Cover Story                       

Project tour: (Toposiomerase 2) and (Topoisomerase 1)

Summary:

Topoisomerases (type I: EC 5.99.1.2, type II: EC 5.99.1.3) are enzymes that act on the topology of DNA. The double-helical configuration that DNA strands naturally reside in makes them difficult to separate, and yet they must be separated by helicase proteins if other enzymes are to transcribe the sequences that encode proteins, or if chromosomes are to be replicated. In so-called circular DNA, in which double helical DNA is bent around and joined in a circle, the two strands are topologically linked, or knotted. Otherwise identical loops of DNA having different numbers of twists are topoisomers, and cannot be interconverted by any process that does not involve the breaking of DNA strands. Topoisomerases catalyze and guide the unknotting of DNA. The insertion of viral DNA into chromosomes and other forms of recombination can also require the action of topoisomerases. Many drugs operate through interference with the topoisomerases. The broad-spectrum fluoroquinolone antibiotics act by disrupting the function of bacterial type II topoisomerases. Some chemotherapy drugs work by interfering with topoisomerases in cancer cells: type 1 is inhibited by irinotecan and topotecan, while type 2 is inhibited by etoposide and teniposide.

DNA topoisomerase II (Top2) is an essential nuclear enzyme that regulates the topological state of the DNA, and a target of very effective anticancer drugs including anthracycline antibiotics. Even though several aspects of drug activity against Top2 are understood, the drug receptor site is not yet known. Several Top2 mutants have altered drug sensitivity and have provided information of structural features determining drug action. Here, we have revised the published crystal structures of eukaryotic and prokaryotic Top2s and relevant biochemical investigations of enzyme activity and anthracycline action. In particular, we have considered Top2 mutations conferring resistance to anthracyclines and related agents. Following a previous study (Moro et al, Biochemistry, 2004; 43: 7503-13), we have then re-built a molecular model of the entire enzyme in complex with DNA after the cleavage reaction, and used it to define the receptor site of anthracyclines. The results suggest a model wherein the drug specifically contacts the cleaved DNA as well as amino acid residues of the enzyme CAP-like domain. The findings can explain several established structure-activity relationships of antitumour anthracyclines, and provide a framework for further developments of effective Top2 poison.

Selected References:

 D. Dal Ben, G. Capranico, G. Zagotto, M. Palumbo, S. Moro “DNA topoisomerase II structures and anthracycline activity: insights into ternary complex formation.” Curr. Pharm. Des. (2007) in press

 H. Ihmels, D. Otto, A. Faccio, F. Dall'Acqua, G. Viola, S. Moro “Comparative Studies on the DNA-binding Properties of Linear and Angular Dibenzoquinolizinium Ions.” J. Org. Chem. 71, 8401-8411 (2006)

 S. Moro, G.L. Beretta, D. Dal Ben, J. Nitiss, M. Palumbo, G. Caprinico “Interaction model for anthracycline activity against DNA topoisomerase II.” Biochemistry 43, 7503-7513 (2004)