QUANTITATIVE ANALYSIS OF FMO GENE mRNA LEVELS IN HUMAN TISSUES

Abstract

ABSTRACT:The developmentally and tissue-specific expression of flavin-containing monooxygenase (FMO) enzymes has been previously characterized in a number of animal species, including humans, mice, rats, and rabbits. In this study, we used sensitive real-time reverse transcription-polymerase chain reaction methodology to systematically quantify the steady-state mRNA levels of FMO1, 2, 3, 4, and 5 in human tissues. We examined the developmental regulation of these enzymes in brain tissue. FMO1 was found to be downregulated in human adult brain. The amount of other FMO mRNAs in human brains in different age groups was not significantly different. The study also provided a systematic quantitative comparison of the steady-state mRNA levels of FMO1 to 5 in several major human organs (i.e., liver, lung, kidney, small intestine, and brain).The nature of the quantitative analysis allowed a comprehensive comparison of each FMO mRNA in different tissues as well as among FMO isoforms in the same tissue. A comparison between fetal liver and adult liver showed that FMO1 was the only FMO that was down-regulated; all other FMOs had greater amounts of mRNA in adult liver. FMO5 was the most prominent FMO form detected in fetal liver. The FMO5 mRNA level was nearly as abundant as FMO3 in adult liver. Whereas other FMOs displayed a significant, dominant tissue-specific mRNA profile (i.e., FMO1 in kidney, FMO2 in lung, FMO3 and FMO5 in adult liver), FMO4 mRNA was observed more broadly at relatively comparable levels in liver, kidney, lung, and small intestine.Flavin-containing monooxygenases (FMOs) (EC 1.14.13.8) are a family of NADPH-and FAD-dependent enzymes that catalyze the oxygenation of a wide variety of compounds containing nucleophilic nitrogen, sulfur, and phosphorous heteroatoms (Ziegler, 1988;Cashman, 1995). The enzyme is recognized as a broad spectrum monooxygenase, and functional diversity of this family is determined by the expression of five FMO genes, named FMO1 to FMO5, and their variants. There are also a large number of FMO splice variants detected in human tissue (Lattard et al., 2004). A number of other FMO pseudogenes have been described, but apparently no other functional proteins are expressed Hernandez et al., 2004).The developmentally and tissue-specific expression of FMOs has been characterized in a number of animal species, including humans, mice, rats, and rabbits (Hines et al., 1994). Methods that have been used to characterize FMO expression include RNase protection assays for FMOs in human kidney and liver (Dolphin et al., 1996) and mouse to quantify mRNA, Western blot analysis in human liver to quantify protein level (Koukouritaki et al., 2002), and microsomal activity measurements for human liver FMO (Overby et al., 1997) and rabbit FMOs (Shehin-Johnson et al., 1995). The functional characterization of FMO previously reported has mainly focused on hepatic-and kidney-mediated metabolism, where the majority of the metabolism of drugs and exogenous chemicals occur. Limited animal studies, mostly us...

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