ER-phagy and proteostasis defects create a pre-malignant injury state in Kras-mediated tumourigenesis (RNA-Seq)

Pancreatic acinar cells are surprisingly resistant to pre-malignant transformation by the mutant Kras oncogene. Here, reporter mice show that an early, transcriptionally-mediated effect of Kras activation is downregulation of ER-phagy. Subsequently, stochastic failure of ER-phagy correlates with pre-malignant transdifferentiation into acinar-ductal metaplasia (ADM). Genetic perturbation of ER-phagy, combined with proteomics and transcriptomics, reveals endogenous injury, micro-inflammation and ADM across the acinar epithelium. Phenotypically, this occurs alongside intra-acinar formation of distinctive insoluble aggregates of ER proteins, including the injury marker REG3B. Mechanistically, REG3B is shown to bind to the ER-phagy receptor CCPG1 to facilitate degradation. Strikingly, spatial transcriptomics shows that oncogenic Kras also drives this cell state in a highly stochastic fashion, again characterised by protein aggregation, partial ductal identity, evidence of injury and propensity for ADM. Importantly, expression of insoluble mutant REG3B shows that protein aggregate formation directly engenders this state. Pancreatic cancer can thus arise from stochastic proteostatic defects that are both made probable by, and co-operate with, oncogenic Kras.