Abstract
Radiation-induced DNA double strand breaks stimulate DNA damage checkpoint through activation of ATM. Recently, we reported that residual DNA damage amplified DNA damage signal through growth of foci of DNA damage checkpoint factors. Although cells with large foci persisted G1 arrest, foci were also detected in divided cells by microcolony assay. To understand the discrepancy we performed time-lapse analysis using live cells-imaging system. Normal human diploid cells transfected with the EGFP-tagged 53BP1 gene were used. Upon irradiation, most foci were formed within 30 minutes after irradiation. However, we found that delayed foci formation over several hours after irradiation, indicating indirect formation of DNA damage. Since these delayed foci were frequently observed in cells exposed to ionizing radiation, it is suggested that the signature of the initial DNA damage persist on damaged chromatin. The results indicated that DNA damage response is highly dynamic process, in which both direct and indirect DNA damage is involved. These must be taken into consideration for understanding entire process of DNA damage response.
