I-44: Mutagenesis during Embryogenesis

We developed several novel tools to genome wide screen for CNVs and SNPs in single cells. When applied to cleavage stage embryos from young fertile couples we discovered, unexpectedly, an extremely high incidence of chromosomal instability, a hallmark of tumorigenesis (Vanneste et al., Nature Medicine, 2009; Vanneste et al., Hum.Reprod., 2011). Not only mosaicisms for whole chromosome aneuploidies and uniparental disomies but also frequent segmental deletions, duplications and amplifications that were reciprocal in sister blastomeres were detected. Based on the copy number changes that were observed in the blastomeres it was hypothesised that chromosome breakages and fusions occur frequently in cleavage stage human embryos and instigate subsequent breakage-fusion-bridge cycles and that the DNA breaks present in spermatozoa might trigger this CIN. To test these hypotheses, we genotyped both parents as well as 93 blastomeres from 24 IVF embryos and developed a novel SNP-array based algorithm to determine the parental origin of (aberrant) loci in single cells. Paternal as well as maternal alleles were commonly rearranged in the blastomeres indicating that sperm-specific DNA-breaks do not explain the majority of these structural variants. The parent- of-origin analyses together with microarray-guided FISH analyses demonstrate the presence of inv dup del chromosomes as well as more complex rearrangements (Voet et al., Hum.Mutation, 2011). These data provide unequivocal evidence for breakage-fusion-bridge cycles in those embryos and suggest that the human cleavage stage embryo is a major source of chromosomal disorders. The type of rearrangements observed can likely explain the majority of constitutional rearrangements seen in miscarriages as well as live births such as deletions, duplications, inverted deletions duplications, ring chromosomes and mosaicisms of all of those rearrange- Abstracts of the 12th Royan International Congress on Reproductive Biomedicine 2 1 International Journal of Fertility & Sterility (IJFS), Vol 5, Suppl 1, Summer 2011 ments. The high frequency of chromosomal imbalances in cleavage stage embryos make it likely that a substantial number of chromosomal rearrangements originate post-zygoticaly.