The small GTPase, Ras, is one of the most frequently mutated oncogenes (20-30%) across all cancers. Ras family members (K-Ras, H-Ras, and N-Ras) regulate diverse biological processes, such as development, growth and protein translation. Dysregulation of Ras contributes fundamentally to the initiation of tumors, the metastatic process and the development of chemoresistance. To this point, Ras oncogenes have stifled direct pharmacological approaches, and inhibitors for the direct effectors of Ras have demonstrated limited or no efficacy within patients. Genetic screens identified point mutations in Kinase Suppressor of Ras (KSR) as potent suppressors of oncogenic Ras, suggesting that KSR could yield novel approaches for Ras-targeted interventions. Although KSR is a member of the protein kinase family, translating KSR mutations into a viable mode of chemical regulation has not yet been practical. KSR belongs to a subclass of protein kinases, termed pseudokinases, which are naturally inactive and therefore belie conventional kinase inhibitor strategies. In recent studies, we identified KSR as a distinct signaling molecule, which functions as a dynamic scaffold of the Ras pathway (Brennan and Dar et al., Nature, 2011). In one state, KSR behaves as an inhibitor of the Ras effector MEK. In another state, KSR is a facilitator of MEK phosphorylation by the kinase immediately downstream of Ras termed RAF. Single point mutations in KSR, which suppress transformation by oncogenic Ras, uncouple the transition between the two states of KSR and suggest novel paths to a chemical suppressor of Ras via KSR. Using structure-based strategies, we will generate small molecules to modulate KSR so that we may stabilize it in distinct conformational states.
We aim to characterize these molecules, and thus the conformational states of KSR, within complex biological systems including engineered cell-based and genetically defined cancer models. Additionally, we will combine our target focused chemistry and profiling with whole animal assays and genetics in the fly. We recently applied this strategy to develop novel therapeutics for Ret-kinase driven cancers (Dar and Das et al., Nature, 2012). Here we will build upon this success to identify molecules that stabilize the Ras-suppressive conformation of KSR as novel treatments for Ras-dependent cancers. Together, this approach will reveal the functional relationship between drug targeting of the pseudokinase KSR and system wide responses within whole animal models of cancer. If successful, this approach will greatly expand opportunities for drug development, reveal regulatory functions and control mechanisms that appear independent of kinase phosphorylation activity, and foster a new paradigm of targeting inactive kinases in diseases including cancer, diabetes, and heart disease.
|Dhawan, Neil S; Scopton, Alex P; Dar, Arvin C (2016) Small molecule stabilization of the KSR inactive state antagonizes oncogenic Ras signalling. Nature 537:112-116|