Dissociation of cytotoxicity and DNA cleavage activity induced by topoisomerase II-reactive intercalating agents in hamster-human somatic cell hybrids

Previous studies using the mutant Chinese hamster ovary cell line Vpm^R-5 indicate that its resistance to mediated through a qualitative change in type II topoisomerase that confers resistance to drug-stimulated DNA cleavage activity. In a further investigation of the genetic basis of drug resistance in Vpm^R-5 cells, we fused a hypoxanthine-guanine phosphoribosyl transferase-deficient subline of Vpm^R-5 (Vtgm-6) with normal human lymphocytes and analyzed the resultant hybrid lines (HL) for altered drug sensitivity. In all, 3 of 16 hybrid clones exhibited partial reconstitution of sensitivity to etoposide, mitoxantrone, doxorubicin, and 5-iminodaunorubicin while retaining complete resistance to m-AMSA. However, enhanced sensitivity to drug-induced DNA cleavage activity was observed only for etoposide. Biochemical and molecular-marker analysis of the hybrids failed to identify human chromosome 17 (the provisional location of TOP2) or any other human chromosome that is consistently and uniquely associated with drug sensitivity. We therefore sought to verify the chromosomal assignment of TOP2 by Southern blot hybridization of TOP2 cDNA on a human hybrid mapping panel and confirmed its location on chromosome 17. However, no hybridizing sequence to the TOP2 cDNA was found in any of the 16 Vtgm-6 hybrid lines. Efforts to select more directly for human chromosome 17 Vpm^R-5 hybrids using microcell fusion of mouse A9 cells carrying human 17 linked to pSV₂neo were unsuccessful. None of the five hybrid clones thus obtained had 17q markers, including the gene for TOP2. Although the mechanism underlying partial reversion to a drug-sensitive phenotype in the original Vtgm-6 hybrid lines has yet to be defined, the data obtained in these lines indicate that anthracycline- and anthracenedione-induced cytotoxic effects can be dissociated from DNA cleavage activity. This suggests that pathways distal to cleavable-complex formation or, alternatively, independent of interactions with topoisomerase II that involve other intracellular targets are important in mediating the cytotoxicity produced by these drugs.