TY - JOUR T1 - SOS1 and KSR1 modulate MEK inhibitor responsiveness to target resistant cell populations based on PI3K and KRAS mutation status JF - bioRxiv DO - 10.1101/2022.12.06.519395 SP - 2022.12.06.519395 AU - Brianna R. Daley AU - Heidi M. Vieira AU - Chaitra Rao AU - Jacob M. Hughes AU - Dianna H. Huisman AU - Deepan Chatterjee AU - Nancy E. Sealover AU - Katherine Cox AU - James W. Askew AU - Zaria M. Beckley AU - Robert A. Svoboda AU - Kurt W. Fisher AU - Robert E. Lewis AU - Robert L. Kortum Y1 - 2023/01/01 UR - http://biorxiv.org/content/early/2023/02/19/2022.12.06.519395.abstract N2 - KRAS 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 is limited both by intrinsic and acquired resistance. Survival of drug-tolerant persister cells 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 enhanced the efficacy of, and prevented resistance to, the MEK inhibitor trametinib in KRAS-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 TICs in KRASG12- or KRASG13-mutated LUAD and COAD cell lines that lacked PIK3CA co-mutations. Cell lines with KRASQ61 and/or PIK3CA mutations were insensitive to combination therapy with trametinib and BI-3406. In contrast, deletion of the RAF/MEK/ERK scaffold protein KSR1 prevented drug-induced TIC upregulation and restored trametinib sensitivity across all tested KRAS mutant cell lines in both PIK3CA-mutated and PIK3CA wildtype cancers. Our findings demonstrate that vertical targeting of RTK/RAS signaling is an effective strategy to target KRAS-mutated cancers, but the specific combination is dependent both on the specific KRAS mutant and underlying co-mutations. Thus, selection of optimal therapeutic combinations in KRAS-mutated cancers will require a detailed understanding of functional dependencies imposed by allele-specific KRAS mutations.Significance Statement We provide an experimental framework for evaluating both adaptive and acquired resistance to RAS pathway-targeted therapies and demonstrate how vertical inhibition of RAS signaling enhances the effectiveness of MEK inhibitors in KRAS-mutated cancer cells. Targeting RAS pathway signaling intermediates SOS1 or KSR1 inhibited tumor initiating cell formation to prevent trametinib resistance. The contribution of either effector to resistance was dependent upon the mutational landscape: SOS1 inhibition synergized with trametinib KRASG12/G13-mutated cells expressing WT PI3K but not in KRASQ61-mutated cells or if PIK3CA is mutated. KSR1 deletion is effective in cells that are unresponsive to SOS1 inhibition. These data show that optimal therapeutic combinations require a detailed understanding of functional dependencies imposed both by allele-specific KRAS mutations and specific co-mutations.Competing Interest StatementThe Kortum laboratory receives funding from Boehringer Ingelheim to study SOS1 as a therapeutic target in RAS-mutated cancers. ER -