Mitochondrial DNA depletion sensitizes cancer cells to PARP inhibitors by translational and post-translational repression of BRCA2

Previous studies have shown that pharmacologic inhibition of poly (ADP-ribose) polymerase (PARP), a nuclear protein that is crucial in signaling single-strand DNA breaks, is synthetically lethal to cancer cells from patients with genetic deficiency in the DNA repair proteins BRCA1 and BRCA2. Herein, we demonstrate that depletion of the mitochondrial genome (mtDNA) in breast, prostate and thyroid transformed cells resulted in elevated steady-state cytosolic calcium concentration and activation of calcineurin/PI3-kinase/AKT signaling leading to upregulation of miR-1245 and the ubiquitin ligase Skp2, two potent negative regulators of the tumor suppressor protein BRCA2, thus resulting in BRCA2 protein depletion, severe reduction in homologous recombination (HR) and increased sensitivity to the PARP inhibitor rucaparib. Treatment of mtDNA-depleted cells with the PI3-kinase inhibitor LY294002, the calmodulin antagonist W-7, the calcineurin inhibitor FK506, the calcium chelator BAPTA-AM, or suppression of AKT activity by AKT small-interfering RNA (siRNA) enhanced BRCA2 protein levels as well as HR. Decreasing the intracellular calcium levels using BAPTA, or direct reconstitution of BRCA2 protein levels either by recombinant expression or by small molecule inhibition of both Skp2 and miR-1245 restored sensitivity to rucaparib to wild-type levels. Furthermore, by studying prostate tissue specimens from prostate carcinoma patients we found a direct correlation between the presence of mtDNA large deletions and loss of BRCA2 protein in vivo, suggesting that mtDNA status may serve as a marker to predict therapeutic efficacy to PARP inhibitors. In summary, our results uncover a novel mechanism by which mtDNA depletion restrains HR, and highlight the role of mtDNA in regulating sensitivity to PARP inhibitors in transformed cells.