Use of the Rad51 promoter for targeted anti-cancer therapy

Recent Research Advances in Double-Strand Break and Mismatch Repair Defects in Prostate Cancer and Potential Clinical Applications

Prostate cancer remains a leading cause of cancer-related death in men worldwide. Recent research advances have emphasized the critical roles of mismatch repair (MMR) and double-strand break (DSB) in prostate cancer development and progression. Here, we provide a comprehensive review of the molecular mechanisms underlying DSB and MMR defects in prostate cancer, as well as their clinical implications. Furthermore, we discuss the promising therapeutic potential of immune checkpoint inhibitors and PARP inhibitors in targeting these defects, particularly in the context of personalized medicine and further perspectives. Recent clinical trials have demonstrated the efficacy of these novel treatments, including Food and Drugs Association (FDA) drug approvals, offering hope for improved patient outcomes. Overall, this review emphasizes the importance of understanding the interplay between MMR and DSB defects in prostate cancer to develop innovative and effective therapeutic strategies for patients.

Applications of tissue-specific and cancer-selective gene promoters for cancer diagnosis and therapy

Current treatment of solid tumors with standard of care chemotherapies, radiation therapy and/or immunotherapies are often limited by severe adverse toxic effects, resulting in a narrow therapeutic index. Cancer gene therapy represents a targeted approach that in principle could significantly reduce undesirable side effects in normal tissues while significantly inhibiting tumor growth and progression. To be effective, this strategy requires a clear understanding of the molecular biology of cancer development and evolution and developing biological vectors that can serve as vehicles to target cancer cells. The advent and fine tuning of omics technologies that permit the collective and spatial recognition of genes (genomics), mRNAs (transcriptomics), proteins (proteomics), metabolites (metabolomics), epiomics (epigenomics, epitranscriptomics, and epiproteomics), and their interactomics in defined complex biological samples provide a roadmap for identifying crucial targets of relevance to the cancer paradigm. Combining these strategies with identified genetic elements that control target gene expression uncovers significant opportunities for developing guided gene-based therapeutics for cancer. The purpose of this review is to overview the current state and potential limitations in developing gene promoter-directed targeted expression of key genes and highlights their potential applications in cancer gene therapy.

Identification of RAD51–BRCA2 Inhibitors Using N-Acylhydrazone-Based Dynamic Combinatorial Chemistry

RAD51 is an ATP-dependent recombinase, recruited by BRCA2 to mediate DNA double-strand breaks repair through homologous recombination and represents an attractive cancer drug target. Herein, we applied for the first-time protein-templated dynamic combinatorial chemistry on RAD51 as a hit identification strategy. Upon design of N-acylhydrazone-based dynamic combinatorial libraries, RAD51 showed a clear templating effect, amplifying 19 N-acylhydrazones. Screening against the RAD51–BRCA2 protein–protein interaction via ELISA assay afforded 10 inhibitors in the micromolar range. Further ¹⁹F NMR experiments revealed that 7 could bind RAD51 and be displaced by BRC4, suggesting an interaction in the same binding pocket of BRCA2. These results proved not only that ptDCC could be successfully applied on full-length oligomeric RAD51, but also that it could address the need of alternative strategies toward the identification of small-molecule PPI inhibitors.

The Emerging Roles of Rad51 in Cancer and Its Potential as a Therapeutic Target

Defects in DNA repair pathways are emerging hallmarks of cancer. Accurate DNA repairs and replications are essential for genomic stability. Cancer cells require residual DNA repair capabilities to repair the damage from replication stress and genotoxic anti-tumor agents. Defective DNA repair also promotes the accumulation of genomic changes that eventually lead to tumorigenesis, tumor progression, and therapeutic resistance to DNA-damaging anti-tumor agents. Rad51 recombinase is a critical effector of homologous recombination, which is an essential DNA repair mechanism for double-strand breaks. Rad51 has been found to be upregulated in many malignant solid tumors, and is correlated with poor prognosis. In multiple tumor types, Rad51 is critical for tumor metabolism, metastasis and drug resistance. Herein, we initially introduced the structure, expression pattern of Rad51 and key Rad51 mediators involved in homologous recombination. Additionally, we primarily discussed the role of Rad51 in tumor metabolism, metastasis, resistance to chemotherapeutic agents and poly-ADP ribose polymerase inhibitors.

Epigenomic Analysis of RAD51 ChIP-seq Data Reveals cis-regulatory Elements Associated with Autophagy in Cancer Cell Lines

Simple Summary

RAD51 is a key enzyme involved in homologous recombination during DNA double-strand break repair. However, recent studies suggest that non-canonical roles of RAD51 may exist. The aim of our study was to assess regulatory roles of RAD51 by reanalyzing RAD51 ChIP-seq data in GM12878, HepG2, K562, and MCF-7 cell lines. We identified 5137, 2611, 7192, and 3498 RAD51-associated cis-regulatory elements in GM12878, HepG2, K562, and MCF-7 cell lines, respectively. Intriguingly, gene ontology analysis revealed that promoters of the autophagy pathway-related genes were most significantly occupied by RAD51 in all four cell lines, predicting a non-canonical role of RAD51 in regulating autophagy-related genes.

Abstract

RAD51 is a recombinase that plays a pivotal role in homologous recombination. Although the role of RAD51 in homologous recombination has been extensively studied, it is unclear whether RAD51 can be involved in gene regulation as a co-factor. In this study, we found evidence that RAD51 may contribute to the regulation of genes involved in the autophagy pathway with E-box proteins such as USF1, USF2, and/or MITF in GM12878, HepG2, K562, and MCF-7 cell lines. The canonical USF binding motif (CACGTG) was significantly identified at RAD51-bound cis-regulatory elements in all four cell lines. In addition, genome-wide USF1, USF2, and/or MITF-binding regions significantly coincided with the RAD51-associated cis-regulatory elements in the same cell line. Interestingly, the promoters of genes associated with the autophagy pathway, such as ATG3 and ATG5, were significantly occupied by RAD51 and regulated by RAD51 in HepG2 and MCF-7 cell lines. Taken together, these results unveiled a novel role of RAD51 and provided evidence that RAD51-associated cis-regulatory elements could possibly be involved in regulating autophagy-related genes with E-box binding proteins.

Chloroxine overrides DNA damage tolerance to restore platinum sensitivity in high-grade serous ovarian cancer

High-grade serous cancer (HGSC) accounts for ~67% of all ovarian cancer deaths. Although initially sensitive to platinum chemotherapy, resistance is inevitable and there is an unmet clinical need for novel therapies that can circumvent this event. We performed a drug screen with 1177 FDA-approved drugs and identified the hydroxyquinoline drug, chloroxine. In extensive validation experiments, chloroxine restored sensitivity to both cisplatin and carboplatin, demonstrating broad synergy in our range of experimental models of platinum-resistant HGSC. Synergy was independent of chloroxine's predicted ionophore activity and did not relate to platinum uptake as measured by atomic absorption spectroscopy. Further mechanistic investigation revealed that chloroxine overrides DNA damage tolerance in platinum-resistant HGSC. Co-treatment with carboplatin and chloroxine (but not either drug alone) caused an increase in γH2AX expression, followed by a reduction in platinum-induced RAD51 foci. Moreover, this unrepaired DNA damage was associated with p53 stabilisation, cell cycle re-entry and triggering of caspase 3/7-mediated cell death. Finally, in our platinum-resistant, intraperitoneal in vivo model, treatment with carboplatin alone resulted in a transient tumour response followed by tumour regrowth. In contrast, treatment with chloroxine and carboplatin combined, was able to maintain tumour volume at baseline for over 4 months. In conclusion, our novel results show that chloroxine facilitates platinum-induced DNA damage to restore platinum sensitivity in HGSC. Since chloroxine is already licensed, this exciting combination therapy could now be rapidly translated for patient benefit.

Xanthatin synergizes with cisplatin to suppress homologous recombination through JAK2/STAT4/BARD1 axis in human NSCLC cells

Xanthatin (Xa) is a bicyclic sesquiterpene lactone identified from the plant Xanthium L. with impressive antitumor activity, but the role of Xa in non‐small cell lung cancer (NSCLC) is not known. Here we found that Xa inhibits proliferation, migration, invasion and induces apoptosis in NSCLC cells. RNA sequencing and Gene set enrichment analysis revealed that Xa significantly activates p53 pathway and suppresses E2F targets, G2M checkpoint and MYC targets in A549 cells. Among these changed genes, the down‐regulated gene BARD1 triggered by Xa was identified as a candidate involved in Xa’s antitumor effect because of its vital role in homologous recombination (HR). Further studies demonstrated that Xa inhibits HR through the BARD1/BRCA1/RAD51 axis, which enhances cell sensitivity to cisplatin. Mechanistic studies showed that Xa inhibits BARD1 through the JAK2/STAT4 pathway. Our study revealed that Xa is a promising drug to treat NSCLC, especially in combination with conventional chemotherapy.

Polymorphic Variants in 5'-UTR Regions of the RAD51 Gene are Associated With RAD51 Expression and Triple-Negative Breast Cancer (TNBC): A Case-Control Study

Breast cancer is a heterogeneous disease at morphologic and molecular levels, which is considered the most commonly occurring cancer in women. RAD51, a DNA-repairing protein, involves homologous recombination and has a vital role in genome stability. Polymorphism of the RAD51 gene, and its overexpression, has been proposed to be associated with the development of breast cancer. Overexpression of RAD51 in many types of human cancer including metastatic breast cancer may signify its potential use as a biomarker. Considering the numerous reports on the role of the 5'-UTR-RAD51 polymorphism in breast cancer, this study aimed to investigate the utility of RAD51 gene expression and its variants G135C and G172T as a possible foretelling factor of breast cancer development. DNA sequencing and immunohistochemistry of RAD51 were conducted on 103 samples from patients diagnosed with sporadic breast cancer and 80 samples from a control group. The results demonstrated that the RAD51 variants, G135C and G172T, were significantly presented in the breast cancer tissue compared with the control group. RAD51 expression was mainly shown in the cytoplasm of malignant cells (56% of cases) and significantly correlated with p53 and G135C, C135C variants. Moreover, the occurrence of the G172T variant was significantly associated with the expression of estrogen receptor. Interestingly, 21/26 (81%) of the triple-negative breast cancer showed G135C and C135C genotypes that were significantly associated with the expression of RAD51 (73%). In conclusion, the G135C and C135C variants together with the cytoplasmic expression of RAD51 may have clinical potential as a prognostic predictor for breast cancer development and aggressiveness.

Regulation and pharmacological targeting of RAD51 in cancer

Regulation of homologous recombination (HR) is central for cancer prevention. However, too little HR can increase cancer incidence, whereas too much HR can drive cancer resistance to therapy. Importantly, therapeutics targeting HR deficiency have demonstrated a profound efficacy in the clinic improving patient outcomes, particularly for breast and ovarian cancer. RAD51 is central to DNA damage repair in the HR pathway. As such, understanding the function and regulation of RAD51 is essential for cancer biology. This review will focus on the role of RAD51 in cancer and beyond and how modulation of its function can be exploited as a cancer therapeutic.

Ralaniten Sensitizes Enzalutamide-Resistant Prostate Cancer to Ionizing Radiation in Prostate Cancer Cells that Express Androgen Receptor Splice Variants

Blocking androgen receptor (AR) transcriptional activity by androgen deprivation therapy (ADT) improves the response to radiotherapy for intermediate and high risk prostate cancer. Unfortunately, ADT, antiandrogens, and abiraterone increase expression of constitutively active splice variants of AR (AR-Vs) which regulate DNA damage repair leading to resistance to radiotherapy. Here we investigate whether blocking the transcriptional activities of full-length AR and AR-Vs with ralaniten leads to enhanced sensitivity to radiotherapy. Combination therapies using ralaniten with ionizing radiation were evaluated for effects on proliferation, colony formation, cell cycle, DNA damage, and Western blot analyses in human prostate cancer cells that express both full-length AR and AR-Vs. Ralaniten and a potent next-generation analog (EPI-7170) decreased expression of DNA repair genes whereas enzalutamide had no effect. FACS analysis revealed a dose-dependent decrease of BrdU incorporation with increased accumulation of γH2AX with a combination of ionizing radiation with ralaniten. An additive inhibitory effect on proliferation of enzalutamide-resistant cells was achieved with a combination of ralaniten compounds with ionizing radiation. Ralaniten and EPI-7170 sensitized prostate cancer cells that express full-length AR and AR-Vs to radiotherapy whereas enzalutamide had no added benefit.

CtBP1 transactivates RAD51 and confers cisplatin resistance to breast cancer cells

Overexpression of RAD51 is found in many cancers including breast cancer and is associated with poor survival. Compared with normal cells, RAD51 promoter is hyperactive in cancer cells indicating that RAD51 is transcriptionally activated. However, little is known about the mechanisms and factors involved in RAD51 transcription regulation. Transcription corepressor, C-terminal binding protein 1 (CtBP1), is an oncogene repressing a panel of tumor suppressors transcription, which contributes to cancer progression. In this study, immunohistochemistry (IHC) revealed that RAD51 expression was positively correlated with CtBP1 expression in breast cancer patient tissues; short hairpin RNA-mediated CtBP1 depletion, chromatin immunoprecipitation, and dual-luciferase reporter assays showed that CtBP1 activated RAD51 transcription in breast cancer cells. Depletion of CtBP1 increased breast cancer cells' sensitivity to cisplatin and, in turn, expression of exogenous RAD51 in the CtBP1-depleted breast cancer cells increased resistance to cisplatin. The results demonstrated that CtBP1 conferred breast cancer cells resistance to cisplatin through transcriptional activation of RAD51.

Cisplatin in Combination with MDM2 Inhibition Downregulates Rad51 Recombinase in a Bimodal Manner to Inhibit Homologous Recombination and Augment Tumor Cell Kill

Dysfunction of p53 and resistance to cancer drugs can arise through mutually exclusive overexpression of MDM2 or MDM4. Cisplatin-resistant cells, however, can demonstrate increased binding of both MDM2 and MDM4 to p53 but in absence of cellular overexpression. Whether MDM2 inhibitors alone can activate p53 in these resistant cells was investigated with the goal to establish the mechanism for potential synergy with cisplatin. Thus, growth inhibition by individual drugs and combinations was assessed by a colorimetric assay. Drug-treated parental A2780 and resistant tumor cells were also examined for protein expression using immunoblot and reverse phase protein array (RPPA) and then subjected to Ingenuity Pathway Analysis (IPA). Gene expression was assessed by real-time polymerase chain reaction, DNA damage by confocal microscopy, cell cycle by flow cytometry, and homologous recombination (HR) by a GFP reporter assay. Our results demonstrate that Nutlin-3 but not RITA (reactivation of p53 and induction of tumor cell apoptosis) effectively disrupted the p53-MDM2-MDM4 complex to activate p53, which increased robustly with cisplatin/Nutlin-3 combination and enhanced antitumor effects more than either agent alone. RPPA, IPA, and confocal microscopy provided evidence for an "apparent" increase in DNA damage resulting from HR inhibition by cisplatin/Nutlin-3. Molecularly, the specific HR protein Rad51 was severely downregulated by the combination via two mechanisms: p53-dependent transrepression and p53/MDM2-mediated proteasomal degradation. In conclusion, Nutlin-3 fully destabilizes the p53-MDM2-MDM4 complex and synergizes with cisplatin to intensify p53 function, which then downregulates Rad51 through a bimodal mechanism. As a result, HR is inhibited and antitumor activity enhanced in otherwise HR-proficient sensitive and resistant tumor cells. SIGNIFICANCE STATEMENT: Rad51 downregulation by the combination of cisplatin and Nutlin-3 inhibits homologous recombination (HR), which leads to persistence in DNA damage but not an increase. Thus, inhibition of HR enhances antitumor activity in otherwise HR-proficient sensitive and resistant tumor cells.

Targeted Diphtheria Toxin-Based Therapy: A Review Article

Cancer remains one of the leading causes of death worldwide. Conventional therapeutic strategies usually offer limited specificity, resulting in severe side effects and toxicity to normal tissues. Targeted cancer therapy, on the other hand, can improve the therapeutic potential of anti-cancer agents and decrease unwanted side effects. Targeted applications of cytolethal bacterial toxins have been found to be especially useful for the specific eradication of cancer cells. Targeting is either mediated by peptides or by protein-targeting moieties, such as antibodies, antibody fragments, cell-penetrating peptides (CPPs), growth factors, or cytokines. Together with a toxin domain, these molecules are more commonly referred to as immunotoxins. Targeting can also be achieved through gene delivery and cell-specific expression of a toxin. Of the available cytolethal toxins, diphtheria toxin (DT) is one of the most frequently used for these strategies. Of the many DT-based therapeutic strategies investigated to date, two immunotoxins, Ontak^TM and Tagraxofusp^TM, have gained FDA approval for clinical application. Despite some success with immunotoxins, suicide-gene therapy strategies, whereby controlled tumor-specific expression of DT is used for the eradication of malignant cells, are gaining prominence. The first part of this review focuses on DT-based immunotoxins, and it then discusses recent developments in tumor-specific expression of DT.

E2F1 facilitates DNA break repair by localizing to break sites and enhancing the expression of homologous recombination factors

The human genome is constantly exposed to both endogenous and exogenous stresses, which can lead to errors in DNA replication and the accumulation of DNA mutations, thereby increasing the risk of cancer development. The transcription factor E2F1 is a key regulator of DNA repair. E2F1 also has defined roles in the replication of many cell cycle-related genes and is highly expressed in cancer cells, and its abundance is strongly associated with poor prognosis in cancers. Studies on colon cancer have demonstrated that the depletion of E2F1 leads to reduced levels of homologous recombination (HR), resulting in interrupted DNA replication and the subsequent accumulation of DNA lesions. Our results demonstrate that the depletion of E2F1 also causes reduced RAD51-mediated DNA repair and diminished cell viability resulting from DNA damage. Furthermore, the extent of RAD51 and RPA colocalization is reduced in response to DNA damage; however, RPA single-stranded DNA (ssDNA) nucleofilament formation is not affected following the depletion of E2F1, implying that ssDNA gaps accumulate when RAD51-mediated DNA gap filling or repair is diminished. Surprisingly, we also demonstrate that E2F1 forms foci with RAD51 or RPA at DNA break sites on damaged DNA. These findings provide evidence of a molecular mechanism underlying the E2F1-mediated regulation of HR activity and predict a fundamental shift in the function of E2F1 from regulating cell division to accelerating tumor development.

Overexpression of Rad51 predicts poor prognosis and silencing of Rad51 increases chemo-sensitivity to doxorubicin in neuroblastoma

Outcome for children with high-risk neuroblastoma (NB) remains suboptimal. Recurrence and metastasis caused by chemo-resistance is an underlying mechanism contributing to the poor prognosis. Aberrant expression of Rad51 is implicated in both radio- and chemo-sensitivity in many human malignancies. However, its clinical significance and relationship with chemo-sensitivity in NB remain undefined. In this study, Rad51 expression was first evaluated in 70 surgically resected NB specimens by immunochemistry using tissue microarray and the correlation with clinic-pathologic features including survival was assessed. We then conducted microarray-based search with the Tumor Neuroblastoma public datasets to validate the immunochemistry results. Furthermore, the role of Rad51 in drug sensitivity was studied by using short hairpin RNA in the human NB SK-N-BE(2) and SH-SY5Y cells with treatment of doxorubicin. Our findings demonstrated for the first time that Rad51 is a prognostic marker in NB and down-regulation of Rad51 can lead to chemo-sensitizing effect in human NB cells.

The VEGF receptor neuropilin 2 promotes homologous recombination by stimulating YAP/TAZ-mediated Rad51 expression

Vascular endothelial growth factor (VEGF) signaling in tumor cells mediated by neuropilins (NRPs) contributes to the aggressive nature of several cancers, including triple-negative breast cancer (TNBC), independently of its role in angiogenesis. Understanding the mechanisms by which VEGF-NRP signaling contributes to the phenotype of such cancers is a significant and timely problem. We report that VEGF-NRP2 promote homologous recombination (HR) in BRCA1 wild-type TNBC cells by contributing to the expression and function of Rad51, an essential enzyme in the HR pathway that mediates efficient DNA double-strand break repair. Mechanistically, we provide evidence that VEGF-NRP2 stimulates YAP/TAZ-dependent Rad51 expression and that Rad51 is a direct YAP/TAZ-TEAD transcriptional target. We also discovered that VEGF-NRP2-YAP/TAZ signaling contributes to the resistance of TNBC cells to cisplatin and that Rad51 rescues the defects in DNA repair upon inhibition of either VEGF-NRP2 or YAP/TAZ. These findings reveal roles for VEGF-NRP2 and YAP/TAZ in DNA repair, and they indicate a unified mechanism involving VEGF-NRP2, YAP/TAZ, and Rad51 that contributes to resistance to platinum chemotherapy.

Optimization of Drug Candidates That Inhibit the D-Loop Activity of RAD51

RAD51 is the central protein in homologous recombination (HR) repair, where it first binds ssDNA and then catalyzes strand invasion via a D-loop intermediate. Additionally, RAD51 plays a role in faithful DNA replication by protecting stalled replication forks; this requires RAD51 to bind DNA but may not require the strand invasion activity of RAD51. We previously described a small-molecule inhibitor of RAD51 named RI(dl)-2 (RAD51 inhibitor of D-loop formation #2, hereafter called 2 h), which inhibits D-loop activity while sparing ssDNA binding. However, 2 h is limited in its ability to inhibit HR in vivo, preventing only about 50 % of total HR events in cells. We sought to improve upon this by performing a structure-activity relationship (SAR) campaign for more potent analogues of 2 h. Most compounds were prepared from 1-(2-aminophenyl)pyrroles by forming the quinoxaline moiety either by condensation with aldehydes, then dehydrogenation of the resulting 4,5-dihydro intermediates, or by condensation with N,N'-carbonyldiimidazole, chlorination, and installation of the 4-substituent through Suzuki-Miyaura coupling. Many analogues exhibited enhanced activity against human RAD51, but in several of these compounds the increased inhibition was due to the introduction of dsDNA intercalation activity. We developed a sensitive assay to measure dsDNA intercalation, and identified two analogues of 2 h that promote complete HR inhibition in cells while exerting minimal intercalation activity.

Melatonin Sensitizes Human Colorectal Cancer Cells to γ-ray Ionizing Radiation In Vitro and In Vivo

Colorectal cancer is the most commonly reported gastrointestinal malignancy, with a recent, rapid increase of the annual incidence all over the world. Enhancing the radiosensitivity of cancer cells while preserving the health of normal cells is one of the most important tasks in clinical radiobiology. However, resistance to radiotherapy for colorectal cancer greatly decreases the therapeutic outcome. Melatonin (N-acetyl-5-methoxytryptamine), a natural secretory product that the pineal gland in the brain normally produces, has been reported to have anticancer properties. In the study, we investigated the combination of melatonin with radiotherapy as a treatment for colorectal cancer. We firstly explored the anti-tumor activity of melatonin combined with ionizing radiation (IR) against colorectal carcinoma in vitro. It was found that melatonin effectively inhibited human colorectal carcinoma cell line HCT 116 cellular proliferation, colony formation rate and cell migration counts following IR. Increasing the radiosensitivity of colorectal cancer cells by melatonin treatment was found to be associated with cell cycle arrest in the G2/M phase, downregulation of proteins involved in DNA double-strand break repair and activation of the caspase-dependent apoptotic pathway. Moreover, we also investigated the combined effect of IR and melatonin on colorectal tumor in vivo. Results from a tumor xenograft showed that melatonin plus IR treatment significantly suppressed tumor cell growth compared with melatonin or IR alone, resulting in a much higher tumor inhibition rate for the combined treatment. The data suggested that melatonin combined with IR could improve the radiosensitivity of colorectal cancer and thus enhance the therapeutic effect of the patients, implying melatonin could function as a potential sensitizer in tumor radiotherapy.

Network Analysis of RAD51 Proteins in Metazoa and the Evolutionary Relationships With Their Archaeal Homologs

The RAD51 (DNA repair protein RAD51) recombinases are essential for homologous recombination, DNA repair, and genome stability. Overexpression of RAD51 proteins has been observed in many cancer cells, such as thyroid carcinoma, breast cancer, pancreatic cancer, and others. In Metazoa, there are multiple members of RAD51 (RAD51, RAD51B, RAD51C, RAD51D, DMC1) (DNA meiotic recombinase 1), XRCC2 (X-ray repair cross-complementing 2), and XRCC3. In this study, we used a protein sequence similarity network (SSN) to analyze the evolutionary relationship within this protein family. The SSN based on the RAD51 proteins from Metazoa indicated that there are several proteins that have yet to be functionally defined. The SSN based on the distribution of the proteins supports the hypothesis that horizontal gene transfer plays an important role in the evolution of RAD51 proteins. Multiple sequence alignments with structural information revealed that the amino acid residues for ATP and Mg²⁺ are highly conserved. The seven RAD51 proteins in humans are under different selective pressure: RAD51 and DMC1 are under stringent negative selection, while other proteins are subject to relatively relaxed negative selection. Furthermore, the expression levels of the seven genes in different tissues showed that the genes in the same cluster in the phylogenetic tree showed similar expression profiles. Finally, the SSN based on the RAD51 proteins from both eukaryotes and prokaryotes suggested that the eukaryotic RAD51 recombinases share a common ancestor with the archaeal homologs, but XRCC2 may have a different origin. These findings expand the understanding of the evolution and diversity of RAD51 recombinases in Metazoa.

Jab1/Cops5 contributes to chemoresistance in breast cancer by regulating Rad51

Jab1 overexpression correlates with poor prognosis in breast cancer patients, suggestting that targeting the aberrant Jab1 signaling in breast cancer could be a promising strategy. In the current study, we investigate the hypothesis that Jab1 positively regulates the DNA repair protein Rad51 and, in turn, the cellular response of breast cancer to chemotherapy with adriamycin and cisplatin. High-throughput mRNA sequencing (RNA-Seq) data from 113 normal and 1109 tumor tissues (obtained from TCGA) were integrated to our analysis to give further support to our findings. We found that Jab1 was overexpressed in adriamycin-resistant breast cancer cell MCF-7R compared with parental MCF-7 cells, and that knockdown of Jab1 expression conferred cellular sensitivity to adriamycin and cisplatin both in vivo and in vitro. By contrast, exogenous Jab1 expression enhanced the resistance of breast cancer cells to adriamycin and cisplatin. Moreover, we discovered that Jab1 positively regulated Rad51 in p53-dependent manner and that overexpression of Rad51 conferred cellular resistance to adriamycin and cisplatin in Jab1-deficient cells. Data from TCGA further validated an correlation between Jab1 and Rad51 in breast cancer, and elevated Jab1 and Rad51 associated with poor survival in breast cancer patients. Our findings indicate that Jab1 association with Rad51 plays an important role in cellular response to chemotherapy in breast cancer.

Promoter-Operating Targeted Expression of Gene Therapy in Cancer: Current Stage and Prospect

The technique of targeted expression of interesting genes, including distinct delivery systems and specific gene promoter-operating expression, is an important strategy for gene therapy against cancers. Up to now, extensive literature documented the efficacy of distinct delivery systems, such as the liposome system, nano-particle system, polyetherimide (PEI) system, and so on, in cancer gene therapy. However, a related document on the potential value of using a specific gene promoter, such as a tumor suppressor, in cancer gene therapy was still scary. The main obstacle might be that the selection of an ideal gene promoter to operate interesting gene expression in cancer gene therapy is still not fully understood. Therefore, many efforts need to be done in order to make it a real power tool for the human clinical treatment of cancer patients. The purpose of this review is to clarify the current state and some problematics in development of promoter-operating targeted expression of interesting genes and highlight its potential in cancer gene therapy.

Use of the XRCC2 promoter for in vivo cancer diagnosis and therapy

The homologous recombination (HR) pathway is a promising target for cancer therapy as it is frequently upregulated in tumors. One such strategy is to target tumors with cancer-specific, hyperactive promoters of HR genes including RAD51 and RAD51C. However, the promoter size and the delivery method have limited its potential clinical applications. Here we identified the ~2.1 kb promoter of XRCC2, similar to ~6.5 kb RAD51 promoter, as also hyperactivated in cancer cells. We found that XRCC2 expression is upregulated in nearly all types of cancers, to a degree comparable to RAD51 while much higher than RAD51C. Further study demonstrated that XRCC2 promoter is hyperactivated in cancer cell lines, and diphtheria toxin A (DTA) gene driven by XRCC2 promoter specifically eliminates cancer cells. Moreover, lentiviral vectors containing XRCC2 promoter driving firefly luciferase or DTA were created and applied to subcutaneous HeLa xenograft mice. We demonstrated that the pXRCC2-luciferase lentivirus is an effective tool for in vivo cancer visualization. Most importantly, pXRCC2-DTA lentivirus significantly inhibited the growth of HeLa xenografts in comparison to the control group. In summary, our results strongly indicate that virus-mediated delivery of constructs built upon the XRCC2 promoter holds great potential for tumor diagnosis and therapy.

STAT5A/B Blockade Sensitizes Prostate Cancer to Radiation through Inhibition of RAD51 and DNA Repair

Purpose: The standard treatment for organ-confined prostate cancer is surgery or radiation, and locally advanced prostate cancer is typically treated with radiotherapy alone or in combination with androgen deprivation therapy. Here, we investigated whether Stat5a/b participates in regulation of double-strand DNA break repair in prostate cancer, and whether Stat5 inhibition may provide a novel strategy to sensitize prostate cancer to radiotherapy.Experimental Design: Stat5a/b regulation of DNA repair in prostate cancer was evaluated by comet and clonogenic survival assays, followed by assays specific to homologous recombination (HR) DNA repair and nonhomologous end joining (NHEJ) DNA repair. For HR DNA repair, Stat5a/b regulation of Rad51 and the mechanisms underlying the regulation were investigated in prostate cancer cells, xenograft tumors, and patient-derived prostate cancers ex vivo in 3D explant cultures. Stat5a/b induction of Rad51 and HR DNA repair and responsiveness to radiation were evaluated in vivo in mice bearing prostate cancer xenograft tumors.Results: Stat5a/b is critical for Rad51 expression in prostate cancer via Jak2-dependent mechanisms by inducing Rad51 mRNA levels. Consistent with this, genetic knockdown of Stat5a/b suppressed HR DNA repair while not affecting NHEJ DNA repair. Pharmacologic Stat5a/b inhibition potently sensitized prostate cancer cell lines and prostate cancer tumors to radiation, while not inducing radiation sensitivity in the neighboring tissues.Conclusions: This work introduces a novel concept of a pivotal role of Jak2-Stat5a/b signaling for Rad51 expression and HR DNA repair in prostate cancer. Inhibition of Jak2-Stat5a/b signaling sensitizes prostate cancer to radiation and, therefore, may provide an adjuvant therapy for radiation to reduce radiation-induced damage to the neighboring tissues. Clin Cancer Res; 24(8); 1917-31. ©2018 AACR.

Inhibition of RAD51 by siRNA and Resveratrol Sensitizes Cancer Stem Cells Derived from HeLa Cell Cultures to Apoptosis

Cervical cancer is the second most frequent tumor type in women worldwide with cases developing clinical recurrence, metastasis, and chemoresistance. The cancer stem cells (CSC) may be implicated in tumor resistance to therapy. RESveratrol (RES), a natural compound, is an antioxidant with multiple beneficial activities. We previously determined that the expression of RAD51 is decreased by RES. The aim of our study was to examine molecular mechanism by which CSC from HeLa cultures exhibit chemoresistance. We hypothesized CSC repair more efficiently DNA breaks and that RAD51 plays an important role in this mechanism. We found that CSC, derived from cervical cancer cell lines, overexpress RAD51 and are less sensitive to Etoposide (VP16). We inhibited RAD51 in CSC-enriched cultures using RES or siRNA against RAD51 messenger RNA and observed a decrease in cell viability and induction of apoptosis when treated simultaneously with VP16. In addition, we found that inhibition of RAD51 expression using RES also sensitizes CSC to VP16 treatment. Our results suggest that resveratrol is effective to sensitize cervical CSC because of RAD51 inhibition, targeting high RAD51 expressing CD49f-positive cells, which supports the possible therapeutic application of RES as a novel agent to treat cancer.

Network Meta-Analysis on the Effects of DNA Damage Response-Related Gene Mutations on Overall Survival of Breast Cancer Based on TCGA Database

The study was conducted for comparing the effects of 12 DNA damage response gene mutations (CHEK1, CHEK2, RAD51, BRCA1, BRCA2, MLH1, MSH2, ATM, ATR, MDC1, PARP1, and FANCF) on the overall survival (OS) of breast cancer (BC) patients. We searched the Cancer Genome Atlas (TCGA) database from inception to September 2016. Studies that investigated the association between 12 DNA damage responses related genes and BC consolidated into this Network meta-analysis, by comparing directly or indirectly to evaluate the hazard rate (HR) value and the surface under the cumulative sequence ranking curves (SUCRA). In total four articles were involved. Our results demonstrated 12 DNA damage response gene mutations were associated to the poor prognosis of BC patients (CHEK1: HR = 9.9, 95%CI = 3.6-26.0; CHEK2: HR = 6.9, 95%CI = 3.1-15.0; RAD51: HR = 5.8, 95%CI = 2.2-15.0; BRCA1: HR = 2.8, 95%CI = 1.3-6.1; BRCA2: HR = 3.9, 95%CI = 2.0-7.7; MLH1: HR = 11.0, 95%CI = 3.4-33.0; MSH2: HR = 6.5, 95%CI = 2.1-20.0; ATM: HR = 5.6, 95%CI = 2.6-12.0; ATR: HR = 2.9, 95%CI = 1.3-6.9; MDC1: HR = 15.0, 95%CI = 5.0-45.0; PARP1: HR = 3.4, 95%CI = 1.8-6.6; FANCF: HR = 6.0, 95%CI = 1.8-20.0). SUCRA results revealed that the mutation of MDC1 gene was related to the worst prognosis in patients with BC (SUCRA = 17.32%). DNA damage response gene mutations were associated to the poor prognosis in patients with BC and the BC patients with MDC1 gene mutation had the worst prognosis. J. Cell. Biochem. 118: 4728-4734, 2017. © 2017 Wiley Periodicals, Inc.

The homologous recombination protein RAD51 is a promising therapeutic target for cervical carcinoma

RAD51 is one of the pivotal enzymes for DNA double-strand break (DSB) repair by the homologous recombination (HR) pathway, which implies it as a promising and novel target for cancer therapy. Recent findings have indicated RAD51 protein is overexpressed in a variety of tumors. The high-expression of RAD51 is related to poor prognosis. RAD51 is involved in the repair of DNA damage and the generation of genetic diversity by an evolutionarily conserved mechanism. However, the exact mechanism of Rad51 in the progression of cervical cancer remains unclear. RI-1 is a small molecule that inhibits the central recombination protein RAD51. In this study, we found that RAD51 was highly expressed in invasive squamous cervical cancer (SCC). The administration of RI-1 inhibited cell growth in vitro and reduced growth of tumor xenografts in vivo with cervical cancer cells (HeLa and SiHa). Further investigation suggested that RAD51 protein significantly promoted the cell cycle transition from the G0/G1 to S phase. In addition, the inhibition of RAD51 reduced the level of the cell cycle related protein cyclin D1, but increased the levels of p21 mRNA and protein. As a DNA DSB repair enzyme, the expression of RAD51 in tumor cells possibly affects their sensitivity to anti-cancer agents. Additionally, in experiments using cisplatin and ionizing radiation, RI-1 treated cervical cancer cells, HeLa and SiHa, were sensitized to a greater extent than the untreated control. Thus, HR inhibition of RAD51 may provide yet another mechanism of therapeutic target for the chemosensitization and radiosensitization of cervical cancer with RI-1. Collectively, our data demonstrated for the first time that inhibition of RAD51 suppressed the cervical cancer cell proliferation and the growth of cervical cancer xenografts by attenuating cell cycle transition, which could be a functional link between RAD51 and cyclin D1 and p21.

RAD51 as a potential surrogate marker for DNA repair capacity in solid malignancies

Targeting deficient mechanisms of cellular DNA repair still represents the basis for the treatment of the majority of solid tumors, and increased DNA repair capacity is a hallmark mechanism of resistance not only to DNA-damaging treatments such as cytotoxic drugs and radiotherapy, but also to small molecule targeted drugs such as inhibitors of poly-ADP ribose polymerase (PARP). Hence, there is substantial medical need for potent and convenient biomarkers of individual response to DNA-targeted treatment in personalized cancer care. RAD51 is a highly conserved protein that catalyzes DNA repair via homologous recombination, a major DNA repair pathway which directly modulates cellular sensitivity to DNA-damaging treatments. The clinical and biological significance of RAD51 protein expression is still under investigation. Pre-clinical studies consistently show the important role of nuclear RAD51 immunoreactivity in chemo- and radioresistance. Validating data from clinical trials however is limited at present, and some clinical studies show controversial results. This review gives a comprehensive overview on the current knowledge about the prognostic and predictive value of RAD51 protein expression and genetic variability in patients with solid malignancies.

The effects of SAHA on radiosensitivity in pancreatic cancer cells by inducing apoptosis and targeting RAD51

Suberoyl anilide hydroxamic acid (SAHA) is one of the most promising Histone deacetylases(HDAC) inhibitors which has shown significant anti-tumor activity for many malignancies. We explored the potential mechanism of the radiosensitivity effect of SAHA in Panc-1 cells and attempted to develop SAHA as a systemic treatment strategy for pancreatic cancer. Growth inhibition was detected by CCK-8 assay. Radiosensitizing enhancement ratio was determined by clonogenic assay. The cell cycle and apoptosis assay was detected using flow cytometry and annexin-V/PI. The level of Bax, Bcl-2, Ku70, Ku86, RAD51, RAD54 protein expression were detected using Western blot analysis. Gene silencing was processed by lentiviral vector and qRT-PCR was performed to detect mRNA expression. The results revealed that SAHA inhibited the proliferation of Panc-1 cells. SAHA enhanced the radiosensitivity with a sensitization enhancement ratio(SER) of 1.10 of the Panc-1 cells. SAHA induced G2-M phase arrest and apoptosis of Panc-1 cells with radiation. SAHA upregulated Bax and downregulated Bcl-2, Ku70, Ku86, RAD51, RAD54 protein expression of irradiated Panc-1 cells. SAHA enhanced the radiosensitivity of Panc-1 cells by modulating RAD51 expression. SAHA enhanced radiosensitivity to pancreatic carcinoma Panc-1 cells. It was associated with the G2-M phase arrest and apoptosis via modulation of Bax and Bcl-2 expression. Downregulation of Ku70, Ku86, RAD51 and RAD54 expression caused suppression of HR-mediated DNA repair. SAHA is a good radiosensitizer for pancreatic cancer treatment.

Gene delivery of TIPE2 inhibits breast cancer development and metastasis via CD8+ T and NK cell-mediated antitumor responses

Breast cancer is the second leading cause of cancer-related deaths in the female patients which was mainly caused by metastasis. Development of target gene therapy for breast cancer to suppress tumor progress and metastasis will improve the therapeutic options and be of great benefit to the patients. Tumor necrosis factor-alpha-induced protein 8-like 2 is a novel molecule for maintaining immune homeostasis and is involved in cancer development. In the present study, we overexpressed TIPE2 in breast cancer cells to investigate the role of TIPE2 in the development of breast cancer. Our results showed that overexpression of TIPE2 significantly inhibited the proliferation of 4T1 cells in vitro and in vivo. We constructed a non-viral targeted gene therapeutic system by using the minicircle plasmids expressing TIPE2. We found that the growth and metastasis of breast cancer was significantly inhibited by hydrodynamic gene delivery of TIPE2 plasmids in vivo. Mechanistically, TIPE2 increased T and NK cells, and decreased MDSCs. Gene delivery of TIPE2 up-regulated the production of IFN-γ and TNF-α by CD8⁺ T and NK cells in spleens and tumor microenvironment, and enhanced the cytotoxic activity of CD8⁺ T and NK cells. Taken together, TIPE2 inhibited breast cancer development and metastasis possibly via promoting CD8⁺ T and NK cell-mediated antitumor immune responses. Thus, the results indicate that TIPE2 may be a potential therapeutic target for breast cancer therapy.

FIGNL1 is overexpressed in small cell lung cancer patients and enhances NCI-H446 cell resistance to cisplatin and etoposide

Abnormal DNA repair plays an important role in tumor occurrence, progression and resistance to therapy. Fidgetin-like 1 (FIGNL1) expression was assayed in 42 small cell lung cancer (SCLC) and 45 normal lung specimens from Chinese patients by qRT-PCR. Notably, FIGNL1 was upregulated by 1.5-fold in the SCLC specimens compared to that noted in the normal counterparts. The SCLC cell line NCI-H446 that overexpresses FIGNL1 was adopted to explore the biological significance of FIGNL1 in SCLC. Even when FIGNL1 expression was suppressed by up to 48.6%, H446 cell growth was increased by only 10–16%. Although no significant changes in cell cycle distribution were observed in the H446 cells, the levels of cyclin E1 and CDK2, key cell cycle regulators, were significantly reduced. After downregulation of FIGNL1 expression by 13.5% in the H446 cells, the cells were 61.8% (24 h) to 29.1% (48 h) more sensitive to etoposide and cisplatin, respectively, consistent with the FIGNL1 function of DNA double-strand repair. The sensitivity of H446 cells to etoposide and cisplatin was negatively correlated with FIGNL1 expression. Meanwhile, an obvious positive correlation between DNA damage severity and the sensitization effect of FIGNL1 knockdown was observed. Since FIGNL1 is essential in the homologous recombination (HR) pathway, these findings suggest that abnormal activation of the HR pathway featured by FIGNL1 overexpression contributes to rapid progression and relapse of SCLC in addition to chemotherapy resistance. Further research assessing the functions and mechanisms of FIGNL1, and other HR pathway genes may disclose unique pathological characteristics of SCLC, and help identify potential therapeutic targets and biomarkers.

Recent Developments Using Small Molecules to Target RAD51: How to Best Modulate RAD51 for Anticancer Therapy?

Homologous recombination (HR) is an evolutionarily conserved DNA repair process. Overexpression of the key HR protein RAD51 is a common feature of malignant cells. RAD51 plays two distinct genome-stabilizing roles, including HR-mediated repair of double-strand breaks (DSBs) and the promotion of replication fork stability during replication stress. Because upregulation of RAD51 in cancer cells can promote tumor resistance to DNA-damaging oncologic therapies, we and others have worked to develop cancer therapeutics that target various aspects of RAD51 protein function. Herein, we provide an overview of recent developments in this field, together with our perspectives on the challenges associated with these evolving anticancer strategies.

Development of Small Molecules that Specifically Inhibit the D-loop Activity of RAD51

RAD51 is the central protein in homologous recombination (HR) DNA repair and represents a therapeutic target in oncology. Herein we report a novel class of RAD51 inhibitors that were identified by high throughput screening. In contrast to many previously reported RAD51 inhibitors, our lead compound 1 is capable of blocking RAD51-mediated D-loop formation (IC50 21.3 ± 7.8 μM) at concentrations that do not influence RAD51 binding to ssDNA. In human cells, 1 inhibits HR (IC50 13.1 ± 1.6 μM) without blocking RAD51's ability to assemble into subnuclear foci at sites of DNA damage. We determined that the active constituent of 1 is actually an oxidized derivative (termed RI(dl)-1 or 8) of the original screening compound. Our SAR campaign also yielded RI(dl)-2 (hereafter termed 9h), which effectively blocks RAD51's D-loop activity in biochemical systems (IC50 11.1 ± 1.3 μM) and inhibits HR activity in human cells (IC50 3.0 ± 1.8 μM).

Progress and problems with the use of suicide genes for targeted cancer therapy

Among various gene therapy methods for cancer, suicide gene therapy attracts a special attention because it allows selective conversion of non-toxic compounds into cytotoxic drugs inside cancer cells. As a result, therapeutic index can be increased significantly by introducing high concentrations of cytotoxic molecules to the tumor environment while minimizing impact on normal tissues. Despite significant success at the preclinical level, no cancer suicide gene therapy protocol has delivered the desirable clinical significance yet. This review gives a critical look at the six main enzyme/prodrug systems that are used in suicide gene therapy of cancer and familiarizes readers with the state-of-the-art research and practices in this field. For each enzyme/prodrug system, the mechanisms of action, protein engineering strategies to enhance enzyme stability/affinity and chemical modification techniques to increase prodrug kinetics and potency are discussed. In each category, major clinical trials that have been performed in the past decade with each enzyme/prodrug system are discussed to highlight the progress to date. Finally, shortcomings are underlined and areas that need improvement in order to produce clinical significance are delineated.

Ewing sarcoma: The clinical relevance of the insulin-like growth factor 1 and the poly-ADP-ribose-polymerase pathway

BACKGROUND

In the last three decades the outcome for patients with localised Ewing sarcoma (ES) has improved significantly since the introduction of multimodality primary treatment. However, for patients with (extra-) pulmonary metastatic and/or non-resectable relapsed disease the outcome remains poor and new treatment options are urgently needed. Currently the insulin-like growth factor 1 receptor (IGF-1R) pathway and the poly-ADP(adenosinediphosphate)-ribose-polymerase (PARP) pathway are being investigated for potential targeted therapies. IGF-1R: The IGF-1R pathway is known to be deregulated by the EWSR1-FLI1 translocation which makes it a potential target for therapy. Clinical trials have been reported in which only ES patients were treated with an IGF-1R inhibitor, either as single agent or in combination. In total 291 ES patients were included in these trials, in which two (0.7%) complete responses, 32 (11%) partial responses of which some durable, and 61 (21%) stable diseases were observed. PARP: In the presence of a PARP inhibitor DNA strand breaks cannot be efficiently repaired, leading to cell death. The first phase II trial with ES patients was recently published and showed no clinical responses, which may have been due to the drug being non-effective as a single agent.

DISCUSSION

The IGF-1R pathway is an interesting target for ES and should be explored further, as biomarkers to select patients that might benefit from treatment are lacking. PARP inhibitors as single agent have so far failed to show improvement in outcome. Future directions include dual insulin receptor/IGF-1R blockade with linsitinib as well as chemotherapy-PARP combinations. Both therapeutic strategies are currently being explored.

A peptide nucleic acid targeting nuclear RAD51 sensitizes multiple myeloma cells to melphalan treatment

RAD51-mediated recombinational repair is elevated in multiple myeloma (MM) and predicts poor prognosis. RAD51 has been targeted to selectively sensitize and/or kill tumor cells. Here, we employed a peptide nucleic acid (PNA) to inhibit RAD51 expression in MM cells. We constructed a PNA complementary to a unique segment of the RAD51 gene promoter, spanning the transcription start site, and conjugated it to a nuclear localization signal (PKKKRKV) to enhance cellular uptake and nuclear delivery without transfection reagents. This synthetic construct, (PNArad51_nls), significantly reduced RAD51 transcripts in MM cells, and markedly reduced the number and intensity of de novo and melphalan-induced nuclear RAD51 foci, while increasing the level of melphalan-induced γH2AX foci. Melphalan alone markedly induced the expression of 5 other genes involved in homologous-recombination repair, yet suppression of RAD51 by PNArad51_nls was sufficient to synergize with melphalan, producing significant synthetic lethality of MM cells in vitro. In a SCID-rab mouse model mimicking the MM bone marrow microenvironment, treatment with PNArad51_nls ± melphalan significantly suppressed tumor growth after 2 weeks, whereas melphalan plus control PNArad4µ_nls was ineffectual. This study highlights the importance of RAD51 in myeloma growth and is the first to demonstrate that anti-RAD51 PNA can potentiate conventional MM chemotherapy.

RAD54 family translocases counter genotoxic effects of RAD51 in human tumor cells

The RAD54 family DNA translocases have several biochemical activities. One activity, demonstrated previously for the budding yeast translocases, is ATPase-dependent disruption of RAD51-dsDNA binding. This activity is thought to promote dissociation of RAD51 from heteroduplex DNA following strand exchange during homologous recombination. In addition, previous experiments in budding yeast have shown that the same activity of Rad54 removes Rad51 from undamaged sites on chromosomes; mutants lacking Rad54 accumulate nonrepair-associated complexes that can block growth and lead to chromosome loss. Here, we show that human RAD54 also promotes the dissociation of RAD51 from dsDNA and not ssDNA. We also show that translocase depletion in tumor cell lines leads to the accumulation of RAD51 on chromosomes, forming complexes that are not associated with markers of DNA damage. We further show that combined depletion of RAD54L and RAD54B and/or artificial induction of RAD51 overexpression blocks replication and promotes chromosome segregation defects. These results support a model in which RAD54L and RAD54B counteract genome-destabilizing effects of direct binding of RAD51 to dsDNA in human tumor cells. Thus, in addition to having genome-stabilizing DNA repair activity, human RAD51 has genome-destabilizing activity when expressed at high levels, as is the case in many human tumors.

Melatonin sensitizes human breast cancer cells to ionizing radiation by downregulating proteins involved in double-strand DNA break repair

Radiation and adjuvant endocrine therapy are nowadays considered a standard treatment option after surgery in breast cancer. Melatonin exerts oncostatic actions on human breast cancer cells. In the current study, we investigated the effects of a combination of radiotherapy and melatonin on human breast cancer cells. Melatonin (1 mm, 10 μm and 1 nm) significantly inhibited the proliferation of MCF-7 cells. Radiation alone inhibited the MCF-7 cell proliferation in a dose-dependent manner. Pretreatment of breast cancer cells with melatonin 1 wk before radiation led to a significantly greater decrease of MCF-7 cell proliferation compared with radiation alone. Melatonin pretreatment before radiation also decreased G2 -M phase arrest compared with irradiation alone, with a higher percentage of cells in the G0 -G1 phase and a lower percentage of cells in S phase. Radiation alone diminished RAD51 and DNA-protein kinase (PKcs) mRNA expression, two main proteins involved in double-strand DNA break repair. Treatment with melatonin for 7 days before radiation led to a significantly greater decrease in RAD51 and DNA-PKcs mRNA expression compared with radiation alone. Our findings suggest that melatonin pretreatment before radiation sensitizes breast cancer cells to the ionizing effects of radiation by decreasing cell proliferation, inducing cell cycle arrest and downregulating proteins involved in double-strand DNA break repair. These findings may have implications for designing clinical trials using melatonin and radiotherapy.

Utilization of Rad51C promoter for transcriptional targeting of cancer cells

Cancer therapy that specifically targets malignant cells with minimal or no toxicity to normal tissue has been a long-standing goal of cancer research. Rad51 expression is elevated in a wide range of cancers and Rad51 promoter has been used to transcriptionally target tumor cells, however, a large size of Rad51 promoter limits its application for gene therapy. To identify novel tumor-specific promoters, we examined expression levels of Rad51 paralogs, Rad51B, Rad51C, and Rad51D as well as Rad52 in a panel of normal and tumor cell lines. We found that Rad51C is significantly overexpressed in cancer cells. The expression was up-regulated by approximately 6-fold at the mRNA level and 9-fold at the protein level. Interestingly, the 2064 bp long Rad51C promoter fragment was approximately 300-fold higher in cancer cells than in normal cells. A construct containing Rad51C promoter driving diphtheria toxin A efficiently killed several types of cancer cells with very mild effect to normal cells. These results underscore the potential of targeting the homologous recombination pathway in cancer cells and provide a proof of principle that the Rad51C promoter fragment can be used to transcriptionally target cancer cells.

Targeting homologous recombination, new pre-clinical and clinical therapeutic combinations inhibiting RAD51

The DNA damage response (DDR) is essential for maintaining genomic stability and cell survival. However, when tumour cells with deficiencies in HR are faced with radio- and chemotherapies they are forced to rely on error-prone, alternative repair pathways or aberrant HR for survival; threatening genome integrity and driving further mutation. Accurate therapeutic targeting of the key drivers of DNA repair can circumvent survival pathways and avoid aggressive therapy resistant mutants. Several studies have identified that stabilization of the cancer genome in HR deficient cells can be achieved by overexpression of the recombinase RAD51. Radio- and chemotherapeutic resistance is associated with overactive HR repair mechanisms. However no clinical trials have directly targeted RAD51, despite RAD51 displaying synergy in several drug screens against multiple cancer types. Currently synthetic lethality targeting the DDR pathways and HR deficiency has had clinical success with BRCA1 functional loss and PARP inhibition. In this review we suggest that clinical outcomes could be improved by additionally targeting RAD51. We examine the latest developments in directly and indirectly targeting RAD51. We scrutinize the potential treatment efficacy and future clinical applications of RAD51 inhibitors as single agents and in combination with other therapies and consider the best therapeutic options.

A new version of targeted minicircle producer system for EBV-positive human nasopharyngeal carcinoma

Targeted gene therapy needs to be implemented for future therapies to ensure efficient activity at the site of patient primary tumors or metastases without causing intolerable side-effects. One of the elements of gene therapy is vector, which includes viral and non-viral vector. In the present study, we constructed a novel non-viral targeted gene therapeutic system by using the new minicircle (MC) producing plasmid for Epstein-Barr virus (EBV)-positive nasopharyngeal carcinoma (NPC). Molecular cloning technique was used to construct plasmids and electrophoretic analysis. Dual-luciferase reporter assay was used to evaluate the expression of luciferase. Fluorescence microscope was used to detect the expression of enhanced green fluorescence protein (EGFP). We constructed a new MC producing system pMC.BESPX-origin of plasmid replication (oriP), and demonstrated that this system could produce highly purified MC-oriP. Furthermore, our results showed that MC-oriP vector produced by the new system could mediate targeted luciferase gene expression in EBV-positive NPC cells. In addition, we verified that MC could mediate enhanced transgene expression compared with parent plasmid through EGFP transfection. The present study constructed a targeted expression vector pMC.BESPX-oriP which could carry diversified therapeutic genes for EBV-positive NPC and provides a new approach for MC-based therapies.

Targeting the homologous recombination pathway by small molecule modulators

During the last decade, the use of small molecule (MW <500 Da) compounds that modulate (inhibit or activate) important proteins of different biological pathways became widespread. Recently, the homologous recombination (HR) pathway emerged as a target for such modulators. Development of small molecule modulators pursues two distinct but not mutually exclusive purposes: to create a research tool to study the activities or functions of proteins of interest and to produce drugs targeting specific pathologies. Here, we review the progress of small molecule development in the area of HR.

Regulation of Rad51 promoter

The DNA double-strand break repair and homologous recombination protein Rad51 is overexpressed in the majority of human cancers. This correlates with therapy resistance and decreased patient survival. We previously showed that constructs containing Rad51 promoter fused to a reporter gene are, on average, 850-fold more active in cancer cells than in normal cells. It is not well understood what factors and sequences regulate the Rad51 promoter and cause its high activity in cancerous cells. Here we characterized regulatory regions and examined genetic requirements for oncogenic stimulation of the Rad51 promoter. We identified specific regions responsible for up- and downregulation of the Rad51 promoter in cancerous cells. Furthermore, we show that Rad51 expression is positively regulated by EGR1 transcription factor. We then modeled the malignant transformation process by expressing a set of oncoproteins in normal human fibroblasts. Expression of different combinations of SV40 large T antigen, oncogenic Ras and SV40 small T antigen resulted in step-wise increase in Rad51 promoter activity, with all the 3 oncoproteins together leading to a 47-fold increase in expression. Cumulatively, these results suggest that Rad51 promoter is regulated by multiple factors, and that its expression is gradually activated as cells progress toward malignancy.

The RAD51-stimulatory compound RS-1 can exploit the RAD51 overexpression that exists in cancer cells and tumors

RAD51 is the central protein that catalyzes DNA repair via homologous recombination, a process that ensures genomic stability. RAD51 protein is commonly expressed at high levels in cancer cells relative to their noncancerous precursors. High levels of RAD51 expression can lead to the formation of genotoxic RAD51 protein complexes on undamaged chromatin. We developed a therapeutic approach that exploits this potentially toxic feature of malignancy, using compounds that stimulate the DNA-binding activity of RAD51 to promote cancer cell death. A panel of immortalized cell lines was challenged with the RAD51-stimulatory compound RS-1. Resistance to RS-1 tended to occur in cells with higher levels of RAD54L and RAD54B, which are Swi2/Snf2-related translocases known to dissociate RAD51 filaments from dsDNA. In PC3 prostate cancer cells, RS-1-induced lethality was accompanied by the formation of microscopically visible RAD51 nuclear protein foci occurring in the absence of any DNA-damaging treatment. Treatment with RS-1 promoted significant antitumor responses in a mouse model, providing proof-of-principle for this novel therapeutic strategy.

Gene expression profiling reveals activation of the FA/BRCA pathway in advanced squamous cervical cancer with intrinsic resistance and therapy failure

Background

Advanced squamous cervical cancer, one of the most commonly diagnosed cancers in women, still remains a major problem in oncology due to treatment failure and distant metastasis. Antitumor therapy failure is due to both intrinsic and acquired resistance; intrinsic resistance is often decisive for treatment response. In this study, we investigated the specific pathways and molecules responsible for baseline therapy failure in locally advanced squamous cervical cancer.

Methods

Twenty-one patients with locally advanced squamous cell carcinoma were enrolled in this study. Primary biopsies harvested prior to therapy were analyzed for whole human gene expression (Agilent) based on the patient’s 6 months clinical response. Ingenuity Pathway Analysis was used to investigate the altered molecular function and canonical pathways between the responding and non-responding patients. The microarray results were validated by qRT-PCR and immunohistochemistry. An additional set of 24 formalin-fixed paraffin-embedded cervical cancer samples was used for independent validation of the proteins of interest.

Results

A 2859-gene signature was identified to distinguish between responder and non-responder patients. ‘DNA Replication, Recombination and Repair’ represented one of the most important mechanisms activated in non-responsive cervical tumors, and the ‘Role of BRCA1 in DNA Damage Response’ was predicted to be the most significantly altered canonical pathway involved in intrinsic resistance (p = 1.86E-04, ratio = 0.262). Immunohistological staining confirmed increased expression of BRCA1, BRIP1, FANCD2 and RAD51 in non-responsive compared with responsive advanced squamous cervical cancer, both in the initial set of 21 cervical cancer samples and the second set of 24 samples.

Conclusions

Our findings suggest that FA/BRCA pathway plays an important role in treatment failure in advanced cervical cancer. The assessment of FANCD2, RAD51, BRCA1 and BRIP1 nuclear proteins could provide important information about the patients at risk for treatment failure.

CD133+ cells contribute to radioresistance via altered regulation of DNA repair genes in human lung cancer cells

BACKGROUND

Radioresistance in human tumors has been linked in part to a subset of cells termed cancer stem cells (CSCs). The prominin 1 (CD133) cell surface protein is proposed to be a marker enriching for CSCs. We explore the importance of DNA repair in contributing to radioresistance in CD133+ lung cancer cells.

MATERIALS AND METHODS

A549 and H1299 lung cancer cell lines were used. Sorted CD133+ cells were exposed to either single 4 Gy or 8 Gy doses and clonogenic survival measured. ϒ-H2AX immunofluorescence and quantitative real time PCR was performed on sorted CD133+ cells both in the absence of IR and after two single 4 Gy doses. Lentiviral shRNA was used to silence repair genes.

RESULTS

A549 but not H1299 cells expand their CD133+ population after single 4 Gy exposure, and isolated A549 CD133+ cells demonstrate IR resistance. This resistance corresponded with enhanced repair of DNA double strand breaks (DSBs) and upregulated expression of DSB repair genes in A549 cells. Prior IR exposure of two single 4 Gy doses resulted in acquired DNA repair upregulation and improved repair proficiency in both A549 and H1299. Finally Exo1 and Rad51 silencing in A549 cells abrogated the CD133+ IR expansion phenotype and induced IR sensitivity in sorted CD133+ cells.

CONCLUSIONS

CD133 identifies a population of cells within specific tumor types containing altered expression of DNA repair genes that are inducible upon exposure to chemotherapy. This altered gene expression contributes to enhanced DSB resolution and the radioresistance phenotype of these cells. We also identify DNA repair genes which may serve as promising therapeutic targets to confer radiosensitivity to CSCs.

Targeting homologous recombination-mediated DNA repair in cancer

INTRODUCTION

DNA is the target of many traditional non-specific chemotherapeutic drugs. New drugs or therapeutic approaches with a more rational and targeted component are mandatory to improve the success of cancer therapy. The homologous recombination (HR) pathway is an attractive target for the development of inhibitors because cancer cells rely heavily on HR for repair of DNA double-strand breaks resulting from chemotherapeutic treatments. Additionally, the discovery that poly(ADP)ribose polymerase-1 inhibitors selectively kill cells with genetic defects in HR has spurned an even greater interest in inhibitors of HR.

AREAS COVERED

HR drives the repair of broken DNA via numerous protein-mediated sequential DNA manipulations. Due to extensive number of steps and proteins involved, the HR pathway provides a rich pool of potential drug targets. This review discusses the latest developments concerning the strategies being explored to inhibit HR. Particular attention is given to the identification of small molecule inhibitors of key HR proteins, including the BRCA proteins and RAD51.

EXPERT OPINION

Current HR inhibitors are providing the basis for pharmaceutical development of more potent and specific inhibitors to be applied in mono- or combinatorial therapy regimes, while novel targets will be uncovered by experiments aimed to gain a deeper mechanistic understanding of HR and its subpathways.

Systematic screen identifies miRNAs that target RAD51 and RAD51D to enhance chemosensitivity

Homologous recombination mediates error-free repair of DNA double-strand breaks (DSB). RAD51 is an essential protein for catalyzing homologous recombination and its recruitment to DSBs is mediated by many factors including RAD51, its paralogs, and breast/ovarian cancer susceptibility gene products BRCA1/2. Deregulation of these factors leads to impaired DNA repair, genomic instability, and cellular sensitivity to chemotherapeutics such as cisplatin and PARP inhibitors. microRNAs (miRNA) are short, noncoding RNAs that posttranscriptionally regulate gene expression; however, the contribution of miRNAs in the regulation of homologous recombination is not well understood. To address this, a library of human miRNA mimics was systematically screened to pinpoint several miRNAs that significantly reduce RAD51 foci formation in response to ionizing radiation in human osteosarcoma cells. Subsequent study focused on two of the strongest candidates, miR-103 and miR-107, as they are frequently deregulated in cancer. Consistent with the inhibition of RAD51 foci formation, miR-103 and miR-107 reduced homology-directed repair and sensitized cells to various DNA-damaging agents, including cisplatin and a PARP inhibitor. Mechanistic analyses revealed that both miR-103 and miR-107 directly target and regulate RAD51 and RAD51D, which is critical for miR-103/107-mediated chemosensitization. Furthermore, endogenous regulation of RAD51D by miR-103/107 was observed in several tumor subtypes. Taken together, these data show that miR-103 and miR-107 overexpression promotes genomic instability and may be used therapeutically to chemosensitize tumors.

IMPLICATIONS

These findings demonstrate a role for miR-103 and -107 in regulating DNA damage repair, thereby identifying new players in the progression of cancer and response to chemotherapy.