A rewiring of DNA replication mediated by MRE11 exonuclease underlies primed-to-naive cell de-differentiation

Mouse embryonic stem cells (mESCs) in the primed pluripotency state, that resembles the post-implantation epiblast, can be de-differentiated in culture to anaivestate that resembles the pre-implantation inner cell mass.We report that primed-to-naive mESC transition entails a significant slowdown of DNA replication forks and the compensatory activation of dormant origins. Using iPOND (“isolation of proteins on nascent DNA”) coupled to mass spectrometry, we identify key changes in replisome composition that are responsible for these effects. Naive mESC forks are enriched in MRE11 nuclease and other DNA repair proteins. MRE11 is recruited to newly synthesized DNA in response to transcription-replication conflicts, and its inhibition or genetic downregulation in naive mESCs is sufficient to restore the fork speed observed in primed cells. Notably, MRE11 is required for the efficient primed-to-naive mESC transition, demonstrating a direct link between DNA replication dynamics and the mESC de-differentiation process RNA-Seq of untreated primed mESCs and after a 6-day 2i treatment, alone or in combination with two different concentrations of a MRE11 inhibitor.