Loss-of-function mutations of flavin-containing monooxygenase 3 (FMO3), the enzyme responsible for trimethylamine N-oxygenation, cause the inherited disorder trimethylaminuria, or fish odor syndrome. The aim of this study was to further investigate the inter-individual variations of FMO3 activity in a Japanese cohort that we had studied previously. The subjects were 640 Japanese volunteers with self-reported trimethylaminuria; genomic DNA was sequenced in those that had 10-70% FMO3 metabolic capacity in urine tests. A heterozygote for the novel single nucleotide substitution p.Ile441Thr (proband 1) and a heterozygote for the novel single nucleotide substitution p.Ser195Leu (proband 2) were identified. The biological parents of probands 1 and 2 were heterozygous and had >90% trimethylamine N-oxygenation metabolic capacity. In addition, single nucleotide substitutions p.Val58Ile, p.Pro70Leu, and p.Gly421Val in FMO3 were found in probands 3-7. In the course of DNA sequencing, another FMO3 variant, p.Thr488Ala, was found in two unrelated heterozygous subjects. Variant FMO3 proteins recombinantly expressed in Escherichia coli membranes exhibited decreased activity toward typical FMO3 substrates. Although the allele frequencies of these six novel variants were low (<1%), the present results suggest that individuals homozygous or heterozygous for any of the six novel missense FMO3 variants or known nonsense mutations such as p.Cys197stop or p.Arg500stop may possess abnormal trimethylamine N-oxygenation.
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