Modulation of the base excision repair (BER) pathway in the treatment of glioblastoma with radiotherapy

Abstract

Glioblastoma is an aggressive brain cancer with a median survival rate of 14.6 months post diagnosis. Treatments for glioblastoma include surgery, radiotherapy, and chemotherapy with the alkylating agent temozolomide (TMZ). In 50% of patients, TMZ treatment is ineffective due to the reparative action of the protein O6- MeG DNA methyltransferase (MGMT). The base excision repair (BER) pathway repairs the most common lesions caused by TMZ. This work reports the characterization of several glioblastoma cell lines in terms of their repair status and sensitivity to traditional therapy of X-ray irradiation and TMZ. We find that the expression of BER proteins differed between cell lines, with alkyladenine-DNA-glycosylase (AAG) showing the greatest variation in expression. Sensitivity to TMZ and X-rays was MGMT dependent. Moreover, our results suggest that cell lines expressing higher AAG levels display increased sensitivity to X-rays and TMZ combination treatment in an MGMT independent fashion. Pharmacological inhibition of BER enzymes AP-endonuclease (APE) and polymerase  (PolB) was examined, intending to enhance sensitivity of the glioblastoma cell lines to TMZ and X-ray or proton treatment. Methoxyamine (MX), an inhibitor of AP-endonuclease (APE) activity, leads to a modest increase in TMZ sensitivity. The combination of X-rays, MX and TMZ sensitised MGMT-negative cell lines, this was not seen in proton radiation. PolB inhibition greatly increased TMZ toxicity in conjunction with radiation in glioblastoma cell lines. Proton irradiation systems were analysed and developed within this work, leading to a high-throughput broadbeam irradiation system. These methodologies lead to differences being detected in response to proton irradiation depending on the method used. This might in future, lead to further understanding of low-dose hypersensitivity. In conclusion, the modulation of BER can enhance glioblastoma sensitivity to current treatment modalities, however, this is in an MGMT dependent fashion. These studies could provide insight for current clinical trials.