9. ASSESSMENT OF ANEUPLOIDY AND MOSAICISM CONCORDANCE BETWEEN DIFFERENT TROPHECTODERM BIOPSY SITES AND THE INNER CELL MASS EVALUATED WITH NEXT-GENERATION SEQUENCING

Abstract

Introduction It is recognized that the biopsy of 6-8 TE cells allows for high-resolution next-generation sequencing. However, concerns have been raised as to whether one biopsy sample can be representative of the whole blastocyst regarding aneuploidy and mosaicism. A recent study estimated the TE confined mosaicism rate at 14% and to be due to discordant chromosomal constitution of ICM and TE (Chuang et al., 2018). Another study showed that one clinical TE biopsy was an excellent representative of blastocyst karyotype in cases of whole chromosome aneuploidy, but not for cases with only segmental aneuploidy (Victor et al., 2019). Material and Methods 101 clinical biopsies were taken from a site opposite to the ICM (TE1) and the second sample was taken from thawed blastocysts previously diagnosed as being aneuploid with the informed consent of the patients (TE2). Five aneuploid blastocysts were also biopsied from within the ICM. Both samples were analyzed by the same platform, ReproSeq on Ion Torrent S5 (Thermo Fisher Scientific) next generation sequencing (NGS) to assess ploidy and mosaicism concordance. Blastocysts of good or top-quality belonging to 78 patients were subjected to TE biopsy for PGT for aneuploidy (PGT-A) with the clinical indications of advanced maternal age, repeated implantation failures or a previous history of abnormal fetal karyotype. Blastocysts were then vitrified as to our routine practice. Samples were analyzed by NGS and blastocysts diagnosed as being aneuploid were thawed, re-biopsied and re-analyzed by NGS. Results A total of 101 TE biopsies with whole chromosome aneuploidies were compared with 96 TE biopsy samples taken at another site of the blastocyst and 5 biopsies of the corresponding ICM. 76 samples out of 101 (75.2%) were found to be fully concordant. 25 diagnoses (24.7%) differ between TE1 and TE2: 13 mosaicisms and 10 segmental aneuploidies were either found in TE1 and not in TE2 or vice versa, without altering the overall aneuploid diagnosis. For four blastocysts, whole chromosome aneuploidies differ between TE1 and TE2, the first diagnosis indicating a chaotic aneuploidy and the second one a simple monosomy/ trisomy or vice versa. Regarding the five ICM samples, they were entirely concordant for full chromosome aneuploidies with their corresponding TE samples. However, in one sample, the 30% mosaicism of chromosomes 1, 13 and 16 was not shared by the ICM. Conclusions The high rate of concordance in two separate TE biopsies and ICM observed in this study validates the reliability of PGT-A by NGS. Blastocysts with segmental aneuploidies only may be re-evaluated if no euploids are found, as results suggest that segmental aneuploidies may not be shared among all TE cells. The study involved blastocysts initially diagnosed as being aneuploid, therefore the findings cannot be extrapolated to euploid or euploid-mosaic embryos.