Can MTHFD1 rs2236225 and MTR rs1805087 variants reduce one‑carbon folate cycle throughput and raise homocysteine and MCV/RDW?

One-carbon metabolism is a foundational biochemical process that supports DNA synthesis, methylation, and homocysteine regulation. Genetic variants in the MTHFD1 and MTR enzymes can alter the efficiency of this cycle, leading to measurable changes in metabolic and hematologic biomarkers.

Mechanistic basis for reduced cycle throughput

The folate cycle requires high-functioning enzymes to maintain the flow of one-carbon units for cellular processes.

• MTHFD1 (rs2236225): This 1958G>A variant affects the trifunctional C1-THF synthase enzyme. This enzyme converts formate into 10-formyl-THF, the primary entry point for one-carbon units into the cycle. While human flux data is limited, mouse models of MTHFD1 deficiency show significantly reduced formate incorporation, confirming its role as a bottleneck for folate-dependent pathways.
• MTR (rs1805087): The A2756G variant impacts methionine synthase, which executes the final remethylation of homocysteine to methionine. The G allele is associated with a reduction in catalytic efficiency, particularly when folate levels are suboptimal, leading to a mechanistic "backup" in the cycle.

Impact on homocysteine and hematologic indices

When throughput in the folate cycle is impaired, the body’s ability to clear homocysteine and synthesize DNA is compromised.

• Hyperhomocysteinemia: Reduced flux through MTR directly hinders the conversion of homocysteine to methionine. Individuals carrying variants in MTR or upstream enzymes like MTHFD1 often exhibit elevated plasma homocysteine levels. This is especially pronounced when combined with other polymorphisms (e.g., MTHFR C677T) or low intake of B vitamins.
• Macrocytosis (MCV and RDW): The folate cycle provides the methyl groups necessary for thymidylate (dTMP) synthesis. If throughput is reduced, DNA replication in erythroid precursors is delayed, while cytoplasmic growth continues—a state known as asynchronous maturation. This results in megaloblastic changes, including an increased Mean Corpuscular Volume (MCV) and greater variation in red cell size (RDW). This "folate trap" mechanism explains how genetic impairments can mimic the hematologic patterns seen in clinical B12 or folate deficiencies.

Bottom line

The claim is supported and mechanistically plausible. Variants in MTHFD1 and MTR can reduce one-carbon cycle throughput, directly contributing to elevated homocysteine and macrocytic red blood cell patterns (high MCV/RDW) by impairing remethylation and DNA synthesis. These effects are most significant in individuals with concurrent nutrient insufficiencies.