Surrogacy 101: PGD Testing and facts
PGD TESTING FACTS AND QUESTIONS
(Pre-implantation Genetic Diagnosis)
Does Pre-implantation Genetic Diagnosis have a role to play in Embryo Selection?
The concept of performing of PGD/ Fluorescence in-situ Hybridization (FISH) to exclude numerical chromosome abnormalities (aneuploidy) in order to select the “best embryos” for transfer is undoubtedly flawed and such practice should be discouraged. In contrast the performance of PGD using comparative genomic hybridization (CGH) for the identification of chromosomally (numerically) normal (euploid) embryos is a completely different consideration and is emerging as a valuable tool that might significantly improve implantation potential, reduce multiple births and , markedly reduce the risk of miscarriage and chromosomal birth defect. The following considerations will assist in better assessing the role of PGD in the selection of embryos for transfer:
1. Trauma to the embryo (through PGD) is always a potential problem. However this is far less likely when PGD is done for CGH than when FISH is performed: Here is why…..PGD for FISH requires maintaining blastomere chromosomes intact for specific analysis. This explains why 2 cells are often biopsied rather than one and why in the hands of the inexperienced, there is a greater potential to traumatize surrounding blastomeres and compromise the embryo. While nothing is more important than expertise (which requires a degree of experience and dexterity that is often lacking), the level of expertise needed to perform an atraumatic single blastomere biopsy for CGH is far less. The reason is that CGH targets DNA and unlike with FISH does not require that the chromosomes be maintained intact. Thus the removal of a single cell for PGD/CGH requires less dexterity and there is a lesser potential for causing trauma to the embryo.
2. FISH does not assess all 23 chromosome pairs for numerical chromosomal abnormalities (aneuploidy): Commercially available FISH, while capable of targeting no more than 12 chromosome pairs, usually only evaluates 8-9 pairs. Moreover, even 12-probe FISH fails to assess several chromosome pairs commonly associated with lethal aneuploidy. CGH on the other hand, accesses all chromosome pairs (i.e. full karyotyping).
Humans have an inordinately high incidence of egg aneuploidy which occurs primarily during meiosis which takes place in the hours leading up to ovulation or egg retrieval. Our own studies where CGH was performed on the first polar body (PB-1) of the mature egg (MII) have shown that in women <35Y, about 2/3 of the eggs are aneuploid (often complex aneuploid, i.e involving >1 chromosome pair). Preliminary data where PB-1 biopsy (with CGH) was performed on the eggs of older women suggests that the incidence of aneuploidy increased progressively as women age beyond 35Y such that at 40Y about 4/5 eggs are aneuploid and at 45Y the incidence of post-meiotic egg aneuploidy might even be as high as 9/10. In addition we have observed that the complexity of the oocyte aneuploidy increases (involving a greater number of chromosome pairs (i.e. chaotic aneuploidy with advancing maternal age.
It has been shown that when 9-probe PGD/FISH performed on embryos derived from the eggs of young women (<35Y) indicates no evidence of aneuploidy, there remains a 47% chance that aneuploidy resides in the untested chromosomes. The comparable error rate in women over 40Y is greater than 55%. This serves to explain why PGD with FISH in order to diagnose embryo aneuploidy, becomes progressively less reliable with advancing maternal age..
3. Performance of PGD when there are few Embryos available is both redundant and unnecessary. The following facts suggest the need for discretion when it comes to PGD/CGH performed to fully karyotype embryos so as to select the best one’s for transfer to the uterus:
· We have already shown that while achieving the blastocyst stage does not exclude embryo aneuploidy, failure to reach blastocyst means that such embryos were almost certainly aneuploid and unworthy of transfer or preservation. Taking embryos to the blastocyst stage automatically culls out many severely aneuploid embryos in the process. Because of the inevitability of an age-related increase in the incidence of egg aneuploidy, women with advancing age beyond 39Y are far less likely to have multiple pregnancies and the likelihood of high order multiple pregnancy (triplets or greater) is negligible beyond the age of 40Y regardless of the number of (unselected) embryos transferred. It follows that where fewer than 6 biopsiable day 3 embryos (i.e 6-9 cells) are obtained from women over 35Y it is probably wiser (in most cases) to allow them to develop to blastocyst and in the process cull out many obviously aneuploid embryos… whereupon those reaching the blastocyst stage can be transferred.