Mitochondrial topoisomerase I (top1mt) is a novel limiting factor of doxorubicin cardiotoxicity.

PURPOSE Doxorubicin is one of the most effective chemotherapeutic agents. However, up to 30% of the patients treated with doxorubicin suffer from congestive heart failure. The mechanism of doxorubicin cardiotoxicity is likely multifactorial and most importantly, the genetic factors predisposing to doxorubicin cardiotoxicity are unknown. On the basis of the fact that mtDNA lesions and mitochondrial dysfunctions have been found in human hearts exposed to doxorubicin and that mitochondrial topoisomerase 1 (Top1mt) specifically controls mtDNA homeostasis, we hypothesized that Top1mt knockout (KO) mice might exhibit hypersensitivity to doxorubicin. EXPERIMENTAL DESIGN Wild-type (WT) and KO Top1mt mice were treated once a week with 4 mg/kg doxorubicin for 8 weeks. Heart tissues were analyzed one week after the last treatment. RESULTS Genetic inactivation of Top1mt in mice accentuates mtDNA copy number loss and mtDNA damage in heart tissue following doxorubicin treatment. Top1mt KO mice also fail to maintain respiratory chain protein production and mitochondrial cristae ultrastructure organization. These mitochondrial defects result in decreased O2 consumption, increased reactive oxygen species production, and enhanced heart muscle damage in animals treated with doxorubicin. Accordingly, Top1mt KO mice die within 45 days after the last doxorubicin injection, whereas the WT mice survive. CONCLUSIONS Our results provide evidence that Top1mt, which is conserved across vertebrates, is critical for cardiac tolerance to doxorubicin and adaptive response to doxorubicin cardiotoxicity. They also suggest the potential of Top1mt single-nucleotide polymorphisms testing to investigate patient susceptibility to doxorubicin-induced cardiotoxicity.