The deubiquitinase USP9X suppresses pancreatic ductal adenocarcinoma

Abstract

An in vivo transposon screen in a pancreatic cancer model identifies frequent inactivation of Usp9x; deletion of Usp9x cooperates with KrasG12D to accelerate rapidly pancreatic tumorigenesis in mice, validating their genetic interaction. An insertional-mutagenesis screen for genes that cooperate with oncogenic KrasG12D in a mouse model of pancreatic cancer has identified the disruption of Usp9x in more than half of the almost 200 distinct tumours analysed. This is surprising, because previous work had indicated that the deubiquitinase USP9X promoted tumour-cell survival. Functional studies show that loss of Usp9x protects pancreatic cancer cells against apoptosis through an effect on multiple oncogenic signalling pathways. In human pancreatic cancers, Usp9x expression is often lost and this correlates with more advanced disease. Approaches that can trigger re-expression of Usp9x may therefore be useful in the treatment of pancreatic cancer. Pancreatic ductal adenocarcinoma (PDA) remains a lethal malignancy despite much progress concerning its molecular characterization. PDA tumours harbour four signature somatic mutations1,2,3,4 in addition to numerous lower frequency genetic events of uncertain significance5. Here we use Sleeping Beauty (SB) transposon-mediated insertional mutagenesis6,7 in a mouse model of pancreatic ductal preneoplasia8 to identify genes that cooperate with oncogenic KrasG12D to accelerate tumorigenesis and promote progression. Our screen revealed new candidate genes for PDA and confirmed the importance of many genes and pathways previously implicated in human PDA. The most commonly mutated gene was the X-linked deubiquitinase Usp9x, which was inactivated in over 50% of the tumours. Although previous work had attributed a pro-survival role to USP9X in human neoplasia9, we found instead that loss of Usp9x enhances transformation and protects pancreatic cancer cells from anoikis. Clinically, low USP9X protein and messenger RNA expression in PDA correlates with poor survival after surgery, and USP9X levels are inversely associated with metastatic burden in advanced disease. Furthermore, chromatin modulation with trichostatin A or 5-aza-2′-deoxycytidine elevates USP9X expression in human PDA cell lines, indicating a clinical approach for certain patients. The conditional deletion of Usp9x cooperated with KrasG12D to accelerate pancreatic tumorigenesis in mice, validating their genetic interaction. We propose that USP9X is a major tumour suppressor gene with prognostic and therapeutic relevance in PDA.