Can MTHFD1 rs2236225 AA reduce red-cell DNA synthesis and raise MCV and RDW despite optimal serum folate?

The MTHFD1 rs2236225 AA genotype represents a genetic variation in cytosolic one-carbon metabolism that directly impacts erythropoiesis and standard red blood cell markers.

Mechanistic pathways

• Enzyme destabilization: The rs2236225 AA (R653Q) variant compromises the stability of the 10-formyl-THF synthetase domain of the trifunctional MTHFD1 enzyme. This variant exhibits increased sensitivity to TRIM21-mediated ubiquitination and subsequent proteasomal degradation.
• Impaired nucleotide synthesis: The resulting reduction in enzyme stability limits the cytosolic flux of serine- and formate-derived one-carbon units. This bottleneck decreases the availability of 10-formyl-THF and methylene-THF, directly impairing de novo purine and thymidylate (dTMP) synthesis.
• Erythroid DNA replication failure: Because rapidly dividing erythroid progenitors depend on a continuous supply of these nucleotide precursors, this metabolic bottleneck compromises DNA replication during red-cell development.

Clinical implications

• Macrocytic indicators: Impaired DNA synthesis during erythropoiesis prevents normal cell division, leading to the formation of larger, irregular red blood cells. The AA genotype is clinically associated with macrocytosis (above-optimal Mean Corpuscular Volume, MCV) and anisocytosis (above-optimal Red Cell Distribution Width, RDW), particularly under metabolic stress.
• Normal serum folate paradox: Because the genetic defect impairs intracellular folate processing and stability rather than systemic absorption, these elevated hematologic markers can persist despite optimal serum folate levels. Standard serum tests may mask a localized, functional folate deficiency in the bone marrow.

Bottom line

• The MTHFD1 rs2236225 AA genotype destabilizes key enzymatic domains required for cytosolic one-carbon flux. This cellular bottleneck impairs red-cell DNA synthesis, explaining why individuals can exhibit elevated MCV and RDW indicative of functional folate deficiency even when circulating serum folate levels are completely optimal.