Targeting Replication Stress for Glioma Stem Cell Specific Cytotoxicity

Abstract AIMS Glioma stem cells (GSC) show high levels of DNA replication stress and exhibit abnormal S phase in comparison to differentiated, non GSC tumour cells. We investigated the DNA replication phenotypes of GSC and mechanism of GSC specific cytotoxicity following DNA replication stress response inhibition with combined ATR/PARP inhibition (CAiPi). METHOD Paired GSC enriched (‘GSC’) and GSC deplete differentiated (‘bulk’) populations were cultured from resected GBM specimens and maintained in neurobasal media with growth factors or serum containing media respectively. Cell viability, neurosphere and clonogenic assays were used to assess cytotoxicity whilst DNA replication was interrogated utilising immunofluorescent repair foci, flow cytometry and DNA fibre assay. CAiPi consisted of VE821 (ATRi) with Olaparib (PARPi). RESULTS GSC show reduced DNA replication rates and alterations in cell cycle profile and demonstrate extreme sensitivity to CAiPi. CAiPi resulted in significantly elevated levels of 53BP1 nuclear bodies in GSC indicating genomic under- replication, however did not preferentially reduce fork speed compared to non-GSC. Analysis of replication fork structures revealed an increase in new origin firing in GSC dependent upon PARP trapping. GSC cytotoxicity was reduced by roscovitine induced inhibition of origin firing, suggesting an important role of dysregulated origin firing in CAiPi response. CAiPi is also potently radiosensitising by clonogenic assay. CONCLUSION Taken together these data suggest that GSC are vulnerable to dysregulation of origin firing which results in under-replication of their genome and reliance upon G1 cell cycle resolution of DNA damage. We propose that CAiPi is an attractive therapeutic strategy for translation to the clinic.