Novel variants and haplotypes of human flavin-containing monooxygenase 3 gene associated with Japanese subjects suffering from trimethylaminuria

1. Flavin-containing monooxygenase 3 (FMO3) in humans is polymorphic in several ethnic groups, including Caucasians, Africans and Asians. Some FMO3 variants are associated with a disorder trimethylaminuria.2. In the current study, we used the results from urinary phenotyping assays to identify 63 subjects with <85% FMO3 metabolic capacity with respect to trimethylamine N-oxidation among 787 Japanese volunteers with self-reported trimethylaminuria. The 63 subjects with reduced FMO3 activity were screened and investigated in detail to identify novel FMO3 variants.3. Homozygous or heterozygous individuals for new single nucleotide substitution variants/haplotypes p.(Pro282Leu), p.[(Glu158Lys; Glu308Gly; Thr329Ala)], p.[(Glu158Lys; Glu308Gly; Asp429Gly)], p.[(Val257Met; Leu473Pro)], p.[(Glu158Lys; Glu308Gly; Ile441Thr)], and p.[(Arg205Cys; Gly503Arg)] were identified in six proband subjects and their family members after pedigree analyses.4. These variant FMO3 proteins recombinantly expressed in Escherichia coli membranes exhibited decreased N-oxygenation activities toward trimethylamine (V-max/K-m < 40% that of the wild-type).5. Although the allele frequencies of the six new variants and/or haplotypes were low, the present results indicated that individuals homozygous or heterozygous for any of these novel missense FMO3 variants or known nonsense mutations such as p.(Cys197Ter) or p.(Arg205Cys) highly found in this self-reported Japanese trimethylaminuria cohort may have reduced FMO3 activity with respect to the N-oxygenation of trimethylamine.