Biomarkers of oxidative stress and their application for assessment of individual radiosensitivity

Abstract: Radiotherapy is one of the most common therapeutic methods for treatment of many types of cancer. Despite many decades of development and experience there is much to improve, both in efficacy of treatment and to decrease the incidences of adverse healthy tissue reactions. Around 20 % of the radiotherapy patients show a broad range in the severity of normal tissue reactions to radiotherapy, and dose limits are governed by severe reactions in the most radiosensitive patients (< 5 %). Identification of patients with low, moderate or high clinical radiosensitivity before commencing of radiotherapy would allow individual adaptation of the maximum dose with an overall increase in the cure rate. Characterization of factors that may modify the biological effects of ionizing radiation has been a subject of intense research efforts. Still, there is no assay currently available that can reliably predict the clinical radiosensitivity. The aim of this work has been to investigate the role of oxidative stress in individual radiosensitivity and evaluate novel markers of radiation response, which could be adapted for clinical use.8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG), a general marker of oxidative stress, is one of the major products of interaction of ionizing radiation with DNA and the nucleotide pool of the cell. As 8-oxo-dG is highly mutagenic due to incorrect base pairing with deoxyadenosine, various repair mechanisms recognize and remove 8-oxo-dG. The repaired lesions are released from cells to the extracellular milieu (serum, urine and cell culture medium) where they can be detected as markers for free radical reactions with the nucleic acids.Significant variations in background levels as well as in radiation induced levels of 8-oxo-dG in urine have been demonstrated in breast cancer patients (paper 1). Two major patterns were observed: high background and no therapy-related increase vs. low background and significant increase during radiotherapy for the radiosensitive and non radiosensitive patients respectively.Studies in paper 2 indicated major contribution of the nucleotide pool to the extracellular 8-oxo-dG levels. The results also implicated induction of prolonged endogenous oxidative stress in the irradiated cells. RNA “knock-down” experiments on the nucleotide pool sanitization enzyme hMTH1 in paper 3 lend further experimental evidence to this assumption.The applicability of 8-oxo-dG as a diagnostic marker of oxidative stress was demonstrated in paper 4. Studies on dialysis patients revealed a good correlation between inflammatory responses (known to be associated with persistent oxidative stress) and extracellular 8-oxo-dG.In summary, our results confirm that extracellular 8-oxo-dG is a sensitive in vivo biomarker of oxidative stress, primarily formed by oxidative damage of dGTP in the nucleotide pool with a potential to become a clinical tool for prediction of individual responses to radiotherapy.