ACCURATE GENOMIC PREDICTION OF MOSAICISM THROUGH CELL DIVISION ORIGIN OF ANEUPLOIDY ANALYSIS IN THE PREIMPLANTATION EMBRYO

Abstract

Simple intermediate copy number thresholds have proven to provide poor predictive value for diagnosing preimplantation mosaicism (both analytically and clinically). This study aims to validate a method to determine the cell division origin of aneuploidy from a single trophectoderm biopsy, and its ability to accurately predict mosaicism in the remaining embryo. A high throughput and PGT-validated DNA array platform was used to analyze over 800,000 SNPs within trophectoderm biopsies from a subset of PGT cases where parental DNA was available. Mendelian inheritance analyses were performed on embryonic genotypes within aneuploid chromosomes to predict their parental and cell division origins. Positive controls included embryos with a male karyotype (n=279) with known maternal origin of the X chromosome, and unbalanced embryos from PGT-SR cases (n=69) with known meiotic and parental origins of aneuploidy. The method was applied to clinical biopsies with aneuploidy and compared to rebiopsies (n=80) to evaluate the accuracy of predicting mosaicism (mitotic) and uniform (meiotic) aneuploidy. All samples were analyzed while blinded to prior results. Following validation, the incidence of mitotic origin of aneuploidy was investigated in 942 embryos. Blinded predictions of the X chromosome origin in male embryos, and parental and cell division origin of aneuploidy in PGT-SR derived unbalanced embryos were 100% concordant with expected results, demonstrating the validity of predicting parental and meiotic cell division origins of aneuploidy. Blinded analysis of rebiopsies showed that all embryos with mitotic origin of aneuploidy predicted in the original biopsy were confirmed as mosaic, and all embryos with meiotic origin were confirmed as uniform aneuploid, demonstrating the validity of predicting mitotic origins of aneuploidy and mosaicism. The overall incidence of mitotic whole chromosome aneuploidy (mosaicism) in embryos was 3%. The overall embryonic aneuploidy rate was 31% (mean maternal age 34.5+4). Aneuploid chromosomes primarily originated from the maternal genome (80%) and maternal whole chromosome aneuploidy primarily originated from meiotic errors (95%), while paternal aneuploidy more often originated from mitotic errors (35%)(p<0.05). These data indicate very high specificity in predicting mosaicism from a single aneuploid trophectoderm biopsy and a low prevalence of mosaicism (3%) in the human blastocyst. This is in stark contrast to existing methods of predicting mosaicism with intermediate copy number thresholds, where rebiopsy confirmation rates are ∼40%, and prevalence is estimated to be ∼20%. Improving the specificity of predicting mosaicism (reducing false positives) from a single biopsy may provide a unique opportunity to investigate the true clinical consequence of transferring bona fide mosaic embryos for the first time.