Poly(ADP-ribose) polymers regulate DNA topoisomerase I (Top1) nuclear dynamics and camptothecin sensitivity in living cells

TDP1 phosphorylation by CDK1 in mitosis promotes MUS81-dependent repair of trapped Top1-DNA covalent complexes

Topoisomerase 1 (Top1) controls DNA topology, relieves DNA supercoiling during replication and transcription, and is critical for mitotic progression to the G1 phase. Tyrosyl-DNA phosphodiesterase 1 (TDP1) mediates the removal of trapped Top1-DNA covalent complexes (Top1cc). Here, we identify CDK1-dependent phosphorylation of TDP1 at residue S61 during mitosis. A TDP1 variant defective for S61 phosphorylation (TDP1-S61A) is trapped on the mitotic chromosomes, triggering DNA damage and mitotic defects. Moreover, we show that Top1cc repair in mitosis occurs via a MUS81-dependent DNA repair mechanism. Replication stress induced by camptothecin or aphidicolin leads to TDP1-S61A enrichment at common fragile sites, which over-stimulates MUS81-dependent chromatid breaks, anaphase bridges, and micronuclei, ultimately culminating in the formation of 53BP1 nuclear bodies during G1 phase. Our findings provide new insights into the cell cycle-dependent regulation of TDP1 dynamics for the repair of trapped Top1-DNA covalent complexes during mitosis that prevents genomic instability following replication stress.

A desthiobiotin labelled NAD+ analogue to uncover Poly(ADP-ribose) polymerase 1 protein targets

ADP-ribosylation is a post-translational modification catalyzed by the enzyme family of polyadenosine diphosphate (ADP)-ribose) polymerases (PARPs). This enzymatic process involves the transfer of single or multiple ADP-ribose molecules onto proteins, utilizing nicotinamide adenine dinucleotide (NAD+ ) as a substrate. It, thus, plays a pivotal role in regulating various biological processes. Unveiling PARP-selective protein targets is crucial for a better understanding of their biological functions. Nonetheless, this task proves challenging due to overlapping targets shared among PARP family members. Therefore, we applied the "bump-and-hole" strategy to modify the nicotinamide binding site of PARP1 by introducing a hydrophobic pocket ("hole"). This PARP1-mutant binds an orthogonal NAD+ (Et-DTB-NAD+ ) containing an ethyl group ("bump") at the nicotinamide moiety. Furthermore, we added a desthiobiotin (DTB) tag directly to the adenosine moiety, enabling affinity enrichment of ADP-ribosylated proteins. Employing this approach, we successfully identified protein targets modified by PARP1 in cell lysate. This strategy expands the arsenal of chemically modified NAD+ analogs available for studying ADP-ribosylation, providing a powerful tool to study these critical post-translational modifications.

Mitochondrial topoisomerase 1 targeted anticancer therapy using irinotecan encapsulated mesoporous MIL-101(Fe) synthesized via a vapour assisted method

Mitochondrial topisomerase 1 (Top1mt) is critical for mtDNA replication, transcription, and energy production. Here, we investigate the carrier-mediated targeted delivery of the anticancer drug irinotecan into the mitochondria to selectively trap Top1mt covalent complexes (Top1mtcc) and its role in anticancer therapeutics. We have designed a biocompatible mesoporous metal-organic framework (MOF) material, namely MIL-101(Fe), as the drug delivery carrier that selectively localizes inside mitochondria. In contrast to the traditional way of synthesising MOFs, here we have employed a vapour-assisted solvothermal method for the synthesis of MIL-101(Fe) using terephthalic acid as the organic linker and Fe(III) as the metal source. The advantage of this method is that it recycles the excess solvent (DMF) and reduces the amount of washing solvent. We demonstrate that MIL-101(Fe)-encapsulated irinotecan (MIL-Iri) was selectively targeted towards the mitochondria to poison Top1mtcc in a dose-dependent manner and was achieved at a low nanomolar drug concentration. We provide evidence that Top1mtcc generated by MIL-Iri leads to mtDNA damage in human colon and breast cancer cells and plays a significant role in cellular toxicity. Altogether, this study provides evidence for a new and effective strategy in anticancer chemotherapy.

Fluorescence-resonance-energy-transfer-based assay to estimate modulation of TDP1 activity through arginine methylation

Tyrosyl DNA phosphodiesterase (TDP1) is a DNA repair enzyme that hydrolyzes the phosphotyrosyl linkage between 3'-DNA-protein crosslinks such as stalled topoisomerase 1 cleavage complexes (Top1cc). Here, we present a fluorescence-resonance-energy-transfer-(FRET) based assay to estimate modulation of TDP1 activity through arginine methylation. We describe steps for TDP1 expression and purification and estimating TDP1 activity using fluorescence-quenched probes mimicking Top1cc. We then detail data analysis of real-time TDP1 activity and screening of TDP1-selective inhibitors. For complete details on the use and execution of this protocol, please refer to Bhattacharjee et al. (2022).¹.

Transcriptomic Analysis of CRISPR/Cas9-Mediated PARP1-Knockout Cells under the Influence of Topotecan and TDP1 Inhibitor

Topoisomerase 1 (TOP1) is an enzyme that regulates DNA topology and is essential for replication, recombination, and other processes. The normal TOP1 catalytic cycle involves the formation of a short-lived covalent complex with the 3' end of DNA (TOP1 cleavage complex, TOP1cc), which can be stabilized, resulting in cell death. This fact substantiates the effectiveness of anticancer drugs-TOP1 poisons, such as topotecan, that block the relegation of DNA and fix TOP1cc. Tyrosyl-DNA phosphodiesterase 1 (TDP1) is able to eliminate TOP1cc. Thus, TDP1 interferes with the action of topotecan. Poly(ADP-ribose) polymerase 1 (PARP1) is a key regulator of many processes in the cell, such as maintaining the integrity of the genome, regulation of the cell cycle, cell death, and others. PARP1 also controls the repair of TOP1cc. We performed a transcriptomic analysis of wild type and PARP1 knockout HEK293A cells treated with topotecan and TDP1 inhibitor OL9-119 alone and in combination. The largest number of differentially expressed genes (DEGs, about 4000 both up- and down-regulated genes) was found in knockout cells. Topotecan and OL9-119 treatment elicited significantly fewer DEGs in WT cells and negligible DEGs in PARP1-KO cells. A significant part of the changes caused by PARP1-KO affected the synthesis and processing of proteins. Differences under the action of treatment with TOP1 or TDP1 inhibitors alone were found in the signaling pathways for the development of cancer, DNA repair, and the proteasome. The drug combination resulted in DEGs in the ribosome, proteasome, spliceosome, and oxidative phosphorylation pathways.

Therapeutic strategies of dual-target small molecules to overcome drug resistance in cancer therapy

Despite some advances in targeted therapeutics of human cancers, curative cancer treatment still remains a tremendous challenge due to the occurrence of drug resistance. A variety of underlying resistance mechanisms to targeted cancer drugs have recently revealed that the dual-target therapeutic strategy would be an attractive avenue. Compared to drug combination strategies, one agent simultaneously modulating two druggable targets generally shows fewer adverse reactions and lower toxicity. As a consequence, the dual-target small molecule has been extensively explored to overcome drug resistance in cancer therapy. Thus, in this review, we focus on summarizing drug resistance mechanisms of cancer cells, such as enhanced drug efflux, deregulated cell death, DNA damage repair, and epigenetic alterations. Based upon the resistance mechanisms, we further discuss the current therapeutic strategies of dual-target small molecules to overcome drug resistance, which will shed new light on exploiting more intricate mechanisms and relevant dual-target drugs for future cancer therapeutics.

A literature review of the promising future of TROP2: a potential drug therapy target

Background and Objective

Previous studies have demonstrated that the oncogene trophoblast cell surface antigen 2 (TROP2) has great application prospects as a therapeutic target. However, few literature reviews have systematically summarized and evaluated its role in cancer therapy. This study aims to summarize the molecular structure, functions, signal transduction pathways, and prognostic value of TROP2, and explore therapeutic agents that target TROP2.

Methods

A total of 1,376 published literatures from PubMed and 614 published literatures from EMBASE were retrieved by searching "TROP2" or "Trophoblast cell surface antigen 2". The search was conducted on December 12, 2020, and updated on November 20, 2022. The cBioportal and GEPIA (Gene Expression Profiling Interactive Analysis) databases were used to analyze the expression, mutation, and prognostic value of TROP2 in different types of cancer.

Key Content and Findings

TROP2 is overexpressed in different tumor tissues and plays roles in cell proliferation, invasion, migration, apoptosis, and treatment resistance by binding to or interacting with several molecules. As a therapeutic target, TROP2 is particularly suitable for antibody-based therapies. Monoclonal antibodies, bispecific antibodies, antibody-drug conjugates (ADCs), virus-like particles, and antibody drugs in combination with traditional chemotherapy, immunotherapy, radioimmunotherapy, photoimmunotherapy, and nanoparticles that target TROP2 have thus far been rapidly developed. For example, sacituzumab govitecan (IMMU-132), a TROP2-targeting ADC, was granted accelerated approval for the treatment of metastatic triple-negative breast cancer (TNBC). Anti-TROP2 antibody-conjugated nanoparticles (ST-NPs) are a promising vehicle for delivering doxorubicin in targeted TNBC therapy.

Conclusions

The availability of TROP2-targeting ADCs makes TROP2 an accessible and promising therapeutic target for advanced metastatic cancers. The present review describes the important role of TROP2 in tumorigenesis and its potential applications as a promising biomarker and therapeutic target that is capable of reversing resistance.

Topoisomerase 1-dependent R-loop deficiency drives accelerated replication and genomic instability

DNA replication is a complex process tightly regulated to ensure faithful genome duplication, and its perturbation leads to DNA damage and genomic instability. Replication stress is commonly associated with slow and stalled replication forks. Recently, accelerated replication has emerged as a non-canonical form of replication stress. However, the molecular basis underlying fork acceleration is largely unknown. Here, we show that mutated HRAS activation leads to increased topoisomerase 1 (TOP1) expression, causing aberrant replication fork acceleration and DNA damage by decreasing RNA-DNA hybrids or R-loops. In these cells, restoration of TOP1 expression or mild replication inhibition rescues the perturbed replication and reduces DNA damage. Furthermore, TOP1 or RNaseH1 overexpression induces accelerated replication and DNA damage, highlighting the importance of TOP1 equilibrium in regulating R-loop homeostasis to ensure faithful DNA replication and genome integrity. Altogether, our results dissect a mechanism of oncogene-induced DNA damage by aberrant replication fork acceleration.

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Increased TOP1 expression by mutated RAS reduces R loops

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Low R-loop levels promote accelerated replication and DNA damage

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TOP1 restoration or mild replication inhibition rescue DNA acceleration and damage

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High TOP1 expression is associated with replication mutagenesis in cancer

Interplay between symmetric arginine dimethylation and ubiquitylation regulates TDP1 proteostasis for the repair of topoisomerase I-DNA adducts

Tyrosyl-DNA phosphodiesterase (TDP1) hydrolyzes the phosphodiester bond between a DNA 3' end and a tyrosyl moiety and is implicated in the repair of trapped topoisomerase I (Top1)-DNA covalent complexes (Top1cc). Protein arginine methyltransferase 5 (PRMT5) catalyzes arginine methylation of TDP1 at the residues R361 and R586. Here, we establish mechanistic crosstalk between TDP1 arginine methylation and ubiquitylation, which is critical for TDP1 homeostasis and cellular responses to Top1 poisons. We show that R586 methylation promotes TDP1 ubiquitylation, which facilitates ubiquitin/proteasome-dependent TDP1 turnover by impeding the binding of UCHL3 (deubiquitylase enzyme) with TDP1. TDP1-R586 also promotes TDP1-XRCC1 binding and XRCC1 foci formation at Top1cc-damage sites. Intriguingly, R361 methylation enhances the 3'-phosphodiesterase activity of TDP1 in real-time fluorescence-based cleavage assays, and this was rationalized using structural modeling. Together, our findings establish arginine methylation as a co-regulator of TDP1 proteostasis and activity, which modulates the repair of trapped Top1cc.

Non-enzymatic function of WRN RECQL helicase regulates removal of topoisomerase-I-DNA covalent complexes and triggers NF-κB signaling in cancer

Mutation in Werner (WRN) RECQL helicase is associated with premature aging syndrome (Werner syndrome, WS) and predisposition to multiple cancers. In patients with solid cancers, deficiency of the WRN RECQL helicase is paradoxically associated with enhanced overall survival in response to treatment with TOP1 inhibitors, which stabilize pathological TOP1‐DNA‐covalent‐complexes (TOP1cc) on the genome. However, the underlying mechanism of WRN in development of chemoresistance to TOP1 inhibitors is not yet explored. Our whole‐genome transcriptomic analysis for ~25,000 genes showed robust activation of NF‐κB‐dependent prosurvival genes in response to TOP1cc. CRISPR‐Cas9 knockout, shRNA silencing, and under‐expression of WRN confer high‐sensitivity of multiple cancers to TOP1 inhibitor. We demonstrated that WRN orchestrates TOP1cc repair through proteasome‐dependent and proteasome‐independent process, unleashing robust ssDNA generation. This in turn ensues signal transduction for CHK1 mediated NF‐κB‐activation through IκBα‐degradation and nuclear localization of p65 protein. Intriguingly, our site‐directed mutagenesis and rescue experiments revealed that neither RECQL‐helicase nor DNA‐exonuclease enzyme activity of WRN (WRN^E84A, WRN^K577M, and WRN^E84A‐K577M) were required for TOP1cc removal, ssDNA generation and signaling for NF‐κB activation. In correlation with patient data and above results, the TOP1 inhibitor‐based targeted therapy showed that WRN‐deficient melanoma tumors were highly sensitive to TOP1 inhibition in preclinical in vivo mouse model. Collectively, our findings identify hitherto unknown non‐enzymatic role of WRN RECQL helicase in pathological mechanisms underlying TOP1cc processing and subsequent NF‐κB‐activation, offering a potential targeted therapy for WRN‐deficient cancer patients.

TDP1 knockout Leishmania donovani accumulate topoisomerase 1-linked DNA damage and are hypersensitive to clinically used antileishmanial drugs

Leishmania donovani, a unicellular protozoan parasite, causes a wide range of human diseases including fatal visceral leishmaniasis. Tyrosyl DNA-phosphodiesterase 1 (TDP1) hydrolyzes the phosphodiester bond between DNA 3'-end and a tyrosyl moiety of trapped topoisomerase I-DNA covalent complexes (Top1cc). We have previously shown Leishmania harbors a TDP1 gene (LdTDP1), however, the biological role of TDP1 remains largely unknown. In the present study, we have generated TDP1 knockout L. donovani (LdTDP1^-/- ) promastigotes and have shown that LdTDP1^-/- parasites are deficient in 3'-phosphodiesterase activities and were hypersensitive to Top1-poison like camptothecin (CPT), DNA alkylation agent like methyl methanesulfonate, and oxidative DNA lesions generated by hydrogen peroxide but were not sensitive to etoposide. We also detected elevated levels of CPT-induced reactive oxygen species triggering cell cycle arrest and cell death in LdTDP1^-/- promastigotes. LdTDP1^-/- promastigotes accumulate a significant change in the membrane morphology with the accumulation of membrane pores, which is associated with oxidative stress and lipid peroxidation. To our surprise, we detected that LdTDP1^-/- parasites were hypersensitive to antileishmanial drugs like amphotericin B and miltefosine, which could be rescued by complementation of wild-type TDP1 gene in the LdTDP1^-/- parasites. Notably, multidrug-resistant L. donovani clinical isolates showed a marked reduction in TDP1 expression and were sensitive to Top1 poisons. Taken together, our study provides a new role of LdTDP1 in protecting L. donovani parasites from oxidative stress-induced DNA damage and resistance to amphotericin B and miltefosine.

TDP1 and TOP1 as targets in anticancer treatment of NSCLC: Activity and protein level in normal and tumor tissue from 150 NSCLC patients correlated to clinical data

OBJECTIVES

Topoisomerase 1 (TOP1) is a drug target used in anticancer treatment of various cancer types. The effect of the TOP1 drugs can be counteracted by the enzymatic activity of tyrosyl-DNA phosphodiesterase 1 (TDP1). Thus, to elucidate the relevance of combining TDP1 and TOP1 as drug targets for anticancer treatment in NSCLC, TDP1 and TOP1 was for the first time quantified in a large cohort of paired normal and tumor tissue from NSCLC patients, and data were correlated between the two enzymes and to clinical data.

MATERIALS AND METHODS

TDP1 and TOP1 activity and protein concentration were measured in paired normal and tumor tissue from 150 NSCLC patients using TDP1 and TOP1 specific biosensors and ELISA. TDP1 and TOP1 activity and protein concentration were correlated to clinical data.

RESULTS

TDP1 and TOP1 activity and protein concentration were significantly upregulated from normal to tumor tissue for the individual patients, but did not correlate to any of the clinical data. TDP1 and TOP1 activity were upregulated in 89.3% and 82.7% of the patients, respectively, and correlated in both normal and tumor tissue. The same tendency was observed for protein concentration with an upregulation of TDP1 and TOP1 in 73.0% and 84.4% of the patients, respectively. The activity and protein concentration correlated in normal and tumor tissue for both TDP1 and TOP1.

CONCLUSION

The upregulations of TDP1 and TOP1 from normal to tumor tissue combined with the observation that TDP1 and TOP1 did not correlate to any of the clinical data indicate that both proteins are important for development or maintenance of the tumor cells in NSCLC. Correlations between TDP1 and TOP1 indicate a biological dependency and potential co-regulation of the enzymes. These observations is encouraging in relation to using TOP1 and TDP1 as targets in anticancer treatment of NSCLC.

MYC assembles and stimulates topoisomerases 1 and 2 in a "topoisome"

High-intensity transcription and replication supercoil DNA to levels that can impede or halt these processes. As a potent transcription amplifier and replication accelerator, the proto-oncogene MYC must manage this interfering torsional stress. By comparing gene expression with the recruitment of topoisomerases and MYC to promoters, we surmised a direct association of MYC with topoisomerase 1 (TOP1) and TOP2 that was confirmed in vitro and in cells. Beyond recruiting topoisomerases, MYC directly stimulates their activities. We identify a MYC-nucleated "topoisome" complex that unites TOP1 and TOP2 and increases their levels and activities at promoters, gene bodies, and enhancers. Whether TOP2A or TOP2B is included in the topoisome is dictated by the presence of MYC versus MYCN, respectively. Thus, in vitro and in cells, MYC assembles tools that simplify DNA topology and promote genome function under high output conditions.

Trapped topoisomerase-DNA covalent complexes in the mitochondria and their role in human diseases

Topoisomerases regulate DNA topology, organization of the intracellular DNA, the transmission of genetic materials, and gene expressions. Other than the nuclear genome, mitochondria also harbor the small, circular DNA (mtDNA) that encodes a critical subset of proteins for the production of cellular ATP; however, mitochondria are solely dependent on the nucleus for all the mitochondrial proteins necessary for mtDNA replication, repair, and maintenance. Mitochondrial genome compiles topological stress from bidirectional transcription and replication, therefore imports four nuclear encoded topoisomerases (Top1mt, Top2α, Top2β, and Top3α) in the mitochondria to relax mtDNA supercoiling generated during these processes. Trapping of topoisomerase on DNA results in the formation of protein-linked DNA adducts (PDAs), which are widely exploited by topoisomerase-targeting anticancer drugs. Intriguingly mtDNA is potentially exposed to DNA damage that has been attributed to a variety of human diseases, including neurodegeneration, cancer, and premature aging. In this review, we focus on the role of different topoisomerases in the mitochondria and our current understanding of the mitochondrial DNA damage through trapped protein-DNA complexes, and the progress in the molecular mechanisms of the repair for trapped topoisomerase covalent complexes (Topcc). Finally, we have discussed how the pathological DNA lesions that cause mtDNA damage,trigger mitochondrial fission and mitophagy, which serve as quality control events for clearing damaged mtDNA.

PARylation prevents the proteasomal degradation of topoisomerase I DNA-protein crosslinks and induces their deubiquitylation

Poly(ADP)-ribosylation (PARylation) regulates chromatin structure and recruits DNA repair proteins. Using single-molecule fluorescence microscopy to track topoisomerase I (TOP1) in live cells, we found that sustained PARylation blocked the repair of TOP1 DNA-protein crosslinks (TOP1-DPCs) in a similar fashion as inhibition of the ubiquitin-proteasome system (UPS). PARylation of TOP1-DPC was readily revealed by inhibiting poly(ADP-ribose) glycohydrolase (PARG), indicating the otherwise transient and reversible PARylation of the DPCs. As the UPS is a key repair mechanism for TOP1-DPCs, we investigated the impact of TOP1-DPC PARylation on the proteasome and found that the proteasome is unable to associate with and digest PARylated TOP1-DPCs. In addition, PARylation recruits the deubiquitylating enzyme USP7 to reverse the ubiquitylation of PARylated TOP1-DPCs. Our work identifies PARG as repair factor for TOP1-DPCs by enabling the proteasomal digestion of TOP1-DPCs. It also suggests the potential regulatory role of PARylation for the repair of a broad range of DPCs.

Validating TDP1 as an Inhibition Target for the Development of Chemosensitizers for Camptothecin-Based Chemotherapy Drugs

Cancer chemotherapy sensitizers hold the key to maximizing the potential of standard anticancer treatments. We have a long-standing interest in developing and validating inhibitors of the DNA repair enzyme tyrosyl-DNA phosphodiesterase 1 (TDP1) as chemosensitizers for topoisomerase I poisons such as topotecan. Herein, by using thieno[2,3-b]pyridines, a class of TDP1 inhibitors, we showed that the inhibition of TDP1 can restore sensitivity to topotecan, results that are supported by TDP1 knockout cell experiments using CRISPR/Cas9. However, we also found that the restored sensitivity towards topoisomerase I inhibitors is likely regulated by multiple complementary DNA repair pathways. Our results showed that one of these pathways is likely modulated by PARP1, although it is also possible that other redundant and partially overlapping pathways may be involved in the DNA repair process. Our work thus raises the prospect of targeting multiple DNA repair pathways to increase the sensitivity to topoisomerase I inhibitors.

Acidic tumor microenvironment-sensitive liposomes enhance colorectal cancer therapy by acting on both tumor cells and cancer-associated fibroblasts

Cancer-associated fibroblasts (CAFs) play a crucial role in facilitating tumor invasion and metastasis, which act as the "soil" in the tumor microenvironment (TME). Accordingly, it would be a promising strategy to enhance the antitumor effect by killing both tumor cells and CAFs simultaneously. Herein, novel TME acid-responsive liposomes for co-delivery of IRI and 398 (IRI&398-s-LPs) were developed, in which the rapid release of both drugs could be triggered under acidic conditions. Notably, a CT-26/3T3 cell co-culture system was used to mimic the real TME both in vitro and in vivo. Cellular immunofluorescence revealed that IRI&398-s-LPs could efficiently decrease the activation of CAFs. In vitro cytotoxicity evaluation demonstrated that IRI&398-s-LPs exhibited higher cytotoxicity than the other liposomal formulations in the CT-26 and CT-26/3T3 cell co-culture system. In vivo NIRF imaging showed that the IRI&398-s-LPs could increase drug accumulation in the tumor sites. Furthermore, IRI&398-s-LPs not only presented superior in vivo anti-tumor activity in CT-26 bearing BALB/c mice, but also enhanced the effect in CT-26/3T3 cell bearing mice with decreased collagen and CAF biomarker expression. Furthermore, IRI&398-s-LPs also presented superior anti-metastatic efficiency in a lung metastasis model. These results indicated that this combinational strategy for eliminating both tumor cells and CAFs provides a new approach for cancer therapy, and the prepared TME-responsive liposomes for co-delivery of drugs hold promising clinical application prospects.

Pinus kesiya Royle ex Gordon induces apoptotic cell death in hepatocellular carcinoma HepG2 cell via intrinsic pathway by PARP and Topoisomerase I suppression

Pinus kesiya Royle ex Gordon (PK), widely found in Southeast Asia, has been traditionally used for the treatment of several illnesses. Our previous studies showed that PK was highly cytotoxicity against liver cancer cells. The detailed mechanism of anticancer action of 50% hydro-ethanolic extract of PK's twig was, therefore, investigated in hepatocellular carcinoma HepG2 cells. Cytotoxicity of PK was determined by using NR assay, followed by determination of the mode of cell death by flow cytometry. The apoptosis-inducing effect was determined based on caspases activity, mitochondria membrane potential change, and expression of proteins related to apoptosis by western blot. The biomolecular alteration in the PK-treated HepG2 cells was investigated by FTIR microspectroscopy. Inhibition of topoisomerase I enzyme was determined by using DNA relaxation assay. Results showed that PK displayed high selective cytotoxicity and induced apoptosis against HepG2. FTIR microspectroscopy indicated that PK altered major biomolecules in HepG2 different from melphalan (a positive control), indicating a different mechanism of anticancer action. PK induced apoptotic cell death through the intrinsic pathway by increasing caspases 9 and 3/7 activity, increasing Bax, and decreasing Bcl-2 expression leading to mitochondrial membrane potential changes. PK also inhibited Top I and PARP activity that triggered an intrinsic apoptotic pathway. The phytochemical test presented terpenoids (i.e., α-pinene confirmed by GC-MS), alkaloids, steroids, xanthone, reducing sugar, and saponin. α-Pinene exhibited low cytotoxicity against HepG2, therefore, several terpene derivatives may work synergistically for inducing apoptosis. Our data demonstrated that PK has the potential for further study with chemotherapeutic purposes.

The role of Trop2 in prostate cancer: an oncogene, biomarker, and therapeutic target

Prostate cancer remains the second leading cause of cancer-associated deaths amongst American men. Trop2, a cell surface glycoprotein, correlates with poor clinical outcome and is highly expressed in metastatic, treatment-resistant prostate cancer. High levels of Trop2 are prognostic for biochemical recurrence. Trop2 regulates tumor growth and metastatic ability of prostate cancer. Moreover, overexpression of Trop2 drives the transdifferentiation to neuroendocrine phenotype in prostate cancer. In addition, Trop2 is overexpressed across epithelial cancers and has emerged as a promising therapeutic target in various solid epithelial cancers. The FDA (Food and Drug Administration) recently approved the use of a Trop2-targeting ADC (antibody-drug conjugate), Sacituzumab Govitecan (IMMU-132), for metastatic, triple-negative breast cancer with at least two prior therapies. Here, we review the role of Trop2 in prostate tumorigenesis and its potential as a promising biomarker and therapeutic target for prostate cancer.

Top1-PARP1 association and beyond: from DNA topology to break repair

Selective trapping of human topoisomerase 1 (Top1) on the DNA (Top1 cleavage complexes; Top1cc) by specific Top1-poisons triggers DNA breaks and cell death. Poly(ADP-ribose) polymerase 1 (PARP1) is an early nick sensor for trapped Top1cc. New mechanistic insights have been developed in recent years to rationalize the importance of PARP1 beyond the repair of Top1-induced DNA breaks. This review summarizes the progress in the molecular mechanisms of trapped Top1cc-induced DNA damage, PARP1 activation at DNA damage sites, PAR-dependent regulation of Top1 nuclear dynamics, and PARP1-associated molecular network for Top1cc repair. Finally, we have discussed the rationale behind the synergy between the combination of Top1 poison and PARP inhibitors in cancer chemotherapies, which is independent of the ‘PARP trapping’ phenomenon.

Thiol-Disulfide Exchange Reaction Promoted Highly Efficient Cellular Uptake of Pyridyl Disulfide Appended Nonionic Polymers

The intracellular transport of molecules, macromolecules or materials is a key step in probing cellular structure and function, as well as regulating a plethora of physical and chemical events for treating disease. This communication reveals direct cellular uptake of pyridyl-disulfide (Py-Ds)-conjugated nonionic and biocompatible macromolecules with the aid of rapid exchange of the highly reactive Py-Ds groups with exofacial cell-surface thiols. Confocal microscopy and flow cytometry analysis confirmed highly efficient cellular uptake of Py-Ds-appended polymers (>50 % in 15 min) by avoiding lysosome as a consequence of thiol-disulfide exchange in the cell surface. In contrast, a control polymer lacking the Py-Ds group followed caveolae-mediated endocytosis. Other control polymers containing either the pyridine group (but not disulfide) or the disulfide group (but not pyridine) revealed significantly low cellular uptake, and thus essential role of the highly reactive Py-Ds group was established beyond doubt.

Inhibition of nicotinamide phosphoribosyltransferase (NAMPT) with OT-82 induces DNA damage, cell death, and suppression of tumor growth in preclinical models of Ewing sarcoma

NAMPT mediates the rate-limiting step of the NAD salvage pathway, which maintains cellular bioenergetics and provides a necessary substrate for functions essential to rapidly proliferating cancer cells. In this study, we evaluated the efficacy and mechanisms of action of OT-82, a novel, high-potency NAMPT inhibitor with a favorable toxicity profile, in preclinical models of Ewing sarcoma (EWS), an aggressive pediatric malignancy with previously reported selective sensitivity to NAMPT inhibition. We show that OT-82 decreased NAD concentration and impaired proliferation of EWS cells in a dose-dependent manner, with IC₅₀ values in the single-digit nanomolar range. Notably, genetic depletion of NAMPT phenocopied pharmacological inhibition. On-target activity of OT-82 was confirmed with the addition of NMN, the product of NAMPT, which rescued NAD concentration and EWS cellular viability. Mechanistically, OT-82 treatment resulted in impaired DNA damage repair through loss of PARP activity, G2 cell-cycle arrest, and apoptosis in EWS cells. Additional consequences of OT-82 treatment included reduction of glycolytic and mitochondrial activity. In vivo, OT-82 impaired tumor growth and prolonged survival in mice bearing EWS xenografts. Importantly, antitumor effect correlated with pharmacodynamic markers of target engagement. Furthermore, combining low-dose OT-82 with low doses of agents augmenting DNA damage demonstrated enhanced antitumor activity in vitro and in vivo. Thus, OT-82 treatment represents a potential novel targeted approach for the clinical treatment of EWS.

Autophagy-Independent Functions of the Autophagy Machinery

Macroautophagy (herein referred to as autophagy) is an evolutionary ancient mechanism that culminates with the lysosomal degradation of superfluous or potentially dangerous cytosolic entities. Over the past 2 decades, the molecular mechanisms underlying several variants of autophagy have been characterized in detail. Accumulating evidence suggests that most, if not all, components of the molecular machinery for autophagy also mediate autophagy-independent functions. Here, we discuss emerging data on the non-autophagic functions of autophagy-relevant proteins.

The molecular landscape of ETMR at diagnosis and relapse

Embryonal tumours with multilayered rosettes (ETMRs) are aggressive paediatric embryonal brain tumours with a universally poor prognosis1. Here we collected 193 primary ETMRs and 23 matched relapse samples to investigate the genomic landscape of this distinct tumour type. We found that patients with tumours in which the proposed driver C19MC2-4 was not amplified frequently had germline mutations in DICER1 or other microRNA-related aberrations such as somatic amplification of miR-17-92 (also known as MIR17HG). Whole-genome sequencing revealed that tumours had an overall low recurrence of single-nucleotide variants (SNVs), but showed prevalent genomic instability caused by widespread occurrence of R-loop structures. We show that R-loop-associated chromosomal instability can be induced by the loss of DICER1 function. Comparison of primary tumours and matched relapse samples showed a strong conservation of structural variants, but low conservation of SNVs. Moreover, many newly acquired SNVs are associated with a mutational signature related to cisplatin treatment. Finally, we show that targeting R-loops with topoisomerase and PARP inhibitors might be an effective treatment strategy for this deadly disease.

Tyrosyl-DNA Phosphodiesterase I N-Terminal Domain Modifications and Interactions Regulate Cellular Function

The conserved eukaryotic DNA repair enzyme Tyrosyl-DNA phosphodiesterase I (Tdp1) removes a diverse array of adducts from the end of DNA strand breaks. Tdp1 specifically catalyzes the hydrolysis of phosphodiester linked DNA-adducts. These DNA lesions range from damaged nucleotides to peptide-DNA adducts to protein-DNA covalent complexes and are products of endogenously or exogenously induced insults or simply failed reaction products. These adducts include DNA inserted ribonucleotides and non-conventional nucleotides, as well as covalent reaction intermediates of DNA topoisomerases with DNA and a Tdp1-DNA adduct in trans. This implies that Tdp1 plays a role in maintaining genome stability and cellular homeostasis. Dysregulation of Tdp1 protein levels or catalysis shifts the equilibrium to genome instability and is associated with driving human pathologies such as cancer and neurodegeneration. In this review, we highlight the function of the N-terminal domain of Tdp1. This domain is understudied, structurally unresolved, and the least conserved in amino acid sequence and length compared to the rest of the enzyme. However, over time it emerged that the N-terminal domain was post-translationally modified by, among others, phosphorylation, SUMOylation, and Ubiquitinoylation, which regulate Tdp1 protein interactions with other DNA repair associated proteins, cellular localization, and Tdp1 protein stability.

SCAN1-TDP1 trapping on mitochondrial DNA promotes mitochondrial dysfunction and mitophagy

A homozygous mutation of human tyrosyl-DNA phosphodiesterase 1 (TDP1) causes the neurodegenerative syndrome, spinocerebellar ataxia with axonal neuropathy (SCAN1). TDP1 hydrolyzes the phosphodiester bond between DNA 3'-end and a tyrosyl moiety within trapped topoisomerase I (Top1)-DNA covalent complexes (Top1cc). TDP1 is critical for mitochondrial DNA (mtDNA) repair; however, the role of mitochondria remains largely unknown for the etiology of SCAN1. We demonstrate that mitochondria in cells expressing SCAN1-TDP1 (TDP1^H493R) are selectively trapped on mtDNA in the regulatory non-coding region and promoter sequences. Trapped TDP1^H493R-mtDNA complexes were markedly increased in the presence of the Top1 poison (mito-SN38) when targeted selectively into mitochondria in nanoparticles. TDP1^H493R-trapping accumulates mtDNA damage and triggers Drp1-mediated mitochondrial fission, which blocks mitobiogenesis. TDP1^H493R prompts PTEN-induced kinase 1-dependent mitophagy to eliminate dysfunctional mitochondria. SCAN1-TDP1 in mitochondria creates a pathological state that allows neurons to turn on mitophagy to rescue fit mitochondria as a mechanism of survival.

Tyrosyl-DNA Phosphodiesterase 1 and Topoisomerase I Activities as Predictive Indicators for Glioblastoma Susceptibility to Genotoxic Agents

Glioblastoma (GBM) patients have an estimated survival of ~15 months with treatment, and the standard of care only modestly enhances patient survival. Identifying biomarkers representing vulnerabilities may allow for the selection of efficacious chemotherapy options to address personalized variations in GBM tumors. Irinotecan targets topoisomerase I (TOP1) by forming a ternary DNA-TOP1 cleavage complex (TOP1cc), inducing apoptosis. Tyrosyl-DNA phosphodiesterase 1 (TDP1) is a crucial repair enzyme that may reduce the effectiveness of irinotecan. We treated GBM cell lines with increasing concentrations of irinotecan and compared the IC₅₀ values. We found that the TDP1/TOP1 activity ratio had the strongest correlation (Pearson correlation coefficient R = 0.972, based on the average from three sets of experiments) with IC₅₀ values following irinotecan treatment. Increasing the TDP1/TOP1 activity ratio by the ectopic expression of wild-type TDP1 increased in irinotecan IC₅₀, while the expression of the TDP1 catalytic-null mutant did not alter the susceptibility to irinotecan. The TDP1/TOP1 activity ratio may be a new predictive indicator for GBM vulnerability to irinotecan, allowing for the selection of individual patients for irinotecan treatment based on risk-benefit. Moreover, TDP1 inhibitors may be a novel combination treatment with irinotecan to improve GBM patient responsiveness to genotoxic chemotherapies.

Formulation of injectable glycyrrhizic acid-hydroxycamptothecin micelles as new generation of DNA topoisomerase I inhibitor for enhanced antitumor activity

To develop a new drug delivery system is one of the useful approaches to break through the limitation of hydroxycamptothecin (HCPT), a typical DNA topoisomerase I (Topo I) inhibitor in clinical appliance. Injectable glycyrrhizic acid-hydroxycamptothecin (GL-HCPT) micelles that were able to dramatically improve the solubility and stability of HCPT were prepared through self-assembly process and evaluated both in vitro and in vivo. With a mean particle size (PS) of 105.7 ± 9.7 nm and a drug loading (DL) of 9.0 ± 1.5%, GL-HCPT micelles were rapidly internalized by HepG2 cells after 1 h, significantly increasing the intracellular accumulation of HCPT. Compared with the current used HCPT injection and HCPT/GL physical mixture, GL-HCPT micelles showed enhanced antitumor activity against liver cancer cells (HepG2 and Huh7) as well as a superior suppression on the tumor growth of HepG2 tumor bearing mice. Interestingly, GL-HCPT micelles gathered in liver and simultaneously reduced the drug accumulation in normal tissues, thereby exhibiting minimal cytotoxicity to human normal liver cells (LO2). Therefore, we offered a convenient and cost-effective strategy to construct an intravenous drug delivery system (GL-HCPT micelles) as new generation of DNA Topo I inhibitor for enhanced cancer chemotherapy.

Recent Advances in Use of Topoisomerase Inhibitors in Combination Cancer Therapy

Inhibitors targeting human topoisomerase I and topoisomerase II alpha have provided a useful chemotherapy option for the treatment of many patients suffering from a variety of cancers. While the treatment can be effective in many patient cases, use of these human topoisomerase inhibitors is limited by side-effects that can be severe. A strategy of employing the topoisomerase inhibitors in combination with other treatments can potentially sensitize the cancer to increase the therapeutic efficacy and reduce resistance or adverse side effects. The combination strategies reviewed here include inhibitors of DNA repair, epigenetic modifications, signaling modulators and immunotherapy. The ongoing investigations on cellular response to topoisomerase inhibitors and newly initiated clinical trials may lead to adoption of novel cancer therapy regimens that can effectively stop the proliferation of cancer cells while limiting the development of resistance.

Effects of SRSF1 on subnuclear localization of topoisomerase I

Subnuclear localization of topoisomerase I (top I) is determined by its DNA relaxation activity and a net of its interactions with in majority unidentified nucleolar and nucleoplasmic elements. Here, we recognized SR protein SRSF1 (Serine/arginine-rich splicing factor 1, previously known as SF2/ASF) as a new element of the net. In HeLa cells, overexpression of SRSF1 recruited top I to the nucleoplasm whereas its silencing concentrated it in the nucleolus. Effect of SRSF1 was independent of top I relaxation activity and was the best pronounced for the mutant inactive in relaxation reaction. In HCT116 cells where top I was not released from the nucleolus upon halting relaxation activity, it was also not relocated by elevated level of SRSF1. Out of remaining SR proteins, SRSF5, SRSF7, and SRSF9 did not influence the localization of top I in HeLa cells whereas overexpression of SRSF2, SRSF3, SRSF6, and partly SRSF4 concentrated top I in the nucleolus, most possibly due to the reduction of the SRSF1 accessibility. Specific effect of SRSF1 was exerted because of its distinct RS domain. Silencing of SRSF1 compensated the deletion of the top I N-terminal region, individually responsible for nucleoplasmic localization of the mutant, and restored the wild-type phenotype of deletion mutant localization. SRSF1 was essential for the camptothecin-induced clearance from the nucleolus. These results suggest a possible role of SRSF1 in establishing partition of top I between the nucleolus and the nucleoplasm in some cell types with distinct combinations of SR proteins levels.

Targeting Topoisomerase I in the Era of Precision Medicine

Irinotecan and topotecan have been widely used as anticancer drugs for the past 20 years. Because of their selectivity as topoisomerase I (TOP1) inhibitors that trap TOP1 cleavage complexes, camptothecins are also widely used to elucidate the DNA repair pathways associated with DNA-protein cross-links and replication stress. This review summarizes the basic molecular mechanisms of action of TOP1 inhibitors, their current use, and limitations as anticancer agents. We introduce new therapeutic strategies based on novel TOP1 inhibitor chemical scaffolds including the indenoisoquinolines LMP400 (indotecan), LMP776 (indimitecan), and LMP744, and on tumor-targeted delivery TOP1 inhibitors using liposome, PEGylation, and antibody-drug conjugates. We also address how tumor-specific determinants such as homologous recombination defects (HRD and BRCAness) and Schlafen 11 (SLFN11) expression can be used to guide clinical application of TOP1 inhibitors in combination with DNA damage response inhibitors including PARP, ATR, CHEK1, and ATM inhibitors.

Discovery and Mechanistic Study of Tailor-Made Quinoline Derivatives as Topoisomerase 1 Poison with Potent Anticancer Activity

To overcome chemical limitations of camptothecin (CPT), we report design, synthesis, and validation of a quinoline-based novel class of topoisomerase 1 (Top1) inhibitors and establish that compound 28 ( N-(3-(1 H-imidazol-1-yl)propyl)-6-(4-methoxyphenyl)-3-(1,3,4-oxadiazol-2-yl)quinolin-4-amine) exhibits the highest potency in inhibiting human Top1 activity with an IC₅₀ value of 29 ± 0.04 nM. Compound 28 traps Top1-DNA cleavage complexes (Top1ccs) both in the in vitro cleavage assays and in live cells. Point mutation of Top1-N722S fails to trap compound 28-induced Top1cc because of its inability to form a hydrogen bond with compound 28. Unlike CPT, compound 28 shows excellent plasma serum stability and is not a substrate of P-glycoprotein 1 (permeability glycoprotein) advancing its potential anticancer activity. Finally, we provide evidence that compound 28 overcomes the chemical instability of CPT in human breast adenocarcinoma cells through generation of persistent and less reversible Top1cc-induced DNA double-strand breaks as detected by γH2AX foci immunostaining after 5 h of drug removal.

Sensitization of Cancer Cells to Radiation and Topoisomerase I Inhibitor Camptothecin Using Inhibitors of PARP and Other Signaling Molecules

Radiation and certain anticancer drugs damage DNA, resulting in apoptosis induction in cancer cells. Currently, the major limitations on the efficacy of such therapies are development of resistance and adverse side effects. Sensitization is an important strategy for increasing therapeutic efficacy while minimizing adverse effects. In this manuscript, we review possible sensitization strategies for radiation and anticancer drugs that cause DNA damage, focusing especially on modulation of damage repair pathways and the associated reactions.

Enhanced anti-hepatocarcinoma efficacy by GLUT1 targeting and cellular microenvironment-responsive PAMAM-camptothecin conjugate

The efficient targeting of drugs to tumor cell and subsequent rapid drug release remain primary challenges in the development of nanomedicines for cancer therapy. Here, we constructed a glucose transporter 1 (GLUT1)-targeting and tumor cell microenvironment-sensitive drug release Glucose–PEG–PAMAM-s-s–Camptothecin-Cy7 (GPCC) conjugate to tackle the dilemma. The conjugate was characterized by a small particle size, spherical shape, and glutathione (GSH)-sensitive drug release. In vitro tumor targeting was explored in monolayer (2D) and multilayer tumor spheroid (3D) HepG2 cancer cell models (GLUT1⁺). The cellular uptake of GPCC was higher than that in the control groups and that in normal L02 cells (GLUT1⁻), likely due to the conjugated glucose moiety. Moreover, the GPCC conjugate exhibited stronger cytotoxicity, higher S arrest and enhanced apoptosis and necrosis rate in HepG2 cells than control groups but not L02 cells. However, the cytotoxicity of GPCC was lower than that of free CPT, which could be explained by the slower release of CPT from the GPCC compared with free CPT. Additional in vivo tumor targeting experiments demonstrated the superior tumor-targeting ability of the GPCC conjugate, which significantly accumulated in tumor meanwhile minimize in normal tissues compared with control groups. The GPCC conjugate showed better pharmacokinetic properties, enabling a prolonged circulation time and increased camptothecin area under the curve (AUC). These features contributed to better therapeutic efficacy and lower toxicity in H22 hepatocarcinoma tumor-bearing mice. The GLUT1-targeting, GSH-sensitive GPCC conjugate provides an efficient, safe and economic approach for tumor cell targeted drug delivery.

PRMT5-mediated arginine methylation of TDP1 for the repair of topoisomerase I covalent complexes

Human tyrosyl-DNA phosphodiesterases (TDP) hydrolyze the phosphodiester bond between DNA and the catalytic tyrosine of Top1 to excise topoisomerase I cleavage complexes (Top1cc) that are trapped by camptothecin (CPT) and by genotoxic DNA alterations. Here we show that the protein arginine methyltransferase PRMT5 enhances the repair of Top1cc by direct binding to TDP1 and arginine dimethylation of TDP1 at residues R361 and R586. Top1-induced replication-mediated DNA damage induces TDP1 arginine methylation, enhancing its 3'- phosphodiesterase activity. TDP1 arginine methylation also increases XRCC1 association with TDP1 in response to CPT, and the recruitment of XRCC1 to Top1cc DNA damage foci. PRMT5 knockdown cells exhibit defective TDP1 activity with marked elevation in replication-coupled CPT-induced DNA damage and lethality. Finally, methylation of R361 and R586 stimulate TDP1 repair function and promote cell survival in response to CPT. Together, our findings provide evidence for the importance of PRMT5 for the post-translational regulation of TDP1 and repair of Top1cc.

A phase 1 dose-escalation study of veliparib with bimonthly FOLFIRI in patients with advanced solid tumours

Background

Veliparib is a potent poly(ADP-ribose) polymerase inhibitor. This phase 1 study aimed to establish the maximum tolerated dose (MTD) and recommended phase 2 dose (RP2D) of veliparib combined with various FOLFIRI regimens in patients with solid tumours.

Methods

Patients received veliparib (10–270 mg BID, days 1–5, 15–19) and FOLFIRI (days 1–3, 15–17) in three regimens containing 5-fluorouracil 2,400 mg/m²: irinotecan 150 mg/m² and folinic acid 400 mg/m² (part 1); irinotecan 180 mg/m², folinic acid 400 mg/m², and 5-fluorouracil 400 mg/m² bolus (part 2), or irinotecan 180 mg/m² (part 3). The RP2D was further evaluated in safety expansion cohorts. Preliminary antitumour activity was also assessed.

Results

Ninety-two patients received ≥1 veliparib dose. MTD was not reached; RP2D was set at 200 mg BID veliparib plus FOLFIRI (without 5-fluorouracil bolus). Most common treatment-emergent adverse events were neutropenia (66.3%), diarrhoea, and nausea (60.9% each). Dose-limiting toxicities (n = 4) were grade 3 gastritis and grade 4 neutropenia and febrile neutropenia. Veliparib exposure was dose-proportional, with no effects on the pharmacokinetics of FOLFIRI components. Fifteen patients had a partial response (objective response rate, 17.6%).

Conclusions

The acceptable safety profile and preliminary antitumour activity of veliparib plus FOLFIRI support further evaluation of this combination.

Neutral Porphyrin Derivative Exerts Anticancer Activity by Targeting Cellular Topoisomerase I (Top1) and Promotes Apoptotic Cell Death without Stabilizing Top1-DNA Cleavage Complexes

Camptothecin (CPT) selectively traps topoisomerase 1-DNA cleavable complexes (Top1cc) to promote anticancer activity. Here, we report the design and synthesis of a new class of neutral porphyrin derivative 5,10-bis(4-carboxyphenyl)-15, 20-bis(4-dimethylaminophenyl)porphyrin (compound 8) as a potent catalytic inhibitor of human Top1. In contrast to CPT, compound 8 reversibly binds with the free enzyme and inhibits the formation of Top1cc and promotes reversal of the preformed Top1cc with CPT. Compound 8 induced inhibition of Top1cc formation in live cells was substantiated by fluorescence recovery after photobleaching (FRAP) assays. We established that MCF7 cells treated with compound 8 trigger proteasome-mediated Top1 degradation, accumulate higher levels of reactive oxygen species (ROS), PARP1 cleavage, oxidative DNA fragmentation, and stimulate apoptotic cell death without stabilizing apoptotic Top1-DNA cleavage complexes. Finally, compound 8 shows anticancer activity by targeting cellular Top1 and preventing the enzyme from directly participating in the apoptotic process.

A novel metallogel based approach to synthesize (Mn, Cu) doped ZnS quantum dots and labeling of MCF-7 cancer cells

Labelling of adenocarcinoma (MCF-7) cells by the Al-PDCA gel based (Mn, Cu) doped QDs.

XRCC1-mediated repair of strand breaks independent of PNKP binding

Repair of DNA-protein crosslinks and oxidatively damaged DNA base lesions generates intermediates with nicks or gaps with abnormal and blocked 3'-phosphate and 5'-OH ends that prevent the activity of DNA polymerases and ligases. End cleaning in mammalian cells by Tdp1 and PNKP produces the conventional 3'-OH and 5'-phosphate DNA ends suitable for completion of repair. This repair function of PNKP is facilitated by its binding to the scaffold protein XRCC1, and phosphorylation of XRCC1 by CK2 at several consensus sites enables PNKP binding and recruitment to DNA damage. To evaluate this documented repair process, a phosphorylation mutant of XRCC1, designed to eliminate PNKP binding, was stably expressed in Xrcc1-/- mouse fibroblast cells. Analysis of PNKP-GFP accumulation at micro-irradiation induced damage confirmed that the XRCC1 phosphorylation mutant failed to support efficient PNKP recruitment, whereas there was rapid recruitment in cells expressing wild-type XRCC1. Recruitment of additional fluorescently-tagged repair factors PARP-1-YFP, GFF-XRCC1, PNKP-GFP and Tdp1-GFP to micro-irradiation induced damage was assessed in wild-type XRCC1-expressing cells. PARP-1-YFP recruitment was best fit to two exponentials, whereas kinetics for the other proteins were fit to a single exponential. The similar half-times of recruitment suggest that XRCC1 may be recruited with other proteins possibly as a pre-formed complex. Xrcc1-/- cells are hypersensitive to the DNA-protein cross-link inducing agent camptothecin (CPT) and the DNA oxidative agent H2O2 due in part to compromised PNKP-mediated repair. However, cells expressing the PNKP interaction mutant of XRCC1 demonstrated marked reversal of CPT hypersensitivity. This reversal represents XRCC1-dependent repair in the absence of the phosphorylation-dependent PNKP recruitment and suggests either an XRCC1-independent mechanism of PNKP recruitment or a functional back-up pathway for cleaning of blocked DNA ends.

The Relationship Between Single-Nucleotide Polymorphisms, the Expression of DNA Damage Response Genes, and Hepatocellular Carcinoma in a Polish Population

The molecular mechanism of hepatocellular carcinoma (HCC) is related to DNA damage caused by oxidative stress products induced by hepatitis B virus (HBV) or C (HCV) infection and exposure to environmental pollutants. Single-nucleotide polymorphisms (SNPs) of DNA damage response (DDR) genes may influence individual susceptibility to environmental risk factors and affect DNA repair efficacy, which, in turn, can influence the risk of HCC. The study evaluates a panel of 15 SNPs in 11 DDR genes (XRCC1, XRCC3, XPD, MUTYH, LIG1, LIG3, hOGG1, PARP1, NFIL1, FEN1, and APEX1) in 65 HCC patients, 50 HBV- and 50 HCV-infected non-cancerous patients, and 50 healthy controls. It also estimates the mRNA expression of nine DDR genes in cancerous and adjacent healthy liver tissues. Two of the investigated polymorphisms (rs1052133 and rs13181) were associated with HCC risk. For all investigated genes, the level of mRNA was significantly lower in HCC cancer tissue than in non-cancerous liver tissue. Seven of the investigated polymorphisms were statistically related to gene expression in cancer tissues. The disruption of DDR genes may be responsible for hepatocellular transformation in HCV-infected patients.

Role of Protein Linked DNA Breaks in Cancer

Topoisomerases are a group of specialized enzymes that function to maintain DNA topology by introducing transient DNA breaks during transcription and replication. As a result of abortive topoisomerases activity, topoisomerases catalytic intermediates may be trapped on the DNA forming topoisomerase cleavage complexes (Topcc). Topoisomerases trapping on the DNA is the mode of action of several anticancer drugs, it lead to formation of protein linked DAN breaks (PDBs). PDBs are now considered as one of the most dangerous forms of endogenous DNA damage and a major threat to genomic stability. The repair of PDBs involves both the sensing and repair pathways. Unsuccessful repair of PDBs leads to different signs of genomic instabilities such as chromosomal rearrangements and cancer predisposition. In this chapter we will summarize the role of topoisomerases induced PDBs, identification and signaling, repair, role in transcription. We will also discuss the role of PDBs in cancer with a special focus on prostate cancer.