Trapping of PARP1 and PARP2 by Clinical PARP Inhibitors

TLDR

Small‑molecule PARP inhibitors are thought to block DNA single‑strand break repair, yet their precise mechanism in cancer cells remains unclear. The study investigates whether PARP inhibitors trap PARP1 and PARP2 enzymes at sites of DNA damage. The authors used 30 genetically modified avian DT40 cell lines lacking specific DNA repair genes to analyze repair pathways for PARP–DNA complexes. Trapped PARP–DNA complexes are more cytotoxic than unrepaired SSBs, with trapping potency varying markedly among inhibitors (niraparib > olaparib > veliparib) independent of catalytic inhibition, and repair requires homologous recombination, post‑replication repair, the Fanconi anemia pathway, polymerase β, and FEN1, establishing a new mechanistic basis for PARP inhibitor use in cancer therapy. Published in Cancer Research 72(21):5588–99; ©2012 AACR.

Abstract

Abstract Small-molecule inhibitors of PARP are thought to mediate their antitumor effects as catalytic inhibitors that block repair of DNA single-strand breaks (SSB). However, the mechanism of action of PARP inhibitors with regard to their effects in cancer cells is not fully understood. In this study, we show that PARP inhibitors trap the PARP1 and PARP2 enzymes at damaged DNA. Trapped PARP–DNA complexes were more cytotoxic than unrepaired SSBs caused by PARP inactivation, arguing that PARP inhibitors act in part as poisons that trap PARP enzyme on DNA. Moreover, the potency in trapping PARP differed markedly among inhibitors with niraparib (MK-4827) > olaparib (AZD-2281) >> veliparib (ABT-888), a pattern not correlated with the catalytic inhibitory properties for each drug. We also analyzed repair pathways for PARP–DNA complexes using 30 genetically altered avian DT40 cell lines with preestablished deletions in specific DNA repair genes. This analysis revealed that, in addition to homologous recombination, postreplication repair, the Fanconi anemia pathway, polymerase β, and FEN1 are critical for repairing trapped PARP–DNA complexes. In summary, our study provides a new mechanistic foundation for the rational application of PARP inhibitors in cancer therapy. Cancer Res; 72(21); 5588–99. ©2012 AACR.