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Path 19.04 > 20.04.15 Structural abnormalities of Y chromosome > Flashcards

20.04.15 Structural abnormalities of Y chromosome Flashcards

1
Q

What are the 3 unique features of Y chromosomes

A
  • Lack of a homologous partner for crossing over
  • Functional specialisation for spermatogenesis
  • High degree of sequence amplification
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2
Q

How is Y chromosome useful for geneaology studies

A

Paternally inherited portion of the nonrecombining Y chromosome retains sequential records of the accumulation of genetic diversity over time

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3
Q

Why is it a difficult chromosome to analyse

A
  • High density of repeated sequences makes physical mapping and sequencing difficult
  • Does not recombine during meiosis so classical linkage mapping is not possible
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4
Q

Why is natural selection less effective in preventing the accumulation of deleterious mutations in the Y chromsome

A
  • The lack of recombination.

- Results in genetic erosion of Y chromosome (will eventually become redundant in function)

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5
Q

What are the 3 azoospermia factor regions on Yq

A

AZFa, AZFb, AZFc

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6
Q

What proportion of Y chromosome is pesudoauatosomal region (1 and 2)

A

5%

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7
Q

What gives variability to Y chromosome length

A
  • Variation in length of Yqh region (Yq12).
  • Inactive heterochromatin.
  • Size doesn’t have a phenotypic consequence
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8
Q

What is a pericentric inversion on Y chromsome

A
  • Inversion that moves centromeric region to the heterochromatin border.
  • No apparent effect on fertility or clinical significance.
  • 1:200
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9
Q

How are sex vesicles formed

A

When X and Y chromosomes synapse at PARs during meiosis.

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10
Q

How prevalent are Y;autosome translocations

A

1:2000

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11
Q

Phenotypic consequences of Y;autosome translocations

A

-Male infertility, due to spermatogenic arrest.

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12
Q

What is the most prevalent Yq autosome translocation

A
  • Acrocentric p arm and Yqh.
  • 70% of Y autosome translocations.
  • Most commonly t(Y;15), possibly due to homology of heterochromatin blocks of 15p and Yq. Also common= t(Y;22)
  • Often familial and has no clinical significance, but could predispose to structural instabilities of Y or secondary chromosomal abnormalities.
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13
Q

Where is Yq-autosome breakpoints usually found

A
  • Acrocentric= p11-13

- Y= Yq12

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14
Q

Can translocations occur between Yq and other autosomes (other than acrocentric p arms)

A
  • Yes but rare.
  • Usually balanced so seen with oligo/azoospermia, hypogonadism
  • Phenotypic abnormalities occur if there is gene disruption at break points, positional effects or unbalanced
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15
Q

Review of autosome Yp translocations

A
  • Can involve SRY translocation to an autosome (usually acrocentric)
  • Phenotypically male with 45 chromosomes, 45,X
  • 45,X,t(Y;15) could lead to PWS if breakpoints are at 15q11-q13
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16
Q

What is a dicentric Yp-acrocentric translocation

A
  • P to P arm translocation
  • 45 chromosome count
  • Almost no genetic material is lost from either acrocentric or Y chromosome
  • may be associated with oligospermia
17
Q

What is the most common X;Y translocation

A

-46,X/Y, der(X) t(X;Y)(p22.3;q11)

18
Q

How do X;Y translocation arise

A
  • NAHR at spermatogenesis of female carrier’s father.

- NAHR between homologous genes (94% sequence similarity) PRKX and PRKY are implicated in 30% cases

19
Q

What is a risk factor for X;Y translocations

A

Polymorphic paracentric inversion on Yq, placing PRKY in the same orientation as PRKX

20
Q

Review of female 46,X,der(X)t(X;Y)

A
  • Typically a fertile female of normal intelligence.
  • Partial monosomy of Xp
  • If SHOX is deleted then patient will have Leri-Weill dyschondrosteosis (skeletal dysplasia)
  • 50% risk of transmitting der(X)
  • XCI tends towards der(X) but can be variable
21
Q

Review of male 46,Y,der(X)t(X;Y)

A
  • Usually son of a der(X)t(X;Y) mother
  • Partial nullisomy for Xp
  • Infertile. Can be cognitively normal. If breakpoint is more proximaly and involves genes like MRX, then there is mental impairment.
22
Q

What translocation accounts for most 46,XX males and some 45,X males

A
  • Cryptic Xp;Yp translocation

- Loss of distal region of X chromosome and transfer of SRY

23
Q

How is a cryptic Xp;Yp translocation detected

A

FISH.

24
Q

Phenotypic consequences of a cryptic Xp;Yp translocation

A
  • Males are infertile

- If loss of SHOX then leads to Leri-Weill dyschondrosteosis

25
Q

When are ring Y chromosomes usually seen

A
  • With 45,X cell line (70% cases)
  • Can be mosaic, e.g. 45,X/46,X,r(Y)
  • Often only a small region of Yp is deleted so SRY region is left intact. i.e. phenotypically male
26
Q

What are the phenotypic characteristics of males with ring y chromosomes

A
  • Sexual infantilism, ambiguous genitalia, hypospadias, azoospermia, short stature
  • varies depending on the genes deleted
27
Q

What is an isochromosome

A

an unbalanced structural abnormality in which the arms of the chromosome are mirror images of each other.

28
Q

Review of i(Yp)

A
  • Cannot be distinguished from a partial Yq deletion
  • characteristics: ambiguous genitalia, TS abnormalities if 45,X cell line is present
  • Otherwise male phenotype with microcephaly, hypogonadism, short stature.
29
Q

Review of i(Yq)

A
  • Does not contain Yp so lacks SRY and RPS4Y, there female. Have sexual infantilism.
  • Features of TS such as streak gonads
30
Q

Is an isodicentric Y chromosome a common abnormality

A
  • Yes, most common Y structural abnormality.
  • 91% are in mosaic form (often lost during cell division) with 45,X
  • Sex-related phenotypes vary depending on percentage of 45,X cells and absence/presence of SRY gene.
31
Q

Review of Idic(Yp)

A
  • 1/3 phenotypically male, azoospermia, short stature, ambiguous genitalia, TS like features
  • 2/3 phenotypically female with streak gonads, short stature and TS like features
32
Q

Review of Idic(Yq)

A
  • 1/3 phenotypically male with azoospermia, small testes, short stature TS like features
  • 1/3 intersex, ambiguous genitalia, short stature gonadoblastoma
  • 1/3 phenotypically females, abdominal testes, TS like features, short stature.
33
Q

Common Y chromosome microdeletion

A

-AZFc deletion at Yq11.23

34
Q

Proportion of infertility cases attributed to microdeletions at AZF regions

A

5-20% of azoo/oligospermia cases

35
Q

How can Y microdeletions be transmitted from father to son

A

Via intracytoplasmic sperm injection (ICSI)

36
Q

Review of AZFa microdeletion

A
  • 0.5-4% of cases
  • Patients have azoospermia and sertoli cell only syndrome (SCOS)
  • Sperm retrieval for ICSI is not possible.
  • Due to spermatogenic failure, caused by loss of USP9Y and DDX3Y genes.
  • Arise due to recombination between repeats.
  • 800kb in size
37
Q

Review of AZFb microdeletion

A
  • 1-5% of cases
  • sertoli cell only syndrome (SCOS) and azoospermia.
  • 6.2Mb in size
38
Q

Review of AZFc microdeletion

A
  • Most common 80% of cases
  • Azoospermia or severe oligospermia.
  • Includes DAZ gene
  • 3.5Mb in size
  • Repeats flank region

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