rs3776467 — MTRR

Intronic MTRR variant with sex-specific association with DNA methylation patterning in one-carbon metabolism

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MTRR rs3776467 — An Intronic Variant Shaping DNA Methylation Under Stress

The MTRR gene encodes methionine synthase reductase, a flavoprotein that keeps the methylation cycle running by reactivating methionine synthase (MTR) after oxidative inactivation. Without functional MTRR, MTR can't recycle homocysteine to methionine, and the entire one-carbon methylation machinery stalls. The well-studied MTRR A66G variant (rs1801394) directly alters the protein, but MTRR also carries intronic variants that appear to influence gene regulation in subtler ways. rs3776467 is one such variant — an intronic substitution at position c.401+827 that has no direct effect on the MTRR protein sequence, yet shows a reproducible association with DNA methylation outcomes in specific contexts.

The Mechanism

Because rs3776467 sits within an intron 11 Intronic variants can influence gene expression through effects on splicing, branch point usage, and regulatory element binding without changing the amino acid sequence of the protein, it does not alter the MTRR protein. Its effects are likely regulatory — modulating the level or splicing of MTRR transcripts rather than the enzyme's catalytic activity. The functional consequence of reduced MTRR expression would parallel the missense A66G effect: less efficient B12 reactivation, impaired MTR activity, and downstream effects on homocysteine remethylation and global DNA methylation. The precise regulatory mechanism has not been characterized.

The Evidence

The primary evidence comes from the Lovelace Smokers Cohort22 Lovelace Smokers Cohort
Flores KG et al. Sex-specific association of sequence variants in CBS and MTRR with risk for promoter hypermethylation in the lung epithelium of smokers. Carcinogenesis, 2012, a study of 907 non-Hispanic white smokers examining promoter hypermethylation in lung epithelium. In this cohort, the G allele of rs3776467 was associated with reduced risk for high promoter hypermethylation: OR 0.57 (95% CI: 0.42–0.77, p=0.0003) overall, driven almost entirely by females (females: OR 0.64, 95% CI: 0.49–0.85, p=0.002; males: OR 0.92, 95% CI: 0.57–1.47, p=0.72). The AA genotype — homozygous for the reference allele — was associated with higher methylation risk, while carrying at least one G allele was protective in women.

A colorectal cancer cohort study33 colorectal cancer cohort study
Wang Y et al. The Roles of MTRR and MTHFR Gene Polymorphisms in Colorectal Cancer Survival. Nutrients, 2022 of 532 CRC patients (Newfoundland, median follow-up 6.4 years) identified significant interactions between rs3776467 and pre-diagnostic alcohol consumption: protective alleles of rs3776467 were associated with superior overall survival, but only in patients consuming below-median alcohol. This interaction suggests the variant's effect on one-carbon metabolism is amplified by lifestyle factors that disrupt folate and methyl-donor homeostasis.

The evidence is currently emerging — two studies, both in specific populations (smokers, CRC patients), with no direct functional validation of the intronic variant's molecular effect and no homocysteine association data. The sex-specific nature of the primary finding limits applicability.

Practical Actions

For carriers of the AA genotype (homozygous reference, most common in Europeans), the primary actionable insight is context-dependent: the elevated methylation risk appears most relevant in the setting of smoking or high alcohol intake that disrupts the one-carbon cycle. Optimizing methylation nutrition — particularly active B12 forms and methylfolate — supports MTRR function regardless of this variant. For women who smoke, this SNP adds to the case for prioritizing methylation support.

Alcohol consumption deserves specific mention: the interaction with rs3776467 and colorectal cancer survival suggests that alcohol's known methylation- disrupting effects may be modulated by this variant. Reducing alcohol intake supports both folate-dependent methylation and the protective effect of the G allele.

Interactions

rs3776467 operates in the same MTRR pathway as the coding variant rs1801394 (A66G). Combined impairment — AA at rs3776467 plus GG at rs1801394 — could theoretically reduce MTRR function through both regulatory and catalytic mechanisms. The upstream MTHFR variants (rs1801133) affect methylfolate supply, and MTR variants (rs1805087) affect the reaction MTRR supports; weakness at multiple points compounds the burden on the methylation cycle. No published study has assessed the combined genotype of rs3776467 with rs1801394.

Nutrient Interactions

folate altered_metabolism
vitamin B12 altered_metabolism

Genotype Interpretations

What each possible genotype means for this variant.

AA Standard Methylation Normal

Common genotype; standard methylation patterns

The AA genotype lacks the G allele that was associated with protection against promoter hypermethylation in smokers. The mechanism is not established — this is an intronic variant, so it likely acts through regulatory effects on MTRR expression rather than protein function. The clinical significance is limited and context-dependent: evidence comes from one study in smokers (n=907) and one in colorectal cancer patients, with no data on homocysteine levels or methylation outcomes in healthy non-smoking populations. Standard methylation nutrition support remains appropriate.

GG Enhanced Methylation Protection Beneficial

Two copies of the protective G allele — lowest methylation disruption risk in women

The GG genotype represents homozygosity for the minor allele at this intronic MTRR position. The G allele is notably more common in East Asian populations (~53%), where it may reflect ancestrally distinct regulatory haplotypes in the MTRR gene region. The protective mechanism is not characterized at the molecular level — it may reflect enhanced MTRR expression or altered splicing that supports more efficient B12 recycling and consequently better methylation fidelity under stress conditions. While the evidence is emerging and sex-specific, GG individuals appear to maintain tighter promoter methylation control even under methylation- disrupting lifestyle exposures.

AG Partial Methylation Protection Intermediate

One protective G allele — reduced methylation disruption risk in women

The AG genotype falls between the common AA genotype and the rare fully protective GG. The one-copy dose-response is consistent with a codominant model, where each G allele incrementally reduces the risk for hypermethylation under stressful one-carbon conditions (smoking, alcohol). The effect is sex-specific based on available data — the male-specific OR was 0.92 (non-significant), suggesting hormonal or metabolic differences modulate how this intronic MTRR variant affects methylation outcomes. For women, especially those with other methylation stressors, this partial protection may be meaningful.