Spontaneous and 3-aminobenzamide-induced sister-chromatid exchange frequencies estimated by ring chromosome analysis

Ring chromosomes offer an opportunity to measure sister-chromatid exchange (SCE) frequencies without the use of an agent to differentiate sister chromatids: SCE frequencies can be determined from the number of dicentric rings formed in cells from a cell line carrying a monocentric ring chromosome. Ash is a pseudotetraploid Chinese hamster ovary cell line in which approximately 40% of metaphase cells have a large ring chromosome. We have used this cell line to investigate the spontaneous rate of SCE by determining the rate of dicentric ring formation and have compared this with the rate of loss of the ring chromosomes over time. In the absence of both [3H]thymidine and bromodeoxyuridine, the spontaneous rate of SCE in Ash cells was 0.12 SCEs/ring/cell cycle; this rate was increased by bromodeoxyuridine, by the polyfunctional alkylating agent mitomycin C, and by the poly(ADP-ribose) polymerase inhibitor 3-aminobenzamide. This indicates that spontaneous SCE occurs in this line and that not all 3-amino-benzamide-induced SCEs are dependent upon incorporated bromodeoxyuridine. Ring chromosomes were not lost over time as rapidly as predicted by the SCE frequencies observed. Non-disjunction of the dicentric ring, or anaphase bridge breakage followed by reunion to form one or two monocentric rings, are the most likely explanations for this discrepancy.