Extremely low incidence of mosaicism in human blastocysts mimics occurrence in natural and IVF clincial pregnancies

Abstract

Chromosome mis-segregation during embryonic mitotic cell division is responsible for the generation of mosaicism, which is defined as two or more cell populations with different chromosome constitutions within the same embryo. Estimates of embryonic chromosomal mosaicism in the literature have been inconsistent with potentially high rates influenced by clinical procedures, laboratory protocols, and patient variability. The aim of this multicenter study was to evaluate the prevalence of embryonic chromosomal mosaicism at numerous IVF clinics consisting of variable infertility patient demographics. Multi-center prospective study. Initial diagnosis of chromosome numeration from trophectoderm (TE) biopsies was performed using the VeriSeq™ NGS platform (Illumina) at a single genetics laboratory. A total of six IVF clinics situated across North America were included in the multicenter study. Each geographical location is considered to have a unique population demographic and their own patient management for clinical treatment including, ovarian stimulation protocols. Embryos in the six IVF laboratories were cultured under similar optimized conditions utilizing the same consumables, reagents, equipment and comparable biopsy techniques. Statistical analysis included Student’s t-test, Fisher’s Exact and ANOVA where appropriate, significance at P<0.05. The overall incidence of chromosome mosaicism following TE biopsy of 16,352 human blastocysts was observed at only 2.6%. There was no significant variability observed across the six IVF clinics (Clinic A=2.6%; Clinic B=2.4%; Clinic C=2.4%; Clinic D=2.5%; Clinic E=2.1%; Clinic F=2.8%; ns). Advanced reproductive age was not associated with chromosomal mosaicism (mean maternal age = 36.3±4.2years; mean paternal age = 38.2±6.1years). For all infertility patients that were identified to have a mosaic blastocyst in their embryo cohort, the vast majority, two thirds, were younger than 38 years of age (P<0.05). Neither infertility diagnosis, ovarian reserve, blastocyst morphology, day of biopsy, nor the chromosome involved in the error were associated with chromosomal mosaicism. Blastocysts identified with chromosomal mosaicism were not transferred. Multiple re-analysis of mosaic blastocysts confirmed the original mosaicism diagnosis and the identification of the inverse aneuploidy in the re-biopsy sections with random allocation between the TE and inner cell mass. This novel study suggests that chromosomal mosaicism in human blastocysts is indeed extremely low, more closely resembling the incidence of mosaicism in spontaneous and IVF clinical pregnancies (1-2%). The combination of optimized conditions and robust protocols in both the IVF and genetics laboratories contributes to the most successful clinical outcomes for infertility patients, including the expected biological representation of chromosomal mosaicism.