Human colorectal cancers are known to possess multiple mutations, though how these mutations interact in tumor development and progression has not been fully investigated. We have previously described the FCPIK3ca* murine colon cancer model which expresses a constitutively activated phosphoinositide-3 kinase (PI3K) in the intestinal epithelium. The expression of this dominantly active form of PI3K results in hyperplasia and invasive mucinous adenocarcinomas. These cancers form via a non-canonical mechanism of tumor initiation that is mediated through activation of PI3K and not through aberrations in WNT signaling. Since the Adenomatous Polyposis Coli (APC) gene is mutated in the vast majority of human colon cancers and often occurs simultaneously with PIK3CA mutations, we sought to better understand the interaction between APC and PIK3CA mutations in the mammalian intestine. In this study, we have generated mice in which the expression of a constitutively active PI3K and the loss of APC occur simultaneously in the distal small intestine and colon. Here we demonstrate that expression of a dominant active PI3K synergizes with loss of APC activity resulting in a dramatic changes in tumor multiplicity, size, morphology, and invasiveness. Activation of the PI3K pathway is not able to directly activate WNT signaling through the nuclear localization of CTNNB1 (β-catenin) in the absence of aberrant WNT signaling. Alterations at the transcriptional level, including increased CCND1, may be the etiology of synergy between these activated pathways.

Aberrations in the phosphatidylinositide-3-kinase (PI3K) signaling pathway play a key role in the pathogenesis of numerous cancers by altering cellular growth, metabolism, proliferation, and apoptosis (1). Mutations in the catalytic domain of PI3K that generate a dominantly active kinase are commonly found in human colorectal cancers and have been thought to drive tumor progression, but not initiation (2). However, the effects of constitutively activated PI3K upon the intestinal mucosa have not been previously studied in animal models. Here, we demonstrate that the expression of a dominantly active form of the PI3K protein in the mouse intestine results in hyperplasia and advanced neoplasia. Mice expressing constitutively active PI3K in the epithelial cells of the distal small bowel and colon rapidly developed invasive adenocarcinomas in the colon that spread into the mesentery and adjacent organs. The histological characteristics of these tumors were strikingly similar to invasive mucinous colon cancers in humans. Interestingly, these tumors formed without a benign polypoid intermediary, consistent with the lack of aberrant WNT signaling observed. Together, our findings indicate a non-canonical mechanism of colon tumor initiation that is mediated through activation of PI3K. This unique model has the potential to further our understanding of human disease and facilitate the development of therapeutics through pharmacologic screening and biomarker identification.

The phosphatidylinositide-3-kinase (PI3K) signaling pathway is critical for multiple cellular functions including metabolism, proliferation, angiogenesis, and apoptosis, and is the most commonly altered pathway in human cancers. Recently, we developed a novel mouse model of colon cancer in which tumors are initiated by a dominant active PI3K (FC PIK3ca*). The cancers in these mice are moderately differentiated invasive mucinous adenocarcinomas of the proximal colon that develop by 50 days of age. Interestingly, these cancers form without a benign intermediary or aberrant WNT signaling, indicating a non-canonical mechanism of tumorigenesis. Since these tumors are dependent upon the PI3K pathway, we investigated the potential for tumor response by the targeting of this pathway with rapamycin, an mTOR inhibitor. A cohort of FC PIK3ca* mice were treated with rapamycin at a dose of 6 mg/kg/day or placebo for 14 days. FDG dual hybrid PET/CT imaging demonstrated a dramatic tumor response in the rapamycin arm and this was confirmed on necropsy. The tumor tissue remaining after treatment with rapamycin demonstrated increased pERK1/2 or persistent phosphorylated ribosomal protein S6 (pS6), indicating potential resistance mechanisms. This unique model will further our understanding of human disease and facilitate the development of therapeutics through pharmacologic screening and biomarker identification.

Colorectal cancer (CRC) often arises from adenomatous colonic polyps. Polyps can grow and progress to cancer, but may also remain static in size, regress, or resolve. Predicting which progress and which remain benign is difficult. We developed a novel long-lived murine model of CRC with tumors that can be followed by colonoscopy. Our aim was to assess whether these tumors have similar growth patterns and histologic fates to human colorectal polyps to identify features to aid in risk-stratification of colonic tumors. Long-lived ApcMin/+ mice were treated with dextran sodium sulfate to promote colonic tumorigenesis. Tumor growth patterns were characterized by serial colonoscopy with biopsies obtained for immunohistochemistry and gene expression profiling. Tumors grew, remained static, regressed, or resolved over time with different relative frequencies. Newly developed tumors demonstrated higher rates of growth and resolution than more established tumors that tended to remain static in size. Colonic tumors were hyperplastic lesions (3%), adenomas (73%), intramucosal carcinomas (20%), or adenocarcinomas (3%). Interestingly, the level of β-catenin was higher in adenomas that became intratumoral carcinomas as compared to those that failed to progress. In addition, differentially expressed genes between adenomas and intramucosal carcinomas were identified. This novel murine model of intestinal tumorigenesis develops colonic tumors that can be monitored by serial colonoscopy, mirror growth patterns seen in human colorectal polyps, and progress to CRC. Further characterization of cellular and molecular features are needed to determine which features can be used to risk-stratify polyps for progression to CRC and potentially guide prevention strategies.