rs162040 — MTRR

MTRR rs162040 — Alcohol Amplifies an Intronic Methylation Risk Signal in Colorectal Cancer

Methionine synthase reductase (MTRR) maintains the one-carbon methylation cycle by reactivating methionine synthase (MTR) after its methylcobalamin cofactor becomes oxidized during catalysis. When MTRR function is impaired — whether by coding variants that reduce enzyme efficiency or by regulatory variants that reduce enzyme expression — homocysteine accumulates and global DNA methylation declines, processes linked to genomic instability and tumor suppressor silencing.

rs162040 is an intronic variant at chromosome 5 position 7,887,365 (GRCh38), within an MTRR haplotype block also containing rs3776467, rs326124, and rs3776455. The GRCh38 reference allele at this position is C, but C is the population minority (~11% globally in gnomAD v4, ~31% in East Asian populations). The common A allele (~89% globally) represents the baseline protective state; the C allele marks the risk haplotype.

The Mechanism

As an intronic variant, rs162040 does not alter the MTRR protein sequence. Its biological significance lies in its membership in a functional MTRR regulatory haplotype. The parallel between rs162040 and the neighboring intronic variants rs3776467 and rs326124 — all showing the same alcohol-modulated survival signal in the same dataset — suggests they tag a shared regulatory element or splice-regulatory sequence within MTRR introns. Such elements can influence MTRR transcript levels or alternative splicing¹¹ MTRR transcript levels or alternative splicing
Intronic regulatory elements including branch point sequences, exonic splicing enhancers, and lncRNA binding sites can alter gene expression without changing the protein coding sequence. Reduced MTRR expression would impair B12 reactivation, slowing MTR-catalyzed homocysteine remethylation and depleting SAM (the universal methyl donor) — the same downstream consequence as coding impairments.

The Evidence

The primary evidence comes from the Newfoundland Familial Colorectal Cancer Study²² Newfoundland Familial Colorectal Cancer Study
Wang Y et al. The Roles of MTRR and MTHFR Gene Polymorphisms in Colorectal Cancer Survival. Nutrients, 2022, which followed 532 patients with newly diagnosed colorectal cancer (1999–2003) through April 2010. The study genotyped 33 MTRR and MTHFR tag SNPs and assessed their association with overall survival (OS) and disease-free survival (DFS).

For rs162040, as part of the MTRR haplotype block, a significant interaction was found with pre-diagnostic alcohol consumption: protective A-allele carriers showed superior overall survival, but this benefit was restricted to patients consuming alcohol below the cohort median of 2.17 g/day (roughly one standard drink per week). Among patients with higher alcohol intake, the allele-associated survival difference was abolished. This interaction pattern — replicated across rs162040, rs3776467, rs326124, and rs3776455 in the same analysis — is consistent with alcohol's known disruption of folate absorption and one-carbon methyl-donor homeostasis. Alcohol depletes intestinal folate uptake and increases urinary folate excretion, amplifying any underlying impairment of MTRR-dependent B12 recycling.

The evidence base is limited: one survival cohort study in CRC patients, with rs162040 analyzed as part of a haplotype block rather than as an isolated variant. No population-based homocysteine data, no in vitro MTRR expression studies, and no prospective healthy-population data exist for rs162040 specifically. Evidence level is emerging.

Practical Actions

For C-allele carriers, two modifiable factors interact with the rs162040 haplotype: alcohol intake and B12/folate status. Keeping alcohol intake consistently low removes the primary amplifying factor for MTRR-related methylation disruption. Using active B12 forms (methylcobalamin or hydroxocobalamin rather than cyanocobalamin) and methylfolate instead of synthetic folic acid supports the one-carbon cycle even when MTRR regulatory capacity is reduced.

Monitoring plasma homocysteine provides an objective readout of functional methylation status. Elevated homocysteine (above 10 µmol/L) with normal dietary B12 intake signals that recycling pathway demand exceeds capacity and warrants targeted supplementation.

Interactions

rs162040 sits in the same MTRR intronic haplotype block as rs3776467, rs326124, and rs3776455, all of which showed parallel alcohol-survival interactions in the Wang 2022 cohort. The coding variant rs1801394 (MTRR A66G, p.Ile22Met), which reduces MTRR catalytic efficiency, operates through a complementary mechanism — carriers of both the regulatory risk haplotype (rs162040 C allele) and the coding impairment (rs1801394 G allele) may face compounded MTRR dysfunction. Upstream impairments in MTHFR (rs1801133 C677T, reduced methylfolate production) or MTR (rs1805087 A2756G, reduced methionine synthase activity) further amplify the downstream consequences of impaired MTRR B12 recycling.