Micronuclei induced by aneugens and clastogens in mononucleate and binucleate cells using the cytokinesis block assay

The human in vitro micronucleus (MN) test has become a fast and reliable assay for mutagenicity testing. Currently, this assay is mostly performed with cytochalasin B, which prevents cytokinesis, resulting in polynucleated cells. The number of nuclei per cell indicates the number of nuclear divisions that have occurred since the addition of cytochalasin B. It is recommended that MN are only counted in binucleated lymphocytes, because these cells have finished one nuclear division. Therefore, almost no attention has been paid to MN in mononucleated cells. However, recent studies have indicated that aneugens, but not clastogens, also induce MN in mononucleates. In order to evaluate mononucleates to distinguish between aneugenic and clastogenic effects, we tested some typical aneugens and clastogens in whole blood lymphocyte cultures of four donors with the cytokinesis block micronucleus (CBMN) assay. Results showed that the aneugens diethylstilbestrol (80 microM), griseofulvin (25 microg/ml) and vincristine sulphate (15 microg/ml) increased MN frequencies in mononucleated and binucleated cells, whilst the clastogens mitomycin C (500 ng/ml), bleomycin (6 microg/ml) and doxorubicin (20 microg/ml) increased MN frequency only in binucleates. We also tested the Y heterochromatin decondensing drug berenil (300 microg/ml). Berenil induced an extremely high number of MN in mononucleated as well as in binucleated cells, indicating an aneugenic action. This was confirmed by centromere labelling. The results suggest that MN in mononucleates may be an interesting additional parameter in the CBMN assay. Future studies should clarify whether the micronucleated mononucleate cells have escaped the cytokinesis block and become polyploid.