Paclitaxel is an anti-mitotic drug that, due to its success in the clinic, has become a backbone of first-line chemotherapeutic regimens for many malignancies including non-small cell lung cancer (NSCLC). While paclitaxel-based regimens are efficacious for some NSCLC patients, response is often incomplete, rarely curative and unpredictable, indicating widespread intrinsic resistance in chemo-naive tumors. Thus, there is an unmet need for new combinatorial treatment strategies to better target paclitaxel resistant tumor cells. To study the molecular basis for this resistance, we first established a test bed of NSCLC-derived cell lines that evade cell death from high concentrations of paclitaxel due to an uncoupling of mitotic damage from cell death. We then employed a genome-wide loss-of-function cytotoxic screen to identify the molecular components that can re-engage paclitaxel-mediated cell death programs in an otherwise paclitaxel-resistant background. This screen was performed in the presence and absence of a mitotic damaging, yet sub-lethal, dose of paclitaxel. This approach revealed a cohort of proteins that support tumor cell viability in the presence of mitotic damage. From this study, we find that prolonging a mitotic delay, by inhibition of either the APC or novel mitotic regulators, CASC1 and TRIM69, collaborates with a sub-lethal dose of paclitaxel to engage cell death programs. In particular, we find that CASC1, which is frequently co-amplified with KRAS, is essential for microtubule polymerization and mitotic spindle formation. We also identified TRIM69, an E3 ubiquitin ligase, that we find is recruited to the spindle poles during mitosis to support mitotic fidelity. Importantly, stable depletion of either CASC1, or TRIM69, attenuates tumor cell growth in vivo. Finally, we demonstrate that pharmacological inhibition of the APC collaborates with an otherwise sublethal dose of paclitaxel. We hypothesize that during the course of tumor evolution, cancer cells become dependent on mechanisms that support rapid and inappropriate mitotic exit for cell viability and that these same intrinsic mechanisms are engaged to evade anti-mitotic therapeutics. Thus, therapeutic strategies that can prolong a mitotic delay may enhance patient response to paclitaxel-based therapies.Doctor of Philosoph
