Does the MTRR rs1801394 GG genotype impair regeneration of active vitamin B12 and raise homocysteine?

The MTRR gene encodes methionine synthase reductase (MSR), a flavoprotein essential for maintaining the structural and functional integrity of the folate and methionine cycles. The rs1801394 polymorphism directly alters this regulatory pathway.

Mechanistic pathway

• Enzyme efficiency: The MTRR rs1801394 G allele (specifically the homozygous GG genotype) involves an adenine-to-guanine transition that causes an isoleucine-to-methionine (Ile22Met) substitution. This structural change moderately decreases the biochemical activity and efficiency of the MSR enzyme compared to the wild-type AA genotype.
• Cofactor recycling: Under normal physiological conditions, MSR is responsible for regenerating the active methylcobalamin (vitamin B12) cofactor. This cofactor is necessary to maintain methionine synthase (MTR) in its active state. The reduced-activity MSR enzyme in GG homozygotes impairs this regeneration process, limiting the availability of active methylcobalamin.

Clinical implications

• Homocysteine accumulation: Because the regeneration of active methylcobalamin is functionally compromised, the remethylation of homocysteine to methionine by MTR is inhibited. This biochemical bottleneck directly causes upstream homocysteine to accumulate in the circulation.
• Nutritional interaction: The tendency toward elevated plasma homocysteine levels is heavily influenced by external factors, becoming particularly pronounced in individuals with suboptimal folate or vitamin B12 intake.

Bottom line

• Key takeaway: The MTRR rs1801394 GG genotype decreases methionine synthase reductase activity through an Ile22Met substitution, which impairs the regeneration of active vitamin B12 and leads to higher homocysteine levels, emphasizing the clinical importance of maintaining robust B-vitamin status.