Disabling de novo DNA methylation in embryonic stem cells allows an illegitimate fate trajectory.

Abstract

Genome remethylation is essential for mammalian development but specific reasons are unclear. Here we examined embryonic stem (ES) cell fate in the absence of de novo DNA methyltransferases. We observed that ES cells deficient for both <i>Dnmt3a</i> and <i>Dnmt3b</i> are rapidly eliminated from chimeras. On further investigation we found that in vivo and in vitro the formative pluripotency transition is derailed toward production of trophoblast. This aberrant trajectory is associated with failure to suppress activation of <i>Ascl2</i><i>Ascl2</i> encodes a bHLH transcription factor expressed in the placenta. Misexpression of <i>Ascl2</i> in ES cells provokes transdifferentiation to trophoblast-like cells. Conversely, <i>Ascl2</i> deletion rescues formative transition of <i>Dnmt3a/b</i> mutants and improves contribution to chimeric epiblast. Thus, de novo DNA methylation safeguards against ectopic activation of <i>Ascl2</i> However, <i>Dnmt3a/b</i>-deficient cells remain defective in ongoing embryogenesis. We surmise that multiple developmental transitions may be secured by DNA methylation silencing potentially disruptive genes.