18.05.12 Cytogenetics of miscarriage

Question 1

What is the estimated % of conceptions that fail to implant? are lost following transient implantation??

Answer 1

30% fail to implant

30% lost following transient implantation

Question 2

What is the estimated % of 3 day embryos that are abnormal?

Answer 2

50%

Question 3

How is miscarriage defined?

Answer 3

spontaneous end of a at a stage where the or fetus is incapable of surviving independently, generally defined in humans at prior to 24 weeks of . Miscarriage is the most common complication of early pregnancy.

Question 4

What % of foetuses are identified as abnormal in the first trimester? What % of these pregnancies will miscarry?

Answer 4

10% abnormal
10-15% SA, usually when placenta takes over nourishment and foetus is recognised as abnormal
50% of these will be chromosomally abnormal (60% trisomy)

Question 5

What are the most common chromosomal abnormalities detected in first trimester pregnancy loss?

Answer 5

+16 (1% of all conceptions, 30% of all trisomies) arising from maternal non-disjunction, 100% inviable if full trisomy and about 10% of mosaics can get to second trimester.

+22 (10% of trisomies),

+21 (9%), +15 (7%), 90% due to maternal meiosis errors

+13 (6%), maternal meiosis I, 9/10 spontaneously aborting. Can occur as Robertsonians however familial inheritance of these is rare.

+2 (5%)

+18 (5%), ~100% due to maternal meiosis II errors

45,X (20% of all abns), triploidy (15% of all abns)

Question 6

In POCs from spontaenous miscarriage, what % are autosomal trisomies? 45,X? triploidy?

Answer 6

60% trisomy

20% 45,X

15% trisomy

Question 7

What are the most common trisomies identified in pregnancy loss?

Answer 7

T16 (30%)

T22 (10%)

T21 (9%)

T13 (6%)

T2 (5%)

T18 (5%)

Question 8

What is the % miscarriage rate in the second trimester and the % abnormality rate of these miscarriages?

Answer 8

2-5% miscarriage

20% abnormality rate

Question 9

Which chromosome abnormalities are more frequently detected in second trimester miscarriages?

Answer 9

50% of second trimester losses may have a chromosome abnormality. Usually the viable trisomies

T21 (63%)
T18 (23%)
T13 (13%)

Other structural rearrangements

Question 10

What is the % pregnancy loss in the third trimester?

Answer 10

~0.5%

Question 11

What fraction of pregnancy loss in the third trimester has an underlying chromosome abnormality? What is the most common detected?

Answer 11

~5%
Most common is T18
XO and tetraploid rarer
Structural abnormaltiies and other sex chr abnormalities more frequent
T21 (32%)
T18 (51%)
T13 (17%)

Question 12

What % of T21 pregnancies miscarry?

Answer 12

80%

Question 13

What % of T16 pregnancies miscarry?

Answer 13

100% (non-mosaic)

Question 14

What % of XO pregnancies miscarry?

Answer 14

> 95-99%

Question 15

What % of triploid and tetraploid pregnancies miscarry?

Answer 15

Almost 100%

Question 16

What % of normal karyotype pregnancies miscarry?

Answer 16

8%

Question 17

Is there a parental chromosome effect in XO pregnancies?

Answer 17

Associated with a younger maternal age

80% of cases has loss of paternal contribution

Question 18

What is tetraploidy? What % of miscarriages does it account for?

Answer 18

2-3% of spontaneous abortions

Almost always 92,XXXX or 92,XXYY from failed early mitotic division.

Rare 92,XXXY embryos have been seen due to dispermy.

Question 19

What is triploidy? What % of miscarriages does it account for?

Answer 19

6-7% of spontaneous abortions.

Often seen a partial molar pregnancy with cystic villi.

2/3 shown to have two paternal chromosome sets (diandry), usually arising from dispermy.
1/3 shown to have two maternal chromosome sets (digyny), most often arising from fertilisation of egg that has retained its polar body.

Diandric triploid foetuses have large cystic placentas and rarely survive to the second trimester. If these foetuses survive they show relatively normal foetal proportions (type I phenotype).

Dygynic foetuses have smaller placenta and are more likely to survive to the second trimester, however they are growth retarded with macrocephaly (type II phenotype)

Question 20

What is the link between mosaicism, CPM and pregnancy loss?

Answer 20

Most ‘common’ are 2, 8, 9, 20 and 22. 90% of i(20) mosaic fetuses are completely normal, only 10% are born with abnormalities.

Will depend on the timing of the mutational event, the mosaicism may be found in the placenta and embryo or in only one of them.

CPM may lead to pregnancy loss and intrauterine growth retardation. A cytogenetic abnormality only in the placenta may cause the placenta to grow incorrectly (too large or too small) which then has a physiological effect on the fetus.

Question 21

What is the definition of recurrent miscarriage? What % of couples experience recurrent miscarriage?

Answer 21

Three or more miscarriages before 24 weeks post-menstruation

1%

Question 22

What factors can be causative of recurrent miscarriage?

Answer 22

epidemiological factors

-parental and/or embryonic genetic abnormalities (2-5%)

anatomic/structural abnormalities (e.g. of the uterus, cervical weakness etc)

maternal endocrine and immune response anomalies (eg: Antiphospholipid syndrome, thyroid dysfunction)

infective agents

acquired thrombophilic defects

unexplained

Question 23

What are the most common chromosome abnormalities that can underlay recurrent miscarriage?

Answer 23

Balanced reciprocal or Roberstonian translocations

Question 24

Why was karyotyping for couples experiencing recurrent miscarriage stopped?

Answer 24

Not cost effective.
Barber (2010) showed balanced translocations identified in 2% of couples with RM
4 unbalanced translocations following referral for prenatal testing after balanced translocation identified through karyotyping parents for RM = £1M per unbalanced pregnancy

Question 25

What are the main conclusions from studies investigating the effectiveness of karyotyping couples experiencing RM?

Answer 25

1. Carrier parents will mostly have chromosomally normal or balanced pregnancies.
2. Those that do result in the miscarriage of a chromosomally unbalanced fetus are likely to spontaneously abort a further child with the same imbalance prior to prenatal testing.
3. Carrier parents with an affected liveborn child with an unbalanced rearrangement have a higher risk of having another abnormal liveborn child than most couples experiencing recurrent miscarriage, therefore money may be better spent improving the detection rate of unbalanced structural abnormalities in affected children (providing a diagnosis for these children will clearly have additional benefits also).
4. Testing of fetal material/products of conception of a third fetal loss is a more effective steategy.

Question 26

What are the recommendations from the RCOG ‘green-top’ guidelines for genetic testing in couples experiencing RM?

Answer 26

“Cytogenetic analysis should be performed on products of conception of the third and subsequent consecutive miscarriage(s). Parental peripheral blood karyotyping of both partners should be performed in couples with recurrent miscarriage where testing of products of conception reports an unbalanced structural chromosomal abnormality.”

Testing of products of conception will lead to detection of unbalanced rearrangements due to parental rearrangements but also sporadic chromosome aneuploidy and polyploidy that is associated with miscarriage.

Knowledge of the karyotype of the products of conception allows an informed prognosis for a future pregnancy outcome to be given. If the karyotype of the miscarried pregnancy is abnormal, there is a better prognosis for the next pregnancy.

Question 27

What are the advantages of molecular analysis rather than karyotyping of POC?

Answer 27

No cell culture which has the following issues:
1. Maternal cell contamination can be problematic. Careful selection and dissection is essential, particularly with placental tissues.

2. Higher risk of infection
3. Culture failure common due to lack of viable fetal cells
4. Mosaicism may be present for a number of reasons, both biological (eg: vanishing twin, chimerism, confined placental mosaicism, true constitutional mosaicism) and technical (cultural artefact, maternal contamination). Particular difficulties may be encountered if a normal cell line overgrows that of an abnormal line, or the abnormality (especially an extra marker chromosome) is lost in vitro.

Question 28

What % of additional abnormalities are detected in CMA vs karyotyping?

Answer 28

CMA detected 13% additional abnormalities over karyotype.

Karyotype detected 3% additional abnormalities over CMA (assumed balanced rearrangements)

Incidence of VOUS was 2%.

Question 29

What is one future direction for molecular testing of pregnancy loss samples?

Answer 29

Qiao et al. (2016) performed whole exome sequencing on seven euploid miscarriages from four families with recurrent pregnancy loss. Compound heterozygous mutations in DYNC2H1 and ALOX15 were identified in miscarriages from two families. Argue that findings suggest that collective effect of mutations in relevant biological pathways could be implicated. In view of the size of this study and lack of functional characterisation, all conclusions must be treated with caution but shows that this technology may be applied to RA analysis.