In which cases (for which diseases) is it necessary / rightful to perform preimplantation genetic diagnosis (PGD)? This has always been the question, as science makes progress and provides the technical means to examine increasingly more genes. Initially, PGD was limited to very severe diseases, such as those mentioned above. However, today we can screen genes for a growing number of diseases, even for those deemed less severe (such as deafness) or those occurring later on in a person’s life, which are affected not only by genetic material but also by other factors (e.g. cancer).
Indicatively, please read on the website of HFEA (Human Fertilisation and Embryology Authority), UK, the list of diseases for PGD (according to the UK legislation):
Legislation in Greece allows gender selection only for medical reasons, e.g. the transmission of a gender - linked disease (see above). In other countries, there is an intense debate concerning selection via PGD for non-medical reasons (e.g. selection of a female embryo after 3 boys in the family). Also intense is the debate with regards to the selection of an embryo having tissue compatibility with a brother/ sister who has e.g. leukemia, with the aim to save the older brother/ sister with bone marrow transplant.
It has been suggested that preimplantation genetic screening (PGS) in embryos of couples with a history of recurrent miscarriage might prove beneficiary. The argument supporting this is that a percentage of recurrent miscarriage is due to embryonic chromosomal abnormalities, to which lesser sperm quality and advanced maternal age contribute.
Even today, PGS is recommended in many assisted reproduction centres, with the aim to increase healthy pregnancy achievement rates following recurrent miscarriage or failed IVF cycles. In theory, the selection of embryos to be transferred after being screened for chromosomal abnormalities increases healthy pregnancy achievement rates. According to a recent study, PGS has significantly diminished spontaneous abortions in patients with a history of recurrent miscarriage, especially in patients with 2 or more miscarriages (12.8% miscarried after PGD versus 35.9% who miscarried without PGD).
Please read below the recent (2009) scientific publication of the aforementioned study in “Fertility & Sterility” journal:
“Effect of infertility, maternal age, and number of previous miscarriages on the outcome of preimplantation genetic diagnosis for idiopathic recurrent pregnancy loss”
Volume 92, Issue 1, Pages 288-295 (July 2009)
However, not all studies agree with the conclusion above. Other studies show that PGS doesn’t offer any advantages after all. It has been proved that PGS doesn’t improve healthy children birth rate in mothers aged between 35 and 41 with an increased chromosomal abnormality risk rate.
Please read the study below:
‘No beneficial effect of preimplantation genetic screening in women of advanced maternal age with a high risk for embryonic aneuploidy’
Human Reproduction Vol.23, No.12 pp. 2813–2817, 2008
Researchers have detected some other points that doctors must pay attention to while applying the method:
• Various studies have shown that a lower percentage of embryos, which have undergone PGS, form blastocysts.
• A small amount of genetic material is isolated for biopsy from 1-2 cells; thus in some cases, it may not be sufficient in order to obtain clear results.
• PGD results are affected by the phenomenon of “mosaicism”. When the embryo starts dividing, daughter cells are identical to the stem cell. Sometimes though, things don’t turn out exactly as described above and finally, in an 8-cell embryo, one cell may be slightly different than the others.
For all of the above reasons, Mr. Evripidis Mantoudis believes that PGS must be applied only in carefully selected cases.