Evaluation of a modified micronucleus assay

The relationship between ionizing radiation-induced cell killing and DNA damage measured by the micronucleus assay was determined in three established cell lines (L929, HL-60, and Chang). Our data revealed a dose-dependent increase of cells bearing multiple micronuclei. Cells with the same number of micronuclei were counted separately up to 50 h after irradiation. The counts of these subsets showed a parallel increase and decrease throughout the study. In order to transform the peak of the micronucleus frequency, occurring over only a brief time period into a less time dependent value, we calculated ratios between the different subsets of micronucleated cells. These ratios converged to values which were almost constant beyond 30 h after irradiation. The values showed correlations with cell survival (clonogenic assay) and radiation dose which were comparable with the correlations with the peak of the micronucleus frequency (maximum micronucleus yield) when utilizing the conventional evaluation of the micronucleus assay performed without cytochalasin B. This means that large-scale time kinetics and additional drugs like cytochalasin B can be avoided by changing the evaluation procedure of the conventional micronucleus assay. The modified assay described in this manuscript revealed apoptosis-induced limitations as recently detected for the maximum micronucleus yield assay.