rs4987188 — MSH2 Gly322Asp

MSH2 Gly322Asp — A Germline Fidelity Modifier in the Mismatch Repair Gate

Your genome is copied and transmitted to the next generation in sperm and eggs. Every round of cell division during gametogenesis introduces errors — and the mismatch repair (MMR) system¹¹ mismatch repair (MMR) system
the cellular machinery that corrects base mismatches and insertion/deletion errors after DNA replication catches most of them before they become permanent. MSH2 is the central scaffold of this system: it forms the MutSα heterodimer with MSH6 to detect mismatches, and the MutSβ heterodimer with MSH3 to handle larger insertion/deletion loops. The Gly322Asp variant sits in the connector domain of MSH2, a structural region that mediates the critical handoff between mismatch recognition and downstream repair signaling through MLH1.

The Mechanism

Glycine at position 322 is part of a conserved loop in MSH2's connector domain (domain II), which serves as the physical interface with MutL homologs²² MutL homologs
the MLH1-PMS2 complex that executes strand excision after mismatch recognition. Replacing glycine — a structurally flexible amino acid — with aspartate introduces a bulkier, charged side chain that subtly alters connector domain geometry. Functional studies give mixed results: an in vitro MMR assay using human nuclear extracts found statistically significant reduction in repair efficiency (~10% of wild-type³³ statistically significant reduction in repair efficiency (~10% of wild-type
Andres et al. 2014, PMC4273566), while an earlier study using purified recombinant protein found no impairment in MSH2-MSH6 complex formation, mismatch binding, or repair activity (Kariola et al. 2008)⁴⁴ (Kariola et al. 2008). The discrepancy likely reflects assay sensitivity: purification steps may exclude structurally damaged heterodimers, making cell extract assays more representative of in vivo conditions.

The Evidence

The most definitive evidence comes from a genome-scale study in Iceland. Kristmundsdottir et al. (2023, Nature Communications)⁵⁵ Kristmundsdottir et al. (2023, Nature Communications)
Sequence variants affecting the genome-wide rate of germline microsatellite mutations analyzed whole-genome sequences from 6,084 Icelandic parent-offspring trios and identified rs4987188[A] as a variant that significantly increases transmitted microsatellite de novo mutations: +13.1 mutations in the paternal lineage and +7.8 in the maternal lineage per offspring per generation (P = 3.6×10⁻¹⁰, effect size 0.37 standard deviations). At a baseline of ~64 microsatellite de novo mutations per generation, the A allele represents a ~12–20% increase in germline microsatellite instability, carried symmetrically through both sperm and eggs. Crucially, the similar effect in both sexes indicates that germ cells from both parents are subject to the same MSH2-dependent sequence fidelity process.

ClinVar classification is Benign for Lynch syndrome (expert panel reviewed, InSiGHT September 2013, VCV000001762), based on allele frequency exceeding 1% in the population, lack of disease segregation in Lynch families, and the absence of clear mismatch repair deficiency on standard assays. The germline mutation-rate effect is a distinct phenotype from Lynch syndrome: it does not cause microsatellite instability in tumor DNA, but does modestly increase the per-generation mutation load passed to children.

Smaller case-control studies have explored cancer associations with inconsistent results. A Polish series found MSH2 Gly322Asp associated with colorectal cancer recurrence (p=0.001)⁶⁶ MSH2 Gly322Asp associated with colorectal cancer recurrence (p=0.001)
Plawski et al. 2017 and a separate Polish cohort reported a protective association with triple-negative breast cancer for the Asp allele, though these studies are small (n ≈ 70–144) and have not been replicated in larger populations.

Practical Actions

For AG heterozygotes, the effect on germline mutation rate is modest — a ~10–15% above-baseline increase in transmitted microsatellite mutations. No specific medical intervention is currently indicated. The finding is most relevant for genetic counseling context in families with unexplained de novo diseases or autism spectrum conditions (where microsatellite instability is one contributor to de novo mutation burden), and for couples undergoing IVF where preimplantation genetic testing for aneuploidy (PGT-A) is already planned.

For AA homozygotes (extremely rare, <0.03% globally), the expected additive effect would be roughly double the heterozygote increase in germline mutation rate. Given how rarely this genotype occurs, robust clinical data specific to AA homozygosity are unavailable; the recommendations remain an extrapolation from the per-allele dosage effect.

Interactions

MSH2 works in obligate heterodimer complexes, so the biological output of Gly322Asp depends on the functional state of its partners — particularly MSH6 (rs2303426 in the MSH2-MSH6 interface region) and MLH1 (rs1799977, already in the GeneOps database). The variant in MLH1 (rs1799977, Ile219Val) sits in the MLH1 ATPase domain that docks with MSH2's connector domain; if a person carries both rs4987188 (MSH2 G322D) and rs1799977 (MLH1 I219V), the connector-domain interface is disrupted from both ends. A compound action covering this combination is worth investigating if familial MMR deficiency studies show additive effects.

EXO1 (rs72755295 and rs1635501 in the GeneOps database) acts downstream of MSH2 in the excision step; MSH2-EXO1 pathway efficiency is relevant to meiotic crossover fidelity. Variants in EXO1 combined with MSH2 G322D could compound the germline mutation-rate phenotype through reduced excision capacity after mismatch recognition.