Do MTRR rs1801394 and TCN2 rs1801198 variants affect B12 status and homocysteine?

Genetic variations in the MTRR and TCN2 genes can significantly disrupt vitamin B12 homeostasis and homocysteine metabolism, particularly influencing cellular transport and recycling efficiency.

Mechanistic explanations of cellular impairment

• MTRR rs1801394 (A66G): This polymorphism alters the FMN-binding domain of methionine synthase reductase, reducing its catalytic activity and affinity for methionine synthase (MTR). Homozygous G/G individuals exhibit up to a three-fold reduction in enzymatic recycling, trapping cobalamin in an inactive oxidized state and preventing the reactivation of MTR.
• TCN2 rs1801198 (776C>G, Pro259Arg): This variant alters the transcobalamin II protein structure, decreasing its expression and binding affinity for B12. This significantly reduces levels of plasma holotranscobalamin (holo-TC)—the active fraction of B12—directly hindering receptor-mediated cellular internalization and transport of the vitamin to tissues.

Clinical and biomarker status

• Altered B12 profiles: While total serum B12 can sometimes remain within normal ranges for TCN2 G-allele carriers, their active holo-TC levels are consistently reduced, indicating a functional tissue-level deficiency. Similarly, the MTRR variant is clinically associated with lower total plasma B12 levels.
• Elevated homocysteine: Because both variants restrict intracellular cobalamin availability, MTR cannot efficiently remethylate homocysteine into methionine. This leads to elevated plasma homocysteine levels, a phenotype that is heavily pronounced in individuals with marginal B12 intake or concurrent folate-pathway mutations such as MTHFR C677T.

Bottom line

• Key takeaway: The MTRR rs1801394 and TCN2 rs1801198 variants impair B12 recycling and cellular transport, respectively. These defects lower active B12 (holo-TC) levels and elevate plasma homocysteine, making the clinical evaluation of active B12 status and adequate B-vitamin intake highly critical.