Application of chromosome painting to clastogenicity testing in vitro

To maximise sensitivity, protocols for testing chemicals in chromosomal aberration assays in vitro are designed so that cells are sampled when the peak frequency of aberrations might be expected to occur. They are not designed to measure the frequency of aberrations in cells which survive. Only chromosomal aberrations which are heritable, however, can have any relevance to human health, but the detection of those aberrations most likely to be tolerated (inversions, reciprocal translocations) is notoriously difficult with conventional light microscopy. Current protocol design is justified by arguing that the presence of structural aberrations of any type at early times after treatment indicates a risk that a proportion of aberrations will persist and be maintained in the population. Chromosome painting allows reciprocal exchanges to be relatively easily measured and permits the validity of these assumptions to be tested. To date, the kinetics of induction and dose-response relationships of reciprocal translocations induced by chemicals have been little investigated. We compared the frequency of chromosome-type aberrations in human lymphocytes following treatment with two powerful clastogens, streptonigrin and Trenimon, using conventional staining techniques and chromosome painting. The results show that although reciprocal translocations can be shown to arise and persist in treated populations of human lymphocytes for several days following treatment, their frequency is very low, even at concentrations where large amounts of chromosomal damage are induced, indicating that, at present, the value of using chromosome painting as an adjunct to traditional clastogenicity testing is limited.