MTRR rs1801394 GG reduces methionine synthase reductase activity and raises homocysteine.

The MTRR rs1801394 GG polymorphism presents a well-characterized genetic variant that compromises intracellular vitamin B12 processing, driving elevated homocysteine levels despite normal or high serum cobalamin.

Clinical and effectiveness evidence

• Genotype Impact on Homocysteine: Clinical and biochemical studies indicate that individuals homozygous for the MTRR rs1801394 G allele (the GG genotype) have a significantly higher risk of elevated plasma homocysteine levels (hyperhomocysteinemia) compared to those with the wild-type AA genotype.
• The Serum B12 Paradox: In clinical settings, patients with the MTRR GG genotype often present with elevated homocysteine levels even when routine blood tests show high or normal circulating serum vitamin B12 levels. This is because standard assays measure total serum cobalamin, which fails to reflect intracellular processing and utilization.

Mechanistic explanations

• Enzyme Dysfunction: The MTRR gene encodes methionine synthase reductase, which is responsible for keeping methionine synthase active. The rs1801394 (A66G) mutation is located in the flavin mononucleotide (FMN)-binding domain of the enzyme, reducing its overall catalytic activity.
• Impaired Cobalamin Regeneration: To convert homocysteine to methionine, methionine synthase requires the methylcobalamin cofactor. During this reaction, the cobalamin cofactor occasionally oxidizes to an inactive cob(I)alamin form. Methionine synthase reductase (MTRR) is responsible for regenerating active methylcobalamin.
• Functional B12 Deficiency: When MTRR activity is compromised, the reactivation of cob(I)alamin to methylcobalamin is impaired. This leads to a cellular-level, functional B12 deficiency where methionine synthase remains inactive, halting the remethylation pathway and causing homocysteine to accumulate in tissues and the bloodstream.

Bottom line

The MTRR rs1801394 GG genotype reduces the activity of methionine synthase reductase, impairing intracellular methylcobalamin regeneration. This results in a functional cobalamin deficiency that leads to elevated homocysteine levels, even in individuals with high or normal serum vitamin B12 levels.