Abstract 2043: Synthetic lethality of PARG inhibition in tumors with homologous recombination deficiencies
Authors: Abed Monah , Munoz Diana , Jaipuri Firoz , Ravindran Nandini , Seshadri Vidya , Liimatta Marya , Neilan Claire , Linghu Xin , Nguyen An , Wagle Marie-Claire , Mounir Zineb 
Published: July 2021

Abstract
Background:The enzyme Poly(ADP-ribose) glycohydrolase (PARG) plays an important role during the DNA damage repair process through hydrolysis of poly(ADP-ribose)(PAR) chains and accounts for approximately 90% of dePARylation activity within the cell. PAR chain synthesis is achieved by poly(ADP-ribose) polymerases (PARPs), which use donor NAD+ molecules to link repeating ADP-ribose units. The resulting PAR chains serve as a platform to recruit DNA repair proteins that are critical for single-strand break (SSB) repair. Following DNA repair PARG hydrolyzes the linkages to break down PAR chains, completing the cycle. Recent studies also found that dePARylation is necessary for the translocation of DNA repair proteins such as XRCC1 to SSB suggesting a more direct role for PARG in DNA damage repair. PARP inhibitors (PARPi), demonstrate synthetic lethality in cells with impaired homologous recombination (HR)-mediated DNA repair. Clinical studies have shown that responses to PARPi are often accompanied by high rates of resistance, generating a need for additional therapies for patients with HR-deficient tumors. The ability of PARG inhibition (PARGi) to exacerbate replication deficiencies makes it a promising therapeutic target for a broad range of cancer types with genomic instability. We therefore sought out to evaluate the effects of PARGi in HR-deficient ovarian and breast cancer models.
Methods: To understand the effects of PARGi in cell growth and survival, a panel of HR-deficient cell lines and cell line derived xenografts (CDX) were exposed to PARGi alone or in combination with DNA damage response inhibitors (DDRi). In these studies, PAR chain accumulation was assessed as a marker of target engagement and PARPi was used to assess on target effects in vitro.
Results: Using cellular proliferation assays and xenograft models, we find that PARGi increases the cellular levels of PAR and significantly decreases the viability of HR-deficient cancer cell lines. Furthermore, inhibition of cell proliferation by PARGi is antagonized by PARPi, which is consistent with an on-target cellular mechanism of action (MOA). In conclusion, PARGi induces significant accumulation of PAR chains and decreases cell proliferation both in vitro and in vivo in HR-deficient tumor cells. Ongoing exploratory studies will further delineate the molecular underpinning of the relationship between HR-deficiency and PARGi.
Citation Format: Monah Abed, Diana Munoz, Firoz Jaipuri, Nandini Ravindran, Vidya Seshadri, Marya Liimatta, Claire Neilan, Xin Linghu, An Nguyen, Marie-Claire Wagle, Zineb Mounir. Synthetic lethality of PARG inhibition in tumors with homologous recombination deficiencies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2043.