Abstract IA02: New structural insights into KRAS biology

Abstract

Abstract Oncogenic mutations of RAS proteins are responsible for approximately 30% of all human cancers. Among the three RAS isoforms—HRAS, KRAS, and NRAS—KRAS is the most commonly mutated gene (in 86% of RAS-driven cancers) and the mutations are often present in pancreatic, colorectal, and lung cancers. Despite decades of research, no RAS inhibitors have reached the clinic. The NCI RAS Initiative at Frederick National Laboratory for Cancer Research (FNLCR) comprises multidisciplinary teams of scientists that are exploring innovative approaches for designing inhibitors against RAS-driven cancers. Structural biology efforts within the RAS Initiative are focused on solving structures of protein-small-molecule complexes and unraveling new structural insights into KRAS biology. RAS proteins act as binary molecular switches that cycle between active GTP-bound and inactive GDP-bound states. The conversion from inactive to the active form is stimulated by guanine nucleotide exchange factors (GEFs). Conversion back to the inactive form is mediated by GTPase-activating proteins (GAPs). In the active state, RAS proteins interact with a variety of effector proteins, such as RAF kinase, PI3K, and RalGDS, leading to activation of downstream signaling pathways. Oncogenic RAS mutations are predominantly found at amino acid positions G12, G13, and Q61 and impair intrinsic and GAP-mediated GTPase function. In this meeting, I will present our structural work on wild-type and oncogenic mutants of KRAS in the active state, and their complexes with various regulators. We recently solved structures of the most common oncogenic mutants of KRAS in the active state (bound to GMPPNP, a nonhydrolyzable analog of GTP). Structures of KRAS mutants in state 1 conformation show novel pockets that are likely to be amenable to in silico drug discovery approaches. We also solved structures of wild-type and mutant KRAS in complex with two major RasGAP proteins, RASA1/p120GAP and neurofibromin. Structures of RAS-GAP complexes provide insights into impaired GAP-mediated GTP hydrolysis process in the oncogenic mutants of KRAS. SPRED1, which causes Legius syndrome, functions as a negative regulator of the RAS-MAPK pathway and interacts with neurofibromin. Recent studies have shown that the EVH1 domain of SPRED1 binds to the GTPase activating protein-related domain of neurofibromin without interfering with its catalytic activity. Our structural work on a ternary complex consisting of KRAS, neurofibromin, and SPRED1 provides novel insights into the regulation of the RAS-MAPK pathway by neurofibromin and SPRED1. Structural studies on wild-type and oncogenic mutants of KRAS in the active state, and in complex with various regulators, have provided new insights into the KRAS biology as well as new blueprints for structure-based drug discovery approaches. Citation Format: Dhirendra K. Simanshu. New structural insights into KRAS biology [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr IA02.