MEGABASE PAIR DELETIONS IN MUTANT MAMMALIAN-CELLS FOLLOWING EXPOSURE TO AMSACRINE, AN INHIBITOR OF DNA TOPOISOMERASE-II

Amsacrine, [4'-(9-acridinylamino)-methanesulfon-m-anisidide], belongs to the class of cancer chemotherapeutic agents that target DNA topoiso merase II. We show that, over its cytotoxic range, amsacrine is a pote nt mutagen of the S1 phenotype in the A(L) (human x hamster) hybrid ce ll line. By contrast, amsacrine induction of the HPRT(-) phenotype in A(L) cells is at least two decades less frequent and is not concentrat ion dependent. Such differential mutation frequencies are hypothesized to reflect the concomitant loss of essential genes neighboring the hp rt locus. It may be that some amsacrine cytotoxicity is due to the ina ctivation of essential genes by large deletions. The AL mutation syste m is well suited for the detection and mapping of mutations which are large deletions because its MIC1 locus, which controls the expression of the selectable cell surface antigen S1, is on a single human chromo some. This human chromosome 11 is in addition to the genome of the Chi nese hamster ovary cell and is basically nonessential. Since there are no sister human chromosomes in A(L) cells, deletions which extend bey ond the MIC1 locus may be conveniently and unambiguously mapped. We ha ve detected the presence or absence of 9 different chromosome 11 marke rs in 48 S1(-) mutants cloned from amsacrine-treated cultures. We find that almost all (92%) of the mutants have deletions of at least 1.5-2 megabase pairs in length. The distribution of marker loss frequencies flanking the MIC1 locus does not appear symmetric with respect to dis tance from that locus. We speculate that amsacrine-induced deletions a re mediated by a series of subunit exchanges between overlapping topoi somerase II dimers at the bases of replicons or larger chromosomal str uctures such as replicon clusters or chromosome minibands.