Abstract A22: STAG2, a novel genetic regulator of bladder cancer
Authors: Krishnan Nithya , Aquila Lanni , Vickers Sofia Lage , Ramakrishnan Swathi , Atwood Kristopher , Hu Qiang , Cortes Eduardo Gomez , Wang Jianmin , Woloszynska Anna 
Published: August 2020

Abstract
STAG2 is known as a cohesin complex component, and frequent loss-of-function mutations in bladder cancer (BC) suggest its importance in tumorigenesis. While STAG2 functions in chromatid segregation and colocalizes with CTCF to organize chromatin, we provide evidence that STAG2 may have roles independent of this canonical function. Importantly, unique STAG2 binding sites may correspond to tissue-specific enhancers. We hypothesize that STAG2 acts as a transcription factor that influences BC. Whole- and targeted-exome sequencing of muscle-invasive (MIBC) patient tissues (n=119) revealed a STAG2 mutation in over 10%. Tumor immunohistochemistry confirmed loss of STAG2 protein expression. We observed that patients (n=330) with no STAG2 expression had prolonged median overall (34 vs. 24.5 months, p=.049) and progression-free survival (23 vs. 13.5 months, p=.016) compared to STAG2-positive tumors. To evaluate the functional impact of STAG2 status, we stably knocked down STAG2 in T24 cells. We observed decreased invasion in shSTAG2 cells via transwell migration assays and a significant increase in cisplatin sensitivity. To determine STAG2 interactions with chromatin, we conducted ChIP-seq (WT STAG2) and RNA-seq (upon STAG2 knockdown) in T24 cells. We identified STAG2-dependent dysregulation of epithelial-to-mesenchymal transition genes including MMP2, MMP9, SLUG, and SNAIL, consistent with STAG2's role in invasion. Cistrome pipeline integration of ChIP-seq and RNA-seq data suggests a repressive role for STAG2 (p=.000503). Overlap of STAG2 targets from Cistrome with GeneHancer revealed 2,811 enhancers that STAG2 may regulate. Finally, STAG2 rapid immunoprecipitation mass spectrometry of endogenous protein (RIME) identified 154 potential transcription cofactors that immunoprecipitated with STAG2 in the context of chromatin. As expected, we identified known components of the cohesin complex, such as RAD21, SMC1A, and SMC3. However, many of the proteins identified through RIME have not been previously found to interact with STAG2 as cohesins. Interestingly, some of these STAG2-interacting proteins have been implicated in the regulation of gene expression, such as EIF4A1 and NONO, further supporting our hypothesis that STAG2 is part of a transcriptional repression complex that is important for BC. Our ongoing analyses of STAG2 will reveal if STAG2 enhancer binding is independent of its role in chromatin organization. We suggest that STAG2 may bind to specific enhancers and repress transcription in BC. Loss of STAG2 may push disease towards a less invasive phenotype and may also confer sensitivity to cisplatin treatment. These results are consistent with the idea that loss of STAG2 may be beneficial in a subset of MIBC patients.
Citation Format: Nithya Krishnan, Lanni Aquila, Sofia Lage Vickers, Swathi Ramakrishnan, Kristopher Atwood, Qiang Hu, Eduardo Gomez Cortes, Jianmin Wang, Anna Woloszynska. STAG2, a novel genetic regulator of bladder cancer [abstract]. In: Proceedings of the AACR Special Conference on Bladder Cancer: Transforming the Field; 2019 May 18-21; Denver, CO. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(15_Suppl):Abstract nr A22.