Does the BHMT rs3733890 AA genotype affect the betaine-dependent backup pathway for homocysteine remethylation?

The BHMT rs3733890 (c.742G>A, Arg239Gln) polymorphism, particularly the AA genotype, plays a critical role in modulating the alternative, betaine-dependent pathway for homocysteine remethylation. This metabolic route acts as a vital backup to the primary folate- and vitamin B12-dependent methionine synthase pathway.

Mechanistic pathways

• Enzymatic alteration: The rs3733890 variant alters the physical structure of the BHMT protein, which changes its catalytic efficiency and substrate affinity for homocysteine.
• Epigenetic regulation: This genetic variant is associated with distinct DNA methylation patterns of BHMT and its isoform BHMT_1. This epigenetic variation mediates approximately one-third of the genotype's overall effect on folate therapy response.

Clinical implications

• Folate therapy failure: Carriers of this genetic variant exhibit a significantly higher risk of folate therapy failure, indicating that the genetic alteration directly impacts therapeutic efficacy.
• Homocysteine regulation: Because the BHMT enzyme utilizes betaine to convert homocysteine to methionine, functional changes associated with the AA genotype directly affect plasma homocysteine clearance.
• Dietary context: The clinical impact of the AA genotype is highly context-dependent. Vulnerability to hyperhomocysteinemia increases significantly when the primary folate pathway is compromised or when dietary intake of alternative methyl donors, such as choline and betaine, is insufficient.

Bottom line

• The BHMT rs3733890 AA genotype alters the betaine-dependent backup remethylation pathway through structural and epigenetic modifications, increasing the risk of folate therapy failure and elevated homocysteine levels when dietary methyl donor intake is inadequate.