Flavin-containing monooxygenase 3 (FMO3): genetic variants and their consequences for drug metabolism and disease

The review focuses on genetic variants of human flavin-containing monooxygenase 3 (FMO3) and their impact on enzyme activity, drug metabolism and disease. The majority of FMO-mediated metabolism in adult human liver is catalyzed by FMO3. Some drugs are metabolized in human liver predominantly by FMO3, but most drug substrates of FMO3 are metabolized also by other enzymes, particularly cytochromes P-450, and the FMO3-catalyzed reaction is not the major route of metabolism. Rare variants that severely affect production or activity of FMO3 cause the disorder trimethylaminuria and impair metabolism of drug substrates of FMO3. More common variants, particularly p.[(Glu158Lys);(Glu308Gly)], can moderately affect activity of FMO3 in vitro and reduce metabolism of drug substrates in vivo, in some cases increasing drug efficacy or toxicity. Common variants of FMO3 have been associated with a number of disorders, but additional studies are needed to confirm or refute such associations. Elevated plasma concentrations of trimethylamine N-oxide, a product of an FMO3-catalyzed reaction, have been implicated in certain diseases, particularly cardiovascular disease. However, the evidence is often contradictory and additional work is required to establish whether trimethylamine N-oxide is a cause, effect or biomarker of the disease. Genetic variants of other FMOs are also briefly discussed.