ABSTRACT: Epigenetic memory undergoes drastic reprogramming in intergenerational gamete-to-embryo transition. H3K27me3, a histone mark catalyzed by PRC2 (Polycomb Repressive Complex 2), is an evolutionally conserved histone mark that plays essential roles in stably repressing developmental genes to maintain cell-type-specific programs. In mice, H3K27me3-mediated epigenetic memory, however, undergoes both resetting at classic Polycomb targets and inheritance of broad H3K27me3 domains that mediate noncanonical imprinting (noncanonical H3K27me3 domains) during early embryogenesis. How such erasure and inheritance are achieved simultaneously remains elusive. Here, through temporal profiling of chromatin occupancy, we show that PRC2 associates with H3K27me3 at both Polycomb targets and noncanonical domains in growing oocytes. PRC2 then pre-disassembles and dissociates from chromatin in mature oocytes, before H3K27me3 erasure at Polycomb targets after fertilization. PRC2 reappears at Polycomb targets in blastocysts when H3K27me3 initiates restoration. By contrast, noncanonical H3K27me3 domains persist to blastocysts, where PRC2 rapidly rebinds by the end of 1-cell embryos until blastocysts. Mechanistically, we found that such noncanonical PRC2 action is regulated by EZHIP, a PRC2 inhibitory factor. By doing so, EZHIP plays a key role in both H3K27me3 erasure and intergenerational inheritance, as well as H3K27me3 re-establishment. Ezhip is highly translated around fertilization, which strongly restricts PRC2 activity, leading to gradual decrease of H3K27me3. Upon knockout of maternal Ezhip, PRC2 becomes hyperactive, binds promiscuously in the genome and deposits widespread H3K27me3. Consequently, H3K27me3 allelic bias and noncanonical imprinting, including imprinted X chromosome inactivation, are impaired. Unexpectedly, the widespread ectopic H3K27me3 paradoxically causes de-repression of H3K27me3 targets, possibly due to dilution of H3K27me3 effectors. Loss of Ezhip further attenuates H3K27me3 restoration at Polycomb targets during peri-implantation, and leads to gene dysregulation and sub-lethality after implantation. Taken together, we uncover PRC2 noncanonical chromatin association during the parental-to-embryonic transition and identify Ezhip as a crucial regulator for both erasure and inheritance of intergenerational epigenetic memories.
