Therapeutic Discovery A Role for Homologous Recombination and Abnormal Cell-Cycle Progression in Radioresistance of

Glioblastomamultiforme (GBM) is themost common formofbrain tumorwith apoorprognosis and resistance toradiotherapy.Recentevidencesuggests thatglioma-initiatingcellsplayacentral role inradioresistance through DNAdamage checkpoint activation and enhanced DNA repair. To investigate this inmore detail, we compared the DNA damage response in nontumor forming neural progenitor cells (NPC) and glioma-initiating cells isolated fromGBMpatient specimens.Asobserved forGBMtumors, initial characterization showed that glioma-initiating cells have long-term self-renewal capacity. They expressmarkers identical toNPCsandhave the ability to form tumors in an animalmodel. In addition, these cells are radioresistant to varying degrees,which could not be explained by enhanced nonhomologous end joining (NHEJ). Indeed, NHEJ in glioma-initiating cells was equivalent, or in some cases reduced, as compared with NPCs. However, there was evidence for more efficient homologous recombination repair in glioma-initiating cells. We did not observe a prolonged cell cycle nor enhancedbasal activationof checkpointproteins as reportedpreviously.Rather, cell-cycledefects in theG1–Sand S-phase checkpoints were observed by determining entry into S-phase and radioresistant DNA synthesis following irradiation. These data suggest that homologous recombination and cell-cycle checkpoint abnormal-itiesmaycontribute to the radioresistanceofglioma-initiating cells and thatbothprocessesmaybesuitable targets for therapy. Mol Cancer Ther; 11(9); 1–10. 2012 AACR