Flavin monooxygenase 3 (FMO3) polymorphism in a white population:: Allele frequencies, mutation linkage, and functional effects on clozapine and caffeine metabolism

Aim: The flavin-containing monooxygenase 3 (FMO3) has been shown to be genetically polymorphic. In vitro, the enzyme contributes to the N-oxidation of clozapine, caffeine, and several other drugs. We therefore wanted to analyze population frequencies and allelic linkage of FM03 mutations and their functional effect on the metabolism of clozapine and caffeine. Methods: This study included 204 patients treated with clozapine for schizophrenia and 192 healthy volunteers receiving a 100 mg oral test dose of caffeine. FM03 polymorphisms M66I, P153L, E158K, V257M, E305X, E(308)G, and R492W were analyzed by polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) analysis. Ratios of serum clozapine N-oxide over clozapine and of urine theobromine vers-us paraxanthine were used as in vivo indicators of FM03 activity, Results: From the known FM03 amino acid variants, only K-158 (frequency 0.426), G(308) (0.225), and M-257 (0.069) were found; mutations I-66, L-153, X-305, and W-492 were not found in the 396 subjects. Linkage analysis revealed seven different alleles; the most frequent of these was the wild-type E-158-V-257-E-308 (0.534), followed by K-158-V-257-G(308) (0.199) and K-158-V-257-E-308 (0.192). Subjects with these frequent variants of FMO3, however, did not differ in clozapine N-oxidation or caffeine oxidation compared with the wild-type. Conclusion: There are several genetic polymorphisms for the FM03 enzyme, The effects on the metabolism of caffeine or clozapine could not be shown, indicating that the mutations have only minor functional effects or that substrate affinity is too low to be clinically relevant.