The microbiome-derived metabolite trimethylamine-N-oxide (TMAO) has attracted major interest and controversy both as a diagnostic biomarker and therapeutic target in atherothrombosis.Plasma TMAO increased in mice on 'unhealthy' high-choline diets and notably also on 'healthy' high-fibre diets. Interestingly, TMAO was found to be generated by direct oxidation in the gut in addition to oxidation by hepatic flavin-monooxygenases. Unexpectedly, two well-accepted mouse models of atherosclerosis, ApoE-/- and Ldlr-/- mice, which reflect the development of stable atherosclerosis, showed no association of TMAO with the extent of atherosclerosis. This finding was validated in the Framingham Heart Study showing no correlation between plasma TMAO and coronary artery calcium score or carotid intima-media thickness, as measures of atherosclerosis in human subjects. However, in the tandem-stenosis mouse model, which reflects plaque instability as typically seen in patients, TMAO levels correlated with several characteristics of plaque instability, such as markers of inflammation, platelet activation, and intraplaque haemorrhage.Dietary-induced changes in the microbiome, of both 'healthy' and 'unhealthy' diets, can cause an increase in the plasma level of TMAO. The gut itself is a site of significant oxidative production of TMAO. Most importantly, our findings reconcile contradictory data on TMAO. There was no direct association of plasma TMAO and the extent of atherosclerosis, both in mice and humans. However, using a mouse model of plaque instability we demonstrated an association of TMAO plasma levels with atherosclerotic plaque instability. The latter confirms TMAO as being a marker of cardiovascular risk.There has been keen interest lately in the microbiome as a novel therapeutic target in the prevention and treatment of cardiovascular diseases. Trimethylamine-N-oxide (TMAO) is a microbiome-derived metabolite that has received a lot of interest as a novel biomarker and potential therapeutic target in atherothrombosis. However, there have been conflicting reports of its association with atherosclerosis and thrombosis, with some even showing an inverse association. In spite of this, in the correct context TMAO appears to be an excellent biomarker of subclinical atherosclerosis that can identify those at short-term and long-term risk of myocardial infarction. Herein, we reveal different dietary-microbiome interactions that help explain the apparent contradictions of TMAO's association with atherothrombosis. This insight is critical for the appropriate use of TMAO as a potential diagnostic and therapeutic target. Furthermore, using a unique murine model, we reveal TMAO to be associated with several features of plaque instability, providing insight into its utility as a marker of cardiovascular risk.
