Haploinsufficiency of Casitas B-Lineage Lymphoma Augments the Progression of Colon Cancer in the Background of Adenomatous Polyposis Coli Inactivation.

Casitas B-lineage lymphoma (c-Cbl) is a recently identified ubiquitin ligase of nuclear β-catenin and a suppressor of colorectal cancer (CRC) growth in cell culture and mouse tumor xenografts. We hypothesized that reduction in c-Cbl in colonic epithelium is likely to increase the levels of nuclear β-catenin in the intestinal crypt, augmenting CRC tumorigenesis in an adenomatous polyposis coli (APC) mouse model. Haploinsufficient c-Cbl mice (APC c-Cbl) displayed a significant (threefold) increase in atypical hyperplasia and adenocarcinomas in the small and large intestines; however, no differences were noted in the adenoma frequency. In contrast to the APC c-Cbl mice, APC c-Cbl crypts showed nuclear β-catenin throughout the length of the crypts and up-regulation of Axin2, a canonical Wnt target gene, and SRY-box transcription factor 9, a marker of intestinal stem cells. In contrast, haploinsufficiency of c-Cbl alone was insufficient to induce tumorigenesis regardless of an increase in the number of intestinal epithelial cells with nuclear β-catenin and SRY-box transcription factor 9 in APC c-Cbl mice. This study demonstrates that haploinsufficiency of c-Cbl results in Wnt hyperactivation in intestinal crypts and accelerates CRC progression to adenocarcinoma in the milieu of APC, a phenomenon not found with wild-type APC. While emphasizing the role of APC as a gatekeeper in CRC, this study also demonstrates that combined partial loss of c-Cbl and inactivation of APC significantly contribute to CRC tumorigenesis.