Preimplantation Genetic Diagnosis: Polar Body Biopsy

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Preimplantation genetic diagnosis (PGD) has been performed by polar body (PB) removal or embryo biopsy at the cleavage stage. While the latter involves sampling of one or two blastomeres from the cleaving 4-8 cell embryo and might influence preimplantation and post-implantation developmentare, the first (1) and second (2) PB are by-products of female meiosis extruded during maturation and fertilization of oocytes with no known biological significance for the embryo. We introduced PB approach 10 years ago for testing oocytes for preconception diagnosis in couple at risk for having an affected child with Mendelian disorder. 

The technique presently involves a two-step oocyte analysis, which requires a sequential genetic testing of PB1 and PB2. The method was applied in 827 clinical cycles for PGD for single gene and chromosomal disorders, which is more than half of the world experience on PGD at the present time. Overall, the work involved genetic testing by PCR or FISH of more than 5000 oocytes, from which, approximately 2000 were pre-selection and transferred in 764 cycles, resulting in 179 clinical pregnancies and birth of 135 healthy children [1-3]. These include healthy children born following PGD for several CF mutations, sickle cell disease, the most common thalassemia mutations, age-related aneuploidies and translocations. 

An average pregnancy rate of 23.4% in the whole series seems quite acceptable, taking into consideration that the majority of patients were 35 years and older, representing the major indication for PGD in our series. In the group of patients with latter indication, the average age was, approximately 39 years, suggesting that the aneuploidy testing may not only allow avoiding the birth of children with age related aneuploidies, but can also improve the pregnancy rate in IVF patients of advanced maternal age. Although more data is clearly needed to make the conclusion, there is no doubt that pre-selection of aneuploid oocytes will reduce at least to some extent the transfer of potentially non- viable oocytes. Although the intrinsic genetic parameters cannot explain completely the reason why the majority of oocytes fail to produce a viable embryo, at least proportion of such oocytes could be detected by PB analysis and avoided from fertilization and transfer. 

Together with other predictive factors such as different clinical and epigenetic characteristics, pre-selection of euploid oocytes by PB1 and PB2 sampling, may allow distinguishing in the future those few oocytes with maximum potential to result in a clinical pregnancy and birth of a healthy child. The method may, therefore, be acceptable for clinical practice, as it allows a pre-selection of a sufficient number of unaffected embryos for transfer and has been shown to be accurate, reliable and safe.

References:

References

1. Verlinsly Y, Kuliev A, Preimplantation diagnosis of genetic diseases: a new technique for assisted reproduction (Wiley-Liss: New York, 1993).

2. Verlinsky Y, Rechitsky S, Verlinsky O, et al, Prepregnancy testing of single-gene disorders by polar body analysis, Genet Testing (1999) 3:185-190.

3. Verlinsky Y, Cieslak J, Ivakhnenko V, et al, Prevention of age-related aneuploidies by polar body testing of oocytes, J Assist Reprod Genet (1999) 16:165-169.