A Mouse Model for the Metabolic Effects of the Human Fat Mass and Obesity Associated FTO Gene

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Abstract
Human FTO gene variants are associated with body mass index and type 2 diabetes. Because the obesity-associated SNPs are intronic, it is unclear whether changes in FTO expression or splicing are the cause of obesity or if regulatory elements within intron 1 influence upstream or downstream genes. We tested the idea that FTO itself is involved in obesity. We show that a dominant point mutation in the mouse Fto gene results in reduced fat mass, increased energy expenditure, and unchanged physical activity. Exposure to a high-fat diet enhances lean mass and lowers fat mass relative to control mice. Biochemical studies suggest the mutation occurs in a structurally novel domain and modifies FTO function, possibly by altering its dimerisation state. Gene expression profiling revealed increased expression of some fat and carbohydrate metabolism genes and an improved inflammatory profile in white adipose tissue of mutant mice. These data provide direct functional evidence that FTO is a causal gene underlying obesity. Compared to the reported mouse FTO knockout, our model more accurately reflects the effect of human FTO variants; we observe a heterozygous as well as homozygous phenotype, a smaller difference in weight and adiposity, and our mice do not show perinatal lethality or an age-related reduction in size and length. Our model suggests that a search for human coding mutations in FTO may be informative and that inhibition of FTO activity is a possible target for the treatment of morbid obesity. Geneticists have identified many gene regions that cause human disease by using multiple genetic markers in large populations to find gene regions associated with disease. However, it is often not clear precisely which gene in any given region causes the disease or how the gene exerts its functional effect. For example, a gene variant in the non-coding region of FTO enhances obesity risk, but it is not clear if this is an effect of the FTO gene itself or another gene located nearby. We therefore tested whether FTO regulates body weight in the mouse. We found that a single change (mutation) in the sequence coding for the mouse FTO protein decreases the functional activity of FTO and causes reduced fat mass and body weight. Food intake and activity were normal, but the mutant mice had a higher metabolic rate. In addition, their fat mass was lower than that of normal mice when both were fed a high-fat diet. Our study provides direct evidence that FTO directly affects fat mass and thus is likely to have a role in human obesity. As reduced FTO function decreases body weight in mice, it is worth exploring if pharmaceutical agents that inhibit FTO activity might help reduce human obesity.