Abstract 3850: Understanding PARP inhibitor sensitivity: Analyses of the genetic interactions between specific PARP inhibitor targets and DNA double-strand repair pathways
Authors: Ghosh Rajib , Roy Sanchita , Dantzer Francoise , Franco Sonia 
Published: August 2015

Abstract
Poly (ADP-ribosyl)ation (PARylation) of histones and DNA repair factors is necessary to maintain genomic integrity in mammalian cells. In response to DNA damage, PARylation is primarily catalyzed by two Poly (ADP-ribose) polymerases (PARPs), PARP1 and PARP2. Ablation of PARP1 in the mouse results in embryonic lethality when combined with deficiencies for the DNA repair factors that mediate homologous recombination (HR; such as ATM or H2AX) or Nonhomologous End-joining (NHEJ; such as Ku80). In contrast, the interaction between PARP2 and DNA double-strand break (DSB) repair pathways has not been investigated in detail. In this context, all PARP inhibitors (PARPi) in clinical use block the catalytic domain of both PARP1 and PARP2. Since the two factors clearly play unique, non-overlapping roles in DNA repair and other cellular processes, we sought to further define their specific contribution to the therapeutic effect of PARPi. We find that combined loss of either PARP1 or PARP2 and histone H2AX results in embryonic lethality in the mouse, similar to previous observations with mice deficient for PARP1 or PARP2 and the ATM kinase. Because H2AX and ATM promote double-strand break (DSB) repair throughout the cell cycle, we next investigated whether this synthetic lethal interaction reflected on their functions in NHEJ and/or HR. Treatment of H2AX or ATM-deficient fibroblasts or lymphocytes with PARPi resulted in a marked increase in the frequency of chromatid-type aberrations relative to wild-type controls, suggesting that PARPi hypersensitivity in H2AX or ATM-deficient cells results primarily from defective HR. To further assess an interaction between PARP2 and NHEJ, we generated mice lacking PARP2 and either Ku80, a core NHEJ factor, or DNA-PKcs, a kinase required for a subset of NHEJ reactions. We find that, unlike the early embryonic lethality previously observed in PARP1/Ku80-deficient mice, loss of PARP2 does not result in additional phenotypes in growth and development in Ku80-deficient mice. Likewise, loss of PARP2 has no major effect on growth, development or lymphoma latency in DNA-PKcs-deficient mice, in marked contrast to the severe segmental progeroid syndrome and early-onset tumorigenesis observed in PARP1/DNA-PKcs-deficient mice. Furthermore, PARP1/DNA-PKcs-deficient lymphomas, but not PARP2/DNA-PKcs-deficient lymphomas, harbor marked aneuploidy and oncogenic p53 mutations. Altogether, these genetic analyses suggest that inhibition of both PARP1 and PARP2 contributes to PARPi-induced killing of HR-deficient human tumor cells. This hypothesis is currently being tested in our lab using 11q23-deleted neuroblastoma cells with monoallelic expression of H2AX and breast cancer cells lacking BRCA1 or BRCA2, and may provide a rationale for the design of PARP-specific inhibitors in the future.
Citation Format: Rajib Ghosh, Sanchita Roy, Francoise Dantzer, Sonia Franco. Understanding PARP inhibitor sensitivity: Analyses of the genetic interactions between specific PARP inhibitor targets and DNA double-strand repair pathways. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3850. doi:10.1158/1538-7445.AM2015-3850