Identification and characterization of the developmentally regulated pattern of expression in the testis of a mouse gene exhibiting similarity to the family of phosphodiesterases

Molecular characterization of TUB, TULP1, and TULP2, members of the novel tubby gene family and their possible relation to ocular diseases

Tubby, an autosomal recessive mutation, mapping to mouse chromosome 7, was recently found to be the result of a splicing defect in a novel gene with unknown function. Database searches revealed that sequences corresponding to the C terminus of the tub protein were highly conserved across a number of species including humans, mice, Caenorhabditis elegans, Arabidopsis, rice, and maize, and that tub was a member of a gene family. We describe here, TUB, the human homolog of mouse tub, and two newly characterized family members, TULP1 for tubby like protein 1 and TULP2. These three family members, which differ in the N-terminal half of the protein, share 60-90% amino acid identity across their conserved C-terminal region and have distinct tissue expression patterns. Alternatively spliced transcripts with 5' variable sequences, three of which have been identified for the tubby gene, may mediate tissue specific expression. We also report that TUB, TULP1, and TULP2 map to human chromosomes 11p15.4, 6p21.3, and 19q13.1, respectively. TULP1 and TULP2 map within the minimal intervals identified for retinitis pigmentosa 14 on chromosome 6p21.3 and cone-rod dystrophy on chromosome 19q13.1. TULP1 and TULP2, which are expressed in the retina, make excellent candidates for these ocular diseases as a mutation within the tub gene is known to lead to early progressive retinal degeneration.

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.

A candidate gene for the mouse mutation tubby

A mutation in the tub gene causes maturity-onset obesity, insulin resistance, and sensory deficits. In contrast to the rapid juvenile-onset weight gain seen in diabetes (db) and obese (ob) mice, obesity in tubby mice develops gradually, and strongly resembles the late-onset obesity seen in the human population. Excessive deposition of adipose tissue eventually leads to a twofold increase of body weight. Tubby mice also suffer retinal degeneration and neurosensory hearing loss. The tripartite character of the tubby phenotype shows striking similarity to human obesity syndromes, such as Alström and Bardet-Biedl. Here we report the identification of a G --> T transversion in a candidate gene that abolishes a donor splice site in the 3' coding region and results in a larger transcript containing the unspliced intron. This alteration is predicted to replace the 44-carboxyterminal amino acids with a 20-amino-acid sequence not found in the wide-type protein. Additionally, a second, prematurely truncated transcript with the unspliced intron is observed in testis messenger RNA and a 2-3-fold increase in brain mRNA is observed in tubby mice compared to B6. The phenotype features of tubby mice may be the result of cellular apoptosis triggered by expression of the mutuated tub gene.