Abstract B050: ERK1 splicing switch triggers lethal hyperactivation and overcomes KRAS inhibitor resistance in pancreatic cancer

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is projected to become the second leading cause of cancer death in the United States by 2040. Oncogenic KRAS, present in ∼90% of PDACs, drives tumor growth through MAPK signaling and ERK1/2 activation. While pan-RAS and mutant-selective KRAS inhibitors can produce initial responses, most tumors develop resistance within 6–12 months, frequently through ERK1/2 reactivation via both genetic and non-genetic mechanisms. Direct ERK inhibition has been limited by toxicity and the high similarity between ERK1 and ERK2, motivating alternative strategies. We identified an ERK1-specific regulatory “switch” in which alternative splicing generates a hyperactive ERK1 isoform that is intrinsically lethal to PDAC cells. Deep RNA-seq was performed in PDAC cells with experimentally altered expression of the splicing factor SMNDC1 (overexpressed in ∼16% of PDACs). This revealed an ERK1 transcript lacking exon 4 (E4), which encodes the activation loop. We defined biological and mechanistic differences between canonical ERK1 and the E4-skipped isoform using gain- and loss-of-function studies, biochemical assays, and in silico structural analyses, coupled with manipulation of SMNDC1. To test therapeutic relevance in KRAS-inhibitor resistance, we targeted SMNDC1 with novel degraders and enforced ERK1 E4 skipping using exon 4 splice-switching oligonucleotides (E4-SSOs). Antitumor activity was evaluated in mice bearing orthotopic PDAC tumors with acquired resistance to multiple next-generation KRAS inhibitors using a systemic delivery approach optimized for PDAC. SMNDC1 promoted oncogenic signaling and tumor growth by favoring ERK1 E4 inclusion, preserving the full activation loop. Enforcing E4 skipping with splice-switching morpholinos/SSOs suppressed tumor growth. Mechanistically, E4 exclusion produced an alternative ERK1 isoform that retained a bilobular kinase structure despite lacking the activation loop and was constitutively active, with ∼2-fold higher activity than canonical ERK1. This hyperactive isoform was tightly constrained by proteasome-dependent degradation, and its expression triggered PDAC cell death. Therapeutically, inducing E4 exclusion—via SMNDC1 degradation or E4-SSOs—reduced tumor burden and approximately doubled survival in orthotopic models with acquired resistance to next-generation KRAS inhibitors. Reprogramming ERK1 splicing to enforce exon 4 skipping induces a lethal hyperactive ERK1 state and overcomes KRAS-inhibitor resistance in PDAC. Targeting this ERK1 splicing switch may offer a distinct therapeutic strategy for PDAC and other RAS/MAPK-driven malignancies. Luisa F. Escobar-Hoyos, Md Siraj Afjalus, Deanne Yugawa, Gilbert Giri, Ching Siang Ong, Priyabrata Mukherjee, Resham Bhattacharya, Daniel Dominguez. ERK1 splicing switch triggers lethal hyperactivation and overcomes KRAS inhibitor resistance in pancreatic cancer [abstract]. In: Proceedings of the AACR Conference on Pancreatic Cancer: New Frontiers in Biology and Therapeutic Development; 2026 Sep 25-28; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(18_Suppl_2):Abstract nr B050.