O-209 The incidence of different ploidy alterations in abnormally fertilized oocytes (AFO)-derived embryos
Authors: Girardi L , Cogo F , Zambon P , Pergher I , Castellòn J A , Patassini C , Rubio C 
Published: June 2023

Abstract

Study question
Which is the incidence of de novo ploidy alterations in different abnormally fertilized embryos categories analysed with SNP-array?

Summary answer
Based on the number of pronuclei (PN) identified, 0PN embryos were mostly diploid, while 1PN and &gt;2PN-derived embryos showed increased rates of ploidy abnormalities.

What is known already
Recent development of SNP-array and haplotyping by sequencing approaches are contributing to expanding preimplantation genetic testing (PGT) clinical utility including ploidy level evaluation. When applied to euploid embryos derived from AFO, the identification of abnormal ploidy constitution (i.e., haploidy, triploidy, or tetraploidy) could rescue viable diploid embryos otherwise considered non-transferable in IVF. However, no definitive genetic evidence has been obtained showing the incidence of different ploidy abnormalities in each AFO-derived blastocyst category. Here we present 4 years of experience in the clinical application of a validated SNP-array based protocol and custom-made algorithm to detect ploidy defects in AFO-derived embryos.

Study design, size, duration
Prospective observational study evaluating the incidence of altered ploidy configurations in AFO-derived blastocyst. After PN check, AFO samples were divided as follow: absence of observed pronuclei (0PN), monopronuclear (1PN), more than two pronuclei observed (2.1PN, with one smaller additional PN and 3PN). Genetic classification combining PGT-A and ploidy analysis was performed at Igenomix Italy laboratory between May 2019 and January 2023 on 133 AFO-derived embryos (44 0PN, 59 1PN, 30 &gt;2PN).

Participants/materials, setting, methods
Multi-centre study involving 293 consenting patients of advance maternal age (mean=38.6±3.9) undergoing PGT-A on MDA-WGA using Ion ReproSeq kit and IonTorrent S5 (ThermoFisher). SNP-array-based ploidy test was performed using HumanKaryomap-12 kit and NextSeq550 (Illumina). Proprietary algorithm was based on genome-wide BAF obtained: expected BAFs were 1, 0.5 or 0 for diploids,1 or 0 for haploid and 1, 0.66, 0.33 or 0 for triploids, as determined by the frequency of each allele for 300000 SNPs loci.

Main results and the role of chance
Preclinical validation of SNP-array-based ploidy test included 26 samples with known ploidy status and karyotype: 7 triploid re-biopsies, 7 haploid left-overs and 12 cell lines. Moreover, inter-platform comparison of 72 diploids, 9 triploids, 8 haploids embryos analysed with an alternative targeted-NGS approach, validated by Igenomix, showed 100% (95% CI= 95.94%-100%) concordance. In this study, a total of 318 AFO-derived embryos were collected for PGT-A analysis for different indications. Of these, 58.2% (n = 185/318; 95%CI=52.54-63.66) resulted as aneuploid. The remaining 133 euploid AFO-derived blastocyst where subjected to SNP-array based ploidy assessment. 0PN-derived blastocysts (n = 44/133) were diploid in 93.2% (n = 41/44; 95%CI=81.34-98.57) of cases; only 4.5% (n = 2/44; 95%CI=0.56-15.47) were haploid and 2.3% (n = 1/44; 95%CI=0.06-12.02) were polyploid. 1PN-derived blastocysts showed all possible ploidy configurations in these proportions: 59.3% (n = 35/59; 95%CI=45.74-71.93) haploid, 35.6% (n = 21/59; 95%CI=23.55-49.13) diploid and 5.1% (n = 3/59; 95%CI=1.06-14.15) polyploid. Finally, in the group of AFO where more than 2PN were observed (2.1PN or 3PN), the additional PN resulted in 66.7% (n = 20/30; 95%CI=47.19-82.71) of polyploid configurations, the remaining 33.3% (n = 10/30; 95%CI=17.29-52.81) were diploid. No haploid results were obtained from this category. Chi-square test showed significant overall correlation between PN category and ploidy status configurations (p &lt; 0.05). Collectively, ploidy evaluation of 133 AFO-derived embryos, allowed the clinical use of 72 additional euploid/diploid embryos otherwise not considered for transfer.

Limitations, reasons for caution
Ploidy assessment protocol was not tested to distinguish triploids from tetraploids. Parental origin of each chromosomal set could not be determined without analysing parental DNA. The way of performing PN check could have affected AFO classification. In future studies we will focus on evaluating clinical outcomes following euploid/diploid embryo transfer.

Wider implications of the findings
An altered number of pronuclei is predictive of the correspondent altered ploidy status, while 0PN category doesn’t represent a clear indication for ploidy evaluation. Nevertheless, a significant proportion of euploid/diploid embryos can be rescued from all types of AFO, potentially increasing the overall chance to achieve a live birth.

Trial registration number
n/a