SOS1 and KSR1 modulate MEK inhibitor responsiveness to target resistant cell populations based on PI3K and KRAS mutation status
Authors: Daley Brianna R. , Vieira Heidi M. , Rao Chaitra , Hughes Jacob M. , Beckley Zaria M. , Huisman Dianna H. , Chatterjee Deepan , Sealover Nancy E. , Cox Katherine , Askew James W. , Svoboda Robert A. , Fisher Kurt W. , Lewis Robert E. , Kortum Robert L. 
Published: December 2022

AbstractKRAS is the most commonly mutated oncogene. Targeted therapies have been developed against mediators of key downstream signaling pathways, predominantly components of the RAF/MEK/ERK kinase cascade. Unfortunately, single-agent efficacy of these agents is limited both by intrinsic and acquired resistance. Survival of drug-tolerant persister cells (DTPs) within the heterogeneous tumor population and/or acquired mutations that reactivate receptor tyrosine kinase (RTK)/RAS signaling can lead to outgrowth of tumor initiating cells (TICs) and drive therapeutic resistance. Here, we show that targeting the key RTK/RAS pathway signaling intermediates SOS1 or KSR1 both enhances the efficacy of, and prevents resistance to, the MEK inhibitor trametinib inKRAS-mutated lung (LUAD) and colorectal (COAD) adenocarcinoma cell lines depending on the specific mutational landscape. The SOS1 inhibitor BI-3406 enhanced the efficacy of trametinib and prevented trametinib resistance by targeting spheroid initiating cells (SICs) inKRASG12/G13-mutated LUAD and COAD cell lines that lackedPIK3CAco-mutations. Cell lines withKRASQ61and/orPIK3CAmutations were insensitive to trametinib and BI-3406 combination therapy. In contrast, deletion of the RAF/MEK/ERK scaffold proteinKSR1prevented drug-induced SIC upregulation and restored trametinib sensitivity across all testedKRASmutant cell lines in bothPIK3CA- mutated andPIK3CAwildtype cancers. Our findings demonstrate that vertical inhibition of RTK/RAS signaling is an effective strategy to prevent therapeutic resistance inKRAS- mutated cancers, but therapeutic efficacy is dependent on both the specific KRAS mutant and underlying co-mutations. Thus, selection of optimal therapeutic combinations inKRAS-mutated cancers will require a detailed understanding of functional dependencies imposed by allele-specific KRAS mutations.Significance StatementWe provide an experimental framework for evaluating both adaptive and acquired resistance to RAS pathway-targeted therapies and demonstrate how targeting specific RAS pathway signaling intermediates SOS1 or KSR1 enhanced effectiveness of and prevented resistance to MEK inhibitors inKRAS-mutated cancer cells with genotypic precision. The contribution of either effector was dependent upon the mutational landscape: SOS1 inhibition synergized with trametinib inKRASG12/G13-mutated cells expressing WT PI3K but not inKRASQ61-mutated cells or ifPIK3CAis mutated.KSR1deletion inhibited MEK/ERK complex stability and was effective in cells that are unresponsive to SOS1 inhibition. These data demonstrate how a detailed understanding of functional dependencies imposed both by allele specificKRASmutations and specific co-mutations facilitates the optimization of therapeutic combinations.