Aneuploidy in Oocytes From Women of Advanced Maternal Age: Analysis of the Causal Meiotic Errors and Impact on Embryo Development

Abstract

Many chromosome abnormalities are commonly observed and can lead to early pregnancy loss, miscarriage, or the birth of children with chromosomal defects. Such abnormalities are considered a major factor in the low pregnancy rate after assisted reproductive technology and natural conception. Preimplantation genetic testing for aneuploidy (PGT-A) aims to minimize the transfer of aneuploid embryos. Embryonic aneuploidies arising from errors in meiosis have an incidence of approximately 25% in embryos from women younger than 35 years, to more than half in embryos from women aged older than 35 years. Although these embryos are able to develop to the blastocyst stage, they tend to be of lower morphological quality. A recent multicenter randomized clinical trial (ESTEEM) analyzed polar bodies (PBs) from women after intracytoplasmic sperm injection aged between 36 and 40 years using microarrays in 205 cycles and found that the transfer of embryos from euploid oocytes did not lead to a higher live birth rate but was associated with a reduction in the number of embryo transfers and miscarriages. This study aimed to evaluate all PB results from this RCT and characterize the types of chromosomal abnormalities and the chromosomes most frequently affected. The ESTEEM trial obtained biopsy of first (PB1) and second (PB2) PB in the cohort receiving PGT-A and analyzed them using array comparative genomic hybridization (aCGH). A total of 693 PB pairs had full results available, including 676 confirmed fertilized oocytes. Chromosome segregations, including likely underlying mechanisms, from these pairs are reported here. To estimate the reliability of the aCGH procedure, 72 PB pairs from a single center were reanalyzed using next-generation sequencing (NGS). Embryos were classified into 4 categories based on morphology: good, fair, poor, and degenerated. A comparative analysis was performed to assess the association between chromosome status and embryo quality as well as study group (PGT-A vs control) and embryo quality. A total of 213/676 oocytes were euploid and 413/676 were aneuploid, whereas in the remaining 50 oocytes, an abnormality observed in PB1 was compensated by an abnormality in PB2. A total of 693 PB pairs reported chromatic numbers with results for 15,939 chromosomes. An abnormal segregation, in PB1 and/or PB2, was observed in 1162 chromosomes (7.3%) in 461 PB pairs. Chromosomes 22 (16.7%), 16 (16.6%), 19 (14.4%), 21 (13.7%), and 15 (12.4%) had the highest frequencies for abnormal segregations. The abnormal segregations were compatible with precocious separation of sister chromatids in meiosis 1 (M1) (n = 568; 48.9%), nondisjunction of chromatics in meiosis 2 (M2) or reverse segregation (n = 417; 35.9%), and nondisjunction in M1 (n = 65; 5.6%). However, 112 chromosomes had segregation patterns that could not be categorized into 1 of the 9 known mechanisms causing aneuploidy in oocytes. Concordance between aCGH and NGS was obtained for both PBs for 1650 of 1656 analyzed chromosomes (99.6%). Embryos predicted to be aneuploid had significantly worse quality scores on day 3 (adjusted odds ratio [aOR], 0.62; 95% confidence interval [CI], 0.43–0.90), day 4 (aOR, 0.15; 95% CI, 0.06–0.39), and day 5 (aOR, 0.28; 95% CI, 0.14–0.58). This study represents one of the largest analyses of chromosomal copy number in both PBs to date and highlights the frequent unexplained chromosome copy numbers underscoring the gap of knowledge into the mechanisms causing aneuploidy in oocytes.