IDENTIFICATION OF TUBEROUS-SCLEROSIS-2 MESSENGER-RNA SPLICE VARIANTS THAT ARE CONSERVED AND DIFFERENTIALLY EXPRESSED IN RAT AND HUMAN TISSUES

Tuberous sclerosis 2 (Tsc2) gene is the target of a germline insertion in the Eker rat model of inherited cancer susceptibility. This tumor suppessor gene, when mutated, gives rise to a spectrum of epithelial a nd nonepithelial neoplasms in the rat, as well as multisystem involvem ent of hamartomas in the human. In this study, we characterized the ra t Tsc2 cDNA and found that it is highly homologous with the human gene , including a conserved rap1GAP catalytic domain. Sequence analysis of independent rat clones from a kidney cDNA library revealed distinct b ut related variants of the Tsc2 transcripts stemming from alternative splicing involving two noncontiguous exons within the translated regio n. The first of these, located at amino acids 947 to 990, gives rise t o isoforms with or without the 129-bp exon. There exists another varia nt related to the use of a ''cryptic'' splice acceptor site in the dow nstream exon that results in an in-frame 3-bp deletion. A separate 69- bp exon encoding a novel serine-rich amino acid sequence (1272 to 1295 ) was also alternatively spliced. Together, we have found a minimum of four and potentially eight Tsc2 isoforms that are differentially expr essed in a tissue-specific manner. These splice variants are highly co nserved in the human gene, suggesting a possible functional role of th ese Tsc2 isoforms in various cell regulatory and developmental process es.