Repair Of DNA Single-strand Breaks Research Articles

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

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

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

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

Effects of the PARP Inhibitor Niraparib on the Radiosensitivity of Human Lung and Cervical Cancer Cells

DNA damage repair (DDR) initiated after DNA damage may lead to a decreased radiosensitivity of tumor cells or even radiation resistance. Previous studies indicate that PARP inhibitors block the repair of DNA single-strand breaks (SSBs), and unrepaired SSBs may continuously accumulate and form double-strand breaks (DSBs) that promotes tumor cell death. Studies have also shown that PARP inhibitors act synergistically with RT to inhibit DDR. In this study, we aim to investigate the effect of Niraparib, a clinically approved PARP inhibitor on the radiosensitivity of human lung and cervical cancer cells, and preliminarily explore the underlying mechanism. The human lung cancer cell line A549 and human cervical cancer cell line Siha were both treated with niraparib (for 1h), or X-ray (4Gy), or Niraparib (for 1h) combined with X-ray (4Gy). The effect was examined via measurements of cell proliferation by the cell counting kit-8 assay, and cell viability was detected by clone formation assays. Cell apoptosis and cell cycle distribution were investigated by flow cytometry. In human lung and cervical cancer cell lines, Niraparib combined with radiation therapy significantly inhibited cell proliferation. The proportion of apoptotic cells in cell lines treated with Niraparib plus radiation (the combination group) was significantly higher compared with control, radiation-only, and Niraparib-only groups. Additionally, the proportion of A549 cells in the G2/M phase was significantly increased in the combination group compared with the radiation-only group. The results support that PARP inhibitor Niraparib increases the radiosensitivity of tumor cells, promotes their apoptosis, and induces cell cycle redistribution. The possible mechanism is associated with the inhibition of radiation-induced DNA damage repair.

Double trouble for prostate cancer: synergistic action of AR blockade and PARPi in non-HRR mutated patients.

Prostate cancer (PCa) is the most common cancer in men worldwide. Despite better and more intensive treatment options in earlier disease stages, a large subset of patients still progress to metastatic castration-resistant PCa (mCRPC). Recently, poly-(ADP-ribose)-polymerase (PARP)-inhibitors have been introduced in this setting. The TALAPRO-2 and PROpel trials both showed a marked benefit of PARPi in combination with an androgen receptor signaling inhibitor (ARSI), compared with an ARSI alone in both the homologous recombination repair (HRR)-mutated, as well as in the HRR-non-mutated subgroup. In this review, we present a comprehensive overview of how maximal AR-blockade via an ARSI in combination with a PARPi has a synergistic effect at the molecular level, leading to synthetic lethality in both HRR-mutated and HRR-non-mutated PCa patients. PARP2 is known to be a cofactor of the AR complex, needed for decompacting the chromatin and start of transcription of AR target genes (including HRR genes). The inhibition of PARP thus reinforces the effect of an ARSI. The deep androgen deprivation caused by combining androgen deprivation therapy (ADT) with an ARSI, induces an HRR-like deficient state, often referred to as "BRCA-ness". Further, PARPi will prevent the repair of single-strand DNA breaks, leading to the accumulation of DNA double-strand breaks (DSBs). Due to the induced HRR-deficient state, DSBs cannot be repaired, leading to apoptosis.

Parp Inhibitors and Radiotherapy: A New Combination for Prostate Cancer (Systematic Review).

PARPi, in combination with ionizing radiation, has demonstrated the ability to enhance cellular radiosensitivity in different tumors. The rationale is that the exposure to radiation leads to both physical and biochemical damage to DNA, prompting cells to initiate three primary mechanisms for DNA repair. Two double-stranded DNA breaks (DSB) repair pathways: (1) non-homologous end-joining (NHEJ) and (2) homologous recombination (HR); and (3) a single-stranded DNA break (SSB) repair pathway (base excision repair, BER). In this scenario, PARPi can serve as radiosensitizers by leveraging the BER pathway. This mechanism heightens the likelihood of replication forks collapsing, consequently leading to the formation of persistent DSBs. Together, the combination of PARPi and radiotherapy is a potent oncological strategy. This combination has proven its efficacy in different tumors. However, in prostate cancer, there are only preclinical studies to support it and, recently, an ongoing clinical trial. The objective of this paper is to perform a review of the current evidence regarding the use of PARPi and radiotherapy (RT) in PCa and to give future insight on this topic.

APTX acts in DNA double-strand break repair in a manner distinct from XRCC4.

Aprataxin (APTX), the product of the causative gene for hereditary neurogenerative syndromes Ataxia-oculomotor apraxia 1 and early onset ataxia with oculomotor apraxia and hypoalbuminemia, has an enzymatic activity of removing adenosine monophosphate from DNA 5'-end, which arises from abortive ligation by DNA ligases. It is also reported that APTX physically binds to XRCC1 and XRCC4, suggesting its involvement in DNA single-strand break repair (SSBR) and DNA double-strand break repair (DSBR) via non-homologous end joining pathway. Although the involvement of APTX in SSBR in association with XRCC1 has been established, the significance of APTX in DSBR and its interaction with XRCC4 have remained unclear. Here, we generated APTX knock-out (APTX-/-) cell from human osteosarcoma U2OS through CRISPR/Cas9-mediated genome editing system. APTX-/- cells exhibited increased sensitivity toward ionizing radiation (IR) and Camptothecin in association with retarded DSBR, as shown by increased number of retained γH2AX foci. However, the number of retained 53BP1 foci in APTX-/- cell was not discernibly different from wild-type cells, in stark contrast to XRCC4-depleted cells. The recruitment of GFP-tagged APTX (GFP-APTX) to the DNA damage sites was examined by laser micro-irradiation and live-cell imaging analysis using confocal microscope. The accumulation of GFP-APTX on the laser track was attenuated by siRNA-mediated depletion of XRCC1, but not XRCC4. Moreover, the deprivation of APTX and XRCC4 displayed additive inhibitory effects on DSBR after IR exposure and end joining of GFP reporter. These findings collectively suggest that APTX acts in DSBR in a manner distinct from XRCC4.

Leveraging homologous recombination repair deficiency in sarcoma.

Personalised oncology is at the forefront of cancer research. The goal of personalised oncology is to selectively kill cancer cells while minimising side effects on normal tissue. This can be achieved by identifying and targeting cancer vulnerabilities that distinguish it from normal cells. Many cancers are deficient in high-fidelity DNA repair pathways that maintain genomic stability, such as homologous recombination (HR). Such cancers are highly sensitive to targeted therapies that induce DNA damage or inhibit DNA repair pathways. A notable example and a poster child of personalised oncology are PARP1/2 inhibitors (PARPi) that selectively kill HR-deficient (HRD) cancer cells by preventing repair of DNA gaps or single-strand breaks (SSBs) (Slade, 2020). Inhibitors of cell cycle checkpoints such as CHK1 and WEE1 can also eliminate HRD cancers by pushing cancer cells through the cell cycle despite unrepaired DNA damage and causing death by mitotic catastrophe (Groelly etal, 2022). PARPi have been approved for the treatment of ovarian, breast, pancreatic, and prostate cancer but other cancer types with an HRD signature (HRDness) may also respond to PARPi treatment. Planas-Paz etal (2023) now show that many sarcomas show HRDness and respond to PARP1/2 and WEE1 inhibitors, thus offering a new personalised oncology approach for this treatment-refractory cancer.

Abstract P4-01-08: Efficacy of PARP Inhibitors in Patients With BRCA1/2-related Breast Cancer with Prior Platinum Exposure: A Systematic Review and Meta-Analysis

Abstract Background: Approximately 5% of breast cancer patients carry a deleterious germline BRCA1/2 mutation, which leads malignant cells to be deficient in the repair of DNA double-strand breaks via homologous recombination. The poly(adenosine diphosphate–ribose) polymerase (PARP) enzymes are important on DNA single-strand break repair and PARP inhibitors (PARPi) cause an accumulation of unresolved DNA damage in tumors with BRCA1/2 mutations, resulting in cell death. However, because platinum salts also ultimately cause double-strand DNA breaks and may have overlapping mechanisms of resistance with PARPi, the efficacy of PARPi in patients with prior platinum therapy is unknown. We sought to evaluate the efficacy of PARPi in patients with BRCA1/2-related breast cancer and previous platinum exposure. Methods: We performed a systematic review and meta-analysis of studies that evaluated the efficacy of PARPi in patients with advanced or metastatic breast cancer and germline BRCA1/2 mutations. Two independent investigators identified double-blind, randomized controlled trials (RCTs) that included the subgroup of previous exposure to platinum. PubMed, Embase, and Cochrane databases were searched for papers up to June 26, 2022. Data extraction from published reports and quality assessment were performed under Cochrane recommendations. Hazard ratios (HRs) with a 95% confidence interval (CI) were pooled, and a p-value of < 0.05 was considered statistically significant. The software Review Manager 5.3 was selected for conducting the statistical analysis. The primary endpoint of interest was progression-free survival (PFS). Results: Out of 2069 database results (487 on PubMed; 1376 on Embase; and 206 on Cochrane), 42 studies were selected for full review, and 4 RCTs, with a total of 249 patients, were included in the final investigation. The PARPi included were Olaparib, Talazoparib, Niraparib, and Veliparib. Pooled analysis showed that PARPi improved PFS in breast cancer patients with prior treatment with platinum-based therapy compared to the control group [HR = 0.72; 95% CI, 0.53-0.97; p = 0.03]. In comparison, the population without previous platinum exposure had a similar but slightly greater benefit from PARPi [HR = 0.68; 95% CI, 0.52-0.89; p = 0.005]. A subgroup analysis containing only the three randomized trials with PARPi monotherapy confirmed the central tendency favoring PARPi over physicians’ choice of single-agent chemotherapy in patients with prior platinum-therapy; however, because of small numbers and wide confidence interval it was not statistically significant [HR = 0.72; 95% CI, 0.51-1.01; p = 0.06]. Conclusions: Our findings suggest that PARPi are associated with longer PFS in patients with advanced breast cancer and previous exposure to platinum therapy, decreasing the concerns about cross-resistance between the drug classes. Nonetheless, more extensive studies are still necessary to investigate the efficacy of PARPi in patients with prior platinum exposure and truly platinum-resistant disease. Progression-free survival of included studies Citation Format: Alice D. Marinho, Beatriz Mella S. Pessoa, Gabriela R. Brandao, Caroliny H. Da Silva, Pedro A. Reis, Ana Carolina M. Comini, Felipe Batalini. Efficacy of PARP Inhibitors in Patients With BRCA1/2-related Breast Cancer with Prior Platinum Exposure: A Systematic Review and Meta-Analysis [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-01-08.

PARP inhibitors in the treatment of ARID1A mutant ovarian clear cell cancer: PI3K/Akt1-dependent mechanism of synthetic lethality.

Poly(ADP-ribose) polymerase (PARP) is a nuclear enzyme involved in the repair of DNA single-strand breaks (SSB). The recent development of poly(ADP-ribose) polymerase inhibitors (PARPi) results from over 45 years of studies. When the activity of PARP1 or PARP2 is compromised, DNA SSB lesions are unresolved and can be converted to DNA double-strand breaks (DSBs) by the cellular transcription mechanisms. ARID1A (also called BAF250a) is an important component of the mammalian Switch/Sucrose Non-Fermentable (SWI/SNF) chromatin-remodeling complex. ARID1A gene demonstrates >50% of mutation rate in ovarian clear-cell carcinomas (OCCC). Mutated or downregulated ARID1A significantly compromises the Homologous Recombination Repair (HRR) of DNA DSB. The present study demonstrated that downregulated or mutated ARID1A attenuates DNA HRR through stimulation of the PI3K/Akt1 pathway and makes tumor cells highly sensitive to PARPi and PARPi/ionizing radiation (IR) combination. We showed that PI3K/Akt1 pathway plays an important role in the sensitization of cancer cell lines with compromised function of ARID1A to PARPi treatment. We believe that using of PARPi monotherapy or in combination with radiation therapy is an appealing strategy for treating ARID1A-mutated cancers, as well as many other types of PI3K/Akt1-driven cancers.

Does single-strand DNA break repair capacity influence oocyte maintenance and quality?

Human genome-wide association studies and evidence from animal models link ovarian ageing to double-strand (ds)DNA break repair capacity. Is there a connection between single-strand (ss)DNA repair mechanisms and ovarian function? We hypothesize that endogenous cellular processes subject oocytes to ssDNA lesions, and thus, ssDNA repair capacity is fundamental to their survival and maintenance.

Design, synthesis, biological evaluation and molecular docking study of novel urea-based benzamide derivatives as potent poly(ADP-ribose) polymerase-1 (PARP-1) inhibitors

Poly(ADP-ribose) polymerase-1 (PARP-1) is one of the key members of DNA repair enzymes that is responsible for the repair of DNA single-strand breaks. Inhibition of PARP-1 has been demonstrated to be a promising strategy to selectively kill tumor cells by targeting DNA repair pathway. Herein, a series of novel urea-based benzamide derivatives were designed and synthesized based on the structure-based drug design strategy. The anticancer activities against five human cancer cell lines including HCT116, MDA-MB-231, HeLa, A579 and A375 were evaluated and the preliminary structure-activity relationships were summarized. Among them, compounds 23f and 27f exhibited potent antiproliferative effects against HCT116 cells with IC50 values of 7.87 μM and 8.93 μM, respectively. Moreover, both compounds displayed excellent PARP-1 inhibitory activities with IC50 values of 5.17 nM and 6.06 nM, respectively. Mechanistic investigations showed that 23f and 27f could effectively inhibit colony formation and cell migration of HCT116 cells. Furthermore, 23f and 27f could cause cell cycle arrest at G2/M phase, and induce apoptosis by upregulating the expression of Bax and cleaved Caspase-3 and downregulating the expression of Caspase-3 and Bcl-2 in HCT116 cells. In addition, molecular docking studies provided the rational binding modes of these compounds in complex with PARP-1. Collectively, these results suggested that 23f and 27f could serve as promising drug candidates for further investigation.

DNA single-strand break repair and human genetic disease.

DNA single-strand breaks (SSBs) are amongst the commonest DNA lesions arising in cells, with many tens of thousands induced in each cell each day. SSBs arise not only from exposure to intracellular and environmental genotoxins but also as intermediates of normal DNA metabolic processes, such as the removal of torsional stress in DNA by topoisomerase enzymes and the epigenetic regulation of gene expression by DNA base excision repair (BER). If not rapidly detected and repaired, SSBs can result in RNA polymerase stalling, DNA replication fork collapse, and hyperactivation of the SSB sensor protein poly(ADP-ribose) polymerase 1 (PARP1). The potential impact of unrepaired SSBs is illustrated by the existence of genetic diseases in which proteins involved in SSB repair (SSBR) are mutated, and which are typified by hereditary neurodevelopmental and/or neurodegenerative disease. Here, I review our current understanding of SSBR and its impact on human neurological disease, with a focus on recent developments and concepts.

Homologous Recombination Deficiency in Ovarian, Breast, Colorectal, Pancreatic, Non-Small Cell Lung and Prostate Cancers, and the Mechanisms of Resistance to PARP Inhibitors.

Homologous recombination (HR) is a highly conserved DNA repair mechanism that protects cells from exogenous and endogenous DNA damage. Breast cancer 1 (BRCA1) and breast cancer 2 (BRCA2) play an important role in the HR repair pathway by interacting with other DNA repair proteins such as Fanconi anemia (FA) proteins, ATM, RAD51, PALB2, MRE11A, RAD50, and NBN. These pathways are frequently aberrant in cancer, leading to the accumulation of DNA damage and genomic instability known as homologous recombination deficiency (HRD). HRD can be caused by chromosomal and subchromosomal aberrations, as well as by epigenetic inactivation of tumor suppressor gene promoters. Deficiency in one or more HR genes increases the risk of many malignancies. Another conserved mechanism involved in the repair of DNA single-strand breaks (SSBs) is base excision repair, in which poly (ADP-ribose) polymerase (PARP) enzymes play an important role. PARP inhibitors (PARPIs) convert SSBs to more cytotoxic double-strand breaks, which are repaired in HR-proficient cells, but remain unrepaired in HRD. The blockade of both HR and base excision repair pathways is the basis of PARPI therapy. The use of PARPIs can be expanded to sporadic cancers displaying the “BRCAness” phenotype. Although PARPIs are effective in many cancers, their efficacy is limited by the development of resistance. In this review, we summarize the prevalence of HRD due to mutation, loss of heterozygosity, and promoter hypermethylation of 35 DNA repair genes in ovarian, breast, colorectal, pancreatic, non-small cell lung cancer, and prostate cancer. The underlying mechanisms and strategies to overcome PARPI resistance are also discussed.

Abstract 5399: Synergistic combination nanomedicine of doxorubicin and oligo hyaluronic acid inhibits DNA damage repair and overcomes drug resistance in metastatic triple-negative breast cancer

Abstract Background: Triple-negative breast cancer (TNBC) is a highly aggressive breast cancer subtype. Despite the early response to chemotherapy, high incidence of recurrence leads to short overall survival and poor prognosis. Native hyaluronic acid (HA) interacts with CD44 and receptors for hyaluronan mediated motility (RHAMM) overexpressed in TNBC mediating chemo-resistance and metastasis. In contrast, oligomeric HA (oHA) can disrupt HA-CD44/RHAMM interactions and attenuate oncogenic and pro-metastatic pathways, such as the mitogen-activated protein kinase/extracellular-signal-regulated kinase (MAPK/ERK) pathway, reducing downstream DNA repair and drug resistance markers. Our previous work demonstrated that oHA can exert synergistic anti-tumor and anti-metastatic effects when combined with doxorubicin (DOX) and co-loaded in an integrin-targeted iRGD-modified nanoparticle system (iRGD-DOX-oHA-PLNs). This study aims to investigate the inhibitory effect of iRGD-DOX-oHA-PLNs on both DNA single-strand break (SSB) and double-strand break (DSB) repair proteins and drug efflux pumps responsible for multidrug resistance to enhance DOX efficacy in breast cancer gene 1 (BRCA1) mutant and non-mutant TNBC. Methods: The cytotoxicity of DOX, oHA and their combinations in free solution or in nanoparticles was evaluated by clonogenic assay in human MDA-MB-231-luc-D3H2LN and MDA-MB-436 (BRCA1 mutant) TNBC cells. The in vitro expression of a DNA DSB marker, DNA repair markers, and drug efflux pump P-glycoprotein (P-gp) were measured by Western blot. The in vivo expression level of BRCA1 and Rad51 in an orthotopic TNBC mouse model was determined by immunohistochemical staining. Results: The combination of DOX-oHA showed synergism against both BRCA1 mutant and non-mutant TNBC cells. The iRGD-DOX-oHA-PLNs induced great increases in DNA DSBs demonstrated by the highest γH2AX level compared to other treatment groups. These nanoparticles also showed inhibitory effects on the expression of both DNA SSB repair protein (poly (ADP-ribose) polymerase) and DNA DSB repair proteins (Rad50 and Rad51), contributing to the enhanced efficacy of chemotherapy. The immunohistochemical staining of tumor tissues indicated lower levels of BRCA1 and Rad51 after iRGD-DOX-oHA-PLN treatment than the formulation without oHA, attributable to the effect of intracellularly delivered oHA on limiting the MAPK signaling. Additionally, iRGD-DOX-oHA-PLNs reduced the expression of the drug efflux pump P-gp as compared to DOX treatment groups without oHA. Conclusion: The co-delivery of oHA and DOX in the iRGD-DOX-oHA-PLNs efficiently blocked DNA damage repair and down-regulated the drug efflux pump P-gp, thus improving the efficacy of DOX. Collectively, this nanoparticle system could be a promising option for metastatic TNBC treatment. Citation Format: Pei Zhi, Ibrahim Alradwan, Tian Zhang, HoYin Lip, Abdulmutalib Zetrini, Chunsheng He, Jeffery Henderson, Andrew Michael Rauth, Xiao Yu Wu. Synergistic combination nanomedicine of doxorubicin and oligo hyaluronic acid inhibits DNA damage repair and overcomes drug resistance in metastatic triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5399.

Abstract 2463: 18F-Talazoparib: A novel potential PET imaging agent of PARP

Abstract Background: Poly-(ADP-ribose) polymerases (PARPs) are an enzyme family that catalyze the transfer of ADP-ribose from nicotinamide adenine dinucleotide (NAD+) to an acceptor protein. PARP family members play fundamental roles in single-strand DNA break (SSB) repair, cell signaling of DNA damage and inflammation, cell death, and cellular replication. Various PARP inhibitors have been developed and PARP1 inhibitors are currently used to treat subsets of breast and ovarian cancer patients. Of all the PARP inhibitors studied, Talazoparib (Pfizer), appears to be the most stable in pre-clinical and clinical studies, affords nanomolar affinity to PARP1, and demonstrates the highest affinity trapping of PARP on the DNA-protein complex. Imaging of PARP using a radiolabeled inhibitor has been proposed for patient selection, therapy dose optimization and validation of target engagement. Several PARP imaging agents have been developed and studied as possible tracers, by modifying the structure of an original PARP inhibitor to accommodate an imaging or therapeutic isotope. Herein we used a copper-mediated 18F-radio-fluorination strategy with a novel aryl boronic ester precursor to access 18F-Talazoparib (18F-TZ), a 18F-radiolabeled form of the structurally identical Talazoparib. Methods: A novel boronic ester precursor was synthetized in six steps synthesis with an overall yield of 3.3% and characterized by NMR and mass spectrometry. Radiofluorination was performed using an automatic GE TracerLab system and analytical radioHPLC was used for tracer characterization and for stability analysis. A panel of tumor cell lines representing a variety of PARP expression levels (Hela, SUM149, SUM1315, MDA-MB-436, MDA-MB-231, PSN1, MiaPaca2, Jurkat) were tested for time-dependent uptake and specific binding, utilizing both 4 °C and excess cold TZ blocking. Results: 18F-TZ was synthetized in 5.0±0.80% (n=20) radiochemical yield and a molar activity of 18.9-98.0 GBq/µmol by a copper-mediated radiofluorination of the corresponding pinacolic boronic ester, followed by purification via semi-preparative HPLC and reformulation in ethanol. 18F-TZ was obtained in >99% radiochemical purity, displaying up to 4 hr shelf stability (25 °C and 4 °C) in PBS (10% EtOH). 18F-TZ was also stable in human plasma up to 4 hr at 37 °C. Both shelf and plasma stability tests showed a decrease of the radioHPLC peak consistent with decay half-life, but did not show the formation of secondary species. All cell lines selected for the in cellulo study showed uptake of radioactive 18F-TZ after 30 min of incubation at 37 °C. Cell uptake was blocked >98% by excess cold TZ or incubation of cells at 4 °C. Conclusion: 18F-Talazoparib was synthesized by copper-mediated radiofluorination from the corresponding pinacol ester and demonstrates both uptake and target specificity in PARP-expressing tumor cells. Citation Format: Riccardo Muzzioli, Federica Pisaneschi, Yi Rao, Seth Gammon, David Piwnica-Worms. 18F-Talazoparib: A novel potential PET imaging agent of PARP [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2463.

Abstract 52: Analytical validation of the Resolution HRD plasma assay used to identify mCRPC patients with mutations, including homozygous deletions, in DNA repair genes as a companion diagnostic for niraparib

Abstract Tumor cells of metastatic castration-resistant prostate cancer (mCRPC) patients with homologous recombination deficiency (HRD) are highly sensitive to the blockade of DNA single-strand break repair via the inhibition of the poly(adenosine diphosphate-ribose) polymerase (PARP) family of nuclear proteins. Niraparib is a highly selective PARP inhibitor, with activity against PARP-1/2 DNA-repair polymerases, and the Resolution HRD assay is being developed as a companion diagnostic for niraparib. Detection of HRD in cell free DNA (cfDNA) isolated from blood is minimally invasive and is of special benefit to mCRPC patients, many without accessible lesions. The Resolution HRD assay identifies patients with substitutions, insertions, deletions, and homozygous deletions of the ATM, BRCA1, BRCA2, BRIP1, CDK12 (except homozygous deletions), CHEK2, FANCA, HDAC2, and PALB2 genes by targeted NGS sequencing of cfDNA isolated from plasma. Analytical performance of the Resolution HRD assay was validated using cfDNA from mCRPC patient plasma, cfDNA from healthy donor plasma, and contrived samples with a wide spectrum of technically challenging genetic aberrations along with CRISPR-engineered human cell lines with ATM and BRCA2 gene deletions. The LOD95 was established for substitutions, insertions, deletions, and homozygous deletions for genes represented in the HRD panel with variant types including complex indels, long indels, and challenging genomic settings including homopolymeric and GC-rich regions. mCRPC specimens were used to confirm an acceptable level of precision near 1X LOD and 2-3X LOD concentrations for all 4 variant types. No false-positives were detected in any samples from healthy donors. Resolution HRD has been validated to give consistent results across the 15-30 ng input range. Studies to confirm accuracy of the Resolution HRD test results using a validated orthogonal method will be presented. The Resolution HRD assay offers highly sensitive, specific, and robust test results, and meets analytical requirements for clinical applications. It is intended as a companion diagnostic to niraparib in combination with abiraterone acetate plus prednisone (AAP) for the treatment of mCRPC patients. Citation Format: Julia Pollak, Kristy Potts, PuiYee Chan, Chen-Hsun Tsai, Angela Liao, Carly Garrison, Taylor Brown, Paul Stull, Zhen Li, Christine Baker, Kavita Garg, Ira Pekker, Michael Farabaugh, Michael Gormley, Lesley Farrington, Katherine Bell, Usha Singh. Analytical validation of the Resolution HRD plasma assay used to identify mCRPC patients with mutations, including homozygous deletions, in DNA repair genes as a companion diagnostic for niraparib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 52.

Olaparib Induces RPL5/RPL11-Dependent p53 Activation via Nucleolar Stress.

The poly (ADP-ribose) polymerase (PARP) inhibitor (PARPi) Olaparib is a widely used targeted therapy for a variety of solid tumors with homologous recombination deficiency (HRD) caused by mutation of BRCA1/2 or other DNA repair genes. The anti-tumor activity of Olaparib has been largely attributed to its ability to inhibit PARP enzymes and block DNA single-strand break (SSB) repair, which eventually leads to the most detrimental DNA damage, double-strand breaks (DSB), in HRD cells. Although PARPi was found to induce p53-dependent cell death, the underlying molecular mechanism remains incompletely understood. Here, we report that Olaparib treatment leads to p53 stabilization and activation of its downstream target genes in a dose- and time-dependent manner. Mechanistically, Olaparib triggers nucleolar stress by inhibiting biosynthesis of the precursor of ribosomal RNAs (pre-rRNA), resulting in enhanced interaction between ribosomal proteins (RPs), RPL5 and RPL11, and MDM2. Consistently, knockdown of RPL5 and RPL11 prevents Olaparib-induced p53 activation. More importantly, Olaparib efficiently suppresses breast and colorectal cancer cell survival and proliferation through activation of p53. Altogether, our study demonstrates that Olaparib activates the nucleolar stress-RPs-p53 pathway, suggesting rRNA biogenesis as a novel target for PARPi.

XRCC1 protects transcription from toxic PARP1 activity during DNA base excision repair

Genetic defects in the repair of DNA single-strand breaks (SSBs) can result in neurological disease triggered by toxic activity of the single-strand-break sensor protein PARP1. However, the mechanism(s) by which this toxic PARP1 activity triggers cellular dysfunction are unclear. Here we show that human cells lacking XRCC1 fail to rapidly recover transcription following DNA base damage, a phenotype also observed in patient-derived fibroblasts with XRCC1 mutations and Xrcc1−/− mouse neurons. This defect is caused by excessive/aberrant PARP1 activity during DNA base excision repair, resulting from the loss of PARP1 regulation by XRCC1. We show that aberrant PARP1 activity suppresses transcriptional recovery during base excision repair by promoting excessive recruitment and activity of the ubiquitin protease USP3, which as a result reduces the level of monoubiquitinated histones important for normal transcriptional regulation. Importantly, inhibition and/or deletion of PARP1 or USP3 restores transcriptional recovery in XRCC1−/− cells, highlighting PARP1 and USP3 as possible therapeutic targets in neurological disease.

Phase 1b Trial of Talazoparib and Gemtuzumab Ozogamicin in Adult Patients with CD33+ Relapsed or Refractory Acute Myeloid Leukemia

Phase 1b Trial of Talazoparib and Gemtuzumab Ozogamicin in Adult Patients with CD33+ Relapsed or Refractory Acute Myeloid Leukemia

Methylmercury chronic exposure affects the expression of DNA single-strand break repair genes, induces oxidative stress, and chromosomal abnormalities in young dyslipidemic APOE knockout mice

Mercury (Hg) is one of the most toxic environmental pollutants, especially when methylated, forming methylmercury (MeHg). MeHg affects DNA repair, increases oxidative stress, and predisposes to cancer. MeHg neurotoxicity is well-known, but recently MeHg-associated cardiovascular effects were recognized. This study evaluated circulating lipids, oxidative stress, and genotoxicity after MeHg-chronic exposure (20 mg/L in drinking water) in C57BL/6J wild-type and APOE knockout (ko) mice, the latter, being spontaneously dyslipidemic. Experimental mice were assigned to four groups: non-intoxicated and MeHg-intoxicated wild-type mice and non-intoxicated and MeHg-intoxicated APOE ko mice. Plasma levels of triglycerides, total cholesterol (TC), HDL, and LDL were analyzed. Liver lipid peroxidation and splenic gene expression of xeroderma pigmentosum complementation groups A, C, D, and G (XPA, XPC, XPD, and XPG), X-ray repair cross-complementing protein 1 (XRCC1), and telomerase reverse transcriptase (TERT) were measured. Fur Hg levels confirmed chronic MeHg intoxication. MeHg exposure raises TC levels both in wild-type and APOE ko mice. HDL and LDL-cholesterol levels were increased only in the MeHg-challenged APOE ko mice. MeHg increased liver lipid peroxidation, regardless of the genetic background. Unintoxicated APOE ko mice showed higher expression of TERT than all other groups. APOE deficiency increases XPA expression, regardless of MeHg intoxication. Furthermore, MeHg-intoxicated mice had more cytogenetic abnormalities, effect which was independent of APOE deficiency. More studies are needed to dissect the interactions between circulating lipids, MeHg intoxication, and DNA-repair pathways even at young age, interactions that likely play critical roles in cell senescence and the risk for chronic disorders later in life.