p53 dependence of topoisomerase I recruitment in vivo

DNA damage is attended by rapid recruitment of endogenous type I topoisomer ase (topo I) into covalent cleavage complexes with genomic DNA in vivo. In contrast, endogenous topoisomerase II alpha and beta are not stimulated by DNA damage. We show that topo I and p53 are able to associate at arrested t opo I-genomic DNA covalent complexes in vivo, suggesting that p53 directly stimulates topo I activity and damage to the genome of the afflicted cell. Moreover, cells that express wild-type p53 are most proficient at recruitin g topo I after DNA damage; however, the p53 dependence is conditional becau se topo I recruitment after DNA damage can be restored if p53 mutant cells (containing a single mutant allele) are artificially held in G(1). In contr ast, p53 null mutants do not recruit topo I after DNA damage under any cond itions (although camptothecin-dependent topo I/DNA complexes readily form i n the nulls). These results show that topo I activation after DNA damage de pends on the p53 status of the cell. It also depends upon the cell cycle in a way that is very different from that observed crith DNA replication-depe ndent, camptothecin-mediated DNA breaks. The data suggest a model where p53 activates topo I, which inflicts additional genomic damage after the initi al UV damage events. Topoisomerases therefore contribute to the p53 commitm ent to apoptosis, and topo I might assist in elimination of DNA-damaged cel ls as part of the cellular proofreading function inherent in the p53 pathwa y.