Deleterious mutations in the flavin-containing monooxygenase 3 (FMO3) gene causing trimethylaminuria

The primary genetic form of trimethylaminuria (TMAU) is caused by inherited defects in the flavin-containing monooxygenase 3 (FMO3) gene. Defective FM03 has a decreased ability to catalyze the N-oxygenation of the dietary-derived malodourous amine, trimethylamine. We report two novel deleterious mutations identified in two unrelated individuals affected by the disorder. Sequence analysis of the FM03 coding exons amplified from genomic DNA revealed that the mutation from individual 1 was heterozygous for a G>A missense mutation in exon 2 of the FM03 gene. The mutation changed a GAG encoding Glu at codon 32 to AAG encoding Lys. Wild-type and mutant E32K FM03 were expressed in Escherichia coli as maltose binding-fusion proteins and assayed for their ability to catalyze oxygenation of various FM03 substrates. The results showed that the E32K mutation abrogated the catalytic activity of the enzyme. Individual 2 was identified as heterozygous for the Pi 53L mutation. In addition, individual 2 was also heterozygous for a novel single nucleotide deletion of A191 in exon 3 at codon 64. The deletion resulted in a frame shift and caused premature termination of the FM03 gene immediately after codon 65. Family pedigree analysis revealed that the P153L and the deletion mutation were carried on different alleles for this individual. Therefore, both alleles of the FM03 gene for individual 2 were affected by mutations abolishing the catalytic activity of the enzyme, explaining the severe TMAU condition. The two deleterious mutations reported herein were rare mutations with estimated allelic frequencies of less than 1%. (C) 2003 Lippincott Williams Wilkins.