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

Abstract

Summary Remodeling DNA methylation in mammalian genomes can be global as seen in pre-implantation 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.

Retinoic acid signaling in regulation of meiosis during embryonic development in mice

“…The next stage, referred to as Stage II, begins at E10.5 and involves locus-specific DNA demethylation, resulting in hypomethylated germ cells in an epigenetic ground state by E13.5 (Dawlaty et al, 2013;Hajkova et al, 2008;Hargan-Calvopina et al, 2016). Stage II demethylation involves Tet family enzymes Tet1 and Tet2 in mice.…”

“…Certain sites, including imprinting control regions (ICRs), intracisternal A particles (IAPs, which are endogenous retroviruses), and promoters of meiotic genes, are protected from the global demethylation activity at the conclusion of Stage I (Seisenberger et al, 2012). However, these sites are eventually demethylated by E13.5 (Hargan-Calvopina et al, 2016). It is intriguing why these sites are protected initially?…”

“…it functions independently to protect PGCs from precocious differentiation(Hargan-Calvopina et al, 2016).…”

“…The two-stage DNA demethylation model poses yet more intriguing questions like why there is locus-specific protection at specific sites during Stage I DNA demethylation and what are the exact molecular mechanisms that confer such a protection. Also, there is a pertinent question of whether Dnmt1 functions in association with Uhrf1 or it functions independently to protect PGCs from precocious differentiation(Hargan-Calvopina et al, 2016).9 | CONCLUSIONSWe have discussed the genetic and epigenetic mechanisms operatingin regulation and cell fate decisions of fetal germ cells. RA has been firmly established as a meiosis-inducing factor in mouse.…”

Do gametes woo? Evidence for non-random unions at fertilization

“…[188][189][190] Various mechanisms preserve genomic integrity and developmental capacity of the 'mother of all stem cell lineages' 191 by repairing DNA defects, 192 maintaining cellular conditions, 193 suppressing transposon activity, [194][195][196][197][198][199] and programming epigenetic state. 200,201 Failure of pluripotency control can lead to precocious differentiation of germ cells, 202,203 spontaneous transformation of germ cells during fetal development, 142,185,186,204 , infertility, 49,205,206 and other reproductive disorders. 204 Dysfunction can also lead to inherited epigenetic changes that affect risk for TGCTs and urogenital abnormalities in offspring and later generations in the absence of genes that originally triggered these transgenerational effects.…”