Wild-type RAS signaling is an essential therapeutic target in RAS-mutated cancers.

Mutated RAS proteins activate downstream effector pathways (RAF-MEK-ERK and PI3K-AKT) to drive oncogenic transformation and progression. Because RAS family members differentially engage these pathways, combined inhibition of both pathways is required to effectively treat RAS-mutated cancers. Here, we found that this was due to signaling contributed by wild-type RAS family members that activated an effector pathway that was poorly engaged by the mutant RAS family member. Wild-type KRAS and NRAS promoted RAF-MEK-ERK signaling in cells expressing mutant HRAS, whereas wild-type HRAS and NRAS promoted PI3K-AKT signaling in cells expressing mutant KRAS. Combining inhibitors targeting the poorly engaged RAS effector pathways with inhibitors targeting the mutant RAS resulted in synergistic cytotoxicity in a manner that depended on wild-type RAS expression. The farnesyltransferase inhibitor tipifarnib blocked mutant HRAS-PI3K signaling and synergized with MEK inhibitors in HRAS-mutated cells, whereas KRAS(G12C) inhibitors blocked mutant KRAS-MEK signaling and synergized with PI3K inhibitors in KRAS(G12C)-mutated cells. Synergy was abolished in MEFs lacking all RAS proteins and in cancer cell lines in which nonmutated RAS family members were deleted. Our data highlight the critical role of wild-type RAS family members in supporting mutant RAS signaling and its importance as a therapeutic cotarget in RAS-mutated cancers.