20.01.15 Origins of aneuploidy including recombination

Question 1

What are gametes?

Answer 1

• haploid (n) cells which combined during reproduction result in a zygote with 2n homologous chromosomes (one of which comes from the sperm and one from the egg) - Human cell (except gametes) have 46 homologous chromosomes, one for each parent (diploid, 2n)

Question 2

What is aneuploidy?

Answer 2

• Abnormal number of chromosomes 1) Nullsomy - 44 homolog chromosomes (2n-2); lethal 2) Monosomy - 45 chromosomes (2n−1) 3) Trisomy - 47 chromosomes (2n+1) 4) Tetrasomy/pentasomy - 48/49 chromosomes (2n+2/2n+3)
• Most commonly identified chromosome abnormality in humans, occurring in at least 5% of all clinically recognized pregnancies
• 1 in 300 liveborn infants are aneuploid, most commonly with a missing or additional sex chromosome or an additional chromosome 21
• About 1 in 3 miscarriages are aneuploid, with sex chromosome monosomy (45,X) and trisomy 16 being the most common

Question 3

What is cross-over?

Answer 3

• DNA exchange during recombination between non-sister chromatids generating genetic variation

Question 4

Whats is segregation?

Answer 4

• How the chromosomes are distributed in the cell/which pole they go to (alternate or adjacent)

Question 5

What is Non-disjunction?

Answer 5

• Failure of the chromosomes to segregate normally

Question 6

What is the difference between meiosis 1 and 2?

Answer 6

MI: affects entire chromosomes (92n to 46n) MII: affects sister chromatids (46n to 23n)

Question 7

What is pachytene?

Answer 7

• Third stage of prophase of meiosis I during which recombination (cross-over) occurs

Question 8

What is the centromere?

Answer 8

• Region of a chromosome that links a pair of sister chromatids - During cell division, spindle fibres attach to the centromere via the kinetochore

Question 9

What is a Paracentric inversion?

Answer 9

• Chromosome break and inversion that does NOT include the centromere - When sister chromatids pair in meiosis with one sister chromatid containing the inversion, the inversion region forms an inversion loop

Question 10

What is a Pericentric inversion?

Answer 10

• Chromosome break and inversion that DOES includesthe centromere (breakpoints to either side of the centromere) - It can occur within gene regions, causing disruption to gene expression or gene fusions

Question 11

What are Kinetochores?

Answer 11

• Protein that attaches to a chromatid allowing it to attach to the spindle

Question 12

What are Chiasmata?

Answer 12

• Joins bivalents together at locations along the length of the chromatids

Question 13

What is cohesin?

Answer 13

• Joins sister chromatids together, and also helps to maintain chiasmata

Question 14

What is Homotisomy?

Answer 14

• the occurrence of more than one child with trisomy in the same family

Question 15

What is Female Age related susceptibility?

Answer 15

Oogenesis is paused in diplotene stage increasing susceptibility to MI/MII errors

Question 16

How does aneuploidy arise?

Answer 16

• during mitosis or meiosis - constitutional aneuplodies most common during meiosis and are present in all cells - Somatic or aquired anauplodies occur during mitosis and result in mosaicism (level of mosaicism depends on when error occured after first postzygotic division)

Question 17

What are the 5 main molecular processes which are linked to aneuploidy?

Answer 17

1) Centrosomes copy number
2) Chromosome cohesions
3) Organisation of spindle microtubules
4) Problems at recombination
5) Disruption of cell-cyle regulation

Question 18

1) Centrosomes copy number

Answer 18

• important in spindle pole organisation and accurate cell division
• In each cell cycle the centrosome duplications to ensure daughter cell has the correct number
• Process is dependent on other cell cycle events (e.g. growth signalling pathway)
• Loss of come pathways can lead to centrosome over duplication and DNA replication
• So cell has too many centrosomes and multipolar spindles which causes chromosome missegragation

Question 19

2) Chromosome cohesion

Answer 19

• Required during cell division
• Maintained by the cohesin protein complex during G2 and M phases of the cell cycle, and ends at anaphase by cleavage of the cohesion complex
• Regulatory factors (Pds5, Wapl, and Eco1) coordinate the precise timing for the cleavage of cohesins
• Timing is crucial to ensure that all sister chromosomes/chromatids segregate simultaneously to prevent chromosome lagging
• Can create an anaphase lagging chromosome which can cause the chromosome being incorporated into the wrong cell

Question 20

3) Organisation of spindle microtubules

Answer 20

• The organisation of spindle microtubules is important for the accuracy of chromosome segregation

In normal mitosis, sister kinetochores segregate to opposite poles of the bipolar spindle during anaphase

When this goes wrong during anaphase, incorrectly attached kinetochores give rise to lagging chromosomes, which might lead to missegregation (same idea as from chromosome cohesion sllide)

• See attached slide - explains kinetochore attachement during mitosis.
• Syntelic attachment (where both sister kinetochores interact with microtubules emanating from the same spindle pole) and merotelic attachement (where a single kinetochore is connected to both spindle poles) are incorrect

Question 21

4) Problems at recombination at M1

Answer 21

• Recombination issues at M1 can lead to anauploidy via 3 mechansisms
  1) Issues with Chiasmata
  2) Paracentric inversions
  3) Pericentric inversions

Question 22

Problems at recombination at M1 - Issues with chiasmata

Answer 22

• Failure to establish chiasmata between homologue pairs, having a single distally placed chisma or premature resolution of the chiasmata can result in homologues segregating to the same pole at M1
• Causes non-disjunction

Question 23

Problems at recombination at M1 - Paracentric inversions

Answer 23

• one sister chromatid containing an inversion without centromere involvement
• During Paracentric exchange crossing-over happens within an inversion loop resulting in connecting homologous centromeres in a bridge and simultaneously producing a fragment without a centromere which cannot align and is lost
• The remaining chromosome separates by randomly breaking the bridge, forming two chromosomes with terminal deletions

Question 24

Problems at recombination at M1 - Pericentric inversions

Answer 24

• one sister chromatid containing an inversion containing the centromere region
• During Pericentric exchange, after the cross-over at MI the chromosomes disjoin without the creation of a bridge
• However, after MII, one of the four resulting chromatids contain a duplicated region another a deleted region
• Therefore, a zygote carrying a crossover chromosome will have a genetic imbalance

Question 25

5) Disruption of cell-cycle regulation

Answer 25

• During mitosis or meiosis, the SAC prevents the progression of anaphase until all chromosomes are properly attached to the spindle

Disruption of the cell-cycle regulation at this point could result in the incorrect attachment of the two kinetochores on the sister chromatids to the spindle poles

• This in turn may result in each daughter cell having two or no homologous chromosomes
• Orchestration of the SAC is carried out by numerous signalling pathway components

Question 26

Give some examples of constitutional aneuplody

Answer 26

• 1p36 deletion syndrome
• Wolf-Hirschom syndrome
• Cri du chat syndrome
• 5q deletion syndrome
• AS/PWS syndrome
• Miller-Dieker syndrome
• DiGeorge syndrome
• Trisomy 8, 9, 13, 15, 16, 17, 18, 21, 22

Question 27

What is the incidence of aneuploidies in gametes?

Answer 27

Sperm - 1-2%

Oocytes - 20%

Pre-implantation embryos - 25%

Spontaneous abortions - 35% (most common are 45,X, T16, T21, T22)

Stillbirths - 4% (most common are T13, T18 or T21)

Live births - 0.3% (most common are T13, T18, T21, XXX, XXY, XYY)

Question 28

What is the most common origin of trisomies?

Answer 28

• In general majority are due to maternal non-disjunction (73%), more M1 (50%) than M2 (23%)
• Paternal non-dicjunction only accounts for 6.5% from M1 and 3.2% from M2
• XXY is exception when 51% of cases are due to paternal non-disjunction at M1
• Most 45,X cases are due to loss of paternal X

- Differences in parental origin of aneuploidy likely due to sex specific differences in meiosis

There is at least one crossover event per chromosome in sperm VS. 10% of oocytes with at least one chromosome with no cross overs

In addition, checkpoint control is less stringent in females leading to defects being more tolerated (link to infertility in male translocation carriers).

Question 29

What are the two major risk factors for aneuploidy?

Answer 29

• altered recombination and maternal age

Question 30

Risk factor for aneuploidy - altered recombination

Answer 30

• Reductions in recombination have been observed in maternal MI-derived trisomies 15, 16, 18, and 21 and sex-chromosome trisomies, and in paternal MI-derived XXYs and T21
• Increases in recombination have been observed for maternal MII-derived T21, which indicates that the precipitating event for these cases probably occurred at MI (translocation)
• Abnormally positioned recombination events (too close to, or too far from, the centromere) have been reported for trisomies 16 and 21

Question 31

Risk factor for aneuploidy - maternal age

Answer 31

- Increasing maternal age is the most important aetiological agent associated with aneuploidy

• for women in their 40s, as many as one-third of all clinically recognized pregnancies might be trisomic
• This is commonly associated with abnormal recombination
• The presence of a pericentromeric exchange might increase the likelihood of abnormal recombination. Subsequent segregation at MII would result in a disomic gamete having identical centromeres (although originating at MII, the precipitating event occurs at MI)
• Due to female age susceptibility, advancing maternal age causes decreased cohesion of sister chromatids resulting in early separation during MI
• Failure to for chiasmata (75%) and chromatid cohesion deterioration (25%) have been observed in segregation of chr 16, 18, 21 and X

Question 32

How can altered recombination lead to mosaic aneuploidy and UPD?

Answer 32

• non-disjunction at Meiosis II can lead to mosaic aneuploidy (if viable), or UPD (if the extra chromosome from the one parent is retained but the chromosome from the other parent is lost when the cell attempts to correct the aneuploidy
• Several cyto studies of oocytes and first polar bodies have shown germ line or gonadal mosaicism for trisomies of chromosomes 13 and 21
• Parental gonadal mosaicism affects 6.5% of young couples with a T21 child and accounts for homotrisomy
• Postzygotic mitotic errors are less frequent than meiotic errors, and most are due to anaphase lagging
• Mitotic non-disjunction results in an embryo containing monosomic and trisomic cells for the same chromosome due to reciprocal loss and gain of one chromosome. Embryos are usually mosaic
• Simultaneous non-disjunction: non-disjunction happens twice in the same meiosis leading to double aneuploidy, two cell lines (eg. 47,XXY,+21 + 45,X)