Background and Aims: The incidence of gallbladder cholesterol stone (GCS) in Tibet and other plateau areas is several times higher than that in plain regions, which brings serious harm to the residents in the plateau areas. Previous studies have shown that the disorder of lipid metabolism is the initial factor for the formation of GCS. Flavin monooxygenase 3 (FMO3) and its metabolite trimethylamine-N-oxide (TMAO) are involved in the formation of GCS (plain region). However, the roles of FMO3 and TMAO in the pathogenesis of GCS in plateau areas is not clear. This study was conducted to observe the changes in FMO3 and TMAO levels in mice GCS model created in a plateau region and then analyze the potential clinical significance. 
Methods: Adult male C57BL/6J mice were reared in Naqu, Tibet (4 500 m above sea level) and randomly divided into GCS model group and control group. Mice in model group were fed high-fat diet (basic diet +15% fat + 1% cholesterol + 0.5% cholic acid) to induce GCS, and those in control group were given normal diet. Eight weeks later, the mice were sacrificed, and the samples were harvested (the presence of cholesterol stones in the gallbladder in model group was a standard for successful model creation, and 20 mice were used in each group). Blood lipids and bile biochemical indexes were measured by automatic biochemical analyzer, plasma TMAO level was measured by high performance liquid chromatography (HPLC), the expression of FMO3 mRNA in liver was detected by qRT-PCR. Spearman correlation coefficient was used to analyze the correlation among the observed variables, and multivariate Logistic regression analysis was used to analyze the risk factors.
Results: The serum total cholesterol (TC), triglyceride (TG), low density lipoprotein (LDL), bile TC, bile acid, phospholipid and cholesterol saturation index (CSI) in model group were significantly higher than those in the control group, while the high-density lipoprotein (HDL) in model group was significantly lower than that in the control group (all P<0.05). The plasma level of TMAO and FMO3 mRNA expression level in the liver in model group were significantly higher than those in control group (both P<0.05). Spearman correlation coefficient analysis showed that FMO3 mRNA and TMAO were positively correlated with plasma TC, TG, LDL, bile TC, bile acid, phospholipid and CSI, and negatively correlated with HDL (all P<0.05). Multivariate Logistic regression showed that the increase of FMO3 mRNA and TMAO were the risk factors for GCS in plateau areas.
Conclusion: The high FMO3 and TMAO levels are closely related to the occurrence GCS in plateau areas, and the mechanism may be probably associated with the lipid metabolism disorders caused by their increase. This provides a new strategy and direction for the prevention and treatment of GCS in plateau areas.