RREB1 transcription factor splice variants in urologic cancer.

RREB1 is an alternatively spliced transcription factor implicated in Ras signaling and cancer. Little is known about the expression of RREB1 isoforms in cell lines or human tumors, or about the clinical relevance of the latter. We have developed tools for IHC of RREB1 protein isoform-specific amplification of RREB1 mRNA and selective knockdown of RREB1 isoforms and use these to provide new information by characterizing RREB1 expression in bladder and prostate cancer cell lines and human tissue samples. Previously described splice variants RREB1α, RREB1β, RREB1γ, and RREB1δ were identified, as well as the novel variant RREB1ε. Total and isoform-specific mRNA expression was lower in most but not all tumors, compared with normal tissues. RREB1 IHC performed on a bladder cancer TMA did not indicate a relationship between total RREB1 expression and overall survival after radical cystectomy for invasive bladder cancer. In contrast, in vitro proliferation studies using the UMUC-3 bladder cancer cell line after selective isoform-specific knockdown of expression indicate that RREB1α is not necessary for proliferation, but that RREB1β may be required. These contributions should accelerate progress in the nascent RREB1 field by providing new reagents while also providing clues to the role of RREB1 isoforms in human cancer and raising the possibility of isoform-specific roles in human carcinogenesis and progression.