Topoisomerase II beta mediates the resistance of glioblastoma stem cells to replication stress-inducing drugs

Background: Glioblastoma stem cells (GSC) have been extensively recognized as a plausible cause of glioblastoma resistance to therapy and recurrence resulting in high glioblastoma mortality. Abnormalities in the DNA repair pathways might be responsible for the inability of the currently used chemotherapeutics to eliminate the (GSC) subpopulation. Methods: In this work, we compared the expression of sixty DNA repair related genes between primary glioblastoma cell cultures and the glioblastoma enriched stem cell primary cultures. MTT test was used to analyze the effect of selected drugs and immunofluorescence to evaluate the load of DNA damage. Results: We found several differentially expressed genes and we identified topoisomerase II beta (Top2 beta) as the gene with highest up-regulation in GSC. Also among the tested cell lines the expression of Top2 beta was the highest in NCH421k cells, a well-characterized glioblastoma cell line with all the stemness characteristics. On the other hand, Top2 beta expression markedly decreased upon the induction of differentiation by all trans-retinoic acid. Depletion of Top2 beta increased the sensitivity of NCH421k cells to replication stress inducing drugs, such as cisplatin, methyl-methanesulfonate, hydrogen peroxide, and temozolomide. Consistently, we found an increased load of DNA damage and increased Chk1 activation upon Top2 beta depletion in NCH421k cells. Conclusion: We suggest that Top2 beta may represent a new target for gene therapy in glioblastoma. In addition, the other genes that we found to be up-regulated in GSC versus glioblastoma primary cells should be further investigated as glioblastoma theranostics.