Abstract 3580: Topoisomerase II mediated DNA damage generates unique classes of genome rearrangements

Abstract Topoisomerase 2 (Top2) is the target of active anti-cancer agents such as etoposide and doxorubicin. These drugs interfere with the Top2 catalytic cycle and lead to trapping of the enzyme as a covalent protein: DNA complex. This trapped covalent complex is a unique DNA lesion that includes DNA strand breaks and covalent protein adducts at the site of the breaks. While Top2 mediated DNA damage is the major mechanism for tumor cell killing, it is also responsible for drug-induced translocations that can lead to secondary malignancies. We have developed several novel systems for assessing Top2-mediated genome instability using yeast as a model system. We developed repair proficient yeast strains that accumulate high levels of etoposide and other Top2 targeting drugs. We selected for loss-of-function mutations in the yeast CAN1 gene that arise following etoposide treatment, and examined the induced mutations by DNA sequencing. We identified a unique spectrum of mutations generated by etoposide in yeast. One novel class of mutational events induced by etoposide is relatively large deletions (>300 bp) and tandem duplications. Most of these etoposide-induced events are flanked by 4-8 nucleotide direct repeats. To further characterize the mechanism of Top2 induced genome instability, we took advantage of a novel Top2 allele isolated in our laboratory. This Top2 allele, termed top2AR, exhibits elevated levels of drug independent DNA cleavage in vitro, and inviability when combined with yeast mutants defective in DNA double strand break repair. We found a similar spectrum of large deletions and tandem duplications flanked by 4-8 nucleotide direct repeats. Interestingly, we also found that the recovery of large deletions and insertions was enhanced in yeast cells lacking Tdp1, a gene that participates in the repair of protein:DNA covalent complexes. Our results illustrate a novel type of genome rearrangement that is mediated by covalent Top2 DNA damage. It is noteworthy that the deletions and insertions flanked by short direct repeats have not been previously observed with other classes of DNA damaging agents. We suggest that the unique properties of these lesions may also contribute to oncogenic translocations induced by etoposide and other Top2 targeting drugs. We are currently using next-generation sequencing of etoposide treated yeast cells and cells carrying the top2AR allele to obtain a more complete picture of the types of alterations induced by Top2 mediated DNA damage. Citation Format: Matthew Gilbertson, Radhika Patel, Karin C. Nitiss, John L. Nitiss. Topoisomerase II mediated DNA damage generates unique classes of genome rearrangements. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3580.