Identifying genes that are required for the maintenance of pancreatic ductal adenocarcinoma

Abstract

We searched for genes that are potentially important for the maintenance of Pancreatic Ductal Adenocarcinoma (PDAC). PDAC is the 4th leading cause for cancer-related deaths and exhibits a 5-year survival rate of less than 5%. Since PDAC is a Kras-driven cancer in that greater than 90% of PDACs contain a Kras mutation, we tested genes that are downstream of Kras. We used RNAi technology to inhibit approximately 30 genes in the canonical Kras effector pathways, Mapk, Pi3k, and Ral. These genes were tested in the context of mouse cell lines derived from a genetically engineered mouse model of PDAC with conditional mutations in Kras G12D and p53. An individual gene-by-gene approach and a pooled high-throughput screening strategy were taken to identify important genes. We identified mTOR, and to a lesser extent, Raptor and Rictor, as genes that are important for the maintenance of PDAC both in vitro and in vivo. In addition, we show that inhibition of mTOR and Raptor are synthetic lethal in that PDAC lines are sensitive to their inhibition, while non-tumorigenic cell lines are not as sensitive. Moreover, inhibition of mTOR results in downregulation of mTORCl and mTORC2 targets, while inhibition of Raptor induces downregulation of only mTORC 1 targets. As combination therapies are likely to be more effective, we looked for a drug that could combine effectively with mTOR and Raptor. From screening several small molecule drugs that target the Mapk and Pi3k pathways, we found PDAC lines to be particularly sensitive to AZD6244, while normal cell lines are significantly less sensitive. The combination or either an mTOR or Raptor hairpin and AZD6244 was found to be additive in that the effect on viability is significantly greater than that of each intervention alone. Another approach to combinations is to combine two drugs. AZD6244 and BEZ235, an inhibitor of Pi3k and mTOR, were tested in combination on both PDAC and normal cell lines. The combination was synergistic in PDAC lines but not in normal lines, suggesting that the combination may be effective with low toxicity. In summary, through a screen, we have identified mTOR, Raptor, and Rictor, as being critical components for the maintenance of PDAC.