SYNOPSIS
In vitro expansion of cells is often limited by stress-induced premature senescence, and DNA double strand breaks are the most relevant damage related to the irreversible cell cycle arrest caused by oxidative stress. Recently, it has been shown that DNA damaging checkpoint proteins are activated by phosphorylation at the sites of DNA double strand breaks caused by ionizing radiation. ATM protein, which is the product of the gene mutated in radiation sensitive disorder called ataxia-telangiectasia (AT), functions as the primary sensor for DNA double strand breaks, and when visualized with an antibody recognizing phosphorylated ATM at S1981, phosphorylated ATM appears as nuclear foci after irradiation. The foci are induced dose-dependently, and the number of phosphorylated ATM foci is well correlated with the estimated number of DNA double strand breaks, providing an unique and sensitive tool for the measurement of DNA double strand breaks at the physiological level in situ.

Key words:DNA damage, immunofluorescence, DNA damage checkpoint factors