Dissecting the cell-killing mechanism of the topoisomerase II-targeting drug ICRF-193

Topoisomerase II is an essential enzyme that is targeted by a number of clinically valuable anticancer drugs. One class referred to as topoisomerase II poisons works by increasing the cellular level of topoisomerase II-mediated DNA breaks, resulting in apoptosis. Another class of topoisomerase II-directed drugs, the bis-dioxopiperazines, stabilizes the conformation of the enzyme where it attains an inactive salt-stable closed clamp structure. Bis-dioxopiperazines, similar to topoisomerase II poisons, induce cell killing, but the underlying mechanism is presently unclear. In this study, we use three different biochemically well characterized human topoisomerase IIalpha mutant enzymes to dissect the catalytic requirements needed for the enzyme to cause dominant sensitivity in yeast to the bis-dioxopirazine ICRF193 and the topoisomerase II poison m-AMSA. We find that the clamp-closing activity, the DNA cleavage activity, and even both activities together are insufficient for topoisomerase II to cause dominant sensitivity to ICRF193 in yeast. Rather, the strand passage event per se is an absolute requirement, most probably because this involves a simultaneous interaction of the enzyme with two DNA segments. Furthermore, we show that the ability of human topoisomerase IIalpha to cause dominant sensitivity to m-AMSA in yeast does not depend on clamp closure or strand passage but is directly related to the capability of the enzyme to respond to m-AMSA with increased DNA cleavage complex formation.