Decreased topoisomerase IIα expression confers increased resistance to ICRF-193 as well as VP-16 in mouse embryonic stem cells

To elucidate the relationship between topoisomerase (topo) II expression and sensitivity to anti-topo II drugs in mammalian cells, we generated mouse embryonic stem cell mutants heterozygous for the topo IIα gene by gene targeting. The level of topo IIα in the heterozygous cells reduced to one-half of that found in wild-type cells, while topo IIβ levels were similar in both cell types. Importantly, the heterozygous cells exhibited an increased resistance to ICRF-193 as well as VP-16, suggesting that ICRF-193, like VP-16, exerts its cytotoxicity through converting topo II to a poison.