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

Ring chromosomes offer an opportunity to measure sisterchromatidexchange (SCE) frequencies without the use of an agent to differentiatesister chromatids: SCE frequencies can be determined from thenumber of dicentric rings formed in cells from a cell line carryinga monocentric ring chromosome. Ash is a pseudotetraploid Chinesehamster ovary cell line in which 40% of metaphase cells havea large ring chromosome. We have used this cell line to investigatethe spontaneous rate of SCE by determining the rate of dicentricring formation and have compared this with the rate of lossof the ring chromosomes over time. In the absence of both [³H]thymidineand bromodeoxyuridine, the spontaneous rate of SCE in Ash cellswas 0.12 SCEs/ring/cell cycle; this rate was increased by bromodeoxyuridine,by the polyfunctional alkylating agent mitomycin C, and by thepoly(ADP-ribose) polymerase inhibitor 3-aminobenzamide. Thisindicates that spontaneous SCE occurs in this line and thatnot all 3-amino-benzamide-induced SCEs are dependent upon incorporatedbromodeoxyuridine. Ring chromosomes were not lost over timeas rapidly as predicted by the SCE frequencies observed. Non-disjunctionof the dicentric ring, or anaphase bridge breakage followedby reunion to form one or two monocentric rings, are the mostlikely explanations for this discrepancy. ¹To whom correspondence should be addressed