Poly ADP-ribose Glycohydrolase Research Articles

The Ser/Thr protein kinase FonKin4-poly(ADP-ribose) polymerase FonPARP1 phosphorylation cascade is required for the pathogenicity of watermelon fusarium wilt fungus Fusarium oxysporum f. sp. niveum.

Poly(ADP-ribosyl)ation (PARylation), catalyzed by poly(ADP-ribose) polymerases (PARPs) and hydrolyzed by poly(ADP-ribose) glycohydrolase (PARG), is a kind of post-translational protein modification that is involved in various cellular processes in fungi, plants, and mammals. However, the function of PARPs in plant pathogenic fungi remains unknown. The present study investigated the roles and mechanisms of FonPARP1 in watermelon Fusarium wilt fungus Fusarium oxysporum f. sp. niveum (Fon). Fon has a single PARP FonPARP1 and one PARG FonPARG1. FonPARP1 is an active PARP and contributes to Fon pathogenicity through regulating its invasive growth within watermelon plants, while FonPARG1 is not required for Fon pathogenicity. A serine/threonine protein kinase, FonKin4, was identified as a FonPARP1-interacting partner by LC-MS/MS. FonKin4 is required for vegetative growth, conidiation, macroconidia morphology, abiotic stress response and pathogenicity of Fon. The S_TKc domain is sufficient for both enzyme activity and pathogenicity function of FonKin4 in Fon. FonKin4 phosphorylates FonPARP1 in vitro to enhance its poly(ADP-ribose) polymerase activity; however, FonPARP1 does not PARylate FonKin4. These results establish the FonKin4-FonPARP1 phosphorylation cascade that positively contributes to Fon pathogenicity. The present study highlights the importance of PARP-catalyzed protein PARylation in regulating the pathogenicity of Fon and other plant pathogenic fungi.

DePARylation is critical for S phase progression and cell survival.

Poly(ADP-ribose)ylation or PARylation by PAR polymerase 1 (PARP1) and dePARylation by poly(ADP-ribose) glycohydrolase (PARG) are equally important for the dynamic regulation of DNA damage response. PARG, the most active dePARylation enzyme, is recruited to sites of DNA damage via pADPr-dependent and PCNA-dependent mechanisms. Targeting dePARylation is considered an alternative strategy to overcome PARP inhibitor resistance. However, precisely how dePARylation functions in normal unperturbed cells remains elusive. To address this challenge, we conducted multiple CRISPR screens and revealed that dePARylation of S phase pADPr by PARG is essential for cell viability. Loss of dePARylation activity initially induced S-phase-specific pADPr signaling, which resulted from unligated Okazaki fragments and eventually led to uncontrolled pADPr accumulation and PARP1/2-dependent cytotoxicity. Moreover, we demonstrated that proteins involved in Okazaki fragment ligation and/or base excision repair regulate pADPr signaling and cell death induced by PARG inhibition. In addition, we determined that PARG expression is critical for cellular sensitivity to PARG inhibition. Additionally, we revealed that PARG is essential for cell survival by suppressing pADPr. Collectively, our data not only identify an essential role for PARG in normal proliferating cells but also provide a potential biomarker for the further development of PARG inhibitors in cancer therapy.

DePARylation is critical for S phase progression and cell survival

Poly(ADP-ribose)ylation or PARylation by PAR polymerase 1 (PARP1) and dePARylation by poly(ADP-ribose) glycohydrolase (PARG) are equally important for the dynamic regulation of DNA damage response. PARG, the most active dePARylation enzyme, is recruited to sites of DNA damage via pADPr-dependent and PCNA-dependent mechanisms. Targeting dePARylation is considered an alternative strategy to overcome PARP inhibitor resistance. However, precisely how dePARylation functions in normal unperturbed cells remains elusive. To address this challenge, we conducted multiple CRISPR screens and revealed that dePARylation of S phase pADPr by PARG is essential for cell viability. Loss of dePARylation activity initially induced S-phase-specific pADPr signaling, which resulted from unligated Okazaki fragments and eventually led to uncontrolled pADPr accumulation and PARP1/2-dependent cytotoxicity. Moreover, we demonstrated that proteins involved in Okazaki fragment ligation and/or base excision repair regulate pADPr signaling and cell death induced by PARG inhibition. In addition, we determined that PARG expression is critical for cellular sensitivity to PARG inhibition. Additionally, we revealed that PARG is essential for cell survival by suppressing pADPr. Collectively, our data not only identify an essential role for PARG in normal proliferating cells but also provide a potential biomarker for the further development of PARG inhibitors in cancer therapy.

Targeting PARG induces tumor cell growth inhibition and antitumor immune response by reducing phosphorylated STAT3 in ovarian cancer

BackgroundOvarian cancer is the most lethal gynecological malignancy, with limited treatment options after failure of standard therapies. Despite the potential of poly(ADP-ribose) polymerase inhibitors in treating DNA damage response (DDR)-deficient...

Abstract 4533: Identification of DAT-2000A, a novel potent PARG inhibitor that selectively inhibits PARPi-resistant and homologous-recombination-deficient tumors

Abstract A significant portion of patients with homologous recombination repair deficiencies(HRD) do not respond to PARP inhibitors, and even among those who initially respond, resistance often develops after PARP inhibitor treatment. This underscores the unmet clinical need to address the challenges in the treatment of HRD patients. Poly (ADP-ribose) glycohydrolase (PARG) plays a key role in hydrolyzing PAR chains from PARylated protein substrates, such as PARP1, serving as a critical checkpoint in DNA damage repair and the control of DNA replication stress. Therefore, PARG has emerged as a promising target for cancer therapy. Cancer cells experiencing replication stress, such as HRD breast cancer cells and those resistant to PARPi, exhibit sensitivity to PARG inhibition. DAT-2000A is a potent, selective, orally bioavailable small-molecule inhibitor of PARG, which directly binds to the catalytic domain of PARG, inducing the accumulation of poly-PAR chains at DNA lesions, and exhibits notable anti-proliferative activity across a wide range of HRD tumors both in vitro and in vivo. Biochemical assays confirm DAT-2000A's high potency and selectivity as an inhibitor of PAR-chain hydrolysis. In cellular assays, DAT-2000A demonstrates selective sensitivity in HRD cells, distinguishing itself from PARPi across various indications. Preclinical studies involving CDX and PDX xenograft models reveal that DAT-2000A exerts robust anti-tumor effects in both PARPi-sensitive and -resistant settings. Furthermore, it induces a dose and time-dependent accumulation of PAR chains, which may serve as a reliable pharmacodynamic biomarker indicative of PARG target engagement. In conclusion, DAT-2000A represents an innovative targeted therapy that warrants further investigation in clinical trials for the treatment of HRD cancers and beyond. Citation Format: Dan Yan, Yan Zhang, Zhangqi Yu, Yao Zhang, Jingxi Zhang, Bin Wang, Huixian Chen, Jingxue Shi, Zhiwei Bao, Xiaoling Lin, Jincong Zhuo, Kevin Zhou. Identification of DAT-2000A, a novel potent PARG inhibitor that selectively inhibits PARPi-resistant and homologous-recombination-deficient tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4533.

Abstract 3366: FoRx-06-428 is a novel PARG inhibitor with potent anti-tumor efficacy in preclinical cancer models

Abstract The formation of poly(ADP-ribose) (PAR) chains from NAD+ precursors by PARP enzymes is a characteristic posttranslational modification during DNA damage repair. PAR chains function as docking platforms for DNA repair proteins that are required to resolve DNA lesions. Equally important is the subsequent removal of PAR chains from protein substrates to conclude the repair process and restore genomic integrity. Poly(ADP-ribose) glycohydrolase (PARG) is the key enzyme in this process and has recently garnered significant attention as a novel cancer target. FoRx-06-428 is a novel, orally bioavailable and highly potent, low molecular weight PARG inhibitor. It binds to the catalytic domain of human PARG with high affinity, as evidenced by surface plasmon resonance (SPR), and displays potent inhibition of enzymatic activity. FoRx-06-428 shows selective reduction of viability in a panel of cancer cell lines highlighting therapeutic potential in multiple cancer indications. Pharmacological inhibition of PARG leads to accumulation of PAR chains and is indicative of target engagement, accompanied by modulation of DNA damage response markers, such as phosphorylation of replication protein A (pRPA) and phosphorylation of H2AX (γH2AX). Compelling anti-tumor efficacy as single agent was observed in several xenograft models (breast and gastric cancer) with excellent tolerability. Dose-dependent accumulation of PAR chains was demonstrated in vivo and constitutes a tractable target engagement biomarker for clinical exploration. Sensitivity to PARG inhibition is partly driven by distinct genomic instabilities, such as defects in homologous recombination (HR) repair, however the activity of PARG inhibitors is also influenced by aberrations in other cancer-relevant pathways. This report represents the first disclosure of the pharmacological characterization of FoRx-06-428 and highlights the value of PARG inhibition as a novel targeted approach to treat DNA repair-deficient cancers. Citation Format: Olivier Querolle, Ulrich Lücking, Luca Iacovino, Alena Freudenmann, Giacomo Rossetti, Marina Gysin, Serena Bologna, Vasilis Dionellis, Marta Malattia, Alessandro Potenza, Nicolas Bocquet, Irena Konstantinova, Andreas Goutopoulos, Sotirios Sotiriou, Thanos Halazonetis, Tarig Bashir, Frank Zenke. FoRx-06-428 is a novel PARG inhibitor with potent anti-tumor efficacy in preclinical cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3366.

Abstract 7114: Exploring the landscape of PARP and PARG inhibitor selectivity in live cells using NanoBRET™ target engagement assays

Abstract The 17-member poly(ADP-ribose) polymerase (PARP) family of proteins mediate a number of cellular processes including gene transcription, cell-division and DNA-repair. PARPs catalyze the addition of poly- or mono- ADP-ribose chains on target proteins, modifying their function and/or recruitment in various pathways. Poly(ADP-ribose) Glycohydrolase (PARG) catabolizes PAR units and facilitates the recycling of PARP and other modified effector proteins. A number of PARPS are implicated in human disease, and PARP inhibitors (PARPi’s) have seen major clinical success, however, many PARPs (especially mono PARPs) are understudied and chemical tools for understanding cellular PARPi selectivity and potency across the entire family are lacking. Similarly, several PARG inhibitors have been developed to date, but tools for directly assessing target engagement in a cellular context are sorely needed. Here we describe the development and application of target engagement assays for characterizing PARP and PARG inhibitor selectivity and affinity in live cells. This method is based on the NanoBRET™ assay platform, which is a proximity-based probe displacement assay that operates in live cells. Using NanoBRET™ technology, we have developed a single probe that covers 12 out of 17 PARP family members. We explored the family-wide selectivity of a diverse panel of clinical and preclinical PARP inhibitors in live cells, representing the first family-wide, cell-based method for profiling PARP selectivity. We also describe the development of a second probe that reports on PARG target engagement. Target engagement of PARG was observed for three out of five tested inhibitors, suggesting a potential indirect mode of action for certain inhibitors on PARG glycohydrolase activity. These assays offer a complementary approach for studying target engagement within the PARylation cycle. Citation Format: Ani Michaud, Kelly Teske, Jennifer Wilkinson, Chad Zimprich, James Vasta, Cesear Corona, Min Zhou, Elizabeth Dominguez, Kaitlin Dunn-Hoffman, Matthew Robers. Exploring the landscape of PARP and PARG inhibitor selectivity in live cells using NanoBRET™ target engagement assays [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7114.

Abstract 7129: DNA polymerase beta expression in head & neck cancer modulates the poly-ADP-ribose-mediated replication checkpoint

Abstract Head and Neck Squamous Cell Carcinoma (HNSCC) impose a significant health burden, necessitating innovative therapeutic strategies to enhance treatment efficacy. Current standard treatments, including surgery, radiation therapy, and chemotherapy, often exhibit limited effectiveness and cause substantial side effects. This underscores the critical need for novel therapies that selectively target cancer cells, sparing normal tissue. Survival from HNSCC is poor, and importantly, a racial disparity between African Americans (AfAm) and European Americans (EuAm) has been known for some time. Ancestry Informative Markers (AIMs) are associated with altered mRNA expression of Polβ where higher expression of Polβ was observed among AfAm patients in comparison to EuAm patients. Our research focuses on DNA polymerase beta (Polβ) and its role in replication stress and response to chemotherapy and DNA damage response modifiers. Elevated expression of Polβ is linked to increased DNA damage and genomic instability, contributing to treatment resistance. In this regard, our investigations reveal a regulatory role for base excision repair (BER) proteins, including Polβ, in the cellular response to inhibitors of poly(ADP-ribose) glycohydrolase (PARG), an enzyme involved in poly(ADP-ribose) (PAR) degradation. Inhibition of PARG triggers a robust intra-S phase response, activating ATR and CHK1 signaling. Exploiting the synthetic lethality between PARG inhibition and ATR/CHK1 inhibition, we have demonstrated the ability to overcome Polβ overexpression mediated treatment resistance in HNSCC cells. Specifically, our in vitro studies demonstrate that the response to a combination of an ATR inhibitor and PARG inhibitor is impacted by Polβ expression in HNSCC cells, with synergistic effects surpassing the efficacy of the individual inhibitors. Similarly, the combination of a CHK1 inhibitor and PARG inhibitor proves effective in cells with elevated Polβ expression, offering a potential dual therapeutic strategy. These combinations exhibit promising potential for HNSCC treatment when Polβ expression is elevated. In conclusion, our findings propose a novel therapeutic paradigm for HNSCC, employing combination therapy with ATR or CHK1 inhibitors and PARG inhibitors. This approach offers a targeted strategy, presenting a promising avenue for the development of more effective and less toxic treatment modalities in the management of HNSCC and may help overcome resistance from elevated expression of Polβ. Citation Format: Md Maruf Khan, Anusha Angajala, Denise Y. Gibbs, Jeffrey C. Liu, Camille Ragin, Robert W. Sobol. DNA polymerase beta expression in head & neck cancer modulates the poly-ADP-ribose-mediated replication checkpoint [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7129.

Targeting OGG1 and PARG radiosensitises head and neck cancer cells to high-LET protons through complex DNA damage persistence

Complex DNA damage (CDD), containing two or more DNA lesions within one or two DNA helical turns, is a signature of ionising radiation (IR) and contributes significantly to the therapeutic effect through cell killing. The levels and complexity of CDD increases with linear energy transfer (LET), however, the specific cellular response to this type of DNA damage and the critical proteins essential for repair of CDD is currently unclear. We performed an siRNA screen of ~240 DNA damage response proteins to identify those specifically involved in controlling cell survival in response to high-LET protons at the Bragg peak, compared to low-LET entrance dose protons which differ in the amount of CDD produced. From this, we subsequently validated that depletion of 8-oxoguanine DNA glycosylase (OGG1) and poly(ADP-ribose) glycohydrolase (PARG) in HeLa and head and neck cancer cells leads to significantly increased cellular radiosensitivity specifically following high-LET protons, whilst no effect was observed after low-LET protons and X-rays. We subsequently confirmed that OGG1 and PARG are both required for efficient CDD repair post-irradiation with high-LET protons. Importantly, these results were also recapitulated using specific inhibitors for OGG1 (TH5487) and PARG (PDD00017273). Our results suggest OGG1 and PARG play a fundamental role in the cellular response to CDD and indicate that targeting these enzymes could represent a promising therapeutic strategy for the treatment of head and neck cancers following high-LET radiation.

PARG-deficient tumor cells have an increased dependence on EXO1/FEN1-mediated DNA repair

Targeting poly(ADP-ribose) glycohydrolase (PARG) is currently explored as a therapeutic approach to treat various cancer types, but we have a poor understanding of the specific genetic vulnerabilities that would make cancer cells susceptible to such a tailored therapy. Moreover, the identification of such vulnerabilities is of interest for targeting BRCA2;p53-deficient tumors that have acquired resistance to poly(ADP-ribose) polymerase inhibitors (PARPi) through loss of PARG expression. Here, by performing whole-genome CRISPR/Cas9 drop-out screens, we identify various genes involved in DNA repair to be essential for the survival of PARG;BRCA2;p53-deficient cells. In particular, our findings reveal EXO1 and FEN1 as major synthetic lethal interactors of PARG loss. We provide evidence for compromised replication fork progression, DNA single-strand break repair, and Okazaki fragment processing in PARG;BRCA2;p53-deficient cells, alterations that exacerbate the effects of EXO1/FEN1 inhibition and become lethal in this context. Since this sensitivity is dependent on BRCA2 defects, we propose to target EXO1/FEN1 in PARPi-resistant tumors that have lost PARG activity. Moreover, EXO1/FEN1 targeting may be a useful strategy for enhancing the effect of PARG inhibitors in homologous recombination-deficient tumors.

Bleomycin-induced modulations of PARP 1 activity, NAD+ and PARG content in rat lung nuclei

Bleomycin-induced lung pathology in rodents is a well recognized animal model widely used for evaluation of new therapeutic approaches in treatment of lung inflammation and fibrotic diseases. It is documented that poly(ADP-ribose)polymerase 1 activity has a significant role in development of inflammatory processes in the heart, liver and brain. Herein, we used biochemical and immunochemical methods for estimation of poly(ADP-ribose)polymerase 1 (PARP 1) activity, NAD+ and poly(ADP-ribose)glycohydrolase (PARG) protein content in rat lung nuclei during the inflammatory phase in a bleomycin-induced one-hit rat model. To evaluate the influence of bleomycin – induced alterations in DNA structure on regulation of poly(ADP-ribose)polymerase 1 activation pathways, we isolated DNA from nuclei of lung tissues in the phase of acute lung inflammation induced by bleomycin, and DNA melting profiles were investigated. In the present study we investigated whether naturally occurring water-soluble polyphenol tannic acid with widely accepted anti-fibrotic and anti-inflammatory effects can influence poly(ADP-ribose)polymerase 1 activity, NAD+ and poly(ADP-ribose)glycohydrolase protein content in nuclear fraction isolated from rat lung tissues in a bleomycin-induced acute lung injury model. It was demonstrated that NAD+ level and poly(ADP-ribose)glycohydrolase protein content decreased in rat lung nuclei during the inflammatory phase in the bleomycin-induced acute lung injury model. Treatment of rats with tannic acid enhanced the effects displayed by bleomycin in lung nuclei, thus indicating synergistic interaction with the drug in the field covering PARP 1 activity, poly(ADP-ribose)glycohydrolase (PARG) protein and NAD+ content in lung nuclei. We observed PARP 1 inhibition in nuclei of lung tissue during the inflammatory phase in the bleomycin-induced acute lung injury rat model, which could be coupled with the drop of NAD+ level in nuclei. In the present study we highlighted that bleomycin (BLM) can induce DNA destabilization in lung nuclei. It was proposed that bleomycin-induced modulations in DNA structure could hamper PARP 1 binding with DNA and down-regulate the enzyme activating pathway in lung nuclei. The role of poly(ADP-ribose)glycohydrolase depletion in lung nuclei and sequential accumulation of poly(ADP-ribose)polymers in lung cells, which triggers their destruction and tissues damage, was proposed. It is suggested that in the light of synergistic interaction between bleomycin and tannic acid (TA) the anti-inflammatory role of tannic acid should be repurposed.

Abstract A022: Poly (ADP-Ribose) glycohydrolase (PARG) removes repressive Poly-ADP Ribose marks from DNA polymerase theta (polq) to stimulate DNA double-strand breaks repair in myeloid malignancies

Abstract Poly (ADP-ribose) polymerase 1 (PARP1) dependent poly-(ADP)-ribosylation (PARylation) of proteins is one of the major post-translational modification events involved in the DNA damage response (DDR). PARylation is tightly controlled by the glycohydrolase activity of poly (ADP-ribose) glycohydrolase (PARG). Thus, interplay between PARP1 and PARG is thought to regulate the PARylation status of relevant DDR proteins and multiple studies demonstrated that both PARP1 and PARG contribute to DDR. Here, we investigated whether the interplay of PARP1 and PARG is important for the regulation of TMEJ and the specific activities of Polq in myeloid malignancies harboring OTKs. We found that upon DNA damage OTKs exerted temporal effect on Polq, PARP1 and PARG detection in chromatin fractions. While both Polq and PARG displayed continuous time-dependent accumulation during 120 min. after irradiation, PARP1 levels sharply increased at 20 min. and dissipated at 120 min. post-irradiation confocal microscopy detected spatiotemporal changes of Polq - PARP1 – PARG colocalization and PARylation of Polq. Polq - PARP1 interaction and PARylation of Polq was detected early followed by dissipation of that interaction and enhanced colocalization of Polq and PARG and dePARylation of Polq. At the time when Polq was PARylated by PARP1, it was not detected at DSBs marked by gH2AX whereas PARG-mediated dePARylation of Polq was associated with its colocalization with DSBs. We find that PARP1 binds to and directly PEGylates Polq in vitro and in cells. Biochemical assays revealed that PARP1 recruits Polq to the vicinity of DNA damage via PARylation-dependent formation of biomolecular condensates containing Polq. However, PARylated Polq is unable to perform TMEJ due to its inability to bind directly to DNA. Hence, PARG is needed to reactivate Polq DNA binding and its TMEJ activity by removing repressive PAR marks on Polq. In support of this, cellular studies show that PARP1-PARG – regulated spatiotemporal recruitment of Polq to DSBs is essential for TMEJ activity. In conclusion, using intracellular and biochemical approaches we show here that both PARP1 and PARG are required for TMEJ despite their counteracting enzymatic activities. We also pinpointed a unique dynamic spatiotemporal interplay between Polq, PARP1 and PARG to regulate TMEJ activity in OTK-positive myeloid malignant cells. Our studies support a two-step mechanism of a PARP1-PARG regulatory axis of Polq and TMEJ. In the first step, the rate of PARP1-Polq spatiotemporal recruitment to DNA damage foci supports a rapid step whereby PARP1 PARylates Polq and facilitates its recruitment to the vicinity of DNA damage in an inactive state. The rate of PARG spatiotemporal recruitment supports a second step whereby subsequent recruitment of PARG corresponds to dissipation of PARP1 and PAR at DNA damage sites, PARG-mediated removal of PAR repressive marks on Polq, and activation of TMEJ. Citation Format: Umeshkumar Vekariya, Leonid Minakhin, Mrityunjay Tyagi, Tatiana Kent, Gurushankar Chandramouly, Katherine Sullivan-Reed, Jessica Atkins, Margaret Nieborowska-Skorska, Anna-Mariya Kukuyan, Richard Pomerantz, Tomasz Skorski. Poly (ADP-Ribose) glycohydrolase (PARG) removes repressive Poly-ADP Ribose marks from DNA polymerase theta (polq) to stimulate DNA double-strand breaks repair in myeloid malignancies [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr A022.

Abstract LB_A19: Genome-wide CRISPR/Cas9 screens for the identification of targeted vulnerabilties in PARP inhibitor-resistant PARG;BRCA2-deficient tumor cells: A focus on EXO1/FEN1-mediated DNA repair

Abstract Targeting poly(ADP-ribose) glycohydrolase (PARG) is currently explored as a therapeutic approach to treat various cancer types, but we have a poor understanding of the specific genetic vulnerabilities that would make cancer cells susceptible to such a tailored therapy. Moreover, the identification of such vulnerabilities is of interest to target BRCA2;p53-deficient tumors that have acquired resistance to poly(ADP-ribose) polymerase inhibitors (PARPi), through loss of PARG expression. Here, by performing whole genome CRISPR/Cas9 drop-out screens we identify various genes involved in DNA repair to be essential for the survival of PARG;BRCA2;p53-deficient cells. In particular, our findings reveal EXO1 and FEN1 as major synthetic lethal interactors to PARG loss. We provide evidence that PARG;BRCA2;p53-deficient cells exhibit compromised replication fork progression, DNA single-strand break repair and Okazaki fragment processing, alterations that exacerbate and become lethal upon EXO1/FEN1 inhibition. Since this sensitivity is dependent on BRCA2 defects, we propose EXO1/FEN1 targeting in PARPi-resistant tumors that have lost PARG activity. Moreover, EXO1/FEN1 targeting may be a useful strategy to enhance the effect of PARG inhibitors in HR-deficient tumors. Citation Format: Christina Andronikou, Kamila Burdova, Diego Dibitetto, Cor Lieftink, Roderick L. Beijersbergen, Hana Hanzlikova, Jos Jonkers, Sven Rottenberg. Genome-wide CRISPR/Cas9 screens for the identification of targeted vulnerabilties in PARP inhibitor-resistant PARG;BRCA2-deficient tumor cells: A focus on EXO1/FEN1-mediated DNA repair [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr LB_A19.

Abstract B051: PARG inhibition leads to the formation of toxic nuclear PARP1 aggregates

Abstract Mutations in DNA repair genes, such as BRCA1/2 mutations, occur in 50-70% of breast cancers among others. PARP inhibitors (PARPis) have shown remarkable efficacy against these tumors, however, emerging clinical evidence has revealed that PARPi resistance represents a major concern in these cancers with up to 90% of initial responders eventually developing resistance to the therapy. To overcome the limitations of PARPis, researchers have been working towards developing PARG inhibitors (PARGis) for the treatment of DNA repair-deficient cancers.   In response to DNA damage, PARP1 synthesizes poly(ADP-ribose) (PAR) chains at break sites, which are crucial for the proper release of PARP1 and the recruitment of downstream repair proteins to the break site. PAR glycohydrolase (PARG) is the primary enzyme responsible for the resolution of PAR chains following repair. Owing to their central role in PARP-dependent processes, PARG inhibitors are currently being developed for cancer therapy, however, their precise mechanism of action has yet to be determined. Here we report that PARG loss or inhibition leads to cytotoxic effects through the formation of toxic PARP1 aggregates in response to DNA damage. Using live cell imaging assays, we determined that PARP1 aggregates form at sites distant from the original site of damage, causing a mislocalization of DNA repair factors from the damage site. We also find that these aggregates are formed by and composed of long, branched PAR chains, which are likely sequestering ATP and NAD+ reserves in the nucleus. Finally, we show that these aggregates are associated with increased cell death in BRCA-deficient cell lines suggesting that these aggregates are critical for PARGi mechanism of toxicity. Overall, this work elucidates the mechanism of action of PARG inhibitors, which will help identify patient populations in which these drugs could be used. Citation Format: Sateja Paradkar, Annie Cui, Julia Purcell, Sam Friedman, Ryan Jensen, Ranjit Bindra. PARG inhibition leads to the formation of toxic nuclear PARP1 aggregates [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B051.

Poly(ADP-Ribose) Glycohydrolase (PARG) Removes Repressive Poly-ADP Ribose Marks from DNA Polymerase Theta (PolΘ) to Stimulate DNA Double-Strand Breaks Repair in Myeloid Malignancies

Myeloid malignant cells (AML, MPN, CML) expressing oncogenic tyrosine kinases [OTKs: FLT3(ITD), JAK2(V617F), BCR/ABL1, respectively] accumulate DNA damage but altered DNA repair mechanisms protect them from apoptosis. We reported before that formaldehyde generated by altered serine/one-carbon cycle metabolism in malignant cells harboring OTKs contributed to accumulation of toxic DNA-protein crosslinks (DPCs) which were converted to highly lethal DNA double-strand breaks (DSBs). To counteract the toxicity of DSBs, OTKs enhanced the expression of DNA polymerase theta (PolΘ, encoded by POLQ gene), a unique DNA helicase-DNA polymerase fusion protein that promotes error-prone repair of DSBs by a mechanism referred to as PolΘ-mediated DNA end-joining (TMEJ). PolΘ plays an essential role in the initiation and maintenance of hematological malignancies harboring OTKs. Although the cellular activities of PolΘ have been widely studied, little is known about how PolΘ is regulated at the molecular level. For example, whether post-translational modifications of PolΘ are important for its TMEJ activity and regulation is unknown. Poly(ADP-ribose) polymerase 1 (PARP1) dependent poly-(ADP)-ribosylation (PARylation) of proteins is one of the major post-translational modification events involved in the DNA damage response (DDR). PARP1 - the founding member of the poly(ADP-ribose) polymerase family -transfers adenosine diphosphate (ADP)-ribose from nicotinamide adenine dinucleotide (NAD +) to substrate proteins enabling their PARylation. PARylation is tightly controlled by the glycohydrolase activity of poly(ADP-ribose) glycohydrolase (PARG). Thus, interplay between PARP1 and PARG is thought to regulate the PARylation status of relevant DDR proteins and multiple studies demonstrated that both PARP1 and PARG contribute to DDR. Here, we investigated whether the interplay of PARP1 and PARG is important for the regulation of TMEJ and the specific activities of PolΘ in myeloid malignancies harboring OTKs. We found that upon DNA damage OTKs exerted temporal effect on PolΘ, PARP1 and PARG detection in chromatin fractions. While both PolΘ and PARG displayed continuous time-dependent accumulation during 120 min. after irradiation, PARP1 levels sharply increased at 20 min. and dissipated at 120 min. We find that PARP1 binds to and directly PARylates PolΘ in vitro and in cells. However, PARylated PolΘ is unable to perform TMEJ in vitro due to its inability to bind DNA despite its PARylation dependent recruitment to DNA. Hence, PARG is needed to reactivate PolΘ DNA binding and its TMEJ activity by removing repressive PAR marks on PolΘ. In support of this, cellular studies show that PARG is essential for TMEJ and support a two-step mechanism of a PARP1-PARG regulatory axis of PolΘ and TMEJ. In the first step, the rate of PARP1-PolΘ spatiotemporal recruitment to DNA damage foci supports a rapid step whereby PARP1 PARylates PolΘ and facilitates its recruitment to the vicinity of DNA damage in an inactive state. The rate of PARG spatiotemporal recruitment supports a second step whereby subsequent recruitment of PARG corresponds to dissipation of PARP1 and PAR at DNA damage sites, dePARylation of PolΘ, and activation of TMEJ. These studies elucidate an unprecedented mechanism of PARP1-PARG activation of TMEJ and reveal the molecular basis by which PARP1 and PARG inhibition suppresses TMEJ. In conclusion, using intracellular and biochemical approaches we show here that both PARP1 and PARG are required for TMEJ despite their counteracting enzymatic activities. We also pinpointed a unique dynamic spatiotemporal interplay between PolΘ, PARP1 and PARG to regulate TMEJ activity. During the initial stage, PARP1 activated by DNA damage PARylates PolΘ and facilitates its recruitment to the vicinity of DNA damage. PARG removal of PAR from PolΘ reactivated its interaction with DNA resulting in robust TMEJ activity in vitro and in cells. Thus, DNA/chromatin binding and TMEJ activity of PolΘ were restored upon PARG-mediated removal of PAR repressive marks on PolΘ. This process seems to play a critical role in protecting OTK-positive hematopoietic malignant cells from genotoxic effect of formaldehyde generated by altered serine/one-carbon cycle metabolism.

Dysfunction of poly (ADP-ribose) glycohydrolase suppresses osteoclast differentiation in RANKL-stimulated RAW264 cells

Poly (ADP-ribose) glycohydrolase (PARG) is an enzyme that mainly degrades poly (ADP-ribose) (PAR) synthesized by poly (ADP-ribose) polymerase (PARP) family proteins. Although PARG is involved in many biological phenomena, including DNA repair, cell differentiation, and cell death, little is known about the relationship between osteoclast differentiation and PARG. It has also not been clarified whether PARG is a valuable target for therapeutic agents in the excessive activity of osteoclast-related bone diseases such as osteoporosis. In the present study, we examined the effects of PARG inhibitor PDD00017273 on osteoclast differentiation in RANKL-induced RAW264 cells. PDD00017273 induced the accumulation of intracellular PAR and suppressed the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells. PDD00017273 also downregulated osteoclast differentiation marker genes such as Trap, cathepsin K (Ctsk), and dendrocyte expressed seven transmembrane protein (Dcstamp) and protein expression of nuclear factor of activated T cells 1 (NFATc1), a master regulator of osteoclast differentiation. Taken together, our findings suggest that dysfunction of PARG suppresses osteoclast differentiation via the PAR accumulation and partial inactivation of the NFATc1.

DNAR-01. PAR-MEDIATED DNA DAMAGE SIGNALING IN IDH MUTANT GLIOMAS: COMBINING PARG INHIBITION AND RADIATION FOR SELECTIVE CYTOTOXICITY

Abstract Isocitrate dehydrogenase (IDH) 1 or 2 mutant gliomas have unique metabolic characteristics and DNA damage response. Radiotherapy and chemotherapy have been proven to be effective treatments for IDH mutant gliomas. Poly (ADP-ribose) (PAR) mediates DNA damage response and is regulated by opposing enzymes: poly (ADP-ribose) polymerases (PARPs) synthesize PAR, while poly (ADP-ribose) glycohydrolase (PARG) breaks it down. Combining PARG inhibitor (PARGi) and Temozolomide (TMZ) in IDH mutant cells leads to imbalanced NAD sequestration into PAR, causing metabolic lethality. This study characterizes the combination of PARGi and radiation. PARGi monotherapy showed no difference between IDH mutant and wild-type cells. However, we found that combining PARGi with radiation significantly increased cytotoxicity in IDH mutant glioma models, similar to PARPi and radiation. (Wang et al, ACi.Adv. 2020) Our findings showed that unlike TMZ+PARGi, PARGi and radiation did not consume and deplete NAD. Cytotoxicity was partially reversed by IDH inhibitor, highlighting the contribution of PAR-mediated DNA damage signaling in IDH mutant contexts. This convergence of vulnerabilities in IDH mutant cells identifies PAR-mediated DNA repair mechanisms that operate independently of NAD metabolism. IDH mutant enzyme generates 2 hydroxyglutarate (2HG), which is critical early in tumor development. However, the effect of 2HG on radiation sensitivity varies across cellular contexts. In this study, we found that IDH1 mutation and 2HG production enhance radiosensitivity in glioma cells both in vivo and in in vitro models, facilitated by PAR-dependent mechanisms. This augmentation of radiosensitivity is associated with S-phase blockage of the cell cycle, unveiling a vulnerability in the replication process of these cancer cells. In summary, this study demonstrates that the combination of PARGi and radiation regulates DNA repair pathways, augmenting cytotoxicity specifically in IDH mutant gliomas. Targeting PAR during radiation holds promise for treating IDH mutant gliomas, tailoring therapy to maximize durability while minimizing impact on normal cells.

Abr, a Rho-regulating protein, modulates osteoclastogenesis by enhancing lamellipodia formation by interacting with poly(ADP-ribose) glycohydrolase.

Osteoclasts are multinucleated bone-resorbing cells formed by the fusion of monocyte/macrophage lineage. During osteoclast differentiation, Rho GTPases are involved in various processes, including cell migration, adhesion, and polarity. However, the role of Rho-regulatory molecules in the regulation of osteoclast differentiation remains unclear. In this study, among these genes, we focused on active breakpoint cluster region-related (Abr) protein that is a multifunctional regulator of Rho GTPases. We examined using knockdown and overexpression experiments in RANKL-stimulated RAW-D macrophages whether Abr regulates osteoclast differentiation and cell morphology. We observed an increase in Abr expression during osteoclast differentiation and identified expression of a variant of the Abr gene in osteoclasts. Knockdown of Abr suppressed osteoclast differentiation and resorption. Abr knockdown markedly inhibited the expression of osteoclast markers, such as Nfatc1, c-fos, Src, and Ctsk in osteoclasts. Conversely, overexpression of Abr enhanced the formation of multinucleated osteoclasts, bone resorption activity, and osteoclast marker gene expression. Moreover, Abr overexpression accelerated lamellipodia formation and induced the formation of well-developed actin in osteoclasts. Importantly, the Abr protein interacted with poly(ADP-ribose) glycohydrolase (PARG) and Rho GTPases, including RhoA, Rac1/2/3, and Cdc42 in osteoclasts. Taken together, these results indicate that Abr modulates osteoclastogenesis by enhancing lamellipodia formation via its interaction with PARG.

PARG Promotes Esophagus Cancer Cell Metastasis by Activation of the Wnt/β-Catenin Pathway.

Esophagus cancer (EC) is a highly malignant and metastatic cancer. Poly(ADP-ribose) glycohydrolase (PARG), a DNA replication and repair regulator, inhibits cancer cell replication defects. This study aimed to explore the role of PARG in EC. The biological behaviors were analyzed using MTT assay, Transwell assay, scratch test, cell adhesion assay, and western blot. PARG expression was detected using quantitative PCR and immunohistochemical assay. The regulation of the Wnt/β-catenin pathway was assessed using western blot. The results showed that PARG was highly expressed in EC tissues and cells. Knockdown of PARG suppressed cell viability, invasion, migration, adhesion, and epithelial-mesenchymal transition. Conversely, overexpression of PARG promoted the biological behaviors mentioned above. Moreover, overexpression of PARG promoted the activation of the Wnt/β-catenin pathway rather than the STAT and Notch pathways. XAV939, the Wnt/β-catenin pathway inhibitor, partly abolished the biological behaviors mediated by PARG overexpression. In conclusion, PARG promoted the malignant advancement of EC via activating the Wnt/β-catenin pathway. These findings suggested that PARG might be a new therapeutic target for EC.

Current Evidence and Future Perspectives about the Role of PARP Inhibitors in the Treatment of Thoracic Cancers.

In recent years, poly (ADP-ribose) polymerase (PARP) inhibition has become a promising therapeutic option for several tumors, especially for those harboring a BRCA 1-2 mutation or a deficit in the homologous recombination repair (HRR) pathway. Nevertheless, to date, PARP inhibitors are still not largely used for thoracic malignancies neither as a single agent nor in combination with other treatments. Recently, a deeper understanding of HRR mechanisms, alongside the development of new targeted and immunotherapy agents, particularly against HRR-deficient tumors, traced the path to new treatment strategies for many tumor types including lung cancer and malignant pleural mesothelioma. The aim of this review is to sum up the current knowledge about cancer-DNA damage response pathways inhibition and to update the status of recent clinical trialsinvestigating the use of PARP inhibitors, either as monotherapy or in combination with other agents for the treatment of thoracic malignancies. We will also briefly discuss available evidence on Poly(ADP-Ribose) Glycohydrolase (PARG) inhibitors, a novel promising therapeutic option in oncology.