Prefertilization diagnostics offers the possibility of genetic testing of a woman’s eggs as part of in vitro fertilization (IVF). The tests are performed on the chromosomes of the 1st and 2nd polar bodies, which are formed during the 1st and 2nd maturation division after the introduction of a male sperm into the egg. The method has the advantage that it is not de iure preimplantation genetic diagnosis (PGD) because the examination is performed before the fusion of the female and male nuclei, so that prefertilization genetic diagnosis is allowed in some countries with PGD bans.
What is prefertilization diagnostics?
Prefertilization diagnostics offers the possibility of genetic testing of a woman’s eggs in the context of in vitro fertilization. Prefertilization diagnostics creates the possibility of detecting chromosomal aberrations on the haploid genome of the female oocyte in the context of in vitro fertilization (IVF). In particular, numerical deviations of certain chromosomes (aneuploidy) and abnormalities of certain genes that cause hereditary diseases can be detected. When a male sperm is introduced into the cytoplasm of the egg cell during IVF, this first initiates the 1st and 2nd maturation divisions (meiosis I and II) in the egg cell. In each case, two “superfluous” cells, the polar bodies, which have the identical set of chromosomes as the oocyte itself, are produced as a result of the division. The polar bodies, which would normally be broken down by the body, are removed by polar body biopsy for chromosome analysis. Since prefertility diagnosis is always performed on the polar bodies, the procedure is also called polar body diagnosis (PCD). The advantage of the examination method is that it may be performed even in some countries where pre-implantation genetic diagnosis (PGD) is prohibited, because the examination is performed on the genome of the egg cell when the nuclei of the sperm cell and the egg cell have not yet fused. The disadvantage is that only chromosomal aberrations of the maternal genome can be examined. The chromosomes of the sperm introduced into the cytoplasm of the egg cell cannot be detected by this method. Y-linked diseases cannot be detected because the haploid chromosome set of the oocyte cannot contain a Y chromosome.
Function, effect, and goals
Prefertilization diagnostics in the form of genetic polar body testing can detect numerical abnormalities (aneuploidy) in certain chromosomes of the maternal genome, as well as translocations whereby chromosome segments have been separated and reinserted in the wrong location. In addition, X-linked gene mutations can be diagnosed that are maternally inherited and based on the mutation of a single gene (monogenetic disease). This requires that the potential inherited disease must be known in order to target a specific gene on the X chromosome. In the case of recessive inheritance, there is a chance that the X chromosome of the polar body – and thus also the X chromosome of the fertilized egg – contains the healthy allele of the corresponding gene. The procedure itself involves a polar body biopsy, in which the two haploid polar bodies are removed from the egg and the chromosomes are then subjected to FISH (fluorescence in situ hybridization) testing. The biopsy of the polar bodies poses an enormous challenge to the laboratory performing the test because the identification and isolation of the polar bodies requires a certain amount of experience. For the FISH test procedure, so-called DNA probes are available for selected chromosomes, which combine with the corresponding haploid chromosomes because they have the complementary amino acid sequence. The DNA probes are marked with different fluorescent colors so that the chromosomes can subsequently be identified using special software and counted in an automated procedure. Most chromosomal aberrations such as aneuploidies and chromosomal shifts within a chromosome are lethal. This means that either no zygote forms during IVF, or the embryo is rejected after transplantation into the uterus, or there is early or late miscarriage. Since the frequency of chromosomal aberrations in women’s eggs increases with age, an important goal of prefertility diagnostics is a positive selection of fertilized eggs.Only fertilized eggs with – as far as recognizable – intact genome are re-transplanted into the uterus. Positive selection is intended to increase the pregnancy rate with after IVF and reduce the rate of rejected fertilized eggs and the number of miscarriages. Another goal is to exclude hereditary diseases based on chromosomal aberrations or certain genetic defects in the re-transplanted fertilized egg from the outset by positive selection of the fertilized eggs. Typical inherited diseases that can be excluded by the test include cystic fibrosis, spinal muscular atrophy, and sickle cell anemia.
Risks, side effects, and hazards
Prefertilization diagnostics are performed outside the body and therefore do not pose any additional physical risks to the woman involved. Minor physical risks of injury and infection are associated only with egg retrieval. Unlike PGD, which also involves chromosomal examination by FISH test, in prefertility diagnostics only chromosomal and genetic material from the mother can be examined. This means that if the FISH test is negative and no chromosomal or genetic aberrations are diagnosed, the parents may have overly positive expectations about the course of the pregnancy and subsequent birth. Chromosomal aberrations of the paternal genome and possibly existing anomalies of the Y chromosome, which can cause a sex-related hereditary disease, are not recorded. In this respect, prefertilization diagnostics is even more incomplete than PGD, in which the entire genome of the embryo can be examined at the blastula stage. However, even in the case of a negative PGD, it cannot be excluded that genetic defects are present in the genome of the embryo, which can cause maldevelopment and possibly lead to impairments after birth. The FISH test can only ever refer to selected chromosomes and genes.
