Preimplantation genetic screening / Pre-implantation genetic diagnosis
PRE – IMPLANTATION GENETIC SCREENING
The technique of embryo biopsy is not new and has been a tool for atleast two decades to diagnose inheritable disorders (PGD) or chromosomal screening (PGS) by FISH techniques. But, the need to use this tool as a screening procedure for embryo integrity in terms of chromosomal number is recent intense.
It’s almost given to understanding that if 100 grade-1 embryos were to be examined, almost 60 would be chromosomally incompetent which explains why success rates in ART stagnate or vary between a 35-40% as a global average. This is also dependent on case profiles and correlated factors in infertile couples, sometimes limiting high if the cases are less complicated.
There are several factors controlling the journey of the gametes, right through extraction from its natural environment, through a laboratory process and thereafter into the womb. One of these is epigenetics, whereby there are changes in gene expressions which may not be related or do not share a cause-effect relationship with inherited genetic predispositions.
Embryo biopsy is done at different stages of embryo division, each giving its own piece of vital information on gametes. For example, polar body biopsy entails study of either first or second polar body or if effectively performed between 8-14 hours post fertilization, a study of both can be done (1). Now, studying only the first polar body may give information on the maternal oocyte but not the paternal contribution. Hence it is more suited for diagnosing monogenic disorders. In many others like recessive conditions a polar body biopsy may effectively rule out affection but may not rule out a carrier state thereby allowing for a bias in embryo selection. Also it has limited use in detecting post-meiotic aneuploidies by a conventional FISH or CGH array (2,3).
The cleavage stage biopsy is more informative but also more detrimental to embryo viability post biopsy. It is performed on day 3 embryos typically when there are 8-10 cells and not yet compacted. The concept behind timing a biopsy at a cleavage stage is that performing the procedure too early may lead to missing critical errors. Also the natural mechanism to self correct embryos occurs in the interim period and hence leads to an incorrect assigning of aneuploidy to actually viable embryos. Studies have shown that aneuploidies may not be limited to non dysjunction in meiosis 1 as speculated but in fact could occur in meiosis 2 and paternal meiosis.
To analyze the effect of epigenetics as an independent contributor to aberrations would be almost impossible and hence the use of PGT is almost justified in terms of screening. The current need is to enhance clinical pregnancy rates and thereby carry home baby rates by scrutinizing multiple factors that affect life in vitro namely the quality of gametes, culture medium and culture conditions.
Age related aneuploidy is an independent risk and contributes to higher post transfer failure rates. Trending towards more number of women trying to conceive at an advanced age, these figures are likely to escalate and pose a dilemma for fertility experts. It then becomes essential to achieve a higher clinical pregnancy rate and include technical screening procedures in the form of embryo biopsies to rule out aneuploidies.
Hence it was inferred that biopsies performed at different stages of embryonic development could detect errors that occur at different timings. So this by itself is evidence that it’s better to time a biopsy at a later stage like the blastocyst than limiting to polar body or cleavage stage (4,5,6).
Ofcourse no method is completely foolproof in detecting aneuploidies since mosaicism is another entity that occurs in 29% of all embryos (2). So although chromosomally normal embryos are transferred, the ensuing pregnancy needs to be screened as per norms by noninvasive testing for aneuploidy such as NACE (non-invasive test for chromosomal examination) and by level 2 anatomical survey of fetus by ultrasound. NACE test promises a 99.9% specificity thereby almost ruling out the need for amniocentesis or chorionic villous biopsy.
It is interesting to note that by and large cleavage stage biopsy has been regarded as more harmful to embryos demonstrating a lowered implantation rate as opposed to blastocyst stage biopsy.
It is understood from all the above quoted studies that doing a blastocyst biopsy and vitrifying the resultant biopsied blasts and transfer in a thawed cycle yields best results (4,8).
1) Advanced maternal Age (AMA)
2) Recurrent implantation Failure (RIF)
3) Recurrent miscarriage
4) Previous abnormal child
A skilled team and a treasury laboratory is necessary for a successful PGT program