CD133+ Glioblastoma Stem-like Cells are Radiosensitive with a Defective DNA Damage Response Compared with Established Cell Lines

Purpose: CD133(+) glioblastoma tumor stem-like cells (TSC) have been defined as radioresistant. However, although previously classified relative to CD133(-) cells, the radiosensitivity of CD133(+) TSCs with respect to the standard glioblastoma model, established glioma cell lines, has not been determined. Therefore, to better understand the radio-response of this cancer stem cell, we have used established cell lines as a framework for defining their in vitro radioresponse. Experimental Design: The intrinsic radiosensitivity of CD133(+) TSC cultures and established glioma cell lines was determined by clonogenic assay. The TSCs and established cell lines were also compared in terms of DNA double-strand break (DSB) repair capacity and cell cycle checkpoint activation. Results: Based on clonogenic analysis, each of the six TSC cultures evaluated was more sensitive to radiation than the established glioma cell lines. Consistent with increased radiosensitivity, the DSB repair capacity as defined by neutral comet assay and yH2AX and Rad51 foci was significantly reduced in TSCs compared with the cell lines. Although G2 checkpoint activation was intact, in contrast to the cell lines, DNA synthesis was not inhibited in TSCs after irradiation, indicating the absence of the intra-S-phase checkpoint. Conclusions: These data indicate that the mechanisms through which CD133(+) TSCs respond to radiation are significantly different from those of the traditional glioblastoma in vitro model, established glioma cell lines. If TSCs play a critical role in glioblastoma treatment response, then such differences are likely to be of consequence in the development and testing of radiosensitizing agents. (Clin Cancer Res 2009;15(16):5145-53)