Evaluation of DNA damage induced by proton and lithium beams in murine melanoma cells

The aim of this study was to evaluate the response of melanoma cells to low and high linear energy transfer (LET) radiation. For this purpose, DNA damage and survival were determined as a function of dose in mouse melanoma cells (B16-F0). Irradiations were performed with monoenergetic proton (14 MeV, 3.4 keV/μm and 2.9 MeV, 14 keV/μm) and lithium (14 MeV, 135 keV/μm) beams generated by the TANDAR accelerator (Buenos Aires, Argentina) and with a Cs γ source. Survival curves were obtained by clonogenic assay and fitted to the linear quadratic model. DNA single strand breaks (SSBs) were evaluated by the alkaline comet assay and double strand breaks (DSBs) by the detection of phosphorylated histone H2AX (γH2AX) foci. Results showed an increase in the α parameter and a decrease in the β parameter of the survival curves as a function of LET. The comet assay showed an increase in SSBs as a function of dose for all the radiations evaluated. γH2AX labeling demonstrated that the number of foci increases as a function of dose. A decrease in the number of foci was found for 1 and 2 Gy of γ rays and 14 MeV protons 6 h post-irradiation, revealing the repair capacity of DSBs for low LET radiation. The comparison of γH2AX labeling results for low and high LET radiations demonstrated an increase in the number of foci as a function of LET. Moreover, there was an increase in the size of γH2AX foci in cells irradiated with lithium beams, which could be attributed to the clusters of DSBs induced by high LET radiation. Concluding, our results showed a good correlation between the evaluation of γH2AX and survival, which is consistent with the fact that DSBs are difficult to repair and may be correlated with cell death.