Does the BHMT rs3733890 (R239Q) variant alter BHMT activity and raise homocysteine in some populations?

Clinical and population evidence

Clinical genetics and epidemiological studies show that the BHMT rs3733890 variant acts as a significant modulator of homocysteine levels, particularly under specific physiological stressors or therapeutic interventions.

• In a clinical trial involving a Chinese Han population, carriers of the mutant rs3733890 genotype demonstrated a significantly higher risk of folic acid treatment failure (defined as persistent hyperhomocysteinemia, with homocysteine levels remaining above 15 μmol/L despite standard supplementation) compared to wild-type individuals.
• Population-level studies indicate that the variant's impact on homocysteine accumulation is amplified when interacting with lifestyle and environmental factors. For example, the risk of elevated homocysteine is more pronounced in carriers who smoke or have high alcohol consumption, which further taxes the one-carbon metabolic network.

Mechanistic explanations

• Enzyme kinetics: The rs3733890 variant is a non-synonymous, missense single-nucleotide polymorphism (G→A exchange resulting in an arginine-to-glutamine substitution at codon 239, R239Q). This structural change alters the physical and kinetic properties of the betaine-homocysteine S-methyltransferase enzyme, acting as a modest-effect modifier that reduces overall metabolic efficiency.
• Alternative remethylation pathway: Under standard physiological conditions, homocysteine is cleared via two main remethylation pathways: the folate/B12-dependent methionine synthase pathway and the folate-independent BHMT pathway (which utilizes betaine as a methyl donor). When the BHMT enzyme's activity is altered or reduced due to the rs3733890 variant, the parallel conversion of homocysteine to methionine is compromised, contributing to higher circulating levels of homocysteine.

Bottom line

The BHMT rs3733890 variant (R239Q) alters BHMT enzymatic activity, compromising the folate-independent remethylation of homocysteine to methionine. While it acts as a modest-effect modifier rather than a severe loss-of-function mutation, it significantly increases the risk of elevated homocysteine levels and leads to a higher rate of folate-treatment failure in specific populations.