rs1635501 — EXO1 EXO1 rs1635501

EXO1 rs1635501 — A DNA Repair Variant Linked to Ovarian Aging

The timing of natural menopause is one of the most heritable aspects of female reproductive biology, with an estimated heritability of 50–60%. Genome-wide association studies have repeatedly converged on the same biological theme: DNA repair genes dominate the genetic landscape of ovarian aging. Among these, EXO1 (exonuclease 1)¹¹ EXO1 (exonuclease 1)
a 5′→3′ exonuclease critical for DNA mismatch repair and meiotic recombination stands out for having both a GWAS-level population signal and a well-characterised functional mechanism. The intronic variant rs1635501 at the EXO1 locus tags a nearby functional promoter change, and each copy of the C allele may shift menopause onset approximately 10 weeks earlier.

The Mechanism

EXO1 encodes a multifunctional nuclease with two essential roles in genome maintenance. In DNA mismatch repair (MMR)²² DNA mismatch repair (MMR)
a post-replication proofreading system that corrects base-base mismatches and small insertion/deletion loops introduced during DNA replication, EXO1 is recruited downstream of MutSα/MutLα recognition to excise the mismatch-containing strand, creating a gap that is filled accurately by DNA polymerase. In meiosis, EXO1 is recruited by the MRN complex to double-strand break sites where it resects DNA ends, generating 3′ single-stranded overhangs that are essential for homologous recombination and crossover formation — the chiasmata that physically hold homologous chromosomes together until anaphase I.

The reproductive consequence of EXO1 loss is severe. EXO1 knockout mice are completely infertile in both sexes³³ EXO1 knockout mice are completely infertile in both sexes
Wei et al. 2003, Genes Dev 17:603–614. Exo1−/− oocytes progress normally through prophase I but lose chiasmata at metaphase I, triggering apoptosis. The ovaries of knockout females are smaller than wild-type and show progressive oocyte loss. This dramatic phenotype established that EXO1 activity is not merely beneficial but essential for oocyte viability during meiosis.

The rs1635501 variant is intronic and does not alter the EXO1 protein sequence. Its effect is mediated through a nearby functional polymorphism (rs1776180) in the EXO1 promoter (r² = 0.83 in European panels). The rs1776180 C allele disrupts a binding site for E47, a basic helix-loop-helix transcription factor that acts as a negative regulator of EXO1 transcription⁴⁴ E47, a basic helix-loop-helix transcription factor that acts as a negative regulator of EXO1 transcription
Lunetta et al. 2009, Mech Ageing Dev 130:438–445. Loss of E47 repression leads to higher EXO1 expression. Paradoxically, this higher-expression allele associates with earlier menopause in the population GWAS yet with greater longevity in centenarian studies — likely because the population menopause signal captures the cumulative cost of elevated EXO1 activity on oocyte DNA processing across decades of meiotic arrest.

The Evidence

The primary evidence comes from the Stolk et al. 2012 meta-analysis of 22 genome-wide association studies⁵⁵ Stolk et al. 2012 meta-analysis of 22 genome-wide association studies
Meta-analyses identify 13 loci associated with age at menopause and highlight DNA repair and immune pathways. Nature Genetics 44:260–268, which examined 38,968 women of European descent with replication in 14,435 additional women. At the EXO1 locus (region 1b), rs1635501 reached P = 8.46×10⁻¹⁰ with a beta of −0.188 years (approximately −9.8 weeks) per C allele in the additive model. Among the 13 novel loci identified, EXO1 was one of eight that harbour DNA damage response genes — a striking enrichment that the authors highlighted as evidence for a shared biological mechanism linking DNA repair capacity to ovarian reserve maintenance.

A substantially larger analysis by Ruth et al. 2021⁶⁶ Ruth et al. 2021
Genetic insights into biological mechanisms governing human ovarian ageing. Nature 596:393–397 expanded the GWAS to approximately 200,000 European women and identified 290 loci associated with age at natural menopause, confirming EXO1 and broadly validating the DNA repair pathway enrichment first observed in the Stolk analysis. That study also demonstrated that women in the top 1% of genetic susceptibility — as defined by polygenic scores across all identified loci — carry a risk of premature ovarian insufficiency (POI) comparable to carriers of FMR1 premutations, the best-known monogenic cause of POI.

The mechanistic case is reinforced by a centenarian study by Lunetta et al. 2009⁷⁷ centenarian study by Lunetta et al. 2009
A functional EXO1 promoter variant is associated with prolonged life expectancy in centenarians. Mech Ageing Dev 130:438–445 showing that the promoter variant in LD with rs1635501 is enriched in long-lived women — pointing to EXO1 expression as a pleiotropic regulator of both meiotic competence and somatic DNA maintenance across the lifespan.

EXO1 coding variants have also been explored in women with premature ovarian failure. A variant analysis in 186 Chinese POF patients by Zhu et al. 2016⁸⁸ variant analysis in 186 Chinese POF patients by Zhu et al. 2016
Variation analysis of EXO1 gene in Chinese patients with premature ovarian failure. J Ovarian Res 2016 found no causal coding mutations, suggesting the GWAS signal at rs1635501 acts through the non-coding regulatory mechanism rather than protein-level dysfunction.

Practical Actions

The rs1635501 locus contributes a modest but real additive effect on reproductive lifespan. For women carrying one or two C alleles, the primary clinical implication is earlier consideration of ovarian reserve assessment — particularly for those with other risk factors (family history of early menopause, autoimmune conditions, prior chemotherapy or pelvic radiotherapy). The ~10-week shift per allele is a population average, and serum AMH with antral follicle count provides a direct and individually calibrated measure of current reserve that no polygenic estimate can replace.

Because the biological mechanism involves DNA repair in oocytes, and because EXO1-deficient oocytes are specifically vulnerable to oxidative DNA damage that accumulates across the decades of meiotic arrest, minimising exogenous genotoxic exposures is a plausible and evidence-consistent risk-reduction strategy — particularly tobacco smoke, which delivers polycyclic aromatic hydrocarbons and other oocyte-toxic genotoxins directly to the follicular microenvironment.

Interactions

The ovarian aging signal from DNA repair loci is collectively enriched beyond what individual loci predict, suggesting additive convergence through shared pathways. The most clinically relevant co-variant is rs10183486 (TLK1), another DNA repair gene locus associated with approximately 10 weeks earlier menopause per allele in the same Stolk 2012 analysis. Women carrying C alleles at rs1635501 and T alleles at rs10183486 may have additive reduction in reproductive lifespan, though a formal published compound analysis of this pair has not been reported. The HELQ locus (rs4693089) and the POLG/FANCI locus (rs2307449) identified in the same study operate in overlapping double-strand break repair and mitochondrial replication pathways respectively, and are noted in related_snps for cross-reference.