FMO3

The FMO3 gene provides instructions for making an enzyme that is part of a larger enzyme family called flavin-containing monooxygenases (FMOs). These enzymes break down compounds that contain nitrogen, sulfur, or phosphorus. The FMO3 enzyme, which is made chiefly in the liver, is responsible for breaking down nitrogen-containing compounds derived from the diet. One of these compounds is trimethylamine, which is the molecule that gives fish their fishy smell. Trimethylamine is produced as bacteria in the intestine help digest certain proteins obtained from eggs, liver, legumes (such as soybeans and peas), certain kinds of fish, and other foods. The FMO3 enzyme normally converts fishy-smelling trimethylamine into another compound, trimethylamine-N-oxide, which has no odor. Trimethylamine-N-oxide is then excreted from the body in urine.

Researchers believe that the FMO3 enzyme also plays a role in processing some types of drugs. For example, this enzyme is likely needed to break down the anticancer drug tamoxifen, the pain medication codeine, the antifungal drug ketoconazole, and certain medications used to treat depression (antidepressants). The FMO3 enzyme may also be involved in processing nicotine, an addictive chemical found in tobacco. Normal variations (polymorphisms) in the FMO3 gene may affect the enzyme's ability to break down these substances. Researchers are working to determine whether FMO3 polymorphisms can help explain why people respond differently to certain drugs.

trimethylaminuria - caused by mutations in the FMO3 gene

More than 25 mutations in the FMO3 gene have been identified in people with trimethylaminuria. Most of these mutations lead to the production of a small, nonfunctional version of the FMO3 enzyme. Other mutations change single building blocks (amino acids) used to build the enzyme, which alters its shape and disrupts its function. Without enough functional FMO3 enzyme, the body is unable to convert trimethylamine into trimethylamine-N-oxide effectively. As a result, trimethylamine builds up in the body and is released in an affected person's sweat, urine, and breath. The excretion of this compound is responsible for the strong body odor characteristic of trimethylaminuria. Studies suggest that diet and stress also play a role in determining the intensity of the fish-like odor.