Abstract PR004: Dissecting the critical ERK functions that support KRAS-driven pancreatic cancer

Abstract The KRAS oncogene is mutationally activated in 95% of pancreatic ductal adenocarcinoma (PDAC) cases. PDAC carries a 5-year survival rate of only 8%, and pancreatic cancer remains the third leading cause of cancer-related deaths in the United States. Recent clinical evaluation of RAS inhibitors in PDAC has shown promising efficacy comparable to first-line chemotherapy; however, nearly all initial responders ultimately relapse. Circulating tumor DNA sequencing from relapsed patients has begun to reveal mechanisms of resistance, most of which converge on reactivation of the KRAS effector ERK-MAPK cascade. Supporting this model, we recently demonstrated that ectopic expression of gain-of-function ERK1 or ERK2 mutants induces near-complete resistance to both mutant-selective and pan-RAS inhibitors. Despite ERK’s central role, how ERK functionally supports KRAS-dependent tumor growth remains poorly defined. We recently identified more than 2, 000 ERK-regulated genes and over 2, 000 ERK-regulated phosphoproteins, underscoring the complexity of ERK signaling. To dissect the specific roles of ERK substrates and gene targets in KRAS-driven oncogenesis, we engineered ERK mutants impaired in two substrate-recognition domains: the DEF-binding pocket (DBP) and common-docking (CD) domains. Using constitutively active ERK1 and ERK2 constructs disrupted for DEF- or D-motif interactions, we found that both domains are essential for ERK to rescue KRAS inhibition-induced growth suppression. RNA-sequencing and proteomic analyses revealed that DBP disruption profoundly alters cellular programming, including dysregulation of key transcription factors such as MYC and FRA1. Phosphoproteomic analyses of DBP- and CD-domain mutant models further showed extensive differences in substrate engagement. In parallel with these genetic models, we employed small molecules that selectively target individual ERK substrate-binding domains. Consistent with our genetic data, blocking either the DBP or CD domain with a small molecule is sufficient to inhibit PDAC cell proliferation. In summary, our work contributes to defining and to identifying the critical ERK substrates and downstream pathways that sustain KRAS-mediated PDAC growth and to identify new strategies to overcome resistance to RAS inhibitors. Citation Format: Jeffrey A. Klomp, Carter Griffioen, Samantha Hrbek, Laura E. Herring, Lee M. Graves, Adrienne Cox, Clint A. Stalnecker, Channing J. Der, Jennifer E. Klomp. Dissecting the critical ERK functions that support KRAS-driven pancreatic cancer abstract. In: Proceedings of the AACR Special Conference in Cancer Research: RAS Oncogenesis and Therapeutics; 2026 Mar 5-8; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Res 2026;86 (5Suppl₁): Abstract nr PR004.