The Evolution of Comparative Genomic Hybridization (CGH) – EGG and Embryo Testing at Sher
In 2007, we reported in the journal “Fertility and Sterility” on a study (Click Here to View the Study) where we fully karyotyped mature eggs (M2 oocytes) using metaphase comparative genomic hybridization (mCGH) rather than FISH. Unlike FISH, CGH can identify all the chromosomes, thereby providing a reliable measure of “competency”. This study showed that the transfer of 1-2 embryos/blastocysts derived from CGH-normal eggs (mean=1.7 per woman), resulted in live births >70% of the time ( i.e. > double the national average). In the process, the risk of multiple pregnancies was markedly lower, and chromosomal miscarriages and birth defects were drastically reduced.
Since CGH requires several weeks to be completed, it is not possible to perform the test on day 3 embryos and still be able to transfer them fresh to the uterus. It is therefore necessary to cryobank and store the embryos until mCGH results become available. The problem has been that conventional (slow) freezing damages eggs/embryos because it results in ice formation in the blastomeres (embryo’s component cells), damaging the embryo and reducing viability.
The solution arrived about 5 years ago through a major advance in freezing technology referred to as vitrification, an ultra-rapid freezing technique that eliminates intracellular ice formation. As a result, vitrified embryos can now be banked indefinitely while retaining virtually the same viability as their fresh counterparts. Thus it is possible to defer embryo transfer by vitrifying and cryobanking embryos until CGH results became available. Accordingly, it is presently possible to use CGH on the egg and/or the embryo in order to identify those that are most likely to be competent.
Since, in the face of normal sperm function, 90% of the embryo’s chromosomal integrity is determined by the egg and not by the sperm, egg CGH can indeed be safely employed in cases where there is no sperm dysfunction. In cases of male infertility however, where sperm dysfunction increases the risk of embryo chromosomal aberrations (aneuploidy), we prefer to biopsy the embryo so as to take the sperm’s contribution into account.
The process of splitting the cycle in two ; i.e. performing CGH on the first polar body of the egg or on a blastomere taken from a day 3 embryo, allowing the embryos to go to blastocyst, cryobanking these, and deferring the embryo transfer to another time, is referred to as Staggered -IVF (St-IVF).
Our results using Staggered-IVF have been very encouraging. The transfer of up to two “CGH-normal” embryos has consistently yielded about a 60% chance of a live birth, a 3-4 fold reduction in the risk of miscarriages (which are usually due to aneuploidy), a minimal risk of chromosomal birth defects such as Down syndrome, and a virtual elimination of high order multiple pregnancies (triplets or greater). It also provides an excellent (hitherto unavailable) tool by which to differentiate between embryo and implantation-related causes of IVF failure and pregnancy loss.