Tyrosyl-DNA phosphodiesterase 1 and topoisomerase I activities as predictive indicators for Glioblastoma susceptibility to genotoxic agents

Background Glioblastoma (GBM) patients have an estimated survival of ∼15 months with treatment, and the standard of care only modestly enhances patient survival. Identifying biomarkers representing vulnerabilities may allow for selection of efficacious chemotherapy options to address personalized variations in GBM tumors. Irinotecan, currently in clinical trials for GBM, targets topoisomerase I (TOP1) by forming a ternary DNA-TOP1 cleavage complex (TOP1cc) inducing apoptosis. Tyrosyl-DNA phosphodiesterase 1 (TDP1) is a crucial repair enzyme that may reduce the effectiveness of irinotecan. Methods We treated GBM cell lines with increasing concentrations of irinotecan and compared the IC 50 values. TOP1 and TDP1 protein levels from each cell type as well as GBM patient tumors were determined by Western blot analysis, while activity levels were ascertained by specific enzymatic assays. Cellular TDP1 was elevated by ectopic expression of wild-type or mutant TDP1. Results After comparing cellular susceptibility to TDP1 and TOP1 concentrations and activities, we found that the TDP1/TOP1 activity ratio had the strongest correlation (Pearson correlation coefficient R = 0.92) with IC 50 values following irinotecan treatment. Increasing the TDP1/TOP1 activity ratio by ectopic expression of wild-type TDP1 increased in irinotecan IC 50 , while expression of the TDP1 catalytic-null mutant did not alter the susceptibility to irinotecan. Conclusions TDP1/TOP1 activity ratio may be a new predictive indicator for GBM vulnerability to irinotecan, allowing for selection of individual patients for irinotecan treatment based on risk-benefit. Moreover, TDP1 inhibitors may be a novel combination treatment with irinotecan to improve GBM patient responsiveness to genotoxic chemotherapies. Key Points TDP1/TOP1 activity ratio correlates with irinotecan sensitivity in GBM cell lines. TDP1 and TOP1 protein levels are not reliable predictors for irinotecan activity. TDP1 inhibition is a plausible approach to improve irinotecan effectiveness in GBM. Importance of the Study The current standard of care (surgery, radiation, and chemotherapy) for GBM patients modestly enhances survival beyond ∼15 months. Thus, there is a great need for effective therapies and biomarkers that address personalized variations in GBM tumors to improve treatment outcome. Topoisomerase I (TOP1) is the target of irinotecan. The repair enzyme tyrosyl-DNA phosphodiesterase 1 (TDP1) is known to excise irinotecan-induced TOP1-DNA cleavage complexes (TOP1ccs). Consequently, this study examines the relationship between TOP1 and TDP1 expression and activities in GBM cells and their correlation with irinotecan sensitivity. The results reveal that the TDP1/TOP1 activity ratio predicts irinotecan vulnerability in GBM cell lines. TDP1/TOP1 activity ratio was found to vary among GBM patient tumors. This potential predictive indicator may permit selection of patients responsive to irinotecan based on the capacity to repair TOP1cc. Moreover, inhibitors of TDP1 may represent a promising approach to enhance irinotecan efficacy in GBM patients.