Topoisomerase-mediated chromosomal break repair: an emerging player in many games

Cdc48/p97 segregase: Spotlight on DNA-protein crosslinks

The ATP-dependent molecular chaperone Cdc48 (in yeast) and its human counterpart p97 (also known as VCP), are essential for a variety of cellular processes, including the removal of DNA-protein crosslinks (DPCs) from the DNA. Growing evidence demonstrates in the last years that Cdc48/p97 is pivotal in targeting ubiquitinated and SUMOylated substrates on chromatin, thereby supporting the DNA damage response. Along with its cofactors, notably Ufd1-Npl4, Cdc48/p97 has emerged as a central player in the unfolding and processing of DPCs. This review introduces the detailed structure, mechanism and cellular functions of Cdc48/p97 with an emphasis on the current knowledge of DNA-protein crosslink repair pathways across several organisms. The review concludes by discussing the potential therapeutic relevance of targeting p97 in DPC repair.

Molecular Pathology of Prostate Cancer

Molecular profiling studies have shed new light on the complex biology of prostate cancer. Genomic studies have highlighted that structural rearrangements are among the most common recurrent alterations. In addition, both germline and somatic mutations in DNA repair genes are enriched in patients with advanced disease. Primary prostate cancer has long been known to be multifocal, but recent studies demonstrate that a large fraction of prostate cancer shows evidence of multiclonality, suggesting that genetically distinct, independently arising tumor clones coexist. Metastatic prostate cancer shows a high level of morphologic and molecular diversity, which is associated with resistance to systemic therapies. The resulting high level of intratumoral heterogeneity has important implications for diagnosis and poses major challenges for the implementation of molecular studies. Here we provide a concise review of the molecular pathology of prostate cancer, highlight clinically relevant alterations, and discuss opportunities for molecular testing.

Identification of FTY720 and COH29 as novel topoisomerase I catalytic inhibitors by experimental and computational studies

The development of novel topoisomerase I (TOP1) inhibitors is crucial for overcoming the drawbacks and limitations of current TOP1 poisons. Here, we identified two potential TOP1 inhibitors, namely, FTY720 (a sphingosine 1-phosphate antagonist) and COH29 (a ribonucleotide reductase inhibitor), through experimental screening of known active compounds. Biological experiments verified that FTY720 and COH29 were nonintercalative TOP1 catalytic inhibitors that did not induce the formation of DNA-TOP1 covalent complexes. Molecular docking revealed that FTY720 and COH29 interacted favorably with TOP1. Molecular dynamics simulations revealed that FTY720 and COH29 could affect the catalytic domain of TOP1, thus resulting in altered DNA-binding cavity size. The alanine scanning and interaction entropy identified Arg536 as a hotspot residue. In addition, the bioinformatics analysis predicted that FTY720 and COH29 could be effective in treating malignant breast tumors. Biological experiments verified their antitumor activities using MCF-7 breast cancer cells. Their combinatory effects with TOP1 poisons were also investigated. Further, FTY720 and COH29 were found to cause less DNA damage compared with TOP1 poisons. The findings provide reliable lead compounds for the development of novel TOP1 catalytic inhibitors and offer new insights into the potential clinical applications of FTY720 and COH29 in targeting TOP1.

Irinotecan dosing and pharmacogenomics: a comprehensive exploration based on UGT1A1 variants and emerging insights

Irinotecan is a critical anticancer drug used to treat metastatic colorectal cancer and advanced pancreatic ductal adenocarcinoma by obstructing topoisomerase 1; however, it can cause minor-to-severe and life-threatening adverse effects. UDP glucuronosyltransferase family 1 member A1 (UGT1A1) polymorphisms increase the risk of irinotecan-induced neutropenia and diarrhea. Hence, screening for UGT1A1 polymorphisms before irinotecan-based chemotherapy is recommended to minimize toxicity, whereas liposomes offer the potential to deliver irinotecan with fewer side effects in patients with pancreatic ductal adenocarcinoma. This review presents a comprehensive overview of the effects of genotype-guided dosing of irinotecan on UGT1A1*28 and UGT1A1*6 variants, incorporating pharmacogenomic research, optimal regimens for metastatic colorectal and pancreatic cancer treatment using irinotecan, guidelines for toxicity reduction, and an evaluation of the cost-effectiveness of UGT1A1 genotype testing.

The story of a decade: Genomics, functional genomics, and molecular breeding in Brassica napus

Rapeseed (Brassica napus L.) is one of the major global sources of edible vegetable oil and is also used as a feed and pioneer crop and for sightseeing and industrial purposes. Improvements in genome sequencing and molecular marker technology have fueled a boom in functional genomic studies of major agronomic characters such as yield, quality, flowering time, and stress resistance. Moreover, introgression and pyramiding of key functional genes have greatly accelerated the genetic improvement of important traits. Here we summarize recent progress in rapeseed genomics and genetics, and we discuss effective molecular breeding strategies by exploring these findings in rapeseed. These insights will extend our understanding of the molecular mechanisms and regulatory networks underlying agronomic traits and facilitate the breeding process, ultimately contributing to more sustainable agriculture throughout the world.

TDP1 suppresses chromosomal translocations and cell death induced by abortive TOP1 activity during gene transcription

DNA topoisomerase I (TOP1) removes torsional stress by transiently cutting one DNA strand. Such cuts are rejoined by TOP1 but can occasionally become abortive generating permanent protein-linked single strand breaks (SSBs). The repair of these breaks is initiated by tyrosyl-DNA phosphodiesterase 1 (TDP1), a conserved enzyme that unlinks the TOP1 peptide from the DNA break. Additionally, some of these SSBs can result in double strand breaks (DSBs) either during replication or by a poorly understood transcription-associated process. In this study, we identify these DSBs as a source of genome rearrangements, which are suppressed by TDP1. Intriguingly, we also provide a mechanistic explanation for the formation of chromosomal translocations unveiling an error-prone pathway that relies on the MRN complex and canonical non-homologous end-joining. Collectively, these data highlight the threat posed by TOP1-induced DSBs during transcription and demonstrate the importance of TDP1-dependent end-joining in protecting both gene transcription and genome stability.

Oxidative DNA damage and repair at non-coding regulatory regions

DNA breaks at protein-coding sequences are well-established threats to tissue homeostasis and maintenance. They arise from the exposure to intracellular and environmental genotoxins, causing damage in one or two strands of the DNA. DNA breaks have been also reported in non-coding regulatory regions such as enhancers and promoters. They arise from essential cellular processes required for gene transcription, cell identity and function. One such process that has attracted recent attention is the oxidative demethylation of DNA and histones, which generates abasic sites and DNA single-strand breaks. Here, we discuss how oxidative DNA breaks at non-coding regulatory regions are generated and the recently reported role of NuMA (nuclear mitotic apparatus) protein in promoting transcription and repair at these regions.

Exploration of s new biomarker in osteosarcoma and association with clinical outcomes: TOP2A+ cancer associated fibroblasts

BACKGROUND

Osteosarcoma (OS) is the leading malignant primary bone tumor in young adults and children and has a high mortality rate. Cancer-associated fibroblasts (CAFs) are major components of the tumor microenvironment, influencing cancer progression and metastasis. However, there is no systematic study on the role of CAF in OS.

METHODS

We collected six OS patients' single-cell RNA sequencing data from the TISCH database, which was processed using the Seurat package. We selected gene sets from the well-known MSigDB database and resorted to the clusterprofiler package for gene set enrichment analysis (GSEA). The least absolute shrinkage and selection operator (LASSO) regression model was used for identification of the variables. Receiver operating characteristic and decision curve analyses were utilized for determining the efficacy of the monogram model.

RESULTS

TOP2A+ CAFs was recognized as the carcinogenic CAFs subset, given its intense interaction with OS malignant cells and association with the critical cancer driver pathway. We intersected the differentially expressed genes of TOP2A+ CAFs with the prognostic genes selected from 88 OS samples. The acquired gene set was selected using the LASSO regression model and integrated with clinical factors to obtain a monogram model of high prognosis predicting power (area under the curve of 5 year survival at 0.883). Functional enrichment analysis revealed the detailed difference between two risk groups.

CONCLUSION

We identified TOP2A+ CAFs as a subset of oncogenic CAFs in OS. Based on differentially expressed genes derived from TOP2A+ CAFs, combined with bulk transcriptome prognostic genes, we constructed a risk model that can efficiently predict OS prognosis. Collectively, our study may provide new insights for future studies to elucidate the role of CAF in OS.

Identification of N-(3-(methyl(3-(orotic amido)propyl)amino)propyl) oleanolamide as a novel topoisomerase I catalytic inhibitor by rational design, molecular dynamics simulation, and biological evaluation

DNA topoisomerase I (TOP1) catalytic inhibitors are a promising class of antitumor agents. Oleanolic acid derivatives are potential TOP1 catalytic inhibitors. However, their inhibitory activity still needs to be enhanced, and the stability and hotspot residue sites of their interaction with TOP1 remain to be elucidated. Herein, a novel oleanolic acid derivative, OA4 (N-(3-(methyl(3-(orotic amido)propyl)amino)propyl)oleanolamide), was identified by rational design. Subsequently, molecular dynamics simulations were performed to explore the stability and conformational dynamics of the TOP1-OA4 complex. The molecular mechanics/generalized Born surface area method calculated the binding free energy and predicted Arg488, Ile535, and His632 to be hotspot residues. Biological experiments verified that OA4 is a nonintercalative TOP1 catalytic inhibitor. OA4 exhibits better proliferation inhibitory activity against tumor cells than normal cells. Furthermore, OA4 can induce apoptosis and effectively suppress the proliferation and migration of cancer cells. This work provides new insights for the development of novel TOP1 catalytic inhibitors.

Chemical-induced phase transition and global conformational reorganization of chromatin

Chemicals or drugs can accumulate within biomolecular condensates formed through phase separation in cells. Here, we use super-resolution imaging to search for chemicals that induce phase transition within chromatin at the microscale. This microscopic screening approach reveals that adriamycin (doxorubicin) - a widely used anticancer drug that is known to interact with chromatin - specifically induces visible local condensation and global conformational change of chromatin in cancer and primary cells. Hi-C and ATAC-seq experiments systematically and quantitatively demonstrate that adriamycin-induced chromatin condensation is accompanied by weakened chromatin interaction within topologically associated domains, compartment A/B switching, lower chromatin accessibility, and corresponding transcriptomic changes. Mechanistically, adriamycin complexes with histone H1 and induces phase transition of H1, forming fibrous aggregates in vitro. These results reveal a phase separation-driven mechanism for a chemotherapeutic drug.

Identification of drought-tolerant hub genes in Iranian KC-2226 genotype of Aegilops tauschii using transcriptomic analysis

Aegilops tauschii, as a donor of D genome to the bread wheat with a valuable source of resistance to different biotic and abiotic stresses, is used to improve the quality of wheat cultivars. Every genotype has a specific genetic content, the investigation of which can lead to the identification of useful genes such as stress tolerance genes, including drought. Therefore, 23 genotypes of Ae. tauschii were selected to evaluate their morphological and physiological traits under greenhouse conditions. Among them, a superior tolerant genotype (KC-2226) was chosen for transcriptomic analysis. Our result showed that 5007 and 3489 genes were deferentially up- and downregulated, respectively. Upregulated genes were involved in photosynthesis, glycolysis/gluconeogenesis, and amino acid biosynthesis whereas downregulated genes were often engaged in DNA synthesis, replication, repair and topological changes. The result of protein-protein interaction network analysis showed that AT1G76550 (1.46), AT1G20950 (1.42), IAR4 (1.19), and PYD2 (1.16) among upregulated genes and THY-1 (44), PCNA1 (41) and TOPII (22) among down-regulated genes had the highest interactions with other genes. In conclusion, Ae. tauschii employs elevated transcription of specific genes involved in photosynthesis, glycolysis and gluconeogenesis and amino acid biosynthesis pathways rather than genes active in DNA synthesis and repair to provide the energy needed for the plant to survive under stress conditions.

Overview of the development of HBV small molecule inhibitors

Like tuberculosis and Acquired Immune Deficiency Syndrome (AIDS), hepatitis B is a globally recognized major public health threat. Although there are many small-molecule drugs for the treatment of hepatitis B, the approved drugs cannot eradicate the pathogenic culprit covalently closed circular DNA in patients, so the patients need long-term medication to control HBV amplification. Driven by a high unmet medical need, many pharmaceutical companies and research institutions have been engaged in the development of anti-HBV drugs to achieve a functional cure for chronic hepatitis B as soon as possible. This review summarizes the pathogenesis of hepatitis B virus and the research progress in the development of anti-HBV small molecule drugs, and introduces the cccDNA formation and transcription inhibitors and core inhibitors in detail, especially emphasizes the role of chinese herbal medicine in the treatment of chronic hepatitis B. Furthermore, this review proposes three potential strategies for cccDNA eradication in the future. We believe this review will provide meaningful guidance to achieve a functional cure for viral hepatitis B in the future.

Cold-induced inhibition of photosynthesis-related genes integrated by a TOP6 complex in rice mesophyll cells

Photosynthesis is the most temperature-sensitive process in the plant kingdom, but how the photosynthetic pathway responds during low-temperature exposure remains unclear. Herein, cold stress (4°C) induced widespread damage in the form DNA double-stranded breaks (DSBs) in the mesophyll cells of rice (Oryza sativa), subsequently causing a global inhibition of photosynthetic carbon metabolism (PCM) gene expression. Topoisomerase genes TOP6A3 and TOP6B were induced at 4°C and their encoded proteins formed a complex in the nucleus. TOP6A3 directly interacted with KU70 to inhibit its binding to cold-induced DSBs, which was facilitated by TOP6B, finally blocking the loading of LIG4, a component of the classic non-homologous end joining (c-NHEJ) pathway. The repression of c-NHEJ repair imposed by cold extended DSB damage signaling, thus prolonging the inhibition of photosynthesis in leaves. Furthermore, the TOP6 complex negatively regulated 13 crucial PCM genes by directly binding to their proximal promoter regions. Phenotypically, TOP6A3 overexpression exacerbated the γ-irradiation-triggered suppression of PCM genes and led to the hypersensitivity of photosynthesis parameters to cold stress, dependent on the DSB signal transducer ATM. Globally, the TOP6 complex acts as a signal integrator to control PCM gene expression and synchronize cold-induced photosynthesis inhibition, which modulates carbon assimilation rates immediately in response to changes in ambient temperature.

Nucleophagy contributes to genome stability through degradation of type II topoisomerases A and B and nucleolar components

The nuclear architecture of mammalian cells can be altered as a consequence of anomalous accumulation of nuclear proteins or genomic alterations. Most of the knowledge about nuclear dynamics comes from studies on cancerous cells. How normal healthy cells maintain genome stability, avoiding accumulation of nuclear damaged material, is less understood. Here, we describe that primary mouse embryonic fibroblasts develop a basal level of nuclear buds and micronuclei, which increase after etoposide-induced DNA double-stranded breaks. Both basal and induced nuclear buds and micronuclei colocalize with the autophagic proteins BECN1 and LC3B (also known as MAP1LC3B) and with acidic vesicles, suggesting their clearance by nucleophagy. Some of the nuclear alterations also contain autophagic proteins and type II DNA topoisomerases (TOP2A and TOP2B), or the nucleolar protein fibrillarin, implying they are also targets of nucleophagy. We propose that basal nucleophagy contributes to genome and nuclear stability, as well as in response to DNA damage.

Untangling the roles of TOP2A and TOP2B in transcription and cancer

Type II topoisomerases (TOP2) are conserved regulators of chromatin topology that catalyze reversible DNA double-strand breaks (DSBs) and are essential for maintaining genomic integrity in diverse dynamic processes such as transcription, replication, and cell division. While controlled TOP2-mediated DSBs are an elegant solution to topological constraints of DNA, DSBs also contribute to the emergence of chromosomal translocations and mutations that drive cancer. The central importance of TOP2 enzymes as frontline chemotherapeutic targets is well known; however, their precise biological functions and impact in cancer development are still poorly understood. In this review, we provide an updated overview of TOP2A and TOP2B in the regulation of chromatin topology and transcription, and discuss the recent discoveries linking TOP2 activities with cancer pathogenesis.

Mitochondrial topoisomerase 1 inhibition induces topological DNA damage and T cell dysfunction in patients with chronic viral infection

T cells are crucial for controlling viral infections; however, the mechanisms that dampen their responses during viral infections remain incompletely understood. Here, we studied the role and mechanisms of mitochondrial topoisomerase 1 (Top1mt) inhibition in mitochondrial dysfunction and T cell dysregulation using CD4 T cells from patients infected with HCV or HIV and compared it with CD4 T cells from healthy individuals following treatment with Top1 inhibitor - camptothecin (CPT). We found that Top1mt protein levels and enzymatic activity are significantly decreased, along with Top1 cleavage complex (Top1cc) formation, in mitochondria of CD4 T cells from HCV- and HIV-infected patients. Notably, treatment of healthy CD4 T cells with CPT caused similar changes, including inhibition of Top1mt, accumulation of Top1cc in mitochondria, increase in PARP1 cleavage, and decrease in mtDNA copy numbers. These molecular changes resulted in mitochondrial dysfunction, T cell dysregulation, and programmed cell death through multiple signaling pathways, recapitulating the phenotype we detected in CD4 T cells from HCV- and HIV-infected patients. Moreover, treatment of CD4 T cells from HCV or HIV patients with CPT further increased cellular and mitochondrial reactive oxygen species (ROS) production and cell apoptosis, demonstrating a critical role for Top1 in preventing mtDNA damage and cell death. These results provide new insights into the molecular mechanisms underlying immune dysregulation during viral infection and indicate that Top1 inhibition during chronic HCV or HIV infection can induce mtDNA damage and T cell dysfunction. Thus, reconstituting Top1mt protein may restore the mtDNA topology and T cell functions in humans with chronic viral infection.

Mechanism of action of non-camptothecin inhibitor Genz-644282 in topoisomerase I inhibition

Topoisomerase I (TOP1) controls the topological state of DNA during DNA replication, and its dysfunction due to treatment with an inhibitor, such as camptothecin (CPT), causes replication arrest and cell death. Although CPT has excellent cytotoxicity, it has the disadvantage of instability under physiological conditions. Therefore, new types of TOP1 inhibitor have attracted particular attention. Here, we characterised the effect of a non-camptothecin inhibitor, Genz-644282 (Genz). First, we found that treatment with Genz showed cytotoxicity by introducing double-strand breaks (DSBs), which was suppressed by co-treatment with aphidicolin. Genz-induced DSB formation required the functions of TOP1. Next, we explored the advantages of Genz over CPT and found it was effective against CPT-resistant TOP1 carrying either N722S or N722A mutation. The effect of Genz was also confirmed at the cellular level using a CPT-resistant cell line carrying N722S mutation in the TOP1 gene. Moreover, we found arginine residue 364 plays a crucial role for the binding of Genz. Because tyrosine residue 723 is the active centre for DNA cleavage and re-ligation by TOP1, asparagine residue 722 plays crucial roles in the accessibility of the drug. Here, we discuss the mechanism of action of Genz on TOP1 inhibition.

Topoisomerase IIA in adult NSCs regulates SVZ neurogenesis by transcriptional activation of Usp37

Topoisomerase IIA (TOP2a) has traditionally been known as an important nuclear enzyme that resolves entanglements and relieves torsional stress of DNA double strands. However, its function in genomic transcriptional regulation remains largely unknown, especially during adult neurogenesis. Here, we show that TOP2a is preferentially expressed in neurogenic niches in the brain of adult mice, such as the subventricular zone (SVZ). Conditional knockout of Top2a in adult neural stem cells (NSCs) of the SVZ significantly inhibits their self-renewal and proliferation, and ultimately reduces neurogenesis. To gain insight into the molecular mechanisms by which TOP2a regulates adult NSCs, we perform RNA-sequencing (RNA-Seq) plus chromatin immunoprecipitation sequencing (ChIP-Seq) and identify ubiquitin-specific protease 37 (Usp37) as a direct TOP2a target gene. Importantly, overexpression of Usp37 is sufficient to rescue the impaired self-renewal ability of adult NSCs caused by Top2a knockdown. Taken together, this proof-of-principle study illustrates a TOP2a/Usp37-mediated novel molecular mechanism in adult neurogenesis, which will significantly expand our understanding of the function of topoisomerase in the adult brain.

The Role of Protein Arginine Methyltransferases in DNA Damage Response

In response to DNA damage, cells have developed a sophisticated signaling pathway, consisting of DNA damage sensors, transducers, and effectors, to ensure efficient and proper repair of damaged DNA. During this process, posttranslational modifications (PTMs) are central events that modulate the recruitment, dissociation, and activation of DNA repair proteins at damage sites. Emerging evidence reveals that protein arginine methylation is one of the common PTMs and plays critical roles in DNA damage response. Protein arginine methyltransferases (PRMTs) either directly methylate DNA repair proteins or deposit methylation marks on histones to regulate their transcription, RNA splicing, protein stability, interaction with partners, enzymatic activities, and localization. In this review, we summarize the substrates and roles of each PRMTs in DNA damage response and discuss the synergistic anticancer effects of PRMTs and DNA damage pathway inhibitors, providing insight into the significance of arginine methylation in the maintenance of genome integrity and cancer therapies.

Nanomedicines targeting the Inflammasome as a promising therapeutic approach for cell senescence

Technological advancements in the present era have enhanced drug discovery and development. Nanomedicines are valuable pharmacotherapeutic tools against several diseases and disorders including aging related disorders. The mechanistic association between nanomedicines and molecular modulation have been investigated by many researchers. Notwithstanding the availability of tremendous amount of data, role of nanomedicines in aging related disorders intending inflammasome transfiguration have not been thoroughly reviewed till now. In the present review, we discuss the application of nanomedicines in aging related disorders. Further, we highlight the recent updates on modulated upstream and downstream signalling molecules of inflammasome cascade due to nanomedicines. The review will benefit researchers targeting nanomedicines as a therapeutic approach towards treatment age related disorders through inflammasome inflection.

Phosphonic acid-containing inhibitors of tyrosyl-DNA phosphodiesterase 1

Tyrosyl-DNA phosphodiesterase 1 (TDP1) repairs stalled type I topoisomerase (TOP1)-DNA complexes by hydrolyzing the phosphodiester bond between the TOP1 Y723 residue and the 3′-phosphate of its DNA substrate. Although TDP1 antagonists could potentially reduce the dose of TOP1 inhibitors needed to achieve effective anticancer effects, the development of validated TDP1 inhibitors has proven to be challenging. This may, in part, be due to the open and extended nature of the TOP1 substrate binding region. We have previously reported imidazopyrazines and imidazopyridines that can inhibit TDP1 catalytic function in vitro. We solved the TDP1 crystal structures with bound inhibitors of this class and found that the dicarboxylic acid functionality within the N-(3,4-dicarboxyphenyl)-2-diphenylimidazo [1,2-a]pyridin-3-amine platform overlaps with aspects of phosphoryl substrate recognition. Yet phosphonic acids could potentially better-replicate cognate TOP1-DNA substrate binding interactions than carboxylic acids. As reported herein, we designed phosphonic acid-containing variants of our previously reported carboxylic acid-containing imidazopyrazine and imidazopyridine inhibitors and effected their synthesis using one-pot Groebke–Blackburn–Bienayme multicomponent reactions. We obtained crystal structures of TDP1 complexed with a subset of inhibitors. We discuss binding interactions of these inhibitors within the context of phosphate-containing substrate and carboxylic acid-based inhibitors. These compounds represent a new structural class of small molecule ligands that mimic aspects of the 3′-processed substrate that results from TDP1 catalysis.

Organocatalyzed umpolung addition for synthesis of heterocyclic-fused arylidene-imidazolones as anticancer agents

A strategy of "Nature-to-new" with iterative scaffold-hopping was considered for investigation of privileged ring/functional motif-elaborated analogs of natural aurones. An organocatalyzed umpolung chemistry based method was established for molecular-diversity feasible synthesis of title class of chemotypes i.e. (Z)-2-Arylideneimidazo[1,2-a]pyridinones and (Z)-2-Arylidenebenzo[d]imidazo[2,1-b]thiazol-3-ones. Various biophysical experiments indicated their important biological properties. The analogs showed characteristic anticancer activities with efficiency more than an anticancer drug. The compounds induced apoptosis with arrest in the S phase of the cell cycle regulation. The compounds' significant effect in up/down-regulation of various apoptotic proteins, an apoptosis cascade, and the inhibition of topoisomerases-mediated DNA relaxation process was identified. The analysis of the structure-activity relationship, interference with biological events and the drug-likeness physicochemical properties of the compounds in the acceptable window indicated distinctive medicinal molecule-to-properties of the investigated chemotypes.

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.

Etoposide-induced DNA damage is increased in p53 mutants: identification of ATR and other genes that influence effects of p53 mutations on Top2-induced cytotoxicity

The functional status of the tumor suppressor p53 is a critical component in determining the sensitivity of cancer cells to many chemotherapeutic agents. DNA topoisomerase II (Top2) plays essential roles in DNA metabolism and is the target of FDA approved chemotherapeutic agents. Topoisomerase targeting drugs convert the enzyme into a DNA damaging agent and p53 influences cellular responses to these agents. We assessed the impact of the loss of p53 function on the formation of DNA damage induced by the Top2 poison etoposide. Using human HCT116 cells, we found resistance to etoposide in cell growth assays upon the functional loss of p53. Nonetheless, cells lacking fully functional p53 were etoposide hypersensitive in clonogenic survival assays. This complex role of p53 led us to directly examine the effects of p53 status on topoisomerase-induced DNA damage. A deficiency in functional p53 resulted in elevated levels of the Top2 covalent complexes (Top2cc) in multiple cell lines. Employing genome-wide siRNA screens, we identified a set of genes for which reduced expression resulted in enhanced synthetic lethality upon etoposide treatment of p53 defective cells. We focused on one hit from this screen, ATR, and showed that decreased expression sensitized the p53-defective cells to etoposide in all assays and generated elevated levels of Top2cc in both p53 proficient and deficient cells. Our findings suggest that a combination of etoposide treatment with functional inactivation of DNA repair in p53 defective cells could be used to enhance the therapeutic efficacy of Top2 targeting agents.

Harnessing Synthetic Lethal Interactions for Personalized Medicine

Two genes are said to have synthetic lethal (SL) interactions if the simultaneous mutations in a cell lead to lethality, but each individual mutation does not. Targeting SL partners of mutated cancer genes can kill cancer cells but leave normal cells intact. The applicability of translating this concept into clinics has been demonstrated by three drugs that have been approved by the FDA to target PARP for tumors bearing mutations in BRCA1/2. This article reviews applications of the SL concept to translational cancer medicine over the past five years. Topics are (1) exploiting the SL concept for drug combinations to circumvent tumor resistance, (2) using synthetic lethality to identify prognostic and predictive biomarkers, (3) applying SL interactions to stratify patients for targeted and immunotherapy, and (4) discussions on challenges and future directions.

The protease SPRTN and SUMOylation coordinate DNA-protein crosslink repair to prevent genome instability

DNA-protein crosslinks (DPCs) are a specific type of DNA lesion in which proteins are covalently attached to DNA. Unrepaired DPCs lead to genomic instability, cancer, neurodegeneration, and accelerated aging. DPC proteolysis was recently identified as a specialized pathway for DPC repair. The DNA-dependent protease SPRTN and the 26S proteasome emerged as two independent proteolytic systems. DPCs are also repaired by homologous recombination (HR), a canonical DNA repair pathway. While studying the cellular response to DPC formation, we identify ubiquitylation and SUMOylation as two major signaling events in DNA replication-coupled DPC repair. DPC ubiquitylation recruits SPRTN to repair sites, promoting DPC removal. DPC SUMOylation prevents DNA double-strand break formation, HR activation, and potentially deleterious genomic rearrangements. In this way, SUMOylation channels DPC repair toward SPRTN proteolysis, which is a safer pathway choice for DPC repair and prevention of genomic instability.

Estradiol increases risk of topoisomerase IIβ-mediated DNA strand breaks to initiate Xp11.2 translocation renal cell carcinoma

Background

Xp11.2 translocation renal cell carcinoma (tRCC) is defined by translocation of the transcription factor E3 (TFE3) gene located on chromosome Xp11.2. Due to the high incidence in women, 17β-estradiol (E2) may be a factor influencing TFE3 breaks, and the topoisomerase II (TOP2) poison is considered one of the important risk factors in mediating DNA breaks. In this study, we investigated the potential pathogenesis of Xp11.2 tRCC using the renal epithelial cell line HK-2.

Methods

Immunofluorescence assay was performed to analyze DNA breaks by quantifying phosphorylation of H2AX (γH2AX), and the micronuclei (MN) assay was designed for monitoring chromosome breaks. The chromatin immunoprecipitation (CHIP) was used to detect whether proteins bound to specific DNA site, and the co-immunoprecipitation (Co-IP) was used to confirm whether proteins bound to other proteins. In some experiments, siRNA and shRNA were transfected to knockdown target genes.

Results

Our results demonstrated that DNA double-strand breaks were mediated by TOP2β in HK-2 cells, and this process could be amplified through estrogen receptor α (ERα)-dependent pathway induced by E2. After performing translocation site analysis using target region sequencing data in two Xp11.2 tRCC cell lines and ten Xp11.2 tRCC patients, we confirmed that TOP2β and ERα could both bind to TFE3 translocation sites directly to mediate DNA breaks in a E2-dependent manner. However, TOP2β and ERα were not observed to have direct interaction, indicating that their collaborative may be implemented in other ways. Besides, TFE3 was found to be upregulated through NRF1 with increasing E2 concentration, which could increase the risk of TFE3 breaks.

Conclusion

Our results indicate that E2 amplifies TOP2β-mediated TFE3 breaks by ERα-dependent pathway, and E2 upregulates TFE3 by NRF1 to increase the risk of TFE3 breaks. This suggests that E2 is an important pathogenic factor for Xp11.2 tRCC pathogenesis.

Keeping intracellular DNA untangled: A new role for condensin?

The DNA-passage activity of topoisomerase II accidentally produces DNA knots and interlinks within and between chromatin fibers. Fortunately, these unwanted DNA entanglements are actively removed by some mechanism. Here we present an outline on DNA knot formation and discuss recent studies that have investigated how intracellular DNA knots are removed. First, although topoisomerase II is able to minimize DNA entanglements in vitro to below equilibrium values, it is unclear whether such capacity performs equally in vivo in chromatinized DNA. Second, DNA supercoiling could bias topoisomerase II to untangle the DNA. However, experimental evidence indicates that transcriptional supercoiling of intracellular DNA boosts knot formation. Last, cohesin and condensin could tighten DNA entanglements via DNA loop extrusion (LE) and force their dissolution by topoisomerase II. Recent observations indicate that condensin activity promotes the removal of DNA knots during interphase and mitosis. This activity might facilitate the spatial organization and dynamics of chromatin.

Nonhomologous DNA end joining of nucleosomal substrates in a purified system

The nonhomologous DNA end joining pathway is required for repair of most double-strand breaks in the mammalian genome. Here we use a purified biochemical NHEJ system to compare the joining of free DNA with recombinant mononucleosomal and dinucleosomal substrates to investigate ligation and local DNA end resection. We find that the nucleosomal state permits ligation in a manner dependent on the presence of free DNA flanking the nucleosome core particle. Local resection at DNA ends by the Artemis:DNA-PKcs nuclease complex is completely suppressed in all mononucleosome substrates regardless of flanking DNA up to a length of 14 bp. Like mononucleosomes, dinucleosomes lacking flanking free DNA are not joined. Therefore, the nucleosomal state imposes severe constraints on NHEJ nuclease and ligase activities.

Transcription-associated topoisomerase 2α (TOP2A) activity is a major effector of cytotoxicity induced by G-quadruplex ligands

G-quadruplexes (G4) are non-canonical DNA structures found in the genome of most species including human. Small molecules stabilizing these structures, called G4 ligands, have been identified and, for some of them, shown to induce cytotoxic DNA double-strand breaks. Through the use of an unbiased genetic approach, we identify here topoisomerase 2α (TOP2A) as a major effector of cytotoxicity induced by two clastogenic G4 ligands, pyridostatin and CX-5461, the latter molecule currently undergoing phase I/II clinical trials in oncology. We show that both TOP2 activity and transcription account for DNA break production following G4 ligand treatments. In contrast, clastogenic activity of these G4 ligands is countered by topoisomerase 1 (TOP1), which limits co-transcriptional G4 formation, and by factors promoting transcriptional elongation. Altogether our results support that clastogenic G4 ligands act as DNA structure-driven TOP2 poisons at transcribed regions bearing G4 structures.

Transcription-associated DNA breaks and cancer: A matter of DNA topology

Transcription is an essential cellular process but also a major threat to genome integrity. Transcription-associated DNA breaks are particularly detrimental as their defective repair can induce gene mutations and oncogenic chromosomal translocations, which are hallmarks of cancer. The past few years have revealed that transcriptional breaks mainly originate from DNA topological problems generated by the transcribing RNA polymerases. Defective removal of transcription-induced DNA torsional stress impacts on transcription itself and promotes secondary DNA structures, such as R-loops, which can induce DNA breaks and genome instability. Paradoxically, as they relax DNA during transcription, topoisomerase enzymes introduce DNA breaks that can also endanger genome integrity. Stabilization of topoisomerases on chromatin by various anticancer drugs or by DNA alterations, can interfere with transcription machinery and cause permanent DNA breaks and R-loops. Here, we review the role of transcription in mediating DNA breaks, and discuss how deregulation of topoisomerase activity can impact on transcription and DNA break formation, and its connection with cancer.

High Temperature Drives Topoisomerase Mediated Chromosomal Break Repair Pathway Choice

Simple Summary

Targeting topoisomerases has been widely used as anticancer therapeutics. Exposure to high temperature (hyperthermia) protects cells from the cytotoxic effect of topoisomerase-targeting therapeutics, yet the mechanism remains unknown. Here, we report that hyperthermia inhibits the nucleolytic processing of topoisomerase-induced DNA damage and drives repair to a more faithful pathway mediated by TDP1 and TDP2. We further show that hyperthermia suppresses topoisomerase-induced chromosomal translocation and hallmarks of inflammation, which has broad implications in cancer development and therapy.

Abstract

Cancer-causing mutations often arise from inappropriate DNA repair, yet acute exposure to DNA damage is widely used to treat cancer. The challenge remains in how to specifically induce excessive DNA damage in cancer cells while minimizing the undesirable effects of genomic instability in noncancerous cells. One approach is the acute exposure to hyperthermia, which suppresses DNA repair and synergizes with radiotherapy and chemotherapy. An exception, however, is the protective effect of hyperthermia on topoisomerase targeting therapeutics. The molecular explanation for this conundrum remains unclear. Here, we show that hyperthermia suppresses the level of topoisomerase mediated single- and double-strand breaks induced by exposure to topoisomerase poisons. We further uncover that, hyperthermia suppresses hallmarks of genomic instability induced by topoisomerase targeting therapeutics by inhibiting nuclease activities, thereby channeling repair to error-free pathways driven by tyrosyl-DNA phosphodiesterases. These findings provide an explanation for the protective effect of hyperthermia from topoisomerase-induced DNA damage and may help to explain the inverse relationship between cancer incidence and temperature. They also pave the way for the use of controlled heat as a therapeutic adjunct to topoisomerase targeting therapeutics.

Mechanisms of damage tolerance and repair during DNA replication

Accurate duplication of chromosomal DNA is essential for the transmission of genetic information. The DNA replication fork encounters template lesions, physical barriers, transcriptional machinery, and topological barriers that challenge the faithful completion of the replication process. The flexibility of replisomes coupled with tolerance and repair mechanisms counteract these replication fork obstacles. The cell possesses several universal mechanisms that may be activated in response to various replication fork impediments, but it has also evolved ways to counter specific obstacles. In this review, we will discuss these general and specific strategies to counteract different forms of replication associated damage to maintain genomic stability.

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.

Diversity and Functions of Type II Topoisomerases

The DNA double helix provides a simple and elegant way to store and copy genetic information. However, the processes requiring the DNA helix strands separation, such as transcription and replication, induce a topological side-effect - supercoiling of the molecule. Topoisomerases comprise a specific group of enzymes that disentangle the topological challenges associated with DNA supercoiling. They relax DNA supercoils and resolve catenanes and knots. Here, we review the catalytic cycles, evolution, diversity, and functional roles of type II topoisomerases in organisms from all domains of life, as well as viruses and other mobile genetic elements.

Canonical non-homologous end-joining promotes genome mutagenesis and translocations induced by transcription-associated DNA topoisomerase 2 activity

DNA topoisomerase II (TOP2) is a major DNA metabolic enzyme, with important roles in replication, transcription, chromosome segregation and spatial organisation of the genome. TOP2 is the target of a class of anticancer drugs that poison the DNA-TOP2 transient complex to generate TOP2-linked DNA double-strand breaks (DSBs). The accumulation of DSBs kills tumour cells but can also result in genome instability. The way in which topoisomerase activity contributes to transcription remains unclear. In this work we have investigated how transcription contributes to TOP2-dependent DSB formation, genome instability and cell death. Our results demonstrate that gene transcription is an important source of abortive TOP2 activity. However, transcription does not contribute significantly to apoptosis or cell death promoted by TOP2-induced DSBs. On the contrary: transcription-dependent breaks greatly contribute to deleterious mutations and translocations, and can promote oncogenic rearrangements. Importantly, we show that TOP2-induced genome instability is mediated by mutagenic canonical non-homologous end joining whereas homologous recombination protects cells against these insults. Collectively, these results uncover mechanisms behind deleterious effects of TOP2 abortive activity during transcription, with relevant implications for chemotherapy.

Dual Processing of R-Loops and Topoisomerase I Induces Transcription-Dependent DNA Double-Strand Breaks

Although accumulation of DNA damage and genomic instability in resting cells can cause neurodegenerative disorders, our understanding of how transcription produces DNA double-strand breaks (DSBs) is limited. Transcription-blocking topoisomerase I cleavage complexes (TOP1ccs) are frequent events that prime DSB production in non-replicating cells. Here, we report a mechanism of their formation by showing that they arise from two nearby single-strand breaks (SSBs) on opposing DNA strands: one SSB from the removal of transcription-blocking TOP1ccs by the TDP1 pathway and the other from the cleavage of R-loops by endonucleases, including XPF, XPG, and FEN1. Genetic defects in TOP1cc removal (TDP1, PNKP, and XRCC1) or in the resolution of R-loops (SETX) enhance DSB formation and prevent their repair. Such deficiencies cause neurological disorders. Owing to the high frequency of TOP1cc trapping and the widespread distribution of R-loops, these persistent transcriptional DSBs could accumulate over time in neuronal cells, contributing to the neurodegenerative diseases.

Analysis of plant-derived phytochemicals as anti-cancer agents targeting cyclin dependent kinase-2, human topoisomerase IIa and vascular endothelial growth factor receptor-2

Cancer is caused by a variety of pathways, involving numerous types of enzymes. Among them three enzymes i.e. Cyclin-dependent kinase-2 (CDK-2), Human topoisomerase IIα, and Vascular Endothelial Growth Factor Receptor-2 (VEGFR-2) are three of the most common enzymes that are involved in the cancer development. Although many chemical drugs are already available in the market for cancer treatment, plant sources are known to contain a wide variety of agents that are proved to possess potential anticancer activity. In this experiment, total thirty phytochemicals were analyzed against the mentioned three enzymes using different tools of bioinformatics and in silico biology like molecular docking study, drug likeness property experiment, ADME/T test, PASS prediction, and P450 site of metabolism prediction as well as DFT calculation to determine the three best ligands among them that have the capability to inhibit the mentioned enzymes. From the experiment, Epigallocatechin gallate was found to be the best ligand to inhibit CDK-2, Daidzein showed the best inhibitory activities towards the Human topoisomerase IIα, and Quercetin was predicted to be the best agent against VEGFR-2. They were also predicted to be quite safe and effective agents to treat cancer. However, more in vivo and in vitro analyses are required to finally confirm their safety and efficacy in this regard.

Programmed DNA Damage and Physiological DSBs: Mapping, Biological Significance and Perturbations in Disease States

DNA double strand breaks (DSBs) are known to be the most toxic and threatening of the various types of breaks that may occur to the DNA. However, growing evidence continuously sheds light on the regulatory roles of programmed DSBs. Emerging studies demonstrate the roles of DSBs in processes such as T and B cell development, meiosis, transcription and replication. A significant recent progress in the last few years has contributed to our advanced knowledge regarding the functions of DSBs is the development of many next generation sequencing (NGS) methods, which have considerably advanced our capabilities. Other studies have focused on the implications of programmed DSBs on chromosomal aberrations and tumorigenesis. This review aims to summarize what is known about DNA damage in its physiological context. In addition, we will examine the advancements of the past several years, which have made an impact on the study of genome landscape and its organization.

DNA fragmentation of human spermatozoa: Simple assessment of single- and double-strand DNA breaks and their respective dynamic behavioral response

BACKGROUND

Procedures to detect sperm DNA fragmentation (SDF), like the sperm chromatin dispersion (SCD) test, determine the "global" SDF without discriminating between spermatozoa with single-strand DNA breaks only (SDF-SSBs) and those containing double-strand DNA breaks (SDF-DSBs).

OBJECTIVES

(a) To validate a test to distinguish human spermatozoa with massive DSBs (DSB-SCD assay), (b) to study the baseline SDF-SSBs and SDF-DSBs, and (c) to assess their dynamics in vitro.

MATERIALS AND METHODS

(a) SDF-DSBs were determined by visualization of diffused DNA fragments from spermatozoa lysed under non-denaturing conditions. This was validated by in vitro incubation with DNase I and the comet assay. (b) Baseline SDF-DSBs and SDF-SSBs were determined in ejaculates from 95 males. (c) Their dynamic appearance was studied in samples untreated or exposed to hyperthermia, acidic pH, nitric oxide released by sodium nitroprusside (SNP), and the metabolic energy inhibitors 2-deoxy-D-glucose and antimycin A.

RESULTS

(a) DNase I and comet assay experiments confirmed that the assay successfully determined SDF-DSBs. (b) The higher the SDF of the semen sample, the higher the frequency of SSBs, whereas DSBs behaved independently. Abnormal samples showed higher SDF than normozoospermic, the difference being only significant for SDF-SSBs. (c) During the first hours of incubation, the linear rate of increase in SDF-SSBs was 3.7 X higher than that of SDF-DSBs. All hazardous agents accelerated the SDF rate when compared to untreated spermatozoa, primarily being associated with SDF-SSBs. SNP treatment was the most damaging, rapidly inducing spermatozoa with SSBs which progressively evolved to DSBs. Remarkably, this phenomenon was also evidenced after acute SNP exposure, revealing cryptic sperm damage.

CONCLUSION

The DSBs-SCD is an easy complement for SDF assessment. The dynamic study of SSBs and DSBs may improve the evaluation of sperm quality in clinical settings, particularly "unmasking" the presence of non-specific cryptic sperm damage that might otherwise go undetected.

Tyrosyl-DNA phosphodiesterase 2 (TDP2) repairs topoisomerase 1 DNA-protein crosslinks and 3'-blocking lesions in the absence of tyrosyl-DNA phosphodiesterase 1 (TDP1)

Topoisomerase I (TOP1) resolves DNA topology during replication and transcription. The enzyme forms an intermediate TOP1 cleavage complex (TOP1cc) through transient TOP1-DNA-protein crosslinks. Camptothecin is a frontline anticancer agent that freezes this reaction intermediate, leading to the generation of irreversible TOP1ccs that act as 3'-blocking lesions. It is widely accepted that TOP1cc is repaired via a two-step pathway involving proteasomal degradation of TOP1cc to the crosslinked peptide, followed by removal of the TOP1cc-derived peptide from DNA by tyrosyl-DNA phosphodiesterase 1 (TDP1). In the present study, we developed an assay system to estimate repair kinetics of TOP1cc separately in the first and second steps, using monoclonal antibodies against the TOP1 protein and the TOP1 catalytic site peptide-DNA complex, respectively. Although TDP1-deficient (TDP1^-/-) TK6 cells had normal kinetics of the first step, a delay in the kinetics of the second step was observed relative to that in wild-type cells. Tyrosyl-DNA phosphodiesterase 2 (TDP2) reportedly promotes the repair of TOP1-induced DNA damage in the absence of TDP1. The present assays additionally demonstrated that TDP2 promotes the second, but not the first, step of TOP1cc repair in the absence of TDP1. We also analyzed sensitivities of TK6 cells with deficiencies in TDP1 and/or TDP2 to agents that produce 3' -blocking lesions. These experiments showed that TDP1^-/-TDP2^-/- cells were more sensitive to the agents Azidothymidine (zidovudine), Cytarabine, Abacavir, Gemcitabine, and Trifluridine than TDP1^-/- or TDP2^-/- cells. Taken together, our findings confirm the roles of TDP2 in the repair of 3'-blocking lesions.

Investigation of the role of VHL-HIF signaling in DNA repair and apoptosis in zebrafish

pVHL is a tumor suppressor. The lack of its function leads to various tumors, among which ccRCC (clear cell renal cell carcinoma) has the most serious outcome due to its resistance to chemotherapies and radiotherapies. Although HIF promotes the progression of ccRCC, the precise mechanism by which the loss of VHL leads to tumor initiation remains unclear. We exploited two zebrafish vhl mutants, vhl and vll, and Tg (phd3:: EGFP)ⁱ¹⁴⁴ fish to identify crucial functions of Vhl in tumor initiation. Through the mutant analysis, we found that the role of pVHL in DNA repair is conserved in zebrafish Vll. Interestingly, we also discovered that Hif activation strongly suppressed genotoxic stress induced DNA repair defects and apoptosis in vll and brca2 mutants and in embryos lacking ATM activity. These results suggest the potential of HIF as a clinical modulator that can protect cells from accumulating DNA damage and apoptosis which can lead to cancers and neurodegenerative disorders.

Nonhomologous end joining and homologous recombination involved in luteolin-induced DNA damage in DT40 cells

Luteolin (3',4',5,7-tetrahydroxyflavone), a naturally occurring flavonoid, has been shown to have anticancer activity in many types of cancer cell lines. The anticancer capacity of luteolin may be related to its ability to induce DNA double-strand breaks (DSBs). Here, we used DT40 cells to determine whether nonhomologous end joining (NHEJ) and homologous recombination (HR) are involved in the repair mechanism of luteolin-induced DNA damage. Cells defective in Ku70 (an enzyme associated with NHEJ) or Rad54 (an enzyme essential for HR) were hypersensitive and presented more apoptosis in response to luteolin. Moreover, the sensitivity and apoptosis of Ku70^-/- and Rad54^-/- cells were associated with increased DNA damage when the numbers of γ-H2AX foci and chromosomal aberrations (CAs) were compared with those from WT cells. Additionally, after treatment with luteolin, Ku70^-/- cells presented more Top2 covalent cleavage complexes (Top2cc). These results indicated that luteolin induced DSBs in DT40 cells and demonstrated that both NHEJ and HR participated in the repair of luteolin-induced DSBs, which might be related to the inhibition of topoisomerases. These results imply that simultaneous inhibition of NHEJ and HR with luteolin treatment would provide a powerful protocol in cancer chemotherapy.