De novo DNA methylation suppresses aberrant fate determination during pluripotency transition

Genome remethylation is essential for mammalian development. Here we examined cell fate in the absence of de novo DNA methyltransferases. We found that embryonic stem (ES) cells deficient for Dnmt3a and Dnmt3b are rapidly eliminated from chimaeras. Pluripotency progression is derailed towards extra-embryonic trophoblast. This aberrant trajectory is propelled by failure to methylate and suppress expression of Ascl2 during formative transition. Ascl2 deletion rescues transition and improves contribution to chimaeric epiblast but mutant cells are progressively lost in later development. These findings indicate that methylation constrains transcriptome trajectories during developmental transitions by silencing potentially disruptive genes.