Abstract 391: Evolutionary trajectories and KRAS gene dosage define pancreatic cancer phenotypes

Pancreatic ductal adenocarcinoma (PDAC) has frequent alterations in few genes (KRAS, CDKN2A/TP53/SMAD4) and extensive heterogeneity of cancer drivers beyond. The expectation that mutational landscapes of rare drivers could explain phenotypic diversity has -with few exceptions- not come true. Likewise, PDAC metastasis is not understood, and comparisons of primary/metastasis pairs did not find recurrently mutated “metastasis genes”. Here we show that key aspects of PDAC biology are defined by gene-dosage variation of PDAC signature genes, evolving along distinct evolutionary routes. We found increased gene dosage of the initiating KRAS mutation (KRASMUT-iGD) in human PDAC precursors. Mouse models revealed the importance of KrasMUT-iGD for both, early progression and metastasis, rationalizing the high frequency of PDAC dissemination at diagnosis. To overcome limitations posed to gene dosage studies by PDAC´s stroma-richness, we developed murine cell culture resources comprising 135 primaries/metastases. Integrative analyses of their genomes, transcriptomes and tumor phenotypes, combined with human studies and functional analyses revealed a series of additional KrasMUT-dosage effects: different KrasMUT-levels define distinct cellular morphologies, histopathologies and clinical outcomes, with highest KrasMUT-expression underlying the most aggressive undifferentiated phenotypes. We also observed KrasMUT-dosage-associated cellular plasticity, including epithelial-to-mesenchymal transition. Mechanistically, oncogenic dosage-variation is linked to distinct evolutionary routes, characterized by defined types/states of tumor-suppressor alterations: Phylogenetic tracking studies revealed convergent evolution of KrasMUT-iGD-gains, with dependence on prior homozygous Cdkn2a- or Trp53-loss. By contrast, in Cdkn2aHET cancers, amplifications of known and novel oncogenes (Myc, Yap1, Nfkb2) collaborate with KrasMUT-HET to drive progression, yet with lower metastatic potential. These results also reveal oncogene-selective/context-dependent Cdkn2a-haploinsufficiency, for which Tgfβ pathway alterations provide permissiveness. Our study uncovers universal principles underlying PDAC biology and phenotypic diversification. It describes evolutionary trajectories, identifies their genetic hallmarks and shows how oncogenic dosage-variation is differentially licensed along individual routes to control critical disease characteristics, including early progression, histopathology, metas