Alternative processing events in human FMO genes.

In humans, flavin-containing monooxygenase (FMO) functional diversity is determined by the expression of five FMO genes, named FMO1 to FMO5, and their variants. In this study, we systematically analyzed transcripts of FMO1 to FMO5 in different human tissues by reverse-transcription-polymerase chain reaction and identified a large number of splice variants. Exon skipping was the major splicing event observed. Normally spliced transcripts were generally the prominent transcript detected. For FMO1, FMO2, and FMO3, two to three different splice variants were identified, and their corresponding expression was always low in the tissues examined. For FMO5 and particularly for FMO4, more complex alternative splice patterns were observed, with five and seven splice variants detected, respectively. Most identified FMO splice variants either caused a frame-shift or lacked essential functional sites. The corresponding transcripts were therefore incapable of encoding a functional enzyme and were not analyzed further in this study. However, a common in-frame exon 3- (exon 4 for FMO4) deleted variant, leading to the deletion of 63 amino acids, was identified for FMO1, FMO3, FMO4, and FMO5. To examine the functional importance of exon 3-deletion, FMO1, FMO3, FMO4 and the corresponding exon-deleted proteins were expressed as fusion proteins. Activity studies were done with two selective functional FMO substrates, methimazole, and 10-(N,N-dimethylaminopentyl)-2-(trifluoromethyl)phenothiazine and exon 3- (exon 4 for FMO4) deleted FMOs were not able to catalyze the S- and N-oxygenation of these substrates, respectively. It is not clear whether these FMO splice variants can oxygenate other substrates, including those that remain to be discovered.