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 Dnmt3a and Dnmt3b 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 Ascl2. Ascl2 encodes a bHLH transcription factor expressed in the placenta. Misexpression of Ascl2 in ES cells provokes transdifferentiation to trophoblast-like cells. Conversely, Ascl2 deletion rescues formative transition of Dnmt3a/b mutants and improves contribution to chimeric epiblast. Thus, de novo DNA methylation safeguards against ectopic activation of Ascl2. However, Dnmt3a/b-deficient cells remain defective in ongoing embryogenesis. We surmise that multiple developmental transitions may be secured by DNA methylation silencing potentially disruptive genes.

基因组再甲基化对于哺乳动物的发育至关重要，但具体原因尚不清楚。在这里，我们检查了在缺乏从头 DNA 甲基转移酶的情况下胚胎干 (ES) 细胞的命运。我们观察到同时缺乏Dnmt3a和Dnmt3b的 ES 细胞会迅速从嵌合体中消除。经过进一步研究，我们发现体内和体外的多能性形成转变偏离了滋养层产生的轨道。这种异常轨迹与抑制Ascl2激活失败有关。 Ascl2编码在胎盘中表达的 bHLH 转录因子。 ES 细胞中Ascl2的错误表达会引发向滋养层样细胞的转分化。相反， Ascl2缺失挽救了Dnmt3a/b突变体的形成转变并提高了对嵌合外胚层的贡献。因此，DNA 从头甲基化可防止Ascl2的异位激活。然而， Dnmt3a/b缺陷细胞在正在进行的胚胎发生中仍然存在缺陷。我们推测，多个发育转变可能是通过 DNA 甲基化沉默潜在的破坏性基因来确保的。