rs236114 — MCM8

MCM8 rs236114 — DNA Helicase Variant That Regulates Reproductive Lifespan

The timing of natural menopause is a direct readout of ovarian reserve — the pool of follicles a woman is born with and depletes over decades. Genetic studies have consistently pointed to one biological process as the dominant determinant of how quickly that pool shrinks: DNA repair¹¹ DNA repair
the cellular machinery that detects and fixes breaks in the double helix, preventing follicle apoptosis when damage accumulates. Among the most repeatedly replicated loci in this domain is chromosome 20p12.3, home to MCM8 — a DNA helicase whose role in meiotic repair is so essential that complete loss-of-function causes primary ovarian failure and male infertility.

The intronic variant rs236114 sits 49 bases into intron 3 of MCM8 (c.336+49T>A). It was first identified in the Stolk et al. 2009 menopause GWAS²² Stolk et al. 2009 menopause GWAS
n=2,979 European women; three independent loci identified at p<5×10⁻⁸; rs236114 reached p=9.7×10⁻¹¹ and subsequently confirmed in the large ReproGen meta-analysis³³ ReproGen meta-analysis
Stolk et al. 2012; 22 GWAS cohorts, 38,968 discovery + 14,435 replication women. Each A allele is associated with approximately 0.5 years of delayed menopause. The T allele, carried by about 79% of the global population, is the baseline-risk allele.

The Mechanism

MCM8 encodes a member of the minichromosome maintenance (MCM) protein family. Unlike the canonical MCM2–7 replicative helicase complex, MCM8 functions specifically in homologous recombination repair⁴⁴ homologous recombination repair
a high-fidelity DNA repair pathway that uses a sister chromatid as a template to accurately fix double-strand breaks, essential during meiosis when recombination itself creates programmed breaks. MCM8 forms a heterohexamer with its partner MCM9 and loads at stalled replication forks and programmed meiotic double-strand breaks, where it unwinds DNA to provide the single-stranded template needed for repair synthesis.

The consequences of MCM8 loss are stark. Rare biallelic loss-of-function mutations⁵⁵ loss-of-function mutations
pathogenic variants that ablate MCM8 helicase activity; found in consanguineous families; consistent phenotype across multiple ethnic groups cause premature ovarian failure (POF10, OMIM 613769) in women and primary gonadal failure with azoospermia or oligospermia in men. Patient fibroblasts show dramatically increased chromosomal breakage following DNA-damaging agents, confirming that MCM8 is rate-limiting for chromosomal integrity in germ cells.

The common intronic variant rs236114 does not ablate MCM8 function, but it may subtly modulate expression or splicing of the gene — mechanisms consistent with population-level shifts in menopause timing without causing frank ovarian failure. The protective A allele at this position may increase MCM8 expression or enhance splicing efficiency, maintaining more efficient meiotic DNA repair and thereby slowing the apoptotic depletion of primordial follicles across the reproductive lifespan.

The Evidence

The discovery signal from Stolk et al. 2009⁶⁶ Stolk et al. 2009
GWAS in 2,979 women from Rotterdam Study, TwinsUK, and Breakthrough Generations Study; 6 SNPs in 3 loci at p<5×10⁻⁸ showed the A allele at rs236114 delays menopause by +0.5 years (frequency 21% in Europeans, p=9.7×10⁻¹¹). This locus was confirmed in the ReproGen meta-analysis⁷⁷ ReproGen meta-analysis
22 cohorts, 38,968 European women; 13 new menopause loci identified; enriched for DNA-repair genes, which also named MCM8 alongside EXO1, HELQ, UIMC1, FANCI, TLK1, POLG, and PRIM1 as the DNA-repair gene cluster underlying reproductive aging.

MCM8 coding mutations directly link the gene to reproductive failure: two separate groups simultaneously identified biallelic MCM8 mutations causing primary amenorrhea and ovarian failure with chromosomal instability⁸⁸ chromosomal instability
hypersensitivity to DNA-damaging agents, especially interstrand cross-links caused by mitomycin C, indicating deficient double-strand break repair (Wood et al. 2015, JCI)⁹⁹ (Wood et al. 2015, JCI) (AlAsiri et al. 2015, J Med Genet)¹⁰¹⁰ (AlAsiri et al. 2015, J Med Genet). These Mendelian phenotypes anchor rs236114's population-level association in solid biological mechanism.

The stronger MCM8 coding variant, rs16991615 (p.Glu341Lys, E341K), shows a larger effect on menopause timing (~1 year per allele) and was directly associated with AMH levels in the Ruth et al. 2019 AMH GWAS¹¹¹¹ Ruth et al. 2019 AMH GWAS
pre-menopausal women; rs16991615 reached p=3.48×10⁻¹⁰ for AMH, establishing MCM8 as a determinant of the active follicular pool, not just endpoint menopause timing. rs236114 and rs16991615 are at the same locus but represent partially independent signals — rs236114 (intronic, common, modest effect) versus rs16991615 (missense, rarer at ~6% globally, larger effect).

The cross-ethnic replication in the PAGE study¹²¹² PAGE study
42,251 women of diverse ancestry confirmed the MCM8 signal broadly, though LD structure varies substantially across ancestries and rs236114 specifically has near-zero A allele frequency in East Asian populations (~0.1%).

Practical Implications

With a per-allele effect of ~0.5 years and A allele frequency of ~21% in Europeans, about 4.4% of European women carry the AA genotype (expected +1 year of menopause delay), while 62.5% carry TT (baseline timing, no effect from this locus). The variant's value lies primarily in polygenic risk scoring for ovarian aging — when stacked with other DNA-repair loci (TLK1 rs10183486, MCM8 rs16991615, HELQ rs1046089, EXO1 rs72755295), the cumulative signal becomes more predictive of early reproductive aging than any single variant.

Interactions

rs16991615 (MCM8, E341K): The stronger coding signal at the same gene. Both rs236114 and rs16991615 are MCM8 locus variants but tag different haplotypes. rs16991615 shows larger per-allele effects on both menopause timing and AMH levels. A woman carrying the risk T/T genotype at rs236114 and the risk G/G genotype at rs16991615 carries two independent MCM8-pathway hits — one common-variant intronic signal and one rarer coding signal — which together constitute a strong MCM8-pathway burden. The combined recommendation for dual-risk carriers would be early baseline AMH testing before age 30 and repeat monitoring every 1-2 years.

rs244715 (ZNF346/UIMC1 locus): The chromosome 5q35.2 locus, ~71 kb downstream of UIMC1, operates through BRCA1-mediated DSB repair — a distinct but converging pathway. Carrying risk alleles at both the MCM8 and UIMC1/ZNF346 loci represents two independent DNA-repair hits on reproductive lifespan from different repair pathways (helicase/HR at MCM8; BRCA1 complex recruitment at UIMC1).

rs10183486 (TLK1): TLK1 kinase phosphorylates histone chaperone Asf1 and the DNA damage checkpoint protein RAD9 — a distinct chromatin-assembly/checkpoint pathway. TLK1 and MCM8 risk alleles have been co-identified in the same GWAS meta-analyses and are additive contributors to polygenic ovarian aging burden.

This variant affects both sexes via reproductive lifespan pathways: in women through follicular depletion and menopause timing; in men (supported by the MCM8 pathogenic mutation literature) potentially through spermatogenesis, though the GWAS data for rs236114 derives exclusively from female cohorts.