Comparison of DNA double strand break repair between irradiated normal human lymphocytes and normal human fibroblasts.

Lymphocytes are known to be sensitive to ionizing radiation, but the crucial factor causing this hyper radiosensitivity is still unclear. To define the cause of hyper radiosensitivity in lymphocytes, we examined the repair process at cellular and molecular levels. We measured the difference in rejoining kinetics of chromosome and DNA double strand breaks between normal human lymphocytes and normal fibroblasts irradiated with X-rays using the fusion-based G1-type premature chromosomal condensation (PCC) method and immnostaining. The expressions of NHEJ associated proteins were also investigated by Western blot. To detect chromosome damage, ice-cold normal cells were exposed to 2Gy of X-rays and processed for the PCC assay. When a DSB repair inhibitor wortmannin was applied during the PCC incubation period, the initial number of breaks for lymphocytes was identical to that for fibroblasts. The rejoining of irradiated lymphocyte\u27s chromosomes was significantly delayed when compared to that of fibroblasts. Immnostaining was carried out using an anti-gH2AX antibody and the number of foci was counted under a fluorescence microscope. Although the number of foci was lower in irradiated lymphocytes, the disappearance of these foci was much slower in lymphocytes than in fibroblasts. Western blot analysis reveled that the expression of DNA-PKcs protein was modified in lymphocytes when compared with that of fibroblasts. These results indicate that the reduced repair caused by modified NHEJ protein(s) might be an important contributing factor for the known hyper-radiosensitivity of lymphocytes.52nd Annual Meeting of the Radiation Research Societ