Within the past several months, there have been a number of news reports and press releases on embryo selection methods that enable evaluation of all 23 pairs of chromosomes in the embryo. Indeed, the identification of “competent” embryos requires that all 23 pairs of chromosomes be identified and accounted for. The “gold standard” for achieving this is through performance of Comparative Genomic Hybridization (CGH) a genetic test performed on DNA extracted from the egg polar body (PB), from a single blastomere taken from the early cleaved embryo 3 days post-fertilization, or from multiple cells biopsied from the trophectoderm (TE) of the blastocyst (Day 5-6 advanced embryo that has >100 cells).

As many of you who are familiar with our published research at SIRM are aware, we have been performing CGH embryo selection for nearly 4 years and have been offering it to our patients as a standard IVF option since 2007. There are two different types of CGH currently available:

1) Metaphase CGH (mCGH)
2) Array CGH (aCGH).

I’d like to offer a comparison of these two methods.

First: While mCGH can readily be performed on a single cell, aCGH (at its present state of development), requires a much larger sample of DNA in order to be reliable. That is why several cells must be removed from the blastocyst TE for testing by the aCGH method. This requirement serves to explain why aCGH, an analysis that is much simpler to perform than mCGH, is at present best used on blastocysts where multiple cells are available for biopsy and not on a 6-9 cell, day 3 embryo where the removal of >1 blastomere might be harmful. Thus, at present, mCGH which can reliably be performed on DNA taken from a single blastomere is still the only reliable method by which to evaluate for all the presence of all the chromosomes in early, cleaved, day-3 embryos.

Second: aCGH provides no more information than does metaphase CGH (mCGH).

Third: concerns the issue of mosaicism (mitotic aneuploidy) where some cells are aneuploid and others are not. Recall that aCGH needs to be performed on advanced embryos (blastocysts) because it requires a larger amount of DNA (i.e., several cells). Since almost all blastocysts have some degree of mosaicism, the question arises as to what percentage of the blastocyst’s cells need to be chromosomally normal in order for it to be capable of developing into a healthy baby. The truth is that no one knows! There have simply been too few births to make that determination. We are thus still in the “speculative phase” when it comes to knowing what percentage of aCGH tested TE cells need to be normal versus abnormal in order to safely transfer that embryo.

Fourth: TE biopsy removes cells for CGH testing from an area known as the trophectoderm (TE) which ultimately develops into the placenta. It is however a different part of the blastocyst known as the inner cell mass (ICM) which develops into the baby and this area of the trophoblast is not tested. Presently, we have no way of knowing whether the percentage of normal versus aneuploid cells detected in the TE is reflective of the same ratio in the ICM.

Our first publication on CGH (Fertility & Sterility, May 2007) demonstrated a clear linear propagation of chromosomal configuration from the pre-fertilized egg through the zygote (1 day post fertilization) to the cleaved (divided) day 3 embryo. Based upon results reported in this study as well as on as yet unpublished data, we have been able to confirm that the presence of aneuploidy in a day 3 embryo almost invariably proves lethal to that embryo. It follows that aneuploidy was due to abnormal meiosis and not due to mitotic dysfunction in these cases.

Presently (and probably for the foreseeable future), mCGH analysis of DNA derived from a single blastomere (removed from a day 3 embryo) is still the preferred method by which to identify the embryo that is most capable of propagating a normal pregnancy following IVF/ET. It is both possible and iondeed likely that as improvements take place in aCGH technology, it will become reliable for use in single cell analysis.