversial.Evidence for in situ differentiation ofMerkel cells from epidermal ectodermversus immigration of Merkel cells fromneural crest is supported by studies inwhich 8- and 11-week EGA fetal volarskin that lacked Merkel cells was trans-planted to the nude mouse. Tissue har-vested 8 weeks later contained an abun-dance of human K18-positive Merkelcells within the epidermis, suggestingthat the cells differentiated within thegrafted tissue.101 On the other hand,more recent lineage tracing studies us-ing a neural-crest specific label have sug-gested that Merkel cells indeed have aneural crest origin.51

Dermal and Subcutaneous Development

The origin of the dermis and subcutane-ous tissue is more diverse than that of

SECTION 3■



70

the epidermis, which is exclusively ec-todermally derived. The embryonic tis-sue that forms the dermis depends onthe specific body site.102,103 Dermalmesenchyme of the face and anteriorscalp is derived from neural crest ecto-derm. The limb and ventral body wallmesenchyme is derived from the lateralplate mesoderm. The dorsal body wallmesenchyme derives from the der-momyotomes of the embryonic somite.LIM homeobox transcription factor 1beta (Lmx1B) and Wnt7a are importantin the specification of the dorsallimb.104–106 En1 and BMPs, on the otherhand, specify the volar (ventral) limbmesenchyme (see Table 7-5).88,105

The embryonic dermis, in contrast tothe mature dermis, is cellular and amor-phous, with few organized fibers. Themature dermis contains a complex meshof collagen and elastic fibers embedded ina matrix of PGs, whereas the embryonicmesenchyme contains a large variety ofpluripotent cells in a hydrated gel that isrich in hyaluronic acid. These mesenchy-mal cells are thought to be the progeni-tors of cartilage-producing cells, adiposetissue, dermal fibroblasts, and intramem-branous bone. Dermal fibers exist as finefilaments but not thick fibers. The pro-tein components of the future elastin andcollagen fibers are synthesized duringthis period but not assembled. At thispoint, there is no obvious separation be-tween cells that will become musculo-skeletal elements and those that will giverise to the skin dermis.

Although there is no known inheriteddisorder of dermal development, certainconditions, such focal dermal hypopla-sia (Goltz syndrome) and Proteus syn-drome, exhibit focal defects, probably aresult of genetic mosaicism affectinggenes important in this process (seeChap. 139). Mutations causing a globaldefect in this process would likely be in-compatible with life.

The superficial mesenchyme becomesdistinct from the underlying tissue bythe embryonic–fetal transition (about 60days EGA). By 12 to 15 weeks, the retic-ular dermis begins to take on its charac-teristic fibrillar appearance in contrast tothe papillary dermis, which is morefinely woven. Large collagen fibers con-tinue to accumulate in the reticular der-mis, as well as elastin fibers, beginningaround mid-gestation and continuinguntil birth. By the end of the second tri-mester, the dermis has changed from anon-scarring tissue to one that is capa-ble of forming scars. As the dermis ma-tures, it also becomes thicker and well

organized, such that at birth, it resem-bles the dermis of the adult, although itis still more cellular.

Many well-known clinical syndromesand molecules have been discoveredthat affect this final stage of dermal dif-ferentiation. These diseases include dys-trophic EB (a collagen VII defect) (seeChap. 60), Marfan syndrome (a defectin fibrillin), Ehlers-Danlos syndrome(collagen V), cutis laxa (elastin), PXE, he-reditary hemorrhagic telangiectasia (Os-ler-Weber-Rendu syndrome), and osteo-genesis imperfecta (see Chap. 139). Inmany of these cases, the specific geneticdefect helps to define the many differ-ent manifestations of these diseases, al-though in certain cases (e.g., PXE), theidentity of the gene does not readily ex-plain the mechanism of disease (seeChap. 139).

SPECIALIZED COMPONENTS OF THE DER-MIS Blood Vessels and Nerves. Cutaneousnerves and vessels begin to form earlyduring gestation, but they do not evolveinto those of the adult until a few monthsafter birth. The process of vasculogenesisrequires the in situ differentiation ofthe endothelial cells at the endoderm–mesoderm interface. Originally, horizon-tal plexuses are formed within the sub-papillary and deep reticular dermis, whichare interconnected by groups of verticalvessels. This lattice of vessels is in placeby 45 to 50 days EGA.

At 9 weeks EGA, blood vessels areseen at the dermal–hypodermal junc-tion. By 3 months, the distinct networksof horizontal and vertical vessels haveformed. By the fifth month, furtherchanges in the vasculature derive frombudding and migration of endotheliumfrom pre-existing vessels, the process ofangiogenesis. Depending on the bodyregion, gestational age, and presence ofhair follicles and glands, this pattern canvary with blood supply requirements.

Defects in vascular development havebeen described, as in the Klippel-Trénaunay and Sturge-Weber syndromes(see Chap. 173). In the Klippel-Trénaunay syndrome, unilateral cutane-ous vascular malformations develop,with associated venous varicosities,edema, and hypertrophy of associatedsoft tissue and bone. In Sturge-Webersyndrome, many cutaneous capillarymalformations are seen in the lips,tongue, nasal, and buccal mucosae.Some familial defects in vascular forma-tion result from mutations in the geneencoding Tie-2 receptor tyrosine kinase.Capillary malformations seen in heredi-

tary hemorrhagic telangiectasia havebeen linked to mutations in trans-forming growth factor-β–binding pro-teins, endoglin, and activin receptor-like kinase 1.

Lymphatics. Accumulating evidence sug-gests that lymphatics originate from en-dothelial cells that bud off from veins.The pattern of embryonic lymphaticvessel development parallels that ofblood vessels. Detailed molecular stud-ies into the development of lymphaticsduring embryogenesis and fetal devel-opment have long been hampered bythe lack of lymphatic-specific markers.However, recent studies have identifiednew genes that appear to be specific forsome of the earliest lymphatic precur-sors. LYVE-1 and Prox-1 are genes con-sidered to be critical for earliest lym-phatic specification, whereas VEGF-R3and SLC may be important in later lym-phatic differentiation.107

Nerves. The development of cutaneousnerves parallels that of the vascular sys-tem in terms of patterning, maturation,and organization. Nerves of the skinconsist of somatic sensory and sympa-thetic autonomic fibers, which are pre-dominantly small and unmyelinated. Asthese nerves develop, they become my-elinated, with associated decrease in thenumber of axons. This process maycontinue as long as puberty.

Subcutis

As mentioned in Specialized Compo-nents of the Dermis, by 50 to 60 daysEGA, the hypodermis is separated fromthe overlying dermis by a plane of thin-walled vessels. Toward the end of thefirst trimester, the matrix of the hypo-dermis can be distinguished from themore fibrous matrix of the dermis. Bythe second trimester, adipocyte precur-sors begin to differentiate and accumu-late lipids. By the third trimester, fatlobules and fibrous septae are found toseparate the mature adipocytes. Themolecular pathways that define thisprocess are currently an area of intenseinvestigation. Although few regulatorsimportant in embryonic adipose specifi-cation and development have beenidentified, several factors critical forpreadipocyte differentiation have beendemonstrated, including leptin, a hor-mone important in fat regulation, and theperoxisome proliferator–activated recep-tor family of transcription factors.77,108

CHAPTER 7■


71

Dermal–Epidermal Junction

The dermal–epidermal junction is aninterface where many inductive interac-tions occur that result in the specificationor differentiation of the characteristics ofthe dermis and epidermis. This zone in-cludes specialized basement membrane,basal cell extracellular matrix, the basal-most portion of the basal cells, and thesuperficial-most fibrillar structures of thepapillary dermis. Both the epidermis anddermis contribute to this region.

As early as 8 weeks EGA, a simplebasement membrane separates the der-mis from the epidermis and containsmany of the major protein elements com-mon to all basement membranes, includ-ing laminin 1, collagen IV, heparin sulfate,and PGs. Components specific to the cu-taneous basement membrane zone, suchas proteins of the hemidesmosome andanchoring filaments, are first detected atthe embryonic–fetal transition. By theend of the first trimester, or around thetime of late embryonic development, allbasement membrane proteins are inplace. The α6 and β4 integrin subunitsare expressed earlier than most of theother basement membrane components.However, they are not localized to thebasal surface until 9.5 weeks EGA, coinci-dent with the time that the hemidesmo-somal proteins are expressed and hemi-desmosomes are first observed. At thesame time, anchoring filaments (laminin-332) and anchoring fibrils (collagen VII)begin to be assembled. The actual syn-thesis of collagen VII can be detectedslightly earlier, at 8 weeks EGA.

Many congenital blistering disordershave been demonstrated to be a result ofdefects in proteins of the DEJ (for details,see Chaps. 51 and 60). The severity of thedisease, plane of tissue separation, and in-volvement of non-cutaneous tissues de-pend on the proteins involved and thespecific mutations. These genes are im-portant candidates for prenatal testing.

DEVELOPMENT OF SKIN APPENDAGES

Skin appendages, which include hair,nails, and sweat and mammary glands,are composed of two distinct compo-nents: an epidermal portion, which pro-duces the differentiated product, andthe dermal component, which regulatesdifferentiation of the appendage. Dur-ing embryonic development, dermal–epidermal interactions are critical forthe induction and differentiation ofthese structures (Fig. 7-10). Disruption

of these signals often has profound in-fluences on development of skin ap-pendages. This discussion focuses onhair differentiation as a paradigm for ap-pendageal development, because it isthe appendage that has been studiedmost intensely.6,109,110

Hair (See Chap. 84)

Dermal signals are initially responsiblefor instructing the basal cells of the epi-dermis to begin to crowd at regularlyspaced intervals, starting between days75 and 80 on the scalp. This initialgrouping is known as the follicular pla-code or Anlage. Based on molecular local-ization of β-catenin, it has been impli-

cated as a candidate for one of theeffectors of this “dermal signal.”

From the scalp, follicular placode for-mation spreads ventrally and caudally,eventually covering the skin. The pla-codes then signal back to the underlyingdermis to form a “dermal condensate,”which occurs at 12 to 14 weeks EGA.This process is thought to be a balanceof placode promoters and placode inhib-itors.6,109,110 Wnt family signaling mole-cules are proposed to mediate placodepromoting effects via the molecules LEFand β-catenin, as well as fibroblastgrowth factor, transforming growth fac-tor-α, Msx1 and -2, ectodysplasin A(EDA), and EDAR (EDA receptor). BMPfamily molecules, on the other hand, act

� FIGURE 7-10 Appendageal morphogenesis. Through a series of reciprocal epithelial (epidermal)–mesenchymal (dermal) signals, including Wnt, sonic hedgehog (Shh), and Noggin (Nog), appendagessuch as the hair follicle and eccrine gland begin as epidermal invaginations (placodes), which signal theorganization of specialized dermis (dermal condensate). This dermal condensate subsequently signalsthe differentiation of the epidermal downgrowth into the germ, peg, and mature appendageal structure.Bu = bulge; Derm = dermis; Du = duct; Epi = epidermis; Gld = gland.

SECTION 3■



72

as inhibitors of follicle formation. Inmodel systems, ectopic expression of thisfamily of molecules tends to suppress theformation of follicles. In mice, EDAR andβ-catenin expression are required for ex-pression of BMP4 and Sonic hedgehog(Shh), implicating these molecules in earlyfollicular morphogenesis. Furthermore,EDAR may be important for lateral inhibi-tion of cells surrounding the follicles.

Formation of the dermal papilla isthought to be initiated by the “first epi-thelial signal” that is transmitted fromthe follicle epithelium to the underlyingmesenchyme. Molecules proposed to beinvolved in this signaling process in-clude platelet-derived growth factor αpolypeptide and Shh. After the folliculardifferentiation process begins, the der-mis sends another signal to the epithe-lial placode cells to proliferate and in-vade the dermis. The dermal cellsassociated with the follicle then developinto the dermal papilla. The epithelialcells go on to form the inner root sheathand hair shaft of the mature hair follicle.

In addition to the widened bulge at thebase, two other bulges form along thelength of the developing follicle, termedthe bulbous hair peg. The uppermost bulgeis the presumptive sebaceous gland,whereas the middle bulge serves as thesite for insertion of the arrector pili mus-cle. This middle bulge is also the locationof the multipotent hair stem cells, whichare capable of differentiating into any ofthe cells of the hair follicle, and also havethe potential to replenish the entire epi-dermis, as has seen in cases of extensivesurface wounds or burns.

By 19 to 21 weeks EGA, the hair canalhas completely formed and the hairs onthe scalp are visible above the surface ofthe fetal epidermis. They continue tolengthen until 24 to 28 weeks, at whichtime they shift from the active growth(anagen) phase to the degenerative phase(catagen), then to the resting phase (telo-gen) (see Chap. 84). This completes thefirst hair cycle. With subsequent hair cy-cles, hairs increase in diameter andcoarseness. During adolescence, vellushairs of androgen-sensitive areas matureto terminal-type hair follicles.

Sebaceous Glands (See Chap. 77)

Sebaceous glands mature during thecourse of follicular differentiation. Thisprocess begins between 13 and 16weeks EGA, at which point the pre-sumptive sebaceous gland is first visibleas the most superficial bulge of the ma-turing hair follicle. The outer prolifera-

tive cells of the gland give rise to the dif-ferentiated cells that accumulate lipidand sebum. After they terminally differ-entiate, these cells disintegrate and re-lease their products into the upper por-tion of the hair canal. Sebum productionis accelerated in the second and third tri-mesters, during which time maternalsteroids cause stimulation of the seba-ceous glands. Hormonal activity is onceagain thought to influence the produc-tion of increased sebum during adoles-cence, resulting in the increased inci-dence in acne at this age.

Nail Development (See Chap. 87)

Presumptive nail structures begin to ap-pear on the dorsal digit tip at 8 to 10weeks EGA, slightly earlier than the ini-tiation of hair follicle development. Thefirst sign is the delineation of the flatsurface of the future nail bed. A portionof ectoderm buds inward at the proxi-mal boundary of the early nail field, andgives rise to the proximal nail fold. Thepresumptive nail matrix cells, which dif-ferentiate to become the nail plate, arepresent on the ventral side of the proxi-mal invagination. At 11 weeks, the dor-sal nail bed surface begins to keratinize.By the fourth month of gestation, thenail plate grows out from the proximalnail fold, completely covering the nailbed by the fifth month. Mutations inp63 affect nail development in syn-dromes such as ankyloblepharon, ecto-dermal dysplasia, and cleft lip/palatesyndrome, as well as ectrodactyly, ecto-dermal dysplasia, and cleft lip/palatesyndrome. Functional p63 is requiredfor the formation and maintenance ofthe apical ectodermal ridge, an embry-onic signaling center essential for limboutgrowth and hand plate formation.Wnt7a is thought to be important fordorsal limb patterning, and thus nail for-mation. In contrast to follicular develop-ment, Shh is not required for nail plateformation. Also similar to follicular dif-ferentiation, LMX1b and MSX1 are im-portant for nail specification; LMX1band MSX1 are mutated in nail-patellasyndrome and Witkop syndrome, re-spectively.111–113 Hoxc13 appears to bean important homeodomain-containinggene for both follicular and nail appen-dages, at least in murine models.114

Eccrine and Apocrine Sweat Gland Development (See Chap. 81)

Eccrine glands begin to develop on thevolar surfaces of the hands and feet, be-

ginning as mesenchymal pads between55 and 65 days EGA. By 12 to 14 weeksEGA, parallel ectodermal ridges are in-duced, which overly these pads. The ec-crine glands arise from the ectodermalridge. By 16 weeks EGA, the secretoryportion of the gland becomes detect-able. The dermal duct begins aroundweek 16, but the epidermal portion ofthe duct and opening are not completeuntil 2 weeks EGA.

Interfollicular eccrine and apocrineglands, in contrast, do not begin to buduntil the fifth month of gestation. Apo-crine sweat glands usually bud from theupper portion of the hair follicle. By 7months EGA, the cells of the apocrineglands become distinguishable.

Although not much is known with re-gard to the molecular signals responsi-ble for the differentiation of these struc-tures, the EDA, EDAR, En1, andWnt10b genes have been implicated.Hypohidrotic ectodermal dysplasia re-sults from mutations in EDA or theEDAR (see Chap. 143).

KEY REFERENCES

The full reference list for all chapters is available at www.digm7.com.

6. Blanpain C, Fuchs E: Epidermal stemcells of the skin. Annu Rev Cell Dev Biol22:339, 2006

8. Pellegrini G et al: The control of epider-mal stem cells (holoclones) in the treat-ment of massive full-thickness burnswith autologous keratinocytes culturedon fibrin. Transplantation 68:868, 1999

9. Cotsarelis G, Sun TT, Lavker RM:Label-retaining cells reside in the bulgearea of pilosebaceous unit: Implicationsfor follicular stem cells, hair cycle, andskin carcinogenesis. Cell 61:1329, 1990

11. Rochat A, Kobayashi K, Barrandon Y:Location of stem cells of human hair fol-licles by clonal analysis. Cell 76:1063,1994

14. Tumbar T et al: Defining the epithelialstem cell niche in skin. Science 303:359,2004

16. Morris RJ et al: Capturing and profilingadult hair follicle stem cells. Nat Biotech-nol 22:411, 2004

22. Ghazizadeh S, Taichman LB: Multipleclasses of stem cells in cutaneous epi-thelium: A lineage analysis of adultmouse skin. EMBO J 20:1215, 2001

69. Detmar M: Molecular regulation ofangiogenesis in the skin. J Invest Derma-tol 106:207, 1996

82. Holbrook KA: Structure and function ofthe developing human skin, in Physiol-ogy, Biochemistry, and Molecular Biology ofthe Skin, edited by Goldsmith LA. NewYork, Oxford Press, 1991, p 63

83. Loomis CA: Development and morpho-genesis of the skin. Adv Dermatol17:183, 2001

87. Yang A et al: p63 is essential for regen-

CHAPTER 8■

GENETICS IN RELATION TO THE SKIN

73

erative proliferation in limb, craniofacialand epithelial development. Nature398:714, 1999

94. Happle R: X-chromosome inactivation:Role in skin disease expression. ActaPaediatr Suppl 95:16, 2006

97. Nishimura EK et al: Dominant role ofthe niche in melanocyte stem-cell fatedetermination. Nature 416:854, 2002

107. Oliver G, Detmar M: The rediscoveryof the lymphatic system: Old andnew insights into the development

and biological function of the lym-phatic vasculature. Genes Dev 16:773,2002

109. Millar SE: Molecular mechanisms regu-lating hair follicle development. J InvestDermatol 118:216, 2002

C H A P T E R 8

Genetics in Relation to the SkinJohn A. McGrathW. H. Irwin McLean

THE HUMAN GENOME IN DERMATOLOGY

In the 30 years since the first humangene, placental lactogen, was cloned in1977, huge investments in time, money,and effort have gone into disclosing theinnermost workings of the human ge-nome. The Human Genome Project,which began in 1990, has led to se-quence information on more than 3 bil-lion base pairs (bp) of DNA, with identi-fication of most of the estimated 25,000genes in the entire human genome.1 Al-though a few relatively small gaps re-main, the near completion of the entiresequence of the human genome is hav-ing a huge impact on both the clinicalpractice of genetics and on the strategiesused to identify disease-associatedgenes. Laborious positional cloning ap-proaches and traditional functionalstudies are gradually being transformedby the emergence of new genomic andproteomic databases.2 Some of the ex-citing challenges that clinicians and ge-neticists now face are determining thefunction of these genes and defining dis-ease associations, and, with relevance topatients, correlating genotype with phe-notype. Nevertheless, many discoveriesare already influencing how clinical ge-netics is practiced throughout theworld, particularly for patients and fam-ilies with rare, monogenic inherited dis-orders. The key benefits of dissection ofthe genome thus far have been the doc-umentation of new information aboutdisease causation, improving the accu-racy of diagnosis and genetic counsel-ing, and making DNA-based prenataltesting feasible.3 Indeed, the genetic ba-sis of more than 2000 inherited singlegene disorders has now been deter-mined, of which about 25 percent have

a skin phenotype. These discoveries,therefore, have direct relevance to der-matologists and their patients. Recently,studies in rare inherited skin disordershave also led to new insight into thepathophysiology of more commoncomplex trait skin disorders.4 This newinformation is expected to have signifi-cant implications for the developmentof new therapies and management strat-egies for patients. For the dermatologist,therefore, understanding the basic lan-guage and principles of clinical and mo-lecular genetics has become a vital partof day-to-day practice. The aim of thischapter is to provide an overview of keyterminology in genetics that is clinicallyrelevant to the dermatologist.

THE HUMAN GENOMENormal human beings have a large com-plex genome packaged in the form of 46chromosomes. These consist of 22 pairsof autosomes, numbered in descendingorder of size from the largest (chromo-some 1) to the smallest (chromosome22), in addition to two sex chromo-somes, X and Y. Females possess twocopies of the X chromosome, whereasmales carry one X and one Y chromo-some. The haploid genome consists ofabout 3.3 billion bp of DNA. Of this,only about 1.5 percent corresponds toprotein-encoding exons of genes. Apartfrom genes and regulatory sequences,perhaps as much as 97 percent of the ge-nome is of unknown function, often re-ferred to as “junk” DNA. Caution shouldbe exercised in labeling the non-codinggenome as “junk,” however, becauseother unknown functions may reside inthese regions. Much of the non-codingDNA is in the form of repetitive se-quences, pseudogenes (“dead” copies ofgenes lost in recent evolution) and trans-posable elements of uncertain relevance.Although initial estimates for the num-ber of human genes was in the order of100,000, current predictions, based onthe essentially complete genome se-quence, are in the range of 20,000 to25,000.1 Surprisingly, therefore, the hu-man genome is comparable in size andcomplexity to primitive organisms suchas the fruit fly. It is thought, however,

that the generation of multiple proteinisoforms from a single gene via alternatesplicing of exons, each with a discretefunction, is what contributes to in-creased complexity in higher organisms,including humans. In addition to protein-encoding genes, there are also manygenes encoding untranslated RNA mole-cules, including transfer RNA, ribosomalRNA, and, as recently described, micro-RNA genes. Micro-RNA is thought to beinvolved in the control of a large numberof other genes through the RNA inhibi-tion pathway.

The draft sequence of the human ge-nome was completed in 2003. Subse-quently, small gaps have been filled, andthe sequence has now been extensivelyannotated in terms of genes, repetitiveelements, regulatory sequences, poly-morphisms, and many other featuresrecognizable by in silico data miningmethods informed, wherever possible,by functional analysis. This annotationprocess will continue for some time asmore features are uncovered. The hu-man genome data, and that for an in-creasing number of other species, isfreely available on websites (Table 8-1).Some regions of the genome, particu-larly near the centromeres, consist oflong stretches of highly repetitive se-quences that are difficult or impossibleto clone and/or sequence. These hetero-chromatic regions of the genome areunlikely to be sequenced and arethought to be structural in nature, medi-ating the chromosomal architecture re-quired for cell division, rather than con-tributing to heritable characteristics.

GENETIC AND GENOMIC DATABASES

Given the size and complexity of thehuman genome and other genomesnow available, analysis of these enor-mous datasets in any kind of meaning-ful way is heavily reliant on computers.Even storage and retrieval of the se-quence data associated with mamma-lian genome require considerable com-puter power, and memory and even theassembly of the raw sequence of anymammalian genome would have beenunfeasible without computers. Many

SECTION 3■



74

web browsers for accessing genomedata are available and the most useful ofthese are listed in Table 8-1. Each ofthese interfaces, which are the oneswhich the authors find most useful anduser-friendly, contains a wide variety oftools for analysis and searching of se-quences according to keyword, genename, protein name, and homology toDNA or protein sequence data.

The main source of historical, clinical,molecular, and biochemical data relatingto human genetic diseases is the OnlineMendelian Inheritance in Man (OMIM)(see Table 8-1). All recognized genetic dis-eases and non-pathogenic heritable traits,including common diseases with a geneticcomponent, as well as all known genesand proteins, are listed and reviewed byOMIM number with links to PubMed.

CHROMOSOME AND GENE STRUCTURE

Human chromosomes share commonstructural features (Fig. 8-1). All consistof two chromosomal arms, designatedas “p” and “q.” If the arms are of un-equal length, the short arm is alwaysdesignated as the “p” arm. Chromoso-mal maps to seek abnormalities arebased on the stained, banded appear-ance of condensed chromosomes duringmetaphase of mitosis. During inter-phase, the uncondensed chromosomesare not discernible by normal micros-copy techniques. Genes can now be lo-cated with absolute precision in termsof the range of bp that they span withinthe DNA sequence for a given chromo-some. The bands are numbered fromthe centromere outwards using a sys-tem that has evolved as increasingly dis-criminating chromosome stains, as wellas higher resolution light microscopes,became available. A typical cytogeneticchromosome band is 17q21.2, withinwhich the type I keratin genes reside(see Fig. 8-1).

The ends of the chromosomal armsare known as telomeres, and these consistof multiple tandem repeats of shortDNA sequences. In germ cells and cer-

tain other cellular contexts, additionalrepeats are added to telomeres by a pro-tein-RNA enzyme complex known astelomerase. During each round of cell di-vision in somatic cells, one of the telo-mere repeats is trimmed off as a con-sequence of the DNA replicationmechanism. By measuring the length oftelomeres, the “age” of somatic cells, interms of the number of times they havedivided during the lifetime of the organ-ism, can be determined. Once the telo-mere length falls below a certain thresh-old, the cell undergoes senescence.Thus, telomeres contribute to an impor-tant biological clock function that re-moves somatic cells that have gonethrough too many rounds of replicationand are at a high risk of accumulatingmutations that could lead to tumorigen-esis or other functional aberration.5

The chromosome arms are separatedby the centromere, which is a largestretch of highly repetitious DNA se-quence. The centromere has importantfunctions in terms of the movement andinteractions of chromosomes. The cen-tromeres of sister chromatids are wherethe double chromosomes align and at-tach during the prophase and anaphasestages of mitosis (and meiosis). The cen-tromeres of sister chromatids are alsothe site of kinetochore formation. Thelatter is a multi-protein complex towhich microtubules attach, allowingmitotic spindle formation, which ulti-mately results in pulling apart of thechromatids during anaphase of the celldivision cycle.

The majority of chromosomal DNAcontains genes interspersed with non-coding stretches of DNA of varyingsizes. The density of genes varieswidely across the chromosomes so thatthere are gene-dense regions or, alter-nately, large areas almost devoid offunctional genes. An example of a com-paratively gene-rich region of particularrelevance to inherited skin diseases isthe type I keratin gene cluster on chro-mosome band 17q21.2 (see Fig. 8-1).This diagram also gives an idea of thesizes in bp of DNA of a typical chromo-

some and a typical gene located withinit. This gene cluster spans about 900,000bp of DNA and contains 27 functionaltype I keratin genes, several genes en-coding keratin-associated proteins, anda number of pseudogenes (not shown).Because chromosome 17 is one of thesmaller chromosomes, Fig. 8-1 starts togive some idea of the overall complexityand organization of the genome.

Protein-encoding genes normally con-sist of several exons, which collectivelycode for the amino acid sequence of theprotein (or open reading frame). Theseare separated by non-coding introns. Inhuman genes, few exons are muchgreater than 1000 bp in size, and intronsvary from less than 100 bp up to morethan 1 million bp. A typical exon mightbe 100 to 300 bp in size. The KRT14gene encoding keratin 14 or K14 proteinis one of the genes in which mutationslead to epidermolysis bullosa (EB) sim-plex (see Chap. 60) and is illustrated inFig. 8-1. KRT14 is contained withinabout 7000 bp of DNA and consists ofeight modestly sized exons inter-spersed by seven small introns. Al-though all genes are present in all hu-man cells that contain a nucleus, notevery gene is expressed in all cells of tis-sues. For example, the KRT14 gene isonly active in basal keratinocytes of theepidermis and other stratified epithelialtissues and is essentially silent in allother tissues. When a protein-encodinggene is expressed, the RNA polymeraseII enzyme transcribes the coding strandof the gene, starting from the cap siteand continuing to the end of the finalexon, where various signals lead to ter-mination of transcription. The initialRNA transcript, known as heteronuclearRNA, contains intronic as well as ex-onic sequences. This primary transcriptundergoes splicing to remove the in-trons, resulting in the messenger RNA(mRNA) molecule.6 In addition, thebases at the 5′ end (start) of the mRNAare chemically modified (capping) and alarge number of adenosine bases areadded at the 3′ end, known as the poly-Atail. These post-transcriptional modifica-tions stabilize the mRNA and facilitateits transport within the cell. The maturemRNA undergoes a test round of trans-lation which, if successful, leads to thetransport of the mRNA to the cyto-plasm, where it undergoes multiplerounds of translation by the ribosomes,leading to accumulation of the encodedprotein. If the mRNA contains a non-sense mutation, otherwise known as apremature termination codon mutation, the

TABLE 8-1Websites for Accessing Human Genome Data

WEBSITE URLUniversity of California, Santa Cruz http://genome.ucsc.edu/National Center for Biotechnology Information

http://www.ncbi.nlm.nih.gov

ENSEMBL http://www.ensembl.org/Online Mendelian Inheritance in Man http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=omim

CHAPTER 8■


75

test round of translation fails, and thecell degrades this mRNA via the non-sense-mediated mRNA pathway.7 Thisis a mechanism that the cell has evolvedto remove aberrant transcripts, and itmay also contribute to gene regulation,particularly when very low levels of aparticular protein are required within agiven cell.

Splicing out of introns is a complexprocess. The genes of prokaryotes, suchas bacteria, do not contain introns, andso mRNA splicing is a process that isspecific to higher organisms. In some

more primitive eukaryotes, RNA mole-cules contain catalytic sequences knownas ribozymes, which mediate the self-splicing out of introns without any re-quirement for additional factors. Inmammals, splicing involves a largenumber of protein and RNA factors en-coded by several genes. This allows an-other level of control over gene expres-sion and also facilitates alternativesplicing of exons, so that a single genecan encode several functionally distinctvariants of a protein. These isoforms areoften differentially expressed in differ-

ent tissues. In terms of the gene se-quences important for splicing, a few bpat the beginning and at the end of an in-tron, known as the 5′ splice site (orsplice donor site) and the 3′ splice site(or splice acceptor site) are crucial. Afew other bp within the intron, such asthe branch point site located 18 to 100bp away from the 3′ end, are also criti-cal. Mutations affecting any of the in-variant residues of these splice sites leadto aberrant splicing and either completeloss of protein expression or generationof a highly abnormal protein.

� FIGURE 8-1 Illustration of the complexity of the human genome. On the left, the short (p) and long (q) arms of human chromosome 17 are depicted withtheir cytogenetic chromosome bands. One of these band regions, 17q21.2, is then highlighted to show that it is made up of approximately 900,000 base pairs(bp) and contains several genes, including 27 functional type I keratin genes. Part of this region is then further amplified to show one keratin gene, KRT14, en-coding keratin 14, which is composed of 8 exons.

SECTION 3■



76

The mRNA also contains two un-translated regions, the 5′UTR upstreamof the initiating ATG codon and the3′UTR downstream of the terminator(or stop codon, which can be TGA, TAAor TAG). The 5′UTR can and often doespossess introns, whereas the 3′UTR ofmore than 99 percent of mammaliangenes does not contain introns. Thenonsense-mediated mRNA decay path-way identifies mutant transcripts bymeans of assessing where the termina-tion codon occurs in relation to introns.The natural stop codon is always fol-lowed immediately by the 3′UTR,which in turn does not normally possessany introns. If a stop codon occurs in anmRNA upstream of a site where an in-tron has been excised, this message istargeted for nonsense-mediated decay.The only genes that contain intronswithin their 3′UTR sequences are ex-pressed at extremely low levels. This isone of the ways in which the cell candetermine how much protein is madefrom a particular gene.

Gene complexity is widely variableand not necessarily related to the size ofthe protein encoded. Some genes consistof only a single small exon, such as thoseencoding the connexin family of gapjunction proteins. Such single exongenes are rapid and inexpensive to ana-lyze routinely. In contrast, the type VIIcollagen gene, COL7A1, in which muta-tions lead to the dystrophic forms of EB(see Chap. 60), has 118 exons, meaningthat 118 different parts of the gene needto be isolated and analyzed for molecu-lar diagnosis of each dystrophic EB pa-tient. The filaggrin gene (FLG) on chro-mosome 1, recently shown to be thecausative gene for ichthyosis vulgaris(see Chap. 47) and a susceptibility genefor atopic dermatitis (see Chap. 14), hasonly three exons. However, the thirdexon of FLG is more than 12,000 bp insize; is made up of repeats of a 1000-bpsequence; and varies in size from 12,000to 14,000 bp between different individu-als in the population. This unusual genestructure makes routine sequencing ofgenes such as COL7A1 or FLG difficult,time consuming, and expensive.

GENE EXPRESSIONEach specific gene is generally only ac-tively transcribed in a subset of cells ortissues within the body. Gene expres-sion is largely determined by the pro-moter elements of the gene. In general,the most important region of the pro-moter is the stretch of sequence imme-

diately upstream of the cap site. Thisproximal promoter region contains con-sensus binding sites for a variety oftranscription factors, some of which aregeneral in nature and required for allgene expression, others are specific toparticular tissue or cell lineage, andsome are absolutely specific for a givencell type and/or stage of development ordifferentiation. The size of the pro-moter can vary widely according togene family or between the individualgenes themselves. For example, the ker-atin genes are tightly spaced within twogene clusters on chromosomes 12q and17q, but these are exquisitely tissue spe-cific in two different ways. First, thesegenes are only expressed in epithelialcells, and therefore their promotersmust possess regulatory sequences thatdetermine epithelial expression. Theseregulatory elements are therefore spe-cific for cells of ectodermal origin. Sec-ond, these genes are expressed in veryspecific subsets of epithelial cells, and sothere must be a second level of controlthat specifies which epithelial cell layersexpress specific keratin genes. This isbest illustrated in the hair follicle, wherethere are many different epithelial celllayers, each with a specific pattern ofkeratin gene expression (see Chap. 84).8

Transcription factors are proteins thateither bind to DNA directly or indirectlyby associating with other DNA-bindingproteins. Binding of these factors to thepromoter region of a gene leads to acti-vation of the transcription machineryand transcription of the gene by RNApolymerase II. The transcription factorproteins are encoded by genes that are inturn controlled by promoters that areregulated by other transcription factorsencoded by other genes. Thus, there areseveral tiers of control over gene expres-sion in a given cell type, and the intrica-cies of this can be difficult to fully un-ravel experimentally. Nevertheless, byisolation of promoter sequences fromgenes of interest and placing these infront of reporter genes that can be as-sayed biochemically, such as firefly lu-ciferase that can be assayed by lightemission, the activity of promoters canbe reproduced in cultured cells that nor-mally express the gene. Combining sucha reporter gene system with site-directedmutagenesis to make deletions or altersmall numbers of bp within the pro-moter can help define the extent of thepromoter and the important sequenceswithin it that are required for gene ex-pression. A variety of biochemical tech-niques, such as DNA footprinting, ri-

bonuclease protection, electrophoreticmobility shift assays, or chromatin im-munoprecipitation, can be used to deter-mine which transcription factors bind toa particular promoter and help delineatethe specific promoter sequences bound.Expression of reporter genes under thecontrol of a cloned promoter in trans-genic mice also helps shed light on theimportant sequences that are required torecapitulate the endogenous expressionof the gene under study. Keratin promot-ers are unusual in that, generally, a smallfragment of only 2000 to 3000 bp up-stream of the gene can confer most ofthe tissue specificity. For this reason, ker-atin promoters are widely used in thedermatology field to drive exogenoustransgene expression in the various spe-cific cellular compartments of the epider-mis and its appendages for a wide vari-ety of experiments aimed at determininggene, cell, or tissue function.9

Some promoter or enhancer se-quences act over very long distances. Insome cases, sequences located millionsof bp distant, with several other genes inthe intervening region, somehow influ-ence expression of a target gene. In somegenetic diseases, mutations affectingsuch long-range promoter elements arenow emerging. These types of muta-tions appear to be rare in the geneticsfield as a whole, but since they occur sovery far away from the target gene andare therefore very difficult to find, thisclass of mutation may, in fact, be morecommon than is immediately obvious.In general, relatively few disease-causingmutations have been shown to involvepromoters, but this class of defect isprobably greatly under-represented be-cause the promoters of most genes havebeen poorly characterized at the presenttime in terms of what sequences aretruly important for promoter activity.Prediction of transcription factor bindingsites by computer analysis remains aninexact science and is an area for furtherstudy in the future as the human ge-nome sequence undergoes greater de-tailed scrutiny and annotation.

FINDING DISEASE GENESIn establishing the molecular basis of aninherited skin disease, there are two keysteps. First, the gene linked to a particu-lar disorder must be identified, and sec-ond, pathogenic mutations within thatgene should be determined. Diseases canbe matched to genes either by geneticlinkage analysis or by a candidate geneapproach.10 Genetic linkage involves

CHAPTER 8■


77

studying pedigrees of affected and unaf-fected individuals and isolating whichbits of the genome are specifically associ-ated with the disease phenotype. Thegoal is to identify a region of the genomethat all the affected individuals and noneof the unaffected individuals have incommon; this region is likely to harborthe gene for the disorder, as well as per-haps other non-pathogenic neighboringgenes that have been inherited by link-age disequilibrium. Traditionally, ge-nome-wide linkage strategies make useof variably-sized microsatellite markersscattered throughout the genome, al-though for recessive diseases involvingconsanguineous pedigrees, a more rapidapproach may be to carry out homozy-gosity mapping using single nucleotidepolymorphism (SNP) chip arrays. Bycontrast, the candidate gene approach in-volves first looking for a clue to the likelygene by finding a specific disease abnor-mality, perhaps in the expression (or lackthereof) of a particular protein or RNA,or from an ultrastructural or biochemicaldifference between the disease and con-trol tissue. Nevertheless, the genetic link-age and candidate gene approaches arenot mutually exclusive and are oftenused in combination. For example, toidentify the gene responsible for the au-tosomal recessive disorder, lipoid protei-nosis (see Chap. 137), genetic linkage us-ing microsatellites was first used toestablish a region of linkage on 1q21 thatcontained 68 genes.11 The putative genefor this disorder, ECM1 encoding extra-cellular matrix protein 1, was then iden-tified by a candidate gene approach thatsearched for reduced gene expression(lack of fibroblast complementary DNA)in all these genes. A reduction in ECM1gene expression in lipoid proteinosiscompared with control provided the clueto the candidate gene because therewere no differences in any of the otherpatterns of gene expression.

Having identified a putative gene foran inherited disorder, the next stage is tofind the pathogenic mutation(s). This canbe done by sequencing the entire gene, afeat which is becoming easier as techno-logic advances make automated nucleo-tide sequencing faster, cheaper, and moreaccessible. However, the large size ofsome genes may make comprehensivesequencing impractical, and thereforeinitial screening approaches to identifythe region of a gene that contains themutation may be a necessary first step.There are many mutation detection tech-niques available to scan for sequencechanges in cellular RNA or genomic

DNA, and these include denaturing gra-dient gel electrophoresis, chemical cleav-age of mismatch, single stranded con-formation polymorphism, heteroduplexanalysis, conformation sensitive gelelectrophoresis, denaturing high-perfor-mance liquid chromatography and theprotein truncation test.12 The most criti-cal factor that determines the success ofany gene screening protocol is the sensi-tivity of the detection technique. In addi-tion, when choosing a mutation screen-ing strategy using genomic DNA, thesize of the gene and its number of exonsmust be taken into account. The sensi-tivities of these methods vary greatly,depending on the size of templatescreened. For example, single-strandedconformation polymorphism has a sensi-tivity of > 95 percent for fragments of155 bp, but this is reduced to only 3 per-cent for 600 bp. Once optimized, dena-turing gradient gel electrophoresis has asensitivity of about 99 percent for frag-ments of up to 500 bp, and conformationsensitive gel electrophoresis is expectedto have a sensitivity of 80 percent to 90percent for fragments of up to 600 bp.Chemical cleavage of mismatch, on theother hand, has a sensitivity of 95 per-cent to 100 percent for fragments > 1.5kilobases (kb) in size and is ideal forscreening compact genes where morethan one exon can be amplified togetherusing genomic DNA as the template.All these techniques detect sequencechanges such as truncating and missensemutations as well as polymorphisms;however, the protein truncation testscreens only for truncating mutationsand is predicted to have a sensitivity of> 95 percent and can be used for RNA orDNA fragments in excess of 3 kb.Whichever approach is taken, havingidentified a difference in the patient’sDNA compared with the control sample,the next stage is to determine how thissegregates within a particular family andalso whether it is pathogenic or not.

GENE MUTATIONS AND POLYMORPHISMS

Within the human genome, the geneticcode of two healthy individuals mayshow a number of sequence dissimilari-ties that have no relevance to disease orphenotypic traits. Such changes withinthe normal population are referred to aspolymorphisms (Fig. 8-2). Indeed, evenwithin the coding region of the genome,clinically irrelevant substitutions of onebp, known as SNPs, are common and oc-cur approximately once every 250 bp.13

Oftentimes, these SNPs do not changethe amino acid composition; for exam-ple, a C-to-T transition in the third posi-tion of a proline codon (CCC to CCT)still encodes for proline, and is referredto as a silent mutation. Some SNPs, how-ever, do change the nature of the aminoacid; for example, a C-to-G transversionat the second position of the same pro-line codon (CCC to CGC) changes theresidue to arginine. It then becomes nec-essary to determine whether a missensechange such as this represents a non-pathogenic polymorphism or a patho-genic mutation. Factors favoring the lat-ter include the sequence segregating onlywith the disease phenotype in a particu-lar family, the amino acid change occur-ring within an evolutionarily conservedresidue, the substitution affecting thefunction of the encoded protein (size,charge, conformation, etc.), and the nu-cleotide switch not being detectable in atleast 100 ethnically matched controlchromosomes. Non-pathogenic poly-morphisms do not always involve singlenucleotide substitutions; occasionally,deletions and insertions may also benon-pathogenic.

A mutation can be defined as achange in the chemical composition of agene. A missense mutation changes oneamino acid to another. Mutations mayalso be insertions or deletions of bases,the consequences of which will dependon whether this disrupts the normalreading frame of a gene or not, as wellas nonsense mutations, which lead topremature termination of translation(see Fig. 8-2). For example, a single nu-cleotide deletion within an exon causesa shift in the reading frame, which usu-ally leads to a downstream stop codon,thus giving a truncated protein, or of-ten an unstable mRNA that is readilydegraded by the cell. However, a dele-tion of three nucleotides (or multiplesthereof ) will not significantly perturbthe overall reading frame, and the con-sequences will depend on the nature ofwhat has been deleted. Nonsense muta-tions typically, but not exclusively, oc-cur at CpG dinucleotides, where methy-lation of a cytosine nucleotide oftenoccurs. Inherent chemical instability ofthis modified cytosine leads to a highrate of mutation to thymine. Where thisalters the codon (e.g., from CGA toTGA), it will change an arginine residueto a stop codon. Nonsense mutationsusually lead to a reduced or absent ex-pression of the mutant allele at themRNA and protein levels. In the hetero-zygous state, this may have no clinical

SECTION 3■



78

effect [e.g., parents of individuals withHerlitz junctional EB are typically carri-ers of nonsense mutations in one of thelaminin 332 (laminin 5) genes but haveno skin fragility themselves; see Chap.60], but a heterozygous nonsense muta-tion in the desmoplakin gene, for exam-ple, can result in the autosomal domi-nant skin disorder, striate palmoplantarkeratoderma (see Chap. 48). This phe-nomenon is referred to as haploinsuffi-ciency (i.e., half the normal amount ofprotein is insufficient for function).

Apart from changes in the coding re-gion that result in frameshift, missense,or nonsense mutations, approximately15 percent of all mutations involve alter-ations in the gene sequence close to theboundaries between the intron and ex-ons, referred to as splice site mutations.This type of mutation may abolish theusual acceptor and donor splice sitesthat normally splice out the introns dur-ing gene transcription. The conse-quences of splice site mutations arecomplex; sometimes they lead to skip-

ping of the adjacent exon, and othertimes they result in the generation ofnew mRNA transcripts through utiliza-tion of cryptic splice sites within theneighboring exon or intron.

Mutations within one gene do not al-ways lead to a single inherited disorder.For example, mutations in the ERCC2gene may lead to xeroderma pigmento-sum (type D), trichothiodystrophy, orcerebrofacioskeletal syndrome, depend-ing on the position and type of muta-tion. Other trans-acting factors may fur-ther modulate phenotypic expression.This situation is known as allelic heteroge-neity. Conversely, some inherited dis-eases can be caused by mutations inmore than one gene (e.g., non-Herlitzjunctional EB; see Chap. 60) and can re-sult from mutations in either theCOL17A1, LAMA3, LAMB3, or LAMC2genes. This is known as genetic heteroge-neity. In addition, the same mutation inone particular gene may lead to a rangeof clinical severity in different individu-als. This variability in phenotype pro-duced by a given genotype is referred toas the expressivity. If an individual withsuch a genotype has no phenotypicmanifestations, the disorder is said to benon-penetrant. Variability in expressionreflects the complex interplay betweenthe mutation, modifying genes, epige-netic factors, and the environment anddemonstrates that interpreting what aspecific gene mutation does to an indi-vidual involves more than just detectingone bit of mutated DNA in a single gene.

MENDELIAN DISORDERSThere are approximately 5000 humansingle-gene disorders and, although themolecular basis of less than one-half ofthese has been established, understand-ing the pattern of inheritance is essentialfor counseling prospective parents aboutthe risk of having affected children. Thefour main patterns of inheritance areautosomal dominant, autosomal reces-sive, X-linked dominant, and X-linkedrecessive.

For individuals with an autosomaldominant disorder, one parent is af-fected, unless there has been a de novomutation in a parental gamete. Malesand females are affected in approxi-mately equal numbers, and the disordercan be transmitted from generation togeneration; on average, half the off-spring will have the condition (Fig. 8-3).It is important to counsel affected indi-viduals that the risk of transmitting thedisorder is 50 percent for each of their

� FIGURE 8-2 Examples of nucleotide sequence changes resulting in a polymorphism and a nonsensemutation. A. Two adjacent codons are highlighted. Outlined in purple, the AGG codon encodes arginine andthe blue boxed codon, CAG encodes glutamine. B. The sequence shows two homozygous nucleotide sub-stitutions. In the purple box, AGG now reads AGT (i.e., coding for serine rather than arginine). This is a com-mon sequence variant in the normal population and is referred to as a non-pathogenic missense polymor-phism. In contrast, in the blue box, the glutamine codon CAG now reads TAG, which is a stop codon. This isan example of a homozygous nonsense mutation. C. This sequence is from one of the parents of the sub-ject sequenced in B and shows heterozygosity for both the missense polymorphism AGG > AGT and thenonsense mutation CAG > TAG, indicating that this individual is a carrier of both sequence changes.

A G G A C A G A G G C A G C T G A G G C

A G G A C A G A G T T A G C T G A G G C

A G G A C A G A G N N A G C T G A G G C

A

B

C

CHAPTER 8■


79

children, and that this is not influencedby the number of previously affected orunaffected offspring. Any offspring thatare affected will have a 50 percent riskof transmitting the mutated gene to thenext generation, whereas for any unaf-fected offspring, the risk of the nextgeneration being affected is negligible,providing that the partner does not havethe autosomal dominant condition.

In autosomal recessive disorders,both parents are carriers of one normaland one mutated allele for the samegene and, typically, they are phenotypi-cally unaffected (Fig. 8-4). If both of themutated alleles are transmitted to theoffspring, this will give rise to an auto-somal recessive disorder, the risk ofwhich is 25 percent. If one mutated andone wild-type allele is inherited by theoffspring, the child will be an unaffectedcarrier, similar to the parents. If bothwild-type alleles are transmitted, thechild will be genotypically and pheno-typically normal with respect to an af-fected individual. If the mutations fromboth parents are the same, the individ-ual is referred to as a homozygote, but ifdifferent parental mutations within agene have been inherited, the individualis termed a compound heterozygote. Forsomeone who has an autosomal reces-sive condition, be it a homozygote orcompound heterozygote, all offspringwill be carriers of one of the mutated al-leles but will be unaffected because ofinheritance of a wild-type allele fromthe other, clinically and genetically un-affected, parent. This assumes that the

unaffected parent is not a carrier. Al-though this is usually the case in non-consanguineous relationships, it maynot hold true in first-cousin marriages orother circumstances where there is a fa-milial interrelationship. For example, ifthe partner of an individual with an au-tosomal recessive disorder is also a car-rier of the same mutation, albeit clini-cally unaffected, then there is a 50percent chance of the offspring inherit-ing two mutant alleles and thereforealso inheriting the same autosomal re-cessive disorder. This pattern of inheri-tance is referred to as pseudo-dominant.

In X-linked dominant inheritance,both males and females are affected,and the pedigree pattern may resemblethat of autosomal dominant inheritance(Fig. 8-5). There is, however, one impor-tant difference. An affected male trans-mits the disorder to all his daughtersand to none of his sons. X-linked domi-nant inheritance has been postulated asa mechanism in incontinentia pigmenti(see Chap. 73), Conradi-Hünermannsyndrome, and focal dermal hypoplasia(Goltz syndrome), conditions that arealmost always limited to females. Inmost X-linked dominant disorders withcutaneous manifestations, affected malesmay be aborted spontaneously or diebefore implantation (for example, mostmale patients with incontinentia pig-menti have a postzygotic mutation inNEMO, and no affected mother; in thisdisorder, transmission tends to be fe-male-to-female).

X-linked recessive conditions occur al-most exclusively in males, but the geneis transmitted by carrier females, whohave the mutated gene only on one Xchromosome (heterozygous state). Thesons of an affected male will all be nor-mal (because their single X chromosomecomes from their clinically unaffectedmother) (Fig. 8-6). However, the daugh-ters of an affected male will all be carri-ers (because all had to have received thesingle X chromosome that their fatherhad, and that X chromosome carries themutant copy of the corresponding gene).Some females show clinical abnormali-ties as evidence of the carrier state (suchas in hypohidrotic ectodermal dysplasia;

� FIGURE 8-3 Pedigree illustration of an autosomal dominant pattern of inheritance. Key observationsinclude: the disorder affects both males and females; on average, 50 percent of the offspring of an af-fected individual will be affected; affected individuals have one normal copy and one mutated copy of thegene; affected individuals usually have one affected parent, unless the disorder has arisen de novo. Im-portantly, examples of male-to-male transmission, seen here, distinguish this from X-linked dominantand are therefore the best hallmark of autosomal dominant inheritance. Filled circles indicate affectedfemales; filled squares indicate affected males; unfilled circles/squares represent unaffected individuals.

� FIGURE 8-4 Pedigree illustration of an autosomal recessive pattern of inheritance. Key observationsinclude: the disorder affects both males and females; there are mutations on both inherited copies of thegene; the parents of an affected individual are both heterozygous carriers and are usually clinically unaf-fected; autosomal recessive disorders are more common in consanguineous families. Filled circle indi-cates affected female; half-filled circles/squares represent clinically unaffected heterozygous carriers ofthe mutation; unfilled circles/squares represent unaffected individuals.

SECTION 3■



80

see Chap. 143); the variable extent ofphenotypic expression can be explainedby lyonization, the normally randomprocess that inactivates either the wild-type or mutated X chromosome in eachcell during the first weeks of gestationand all progeny cells.14 Other carriersmay not show manifestations becausethe affected region on the X chromo-some escapes lyonization (as in reces-sive X-linked ichthyosis) or the selectivesurvival disadvantage of cells in whichthe mutated X chromosome is activated(as in the lymphocytes and platelets ofcarriers of Wiskott-Aldrich syndrome;see Mosaicism).

CHROMOSOMAL DISORDERSAberrations in chromosomes are com-mon. They occur in about 6 percent ofall conceptions, although most of theselead to miscarriage, and the frequencyof chromosomal abnormalities in livebirths is about 0.6 percent. Approxi-mately two-thirds of these involve ab-normalities in either the number of sexchromosomes or the number of auto-somes; the remainder are chromosomalrearrangements. The number and ar-rangement of the chromosomes is re-ferred to as the karyotype. The mostcommon numerical abnormality is tri-somy, the presence of an extra chromo-some. This occurs because of non-dis-junction, when pairs of homologouschromosomes fail to separate duringmeiosis, leading to gametes with an ad-ditional chromosome. Loss of a com-plete chromosome, monosomy, can af-

fect the X chromosome but is rarelyseen in autosomes because of non-via-bility. A number of chromosomal disor-ders are also associated with skin abnor-malities, as detailed in Table 8-2.

Structural aberrations (fragility breaks)in chromosomes may be random, al-though some chromosomal regions ap-pear more vulnerable. Loss of part of achromosome is referred to as a deletion. Ifthe deletion leads to loss of neighboringgenes this may result in a contiguousgene disorder, such as a deletion on the Xchromosome giving rise to X-linked ich-thyosis (see Chap. 47) and Kallman syn-

drome. If two chromosomes break, thedetached fragments may be exchanged,known as reciprocal translocation. If thisprocess involves no loss of DNA it is re-ferred to as a balanced translocation. Otherstructural aberrations include duplicationof sections of chromosomes, two breakswithin one chromosome leading to inver-sion, and fusion of the ends of two bro-ken chromosomal arms, leading to join-ing of the ends and formation of a ringchromosome.

A further possible chromosomal ab-normality is the inheritance of both cop-ies of a chromosome pair from just oneparent (paternal or maternal), known asuniparental disomy.15 Uniparental heterodi-somy refers to the presence of a pair ofchromosome homologs, whereas unipa-rental isodisomy describes two identicalcopies of a single homolog, and meroisodi-somy is a mixture of the two. Uniparentaldisomy with homozygosity of recessivealleles is being increasingly recognized asthe molecular basis for several autosomalrecessive disorders, and there have beenmore than 35 reported cases of recessivediseases, including junctional and dystro-phic EB (see Chap. 60), resulting fromthis type of chromosomal abnormality.For certain chromosomes, uniparental di-somy can also result in distinct pheno-types depending on the parental originof the chromosomes, a phenomenonknown as genomic imprinting.16,17 This par-ent-of-origin, specific gene expression isdetermined by epigenetic modificationof a specific gene or, more often, a groupof genes, such that gene transcription is

� FIGURE 8-5 Pedigree illustration of an X-linked dominant pattern of inheritance. Key observationsinclude: affected individuals are either hemizygous males or heterozygous females; affected males willtransmit the disorder to their daughters but not to their sons (no male-to-male transmission); affected fe-males will transmit the disorder to half their daughters and half their sons; some disorders of this typeare lethal in hemizygous males and only heterozygous females survive. Filled circles indicate affected fe-males; filled squares indicate affected males; unfilled circles/squares represent unaffected individuals.

� FIGURE 8-6 Pedigree illustration of an X-linked recessive pattern of inheritance. Key observationsinclude: usually affects only males but females can show some features because of lyonization (X-chro-mosome inactivation); transmitted through female carriers, with no male-to-male transmission; for af-fected males, all daughters will be heterozygous carriers; female carrier will transmit the disorder to halfher sons, and half her daughters will be heterozygous carriers. Dots within circles indicate heterozygouscarrier females who may or may not display some phenotypic abnormalities; filled squares indicate af-fected males; unfilled circles/squares represent unaffected individuals.

CHAPTER 8■


81

TABLE 8-2Chromosomal Disorders with a Skin Phenotype

CHROMOSOMAL ABNORMALITY SYNONYM GENERAL FEATURES SKIN MANIFESTATIONS

Trisomy 21 Down syndrome

Small head with flat face 1–10 yr: dry skin, xerosis, lichenificationNose short and squat 10+ yr: increased frequency of atopic dermatitis, alopecia areata, single

crease in palm and fifth fingerEars small and misshapenSlanting palpebral fissures Other associations: skin infections, angular cheilitis, geographic tongue,

blepharitis, red cheeks, folliculitis, seborrheic dermatitis, boils, onycho-mycosis, fine hypopigmented hair, vitiligo, delayed dentition and hypo-plastic teeth, acrocyanosis, livedo reticularis, cutis marmorata, calcinosis cutis, palmoplantar keratoderma, pityriasis rubra pilaris, syringomas, elastosis perforans serpiginosa, anetoderma, hyperkera-totic form of psoriasis, collagenoma, eruptive dermatofibromas, urticaria pigmentosa, leukemia cutis, keratosis follicularis spinulosa decalvans

Thickened eyelidsEyelashes short and sparseShortened limbs, lax jointsFingers short, sometimes webbedHypoplastic iris, lighter outer zone (Brushfield’s spots)

Trisomy 18 Edwards syndrome

Severe mental deficiency Cutis laxa (neck), hypertrichosis of forehead and back, superficial hemangiomas, abnormal dermatoglyphics, single palmar crease, hyper-pigmentation, ankyloblepharon filiforme adnatum

Abnormal skull shapeSmall chin, prominent occiputLow-set, malformed ears“Rocker bottom” feetShort sternumMalformations of internal organsOnly 10% survive beyond first year

Trisomy 13 Patau syn-drome

Mental retardation Vascular anomalies (especially on forehead)Sloping forehead due to forebrain maldevelopment (holoprosencephaly)

Hyperconvex nailsLocalized scalp defects

Microphthalmia or anophthalmia Cutis laxa (neck)Cleft palate/cleft lip Abnormal palm print (distal palmar axial triradius)Low-set ears“Rocker bottom” feetMalformations of internal organsSurvival beyond 6 mo is rare

Chromosome 4, short arm deletion

Microcephaly Scalp defectsMental retardationHypospadiasCleft lip/palateLow-set ears, preauricular pits

Chromosome 5, short arm deletion

Mental retardation Premature graying of hairMicrocephalyCat-like cryLow-set ears, preauricular skin tag

Chromosome 18, long arm deletion

Hypoplasia of mid-face Eczema in 25% of casesSunken eyesProminent ear anti-helixMultiple skeletal and ocular abnormalities

45 XO Turner syndrome

Early embryonic loss; prenatal ultra-sound findings of cystic hygroma, chy-lothorax, ascites and hydrops

Redundant neck skin and peripheral edemaWebbed neck, low posterior hairlineCutis laxa (neck, buttocks)

Short stature, amenorrhea Hypoplastic, soft up-turned nailsBroad chest, widely spaced nipples Increased incidence of keloidsWide carrying angle of arms Increased number of melanocytic nevi and halo neviLow misshapen ears, high arched palate Failure to develop full secondary sexual characteristicsShort fourth/fifth fingers and toes Lymphatic hypoplasia/lymphedemaSkeletal abnormalities, coarctation of aorta

47 XXY Klinefelter syndrome

No manifestations before puberty May develop gynecomastiaSmall testes, poorly developed secon-dary sexual characteristics

Sparse body and facial hairIncreased risk of leg ulcers

Infertility Increased incidence of systemic lupus erythematosusTall, obese, osteoporosis

(continued)

SECTION 3■



82

altered, and only one inherited copy ofthe relevant imprinted gene(s) is expressedin the embryo. This means that, duringdevelopment, the parental genomes func-tion unequally in the offspring. The mostcommon examples of genomic imprintingare Prader–Willi (OMIM #176270) andAngelman (OMIM #105830) syndromes,which can result from maternal or pater-nal uniparental disomy for chromosome15, respectively. Three phenotype abnor-malities commonly associated with uni-parental disomy for chromosomes withimprinting are intrauterine growth retar-dation, developmental delay, and reducedstature.18

MITOCHONDRIAL DISORDERSIn addition to the 3.3 billion bp nucleargenome, each cell contains hundreds orthousands of copies of a further 16.5-kbmitochondrial genome, which is inher-ited solely from an individual’s mother.This closed, circular genome contains 37genes, 13 of which encode proteins of therespiratory chain complexes, whereas theother 24 genes generate 22 transfer RNAsand two ribosomal RNAs used in mito-chondrial protein synthesis.19 Mutationsin mitochondrial DNA were first reportedin 1988, and more than 250 pathogenicpoint mutations and genomic rearrange-ments have been shown to underlie anumber of myopathic disorders and neu-rodegenerative diseases, some of whichshow skin manifestations, including lipo-mas, abnormal pigmentation or ery-thema, and hypo- or hypertrichosis.20 Mi-tochondrial DNA has the capacity toform a mixture of both wild-type andmutant DNA within a cell, leading to cel-lular dysfunction only when the ratio ofmutated to wild-type DNA reaches a cer-

tain threshold. The phenomenon of hav-ing mixed mitochondrial DNA specieswithin a cell is known as heteroplasmy. Mi-tochondrial mutations can induce, or beinduced by, reactive oxygen species, andmay be found in, or contribute to, bothchronologic aging and photoaging. So-matic mutations in mitochondrial DNAhave also been reported in several pre-malignant and malignant tumors, includ-ing malignant melanoma, although it isnot yet known whether these mutationsare causally linked to cancer developmentor simply a secondary bystander effect asa consequence of nuclear DNA instability.Indeed, currently there is little under-standing of the interplay between the nu-clear and mitochondrial genomes in bothhealth and disease. Nevertheless, it is evi-dent that the genes encoded by the mito-chondrial genome have multiple biologicfunctions linked to energy production,cell proliferation, and apoptosis.21

COMPLEX TRAIT GENETICSFor Mendelian disorders, identifyinggenes that harbor pathogenic mutationshas become relatively straightforward,with hundreds of disease-associatedgenes being discovered through a com-bination of linkage, positional cloning,and candidate gene analyses. By con-trast, for complex traits, such as psoria-sis and atopic dermatitis, these tradi-tional approaches have been largelyunsuccessful in mapping genes influenc-ing the disease risk or phenotype be-cause of low statistical power and otherfactors.22,23 Complex traits do not dis-play simple Mendelian patterns of in-heritance, although genes do have an in-fluence, and close relatives of affectedindividuals may have an increased risk.

To dissect out genes that contribute to,and influence susceptibility to, complextraits, several stages may be necessary,including establishing a genetic basis forthe disease in one or more populations;measuring the distribution of gene ef-fects; studying statistical power usingmodels; and carrying out marker-basedmapping studies using linkage or associ-ation. It is possible to establish quantita-tive genetic models to estimate the heri-tability of a complex trait, as well as topredict the distribution of gene effectsand to test whether one or more quanti-tative trait loci exist. These models canpredict the power of different mappingapproaches, but often only provide ap-proximate predictions. Moreover, lowpower often limits other strategies suchas transmission analyses, associationstudies, and family-based associationtests. Another potential pitfall of asso-ciation studies is that they can generatespurious associations due to popula-tion admixture. To counter this, alter-native strategies for association map-ping include the use of recent founderpopulations or unique isolated popula-tions that are genetically homoge-neous, and the use of unlinked markers(so-called genomic controls) to assign dif-ferent regions of the genome of an ad-mixed individual to particular sourcepopulations. In addition, and relevantto several studies on psoriasis, linkagedisequilibrium observed in a sample ofunrelated affected and normal individ-uals can also be used to fine-map a dis-ease susceptibility locus in a candidateregion.

Recently, however, a conventional ge-netics approach has revealed fascinatingnew insight into the pathophysiology ofone particular complex trait, namely

48 XXYY Similar to Klinefelter syndrome Multiple cutaneous angiomasAcrocyanosis, peripheral vascular disease

47 XYY Phenotypic males (tall) Severe acneMental retardationAggressive behavior

49 XXXXY Low birth weight Hypotrichosis (variable)Slow mental and physical developmentLarge, low-set, malformed earsSmall genitalia

Fragile X syndrome

Mental retardation —Mild dysmorphismHyperextensible joints, flat feet

TABLE 8-2Chromosomal Disorders with a Skin Phenotype (Continued)

CHROMOSOMAL ABNORMALITY SYNONYM GENERAL FEATURES SKIN MANIFESTATIONS

CHAPTER 8■


83

atopic dermatitis. This finding emanatedfrom the discovery that the disorder ich-thyosis vulgaris was due to loss-of-func-tion mutations in the gene encodingthe skin barrier protein filaggrin (seeChaps. 14 and 47).24 To dermatologists,the clinical association between thiscondition and atopic dermatitis is wellknown, and the same loss-of-functionmutations in filaggrin have subsequentlybeen shown to be a major susceptibilityrisk factor for atopic dermatitis, as wellas asthma associated with atopic der-matitis, but not asthma alone.4 Thissuggests that asthma in individuals withatopic dermatitis may be secondary toallergic sensitization, which developsbecause of the defective epidermal bar-rier that allows allergens to penetratethe skin to make contact with antigen-presenting cells. Indeed, transmission-disequilibrium tests have demonstratedan association between filaggrin genemutations and extrinsic atopic dermati-tis associated with high total serum im-munoglobulin E levels and concomitantallergic sensitizations.25 These recentdata on the genetics of atopic dermatitisdemonstrate how the study of a “sim-ple” genetic disorder can also providenovel insight into a complex trait. Men-delian disorders, therefore, may be use-ful in the molecular dissection of morecomplex traits.26

MOSAICISMThe presence of a mixed population ofcells bearing different genetic or chro-mosomal characteristics leading to phe-notypic diversity is referred to as mosa-icism. There are several different typesof mosaicism, including single gene,chromosomal, functional, and revertantmosaicism.27

Mosaicism for a single gene, referredto as somatic mosaicism, indicates a muta-tional event occurring after fertilization.The earlier this occurs, the more likely itis there will be clinical expression of adisease phenotype as well as involve-ment of gonadal tissue (gonosomal mo-saicism); for example, when individualswith segmental neurofibromatosis sub-sequently have offspring with full-blown neurofibromatosis (see Chap.142). However, in general, if the muta-tion occurs after generation of cellscommitted to gonad formation, then themosaicism will not involve the germ-line, and the reproductive risk of trans-mission is negligible. Gonosomal mosa-icism refers to involvement of bothgonads and somatic tissue, but mosa-

icism can occur exclusively in gonadaltissue, referred to as gonadal mosaicism.Clinically, this may explain recurrencesamong siblings of autosomal dominantdisorders such as tuberous sclerosis orneurofibromatosis, when none of theparents has any clinical manifestationsand gene screening using genomic DNAfrom peripheral blood samples yields nomutation. Segmental mosaicism for au-tosomal dominant disorders is thoughtto occur in one of two ways: eitherthere is a postzygotic mutation with theskin outside the segment and genomicDNA being normal (type 1), or there is aheterozygous genomic mutation that isthen exacerbated by loss of heterozy-gosity within a segment or along thelines of Blaschko (type 2). This patternhas been described in several autosomaldominant disorders, including Darierdisease, Hailey-Hailey disease (see Chap.49), superficial actinic porokeratosis (seeChap. 50), and tuberous sclerosis (seeChap. 141).

The lines of Blaschko were delineatedover 100 years ago; the pattern is attrib-uted to the lines of migration and prolif-eration of epidermal cells during em-bryogenesis (i.e., the bands of abnormalskin represent clones of cells carrying amutation in a gene expressed in theskin).28 Apart from somatic mutations[either in dominant disorders, such asbullous ichthyosiform erythrodermaleading to linear epidermolytic hyper-keratosis (see Chap. 47), or in condi-tions involving mutations in lethaldominant genes such as in McCune-Albright syndrome], mosaicism follow-ing Blaschko’s lines is also seen in chro-mosomal mosaicism and functionalmosaicism (random X-chromosomeinactivation through lyonization). Chro-mosomal mosaicism results from non-disjunction events that occur after fertil-ization. Clinically, this is found in thelinear pigmentary disorders hypomel-anosis of Ito (see Chap. 73) and linearand whorled hyperpigmentation. It isimportant to point out that hypomel-anosis of Ito is not a specific diagnosisbut may occur as a consequence of sev-eral different chromosomal abnormali-ties that perturb various genes relevantto skin pigmentation. Functional mosa-icism relates to genes on the X chromo-some, because during embryonic devel-opment in females, one of the Xchromosomes, either the maternal orthe paternal, is inactivated. For X-linkeddominant disorders, such as focal der-mal hypoplasia (Goltz syndrome) or in-continentia pigmenti (see Chap. 73), fe-

males survive because of the presence ofsome cells in which the X chromosomewithout the mutation is active and ableto function. For males, these X-linkeddominant disorders are typically lethal,unless associated with an abnormal kary-otype (e.g., Klinefelter syndrome; 47,XXY) or if the mutation occurs duringembryonic development. For X-linkedrecessive conditions, such as X-linked re-cessive hypohidrotic ectodermal dyspla-sia (see Chap. 143), the clinical featuresare evident in hemizygous males (whohave only one X chromosome), but fe-males may show subtle abnormalitiesdue to mosaicism caused by X-inactiva-tion, such as decreased sweating or re-duced hair in areas of the skin in whichthe normal X is selectively inactivated.There are 1317 known genes on the Xchromosome, and most undergo randominactivation but a small percentage (ap-proximately 27 genes on Xp, includingthe steroid sulfatase gene, and 26 geneson Xq) escape inactivation.

Revertant mosaicism, also known asnatural gene therapy, refers to genetic cor-rection of an abnormality by presenceof a second mutation that either correctsa mutant gene or silences it.29,30 Suchevents have been described in a fewgenes expressed in the skin, includingthe keratin 14, laminin 332, and collagenXVII genes in different forms of EB (Fig.8-7; see Chap. 60). However, the clinicalrelevance of the conversion process de-pends on several factors, including thenumber of cells involved, how much re-versal actually occurs, and at what stagein life the reversion takes place.

Apart from mutations in nuclearDNA, mosaicism can also be influencedby environmental factors, such as viralDNA sequences (retrotransposons) thatcan be incorporated into nuclear DNA,replicate, and activate or silence genesthrough methylation or demethylation.This phenomenon is known as epigeneticmosaicism; such events may be impli-cated in tumorigenesis but have notbeen associated with any genetic skindisorder.

EPIGENETICSDisease phenotypes reflect the result ofthe interaction between a particulargenotype and the environment, but it isevident that some variation, for example,in monozygotic twins, is attributable toneither. Additional influences at the bio-chemical, cellular, tissue, and organismlevels occur, and these are referred to asepigenetic phenomena.31 Single genes are

SECTION 3■



84

not solely responsible for each separatefunction of a cell. Genes may collaboratein circuits, be mobile, exist in plasmidsand cytoplasmic organelles, and can beimported by nonsexual means fromother organisms or as synthetic prod-ucts. Even prion proteins can simulatesome gene properties. Epigenetic effectsreflect chemical modifications to DNAthat do not alter DNA sequence but doalter the probability of gene transcrip-tion. Analysis of these changes is knownas epigenomics.32 Examples of modifica-tions include direct covalent modifica-tion of DNA by methylation of cy-tosines and alterations in proteins thatbind to DNA. Such changes may affectDNA accessibility to local transcriptionalcomplexes as well as influencing chro-matin structure at regional and genome-wide levels, thus providing a link be-tween genome structure and regulationof transcription. Indeed, epigenomeanalysis is now being carried out in par-allel with gene expression to identify ge-nome-wide methylation patterns andprofiles of all human genes. For example,there is considerable interindividual vari-ation in cytosine methylation of CpG di-nucleotides within the major histocom-patibility complex (MHC) region genes,although whether this has any bearingon the expression of skin disorders suchas psoriasis remains to be seen. A furtherbut as yet unresolved issue is whetherthere is heritability of epigenetic charac-teristics. Likewise, it is unclear whetherenvironmentally induced changes in epi-genetic status, and hence gene transcrip-tion and phenotype, can be transmittedthrough more than one generation. Sucha phenomenon might account for the

cancer susceptibility of grandchildren ofindividuals who have been exposed todiethylstilbestrol, but this has not beenproved. Germline epimutations, how-ever, have been identified in other hu-man diseases, such as colorectal cancerscharacterized by microsatellite instabil-ity and hypermethylation of the MLH1DNA mismatch repair gene, althoughthe risk of transgenerational epigeneticinheritance of cancer from such a muta-tion is not well established and proba-bly small. Over the course of an indi-vidual’s lifespan, epigenetic mutationsmay occur more frequently than DNAmutations, and it is expected that, overthe next decade, the role of epigeneticphenomena in influencing phenotypicvariation will gradually become betterunderstood.33

HISTOCOMPATABILITY ANTIGEN DISEASE ASSOCIATION

HLA molecules are glycoproteins that areexpressed on almost all nucleated cells.The HLA region is located on the shortarm of chromosome 6, at 6p21, referredto as the MHC. There are three classicloci at HLA class I: HLA-A, -B, and -Cw,and five loci at class II: HLA-DR, -DQ,-DP, -DM, and -DO. The HLA moleculesare highly polymorphic, there beingmany alleles at each individual locus.Thus, allelic variation contributes to de-fining a unique “fingerprint” for each per-son’s cells, which allows an individual’simmune system to define what is foreignand what is self. The clinical significanceof the HLA system is highlighted in hu-man tissue transplantation, especially inkidney and bone marrow transplanta-

tion, where efforts are made to match atthe HLA-A, -B, and -DR loci. MHC classI molecules, complexed to certain pep-tides, act as substrates for CD8+ T-cellactivation, whereas MHC class II mole-cules on the surface of antigen-present-ing cells display a range of peptides forrecognition by the T-cell receptors ofCD4+ T helper cells (see Chap. 10). MHCmolecules, therefore, are central to effec-tive adaptive immune responses. Con-versely, however, genetic and epide-miologic data have implicated thesemolecules in the pathogenesis of variousautoimmune and chronic inflammatorydiseases. Several skin diseases, such aspsoriasis (see Chap. 18), psoriatic arthrop-athy (central and peripheral), dermatitisherpetiformis, pemphigus, reactive ar-thritis syndrome (see Chap. 20), and Beh-çet disease (see Chap. 167), all show anassociation with inheritance of certainHLA haplotypes (i.e., there is a higher in-cidence of these conditions in individualsand families with particular HLA alleles).However, the molecular mechanisms bywhich polymorphisms in HLA moleculesconfer susceptibility to certain disordersare still unclear. This situation is furthercomplicated by the fact that, for mostdiseases, it is unknown which autoanti-gens (presented by the disease-associatedMHC molecules) are primarily involved.For many diseases, the MHC class asso-ciation is the main genetic association.Nevertheless, for most of the MHC-asso-ciated diseases, it has been difficult to un-equivocally determine the primary dis-ease-risk gene(s), owing to the extendedlinkage disequilibrium in the MHC re-gion. However, recent genetic and func-tional studies support the long-held as-sumption that common MHC class I andII alleles themselves are responsible formany disease associations, such as theHLA cw6 allele in psoriasis.

GENETIC COUNSELINGIn 2006, the National Society of GeneticCounselors (http://www.nsgc.org) de-fined genetic counseling as “the processof helping people understand and adaptto the medical, psychological and famil-ial implications of genetic contributionsto disease.” Genetic counseling shouldinclude: (1) interpretation of family andmedical histories to assess the chance ofdisease occurrence or recurrence; (2) ed-ucation about inheritance, testing, man-agement, prevention, resources, and re-search; and (3) counseling to promoteinformed choice and adaptation to therisk or condition.34

� FIGURE 8-7 Revertant mosaicism in an individual with non-Herlitz junctional epidermolysis bullosa.The subject has loss-of-function mutations on both alleles of the type XVII collagen gene, COL17A1, butspontaneous genetic correction of the mutation in some areas has led to patches of normal-appearingskin (areas within black marker outline) that do not blister. (From Jonkman MF et al: Revertant mosa-icism in epidermolysis bullosa caused by mitotic gene conversion. Cell 88:543, 1997, with permission.)

CHAPTER 8■


85

Genetic counseling is an integral partof the practice of dermatology. Once thediagnosis of an inherited skin disease isestablished and the mode of inheritanceis known, every dermatologist shouldbe able to advise patients correctly andappropriately, although additional sup-port from specialists in medical geneticsis often necessary. Genetic counselingmust be based on an understanding ofgenetic principles and on a familiaritywith the usual behavior of hereditaryand congenital abnormalities. It is alsoimportant to be familiar with the rangeof clinical severity of a particular dis-ease, the social consequences of the dis-order, the availability of therapy (if any),and the options for mutation detectionand prenatal testing in subsequent preg-nancies at risk for recurrence (one usefulsite is http://www.genetests.com).

A key component of genetic counsel-ing is to help parents, patients, and fami-lies know about the risks of recurrence ortransmission for a particular condition.This information is not only practical butoften relieves guilt and can allay ratherthan increase anxiety. For example, itmay not be clear to the person that he orshe cannot transmit the given disorder.The unaffected brother of a patient withan X-linked recessive disorder such asFabry disease (see Chap. 136), X-linkedichthyosis (see Chap. 47), Wiskott-Aldrich syndrome (see Chap. 144), orMenkes syndrome (see Chap. 86) neednot worry about his children being af-

fected or even carrying the abnormal al-lele, but he may not know this.

Prognosis and counseling for condi-tions such as psoriasis in which the ge-netic basis is complex or still unclear ismore difficult (see Chap. 18). Persons canbe advised, for example, that if both par-ents have psoriasis, the probability is 60percent to 75 percent that a child willhave psoriasis; if one parent and a child ofthat union have psoriasis, then thechance is 30 percent that another childwill have psoriasis; and if two normalparents have produced a child with psori-asis, the probability is 15 percent to 20percent for another child with psoriasis.35

PRENATAL DIAGNOSISIn recent years, there has been consider-able progress in developing prenatal test-ing for severe inherited skin disorders(Fig. 8-8). Initially, ultrastructural exami-nation of fetal skin biopsies was estab-lished in a limited number of conditions.In the late 1970s, the first diagnostic ex-amination of fetal skin was reported forepidermolytic hyperkeratosis and Herlitzjunctional EB (see Chap. 60).36,37 Theseinitial biopsies were performed with theaid of a fetoscope to visualize the fetus.However, with improvements in sono-graphic imaging, biopsies of fetal skin arenow taken under ultrasound guidance.The fetal skin biopsy samples obtainedduring the early 1980s could be exam-ined only by light microscopy and trans-

mission electron microscopy. However,the introduction of a number of mono-clonal and polyclonal antibodies to vari-ous basement membrane componentsduring the mid-1980s led to the develop-ment of immunohistochemical tests tohelp complement ultrastructural analysisin establishing an accurate diagnosis, es-pecially in cases of EB.38 Fetal skin biop-sies are taken during the mid-trimester.For disorders such as EB, testing at 16weeks’ gestation is appropriate. How-ever, for some forms of ichthyosis, thedisease-defining structural pathology maynot be evident at this time, and fetal skinsampling may need to be deferred until20 to 22 weeks of development.

Nevertheless, since the early 1990s, asthe molecular basis of an increasingnumber of genodermatoses has been elu-cidated, fetal skin biopsies have gradu-ally been superseded by DNA-based di-agnostic screening using fetal DNA fromamniotic fluid cells or samples of chori-onic villi; the latter are usually taken at10 to 12 weeks’ gestation (i.e., at the endof the first trimester).39,40 In addition, ad-vances with in vitro fertilization and em-bryo micromanipulation have led to thefeasibility of even earlier DNA-based as-sessment through preimplantation ge-netic diagnosis, an approach first suc-cessfully applied in 1990, for risk ofrecurrence of cystic fibrosis.41 Successfulpreimplantation testing has also been re-ported for severe inherited skin disor-ders.42 This is likely to become a more

� FIGURE 8-8 Options for prena-tal testing of inherited skin diseases.A. Fetal skin biopsy, here shown at18 weeks’ gestation. B. Chorionicvilli sampled at 11 weeks’ gestation.C. Preimplantation genetic diagno-sis. A single cell is being extractedfrom a 12-cell embryo using a suc-tion pipette.

A

BC

SECTION 3■



86

popular, though still technically challeng-ing, option for some couples, in view ofrecent advances in amplifying the wholegenome in single cells and the applica-tion of multiple linkage markers in an ap-proach termed preimplantation genetic hap-lotyping.43 For some disorders, alternativeless invasive methods of testing are nowalso being developed, including analysisof fetal DNA from within the maternalcirculation and the use of three-dimen-sional ultrasonography.

In the current absence of effectivetreatment for many hereditary skin dis-eases, prenatal diagnosis can providemuch appreciated information to cou-ples at risk of having affected children,although detailed and supportive ge-netic counseling is also a vital elementof all prenatal testing procedures.

GENE THERAPYThe field of gene therapy can be subdi-vided in different ways.44 First, there areapproaches aimed at treatment of reces-sive genetic diseases where homozygousor compound heterozygous loss-of-func-tion mutations lead to complete absenceor complete functional ablation of a vitalprotein. These types of diseases are ame-nable to gene replacement therapy, and itis this form of gene therapy that hastended to predominate because it is gen-erally technically more feasible thantreatment of dominant genetic condi-tions.45 In dermatology, these includediseases such as lamellar ichthyosis (seeChap. 47), where in most cases, there ishereditary absence of transglutaminase-1activity in the outer epidermis, or the se-vere Hallopeau-Siemens form of reces-sive dystrophic EB, where there is com-plete absence of type VII collagenexpression due to recessive mutations.46

The second form of gene therapy, inbroad terms, is aimed at treatment ofdominant-negative genetic disorders andis known as gene inhibition therapy. Here,there is a completely different type ofproblem to be tackled because these pa-tients already carry one normal copy ofthe gene and one mutated copy. The dis-ease results because an abnormal proteinproduct produced by the mutant allele,dominant-negative mutant protein, bindsto and inhibits the function of the normalprotein produced by the wild-type allele.In many cases, it can be shown from thestudy of rare recessive variants of domi-nant diseases that one allele is sufficientfor normal skin function, and so if ameans could be found of specifically in-hibiting the expression of the mutant al-lele, this should be therapeutically benefi-

cial. However, finding a gene therapyagent that is capable of discriminating thewild-type and mutant alleles, which candiffer by as little as one bp of DNA, ischallenging , until recently, has resulted inlittle success. A typical dominant-negativegenetic skin disease is EB simplex (seeChap. 60), caused by mutations in eitherof the genes encoding keratins 5 or 14.The vast majority of cases are caused bydominant-negative missense mutations,changing only a single amino acid, carriedin a heterozygous manner on one allele.47

The other way that gene therapy ap-proaches can be broadly subdivided is ac-cording to whether they involve in vivoor ex vivo strategies.44 Using an in vivoapproach, the gene therapy agent wouldbe applied directly to the patient’s skin oranother tissue. In an ex vivo approach, askin biopsy would be taken, kerati-nocytes or fibroblasts would be grownand expanded in culture, treated with thegene therapy agent, and then grafted ontoor injected back into the patient. The skinis a good organ system for both these ap-proaches because it is very accessible forin vivo applications. In addition, the skincan be readily biopsied, and cell cultureand re-grafting of keratinocytes can beadapted for ex vivo gene therapy.

A disadvantage of the skin as a targetorgan for gene therapy is that it is a bar-rier tissue that is fundamentally de-signed to prevent entry of foreign nu-cleic acid in the form of viruses or otherpathogenic agents. This is an impedi-ment to in vivo gene therapy develop-ment but is not insurmountable due todevelopments in liposome technologyand other methods for cutaneous mac-romolecule delivery.48

Gene replacement therapy systemshave been developed for lamellar ichthyo-sis (see Chap. 47) and the recessive formsof EB (see Chap. 60), among other dis-eases. These mostly consist of expressingthe normal complementary DNA encod-ing the gene of interest from some form ofgene therapy vector adapted from virusesthat can integrate their genomes stablyinto the human genome. Such viral vec-tors can therefore lead to long-term stableexpression of the replacement gene.49

Early studies tended to use retroviral vec-tors or adeno-associated viral vectors, butthese have a number of limitations. Forexample, retroviruses only transduce di-viding cells and therefore fail to targetstem cells; consequently, gene expressionis quickly lost due to turnover of the epi-dermis through keratinocyte differentia-tion. Furthermore, there have been somesafety issues in small-scale human trialsfor both retroviral and adeno-associated

viral vectors. Lentiviral vectors, derivedfrom short integrating sequences found ina number of immunodeficiency viruses,have the advantage of being able to stablytransduce dividing and non-dividing cells,with close to 100 percent efficiency and atlow copy number. These may be the cur-rent vector of choice, but they also havepotential problems because their pre-ferred integration sites in the human ge-nome are currently ill-defined and maylead to concerns about safety. However,with a wide variety of vectors under de-velopment and testing, it should becomeclear in future years which vectors are ef-fective and safe for human use. Ulti-mately, like all novel therapeutics, animaltesting can only act as a guide because thehuman genome is quite different and mayreact differently to foreign DNA integra-tion, so that phase I, II, and III human tri-als or adaptations thereof will ultimatelyhave to be performed to determine effi-cacy and safety. Currently, small-scaleclinical trials are ongoing for junctional EBand are planned for a number of othergenodermatoses, mainly concentrating onthe more severe recessive conditions.

Treatment of dominant-negative disor-ders has recently started to receive a greatdeal of attention with the discovery of theRNA inhibition pathway in humans andthe finding that small synthetic double-stranded RNA molecules of 19 to 21 bp,known as short inhibitory RNA (siRNA), canefficiently inhibit expression of humangenes in a sequence-specific, user-definedmanner.47,50 There is currently a great dealof attention being focused on develop-ment of siRNA inhibitors to selectively si-lence mutant alleles in dominant-negativegenetic diseases, such as the keratin disor-ders EB simplex or pachyonychia congen-ita. Because siRNAs can be designedagainst many different mRNA sequenceswith ease, and because they are muchsmaller molecules than gene therapy vec-tors, this new, rapidly evolving technol-ogy fits in between small molecule ther-apy and gene therapy. In a number ofcases, siRNAs have been identified thatcan discriminate between normal andmutant alleles that differ by only a singlebp mutation. In parallel, a number ofgroups are working on means of deliver-ing siRNA to skin and other organ sys-tems, and there is currently much opti-mism about these developing intoclinically applicable agents in the near fu-ture. Some small scale clinical trials areunder way and others are planned, includ-ing for keratinizing disorders. A numberof animal models have shown positive re-sults with low toxicity, including in non-human primates. However, at least one

CHAPTER 9■

BASIC SCIENCE APPROACHES TO THE PATHOPHYSIOLOGY OF SKIN DISEASE

87

study has shown liver toxicity with cer-tain sequences but not others.

KEY REFERENCES

The full reference list for all chapters is available at www.digm7.com.

1. Hsu F et al: The UCSC known genes.Bioinformatics 22:1036, 2006

2. Tsongalis GJ, Silverman LM: Molecular

diagnostics: A historical perspective.Clin Chim Acta 369:188, 2006

14. Happle R: X-chromosome inactivation:Role in skin disease expression. ActaPaediatr Suppl 95:16, 2006

20. Schapira AH: Mitochondrial disease.Lancet 368:70, 2006

26. Antonarakis SE, Beckmann JS: Mende-lian disorders deserve more attention.Nat Rev Genet 7:277, 2006

32. Callinan PA, Feinberg AP: The emergingscience of epigenomics. Hum Mol Genet15:R95, 2006

34. Resta RG: Defining and redefining thescope and goals of genetic counseling.Am J Med Genet C Semin Med Genet142:269, 2006

44. Hengge UR: Progress and prospects ofskin gene therapy: A ten year history.Clin Dermatol 23:107, 2005

45. Ferrari S et al: Gene therapy in combi-nation with tissue engineering to treatepidermolysis bullosa. Expert Opin BiolTher 6:367, 2006

C H A P T E R 9

Basic Science Approaches to the Pathophysiology of Skin DiseasePaul R. Bergstresser

TOOLS TO INVESTIGATE SKIN DISEASE

Dermatologists and skin biologists, likescientists in other disciplines, use asmany tools as possible to unravel mech-anisms of disease. They even inventtools to address the unique features ofskin. Contemporary medical science isvirtually universal in its techniques, andinvestigators from diverse fields com-monly use the same cutting-edge methodsto address the pathogenesis of disease.A significant portion of ground-breakingactivity does not occur in conventionalbasic science or clinical departments,but, rather, in department-independent“centers” (or “centers of excellence”),which combine the resources and spe-cial expertise of investigators from sev-eral disciplines.

BASIC SCIENCE APPROACHES: A HISTORICAL PERSPECTIVE

With this preamble about the universal-ity of methods used by investigators,this chapter continues with a simplequestion: “How have basic science ap-proaches to the pathophysiology of skin dis-ease fit into contemporary models ofbiomedical investigation?” This ques-tion is addressed with an assertion andfour principles (Table 9-1).

The best way to illustrate how newknowledge has been generated for skindisease is through examples of successfulachievement. What follows are three ex-amples of effective laboratory research,each with important contemporary ef-fects. These examples begin 40 yearsago, and they illustrate that the principlesfor scientific success have not changed.

Repair of Ultraviolet Radiation-Induced Damage: A Model of Discovery

BACKGROUND Treatment of skin cancerhas occupied an increasingly large por-tion of dermatologists’ clinical activitysince the 1950s, as life expectancieshave increased, and as leisure time hasled to more outdoor recreation. Impor-tantly, ultraviolet radiation (UVR) re-tains its position as the major relevantetiologic factor. Because of the centralrole of skin cancer and skin cancer ther-apy in dermatologic practice, detailedknowledge about the very rare geneticdisorder xeroderma pigmentosum servesas a useful model for how basic scienceand clinical observations have cometogether.

KNOWLEDGE LINKS TO THE LABORATORYThe seminal discovery in understandingthe pathogenesis of xeroderma pigmen-tosum was made by James Cleaverwho, as a Ph.D. scientist, headed a basicscience laboratory at the University ofCalifornia in San Francisco.1 His workhad included the development of tech-niques to study DNA repair in cells invitro. Making use of the rich intellectualenvironment of an academic medicalcenter, he introduced himself to one ofthe dermatologists, John Epstein, ask-ing whether there might be a genetichuman photosensitive skin disease inwhich there was excess of skin cancer.Dr. Epstein told him that xerodermapigmentosum would be his best choice.

STUDIES IN HUMANS The advice to exam-ine patients with xeroderma pigmento-sum led to the first set of observationsmade in vitro, that the repair of UVR-induced damage in fibroblasts taken frompatients with the disease was defective.1

Cleaver’s work was followed rapidly bya second paper from John Epstein and hisassociates, in which the repair of DNAafter ultraviolet irradiation in vivo wasobserved to be defective in both kerati-nocytes and fibroblasts.2 This work setthe stage for a series of discoveries madeby an increasingly large number of scien-tists, who demonstrated that the geneticerror in xeroderma pigmentosum was aloss in the ability to excise DNA that hadbeen damaged through irradiation.

SUBSEQUENT OBSERVATIONS Subse-quently, cell fusion studies based on theconcept of “complementation” demon-

TABLE 9-1Assertion and Principles about Basic Science Approaches to Skin Disease

• Assertion: Basic science research in derma-tology at academic medical centers is suc-cessful to the extent that it “fits” the resources and core values of that institution.

• Principle 1 (core values): Highly effective laboratory research in skin disease is more likely to occur when conducted in centers with core values and resources that pro-mote research.

• Principle 2 (people): Institutions do not con-duct research; people do. Well-trained, energetic, optimistic, and enthusiastic peo-ple do the best research, although intelli-gence helps as well.

• Principle 3 (collaboration): Research is now so complex that scientific collaboration is required. (Scientists work, not as individu-als, but in groups. Many groups are inter-national in scope.)

• Principle 4 (resources): It takes time to do research, and time costs money.

SECTION 3■



88

strated that there was a family of exci-sion defects, any one of which could beresponsible for the disease.3 Comple-mentation through cell fusion was anestablished genetic technique, and itsuse in defining xeroderma pigmentosumillustrated the application of a generalgenetic concept to skin disease. Track-ing down the specific steps in the DNAexcision process opened an entirely newfield of DNA repair,4 and a journal isnow devoted entirely to this field.5 Animportant intellectual circle has beenclosed recently: James Cleaver deliveredthe Lila Gruber Cancer Research Lectureat the Annual Meeting of the AmericanAcademy of Dermatology in 1976 inwhich he described his initial work, andin 2007 Errol Friedberg did so again, 31years later, with a presentation entitled“DNA Repair and DNA Damage Toler-ance: Fundamental Mechanisms ThatProtect Against Cancer.”

These experiments were not con-ducted in a vacuum, without attention tothe possibility of benefiting patients.In addition to preventing skin cancerthrough protection against UVR, phar-maceutical enhancement of DNA repairhas now been developed.6 It appears todiminish the rate of cancer development,at least in patients with xeroderma pig-mentosum. This line of investigation be-gan with a series of questions: (1) Is itpossible for repair enzymes from a bacte-rial source to repair UVR-induced dam-age in human cells? (2) Is it possible todevelop a liposome system that allowsone to introduce the enzymes into cells?(3) Can one obtain an effect by topicalapplication of liposomes containing theenzymes? (4) Is it possible to developand manufacture a clinical product (drug)that repairs UVR-induced damage in hu-mans? and (5) Does this product reducethe incidence of malignancy in patientswith xeroderma pigmentosum? Throughan extensive series of experiments andclinical trials, the answer to each ques-tion appears to be “yes.”

Each of these developments was rootedin the original observation of JamesCleaver, and returning to that observa-tion, what were the critical elements?

Principle 1 (core values): The Univer-sity of California in San Franciscohas been an institution where basicscience and clinical science havebeen valued and promoted for sev-eral generations.

Principle 2 (people): Dr. John Epsteinstudied photosensitive diseases,knew how to find patients withxeroderma pigmentosum, and had

energy, inventiveness, and ability.James Cleaver then took the rightquestions to the laboratory, usingcontemporary genetic techniques.

Principle 3 (collaboration): Increasingnumbers of investigators havebeen involved with each step ofthis process.

Principle 4 (resources): Talking, plan-ning, thinking, and learning all taketime. The amount of personalinvestment of time and talent hasbeen considerable.

Recessive Dystrophic Epidermolysis Bullosa and Skin Cancer: A Second Model of Discovery

BACKGROUND A related sequence of ob-servations may be found in a paper pub-lished, not 30 years ago, but 2 years ago,in which clinical insight led to a criticalset of observations.7 It began with stud-ies of recessive dystrophic epidermolysisbullosa (RDEB), which is caused by anyone of several collagen VII defects; somemutations produce no protein, and someproduce a truncated protein. This workwas derived from ongoing general stud-ies of epidermolysis bullosa at StanfordUniversity, with the ultimate goal of de-veloping techniques of gene therapy.

THE PROBLEM OF CANCER It was alsoknown that patients with RDEB com-monly develop lethal squamous cell car-cinomas. This clinical knowledge waskey to the subsequent paper. However,this work could only be conducted inlaboratories in which in vitro models ofcutaneous carcinogenesis were alreadyin development, as was happening inthe department of dermatology at Stan-ford University.

THE CRITICAL EXPERIMENTS The investi-gators examined Ras-driven tumorigene-sis in keratinocytes taken from patientswith RDEB. It was noted that cells en-tirely devoid of collagen VII did not formtumors (in mice), whereas those retainingspecific collagen VII fragments were tu-morigenic. Importantly, the forced ex-pression of fragments restored tumorige-nicity to the collagen VII-null epidermis.Finally, fibronectin-like sequences in thatportion of the fragment promoted tumorcell invasion. Thus, tumor-stroma interac-tions mediated by collagen VII appearedto promote neoplasia. The conclusion isthat the retention of collagen VII se-quences in a subset of RDEB patientsmay contribute to their increased suscep-tibility for squamous cell carcinoma.

But the critical issue was the intellec-tual context in which this work tookplace. Ultimately, it was the knowledgethat carcinogenesis is common in pa-tients with RDEB, combined with ongo-ing laboratory studies in cutaneous carci-nogenesis and in the blistering diseasesthat led to this important discovery. Ofcourse, one should not ignore the impactof energetic and inquisitive investigators.

Principle 1 (core values): Stanford Uni-versity has been an institution wherebasic science and clinical sciencehave been valued and promoted.

Principle 2 (people): Dating back to the1980s, investigators at Stanford, ledinitially by Dr. Eugene Bauer, wereinterested in epidermolysis bullosa,stemming from his work on collage-nase at Washington University inSt. Louis. With the arrival of Drs.Khavari and Marinkovich, models ofcarcinogenesis and models of blister-ing skin diseases were developed.One of the investigators developedthe idea that type VII might be a crit-ical element in carcinogenesis.

Principle 3 (collaboration): Increasingnumbers of investigators have beeninvolved with each step of thisprocess.

Principle 4 (resources): Talking, plan-ning, thinking, and learning alltake time.

Of course, success is not limited to lab-oratories in the San Francisco Bay area ofCalifornia; similar events occur in aca-demic medical centers around the world.

Pemphigus Vulgaris: One Observation Opens a New Field

BACKGROUND A landmark study in char-acterizing the pathogenesis of pemphi-gus vulgaris illustrates how one studycan precipitate decades of investigation.In the early 1960s, under the leadershipof Ernst Witebsky, Ernst Beutner and hiscolleagues at the University of Buffalohad been studying diseases in which cir-culating autoantibodies might cause in-jury to organs such as the thyroid. At thesame time, the chair of dermatology atthe medical center, James Jordon, over-saw the care of patients with pemphigus.It turned out that his son, Robert Jordan,worked as a medical student in ErnstBeutner’s immunologic laboratory ondiseases affecting organs other than skin.It should be noted that the observationthat follows did not take place in a vac-uum. Walter Lever had studied pemphi-gus for some time, and he had already

CHAPTER 9■


89

differentiated pemphigus and pemphi-goid on the basis of classical histopatho-logic observations.8 On the other hand, itwas not an accident that the observationwas made in Buffalo. Ernst Beutner hadbeen interested in fundamental aspectsof autoimmunity for many years, and hewas a participant in an active group of in-vestigators put together by Witebsky, be-ginning in 1935.

THE OBSERVATION Thus, given a conver-gence among clinical responsibilities(James Jordon), basic immunologic sci-ence (Ernst Beutner), and an enterprisingmedical student (Robert Jordon), theresearchers demonstrated by directimmunofluorescence microscopy thatpatients with pemphigus possessed au-toantibodies directed against an epider-mal intercellular substance.8,9 Antibod-ies were found precisely where thepathology develops.

SUBSEQUENT STUDIES This observationwas followed over the next 40 years by aseries of studies identifying the molecu-lar characteristics of such antibodies,their specific targets, and increasinglynovel therapies for patients with the dis-ease. In fact, there are too many investi-gators who have played important rolesin extending the original observation torecognize any one in particular, althoughthe summary of a recent conferencewritten in 2005 by Goldsmith,10 as wellas the relevant chapters in this text, aresufficient to assign credit appropriately.

So, how does this observation con-form with the rules cited previously:

Principle 1 (core values): In 1964, theUniversity of Buffalo was a rec-ognized center for immunologicresearch. Dr. Beutner had establisheda highly effective laboratory; Dr.James Jordon had access to a suitablecohort of patients with the disease,and the future Dr. Robert Jordon didthe major portion of the work.

Principle 2 (people): Drs. James Jor-don and Ernst Beutner included allthat was necessary, once the energyand enthusiasm of Robert Jordonwere harnessed.

Principle 3 (collaboration): Even 40years ago, this work could not havebeen accomplished without thecollaboration cited above.

Principle 4 (resources): This work didtake time, and the time of Robert Jor-don was offered without compen-sation. Moreover, Dr. Beutner’s labo-ratory was appropriately funded.

This last example introduces a morecomplicated set of issues, because thedisease under study is not caused by asingle gene defect. Rather, the exampleshows that investigators must addresscomplex diseases, such as psoriasis,atopic dermatitis, cutaneous T-cell lym-phoma, and toxic epidermolytic necroly-sis, in which many genes play roles. Re-cent progress in all of these diseases willbe found in the chapters that follow, andperhaps there will be even more to say inthe next edition of this book.

These examples also presage a trendtoward increased collaboration acrosslaboratories and across cities and na-tions. As a result, contemporary cutane-ous research has become international inscope. In the three examples cited abovein the sections Repair of Ultraviolet Ra-diation-Induced Damage: A Model ofDiscovery; Recessive Dystrophic Epider-molysis Bullosa and Skin Cancer: A Sec-ond Model of Discovery; and Pemphi-gus Vulgaris: One Observation Opens aNew Field; collaboration occurred pri-marily within one institution. It shouldbe noted, however, that the expertise re-quired for cutting-edge research now of-ten requires collaboration by investiga-tors in more than one institution andeven among investigators in more thanone country. This may be seen in theJournal of Investigative Dermatology (JID),which has served as one of the preferredrepositories of cutaneous research re-sults for more than 50 years. Impor-tantly, the extent of collaboration amonginvestigators has increased substantiallyover that time. Two 6-month periodsseparated by 50 years in the JID wereexamined (Table 9-2). The unequivocaland dramatic results make two points.First, the number of authors per paperhas increased four-fold in 50 years, re-flecting the increasing requirement forcollaboration among investigators. Sec-ond, collaboration now includes investi-

gators located in different institutions,sometimes in the same country (36 per-cent) and often in different countries (38percent), even from different continents(24 percent). Fully one-fourth of themanuscripts in 2006 included collaborat-ing investigators from two different con-tinents. The countries represented in1956 were six in number: the UnitedStates, Israel, Brazil, Hungary, Spain, andthe Netherlands. By 2006, 16 countrieswere represented (11 in Europe, 4 inAsia, and 1 in North America). Theworld of science is becoming smaller.

THE FUTURE OF LABORATORY INVESTIGATION IN DERMATOLOGY

A Resurgence of Clinical Science Can Be Anticipated

Although it is likely that the pace in thedevelopment of scientific technologieswill only accelerate with time, access tosuch technologies may not be the rate-limiting step in cutaneous research. Infact, a strong case may be made that thelimiting step will be the availability ofwell-characterized patient populationsfor study.11

In nearly 70 years of publication, onecan observe in the JID several transitionsin emphasis, each reflecting changes inthe scientific communities that it repre-sents. In the first half of its life, clinicalobservation was gradually replaced byexperimentation. By the 1970s, therewas increasing emphasis on laboratoryinvestigation, as was required by the cu-taneous scientists who laid the founda-tion of scientific dermatology. Of course,this was aided by knowledge that com-petition among the specialties for na-tional and international funding requiredexcellence in the laboratory. And, sincethe 1980s, one can observe continuinginterest in biochemistry and physiology,

TABLE 9-2Authors, Institutions, and Countries of Origin for Manuscriptsa

1956 (27:1–469, 1956) (JULY THROUGH DECEMBER)

2006 (126:1429–1921) (JULY AND AUGUST)

Manuscripts (total) 46 42Authors

Authors/manuscript (mean) 2.2 8.1Single-author manuscripts 14 0

Origin of manuscriptsOne institution 40 11Two institutions (one country) 6 15Two or more countries (two continents) 0 16 (10)

aPublished in two volumes of the Journal of Investigative Dermatology, separated by 50 years.

SECTION 3■



90

with a growing interest in genetics andbioinformatics. An astounding array ofgenetic characterizations and identifica-tions have been reported, however pri-marily for single gene “defects.”

More recently, however, there hasbeen growing emphasis on clinical inves-tigation and clinical science. These stud-ies have required well-characterized anduniform patient populations, and the pa-pers reveal a trend in which cohorts ofpatients are required for the cutting-edgelaboratory studies that are now reportedin the JID. It is not to say that biochemis-try and physiology and single-patientstudies have been abandoned; rather, thishas been a process of supplementation.Four examples can be cited:

1. Winter and colleagues identified asingle nucleotide polymorphism chang-ing the sequence of keratin K6HFgene, which was “required” for theexpression of pseudofolliculitis bar-bae in African American men.12 Theirdiscovery began with access to awell-defined population of Ameri-can servicemen in Germany.

2. Alamartine and colleagues reportedthat interleukin 10 promoter poly-morphisms conferred susceptibility tocutaneous squamous cell carcinomasin recipients of renal transplants.13

This discovery required access to awell-defined cohort of renal trans-plant patients.

3. Warren and colleagues reported thatautoantibodies to desmoglein 1 ofthe immunoglobulin G4 subclassalone were required for the expres-sion of endemic pemphigus foliaceusin Brazil.14 Access to multiple serasamples from patients and controlsubjects in a unique region of theworld was required for this work.

4. Palmer and colleagues studied the ef-fect of solar-simulated radiation onelicitation phases of contact sensi-tivity (contact allergic dermatitis).15

These investigators tested the hy-pothesis that patients with polymor-phic light eruption are resistant tothe expected suppression of contactsensitivity elicitation reactions byUVR. Importantly, their hypothesisarose from the earlier observationthat patients with this disease wereresistant to the UVR-induced sup-pression that occurs normally duringsensitization. Their ultimate conclu-sion that mechanisms of immuno-logic sensitization and elicitationdiverge substantially depended onthe availability of patients with aunique cutaneous disorder.

Impact of Priorities Set By the National Institutes of Health

The largest source of funding for skinresearch in the United States is the Na-tional Institute of Arthritis and Muscu-loskeletal and Skin Diseases (NIAMS), 1of 27 institutes and centers under theumbrella of the National Institutes ofHealth. The leaders at NIAMS pub-lished recently a “Long-Range Plan forResearch” (http://www.niams.nih.gov/an/stratplan/index.htm), which is useful inexamining the future of basic scienceapproaches to the pathophysiology ofskin disease. Although other instituteswithin the National Institutes of Health,especially the National Cancer Institute,also fund cutaneous research, becauseof the size of its funding, NIAMS takesthe lead in setting priorities. It should benoted that reports of this sort possesssubstantial influence on the direction ofbiomedical research, including the typesof laboratory science that are fundedand the priorities for funding. Althoughthe entire report should be examined, abrief summary provides considerable in-sight into the future. Leaders of NIAMSanticipate the following:

1. New emphasis on biologic mecha-nisms of disease, genetic and environ-mental influences, and neuroimmuneand neuroendocrine pathways;

2. A search for biomarkers of skin dis-eases;

3. Increasing emphasis on clinical re-search;

4. A search for complex genetic influ-ences on disease;

5. Interest in new animal models;6. Emphasis on research infrastructure;

and7. Emphasis on disease-specific research.

Finally, research needs and opportuni-ties are predicted by NIAMS to be in thedisciplines of:

Developmental and molecular biologyPercutaneous penetration and absorp-

tionWound healingInflammatory and immune skin dis-

easesMolecular genetics of skin diseasesTechnology researchDrug therapies and biologic agents for

skin diseasesGene therapies for skin diseases or

gene therapies that use skinRegenerative medicineClinical and outcomes researchSkin disease prevention and aging skin

Cutting-edge laboratory investigationtoday, and for the foreseeable future, willcontinue to follow the principles enu-merated in Table 9-1. It will most likelybe conducted in academic centers withcore values and resources that promoteresearch, by people who are motivatedand well trained, commonly by investi-gators who collaborate with others, andwhere financial resources are available.Finally, clinicians who care for patientswill have opportunities with increasingfrequency to play critical roles in patient-centered investigation. National bordershave virtually disappeared in cutaneousscience, as investigators collaborate of-ten, and with whomever they believe tobe appropriate. Importantly, collabora-tion with scientists in other countriesmeans that there is a chance for a livelyexchange of personal and cultural infor-mation; simply put, doing science can beinformative and rewarding.

REFERENCES1. Cleaver JE: Xeroderma pigmentosum: A

human disease in which the initial stageof DNA repair is defective. Proc NatlAcad Sci U S A 63:428, 1969

2. Epstein JH et al: Defect in DNA synthe-sis in skin of patients with xerodermapigmentosum demonstrated in vivo.Science 168:1477, 1970

3. Robbins JH, Moshell AN: DNA repairprocesses protect human beings frompremature solar skin damage—Evidencefrom studies on xeroderma pigmento-sum. J Invest Dermatol 73:102, 1979

4. Marchetto MCN et al: Gene transduc-tion in skin cells: Preventing cancer inxeroderma pigmentosum mice. ProcNatl Acad Sci U S A 101:17759, 2004

5. Friedberg EC: Growth of a journal.DNA Repair (Amst) 4:1, 2005

6. Yarosh D et al: Effect of topicallyapplied T4 endonuclease V in liposomeson skin cancer in xeroderma pigmen-tosum: A randomised study. Xero-derma Pigmentosum Study Group. Lancet357:926, 2001

7. Ortiz-Urda S et al: Type VII collagen isrequired for Ras-driven human epider-mal tumorigenesis. Science 307:1773,2005

8. Levene GM: The treatment of pemphi-gus and pemphigoid. Clin Exp Dermatol7:643, 1982

9. Beutner EH, Jordon RE: Demonstrationof skin autoantibodies in sera of pem-phigus vulgaris patients by indirectimmunofluorescent staining. Proc SocExp Biol Med 117:505, 1965

10. Goldsmith LA: Pemphigus: Pathogene-sis, pharmacology and progress. J InvestDermatol 125:vii, 2005

11. Bergstresser PR: Resurgent clinical sci-ence (and it’s all about health). J InvestDermatol 124:xvii, 2005

12. Winter H et al: An unusual Ala12Thrpolymorphism in the 1A alpha-helicalsegment of the companion layer-spe-cific keratin K6hf: Evidence for a risk

CHAPTER 9■


91

factor in the etiology of the commonhair disorder pseudofolliculitis barbae. JInvest Dermatol 122:652, 2004

13. Alamartine E et al: Interleukin-10 pro-moter polymorphisms and susceptibil-ity to skin squamous cell carcinoma

after renal transplantation. J Invest Der-matol 120:99, 2003

14. Warren SJP et al: The role of subclassswitching in the pathogenesis of endemicpemphigus foliaceus. J Invest Dermatol120:104, 2003

15. Palmer RA et al: The effect of solar-sim-ulated radiation on the elicitation phaseof contact hypersensitivity does not dif-fer between controls and patients withpolymorphic light eruption. J Invest Der-matol 124:1308, 2005

92—blank

section overview of biology, development,...

Documents