Frequent truncating mutations of STAG2 in bladder cancer

Abstract

Here we report the discovery of truncating mutations of the gene encoding the cohesin subunit STAG2, which regulates sister chromatid cohesion and segregation, in 36% of papillary non-invasive urothelial carcinomas and 16% of invasive urothelial carcinomas of the bladder. Our studies suggest that STAG2 has a role in controlling chromosome number but not the proliferation of bladder cancer cells. These findings identify STAG2 as one of the most commonly mutated genes in bladder cancer.

Figure 1

Frequent truncating mutations of STAG2 in urothelial carcinoma of the bladder. (A) Diagram of STAG2 protein with location of mutations in urothelial carcinomas identified in this study. STAG, stromal antigen domain; SCD, stromalin conserved domain. (B) Examples of complete somatic loss of STAG2 expression by immunohistochemistry in two urothelial carcinomas harboring truncating mutations of STAG2 (nonsense mutation in MDACC 10059 and canonical splice acceptor mutation in MDACC 20711, see Supplementary Figure 10). There is retained expression within the non-neoplastic fibrovascular stroma in each case.

Figure 2

Effects of STAG2 inactivation on proliferation and chromosomal stability in urothelial cancer cells. (A) Western blot demonstrating lentiviral re-expression of wild-type STAG2 in UM-UC-3 cells, which harbor an endogenous truncating mutation. The level of re-expression is comparable to the endogenous level of STAG2 protein in RT4 cells, which harbor a wild-type STAG2 gene. (B) Proliferation of UM-UC-3 pooled clones infected with either lentiviral-STAG2 or lentiviral-empty vector after five days of selection in puromycin, measured via CellTiter-Glo assay. (C) Western blot depicting expression of endogenous STAG2 in individual clones of RT4 cells following infection with either lentiviral-empty vector or lentiviral-STAG2 shRNA. (D) Gaussian distribution plots depicting chromosome numbers per cell in the individual RT4 cell clones depicted in C. Chromosome counts for 100 cells were determined in metaphase spreads for each clone. Individual chromosome numbers are listed in Supplementary Figure 18.