Abstract 307: KSR1-dependent modulation of the translational landscape in Ras-driven colorectal cancer

Abstract KRAS is mutated and activated in an estimated 40% of colon cancers but has proven difficult to target directly due to the lack of drug-binding pockets on the surface of Ras. Thus, identifying effectors that transmit signals from oncogenic Ras is a critical step in targeting vulnerabilities in the tumors of patients with colorectal cancer (CRC). Kinase Suppressor of Ras 1 (KSR1) is a molecular scaffold that coordinates the interaction of effectors in the Raf/MEK/ERK signaling cascade downstream of Ras. KSR1 is necessary for oncogenic transformation of cells expressing mutated Ras but is dispensable for normal cell growth making KSR1 an attractive therapeutic target for Ras-driven colon cancers. In CRC cells, KSR1 and ERK mediate Ras-dependent effects on protein translation, notably driving cap-dependent and cap-independent translation of the Myc oncogene. Depletion of KSR1 or treatment with ERK inhibitor (SCH772984) leads to a decrease in phophorylated 4EBP1 and a decrease in PDCD4, which collectively impair protein translation, notably leading to a decrease in Myc translation. Patient-derived CRC organoids with deletion of APC, p53, and SMAD4, and an activating G12D mutation in KRAS treated with ERK inhibitor show a marked decrease in Myc protein expression, coincident with changes in PDCD4 and 4EBP1 that impair protein translation. These observations indicate that the KSR1-dependent ERK activation observed in CRC cell lines is present in pre-clinical colon tumoroid models. Genome-wide polysome profiling in CRC cell lines HCT116 and HCT15 depleted of KSR1 identified mRNAs that were preferentially translated in a KSR1 and ERK-dependent manner. Several of these mRNAs were previously predicted by our Functional Signature Ontology (FUSION) screen to be critical to CRC viability but dispensable for normal cell growth. GSEA identified functional classes of mRNAs whose translation is KSR1-dependent, including mediators of oncogenic signaling in pathways that regulate mTOR, MAPK, WNT and JAK-STAT. Our data suggest that an essential component of oncogenic Ras-induced and KSR1-dependent signaling is the preferential translation of mRNAs supporting the transformed phenotype of CRC cells. Citation Format: Heidi Vieira, Chaitra Rao, Adrian R. Black, Siddesh Southekal, Tomohiro Mizutani, Babu Guda, Hans Clevers, Jennifer D. Black, Robert E. Lewis. KSR1-dependent modulation of the translational landscape in Ras-driven colorectal cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 307.