Can TCN2 and FUT2 variants affect vitamin B12 transport and measured B12 status?

Genetic variants and B12 transport

• The TCN2 rs1801198 (776C>G, Pro259Arg) variant alters the secondary structure of transcobalamin II, reducing its B12-loading efficiency. This structural change lowers circulating levels of active holotranscobalamin (holo-TC), directly restricting the delivery and cellular availability of active vitamin B12 to target tissues.
• The FUT2 rs602662 variant (strongly associated with non-secretor status via rs601338) alters mucosal fucosylation and gastric intrinsic factor secretion. This impairment in gastrointestinal absorption reduces the overall pool of circulating vitamin B12, lowering total serum B12 concentrations by approximately 20% to 25%.

Metabolic markers of cellular deficiency

• Elevated methylmalonic acid (MMA) serves as a highly sensitive, direct marker of intracellular B12 deficiency. When cellular B12 transport is impaired, the B12-dependent mitochondrial enzyme methylmalonyl-CoA mutase loses its required cofactor, resulting in the systemic accumulation of MMA.
• Elevated homocysteine levels occur when cellular B12 availability is compromised, as B12 is an essential cofactor for methionine synthase. Impairment of this remethylation pathway prevents the conversion of homocysteine to methionine, raising circulating homocysteine levels.

Mechanistic distinctions between TCN2 and FUT2

[FUT2 Variant (rs602662)] ──> Decreased Mucosal Fucosylation ──> Reduced Gastrointestinal B12 Absorption ──> Lower Total Serum B12
                                                                                                           (Primarily Holo-Haptocorrin)

[TCN2 Variant (rs1801198)] ──> Altered Transcobalamin II Structure ──> Lower Active Holo-TC ──> Reduced Cellular B12 Delivery ──> Elevated MMA & Homocysteine

• TCN2 variants drive functional deficiency: Because TCN2 governs the active transport carrier (holo-TC) into cells, carrying the G allele (especially the GG genotype) directly correlates with functional intracellular deficiency, characterized by elevated MMA and homocysteine.
• FUT2 variants primarily affect inert pools: The reduction in serum B12 associated with FUT2 variants is largely driven by decreases in haptocorrin-bound B12 (holo-haptocorrin), which is biologically inactive for tissue uptake. Consequently, FUT2 variants lower total serum B12 measurements without necessarily causing cellular-level deficiency or elevating MMA and homocysteine.

Bottom line

The claim is biochemically and genetically accurate. TCN2 variants (rs1801198) impair active transport, causing true cellular B12 deficiency that presents with elevated MMA and homocysteine. Conversely, FUT2 variants (rs602662) reduce overall circulating B12 and lower total serum B12 measurements, but because they primarily affect inactive carrier pools, they do not automatically result in cellular or tissue-level deficiency unless accompanied by other dietary or clinical risks.