Development of a high-throughput fluorescence polarization DNA cleavage assay for the identification of FEN1 inhibitors

High-throughput kinetics in drug discovery

The importance of a drug's kinetic profile and interplay of structure-kinetic activity with PK/PD has long been appreciated in drug discovery. However, technical challenges have often limited detailed kinetic characterization of compounds to the latter stages of projects. This review highlights the advances that have been made in recent years in techniques, instrumentation, and data analysis to increase the throughput of detailed kinetic and mechanistic characterization, enabling its application earlier in the drug discovery process.

Construction of dual exponential amplification accompanied by multi-terminal signal output method for convenient detection of tumor biomarker FEN1 activity

As an important 5'-nuclease in DNA replication and damage repair, Flap endonuclease 1 (FEN1) has been considered as a potential tumor biomarker due to its overexpression in different human cancer cells. Here, we developed a convenient fluorescent method based on dual enzymatic repairing exponential amplification accompanied by multi-terminal signal output to realize the rapid and sensitive detection of FEN1. In the presence of FEN1, the double-branched substrate could be cleaved to produce 5' flap single strand DNA (ssDNA) which subsequently was used as a primer to initiate the dual exponential amplification (EXPAR) to generate abundant ssDNAs (X' and Y'), then the ssDNAs can respectively hybridize with the 3' and 5' ends of the signal probe to form partially complementary double strands (dsDNAs). Subsequently, the signal probe on the dsDNAs could be digested under the assistance of Bst. polymerase and T7 exonuclease, as well as releasing the fluorescence signals. The method displayed high sensitivity with the detection limit of 9.7 × 10^-3 U mL^-1 (1.94 × 10^-4 U) and also exhibited good selectivity towards FEN1 under the challenge from complicated samples including extracts of normal and cancer cells. Furthermore, it was successfully applied to screen FEN1 inhibitors, holding great promise in the screening of potential drugs targeting FEN1. This sensitive, selective and convenient method could be used for FEN1 assay without the complicated nanomaterial synthesis/modification, showing great potential in FEN1- related prediction and diagnosis.

Detection speed optimization of the OI-RD microscope for ultra-high throughput screening

The oblique-incidence reflectivity difference (OI-RD) microscope is a label-free detection system for microarrays that has many successful applications in high throughput drug screening. The increase and optimization of the detection speed of the OI-RD microscope will enable it to be a potential ultra-high throughput screening tool. This work presents a series of optimization methods that can significantly reduce the time to scan an OI-RD image. The wait time for the lock-in amplifier was decreased by the proper selection of the time constant and development of a new electronic amplifier. In addition, the time for the software to acquire data and for translation stage movement was also minimized. As a result, the detection speed of the OI-RD microscope is 10 times faster than before, making the OI-RD microscope suitable for ultra-high throughput screening applications.

Detection of Cancer Marker Flap Endonuclease 1 Using One-Pot Transcription-Powered Clustered Regularly Interspaced Short Palindromic Repeat/Cas12a Signal Expansion

As a critical functional protein in DNA replication and genome stability, flap endonuclease 1 (FEN1) has been considered a promising biomarker and druggable target for multiple cancers. We report here a transcription-powered clustered regularly interspaced short palindromic repeat (CRISPR)/Cas12a signal expansion platform for rapid and sensitive detection of FEN1. In this method, the probe cleavage by FEN1 generated a free 5' flap single-stranded DNA which could hybridize with the single-stranded T7 promoter-bearing template and trigger the extension. Then, the CRISPR guide RNA (crRNA) transcribed from the extended template activated the collateral DNase activity of Cas12a, releasing the fluorophore from the quenched DNA signal probe to report the FEN1 detection result. The high specificity for FEN1 was validated by comparing with other repair-relevant proteins. The limit of detection (LOD) could be as low as 0.03 mU, which is sensitive enough to detect the FEN1 activity in biological samples. In addition, the inhibition assay of FEN1 was also successfully achieved with this platform, proving its potential in inhibitor screening. In summary, this study provides a novel biosensor for FEN1 activity analysis and provides new insights into the development of CRISPR-based biosensors for non-nucleic acid targets.

Cross-Species Complementation of Nonessential Yeast Genes Establishes Platforms for Testing Inhibitors of Human Proteins

Cross-species complementation can be used to generate humanized yeast, which is a valuable resource with which to model and study human biology. Humanized yeast can be used as an in vivo platform to screen for chemical inhibition of human protein drug targets. To this end, we report the systematic complementation of nonessential yeast genes implicated in chromosome instability (CIN) with their human homologs. We identified 20 human-yeast complementation pairs that are replaceable in 44 assays that test rescue of chemical sensitivity and/or CIN defects. We selected a human-yeast pair (hFEN1/yRAD27), which is frequently overexpressed in cancer and is an anticancer therapeutic target, to perform in vivo inhibitor assays using a humanized yeast cell-based platform. In agreement with published in vitro assays, we demonstrate that HU-based PTPD is a species-specific hFEN1 inhibitor. In contrast, another reported hFEN1 inhibitor, the arylstibonic acid derivative NSC-13755, was determined to have off-target effects resulting in a synthetic lethal phenotype with yRAD27-deficient strains. Our study expands the list of human-yeast complementation pairs to nonessential genes by defining novel cell-based assays that can be utilized as a broad resource to study human drug targets.

Flap endonuclease overexpression drives genome instability and DNA damage hypersensitivity in a PCNA-dependent manner

Overexpression of the flap endonuclease FEN1 has been observed in a variety of cancer types and is a marker for poor prognosis. To better understand the cellular consequences of FEN1 overexpression we utilized a model of its Saccharomyces cerevisiae homolog, RAD27. In this system, we discovered that flap endonuclease overexpression impedes replication fork progression and leads to an accumulation of cells in mid-S phase. This was accompanied by increased phosphorylation of the checkpoint kinase Rad53 and histone H2A-S129. RAD27 overexpressing cells were hypersensitive to treatment with DNA damaging agents, and defective in ubiquitinating the replication clamp proliferating cell nuclear antigen (PCNA) at lysine 164. These effects were reversed when the interaction between overexpressed Rad27 and PCNA was ablated, suggesting that the observed phenotypes were linked to problems in DNA replication. RAD27 overexpressing cells also exhibited an unexpected dependence on the SUMO ligases SIZ1 and MMS21 for viability. Importantly, we found that overexpression of FEN1 in human cells also led to phosphorylation of CHK1, CHK2, RPA32 and histone H2AX, all markers of genome instability. Our data indicate that flap endonuclease overexpression is a driver of genome instability in yeast and human cells that impairs DNA replication in a manner dependent on its interaction with PCNA.

Identification of human flap endonuclease 1 (FEN1) inhibitors using a machine learning based consensus virtual screening

Human Flap endonuclease1 (FEN1) is an enzyme that is indispensable for DNA replication and repair processes and inhibition of its Flap cleavage activity results in increased cellular sensitivity to DNA damaging agents (cisplatin, temozolomide, MMS, etc.), with the potential to improve cancer prognosis. Reports of the high expression levels of FEN1 in several cancer cells support the idea that FEN1 inhibitors may target cancer cells with minimum side effects to normal cells. In this study, we used large publicly available, high-throughput screening data of small molecule compounds targeted against FEN1. Two machine learning algorithms, Support Vector Machine (SVM) and Random Forest (RF), were utilized to generate four classification models from huge PubChem bioassay data containing probable FEN1 inhibitors and non-inhibitors. We also investigated the influence of randomly selected Zinc-database compounds as negative data on the outcome of classification modelling. The results show that the SVM model with inactive compounds was superior to RF with Matthews's correlation coefficient (MCC) of 0.67 for the test set. A Maybridge database containing approximately 53 000 compounds was screened and top ranking 5 compounds were selected for enzyme and cell-based in vitro screening. The compound JFD00950 was identified as a novel FEN1 inhibitor with in vitro inhibition of flap cleavage activity as well as cytotoxic activity against a colon cancer cell line, DLD-1.

Synergistic antitumor effect of combined paclitaxel with FEN1 inhibitor in cervical cancer cells

Studies on cervical cancer are urgently required to improve clinical outcomes. As a major anticancer drug for cervical cancer, paclitaxel has been used for many years in clinical therapy but its therapeutic efficacy is limited by common obstacle from cancer cells. The enhanced DNA repair pathways of cancer cells have been proved to survive DNA damage induced by chemotherapeutic drug. Inhibitors of specific DNA repair pathway can sensitize cancer cells to the treatment of chemotherapeutic drugs. In this paper we found that the effect of paclitaxel can be significantly improved when used in combination with FEN1 inhibitor SC13, suggesting a synergistic mechanism between the two compounds. Our studies suggest that FEN1 inhibition could be a novel strategy of tumor-targeting therapy for cervical cancer. Our work also revealed that paclitaxel demonstrates stronger synergistic effect with SC13 than other common used chemical drugs such as doxorubicin, carboplatin or camptothecin on cervical cancer cells.

Small molecule inhibitors uncover synthetic genetic interactions of human flap endonuclease 1 (FEN1) with DNA damage response genes

Flap endonuclease 1 (FEN1) is a structure selective endonuclease required for proficient DNA replication and the repair of DNA damage. Cellularly active inhibitors of this enzyme have previously been shown to induce a DNA damage response and, ultimately, cell death. High-throughput screens of human cancer cell-lines identify colorectal and gastric cell-lines with microsatellite instability (MSI) as enriched for cellular sensitivity to N-hydroxyurea series inhibitors of FEN1, but not the PARP inhibitor olaparib or other inhibitors of the DNA damage response. This sensitivity is due to a synthetic lethal interaction between FEN1 and MRE11A, which is often mutated in MSI cancers through instabilities at a poly(T) microsatellite repeat. Disruption of ATM is similarly synthetic lethal with FEN1 inhibition, suggesting that disruption of FEN1 function leads to the accumulation of DNA double-strand breaks. These are likely a result of the accumulation of aberrant replication forks, that accumulate as a consequence of a failure in Okazaki fragment maturation, as inhibition of FEN1 is toxic in cells disrupted for the Fanconi anemia pathway and post-replication repair. Furthermore, RAD51 foci accumulate as a consequence of FEN1 inhibition and the toxicity of FEN1 inhibitors increases in cells disrupted for the homologous recombination pathway, suggesting a role for homologous recombination in the resolution of damage induced by FEN1 inhibition. Finally, FEN1 appears to be required for the repair of damage induced by olaparib and cisplatin within the Fanconi anemia pathway, and may play a role in the repair of damage associated with its own disruption.

Targeting DNA Flap Endonuclease 1 to Impede Breast Cancer Progression

DNA flap endonuclease 1 (FEN1) plays critical roles in maintaining genome stability and integrity by participating in both DNA replication and repair. Suppression of FEN1 in cells leads to the retardation of DNA replication and accumulation of unrepaired DNA intermediates, resulting in DNA double strand breaks (DSBs) and apoptosis. Therefore, targeting FEN1 could serve as a potent strategy for cancer therapy. In this study, we demonstrated that FEN1 is overexpressed in breast cancers and is essential for rapid proliferation of cancer cells. We showed that manipulating FEN1 levels in cells alters the response of cancer cells to chemotherapeutic drugs. Furthermore, we identified a small molecular compound, SC13 that specifically inhibits FEN1 activity, thereby interfering with DNA replication and repair in vitro and in cells. SC13 suppresses cancer cell proliferation and induces chromosome instability and cytotoxicity in cells. Importantly, SC13 sensitizes cancer cells to DNA damage-inducing therapeutic modalities and impedes cancer progression in a mouse model. These findings could establish a paradigm for the treatment of breast cancer and other cancers as well.

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FEN1 is overexpressed in cancer cells and essential for cancer cell growth;

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Down regulation of FEN1 leads to retarded cell growth and sensitizes cancer cells to chemotherapeutic agents;

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SC13, a FEN1 specific inhibitor, inhibits cancer growth in vitro and in xenograft tumor mice.

Most anticancer agents used in clinic today kill cells by interfering DNA replication or inducing DNA damage, which in turn lead to cell apoptosis. However, cancer cells have evolved a compilation of highly effective DNA replication and repair systems to meet up the requirement of rapidly dividing of cancer cells and protect DNA against both endogenous and exogenous DNA damage. FEN1 has been shown to be an important factor in both DNA replication and repair pathways, making FEN1 a logical target for developing anticancer drugs as stand-alone agents for treating cancers that rely on its activity and as a therapy in combination with chemotherapeutic agents that cause DNA damage.

Cellularly active N-hydroxyurea FEN1 inhibitors block substrate entry to the active site

The structure-specific nuclease human flap endonuclease-1 (hFEN1) plays a key role in DNA replication and repair and may be of interest as an oncology target. We present the crystal structure of inhibitor-bound hFEN1, which shows a cyclic N-hydroxyurea bound in the active site coordinated to two magnesium ions. Three such compounds had similar IC50 values but differed subtly in mode of action. One had comparable affinity for protein and protein-substrate complex and prevented reaction by binding to active site catalytic metal ions, blocking the necessary unpairing of substrate DNA. Other compounds were more competitive with substrate. Cellular thermal shift data showed that both inhibitor types engaged with hFEN1 in cells, and activation of the DNA damage response was evident upon treatment with inhibitors. However, cellular EC50 values were significantly higher than in vitro inhibition constants, and the implications of this for exploitation of hFEN1 as a drug target are discussed.

Direct observation of DNA threading in flap endonuclease complexes

Maintenance of genome integrity requires that branched nucleic acid molecules are accurately processed to produce double-helical DNA. Flap endonucleases are essential enzymes that trim such branched molecules generated by Okazaki fragment synthesis during replication. Here, we report crystal structures of bacteriophage T5 flap endonuclease in complexes with intact DNA substrates, and products, at resolutions of 1.9–2.2 Å. They reveal single-stranded DNA threading through a hole in the enzyme enclosed by an inverted V-shaped helical arch straddling the active site. Residues lining the hole induce an unusual barb-like conformation in the DNA substrate juxtaposing the scissile phosphate and essential catalytic metal ions. A series of complexes and biochemical analyses show how the substrate’s single-stranded branch approaches, threads through, and finally emerges on the far side of the enzyme. Our studies suggest that substrate recognition involves an unusual “fly-casting, thread, bend and barb” mechanism.

Adapting capillary gel electrophoresis as a sensitive, high-throughput method to accelerate characterization of nucleic acid metabolic enzymes

Detailed biochemical characterization of nucleic acid enzymes is fundamental to understanding nucleic acid metabolism, genome replication and repair. We report the development of a rapid, high-throughput fluorescence capillary gel electrophoresis method as an alternative to traditional polyacrylamide gel electrophoresis to characterize nucleic acid metabolic enzymes. The principles of assay design described here can be applied to nearly any enzyme system that acts on a fluorescently labeled oligonucleotide substrate. Herein, we describe several assays using this core capillary gel electrophoresis methodology to accelerate study of nucleic acid enzymes. First, assays were designed to examine DNA polymerase activities including nucleotide incorporation kinetics, strand displacement synthesis and 3′-5′ exonuclease activity. Next, DNA repair activities of DNA ligase, flap endonuclease and RNase H2 were monitored. In addition, a multicolor assay that uses four different fluorescently labeled substrates in a single reaction was implemented to characterize GAN nuclease specificity. Finally, a dual-color fluorescence assay to monitor coupled enzyme reactions during Okazaki fragment maturation is described. These assays serve as a template to guide further technical development for enzyme characterization or nucleoside and non-nucleoside inhibitor screening in a high-throughput manner.

Inhibition of the ERCC1-XPF structure-specific endonuclease to overcome cancer chemoresistance

ERCC1-XPF is a structure-specific endonuclease that is required for the repair of DNA lesions, generated by the widely used platinum-containing cancer chemotherapeutics such as cisplatin, through the Nucleotide Excision Repair and Interstrand Crosslink Repair pathways. Based on mouse xenograft experiments, where ERCC1-deficient melanomas were cured by cisplatin therapy, we proposed that inhibition of ERCC1-XPF could enhance the effectiveness of platinum-based chemotherapy. Here we report the identification and properties of inhibitors against two key targets on ERCC1-XPF. By targeting the ERCC1-XPF interaction domain we proposed that inhibition would disrupt the ERCC1-XPF heterodimer resulting in destabilisation of both proteins. Using in silico screening, we identified an inhibitor that bound to ERCC1-XPF in a biophysical assay, reduced the level of ERCC1-XPF complexes in ovarian cancer cells, inhibited Nucleotide Excision Repair and sensitised melanoma cells to cisplatin. We also utilised high throughput and in silico screening to identify the first reported inhibitors of the other key target, the XPF endonuclease domain. We demonstrate that two of these compounds display specificity in vitro for ERCC1-XPF over two other endonucleases, bind to ERCC1-XPF, inhibit Nucleotide Excision Repair in two independent assays and specifically sensitise Nucleotide Excision Repair-proficient, but not Nucleotide Excision Repair-deficient human and mouse cells to cisplatin.

DNA Repair Endonucleases: Physiological Roles and Potential as Drug Targets

Genomic DNA is constantly exposed to endogenous and exogenous damaging agents. To overcome these damaging effects and maintain genomic stability, cells have robust coping mechanisms in place, including repair of the damaged DNA. There are a number of DNA repair pathways available to cells dependent on the type of damage induced. The removal of damaged DNA is essential to allow successful repair. Removal of DNA strands is achieved by nucleases. Exonucleases are those that progressively cut from DNA ends, and endonucleases make single incisions within strands of DNA. This review focuses on the group of endonucleases involved in DNA repair pathways, their mechanistic functions, roles in cancer development, and how targeting these enzymes is proving to be an exciting new strategy for personalized therapy in cancer.

Therapeutic opportunities within the DNA damage response

The DNA damage response (DDR) is essential for maintaining the genomic integrity of the cell, and its disruption is one of the hallmarks of cancer. Classically, defects in the DDR have been exploited therapeutically in the treatment of cancer with radiation therapies or genotoxic chemotherapies. More recently, protein components of the DDR systems have been identified as promising avenues for targeted cancer therapeutics. Here, we present an in-depth analysis of the function, role in cancer and therapeutic potential of 450 expert-curated human DDR genes. We discuss the DDR drugs that have been approved by the US Food and Drug Administration (FDA) or that are under clinical investigation. We examine large-scale genomic and expression data for 15 cancers to identify deregulated components of the DDR, and we apply systematic computational analysis to identify DDR proteins that are amenable to modulation by small molecules, highlighting potential novel therapeutic targets.

Using the BioAssay Ontology for analyzing high-throughput screening data

High-throughput screening (HTS) is the main starting point for hit identification in drug discovery programs. This has led to a rapid increase of available screening data both within pharmaceutical companies and the public domain. We have used the BioAssay Ontology (BAO) 2.0 for assay annotation within AstraZeneca to enable comparison with external HTS methods. The annotated assays have been analyzed to identify technology gaps, evaluate new methods, verify active hits, and compare compound activity between in-house and PubChem assays. As an example, the binding of a fluorescent ligand to formyl peptide receptor 1 (FPR1, involved in inflammation, for example) in an in-house HTS was measured by fluorescence intensity. In total, 155 active compounds were also tested in an external ligand binding flow cytometry assay, a method not used for in-house HTS detection. Twelve percent of the 155 compounds were found active in both assays. By the annotation of assay protocols using BAO terms, internal and external assays can easily be identified and method comparison facilitated. They can be used to evaluate the effectiveness of different assay methods, design appropriate confirmatory and counterassays, and analyze the activity of compounds for identification of technology artifacts.