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Identification of the coding sequences responsible for Tsc2-mediated tumor suppression using a transgenic rat system.

Hereditary renal carcinomas in the Eker rat are caused by germline retrotransposon insertion in the tuberous sclerosis-2 (Tsc2) gene. We established previously a transgenic Eker rat model into which was introduced a wild-type Tsc2 gene. The embryonic lethality of mutant homozygotes and renal carcinogenesis of heterozygotes were completely suppressed by this transgene (Tg). The function of the Tsc2 product (tuberin) is not fully understood, although several findings have been obtained mainly in vitro. Therefore, to elucidate the functional domains of Tsc2 in vivo, we generated transgenic Eker rats carrying deletion mutants of the Tsc2 gene. A Tg coding for the C-terminal region (amino acids 1425-1755) suppressed renal carcinogenesis in the Eker rat and interestingly the degree of this suppression correlated with the level of expression of the Tg. Notably, the product of this Tg lacks the ability to bind to the Tsc1 product (hamartin). Surprisingly, while a Tg lacking the C-terminus of tuberin (amino acids 1-1755) completely suppressed renal carcinogenesis, it partially rescued homozygous mutants from embryonic lethality. In conclusion, we have determined the minimal region of tuberin necessary for tumor suppression but the suppressive effect was quantitative. Tuberin could function as a tumor suppressor without binding to hamartin. The requirement of the functional domain(s) of tuberin might differ for prevention of embryonic lethality and for suppression of renal carcinogenesis.