Production of early and late nuclear DNA damage and extracellular 8-oxodG in normal human skin fibroblasts after carbon ion irradiation compared to X-rays.

This work aims to evaluate genotoxicity of C-ion vs. X-ray irradiation in normal human skin fibroblasts. Clonogenic cell survival was first evaluated using a linear-quadratic model. Then, early and late genotoxicity was quantified by alkaline comet assay, micronucleus test and 8-oxodG extracellular measurement. Survival strongly decreased after C-ions compared to X-rays with a 4.8-fold decrease at D0 – irradiation dose corresponding to 37% of survival. The level of immediate DNA damage was approximately the same after C-ions or X-rays. However, half-time of DNA repair was 1.3-fold decreased after C-ions compared to X-rays leading to a 2.2-fold increase in remaining damage. In the same way, micronucleus formation was 1.7-fold increased 24 h after irradiation and remained 1.8-fold increased 2 weeks after C-ions vs. X-rays when secondary oxidative stress wave occurred. This was related to a 2.6-fold increase in binucleated cells percentage in carbon- vs. X-ray-irradiated fibroblasts. Excretion of 8-oxodG was also 1.5–fold increased after C-ions vs. X-rays. 8-oxodG excretion could therefore participate to the appearance of late waves of oxidative stress. These results showed a stronger genotoxicity of C-ions in skin fibroblasts. More investigations are needed to clarify the discrepancy between the two types of radiations.