EMBRYONIC MOSAICISM IN PREIMPLANTATION GENETIC TESTING (PGT): EVALUATION OF VARIABLES THAT COULD AFFECT ITS FREQUENCY

Abstract

Next-generation sequencing (NGS) for PGT has improved the resolution for detecting mosaicism, which creates challenging embryo transfer decisions. Currently, there is a growing bulk of reports about mosaic embryo transfers and their clinical outcomes. We consider that the evidence is still limited regarding the factors that could be related to the risk of mosaicism and the clinical outcome after mosaic transfer. This study sought to understand if mosaic embryos frequency are related to different variables and to report their clinical outcomes. This single-center retrospective study included 2174 blastocysts tested between 2016 and 2020 by NGS (Veriseq, Illumina). DNA obtained from trophectoderm biopsies was tested in our laboratory and embryos were classified in euploid, aneuploid, or mosaic. The mosaic embryos were divided into groups based on mosaic levels: low grade (20-40%), high grade (41-80%), and chromosomal constitution (single, double, or complex mosaic aneuploidy). The frequency of mosaic was analyzed according to female age (<30, 30-34, 35-39, >40 years), PGT indication (advanced maternal age, recurrent miscarriages, repeated IVF failure, patient request, translocations, others), day of the biopsy (5, 6 or 7), male factor, and embryo quality: high (3-6AA and 4-6AB, 3-6BA), mid (any BB, 1-3AB, 1-2AA and 1-2BA) and low (any AC, CA, BC, CB or CC). A Chi-square test was applied for statistical analysis. The distributions of the mosaic groups according to the variables studied were determined. In addition, the reproductive outcomes of the mosaic embryos transferred to date were analyzed regarding positive serum βhCG, ongoing pregnancy, and live births. 891 (41%) out of the 2,174 embryos were euploid, 702 (32%) aneuploid, 521 (24%) mosaic, and in 60 cases (3%) the amplification failed. When the embryonic mosaic distributions were analyzed according to the study variables, significant differences (p <0.05) were found for the age of the oocyte and the indication of the study. When we divided mosaic embryos into low grade, high grade or complex, we could observe that they were distributed across all the variables studied and low grade mosaics were the most frequent ones in each variable. Regarding the reproductive outcomes, in 50% of the 22 transferred mosaic embryos, there was a positive serum βhCG, 23% had an ongoing pregnancy and this resulted in a live birth. On the other hand, considering the 391 euploid embryo transfers in our center, 56% had a positive serum βhCG, 42% an ongoing pregnancy, and 33% resulted in a live birth. Our results suggested that the frequency of mosaics could be associated with female age and indication for PGT and that the lower grade embryos are the most frequent. However, the identification of mosaic embryos, mainly the low-grade ones, could be due to methodological artifacts, therefore a prospective study including more samples will validate these results.