The potential value of the neutral comet assay and the expression of genes associated with DNA damage in assessing the radiosensitivity of tumor cells

The assessment of tumor radiosensitivity would be particularly useful in optimizing the radiation dose during radiotherapy. Therefore, the degree of correlation between radiation-induced DNA damage, as measured by the alkaline and the neutral comet assays, and the clonogenic survival of different human tumor cells was studied. Further, tumor radiosensitivity was compared with the expression of genes associated with the cellular response to radiation damage. Five different human tumor cell lines were chosen and the radiosensitivity of these cells was established by clonogenic assay. Alkaline and neutral comet assays were performed in γ-irradiated cells (2-8Gy; either acute or fractionated). Quantitative PCR was performed to evaluate the expression of DNA damage response genes in control and irradiated cells. The relative radiosensitivity of the cell lines assessed by the extent of DNA damage (neutral comet assay) immediately after irradiation (4Gy or 6Gy) was in agreement with radiosensitivity pattern obtained by the clonogenic assay. The survival fraction of irradiated cells showed a better correlation with the magnitude of DNA damage measured by the neutral comet assay (r=-0.9; P<0.05; 6Gy) than evaluated by alkaline comet assay (r=-0.73; P<0.05; 6Gy). Further, a significant correlation between the clonogenic survival and DNA damage was observed in cells exposed to fractionated doses of radiation. Of 15 genes investigated in the gene expression study, HSP70, KU80 and RAD51 all showed significant positive correlations (r=0.9; P<0.05) with tumor radiosensitivity. Our study clearly demonstrated that the neutral comet assay was better than alkaline comet assay for assessment of radiosensitivities of tumor cells after acute or fractionated doses of irradiation.