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BACKGROUND Investigation of mitochondrial DNA (mtDNA) content in glioblastoma has gathered speed in recent years. Our study focusses on the effect of mtDNA content depletion on treatment response in glioblastoma. MATERIALS AND METHODS The study consists of a survival cohort (n=130), a recurrent cohort (32 pairs) and 35 non neoplastic control brain tissues. Malignant glioma cell lines, U87 and LN229 were utilized for radiation biology experiments. Patient tumor tissue was molecularly characterized for IDH, ATRX and TERT promoter mutations, MGMT promoter methylation and EGFR amplification using established methods. Relative quantification of mtDNA content was performed using quantitative real time PCR. Ethidium bromide was used to deplete mtDNA in U87 and LN229 cells. Clonogenic assay and MTT assay were performed to assess sensitivity to radiation and chemotherapy. RESULTS mtDNA content was lower in glioblastoma than in controls. Lower range of mtDNA copy number was associated with poor overall survival (p=0.033) and with markers of tumor aggressiveness like wild type IDH (p=0.02), unmethylated MGMT promoter, amplified EFGR and mutated TERT promoter. Out of 32 recurrent GBM patients, only 19 had received radiation treatment (RT) between the two surgeries and in these patients, mtDNA content was significantly higher in the recurrent tumor when compared to the primary tumor(p=0.01). U87 and LN229 cells showed a higher mtDNA copy number post irradiation (p=0.002). mtDNA depleted U87 and LN229 cells showed higher survival fraction post radiation exposure when compared to parent lines. The IC50 of TMZ was also higher for mtDNA depleted U87 and LN229 cells. CONCLUSION Low mtDNA content is associated with poor survival and aggressive tumor markers in glioblastoma. Lowering the mtDNA content in GBM cells induces radiation resistance as well as resistance to chemotherapy. Irradiation leads to increase in mitochondrial DNA content in both patient samples as well as malignant glioma cell lines.

RDNA-11. LOW MITOCHONDRIAL DNA CONTENT INDUCES TREATMENT RESISTANCE AND CONFERS POOR PROGNOSIS IN GLIOBLASTOMA