rs16991615 — MCM8 E341K

MCM8 E341K — Your DNA Repair Helicase and Ovarian Clock

Every egg in a woman's ovaries was formed before birth and has been waiting, arrested mid-way through meiosis, ever since. Keeping those eggs healthy over decades requires continuous DNA repair — and MCM8 is one of the key proteins doing that work. The E341K variant (rs16991615) in this gene is one of the most robustly replicated genetic influences on ovarian reserve and the timing of natural menopause identified to date.

The Mechanism

MCM8 encodes a DNA helicase¹¹ DNA helicase
an enzyme that unwinds double-stranded DNA to allow repair machinery access that forms a complex with MCM9. Together, MCM8/MCM9 repair double-strand breaks²² double-strand breaks
the most dangerous type of DNA damage, where both strands of the helix are cut during meiosis I — precisely the stage at which oocytes are arrested in human ovaries. When double-strand breaks accumulate without repair, oocytes undergo programmed death (atresia), shrinking the follicle pool over time.

The E341K substitution changes glutamic acid (negatively charged) to lysine (positively charged) at position 341 of the protein. Research has shown that this amino acid change impairs MCM8's ability to resolve R-loops³³ R-loops
RNA-DNA hybrid structures that form during transcription and can block DNA repair if not cleared, suggesting a subtle reduction in helicase efficiency rather than complete loss of function. This partial impairment may slow the rate of DNA repair in oocytes, increasing cumulative damage over years of follicular dormancy.

The Evidence

The landmark GWAS evidence comes from He et al. 2009⁴⁴ He et al. 2009
He C, et al. Genome-wide association studies identify loci associated with age at menarche and age at natural menopause. Nature Genetics, 2009, a joint analysis of 17,438 women (Nurses' Health Study + Women's Genome Health Study). The MCM8 locus produced the smallest p-value in the entire analysis (p = 1.2 × 10⁻²¹), with each copy of the A allele associated with approximately 1.07 years later menopause onset. This effect was replicated by Stolk et al. 2012⁵⁵ Stolk et al. 2012
Stolk L, et al. Meta-analyses identify 13 loci associated with age at menopause and highlight DNA repair and immune pathways. Nature Genetics, 2012, a meta-analysis confirming MCM8 among 13 genome-wide significant menopause loci and noting striking enrichment of DNA repair genes across the top hits.

The connection to ovarian reserve — not just menopause age — was established by Schuh-Huerta et al. 2012⁶⁶ Schuh-Huerta et al. 2012
Schuh-Huerta SM, et al. Genetic markers of ovarian follicle number and menopause in women of multiple ethnicities. Human Genetics, 2012, who found that rs16991615 A allele carriers had approximately 2.79 more antral follicles counted by ultrasound — a direct measure of the remaining egg supply. The anti-Müllerian hormone (AMH) link was confirmed by Ruth et al. 2019⁷⁷ Ruth et al. 2019
Ruth KS, et al. Genome-wide association study of anti-Müllerian hormone levels in pre-menopausal women. Human Molecular Genetics, 2019, which found a 0.26 SD increase in AMH per A allele (p = 3.48 × 10⁻¹⁰) in 3,344 pre-menopausal women — providing the clearest link between this variant and a clinically measurable ovarian reserve biomarker.

Effect sizes are modest at the population level (roughly 1 year of menopause timing per allele), but the variant's association with AMH gives it direct clinical relevance: women whose AMH appears lower than expected for their age may benefit from knowing whether this genetic component is contributing.

Note on population variation: the A allele is substantially more common in European-ancestry women (~7%) than in African-ancestry women (~1%), meaning the genetic contribution to ovarian reserve timing differs across populations. Results from GWAS performed predominantly in European cohorts should be interpreted with appropriate caution in other ancestry groups.

Practical Actions

For women carrying the more common GG genotype, this SNP provides no protective signal for ovarian reserve — baseline monitoring of AMH and antral follicle count is appropriate, particularly if fertility is being planned for the mid-to-late 30s. The absence of the A allele is not predictive of early menopause by itself; it simply means this particular genetic advantage is absent.

For A allele carriers (AG or AA), the genetic data suggests a tendency toward somewhat better-preserved ovarian reserve — but genotype alone cannot predict individual AMH levels or fertility outcomes. AMH testing remains the most useful clinical tool to confirm whether this genetic signal translates to a measurable advantage in any individual.

For male carriers: animal models of MCM8 deficiency (knockout mice) show complete male sterility through meiotic arrest, and Tenenbaum-Rakover et al. 2015⁸⁸ Tenenbaum-Rakover et al. 2015
Tenenbaum-Rakover Y, et al. MCM8 gene mutations result in primary gonadal failure. Journal of Medical Genetics, 2015 documented azoospermia in human males with homozygous loss-of-function MCM8 mutations. The E341K common variant is far milder than these rare mutations; no published studies have specifically examined semen parameters in male rs16991615 carriers, so male relevance at this allele frequency remains speculative.

Interactions

MCM8 rs16991615 + FNDC4 rs2303369 (dual reproductive aging loci): FNDC4 encodes a secreted protein related to irisin, associated with age at natural menopause through GWAS. MCM8 acts in DNA repair pathways while FNDC4 modulates follicular granulosa cell metabolism. Women carrying the MCM8 rs16991615 GG genotype (absent protective allele) together with a risk genotype at FNDC4 rs2303369 may have compounding risk from two independent biological pathways (DNA repair helicase insufficiency and follicular metabolic signaling), potentially accelerating follicle depletion beyond what either variant predicts alone. A compound action for this combination — emphasizing early AMH baseline testing and proactive fertility timeline discussion — is warranted. See related SNP rs2303369.

MCM8 rs16991615 + PRRC2A rs1046089 (multiple menopause-timing loci): PRRC2A (proline-rich coiled-coil 2A) at chromosome 6p21.33 is another replicated GWAS locus for age at natural menopause, operating through a different mechanism (immune modulation via HLA class II expression in the MHC region). Carrying the risk configuration at both MCM8 and PRRC2A represents two independent hits on reproductive lifespan from distinct biological pathways (DNA repair and immune-mediated follicle depletion). Women with risk genotypes at both loci may have a meaningfully earlier expected menopause onset and should be counseled about earlier fertility assessment. A compound action emphasizing proactive reproductive timeline planning is warranted. See related SNP rs1046089.