The tumor suppressor STAG2 cooperates with DREAM to maintain quiescence in the urothelium

The maintenance of quiescence is essential for tissue homeostasis. STAG2 is one of the few genes mutated in the normal urothelium of organ donors, with mutant cells undergoing positive selection 1. STAG2 is also a major tumour suppressor gene 2–4 and its inactivation is an early event in bladder carcinogenesis 1,3. However, the mechanisms through which STAG2, a cohesin component, disrupt urothelial homeostasis remain largely unknown. Here, we show that, in normal murine urothelial cells, Stag2 inactivation disrupts differentiation programs, induces a transient cell cycle entry, and primes cells for clonal expansion under stress conditions. In addition, STAG2 loss in urothelial cells expressing mutant FGFR3 —the most important bladder cancer oncogene 5— enhances tumour formation. For the first time, we show that STAG2 co-operates with the transcriptional complex DREAM, a master regulator of quiescence 6,7, binding to common genomic sites, including genes involved in cell cycle control. STAG2 loss results in changes in the expression of DREAM targets, its composition and chromatin distribution, and in the rewiring of chromatin interactions involving DREAM binding motifs in genes critical for cell cycle entry. Our findings provide strong evidence that STAG2 loss leads to changes in 3D genome organization through a novel mechanism involving the DREAM complex, leading to the disruption of homeostatic quiescence and oncogenic sensitivity. Overall design: To dissect the molecular mechanisms underlying transcriptional changes upon Stag2 depletion in quiescent urothelial cells in vivo, we performed ATAC-Seq on peeled urothelium from Stag2 WT and KO mice one week after TMX administration.