Distinct roles for PARP-1 and PARP-2 in c-Myc-driven B-cell lymphoma in mice

The dynamics and regulation of PARP1 and PARP2 in response to DNA damage and during replication

DNA strand breaks activate Poly(ADP-ribose) polymerase (PARP) 1 and 2, which use NAD+ as the substrate to covalently conjugate ADP-ribose on themselves and other proteins (e.g., Histone) to promote chromatin relaxation and recruit additional DNA repair factors. Enzymatic inhibitors of PARP1 and PARP2 (PARPi) are promising cancer therapy agents that selectively target BRCA1- or BRCA2- deficient cancers. As immediate early responders to DNA strand breaks with robust activities, PARP1 and PARP2 normally form transient foci (<10 minutes) at the micro-irradiation-induced DNA lesions. In addition to enzymatic inhibition, PARPi also extend the presence of PARP1 and PARP2 at DNA lesions, including at replication forks, where they may post a physical block for subsequent repair and DNA replication. The dynamic nature of PARP1 and PARP2 foci made live cell imaging a unique platform to detect subtle changes and the functional interaction among PARP1, PARP2, and their regulators. Recent imaging studies have provided new understandings of the biological consequence of PARP inhibition and uncovered functional interactions between PARP1 and PARP2 and new regulators (e.g., histone poly(ADP-ribosylation) factor). Here, we review recent advances in dissecting the temporal and spatial Regulation of PARP1 and PARP2 at DNA lesions and discuss their physiological implications on both cancer and normal cells.

A PARP2-specific active site α-helix melts to permit DNA damage-induced enzymatic activation

PARP1 and PARP2 recognize DNA breaks immediately upon their formation, generate a burst of local PARylation to signal their location, and are co-targeted by all current FDA-approved forms of PARP inhibitors (PARPi) used in the cancer clinic. Recent evidence indicates that the same PARPi molecules impact PARP2 differently from PARP1, raising the possibility that allosteric activation may also differ. We find that unlike for PARP1, destabilization of the autoinhibitory domain of PARP2 is insufficient for DNA damage-induced catalytic activation. Rather, PARP2 activation requires further unfolding of an active site α-helix absent in PARP1. Only one clinical PARPi, Olaparib, stabilizes the PARP2 active site α-helix, representing a structural feature with the potential to discriminate small molecule inhibitors. Collectively, our findings reveal unanticipated differences in local structure and changes in activation-coupled backbone dynamics between PARP1 and PARP2.

Specific and shared biological functions of PARP2 - is PARP2 really a lil' brother of PARP1?

PARP2, that belongs to the family of ADP-ribosyl transferase enzymes (ART), is a discovery of the millennium, as it was identified in 1999. Although PARP2 was described initially as a DNA repair factor, it is now evident that PARP2 partakes in the regulation or execution of multiple biological processes as inflammation, carcinogenesis and cancer progression, metabolism or oxidative stress-related diseases. Hereby, we review the involvement of PARP2 in these processes with the aim of understanding which processes are specific for PARP2, but not for other members of the ART family. A better understanding of the specific functions of PARP2 in all of these biological processes is crucial for the development of new PARP-centred selective therapies.

PARP Inhibitors for Breast Cancer Treatment: A Review

Importance

Poly(adenosine diphosphate-ribose) polymerase (PARP) inhibitors have revolutionized the treatment of patients with germline BRCA1/2-associated breast cancer, representing the first targeted therapy capable of improving outcomes in patients with hereditary tumors. However, resistance to PARP inhibitors occurs in almost all patients.

Observations

This narrative review summarizes the biological rationale behind the use of PARP inhibitors in breast cancer, as well as the available evidence, recent progress, and potential future applications of these agents. Recent studies have shown that the benefit of PARP inhibitors extends beyond patients with germline BRCA1/2-associated metastatic breast cancer to patients with somatic BRCA1/2 variants and to those with germline PALB2 alterations. Moreover, these agents proved to be effective both in the metastatic and adjuvant settings. However, patients with metastatic breast cancer usually do not achieve the long-term benefit from PARP inhibitors observed in other tumor types. Mechanisms of resistance have been identified, but how to effectively target them is largely unknown. Ongoing research is investigating both novel therapeutics and new combination strategies to overcome resistance. PARP1-selective inhibitors, by sparing the hematological toxic effects induced by the PARP2 blockade, are promising agents to be combined with chemotherapy, antibody-drug conjugates, and other targeted therapies.

Conclusions and Relevance

Although the efficacy of PARP inhibitors is well established, many questions persist. Future research should focus on identifying predictive biomarkers and therapeutic strategies to overcome resistance. Integrating well-designed translational efforts into all clinical studies is thereby crucial to laying the groundwork for future insights from ongoing research.

PARP2 promotes Break Induced Replication-mediated telomere fragility in response to replication stress

PARP2 is a DNA-dependent ADP-ribosyl transferase (ARTs) enzyme with Poly(ADP-ribosyl)ation activity that is triggered by DNA breaks. It plays a role in the Base Excision Repair pathway, where it has overlapping functions with PARP1. However, additional roles for PARP2 have emerged in the response of cells to replication stress. In this study, we demonstrate that PARP2 promotes replication stress-induced telomere fragility and prevents telomere loss following chronic induction of oxidative DNA lesions and BLM helicase depletion. Telomere fragility results from the activity of the break-induced replication pathway (BIR). During this process, PARP2 promotes DNA end resection, strand invasion and BIR-dependent mitotic DNA synthesis by orchestrating POLD3 recruitment and activity. Our study has identified a role for PARP2 in the response to replication stress. This finding may lead to the development of therapeutic approaches that target DNA-dependent ART enzymes, particularly in cancer cells with high levels of replication stress.

Advances in Development of Selective Antitumor Inhibitors That Target PARP-1

Cancer is a major threat to the lives and health of people around the world, and the development of effective antitumor drugs that exhibit fewer toxic effects is an important aspect of cancer treatment. PARP inhibitors are antitumor drugs that target pathways involved in DNA-damage repair. The currently approved PARP inhibitors include olaparib, niraparib, rucaparib, talazoparib, fuzuloparib, and pamiparib. Hematological toxicities associated with the simultaneous inhibition of PARP-1 and PARP-2 have limited the clinical applications of these drugs. The present review introduces the necessity for research on the development of selective PARP-1 inhibitors from the perspective of structural and functional mechanisms of PARP-1 inhibition. A review of recently reported selective PARP-1 inhibitors provides the foundation for exploring novel strategies for designing selective PARP-1 inhibitors from the perspective of structure-activity relationships combined with computer simulations.

PARP-1 inhibits DNMT1-mediated promoter methylation and promotes linc01132 expression in benzene-exposed workers and hydroquinone-induced malignant transformed cells

Hydroquinone (HQ), one of the main active metabolites of benzene, can induce the abnormal expression of long non-coding RNA (lncRNA). Studies have shown that lncRNA plays an important role in the occurrence of hematologic tumors induced by benzene or HQ. However, the molecular mechanism remains to be elucidated. Here, we investigated the molecular mechanism by which poly(ADP-ribose)polymerase 1 (PARP-1) interacts with DNA methyltransferase 1 (DNMT1) to regulate promoter methylation mediated linc01132 expression in HQ-induced TK6 malignant transformed cells (HQ-MT). The results revealed that the expression of linc01132 was increased in benzene-exposed workers and HQ-MT cells. The methylation of linc01132 promoter region was inhibited. Furthermore, in HQ-MT cells treated with 5-Aza-2'-deoxycytidine (5-AzaC) (DNA methyltransferase inhibitor) or trichostatin A (TSA) (histone deacetylation inhibitor), the expression of linc01132 was increased due to the regulation of DNA promoter methylation level by inhibiting DNMT1 expression. The methylation level of linc01132 promoter was correlated negatively with the expression of linc01132 in benzene-exposed workers, indicating that DNA methylation may contribute the expression of linc01132. Knockout of DNMT1, not DNMT3b, increased the expression of linc01132 as well as the demethylation of linc01132 promoter in HQ-MT cells. It was found that by knockdown PARP-1, the expression of DNMT1 in the nucleus was increased by immunofluorescence confocal microscopy, leading to the inhibition of hypermethylation in the promoter region of linc01132. Therefore, PARP-1 inhibits DNA methyltransferase (DNMT)-mediated promoter methylation and plays a role in linc01132 expression in benzene-exposed workers or HQ-MT cells, and is associated with benzene or HQ induced leukemia progression.

PARP1 Inhibition Halts EBV+ Lymphoma Progression by Disrupting the EBNA2/MYC Axis

PARP1 has been shown to regulate EBV latency. However, the therapeutic effect of PARP1 inhibitors on EBV+ lymphomagenesis has not yet been explored. Here, we show that PARPi BMN-673 has a potent anti-tumor effect on EBV-driven LCL in a mouse xenograft model. We found that PARP1 inhibition induces a dramatic transcriptional reprogramming of LCLs driven largely by the reduction of the MYC oncogene expression and dysregulation of MYC targets, both in vivo and in vitro. PARP1 inhibition also reduced the expression of viral oncoprotein EBNA2, which we previously demonstrated depends on PARP1 for activation of MYC. Further, we show that PARP1 inhibition blocks the chromatin association of MYC, EBNA2, and tumor suppressor p53. Overall, our study strengthens the central role of PARP1 in EBV malignant transformation and identifies the EBNA2/MYC pathway as a target of PARP1 inhibitors and its utility for the treatment of EBNA2-driven EBV-associated cancers.

Lessons from Using Genetically Engineered Mouse Models of MYC-Induced Lymphoma

Oncogenic overexpression of MYC leads to the fatal deregulation of signaling pathways, cellular metabolism, and cell growth. MYC rearrangements are found frequently among non-Hodgkin B-cell lymphomas enforcing MYC overexpression. Genetically engineered mouse models (GEMMs) were developed to understand MYC-induced B-cell lymphomagenesis. Here, we highlight the advantages of using Eµ-Myc transgenic mice. We thoroughly compiled the available literature to discuss common challenges when using such mouse models. Furthermore, we give an overview of pathways affected by MYC based on knowledge gained from the use of GEMMs. We identified top regulators of MYC-induced lymphomagenesis, including some candidates that are not pharmacologically targeted yet.

Studying the Immunomodulatory Functions of PARP1 and PARP2 in Mouse Models of Cancer

Poly-ADP-ribosylation of proteins, mediated by the two ADP-ribosyltransferases PARP1 and PARP2 in response to DNA damage, has emerged as a critical mediator of the DNA damage response (DDR). Accordingly, considering the critical role of DDR in cancer, PARP inhibitors (PARPi) have become an important class of therapeutics. PARPi have largely been considered for their intrinsic actions to tumor cells per se. However, these compounds also affect the immune response to tumors. It is now an emerging evidence supporting immunomodulatory roles of PARP1 and PARP2 which can facilitate or impede tumor progression. In this chapter, we describe some protocols to study the immunomodulatory functions of PARP1 and PARP2 in mouse tumor models.

Hematological disorders after salvage PARPi treatment for ovarian cancer: Cytogenetic and molecular defects and clinical outcomes

Inhibitors of poly(ADP-ribose) polymerase (PARPi) are increasingly employed as salvage therapy in epithelial ovarian cancer (EOC), but cytotoxic drug exposure along with PARP inhibition may favor development of hematological disorders. In our study, of 182 women with EOC treated with PARPi, 16 (8.7%) developed therapy-related myeloid neoplasms (t-MNs), with 12 cases of myelodysplasia and 4 of acute myeloid leukemia. All experienced persistent cytopenia after PARPi discontinuation. Seven patients had del(5q)/-5 and/or del(7q)/-7, nine had a complex karyotype and TP53 mutations, recently reported as risk factor for t-MNs in EOC post-PARPi, were found in 12 out of 13 tested patients. Four patients had a rapid and fatal outcome, one had stable disease, eleven underwent induction therapy, followed by allogeneic hematopoietic cell transplantation in seven. Three of these 11 patients experienced refractory disease, and 8 had complete remission. During a 6.8 months (range 2.3-49) median observation time, 3 out of 16 patients were alive, with one surviving patient free of both solid and hematological tumors. Ten patients died because of leukemia, two because of transplant-related events, one from heart failure. Five more patients experienced persistent cell blood count abnormalities following PARPi discontinuation, without reaching MDS diagnostic criteria. A customized Myelo-panel showed clonal hematopoiesis in all five patients. These findings confirm the actual risk of t-MNs in EOC patients after chemotherapy and prolonged PARPi therapy. The management of these patients is complex and outcomes are extremely poor. Careful diagnostic procedures are strongly recommended whenever unusual cytopenias develop in patients receiving PARPi therapy.

LINC00958: A promising long non-coding RNA related to cancer

Long non-coding RNAs (lncRNAs), a class of RNA transcripts longer than 200 nucleotides, do not encode proteins; however, they encode small peptides and micropeptides that act as bioactive peptides with notable effects in regulating the progression of malignant tumors, such as lung and colorectal cancers, and affecting patient prognosis. lncRNAs are important intracellular regulators, particularly in tumorigenesis and tumor progression. Long intergenic non-protein coding RNA958 (LINC00958), which has received increasing attention in recent years, is highly expressed in various malignancies, including head and neck squamous cell carcinoma (HNSC), non-small-cell lung cancer (NSCLC), gastric cancer, hepatocellular carcinoma (HCC), colorectal cancer, bladder cancer, and breast cancer. Here, we reviewed the recent studies on LINC00958 as well as its closely related clinical features and functional regulation in cancers. We systematically expounded the molecular mechanisms underlying the biological functions of LINC00958 in inhibiting cell apoptosis and enhancing the chemoradiotherapy resistance of tumor cells. The upregulation of LINC00958 enhances the resistance of tumor cells to radiotherapy and chemotherapy and induces lymphangiogenesis. Moreover, it is involved in tumor glycolytic metabolism, which plays a crucial role in facilitating the proliferation, invasion, and migration of tumor cells. Additionally, analysis of various studies revealed that LINC00958 acts as an endogenous competitive RNA (ceRNA) and regulates the malignant behavior of tumor cells through the miRNA-mRNA axis. Collectively, the use of LINC00958 as a novel biomarker and therapeutic target for the clinical diagnosis and treatment of different cancers has bright prospects in the future.

Attenuation of Muscle Damage, Structural Abnormalities, and Physical Activity in Respiratory and Limb Muscles following Treatment with Rucaparib in Lung Cancer Cachexia Mice

Simple Summary

Muscle wasting and cachexia are common in patients with cancer. Several mechanisms underlie muscle physiological and structural alterations in cancer-induced cachexia. Poly (ADPribose) polymerases (PARPs) are involved in muscle metabolism and in cancer. Selective inhibitors of PARP activity improve muscle function and structure. This study sought to investigate whether rucaparib (PARP inhibitor) may attenuate muscle damage in a mouse model of lung-cancer-induced cachexia. Rucaparib was administered to cancer-cachectic mice. Physiological and biological parameters were determined in the respiratory and limb muscles of the animals. In cancer cachexia mice compared to non-cachexia controls, body weight and body weight gain, muscle weight, limb strength, physical activity, and muscle fiber size significantly declined, while levels of PARP activity, plasma troponin I, muscle damage, and proteolytic and autophagy markers increased. Treatment with rucaparib elicited a significant improvement in body weight gain, tumor size and weight, physical activity, muscle damage, troponin I, and proteolytic and autophagy levels.

Abstract

Overactivation of poly (ADPribose) polymerases (PARPs) is involved in cancer-induced cachexia. We hypothesized that the PARP inhibitor rucaparib may improve muscle mass and reduce damage in cancer cachexia mice. In mouse diaphragm and gastrocnemius (LP07 lung adenocarcinoma) treated with PARP inhibitor (rucaparib,150 mg/kg body weight/24 h for 20 days) and in non-tumor control animals, body, muscle, and tumor weights; tumor area; limb muscle strength; physical activity; muscle structural abnormalities, damage, and phenotype; PARP activity; and proteolytic and autophagy markers were quantified. In cancer cachexia mice compared to non-cachexia controls, body weight and body weight gain, muscle weight, limb strength, physical activity, and muscle fiber size significantly declined, while levels of PARP activity, plasma troponin I, muscle damage, and proteolytic and autophagy markers increased. Treatment with the PARP inhibitor rucaparib elicited a significant improvement in body weight gain, tumor size and weight, physical activity, muscle damage, troponin I, and proteolytic and autophagy levels. PARP pharmacological inhibition did not exert any significant improvements in muscle weight, fiber size, or limb muscle strength. Treatment with rucaparib, however, improved muscle damage and structural abnormalities and physical activity in cancer cachexia mice. These findings suggest that rucaparib exerts its beneficial effects on cancer cachexia performance through the restoration of muscle structure.

PARP inhibitors trap PARP2 and alter the mode of recruitment of PARP2 at DNA damage sites

Dual-inhibitors of PARP1 and PARP2 are promising anti-cancer drugs. In addition to blocking PARP1&2 enzymatic activity, PARP inhibitors also extend the lifetime of DNA damage-induced PARP1&2 foci, termed trapping. Trapping is important for the therapeutic effects of PARP inhibitors. Using live-cell imaging, we found that PARP inhibitors cause persistent PARP2 foci by switching the mode of PARP2 recruitment from a predominantly PARP1- and PAR-dependent rapid exchange to a WGR domain-mediated stalling of PARP2 on DNA. Specifically, PARP1-deletion markedly reduces but does not abolish PARP2 foci. The residual PARP2 foci in PARP1-deficient cells are DNA-dependent and abrogated by the R140A mutation in the WGR domain. Yet, PARP2-R140A forms normal foci in PARP1-proficient cells. In PARP1-deficient cells, PARP inhibitors - niraparib, talazoparib, and, to a lesser extent, olaparib - enhance PARP2 foci by preventing PARP2 exchange. This trapping of PARP2 is independent of auto-PARylation and is abolished by the R140A mutation in the WGR domain and the H415A mutation in the catalytic domain. Taken together, we found that PARP inhibitors trap PARP2 by physically stalling PARP2 on DNA via the WGR-DNA interaction while suppressing the PARP1- and PAR-dependent rapid exchange of PARP2.

Impact of DNA Damage Response-Targeted Therapies on the Immune Response to Tumours

The DNA damage response (DDR) maintains the stability of a genome faced with genotoxic insults (exogenous or endogenous), and aberrations of the DDR are a hallmark of cancer cells. These cancer-specific DDR defects present new therapeutic opportunities, and different compounds that inhibit key components of DDR have been approved for clinical use or are in various stages of clinical trials. Although the therapeutic rationale of these DDR-targeted agents initially focused on their action against tumour cells themselves, these agents might also impact the crosstalk between tumour cells and the immune system, which can facilitate or impede tumour progression. In this review, we summarise recent data on how DDR-targeted agents can affect the interactions between tumour cells and the components of the immune system, both by acting directly on the immune cells themselves and by altering the expression of different molecules and pathways in tumour cells that are critical for their relationship with the immune system. Obtaining an in-depth understanding of the mechanisms behind how DDR-targeted therapies affect the immune system, and their crosstalk with tumour cells, may provide invaluable clues for the rational development of new therapeutic strategies in cancer.