The influence of DNA ploidy of a human tumor cell line on the frequencies of micronuclei or chromosome aberrations after irradiation.

To develop methods for assessing the intrinsic cellular radiosensitivity, it is important to evaluate the relationship between DNA ploidy of cells and frequencies of micronuclei (MN) or chromosome aberrations after irradiation. From the original human fibrosarcoma cell line HT-1080, we isolated two clones which have different chromosome ploidy: clone 5 is pseudodiploid and clone 1 is heteroploid. We examined the radiosensitivity of the two clones using a clonogenic cell survival assay and a cytokinesis-block MN assay, and by scoring chromosome aberrations using the premature chromosome condensation (PCC) method combined with a fluorescence in situ hybridization (FISH) procedure immediately and at 24 h after irradiation. The MN frequency increased according to the irradiation dose in both clones. The MN frequency of clone 1 was significantly higher than that of clone 5 regardless of whether the assay was performed immediately or 24 h after irradiation. However, when the numbers of MN were normalized by the DNA index of each clone, a significant difference in the frequency of MN was not observed. In the PCC and FISH studies, there was a linear relationship between the radiation dose and the initial breaks of chromosome 4, but the breaks of clone 1 were much more frequent than those of clone 5. Twenty-four h after irradiation, the chromosome 4 breaks of clone 1 were observed much more frequently than those of clone 5 at the same radiation dose. When the numbers of chromosome 4 breaks were normalized by the number of chromosome 4 in each clone without radiation, no such difference in the number of breaks was observed. These findings demonstrated that the DNA content or chromosome ploidy influenced the induction of the MN or chromosome aberrations in HT-1080 cells after irradiation.