Clinical utility gene card for: Trimethylaminuria - update 2014

Diagnosis of Primary Trimethylaminuria in an Affected Patient With a Rare Genotype in Sub-Saharan Africa

Primary trimethylaminuria (TMAU) is characterized by systemic accumulation of trimethylamine (TMA) due to the deficient activity of flavin-containing monooxygenase 3 (FMO3). The disorder does not have detrimental pathophysiological consequences, but patients develop psychological symptoms due to the emotionally debilitating bodily odor defined as decaying fish that affects their quality of life. Here, we illustrate the utility of a diagnostic workup on an adolescent with primary TMAU, including biochemical and genetic investigations that confirm the diagnosis. A direct substrate (TMA) loading protocol was used, followed by the collection of urine samples at predetermined intervals. The conversion of TMA to trimethylamine oxide (TMAO), monitored by 1H-NMR spectrometry, showed a compromised FMO3 metabolic capacity at baseline, becoming more pronounced after loading commenced. The eight coding exons of the FMO3 gene were Sanger sequenced, revealing a homozygous missense variant, c.23T>C (p.Ile8Thr), as well as two known homozygous variants, c.472G>A (p.Glu158Lys) and c.923A>G (pGlu308Gly), associated with no to mild presentation of TMAU. The advantage of direct substrate-to-product monitoring is the elimination of alternative contributors to the odor that would result in the diagnosis of secondary TMAU. The combined functional and genetic approach provided adequate evidence to describe the first primary TMAU patient reported in sub-Saharan Africa with a genotype not yet described in a homozygous state. Our findings motivate a comprehensive biochemical and genetic approach to discriminate between primary and secondary TMAU. Subsequently, this targeted approach can provide advice on therapeutic management for optimal emotional well-being.

First Report from Saudi Arabia of Trimethylaminuria Caused by a Premature Stop Codon Mutation in the FMO3 Gene

Background

Trimethylaminuria (TMAU) is a rare recessive genetic disorder with limited global prevalence. To date, there have been no official reports of TMAU cases documented in Saudi Arabia.

Purpose

In this study, we developed a liquid chromatography-mass spectrometry (LC-MS) method for the analysis of trimethylamine (TMA) and Trimethylamine N-Oxide (TMAO) in urine and plasma samples for the first reported case of TMAU in Saudi Arabia.

Patients and Methods

A 41-year-old Saudi man was diagnosed with TMAU in National Guard Hospital. Blood and urine samples were collected to confirm the diagnosis of TMAU. In this study, we have studied LC-MS, cell culture, flow cytometry, adhesion assay and Sanger sequencing analysis. Additionally, in this study, we have selected 5 healthy controls.

Results

The results have revealed elevated TMA levels were present in both urine and plasma samples, while TMAO levels were significantly lower compared to control group. Further, we utilized plasma sample from the TMAU patient as novel model to investigate the potential effect of low TMAO on monocyte and endothelial cell function in vitro. DNA sequencing analysis identified a c.622G >T (p.Glu208*) which creates a premature stop codon in FMO3 gene.

Conclusion

Our findings revealed differential responses in monocytes and endothelial cells stimulated with plasma from the TMAU patient compared to plasma from non-TMAU patients. These distinct responses may be key modulators of endothelial function and contributes to vascular damage.

Exploring Trimethylaminuria: Genetics and Molecular Mechanisms, Epidemiology, and Emerging Therapeutic Strategies

Trimethylaminuria (TMAU) is a rare metabolic syndrome caused by the accumulation of trimethylamine in the body, causing odor emissions similar to rotten fish in affected patients. This condition is determined by both genetic and environmental factors, especially gut dysbiosis. The multifactorial nature of this syndrome makes for a complex and multi-level diagnosis. To date, many aspects of this disease are still unclear. Recent research revealed the FMO3 haplotypes' role on the enzyme's catalytic activity. This could explain why patients showing only combined polymorphisms or heterozygous causative variants also manifest the TMAU phenotype. In addition, another research hypothesized that the behavioral disturbances showed by patients may be linked to gut microbiota alterations. Our review considers current knowledge about TMAU, clarifying its molecular aspects, the therapeutic approaches used to limit this condition, and the new therapies that are under study.

Living with trimethylaminuria and body and breath malodour: personal perspectives

BACKGROUND

Many people suffer from body and breath malodour syndromes. One of these is trimethylaminuria, a condition characterized by excretion in breath and bodily fluids of trimethylamine, a volatile and odorous chemical that has the smell of rotting fish. Trimethylaminuria can be primary, due to mutations in the gene encoding flavin-containing monooxygenase 3, or secondary, due to various causes. To gain a better understanding of problems faced by United Kingdom residents affected by body and breath malodour conditions, we conducted a survey.

METHODS

Two anonymous online surveys, one for adults and one for parents/guardians of affected children, were conducted using the Opinio platform. Participants were invited via a trimethylaminuria advisory website. Questions were a mix of dropdown, checkbox and open-ended responses. Forty-four adults and three parents/guardians participated. The dropdown and checkbox responses were analysed using the Opinio platform.

RESULTS

All participants reported symptoms of body/breath odour. However, not all answered every question. Twenty-three respondents experienced difficulties in being offered a diagnostic test for trimethylaminuria. Problems encountered included lack of awareness of the disorder by medical professionals and reluctance to recognise symptoms. Of those tested, 52% were diagnosed with trimethylaminuria. The main problems associated with living with body/breath malodours were bullying, harassment and ostracism in either the workplace (90%) or in social settings (88%). All respondents thought their condition had disadvantaged them in their daily lives. Open-ended responses included loss of confidence, stress, exclusion, isolation, loneliness, depression and suicidal thoughts. Respondents thought their lives could be improved by greater awareness and understanding of malodour conditions by medical professionals, employers and the general public, and appreciation that the malodour was due to a medical condition and not their fault.

CONCLUSIONS

Breath and body malodour conditions can cause immense hardship and distress, both mentally and socially, having devastating effects on quality of life. It would be advantageous to establish a standardised pathway from primary care to a specialist unit with access to a robust and reliable test and diagnostic criteria. There is a need to recognise malodour disorders as a disability, giving affected individuals the same rights as those with currently recognised disabilities.

Simple confirmation methods for rare but impaired variants of human flavin-containing monooxygenase 3 (FMO3) found in an updated genome resource databank

Forty-seven new nonsense or missense human flavin-containing monooxygenase 3 (FMO3) variants were recently identified in an updated Japanese population reference panel. Of these, 20 rare single-nucleotide substitutions resulted in moderately or severely impaired FMO3 activity. To easily identify these 20 FMO3 variants (2 stop codon mutations, 2 frameshifts, and 16 amino-acid substitutions) in the clinical setting, simple confirmation methods for impaired FMO3 variants are proposed using polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) or allele-specific PCR methods. Using PCR-RFLP, FMO3 variants p.Arg51Gly, p.Met66Lys, p.Asn80Lys, p.Val151Glu, p.Val187fsTer25, p.Gly193Arg, p.Val283Ala, p.Asp286His, p.Val382Ala, and p.Phe451Leu were digested by the designated restriction enzymes and confirmed using reference cDNAs. In contrast, the FMO3 variants p.Gly39Val, p.Arg238Ter, p.Arg387Cys, p.Arg387His, p.Leu457Trp, and p.Met497Arg were not digested, whereas the wild type was digested. FMO3 variants p.Gly11Asp, p.Lys416fsTer72, p.Gln427Ter, and p.Thr453Pro were confirmed using allele-specific PCR systems. The previously identified FMO3 p.Arg500Ter variant has a relatively high frequency and was differentiated from p.Arg500Gln in two steps, i.e., enzyme restriction followed by allele-specific PCR, similar to the method for p.Arg387Cys and p.Arg387His. These systems should facilitate easy detection in the clinical setting of FMO3 variants in Japanese subjects susceptible to low drug clearance possibly caused by impaired FMO3 function.

Balancing the Equation: A Natural History of Trimethylamine and Trimethylamine-N-oxide

Trimethylamine (TMA) and its N-oxide (TMAO) are ubiquitous in prokaryote and eukaryote organisms as well as in the environment, reflecting their fundamental importance in evolutionary biology, and their diverse biochemical functions. Both metabolites have multiple biological roles including cell-signaling. Much attention has focused on the significance of serum and urinary TMAO in cardiovascular disease risk, yet this is only one of the many facets of a deeper TMA-TMAO partnership that reflects the significance of these metabolites in multiple biological processes spanning animals, plants, bacteria, and fungi. We report on analytical methods for measuring TMA and TMAO and attempt to critically synthesize and map the global functions of TMA and TMAO in a systems biology framework.

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.

Non-P450 Drug-Metabolizing Enzymes: Contribution to Drug Disposition, Toxicity, and Development

Most clinically used drugs are metabolized in the body via oxidation, reduction, or hydrolysis reactions, which are considered phase I reactions. Cytochrome P450 (P450) enzymes, which primarily catalyze oxidation reactions, contribute to the metabolism of over 50% of clinically used drugs. In the last few decades, the function and regulation of P450s have been extensively studied, whereas the characterization of non-P450 phase I enzymes is still incomplete. Recent studies suggest that approximately 30% of drug metabolism is carried out by non-P450 enzymes. This review summarizes current knowledge of non-P450 phase I enzymes, focusing on their roles in controlling drug efficacy and adverse reactions as an important aspect of drug development. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Genetic variants of flavin-containing monooxygenase 3 (FMO3) in Japanese subjects identified by phenotyping for trimethylaminuria and found in a database of genome resources

The oxygenation of food-derived trimethylamine to its N-oxide is a representative reaction mediated by human flavin-containing monooxygenase 3 (FMO3). Impaired FMO3 enzymatic activity is associated with trimethylaminuria (accumulation of substrate), whereas trimethylamine N-oxide (metabolite) is associated with arteriosclerosis. We previously reported FMO3 single-nucleotide and/or haplotype variants with low FMO3 metabolic capacity using urinary phenotyping and the whole-genome sequencing of Japanese populations. Here, we further analyze Japanese volunteers with self-reported malodor and interrogate an updated Japanese database for novel FMO3 single-nucleotide and/or haplotype variants. After 3 years of follow up, seven probands were found to harbor the known impaired FMO3 variant p.(Gly191Cys) identified in the database or novel variants/haplotypes including p.(Met66Val), p.(Arg223Gln), p.(Glu158Lys;Glu308Gly;Arg492Trp), and p.(Glu158Lys;Glu308Gly;Pro496Ser). The known severe mutation p.(Cys197Ter) (a TG deletion) and four variants including p.(Tyr269His) and p.(Pro496Ser) were first detected in the updated genome panel. Among previously unanalyzed FMO3 variants, the trimethylamine/benzydamine N-oxygenation activities of recombinant p.(Met66Val), p.(Arg223Gln), p.(Tyr269His), p.(Glu158Lys;Glu308Gly;Arg492Trp), and p.(Glu158Lys;Glu308Gly;Pro496Ser) FMO3 variant proteins were severely decreased (V_max/K_m <10% of wild-type). Although the present novel mutations or alleles were relatively rare, both in self-reported Japanese trimethylaminuria sufferers and in the genomic database panel, three common FMO3 missense or deletion variants severely impaired FMO3-mediated N-oxygenation of trimethylamine.

Impact of trimethylaminuria on daily psychosocial functioning

BACKGROUND

Trimethylaminuria (TMAU) (OMIM #602079) is a rare inherited metabolic condition. TMAU is associated with decreased hepatic trimethylamine N-oxidation, which leads to an excess of the volatile trimethylamine (TMA) instead of substrate conversion to trimethylamine N-oxide (TMAO). TMA is a tertiary amine derived from the enterobacterial metabolism of precursors such as choline and phosphatidylcholine present in the diet, and is also a bacterial metabolite of TMAO, a normal constituent of saltwater fish. When the involved enzyme flavin mono-oxygenase 3 is deficient, TMA builds up and is released in the person's sweat, urine, and breath, giving off a strong body odor. We have recently reported the biochemical and genetic characteristics of 13 Irish adult patients with TMAU attending the main Irish Reference Center. Research on the behavioral and psychosocial aspects of this condition is limited. This study explores the patients' perspectives of living with TMAU in Ireland.

METHODS

A qualitative descriptive phenomenological approach was used. Six adults participated in this study. Data were gathered through semi-structured interviews, which were transcribed and analyzed.

RESULTS

The results suggest that the participants experienced a negative journey to diagnosis. Fear, anxiety, paranoia, and dysfunctional thinking are a constant struggle. Participants reported using avoidant coping mechanisms and strategic planning to navigate daily life.

CONCLUSION

It is considered that the results from this study will inform future interventions with this unique patient cohort.

In vivo drug interactions of itopride and trimethylamine mediated by flavin-containing monooxygenase 3 in humanized-liver mice

Flavin-containing monooxygenase (FMO) catalyzes the oxygenation of a wide variety of medicines and dietary-derived compounds. However, little information is available regarding drug interactions mediated by FMO3 in vivo. Consequently, we investigated interactions between FMO substrates in humanized-liver mice. Trimethylamine-d₉ and itopride were, respectively, intravenously and orally administered to humanized-liver mice (n = 5-7). The pharmacokinetic profiles of itopride (the victim drug) in the presence of trimethylamine (the perpetrator drug) were determined for 24 h after co-administration using liquid chromatography/tandem mass spectrometry. Itopride (10 mg/kg) was extensively oxygenated in humanized-liver mice to its N-oxide. The plasma concentrations of itopride N-oxide after co-administration of itopride and trimethylamine (10 and 100 mg/kg) were significantly suppressed in a dose-dependent manner, but only during the early phase, i.e., up to 2 h after co-administration. With the higher dose of trimethylamine, the areas under the concentration-time curves of itopride and its N-oxide significantly increased (1.6-fold) and decreased (to 60%), respectively; modeling suggested that these modified pharmacokinetics resulted from suppression of the in vivo hepatic intrinsic clearance (to 67%). These results suggest that food-derived trimethylamine may result in interactions with FMO drug substrates immediately after administration; however, the potential for this to occur in vivo may be limited.

Trimethylamine N-oxygenation in cynomolgus macaques genotyped for flavin-containing monooxygenase 3 (FMO3)

Polymorphic human and cynomolgus macaque flavin-containing monooxygenases (FMO) 3 are important oxygenation enzymes for nitrogen-containing drugs. Inter-animal variability of FMO3-dependent drug oxygenations in vivo is suspected in cynomolgus macaques because such variability is evident in humans. Therefore, this follow-up study was performed to investigate the pharmacokinetics of orally administered deuterium-labeled trimethylamine in three cynomolgus macaques genotyped for FMO3. Trimethylamine-d₉ was rapidly absorbed and attained plasma concentrations greater than the background levels of non-labeled trimethylamine. Trimethylamine-d₉ was then converted to trimethylamine-d₉N-oxide. The half-lives, maximum plasma concentrations, and areas under the curve for trimethylamine-d₉ and its N-oxygenated metabolite and the total clearance for orally administered trimethylamine-d₉ were not different among the heterozygote for Q506K FMO3, the heterozygote for V325I FMO3, and the heterozygote for both S99N and F510S FMO3. Trimethylamine N-oxygenation activities mediated by liver microsomes prepared from the same three animals were not substantially different. However, recombinant proteins of the corresponding cynomolgus FMO3 variants showed apparent reduced trimethylamine N-oxygenation activities compared with the wild-type proteins. This study suggests only limited polymorphic effects on the in vivo catalytic function of cynomolgus FMO3. These findings yield important insights in terms of both quantitative and qualitative variations of polymorphic FMO3 in cynomolgus liver.

Novel variants in outer protein surface of flavin-containing monooxygenase 3 found in an Argentinian case with impaired capacity for trimethylamine N-oxygenation

Flavin-containing monooxygenase 3 (FMO3) is a polymorphic drug metabolizing enzyme associated with the genetic disorder trimethylaminuria. We phenotyped a white Argentinian 11-year-old girl by medical sensory evaluation. After pedigree analysis with her brother and parents, this proband showed to harbor a new allele p.(P73L; E158K; E308G) FMO3 in trans configuration with the second new one p.(F140S) FMO3. Recombinant FMO3 proteins of the wild-type and the novel two variants underwent kinetic analyses of their trimethylamine N-oxygenation activities. P73L; E158K; E308G and F140S FMO3 proteins exhibited moderately and severely decreased trimethylamine N-oxygenation capacities (~50% and ~10% of wild-type FMO3, respectively). Amino acids P73 and F140 were located on the outer surface region in a crystallographic structure recently reported of a FMO3 analog. Changes in these positions would indirectly impact on key FAD-binding residues. This is the first report and characterization of a patient of fish odor syndrome caused by genetic aberrations leading to impaired FMO3-dependent N-oxygenation of trimethylamine found in the Argentinian population. We found novel structural determinants of FAD-binding domains, expanding the list of known disease-causing mutations of FMO3. Our results suggest that individuals homozygous for any of these new variants would develop a severe form of this disorder.

Endogenous Roles of Mammalian Flavin-Containing Monooxygenases

Flavin-containing monooxygenases (FMOs) catalyze the oxygenation of numerous foreign chemicals. This review considers the roles of FMOs in the metabolism of endogenous substrates and in physiological processes, and focuses on FMOs of human and mouse. Tyramine, phenethylamine, trimethylamine, cysteamine, methionine, lipoic acid and lipoamide have been identified as endogenous or dietary-derived substrates of FMOs in vitro. However, with the exception of trimethylamine, the role of FMOs in the metabolism of these compounds in vivo is unclear. The use, as experimental models, of knockout-mouse lines deficient in various Fmo genes has revealed previously unsuspected roles for FMOs in endogenous metabolic processes. FMO1 has been identified as a novel regulator of energy balance that acts to promote metabolic efficiency, and also as being involved in the biosynthesis of taurine, by catalyzing the S-oxygenation of hypotaurine. FMO5 has been identified as a regulator of metabolic ageing and glucose homeostasis that apparently acts by sensing or responding to gut bacteria. Thus, FMOs do not function only as xenobiotic-metabolizing enzymes and there is a risk that exposure to drugs and environmental chemicals that are substrates or inducers of FMOs would perturb the endogenous functions of these enzymes.

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.

Development and feasibility of the use of an assessment tool measuring treatment efficacy in patients with trimethylaminuria: A mixed methods study

Trimethylaminuria (TMAU) is a rare metabolic condition characterised by an unpleasant smell resembling rotting fish. Currently, the only measure of treatment efficacy is urine trimethylamine levels which do not always reflect the patient's experience of symptoms. A literature review did not find a specific tool to assess treatment efficacy from the patient's perspective. The aim of this study was to develop an assessment tool to provide a quantitative measure of treatment efficacy in patients diagnosed with TMAU before and after treatment and assess its acceptability (feasibility of use and face and content validity) to people living with TMAU. Mixed methods; a modified, four-round Delphi by email and semi-structured interviews conducted after clinical appointments. Delphi; Eight individuals living with TMAU from the TMAU forum, six medical consultants, and four dieticians in Metabolic Medicine in four National Health Service hospitals in England. Semi-structured interviews; three patients with TMAU in two National Health Service hospitals, United Kingdom. The assessment tool contains 27 items distributed across four domains; Odour characteristics with 6 items, mental well-being with 13 items, social well-being with 5 items, and healthcare professionals support with 3 items. Semi-structured interviews; views on the content and design of the tool. The co-produced tool was successfully developed and considered acceptable to people living with TMAU. While further testing is needed to evaluate the validity and reliability of the assessment tool, the tool may serve as a prompt for questioning for clinicians diagnosing and treating TMAU.

Effect of Flavin-Containing Monooxygenase Genotype, Mouse Strain, and Gender on Trimethylamine N-oxide Production, Plasma Cholesterol Concentration, and an Index of Atherosclerosis

The objectives of the study were to determine the contribution, in mice, of members of the flavin-containing monooxygenase (FMO) family to the production of trimethylamine (TMA) N-oxide (TMAO), a potential proatherogenic molecule, and whether under normal dietary conditions differences in TMAO production were associated with changes in plasma cholesterol concentration or with an index of atherosclerosis (Als). Concentrations of urinary TMA and TMAO and plasma cholesterol were measured in 10-week-old male and female C57BL/6J and CD-1 mice and in mouse lines deficient in various Fmo genes (Fmo1-/- , 2-/- , 4-/- , and Fmo5-/- ). In female mice most TMA N-oxygenation was catalyzed by FMO3, but in both genders 11%-12% of TMA was converted to TMAO by FMO1. Gender-, Fmo genotype-, and strain-related differences in TMAO production were accompanied by opposite effects on plasma cholesterol concentration. Plasma cholesterol was negatively, but weakly, correlated with TMAO production and urinary TMAO concentration. Fmo genotype had no effect on Als. There was no correlation between Als and either TMAO production or urinary TMAO concentration. Our results indicate that under normal dietary conditions TMAO does not increase plasma cholesterol or act as a proatherogenic molecule.

Trimethylamine and Trimethylamine N-Oxide, a Flavin-Containing Monooxygenase 3 (FMO3)-Mediated Host-Microbiome Metabolic Axis Implicated in Health and Disease

Flavin-containing monooxygenase 3 (FMO3) is known primarily as an enzyme involved in the metabolism of therapeutic drugs. On a daily basis, however, we are exposed to one of the most abundant substrates of the enzyme trimethylamine (TMA), which is released from various dietary components by the action of gut bacteria. FMO3 converts the odorous TMA to nonodorous TMA N-oxide (TMAO), which is excreted in urine. Impaired FMO3 activity gives rise to the inherited disorder primary trimethylaminuria (TMAU). Affected individuals cannot produce TMAO and, consequently, excrete large amounts of TMA. A dysbiosis in gut bacteria can give rise to secondary TMAU. Recently, there has been much interest in FMO3 and its catalytic product, TMAO, because TMAO has been implicated in various conditions affecting health, including cardiovascular disease, reverse cholesterol transport, and glucose and lipid homeostasis. In this review, we consider the dietary components that can give rise to TMA, the gut bacteria involved in the production of TMA from dietary precursors, the metabolic reactions by which bacteria produce and use TMA, and the enzymes that catalyze the reactions. Also included is information on bacteria that produce TMA in the oral cavity and vagina, two key microbiome niches that can influence health. Finally, we discuss the importance of the TMA/TMAO microbiome-host axis in health and disease, considering factors that affect bacterial production and host metabolism of TMA, the involvement of TMAO and FMO3 in disease, and the implications of the host-microbiome axis for management of TMAU.

Drug metabolism by flavin-containing monooxygenases of human and mouse

INTRODUCTION

Flavin-containing monooxygenases (FMOs) play an important role in drug metabolism. Areas covered: We focus on the role of FMOs in the metabolism of drugs in human and mouse. We describe FMO genes and proteins of human and mouse; the catalytic mechanism of FMOs and their significance for drug metabolism; differences between FMOs and CYPs; factors contributing to potential underestimation of the contribution of FMOs to drug metabolism; the developmental and tissue-specific expression of FMO genes and differences between human and mouse; and factors that induce or inhibit FMOs. We discuss the contribution of FMOs of human and mouse to the metabolism of drugs and how genetic variation of FMOs affects drug metabolism. Finally, we discuss the utility of animal models for FMO-mediated drug metabolism in humans. Expert opinion: The contribution of FMOs to drug metabolism may be underestimated. As FMOs are not readily induced or inhibited and their reactions are generally detoxifications, the design of drugs that are metabolized predominantly by FMOs offers clinical advantages. Fmo1^(-/-),Fmo2^(-/-),Fmo4^(-/-) mice provide a good animal model for FMO-mediated drug metabolism in humans. Identification of roles for FMO1 and FMO5 in endogenous metabolism has implications for drug therapy and initiates an exciting area of research.

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.