Do common TCN2 and FUT2 variants lower vitamin B12 and raise homocysteine?

Genetic variants in the TCN2 and FUT2 genes are well-established determinants of vitamin B12 status and, consequently, homocysteine levels. These associations are driven by the critical roles these genes play in the absorption and transport of B12.

Clinical and effectiveness evidence

Extensive genome-wide association studies (GWAS) and candidate gene analyses have confirmed the impact of specific polymorphisms:

• FUT2 (rs602662): This variant is one of the strongest genetic predictors of serum B12 concentrations. The "A" allele (non-secretor status) is consistently associated with significantly lower plasma B12 levels compared to the "G" allele (p < 10⁻¹⁵ in large cohorts like the Nurses' Health Study).
• TCN2 (rs1801198): The "G" allele (Pro259Arg) of this variant is linked to lower levels of holotranscobalamin, the active form of B12 available for cellular use. This association is particularly pronounced in elderly populations or those with high folate intake, where it may exacerbate markers of deficiency.
• Homocysteine Impact: Because B12 is a required cofactor for the enzyme methionine synthase, which converts homocysteine to methionine, these genetic reductions in B12 availability lead to secondary elevations in plasma homocysteine levels.

Mechanistic explanations

The biological pathways for these associations are distinct:

• Absorption (FUT2): The FUT2 gene dictates "secretor status" by encoding an enzyme that places blood group antigens on mucosal surfaces. Non-secretors (A allele) lack these antigens, which alters the gut microbiota composition and intestinal environment, leading to reduced B12 absorption efficiency.
• Transport (TCN2): The TCN2 gene encodes transcobalamin II, the primary protein that binds and delivers B12 to tissues. The rs1801198 polymorphism decreases the circulating concentration and transport efficiency of this protein, reducing the amount of "active" B12 reaching the cells.

Bottom line

Common variants in TCN2 (rs1801198) and FUT2 (rs602662) are scientifically validated markers for lower vitamin B12 status and higher homocysteine. Individuals carrying these risk alleles may have a higher physiological requirement for B12 to maintain optimal metabolic function.