Mutation of the Retinoblastoma Tumor Suppressor Gene Sensitizes Cancers to Mitotic Inhibitor Induced Cell Death

The retinoblastoma gene Rb is a prototype tumor suppressor, which encodes a protein that is inactivated in a broad range of human cancers through different mechanisms. Rb functions to regulate cell proliferation, differentiation, as well as cell death. Therefore, even though Rb inactivation promotes cancer development, this may also open up certain vulnerabilities of cancers that can potentially be targeted with drug intervention. Based on the assumption that cancers that have mutation, deletion, or rearrangement in the Rb locus represent strong loss of Rb function while cancers with WT Rb on average retain some Rb function, we searched Genomics of Drug Sensitivity in Cancer database to identify cancer drugs that are particularly effective to cancers with Rb genomic alterations. Three mitotic inhibitors were identified from this analysis. We further tested the effects of two mitotic inhibitors, Taxol and STLC, on prostate and breast cancer cells. We demonstrate that the Rb status affects cancer cell sensitivity to these mitotic drugs and that the sensitizing effects of Rb are mediated in part by its regulation of the cell cycle checkpoint protein Mad2. Since the mitotic inhibitors identified in our analysis inhibit mitosis through distinct targets, it is possible that the Rb functional status may serve as a general biomarker for cancer sensitivity to mitotic inhibitors. Because the Rb pathway is inactivated in a large number of human cancers, identification of agents that are particularly effective or ineffective based on the Rb status in cancers can potentially be used generally to matching patients with appropriate treatments to achieve better therapeutic outcome.