Oncogenic Kras  cells override homeostatic cell elimination mechanisms in adult pancreas via Wnt signalling and cell dormancy.
Epithelial tissues use homeostatic defence mechanisms to actively remove aberrant or poorly functioning cells and maintain tissue health. We show that when present in tissues in low numbers, cells expressing KrasG12D or p53R172H  mutations compete with normal cells for survival and are often eliminated. This implies that tumour initiation would require mechanisms whereby mutant cells override tissue homeostasis and remain in a tissue; however, the biology of these initial events is poorly understood. Here, we use an in vivo model of sporadic tumorigenesis in the adult pancreas to show that cell elimination processes are inefficient and a proportion of KrasG12D or p53R172H expressing cells are never eliminated from the epithelium. Using RNA sequencing of non-eliminated mutant populations, we reveal a general activation of the non-canonical  Wnt pathway. We demonstrate that non-eliminated KrasG12D cells express cell dormancy and diapause gene signatures, suggesting non-eliminated KRasG12D cells adopt a dormant cell state in vivo. Using cell competition assays in vitro, we show that active Wnt or cell cycle arrest blocks elimination of RasV12 mutant cells from normal epithelial cell sheets. Importantly, we demonstrate that Wnt signalling is a general mechanism required to promote retention of KrasG12D and p53R172H cells in tissues in vivo . Our results suggest that mutant cells activate Wnt and/or a dormant cell state to avoid homeostatic tissue defence mechanisms and survive in the adult pancreas. Overall design: To gain an unbiased insight into how non-eliminated mutant cells survive in tissues we performed bulk RNA sequencing. Whole pancreata were harvested from experimental (KC/PC/KPC) and control mice (Control) at 35 days p.i. and digested to cell suspensions. We used anti-leptin antibodies to enrich for acinar/ductal epithelial cells and FACS sorted leptin positive RFP positive cells. We pooled cell suspensions from three mice per genotype to obtain a minimum of 5,000 viable leptin positive RFP positive cells, with n=3 replicates per genotype.