Bleomycin-induced chromosomal aberrations as measured by chromosome painting

The utility of fluorescent in situ hybridization (FISH) for measurement of the genomic frequencies of translocations has been demonstrated for radiobiological applications under the assumption that the breaks leading to translocations are randomly distributed throughout the genome. Under these circumstances, whole chromosome composite DNA probes "paint" according to their relative DNA content or length and genomic frequencies of translocations can be calculated from measurements of a small fraction of the genome. Here, we examine the utility of FISH for in vitro chemical assays and test the hypothesis that chemically-induced breaks are distributed randomly throughout the genome. Bleomycin, a radiomimetic compound and efficient inducer of chromosomal aberrations, was used as a test chemical to treat human peripheral lymphocytes at different doses during the Go phase. A whole chromosome composite probe mix specific for chromosomes 1, 2, and 4 was used for FISH detection of chromosomal aberrations. As a test for the randomness of induced breaks, exchange aberrations detected by FISH were classified according to individual probe involvement and compared to the expected frequencies of events as calculated from the relative DNA content of the probes. Dose-response curves were also calculated for measured exchange events and dicentric frequencies measures by FISH were compared to the frequency of dicentrics measured by conventional staining. The main aberration types detected after exposure in Go were translocations, dicentric chromosomes, ring chromosomes and broken chromosomes. A linear doseresponse was calculated for dicentrics, total aberrations, and for translocations and dicentrics combined. These compare with cytogenetic results where dose-response curves for bleomycin-induced chromosomal aberrations were generated from measurements by conventional staining. Genomic dicentric frequencies estimated from FISH measurements compared well to dicentric frequencies measured using conventional staining. Both chi-square analyses and linear regression results provide evidence that bleomycin induces breaks randomly. Based on its similarity to radiation, bleomycin might serve as a control for the in vitro analyses of clastogenic chemicals where FISH painting is used in a manner similar to that for radiobiological applications.