MTHFR C677T

The MTHFR enzyme catalyzes the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate (5-MTHF), the circulating form of folate that donates the methyl group needed to re-methylate homocysteine back into methionine. The C677T variant, described by Frosst and colleagues in 1995 (Nature Genetics), swaps one amino acid (alanine for valine at codon 222), making the enzyme thermolabile and reducing its catalytic activity.

The TT genotype affects roughly 10-12% of Europeans, with marked geographic variation. Under low-folate conditions, TT carriers show significantly higher plasma homocysteine, a factor linked to cardiovascular risk, stroke and accelerated brain atrophy. Clinical relevance, however, is modest and depends on nutritional status: with adequate folate, the between-genotype difference in homocysteine narrows.

Practical longevity implication: TT carriers may benefit from active B-vitamin forms — methylfolate (5-MTHF) instead of synthetic folic acid, and methylcobalamin — alongside riboflavin, a direct cofactor of the MTHFR enzyme. An important note of scientific honesty: the American College of Medical Genetics (ACMG) advises against routine MTHFR genotype screening, because the actionable biomarker is homocysteine itself, not the genotype.