DNA-SYNTHESIS INHIBITION AS AN INDIRECT MECHANISM OF CHROMOSOME-ABERRATIONS - COMPARISON OF DNA-REACTIVE AND NON-DNA-REACTIVE CLASTOGENS

Positive results in the in vitro assay for chromosome aberrations some times occur with test chemicals that apparently do not react with DNA, being negative in tests for mutation in bacteria, for DNA strand brea ks, and for covalent binding to DNA. These chromosome aberrations typi cally occur over a narrow concentration range at toxic doses, and with mitotic inhibition. Indirect mechanisms, including oxidative damage, cytotoxicity and inhibition of DNA synthesis induced by chemical expos ure, may be involved. Understanding when such mechanisms are operating is important in evaluating potential mutagenic hazards, since the eff ects may occur only above a certain threshold dose. Here, we used two- parameter flow cytometry to assess DNA synthesis inhibition (uptake of bromodeoxyuridine [BrdUrd]) associated with the induction of aberrati ons in CHO cells by DNA-reactive and non-reactive chemicals, and to fo llow cell cycle progression. Aphidicolin (APC), a DNA polymerase inhib itor, induces aberrations without reacting with DNA; 50 mu M APC suppr essed BrdUrd uptake during a 3-h treatment to < 10% of control levels. Several new drug candidates induced aberrations concomitant with mark ed reductions in cell counts at 20 h (to 50-60% of controls) and suppr ession of BrdUrd uptake (< 15% of control). Several non-mutagenic chem icals and a metabolic poison, which induce DNA double strand breaks an d chromosome aberrations at toxic dose levels, also suppressed DNA syn thesis. In contrast, the alkylating agents 4-nitroquinoline-1-oxide, m itomycin C, methylnitrosourea, ethylnitrosourea, methylmethane sulfona te and ethylmethane sulfonate, and a topoisomerase II inhibitor, etopo side, produced many aberrations at concentrations that were less toxic (cell counts greater than or equal to 73% of controls) and gave littl e inhibition of DNA synthesis during treatment (BrdUrd uptake greater than or equal to 85% of controls), although cell cycle delay was seen following the 3-h treatment. Thus, inhibition of DNA synthesis at the time of treatment is supporting evidence for an indirect mechanism of aberrations, when there is no direct DNA reactivity. (C) 1998 Elsevier Science B.V. All rights reserved.