Can elevated homocysteine and MTRR rs1801394 GG impair endothelial function?

The one-carbon metabolism pathway plays a critical role in cardiovascular health by regulating homocysteine levels and maintaining endothelial function.

Genetic mechanisms of remethylation

• The MTRR rs1801394 GG genotype (A66G) causes an Ile22Met substitution in the flavodoxin-binding domain of methionine synthase reductase (MTRR).
• This mutation results in a three-fold reduction in MTRR enzymatic activity, impairing its capacity to regenerate the active methylcobalamin (vitamin B12) cofactor needed to reactivate methionine synthase.
• Cells carrying this homozygous variant exhibit a four-fold increase in the homocysteine-to-methionine ratio, demonstrating significantly less efficient remethylation. This biochemical deficit is highly dependent on nutritional status, becoming most pronounced during vitamin B12 or folate insufficiency.

Endothelial pathophysiology and oxidative stress

• When remethylation is impaired, accumulating homocysteine drives endothelial oxidative stress. This occurs through metal-catalyzed autoxidation, upregulation of endothelial NADPH oxidases (NOX), and induction of mitochondrial and endoplasmic reticulum stress, alongside the inhibition of antioxidant enzymes like glutathione peroxidase.
• This elevated oxidative stress directly impairs nitric oxide (NO) signaling. Excess superoxide quenches available NO to form peroxynitrite, which in turn oxidizes tetrahydrobiopterin (BH4).
• This cofactor depletion uncouples endothelial nitric oxide synthase (eNOS), shifting it from producing protective NO to generating further superoxide. This destructive feedback loop is compounded by protein kinase C (PKC) activation and the accumulation of asymmetric dimethylarginine (ADMA), a competitive inhibitor of eNOS.

Bottom line

• The MTRR rs1801394 GG genotype genetically compromises homocysteine clearance—particularly under conditions of B-vitamin insufficiency—leading to elevated homocysteine levels that directly drive endothelial dysfunction through oxidative stress, eNOS uncoupling, and impaired nitric oxide bioavailability.