Trimethylaminuria is caused by mutations of the FMO3 gene in a North American cohort

Spontaneously hypertensive rats exhibit increased liver flavin monooxygenase expression and elevated plasma TMAO levels compared to normotensive and Ang II-dependent hypertensive rats

Background: Flavin monooxygenases (FMOs) are enzymes responsible for the oxidation of a broad spectrum of exogenous and endogenous amines. There is increasing evidence that trimethylamine (TMA), a compound produced by gut bacteria and also recognized as an industrial pollutant, contributes to cardiovascular diseases. FMOs convert TMA into trimethylamine oxide (TMAO), which is an emerging marker of cardiovascular risk. This study hypothesized that blood pressure phenotypes in rats might be associated with variations in the expression of FMOs. Methods: The expression of FMO1, FMO3, and FMO5 was evaluated in the kidneys, liver, lungs, small intestine, and large intestine of normotensive male Wistar-Kyoto rats (WKY) and two distinct hypertensive rat models: spontaneously hypertensive rats (SHRs) and WKY rats with angiotensin II-induced hypertension (WKY-ANG). Plasma concentrations of TMA and TMAO were measured at baseline and after intravenous administration of TMA using liquid chromatography-mass spectrometry (LC-MS). Results: We found that the expression of FMOs in WKY, SHR, and WKY-ANG rats was in the descending order of FMO3 > FMO1 >> FMO5. The highest expression of FMOs was observed in the liver. Notably, SHRs exhibited a significantly elevated expression of FMO3 in the liver compared to WKY and WKY-ANG rats. Additionally, the plasma TMAO/TMA ratio was significantly higher in SHRs than in WKY rats. Conclusion: SHRs demonstrate enhanced expression of FMO3 and a higher plasma TMAO/TMA ratio. The variability in the expression of FMOs and the metabolism of amines might contribute to the hypertensive phenotype observed in SHRs.

Mice, rats, and guinea pigs differ in FMOs expression and tissue concentration of TMAO, a gut bacteria-derived biomarker of cardiovascular and metabolic diseases

INTRODUCTION

Increased plasma trimethylamine oxide (TMAO) is observed in cardiovascular and metabolic diseases, originating from the gut microbiota product, trimethylamine (TMA), via flavin-containing monooxygenases (FMOs)-dependent oxidation. Numerous studies have investigated the association between plasma TMAO and various pathologies, yet limited knowledge exists regarding tissue concentrations of TMAO, TMAO precursors, and interspecies variability.

METHODS

Chromatography coupled with mass spectrometry was employed to evaluate tissue concentrations of TMAO and its precursors in adult male mice, rats, and guinea pigs. FMO mRNA and protein levels were assessed through PCR and Western blot, respectively.

RESULTS

Plasma TMAO levels were similar among the studied species. However, significant differences in tissue concentrations of TMAO were observed between mice, rats, and guinea pigs. The rat renal medulla exhibited the highest TMAO concentration, while the lowest was found in the mouse liver. Mice demonstrated significantly higher plasma TMA concentrations compared to rats and guinea pigs, with the highest TMA concentration found in the mouse renal medulla and the lowest in the rat lungs. FMO5 exhibited the highest expression in mouse liver, while FMO3 was highly expressed in rats. Guinea pigs displayed low expression of FMOs in this tissue.

CONCLUSION

Despite similar plasma TMAO levels, mice, rats, and guinea pigs exhibited significant differences in tissue concentrations of TMA, TMAO, and FMO expression. These interspecies variations should be considered in the design and interpretation of experimental studies. Furthermore, these findings may suggest a diverse importance of the TMAO pathway in the physiology of the evaluated species.

Dietary phenolics and their microbial metabolites are poor inhibitors of trimethylamine oxidation to trimethylamine N-oxide by hepatic flavin monooxygenase 3

High circulating levels of trimethylamine N-oxide (TMAO) have been associated with cardiovascular disease risk. TMAO is formed through a microbiome-host pathway utilizing primarily dietary choline as a substrate. Specific gut microbiota transform choline into trimethylamine (TMA), and, when absorbed, host hepatic flavin-containing monooxygenase 3 (FMO3) oxidizes TMA into TMAO. Chlorogenic acid and its metabolites reduce microbial TMA production in vitro. However, little is known regarding the potential for chlorogenic acid and its bioavailable metabolites to inhibit the last step: hepatic conversion of TMA to TMAO. We developed a screening methodology to study FMO3-catalyzed production of TMAO from TMA. HepG2 cells were unable to oxidize TMA into TMAO due to their lack of FMO3 expression. Although Hepa-1 cells did express FMO3 when pretreated with TMA and NADPH, they lacked enzymatic activity to produce TMAO. Rat hepatic microsomes contained active FMO3. Optimal reaction conditions were: 50 µM TMA, 0.2 mM NADPH, and 33 µL microsomes/mL reaction. Methimazole (a known FMO3 competitive substrate) at 200 µM effectively reduced FMO3-catalyzed conversion of TMA to TMAO. However, bioavailable chlorogenic acid metabolites did not generally inhibit FMO3 at physiological (1 µM) nor supra-physiological (50 µM) doses. Thus, the effects of chlorogenic acid in regulating TMAO levels in vivo are unlikely to occur through direct FMO3 enzyme inhibition. Potential effects on FMO3 expression remain unknown. Intestinal inhibition of TMA production and/or absorption are thus likely their primary mechanisms of action.

Microbiota Effect on Trimethylamine N-Oxide Production: From Cancer to Fitness-A Practical Preventing Recommendation and Therapies

Trimethylamine N-oxide (TMAO) is a microbial metabolite derived from nutrients, such as choline, L-carnitine, ergothioneine and betaine. Recently, it has come under the spotlight for its close interactions with gut microbiota and implications for gastrointestinal cancers, cardiovascular disease, and systemic inflammation. The culprits in the origin of these pathologies may be food sources, in particular, high fat meat, offal, egg yolk, whole dairy products, and fatty fish, but intercalated between these food sources and the production of pro-inflammatory TMAO, the composition of gut microbiota plays an important role in modulating this process. The aim of this review is to explain how the gut microbiota interacts with the conversion of specific compounds into TMA and its oxidation to TMAO. We will first cover the correlation between TMAO and various pathologies such as dysbiosis, then focus on cardiovascular disease, with a particular emphasis on pro-atherogenic factors, and then on systemic inflammation and gastrointestinal cancers. Finally, we will discuss primary prevention and therapies that are or may become possible. Possible treatments include modulation of the gut microbiota species with diets, physical activity and supplements, and administration of drugs, such as metformin and aspirin.

Gut microbial metabolites in Parkinson's disease: Association with lifestyle, disease characteristics, and treatment status

There is growing appreciation of the importance of the intestinal microbiota in Parkinson’s disease (PD), and one potential mechanism by which the intestinal microbiota can communicate with the brain is via bacteria-derived metabolites. In this study, plasma levels of bacterial-derived metabolites including trimethylamine-N-oxide (TMAO), short chain fatty acids (SCFA), the branched chain fatty acid isovalerate, succinate, and lactate were evaluated in PD subjects (treatment naïve and treated) which were compared to (1) population controls, (2) spousal / household controls (similar lifestyle to PD subjects), and (3) subjects with multiple system atrophy (MSA). Analyses revealed an increase in the TMAO pathway in PD subjects which was independent of medication status, disease characteristics, and lifestyle. Lactic acid was decreased in treated PD subjects, succinic acid positively correlated with disease severity, and the ratio of pro-inflammatory TMAO to the putative anti-inflammatory metabolite butyric acid was significantly higher in PD subjects compared to controls indicating a pro-inflammatory shift in the metabolite profile in PD subjects. Finally, acetic and butyric acid were different between PD and MSA subjects indicating that metabolites may differentiate these synucleinopathies. In summary, (1) TMAO is elevated in PD subjects, a phenomenon independent of disease characteristics, treatment status, and lifestyle and (2) metabolites may differentiate PD and MSA subjects. Additional studies to understand the potential of TMAO and other bacterial metabolites to serve as a biomarker or therapeutic targets are warranted.

Dietary Regulation of Gut-Brain Axis in Alzheimer's Disease: Importance of Microbiota Metabolites

Alzheimer's disease (AD) is a neurodegenerative disease that impacts 45 million people worldwide and is ranked as the 6th top cause of death among all adults by the Centers for Disease Control and Prevention. While genetics is an important risk factor for the development of AD, environment and lifestyle are also contributing risk factors. One such environmental factor is diet, which has emerged as a key influencer of AD development/progression as well as cognition. Diets containing large quantities of saturated/trans-fats, refined carbohydrates, limited intake of fiber, and alcohol are associated with cognitive dysfunction while conversely diets low in saturated/trans-fats (i.e., bad fats), high mono/polyunsaturated fats (i.e., good fats), high in fiber and polyphenols are associated with better cognitive function and memory in both humans and animal models. Mechanistically, this could be the direct consequence of dietary components (lipids, vitamins, polyphenols) on the brain, but other mechanisms are also likely to be important. Diet is considered to be the single greatest factor influencing the intestinal microbiome. Diet robustly influences the types and function of micro-organisms (called microbiota) that reside in the gastrointestinal tract. Availability of different types of nutrients (from the diet) will favor or disfavor the abundance and function of certain groups of microbiota. Microbiota are highly metabolically active and produce many metabolites and other factors that can affect the brain including cognition and the development and clinical progression of AD. This review summarizes data to support a model in which microbiota metabolites influence brain function and AD.

A series of simple detection systems for genetic variants of flavin-containing monooxygenase 3 (FMO3) with impaired function in Japanese subjects

Increasing numbers of single-nucleotide substitutions of the human flavin-containing monooxygenase 3 (FMO3) gene are being recorded in mega-databases. Phenotype-gene analyses revealed impaired FMO3 variants associated with the metabolic disorder trimethylaminuria. Here, a series of reliable FMO3 genotyping confirmation methods was assembled and developed for 45 impaired FMO3 variants, mainly found in Japanese populations, using singleplex or duplex polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) methods and singleplex, duplex, or tetraplex allele-specific PCR methods. Nine PCR-RFLP procedures with single restriction enzymes and fourteen duplex PCR-RFLP procedures (for p.Trp41Ter and p.Thr329Ala, p.Met66Val and p.Leu163Pro, p.Pro70Leu and p.Glu308Gly, p.Asn114Ser and p.Ser195Leu, p.Glu158Lys and p.Ile441Thr, p.Cys197Ter and p.Trp388Ter, p.Arg205Cys and p.Val257Met, p.Arg205His and p.Cys397Ser, p.Met211ArgfsTer10 and p.Arg492Trp, p.Arg223Gln and p.Leu473Pro, p.Met260Val and p.Thr488Ala, p.Tyr269His and p.Ala311Pro, p.Ser310Leu and p.Gly376Glu, and p.Gln470Ter and p.Arg500Ter) were newly established along with eight singleplex (for p.Pro153GlnfsTer14, p.Gly191Cys, p.Pro248Thr, p.Ile486Met, and p.Pro496Ser, among others), one duplex (p.Ile199Ser and p.Asp286Tyr), and one tetraplex (p.Ile7Thr, p.Val58Ile, p.Thr201Lys, and p.Gly421Val) allele-specific PCR systems. This series of systems should facilitate the easy detection in a clinical setting of FMO3 variants in Japanese subjects susceptible to low drug clearances or drug reactions possibly caused by impaired FMO3 function.

The genetic and biochemical basis of trimethylaminuria in an Irish cohort

BACKGROUND

Inherited trimethylaminuria (TMAU), a rare genetic disorder of hepatic metabolism of trimethylamine (TMA) causing excessive accumulation of malodorous trimethylamine (TMA), is a socially distressing disorder. Diagnosis is made by biochemical analysis of urine, with the calculation of flavin monooxygenase trimethylamine conversion capacity. Genetic testing, sequencing the entire coding region of the FMO3 gene has been recommended for affected individuals who convert less than 90% of the total TMA load to TMAO.

METHODS

Genetic analysis was undertaken for 13 Irish patients with TMAU of varying phenotypic severity (three severe, six moderate, and four mild).

RESULTS

A genetic diagnosis was made for seven patients, including for five of the nine moderate to severely affected cases. We noted the c.913G>T;p.(Glu305*) and c.458C>T;p.(Pro153Leu) mutations in this Irish population with severe TMAU which is consistent with our earlier findings in Australian and North American families of Irish and British descent.Three individuals were noted to be homozygous for the common variant haplotype c.472G>A;923A>G;p.(Glu158Lys);(Glu308Gly). We also identified three novel variants in this population, which are likely to be pathogenic: c.682G>A;p(Gly228Ser), c.694G>T:p(Asp232Tyr), and c.989G>A;p.(Gly330Glu).

CONCLUSION

Urinary biochemical analysis probably remains the first line diagnostic approach to classify the various types of TMAU. FMO3 gene analysis is likely only to be informative for certain presentations of TMAU.

Genetic analysis of impaired trimethylamine metabolism using whole exome sequencing

Background

Trimethylaminuria (TMAU) is a genetic disorder whereby people cannot convert trimethylamine (TMA) to its oxidized form (TMAO), a process that requires the liver enzyme FMO3. Loss-of-function variants in the FMO3 gene are a known cause of TMAU. In addition to the inability to metabolize TMA precursors like choline, patients often emit a characteristic odor because while TMAO is odorless, TMA has a fishy smell. The Monell Chemical Senses Center is a research institute with a program to evaluate people with odor complaints for TMAU.

Methods

Here we evaluated ten subjects by (1) odor evaluation by a trained sensory panel, (2) analysis of their urine concentration of TMA relative to TMAO before and after choline ingestion, and (3) whole exome sequencing as well as subsequent variant analysis of all ten samples to investigate the genetics of TMAU.

Results

While all subjects reported they often emitted a fish-like odor, none had this malodor during sensory evaluation. However, all were impaired in their ability to produce >90% TMAO/TMA in their urine and thus met the criteria for TMAU. To probe for genetic causes, the exome of each subject was sequenced, and variants were filtered by genes with a known (FMO3) or expected effect on TMA metabolism function (other oxidoreductases). We filtered the remaining variants by allele frequency and predicated functional effects. We identified one subject that had a rare loss-of-function FMO3 variant and six with more common decreased-function variants. In other oxidoreductases genes, five subjects had four novel rare single-nucleotide polymorphisms as well as one rare insertion/deletion. Novel in this context means no investigators have previously linked these variants to TMAU although they are in dbSNP.

Conclusions

Thus, variants in genes other than FMO3 may cause TMAU and the genetic variants identified here serve as a starting point for future studies of impaired TMA metabolism.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-017-0369-8) contains supplementary material, which is available to authorized users.

Association of FMO3 Variants and Trimethylamine N-Oxide Concentration, Disease Progression, and Mortality in CKD Patients

Elevated levels of circulating pro-atherogenic uremic solutes, particularly trimethylamine N-oxide (TMAO), have been implicated in cardiovascular disease development in patients with chronic kidney disease (CKD). TMAO is generated from trimethylamine (TMA) via metabolism by hepatic flavin-containing monooxygenase isoform 3 (FMO3). We determined the functional effects of three common FMO3 variants at amino acids 158, 308, and 257 on TMAO concentrations in a prospective cohort study and evaluated associations of polymorphisms with CKD progression and mortality. Each additional minor allele at amino acid 158 was associated with a 0.38 μg/mL higher circulating TMAO (p = 0.01) and with faster rates of annualized relative eGFR decline. Participants with 0, 1 and 2 variant alleles averaged an eGFR loss of 8%, 12%, and 14% per year, respectively (p-for trend = 0.05). Compared to participants with the homozygous reference allele, heterozygous and homozygous variant participants had a 2.0-fold (95% CI: 0.85, 4.6) and 2.2-fold (95% CI: 0.89, 5.48) higher risk of mortality, respectively (p-for-trend = 0.04). No associations with clinical outcomes were observed for allelic variants at amino acids 257 or 308. Understanding the contribution of genetic variation of FMO3 to disease progression and all-cause mortality can guide recommendations for diet modification or pharmacotherapy in CKD patients at increased risk of adverse outcomes.

Analysis of six novel flavin-containing monooxygenase 3 (FMO3) gene variants found in a Japanese population suffering from trimethylaminuria

Polymorphic human flavin-containing monooxygenase 3 (FMO3) is associated with the inherited disorder trimethylaminuria. Several FMO3 variants have been observed in a variety of ethnic groups, including a Japanese cohort suffering from trimethylaminuria. The aim of this study was to screen another self-reported Japanese trimethylaminuria cohort for novel FMO3 variants and to investigate these new variants. Subjects with low FMO3 metabolic capacities were identified by measuring the urinary trimethylamine and trimethylamine N-oxide concentrationsin171 Japanese volunteers. The FMO3 genes from these subjects and their family members were then sequenced. Heterozygotes or homozygotes for novel single-nucleotide polymorphisms c.20 T > C p.(Ile7Thr), c.122 G > A p.(Trp41Ter), c.127T > A p.(Phe43Ile), c.488 T > C p.(Leu163Pro), and c.1127G > A p.(Gly376Glu) and a heterozygote for the novel duplication c.850_860dupTTTAACGATGA p.(Glu287AspfsTer17) were identified. In addition, the known (but as yet uncharacterized) single-nucleotide polymorphism c.929 C > T p.(Ser310Leu) was found. Pedigree analysis revealed the p.(Ser310Leu) FMO3 allele in cis configuration with c.929 C > T p.(Glu158Lys). These variant FMO3 proteins recombinantly expressed in Escherichia coli membranes exhibited decreased N-oxygenation activities toward trimethylamine and benzydamine. Although the allele frequencies of these seven variants were low, the present results suggest that individuals homozygous or heterozygous for any of these novel missense or duplicationFMO3 variants or known nonsense mutations such as p.(Cys197Ter) may possess abnormal activities toward trimethylamine N-oxygenation.

Emerging roles of flavin monooxygenase 3 in cholesterol metabolism and atherosclerosis

PURPOSE OF REVIEW

Atherosclerosis and associated cardiovascular disease still remain the largest cause of mortality worldwide. Several recent studies have discovered that metabolism of common nutrients by gut microbes can produce a proatherogenic metabolite called trimethylamine-N-oxide (TMAO). The goal of this review is to discuss emerging evidence that the hepatic enzyme that generates TMAO, flavin monooxygenase 3 (FMO3), plays a regulatory role in maintaining whole body cholesterol balance and atherosclerosis development.

RECENT FINDINGS

Several independent studies have recently uncovered a link between either FMO3 itself or its enzymatic product TMAO with atherosclerosis and hepatic insulin resistance. These recent studies show that inhibition of FMO3 stimulates macrophage reverse cholesterol transport and protects against atherosclerosis in mice.

SUMMARY

A growing body of work demonstrates that nutrients present in high-fat foods (phosphatidylcholine, choline and L-carnitine) can be metabolized by the gut microbial enzymes to generate trimethylamine, which is then further metabolized by the host enzyme FMO3 to produce proatherogenic TMAO. Here, we discuss emerging evidence that the TMAO-producing enzyme FMO3 is centrally involved in the pathogenesis of atherosclerosis by regulating cholesterol metabolism and insulin resistance, and how these new insights provide exciting new avenues for cardiovascular disease therapies.

The Flavin-Containing Monooxygenase 3 Gene and Essential Hypertension: The Joint Effect of Polymorphism E158K and Cigarette Smoking on Disease Susceptibility

Gene encoding flavin-containing monooxygenase 3 (FMO3), a microsomal antioxidant defense enzyme, has been suggested to contribute to essential hypertension (EH). The present study was designed to investigate whether common functional polymorphism E158K (rs2266782) of the FMO3 gene is associated with EH susceptibility in a Russian population. A total of 2 995 unrelated subjects from Kursk (1 362 EH patients and 843 healthy controls) and Belgorod (357 EH patients and 422 population controls) regions of Central Russia were recruited for this study. DNA samples from all study participants were genotyped for the FMO3 gene polymorphism through PCR followed by RFLP analysis. We found that the polymorphism E158K is associated with increased risk of essential hypertension in both discovery population from Kursk region (OR 1.36 95% CI 1.09-1.69, P = 0.01) and replication population from Belgorod region (OR 1.54 95% CI 1.07-1.89, P = 0.02) after adjustment for gender and age using logistic regression analysis. Further analysis showed that the increased hypertension risk in carriers of genotype 158KK gene occurred in cigarette smokers, whereas nonsmoker carriers of this genotype did not show the disease risk. This is the first study reporting the association of the FMO3 gene polymorphism and the risk of essential hypertension.

Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease

Some individuals with a particular disease-causing mutation or genotype fail to express most if not all features of the disease in question, a phenomenon that is known as 'reduced (or incomplete) penetrance'. Reduced penetrance is not uncommon; indeed, there are many known examples of 'disease-causing mutations' that fail to cause disease in at least a proportion of the individuals who carry them. Reduced penetrance may therefore explain not only why genetic diseases are occasionally transmitted through unaffected parents, but also why healthy individuals can harbour quite large numbers of potentially disadvantageous variants in their genomes without suffering any obvious ill effects. Reduced penetrance can be a function of the specific mutation(s) involved or of allele dosage. It may also result from differential allelic expression, copy number variation or the modulating influence of additional genetic variants in cis or in trans. The penetrance of some pathogenic genotypes is known to be age- and/or sex-dependent. Variable penetrance may also reflect the action of unlinked modifier genes, epigenetic changes or environmental factors. At least in some cases, complete penetrance appears to require the presence of one or more genetic variants at other loci. In this review, we summarize the evidence for reduced penetrance being a widespread phenomenon in human genetics and explore some of the molecular mechanisms that may help to explain this enigmatic characteristic of human inherited disease.

Trimethylaminuria (fish odor syndrome): genotype characterization among Portuguese patients

Trimethylaminuria (TMAu) or "fish odor syndrome" is a metabolic disorder characterized by the inability to convert malodorous dietarily-derived trimethylamine (TMA) to odorless TMA N-oxide by the flavin-containing monooxygenase 3 (FMO3). Affected individuals unable to complete this reaction exude a "fishy" body odor due to the secretion of TMA in their corporal fluids leading to a variety of psychosocial problems. Interindividual variability in the expression of FMO3 gene may affect drug and foreign chemical metabolism in the liver and other tissues. Therefore, it is important to screen for common TMAu mutations but also extend the search to other genetic variants in order to correlate genotype and disease-associated phenotypes. In this study, 25 Portuguese patients with phenotype suggestive of TMAu were evaluated for molecular screening of the FMO3 gene. Herein, we found 16 variants in the FMO3 coding region, some of which had not been previously documented (Gly38Trp, Asp232Val, Thr307Pro, Ser310Leu). Whenever common variants (Glu158Lys, Glu308Gly) were considered in combination a distinct pattern between the control population and patients was observed, mainly in what concerns the presence of Lys158 and Gly308 in homozygous state. Further studies are necessary to clarify the pathogenicity of novel variants identified in this study, as well as the effect of the common single nucleotide polymorphisms, which may play an important role in disease presentation and/or protective mechanism to xenobiotics drugs or environment.

FMO3 allelic variants in Sicilian and Sardinian populations: trimethylaminuria and absence of fish-like body odor

The N-oxygenation of amines by the human flavin-containing monooxygenase (form 3) (FMO3) represents an important means for the conversion of lipophilic nucleophilic heteroatom-containing compounds into more polar and readily excreted products. In healthy individuals, virtually all Trimethylamine (TMA) are metabolized to Trimethylamine N-oxide (TMAO). Several single nucleotide polymorphisms (SNPs) of the FMO3 gene have been described and result in an enzyme with decreased or abolished functional activity for TMA N-oxygenation thus leading to TMAU, or fish-like odor syndrome. Three coding region variants, c. G472A (p.E158K) in exon 4, c. G769A (p.V257M) in exon 6, and c.A923G (p.E308G) in exon 7, are common polymorphisms identified in all population examined so far and are associated with normal or slightly reduced TMA N-oxygenation activity. However, simultaneous occurrence of 158K and 308G variants results in a more pronounced decrease in FMO3 activity. A fourth polymorphism, c. G1424A (p.G475D) in exon 9, less common in the general population, was observed in individuals suffering severe or moderate trimethylaminuria. The aim of this study was to determine the allelic and genotypic distributions of these four FMO3 variants in 528 healthy individuals collected from the Sicilian and Sardinian populations together with haplotype and linkage analyses. Finally, we present data on the genotype-phenotype correlation by ESI-MS/MS TMA/TMAO urinary determination in 158KK/308EG individuals. Variant 158K shows the same frequency in Sicilian and Sardinian populations while variant 257M was not observed in the Sardinian sampling. No significant differences were found for 308G and 475D variants among two populations. Cis-linkage between 158K and 308G was confirmed with the compound variant (158K-308G) being found in a proportion of 0.9% and 0.3% of Sicilian subjects, and 0.01% and 0.5% in Sardinian population. Urinary determination of TMA/TMAO ratio in 158KK/308EG individuals showed a considerable reduction in FMO3 activity although they do not show the classical features of trimethylaminuria as a strong body odor and breath. Our data support the conclusion that trimethylaminuria is not always accompanied by a fish-like odor, despite the coexistence in the same individual of the two variants 158K and 308G, and other factors account for the expression of that phenotype.

[Primary trimethylaminuria: the fish odor syndrome]

Primary trimethylaminuria, or fish odor syndrome, is a congenital metabolic disorder characterized by a failure in the hepatic trimethylamine (TMA) oxidation route to trimethylamine N-oxide (TMANO). TMA is mostly derived from dietary precursors such as choline, carnitine and TMANO. The presence of abnormal amounts of TMA in the urine, sweat, exhaled air and other body secretions confers a very unpleasant body odor resembling that of decaying fish. As a consequence, patients can suffer from serious psychosocial sequelae. We present a case of primary trimethylaminuria with the aim of raising awareness about this condition.

Novel variants of the human flavin-containing monooxygenase 3 (FMO3) gene associated with trimethylaminuria

The disorder trimethylaminuria (TMAu) often manifests itself in a body odor for individuals affected. TMAu is due to decreased metabolism of dietary-derived trimethylamine (TMA). In a healthy individual, 95% or more of TMA is converted by the flavin-containing monooxygenase 3 (FMO3, EC 1.14.13.8) to non-odorous trimethylamine N-oxide (TMA N-oxide). Several single nucleotide polymorphisms (SNPs) of the FMO3 gene have been described and result in an enzyme with decreased or abolished functional activity for TMA N-oxygenation thus leading to TMAu. Herein, we report two novel mutations observed from phenotyping and genotyping two self-reporting individuals. Sequence analysis of the exon regions of the FMO3 gene of a young woman with severe TMAu revealed heterozygous mutations at positions 187 (V187A), 158 (E158K), 308 (E308G), and 305 (E305X). Familial genetic analysis showed that the E158K/V187A/E308G derived from the same allele from the mother, and the E305X was derived from the father. FMO3 variants V187A and V187A/E158K were characterized for oxygenation of several common FMO3 substrates (i.e., 5- and 8-DPT, mercaptoimidazole (MMI), TMA, and sulindac sulfide) and for its thermal stability. Our findings show that with the combination of V187A/E158K mutations in FMO3, the enzyme activity is severely affected and possibly contributes to the TMAu observed. In another study, genotyping analysis of a 17 year old female revealed a mutation that caused a frame shift after K415 and resulted in a protein variant with only 486 amino acid residues that was associated with severe TMAu.

Trimethylaminuria (fish-odour syndrome) and oral malodour

A small but important percentage of oral malodour cases have an extra-oral aetiology and certain of these fall into the category of 'blood-borne halitosis'. Odoriferous substances generated within the body and transported to the lungs via the circulatory system may, if sufficiently volatile, leave with the exhaled air and impart a foetid odour to the breath. The aliphatic tertiary amine, trimethylamine, is such a volatile compound that is generated to excess in patients with a metabolic disorder known as trimethylaminuria (fish-odour syndrome). This article highlights this condition and draws attention to its potential role in the causation of recalcitrant oral malodour.

Flavin-containing monooxygenase 3 and human disease

This review summarizes information concerning the association of the human flavin-containing monooxygenase 3 (FMO3) and human diseases. Human FMO3 oxygenates a wide variety of nucleophilic heteroatom-containing xenobiotics, including endogenous substrates and various clinically important drugs. In this article, the authors discuss the association of FMO3 with human disease, including: i) direct association of FMO3 genetic mutations to human genetic disease; ii) association of FMO3 genetic polymorphism to altered drug metabolism and, therefore, indirect association of FMO3 with drug therapeutic efficacy of human disease; and iii) the potential impact and/or effect of FMO3 transcriptional regulation during disease states. Even though many studies discussed for the latter two points are at a preliminary stage and require much more research to bring to a definite conclusion, the authors include these studies to stimulate general interest and invite further discussion.

Functional characterization of genetic variants of human FMO3 associated with trimethylaminuria

Impaired conversion of trimethylamine to trimethylamine N-oxide by human flavin containing monooxygenase 3 (FMO3) is strongly associated with primary trimethylaminuria, also known as 'fish-odor' syndrome. Numerous non-synonymous mutations in FMO3 have been identified in patients suffering from this metabolic disorder (e.g., N61S, M66I, P153L, and R492W), but the molecular mechanism(s) underlying the functional deficit attributed to these alleles has not been elucidated. The purpose of the present study was to determine the impact of these disease-associated genetic variants on FMO3 holoenzyme formation and on steady-state kinetic parameters for metabolism of several substrates, including trimethylamine. For comparative purposes, several common allelic variants not associated with primary trimethylaminuria (i.e., E158K, V257M, E308G, and the E158K/E308G haplotype) were also analyzed. When recombinantly expressed in insect cells, only the M66I and R492W mutants failed to incorporate/retain the FAD cofactor. Of the remaining mutant proteins P153L and N61S displayed substantially reduced (<10%) catalytic efficiencies for trimethylamine N-oxygenation relative to the wild-type enzyme. For N61S, reduced catalytic efficiency was solely a consequence of an increased K(m), whereas for P153L, both K(m) and k(cat) were altered. Similar results were obtained when benzydamine N-oxygenation was monitored. A homology model for FMO3 was constructed based on the crystal structure for yeast FMO which places the N61 residue alone, of the mutants analyzed here, in close proximity to the FAD catalytic center. These data demonstrate that primary trimethylaminuria is multifactorial in origin in that enzyme dysfunction can result from kinetic incompetencies as well as impaired assembly of holoprotein.

A spectrum of molecular variation in a cohort of Italian families with trimethylaminuria: identification of three novel mutations of the FM03 gene

Fish-odor syndrome or trimethylaminuria, is a rare inborn error of metabolism inherited in an autosomal recessive fashion, involving the dysfunction of hepatic enzyme flavin-containing monooxygenase 3 (FMO3) that converts fishy-smelling trimethylamine (TMA) into odorless trimethylamine-N-oxide (TMAO). This confers, to the affected individual a very unpleasant body odor resembling that of rotting fish. This disorder has been relatively well-documented in British, Australian, and American populations and reports have appeared regarding patients in Thailand and Hong Kong, but no Italian families affected by trimethylaminuria have been reported in the literature. We have collected a cohort of Italian families and investigated the genetic basis of the disorder in these Italian pedigrees disclosing a spectrum of molecular variation in the FM03 gene comprising three novel deleterious mutations: the first documented de novo missense mutation causative of trimethylaminuria; a guanidine nucleotide deletion (G1182del) at codon 394 and a novel missense mutation (R238P) that altered highly conserved amino acid in the exon 6. Moreover, we investigated by aplotype analysis a family with mild TMAuria identifying a putative causative aplotype. Finally, we failed to detect any variation in other Italian families suggesting that this gene is not associated with all clinical form of trimethylaminuria or that polymorphisms in this gene could be susceptibility factors for developing the disease. Our findings support the hypothesis that TMAuria is not a rare recessive disorder but rather a spectrum of malodour phenotypes in which diet and environmental exposures can play a role in triggering symptoms.

Human flavin-containing monooxygenases

This review summarizes recent information concerning the pharmacological and toxicological significance of the human flavin-containing monooxygenase (FMO, EC 1.14.13.8). The human FMO oxygenates nucleophilic heteroatom-containing chemicals and drugs and generally converts them into harmless, polar, readily excreted metabolites. Sometimes, however, FMO bioactivates chemicals into reactive materials that can cause toxicity. Most of the interindividual differences of FMO are due to genetic variability and allelic variation, and splicing variants may contribute to interindividual and interethnic variability observed for FMO-mediated metabolism. In contrast to cytochrome P450 (CYP), FMO is not easily induced nor readily inhibited, and potential adverse drug-drug interactions are minimized for drugs prominently metabolized by FMO. These properties may provide advantages in drug design and discovery, and by incorporating FMO detoxication pathways into drug candidates, more drug-like materials may be forthcoming. Although exhaustive examples are not available, physiological factors can influence FMO function, and this may have implications for the clinical significance of FMO and a role in human disease.

Mutation, polymorphism and perspectives for the future of human flavin-containing monooxygenase 3

Flavin-containing monooxygenases (FMOs) catalyze NADPH-dependent monooxygenation of soft-nucleophilic nitrogen, sulfur, and phosphorous atoms contained within various drugs, pesticides, and xenobiotics. Flavin-containing monooxygenase 3 (FMO3) is responsible for the majority of FMO-mediated xenobiotic metabolism in the adult human liver. Mutations in the FMO3 gene can result in defective trimethylamine (TMA) N-oxygenation, which gives rise to the disorder known as trimethylaminuria (TMAU) or "fish-odour syndrome". To date 18 mutations of FMO3 gene have been reported that cause TMAU, and polymorphic variants of the gene have also been identified. Interindividual variability in the expression of FMO3 may affect drug and foreign chemical metabolism in the liver and other tissues. It is important therefore to study how base sequence variation of the FMO3 gene might affect the ability of individuals and different ethnic population groups to deal with the variety of environmental chemicals and pharmaceutical products that are substrates for FMO3.

Diagnosis and management of trimethylaminuria (FMO3 deficiency) in children

Persistent trimethylaminuria in children is caused by autosomal recessively inherited impairment of hepatic trimethylamine (TMA) oxidation due to deficiency of flavin monooxygenase 3 (FMO3) secondary to mutations in the FMO3 gene. Trimethylaminuria or 'fish odour syndrome' is due to excessive excretion into body fluids and breath of TMA derived from the enterobacterial metabolism of dietary precursors. The disorder is present from birth but becomes apparent as foods containing high amounts of choline or of trimethylamine N-oxide (TMAO) from marine (sea or saltwater) fish are introduced into the diet. In our experience, trimethylaminuria (FMO3 deficiency) in children is rare. We have compared the dynamics and diagnostic efficacy of choline loading with marine fish meals in six children with trimethylaminuria. Loading with a marine fish meal provides a simple and acceptable method for confirmation of diagnosis of suspected trimethylaminuria in children, with the effects being cleared more quickly than with a choline load test. However, oral loading with choline bitartrate allows estimation of residual oxidative capacity in vivo and is a useful adjunct to molecular studies. Patients homozygous for the 'common' P153L mutation in the FMO3 gene showed virtual complete lack of residual TMA N-oxidative capacity, consistent with a nonfunctional or absent FMO3 enzyme, whereas a patient with the M82T mutation showed some residual oxidative capacity. A patient compound heterozygous for two novel mutations, G193E and R483T, showed considerable residual N-oxidative capacity. A further patient, heterozygous for two novel sequence variations in the FMO3 gene, consistently showed malodour and elevated urinary TMA/TMAO ratios under basal conditions but a negative response to both choline and marine fish meal loading. Comparison of the effects of administration of antibiotics (metronidazole, amoxicillin, neomycin) on gut bacterial production of trimethylamine from choline showed they all reduced TMA production to a limited extent, with neomycin being most effective. 'Best-practice' diagnostic and treatment guidelines are summarized.

Polymorphisms of the Flavin containing monooxygenase 3 (FMO3) gene do not predispose to essential hypertension in Caucasians

BACKGROUND

The recessive disorder trimethylaminuria is caused by defects in the FMO3 gene, and may be associated with hypertension. We investigated whether common polymorphisms of the FMO3 gene confer an increased risk for elevated blood pressure and/or essential hypertension.

METHODS

FMO3 genotypes (E158K, V257M, E308G) were determined in 387 healthy subjects with ambulatory systolic and diastolic blood pressure measurements, and in a cardiovascular disease population of 1649 individuals, 691(41.9%) of whom had a history of hypertension requiring drug treatment. Haplotypes were determined and their distribution noted.

RESULTS

There was no statistically significant association found between any of the 4 common haplotypes and daytime systolic blood pressure in the healthy population (p = 0.65). Neither was a statistically significant association found between the 4 common haplotypes and hypertension status among the cardiovascular disease patients (p = 0.80).

CONCLUSION

These results suggest that the variants in the FMO3 gene do not predispose to essential hypertension in this population.

Flavin-containing monooxygenase genetic polymorphism: impact on chemical metabolism and drug development

The flavin-containing monooxygenases (FMOs) metabolize a broad range of therapeutics. Consisting of five gene products in humans (FMO1–5), the different FMO family members exhibit pronounced tissue- and temporal-specific expression patterns. Substantial interindividual differences are also observed, and the inability to modulate with exogenous agents is consistent with an important role for genetic variation. Several rare FMO3 alleles causative for trimethylaminuria have been well characterized. However, the identification and characterization of functional FMO1–5 polymorphisms has been more recent. Although none of these polymorphisms has been associated with an adverse drug reaction, the continued broadening of our therapeutic armamentarium makes such an event likely in the future. Furthermore, at least one example has been reported for a direct association between FMO3 polymorphism and therapeutic efficacy. Thus, it is anticipated that knowledge regarding functionally-relevant FMO genetic variability will become increasingly important for making drug development and patient therapeutic choices.

Mammalian flavin-containing monooxygenases: structure/function, genetic polymorphisms and role in drug metabolism

Flavin-containing monooxygenase (FMO) oxygenates drugs and xenobiotics containing a "soft-nucleophile", usually nitrogen or sulfur. FMO, like cytochrome P450 (CYP), is a monooxygenase, utilizing the reducing equivalents of NADPH to reduce 1 atom of molecular oxygen to water, while the other atom is used to oxidize the substrate. FMO and CYP also exhibit similar tissue and cellular location, molecular weight, substrate specificity, and exist as multiple enzymes under developmental control. The human FMO functional gene family is much smaller (5 families each with a single member) than CYP. FMO does not require a reductase to transfer electrons from NADPH and the catalytic cycle of the 2 monooxygenases is strikingly different. Another distinction is the lack of induction of FMOs by xenobiotics. In general, CYP is the major contributor to oxidative xenobiotic metabolism. However, FMO activity may be of significance in a number of cases and should not be overlooked. FMO and CYP have overlapping substrate specificities, but often yield distinct metabolites with potentially significant toxicological/pharmacological consequences. The physiological function(s) of FMO are poorly understood. Three of the 5 expressed human FMO genes, FMO1, FMO2 and FMO3, exhibit genetic polymorphisms. The most studied of these is FMO3 (adult human liver) in which mutant alleles contribute to the disease known as trimethylaminuria. The consequences of these FMO genetic polymorphisms in drug metabolism and human health are areas of research requiring further exploration.

The implications of polymorphisms in mammalian flavin-containing monooxygenases in drug discovery and development

Use of the human flavin-containing monooxygenases (FMOs) in drug design and discovery could represent a paradigm shift in drug development and basic research. Although FMOs have been previously viewed as minor contributors to drug metabolism, the advantages associated with using FMOs to diversify the metabolism of a drug are now being recognized. Because FMOs typically oxygenate a wide variety of nucleophilic compounds to polar, benign metabolites, and because drugs do not induce expression of FMOs or inhibit their activity, potential drug-drug interactions are minimized. Interindividual variation for this class of enzyme is largely dependent on genetic variation. Examples of FMO allelic variation and splicing variants suggest that these genetic mutations could contribute to the interindividual and interethnic variability of FMO-mediated metabolism.

Effects of the dietary supplements, activated charcoal and copper chlorophyllin, on urinary excretion of trimethylamine in Japanese trimethylaminuria patients

Trimethylaminuria (TMAU) is a metabolic disorder characterized by the inability to oxidize and convert dietary-derived trimethylamine (TMA) to trimethylamine N-oxide (TMAO). This disorder has been relatively well-documented in European and North American populations, but no reports have appeared regarding patients in Japan. We identified seven Japanese individuals that showed a low metabolic capacity to convert TMA to its odorless metabolite, TMAO. The metabolic capacity, as defined by the concentration of TMAO excreted in the urine divided by TMA concentration plus TMAO concentration, in these seven individuals ranged from 70 to 90%. In contrast, there were no healthy controls examined with less than 95% of the metabolic capacity to convert TMA to TMAO. The intake of dietary charcoal (total 1.5 g charcoal per day for 10 days) reduced the urinary free TMA concentration and increased the concentration of TMAO to normal values during charcoal administration. Copper chlorophyllin (total 180 mg per day for 3 weeks) was also effective at reducing free urinary TMA concentration and increasing TMAO to those of concentrations present in normal individuals. In the TMAU subjects examined, the effects of copper chlorophyllin appeared to last longer (i.e., several weeks) than those observed for activated charcoal. The results suggest that the daily intake of charcoal and/or copper chlorophyllin may be of significant use in improving the quality of life of individuals suffering from TMAU.

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 FMO3 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 FMO3 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 FMO3 gene. The mutation changed a GAG encoding Glu at codon 32 to AAG encoding Lys. Wild-type and mutant E32K FMO3 were expressed in Escherichia coli as maltose binding-fusion proteins and assayed for their ability to catalyze oxygenation of various FMO3 substrates. The results showed that the E32K mutation abrogated the catalytic activity of the enzyme. Individual 2 was identified as heterozygous for the P153L 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 FMO3 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 FMO3 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%.

Fish odour syndrome with features of both primary and secondary trimethylaminuria

We report a patient with the fish odour syndrome who has both primary and secondary trimethylaminuria. The diagnosis was made using biochemical and genetic analysis in the apparent absence of any characteristic smell. Differentiation of primary and secondary trimethylaminuria is usually made on urinary analysis of trimethylamine and its metabolite trimethylamine N-oxide, with different, characteristic patterns of both compounds in primary and secondary trimethylaminuria. Our patient had biochemical analysis consistent with a diagnosis of secondary trimethylaminuria, while analysis of the flavin-containing mono-oxygenase 3 gene, the causative gene in primary trimethylaminuria, demonstrated three sequence polymorphisms, two of which are known to reduce enzyme activity. The patient showed temporary clinical and biochemical response to treatment with metronidazole and neomycin. It is important to be aware of this diagnosis in patients without obvious clinical signs, and of the subjective benefits of treatment.

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.

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.

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.

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.

A Mutation in the Flavin-containing Monooxygenase 3 Gene and its Effects on Catalytic Activity for N-oxidation of Trimethylamine In Vitro

To clarify the mutation of the flavin-containing monooxygenase (FMO) 3 gene causing fish-odor syndrome, we analyzed the FMO3 gene of a Thai subject who possibly suffered from fish-odor syndrome. A novel mutation, a single-base substitution from G to A at the position of 265 (G265A), was identified in exon 3. The mutation caused an amino acid substitution from valine to isoleucine at residue 58 (V58I). The mutated FMO3 protein with V58I exhibited the reduced trimethylamine N-oxidase activity when it was expressed in E. coli. The V(max)/K(m) value for the activity of the mutant-type FMO3 was about 5 times lower than that for the wild-type FMO3.

In vivo variability of TMA oxidation is partially mediated by polymorphisms of the FMO3 gene

Trimethylaminuria (TMAU) results from an accumulation of an excessive amount of unoxidized trimethylamine that is excreted in urine and body secretions. Mutations of the flavin-containing monooxygenase 3 (FMO3) gene (a hepatic phase I drug-metabolizing enzyme) account for the severe recessively encoded form of this condition. We have previously described a number of FMO3 polymorphisms which in vitro exhibit reduced substrate affinity for several FMO substrates. Here we show that three prevalent polymorphisms (E158K, V257M, and E308G) inherited in particular combinations confer a slight decrease in TMA oxidation under normal physiological conditions, which may be clinically "silent." With the use of substrate loading or with the interaction of other known modulators of FMO3 activity such as hormonal influences, these genotypes may predispose to mild TMAU.

A novel deletion in the flavin-containing monooxygenase gene (FMO3) in a Greek patient with trimethylaminuria

Mutations of the flavin-containing monooxygenase type 3 gene (FMO3) that encode the major functional form present in adult human liver, have been shown to cause trimethylaminuria. We now report a novel homozygous deletion of exons 1 and 2 in an Australian of Greek ancestry with TMAuria, the first report of a deletion causative of trimethylaminuria. The deletion occurs 328 bp upstream from exon 1. The 3'-end of the deletion occurs in intron 2, 10013 base pairs downstream from the end of exon 2. The deletion is 12226 bp long. For the proband homozygous for the human FMO3 gene deletion, it is predicted that in addition to loss of monooxygenase function for human FMO3 substrates, such as TMA and other amines, the proband will exhibit decreased tolerance of biogenic amines, both medicinal and those found in foods.

Benzydamine N-oxidation as an index reaction reflecting FMO activity in human liver microsomes and impact of FMO3 polymorphisms on enzyme activity

AIMS

The role of flavin containing monooxygenases (FMO) on the disposition of many drugs has been insufficiently explored. In vitro and in vivo tests are required to study FMO activity in humans. Benzydamine (BZD) N-oxidation was evaluated as an index reaction for FMO as was the impact of genetic polymorphisms of FMO3 on activity.

METHODS

BZD was incubated with human liver microsomes (HLM) and recombinant enzymes. Human liver samples were genotyped using PCR-RFLP.

RESULTS

BZD N-oxide formation rates in HLM followed Michaelis-Menten kinetics (mean Km = 64.0 microM, mean Vmax = 6.9 nmol mg-1 protein min-1; n = 35). N-benzylimidazole, a nonspecific CYP inhibitor, and various CYP isoform selective inhibitors did not affect BZD N-oxidation. In contrast, formation of BZD N-oxide was almost abolished by heat treatment of microsomes in the absence of NADPH and strongly inhibited by methimazole, a competitive FMO inhibitor. Recombinant FMO3 and FMO1 (which is not expressed in human liver), but not FMO5, showed BZD N-oxidase activity. Respective Km values for FMO3 and FMO1 were 40.4 microM and 23.6 microM, and respective Vmax values for FMO3 and FMO1 were 29.1 and 40.8 nmol mg-1 protein min-1. Human liver samples (n = 35) were analysed for six known FMO3 polymorphisms. The variants I66M, P135L and E305X were not detected. Samples homozygous for the K158 variant showed significantly reduced Vmax values (median 2.7 nmol mg-1 protein min-1) compared to the carriers of at least one wild type allele (median 6.2 nmol mg-1 protein min-1) (P < 0.05, Mann-Whitney-U-test). The V257M and E308G substitutions had no effect on enzyme activity.

CONCLUSIONS

BZD N-oxidation in human liver is mainly catalysed by FMO3 and enzyme activity is affected by FMO3 genotype. BZD may be used as a model substrate for human liver FMO3 activity in vitro and may be further developed as an in vivo probe reflecting FMO3 activity.

Compound heterozygosity for missense mutations in the flavin-containing monooxygenase 3 (FM03) gene in patients with fish-odour syndrome

Fish-odour syndrome is a highly unpleasant disorder of hepatic trimethylamine (TMA) metabolism characterized by a body odour reminiscent of rotting fish, due to excessive excretion of the malodorous free amine. Although fish-odour syndrome may exhibit as sequelae with other conditions (e.g. liver dysfunction), many patients exhibit an inherited, more persistent form of the disease. Ordinarily, dietary-derived TMA is oxidized to the nonodorous N-oxide by hepatic flavin-containing monooxygenase 3 (FMO3). Our previous demonstration that a mutation, P153L (C to T), in the FMO3 gene segregated with the disorder and inactivated the enzyme confirmed that defects in FMO3 underlie the inherited form of fish-odour syndrome. We have investigated the genetic basis of the disorder in two further affected pedigrees and report that the three propositi are all compound heterozygotes for causative mutations of FMO3. Two of these individuals possess the P153L (C to T) mutation and a novel mutation, N61S (A to G). The third is heterozygous for novel, M4341 (G to A), and previously reported, R492W (C to T), mutations. Functional characterization of the S61, 1434 and W492 variants, via baculovirus-mediated expression in insect cells, confirmed that all three mutations either abolished, or severely attenuated, the capacity of the enzyme to catalyse TMA N-oxidation. Although 1434 and W492 were also incapable of catalysing the S-oxidation of methimazole, S61 was fully active with this sulphur-containing substrate. Since an asparagine is conserved at the equivalent position to N61 of FMO3 in mammalian, yeast and Caenorhabditis elegans FMOs, the characterization of the naturally occurring N61S (A to G) mutation may have identified this asparagine as playing a critical role specifically in FMO-catalysed N-oxidation.

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

The flavin-containing monooxygenases (FMOs) are a family of xenobiotic-metabolizing enzymes that are expressed in a species- and tissue-specific manner. FMO2 expression has been observed in pulmonary tissue from several species, but not human. Two human FMO2 point mutations have been reported: a cytosine to thymidine transition at position 1414 resulting in a premature stop codon and a thymidine insertion at position 1589 resulting in a frameshift. To define the frequency of these sequence variations and explore their significance, unrelated African-American, Caucasian, and Korean individuals were genotyped. In the African-American population tested (n = 180), the 1414C allele occurred at a 13% frequency; however, all of the tested Caucasians (n = 52) and Koreans (n = 100) were homozygous for the 1414T allele. The T1589 allele occurred at frequencies of 6.9 and 13.0% in African-Americans (n = 175) and Caucasians (n = 23), respectively, and appears to segregate with the 1414T allele. Thus, it would have no further impact on FMO2 activity. Western blot analysis of pulmonary microsomes failed to detect immunoreactive protein in 1414T homozygotes. A heterozygotic individual did exhibit a single band of the expected size, but no detectable FMO activity in the corresponding lung microsomes. Sequence analysis, however, was consistent with the 1414C allele encoding an active FMO2 enzyme. FMO2 mRNA expression was observed in most individuals, but failed to correlate with genotype or protein expression. In summary, functional FMO2 is expressed in only a small percentage of the overall population. However, in certain ethnic groups, active pulmonary FMO2 enzyme will be present in a significant number of individuals.