Does the MTRR rs1801394 GG genotype contribute to macrocytosis?

The MTRR rs1801394 GG genotype is a significant genetic factor in one-carbon metabolism, specifically affecting how the body maintains and utilizes vitamin B12. Research supports the claim that this genotype reduces the efficiency of the methionine synthase reductase (MTRR) enzyme, creating a functional strain on B12 recycling that can contribute to macrocytosis (enlarged red blood cells) when combined with other metabolic constraints.

Clinical and enzymatic evidence

The rs1801394 polymorphism (A66G) results in an amino acid substitution (isoleucine to methionine) that alters the enzyme's flavodoxin-like domain.

• Reduced Activity: Biochemical and kinetic studies indicate that the GG genotype results in a 60% to 80% reduction in MTRR enzymatic activity compared to the wild-type AA genotype.
• Metabolic Impact: This reduction is clinically associated with elevated homocysteine levels and an increased risk of developmental and cardiovascular conditions (e.g., neural tube defects with an OR of 1.58).
• Synergistic Risks: The impact of the GG genotype is most pronounced when paired with other constraints, such as the MTHFR C677T variant or low dietary intake of folate and B12.

Mechanistic explanations

MTRR is the primary enzyme responsible for "re-priming" methionine synthase (MTR) by regenerating its essential B12 cofactor.

• B12 Oxidation: During normal metabolism, the active cobalt center of the B12 cofactor (methylcobalamin) occasionally oxidizes into an inactive state [cob(II)alamin].
• Reductive Activation: MTRR uses NADPH to reduce this inactive B12 back to its functional form. The GG genotype slows this process, causing a "functional B12 deficiency" where B12 is present in the cell but trapped in an unusable state.
• Macrocytosis Pathway: When B12 recycling is sluggish, the "methyl trap" occurs—folate becomes sequestered as 5-methyltetrahydrofolate, which cannot be used for DNA synthesis. Impaired DNA synthesis in developing red blood cells is the direct mechanistic cause of megaloblastic changes and macrocytosis (elevated Mean Corpuscular Volume or MCV).

Bottom line

The MTRR rs1801394 GG genotype significantly impairs the recycling of vitamin B12 into its active form. While this may not cause macrocytosis in isolation, it creates a metabolic vulnerability that likely leads to enlarged red blood cells when nutritional B12/folate levels are borderline or other genetic variants are present.