Oncogenic K-Ras Alleles and Their Genetic Interactors in Mouse Models of Colorectal Cancer

Abstract

KRAS (Kirsten rat sarcoma) oncoprotein is mutationally activated in ~40% of colorectal cancers (CRC) and KRAS mutation state is an excellent predictor of the failure of a cancer to respond to conventional chemotherapy or targeted therapy. The colonic epithelium provides a biologically relevant context in which to study allele-specific characteristics of KRAS because CRC displays a broader spectrum of activating mutations than KRAS-mutant tumors in other organs. For example, G13D (10-20%) and A146T (5-10%) are activating mutations that occur much more frequently in CRC than in other cancers. Both mutants were identified in historical studies as oncogenic variants of RAS, but the oncogenic mechanism and biological relevance for the tissue-specific prevalence remains to be elucidated. Understanding the biological properties of different mutant forms of KRAS is especially important for informing cancer therapy, as KRAS remains largely undruggable. I established conditional mouse models of K-RasG13D and K-RasA146T to compare their effects on murine colonic homeostasis and neoplasia to the previously characterized K-RasG12D allele. In homeostasis – when K-Ras is mutated on its own, in the absence of secondary oncogenic mutations – I found that K-RasG13D exhibits a lower level of GTP-binding that correlates with intermediate crypt hyperplasia and downstream Erk activation. In combination with conditional loss of the tumor suppressor Apc, K-RasG13D is capable of driving murine CRC development, albeit with a better survival prognosis than K-RasG12D tumors. Integrating transcriptomic data from our murine tumors and CRC patients, we found that an allele-specific Krasmutant expression signature from genetically simple murine tumors was capable of clustering CRC patient samples by KRAS allele. Finally, we introduced a novel workflow for in vivo validation of allele-specific synthetic lethal hits curated from publicly available data from the Achilles Project. In parallel work, I showed that KrasA146T is capable of cooperating with secondary tissue-specific genetic hits (Apc in colon and Tp53 in pancreas) to promote carcinogenesis, even in pancreatic ductal adenocarcinoma (PDAC) where the mutation is exceedingly rare in humans. Ultimately, I aim to elucidate mutant-specific drug sensitivities by deeply understanding K-Ras biology in the genetically controlled setting of our mouse models.