Defining the oncogenic potential and therapeutic dependencies of PDAC-associated KRAS variants - PROJECT SUMMARY Pancreatic ductal adenocarcinoma (PDAC) is a deadly cancer, often diagnosed in late stages and with few to no effective treatment options. Understanding the molecular and cellular changes that drive the genesis of PDAC will help define therapeutic opportunities to improve clinical outcomes. The current dogma of PDAC initiation posits that activating mutations in the KRAS oncogene drive cell lineage transformation and proliferation to convert normal pancreatic acini into ductal-like pancreatic intraepithelial neoplasia (PanIN). Our understanding of this process is derived largely from the KrasLSL-G12D mouse model that mimics this near-ubiquitous mutational activation of KRAS in PDAC. However, KRASG12D mutations account for only half of all KRAS mutations. The other cases are driven by KRASG12V (~25%) and KRASG12R (~15-20%) mutations; interestingly, KRASG12R mutations are rarely observed in other cancer types and could reveal PDAC-specific vulnerabilities. In contrast to the commonly accepted disease initiation paradigm, we recently showed – using novel KRAS mouse models - that induction of KRASG12R mutations in the pancreas do not drive acinar-to-ductal metaplasia and PanIN development like KRASG12D, but in the context of cooperating genetic alterations (e.g., p53 loss) in ductal-derived organoids can promote aggressive PDAC, suggesting there are either non-canonical or cell-of-origin-dependent routes to PDAC in G12R mutant cancers. In parallel, we have also used temporally regulated KRASG12D murine models to reveal the requisite chromatin and transcriptomic changes that drive lineage plasticity in nascent pancreatic neoplastic lesions, suggesting that the G12R mutant is deficient in its ability to initiate this epigenetic reprogramming. Molecular characterization of the G12R mutant points to a specific defect in RAC1 signaling because of an inability to upregulate the key RAC1 GEF Vav1. Translational studies from our group have also identified an enrichment of G12R mutations in early-stage disease, with these tumors featuring less frequent lymph node metastases and improved clinical outcomes, including both disease-free and overall survival. Given these data, we hypothesize that the KRASG12R mutation in PDAC has distinct molecular features, oncogenic potential, cooperativity with tumor suppressor loss, and therapeutic vulnerabilities. In this proposal, we will define the role of PDAC-associated KRAS variants by (1) deciphering how cell lineage impacts neoplastic transformation and molecular evolution of epithelial cells in the KRASG12R and KRASG12V mutants; (2) defining the oncogenic factors required for KRASG12R mutant PDAC; and (3) delineating the molecular dependencies and therapeutic responses in KRASG12R and KRASG12V mutant disease. By exploiting our animal models systems, molecular framework for understanding cell lineage and transformation in the pancreas, patient- derived organoids, and in vivo editing technologies this work will define how specific KRAS mutations shape the progression to malignancy and the responses to therapeutic interception.
