Glioblastoma multiforme (GBM) is an aggressive brain tumor that is resistant to all known therapies. Within these tumors, a CD133-positive cancer-initiating neural stem cell (NSC) population was shown to be resistant to gamma radiation through preferential activation of the DNA double-strand break (DSB) response machinery, including the ataxia-telangiectasia-mutated (ATM) kinase. The polycomb group protein BMI1 is enriched in CD133-positive GBM cells and required for their self-renewal in anINK4A/ARF-independent manner through transcriptional repression of alternate tumor suppressor pathways. We report here that BMI1 copurifies with DNA DSB response and nonhomologous end joining (NHEJ) repair proteins in GBM cells. BMI1 was enriched at the chromatin after irradiation and colocalized and copurified with ATM and the histone γH2AX. BMI1 also preferentially copurified with NHEJ proteins DNA-PK, PARP-1, hnRNP U, and histone H1 in CD133-positive GBM cells. BMI1 deficiency in GBM cells severely impaired DNA DSB response, resulting in increased sensitivity to radiation. In turn, BMI1 overexpression in normal NSCs enhanced ATM recruitment to the chromatin, the rate of γH2AX foci resolution, and resistance to radiation. BMI1 thus displays a previously uncharacterized function in controlling DNA DSB response and repair. Pharmacological inhibition of BMI1 combined with radiation therapy may provide an effective mean to target GBM stem cells.

Glioblastoma multiforme (GBM) is

Glioblastoma multiforme (GBM), an aggressive brain tumor of astrocytic/neural stem cell origin, represents one of the most incurable cancers. GBM tumors are highly heterogeneous. However, most tumors contain a subpopulation of cells that display neural stem cell characteristics in vitro and that can generate a new brain tumor upon transplantation in mice. Hence, previously identified molecular pathways regulating neural stem cell biology were found to represent the cornerstone of GBM stem cell self-renewal mechanism. GBM tumors are also notorious for their resistance to radiation therapy. Notably, GBM “cancer stem cells” were also found to be responsible for this radioresistance. Herein, we will analyze the data supporting or not the cancer stem cell model in GBM, overview the current knowledge regarding GBM stem cell self-renewal and radioresistance molecular mechanisms, and discuss the potential therapeutic application of these findings.

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