Does the MTRR rs1801394 AG genotype reduce methionine synthase reductase efficiency?

The MTRR gene encodes methionine synthase reductase, a flavoprotein essential for maintaining the active state of methionine synthase (MTR), which drives the remethylation of homocysteine to methionine.

Mechanistic pathway

• Enzyme binding affinity: The MTRR rs1801394 (A66G) polymorphism causes an Ile22Met amino acid substitution in the flavin mononucleotide (FMN)-binding domain of the enzyme. This structural change physically reduces the binding affinity of MTRR for its target partner, MTR.
• Impaired B12 recycling: Reduced binding affinity slows down the vital re-reduction of oxidized cobalamin (vitamin B12) to its active methylcobalamin state. Without active cobalamin, MTR cannot be efficiently reactivated, which directly impairs the rate of B12-dependent homocysteine remethylation.

Clinical and metabolic implications

• Intermediate efficiency: The heterozygous AG genotype results in intermediate MTRR efficiency compared to the wild-type AA genotype, carrying one copy of this lower-efficiency G allele.
• Increased cofactor dependence: Individuals with the AG genotype typically maintain normal fasting plasma homocysteine levels when nutritional intake is optimal. However, the underlying enzymatic inefficiency increases metabolic reliance on exogenous folate and B12 recycling. Suboptimal levels of these vitamins unmask the genetic deficit, leading to elevated homocysteine levels.

Bottom line

• Bottom line: The MTRR rs1801394 AG genotype reduces methionine synthase reductase efficiency via an Ile22Met substitution that impairs binding to MTR, slowing B12-dependent remethylation and increasing the physiological requirement for adequate folate and vitamin B12 to maintain homocysteine balance.