Abstract A197: Oncogenic KRAS is dependent on the scaffold protein CNKSR1 for cell growth and signaling.
Authors: Ihle Nathan T. , Grandjean Geoff , Kingston John , Zhang Shuxing , Du-Curry Lei , Indarte Martin , Kirkpatrick Lynn , Powis Garth 
Published: June 2012

KRAS is the predominant form of mutated RAS (mut-KRAS) and is found in 25% of patient tumors across many cancer types. Mut-KRAS is well known to play a critical role in driving tumor growth and resistance to therapy, and its effects are so powerful that it overrides the activity of many of the new molecularly targeted signaling drugs being developed for cancer today such that they cannot be used in patients with mut-KRAS. However, despite extensive effort, there is no effective treatment for mut-KRAS. The effects of mut-KRAS are mediated through multiple downstream signaling pathways which have been independently associated with tumorigenesis, including RAF1, RALGDS, and PI3K. Current efforts to treat mut-KRAS tumors employ concurrent treatment with inhibitors of the RAF and PI3K pathways but this does not address the potential contribution of other pathways for which there are currently no inhibitors. Using a global siRNA screen we searched for genes that when inhibited would block the growth of mut-KRAS cancer cells without affecting wild type-KRAS (wt-KRAS) cell growth using an isogenic MiaPaCa-2 pancreatic cell line with and without oncogenic KRAS, and validated these hits in a similar isogenic HCT-116 colon cell line. From the screen we identified CNKSR1 (connector enhancer of kinase suppressor of Ras 1) as a top hit. CNKSR1 is found associated with KRAS in the RAS membrane associated signaling nanocluster that KRAS has to be associated with to provide growth signals. Knockdown of CNKSR1 with siRNA inhibited the growth of a panel of 10 mut-KRAS non small cell lung cancer (NSCLC) cell lines but not of 4 NSCLC cell lines with wt-KRAS. CNKSR1 is a multidomain protein that has a potentially druggable plekstrin homology (PH) domain responsible for binding to membrane phosphatidylinositols-3-phosphates. In order to demonstrate whether the PH domain of CNKSR1 is necessary for mut-KRAS activity we over expressed the PH domain in H1373 mut-KRAS NSCLC cells and found that it acted as a dominant negative and inhibited cell growth. We suggest that the PH domain fragment competes with the full length CNKSR1 in the cell. We also showed that knockdown of CNKSR1 inhibited KRAS dependent phosphorylation of RAF1 in multiple non small cell lung cancer cell lines. Together our results suggest that the CNKSR1 protein, acting through it's PH domain, is necessary for cell growth and down stream signaling by the KRAS oncogene. The PH domain can be targeted by inhibitors thus potentially providing agents that will selectively block mut-KRAS signaling and cell growth, creating a therapeutic potential for patients with oncogenic KRAS for which there is currently no effective therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A197.