Involvement of homologous recombination repair after proton-induced DNA damage

Abstract

Protection from chronic exposure to cosmic radiation, which is primarily composed of protons, in future manned missions to Mars and beyond is considered to be a key unresolved issue. To model the effects of cosmic radiation on a living cell, we used Saccharomyces cerevisiae cells harboring various deletions of DNA repair genes to investigate the response of cells to DNA strand breaks caused by exposure to 250 MeV proton irradiation (linear energy transfer of 0.41 keV/μm). In our study, DNA strand breaks induced by exposure to protons were predominantly repaired via the homologous recombination and postreplication repair pathways. We simulated chronic exposure to proton irradiation by treating cells from colonies that survived proton treatment, after several rounds of subculturing, to a second proton dose, as well as additional cell stressors. In general, cells cultured from proton surviving colonies were not more sensitive to secondary cell stressors. However, cells from rad52Δ colonies that survived proton treatment showed increased resistance to secondary stressors, such as γ-rays (1.17 and 1.33 MeV; 0.267 keV/μm), ultraviolet (UV) and proton irradiation and elevated temperatures. Resistance to secondary stressors was also observed in rad52Δ cells that survived exposure to γ-rays, rather than protons, but this was not observed to occur in rad52Δ cells after UV irradiation. rad52Δ cells that survived exposure to protons, followed by γ-rays (proton surviving colonies were cultured prior to γ-ray exposure), exhibited an additive effect, whereby these cells had a further increase in stress resistance. A genetic analysis indicated that increased stress resistance is most likely due to a second-site mutation that suppresses the rad52Δ phenotype. We will discuss possible origins of these second-site mutations. © The Author 2008. Published by Oxford University Press on behalf of the UK Environmental Mutagen Society. All rights reserved.