Preimplantation Genetic Screening
Preimplantation genetic diagnosis is a highly specialised technique designed to identify chromosomal aberrations (deviations, changes) or monogenic hereditary diseases before an embryo is transferred to the uterus. During studies of the chromosomal structure of embryos it was determined that an incidence of aneuploidies (deviations in the number of chromosomes) is one of the most common causes of infertility. Examining the chromosomes of an embryo can therefore increase the likelihood of a successful artificial insemination.
The objective of preimplantation genetic diagnosis is to select for transfer only those embryos for which no abnormalities have been detected within the scope of PGD.
Indications for preimplantation genetic diagnosis are particularly the following:
- high risk of chromosomal aberration or severe monogenic hereditary disease in children,
- genetic defects linked to the child's gender,
- repeated miscarriages,
- premature birth or miscarriage with chromosomal disorder in the medical history,
- numerous unsuccessful IVF cycles.
Preimplantation genetic diagnosis is conducted in the early stage of embryo development. A single independent cell (blastomere) is retrieved from the embryo via the mechanical opening of the zona pellucida, which is then examined with the help of the methods indicated below. A biopsy of the embryo – the retrieval of a single independent cell – is most often conducted 3 days after fertilisation, when the embryo has 6-8 cells. Only those embryos which are healthy from the perspective of the preimplantation genetic diagnosis are subsequently selected for transfer to the uterus.
Preimplantation genetic aneuploidy screening
Preimplantation genetic aneuploidy screening (PGS) consists of examining the most common aneuploidies (numerical chromosome abnormalities) in an embryo. PGS uses the technique of fluorescence in situ hybridisation (FISH) and comparative genome hybridisation (CGH), using fluorescent markings of the chromosome for their identification.
The method of fluorescence in situ hybridisation (FISH) uses specific fluorescent marked DNA probes to identify numerical and structural aberrations of the chromosomes. The screening usually takes place in chromosomes 13, 18, 21, X and Y, which most often cause chromosomal aberrations, and is often supplemented with an examination of chromosomes 15, 16 and 22, whose numerical abnormalities are more frequently found in miscarried foetuses.
The method of comparative genome hybridisation (CGH) examines chromosomal aberration by comparing the intensity of the fluorescence of two different fluorescent marked DNA probes – the DNA of the patient and the DNA of a healthy individual. CGH enables detection of quantitative changes to the genome, loss and surplus of genetic material, particularly in patients with oncological illnesses.
In cases where one or both partners are carriers of monogenic diseases, examination of embryos using the PCR method can help to eliminate this monogenic disease for their children. Examination with the help of polymerase chain reaction (PCR) can diagnose mutations of genetic information in carriers of monogenic diseases including cystic fibrosis, Huntington's disease or diseases linked to the X chromosome.
Examination of embryos for unusual arrangement of chromosome segments (translocation) occurring during oocyte or spermatozoa development, or inherited from one of the parents. Indications for this type of exam are determined on the basis of individual and family medical history. There exist two primary types of translocation: reciprocal translocation (exchange of material between two chromosomes) and Robertsonian translocation (fusing of two chromosomes to each other) which can be identified with the help of fluorescence in situ hybridisation.