Mosaic human embryos – our experiences

Introduction Embryonic mosaicism is described as a presence of 2 or more distinct cell lines in embryo. This phenomenon is common and more prevalent in human preimplantation embryos than in later prenatal samples, affecting 15-90% of cleavage stage embryos and 30-40% of blastocyst stage embryos. Mosaic embryos have lower implantation potential than euploid. Material and Methods 98 infertile couples from 4 IVF centres from Czech Republic had undergone PGT for aneuploidy and structural imbalances of chromosomes. Preimplantation genetic testing was performed on NGS platform MiSeq using VeriSeq PGS Kit (Illumina) and analyzed on BlueFuse Multi software (Illumina). Scoring of aneuploidy and mosacism was done according to manufacturer's instructions. Results From August 2016 to December of 2017 the 387 trophectoderm and blastomere samples from 98 couples were tested for aneuploidies and rearrangements of chromosomes. Results were obtained from 95.3% of samples (369/387). No imbalanceswere found in 26.9% of samples (104/387), conditioned embryotransferwas recommended in 7.2% of samples (28/387), because analysed couple´s embryos were mosaic or aneuploid. The mosaicism was detected in 32.8% of samples in the study (127/387). Mosaicism of 1 chromosome was found only in 5.7% of samples (22/387). In 17.1% of samples (66/387) mosaicism more than 1 chromosome was found. Subchromosomal changes of chromosomes in mosaic were detected in 10.1% of samples (39/387). PGT of structural rearrangements was performed in 17.6% of samples (68/387) from 14 couples. 17.6% of samples (12/68) had balanced karyotype. In the study only 2 couples decided to transfer mosaic embryos after genetic counselling and in both cases positive biochemical pregnancies were recorded. Conclusion PGT by NGS was performed for diagnosis of aneuploidies and structural abnormalities in early human embryos. In 28 cases only mosaic embryos could be transferred. Only 2 couples decided to transfer mosaic embryo. For many couples, the decision to transfer mosaic embryo is complicated by fear about the baby's health, because the presence and significance of mosaicism in early human embryos is still poorly understood and interpretation of mosaic result is difficult with respect to which chromosome or chromosomes are involved in mosaic and with respect to accuracy of testing.