Does the MTRR rs1801394 G allele raise MCV by impairing B12 recycling?

The MTRR rs1801394 polymorphism is a well-characterized genetic variant that directly influences the efficiency of the folate and methionine cycles. By altering the structure of the methionine synthase reductase enzyme, this variant impacts the cellular utilization of vitamin B12, which can have cascading effects on red blood cell development.

Mechanistic explanations

• Enzyme Dysfunction: The rs1801394 (A66G) polymorphism causes an isoleucine-to-methionine substitution (Ile22Met) in the flavodoxin-like domain of the MTRR enzyme. Bioinformatics tools (PolyPhen-2, SIFT) predict this change is damaging to protein stability.
• B12 Recycling: MTRR is responsible for the "reductive activation" of methionine synthase (MTR). During normal metabolism, the cobalamin (B12) cofactor in MTR occasionally becomes oxidized to an inactive cob(II)alamin state. MTRR transfers electrons to regenerate active methylcob(III)alamin. The G allele impairs this electron transfer, leading to a reduced rate of B12 recycling.
• Megaloblastic Erythropoiesis: Impaired B12 recycling reduces the availability of methylcobalamin required for the conversion of homocysteine to methionine. This disruption traps folate in the 5-methyltetrahydrofolate form (the "folate trap"), preventing the synthesis of thymidine for DNA. In rapidly dividing erythroid precursors, this leads to nuclear-cytoplasmic dyssynchrony—where the cell grows but cannot divide its DNA—resulting in the large, immature cells characteristic of megaloblastic erythropoiesis.

Clinical and hematological evidence

• Metabolic Markers: The GG (Met/Met) genotype is consistently associated with markers of suboptimal one-carbon metabolism, including elevated plasma homocysteine levels (p < 0.05 in several cohorts) and increased risks for neural tube defects and congenital heart disease (OR = 1.58).
• MCV and Anemia: While direct epidemiological studies specifically linking the rs1801394 G allele to Mean Corpuscular Volume (MCV) are less common than those for other variants (like MTHFR), the biochemical pathway is established. Animal models of MTRR deficiency demonstrate macrocytic anemia, and human studies indicate that the GG variant significantly increases the risk of functional folate deficiency, a primary driver of elevated MCV (>100 fL).

Bottom line

The MTRR rs1801394 G allele reduces the efficiency of vitamin B12 recycling by impairing electron transfer within the MTRR enzyme. This biochemical defect is a mechanistically supported cause of megaloblastic changes in red blood cells, which can contribute to elevated Mean Corpuscular Volume (MCV).