Methylome inheritance and enhancer dememorization reset an epigenetic gate safeguarding embryonic programs

Drastic epigenetic reprogramming is essential to convert terminally-differentiated gametes to totipotent embryos. However, it remains puzzling why post-fertilization global DNA reprogramming occurs only in mammals but not in non-mammalian vertebrates. In zebrafish, global methylome inheritance is however accompanied by sweeping enhancer "dememorization" as they become fully methylated. By depleting maternal dnmt1 using oocyte microinjection in situ, we eliminated DNA methylation in zebrafish early embryos, which died around gastrulation with severe differentiation defects. Strikingly, methylation deficiency leads to extensive derepression of adult tissue-specific genes and CG-rich enhancers, which acquire ectopic TF binding and, unexpectedly, H3K4me3. By contrast, embryonic enhancers are generally CG-poor and evade DNA methylation repression. Hence, global DNA hypermethylation inheritance coupled with enhancer dememorization installs an epigenetic gate that safeguards embryonic programs and ensures temporally ordered gene expression. We propose that "enhancer dememorization" underlies and unifies distinct epigenetic reprogramming modes in early development between mammals and non-mammals.