Non-destructive preimplantation diagnostics

Diagnosis before birth

Alexandra Bruter, <url>Chinese scientists have come up with a new way to diagnose genetic diseases at the earliest stages of embryonic development.

It is applicable in the case of in vitro fertilization. (Article by Yu Hou et al., Genome Analyses of Single Human Oocytes published in the journal Cell.)

Previously, genetic diagnostics of embryos obtained by in vitro fertilization was also available. They could be tested for specific diseases, and implanted with one that did not carry the defective gene. This technology was used if it was known that there was a high risk of any genetic diseases in the family. For example, one of the parents has cystic fibrosis, and the other is a carrier of a mutant gene. Cystic fibrosis is a recessive disease. This means that in order for a person to be sick, he must have mutations in both copies of the gene. A person with one copy of a mutant gene is called a carrier. He does not get sick himself, but with a 1/2 probability he will give a mutant copy to children. Thus, in the case described by us, when one parent is sick and the other is a carrier, parents can resort to preimplantation testing of embryos. The future mother has eggs taken, the father has sperm, fertilization is performed. In the case described above, half of the embryos will have two mutant copies, and the other half will have only one, people who do not get sick themselves, but are carriers of the mutation, can grow out of such embryos. They are implanted into the uterus.

We have given a rather harsh example, there are easier cases. For example, if both parents are carriers of a mutation associated with a recessive disease, then you can choose an embryo that does not carry mutations at all. It is possible to resort to preimplantation diagnostics, when the child is not directly in danger, he can be born either without a mutation, or only as a carrier. Choosing an embryo for implantation in this case will exclude the mutant gene from inheritance in this family.

Recently, pre–implantation diagnostics have also been resorted to in the case of mutations with incomplete penetrance - that is, when a mutation increases the risk of developing the disease, but does not make it one hundred percent. This is done, for example, in the case of BRCA gene mutations that are strongly associated with the development of breast cancer. The blurring of indications for preimplantation diagnostics has given rise to a public discussion about the ethics of embryo selection on some basis. If almost no one doubts that such selection is fully justified in the case of serious diseases, then the possibility of selecting embryos of a certain sex, hair color or, for example, with a higher IQ, if the genes responsible for IQ are ever discovered, seems unethical to many. So the procedure is under the close attention of the ethics committees.

Until now, more than two embryo cells were needed to make a diagnosis. Theoretically, in the early stages of development, when the cells are not yet different from each other, the embryo has an amazing ability to self-regulate. For example, if you divide it mechanically into two parts, two identical organisms will develop. This separation sometimes occurs spontaneously, and then twins are born. Even more interesting is another property of embryos at an early stage of development – if two different embryos are mechanically combined, then they will turn out to be one organism, in part of which there will be cells with one genotype, and in part with another. This phenomenon is called chimerism. It happens even in humans. The case of Lydia Fairchild, accused of not having her children, is widely known. The matter went far, the children were taken away by custody, the woman was tried for fraud or for surrogacy prohibited in her state, and it did not immediately become clear that cells with two different genotypes were present in the woman's body. Cells of one type gave rise to her eggs and tissues of the reproductive system, and of another type to hair and skin cells, which were taken for analysis.

But back to the embryo biopsy. Previously, in order to read the DNA sequence – scientists say "sequenced" – it was necessary to have more than one cell. Preferably at least three. Usually, an embryo is taken for research at a stage when there are 8 cells in it, the required number of cells is separated from it, and an analysis is performed. Indeed, some embryos normally tolerate such manipulation, but still not all. If we consider that further tested embryos are waiting for the "bottleneck" of in vitro fertilization – implantation into the uterine cavity – it is undesirable to worsen the condition of the embryo, and without that, with all the efforts of doctors, about a third of the embryos take root.

In the work described now, the authors have proposed a method that allows at least partially to establish the genotype of the embryo without destroying it. Firstly, the development of technology has led to the fact that it has become possible to read the DNA sequence with a single molecule. You can read more about the technology in an interview with Egor Prokhorchuk published by ProScience.

Secondly, they came up with an original method for calculating which copy of the gene got into the embryo, according to polar corpuscles. All cells of the body, except mature germ cells, have a double set of chromosomes. Germ cells are single. How does it work? Germ cells are formed in the body as a result of meiosis. At the very beginning there is one precursor cell. She has a double set of chromosomes, the chromosomes in her are the same as they were at the time of fertilization by the sperm of the egg, from which the organism subsequently developed. First, there is a crossing–over in it - the chromosomes inherited from the father and from the mother are exchanged with the corresponding sections. New chromosomes are obtained, each of which has something from the paternal and something from the maternal chromosome. In this case, the number of chromosomes doubles and the cell divides into two cells with a double set of chromosomes. After that, both resulting cells divide again, without DNA doubling. This results in four cells with a single set of chromosomes. In men, all four cells become spermatozoa.

It's not like that with women. One of the cells gets most of the cytoplasm – it becomes an egg. The other three are much smaller, they are called polar corpuscles. They will continue to be near the egg. In rare cases, polar corpuscles can be fertilized, but they lack the cytoplasm and nutrients to make a developing embryo. Polar corpuscles can be separated from the embryo without harm to it. And thanks to modern sequencing techniques, it is possible to read the genotype of each of the three cells. And now, attention: in the progenitor cell of the egg before it began to divide, there were two copies of the gene inherited from the father, and two copies of the gene inherited from the mother. Of these four variants, three turned out to be in polar corpuscles. So, the fourth option is in the developing embryo.

Diagram from the article in Cell – VM

In addition to mutations, the technology allows you to determine the presence of chromosomal aberrations, for example, to seriously suspect a trisomy on the 21st chromosome - Down syndrome. If, for example, one of the polar bodies lacks the 21st chromosome or part of it, then this is a reason to suspect the presence of an extra one in the embryo.

This technique does not allow you to find out anything about which copy of the gene was inherited from the father, but it allows you to get half of the information. Which is very important, without risking harm to the embryo. This is a very promising technique, its clinical trials have already begun, and in the future it will most likely allow many women to give birth to healthy children.

Portal "Eternal youth" http://vechnayamolodost.ru23.12.2013