27 July 2020

Why are embryo genes studied?

Noninvasive prenatal diagnosis

Denis Rebrikov, Post-science

Preimplantation genetic diagnostics, which is carried out during in vitro fertilization, makes it possible to check the genetics of the embryo even before its transfer to the mother's body. But since IVF is a technically complex and expensive procedure, they resort to an alternative method – noninvasive prenatal diagnosis.

New generation sequencing

At an early stage of development, around the seventh to tenth week, the embryo's body is intensively rebuilt: there is a process of rapid division of individual cell types, but also their self-destruction due to apoptosis – programmed cell death. The rapid generation of cells and their destruction lead to the fact that DNA molecules of those cells that divide and break down in it enter the bloodstream of the embryo. Some of these molecules pass the placental barrier and end up in the mother's bloodstream. In the blood that we take from a woman at 10-12 weeks of pregnancy, it is possible to detect fetal DNA molecules - from 5 to 10% of the entire extracellular DNA fraction.

Powerful genetic testing methods, such as high-throughput sequencing (NGS), have made it possible to determine the DNA that entered the mother's bloodstream from a developing embryo. At the stage of 10-12 weeks of pregnancy, we can detect aneuploidy – when the number of chromosomes in cells is not a multiple of the haploid set. Each person should have 23 pairs of chromosomes, each pair is formed by chromosomes that came from parents – one from the father and mother. But there are situations when there is one missing in the chromosome set of an organism (this is called a monosomy) or, on the contrary, there is an additional chromosome (trisomy). Or when there is no genetic diversity inside a pair of chromosomes, since both came from the mother or from the father, which is also bad from the point of view of the development of the future organism, even if the number of pairs of chromosomes is correct.

We can determine all cases of aneuploidy at the stage of 10-12 weeks of pregnancy by simply taking venous blood from a woman and analyzing it using high-performance sequencing. Thus, the number of chromosomes is established and their combination is determined in the unborn child.

Noninvasive prenatal testing

Screening technologies may not necessarily be genetic. Today, one of the main methods of finding problems with the number of chromosomes is screening of the first trimester – ultrasound examination (ultrasound), when a specialist can pre-determine abnormalities in the development of the embryo leading to genetic diseases. After that, a genetic analysis is already carried out, which either confirms or refutes the presumed diagnosis – a non-invasive prenatal test (NIPT). It is quite complicated and costs about 20 thousand rubles, but its advantage is that it is performed noninvasively, without any biopsy of the fetus itself. This is a procedure that is not associated with any complications, and thanks to it you can tell for sure if everything is OK with the number of chromosomes.

There are methods by which they try to bioinformatically analyze the resulting array of DNA data. However, analyzing extracellular DNA from the mother's blood, we have 95% of the mother and 5% of the fetus, that is, bioinformatics breeds these pools. In addition, there are techniques when we can not only count the number of chromosomes, but also look at the specific genetics of a child, identify certain mutations, polymorphisms, disorders in genes on his chromosomes. Such methods have not yet entered into ordinary medical practice, but there is a chance that they will be included there in the future for several years. Then a woman, donating venous blood as part of a routine screening of the first trimester, will receive information not only about the number of chromosomes, but also about the genetics of the unborn child.

Advantages of noninvasive prenatal diagnosis

Thus, there are two blocks of genetic testing: preimplantation, when embryos are selected without genetic disorders as part of IVF, and prenatal, when we analyze genetics from the mother's blood. These two tests over the past 10 years have significantly reduced the risk of having a child with genetic problems and chromosomal pathologies. What is the best non-invasive approaches (NIPT) compared to the classical option when we take an embryo biopsy? Despite the fact that the invasive method is quite well developed, in about 0.5% of cases, the procedure of cell sampling (biopsy) itself leads to a complication of pregnancy. At first glance, the risk is small, but if we take pregnancies passing through our center of obstetrics and gynecology – and this is about 9 thousand per year – then this is a fairly large number of women who may be referred for an invasive test and who may develop complications associated with the procedure of taking the embryo biomaterial.

Therefore, non-invasive approaches are positioned as the first line of screening. As a rule, after detecting some violations at this stage, the doctor sends for an invasive test. Thus, the number of cases of invasive testing is significantly reduced. 

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