Ethnic differences in human flavin-containing monooxygenase 2 (FMO2) polymorphisms: detection of expressed protein in African-Americans

Identification of novel variants of the flavin-containing monooxygenase gene family in African Americans

Sequence polymorphisms in enzymes involved in drug metabolism have been widely implicated in the differences observed in the sensitivity to various xenobiotics. The flavin-containing monooxygenase (FMO) gene family in humans catalyzes the monooxygenation of numerous N-, P- and S-containing drugs, pesticides, and environmental toxicants. Six genes (FMO1-6) have been identified so far, but the major alleles of FMO2 and FMO6 encode nonfunctional proteins due to a nonsense mutation and splice-site abnormalities, respectively. Data on structural variants exist for human FMO2 and 3, whereas very little is known about the other FMO genes. FMO1-6 were scanned in 50 individuals of African-American descent using the method, detection of virtually all mutations-single-strand conformational polymorphism. A total of 49 sequence variants were identified in a total 1.35 megabases of scanned sequence, of which 29 were variants affecting protein structure or expression. Some of these are expected to affect the activity of the protein, including a nonsense mutation in FMO1 (R502X) and missense mutations in FMO1 (I303T), FMO4 (E339Q), and FMO5 (P457L) that occur in highly conserved amino acids. Additional deleterious substitutions in FMO2 (del337G) and FMO6 (Q105X) were also identified. Multiple structural variants in the FMO gene family were observed in this African-American sample. Some of the substitutions identified in this study might be useful markers in future association studies assessing sensitivity to environmental toxicants and common disease.

Genotyping and site-directed mutagenesis of a cytochrome P450 meander Pro-X-Arg motif critical to CYP4B1 catalysis

CYP4B1 isoforms from rodents and other common laboratory animals are involved in the bioactivation of a range of protoxins, including 2-aminofluorene, 4-ipomeanol, and valproic acid. However, an earlier study provided evidence for a human allele encoding a nonfunctional CYP4B1 enzyme due to a Pro427Ser transversion in the meander region of the protein. In the present study, the CYP4B1 gene from several racial groups, Caucasians, African-Americans, and Hispanics, and from six nonhuman primate species was genotyped using a PCR-Hinf1 restriction enzyme fragment length polymorphism assay or by direct sequencing. All human populations examined were found to possess only the Ser allele at codon 427 ((1279)TCT) and all of the nonhuman primate species possessed only the Pro (CCT) allele. Therefore, an inactivating (1279)C-->T mutation in the human CYP4B1 gene likely arose following divergence of the Homo and Pan clades. Amino acid sequence alignments revealed further that this key Pro residue is located two amino acid residues N-terminal to the distal Arg of a Glu-Arg-Arg triad thought to participate in heme binding and/or redox partner interactions. Mutation of the corresponding Arg424 residue in rabbit CYP4B1 to Leu, but not His, resulted in a loss of lauric acid hydroxylase activity and ability to generate a reduced-CO binding spectrum. These data provide additional evidence for the importance of this meander region Pro-X-Arg motif in CYP4B1 heme binding and catalytic function.

Human kidney flavin-containing monooxygenases and their potential roles in cysteine s-conjugate metabolism and nephrotoxicity

The potential roles of human hepatic and renal flavin-containing monooxygenases (FMOs) in the metabolism of the cysteine S-conjugates S-allyl cysteine (SAC) and S-(1,2-dichlorovinyl)-L-cysteine (DCVC) were investigated. Incubations of human cDNA-expressed FMO1, FMO3, FMO4, and FMO5 with SAC resulted in detection of SAC sulfoxide, with FMO3 exhibiting approximately 3-, 4-, and 10-fold higher activity than FMO1, FMO4, and FMO5, respectively. DCVC sulfoxide formation was only detected with FMO3 and was 59-fold lower than SAC sulfoxide formation. Incubations of human liver microsomes with SAC or DCVC resulted in detection of the corresponding sulfoxides and provided evidence for the involvement of FMO3. Incubations of SAC or DCVC with human kidney microsomes, however, led only to the detection of SAC sulfoxide. Immunoblots with monospecific antibodies to FMO1, FMO3, and FMO5 in kidney microsomes from 26 humans showed that the average expression levels for FMO1, FMO3, and FMO5 were 5.8 +/- 2.3, 0.5 +/- 0.4, and 2.4 +/- 1.4 pmol/mg (means +/- S.D.), respectively. Interestingly, African-American kidney samples (n = 8) exhibited significantly higher FMO1 levels than Caucasian samples (n = 17), whereas no difference in expression level between males and females was observed with any of the examined FMO isoforms. Collectively, the results provide evidence for the expression of three FMO isoforms in the human kidney and show that the contribution of renal FMOs in cysteine S-conjugate metabolism is likely to vary depending upon the cysteine S-conjugate and the relative expression levels of the active FMOs.

Interindividual differences of human flavin-containing monooxygenase 3: genetic polymorphisms and functional variation

The human flavin-containing monooxygenase (form 3) (FMO3) participates in the oxygenation of nucleophilic heteroatom-containing drugs, xenobiotics, and endogenous materials. Currently, six forms of the FMO gene are known, but it is FMO3 that is the major form in adult human liver that is likely responsible for the majority of FMO-mediated metabolism. The substrate structural feature requirements for human FMO3 is beginning to become known to a greater extent and a few chemicals extensively metabolized by FMO3 have been reported. Expression of FMO3 is species- and tissue-specific, but unlike human cytochrome p450, mammalian FMO3 does not appear to be inducible. Interindividual variation in FMO3-dependent metabolism of drugs, chemicals, and endogenous material is therefore more likely due to genetic effects and not environmental ones. Examples of such interindividual variation come from the study of very rare mutations of the human FMO3 gene that have been associated with deficient N-oxygenation of dietary trimethylamine. Defective trimethylamine N-oxygenation causes trimethylaminuria or "fish-like odor syndrome". Information on human FMO3 mutations from individuals suffering from the condition of trimethylaminuria has provided knowledge about the underlying molecular mechanism(s) for trimethylaminuria. A number of common variants of human FMO3 have been reported. Diversification of the FMO3 gene may have led to selective advantages and new functions. As more examples of human FMO3-mediated metabolism of drugs or new chemical entities are discovered in the future, it is possible that FMO3 allelic variation may be shown to contribute to interindividual and interethnic variability of FMO-mediated metabolism. Human FMO3 may be another example of an environmental gene that participates in a protective mechanism to help humans ward off potentially toxic exposure of chemicals.

Biomonitoring of arylamines and nitroarenes

Arylamines and nitroarenes are very important intermediates in the industrial manufacture of dyes, pesticides and plastics, and are significant environmental pollutants. The metabolic steps of N-oxidation and nitroreduction to yield N-hydroxyarylamines are crucial for the toxic properties of arylamines and nitroarenes. Nitroarenes are reduced by microorganisms in the gut or by nitroreductases and aldehyde dehydrogenase in hepatocytes to nitrosoarenes and N-hydroxyarylamines. N-Hydroxyarylamines can be further metabolized to N-sulphonyloxyarylamines, N-acetoxyarylamines or N-hydroxyarylamine N-glucuronide. These highly reactive intermediates are responsible for the genotoxic and cytotoxic effects of this class of compounds. N-Hydroxyarylamines can form adducts with DNA, tissue proteins, and the blood proteins albumin and haemoglobin in a dose-dependent manner. DNA and protein adducts have been used to biomonitor humans exposed to such compounds. All these steps are dependent on enzymes, which are present in polymorphic forms. This article reviews the metabolism of arylamines and nitroarenes and the biomonitoring studies performed in animals and humans exposed to these substances.

Genetic polymorphisms of flavin-containing monooxygenase (FMO)

Mammalian flavin-containing monooxygenase (FMO) exists as six gene families and metabolizes a plethora of drugs and xenobiotics. The major FMO in adult human liver, FMO3, is responsible for trimethylamine (TMA) N-oxygenation. A number of FMO3 mutant alleles have been described and associated with a disease termed trimethylaminuria (TMAU). The TMAU patient excretes large amounts of TMA in urine and sweat. A more recent ethnically related polymorphism in expression of the major FMO in lung, FMO2, has been described. All Caucasians and Asians genotyped to date are homozygous for a CAG --> TAG amber mutation resulting in a premature stop codon and a nonfunctional protein truncated at AA 472 (wildtype FMO2 is 535 AA). This allele has been designated hFMO2*2A. Twenty-six percent of individuals of African descent and 5% of Hispanics genotyped to date carry at least one allele coding for full-length FMO2 (hFMO2*1 allele). Preliminary evidence indicates that FMO2.1 is very active toward the S-oxygenation of low MW thioureas, including the lung toxicant ethylene thiourea. Polymorphic expression of functional FMO2 in the individuals of African and Hispanic descent may markedly influence drug metabolism and/or xenobiotic toxicity in the lung.

Sequencing, expression, and characterization of cDNA expressed flavin-containing monooxygenase 2 from mouse

The cDNA clone of mouse flavin-containing monooxygenase 2 (FMO2) was obtained as an expressed sequence tag (EST) isolated from a female mouse kidney cDNA library from the I.M.A.G.E. consortium (I.M.A.G.E. CloneID 1432164). Complete sequencing of the EST derived a nucleotide sequence for mouse FMO2, which contains 112 bases of 5' flanking region, 1607 bases of coding region, and 309 bases of 3' flanking region. This FMO2 sequence encodes a protein of 535 amino acids including two putative pyrophosphate binding sequences (GxGxxG/A) beginning at positions 9 and 191. Additionally, this mouse FMO protein sequence shows 87 and 86% homology to rabbit and human FMO2 respectively. The mouse FMO2 sequence was subcloned into the expression vector pJL-2, a derivative of pKK233-2 and used to transform XL1-Blue Escherichia coli. FMO activity in particulate fractions isolated from isopropyl-beta-D-thiogalactopyanoside (IPTG) induced cells was heat stable (45 degrees C for 5 min) and demonstrated optimal activity at a relatively high pH of 10.5. The expressed FMO2 enzyme showed catalytic activity towards the FMO substrate methimazole and further analysis of E. coli fractions utilizing NADPH oxidation demonstrated that the mouse FMO2 enzyme also exhibits catalytic activity towards thiourea, trimethylamine, and the insecticide phorate.

Human flavin-containing monooxygenase (form 3): polymorphisms and variations in chemical metabolism

The human flavin-containing monooxygenases catalyze the oxygenation of nucleophilic heteroatom-containing drugs, xenobiotics and endogenous materials. Evidence for six forms of the FMO gene exist but it is FMO form 3 (FMO3) that is the prominent form in adult human liver that is likely to be associated with the bulk of FMO-mediated metabolism. An understanding of the substrate specificity of human FMO3 is beginning to emerge and several examples of drugs and chemicals extensively metabolized by FMO3 have been reported. Expression of FMO3 is species- and tissue-specific, but unlike human cytochrome P450 (CYP450), mammalian FMO3 does not appear to be inducible. Interindividual variation in FMO3-dependent metabolism of drugs, chemicals and endogenous materials is therefore more likely to be due to genetic and not environmental effects. Certain mutations of the human FMO3 gene have been associated with abnormal N-oxygenation of trimethylamine. Deficient N-oxygenation of trimethylamine results in a condition called trimethylaminuria. Some treatment strategies for this inborn error of metabolism are discussed. Other common variants of the FMO3 gene including E158K, V257M and E308G have been observed. It is possible that allelic variation of human FMO3 causes abnormal metabolism of chemicals and has clinical implications for human drug metabolism, but this is an understudied area. Human FMO3 allelic variation may eventually be shown to contribute to interindividual and interethnic variability in FMO3-mediated metabolism. Human FMO3 may be another example of an environmental gene that participates in a protective mechanism to help shield humans from potentially toxic exposure to chemicals. Heterogeneity in the relative frequencies of single and multiple site alleles, haplotypes and genotypes of the human FMO3 amongst various ethnic groups suggests population differences.

The FMO2 gene of laboratory rats, as in most humans, encodes a truncated protein

We describe the isolation and characterization of cDNAs for FMO2 from the laboratory rat. In contrast to FMO2 in other animals, each of which contain 535 amino acid residues, analysis of the sequence of the cDNAs and of a section of the corresponding gene revealed that the ORF of the laboratory rat FMO2 encodes a polypeptide of only 432 residues. This truncated protein is due to the presence of a double deletion corresponding to 1263 and 1264 nucleotides of the orthologous FMO2 cDNAs. This double deletion provokes a frame-shift, with the appearance of a premature stop codon in position 1297-1299. By Northern blotting, the probe for FMO2 hybridized a 2.5-kb transcript in lung and kidney samples only. Heterologous expression of the cDNA revealed that the truncated protein was catalytically inactive. By Western blotting, FMO2 was faintly detected at approximately 50 kDa in laboratory rat lung.

Human hepatic flavin-containing monooxygenases 1 (FMO1) and 3 (FMO3) developmental expression

The flavin-containing monooxygenases (FMOs) are important for the metabolism of numerous therapeutics and toxicants. Six mammalian FMO genes (FMO1-6) have been identified, each exhibiting developmental and tissue- and species-specific expression patterns. Previous studies demonstrated that human hepatic FMO1 is restricted to the fetus whereas FMO3 is the major adult isoform. These studies failed to describe temporal expression patterns, the precise timing of the FMO1/FMO3 switch, or potential control mechanisms. To address these questions, FMO1 and FMO3 were quantified in microsomal fractions from 240 human liver samples representing ages from 8 wk gestation to 18 y using Western blotting. FMO1 expression was highest in the embryo (8-15 wk gestation; 7.8 +/- 5.3 pmol/mg protein). Low levels of FMO3 expression also were detectable in the embryo, but not in the fetus. FMO1 suppression occurred within 3 d postpartum in a process tightly coupled to birth, but not gestational age. The onset of FMO3 expression was highly variable, with most individuals failing to express this isoform during the neonatal period. FMO3 was detectable in most individuals by 1-2 y of age and was expressed at intermediate levels until 11 y (12.7 +/- 8.0 pmol/mg protein). These data suggest that birth is necessary, but not sufficient for the onset of FMO3 expression. A gender-independent increase in FMO3 expression was observed from 11 to 18 y of age (26.9 +/- 8.6 pmol/mg protein). Finally, 2- to 20-fold interindividual variation in FMO1 and FMO3 protein levels were observed, depending on the age bracket.

Identification of active flavin-containing monooxygenase isoform 2 in human lung and characterization of expressed protein

Full-length human (hFMO2.1) and monkey (mFMO2) flavin-containing monooxygenase proteins, which share 97% sequence identity, were produced by baculovirus-mediated expression in insect cells and assayed for S-oxygenation under conditions known to affect FMO activity. Both enzymes demonstrated maximal activity at pH 9.5; but hFMO2.1 retained significantly more activity than mFMO2 did at pH 9.0 and higher. hFMO2.1 also retained significantly more activity than mFMO2 did in the presence of magnesium and all detergents tested. Although hFMO2.1 had more residual activity after heating at 45 degrees C than mFMO2, under some conditions, both had less than 10% of control activity, whereas expressed rabbit FMO2 retained over 50% activity. Screening for NADPH-oxygenation by hFMO2.1, indicated that substituted thioureas with a small cross-sectional area (2.4-4.3 A) are good substrates, whereas 1,3-diphenylthiourea (11.2 A) was not oxygenated. We confirmed the presence of hFMO2.1 in lung tissue from a heterozygous individual (hFMO2*1/hFMO2*2A) by Western analysis and confirmed activity by S-oxygenation. These microsomes also demonstrated a heat-associated loss of activity similar to expressed hFMO2.1. The heat sensitivity of hFMO2.1 may partially explain why activity in post mortem human lung samples has previously been unreported. Individuals that have the FMO2*1 allele-encoding full-length hFMO2.1 may exhibit altered drug metabolism in the lung.

Regulation of flavin-containing monooxygenase 1 expression by ying yang 1 and hepatic nuclear factors 1 and 4

The flavin-containing monooxygenases (FMOs) are important for the oxidation of a variety of environmental toxicants, natural products, and therapeutics. Consisting of six family members (FMO1-5), these enzymes exhibit distinct but broad and overlapping substrate specificity and are expressed in a highly tissue- and species-selective manner. Corresponding to previously identified regulatory domains, a YY1 binding site was identified at the major rabbit FMO1 promoter, position -8 to -2, two overlapping HNF1alpha sites, position -132 to -105, and two HNF4alpha sites, position -467 to -454 and -195 to -182. Cotransfection studies with HNF1alpha and HNF4alpha expression vectors demonstrated a major role for each of these factors in enhancing FMO1 promoter activity. In contrast, YY1 was shown by site-directed mutagenesis to be dispensable for basal promoter activity but suppressed the ability of the upstream domains to enhance transcription. Finally, comparisons between rabbit and human FMO1 demonstrated conservation of each of these regulatory elements. With the exception of the most distal HNF4alpha site, each of the orthologous human sequences also was able to compete with rabbit FMO1 cis-elements for specific protein binding. These data are consistent with these same elements being important for regulating human FMO1 developmental- and tissue-specific expression.