rs3776467 — MTRR

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.