Radiosensitivity enhancement of glioblastoma multiforme by an HSV vector.

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Ionizing radiation (IR) is the primary adjuvant treatment for glioblastoma multiforme (GBM), the most aggressive primary brain tumor in adults. The repair of double strand DNA (dsDNA) breaks after IR proceeds along two pathways, nonhomologous end-joining (NHEJ) and homologous repair (HR). The herpes simplex virus (HSV) immediate-early protein, ICP0, has been shown to induce the degradation of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs). DNA-PKcs is involved in NHEJ, the main dsDNA repair pathway in mammalian cells. Two HSV-1 vectors defective for immediate-early (IE) proteins were used to infect human GBM (U87-MG ) cells at a multiplicity of infection (MOI) of 10 PFU/cell. The HSV-1 vector, d106, solely expresses ICP0 while d109 is defective for all IE proteins. Both infected and mock infected cells were treated with escalating single doses of IR (0, 5, 10, and 20 Gy). Cell viability and proliferation after IR was assessed with an MTT cell proliferation assay. Cells infected with d106 showed a dose-dependent decrease in cell survival and proliferation after IR compared to d109 and mock infected cells. Clonogenic survival assays were also performed confirming the effect of d106 infection on U87-MG cell survival after IR. Western blot analysis revealed that DNA-PKcs was degraded between 6 and 24 hours postinfection (hpi) of U87-MG cells by the ICP0-expressing virus d106 but not by d109. These results suggest that increased radiosensitivity of GBM cells may be due to the degradation of DNA-PKcs by ICP0.