A wave of minorde novoDNA methylation initiates in mouse 8-cell embryos and co-regulates imprinted X- chromosome inactivation

DNA methylation is extensively reprogrammed during early stage of mammalian development and is essential for normal embryogenesis. It is well established that mouse embryos acquire genome-wide DNA methylation during implantation, referred to asde novoDNA methylation, from globally hypomethylated blastocysts. However, the fact that the mainde novoDNA methyltransferase 3B (DNMT3B) is initially expressed as early as the 8-cell stage during preimplantation development, contradicts the current knowledge about timing of initiation ofde novoDNA methylation. Here, we reported that a previously overlooked minor wave ofde novoDNA methylation initially occurs during the transition from the 8-cell to blastocyst stage, before the well-known large-scalede novoDNA methylation during implantation. Bioinformatic and functional analyses indicated that minorde novoDNA methylation preferentially occurs on the X chromosome and co-regulates imprinted X-chromosome inactivation via the interaction between DNMT3B and polycomb repressive complexes 2 core components during blastocyst formation. Furthermore, minorde novoDNA methylation also finetunes proliferation, lineage differentiation and metabolic homeostasis of preimplantation embryos, and is critical for embryonic developmental potential and pregnancy outcomes. Thus, our study updates the current knowledge of embryonicde novoDNA methylation, thereby providing a novel insight of early embryonic epigenetic reprogramming.