Stage-Specific Demethylation in Primordial Germ Cells Safeguards against Precocious Differentiation

Remodeling DNA methylation in mammalian genomes can be global, as seen in preimplantation embryos and primordial germ cells (PGCs), or locus specific, which can regulate neighboring gene expression. In PGCs, global and locus-specific DNA demethylation occur in sequential stages, with an initial global decrease in methylated cytosines (stage I) followed by a Tet methylcytosine dioxygenase (Tet)-dependent decrease in methylated cytosines that act at imprinting control regions (ICRs) and meiotic genes (stage II). The purpose of the two-stage mechanism is unclear. Here we show that Dnmt1 preserves DNA methylation through stage I at ICRs and meiotic gene promoters and is required for the pericentromeric enrichment of 5hmC. We discovered that the functional consequence of abrogating two-stage DNA demethylation in PGCs was precocious germline differentiation leading to hypogonadism and infertility. Therefore, bypassing stage-specific DNA demethylation has significant consequences for progenitor germ cell differentiation and the ability to transmit DNA from parent to offspring. [Display omitted] •Dnmt1 is responsible for maintaining DNA methylation in PGCs•Dnmt1 has no role in transposon repression in PGCs•Precocious germline differentiation is restrained by Dnmt1•The 5hmC epigenetic mark is not required for meiotic entry Hargan-Calvopina et al. shed light on functional significance of two-stage DNA demethylation—a global decrease followed by locus-specific loss—in primordial germ cells. Dnmt1 preserves DNA methylation in stage I at specific loci, including pericentromeric regions. Abrogating the two stages results in precocious germline differentiation, leading to hypogonadism and infertility.