DNA Decatenation in Breast Cancer by Metnase

We and others identified a human SET and transposase domain protein termed Metnase also called SETMAR that methylates histone H3, and promotes DNA double-strand break repair. We postulated that Metnase could improve decatenation in breast cancer cells. In the funded period, we initially characterized the decatenation activity of Metnase. We showed that Metnase physically interacts and co-localizes with Topoisomerase IIalpha Topo IIalpha , the key chromosome decatenating enzyme. Metnase promotes progression through decatenation, and increases resistance to the Topo IIalpha inhibitors ICRF-193 and VP-16. Pure Metnase greatly enhanced Topo IIalpha decatenation of kinetoplast DNA to relaxed circular forms. Nuclear extracts containing Metnase decatenated kDNA more rapidly than those without Metnase, and neutralizing anti-sera against Metnase reversed that enhancement of decatenation. Metnase automethylates at K485, and the presence of a methyl donor blocked the enhancement of Topo IIalpha decatenation by Metnase, implying an internal regulatory inhibition. Importantly, we showed that inhibiting Metnase in breast cancer cells did block decatenation. Reducing Metnase levels increased breast cancer cell chemosensitivity to the Topo IIalpha inhibitor VP-16. Thus, Metnase serves as an enhancer of Topo IIalpha decatenation, but can automethylate to repress this enhancement. In addition, these data suggest that cancer cells could subvert Metnase to mediate clinically relevant resistance to Topo IIalpha inhibitors.