review article genetics of keratoconus: where do...

Review ArticleGenetics of Keratoconus Where Do We Stand

Khaled K Abu-Amero123 Abdulrahman M Al-Muammar1 and Altaf A Kondkar12

1 Ophthalmic Genetics Laboratory Department of Ophthalmology College of Medicine King Saud University PO Box 245Riyadh 11411 Saudi Arabia

2 Glaucoma Research Chair Department of Ophthalmology College of Medicine King Saud University PO Box 245Riyadh 11411 Saudi Arabia

3 Department of Ophthalmology College of Medicine University of Florida Jacksonville FL 32209 USA

Correspondence should be addressed to Khaled K Abu-Amero abuamerogmailcom

Received 12 June 2014 Revised 7 August 2014 Accepted 20 August 2014 Published 28 August 2014

Academic Editor Kyoung Y Seo

Copyright copy 2014 Khaled K Abu-Amero et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Keratoconus is a progressive thinning and anterior protrusion of the cornea that results in steepening and distortion of the corneaaltered refractive powers and reduced vision Keratoconus has a complex multifactorial etiology with environmental behavioraland multiple genetic components contributing to the disease pathophysiology Using genome-wide and candidate gene approachesseveral genomic loci and genes have been identified that highlight the complexmolecular etiology of this diseaseThe review focuseson current knowledge of these genetic risk factors associated with keratoconus

1 Introduction

Keratoconus is a corneal ectatic disease that results in bilateraland asymmetrical corneal distortion altered refractive pow-ers and reduced vision The disease usually manifests itselfduring the late teens or early twenties and shows a slow pro-gression for the next decade or two The clinical signs of ker-atoconus are highly variable depending on the stage of pro-gression of the disease and may include stromal thinningconical protrusion Fleischerrsquos ring Vogtrsquos striae increasedvisibility of nerve fibres and rupture in Bowmanrsquos layer [1ndash3]

Keratoconus is known to affect all ethnicities but its inci-dence exhibits geographical variability plausibly due to formefruste or subclinical forms of the disease differences in diag-nostic methods and criteria or differences in genetic vari-ations in such populations [4] Different population-basedstudies have evaluated the prevalence of keratoconus amongdifferent ethnic groups and the rates are found to be variableThe estimated incidence of keratoconus varies between 1 in500 and 1 in 2000 individuals in the general population andthe estimated prevalence is reported to be 545 per 100000 [15] Studies suggest that the prevalence and incidence rates aremuch higher among Asians than the Caucasian population[6 7] In a study undertaken in the Midlands UK a

prevalence of 4 1 and an incidence of 44 1 were reported inAsians compared to the white Caucasians [6] while anotherUK study conducted in Yorkshire found that the incidencewas 75 times higher in Asians as compared to the whites [7]Population studies in the Middle East (including the Arabsand non-Arabs) suggest that the incidence of keratoconus isbetween 20100000 and 249100000 [8 9] which is com-parable to that observed in the Asian-Pacific population [67] However in one study from India the prevalence wasreported to be as high as 23 (2300 per 100000) [10] Themean age of onset of keratoconus is reported to be 393 years[11] The severity of the disease also varies with race andwas reported to be 44 and 75 times greater among Asiangroup compared to white Caucasians [6 7] And althoughexact figures are not available keratoconus is believed tobe overrepresented and more aggressive in the Maori andPolynesian populations of New Zealand [12]

Keratoconus is amultifactorial disease involving complexinteraction of both genetic and environmental factors thatcontribute to the disease manifestation As with any otherdiseases of complex etiology differentiation between associa-tion cause and effect is very challenging and varies betweenindividuals Although keratoconus affects both genders mostof the studies suggest a preponderance of men over women

Hindawi Publishing CorporationJournal of OphthalmologyVolume 2014 Article ID 641708 11 pageshttpdxdoiorg1011552014641708

2 Journal of Ophthalmology

Table 1 Loci identified in keratoconus patients by linkage analysis and identity-by-descent approach

Locus LOD score Candidate genes excludedDagger Population Reference

1p36 8q13-q21 34 ENO1 CTNNBIP1 PLOD1UBIAD1 SPSB1 and TCEB1 Australian [18]

2p24 513 mdash Caucasian and Arab [40]3p14-q13 309 COL8A1 Italian [43]4q 5q 12p and 14q(suggestive) mdash mdash Caucasian and Hispanic [41]

5q141-q213 353 mdash Americans [39]5q21 5q32-q33 and 14q11(suggestive) mdash mdash Italian [24]

9p34 45 mdash Caucasian and Hispanic [41]14q243 358 VSX2 Mixed [42]16q22-q23 41 mdash Finnish [23]20q12dagger mdash MMP9 Australian (UK descent) [38]daggerMapped using identity-by-descent approachDaggerNo mutations were found in these screened candidate genes

[13ndash15] In the study by Georgiou et al keratoconus was 26times more common in males than in females [7] and fivetimes more common in men than women by Millodot et al[14] However there are studies that either reported no differ-ences among genders [16] or showed a greater prevalence infemales [17] In some individuals keratoconusmay be entirelyassociated with currently well recognized environmentalinfluences such as contact lens wear eye rubbing atopy of theeye and those related to increased oxidative damage such asultraviolet light and yet in another may be solely controlledby genetic mechanisms exhibiting Mendelian inheritancepattern [1 18] Studies supporting the role of consanguinityas a risk factor for keratoconus have also been documented[7 19] providing strong evidence of genetic contribution tothe disease and may be relevant in communities practicingconsanguineousmarriages such as in SaudiArabiaHoweveramajority of keratoconus cases occur as a result of the geneticpredisposition triggered by environmental factors [20] Thisreview summarizes the current knowledge on genetic aspectsof keratoconus

2 Role of Heredity in Keratoconus

Amajority of keratoconus cases are sporadic however auto-somal dominant with reduced penetrance and autosomalrecessive mode of inheritance have also been documented[21ndash24] First-degree relatives are at much higher risk of thedisease than the general population [5 25] Monozygotictwins show a high concordance of keratoconus with a greatersimilarity of phenotypes indicating a strong role of geneticcomponent(s) in the disease phenotype [26] These dataprovide strong evidence to support the role of heredity inkeratoconus

3 Genetic Studies in Keratoconus

Corneal topographical assessment in vivo confocal micros-copy Placido disk analysis and slit lamp biomicroscopy have

greatly aided the diagnosis of keratoconus [27] but are oftenhighly variable and difficult to interpret in young individ-uals with mild symptoms Identification of specific geneticmarkers may thus be a valuable tool in clinical diagnosticsMultiple genomic approaches have been used to identifychromosomal loci and genes involved in keratoconus

4 Linkage Studies

Linkage analysis is a powerful tool to map susceptiblegenetic loci and has been utilized at genome-wide level inkeratoconus To date 17 distinct genomic loci have beenmapped for keratoconus indicating that there exists highdegree of genetic heterogeneity in this disease [28] (Table 1)Unfortunately only three of these loci namely 5q21 5q32and 14q11 have been replicated independently [24] Howeverusing this approach in large extended pedigrees has led to theidentification of two potential genes (MIR184 and DOCK9)associated with keratoconus as described below (Table 2)

Using linkage analysis [21] and finemapping [29] a 55Mbregion of chromosome 15q22-q24a was identified in a singleextended pedigree from Northern Ireland with hereditarykeratoconus related with anterior capsular cataract Withdeep sequencing using next-generation sequencing technol-ogy three novel variants were identified recently in DNAJA4IREB2 andMIR184 genes that segregated with the phenotypein this family A mutation (r57 cgtu) in the micro-RNA(miRNA) geneMIR184 (OMIM 613146) was considered to bethe most likely cause [30] miRNA regulate more than 60 ofknown genes by the mechanism of mRNA degradation andinhibition of protein translation by binding to the 31015840 UTRregion of the gene and are known to affect disease phenotypes[31] MIR184 is abundantly expressed in the cornea and lensepithelia and mutation in the seed region ofMIR184 is likelyto affect its function Consistent with this finding subsequentstudies identified the same mutation in families with EDICTsyndrome (endothelial dystrophy iris hypoplasia congenitalcataract and stromal thinning) [32] and dominant congenitalcataract associated with corneal phenotype [33] However

Journal of Ophthalmology 3

Table 2 Some of the genes reported in keratoconus using different genomic approaches

Genes Method Population ReferenceMIR184 Linkage Northern Irish [30]DOCK9 Linkage Ecuadorian [36]VSX1 Candidate gene European [49]ZEB1 Candidate gene European [62]TGFB1 Candidate gene Chinese [64]COL4A3COL4A4 Candidate gene European [65]FLG Candidate gene European [70]IL1A Candidate gene Korean Chinese [54 71]IL1B Candidate gene Korean Japanese [71 72]CAST Linkagecandidate gene Americans [73]SOD1 Candidate gene Americans [77]HGF GWAS Australian Americans [102]RAB3GAP1 GWAS Americans Australian [103 104]LOX LinkageGWAS Americans [107]MPDZ-NFIBBANP-ZNF469 GWAScandidate gene EuropeanAsian and Australian [109 111]

COL5A1 LinkageGWAS Americans [112]KRT72 Gene expression European [114]TIMP1 TIMP3 BMP4andCFL1 Gene expression Korean [74]

a study in 780 keratoconus patients could identify MIR184variants in only 025 (two) patients indicating that variantsin MIR184 gene may not account for isolated keratoconus[34]

In another study in large keratoconus family of Ecuado-rian origin Gajecka and colleagues identified a region of559 Mb on chromosome 13q32 consisting of 25 genes usingwhole-genome single nucleotide polymorphism (SNP) 250Karray [35] Sequencing of eight candidate genes led to theidentification of a novel variant c2262agtc (Gln745His) inthe dedicator of cytokinesis 9 (DOCK9 OMIM 607325) genethat segregated in the family and was absent in ethnicallymatched controls [36]DOCK9 is expressed in the cornea andspecifically activates CDC42 a G-protein Although the exactmechanism by which this mutationmay cause keratoconus isnot clear it is known that the mutation is located in DHR1domain which binds phospholipids and may affect recruit-ment of the protein to the membrane and it is predicted to beldquopossibly damagingrdquo [36] Furthermore this locus containsadditional genes IPO5 (importin 5 OMIM 602008) andSTK24 (serinethreonine kinase 24 OMIM 604984) the exactrole of which in keratoconus pathogenesis is not known yetA recent study in Polish patients with sporadic keratoconushowever reported that keratoconus-related sequence variantsin these genes (DOCK9 IPO5 and STK24) were present onlyin small number of studied patients indicating that thesegenes may have minor roles to play [37]

Several other loci have been reported in keratoconusfamilies among different population (listed in Table 1) [1823 24 38ndash43] A locus on 17p13 was reported in a two-generation Pakistani family with autosomal recessive Lebercongenital amaurosis and keratoconus [44] Li et al reportedseveral regions of linkage on chromosomes 4 5 9 12 and

14 using two-stage genome-wide linkage scan in keratoconussib pair families of white and Hispanic origin [41] Similarlya genome-wide linkage scan in a large Australian pedigreeidentified two regions of linkage at chromosomal regions1p3623-3621 and 8q131-q2111 [18] Moreover Bisceglia et alprovided evidence of linkage replication in keratoconus atchromosome5q212 and suggestive linkage at chromosomalregions 5q32-q33 14q112 and 15q232 [24] Despite efforts toscreen numerous candidate genes in these loci identificationof causative genes (and mutations) however remains elusiveto a large extent This may be due to the several limitationsposed by linkage studies of complex diseases The size ofgenetic effect power of analysis to identify genes with smalleffects presence of phenocopies with reduced penetranceand various subclinical forms of keratoconus may be thereasons in many instances [45 46] With the advent of high-density SNP arrays and whole-genomeexome sequencingusing the next-generation sequencing technologies it maynow be possible to identify causal genetic variants in chromo-somal regions exhibiting linkage in families of keratoconus

5 Candidate Gene Analysis

Genetic risk factors for keratoconus have been difficult toidentify because of the complex etiology of the diseaseAmong other approaches used to identify genetic compo-nents in families with suspected dominant forms of kera-toconus candidate gene analysis has also been employed tostudy keratoconus cohorts Based on the underlying biologi-cal traits of the disease candidate genes are predicted depend-ing on their known biological functions and expressionpatterns relevant to the disease Candidate gene approaches


Table 3 Common VSX1 variants reported in patients with keratoconus

SNP ID Amino acid change Clinical significance Population Referencers74315436 Leu17Pro Pathogenic European [51 55]rs6050307 Arg131Ser Pathogenic Han Chinese [54]rs140122268 Asp144Glu Unknown European [56]rs74315434 Leu150Met Unknown Canadian Americans and European [49 51 60]rs74315433 Gly160Asp Pathogenic European [55 56]mdash Gly160Val Pathogenic Korean [57]rs74315432 Arg166Trp Pathogenic Canadian Iranian [49 52]rs148957473 His244Arg Unknown Canadian Americans and Iranian [49 52 60]VAR 014248 Pro247Arg Unknown Canadian European [49 55]

are particularly useful in studying complexmultifactorial dis-eases and enables us to identify even small gene effects usinglarge case-control cohorts Similar approach has been com-monly employed in keratoconus among different populationstargeting potential genes involved in craniofacial and oculardevelopment extracellular matrix collagens apoptosis andoxidative stress related pathways (Table 2)

Keratoconus exhibits overlapping pathophysiology withother corneal dystrophies [47] Although clinically distinctposterior polymorphous corneal dystrophy (PPCD) has beenassociated with keratoconus suggesting a common geneticpathway [48] Mutations in visual system homeobox 1 (VSX1OMIM 605020) and zinc-finger E-box binding homeobox 1(ZEB1 OMIM 189909) genes have been previously implicatedin PPCD [42 49] and hence their role in keratoconus hasbeen investigated VSX1 is a member of the paired-like ho-meodomain transcription factors family that plays a role incraniofacial and ocular development VSX1 is expressed bykeratocytes only in injured corneas and is associated withfibroblastic transformation suggesting its role in response towound and making it a potential candidate involved incorneal diseases [50] The gene is mapped to chromosome20p1121 Since the first report by Heon and colleagues [49]numerous studies have evaluated the association of variantsin VSX1 and keratoconus [4 51ndash56] Two synonymous(rs56157240 and rs12480307) and one missense tag SNP(rs6050307) in the VSX1 gene were found to be significantlyassociated with keratoconus in Han Chinese population [54]Similarly two heterozygous mutations (N151S and G160V)and an intragenic polymorphism (IVS1-11lowasta allele) weresignificantly associated with increased risk of keratoconusin Korean patients [57] The G160V accounted for 53(13249) of the study population However the results havebeen contradictory so far and this includes our own studyin Saudi cohorts that failed to show any association [58ndash61] Some of these common variants reported in VSX1 arelisted in Table 3 AlthoughVSX1 remains themost commonlystudied gene the data from various populations suggests thatvariations in VSX1 account for only 2-3 of keratoconuscases and may not have a major role in the molecularetiology of keratoconus reflecting the polygenic nature of thisdisease To date there is only one recent study in unrelatedEuropean patients supporting the role of ZEB1 mutations inkeratoconus [34 62] that needs to be replicated Another gene

associated with multiple types of corneal dystrophy includesan extracellular matrix gene transforming growth factor beta-induced (TGFBI OMIM 190180) [63] A novel nonsensemutation (G535X) has been reported in a Chinese patientwith keratoconus [64] but replication studies to confirm therole of TGFBI in keratoconus are lacking

Based on disease pathogenesis of keratoconus geneticalterations in collagen pathway that may affect corneal col-lagen structurefunction have also been investigated withoutmuch success No pathological mutations were reported in104 European keratoconus patients in the COL4A3 (OMIM120070) and COL4A4 (OMIM 120131) genes however signif-icant allelic effects were seen for D326Y variant in COL4A3and M1237V and F1644F variants in COL4A4 [65] Howevervariants in COL4A3 and COL4A4 failed to show any asso-ciation in Han Chinese population [54] Similarly variantsin COL4A1 (OMIM 120130) and COL4A2 (OMIM 120090)genes failed to segregate with the disease in Ecuadorian fami-lies [66] and the role ofCOL8A1 (OMIM120251) andCOL8A2(OMIM 120252) in keratoconus has also been excluded [67]Keratoconus is associated with eye rubbing in atopic patients[68] Filaggrin (FLG OMIM 135940) mutations are a stronggenetic risk factor for atopic dermatitis Mechanical injuryto the keratoconic corneal epithelium results in keratocyteapoptosis via the interleukin 1 (IL1) pathway causing stromalthinning [69] For these reasons FLG and IL1 have beensuggested as candidate genes for keratoconus Only fivepatients (56) were found to be carriers of at least oneFLG mutation of the two prevalent loss-of-function FLGalleles (R501X and 2282del4) indicating the need to lookfor other FLG mutations andor other candidate genes [70]IL1B (OMIM 147720) promoter polymorphisms rs1143627(minus31tgtc) and rs16966 (minus511cgtt) were first reported to beassociated with increased risk of keratoconus in Koreanpatients byKim et al [71]The study also reported a decreasedrisk of the disease with an intronic IL1A (OMIM 147760)+376cgta polymorphism (rs2071376) Recently Mikami andcolleagues replicated the association of polymorphisms inIL1B (rs1143627 minus31tgtc and rs16944 minus511cgtt) and reportedthat the haplotypewas associatedwith 172-fold increased riskof disease in Japanese keratoconus patients [72] There areno functional studies yet to identify the role of these SNPsin the development of keratoconus However the data doessuggest that polymorphisms in IL1B are important risk factors


for susceptibility to keratoconus and must be investigated inother keratoconic populations

In yet another recent study using linkage and candidategene approach Li and colleagues identified a region of5Mb on chromosome 5q15 consisting of gene encodingcalpastatin (CAST OMIM 114090) an endogenous inhibitorof calpains [73] SNP (rs4434401) located in CAST genewas associated with both familial and sporadic keratoconusmaking it an interesting candidate to be tested in other ethnicgroups Other candidate genes screened in keratoconuscohorts include TIMP3 (OMIM 188826) SPARC (OMIM182120) and SOD1 (OMIM 147450) The tissue inhibitors ofmetalloproteinases (TIMPs) are natural inhibitors of matrixmetalloproteinases and the balance between the two reg-ulates remodeling of the extracellular matrix TIMP3 hasbeen shown to be differentially expressed in keratoconiccorneas [74] However no pathogenic variants have beenidentified in keratoconus patients [51] SPARC is localizedto a chromosomal region 5q313-g32 that was suggestive oflinkage in familial keratoconus [41] making it an interestingcandidate Three novel missense mutations (E63K c187ggtaM92I c276ggta and D219E c657cgta) have been reportedin Italian patients with keratoconus that were not detectedin 200 controls however it is not clear yet whether theseare rare polymorphisms or causative mutations and needfurther investigation [51] Since oxidative stress has beenhypothesized to play a role in the etiology of keratoconus[75 76] and given the association of trisomy 21 (Downsyndrome) with keratoconus association of variants in SOD1gene localized on chromosome 21 has also been investigatedAlthough an intronic deletion in SOD1was found to segregatein two small families with keratoconus [77] this findinghas not been consistent in additional cohorts including inSaudi cohorts [78] and so far accounts for less than 1 ofkeratoconus cases

6 Keratoconus andthe Mitochondrial Connection

It was previously reported that mitochondrial oxidative stressinTet-mev-1mice causes excessive apoptosis in several tissuesleading to precocious age-dependent corneal physiologicalchanges delayed corneal epithelialization decreased cornealendothelial cells thickened Descemetrsquos membrane and thin-ning of parenchyma with corneal pathological dysfunctionssuch as keratitis Fuchsrsquo corneal dystrophy (FCD) and prob-ably keratoconus [79] Under TEM swelling of the mito-chondria was observed in keratoconus corneal tissues [80]Keratoconus corneas exhibited more mitochondrial DNA(mtDNA) damage than normal corneas [75] Keratoconusfibroblasts had increased basal generation of reactive oxygenspecies and were more susceptible to stressful challenges(low-pH andor H2O2 conditions) than were normal fibrob-lasts [81] Additionally cultured keratoconus fibroblasts havean inherent hypersensitive response to oxidative stress thatinvolves mitochondrial dysfunction and mtDNA damage[81] As a result it was suggested that keratoconus fibroblasthypersensitivity may play a role in the development and

progression of keratoconus [82] A large number of variationsincluding two novel frameshift mutations in mitochondrialcomplex I (ND1-6) gene of the mitochondrial genome havebeen reported in keratoconus patients negative for VSX1mutations [83] Similarly Abu-Amero and colleagues hadrecently shown that mtDNA mutations were present inkeratoconus patients from Saudi Arabia [84] In additionour recent study showed that the mean relative mtDNAcontent was found to be significantly higher in patients withkeratoconus than the normal control subjects [85] Futurestudies in large cohorts of multiple ethnic populations areneeded to establish the role of oxidative stress in keratoconusand the relevance of mitochondrial genes which in parts mayprovide insights into the pathophysiological mechanisms ofthis complex disease

7 Keratoconus Associated withOther Disorders

Keratoconus has been associated with numerous systemicand ocular disorders including Leber congenital amaurosis[86 87] X-linked hypohidrotic ectodermal dysplasia withmutation in EDA (OMIM 300451) gene [88] Williams-Beuren syndrome [89] brittle cornea syndrome [90]Costello syndrome [91] intellectual disability [92] and othercorneal dystrophies (PPCD and Fuchsrsquo endothelial dystro-phy) [62 93] Individuals with Down syndrome have 10ndash300-fold higher prevalence of keratoconus [1 94] Otherchromosomal abnormalities include Turner syndrome [95]chromosome 13 ring anomaly [96] and translocation 711[97] Similarly individuals with connective tissue disorderssuch as Marfan syndrome [98] Ehlers-Danlos syndrome[99] and mitral valve prolapse [100] also show an increasedprevalence of keratoconus Such observations imply thateither these disorders may provide environmental triggersfor manifestation of keratoconus or they share a commonunderlying genetic mechanism(s) However the caveat tothis plausibility is that the mutations in the causative genesfor such disorders have not been found to be enriched orcausative in isolated keratoconus cases Therefore the exactrelevance of increased prevalence of keratoconus associatedwith these genetic disorders is still not known

8 Genome-Wide Association Studies

Genome-wide association studies (GWAS) in case-controlcohorts provide a powerful platform to identify common riskvariants in complex genetic disease These studies identifySNP(s) that are in linkage disequilibriumwith causative vari-ants and require large population-based samples to achievea genome-wide significance (119875 lt 5 times 10minus8) To this endGWAShas been performed in keratoconus albeit in relativelysmall number of patients A recent segregation analysisin unrelated sporadic keratoconus families indicated thatkeratoconus is a complex non-Mendelian disease with lowgenotype-phenotype correlation and suggested the use of


GWAS epigenetics and pathway analyses in nonfamilialkeratoconus [101] UsingGWAS Burdon and colleagues iden-tified a SNP (rs3735520) at the HGF (OMIM 142409) locusto be associated with keratoconus in cohorts from AustraliaUSA and Northern Ireland [102] In another GWAS Liand colleagues identified a SNP (rs4954218) located nearRAB3GAP1 (OMIM 602536) as a potential susceptibilitylocus for keratoconus in Caucasian cohorts from USA [103]In both these GWAS the results did not achieve a genome-wide significant 119875 value The findings of SNP rs4954218 havebeen replicated in an Australian Caucasian cohort and repre-sent a strong candidate for keratoconus [104]

Li and colleagues performed a two-stage genome-widelinkage scan in keratoconus families and identified a locusat chromosome 5q232 overlapping the gene encoding lysyloxidase (LOX OMIM153455) [41]LOX is involved in cornealcollagen and elastin cross-linking [105] Artificial collagenfibre cross-linking following riboflavin and UV light expo-sure is a procedure currently being tested for the treatmentof keratoconus and therefore this gene may have therapeuticimplications [106] Two SNPs (rs10519694 and rs2956540)in LOX showing nominal genome-wide significance wereassociated with keratoconus by family-based associationtesting that were also found to be significantly associated withkeratoconus in case-control cohorts [107] Other studies sofar have provided conflicting evidence for the role of LOX inkeratoconus [51 108]

An alternative approach to case-control GWAS wouldbe to test for association with intermediate phenotypesand identify the heritable quantitative trait loci An elegantexample of this approach was demonstrated by Lu andcolleagues to identify multiple loci associated with centralcornea thickness (CCT) and keratoconus [109] In a large(gt20000) European and Asian population-based samplethe study reported 16 new loci associated with CCT atgenome-wide significance The authors then investigated theassociation of these CCT-linked loci with risk of kerato-conus in 874 cases and 6085 controls This meta-analysisidentified six SNPs within or nearby FOXO1 (rs2721051)FNDC3B (rs4894535) RXRA-COL5A1 (rs1536482) MPDZ-NFIB (rs1324183) COL5A1 (rs7044529) and BANP-ZNF469(rs9938149) genesloci that were strongly associated with therisk of keratoconus Interestingly a deleterious mutation inZNF469 (OMIM 612078) is known to cause brittle corneasyndrome which is associated with extremely thin and fragilecornea [110] However the GWAS showed an unexpectedeffect direction for SNP rs9938149 in BANP-ZNF469 withthe CCT-increasing allele leading to increased risk for kerato-conus implying that in addition to alternative mechanismswhich may be specific for keratoconus part of the geneticpredisposition to keratoconus is mediated through the genesunderlying CCT [109]The association of SNPs rs1324183 andrs9938149 with keratoconus has been replicated in an inde-pendent Australian cohort however their associationwas viaa noncorneal curvature route [111] Using a similar approachLi and colleagues identified polymorphisms (rs1536482 andrs7044529) in the COL5A1 (OMIM 120215) region whichregulate normal variation in CCT and may play a role incorneal thinning associated with keratoconus [112]

9 Chromosomal Copy Number Alterations

Copy number variation (CNV) is also a significant molecularmechanism in Mendelian diseases These variants can bedetected on a genome-wide level using microarray-basedcomparative genomic hybridization In our previous studyin 20 Saudi patients with isolated nonfamilial keratoconuswe did not detect any chromosomal variations that wouldaccount for the disease [113]

10 Gene Expression Studies

In addition to the numerous studies focused on identifyinggenes using DNA-based approaches few studies have alsoinvestigated the mRNA transcriptome to identify functionalgenes and the disease pathwaysMicroarray analysis of 11 ker-atoconus and 8 normal corneal epithelium samples resultedin identification of 47 genes thatwere upregulated and 9 genesthat were downregulated The keratin 72 (KRT72 OMIM608246) gene was the most upregulated (52-fold) gene iden-tified in this study [114] Another study identified genes inapoptosis pathway that were differentially expressed suggest-ing a role in stromal thinning [115] Mootha and colleaguesreported a 212-fold decrease in alcohol dehydrogenase 1B (classI) beta polypeptide (ADH1B OMIM 103720) mRNA levels incultured keratocytes [116] Similarly to identify differentiallyexpressed genes in human keratocytes Lee et al performedPCR-based differential analysis on cultured corneal stro-mal fibroblasts from normal and keratoconic corneas Thestudy reported upregulation of bone morphogenetic protein 4(BMP4 OMIM 112262) cofilin 1 (CFL1 OMIM 601442) andJAW1-related protein (MRVI1 604673) and downregulationof actin alpha 2 (ACTA2 OMIM 102620) gene rich clusterC 10 gene (GRCC10 OMIM 615140) tissue inhibitor of met-alloproteinases 1 and 3 (TIMP1 OMIM 305370 and TIMP3)and somatostatin receptor 1 (SSTR1 OMIM 182451) [74] Anovel cornea-expressed gene aquaporin 5 (AQP5 OMIM600442) was found to be downregulated in keratoconiccorneas as compared to controls [117] In another studyusing pooled patients corneas microarray analysis identified87 differentially expressed genes that belonged to pathwaysof apoptosis and regulation of cellular differentiation andproliferationOf these amajority of 69 geneswere found to bedownregulated controlled by transcription factor AP1 [118]

Overall the data from gene expression studies suggest arole of genes involved in apoptosis cellular differentiationand proliferation pathways which supports the proteomicdata Unfortunately none of the studies help validate genesidentified from DNA-based genetic studies with the excep-tion of extracellular matrix genes (TIMP1 and TIMP3) Inaddition the identification of AQP5 could not be replicatedin another study [119] However the role of other genes (egBMP4 CFL1) identified by differential expression analysis asa potential mediator of keratoconus requires further studiesGene expression is known to be tissue and cell specificThe use of different tissue and cell types forme fruste ofkeratoconus and methods used for expression studies maybe the likely cause(s)


11 Conclusion

There is sufficient data to suggest that keratoconus has amajorgenetic predisposition Current understanding supports acomplex etiology involving both genetic and environmentalfactors A number of genetic susceptibility loci have beenimplicated in keratoconus and there exists genetic hetero-geneity rather than a single major gene-effect responsiblefor development and progression of keratoconus Family-based studies have led to the identification of MIR184 andDOCK9 genes in cases with familial keratoconus howeverthe replication of locus identified in a specific keratoconusfamily in other families (even in homogeneous popula-tion) has been very limited Identification of a mutationin MIR184 in keratoconus opens a new area of research tostudy miRNA regulation in eye diseases Despite numerousefforts potentially pathogenic variants have been identi-fied in only small number of keratoconus patients and amajority of causative genes and mutations remain to beunraveled Although VSX1 only accounts for rare cases itstill remains the most studied keratoconus gene However itis unclear whether and how mutations in VSX1 contributeto the pathogenesis of keratoconus Other candidate genesincluding SOD1ZEB1TGFB1 FLG interleukin and collagenmay have a role to play in the pathogenesis of keratoconusand need further elucidations Similarly other candidategenes identified by GWAS includingHGF RAB3GAP1 LOXMPDZ NFIB BANP and ZNF469 may be important riskfactors for keratoconus and require replication studies inother populations Gene expression analyses are a valuabletool in addition to other approaches in identifying candidategenes and to elucidate the pathogenesis of keratoconus Inaddition identification of heritable quantitative trait locican provide an alternative approach for selection and anal-ysis of candidate genes in keratoconus [109] and has beensuccessfully demonstrated in Australian cohort [111] Thesubclinical forms of keratoconusmay be a confounding factorand complicate the identification of common genetic defectsIt is therefore important to include clinically well-definedand accurately classified patients to minimize phenotypicdifferences With increased utilization of high-density SNParrays and whole-genomeexome sequencing technologiesit will now be possible to apply genome-wide approach toidentify causal genetic variants in both familial and sporadicforms of keratoconus and at a more rapid pace until thenthe molecular pathogenesis of keratoconus remains hetero-geneous

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgment

Drs Khaled K Abu-Amero and Altaf A Kondkar aresupported by the Glaucoma Research Chair at Departmentof Ophthalmology (Supervisor Dr Saleh A Al-Obeidan)

College of Medicine King Saud University Riyadh SaudiArabia

References

[1] Y S Rabinowitz ldquoKeratoconusrdquo Survey of Ophthalmology vol42 no 4 pp 297ndash319 1998

[2] X Li H Yang and Y S Rabinowitz ldquoLongitudinal study ofkeratoconus progressionrdquo Experimental Eye Research vol 85no 4 pp 502ndash507 2007

[3] M Stabuc-Silih M Strazisar M Ravnik-Glavac M Hawlinaand D Glavac ldquoGenetics and clinical characteristics of ker-atoconusrdquo Acta Dermatovenerologica Alpina Panonnica etAdriatica vol 19 no 2 pp 3ndash10 2010

[4] A Bialasiewicz and D P Edward ldquoCorneal ectasias studycohorts and epidemiologyrdquo Middle East African Journal ofOphthalmology vol 20 no 1 pp 3ndash4 2013

[5] R H KennedyWM Bourne and J A Dyer ldquoA 48-year clinicaland epidemiologic study of keratoconusrdquo American Journal ofOphthalmology vol 101 no 3 pp 267ndash273 1986

[6] A R Pearson B Soneji N Sarvananthan and J H Sanford-Smith ldquoDoes ethnic origin influence the incidence or severityof keratoconusrdquo Eye vol 14 no 4 pp 625ndash628 2000

[7] T Georgiou C L Funnell A Cassels-Brown and R OrsquoConorldquoInfluence of ethnic origin on the incidence of keratoconus andassociated atopic disease in Asians and white patientsrdquo Eye vol18 no 4 pp 379ndash383 2004

[8] A A Assiri B I Yousuf A J Quantock P J Murphy and A AAssiri ldquoIncidence and severity of keratoconus in Asir provinceSaudi Arabiardquo British Journal of Ophthalmology vol 89 no 11pp 1403ndash1406 2005

[9] H Ziaei M R Jafarinasab M A Javadi et al ldquoEpidemiologyof keratoconus in an Iranian populationrdquo Cornea vol 31 no 9pp 1044ndash1047 2012

[10] J B Jonas V Nangia A Matin M Kulkarni and K BhojwanildquoPrevalence and associations of keratoconus in rural maharash-tra in central India the central India eye and medical studyrdquoAmerican Journal of Ophthalmology vol 148 no 5 pp 760ndash7652009

[11] K Zadnik J T Barr T B Edrington et al ldquoBaseline findingsin the collaborative longitudinal evaluation of keratoconus(CLEK) studyrdquo Investigative Ophthalmology and Visual Sciencevol 39 no 13 pp 2537ndash2546 1998

[12] H Owens G D Gamble M C Bjornholdt N K Boyce andL Keung ldquoTopographic indications of emerging keratoconusin teenage New Zealandersrdquo Cornea vol 26 no 3 pp 312ndash3182007

[13] A Ertan and O Muftuoglu ldquoKeratoconus clinical findingsaccording to different age and gender groupsrdquo Cornea vol 27no 10 pp 1109ndash1113 2008

[14] M Millodot E Shneor S Albou E Atlani and A Gordon-Shaag ldquoPrevalence and associated factors of keratoconus injerusalem a cross-sectional studyrdquo Ophthalmic Epidemiologyvol 18 no 2 pp 91ndash97 2011

[15] KHWeed C JMacEwen TGiles J Low andCN JMcGheeldquoThe Dundee University Scottish Keratoconus study demo-graphics corneal signs associated diseases and eye rubbingrdquoEye vol 22 no 4 pp 534ndash541 2008

[16] X Li Y S Rabinowitz K Rasheed and H Yang ldquoLongitudinalstudy of the normal eyes in unilateral keratoconus patientsrdquoOphthalmology vol 111 no 3 pp 440ndash446 2004


[17] J H Krachmer R S Feder and M W Belin ldquoKeratoconus andrelated noninflammatory corneal thinning disordersrdquo Survey ofOphthalmology vol 28 no 4 pp 293ndash322 1984

[18] K P Burdon D J Coster J C Charlesworth et al ldquoApparentautosomal dominant keratoconus in a large Australian pedigreeaccounted for by digenic inheritance of two novel locirdquoHumanGenetics vol 124 no 4 pp 379ndash386 2008

[19] A Gordon-Shaag M Millodot M Essa J Garth M Gharaand E Shneor ldquoIs consanguinity a risk factor for keratoconusrdquoOptometry and Vision Science vol 90 no 5 pp 448ndash454 2013

[20] A E Davidson S Hayes A J Hardcastle and S J Tuft ldquoThepathogenesis of keratoconusrdquo Eye vol 28 no 2 pp 189ndash1952014

[21] A E Hughes D P Dash A J Jackson D G Frazer and GSilvestri ldquoFamilial keratoconus with cataract linkage to thelong arm of chromosome 15 and exclusion of candidate genesrdquoInvestigative Ophthalmology and Visual Science vol 44 no 12pp 5063ndash5066 2003

[22] Y Wang Y S Rabinowitz J I Rotter and H Yang ldquoGeneticepidemiological study of keratoconus evidence for major genedeterminationrdquo American Journal of Medical Genetics vol 93no 5 pp 403ndash409 2000

[23] H Tyynismaa P Sistonen S Tuupanen et al ldquoA locus for autos-omal dominant keratoconus linkage to 16q223-q231 in Finnishfamiliesrdquo Investigative Ophthalmology and Visual Science vol43 no 10 pp 3160ndash3164 2002

[24] L Bisceglia P de Bonis C Pizzicoli et al ldquoLinkage analysisin keratoconus replication of locus 5q212 and identification ofother suggestive locirdquo Investigative Ophthalmology and VisualScience vol 50 no 3 pp 1081ndash1086 2009

[25] Y S Rabinowitz ldquoThe genetics of keratoconusrdquoOphthalmologyClinics of North America vol 16 no 4 pp 607ndash620 2003

[26] S J Tuft H Hassan S George D G Frazer C E Willoughbyand P Liskova ldquoKeratoconus in 18 pairs of twinsrdquoActa Ophthal-mologica vol 90 no 6 pp e482ndashe486 2012

[27] C A Jordan A Zamri C Wheeldon D V Patel R Johnsonand C N J McGhee ldquoComputerized corneal tomographyand associated features in a large New Zealand keratoconicpopulationrdquo Journal of Cataract and Refractive Surgery vol 37no 8 pp 1493ndash1501 2011

[28] K P Burdon and A L Vincent ldquoInsights into keratoconus froma genetic perspectiverdquo Clinical and Experimental Optometryvol 96 no 2 pp 146ndash154 2013

[29] D P Dash G Silvestri and A E Hughes ldquoFine mapping ofthe keratoconus with cataract locus on chromosome 15q andcandidate gene analysisrdquoMolecular Vision vol 12 pp 499ndash5052006

[30] A E Hughes D T Bradley M Campbell et al ldquoMutation alter-ing the miR-184 seed region causes familial keratoconus withcataractrdquoThe American Journal of Human Genetics vol 89 no5 pp 628ndash633 2011

[31] D P Bartel ldquoMicroRNAs genomics biogenesis mechanismand functionrdquo Cell vol 116 no 2 pp 281ndash297 2004

[32] B W Iliff S Amer Riazuddin and J D Gottsch ldquoA single-base substitution in the seed region of miR-184 causes EDICTsyndromerdquo Investigative Ophthalmology and Visual Science vol53 no 1 pp 348ndash353 2012

[33] B W Iliff S A Riazuddin and J D Gottsch ldquoDocumentingthe corneal phenotype associated with the MIR184 c57CgtTmtationrdquoThe American Journal of Human Genetics vol 90 no5 p 934 2012

[34] C EWilloughby J Lechner H A Bae et al ldquoMutational analy-sis ofMIR184 in sporadic keratoconus andmyopiardquo InvestigativeOphthalmology and Visual Science vol 54 no 8 pp 5266ndash52722013

[35] M Gajecka U Radhakrishna DWinters et al ldquoLocalization ofa gene for keratoconus to a 56-Mb interval on 13q32rdquo Investiga-tive Ophthalmology and Visual Science vol 50 no 4 pp 1531ndash1539 2009

[36] M Czugala J A Karolak D M Nowak et al ldquoNovel mutationand three other sequence variants segregating with phenotypeat keratoconus 13q32 susceptibility locusrdquo European Journal ofHuman Genetics vol 20 no 4 pp 389ndash397 2012

[37] J A Karolak P Polakowski J Szaflik J P Szaflik and MGajecka ldquoMolecular screening of keratoconus susceptibilitysequence variants in VSX1 TGFBI DOCK9 STK24 and IPO5genes in Polish patients and novelTGFBI variant identificationrdquoOphthalmic Genetics vol 18 pp 1ndash7 2014

[38] J Fullerton P Paprocki S Foote D A Mackey R Williamsonand S Forrest ldquoIdentity-by-descent approach to gene localisa-tion in eight individuals affected by keratoconus from north-west Tasmania Australiardquo Human Genetics vol 110 no 5 pp462ndash470 2002

[39] Y G Tang Y S Rabinowitz K D Taylor et al ldquoGenomewidelinkage scan in a multigeneration Caucasian pedigree identifiesa novel locus for keratoconus on chromosome 5q143-q211rdquoGenetics in Medicine vol 7 no 6 pp 397ndash405 2005

[40] H Hutchings H Ginisty M Le Gallo et al ldquoIdentification of anew locus for isolated familial keratoconus at 2p24rdquo Journal ofMedical Genetics vol 42 no 1 pp 88ndash94 2005

[41] X Li Y S Rabinowitz Y G Tang et al ldquoTwo-stage genome-wide linkage scan in keratoconus sib pair familiesrdquo InvestigativeOphthalmology and Visual Science vol 47 no 9 pp 3791ndash37952006

[42] P LiskovaM Filipec SMerjava K Jirsova and S J Tuft ldquoVari-able ocular phenotypes of posterior polymorphous cornealdystrophy caused by mutations in the ZEB1 generdquo OphthalmicGenetics vol 31 no 4 pp 230ndash234 2010

[43] F Brancati E M Valente A Sarkozy et al ldquoA locus for autoso-mal dominant keratoconus maps to human chromosome 3p14-q13rdquo Journal ofMedical Genetics vol 41 no 3 pp 188ndash192 2004

[44] A Hameed S Khaliq M Ismail et al ldquoA novel locus forLeber congenital amaurosis (LCA4) with anterior keratoconusmapping to chromosome 17p13rdquo Investigative Ophthalmologyand Visual Science vol 41 no 3 pp 629ndash633 2000

[45] J Altmuller L J Palmer G Fischer H Scherb and M WjstldquoGenomewide scans of complex human diseases true linkage ishard to findrdquoThe American Journal of Human Genetics vol 69no 5 pp 936ndash950 2001

[46] D Altshuler M J Daly and E S Lander ldquoGenetic mapping inhuman diseaserdquo Science vol 322 no 5903 pp 881ndash888 2008

[47] F A Cremona F R Ghosheh C J Rapuano et al ldquoKeratoconusassociated with other corneal dystrophiesrdquo Cornea vol 28 no2 pp 127ndash135 2009

[48] H Y Lam J L Wiggs and U V Jurkunas ldquoUnusual presenta-tion of presumed posterior polymorphous dystrophy associatedwith iris heterochromia band keratopathy and keratoconusrdquoCornea vol 29 no 10 pp 1180ndash1185 2010

[49] E Heon A Greenberg K K Kopp et al ldquoVSX1 a gene forposterior polymorphous dystrophy and keratoconusrdquo HumanMolecular Genetics vol 11 no 9 pp 1029ndash1036 2002


[50] V Barbaro E di Iorio S Ferrari et al ldquoExpression of VSX1 inhuman corneal keratocytes during differentiation into myofi-broblasts in response to wound healingrdquo Investigative Ophthal-mology and Visual Science vol 47 no 12 pp 5243ndash5250 2006

[51] P de Bonis A Laborante C Pizzicoli et al ldquoMutational screen-ing of VSX1 SPARC SOD1 LOX and TIMP3 in keratoconusrdquoMolecular Vision vol 17 pp 2482ndash2494 2011

[52] S Saee-Rad H Hashemi M Miraftab et al ldquoMutation analysisof VSX1 and SOD1 in Iranian patients with keratoconusrdquoMolecular Vision vol 17 pp 3128ndash3136 2011

[53] P Paliwal R Tandon D Dube P Kaur and A Sharma ldquoFamil-ial segregation of a VSX1mutation adds a new dimension to itsrole in the causation of keratoconusrdquo Molecular Vision vol 17pp 481ndash485 2011

[54] Y Wang T Jin X Zhang et al ldquoCommon single nucleotidepolymorphisms and keratoconus in the Han Chinese popula-tionrdquo Ophthalmic Genetics vol 34 no 3 pp 160ndash166 2013

[55] L Bisceglia M Ciaschetti P de Bonis et al ldquoVSX1 mutationalanalysis in a series of Italian patients affected by keratoconusdetection of a novel mutationrdquo Investigative Ophthalmology andVisual Science vol 46 no 1 pp 39ndash45 2005

[56] D P Dash S George D Orsquoprey et al ldquoMutational screening ofVSX1 in keratoconus patients from the European populationrdquoEye vol 24 no 6 pp 1085ndash1092 2010

[57] J-W Mok S-J Baek and C-K Joo ldquoVSX1 gene variantsare associated with keratoconus in unrelated Korean patientsrdquoJournal of Human Genetics vol 53 no 9 pp 842ndash849 2008

[58] M Stabuc-Silih M Strazisar M Hawlina and D GlavacldquoAbsence of pathogenic mutations in VSX1 and SOD1 genes inpatients with keratoconusrdquo Cornea vol 29 no 2 pp 172ndash1762010

[59] K K Abu-Amero H Kalantan andAM Al-Muammar ldquoAnal-ysis of the VSX1 gene in keratoconus patients from SaudiArabiardquoMolecular Vision vol 17 pp 667ndash672 2011

[60] Y G Tang Y Picornell X Su X Li H Yang and Y S Rabi-nowitz ldquoThree VSX1 gene mutations L159M R166W andH244R are not associated with keratoconusrdquo Cornea vol 27no 2 pp 189ndash192 2008

[61] A Verma M Das M Srinivasan N V Prajna and P Sundare-san ldquoInvestigation of VSX1 sequence variants in South Indianpatients with sporadic cases of keratoconusrdquo BMC ResearchNotes vol 6 no 1 article 103 2013

[62] J Lechner D P Dash D Muszynska et al ldquoMutational spec-trum of the ZEB1 gene in corneal dystrophies supports agenotype-phenotype correlationrdquo Investigative Ophthalmologyand Visual Science vol 54 no 5 pp 3215ndash3223 2013

[63] F L Munier B E Frueh P Othenin-Girard et al ldquoBIGH3mutation spectrum in corneal dystrophiesrdquo Investigative Oph-thalmology and Visual Science vol 43 no 4 pp 949ndash954 2002

[64] T Guan C Liu Z Ma and S Ding ldquoThe point mutation andpolymorphism in keratoconus candidate gene TGFBI in Chi-nese populationrdquo Gene vol 503 no 1 pp 137ndash139 2012

[65] M Stabuc-Silih M Ravnik-Glavac D Glavac M Hawlina andM Strazisar ldquoPolymorphisms in COL4A3 and COL4A4 genesassociated with keratoconusrdquo Molecular vision vol 15 pp2848ndash2860 2009

[66] J A Karolak K Kulinska D M Nowak et al ldquoSequencevariants in COL4A1 and COL4A2 genes in ecuadorian familieswith keratoconusrdquoMolecular Vision vol 17 pp 827ndash843 2011

[67] A J Aldave N Bourla V S Yellore et al ldquoKeratoconus is notassociated with mutations in COL8A1 and COL8A2rdquo Corneavol 26 no 8 pp 963ndash965 2007

[68] A M Bawazeer W G Hodge and B Lorimer ldquoAtopy andkeratoconus amultivariate analysisrdquoBritish Journal of Ophthal-mology vol 84 no 8 pp 834ndash836 2000

[69] S E Wilson Y-G He J Weng et al ldquoEpithelial injury induceskeratocyte apoptosis hypothesized role for the interleukin-1system in the modulation of corneal tissue organization andwound healingrdquo Experimental Eye Research vol 62 no 4 pp325ndash337 1996

[70] C Droitcourt D Touboul C Ged et al ldquoA prospective study offilaggrin null mutations in keratoconus patients with or withoutatopic disordersrdquoDermatology vol 222 no 4 pp 336ndash341 2011

[71] S H Kim J W Mok H S Kim and C K Joo ldquoAssociation ofminus31TgtC and minus511 CgtT polymorphisms in the interleukin 1 beta(IL1B) promoter in Korean keratoconus patientsrdquo MolecularVision vol 14 pp 2109ndash2116 2008

[72] T Mikami A Meguro T Teshigawara et al ldquoInterleukin 1 betapromoter polymorphism is associated with keratoconus in aJapanese populationrdquo Molecular Vision vol 19 pp 845ndash8512013

[73] X Li Y Bykhovskaya Y G Tang et al ldquoAn association betweenthe calpastatin (CAST) gene and keratoconusrdquo Cornea vol 32no 5 pp 696ndash701 2013

[74] J-E Lee B S Oum H Y Choi S U Lee and J S Lee ldquoEvalua-tion of differentially expressed genes identified in keratoconusrdquoMolecular Vision vol 15 pp 2480ndash2487 2009

[75] S R Atilano P CoskunM Chwa et al ldquoAccumulation ofmito-chondrial DNA damage in keratoconus corneasrdquo InvestigativeOphthalmology and Visual Science vol 46 no 4 pp 1256ndash12632005

[76] K A Wojcik A Kaminska J Blasiak J Szaflik and J P SzaflikldquoOxidative stress in the pathogenesis of keratoconus and Fuchsendothelial corneal dystrophyrdquo International Journal of Molecu-lar Sciences vol 14 no 9 pp 19294ndash19308 2013

[77] N Udar S R Atilano D J Brown et al ldquoSOD1 a candidategene for keratoconusrdquo Investigative Ophthalmology and VisualScience vol 47 no 8 pp 3345ndash3351 2006

[78] A M Al-Muammar H Kalantan T A Azad T Sultan andK K Abu-Amero ldquoAnalysis of the SOD1 gene in keratoconuspatients from Saudi Arabiardquo Ophthalmic Genetics 2014

[79] T Ishii M Miyazawa H Onouchi K Yasuda P S Hartmanand N Ishii ldquoModel animals for the study of oxidative stressfrom complex IIrdquo Biochimica et Biophysica Acta vol 1827 no 5pp 588ndash597 2013

[80] M Aktekin M F Sargon P Cakar H H Celik and E FiratldquoUltrastructure of the cornea epithelium in keratoconusrdquo Oka-jimas Folia Anatomica Japonica vol 75 no 1 pp 45ndash53 1998

[81] M Chwa S R Atilano V Reddy N Jordan D W Kim andM C Kenney ldquoIncreased stress-induced generation of reactiveoxygen species and apoptosis in human keratoconus fibrob-lastsrdquo Investigative Ophthalmology and Visual Science vol 47no 5 pp 1902ndash1910 2006

[82] M Chwa S R Atilano D Hertzog et al ldquoHypersensitiveresponse to oxidative stress in keratoconus corneal fibroblastsrdquoInvestigative Ophthalmology and Visual Science vol 49 no 10pp 4361ndash4369 2008

[83] D Pathak B Nayak M Singh N Sharma J S Titiyal and RDada ldquoMitochondrial complex 1 gene analysis in keratoconusrdquoMolecular Vision vol 17 pp 1514ndash1525 2011

[84] K K Abu-Amero T A Azad H Kalantan T Sultan and AM Al-Muammar ldquoMitochondrial sequence changes in kerato-conus patientsrdquo Investigative Ophthalmology amp Visual Sciencevol 55 no 3 pp 1706ndash1710 2014


[85] K K Abu-Amero A A Kondkar T A Azad T Sultan HKalantan and A M Al-Muammar ldquoKeratoconus increases thecopy number of mitochondrial DNArdquo Molecular Vision Inpress

[86] M J Elder ldquoLeber congenital amaurosis and its associationwithkeratoconus and keratoglobusrdquo Journal of Pediatric Ophthal-mology and Strabismus vol 31 no 1 pp 38ndash40 1994

[87] T T McMahon L S Kim G A Fishman et al ldquoCRB1 genemutations are associatedwith keratoconus in patients with lebercongenital amaurosisrdquo Investigative Ophthalmology and VisualScience vol 50 no 7 pp 3185ndash3187 2009

[88] M Piccione G Serra C Sanfilippo E Andreucci I Sani andG Corsello ldquoA new mutation in EDA gene in X-linked hypo-hidrotic ectodermal dysplasia associated with keratoconusrdquoMinerva Pediatrica vol 64 no 1 pp 59ndash64 2012

[89] MM VianaM Frasson L L Leao et al ldquoA new case of kerato-conus associated with williams-beuren syndromerdquo OphthalmicGenetics vol 34 no 3 pp 174ndash177 2013

[90] H Al-Hussain S M Zeisberger P R Huber C Giunta and BSteinmann ldquoBrittle cornea syndrome and its delineation fromthe kyphoscoliotic type of Ehlers-Danlos syndrome (EDS VI)report on 23 patients and review of the literaturerdquo AmericanJournal of Medical Genetics vol 124A no 1 pp 28ndash34 2004

[91] K W Gripp and L A Demmer ldquoKeratoconus in CostellosyndromerdquoAmerican Journal of Medical Genetics A vol 161 no5 pp 1132ndash1136 2013

[92] B Koller T F Neuhann and I M Neuhann ldquoKeratoplasty inpatients with intellectual disabilityrdquo Cornea vol 33 no 1 pp10ndash13 2014

[93] R M Lipman J B Rubenstein and E Torczynski ldquoKerato-conus and Fuchsrsquo corneal endothelial dystrophy in a patient andher familyrdquo Archives of Ophthalmology vol 108 no 7 pp 993ndash994 1990

[94] A L Vincent B AWeiserMCupryn RM SteinMAbdolelland A V Levin ldquoComputerized corneal topography in apaediatric population with Down syndromerdquo Clinical andExperimental Ophthalmology vol 33 no 1 pp 47ndash52 2005

[95] MMacsai E Maguen and P Nucci ldquoKeratoconus and Turnerrsquossyndromerdquo Cornea vol 16 no 5 pp 534ndash536 1997

[96] C J Heaven F Lalloo and E McHale ldquoKeratoconus associatedwith chromosome 13 ring abnormalityrdquo British Journal ofOphthalmology vol 84 no 9 p 1079 2000

[97] D A Morrison E M Rosser and C Claoue ldquoKeratoconusassociated with a chromosome 7 11 translocationrdquo Eye vol 15no 4 pp 556ndash557 2001

[98] H N Bass R S Sparkes B F Crandall and S MMarcy ldquoCon-genital contractural arachnodactyly keratoconus and probableMarfan syndrome in the same pedigreerdquo Journal of Pediatricsvol 98 no 4 pp 591ndash593 1981

[99] M Rohrbach A Vandersteen U Yi et al ldquoPhenotypic vari-ability of the kyphoscoliotic type of Ehlers-Danlos syndrome(EDS VIA) clinical molecular and biochemical delineationrdquoOrphanet Journal of Rare Diseases vol 6 no 1 article 46 2011

[100] E K Akcay M Akcay B S Uysal et al ldquoImpaired cornealbiomechanical properties and the prevalence of keratoconusin mitral valve prolapserdquo Journal of Ophthalmology vol 2014Article ID 402193 6 pages 2014

[101] A Kriszt G Losonczy A Berta G Vereb and L Takacs ldquoSeg-regation analysis suggests that keratoconus is a complex non-mendelian diseaserdquo Acta Ophthalmologica 2014

[102] K P Burdon S Macgregor Y Bykhovskaya et al ldquoAssocia-tion of polymorphisms in the hepatocyte growth factor genepromoter with keratoconusrdquo Investigative Ophthalmology andVisual Science vol 52 no 11 pp 8514ndash8519 2011

[103] X Li Y Bykhovskaya T Haritunians et al ldquoA genome-wideassociation study identifies a potential novel gene locus forkeratoconus one of the commonest causes for corneal trans-plantation in developed countriesrdquoHuman Molecular Geneticsvol 21 no 2 pp 421ndash429 2012

[104] H A Bae R A D Mills R G Lindsay et al ldquoReplicationand meta-analysis of candidate loci identified variation atRAB3GAP1 associated with keratoconusrdquo Investigative Ophthal-mology and Visual Science vol 54 no 7 pp 5132ndash5135 2013

[105] L Dudakova P Liskova T Trojek M Palos S Kalasova andK Jirsova ldquoChanges in lysyl oxidase (LOX) distribution andits decreased activity in keratoconus corneasrdquo Experimental EyeResearch vol 104 pp 74ndash81 2012

[106] P T Ashwin and P J McDonnell ldquoCollagen cross-linkagea comprehensive review and directions for future researchrdquoBritish Journal of Ophthalmology vol 94 no 8 pp 965ndash9702010

[107] Y Bykhovskaya X Li I Epifantseva et al ldquoVariation in the lysyloxidase (LOX) gene is associated with keratoconus in family-based and case-control studiesrdquo Investigative Ophthalmologyand Visual Science vol 53 no 7 pp 4152ndash4157 2012

[108] F Hasanian-Langroudi R Saravani M H Validad G Bahariand D Yari ldquoAssociation of Lysyl oxidase (LOX) polymorphismswith the risk of Keratoconus in an Iranian populationrdquo Oph-thalmic Genetics 2014

[109] Y Lu V Vitart K P Burdon et al ldquoGenome-wide associationanalyses identify multiple loci associated with central cornealthickness and keratoconusrdquo Nature Genetics vol 45 no 2 pp155ndash163 2013

[110] A Abu M Frydman D Marek E Pras U Nir and H Reznik-Wolf ldquoDeleterious mutations in the Zinc-Finger 469 gene causebrittle cornea syndromerdquo The American Journal of HumanGenetics vol 82 no 5 pp 1217ndash1222 2008

[111] S Sahebjada M Schache A J Richardson et al ldquoEvaluatingthe association between keratoconus and the corneal thicknessgenes in an independent Australian populationrdquo InvestigativeOphthalmology amp Visual Science vol 54 no 13 pp 8224ndash82282013

[112] X Li Y Bykhovskaya A L C Canedo et al ldquoGenetic associa-tion ofCOL5A1 variants in keratoconus patients suggests a com-plex connection between corneal thinning and keratoconusrdquoInvestigative Ophthalmology and Visual Science vol 54 no 4pp 2696ndash2704 2013

[113] K K Abu-Amero A M Hellani S M Al Mansouri H Kalan-tan and A M Al-Muammar ldquoHigh-resolution analysis ofDNA copy number alterations in patients with isolated sporadickeratoconusrdquoMolecular Vision vol 17 pp 822ndash826 2011

[114] K Nielsen K Birkenkamp-Demtroder N Ehlers and T FOrntoft ldquoIdentification of differentially expressed genes inkeratoconus epithelium analyzed on microarraysrdquo InvestigativeOphthalmology andVisual Science vol 44 no 6 pp 2466ndash24762003

[115] T H Nguyen K Nakayasu A Murakami K Ishidoh and AKanai ldquoMicroarray analysis identified differentially expressedgenes in keratocytes from keratoconus patientsrdquo Current EyeResearch vol 28 no 6 pp 373ndash379 2004


[116] V V Mootha J M Kanoff J Shankardas and S DimitrijevichldquoMarked reduction of alcohol dehydrogenase in keratoconuscorneal fibroblastsrdquoMolecular Vision vol 15 pp 706ndash712 2009

[117] Y S Rabinowitz L Dong and G Wistow ldquoGene expressionprofile studies of human keratoconus cornea for NEIBank anovel cornea-expressed gene and the absence of transcripts foraquaporin 5rdquo Investigative Ophthalmology and Visual Sciencevol 46 no 4 pp 1239ndash1246 2005

[118] MMace S DGaliacy A Erraud et al ldquoComparative transcrip-tome and network biology analyses demonstrate antiprolifer-ative and hyperapoptotic phenotypes in human keratoconuscorneasrdquo Investigative Ophthalmology and Visual Science vol52 no 9 pp 6181ndash6191 2011

[119] Y Garfias A Navas H J Perez-Cano J Quevedo L Vil-lalvazo and J C Zenteno ldquoComparative expression analysisof aquaporin-5 (AQP5) in keratoconic and healthy corneasrdquoMolecular Vision vol 14 pp 756ndash761 2008

Submit your manuscripts athttpwwwhindawicom

Stem CellsInternational

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014


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Behavioural Neurology

EndocrinologyInternational Journal of



Disease Markers


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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Immunology ResearchHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Parkinsonrsquos Disease

Evidence-Based Complementary and Alternative Medicine

Volume 2014Hindawi Publishing Corporationhttpwwwhindawicom


Table 1 Loci identified in keratoconus patients by linkage analysis and identity-by-descent approach

Locus LOD score Candidate genes excludedDagger Population Reference

1p36 8q13-q21 34 ENO1 CTNNBIP1 PLOD1UBIAD1 SPSB1 and TCEB1 Australian [18]

2p24 513 mdash Caucasian and Arab [40]3p14-q13 309 COL8A1 Italian [43]4q 5q 12p and 14q(suggestive) mdash mdash Caucasian and Hispanic [41]

5q141-q213 353 mdash Americans [39]5q21 5q32-q33 and 14q11(suggestive) mdash mdash Italian [24]

9p34 45 mdash Caucasian and Hispanic [41]14q243 358 VSX2 Mixed [42]16q22-q23 41 mdash Finnish [23]20q12dagger mdash MMP9 Australian (UK descent) [38]daggerMapped using identity-by-descent approachDaggerNo mutations were found in these screened candidate genes

[13ndash15] In the study by Georgiou et al keratoconus was 26times more common in males than in females [7] and fivetimes more common in men than women by Millodot et al[14] However there are studies that either reported no differ-ences among genders [16] or showed a greater prevalence infemales [17] In some individuals keratoconusmay be entirelyassociated with currently well recognized environmentalinfluences such as contact lens wear eye rubbing atopy of theeye and those related to increased oxidative damage such asultraviolet light and yet in another may be solely controlledby genetic mechanisms exhibiting Mendelian inheritancepattern [1 18] Studies supporting the role of consanguinityas a risk factor for keratoconus have also been documented[7 19] providing strong evidence of genetic contribution tothe disease and may be relevant in communities practicingconsanguineousmarriages such as in SaudiArabiaHoweveramajority of keratoconus cases occur as a result of the geneticpredisposition triggered by environmental factors [20] Thisreview summarizes the current knowledge on genetic aspectsof keratoconus

2 Role of Heredity in Keratoconus

Amajority of keratoconus cases are sporadic however auto-somal dominant with reduced penetrance and autosomalrecessive mode of inheritance have also been documented[21ndash24] First-degree relatives are at much higher risk of thedisease than the general population [5 25] Monozygotictwins show a high concordance of keratoconus with a greatersimilarity of phenotypes indicating a strong role of geneticcomponent(s) in the disease phenotype [26] These dataprovide strong evidence to support the role of heredity inkeratoconus

3 Genetic Studies in Keratoconus

Corneal topographical assessment in vivo confocal micros-copy Placido disk analysis and slit lamp biomicroscopy have

greatly aided the diagnosis of keratoconus [27] but are oftenhighly variable and difficult to interpret in young individ-uals with mild symptoms Identification of specific geneticmarkers may thus be a valuable tool in clinical diagnosticsMultiple genomic approaches have been used to identifychromosomal loci and genes involved in keratoconus

4 Linkage Studies

Linkage analysis is a powerful tool to map susceptiblegenetic loci and has been utilized at genome-wide level inkeratoconus To date 17 distinct genomic loci have beenmapped for keratoconus indicating that there exists highdegree of genetic heterogeneity in this disease [28] (Table 1)Unfortunately only three of these loci namely 5q21 5q32and 14q11 have been replicated independently [24] Howeverusing this approach in large extended pedigrees has led to theidentification of two potential genes (MIR184 and DOCK9)associated with keratoconus as described below (Table 2)

Using linkage analysis [21] and finemapping [29] a 55Mbregion of chromosome 15q22-q24a was identified in a singleextended pedigree from Northern Ireland with hereditarykeratoconus related with anterior capsular cataract Withdeep sequencing using next-generation sequencing technol-ogy three novel variants were identified recently in DNAJA4IREB2 andMIR184 genes that segregated with the phenotypein this family A mutation (r57 cgtu) in the micro-RNA(miRNA) geneMIR184 (OMIM 613146) was considered to bethe most likely cause [30] miRNA regulate more than 60 ofknown genes by the mechanism of mRNA degradation andinhibition of protein translation by binding to the 31015840 UTRregion of the gene and are known to affect disease phenotypes[31] MIR184 is abundantly expressed in the cornea and lensepithelia and mutation in the seed region ofMIR184 is likelyto affect its function Consistent with this finding subsequentstudies identified the same mutation in families with EDICTsyndrome (endothelial dystrophy iris hypoplasia congenitalcataract and stromal thinning) [32] and dominant congenitalcataract associated with corneal phenotype [33] However






































11 Conclusion




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11 Conclusion




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OncologyJournal of





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OncologyJournal of





PPAR Research



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ObesityJournal of
















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