OP34 * TARGETING RAD51 TO RADIOSENSITISE GLIOBLASTOMA STEM CELLS

Abstract

INTRODUCTION: The capacity of a sub-population of stem-like cells (Glioma stem cells, GSC) to withstand treatment is thought to play a major role in tumour recurrence. It is becoming increasingly apparent that these GSC express elevated levels of DNA repair proteins including the homologous recombination protein Rad51. We have investigated whether targeting Rad51 is a viable mehod of radio-sensitising GSC. METHOD: Patient derived GSC were characterised for repair protein levels using western blot and IF microscopy when cells were cultured in stem cell permissive media and in differentiating conditions (exposure to BMP-4 and/or serum). The expression of Rad51 and its co-expression with stem cell markers in tumour samples was determined using IHC. We used 2 pharmacological inhibitors of Rad51 (RI-1 and B02) on undifferentiated cells to investigate the effect of inhibition on sensitivity to clinically relevant radiation doses RESULTS: Levels of Rad51 were elevated in GSC compared to normal human astrocytes. Differentiation of these cells reduces the expression of Rad51 in parallel with the stem cell markers SOX2 and Nestin. Rad51 was also found to preferentially co-localise with Nestin in tumour samples. The Rad51 inhibitors reduced Rad51 foci formation following irradiation in GSC without affecting γH2AX foci. Both inhibitors caused radiosensitisation when combined with single radiation doses between 1 and 5 Gy, with dose modifying factors of between 1.33 and 1.46 CONCLUSION: The increased expression of Rad51 in GSC and the ability of Rad51 inhibitors to radiosensitise these cells suggest that Rad51 may be a specific target for GSC and that targeting this repair pathway may lead to improved response to radiotherapy in GBM.