Chromosomal Mosaicism in Cleavage Stage Embryos vs. Blastocyst Stage Embryos

El-Bisari S, Alyounis D, Sayed


Introduction: Chromosomal mosaicism is characterized by the presence of more than one chromosomally different cell line in an individual. Preimplantation chromosomal mosaicism is characterized by the presence of a mixture of chromosomally different cell lines in an embryo. Studies show that mosaicism for whole chromosomes (aneuploidies) in one or more cells (blastomeres) occurred in more than 75% of cleavage stage embryos, whilst 3%-24% of blastocyst stage embryos are chromosomally mosaic. Aim: The purpose of this study was to standardize and validate a Next Generation Sequencing (NGS) method for comprehensive chromosome testing for aneuploidies and to study the level of mosaicism in cleavages stage vs. blastocyst stage embryos. Methods: The validation involved a retrospective blind assessment of whole genome amplification (WGA) products from 14 cleavages stage embryo biopsies (blastomeres), 6 blastocyst stage embryo biopsies (TE), in addition to their 20 discarded blastocyst stage whole embryos. 42.8% of the cleavages stage embryos showed mosaicism, whilst results between the trophectoderm (TE) biopsies (TEB) and their whole embryos at blastocyst stage showed total concordance as no mosaicism was observed. NGS sensitivity and specificity for calling aneuploidy was found to be 100%. Conclusion: This is the first study reporting preclinical validation and accuracy assessment of the Ion semiconductor sequencing technology in studying the level of mosaicism in cleavage stage and TE biopsies blastocyst stage embryos vs. their whole embryos. The high level of mosaicism in cleavages stage embryos compared to blastocyst stage embryos does not recommend the PGT-A to be performed on cleavage stage embryos. The NGS proved to be a robust methodology in clinical application of PGT-A.

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