Cytogenetics IICytogenetics IIStructural chromosomal Structural chromosomal

aberrationsaberrations

RNDr Z.PolívkováRNDr Z.Polívková

Lecture No 432Lecture No 432 - - coursecourse:: Heredity Heredity

Causes of structural aberrations:

external mutagens (except Robertsonian translocations)

origin in: G1,S, G2, mitosis, meiosis

Structural CHA:

• unbalanced – loss or gain of chromosomal material

• balanced – abnormal rearrangement without loss or gain of chromosomal material

Unbalanced• Deletion (del) = partial monosomy – terminal

- interstitial - break and loss of chromosomal segment in G1

- unequal crossing over in meiosis

- segregation of balanced aberration in meiosis → unbalanced product

• Duplication (dup) = partial trisomy

- duplication in S phase,

- insertion of a segment of sister chromatid

- unequal crossing over in meiosis,

- segregation of balanced aberration in meiosis

• Ring chromosome (r) – partial monosomy of segments distal to

breaks on short and long arms - reunion of broken chromosome to a ring formation

• Dicentric chromosome (dic) – abnormal chromosome with 2 centromeres - 2 breaks of 2 chromosomes or 2 chromatids and

reunion of broken ends, in G1, G2

• Isochromosome – partial monosomy of one arm and partial trisomy of other arm

- misdivision (transverse splitting) of centromere in MI, MII, mitosis

• Additional marker chromosome (+mar) small chromosome of unknown origin – supernumerrary - if heterochromatic – mostly without clinical consequences

Detection of origin of marker chromosome – FISH method

Balanced CHA:

• Robertsonian translocation (Rob t)– fusion of 2 acrocentrics near centromere – origin in meiosis-by nonhomologous pairing and exchange similar to crossing over

• Reciprocal translocation (rcp t) – reciprocal exchange of 2 segments of 2 chromosomes – in G1,G2 – breaks and exchanges

• Inversion (inv) - pericentric – 2 breaks on p (short arms) and q (long arms) and reconstitutions of inverted segment between breaks – in G1

- paracentric – 2 breaks on one arm and

reconstitutions of inverted segment between breaks – in G1

• Insertion – segment removed from 1 chromosome is inserted into another chromosome – 3 breaks rearrangement

Deletions

terminal interstitial

break, (2 breaks) and loss of terminal (interstitial) segment

in G1

Terminal deletion Xp – short stature

Terminal deletion Xq -steriliy

Origin of interstitial deletion and duplication

unequal crossing over between homologs or unequal exchange between sister chromatids

Origin of interstitial deletion and duplication

insertion of a segment of sister chromatid in G2

Interstitial deletion 16q – congenital abnormalities + MR

Interstitial deletion 16q

Interstitial duplication 2q – congenital abnormalities + MR

dup2q

Ring chromosome

breaks on p and q and joining of broken ends

Ring chromosome X – patient with TS – mosaic with 45,X

Dicentric chromosome

translocation dicentric isodicentric + acentric fragment breaks on both chromatids and chromatid reunion

Breaks of 2 chromosomes and fusion inactivation of 1 centromere = pseudodicentricof centric (and acentric) fragmens

SS

dicentric (pseudodicentric) chromosome X from 2 cells, C-band

Isochromosome

missdivision of centromere – loss of one arm and duplication of the second arm – i(q)

patient with clinical features of TS but fertile

Robertsonian translocation

break on 2 acrocentrics near centromere and fusion of long arms (fused product of short arms is lost) – origin usually in meiosis

balanced Robertsonian translocation

unbalanced Robertsonian translocation - DS

Origin of reciprocal translocation

chromatid exchange in G2 breaks and exchange in G1

chromatid breaks on 2 chromosomes

and exchange

t(4;6) reciprocal translocation-balanced

der(6)t(4;6) derivative chromosome 6 – congenital abnormalities + MR

t(4;6) der (6)

Origin of inversions

pericentric paracentric

2 breaks on p and q 2 breaks on one arm

inversion and joining of inverted segment

pericentric inversion 6 - balanced

recombinant chromosome 6 – congenital abnormalities + MR

Inversion 6 recombinant chromosome 6

paracentric inversion 1q

balanced

Origin of insertion

insertion of interstitial segment of one chromosome to the site of break on another chromosome - in G1

ins(10;14) - insertion of a part of 14q to 10q - balanced

ins(10;14)

CCR(1;6;14;18) - complex chromosomal rearrangement - balanced

Risk of balanced structural aberration:

Carrier of balanced structural aberration

usually without clinical signs

risk of unbalanced aberration in progeny

+

Carrier ofbalanc. Rob.t normal

Nonhomologous Rob t

gametes

after MI

Zygotes after S phase

transl.trisomy carrier of balanc.t normal monosomy 21 1/3 1/3 1/3 letal theoretic risk

Theoretic risk of translocation form of Down syndrome (DS)

in parent - carrier = 1/3

Actual risk: 10-15% for woman - carrier of t21/14

2% for man - carrier of t21/14

For other nonhomologous translocations lower risks (21/22, 13/14)

Mechanisms of selection against chromosomal anomaly:• some types of segregation are less probable (influenced by morphology of rearranged chromosomes)

• in ♀ meiosis= cell with CHA more probably become polar body

• in ♂ meiosis – irregularities in pairing of rearranged chromosomes → poor sperm development → oligospermia, azoospermia

• gamete with CHA – selectional disadvantage in fertilisation (in sperm)

• selection against abnormal zygote = spontaneous abortion

46,XX

22,X-21

23,Xt21/21

23,X

45,XX

t21/21

23,Xt21/21

23,Xt21/21

23,Y

23,X

23,Xt21/21

22,X-21

46,XY

t21/21

46,XY

t21/21

+ +

100% (DS or abortion) 0%

Homologous Rob t

21/21 (13/13)

de novo origin

risk risk

Carrier of balanced Robt

Normal karyotype

translocation form of DS (homologous fusion 21/21)

Reciprocal translocation

N1 T1 T2 N2

T2

N1

N2

T2

in M I prophase - kvadrivalent

Balanced reciprocal translocation

Segregation 2:2

Types of segregation:

T1,T2 - N1, N2 = alternate → balanced + normal gametes

T1, N2 - T2, N1 = adjacent 1 unbalanced gametes

T1, N1 - T2, N2 = adjacent 2 with duplication a deletion

Empiric risk: 10% for woman - carrier

2-5% for man - carrier

But risk is dependent on the type of translocation (length of translocated segments)

Segregation 3:1 = tercial trisomy

One of the translocated chromosomes is small and its trisomy is compatible with life

t(4;6) der(6)

Pericentric inversion – mechanism of meiotic recombination

crossing over inside meiotic loop→ a) duplication of p and deletion of q or b) duplication of q and deletion of p

a) b)

Risk of meiotic recombination in a carrier of pericentric inversion

depends on the length of inverted segment

in average: for woman – carrier - 10%

for man - carrier - 5%

Paracentric inversion

Crossing over inside loop → dicentric chromosome and acentric fragment

Origin of interstitial deletion and duplication

Unequal crossing over outside of meiotic loop

– in both inversions

Consequences of balanced aberrations• Segregation of unbalanced genome (affected child,

abortion)• Sterility (esp.in men)• Effect on phenotype – MR, anomalies (rcp t, inv de novo)

Balanced CHA with phenotypic effect :

1. Small deletion

2. Small mosaic?

3. Inactivation of gene (site of break inside gene) → manifestation of recessive alleles on homologous chromosome (described in X/A translocations)

4. Posion effect: incorrect gene order, impairment of gene regulation, neighbourhood of heterochromatine (spreading effect)

Consequences of unbalanced aberrations

• congenital malformations

• mental retardation

Partial trisomy – less severe than partial monosomy of the same segment

Lack of chromosomal material = more severe then excess

Degree of expression – depends of the length of trisomic and monosomic segment and its gene content

Indications to chromosomal examination !!!

Postnatal (from peripheral blood – lymphocytes):

1. Specific phenotype (MD……)

2. Psychomotoric retardation (PMR), growth retardation, dysmorpfic features, congenital malformations, small stature in girls, oedema in newborns (TS)

3. Dysfertility (repeated spontaneous abortions, sterility – chromosomal examination in both partners)

4. Amenorrhea, delayed puberty, genital malformations

Indication of prenatal cytogenetic examination

From cells of amniotic fluid, chorionic villi, fetal blood

1. Increased maternal age (≥ 35 years)

2. Patological values of biochemical markers

3. Abnormality on ultrasound

4. One parent is a carrier of balanced chromosomal aberration

Indication of prenatal cytogenetic examination

From cells of amniotic fluid, chorionic villi, fetal blood

1. Increased maternal age (≥ 35 years)

2. Patological values of biochemical markers

3. Abnormality on ultrasound

4. One parent is a carrier of balanced chromosomal aberration

http://dl1.cuni.cz/course/view.php?id=324 presentation

http://dl1.cuni.cz/course/view.php?id=324 supplementary text to cytogenetics

Thompson &Thompson: Genetics in medicine, 7th ed.

Chapter 5: Principles of clinical cytogenetics

Chapter 6: Clinical cytogenetics: Disorders of the autosomes and the sex chromosomes

+ informations from presentation