Ariel (2020) 'Unravelling the role of epigenetics in reproductive adaptations to early-life environment.', Nature reviews endocrinology., 16 . pp. 519-533.

Background Women facing increased energetic demands in childhood commonly have altered adult ovarian activity and shorter reproductive lifespan, possibly comprising a strategy to optimize reproductive success. Here, we sought to understand the mechanisms of early-life programming of reproductive function, by integrating analysis of reproductive tissues in an appropriate mouse model with methylation analysis of proxy tissue DNA in a well-characterized population of Bangladeshi migrants in the UK. Bangladeshi women whose childhood was in Bangladesh were found to have later pubertal onset and lower age-matched ovarian reserve than Bangladeshi women who grew-up in England. Subsequently, we aimed to explore the potential relevance to the altered reproductive phenotype of one of the genes that emerged from the screens. Results Of the genes associated with differential methylation in the Bangladeshi women whose childhood was in Bangladesh as compared to Bangladeshi women who grew up in the UK, 13 correlated with altered expression of the orthologous gene in the mouse model ovaries. These mice had delayed pubertal onset and a smaller ovarian reserve compared to controls. The most relevant of these genes for reproductive function appeared to be SRD5A1, which encodes the steroidogenic enzyme 5α reductase-1. SRD5A1 was more methylated at the same transcriptional enhancer in mice ovaries as in the women’s buccal DNA, and its expression was lower in the hypothalamus of the mice as well, suggesting a possible role in the central control of reproduction. The expression of Kiss1 and Gnrh was also lower in these mice compared to controls, and inhibition of 5α reductase-1 reduced Kiss1 and Gnrh mRNA levels and blocked GnRH release in GnRH neuronal cell cultures. Crucially, we show that inhibition of this enzyme in female mice in vivo delayed pubertal onset. Conclusions SRD5A1/5α reductase-1 responds epigenetically to the environment and its downregulation appears to alter the reproductive phenotype. These findings help to explain diversity in reproductive characteristics and how they are shaped by early-life environment and reveal novel pathways that might be targeted to mitigate health issues caused by life-history trade-offs.

Reproductive function and duration of the reproductive life span are phenotypically plastic and programmed in response to the early-life environment. Such adaptive responses are described and rationalized in life history theory in the context of resource availability, but the molecular mechanisms responsible have remained enigmatic. In this study, we hypothesized that epigenetic modifications underlie adaptive reproductive strategies, and found distinct methylation patterns in buccal DNA of Bangladeshi women who grew up in Bangladesh or the UK. The later pubertal onset and lower ovarian reserve associated with Bangladeshi childhood was seen to correlate with more numerous childhood infections, so we adopted a mouse model of pre-pubertal colitis to mimic these conditions. These mice have a similarly-altered reproductive phenotype, which enabled us to determine its mechanistic basis. Several genes encoding proteins with known functions in follicle recruitment were differentially expressed in the mice ovaries, and were also differentially methylated in the women’s buccal DNA. One of these, SRD5A1 which encodes the steroidogenic enzyme 5α reductase-1, was down-regulated in the mice ovaries and hyper methylated at the same putative transcriptional enhancer as in the women’s DNA; the levels of methylation correlating with gene expression levels. Srd5a1 expression was down-regulated also in the hypothalamus where 5α reductase-1 catalyzes production of neurosteroids that regulate gonadotropin releasing hormone (GnRH). Chemical inhibition of this enzyme affected both GnRH synthesis and release, and resulted in delayed pubertal onset in vivo. The activity of 5α reductase-1 in hypothalamus and ovary and the sensitivity of SRD5A1 to epigenetic regulation attest to its role in directing long-term physiological strategies in response to environmental conditions. In the reproductive axis, this includes timing of pubertal onset, adult reproductive function and duration of the reproductive lifespan.

5α-reductase-1 catalyzes production of various steroids, including neurosteroids. We reported previously that expression of its encoding gene, Srd5a1, drops in murine ovaries and hypothalamic pre-optic area (POA) after early-life immune stress, seemingly contributing to delayed puberty and ovarian follicle depletion, while in their ovaries the first intron was more methylated at two CpGs. Here we hypothesized that this CpG-containing locus comprises a methylation-sensitive transcriptional enhancer for Srd5a1. We found that ovarian Srd5a1 mRNA increased 8-fold and methylation of the same two CpGs decreased upto 75% between post-natal days (PND) 10-30. Estradiol (E2) levels rise during this pre-pubertal stage, and exposure of ovarian cells to E2 increased Srd5a1 expression. Chromatin immunoprecipitation (ChIP) in an ovarian cell line confirmed ESR1 binding to this differentially-methylated genomic region, and also enrichment of the enhancer modification, H3K4me1. Targeting dCas9-DNMT3 to this locus increased CpG2 methylation 2.5-fold and abolished the Srd5a1 response to E2. In the POA, Srd5a1 mRNA levels decreased 70% between PND 7-10 and then remained constant without correlation to CpG methylation levels. Srd5a1 mRNA levels did not respond to E2 in hypothalamic GT1-7 cells, even after dCas9-TET1 reduced CpG1 methylation by 50%. The neonatal drop in POA Srd5a1 expression occurs at a time of increasing glucocorticoids, and treatment of GT1-7 cells with dexamethasone reduced Srd5a1 mRNA levels, while ChIP confirmed GR binding at the enhancer. Our findings on the tissue-specific regulation of Srd5a1 and its methylation-sensitive control by E2 in the ovaries illuminate epigenetic mechanisms underlying reproductive phenotypic variation which impact life-long health.

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