Segmental aneuploidies predominantly affect paternal chromosomes, whereas whole chromosomal aneuploidies affect mainly maternal chromosomes in human IVF-derived embryos.

Chromosomal mosaicism detected during preimplantation genetic testing for aneuploidy (PGT-A) and its impact on embryo implantation have been widely discussed, and healthy live births from mosaic embryos were reported by many groups. On the other hand, only very few studies have focused on segmental chromosome aneuploidies and their clinical impact. Eighty-nine embryos with various PGT-A results (trophectoderm 1: TE1) were re-analysed using a second trophectoderm biopsy (TE2) and the rest of the embryo (RE) for testing. Of 19 euploid TE1 biopsies, 18 were concordant across TE2 and RE. Similarly, whole chromosomal aneuploidies were concordant in 59 of 62 TE1-TE2 and 58 TE1-RE. In contrast, from 31 segmental aneuploidies detected in TE1, only 15 were observed again in TE2 and 14 in RE. If a TE1 segmental abnormality appeared again in TE2, it was almost always present in RE (17/18) as well. Moreover, when a TE1 segmental abnormality was not detected in TE2, in 12 out of 13 cases RE was also unaffected. Similarly, only 1 of 26 TE1 whole chromosome mosaics were repeated in TE2 and 7 in RE. Our study confirms that euploid and whole chromosomal aneuploidy results are highly predictive of the embryo. In contrast, mosaicism has a very low concordance rate. Most importantly, re-biopsy of embryos with segmental aneuploidies demonstrated that they are mostly not uniform across the embryo. Finally, in the case of segmental aneuploidy, the second biopsy enables an accurate prediction of the real status of the embryo and could be offered to patients undergoing PGT-A.

Bovine embryos are now routinely used in agricultural systems as a means of disseminating superior genetics worldwide, ultimately with the aim of feeding an ever-growing population. Further investigations, common for human IVF embryos, thus have priority to improve cattle IVF, as has screening for aneuploidy (abnormal chromosome number). Although the incidence and consequences of aneuploidy are well documented in human preimplantation embryos, they are less well known for the embryos of other animals. To address this, we assessed aneuploidy levels in thirty-one 2-cell bovine embryos derived from early- and late-cleaving zygotes. Contemporary approaches ( Whole Genome Amplification and next-generation sequencing) allowed aneuploidy assessment for all chromosomes in oocytes from donors aged 4-7 years. We also quantified mitochondrial DNA (mtDNA) levels in all blastomeres assessed, thereby testing the hypothesis that they are related to levels of aneuploidy. The overall incidence of aneuploidy in this cohort of bovine embryos was 41.1% and correlated significantly with the timing of cleavage (77.8% in late-cleaving vs. 31.8% in early-cleaving embryos). Moreover, based on mtDNA sequence read counts, we observed that the median mtDNA quantity is significantly lower in late-cleaving embryos. These findings further reinforce the use of the bovine system as a model for human IVF studies.

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