Patrick W. Kleyn

Identification and Characterization of the Mouse Obesity Gene tubby: A Member of a Novel Gene Family

The mutated gene responsible for the tubby obesity phenotype has been identified by positional cloning. A single base change within a splice donor site results in the incorrect retention of a single intron in the mature tub mRNA transcript. The consequence of this mutation is the substitution of the carboxy-terminal 44 amino acids with 24 intron-encoded amino acids. The normal transcript appears to be abundantly expressed in the hypothalamus, a region of the brain involved in body weight regulation. Variation in the relative abundance of alternative splice products is observed between inbred mouse strains and appears to correlate with an intron length polymorphism. This allele of tub is a candidate for a previously reported diet-induced obesity quantitative trait locus on mouse chromosome 7.

Targeted deletion of the tub mouse obesity gene reveals that tubby is a loss-of-function mutation.

ABSTRACT The mouse tubby phenotype is characterized by maturity-onset obesity accompanied by retinal and cochlear degeneration. A positional cloning effort to find the gene responsible for this phenotype led to the identification of tub, a member of a novel gene family of unknown function. A splice defect mutation in the 3′ end of the tub gene, predicted to disrupt the C terminus of the Tub protein, has been implicated in the genesis of the tubby phenotype. It is not clear, however, whether the Tub mutant protein retains any biological activity, or perhaps has some dominant function, nor is it established that thetubby mutation is itself responsible for all of the observed tubby phenotypes. To address these questions, we generated tub-deficient mice and compared their phenotype to that of tubby mice. Our results demonstrate thattubby is a loss-of-function mutation of the tubgene and that loss of the tub gene is sufficient to give rise to the full spectrum of tubby phenotypes. We also demonstrate that loss of photoreceptors in the retina oftubby and tub-deficient mice occurs by apoptosis. In addition, we show that Tub protein expression is not significantly altered in the ob, db, or melanocortin 4 receptor-deficient mouse model of obesity.