Abstract: The poor correlation of mutational landscapes with phenotypes limits our understanding of the pathogenesis and
metastasis of pancreatic ductal adenocarcinoma (PDAC). Here we show that oncogenic dosage-variation has a critical
role in PDAC biology and phenotypic diversification. We find an increase in gene dosage of mutant KRAS in human
PDAC precursors, which drives both early tumorigenesis and metastasis and thus rationalizes early PDAC dissemination.
To overcome the limitations posed to gene dosage studies by the stromal richness of PDAC, we have developed large
cell culture resources of metastatic mouse PDAC. Integration of cell culture genomes, transcriptomes and tumour
phenotypes with functional studies and human data reveals additional widespread effects of oncogenic dosage variation
on cell morphology and plasticity, histopathology and clinical outcome, with the highest KrasMUT levels underlying
aggressive undifferentiated phenotypes. We also identify alternative oncogenic gains (Myc, Yap1 or Nfkb2), which
collaborate with heterozygous KrasMUT in driving tumorigenesis, but have lower metastatic potential. Mechanistically,
different oncogenic gains and dosages evolve along distinct evolutionary routes, licensed by defined allelic states and/or
combinations of hallmark tumour suppressor alterations (Cdkn2a, Trp53, Tgf?-pathway). Thus, evolutionary constraints
and contingencies direct oncogenic dosage gain and variation along defined routes to drive the early progression of PDAC
and shape its downstream biology. Our study uncovers universal principles of Ras-driven oncogenesis that have potential
relevance beyond pancreatic cancer.
