Can MTHFD1 rs2236225 AA reduce one-carbon transfer and lead to higher homocysteine and macrocytosis even with normal serum folate?

The MTHFD1 rs2236225 (G1958A) polymorphism is a missense mutation in the gene encoding the cytosolic methylenetetrahydrofolate dehydrogenase 1 enzyme, a trifunctional enzyme crucial for one-carbon transfer. This polymorphism results in an arginine-to-glutamine substitution (R653Q) that directly compromises the enzyme's structural stability and catalytic capacity.

Mechanistic explanations

• Protein Destabilization: Structural modeling demonstrates that the rs2236225 AA genotype significantly destabilizes the MTHFD1 enzyme. This structural vulnerability alters enzyme kinetics and reduces the overall folate-dependent one-carbon transfer capacity (metabolic flux) within the cell.
• Intracellular vs. Extracellular Folate: Because this genetic variant impairs intracellular enzymatic processing rather than cellular uptake, the reduction in metabolic capacity can occur even when extracellular markers, such as serum folate, appear normal.
• Downstream Methylation Dynamics: While overall metabolic capacity is strained, studies show that carriers of the A allele (including AA homozygotes) exhibit higher global DNA methylation capacity in blood cells than those with the GG genotype, indicating a complex rerouting of cellular one-carbon intermediates under genetic stress.

Clinical and effectiveness evidence

• Homocysteine Elevation: Reduced one-carbon transfer capacity directly limits the remethylation of homocysteine back to methionine. When folate-dependent pathway capacity is compromised (especially in states of marginal folate intake), systemic homocysteine levels systematically rise.
• Macrocytosis Development: The loss of one-carbon transfer efficiency impairs the synthesis of thymidylate and purines. This reduction in nucleotide availability disrupts cellular DNA replication, which is the classical biochemical trigger for megaloblastic macrocytic anemia and an elevated mean corpuscular volume (MCV).

Bottom line

• The MTHFD1 rs2236225 AA genotype destabilizes the MTHFD1 enzyme and reduces cellular one-carbon transfer capacity, which can biochemically elevate homocysteine and impair DNA synthesis. This impairment can theoretically contribute to macrocytosis even in the presence of normal serum folate levels, although direct clinical isolation of this genetic variant as an independent cause of elevated MCV remains rare in the literature.