Ras inhibition by a novel bacterial protease

Aberrant function and regulation of small GTPases is often associated with the development of human malignancies, thus rendering these proteins highly interesting targets in drug discovery. Ras is small GTPase that centrally regulates cell growth. Ras mutations often drive constitutive cell proliferation and are found in 30% of cancers, including lung, colorectal, and pancreatic cancers. In particular, specific mutations in KRas isoform are directly involved in the initiation and progression of 90% of pancreatic cancers. Ras has been deemed as “undruggable”, since three decade of effort did not yield clinically relevant direct inhibitors. Several bacterial toxins are known to irreversibly inactivate small GTPase signaling to effectively block host immune response and enable survival of the bacterium within the hostile environment, but their exploitation as therapeutics against cancer has yet to be realized. Recently, we found that RRSP, an effector domain of bacterial MARTX toxins, specifically cleaves the common oncogenic Ras and thereby kills cancer cells. Moreover, targeting Ras downstream effectors has failed as a therapeutic strategy due to acquired resistance of cancer cells. Thus, for drug development against Ras, radically novel approaches are urgently needed. We hypothesize this RRSP enzymatic function and/or its effects on Ras structure can be harnessed for novel cancer treatments. Here we propose to investigate first the structure of cleaved KRas to identify conformational differences with intact KRas and oncogenic mutants. Second we will conduct biochemical studies to elucidate functional variation induced by RRSP in commonly occurring mutant forms of KRas in pancreatic cancers. The planned research has the promise to reveal new structural features that could be exploited for development of novel drugs. Further, knowledge of the RRSP processing mechanism can facilitate development of RRSP or RRSP mimics as therapeutics. The RRSP enzyme has been designed by thousands of years of evolution in bacteria to specifically shut down Ras function to inactivate host cell innate immune responses, increasing bacteria virulence. With this research project we have the great opportunity to understand how RRSP destroys Ras signaling, the prominent cellular pathway involved in the cellular proliferation of the most aggressive malignancies. In the future, this study might represent the first step for the development of novel therapeutics for pancreatic cancer patients. This project matches with the RAS Initiative of NCI to attract cutting-edge ideas to the field of Pancreatic Cancer Action Network. The proposed work can benefit enormously from expertise and collaborations with at FNLCR. In particular, this project can impact directly the Structural Biology and Biochemistry project area, giving a new insight about oncogenic KRas structure and function.