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FRC Path 2023 > Session 5: Infertility, Sex Chromosomes > Flashcards

Session 5: Infertility, Sex Chromosomes Flashcards

1
Q

What gene is required for male sex determination?

A

SRY gene- acts as a mammalian switch in male sex determination
- Presence results in development of male sex characteristics and absence results in the formation of female sex organs e.g. ovaries even in the presence of the Y chromosome

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2
Q

What signalling cascade is activated by the SRY gene?

A

SRY is thought to activate SOX-9 signalling and downregulates the female sex pathway

Sertoli cells develop into testis cords and stimulate development of germ cell, leydig cell, vascular cells etc = testes develop

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3
Q

What is the female sex determining pathway?

A

In the absence of SRY female sexual development takes place.
Wnt and Foxl2 genes are expressed and male genes are downregulated.
Results in the differentiation of granulosa cells, theca cells, oocyte production and ovarian follicle formation.

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4
Q

What is the function of the SRY gene

A

Thought to act as a transcription factor in the nucleaus- has a DNA binding domain and a nuclear localisation signal, disruption of which results in XY sex reversal

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5
Q

What is the SOX9 gene?

A

Transcription factor – has been shown to be sufficient for testes determination
Once expression has been activated by SRY it is able to sustain its own expression
-Dup of SOX 9 reported in XX sex reversal
-SOX9 mutation in skeletal dysplasia syndrome campomelic dysplasia in which ~75% have complete or partial XY sex reversal

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6
Q

What is the genetic pathway controlling female ovarian development?

A

Female development is the default and occurs in the absence of SRY
In the absence of SRY Wnt and Foxl2 are expressed activate the expression of downstream genes in a female specific manner
RPSO1 (R-spondin 1) is also key
RPSO1 and Wnt are though to act together to activate the beta-catenin signalling pathway. The Foxl2 gene is independently expressed from the RPSO1/Wnt4 genes

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7
Q

What phenotype is associated with a mutation in Wnt?

A

Dominant Wnt mutations have been reported in women with varying degrees of virilisation incl androgen excess and abnormal development of the mullerian ducts.

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8
Q

What occurs in the male testes during sexual development?

A
  • SRY is expressed
  • Sertoli cells express AMH which results in regression of the mullerian ducts
  • Leydig cells express testosterone- causes the wolffian ducts to develop into the male gonads (epididymis, vas deferens, seminal vesicles)
  • Testosterone from the leydig cells is also converted to dihydrotestosterone by alpha-reductase- this acts on the androgen receptor resulting in the formation of the male external genitalia
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9
Q

What happens in the ovary during female sexual development?

A

In the absence of AMH and androgen action the wolfian ducts regresses and the mullerian duct develop into the fallopian tubes, uterus and upper vagina

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10
Q

What is the definition of a DSD?

A

Congenital condition where there is atypical development of the chromosomal, anatomical or gonadal sex.

Use of the terminology intersex and hermaphrodite is no longer used following a meeting in Chicago in 2015 as it was considered outdated

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11
Q

What is the range of phenotype associated with DSD?

A

Can range from mild with hypospadias, undescended testes and labia fusion to complete AR insensitivity and complete gonadal dysgenesis

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12
Q

What is a 46,XX phenotypic male?

A

Female karyotype with male phenotype

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13
Q

What is the most common cause of 46,XX sex reversal?

A
  • 80-90% is due to an X;Y translocation with a small portion of the Y chromosome including the SRY gene translocated onto the distal X
  • t(X;Y)(p22.33;p)- not usually visible by karyotype so need to do FISH
  • sporadic as 46,XX males are infertile
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14
Q

What are the other causes of 46,XX male?

A

Male phenotype despite the absence of SRY
- SOX9 dup,
- RPSO1 mutation,
- SOX3 mutation,
- Wnt4 mutation

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15
Q

What causes the 46,XY female?

A

Mutation in
- SRY- Swyer syndrome
- AR gene - CAIS (Complete Androgen insensitivity syndrome)

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16
Q

What cause Swyer syndrome and what is the phenotype?

A

Due to mutation in the SRY gene or another gene in the male sex determination syndrome
- pure/complete gonadal dysgenesis
- streak gonads
- female external genitalia
- increased risk of gonadoblastomas so streak gonads removed in 1st decade
- failure of pubertal development
- infertile

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17
Q

What cause CAIS syndrome and what is the phenotype?

A

Complete Androgen Insensitivity Syndrome
AR mutation in the androgen receptor gene = does not respond to testosterone
- female external genitalia but short or absent vagina
- normal breast development and female characteristics at puberty
- infertility
- primary amenorrhea
- intra-abdominal testes
- low risk of gonadoblastoma

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18
Q

What are the features of Androgen resistance syndromes/defects in androgen biosynthesis?

A

Caused by defect at various points in the pathway involved in the biosynthesis 5α-reductase or 17α-hydroxlyase
- 46,XY
- Female extertnal genitalia
- Male breasts
- Reduced spermatogenesis
- Bilateral testes

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19
Q

What genes may be disrupted by autosomal rearrangements in 46,XY females?

A

WT1 11p13,
SF1 9q33,
LHX 1q31-32,
SOX9 17q24-25,
DAX1 Xp21,
SRY Y
- often have other phenotypic features such as mental retardation and dysmorphism.

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20
Q

What genes may be disrupted by X chr rearrangements in 46,XY females?

A

Duplication of of DAX1 on the X chr has been shown to disrupt testicular development

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21
Q

What are the features of 46,XX DSD with female phenotype?

A

Female phenotype and karyotype but fail to enter puberty and develop secondary sexula characteristics
- elevated gonadotropins and streak gonads
- do not have classic TS stigmamta e.g. short stature
e.g. Mutations in the FSH receptor gene (FSHR) cause hypergonadotropic hypogonadism.

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22
Q

Name a disorder of Androgen Synthesis/Action

A

Congenital adrenal hyperplasia CAH

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23
Q

What causes CAH?

A

Due to enzyme deficiencies affecting steroidogenesis
Classic CAH e.g. 21-hydroxylase deficiency

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24
Q

What causes CAH? What are the features of CAH in males and females?

A

CAH results in deficiency of enzyme e.g. 21-hydroxylase involved in cortisol biosynthesis = build-up of precursors which are also part of the androgen synthesis pathway resulting in increased androgen production.
- Males have normal genitalia
- In females there is masculinisation of external genitalia

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25
Q

What should CH be diagnosed early? What treatment should be given?

A
  • The glucocorticoid deficiency resulting from mutations in these enzymes also results in increased ACTH secretion, which increases build-up of cortisol precursers further = adrenal androgen excess and adrenal hyperplasia.
  • If left untreated, patients are known as ‘salt-wasters’ and will often die in the neonatal period.
  • For both sexes it is important to treat the mother with dexamethosone (a corticosteroid) during the pregnancy and after birth the child should be treated with hormones.
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26
Q

Name 3 more conditions associated with a deficiency in androgens?

A
  1. Testosterone secretion defect - impaired Leydig cell diffn (complete & partial forms)
  2. Defects in testosterone metabolism 5-reductase deficiency – this enzyme is required to convert testosterone to DHT – leads to incomplete virilisation of the genitalia
  3. Defects in androgen action- CAIS, MAIS and PAIS
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27
Q

What is the role of gene dosage in DSD? Give an example of a DSD due to gene deletion and gene duplication

A

Deletion of SOX9, NF5A1 or WT1 can lead to 46XY gonadal dysgenesis (lack of male genes)
Duplication of DAX1 or WNT4 can lead to 46XY gonadal dysgenesis (excess of female genes)

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28
Q

What are the important factors in DSD management?

A

Considered a medical emergency because:
- Risk of salt wasting in CAH
- Parental anxiety to assign the correct sex
Longer term treatment includes clinical and diagnostic evaluation (which will assist with gender assignment), surgical management, hormone replacement therapy and psychosocial care.

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29
Q

How are DSDs investigated?

A

Biochemical and clinical examination
PCR/FISH or karyotype may be required to determine the genetic sex

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30
Q

Name 2 rare monogenic causes of 46,XY DSD

A

Campomelic dysplasia (CMPD)- SOX9 (AD) important for sex and skeletal development; CMPD is often lethal in the first year of life, due to respiratory insufficiency related to small chest size and tracheobronchial hypoplasia

WT1- Wilms tumour suppressor: WT1 (11p13) is involved in the development of the kidneys and gonads.
WT1 mutations cause 3 syndromes:- WAGR, Frasier syndrome & Denys-Drash syndrome

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31
Q

Name 1 rare monogenic causes of 46,XX DSD

A
  1. Aromatase Deficiency = increased prenatal exposure to androgens because the steroid hormones produced in the placenta are not converted to oestrogens.
    *ambiguous genitalia, elevated androgens and undetectable estrogens at birth
    *lack of breast development, amenorrhea, tall stature and multicystic ovaries at puberty
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32
Q

What is the definition of infertility?

A

Failure to achieve a pregnancy after 2 years of unprotected sex
Needs to be seen in respect to a couple e.g. a male with a low sperm count may get a hyperfetile women pregnant

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33
Q

What percentage of couple fail to conceive?

A
  • 80% of couples will conceive after 1 yr of trying and 50% of those remaining will conceive after 2 yrs (cumulative rate of 90%)
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34
Q

What percentage of infertility goes undiagnosed?

A

25-30%

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35
Q

What are the non-genetic causes of infertility?

A
  • Medication e.g. chemotherapy
  • Anatomical – uterine abnormalities
  • Lifestyle- smoking, weight, alcohol consumption
  • Sperm- low sperm count, low motility, abnormal morphology
  • Maternal age
  • Endometriosis
  • Ovulation disorders- PCOS, thyroid problems, POI/POF
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36
Q

What is the benefit in identifying the cause of infertility?

A
  • Can ID the type of ART that is most appropriate
  • Can make lifestyle changes
  • If a genetic factor is the cause can inform on the recurrence risk and possibility of affected offspring e.g. balanced rearrangement and the risk of unbalanced offspring
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37
Q

What are the main causes of male infertility?

A
  • Genetic factors account for 15%, 25% if they are azoospermic (CF?)
  • Qualitative sperm defects- teratospermia (abn morphology)
  • Quantitative sperm defects – azoospermia or oligospermia
  • Reduce sperm motility- athenospermia
  • Ductual obstruction or dysfunction
  • Hypothalamic- pituatry axis abn
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38
Q

What are the male chromosomal causes of infertility?

A
  • Klinefelters syndrome
  • Iso(Y)
  • X;Y translocations
  • X;A translocations
  • Y;A translocations
  • 45,X/46,XY mosaicism
39
Q

What are the clinical features of XXY?

A
  • Infertility- hypogonadism, low testosterone and high gonadotropins
  • Tall, long limbs
  • Gynecomastia
  • IQ reduced compared to siblings
  • Extra X silenced
40
Q

What is the relevance of XXY to infertility?

A
  • Most frequent genetic cause of male infertility
  • ~1/600 in gen pop but as high as 1/7 in males with on-obstructive azoospermia
  • If sperm present testosterone treatment and testicular sperm extraction followed by ICSI is possible
41
Q

When is XXY diagnosed?

A
  • Majority diagnosed in adulthood due to fertility problems
  • Some diagnosed in prenatal diagnosis or in childhood due to investigations into mild MR
42
Q

What is the phenotype associated with XXY variants? XXYY, XXXY, XXXXY etc

A
  • Severity of effect on physical and mental development increases with each additional X due to increases dosage of PAR genes
  • Male sex regardless of number of X as long as Y (SRY) present
  • Rare cases of XXY females due to mutation of SRY
43
Q

What is the phenotype of 45,X/46,XY in terms of infertility?

A
  • 90% have male external genital, 10% ambiguous of female genital
  • Usually infertile
  • Phenotype and fertility dependent on level of XY cell line
  • In phenotypic female there are streak gonads and risk of gonadoblastoma so removed
44
Q

What are the possible causes of XX male DSD? SRY present

A
  • X;Y rearrangement with SRY present on X chromosome
  • Mutation in gene involved in male sex cascade e.g. SRY or SOX-9
  • 75% have SRY present due to X;Y rearrangement
  • 90% present after puberty with normal pubic hair and penile size but small testes, gynaecomastia and azoospermic infertility
45
Q

What are the possible causes of XX male DSD? SRY absent

A
  • Due to a mutation in a gene resulting in inappropriate activation of male sex e.g. SOX-9 or inactivating mutation in a gene required for female sex
  • Generally present at birth with ambiguous genitalia
46
Q

What is the relevance of iso(y) to male infertility?

A
  • Often seen in a mosaic form with 45,X due to loss of the isochromosome Y
  • Phenotype varies depending on level of 45,X cell line
  • Range from infertile males to females with TS features and ambiguous genitalia
47
Q

What is the impact of X;A translocation on male infertility?

A

Almost always lead to azoospermic infertility due to spermatogenic arrest as a result of disruption of the sex vesicle

48
Q

What is the impact of X;Y translocation on male infertility?

A
  • some show infertility due to disruption of sex vesicle or AZF loci if breakpoint in critical Yq region
  • Majority are rearrangements between the Yqh and short arm of an acrocentric (usually 15 or 22) and is not associated with infertility
49
Q

What is the impact of A;A rearrangement in male infertility?

A
  • Rearrangements can result in failure of synapsis and activate checkpoint arrest and cell death = spermatogenic arrest
  • 4-10x more common in infertile males
50
Q

Why is an A;A rearrangement less likely to result in infertility in a female?

A
  • Failure of synapsis of rearrangements is less likely to activate pachytene checkpoint and oocytes continue in meiosis. But are hence more likely to produce unbalance offspring
51
Q

How does the X;Y synapse in meiosis and relevance of acrocentrics?

A
  • Form a bivalent via interaction between the PAR regions and are sequestered in a sex vesicle
  • Pairing is incomplete and unpaired segments can pair with unpaired region of autosomes e.g. acrocentric p-arms in rob translocations. Interaction with autosome can interfere with formation of the sex vesicle resulting in spermatogenic arrest
  • Rob rearrangements are 9x more common in infertile males than general population
52
Q

What are the molecular genetic causes of infertility?

A
  • AZF microdeletions
  • CF and CFTR-RD mutations
  • Myotonic dystrophy- DMPK
  • Kallman syndrome KALL1 on Xq
  • AR mutations on CAIS, MAIS or PAIS
  • ADPKD
53
Q

What are Y microdels?

A
  • Microdeletions in the long arm of the Y chromosome affecting 1 or all of the AZF loci (AZFa,b,c)
  • AZF are critical regions for spermatogenesis
  • AZFa most severe, for AZFa and AZFb there is azoospermia and intratesticular sperm retrieval is not possible
  • AZFc- may have some spermatogenesis
54
Q

What is the relevance of CFTR mutations in male infertility?

A
  • Associated with Congenital absence of the vas deferens (CAVD)- can be bilateral or unilateral
  • CUAVD may be fertile
  • CBAVD- infertile and agenesis of seminal vesical but can be treated by sperm retrieval and ICSI/IVF
  • Carrier screen partner for risk of CF affected offspring
  • Isolated CAVD associated with mild non-classical CF muts e.g. R117H and 5T
55
Q

What are the features of Kallman syndrome?

A
  • Anosmia with congenital hypogonadotropic hypogonadism
  • KALL1 is on the X chromosome
56
Q

What is the definition of POF/POI

A

Cessation of menses prior to 40, may be associated with a fragile PM

57
Q

What are the chromosomal causes of female infertility?

A
  • TS
  • X;A translocations
  • X;X translocations
58
Q

What are the features of TS and relevance to infertility?

A
  • Frequently mosaic
  • Infertile- mosaic cases may be fertile with only minor menstrual abn
  • mosaic commonly have 45,X/46,X+structurally abn X
  • Short stature (associated with loss of SHOX in XpPAR region)
  • Webbed neck, hypoplastic nipples
  • Delayed puberty
  • Ovarian dysgenesis
  • Hypergonadotropic hypogonadism
  • Congenital heart defect
  • Prenatal – cystic hygroma, raised NT, oedema, 99% miscarry
  • Cardiac abnormalities
  • Most have normal intelligence
59
Q

How common is mosaicism in TS?

A

50%- suggested that all TS that result in livebirth are mosaic to some level but may be tissue specific and not detected in blood
- Numerical 45,X/46,XX/47,XXX
- 45,X/46XY- streak gonads and risk of gonadoblastoma
- Structural: 45,X/46,X,i(X)(q10)
- 46,X,r(X)- if Xist not present phenotype may be more severe
- 46,X+mar (check for Y for gonadoblastoma risk)
Less severe phenotype the more normal cell line is present

60
Q

What are the treatments for TS?

A

Hormone replacement therapy can be given at puberty to enable normal secondary sexual characteristics to develop e.g. breasts
- Pregnancy: oocyte donation, gamete or embryo transplant

61
Q

What is the recurrence risk of TS?

A
  • Generally sporadic with no increased recurrence risk.
  • May be inherited if parent caries an X chr rearrangement or is germline mosaic
62
Q

What are the features of X;A rearrangement and relevance to infertility?

A
  • May result in expression of an XLR phenotype due to skewed X inactivation
  • May be infertile in critical region for oogenesis affected Xq21 and Xq23q28
  • If fertile risk of unbalanced offspring- unbalanced females will silence the der(X)
63
Q

What are the features of X;X rearrangement and relevance to infertility?

A
  • 46,(X),der(X)t(Xp;Xq)
  • Infertile, usually present with menstrual abnormality
64
Q

What is the genetic cause and phenotype of Swyer syndrome?

A
  • 46,XY female
  • Normal external genitalia
  • Streak gonads and infertile so streak gonads are removed and presence of Y is associated with a high risk of gonadoblastoma
  • 15% have SRY pathogenic mutation
65
Q

What is the cause and phenotype of XX gonadal dysgenesis?

A
  • Hypergonadotropic hypogonadism- impaired response to gonadotropins
  • Symptoms range from primary amenorrhea to lack pf pubertal development, uterine hypoplasia, streak gonads, POF
  • Associated with mutations in FSH receptor (AR), BMP15 (XL) etc
66
Q

What is the relevance of CFTR mutations in female infertility?

A

Women with CF have thicker cervical mucus making it harder for sperm to penetrate the cervix and poor nutrition can result in abnormal menses
- Most women are fertile but up to 30% may fail to conceive

67
Q

What is the relevance of Fragile PM in female infertility?

A
  • Increased risk of POI- 20% compared to pop risk of 1%
  • 59-200 rpts, not methylated
  • Suggested GOF mechanism
  • Non-linear relationship between expansion size and risk of POI- highest risk is for 80-100 and reduces for rpts >100
68
Q

What are other genetic causes of infertility?

A

DM1
- males = testicular atrophy and oligo/azoospermia
- females = uterine dysfunction and labour complications, POF, increased miscarriage rate

CAH- females have virulisation of genitals ranging from ambiguous to normal male external genitals with small testes
- 21 hydroxylase deficiency

PWS
- Males have cryptorchidism
- Females have hypogonadotropic hypogonadism and most have primary amenorrhea

69
Q

What is the strategy for genetic testing in infertility?

A
  • CF, Ydel and karyotype in males
  • Karyotype and FMR1 for POI/POF
  • DSD = medical emergency due to risk of death from salt wasting and need to assign correct gender. May be tested for specifically although DSD is now likely to be tested for by array and WES panels
70
Q

What is the phenotype associated with X chr aneuploidy?

A
  • Associated with reduced IQ, facial dysmorphism and physical abn
  • Severity gets worse with each subsequent additional X due to increased dosage of PAR genes
  • Additional X will be silenced
71
Q

What are the key features of the Y chromosome? (recombination)

A
  • No partner with which to recombine during meiosis means it accumulates genetic variation over time and I therefore useful for genealogy studies
  • Lack of recombination means Y cannot be used for linkage
  • Lack of recombination also means it is more susceptible to mutation and functionality is progressively being reduced and it will eventually become redundant
  • Lots of repetitive sequences make designing array probes and sequencing difficult
72
Q

What is the structure of the Y chromosome? Location of key genes

A
  • SRY on P arm just below PAR region
  • SHOX Y PAR P
  • AZF loci in q arm
  • DAZ1/DAZ2 also on q arm and deletion associated with azoospermia
73
Q

What are the different variant Y chromosomes and there clinical significance?

A
  • Yqh can vary in length with no phenotypic consequence
  • Pericentric Y - inv moves centromere to het border, approx. 1 in 200 and not associated with infertility
  • Satellited Yq due to translocation with p arm of acrocentric (usually 15 or 22) no phenotypic consequence
  • Y het translocation- het material from y on p arm of acrocentric- no phenotypic consequence
74
Q

What pathogenic Y chromosome variants are associated with oligozoospermia?

A
  • isoYp
  • isoYq (no SRY)
  • Yp-
  • Yq-
  • Often seen as mosaic with 45,X an genital development may be male, female or ambiguous
75
Q

What is the clinical significance of a Y;A translocation?

A
  • Often associated with infertility due to disruption of the sex vesicle
  • Infertility also seen if breakpoint disrupts AZF loci in Yq
  • Differing levels of fertility in male members of the same family
  • Translocation of Yp to an autosome can result in a 45,X male
76
Q

What is the clinical significance of a X;Y translocation?

A
  • Most frequent is t(X;Y)(p22.3;q11.1)
  • Majority of cases are familial and arise at spermatogenesis of female carriers father
  • Most clinically significant factor is the loss of PAR on Y including SHOX- leri-weill dyschondrosteosis, KALL1- kallman syndrome , STS- icthyosis, MRX- mental retardation
77
Q

What is the clinical implication of the t(X;Y)(p22.33;q11.1)

A
  • Cryptic rearrangement
  • can cause XX male in the 46,X,der(X)t t(X;Y)(p22.33;q11.1) male has SRY on 1 X chromosome
78
Q

What is the clinical significance of a Y;Y translocation?

A
  • Rare and difficult to distinguish from iso(Yq) = phenotypic female
79
Q

What is the clinical significance of a cryptic Xp;Xp translocation?

A

46,XX males and 45,X males due to translocation of terminal Y region including PAR and SRY onto the X chromosome
- All males infertile
- Loss of SHOX result in leri-weill dyschondrosteosis

80
Q

What is the clinical significance of a r(Y)?

A
  • Associated with 45,X cell line due to instability of the ring
  • Most cases phenotypically male – only small region of Yp lost leaving SRY intact
81
Q

What is the clinical significance of iso(Yp)?

A
  • Ambiguous genitalia, TS abnormalities and normal stature especially when a 45,X cell line is present
  • Risk of gonadoblastoma in females
  • Sex-related phenotype is due to the level and distribution of 45,X and presence or absence of SRY
82
Q

What is the clinical significance of iso(Yq)?

A
  • 1/3 male, 1/3 ambiguous genital and 1/3 female
83
Q

What is the clinical significance of a deletion of Yp or Yq translocation?

A
  • Yp del = phenotypically female with TS features except short stature, streak gonads and risk of gonadoblastoma
84
Q

What is the location if XIC?

A

Xq13

85
Q

What are the locations of the POF critical regions?

A

POF 12 Xq13q21
POF 2 Xq23q28
Breakpoints in these regions in X chromosome rearrangements can be associated with POF
Symptoms range from POF to primary amenorhea

86
Q

What other X chromosome regions are key for microdel phenoytpes?

A

PAR- SHOX, STS
Xp11.2 – gene rich del associated with ID
Xp21- DMD

87
Q

What X chromosome regions are key for microdup phenotypes?

A
  • DAX1 dup Xp21.22 = dose dependent sex reversal
  • Xq28- dup results in mod to severe ID in males. Region contains the MECP2 gene which is mutated in females with Rett Syndrome
88
Q

What is the role of X inactivation?

A
  • To balance dosage of genes on the X chromosome in females
  • Not complete and PAR region escape inactivation
89
Q

When does X inactivation occur?

A
  • At 5000 cell stage indicating that 2 active copies of X is important in early embryonic female development
  • Occurs randomly but once established patern is maintained in daughter cells.
90
Q

What is the role of the XIC?

A
  • Contains critical gene XIST
  • Need 2 copies of XIST if not inactivation will not occur
  • XIST is transcribed into an RNA molecule which coats the X chr and activates inactivation
91
Q

What is the process of X inactivation?

A
  • XIST is transcribed into an RNA molecule which coats the X chr and activates inactivation
  • Inactivation spreads through the X from the XIC- inactive X is later replicating and can be seen as a Barr body
  • XISt is required for the initiation but not maintenance of X inactivation
  • X inactivation is maintained by epigenetic modifcation e.g. methylation, histone acetylation
92
Q

What is likely to be silenced in X;A rearrangements?

A
  • In balanced carrier normal X will be silenced
  • In unbalanced offspring der X will be silenced
  • Required for functional balance- skewed inactivation can result in cells with opposite inactivation being unviable and not contributing to the embryo
  • X inactivation can spread to autosomal regions
  • Skewed X inactivation can result in the expression of a XLR phenotype
93
Q

Describe the features of iso(X)- which region, and phenotype?

A

Iso(Xq) is one of the most common sex chromosome abn, associated with TS like phenotype due to loss of SHOX on Xp and is commonly found in a mosaic state.
- May also be seen in variant klinefelters

Iso(Xp) is not viable as there is no Xist

94
Q

What is a ring chromosome and what phenotyoe is r(X) associated with?

A
  • A ring is a chromosome whose ends have fused together.
  • Phenotype is dependent on how much material is lost as the breakpoints and whether the ring is replacing an chromosome 46,X,r(X) or in addition to the normal chr set 47,XX+r
  • May be inherited
  • Often present in mosaic state- dynamic mosaicism can result in generalised ring syndrome
  • May be present in females with TS (rX)- if the ring is small should check to see if it contains Y chr material due to the increased risk of gonadoblastoma
  • Rings with XIST will generally undergo skewed X inactivation
  • If Xist is absence the phnotype may be more severe due to increased dosage of genes present on the ring

FRC Path 2023 (15 decks)

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