O-302 Triploid conceptions are predominantly caused by female meiosis II errors and their risk increases with advancing maternal age
Authors: Picchetta L , Figliuzzi M , Poli M , Zhan Y , Caroselli S , Tao X , Jalas C , Capalbo A 
Published: June 2023

Abstract

Study question
What are the incidence and origin of ploidy abnormalities in embryos derived from 2 pronuclei (2PN) zygotes, and is their risk linked to maternal age?

Summary answer
While embryo haploidy is usually due to male meiosis errors, triploidy is mainly caused by female meiosis II errors, whose incidence increases with maternal age.

What is known already
Normal fertilization is denoted by the appearance of 2PN 16-18 hours after insemination. Deviation from 2PN is considered evidence of abnormal fertilization and ploidy anomalies. Nonetheless, even 2PN embryos can be diagnosed with ploidy abnormalities during preimplantation genetic testing (PGT). The incidence of 3PN has already been linked to advanced maternal age. However, no conclusive and well-powered studies have yet investigated the incidence, origin, and maternal age correlation of genetically-diagnosed ploidy abnormalities. A targeted NGS-based approach that simultaneously analyzes copy number and genotyping data provides a comprehensive ploidy status assessment and helps address numerous basic and clinical research questions.

Study design, size, duration
Retrospective study including 96,660 trophectoderm biopsies analyzed between 2020 and 2022 using a targetedNGS-based PGT platform. A total of 1,063 embryos carrying haploidy or triploidy were used to investigate maternal age correlation, and parental/meiotic origin of the anomaly. Of these, 57 embryos were from concomitant PGT-M (PGT for monogenic disorders) cycles where also parental DNA was available. These trios (embryo-parents) were used to independently estimate parental/meiotic contribution to the ploidy abnormality and genome-wide recombination events.

Participants/materials, setting, methods
Targeted-NGS’s accuracy in genotyping and copy number (CN) detection were previously validated on triploid cell lines and multifocal blastocyst biopsies with known ploidy status derived from abnormally fertilized oocytes. Parental and meiotic origins were estimated using two independent approaches. First, they were inferred using sex chromosomes CN from 1,063 embryos with altered ploidy status. Secondly, they were directly calculated using genotyping data from 57 trios (embryo and parents) from PGT-M cycles.

Main results and the role of chance
The prevalence of ploidy abnormalities in 2PN-derived embryos was 1.1% (n = 1,063/96,660), with triploids accounting for 83.0% (n = 882/1,063) and haploids for 17.0% (n = 181/1,063). Remarkably, the incidence of ploidy anomalies is positively correlated with maternal age (OR = 1.046 per year; p &lt; 0.001). Triploidy showed strong correlation with age (OR = 1.059 per year; p &lt; 0.001), while haploidy did not (OR = 0.96 per year; p = 0.1). Based on sex chromosomes CN analysis, the extra haploid set of triploid embryos was almost completely of maternal origin (94.6%; 95%CI:93.0-96.1), with male errors accounting for only 5.4% (95%CI:4.0-7.1). On the other hand, haploid embryos were the result of paternal errors in 97.8% of cases (95%CI:94.4-99.4), with the missing haploid set being of maternal origin in the remaining 2.2% (95%CI:0.6-5.6). In terms of triploidy’s meiotic origin, two-thirds of the errors occurred during MII (95%CI:63.4-69.8), while one-third occurred during MI (95%CI:30.2-36.5). An independent method using genotyping data of 57 trios confirmed the predominance of paternal error in haploidy (n = 12/14) and the exclusively maternal origin of all embryonic triploidies (n = 43/43). The extra haploid set resulted from an error during MI in 27.9% (n = 12/43) and during MII in 72.1% (n = 31/43) of cases. Interestingly, 16.3% of triploids (n = 7/43) showed no genome-wide recombination events.

Limitations, reasons for caution
Despite the uniquely large sample size, the inference based on sex chromosomes CN suffers the limitations of the modeling assumptions (independence between parental and meiotic errors), which require further validation. Due to the low prevalence of paternal errors, clinical correlation with male factor could not be investigated with sufficient power.

Wider implications of the findings
Thanks to the exceedingly high sample size, this is the first study to reveal an increased risk of triploid conception with advancing female age (76% higher at age 40 than at age 30), providing meaningful insights for patients counseling. Importantly, the meiotic origin of ploidy anomalies in embryos were unveiled.

Trial registration number
not applicable