Advanced cytogenetic biodosimetry using DNA damage markers

Chromosome aberrations have been used as a valuable marker for radiation dose assessment. However, as DNA double strand breaks are rapidly rejoined by DNA repair pathways, the initial number of DNA breaks is practically unable to measure by this method. Consequently, the numbers of dicentrics and translocations, which are caused by mis-rejoining of the initial DNA breaks, are counted for dose estimation. Thus, in order to increase the sensitivity of cytogenetic biodosimetry, some markers reflecting the initial number of breaks are required. Recently, it has been well established that DNA double strand breaks initiate activation of ATM, which phosphorylates various DNA damage checkpoint factors. For example, histone H2AX, a member of histone H2A, is phosphorylated at serine 139, and phosphorylation of histone H2AX results in a sequential accumulation of various DNA damage checkpoint factors at DNA damage sites. They form multiprotein complex, called ionizing radiation-induced foci, which are detectable under microscope. Recently, we found that the foci of phosphorylated H2AX and MDC1 persist into mitosis. Therefore, we established a method that visualizes phosphorylated H2AX foci on methaphase chromosome spreads. We confirmed that the foci were detectable on more than 95% of chromatid breaks. An application of this method for cytogenetic biodosimetry will be discussed.