Low oxygen tension and euploidy after preimplantation genetic screening

Abstract

In 2002 Bean and colleagues suggested a higher rate of mitotic non-disjunction in 5% CO2 in air. However when cultured in a 5% oxygen (O2) concentration the rate of mitotic induced mosaicism returned to the in vivo control models. The purpose of this study is to determine if the introduction of reduced O2 culture microenvironment would yield a higher rate of euploidy in preimplantation genetic screening (PGS) patients. Prospective cohort study of PGS patients where embryos were cultured in a 20% O2 and 5% O2 microenvironment. Embryos were cultured individually in a Sage sequential culture medium in 6% CO2 and air or lowered O2 (5%) balanced with nitrogen. On day 3 embryos >4 cells were subjected to biopsy of one cell in a Sage Ca/Mg free medium. The rate of blastocyst formation was evaluated in the two culture systems. Euploidy was determined from the reference laboratory report as normal or abnormal. Additionally, the rate of blastocyst formation was evaluated for euploid and aneuploid embryos. A total of 933 embryos were subjected to embryo biopsy on day 3 post insemination. Where 456 embryos were biopsied and cultured in the low (5%) O2 group and 477 embryos in the elevated (20%) O2. In the low oxygen microenvironment there was no significant (P=0.27) increase in the rate of euploidy in embryos as determined by FISH. There was a euploidy rated of 152/456 (33%) in low O2 and 143/477 (30%) in the CO2 in air culture system (not significant). In the low O2 culture system there was a significantly (P=0.004) higher rate of blastocyst formation for euploidy embryos in the low O2 as compared to elevated O2. Euploid embryos developed to blastocyst at a rate of 115/152 (76%) in low O2 vs. 86/143 (43%) in elevated O2. Where as aneuploid embryos in the low O2 vs. the elevated O2 was 141/304 (46%) and 138/334 (41%) respectively (not significant). Results show there was no decrease in mitotic non-disjunction in the human with variable oxygen tensions on the embryo. Interestingly we did show there were significantly more euploid blastocysts available for transfer when culturing in a lower O2 environment. Additionally, there was no significant increase in aneuploid embryos that developed to the blastocyst stage in low O2 vs. higher oxygen levels. Therefore showing there is an advantage to culturing embryos in low O2 in order to yield the highest number of good quality embryos available for embryo transfer.