What is a genetic compatibility test when planning a pregnancy and why it is needed – we understand together with the geneticist.
Ask if any of your friends did a genetic compatibility test when planning a pregnancy? Most likely, the answer will be “no”. But such a test today is the only opportunity for parents to find out if they have a risk of having a baby with a serious congenital disease, such as cystic fibrosis, phenylketonuria, hemophilia and others. Such diseases (and there are more than 10,000 of them in total) are called monogenic, and the cause of each of these diseases is damage or mutation of one gene.
Everyone is familiar with diseases caused by chromosomal abnormalities. The most striking example is Down syndrome. Most embryos with chromosomal abnormalities die in the early stages of pregnancy. At the same time, chromosomal diseases are rarely inherited, and in more than 95% of cases, the risk of re-birth in the family of a sick child with chromosomal pathology does not exceed the general population level. To diagnose chromosomal abnormalities in IVF programs, preimplantation genetic diagnosis is used, or prenatal screening, including biochemical studies, ultrasound diagnostics, noninvasive genetic test, analysis of chorionic villi cells or amniotic fluid.
The situation with monogenic diseases is quite different. It is not possible to detect them in the early stages of pregnancy by standard screenings, and natural selection does not work for them. Fortunately, monogenic diseases are quite rare. The most well-known monogenic diseases include phenylketonuria, cystic fibrosis, galactosemia, adrenogenital syndrome, hemophilia A and B, Duchenne/Becker myodystrophy, proximal spinal muscular atrophy, and many other diseases.
Depending on the localization of the mutant gene and the nature of inheritance, genetics distinguish autosomal and sex-linked diseases, which can also be dominant or recessive. In the first case, the disease will be transmitted if one of the parents is sick, with a frequency of 50%. Such diseases, for example, include osteogenesis imperfecta — bone fragility. More insidious is the autosomal recessive type of inheritance. After all, a sick child can in this case be born to healthy parents, if both of them are only carriers of a mutant gene. In such couples, the risk of having a sick child is 25%, in 50% of cases the child will be a carrier of a gene with a mutation and in 25% of cases a normal child will be born. Phenylcutonuria and cystic fibrosis are just such diseases.
Also today, more than 500 genes linked to the X chromosome are known, that is, localized in it. Many of them cause the appearance of hereditary diseases (hemophilia, muscular dystrophy, etc.), which manifest themselves mainly in boys, since girls receive one X chromosome from their father, and the other from their mother, and boys — only from their mother. The most widespread X-linked disease with a recessive nature of inheritance is hemophilia (non-coagulability of blood). Another example is Duchenne muscular dystrophy, which affects boys aged 2 to 6 years. The disease has a recessive type of inheritance, and the gene is localized on the X chromosome.
The risk of having children with monogenic diseases is especially high in populations with closely related marriages. Fortunately, in Russia, due to the diverse national composition and a large number of mixed marriages, the problem of congenital genetic diseases is not so acute. But still, most parents would prefer not to risk the health of the unborn child.
It is for such couples that a test for genetic compatibility has been developed. Who should do this test? It is definitely shown to any couple if there have been serious congenital pathologies associated with genetic mutations in the family history of the husband or wife. Also, it will not be superfluous to make it for those who are just planning their first pregnancy. After all, unfortunately, most carriers of genetic mutations do not have a history of relatives with pronounced clinical syndromes caused by monogenic diseases, and do not suspect the carrier of genes with mutations. At the same time, 10 out of 1000 births end with the birth of a child with a monogenic disease associated with a recessive type of genetic inheritance (that is, the disease manifests itself when both the father and mother are carriers of the mutation).
By the way, in some countries, genetic screening for the most common diseases found in the population is mandatory. Why is the study called a compatibility test? The test allows for high-precision screening of known polymorphisms and mutations (40,000 variants, 549 genes) responsible for the development of more than 600 genetic diseases during family planning and pregnancy, to identify their carrier and possible coincidence of the detected mutations in spouses. It is known that each of us carries an average of 5-10 mutations in our genes. But in this case we will be interested in whether these mutations coincide with mutations in the genes of the spouse.
To optimize the cost of testing, first of all, it is recommended that a woman undergo a study. If mutations are detected in one or more genes, the spouse can already be examined specifically for their presence in their chromosomes. At the moment, the cost of such tests in Russia ranges from 50 to 60 thousand rubles, depending on the number of diseases and syndromes studied, and is performed in a period of 40 to 60 days. It is also possible to undergo a study selectively for only a few of the most common diseases in the population, which significantly reduces the cost of the examination.
What should I do if matches are found? This occurs, and means only one thing — specifically, this couple has a high risk of giving birth to a sick child. Knowing this, the couple can go two ways. In developed countries, it is most often recommended to resort to IVF with preimplantation genetic diagnosis of the embryo, which is associated with significant costs, but gives the opportunity to choose a healthy embryo. If a couple is not considering IVF for themselves, then it is definitely recommended that they undergo a prenatal diagnosis to determine whether the child is sick or not. It is also important to understand that, unfortunately, conventional ultrasound and biochemical screening will not show the presence of a congenital genetic disease.