18.05.12 Cytogenetics of miscarriage Flashcards
What is the estimated % of conceptions that fail to implant? are lost following transient implantation??
30% fail to implant
30% lost following transient implantation
What is the estimated % of 3 day embryos that are abnormal?
50%
How is miscarriage defined?
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.
What % of foetuses are identified as abnormal in the first trimester? What % of these pregnancies will miscarry?
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)
What are the most common chromosomal abnormalities detected in first trimester pregnancy loss?
+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)
In POCs from spontaenous miscarriage, what % are autosomal trisomies? 45,X? triploidy?
60% trisomy
20% 45,X
15% trisomy
What are the most common trisomies identified in pregnancy loss?
T16 (30%)
T22 (10%)
T21 (9%)
T13 (6%)
T2 (5%)
T18 (5%)
What is the % miscarriage rate in the second trimester and the % abnormality rate of these miscarriages?
2-5% miscarriage
20% abnormality rate
Which chromosome abnormalities are more frequently detected in second trimester miscarriages?
50% of second trimester losses may have a chromosome abnormality. Usually the viable trisomies
T21 (63%)
T18 (23%)
T13 (13%)
Other structural rearrangements
What is the % pregnancy loss in the third trimester?
~0.5%
What fraction of pregnancy loss in the third trimester has an underlying chromosome abnormality? What is the most common detected?
~5% Most common is T18 XO and tetraploid rarer Structural abnormaltiies and other sex chr abnormalities more frequent T21 (32%) T18 (51%) T13 (17%)
What % of T21 pregnancies miscarry?
80%
What % of T16 pregnancies miscarry?
100% (non-mosaic)
What % of XO pregnancies miscarry?
> 95-99%
What % of triploid and tetraploid pregnancies miscarry?
Almost 100%
What % of normal karyotype pregnancies miscarry?
8%
Is there a parental chromosome effect in XO pregnancies?
Associated with a younger maternal age
80% of cases has loss of paternal contribution
What is tetraploidy? What % of miscarriages does it account for?
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.
What is triploidy? What % of miscarriages does it account for?
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)
What is the link between mosaicism, CPM and pregnancy loss?
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.
What is the definition of recurrent miscarriage? What % of couples experience recurrent miscarriage?
Three or more miscarriages before 24 weeks post-menstruation
1%
What factors can be causative of recurrent miscarriage?
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
What are the most common chromosome abnormalities that can underlay recurrent miscarriage?
Balanced reciprocal or Roberstonian translocations
Why was karyotyping for couples experiencing recurrent miscarriage stopped?
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
What are the main conclusions from studies investigating the effectiveness of karyotyping couples experiencing RM?
- Carrier parents will mostly have chromosomally normal or balanced pregnancies.
- 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.
- 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).
- Testing of fetal material/products of conception of a third fetal loss is a more effective steategy.
What are the recommendations from the RCOG ‘green-top’ guidelines for genetic testing in couples experiencing RM?
“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.
What are the advantages of molecular analysis rather than karyotyping of POC?
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.
- Higher risk of infection
- Culture failure common due to lack of viable fetal cells
- 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.
What % of additional abnormalities are detected in CMA vs karyotyping?
CMA detected 13% additional abnormalities over karyotype.
Karyotype detected 3% additional abnormalities over CMA (assumed balanced rearrangements)
Incidence of VOUS was 2%.
What is one future direction for molecular testing of pregnancy loss samples?
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.
