Germline DNA Demethylation Dynamics and Imprint Erasure Through 5-Hydroxymethylcytosine

TLDR

Mouse primordial germ cells undergo sequential epigenetic changes and genome‑wide DNA demethylation to reset the epigenome for totipotency. The study shows that CpG methylation erasure in mouse PGCs proceeds through TET1/2‑mediated conversion to 5hmC, which appears asynchronously at E9.5–E10.5, is subsequently diluted by replication, forms part of redundant reprogramming mechanisms, and that a few regulatory elements escape this demethylation, suggesting a mechanism for transgenerational epigenetic inheritance.

Abstract

Mouse primordial germ cells (PGCs) undergo sequential epigenetic changes and genome-wide DNA demethylation to reset the epigenome for totipotency. Here, we demonstrate that erasure of CpG methylation (5mC) in PGCs occurs via conversion to 5-hydroxymethylcytosine (5hmC), driven by high levels of TET1 and TET2. Global conversion to 5hmC initiates asynchronously among PGCs at embryonic day (E) 9.5 to E10.5 and accounts for the unique process of imprint erasure. Mechanistically, 5hmC enrichment is followed by its protracted decline thereafter at a rate consistent with replication-coupled dilution. The conversion to 5hmC is an important component of parallel redundant systems that drive comprehensive reprogramming in PGCs. Nonetheless, we identify rare regulatory elements that escape systematic DNA demethylation in PGCs, providing a potential mechanistic basis for transgenerational epigenetic inheritance.