Health, Medicine
Description of the method of preimplantation genetic diagnosis (PGD): features and consequences
The method of preimplantation genetic diagnosis is used to assess the health status of an embryo obtained by in vitro fertilization before its implantation into the uterine cavity. The method allows not only to choose a healthy future of the baby, but also to increase the chance of pregnancy. The technique is widely used in all world clinics engaged in in vitro fertilization.
General concepts
Preimplantation genetic diagnosis allows for a comprehensive examination and excludes the presence of chromosomal abnormalities in general or any one specific deviation. Recently, in parallel with early vices, genes are screened, which can provoke the appearance of tumors in adulthood (ovaries, intestines, chest).
In the case of carriage of genomic diseases from the mother's side, several polar bodies of the oocyte are biopsy before the fertilization is carried out. The described methods are an alternative to early prenatal diagnosis, which allows you to warn against interruptions of pregnancy, if suddenly the fetus will have genetic pathologies.
Preimplantation genetic diagnosis makes it possible to determine the sex of the future baby already in the process of screening. This is used to prevent the development of anomalies linked to sex.
How are genetic diseases inherited?
Distinguish hereditary pathologies that have arisen according to the dominant, recessive and x-linked type.
| Inheritance type | Features | Risk of a sick child's birth |
| Dominant | One of the partners has one defective gene, dominant in its pair | 50% inherit the gene |
| Recessive | Both partners are carriers of one defective gene, but they have a normal pair | 50% become a carrier, 25% get a disease |
| X-linked | Women are protected from pathology by their set of chromosomes and are exclusively carriers. For men, development of anomaly is characteristic | A man can inherit a gene and a disease - 50%, a woman can become a carrier, like her mother - 50% |
Who is screened?
Preimplantation genetic diagnosis (PGD) is performed in the following cases:
- The presence of hereditary diseases or changes in the karyotype in one or both of the married couple. Often, the partners themselves know about their problems, less often - they learn in the process of screening.
- Habitual miscarriage of pregnancy. The problem of this is not the anomalies from the parents (they may not be), but the genetic mutations of the fetus that cause miscarriages.
- The age of persons undergoing the IVF program is more than 38 years.
- More than 3 unsuccessful attempts at in vitro fertilization in the anamnesis.
- Male infertility.
- Inclusion of ICSI (intracytoplasmic sperm injection) in the fertilization program. The process is fraught with the development of genetic abnormalities in the embryo.
- Cases of a history of pedestrian skidding .
PGD Features
Studies in the field of preimplantation genetic diagnosis indicate that the method has the right to exist, helps to diagnose and prevent chromosomal abnormalities, however, requires further improvement.
The features of PGD are as follows:
- The frequency of miscarriage at screening is lower than without it;
- At the moment there is no evidence that the possibility of pregnancy is increasing;
- There is a possibility of a false result: the embryo is healthy, and the result shows deviations, the embryo is sick, and the study says the opposite (5-10% of cases).
Conducting preimplantation genetic diagnosis (blastomere examination)
After the agreement on in-vitro fertilization, the reproductive expert assesses the need for PGD to have a married couple. If there is evidence, the partners may agree or refuse the procedure. Then a document confirming the permission to conduct the survey is signed.
On the third day from the moment of fertilization of the embryo, the geneticist takes one of the formed blastomer cells for analysis. It is believed that for the fetus this has no consequences, since all cells are interchangeable.
The specialist conducts research on one of the previously selected areas: studying the structure of the cell under a special fluorescent microscope, DNA testing using the polymerase chain reaction method to determine the presence of a mutation.
Based on the result, embryos having anomalies will not be used in the implantation process. Choose healthy and quality. Then one or more of them are transferred to the mother's uterus.
When a polar body is biopsy, the mother's chromosomal set is examined. It is carried out in the event that the presence of genetically determined pathology is revealed on the female line. The procedure allows you to determine whether the egg is healthy, which will be used for fertilization, without violating its structure. If anomalies are not identified, then it can be used for fertilization and transfer to the uterine cavity.
There are cases when screening of polar bodies, and then of blastomeres, is carried out. This allows you to get a more reliable result. Scheme, which will be screened, selects a geneticist.
Methods used
Numerical and structural chromosome abnormalities are determined by fluorescence hybridization. This method allows you to refine the DNA sequence of the cell. Special DNA probes are used, which in their structure are complementary areas for blastomere DNA zones.
The probe has a nucleotide and a fluorophore (a molecule capable of fluorescence). After the DNA target interacts with the DNA probe, the illuminated areas are formed, which are viewed under a fluorescent microscope.
The PCR (polymerase chain reaction) method is also used . It consists in copying a section of DNA with enzymes in certain laboratory conditions.
Diagnosis of aneuploidy
This is one of the methods of preimplantation genetic diagnosis, which makes it possible to identify the presence of numerical chromosomal abnormalities (to identify unnecessary or missing chromosomes).
The genetic material consists of 46 chromosomes, half of which is given by the father, and the second half by the mother. If the embryo gets an extra chromosome, this is called trisomy, and the loss of any is a monosomy. These defects can lead to consequences:
- Absence of embryo implantation;
- Spontaneous termination of pregnancy;
- Hereditary pathologies (Down syndrome, Klinefelter, Shereshevsky-Turner).
Male infertility and the age of a married couple are frequent factors in the development of such anomalies.
PGD of monogenic pathologies
Preimplantation genetic diagnosis of embryos in order to determine the presence of monogenic diseases is carried out in families where hereditary anomalies are established. If a married couple already has a baby with a homogeneous disease, the opportunity to give birth to another sick child rises several times.
The reason is a violation of the sequence of chromosomes in a certain region of DNA. Such diseases include:
- Cystic fibrosis;
- Adrenogenital syndrome;
- Phenylketonuria;
- hemophilia;
- Sickle-cell anemia;
- The Wernig-Hoffman syndrome.
Advantages and risks of the method
Preimplantation genetic diagnosis, which is considered to be ambiguous, has certain advantages in the field of prenatal screening:
- Only qualitative and healthy embryos are transferred to the uterus;
- Reduces the risk of having a baby with genetic abnormalities;
- The risk of miscarriage is halved;
- The risk of multiplicity is halved;
- The chance of embryo implantation increases by 10%;
- 20% increase the chance of a safe birth of a baby.
The risk of accidental damage to the embryo in PGD is 1%. The probability of an erroneous result is 1:10, in addition, 3% of the possibility that an embryo with any anomaly will be evaluated as healthy.
The probability that a healthy embryo will have a deviant result is 1:10. Chance 1: 5, that the transfer of embryos into the uterine cavity will be canceled because of the anomalies found in each of them.
Contraindications
Preimplantation genetic diagnosis (PGD), whose consequences for the embryo are not accurately identified, has a number of contraindications to the conduct of:
- Fragmentation of the embryo more than 30% (the norm is considered the formation of fragments up to 25%);
- The presence of multinucleated blastomeres in the embryo;
- The embryo has less than 6 blastomeres on the third day of its development.
Safety of the method
Diagnosis is carried out at an early stage of the creation of the future organism, when all its cells are polypotent, that is, a fully-fledged healthy organism can form from each.
All the stages of PGD do not affect the embryo, the fetus and the future baby. After the sample for the study is obtained, it is prepared in a special way. Cells are placed on a slide on which they are fixed, or in a buffer solution.
Cost of the procedure
Preimplantation genetic diagnosis, the cost of which varies from 50 to 120 thousand rubles, at first glance, seems costly manipulation. However, the health and treatment of a child with genetically determined pathologies will cost much more.
The procedure gives all the advantages to women who were able to conceive a child naturally, since they are not available for this method of diagnosis. Preimplantation genetic diagnosis, the price of which is negligible in comparison with extracorporeal fertilization itself, serves only as an addition to the general program, which allows not only to bear and give birth, but also to grow a healthy baby.
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