The genetic abnormalities behind octopus miscarriages

This number, which was the starting point of a recent American-Canadian study, reminds us of the importance of better understanding the causes of miscarriage. The researchers were specifically trying to determine the type and frequency of chromosomal abnormalities, explains Dr. Rina Slim of the Child Health and Human Development Program at McGill University Health Center Research Institute (IR-MUHC) and co-author of this recent study.

A wide range of genomic imbalances could therefore lead to recurrent miscarriages. The authors analyzed samples from 24,900 miscarriages with a new technology using chromosomal microarrays (SNP-CMA), which are chromosomal microarray analysis tests.

“We don’t know much about the genes that cause spontaneous abortions. This study tells us that some genetic abnormalities are not found in the parents but occur at conception – a chromosomal defect in the fertilized egg. The fetus is not viable,” explains the researcher.

Miscarriages are indeed very different, as are their causes, such as the age of the future mother – the risk would be one in three from the age of 40 – the maternal reproductive anatomy – abnormalities of the uterus for example – previous miscarriages or environmental factors – among other tobacco, alcohol, drug addiction, but also medication, occupational hazards, infections.

Genes from mother, father or embryo

Many researchers also point out that a large proportion of recurrent miscarriages could have an underlying genetic cause – which would affect almost 5% of couples. Such genetic abnormalities can lie in the genome of the mother, father or embryo (the fetal genome).

This study also showed that abnormalities were observed in almost 56% of miscarriage samples, of which almost 8% had significant polyploid imbalances – these are changes in the number of chromosomes, missing or excess.

In humans, genes are grouped into 46 chromosomes, 23 from the father and 23 from the mother — together they make up the equivalent of a book with various instruction pages where mistakes can happen: missing pages, too many, or duplicates that are still difficult to spot .

Above all, a so-called “chromosomal translocation” is aimed at: More than a loss or gain of certain genes, these are changes within the structure of one or more chromosomes – for example abnormal breaks or adhesions – that are likely to have an impact on the design.

These changes in the structure of the chromosomes go under the doctors’ radar. “Spontaneous death occurs in 90% of cases with such anomalies,” says Dr. Slim.

The researcher also notes that “aneuploidy” – an abnormal number of chromosomes in a cell – is observed in almost 13% of cases.

The analysis of samples from 1103 patients with multiple miscarriages thus provides information about possible predispositions. And it turned out that any termination of pregnancy would increase the risk of miscarriage again – 15-20% after the first miscarriage, 25-40% after the second, up to almost 45% from the 3rd pregnancy that was not completed.

In a previous study, the researcher investigated the cause of these repeated events in a patient who had suffered 16 miscarriages. Genetic sequencing of spontaneously broken off cells had shown that behind these embryo losses there would be a mutation in the CCNB3 gene, which contributes to the cell division of the egg cell.

15 years ago, the research team also identified the first of the genes (NLRP7) responsible for recurrent miscarriages. It is a gene transmitted by the mothers in the egg that affects the viability of the embryo due to the presence of chromosomal abnormalities.

How do you prevent the worst?

Alarmed by cramps, bleeding and pain, the woman, who may be experiencing pregnancy loss, first has an ultrasound to detect the fetal heartbeat and blood tests to determine the level of pregnancy hormones (beta-hCG) in the blood: a low one Value points out the death of the fetus.

Fetal karyotype analysis might be suggested to try to trace chromosomal abnormalities. It should be noted that some do not prevent the viability of an embryo, such as B. Trisomy 21 with the presence of a supernumerary chromosome 21 or a supernumerary fragment of chromosome 21.

If the couple repeatedly fails in their attempts to conceive, Dr. Slim genetic testing to find part of the explanation, especially for the 5% of those who fail to conceive. “This is another clue for these couples in distress, but it won’t explain all the gray areas,” says the researcher.

Not yet available in Quebec

The strength of this study is that this new American technology shows great promise for revealing genetic abnormalities underlying failed pregnancies, comments Dr. Camille Sylvestre, associate professor in the Department of Obstetrics and Gynecology at the University of Montreal, who did not participate in this study.

This makes it possible to examine the genome of the fetus in more detail. “It’s like looking at the letters in a book, but it’s not available in Quebec yet. An intermediate technology, analysis by comparative genomic hybridization (Comparative genomic hybridization or CGH) instead suggests looking at the pages of the book — the amount of DNA in our chromosomes. But what is most commonly used as a screening in Quebec is the analysis of the karyotype – which counts the chromosomes: it is like looking at the chapters of this book,” explains the fertility specialist, who is also affiliated with CHU Sainte-Justine .

In Quebec, three labs — two in Montreal and one in Sherbrooke — offer CGH technology, but only after three miscarriages. “Our recommendations would be to do it after two miscarriages, because waiting so long to better understand the reasons for the failures carries a disproportionate financial and emotional cost. Not to mention that it also takes between 3 and 4 months to get a result,” specifies the expert, a member of the Canadian Fertility and Andrology Society of Canada’s Guidance Committee.

Ordinary screening by karyotype analysis “doesn’t provide all the answers, so you might want to push the analysis further and insist on doing these CGH tests. The companies that offer them are currently very present at fertility congresses and this would be my only criticism of the study, so there is a commercial bias, while this study, as we learn from this study, much more about the anomalies related to sporadic miscarriages (64%) than recurrent miscarriages (5%). We don’t always end up with a diagnosis that explains everything, and we have to take the mother’s age into account,” the expert reiterates.

Many expectant mothers are already old – in Canada, the average age for a first pregnancy is 30 – “the average age for consultation at the clinic is 38 in Quebec” – and with age the risk of genetic abnormalities increases rapidly.

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