THE MICRONUCLEUS ASSAY - AN EVALUATION OF ITS USE IN DETERMINING RADIOSENSITIVITY IN-VITRO

The cytokinesis-block micronucleus assay was used to measure radiosens itivity in vitro in a panel of seven cell lines. Six of these cell lin es were used to study the major parameters of this assay. We observed varying sensitivities following cytochalasin-B exposure. Treatment wit h 1 mu g/ml cytochalasin-B for 24 h reduced cell survival in four of t he six cell lines by >60%, Cytochalasin-B concentration and post-irrad iation culture time were both found to influence cell-response. In thr ee cell lines (V39, V134 and HX142), a decrease in cytochalasin-B conc entration (2-0.5 mu g/ml) resulted in an increase in the frequency of radiation-induced micronuclei per binucleate cell. In other cell lines , either the opposite (V7M, CHO-K1) or no effect (WiDr) was seen. A li near dose-response was observed between induced damage expressed as th e frequency of micronuclei and radiation dose in all but one melanoma (V39) cell line. Evidence for radiation-induced division-delay, with t he maximum frequency of binucleation in irradiated cultures occurring 24-48 h after that of controls, was only seen in two cell lines. Of pa rticular note, and in contrast to some other published reports, was th e lack of a general correlation between cell-response measured in the clonogenic and the cytokinesis-block micronucleus assays. Consideratio n of lethal lesions, determined from the clonogenic dose-response curv e, with respect to micronucleus frequency showed a complex relationshi p, with one micronucleus per binucleate cell corresponding to a wide r ange of lethal lesions depending on the cell line. It has been postula ted that the binucleate cell with no micronuclei may represent the sur viving cell; however, we found no correlation between the slope of the frequency of these cells with respect to radiation dose and the clono genic alpha slope. These observations should be considered prior to at tempting to use the cytokinesis-block micronucleus assay to measure in vitro radiosensitivity in human tumour cells.