Allelic reprogramming of the histone modification H3K4me3 in early mammalian development

Interestingly, we found that upon fertilization, extensive reprogramming occurred on the paternal genome, as few H3K4me3 peaks were observed in zygotes before they reappeared after major zygotic genome activation at the late 2-cell stage. On the maternal genome, we unexpectedly found a non-canonical form of H3K4me3 (ncH3K4me3) in full-grown and mature oocytes, which exists as broad peaks at promoters and a large number of distal loci. Such broad H3K4me3 peaks are in striking contrast to typical sharp H3K4me3 peaks restricted in CpG rich regions of promoters. Interestingly, ncH3K4me3 in oocytes overlaps almost exclusively with partially methylated DNA domains (PMDs). It is then inherited in preimplantation embryos before being erased in the late 2-cell embryos, when canonical H3K4me3 starts to be established. The removal of ncH3K4me3 requires zygotic transcription but is independent of DNA replication mediated passive dilution. Finally, downregulation of H3K4me3 in full-grown oocytes by overexpressing an H3K4me3 demethylase KDM5B is associated with defects in genome silencing. Taken together, these data unveiled inheritance and highly dynamic reprogramming of the epigenome in early mammalian development.