Expression of a mutant DNA topoisomerase II in CCRF-CEM human leukemic cells selected for resistance to teniposide.

Abstract

Nuclear extracts from teniposide (VM-26)-resistant sublines of the human leukemic cell line CCRF-CEM have decreased levels of DNA topoisomerase II catalytic activity and decreased capacity to form drug-stabilized covalent protein-DNA complexes. The ATP concentration required for equivalent activity in a DNA-unknotting assay is 2- to 8-fold higher in nuclear extracts from drug-resistant cell lines as compared with the parental line. When adenosine 5'-[beta,gamma-imido]triphosphate is substituted for ATP in complex-formation assays, no significant change is seen with drug-sensitive cells, but a 50-65% reduction is seen with VM-26-resistant cells. Collectively, these results indicate that an alteration in ATP binding may be involved in the resistance phenotype. Therefore, we identified regions of the topoisomerase II sequence that conform to previously identified nucleotide-binding sites. Starting with cDNA as the template we determined the sequence of the topoisomerase II mRNA surrounding these sites by sequencing DNA fragments produced by the polymerase chain reaction. In the region corresponding to the consensus B ATP-binding sequence described by Walker et al. [Walker, J. E., Saraste, M., Runswick, M. J. & Gay, N. J. (1982) EMBO J. 1, 945-951], the cDNA from the two VM-26-resistant sublines contained an altered sequence having a G----A base change. This base substitution results in the replacement of the conserved arginine at position 449 with a glutamine. Hybridization with allele-specific oligonucleotides confirmed the presence of both the normal and the altered sequence in the resistant cell lines, whereas only the normal sequence was found in the sensitive CEM cells. A chemical mismatch cleavage procedure for the detection of mispaired bases in DNA duplexes identified no other alterations in the 5' third of the mRNA coding sequence, which contains the complete ATP-binding domain of topoisomerase II. The presence of mRNA encoding topoisomerase II with Gln449 correlates both with the presence of a topoisomerase II protein whose interaction with ATP is altered and with increased resistance to the cytotoxicity of VM-26.