Due to the fact that IVF specialists can observe the eggs and embryos, the opportunity to explore their chromosomes and genes and thus diagnose many genetic diseases.
Since the diagnosis in this case is placed before the embryo transfer into the uterus, these studies were called "preimplantation genetic diagnosis" (Preimplantation Genetic Diagnostics - PGD) - diagnosis prior to attachment (implantation) of the embryo (in the endometrium), t. E. Before pregnancy.
Preimplantation genetic diagnosis (PGD) - an early form of prenatal diagnosis, developed in the late 1980s to help couples who are at risk of possible transmission of hereditary diseases offspring. If these pairs have the desire to get healthy offspring, the only addition to the PGA, a possible option for them is a prenatal diagnosis using amniocentesis or chorionic villus cytogenetic studies (TSIVH). The main disadvantage of prenatal diagnosis is that the detection of the disease in the fetus before the pair dilemma, whether to interrupt the pregnancy or prolong it, knowing that the child will suffer from a hereditary disease. PGD provides some alternative pairs as diagnosis is made on preimplatatsionnyh embryos and the patient tolerated only healthy embryos. Thus, pregnancy begins with the awareness of the fact that the fetus will not be affected.
Indications for preimplantation genetic diagnosis:
- the woman's age over 40 years;
- male infertility, in which confirmed aneuploidy in spermatozoa;
- recurrent miscarriages;
- several failed IVF attempts;
- sex determination, including the presence of diseases related to sex;
- the presence of one of the parents of chromosomal translocations.
How is pre-implantation diagnosis of hereditary diseases?
In oocytes and embryos biopsied cell can be carried out at three different stages:
1. The oocyte / zygote (polar body biopsy).
2. 6-10-cell cleavage stage embryos (cleavage stage biopsy).
3. The embryos at the blastocyst stage (blastocyst biopsy).
Biopsy cleavage stage is the most widely used technique in the world. Polar body biopsy investigated only maternal chromosomes.
From each parent the child receives only half the number of chromosomes. Each cell contains a complete set of chromosomes, and immature germ cells are no exception. How, then, it appears that at the time of the merger of sex cells have only half the number of chromosomes?
Y-sperm is derived from the fact that they share at maturation, and each receives a half of the full set. Egg in the process of maturation discards exactly half of its chromosomes in the "wastebasket" - the so-called polar body. Polar body can pick up and examine its genes and chromosomes, and as a result of studies to determine the egg is healthy or not, without damaging the egg itself. Fertilize only healthy eggs. It also ensures the development of a healthy fetus.
In some cases, pre-implantation diagnosis is a biopsy of the embryo. Biopsy of 6-8 cell embryos removed one or two cells (blastomeres), which was subjected to. It is proved that all these manipulations are absolutely harmless to the unborn child and have no effect on its development and health.
PGD can be compared with prenatal diagnosis. Embryo biopsy or amniocentesis equivalent TSIVH and diagnostics blastomeres can be equated to prenatal diagnosis. Because here we have the use of two techniques, biopsy and diagnosis, PGD group consists of a group and ICSI group of geneticists. Embryo biopsy technique should be performed by a competent embryologist, but the diagnosis should be carried department of genetics.
What hereditary diseases can be avoided by pre-implantation diagnosis?
Today, thanks to the existing techniques, it is possible diagnosis of a large number of genetic diseases: cystic fibrosis, hemophilia, thalassemia, Tay-Sachs disease, 1-antitrypsin deficiency and others.
In addition, pre-implantation diagnosis can prevent the birth of a child with malformations and Down's syndrome in women older than 35 years who due to age are at high risk of having a child with disabilities.
The important point is also the cost of the procedure, as the number of cell diagnostics technologies are very expensive. For example, the cost of FISH probes can equal the cost of the IVF cycle, which makes PGD very expensive manipulation. In the UK, some health authorities are funding cycles of PGD in other countries also noted the use of public funds or assets of insurance companies for this purpose; if you have to pay for the procedure by patients, they can choose prenatal diagnosis.
The most logical and most motivated is the use of PGD in the group of patients suffering from infertility or recurrent miscarriage. Patients who are carriers of chromosomal abnormalities, represent one of the most difficult groups to treat due to the limited number of available probes and notes they have a high level of abnormal embryos.
Ethics and legislation
Legislation regarding PGD varies in different countries. Many countries have laws regulating PGD or PGD and embryo research, while others have no such laws. Nevertheless, several countries have banned PGD? What is striking, considering the purpose of PGD - eliminate the need for abortion. The main argument against PGD is a possibility of abuse, for example, determine the sex of embryos for balancing family, or to select specific characteristics of the child, so-called "designer children". Abuse of prenatal diagnosis was noted in some countries for many years, but, as with all medical procedures, positive should outweigh the negative; Technology should not be banned just because of the risk of abuse. Legislation regulating the use of PGD must address these issues. In the UK Office of the Human Fertilisation and Embryology Authority, the licensing all IVF centers, banned sex determination of embryos for the purpose of balancing the composition of the family; it also issues licenses to all centers PGD. It should be remembered that PGD is not a simple procedure, primarily because of its high invasiveness and low pregnancy rates. For couples who want a "designer baby", prenatal diagnosis is an easier way in which the possible one-time diagnosis of many diseases.
The future of PGD
The development of PGD in the last 10 years has been very slow. Still can not make a diagnosis of cystic fibrosis or Down's syndrome on the same cell, and a list of diagnosed diseases is very low. PGD is more complex manipulation than expected: the concept of a single cell as a representative in relation to the whole embryo has been blurred and complicated by the discovery of high frequency human embryos chromosomal mosaicism. Unfortunately, there have been a few times diagnostic errors, mainly due to chromosomal mosaicism, allele loss or contamination (perhaps cumulus cells or sperm). However, recent studies have shown that the probability of erroneous diagnosis mukovitsidoza was 0,3-5,6%, depending on whether one or two cells were examined; This risk can be reduced to 0,015-1,25%, considering as useful to the transfer of only unaffected embryos homozygous.