MO14-2 KRAS Q61H mutation evades the regulation of tyrosyl phosphorylation and renders cancer cells resistant to SHP2 inhibitor

KRAS is a small GTPase that functions as a molecular switch to regulate vital eukaryotic cellular functions including proliferation, differentiation and survival. Mutations in KRAS and various other components of the KRAS signaling pathways are among the most common genetic alterations in human cancers. A recent series of reports suggest that SHP2 inhibition (SHP2i) is effective to KRAS-mutated cancers including pancreatic ductal adenocarcinoma (PDAC). However, a lack of understanding of the underlying mechanisms is hindering the clinical advancement of SHP2i for cancer. Here we report that cancer cells that harbor KRAS Q61H mutation (found in about 5% of PDAC patients) are resistant to SHP2i. Our biochemical studies show that the Q61H mutation impairs intrinsic GTP hydrolysis and impedes stimulation of the GTPase cycle by both SOS1 and RASA1, but has negligible impact on binding to BRAF-RBD. Similar to WT-KRAS, the Q61H mutant can be phosphorylated by Src at Y32 and Y64, and both sites can be dephosphorylated by SHP2, although SHP2i does not reduce ERK phosphorylation in KRAS Q61H cells. Whereas phosphorylation of WT and G12 mutants impairs binding to BRAF-RBD, the uncompromised ability of phosphorylated KRAS Q61H to activate MAPK signaling. The independence of KRAS Q61H from the stimulatory roles of SHP2 in reversing Src phosphorylation confers resistance to SHP2i. Together these insights provide an unprecedented mechanistic understanding of oncogenic KRAS mutants that can guide clinical trials of SHP2i for patients with pancreatic and other cancers bearing KRAS Q61H.