20.01.17 Segregation of Structural Abnormalities Flashcards
What percentage of pregnancies miscarry
15-20%
Proportion of miscarriages with a chromosomal abnormally
50%
Percentage of women with 2+ miscarriages
2-5%
Common autosomal trisomies in miscarriages:
- 13 (95% will miscarry)
- 18 (95% miscarry)
- 21 (80% miscarry)
What is the recurrence risk following autosomal trisomy
1%. If there is recurrence, then likely due to gonadal mosaicism
5% of couples with recurrent miscarriage have
balanced rearrangements
Types of balanced rearrangements
- Translocations
- Inversions
- Insertions
What is a reciprocal translocation
- When 2 nonhomologous chromosomes exchange segments. Balanced if there is no loss or gain
- 1:500 frequency in general population
WHat happens to a reciprocal translocation during cell cycle
- At meiosis 1, a quadrivalent forms to achieve maximum homology, called a pachytene cross
- Distribution of 4 homologues to two daughter cell is determined by segregation
How many outcomes of segregation are there for reciprocal translocations
- 16 outcomes
- 2:2 segregation (6 outcomes)
- 3:1 segregation (8 outcomes)
- 4:0 (2 outcomes)
What are the different types of 2:2 segregation in reciprocal translocations
- Alternate: only mode leading to balanced or normal gametes
- Adjacent 1: non homologous centromeres travel together
- Adjacent 2: homologous centromeres travel together
What are the different types of 3:1 segregation in reciprocal translocations
- Tertiary trisomy: 2 normal, 1 derivative
- Interchange trisomy: 2 derivatives, 1 normal
In reciprocal translocations, when is 2:2 adjacent 1 more likely
When translocated segments are shorter than centric ones.
In reciprocal translocations, when is 3:1 adjacent 1 more likely
-When one of the derivative chromosomes is small
What is haploid autosomal length (HAL)
-Correlation of quantitative chromatin imbalance with risk of an abnormal live-born child
How do you calculate HAL (haploid autosomal length)
- Measure length of chromosome (mm)
- Measure length of imbalanced segment (mm).
- Determine % of imbalance for the chromosome
- Bear in mind chromosome dependent. Imbalances are less tolerated in G-band negative regions.
What is a Robertsonian translocation
- When 2 acrocentric chromosomes fuse at centromere
- Acrocentric chromosomes= 13, 14, 15, 21, 22.
What is the incidence of Robertsonian translocations in general population
0.1% i
What is the incidence of Robertsonian translocations in patients with recurrent pregnancy loss
1.1%
What is the incidence of Robertsonian translocations in infertile men
3%
Which chromosomes account for 75% of Robertsonian translocations
Chr 13 and 14
What do Robertsonian translocations form during meiosis
Trivalent
What are the possible segregation outcomes for Robertsonian translocations if heterologous
- 6 possible outcomes
- 2:1 alternate (normal and carrier gametes)
- 2:1 adjacent (disomic and nullisomic gametes)
- 3:0 very rare
What are the possible segregation outcomes for Robertsonian translocations if homologous
- 2 segregation outcomes
- 1:0 disomic gamete
- 1:0 nullisomic gamete
- 100% chance of imbalanced transmission
What are sex chromosome translocations
- Females with an X-autosome translocation.
- Form a quadrivalent in meiosis.
- Males with X-autosome translocation cause spermatogenic arrest
- Males with Y-autosome translocation, can disrupt sex vesicles, causing infertility. But can also have normal fertility if invovles Yqh
Why are a greater number of conceptuses potentially viable in an X-autosome translocation
-X inactivation
What is an inversion
-2 break rearrangement where segment rotatoes 180 degrees, reinserts and breaks unite
What is a pericentric inversion
- includes the centromere
- 0.12-0.7% frequency
What is a paracentric inversion
- Does not include the centromere
- 0.1-0.5% frequency
What correlates with the size of an inverted segment
- Chances of a meiotic imbalance
- Size of inverted segment in proportion to length of chromosome.
What 2 ways do inversion act during meiosis
- Inversion loop (classic)
- Asynapsis/heterosynapsis (alternative)
What is inversion loop
- Allows maximum pairing
- Cross over outside the inversion to give normal or balanced gametes
- unequal number of crossovers within inversion gives normal, balanced and unbalanced gametes.
What is asynapsis/heterosynapsis
-Asynapsis is the complete failure of homologous chromosomes to pair or synapse during the first meiotic division,
What happens during asynapsis of small inverted segments
-No crossing over in inverted segment so no recombinant products
What happens during asynapsis of large inverted segments
- Crossing over can occur in inverted segment
- formation of recombinant products
- Paracentric: creates loop?
Two mechanisms reduce the number of recombinant products among the progeny of inversion heterozygotes
1) elimination of the products of crossovers in the inversion loop
2) inhibition of pairing in the region of the inversion.
What is an insertion
- A three break rearrangement
- Can be inter or intra-chromosomal
Review of interchromosomal insertions
- 1 in 80,000
- 2 types of meiotic behavior
1) Independent synapsing (most likely with small insertions). Segment loops out
2) Formation of quadrivalent. Less common and more likely with larger insertions. Form recombinant chromosomes
Review of intrachromosomal insertions
- Very rare
- Incomplete synapsis most likely, producing odd number of cross overs in centromeric segment. Results in recombinant chromosomes.
Review of chromosomal rings
- Occur when two breaks in one chromosome are fused
- 1 in 50,000
- 99% are sporadic
- 46,(r): expect symmetric segregation
- Risk for a 46(r) parent is 40% that offspring will have same or more severe phenotype.
What are supernumary chromosomes
Structurally abnormal chromosome fragments that cannot be fully characterised fully by conventional techniques.
Review of supernmary chromosomes
- Pathogenicity depends on gene content
- Small markers prone to loss during cell division
- Can interfere with segregation if engages in meiotic pairing, so could cause infertility
- Recurrence risk low- 1%
- e.g. i(12p) in Pallister Kilian syndrome.
what are isochromosomes
- Mirror image chromosome with identical arms either side of centromere.
- Can present as supernumary chromosomes.
- Usually denovo
- e.g. i(Xq) in Turner syndrome
