27 January 2009
The first pregnancy following the use of a new prenatal genetic screening technique has been announced by the CARE Fertility Group in Nottingham (see press release). The new technique involves using array comparative genomic hybridisation (array CGH) to screen chromosomes biopsied from the polar body of a fertilised egg following in vitro fertilisation (IVF).
A polar body is the small cell resulting from meiosis during gamete formation, which contains a complementary number of chromosomes to the gamete itself. Two polar bodies are produced – one (entirely maternal in origin) from the egg prior to fertilisation, and one following penetration of the egg by the sperm. These have no known function, and are generally regarded as ‘by-products’ of meiosis. However, by testing both the first and second polar bodies, the chromosomal content of the resultant embryo can be inferred. Even though the majority of aneuploidies are incompatible with fetal development, up to half of eggs in younger women, and up to 75% in women approaching 40, are chromosomally abnormal. Therefore, this technique allows the identification of the most viable embryos for transfer, thereby significantly increasing the chances of a successful pregnancy.
To date, analysis of genetic material in polar bodies has been used for purposes of prenatal genetic diagnosis (PGD) of monogenic diseases (through detection of the disease causing mutation in the polar body, and hence its exclusion from the embryo itself) as well as some aneuploidies of maternal origin. Techniques for pre-implantation genetic screening (PGD) have been developing rapidly in recent times and they have all been based on the use of array CGH (see previous news). However, the disadvantage of these techniques is that they involved the removal of a single cell from the embryo and its freezing during the analysis procedure, which may be detrimental. The improved technique developed by the CARE Group has the advantage that it can be carried out within 24-48 hours negating the need for embryos to be frozen prior to transfer. In addition, analysis of chromosomes in the polar body is performed at an early stage of development and precludes the need for removal of a cell from the embryo.
Although the Human Embryology and Fertility Authority (HFEA) have agreed that the Care Group may offer the treatment to their patients, it is unlikely to become widely available in the NHS for some years, as further research is required to ascertain its benefits, efficacy and safety.