A novel approach for organelle-specific DNA damage targeting reveals different susceptibility of mitochondrial DNA to the anticancer drugs camptothecin and topotecan

DNA is susceptible of being damaged by chemicals, UV light or gamma irradiation. Nuclear DNA damage invokes both a checkpoint and a repair response. By contrast, little is known about the cellular response to mitochondrial DNA damage. We designed an experimental system that allows organelle-specific DNA damage targeting in Saccharomyces cerevi- siae. DNA damage is mediated by a toxic topo- isomerase I allele which leads to the formation of persistent DNA single-strand breaks. We show that organelle-specific targeting of a toxic topoisom- erase I to either the nucleus or mitochondria leads to nuclear DNA damage and cell death or to loss of mitochondrial DNA and formation of respiration- deficient 'petite' cells, respectively. In wild-type cells, toxic topoisomerase I-DNA intermediates are formed as a consequence of topoisomerase I inter- action with camptothecin-based anticancer drugs. We reasoned that targeting of topoisomerase I to the mitochondria of top1" cells should lead to petite formation in the presence of camptothecin. Interestingly, camptothecin failed to generate petite; however, its derivative topotecan accumu- lates in mitochondria and induces petite formation. Our findings demonstrate that drug modifications can lead to organelle-specific DNA damage and thus opens new perspectives on the role of mito- chondrial DNA-damage in cancer treatment.