Cell cycle checkpoint function in bladder cancer

Background: Cell cycle checkpoints function to maintain genetic stability by providing additional time for repair of DNA damage and completion of events that are necessary for accurate cell division. Some checkpoints, such as the DNA damage G₁ checkpoint, are dependent on p53, whereas other checkpoints, such as the decatenation G₂ checkpoint, are not. Because bladder transitional cell carcinomas (TCCs) often contain numerous chromosomal aberrations and appear to have highly unstable genomes, we analyzed cell cycle checkpoint functions in a panel of TCC lines. Methods: Cell cycle arrest was induced in normal human fibroblasts (NHF1-hTERT) and normal human uroepithelial cells (HUCs), and TCC lines and checkpoint functions were quantified using flow cytometry and fluorescence microscopy. The inducers and checkpoints were ionizing radiation (i.e., DNA damage) (G₁ and G₂ checkpoints), the mitotic inhibitor colcemid (polyploidy checkpoint), or the topoisomerase II catalytic inhibitor ICRF-193 (decatenation G₂ checkpoint). Four of the five TCC lines expressed mutant p53. Results: HUCs had an effective G₁ checkpoint response to ionizing radiation, with 68% of cells inhibited from moving from G₁ into S phase. By contrast, G₁ checkpoint function was severely attenuated (<15% inhibition) in three of the five TCC lines and moderately attenuated (<50% inhibition) in the other two lines. NHF1-hTERT had an effective polyploidy checkpoint response, but three of five TCC lines were defective in this checkpoint. HUCs had effective ionizing radiation and decatenation G₂ checkpoint responses. All TCC lines had a relatively effective G₂ checkpoint response to DNA damage, although the responses of two of the TCC lines were moderately attenuated relative to HUCs. All TCC lines had a severe defect in the decatenation G₂ checkpoint response. Conclusion: Bladder TCC lines have defective cell cycle checkpoint functions, suggesting that the p53-independent decatenation G₂ checkpoint may cooperate with the p53-dependent G₁ checkpoints to preserve chromosomal stability and suppress bladder carcinogenesis.