14 November 2008
An alternative technique for PGS is the use of comparative genomic hybridisation (CGH) (see previous news). CGH allows regions on every chromosome to be analysed for aneuploidies compared with current techniques, which are restricted to specific chromosomal defects. A number of studies have indicated that this technique may increase the success rate of IVF and the results of the first clinical study of this technique were presented at the annual meeting of the American Society of Reproductive Medicine. The study carried out by scientists at the University of Oxford and the Colorado Centre for Reproductive Medicine in the US has shown that CGH may be a better approach for pre-implantation genetic screening (reported by BBC news). The clinical trials were undertaken on 23 women with an average age of 37, in order to assess the effectiveness of CGH; chromosome screens were obtained for 91% of embryos tested and 70% percent of the cycles that went to embryo transfer resulted in clinical pregnancy (see press release). The results of this study suggest that the rate of implantation following CGH is dramatically greater than that achieved using other screening methods (BBC news). Dr. Dagan Wells from the University of Oxford is currently applying for a licence for its use in the UK. Although work on CGH has been on-going for a number of years, it has not been widely adopted as it is requires expertise, is labour intensive and takes several days. However, technological advances are making it a more amenable method of screening and if conjugated with microarray platforms (referred to as array CGH) it can allow high-throughput processing of multiple samples simultaneously.
Techniques for pre-implantation genetic diagnosis (PGD) or pre-implantation haplotyping (see previous news) are used for the diagnosis of mutations that cause specific serious genetic disorders. In contrast, techniques such as array CGH and karyomapping (see previous news) are applied to the whole genome and can be used to identify multiple genetic abnormalities as well as specific mutations. The advantage of these techniques is that they allow more thorough screening in order to identify chromosomally normal embryos thereby increases the chances of a healthy pregnancy - one of the purposes of IVF. However, the clinical significance of some mutations or chromosomal changes (i.e. deletions/duplications) identified by these techniques is not yet known nor is their influence on the successful establishment of a healthy pregnancy. Consequently, these techniques raise a number of ethical issues; a European ethics task force is already assessing the use of microarrays in PGS in order to develop a code of practice and regulate their use (see previous news).