STAG2 and PPARg as drivers of luminal-type bladder cancer

Introduction Several groups, including ours, have identified STAG2 as a new bladder tumor suppressor gene. STAG2 is part of cohesin, a complex involved in chromosome segregation, DNA repair and chromatin organization. A role for STAG2 in hematopoietic stem cell self-renewal and differentiation has been shown. The mechanisms through which STAG2 contributes to UBC have not been elucidated, but we proposed that they involve processes different from altered chromosome segregation. STAG2 inactivation is associated with a class of differentiated, luminal tumours significantly enriched in the PPARg pathway, a driver of urothelial differentiation. We have set out to determine whether STAG2 inactivation alters stemness and differentiation balance in UBC. Methods We have established 2D cultures of primary murine urothelial cells (NU-1), applied integrated ChIPseq and RNAseq data, and generated organoids from conditional Stag2 knockout mice. Results Stag2 loss of expression is significantly associated with the luminal subtype, both in NMIBC and MIBC - Spontaneously immortalized NU-1 cells undergo urothelial differentiation upon PPARg activation and EGFR inhibition - In differentiated NU-1 cells, STAG2 localizes predominantly at promoters and enhancers and is important for the transcriptional regulation of the associated genes - ChIPseq and co-IP data indicate that PPARg and STAG2 - but not STAG1 - are present in the same complex and cooperate in the regulation of tissue-specific transcription - Stag2 knock out urothelial cells show higher clonogenic potential - Concomitant STAG2 loss and urothelial damage lead to hyperplasia, suggesting a role in homeostatic regeneration Conclusions STAG2 loss of expression is associated with luminal tumors. Our genomic and biochemical data suggest that STAG2 cooperates with PPARg in regulating urothelial differentiation and its loss alters gene expression, priming cells to proliferate. ATAC-seq experiments will shed light on the effects of Stag2 inactivation on genome accessibility and regulation of gene expression in proliferating and differentiated urothelial cells. Organoids and 2D cultures derived from conditional Stag2 knockout mice provide a novel platform for functional analyses and suggest a possible role of Stag2 in stem cell homeostasis.