Replication stress in the human preimplantation embryo. Homo sapiens

Human cleavage stage embryos frequently acquire chromosomal aneuploidies during mitosis. We recently showed that the failure to repair double strand breaks (DSBs) introduced by Cas9 results in both segmental as well as whole chromosome aneuploidies. Here we show that DSBs arise de novo during embryonic DNA replication. Recurrent spontaneous breaks in human embryos occur primarily, but not exclusively in gene-poor areas with long genes implicated in structural morphogenesis and neurodevelopmental disorders, including CNTN5, IMMP2L, LRP1B, NBPF9, RYR2, as well as in centromeric and pericentromeric areas. More than half of these regions complete replication late in G2 phase, hours after most of the genome has been duplicated. Interference with replication completion in G2 phase in both human and mouse zygotes, results in DSBs, whole chromosome errors as well as segmental errors, micronucleation, chromothripsis and poor embryo quality; all characteristic of spontaneous abnormalities in human embryos. In human embryos, replication-dependent segregation errors are common and spontaneous, while in mice, they are rare and development is efficient. Thus, failure to accurately duplicate the genome during cleavage development is a mechanism of developmental failure, and a source of detrimental genetic alterations in humans.