Cell size matters in gamma-H2AX assay for low-dose alpha particle effect assessment

Gamma-H2AX assay is an immuno-fluorescence experiment that enables detecting the location and number of DNA double strand breaks (DSBs) in cells. Under uniform radiation beam intensity, cells would respond with similar numbers of gamma-H2AX if they are similar in cross section. If cells are not represented by a common size, however, a larger cell has a greater chance of radiation exposure and has a better chance of counting a greater number of foci. In other words, the cell size distribution would be reflected in the FPC distribution. In the conventional gamma-H2AX assay, the mean FPC value solely indicates the level of cellular damage under a certain radiation exposure. The purpose of this study is to investigate the FPC distribution in connection with the cell size distribution. The high-LET alpha beam was employed for radiation exposure so that a single track of radiation leaves a meaningful amount of energy in the cell. Gamma-H2AX is a powerful tool for investigating the cellular response at low-dose exposure. If the gamma-H2AX assay is performed with cells of the same size, 'the average number of foci per cell' may accord with the overall response of sample cells to radiation exposure. With cells of non-uniform size, however, one should be cautious in taking the value as an index of the severity in cellular effect of radiation exposure. According to our experiments, a portion of sample cells carried DSBs of more than 5 times greater number than the mean FPC value and might play a critical role in radio-response