Involvement of poly(ADP-ribose) polymerase in base excision repair

A comparative study of different antiviral treatment protocols in HCV related cryoglobulinemic vasculitis

The treatment of HCV and its sequelae are used to be predominantly based on Interferon (IFN). However, this was associated with significant adverse events as a result of its immunostimulant capabilities. Since their introduction, the directly acting antiviral drugs (DAAs), have become the standard of care to treat of HCV and its complications including mixed cryoglobulinemic vasculitis (MCV). In spite of achieving sustained viral response (SVR), there appeared many reports describing unwelcome complications such as hepatocellular and hematological malignancies as well as relapses. Prolonged inflammation induced by a multitude of factors, can lead to DNA damage and affects BAFF and APRIL, which serve as markers of B-cell proliferation. We compared, head-to-head, three antiviral protocols for HCV-MCV treatment As regards the treatment response and relapse, levels of BAFF and APRIL among pegylated interferon α-based and free regimens (Sofosbuvir + Ribavirin; SOF-RIBA, Sofosbuvir + Daclatasvir; SOF-DACLA). Regarding clinical response HCV-MCV and SVR; no significant differences could be identified among the 3 different treatment protocols, and this was also independent form using IFN. We found no significant differences between IFN-based and free regimens DNA damage, markers of DNA repair, or levels of BAFF and APRIL. However, individualized drug-to-drug comparisons showed many differences. Those who were treated with IFN-based protocol showed decreased levels of DNA damage, while the other two IFN-free groups showed increased DNA damage, being the worst in SOF-DACLA group. There were increased levels of BAFF through follow-up periods in the 3 protocols being the best in SOF-DACLA group (decreased at 24 weeks). In SOF-RIBA, CGs relapsed significantly during the follow-up period. None of our patients who were treated with IFN-based protocol had significant clinico-laboratory relapse. Those who received IFN-free DAAs showed a statistically significant relapse of constitutional manifestations. Our findings suggest that IFN-based protocols are effective in treating HCV-MCV similar to IFN-free protocols. They showed lower levels of DNA damage and repair. We believe that our findings may offer an explanation for the process of lymphoproliferation, occurrence of malignancies, and relapses by shedding light on such possible mechanisms.

A review of poly(ADP-ribose)polymerase-1 (PARP1) role and its inhibitors bearing pyrazole or indazole core for cancer therapy

Cancer is a disease with a high mortality rate characterized by uncontrolled proliferation of abnormal cells. The hallmarks of cancer evidence the acquired cells characteristics that promote the growth of malignant tumours, including genomic instability and mutations, the ability to evade cellular death and the capacity of sustaining proliferative signalization. Poly(ADP-ribose) polymerase-1 (PARP1) is a protein that plays key roles in cellular regulation, namely in DNA damage repair and cell survival. The inhibition of PARP1 promotes cellular death in cells with homologous recombination deficiency, and therefore, the interest in PARP protein has been rising as a target for anticancer therapies. There are already some PARP1 inhibitors approved by Food and Drug Administration (FDA), such as Olaparib and Niraparib. The last compound presents in its structure an indazole core. In fact, pyrazoles and indazoles have been raising interest due to their various medicinal properties, namely, anticancer activity. Derivatives of these compounds have been studied as inhibitors of PARP1 and presented promising results. Therefore, this review aims to address the importance of PARP1 in cell regulation and its role in cancer. Moreover, it intends to report a comprehensive literature review of PARP1 inhibitors, containing the pyrazole and indazole scaffolds, published in the last fifteen years, focusing on structure-activity relationship aspects, thus providing important insights for the design of novel and more effective PARP1 inhibitors.

Cleaved caspase-3 is present in the majority of glial cells in the intact rat spinal cord during postnatal life

Cell death is an essential process that occurs during the development of the central nervous system. Despite the availability of a wide range of commercially produced antibodies against various apoptotic markers, data regarding apoptosis in intact spinal cord during postnatal development and adulthood are mostly missing. We investigated apoptosis in rat spinal cord at different stages of ontogenesis (postnatal days 8, 29, and 90). For this purpose, we applied immunofluorescent detection of two widely used apoptotic markers, cleaved caspase-3 (cC3) and cleaved poly(ADP-ribose) polymerase (cPARP). Surprisingly, we found significant discrepancy between the number of cC3+ cells and PARP+ cells, with a ratio between 500:1 and 5000:1 in rat spinal cord at all postnatal time points. The majority of cC3+ cells were glial cells and did not exhibit an apoptotic phenotype. In contrast with in vivo results, in vitro analysis of primary cell cultures derived from neonatal rat spinal cord and treated with the apoptotic inductor staurosporine revealed a similar onset of occurrence of both cC3 and cPARP in cells subjected to apoptosis. Gene expression analysis of spinal cord revealed elevated expression of the Birc4 (XIAP), Birc2, and Birc5 (Survivin) genes, which are known potent inhibitors of apoptosis. Our data indicate that cC3 is not an exclusive marker of apoptosis, especially in glial cells, owing its possible presence in inhibited forms and/or its participation in other non-apoptotic roles. Therefore, cPARP appears to be a more appropriate marker to detect apoptosis.

A Knockout of Poly(ADP-Ribose) Polymerase 1 in a Human Cell Line: An Influence on Base Excision Repair Reactions in Cellular Extracts

Base excision repair (BER) is the predominant pathway for the removal of most forms of hydrolytic, oxidative, and alkylative DNA lesions. The precise functioning of BER is achieved via the regulation of each step by regulatory/accessory proteins, with the most important of them being poly(ADP-ribose) polymerase 1 (PARP1). PARP1's regulatory functions extend to many cellular processes including the regulation of mRNA stability and decay. PARP1 can therefore affect BER both at the level of BER proteins and at the level of their mRNAs. Systematic data on how the PARP1 content affects the activities of key BER proteins and the levels of their mRNAs in human cells are extremely limited. In this study, a CRISPR/Cas9-based technique was used to knock out the PARP1 gene in the human HEK 293FT line. The obtained cell clones with the putative PARP1 deletion were characterized by several approaches including PCR analysis of deletions in genomic DNA, Sanger sequencing of genomic DNA, quantitative PCR analysis of PARP1 mRNA, Western blot analysis of whole-cell-extract (WCE) proteins with anti-PARP1 antibodies, and PAR synthesis in WCEs. A quantitative PCR analysis of mRNAs coding for BER-related proteins-PARP2, uracil DNA glycosylase 2, apurinic/apyrimidinic endonuclease 1, DNA polymerase β, DNA ligase III, and XRCC1-did not reveal a notable influence of the PARP1 knockout. The corresponding WCE catalytic activities evaluated in parallel did not differ significantly between the mutant and parental cell lines. No noticeable effect of poly(ADP-ribose) synthesis on the activity of the above WCE enzymes was revealed either.

A multi-site phase I trial of Veliparib with standard radiation and temozolomide in patients with newly diagnosed glioblastoma multiforme (GBM)

PURPOSE

A multi-site Phase I trial was conducted to determine the safety, maximum tolerated dose, and pharmacokinetics (PK) of Veliparib, a Poly (ADP-ribose) polymerase [PARP] enzyme inhibitor, when administered with temozolomide (TMZ) alone and then with temozolomide and radiation (RT) in patients with newly diagnosed glioblastoma.

METHODS

Given the potential for myelosuppression when a PARP inhibitor is combined with chemotherapy, the first 6 patients accrued were given Veliparib 10 mg bid and TMZ 75 mg/m2/d daily for six weeks. If this was well tolerated, the same doses of Veliparib and TMZ would be tested along with standard radiation with plans to dose escalate the Veliparib in subsequent patient cohorts. Once a maximal tolerated dose was determined, a 78 patient phase II study was planned. Peripheral blood pharmacokinetics were assessed.

RESULTS

Twenty-four patients were enrolled. In the first 6 patients who received 6 weeks of TMZ with Veliparib only one dose limiting toxicity (DLT) occurred. The next 12 patients received 6 weeks of RT + TMZ + veliparib and 4/12 (33%) had dose limiting hematologic toxicities. As a result, Veliparib was reduced by 50% to 10 mg BID every other week, but again 3/3 patients had dose limiting hematologic toxicities. The trial was then terminated. The mean clearance (± SD) CL/F of Veliparib for the initial dose (27.0 ± 9.0 L/h, n = 16) and at steady-state for 10 mg BID (23.5 ± 10.4 L/h, n = 18) were similar. Accumulation for BID dosing was 56% (± 33%).

CONCLUSIONS

Although Veliparib 10 mg BID administered with TMZ 75 mg/m2 for six weeks was well tolerated, when this regimen was combined with standard partial brain irradiation it was severely myelosuppressive even when the dose was reduced by 50%. This study again highlights the potential of localized cranial radiotherapy to significantly increase hematologic toxicity of marginally myelosuppressive systemic therapies.

A Multi-Site Phase I Trial of Veliparib with Standard Radiation and Temozolomide in Patients with Newly Diagnosed Glioblastoma Multiforme (GBM)

Purpose

A multi-site Phase I trial was conducted to determine the safety, maximum tolerated dose, and pharmacokinetics (PK) of Veliparib, a Poly (ADP-ribose) polymerase [PARP] enzyme inhibitor, when administered with temozolomide (TMZ) alone and then with temozolomide and radiation (RT) in patients with newly diagnosed glioblastoma.

Methods

Given the potential for myelosuppression when a PARP inhibitor is combined with chemotherapy, the first 6 patients accrued were given Veliparib 10 mg bid and TMZ 75 mg/m2/d daily for six weeks. If this was well tolerated, the same doses of Veliparib and TMZ would be tested along with standard radiation with plans to dose escalate the Veliparib in subsequent patient cohorts. Once a maximal tolerated dose was determined, a 78 patient phase II study was planned. Peripheral blood pharmacokinetics were assessed.

Results

Twenty-four patients were enrolled. In the first 6 patients who received 6 weeks of TMZ with Veliparib only one dose limiting toxicity (DLT) occurred. The next 12 patients received 6 weeks of RT + TMZ + veliparib and 4/12 (33%) had dose limiting hematologic toxicities. As a result, Veliparib was reduced by 50% to 10 mg BID every other week, but again 3/3 patients had dose limiting hematologic toxicities. The trial was then terminated. The mean clearance (± SD) CL/F of Veliparib for the initial dose (27.0 ± 9.0 L/h, n = 16) and at steady-state for 10 mg BID (23.5 ± 10.4 L/h, n = 18) were similar. Accumulation for BID dosing was 56% (± 33%).

Conclusions

Although Veliparib 10 mg BID administered with TMZ 75 mg/m2 for six weeks was well tolerated, when this regimen was combined with standard partial brain irradiation it was severely myelosuppressive even when the dose was reduced by 50%. This study again highlights the potential of localized cranial radiotherapy to significantly increase hematologic toxicity of marginally myelosuppressive systemic therapies.

Unleashing the Power of Synthetic Lethality: Augmenting Treatment Efficacy through Synergistic Integration with Chemotherapy Drugs

Cancer is the second leading cause of death in the world, and chemotherapy is one of the main methods of cancer treatment. However, the resistance of cancer cells to chemotherapeutic drugs has always been the main reason affecting the therapeutic effect. Synthetic lethality has emerged as a promising approach to augment the sensitivity of cancer cells to chemotherapy agents. Synthetic lethality (SL) refers to the specific cell death resulting from the simultaneous mutation of two non-lethal genes, which individually allow cell survival. This comprehensive review explores the classification of SL, screening methods, and research advancements in SL inhibitors, including Poly (ADP-ribose) polymerase (PARP) inhibitors, Ataxia telangiectasia and Rad3-related (ATR) inhibitors, WEE1 G2 checkpoint kinase (WEE1) inhibitors, and protein arginine methyltransferase 5 (PRMT5) inhibitors. Emphasizing their combined use with chemotherapy drugs, we aim to unveil more effective treatment strategies for cancer patients.

TRAF6 regulates autophagy and apoptosis of melanoma cells through c-Jun/ATG16L2 signaling pathway

Autophagy and apoptosis are essential processes that participate in cell death and maintain cellular homeostasis. Dysregulation of these biological processes results in the development of diseases, including cancers. Therefore, targeting the interaction between apoptosis and autophagy offers a potential strategy for cancer therapy. Melanoma is the most lethal skin cancer. We previously found that tumor necrosis factor receptor-associated factor 6 (TRAF6) is overexpressed in melanoma and benefits the malignant phenotype of melanoma cells. Additionally, TRAF6 promotes the activation of cancer-associated fibroblasts in melanoma. However, the role of TRAF6 in autophagy and apoptosis remains unclear. In this study, we found that knockdown of TRAF6 induced both apoptosis and autophagy in melanoma cells. Transcriptomic data and real-time PCR analysis demonstrated reduced expression of autophagy related 16 like 2 (ATG16L2) in TRAF6-deficient melanoma cells. ATG16L2 knockdown resulted in increased autophagy and apoptosis. Mechanism studies confirmed that TRAF6 regulated ATG16L2 expression through c-Jun. Importantly, targeting TRAF6 with cinchonine, a TRAF6 inhibitor, effectively suppressed the growth of melanoma cells by inducing autophagy and apoptosis through the TRAF6/c-Jun/ATG16L2 signaling pathway. These findings highlight the pivotal role of TRAF6 in regulating autophagy and apoptosis in melanoma, emphasizing its significance as a novel therapeutic target for melanoma treatment.

Lamin A/C impairments cause mitochondrial dysfunction by attenuating PGC1α and the NAMPT-NAD+ pathway

Mutations in the lamin A/C gene (LMNA) cause laminopathies such as the premature aging Hutchinson Gilford progeria syndrome (HGPS) and altered lamin A/C levels are found in diverse malignancies. The underlying lamin-associated mechanisms remain poorly understood. Here we report that lamin A/C-null mouse embryo fibroblasts (Lmna-/- MEFs) and human progerin-expressing HGPS fibroblasts both display reduced NAD+ levels, unstable mitochondrial DNA and attenuated bioenergetics. This mitochondrial dysfunction is associated with reduced chromatin recruitment (Lmna-/- MEFs) or low levels (HGPS) of PGC1α, the key transcription factor for mitochondrial homeostasis. Lmna-/- MEFs showed reduced expression of the NAD+-biosynthesis enzyme NAMPT and attenuated activity of the NAD+-dependent deacetylase SIRT1. We find high PARylation in lamin A/C-aberrant cells, further decreasing the NAD+ pool and consistent with impaired DNA base excision repair in both cell models, a condition that fuels DNA damage-induced PARylation under oxidative stress. Further, ATAC-sequencing revealed a substantially altered chromatin landscape in Lmna-/- MEFs, including aberrantly reduced accessibility at the Nampt gene promoter. Thus, we identified a new role of lamin A/C as a key modulator of mitochondrial function through impairments of PGC1α and the NAMPT-NAD+ pathway, with broader implications for the aging process.

Tolerability, safety, and preliminary antitumor activity of fuzuloparib in combination with SHR-1316 in patients with relapsed small cell lung cancer: a multicenter, open-label, two-stage, phase Ib trial

Background

Second-line treatment options for small cell lung cancer (SCLC) are limited. Preclinical research shows that inhibition of poly (ADP-ribose) polymerase (PARP) could upregulate programmed death-ligand 1 (PD-L1), and thus render cancer cells more sensitive to immune checkpoint inhibitors. This study investigated the tolerability, safety, and preliminary antitumor activity of fuzuloparib (a PARP inhibitor) plus SHR-1316 (a PD-L1 inhibitor) for relapsed SCLC.

Methods

Patients with SCLC who failed previous first-line platinum-based therapy were enrolled in this two-stage phase Ib trial. In stage 1, 2 dose levels were designed: fuzuloparib 100 mg or 150 mg twice daily plus SHR-1316 600 mg every 2 weeks, with 6 patients in each dose level. Based on the tolerability during the first 28-day cycle and the preliminary antitumor activity in stage 1, a recommended phase II dose (RP2D) was determined and introduced in the stage 2 expansion phase. The primary endpoints were safety and RP2D in stage 1 and objective response rate (ORR) in stage 2.

Results

A total of 23 patients were enrolled, with 16 receiving fuzuloparib 100 mg plus SHR-1316 and 7 receiving fuzuloparib 150 mg plus SHR-1316. At data cutoff on April 23, 2021, the median follow-up duration was 6.4 months (IQR, 3.0–9.7 months). All patients discontinued study treatment. One patient receiving fuzuloparib 150 mg plus SHR-1316 had clinically significant toxicities, and fuzuloparib 100 mg plus SHR-1316 was considered as the RP2D. In the RP2D cohort, the confirmed ORR was 6.3% (95% CI: 0.2–30.2%), and the disease control rate was 37.5% (95% CI: 15.2–64.6%). The median progression-free survival was 1.4 months (95% CI: 1.3–2.8 months), and the median overall survival was 5.6 months (95% CI: 3.0–16.7 months). Grade ≥3 treatment-related adverse events (TRAE) occurred in 8 patients (34.8%). No treatment-related death occurred, and no patients discontinued treatment due to TRAEs.

Conclusions

Fuzuloparib combined with SHR-1316 failed to improve the outcomes in unselected patients with relapsed SCLC. Future studies with biomarker analysis are warranted to select patients most likely to benefit from this combination treatment. Fuzuloparib 100 and 150 mg plus SHR-1316 were both tolerable with no new signals observed.

Olaparib and Doxorubicin Co-Loaded Polypeptide Nanogel for Enhanced Breast Cancer Therapy

Although great progress has been made in improving the efficacy of cancer treatment through combination treatment using drug agents, there are still challenges in improving the efficiency of drug delivery. In this study, olaparib and doxorubicin were co-loaded on disulfide bond cross-linked polypeptide nanogels for the treatment of breast cancer in mouse models. Under stimulation of a high glutathione environment in cancer cells, the drug is quickly released from the nanogel to target cancer cells. In addition, compared with free drugs and single-drug-loaded nanogels, dual-drug- co-loaded nanogels exhibit the best anti-cancer effect and demonstrated excellent biological safety. Therefore, the co-delivery of olaparib and doxorubicin through polypeptide nanogels presents good prospects for application as anti-cancer treatment.

PARP Inhibitors and Myeloid Neoplasms: A Double-Edged Sword

Simple Summary

Poly(ADP-ribose) polymerase (PARP) inhibitors, which are medications approved to treat various solid tumors, including breast, prostate, ovarian, and prostate cancers, are being examined in hematological malignancies. This review summarizes the potential role of PARP inhibitors in the treatment of myeloid diseases, particularly acute myeloid leukemia (AML). We review ongoing clinical studies investigating the safety and efficacy of PARP inhibitors in the treatment of AML, focusing on specific molecular and genetic AML subgroups that could be particularly sensitive to PARP inhibitor treatment. We also discuss reports describing an increased risk of treatment-related myeloid neoplasms in patients receiving PARP inhibitors for solid tumors.

Abstract

Despite recent discoveries and therapeutic advances in aggressive myeloid neoplasms, there remains a pressing need for improved therapies. For instance, in acute myeloid leukemia (AML), while most patients achieve a complete remission with conventional chemotherapy or the combination of a hypomethylating agent and venetoclax, de novo or acquired drug resistance often presents an insurmountable challenge, especially in older patients. Poly(ADP-ribose) polymerase (PARP) enzymes, PARP1 and PARP2, are involved in detecting DNA damage and repairing it through multiple pathways, including base excision repair, single-strand break repair, and double-strand break repair. In the context of AML, PARP inhibitors (PARPi) could potentially exploit the frequently dysfunctional DNA repair pathways that, similar to deficiencies in homologous recombination in BRCA-mutant disease, set the stage for cell killing. PARPi appear to be especially effective in AML with certain gene rearrangements and molecular characteristics (RUNX1-RUNX1T1 and PML-RARA fusions, FLT3- and IDH1-mutated). In addition, PARPi can enhance the efficacy of other agents, particularly alkylating agents, TOP1 poisons, and hypomethylating agents, that induce lesions ordinarily repaired via PARP1-dependent mechanisms. Conversely, emerging reports suggest that long-term treatment with PARPi for solid tumors is associated with an increased incidence of myelodysplastic syndrome (MDS) and AML. Here, we (i) review the pre-clinical and clinical data on the role of PARPi, specifically olaparib, talazoparib, and veliparib, in aggressive myeloid neoplasms and (ii) discuss the reported risk of MDS/AML with PARPi, especially as the indications for PARPi use expand to include patients with potentially curable cancer.

With Our Powers Combined: Exploring PARP Inhibitors and Immunotherapy

ABSTRACT

The use of poly(ADP-ribose) polymerase inhibitors and immune checkpoint inhibitor therapies has seen substantial clinical success in oncology therapeutic development. Although multiple agents within these classes have achieved regulatory approval globally-in several malignancies in early and advanced stages-drug resistance remains an issue. Building on preclinical evidence, several early trials and late-phase studies are underway. This review explores the therapeutic potential of combination poly(ADP-ribose) polymerase inhibitors and immune checkpoint inhibitor therapy in solid tumors, including the scientific and therapeutic rationale, available clinical evidence, and considerations for future trial and biomarker development across different malignancies using ovarian and other solid cancer subtypes as key examples.

Targeting DNA repair pathway in cancer: Mechanisms and clinical application

Over the last decades, the growing understanding on DNA damage response (DDR) pathways has broadened the therapeutic landscape in oncology. It is becoming increasingly clear that the genomic instability of cells resulted from deficient DNA damage response contributes to the occurrence of cancer. One the other hand, these defects could also be exploited as a therapeutic opportunity, which is preferentially more deleterious in tumor cells than in normal cells. An expanding repertoire of DDR-targeting agents has rapidly expanded to inhibitors of multiple members involved in DDR pathways, including PARP, ATM, ATR, CHK1, WEE1, and DNA-PK. In this review, we sought to summarize the complex network of DNA repair machinery in cancer cells and discuss the underlying mechanism for the application of DDR inhibitors in cancer. With the past preclinical evidence and ongoing clinical trials, we also provide an overview of the history and current landscape of DDR inhibitors in cancer treatment, with special focus on the combination of DDR-targeted therapies with other cancer treatment strategies.

Simulated microgravity triggers DNA damage and mitochondria-mediated apoptosis through ROS generation in human promyelocytic leukemic cells

The weightlessness or microgravity, a physical factor in space, may adversely affect the health of the space travellers or astronauts. The knowledge about the effect of microgravity on human cancer cells is very limited and poorly understood. Here, we employed rotary cell culture system (RCCS) to induce simulated microgravity (SMG) and examined its effects on human promyelocytic leukemic HL-60 cells. These cells were grown in normal gravity condition (1g) for control purpose. The 72 h exposure of cells to SMG decreased cell proliferation and viability which were accompanied by the reduced expression of PCNA and phosphorylated ERK1/2 and AKT proteins. SMG increased the DNA damage as well as the expression of DNA damage sensing proteins including ATM, ATR, Chk1, Chk2 and γH2A.X. The expression of AP1, XRCC1 and APEX1 regulating BER, XPC regulating NER and MLH1 and PMS2 regulating MMR were downregulated. However, SMG increased the expression of Ku70/80, DNA-PK and Rad51, regulating NHEJ and HR. SMG induced apoptosis and increased the levels of cleaved-poly-(ADP-ribose) polymerase and cleaved-caspase-3. An increase in Bax/Bcl-2 ratio and dissipation of mitochondrial membrane potential were also observed. SMG enhanced reactive oxygen species (ROS) formation which led to the enhanced DNA damage and apoptotic cell death. Overall, SMG induced ROS, DNA damage and differential expression of DNA repair genes, and altered the overall DNA repair capacity which may activate ATM/ATR-Chk1/2 and Ku70/80 and DNA-PK-mediated apoptotic cell death.

Niraparib Suppresses Cholangiocarcinoma Tumor Growth by Inducing Oxidative and Replication Stress

Simple Summary

Cholangiocarcinoma (CCA) is a rare and highly aggressive tumor with limited therapeutic options, thus underscoring the need to develop novel therapeutic approaches. We analyzed a publicly available CCA patient database to identify mutations in DNA damage response (DDR) genes. Mutations in DDR genes were prevalent, thus rendering these tumors potentially susceptible to poly-ADP-ribose polymerase (PARP) inhibition. PARP genes are critical to DNA repair and genomic stability. The role of PARP inhibitors in CCA was investigated by employing a series of in vitro functional assays and in vivo patient-derived xenograft models. This study highlights the therapeutic potential of PARP inhibitors alone or in combination with the chemotherapeutic agent gemcitabine for the treatment of CCA.

Abstract

Cholangiocarcinoma (CCA) is the second most common hepatobiliary cancer, an aggressive malignancy with limited therapeutic options. PARP (poly (ADP-ribose) polymerase) 1 and 2 are important for deoxyribonucleotide acid (DNA) repair and maintenance of genomic stability. PARP inhibitors (PARPi) such as niraparib have been approved for different malignancies with genomic alteration in germline BRCA and DNA damage response (DDR) pathway genes. Genomic alterations were analyzed in DDR genes in CCA samples employing The Cancer Genome Atlas (TCGA) database. Mutations were observed in various DDR genes, and 35.8% cases had alterations in at least one of three genes (ARID1A, BAP1 and ATM), suggesting their susceptibility to PARPi. Niraparib treatment suppressed cancer cell viability and survival, and also caused G2/M cell cycle arrest in patient-derived xenograft cells lines (PDXC) and established CCA cells harboring DDR gene mutations. PARPi treatment also induced apoptosis and caspase3/7 activity in PDXC and CCA cell lines, and substantially reduced expression of BCL2, BCL-XL and MCL1 proteins. Niraparib caused a significant increase in oxidative stress, and induced activation of DNA damage markers, phosphorylation of CHK2 and replication fork stalling. Importantly, niraparib, in combination with gemcitabine, produced sustained and robust inhibition of tumor growth in vivo in a patient-derived xenograft (PDX) model more effectively than either treatment alone. Furthermore, tissue samples from mice treated with niraparib and gemcitabine display significantly lower expression levels of pHH3 and Ki-67, which are a mitotic and proliferative marker, respectively. Taken together, our results indicate niraparib as a novel therapeutic agent alone or in combination with gemcitabine for CCA.

The trans cell cycle effects of PARP inhibitors underlie their selectivity toward BRCA1/2-deficient cells

In this study, Simoneau et al. investigated why PARPi is more effective than other DNA-damaging drugs when used to treat BRCA1/2-deficient tumors. They show that PARPi induces DSBs progressively through trans-cell-cycle ssDNA gaps, and BRCA1/2-deficient cells fail to slow down and repair DSBs over multiple cell cycles, explaining the unique efficacy of PARPi in BRCA1/2-deficient cells.

PARP inhibitor (PARPi) is widely used to treat BRCA1/2-deficient tumors, but why PARPi is more effective than other DNA-damaging drugs is unclear. Here, we show that PARPi generates DNA double-strand breaks (DSBs) predominantly in a trans cell cycle manner. During the first S phase after PARPi exposure, PARPi induces single-stranded DNA (ssDNA) gaps behind DNA replication forks. By trapping PARP on DNA, PARPi prevents the completion of gap repair until the next S phase, leading to collisions of replication forks with ssDNA gaps and a surge of DSBs. In the second S phase, BRCA1/2-deficient cells are unable to suppress origin firing through ATR, resulting in continuous DNA synthesis and more DSBs. Furthermore, BRCA1/2-deficient cells cannot recruit RAD51 to repair collapsed forks. Thus, PARPi induces DSBs progressively through trans cell cycle ssDNA gaps, and BRCA1/2-deficient cells fail to slow down and repair DSBs over multiple cell cycles, explaining the unique efficacy of PARPi in BRCA1/2-deficient cells.

XRCC1 prevents toxic PARP1 trapping during DNA base excision repair

Mammalian DNA base excision repair (BER) is accelerated by poly(ADP-ribose) polymerases (PARPs) and the scaffold protein XRCC1. PARPs are sensors that detect single-strand break intermediates, but the critical role of XRCC1 during BER is unknown. Here, we show that protein complexes containing DNA polymerase β and DNA ligase III that are assembled by XRCC1 prevent excessive engagement and activity of PARP1 during BER. As a result, PARP1 becomes "trapped" on BER intermediates in XRCC1-deficient cells in a manner similar to that induced by PARP inhibitors, including in patient fibroblasts from XRCC1-mutated disease. This excessive PARP1 engagement and trapping renders BER intermediates inaccessible to enzymes such as DNA polymerase β and impedes their repair. Consequently, PARP1 deletion rescues BER and resistance to base damage in XRCC1^-/- cells. These data reveal excessive PARP1 engagement during BER as a threat to genome integrity and identify XRCC1 as an "anti-trapper" that prevents toxic PARP1 activity.

BRCA Genetic Test and Risk-Reducing Salpingo-Oophorectomy for Hereditary Breast and Ovarian Cancer: State-of-the-Art

In the field of gynecology, the approval of the PARP inhibitors (PARPi) has been changing the treatment of ovarian cancer patients. The BRCA genetic test and the HRD test are being used as a companion diagnosis before starting PARPi treatment. BRACAnalysis CDx^® and Myriad myChoice^® HRD test are widely used as a BRCA genetic test and HRD test, respectively. In addition, FoundationOne^®CDx is sometimes used as a tumor BRCA test and HRD test. In clinical practice, gynecologists treating ovarian cancer are faced with making decisions such as whether to recommend the gBRCA test to all ovarian cancer patients, whether to perform the gBRCA test first or HRD test first, and so on. Regarding the judgment result of the HRD test, the cutoff value differs depending on the clinical trial, and the prevalence of gBRCA pathogenic variant rate is different in each histological type and country. A prospective cohort study showed that RRSO reduced all-cause mortality in both pre- and postmenopausal women; however, RRSO significantly reduced the risk of breast cancer for BRCA2 pathogenic variant carriers, but not for BRCA1 pathogenic variant carriers. Moreover, salpingectomy alone is said to not decrease the risk of developing ovarian or breast cancer, so further discussion is evidently required. We discuss the current situation and problems in doing BRCA genetic test and RRSO in this review article.

In silico screening identifies a novel small molecule inhibitor that counteracts PARP inhibitor resistance in ovarian cancer

Poly ADP-ribose polymerase (PARP) inhibitors are promising targeted therapy for epithelial ovarian cancer (EOC) with BRCA mutations or defective homologous recombination (HR) repair. However, reversion of BRCA mutation and restoration of HR repair in EOC lead to PARP inhibitor resistance and reduced clinical efficacy of PARP inhibitors. We have previously shown that triapine, a small molecule inhibitor of ribonucleotide reductase (RNR), impaired HR repair and sensitized HR repair-proficient EOC to PARP inhibitors. In this study, we performed in silico screening of small molecule libraries to identify novel compounds that bind to the triapine-binding pocket on the R2 subunit of RNR and inhibit RNR in EOC cells. Following experimental validation of selected top-ranking in silico hits for inhibition of dNTP and DNA synthesis, we identified, DB4, a putative RNR pocket-binding inhibitor markedly abrogated HR repair and sensitized BRCA-wild-type EOC cells to the PARP inhibitor olaparib. Furthermore, we demonstrated that the combination of DB4 and olaparib deterred the progression of BRCA-wild type EOC xenografts and significantly prolonged the survival time of tumor-bearing mice. Herein we report the discovery of a putative small molecule inhibitor of RNR and HR repair for combination with PARP inhibitors to treat PARP inhibitor-resistant and HR repair-proficient EOC.

Tumor suppressive function of Matrin 3 in the basal-like breast cancer

BACKGROUND

Basal-like breast cancer (BLBC) or triple-negative breast cancer (TNBC) is an aggressive and highly metastatic subtype of human breast cancer. The present study aimed to elucidate the potential tumor-suppressive function of MATR3, an abundant nuclear protein, in BLBC/TNBC, whose cancer-relevance has not been characterized.

METHODS

We analyzed in vitro tumorigenecity by cell proliferation and soft agar colony formation assays, apoptotic cell death by flow cytometry and Poly (ADP-ribose) polymerase (PARP) cleavage, epithelial-mesenchymal transition (EMT) by checking specific EMT markers with real-time quantitative PCR and in vitro migration and invasion by Boyden Chamber assays. To elucidate the underlying mechanism by which MATR3 functions as a tumor suppressor, we performed Tandem affinity purification followed by mass spectrometry (TAP-MS) and pathway analysis. We also scrutinized MATR3 expression levels in the different subtypes of human breast cancer and the correlation between MATR3 expression and patient survival by bioinformatic analyses of publicly available transcriptome datasets.

RESULTS

MATR3 suppressed in vitro tumorigenecity, promoted apoptotic cell death and inhibited EMT, migration, and invasion in BLBC/TNBC cells. Various proteins regulating apoptosis were identified as MATR3-binding proteins, and YAP/TAZ pathway was suppressed by MATR3. MATR3 expression was inversely correlated with the aggressive and metastatic nature of breast cancer. Moreover, high expression levels of MATR3 were associated with a good prognosis of breast cancer patients.

CONCLUSIONS

Our data demonstrate that MATR3 functions as a putative tumor suppressor in BLBC/TNBC cells. Also, MATR3 potentially plays a role as a biomarker in predicting chemotherapy-sensitivity and patient survival in breast cancer patients.

PARP Inhibitors: Clinical Relevance, Mechanisms of Action and Tumor Resistance

The Poly (ADP-ribose) polymerase (PARP) family has many essential functions in cellular processes, including the regulation of transcription, apoptosis and the DNA damage response. PARP1 possesses Poly (ADP-ribose) activity and when activated by DNA damage, adds branched PAR chains to facilitate the recruitment of other repair proteins to promote the repair of DNA single-strand breaks. PARP inhibitors (PARPi) were the first approved cancer drugs that specifically targeted the DNA damage response in BRCA1/2 mutated breast and ovarian cancers. Since then, there has been significant advances in our understanding of the mechanisms behind sensitization of tumors to PARP inhibitors and expansion of the use of PARPi to treat several other cancer types. Here, we review the recent advances in the proposed mechanisms of action of PARPi, biomarkers of the tumor response to PARPi, clinical advances in PARPi therapy, including the potential of combination therapies and mechanisms of tumor resistance.

PARP Inhibitor Resistance Mechanisms and Implications for Post-Progression Combination Therapies

The use of PARP inhibitors (PARPi) is growing widely as FDA approvals have shifted its use from the recurrence setting to the frontline setting. In parallel, the population developing PARPi resistance is increasing. Here we review the role of PARP, DNA damage repair, and synthetic lethality. We discuss mechanisms of resistance to PARP inhibition and how this informs on novel combinations to re-sensitize cancer cells to PARPi.

PARP inhibition and immune modulation: scientific rationale and perspectives for the treatment of gynecologic cancers

Poly[adenosine diphosphate (ADP) ribose]polymerase (PARP) has multifaceted roles in the maintenance of genomic integrity, deoxyribonucleic acid (DNA) repair and replication, and the maintenance of immune-system homeostasis. PARP inhibitors are an attractive oncologic therapy, causing direct cancer cell cytotoxicity by propagating DNA damage and indirectly, by various mechanisms of immunostimulation, including activation of the cGAS/STING pathway, paracrine stimulation of dendritic cells, increased T-cell infiltration, and upregulation of death-ligand receptors to increase susceptibility to natural-killer-cell killing. However, these immunostimulatory effects are counterbalanced by PARPi-mediated upregulation of programmed cell-death-ligand 1 (PD-L1), which leads to immunosuppression. Combining PARP inhibition with immune-checkpoint blockade seeks to exploit the immune stimulatory effects of PARP inhibition while negating the immunosuppressive effects of PD-L1 upregulation.

Evaluation of the Efficacy and Safety of PARP Inhibitors in Advanced-Stage Epithelial Ovarian Cancer

Purpose: PARP inhibitors are a novel targeted anti-cancer drug and a large number of clinical studies on PARP inhibitors have been accomplished. This updated meta-analysis was conducted to evaluate the efficacy and safety of PARP inhibitors in advanced-stage epithelial ovarian cancer.

Methods: Medline (PubMed), Embase, Cochrane Central Register of Controlled Trials, Web of Science, and Scopus were searched to identify the eligible trials up to April 2020. ClinicalTrials.gov was also screened for additional unpublished trials. Data extraction and risk of bias assessment were performed by two independent investigators, respectively. The hazard ratios (HRs) and its 95% confidence intervals (CI) for time-to-event data of progression-free survival (PFS) and overall survival (OS), and the risk ratios (RRs) with 95% CI for dichotomous data of overall response rate (ORR) and occurrence of adverse events (AEs) were calculated by Review Manager 5.3 and Stata 12.0 software.

Results: A total of 12 trials with 5,347 patients were included in this meta-analysis. Compared with the control group, PARP inhibitors significantly improved PFS (HR, 0.51; 95% CI, 0.40–0.65; P < 0.00001) and ORR (RR, 1.26; 95% CI, 1.11–1.43; P = 0.0003). Specifically, PFS was improved regardless of BRCA genes mutations and homologous-recombination status. However, no difference was observed in OS between the PARP inhibitors group and the control group (95% CI, 0.73–1.01; P = 0.06). PARP inhibitors were associated with a statistically significant higher risk of hematologic events and different PARP inhibitors had different toxicities profiles.

Conclusion: PARP inhibitors are an effective and well-tolerated treatment for patients with advanced-stage epithelial ovarian cancer.

MacroH2A1 Regulation of Poly(ADP-Ribose) Synthesis and Stability Prevents Necrosis and Promotes DNA Repair

Through its ability to bind the ends of poly(ADP-ribose) (PAR) chains, the function of the histone variant macroH2A1.1, including its ability to regulate transcription, is coupled to PAR polymerases (PARPs). PARP1 also has a major role in DNA damage response (DDR) signaling, and our results show that macroH2A1 alters the kinetics of PAR accumulation following acute DNA damage by both suppressing PARP activity and simultaneously protecting PAR chains from degradation.

Through its ability to bind the ends of poly(ADP-ribose) (PAR) chains, the function of the histone variant macroH2A1.1, including its ability to regulate transcription, is coupled to PAR polymerases (PARPs). PARP1 also has a major role in DNA damage response (DDR) signaling, and our results show that macroH2A1 alters the kinetics of PAR accumulation following acute DNA damage by both suppressing PARP activity and simultaneously protecting PAR chains from degradation. In this way, we demonstrate that macroH2A1 prevents cellular NAD⁺ depletion, subsequently preventing necrotic cell death that would otherwise occur due to PARP overactivation. We also show that macroH2A1-dependent PAR stabilization promotes efficient repair of oxidative DNA damage. While the role of PAR in recruiting and regulating macrodomain-containing proteins has been established, our results demonstrate that, conversely, macrodomain-containing proteins, and specifically those containing macroH2A1, can regulate PARP1 function through a novel mechanism that promotes both survival and efficient repair during DNA damage response.

Poly (ADP-Ribose) Polymerases (PARPs) and PARP Inhibitor-Targeted Therapeutics

BACKGROUND

Poly-ADP-ribosylation, that is, adding ADP-ribose moieties to a protein, is a unique type of protein post-translational modification that regulates various cellular processes such as DNA repair, mitosis, transcription, and cell growth. Small-molecule inhibitors of poly-ADP-ribose polymerase 1 (PARP1) have been developed as anticancer agents because inhibition of PARP enzymes may be a synthetic lethal strategy for cancers with or BRCA2 mutations. However, there are still questions surrounding PARP inhibitors.

METHODS/RESULTS

Data were collected from Pubmed, Medline, through searching of these keywords: "PARP", "BRCA", "Synthetic lethal" and "Tankyrase inhibitors". We describe the current knowledge of PARP inhibition and its effects on DNA damage; mechanisms of resistance to PARP inhibitors; the evolution of PARP inhibitors; and the potential use of PARP5a/b (tankyrases) inhibitors in cancer treatment.

CONCLUSION

PARP inhibitors are already showing promise as therapeutic tools, especially in the management of BRCA-mutated breast and ovarian cancers but also in tumors with dysfunctional BRCA genes. Small-molecule tankyrase inhibitors are important for increasing our understanding of tankyrase biology.

DNA Base Excision Repair in Plants: An Unfolding Story With Familiar and Novel Characters

Base excision repair (BER) is a critical genome defense pathway that deals with a broad range of non-voluminous DNA lesions induced by endogenous or exogenous genotoxic agents. BER is a complex process initiated by the excision of the damaged base, proceeds through a sequence of reactions that generate various DNA intermediates, and culminates with restoration of the original DNA structure. BER has been extensively studied in microbial and animal systems, but knowledge in plants has lagged behind until recently. Results obtained so far indicate that plants share many BER factors with other organisms, but also possess some unique features and combinations. Plant BER plays an important role in preserving genome integrity through removal of damaged bases. However, it performs additional important functions, such as the replacement of the naturally modified base 5-methylcytosine with cytosine in a plant-specific pathway for active DNA demethylation.

DNA Damage by an essential enzyme: A delicate balance act on the tightrope

DNA topoisomerases are essential for DNA metabolic processes such as replication and transcription. Since DNA is double stranded, the unwinding needed for these processes results in DNA supercoiling and catenation of replicated molecules. Changing the topology of DNA molecules to relieve supercoiling or resolve catenanes requires that DNA be transiently cut. While topoisomerases carry out these processes in ways that minimize the likelihood of genome instability, there are several ways that topoisomerases may fail. Topoisomerases can be induced to fail by therapeutic small molecules such as by fluoroquinolones that target bacterial topoisomerases, or a variety of anti-cancer agents that target the eukaryotic enzymes. Increasingly, there have been a large number of agents and processes, including natural products and their metabolites, DNA damage, and the intrinsic properties of the enzymes that can lead to long-lasting DNA breaks that subsequently lead to genome instability, cancer, and other diseases. Understanding the processes that can interfere with topoisomerases and how cells respond when topoisomerases fail will be important in minimizing the consequences when enzymes need to transiently interfere with DNA integrity.

A Single-Molecule Atomic Force Microscopy Study of PARP1 and PARP2 Recognition of Base Excision Repair DNA Intermediates

Nuclear poly(ADP-ribose) polymerases 1 and 2 (PARP1 and PARP2) catalyze the synthesis of poly(ADP-ribose) (PAR) and use NAD⁺ as a substrate for the polymer synthesis. Both PARP1 and PARP2 are involved in DNA damage response pathways and function as sensors of DNA breaks, including temporary single-strand breaks formed during DNA repair. Consistently, with a role in DNA repair, PARP activation requires its binding to a damaged DNA site, which initiates PAR synthesis. Here we use atomic force microscopy to characterize at the single-molecule level the interaction of PARP1 and PARP2 with long DNA substrates containing a single damage site and representing intermediates of the short-patch base excision repair (BER) pathway. We demonstrated that PARP1 has higher affinity for early intermediates of BER than PARP2, whereas both PARPs efficiently interact with the nick and may contribute to regulation of the final ligation step. The binding of a DNA repair intermediate by PARPs involved a PARP monomer or dimer depending on the type of DNA damage. PARP dimerization influences the affinity of these proteins to DNA and affects their enzymatic activity: the dimeric form is more effective in PAR synthesis in the case of PARP2 but is less effective in the case of PARP1. PARP2 suppresses PAR synthesis catalyzed by PARP1 after single-strand breaks formation. Our study suggests that the functions of PARP1 and PARP2 overlap in BER after a site cleavage and provides evidence for a role of PARP2 in the regulation of PARP1 activity.

Studies Towards Hypoxia-Activated Prodrugs of PARP Inhibitors

Poly(ADP-ribose)polymerase (PARP) inhibitors (PARPi) have recently been approved for the treatment of breast and ovarian tumors with defects in homologous recombination repair (HRR). Although it has been demonstrated that PARPi also sensitize HRR competent tumors to cytotoxic chemotherapies or radiotherapy, normal cell toxicity has remained an obstacle to their use in this context. Hypoxia-activated prodrugs (HAPs) provide a means to limit exposure of normal cells to active drug, thus adding a layer of tumor selectivity. We have investigated potential HAPs of model PARPi in which we attach a bioreducible “trigger” to the amide nitrogen, thereby blocking key binding interactions. A representative example showed promise in abrogating PARPi enzymatic activity in a biochemical assay, with a ca. 160-fold higher potency of benzyl phthalazinone 4 than the corresponding model HAP 5, but these N-alkylated compounds did not release the PARPi upon one-electron reduction by radiolysis. Therefore, we extended our investigation to include NU1025, a PARPi that contains a phenol distal to the core binding motif. The resulting 2-nitroimidazolyl ether provided modest abrogation of PARPi activity with a ca. seven-fold decrease in potency, but released the PARPi efficiently upon reduction. This investigation of potential prodrug approaches for PARPi has identified a useful prodrug strategy for future exploration.

Recent advances in the molecular mechanism of sex disparity in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is more frequently observed and aggressive in men compared with women. Increasing evidence demonstrates that the sex disparity appears to be mediated by the stimulatory effects of androgens and the protective effects of estrogen in the development and progression of HCC. In the past few decades, studies on the sex difference of HCC mainly focused on the effect of sex hormones on the transactivation of hepatitis B virus X protein and the release of inflammatory cytokines, and these studies have further intensified in recent years. Sex hormones are also involved in genetic alterations and DNA damage repair in hepatocytes through binding to their specific cellular receptors and affecting the corresponding signaling pathways. Furthermore, the theory of sex chromosomes participating in HCC has been considered. The present review discussed the recent advances in the molecular mechanisms of sex disparity in HCC, with the aim of improving the understanding of the underlying critical factors and exploring more effective methods for the prevention and treatment of HCC.

PARP1 is required for preserving telomeric integrity but is dispensable for A-NHEJ

Poly-ADP ribose polymerase 1 (PARP1) is clinically important because of its synthetic lethality with breast cancer allele 1 and 2 mutations, which are causative for inherited breast and ovarian cancers. Biochemically, PARP1 is a single-stranded DNA break repair protein that is needed for preserving genomic integrity. In addition, PARP1 has been implicated in a veritable plethora of additional cellular pathways and thus its precise contribution(s) to human biology has remained obscure. To help address this deficiency, we utilized gene editing to construct genetically-null PARP1 human cancer cells. We found a minor role for PARP1 in an alternative form of DNA double-strand break (DSB) repair, but only when these cells were deficient for the classical form of DSB repair. Despite being proficient for DSB repair, however, cell cycle progression defects and elevated endogenous DNA damage signaling were observed. These deficiencies were instead linked to telomere defects, where PARP1 -/- cells had short telomeres that co-localized with markers of endogenous DNA damage and were compromised in their ability to escape a telomere-driven crisis. Our data suggest that while PARP1 does not participate significantly in DNA DSB repair itself, it does prevent the incidence of telomeric DSBs, which, in turn, can drive genomic instability.

Metabolic Enhancer Piracetam Attenuates the Translocation of Mitochondrion-Specific Proteins of Caspase-Independent Pathway, Poly [ADP-Ribose] Polymerase 1 Up-regulation and Oxidative DNA Fragmentation

Piracetam, a nootropic drug, has been clinically used for decades; however, its mechanism of action still remains enigmatic. The present study was undertaken to evaluate the role of mitochondrion-specific factors of caspase-independent pathway like apoptotic-inducing factor (AIF) and endonuclease-G (endo-G) in piracetam-induced neuroprotection. N2A cells treated with lipopolysaccharide (LPS) exhibited significant cytotoxicity, impaired mitochondrial activity, and reactive oxygen species generation which was significantly attenuated with piracetam co-treatment. Cells co-treated with LPS and piracetam exhibited significant uptake of piracetam in comparison to only piracetam-treated cells as estimated by liquid chromatography-mass spectrometry (LC-MSMS). LPS treatment caused significant translocation of AIF and endonuclease-G in neuronal N2A cells which were significantly attenuated with piracetam co-treatment. Significant over-expression of proinflammatory cytokines was also observed after treatment of LPS to cells which was inhibited with piracetam co-treatment demonstrating its anti-inflammatory property. LPS-treated cells exhibited significant oxidative DNA fragmentation and poly [ADP-ribose] polymerase-1 (PARP-1) up-regulation in nucleus, both of which were attenuated with piracetam treatment. Antioxidant melatonin but not z-VAD offered the inhibited LPS-induced DNA fragmentation indicating the involvement of oxidative DNA fragmentation. Further, we did not observe the altered caspase-3 level after LPS treatment initially while at a later time point, significantly augmented level of caspase-3 was observed which was not inhibited with piracetam treatment. In total, our findings indicate the interference of piracetam in mitochondrion-mediated caspase-independent pathway, as well as its anti-inflammatory and antioxidative properties. Graphical Abstract Graphical abstract indicating the novel interference of metabolic enhancer piracetam (P) in neuronal death mechanisms.

Role of novel histone modifications in cancer

Oncogenesis is a multistep process mediated by a variety of factors including epigenetic modifications. Global epigenetic post-translational modifications have been detected in almost all cancers types. Epigenetic changes appear briefly and do not involve permanent changes to the primary DNA sequence. These epigenetic modifications occur in key oncogenes, tumor suppressor genes, and transcription factors, leading to cancer initiation and progression. The most commonly observed epigenetic changes include DNA methylation, histone lysine methylation and demethylation, histone lysine acetylation and deacetylation. However, there are several other novel post-translational modifications that have been observed in recent times such as neddylation, sumoylation, glycosylation, phosphorylation, poly-ADP ribosylation, ubiquitination as well as transcriptional regulation and these have been briefly discussed in this article. We have also highlighted the diverse epigenetic changes that occur during the process of tumorigenesis and described the role of histone modifications that can occur on tumor suppressor genes as well as oncogenes, which regulate tumorigenesis and can thus form the basis of novel strategies for cancer therapy.

Precision Immuno-Oncology: Prospects of Individualized Immunotherapy for Pancreatic Cancer

Pancreatic cancer, most commonly referring to pancreatic ductal adenocarcinoma (PDAC), remains one of the most deadly diseases, with very few effective therapies available. Emerging as a new modality of modern cancer treatments, immunotherapy has shown promises for various cancer types. Over the past decades, the potential of immunotherapy in eliciting clinical benefits in pancreatic cancer have also been extensively explored. It has been demonstrated in preclinical studies and early phase clinical trials that cancer vaccines were effective in eliciting anti-tumor immune response, but few have led to a significant improvement in survival. Despite the fact that immunotherapy with checkpoint blockade (e.g., anti-cytotoxic T-lymphocyte antigen 4 [CTLA-4] and anti-programmed cell death 1 [PD-1]/PD-L1 antibodies) has shown remarkable and durable responses in various cancer types, the application of checkpoint inhibitors in pancreatic cancer has been disappointing so far. It may, in part, due to the unique tumor microenvironment (TME) of pancreatic cancer, such as existence of excessive stromal matrix and hypovascularity, creating a TME of strong inhibitory signaling circuits and tremendous physical barriers for immune agent infiltration. This informs on the need for combination therapy approaches to engender a potent immune response that can translate to clinical benefits. On the other hand, lack of effective and validated biomarkers to stratify subgroup of patients who can benefit from immunotherapy poses further challenges for the realization of precision immune-oncology. Future studies addressing issues such as TME modulation, biomarker identification and therapeutic combination are warranted. In this review, advances in immunotherapy for pancreatic cancer were discussed and opportunities as well as challenges for personalized immune-oncology were addressed.

PARP1 changes from three-dimensional DNA damage searching to one-dimensional diffusion after auto-PARylation or in the presence of APE1

PARP1-dependent poly-ADP-ribosylation (PARylation) participates in the repair of many forms of DNA damage. Here, we used atomic force microscopy (AFM) and single molecule fluorescence microscopy to examine the interactions of PARP1 with common DNA repair intermediates. AFM volume analysis indicates that PARP1 binds to DNA at nicks, abasic (AP) sites, and ends as a monomer. Single molecule DNA tightrope assays were used to follow the real-time dynamic behavior of PARP1 in the absence and presence of AP endonuclease (APE1) on AP DNA damage arrays. These experiments revealed that PARP1 conducted damage search mostly through 3D diffusion. Co-localization of APE1 with PARP1 on DNA was found capable of inducing 1D diffusion of otherwise nonmotile PARP1, while excess APE1 also facilitated the dissociation of DNA-bound PARP1. Moreover, auto-PARylation of PARP1 allowed the protein to switch its damage search strategy by causing a 3-fold increase in linear diffusion. Finally, we demonstrated that PARP inhibitor olaparib did not significantly alter the rate of PARP1 dissociation from DNA, but instead resulted in more motility of DNA-bound PARP1 molecules.

Therapeutic options for leukemic transformation in patients with myeloproliferative neoplasms

Approximately 5-10% of patients with Philadelphia chromosome negative myeloproliferative neoplasms (MPN) comprising of essential thrombocythemia, polycythemia vera and primary myelofibrosis) experience transformation to acute myeloid leukemia (AML, ≥20% blasts). Treatment options for post-MPN AML patients are limited, as conventional approaches like standard chemotherapy, fail to offer long-term benefit. Median survival for secondary AML is ∼2.4 months. Post-MPN AML therefore represents an area of urgent clinical need. At present, allogeneic stem cell transplant (ASCT) following induction therapy is the best therapeutic option. Patients ineligible for ASCT are treated with hypomethylating agents. New agents under investigation include histone deacetylase inhibitors, JAKinhibitors and agents targeting the BRD4 protein. Combined treatment strategies involving these novel agents are being tested. In this review we present the current evidence regarding treatment options for post-MPN AML patients.

Alkylation induced cerebellar degeneration dependent on Aag and Parp1 does not occur via previously established cell death mechanisms

Alkylating agents are ubiquitous in our internal and external environments, causing DNA damage that contributes to mutations and cell death that can result in aging, tissue degeneration and cancer. Repair of methylated DNA bases occurs primarily through the base excision repair (BER) pathway, a multi-enzyme pathway initiated by the alkyladenine DNA glycosylase (Aag, also known as Mpg). Previous work demonstrated that mice treated with the alkylating agent methyl methanesulfonate (MMS) undergo cerebellar degeneration in an Aag-dependent manner, whereby increased BER initiation by Aag causes increased tissue damage that is dependent on activation of poly (ADP-ribose) polymerase 1 (Parp1). Here, we dissect the molecular mechanism of cerebellar granule neuron (CGN) sensitivity to MMS using primary ex vivo neuronal cultures. We first established a high-throughput fluorescent imaging method to assess primary neuron sensitivity to treatment with DNA damaging agents. Next, we verified that the alkylation sensitivity of CGNs is an intrinsic phenotype that accurately recapitulates the in vivo dependency of alkylation-induced CGN cell death on Aag and Parp1 activity. Finally, we show that MMS-induced CGN toxicity is independent of all the cellular events that have previously been associated with Parp-mediated toxicity, including mitochondrial depolarization, AIF translocation, calcium fluxes, and NAD+ consumption. We therefore believe that further investigation is needed to adequately describe all varieties of Parp-mediated cell death.

Niraparib in ovarian cancer: results to date and clinical potential

Ovarian cancer is the first cause of death from gynaecological malignancy. Germline mutation in BRCA1 and 2, two genes involved in the mechanisms of reparation of DNA damage, are showed to be related with the incidence of breast and ovarian cancer, both sporadic and familiar. PARP is a family of enzymes involved in the base excision repair (BER) system. The introduction of inhibitors of PARP in patients with BRCA-mutated ovarian cancer is correlated with the concept of synthetic lethality. Among the PARP inhibitors introduced in clinical practice, niraparib showed interesting results in a phase III trial in the setting of maintenance treatment in ovarian cancer, after platinum-based chemotherapy. Interestingly, was niraparib showed to be efficacious not only in BRCA-mutated patients, but also in patients with other alterations of the homologous recombination (HR) system and in patients with unknown alterations. These results position niraparib as the first PARP-inhibitor with clinically and statistically significant results also in patients with no alterations in BRCA 1/2 and other genes involved in the DNA repair system. Even if the results are potentially practice-changing, the action of niraparib must be further studied and deepened.

The multifaceted roles of PARP1 in DNA repair and chromatin remodelling

Cells are exposed to various endogenous and exogenous insults that induce DNA damage, which, if unrepaired, impairs genome integrity and leads to the development of various diseases, including cancer. Recent evidence has implicated poly(ADP-ribose) polymerase 1 (PARP1) in various DNA repair pathways and in the maintenance of genomic stability. The inhibition of PARP1 is therefore being exploited clinically for the treatment of various cancers, which include DNA repair-deficient ovarian, breast and prostate cancers. Understanding the role of PARP1 in maintaining genome integrity is not only important for the design of novel chemotherapeutic agents, but is also crucial for gaining insights into the mechanisms of chemoresistance in cancer cells. In this Review, we discuss the roles of PARP1 in mediating various aspects of DNA metabolism, such as single-strand break repair, nucleotide excision repair, double-strand break repair and the stabilization of replication forks, and in modulating chromatin structure.

Ovarian Cancers Harbor Defects in Nonhomologous End Joining Resulting in Resistance to Rucaparib

Purpose: DNA damage defects are common in ovarian cancer and can be used to stratify treatment. Although most work has focused on homologous recombination (HR), DNA double-strand breaks are repaired primarily by nonhomologous end joining (NHEJ). Defects in NHEJ have been shown to contribute to genomic instability and have been associated with the development of chemoresistance.Experimental Design: NHEJ was assessed in a panel of ovarian cancer cell lines and 47 primary ascetic-derived ovarian cancer cultures, by measuring the ability of cell extracts to end-join linearized plasmid monomers into multimers. mRNA and protein expression of components of NHEJ was determined using RT-qPCR and Western blotting. Cytotoxicities of cisplatin and the PARP inhibitor rucaparib were assessed using sulforhodamine B (SRB) assays. HR function was assessed using γH2AX/RAD51 foci assay.Results: NHEJ was defective (D) in four of six cell lines and 20 of 47 primary cultures. NHEJ function was independent of HR competence (C). NHEJD cultures were resistant to rucaparib (P = 0.0022). When HR and NHEJ functions were taken into account, only NHEJC/HRD cultures were sensitive to rucaparib (compared with NHEJC/HRC P = 0.034, NHEJD/HRC P = 0.0002, and NHEJD/HRD P = 0.0045). The DNA-PK inhibitor, NU7441, induced resistance to rucaparib (P = 0.014) and HR function recovery in a BRCA1-defective cell line.Conclusions: This study has shown that NHEJ is defective in 40% of ovarian cancers, which is independent of HR function and associated with resistance to PARP inhibitors in ex vivo primary cultures. Clin Cancer Res; 23(8); 2050-60. ©2016 AACR.

A new sub-pathway of long-patch base excision repair involving 5' gap formation

Base excision repair (BER) is one of the most frequently used cellular DNA repair mechanisms and modulates many human pathophysiological conditions related to DNA damage. Through live cell and in vitro reconstitution experiments, we have discovered a major sub-pathway of conventional long-patch BER that involves formation of a 9-nucleotide gap 5' to the lesion. This new sub-pathway is mediated by RECQ1 DNA helicase and ERCC1-XPF endonuclease in cooperation with PARP1 poly(ADP-ribose) polymerase and RPA The novel gap formation step is employed during repair of a variety of DNA lesions, including oxidative and alkylation damage. Moreover, RECQ1 regulates PARP1 auto-(ADP-ribosyl)ation and the choice between long-patch and single-nucleotide BER, thereby modulating cellular sensitivity to DNA damage. Based on these results, we propose a revised model of long-patch BER and a new key regulation point for pathway choice in BER.

Targeting Plk1 to Enhance Efficacy of Olaparib in Castration-Resistant Prostate Cancer

Olaparib is an FDA-approved PARP inhibitor (PARPi) that has shown promise as a synthetic lethal treatment approach for BRCA-mutant castration-resistant prostate cancer (CRPC) in clinical use. However, emerging data have also shown that even BRCA-mutant cells may be resistant to PARPi. The mechanistic basis for these drug resistances is poorly understood. Polo-like kinase 1 (Plk1), a critical regulator of many cell-cycle events, is significantly elevated upon castration of mice carrying xenograft prostate tumors. Herein, by combination with Plk1 inhibitor BI2536, we show a robust sensitization of olaparib in 22RV1, a BRCA1-deficient CRPC cell line, as well as in CRPC xenograft tumors. Mechanistically, monotherapy with olaparib results in an override of the G1-S checkpoint, leading to high expression of Plk1, which attenuates olaparib's overall efficacy. In BRCA1 wild-type C4-2 cells, Plk1 inhibition also significantly increases the efficacy of olaparib in the presence of p53 inhibitor. Collectively, our findings not only implicate the critical role of Plk1 in PARPi resistance in BRCA-mutant CRPC cells, but also shed new light on the treatment of non-BRCA-mutant patient subgroups who might also respond favorably to PARPi. Mol Cancer Ther; 16(3); 469-79. ©2017 AACR.

Phase I, Dose-Escalation, Two-Part Trial of the PARP Inhibitor Talazoparib in Patients with Advanced Germline BRCA1/2 Mutations and Selected Sporadic Cancers

Talazoparib inhibits PARP catalytic activity, trapping PARP1 on damaged DNA and causing cell death in BRCA1/2-mutated cells. We evaluated talazoparib therapy in this two-part, phase I, first-in-human trial. Antitumor activity, MTD, pharmacokinetics, and pharmacodynamics of once-daily talazoparib were determined in an open-label, multicenter, dose-escalation study (NCT01286987). The MTD was 1.0 mg/day, with an elimination half-life of 50 hours. Treatment-related adverse events included fatigue (26/71 patients; 37%) and anemia (25/71 patients; 35%). Grade 3 to 4 adverse events included anemia (17/71 patients; 24%) and thrombocytopenia (13/71 patients; 18%). Sustained PARP inhibition was observed at doses ≥0.60 mg/day. At 1.0 mg/day, confirmed responses were observed in 7 of 14 (50%) and 5 of 12 (42%) patients with BRCA mutation-associated breast and ovarian cancers, respectively, and in patients with pancreatic and small cell lung cancer. Talazoparib demonstrated single-agent antitumor activity and was well tolerated in patients at the recommended dose of 1.0 mg/day.Significance: In this clinical trial, we show that talazoparib has single-agent antitumor activity and a tolerable safety profile. At its recommended phase II dose of 1.0 mg/day, confirmed responses were observed in patients with BRCA mutation-associated breast and ovarian cancers and in patients with pancreatic and small cell lung cancer. Cancer Discov; 7(6); 620-9. ©2017 AACR.This article is highlighted in the In This Issue feature, p. 539.

Poly(ADP-ribose) polymerase activity and inhibition in cancer

Genomic instability resultant from defective DNA repair mechanisms is a fundamental hallmark of cancer. The poly(ADP-ribose) polymerase (PARP) proteins 1, 2 and 3 catalyze the polymerization of poly(ADP-ribose) and covalent attachment to proteins in a phylogenetically ancient form of protein modification. PARPs play a role in base excision repair, homologous recombination, and non-homologous end joining. The discovery that loss of PARP activity had cytotoxic effects in cells deficient in homologous recombination has sparked a decade of translational research efforts that culminated in the FDA approval of an oral PARP inhibitor for clinical use in patients with ovarian cancer and defective homologous recombination. Five PARP inhibitors are now in late-stage development in clinical trials that are seeking to expand the understanding of targeted therapies and DNA repair defects in human cancer. This review examines the cell biology of PARP, the discovery of synthetic lethality with HR deficiency, the clinical development of PARP inhibitors, and the role of PARP inhibitors in ongoing clinical trials and clinical practice.