with maternal diabetes predisposing offspring to metabolic dysfunction in later life through fetal hyperinsulinemia [33]. A recent mouse model suggests that increasing AMH levels in pregnancy (as seen in PCOS women) can promote both LH-mediated T excess and reduced placental aromatization of maternal androgens [28], thereby contributing to fetal hyperandrogenism in their daughters (Fig. 1), although such a mechanism in humans remains to be confirmed.
Post-natal consequences of in utero androgen excess are found as early as the newborn for women with PCOS. Infant daughters not only exhibit transient facial sebum [34], a biomarker of prior T exposure, but also demonstrate an elongated anogenital distance [35], a reliable biomarker for early-to-mid gestation androgen excess [36]. Newborn daughters of women with PCOS also exhibit elevated AMH levels indicative of increased numbers of ovarian antral follicles, a PCOS trait. In adulthood, women with PCOS retain an elongated anogenital distance ([37, 38], typical of in utero, T-exposed, PCOS-like female monkeys [36] and sheep [23]. A diminished or exaggerated 2D:4D finger length ratio is also associated with both in utero androgen excess and PCOS, in women [39], their prepubertal daughters [24] and adult, early-to-mid gestation, T-exposed PCOS-like monkeys [36], since similar T- and E2-regulated genes control differentiation of gonads, hands, and feet. Prepubertal daughters of women with PCOS also excrete increased concentrations of dihydrotestosterone (DHT) metabolites in their urine compared to prepubertal girls of women without PCOS [40], indicating increased 5-alpha reductase activity, and perhaps amplified target tissue androgen action, well before the onset of PCOS signs and symptoms at puberty.
Mixed umbilical cord blood androgen levels from human female fetuses at term, however, have yielded inconsistent results in support of late gestation fetal hyperandrogenism in daughters of PCOS women, regardless of whether or not “gold standard” liquid chromatography-tandem mass spectrometry assays are employed. Increased T or androstenedione levels are reported in two studies [41, 42], equivalent levels in one [43], and diminished levels in a further two studies [29, 44]. Labor onset and duration, together with increasing term gestational age, however, diminish umbilical cord androgen levels and likely often confound understanding of late gestation female androgenic state from this measure [45]. Moreover, no sex differences remain between circulating T levels in male and female human fetuses by late gestation [21], suggesting term birth is inauspicious for exploration of developmental hyperandrogenism.
Genetic Origins: PCOS Risk Genes Are Compatible with in utero Hyperandrogenic Pathogenesis for PCOS
Family-based and extensive genome-wide association studies have yielded 17 replicated PCOS risk genes, regulating gonadotropin secretion (FSHB), gonadotropin action and ovarian function (LHCGR, FSHR, DENND1A, RAB5/SUOX, HMGA2, C9orf3, YAP1, TOX3, RAD50, FBN3), and various metabolic functions (THADA, GATA4/NEIL2, ERBB4, SUMO1P1, INSR,and KRR1) [46–48]. Of the genes regulating gonadotropin and ovarian function, a substantial number have been proposed as enabling ovarian hyperandrogenism [46, 49]. A recent alternative approach, employing rare gene variant association testing, followed by targeted resequencing of AMH in a replication cohort, identified an additional 17 PCOS-specific, rare coding and splice-site variants in AMH that diminish AMH signaling [49]. Ovarian hyperandrogenism is a potential outcome of reduced ovarian AMH inhibition of CYP17A1 expression [49]. While progress toward understanding gene variant-based PCOS heritability has clearly advanced, a heritability gap between low incidence of PCOS risk genes (∼10%) and the high heritability of PCOS (∼70%) [50], indicates a pressing need to identify (1) more PCOS risk genes, as each may confer a small degree of disease risk, (2) rare gene variants, as each may confer unduly large degrees of PCOS risk, and/or (3) epigenetic mechanisms altering a wide range of gene expression that confer considerable risk for PCOS. Current thinking embraces a combination of polygenic, epigenetic, and developmental contributions to PCOS pathogenesis that are ameliorated or exaggerated by lifestyle [1, 47].
Epigenetic Origins: Developmental Contribution to PCOS
T and its biopotent metabolites are highly effective regulators of DNA methylation during fetal development. They enable the majority of phenotypic sexual differentiation in multiple organ systems and tissues, including the brain [51]. Increased or decreased DNA methylation can diminish or enhance, respectively, mRNA transcription of inherited gene variants [52]. Different patterns and degrees of DNA methylation at any single gene locus, however, are specific to each organ system or cell type in each individual. Unlike GWAS, therefore, there is less certainty as to how genome-wide methylation studies generalize beyond an organ system or cell type. DNA is differentially methylated in a variety of organ systems in women with PCOS [53, 54]. Gene-targeted DNA methylation studies of LHCGR have reported its hypomethylation in blood cells and subcutaneous (SC) adipose of women with PCOS, concurrent with increased LHCGR mRNA expression in these tissues [55–57]. If comparable DNA hypomethylation of LHCGR occurs in PCOS ovarian theca cells, it would likely increase androgenic responses to LH pulses, causing or amplifying ovarian hyperandrogenism. GWMS and bioinformatic pathway analyses have identified clusters of differentially methylated genes in PCOS women that may alter a variety of cellular functions, including immune response pathways (including autoimmunity), ovarian steroidogenic and metabolic functions, and cancer-related pathways [56, 58, 59]. Notably, there are commonalties between identified PCOS risk genes and differentially methylated
