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Invasive bladder cancer has high morbidity and nearly uniform mortality when metastatic, with no therapeutic improvement in many years. Although chemotherapy combined with Chk1 inhibition has been investigated in several cancer types in which TP53 mutation is seen, this combination treatment approach has not been studied in bladder cancer. Recently, cancer genome sequencing efforts have identified CDKN1A (p21) mutations at 14% frequency in invasive bladder cancer, co-occurring half the time with TP53 mutations. We hypothesized that combined CDKN1A-TP53 loss would make bladder cancer sensitive to combined treatment with gemcitabine and Chk1 inhibitor. Here, we show that TP53-CDKN1A double-mutant bladder cancer cell lines, 647V and RT-112, have a remarkable increase in p-Chk1 levels and G2-M arrest in response to gemcitabine treatment, with a heightened sensitivity to combination treatment with gemcitabine and either Chk1 inhibitor PF477736 or AZD7762, in comparison with other bladder cancer cell lines (either TP53 or p21 deficient). In addition, CDKN1A restoration in p21-deficient bladder cancer cells significantly reduced their sensitivity to combined treatment by protecting them from DNA damage and apoptosis. Furthermore, xenograft studies using RT-112 showed a significant synergistic effect of combined gemcitabine-PF477736 treatment on tumor growth. Our findings suggest that TP53/CDKN1A double-mutant bladder cancer cells have a unique dependence on Chk1 activity for the G2-M cell-cycle checkpoint in response to chemotherapy-induced DNA damage. This combination or others involving genotoxic agents and Chk kinase inhibitors is a promising therapeutic approach for bladder cancer with these mutations.

Combined CDKN1A/TP53 Mutation in Bladder Cancer Is a Therapeutic Target