Preimplantation genetic diagnosis is the study of the chromosomal set of an embryo before its implantation during after conducting vitro fertilization. It is resorted to in order to ensure the absence of chromosomal abnormalities in a fetus.
Nowadays, two methods of genetic research are widely used, such as aCGH and FISH. The first one allows you to explore all 23 pairs of chromosomes, and the second one only part of the genome.
When is a study prescribed?
In the course of genetic counseling, a geneticist collects a history and anamnesis, compiles a pedigree, assigns karyotyping to future parents. Analyzing the collected information, the doctor decides on the need for pre-implantation diagnosis, determines the method that is suitable for a pair. Pre-implantation diagnosis can be prescribed if:
- there is a history of at least one still pregnancy;
- age of the biological mother is over 37;
- parents have a child with a genetic abnormality or had a pregnancy with a chromosomal abnormality;
- biological parents or their blood relatives have genetic diseases;
- there is a history of more than two unsuccessful in vitro fertilization cycles.
Comparative genomic hybridization on aCGH microchips is currently the most comprehensive study of the number and structure of chromosomes. All pairs of the chromosome set are examined.
For analysis on the fifth day of development, all embryos grown in the laboratory are sampled, after which the zygotes are frozen. Freezing is performed because embryos of the fifth or sixth day of development are suitable for implantation, and the analysis is carried out for about three days. Further, special equipment is tested for DNA. According to the results of the analysis of the chromosome set, only those embryos are selected for which no anomalies have been detected.
The pre-implantation diagnosis of aCGH makes it possible to identify and discard embryos, implanting of which could lead to a spontaneous abortion, as well as to ensure that the unborn child will not suffer from genetic diseases.
This research technique allows screening of no more than 12 pairs of chromosomes out of 23. However, it is quite informative if a number of gene abnormalities are suspected and may be quite sufficient.
For analysis using the FISH method, a cell biopsy is performed on the third day of embryo development. Next, geneticists carry out a qualitative and quantitative analysis of certain pairs of chromosome set.
This method also makes it possible to identify genome abnormalities at the preimplantation stage and prevent the implantation of genetically insignificant embryos.
FISH or aCGH?
The answer to this question depends on several aspects. Given that aCGH is much more informative, the price for this study is much higher, which does not always fit into the budget of future parents. According to statistics, the extraction of one of 8 cells on the third day of embryo development during FISH reduces the chance of survival by 22%. At the same time, there are cases when the aCGH test cannot be performed, because none of the embryos on the fifth day reached the required quality for the study.
Both methods have their strengths and weaknesses. It is possible to determine unequivocally which of them can only be used by a geneticist, relying on medical genetic indications and circumstances in each specific case.