DNA structural elements required for ERCC1-XPF endonuclease activity

Repair of genomic interstrand crosslinks

Genomic interstrand crosslinks (ICLs) are formed by reactive species generated during normal cellular metabolism, produced by the microbiome, and employed in cancer chemotherapy. While there are multiple options for replication dependent and independent ICL repair, the crucial step for each is unhooking one DNA strand from the other. Much of our insight into mechanisms of unhooking comes from powerful model systems based on plasmids with defined ICLs introduced into cells or cell free extracts. Here we describe the properties of exogenous and endogenous ICL forming compounds and provide an historical perspective on early work on ICL repair. We discuss the modes of unhooking elucidated in the model systems, the concordance or lack thereof in drug resistant tumors, and the evolving view of DNA adducts, including ICLs, formed by metabolic aldehydes.

Analysis of cytosine deamination events in excision repair sequencing reads reveals mechanisms of incision site selection in NER

Nucleotide excision repair (NER) removes helix-distorting DNA lesions and is therefore critical for genome stability. During NER, DNA is unwound on either side of the lesion and excised, but the rules governing incision site selection, particularly in eukaryotic cells, are unclear. Excision repair-sequencing (XR-seq) sequences excised NER fragments, but analysis has been limited because the lesion location is unknown. Here, we exploit accelerated cytosine deamination rates in UV-induced CPD (cyclobutane pyrimidine dimer) lesions to precisely map their locations at C to T mismatches in XR-seq reads, revealing general and species-specific patterns of incision site selection during NER. Our data indicate that the 5' incision site occurs preferentially in HYV (i.e. not G; C/T; not T) sequence motifs, a pattern that can be explained by sequence preferences of the XPF-ERCC1 endonuclease. In contrast, the 3' incision site does not show strong sequence preferences, once truncated reads arising from mispriming events are excluded. Instead, the 3' incision is partially determined by the 5' incision site distance, indicating that the two incision events are coupled. Finally, our data reveal unique and coupled NER incision patterns at nucleosome boundaries. These findings reveal key principles governing NER incision site selection in eukaryotic cells.

Saccharomyces cerevisiae DNA polymerase IV overcomes Rad51 inhibition of DNA polymerase δ in Rad52-mediated direct-repeat recombination

Saccharomyces cerevisiae DNA polymerase IV (Pol4) like its homolog, human DNA polymerase lambda (Polλ), is involved in Non-Homologous End-Joining and Microhomology-Mediated Repair. Using genetic analysis, we identified an additional role of Pol4 also in homology-directed DNA repair, specifically in Rad52-dependent/Rad51-independent direct-repeat recombination. Our results reveal that the requirement for Pol4 in repeat recombination was suppressed by the absence of Rad51, suggesting that Pol4 counteracts the Rad51 inhibition of Rad52-mediated repeat recombination events. Using purified proteins and model substrates, we reconstituted in vitro reactions emulating DNA synthesis during direct-repeat recombination and show that Rad51 directly inhibits Polδ DNA synthesis. Interestingly, although Pol4 was not capable of performing extensive DNA synthesis by itself, it aided Polδ in overcoming the DNA synthesis inhibition by Rad51. In addition, Pol4 dependency and stimulation of Polδ DNA synthesis in the presence of Rad51 occurred in reactions containing Rad52 and RPA where DNA strand-annealing was necessary. Mechanistically, yeast Pol4 displaces Rad51 from ssDNA independent of DNA synthesis. Together our in vitro and in vivo data suggest that Rad51 suppresses Rad52-dependent/Rad51-independent direct-repeat recombination by binding to the primer-template and that Rad51 removal by Pol4 is critical for strand-annealing dependent DNA synthesis.

Perspectives of ERCC1 in early-stage and advanced cervical cancer: From experiments to clinical applications

Cervical cancer is a public health problem of extensive clinical importance. Excision repair cross-complementation group 1 (ERCC1) was found to be a promising biomarker of cervical cancer over the years. At present, there is no relevant review article that summarizes such evidence. In this review, nineteen eligible studies were included for evaluation and data extraction. Based on the data from clinical and experimental studies, ERCC1 plays a key role in the progression of carcinoma of the uterine cervix and the therapeutic response of chemoradiotherapy. The majority of the included studies (13/19, 68%) suggested that ERCC1 played a pro-oncogenic role in both early-stage and advanced cervical cancer. High expression of ERCC1 was found to be associated with the poor survival rates of the patients. ERCC1 polymorphism analyses demonstrated that ERCC1 might be a useful tool for predicting the risk of cervical cancer and the treatment-related toxicities. Experimental studies indicated that the biological effects exerted by ERCC1 in cervical cancer might be mediated by its associated genes and affected signaling pathways (i.e., XPF, TUBB3, and. To move towards clinical applications by targeting ERCC1 in cervical cancer, more clinical, in-vitro, and in-vivo investigations are still warranted in the future.

Modulation of ERCC1-XPF Heterodimerization Inhibition via Structural Modification of Small Molecule Inhibitor Side-Chains

Inhibition of DNA repair enzymes is an attractive target for increasing the efficacy of DNA damaging chemotherapies. The ERCC1-XPF heterodimer is a key endonuclease in numerous single and double strand break repair processes, and inhibition of the heterodimerization has previously been shown to sensitize cancer cells to DNA damage. In this work, the previously reported ERCC1-XPF inhibitor 4 was used as the starting point for an in silico study of further modifications of the piperazine side-chain. A selection of the best scoring hits from the in silico screen were synthesized using a late stage functionalization strategy which should allow for further iterations of this class of inhibitors to be readily synthesized. Of the synthesized compounds, compound 6 performed the best in the in vitro fluorescence based endonuclease assay. The success of compound 6 in inhibiting ERCC1-XPF endonuclease activity in vitro translated well to cell-based assays investigating the inhibition of nucleotide excision repair and disruption of heterodimerization. Subsequently compound 6 was shown to sensitize HCT-116 cancer cells to treatment with UVC, cyclophosphamide, and ionizing radiation. This work serves as an important step towards the synergistic use of DNA repair inhibitors with chemotherapeutic drugs.

RETRACTED: Human DNA polymerase θ harbors DNA end-trimming activity critical for DNA repair

Cancers with hereditary defects in homologous recombination rely on DNA polymerase θ (pol θ) for repair of DNA double-strand breaks. During end joining, pol θ aligns microhomology tracts internal to 5'-resected broken ends. An unidentified nuclease trims the 3' ends before synthesis can occur. Here we report that a nuclease activity, which differs from the proofreading activity often associated with DNA polymerases, is intrinsic to the polymerase domain of pol θ. Like the DNA synthesis activity, the nuclease activity requires conserved metal-binding residues, metal ions, and dNTPs and is inhibited by ddNTPs or chain-terminated DNA. Our data indicate that pol θ repurposes metal ions in the polymerase active site for endonucleolytic cleavage and that the polymerase-active and end-trimming conformations of the enzyme are distinct. We reveal a nimble strategy of substrate processing that allows pol θ to trim or extend DNA depending on the DNA repair context.

Coordinated roles of SLX4 and MutSβ in DNA repair and the maintenance of genome stability

SLX4 provides a molecular scaffold for the assembly of multiple protein complexes required for the maintenance of genome stability. It is involved in the repair of DNA crosslinks, the resolution of recombination intermediates, the response to replication stress and the maintenance of telomere length. To carry out these diverse functions, SLX4 interacts with three structure-selective endonucleases, MUS81-EME1, SLX1 and XPF-ERCC1, as well as the telomere binding proteins TRF2, RTEL1 and SLX4IP. Recently, SLX4 was shown to interact with MutSβ, a heterodimeric protein involved in DNA mismatch repair, trinucleotide repeat instability, crosslink repair and recombination. Importantly, MutSβ promotes the pathogenic expansion of CAG/CTG trinucleotide repeats, which is causative of myotonic dystrophy and Huntington's disease. The colocalization and specific interaction of MutSβ with SLX4, together with their apparently overlapping functions, are suggestive of a common role in reactions that promote DNA maintenance and genome stability. This review will focus on the role of SLX4 in DNA repair, the interplay between MutSβ and SLX4, and detail how they cooperate to promote recombinational repair and DNA crosslink repair. Furthermore, we speculate that MutSβ and SLX4 may provide an alternative cellular mechanism that modulates trinucleotide instability.

ERCC1 mutations impede DNA damage repair and cause liver and kidney dysfunction in patients

ERCC1-XPF is a multifunctional endonuclease involved in nucleotide excision repair (NER), interstrand cross-link (ICL) repair, and DNA double-strand break (DSB) repair. Only two patients with bi-allelic ERCC1 mutations have been reported, both of whom had features of Cockayne syndrome and died in infancy. Here, we describe two siblings with bi-allelic ERCC1 mutations in their teenage years. Genomic sequencing identified a deletion and a missense variant (R156W) within ERCC1 that disrupts a salt bridge below the XPA-binding pocket. Patient-derived fibroblasts and knock-in epithelial cells carrying the R156W substitution show dramatically reduced protein levels of ERCC1 and XPF. Moreover, mutant ERCC1 weakly interacts with NER and ICL repair proteins, resulting in diminished recruitment to DNA damage. Consequently, patient cells show strongly reduced NER activity and increased chromosome breakage induced by DNA cross-linkers, while DSB repair was relatively normal. We report a new case of ERCC1 deficiency that severely affects NER and considerably impacts ICL repair, which together result in a unique phenotype combining short stature, photosensitivity, and progressive liver and kidney dysfunction.

CRISPR/Cas-based precision genome editing via microhomology-mediated end joining

Gene editing and/or allele introgression with absolute precision and control appear to be the ultimate goals of genetic engineering. Precision genome editing in plants has been developed through various approaches, including oligonucleotide-directed mutagenesis (ODM), base editing, prime editing and especially homologous recombination (HR)-based gene targeting. With the advent of CRISPR/Cas for the targeted generation of DNA breaks (single-stranded breaks (SSBs) or double-stranded breaks (DSBs)), a substantial advancement in HR-mediated precise editing frequencies has been achieved. Nonetheless, further research needs to be performed for commercially viable applications of precise genome editing; hence, an alternative innovative method for genome editing may be required. Within this scope, we summarize recent progress regarding precision genome editing mediated by microhomology-mediated end joining (MMEJ) and discuss their potential applications in crop improvement.

Human XPG nuclease structure, assembly, and activities with insights for neurodegeneration and cancer from pathogenic mutations

DNA repair is essential to life and to avoidance of genome instability and cancer. Xeroderma pigmentosum group G (XPG) protein acts in multiple DNA repair pathways, both as an active enzyme and as a scaffold for coordinating with other repair proteins. We present here the structure of the catalytic domain responsible for its DNA binding and nuclease activity. Our analysis provides structure-based hypotheses for how XPG recognizes its bubble DNA substrate and predictions of the structural impacts of XPG disease mutations associated with two phenotypically distinct diseases: xeroderma pigmentosum (XP, skin cancer prone) or Cockayne syndrome (XP/CS, severe progressive developmental defects).

Xeroderma pigmentosum group G (XPG) protein is both a functional partner in multiple DNA damage responses (DDR) and a pathway coordinator and structure-specific endonuclease in nucleotide excision repair (NER). Different mutations in the XPG gene ERCC5 lead to either of two distinct human diseases: Cancer-prone xeroderma pigmentosum (XP-G) or the fatal neurodevelopmental disorder Cockayne syndrome (XP-G/CS). To address the enigmatic structural mechanism for these differing disease phenotypes and for XPG’s role in multiple DDRs, here we determined the crystal structure of human XPG catalytic domain (XPGcat), revealing XPG-specific features for its activities and regulation. Furthermore, XPG DNA binding elements conserved with FEN1 superfamily members enable insights on DNA interactions. Notably, all but one of the known pathogenic point mutations map to XPGcat, and both XP-G and XP-G/CS mutations destabilize XPG and reduce its cellular protein levels. Mapping the distinct mutation classes provides structure-based predictions for disease phenotypes: Residues mutated in XP-G are positioned to reduce local stability and NER activity, whereas residues mutated in XP-G/CS have implied long-range structural defects that would likely disrupt stability of the whole protein, and thus interfere with its functional interactions. Combined data from crystallography, biochemistry, small angle X-ray scattering, and electron microscopy unveil an XPG homodimer that binds, unstacks, and sculpts duplex DNA at internal unpaired regions (bubbles) into strongly bent structures, and suggest how XPG complexes may bind both NER bubble junctions and replication forks. Collective results support XPG scaffolding and DNA sculpting functions in multiple DDR processes to maintain genome stability.

Acetylation of XPF by TIP60 facilitates XPF-ERCC1 complex assembly and activation

The XPF-ERCC1 heterodimer is a structure-specific endonuclease that is essential for nucleotide excision repair (NER) and interstrand crosslink (ICL) repair in mammalian cells. However, whether and how XPF binding to ERCC1 is regulated has not yet been established. Here, we show that TIP60, also known as KAT5, a haplo-insufficient tumor suppressor, directly acetylates XPF at Lys911 following UV irradiation or treatment with mitomycin C and that this acetylation is required for XPF-ERCC1 complex assembly and subsequent activation. Mechanistically, acetylation of XPF at Lys911 disrupts the Glu907-Lys911 salt bridge, thereby leading to exposure of a previously unidentified second binding site for ERCC1. Accordingly, loss of XPF acetylation impairs the damage-induced XPF-ERCC1 interaction, resulting in defects in both NER and ICL repair. Our results not only reveal a mechanism that regulates XPF-ERCC1 complex assembly and activation, but also provide important insight into the role of TIP60 in the maintenance of genome stability.

The XPF-ERCC1 heterodimer is an endonuclease involved in nucleotide excision (NER) and interstrand crosslink (ICL) repair in mammalian cells. Here, the authors provide insights into its regulation by revealing that TIP60 regulates XPF-ERCC1 complex assembly and activation.

Distinct DNA repair pathways cause genomic instability at alternative DNA structures

Alternative DNA structure-forming sequences can stimulate mutagenesis and are enriched at mutation hotspots in human cancer genomes, implicating them in disease etiology. However, the mechanisms involved are not well characterized. Here, we discover that Z-DNA is mutagenic in yeast as well as human cells, and that the nucleotide excision repair complex, Rad10-Rad1(ERCC1-XPF), and the mismatch repair complex, Msh2-Msh3, are required for Z-DNA-induced genetic instability in yeast and human cells. Both ERCC1-XPF and MSH2-MSH3 bind to Z-DNA-forming sequences, though ERCC1-XPF recruitment to Z-DNA is dependent on MSH2-MSH3. Moreover, ERCC1-XPF^-dependent DNA strand-breaks occur near the Z-DNA-forming region in human cell extracts, and we model these interactions at the sub-molecular level. We propose a relationship in which these complexes recognize and process Z-DNA in eukaryotes, representing a mechanism of Z-DNA-induced genomic instability.

Recognition and processing of branched DNA substrates by Slx1-Slx4 nuclease

Structure-selective endonucleases cleave branched DNA substrates. Slx1 is unique among structure-selective nucleases because it can cleave all branched DNA structures at multiple sites near the branch point. The mechanism behind this broad range of activity is unknown. The present study structurally and biochemically investigated fungal Slx1 to define a new protein interface that binds the non-cleaved arm of branched DNAs. The DNA arm bound at this new site was positioned at a sharp angle relative to the arm that was modeled to interact with the active site, implying that Slx1 uses DNA bending to localize the branch point as a flexible discontinuity in DNA. DNA binding at the new interface promoted a disorder-order transition in a region of the protein that was located in the vicinity of the active site, potentially participating in its formation. This appears to be a safety mechanism that ensures that DNA cleavage occurs only when the new interface is occupied by the non-cleaved DNA arm. Models of Slx1 that interacted with various branched DNA substrates were prepared. These models explain the way in which Slx1 cuts DNA toward the 3' end away from the branch point and elucidate the unique ability of Slx1 to cleave various DNA structures.

The balancing act of R-loop biology: The good, the bad, and the ugly

An R-loop is a three-stranded nucleic acid structure that consists of a DNA:RNA hybrid and a displaced strand of DNA. R-loops occur frequently in genomes and have significant physiological importance. They play vital roles in regulating gene expression, DNA replication, and DNA and histone modifications. Several studies have uncovered that R-loops contribute to fundamental biological processes in various organisms. Paradoxically, although they do play essential positive functions required for important biological processes, they can also contribute to DNA damage and genome instability. Recent evidence suggests that R-loops are involved in a number of human diseases, including neurological disorders, cancer, and autoimmune diseases. This review focuses on the molecular basis for R-loop-mediated gene regulation and genomic instability and briefly discusses methods for identifying R-loops in vivo It also highlights recent studies indicating the role of R-loops in DNA double-strand break repair with an updated view of much-needed future goals in R-loop biology.

Optimised oligonucleotide substrates to assay XPF-ERCC1 nuclease activity for the discovery of DNA repair inhibitors

We report the design and optimisation of novel oligonucleotide substrates for a sensitive fluorescence assay for high-throughput screening and functional studies of the DNA repair enzyme, XPF-ERCC1, with a view to accelerating inhibitor and drug discovery.

Coordination of Rad1-Rad10 interactions with Msh2-Msh3, Saw1 and RPA is essential for functional 3' non-homologous tail removal

Double strand DNA break repair (DSBR) comprises multiple pathways. A subset of DSBR pathways, including single strand annealing, involve intermediates with 3' non-homologous tails that must be removed to complete repair. In Saccharomyces cerevisiae, Rad1-Rad10 is the structure-specific endonuclease that cleaves the tails in 3' non-homologous tail removal (3' NHTR). Rad1-Rad10 is also an essential component of the nucleotide excision repair (NER) pathway. In both cases, Rad1-Rad10 requires protein partners for recruitment to the relevant DNA intermediate. Msh2-Msh3 and Saw1 recruit Rad1-Rad10 in 3' NHTR; Rad14 recruits Rad1-Rad10 in NER. We created two rad1 separation-of-function alleles, rad1R203A,K205A and rad1R218A; both are defective in 3' NHTR but functional in NER. In vitro, rad1R203A,K205A was impaired at multiple steps in 3' NHTR. The rad1R218A in vivo phenotype resembles that of msh2- or msh3-deleted cells; recruitment of rad1R218A-Rad10 to recombination intermediates is defective. Interactions among rad1R218A-Rad10 and Msh2-Msh3 and Saw1 are altered and rad1R218A-Rad10 interactions with RPA are compromised. We propose a model in which Rad1-Rad10 is recruited and positioned at the recombination intermediate through interactions, between Saw1 and DNA, Rad1-Rad10 and Msh2-Msh3, Saw1 and Msh2-Msh3 and Rad1-Rad10 and RPA. When any of these interactions is altered, 3' NHTR is impaired.

Targeting DNA Repair in Tumor Cells via Inhibition of ERCC1-XPF

The ERCC1-XPF heterodimer is a 5'-3' structure-specific endonuclease, which plays an essential role in several DNA repair pathways in mammalian cells. ERCC1-XPF is primarily involved in the repair of chemically induced helix-distorting and bulky DNA lesions, such as cyclobutane pyrimidine dimers (CPDs), and DNA interstrand cross-links. Inhibition of ERCC1-XPF has been shown to potentiate cytotoxicity of platinum-based drugs and cyclophosphamide in cancer cells. In this study, the previously described ERCC1-XPF inhibitor 4-((6-chloro-2-methoxyacridin-9-yl)amino)-2-((4-methylpiperazin-1-yl)methyl)phenol (compound 1) was used as a reference compound. Following the outcome of docking-based virtual screening (VS), we synthesized seven novel derivatives of 1 that were identified in silico as being likely to have high binding affinity for the ERCC1-XPF heterodimerization interface by interacting with the XPF double helix-hairpin-helix (HhH2) domain. Two of the new compounds, 4-((6-chloro-2-methoxyacridin-9-yl)amino)-2-((4-cyclohexylpiperazin-1-yl)methyl)phenol (compound 3) and 4-((6-chloro-2-methoxyacridin-9-yl)amino)-2-((4-(2-(dimethylamino)ethyl) piperazin-1-yl) methyl) phenol (compound 4), were shown to be potent inhibitors of ERCC1-XPF activity in vitro. Compound 4 showed significant inhibition of the removal of CPDs in UV-irradiated cells and the capacity to sensitize colorectal cancer cells to UV radiation and cyclophosphamide.

Structure-specific endonuclease activity of SNM1A enables processing of a DNA interstrand crosslink

DNA interstrand crosslinks (ICLs) covalently join opposing strands, blocking both replication and transcription, therefore making ICL-inducing compounds highly toxic and ideal anti-cancer agents. While incisions surrounding the ICL are required to remove damaged DNA, it is currently unclear which endonucleases are needed for this key event. SNM1A has been shown to play an important function in human ICL repair, however its suggested role has been limited to exonuclease activity and not strand incision. Here we show that SNM1A has endonuclease activity, having the ability to cleave DNA structures that arise during the initiation of ICL repair. In particular, this endonuclease activity cleaves single-stranded DNA. Given that unpaired DNA regions occur 5′ to an ICL, these findings suggest SNM1A may act as either an endonuclease and/or exonuclease during ICL repair. This finding is significant as it expands the potential role of SNM1A in ICL repair.

Capsaicin enhances erlotinib-induced cytotoxicity via AKT inactivation and excision repair cross-complementary 1 (ERCC1) down-regulation in human lung cancer cells

Capsaicin, a natural active ingredient of green and red peppers, has been demonstrated to exhibit anti-cancer properties in several malignant cell lines. Excision repair cross-complementary 1 (ERCC1) has a leading role in the nucleotide excision repair (NER) process because of its involvement in the excision of DNA adducts. Erlotinib (Tarceva^R) is a selective epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor that has demonstrated clinical activity in non-small cell lung cancer (NSCLC) cells. However, whether capsaicin and erlotinib could induce synergistic cytotoxicity in NSCLC cells through modulating ERCC1 expression is unknown. In this study, capsaicin decreased the ERCC1 expression in an AKT inactivation dependent manner in two human lung adenocarcinoma cells, namely, A549 and H1975. Enhancement of AKT activity by transfection with constitutive active AKT vectors increased the ERCC1 protein level as well as the cell survival by capsaicin. Moreover, capsaicin synergistically enhanced the cytotoxicity and cell growth inhibition of erlotinib in NSCLC cells, which were associated with the down-regulation of ERCC1 expression and inactivation of AKT in A549 and H1975 cells. Together, these results may provide a rationale to combine capsaicin with erlotinib for lung cancer treatment.

Function and Interactions of ERCC1-XPF in DNA Damage Response

Numerous proteins are involved in the multiple pathways of the DNA damage response network and play a key role to protect the genome from the wide variety of damages that can occur to DNA. An example of this is the structure-specific endonuclease ERCC1-XPF. This heterodimeric complex is in particular involved in nucleotide excision repair (NER), but also in double strand break repair and interstrand cross-link repair pathways. Here we review the function of ERCC1-XPF in various DNA repair pathways and discuss human disorders associated with ERCC1-XPF deficiency. We also overview our molecular and structural understanding of XPF-ERCC1.

R-loop generation during transcription: Formation, processing and cellular outcomes

R-loops are structures consisting of an RNA-DNA duplex and an unpaired DNA strand. They can form during transcription upon nascent RNA "threadback" invasion into the DNA duplex to displace the non-template strand. Although R-loops occur naturally in all kingdoms of life and serve regulatory roles, they are often deleterious and can cause genomic instability. Of particular importance are the disastrous consequences when replication forks or transcription complexes collide with R-loops. The appropriate processing of R-loops is essential to avoid a number of human neurodegenerative and other clinical disorders. We provide a perspective on mechanistic aspects of R-loop formation and their resolution learned from studies in model systems. This should contribute to improved understanding of R-loop biological functions and enable their practical applications. We propose the novel employment of artificially-generated stable R-loops to selectively inactivate tumor cells.

Distinct roles of XPF-ERCC1 and Rad1-Rad10-Saw1 in replication-coupled and uncoupled inter-strand crosslink repair

Yeast Rad1-Rad10 (XPF-ERCC1 in mammals) incises UV, oxidation, and cross-linking agent-induced DNA lesions, and contributes to multiple DNA repair pathways. To determine how Rad1-Rad10 catalyzes inter-strand crosslink repair (ICLR), we examined sensitivity to ICLs from yeast deleted for SAW1 and SLX4, which encode proteins that interact physically with Rad1-Rad10 and bind stalled replication forks. Saw1, Slx1, and Slx4 are critical for replication-coupled ICLR in mus81 deficient cells. Two rad1 mutations that disrupt interactions between Rpa1 and Rad1-Rad10 selectively disable non-nucleotide excision repair (NER) function, but retain UV lesion repair. Mutations in the analogous region of XPF also compromised XPF interactions with Rpa1 and Slx4, and are proficient in NER but deficient in ICLR and direct repeat recombination. We propose that Rad1-Rad10 makes distinct contributions to ICLR depending on cell cycle phase: in G1, Rad1-Rad10 removes ICL via NER, whereas in S/G2, Rad1-Rad10 facilitates NER-independent replication-coupled ICLR.

A meiotic XPF-ERCC1-like complex recognizes joint molecule recombination intermediates to promote crossover formation

De Muyt et al. identified the ZZS (Zip2–Zip4–Spo16) complex, required for crossover formation, which carries two distinct activities: one provided by Zip4, which acts as hub through physical interactions with components of the chromosome axis and the crossover machinery, and the other carried by Zip2 and Spo16, which preferentially bind branched DNA molecules in vitro.

Meiotic crossover formation requires the stabilization of early recombination intermediates by a set of proteins and occurs within the environment of the chromosome axis, a structure important for the regulation of meiotic recombination events. The molecular mechanisms underlying and connecting crossover recombination and axis localization are elusive. Here, we identified the ZZS (Zip2–Zip4–Spo16) complex, required for crossover formation, which carries two distinct activities: one provided by Zip4, which acts as hub through physical interactions with components of the chromosome axis and the crossover machinery, and the other carried by Zip2 and Spo16, which preferentially bind branched DNA molecules in vitro. We found that Zip2 and Spo16 share structural similarities to the structure-specific XPF–ERCC1 nuclease, although it lacks endonuclease activity. The XPF domain of Zip2 is required for crossover formation, suggesting that, together with Spo16, it has a noncatalytic DNA recognition function. Our results suggest that the ZZS complex shepherds recombination intermediates toward crossovers as a dynamic structural module that connects recombination events to the chromosome axis. The identification of the ZZS complex improves our understanding of the various activities required for crossover implementation and is likely applicable to other organisms, including mammals.

Effects of indirubin and isatin on cell viability, mutagenicity, genotoxicity and BAX/ERCC1 gene expression

CONTEXT

Indigofera suffruticosa Miller (Fabaceae) and I. truxillensis Kunth produce compounds, such as isatin (ISA) and indirubin (IRN), which possess antitumour properties. Their effects in mammalian cells are still not very well understood.

OBJECTIVE

We evaluated the activities of ISA and/or IRN on cell viability and apoptosis in vitro, their genotoxic potentials in vitro and in vivo, and the IRN- and ISA-induced expression of ERCC1 or BAX genes.

MATERIALS AND METHODS

HeLa and/or CHO-K1 cell lines were tested (3 or 24 h) in the MTT, Trypan blue exclusion, acridine orange/ethidium bromide, cytokinesis-blocked micronucleus (CBMN) and comet (36, 24 and 72 h) tests after treatment with IRN (0.1 to 200 μM) or ISA (0.5 to 50 μM). Gene expression was measured by RT-qPCR in HeLa cells. Swiss albino mice received IRN (3, 4 or 24 h) by gavage (50, 100 and 150 mg/kg determined from the LD₅₀ - 1 g/kg b.w.) and submitted to comet assay in vivo.

RESULTS

IRN reduced the viability of CHO-K1 (24 h; 5 to 200 μM) and HeLa cells (10 to 200 μM), and was antiproliferative in the CBMN test (CHO-K1: 0.5 to 10 μM; HeLa: 5 and 10 μM). The drug did not induce apoptosis, micronucleus neither altered gene expression. IRN and ISA were genotoxic for HeLa cells (3 and 24 h) at all doses tested. IRN (100 and 150 mg/kg) also induced genotoxicity in vivo (4 h).

CONCLUSION

IRN and ISA have properties that make them candidates as chemotherapeutics for further pharmacological investigations.

RPA activates the XPF-ERCC1 endonuclease to initiate processing of DNA interstrand crosslinks

During replication-coupled DNA interstrand crosslink (ICL) repair, the XPF-ERCC1 endonuclease is required for the incisions that release, or "unhook", ICLs, but the mechanism of ICL unhooking remains largely unknown. Incisions are triggered when the nascent leading strand of a replication fork strikes the ICL Here, we report that while purified XPF-ERCC1 incises simple ICL-containing model replication fork structures, the presence of a nascent leading strand, modelling the effects of replication arrest, inhibits this activity. Strikingly, the addition of the single-stranded DNA (ssDNA)-binding replication protein A (RPA) selectively restores XPF-ERCC1 endonuclease activity on this structure. The 5'-3' exonuclease SNM1A can load from the XPF-ERCC1-RPA-induced incisions and digest past the crosslink to quantitatively complete the unhooking reaction. We postulate that these collaborative activities of XPF-ERCC1, RPA and SNM1A might explain how ICL unhooking is achieved in vivo.

Recruitment and positioning determine the specific role of the XPF-ERCC1 endonuclease in interstrand crosslink repair

XPF‐ERCC1 is a structure‐specific endonuclease pivotal for several DNA repair pathways and, when mutated, can cause multiple diseases. Although the disease‐specific mutations are thought to affect different DNA repair pathways, the molecular basis for this is unknown. Here we examine the function of XPF‐ERCC1 in DNA interstrand crosslink (ICL) repair. We used Xenopus egg extracts to measure both ICL and nucleotide excision repair, and we identified mutations that are specifically defective in ICL repair. One of these separation‐of‐function mutations resides in the helicase‐like domain of XPF and disrupts binding to SLX4 and recruitment to the ICL. A small deletion in the same domain supports recruitment of XPF to the ICL, but inhibited the unhooking incisions most likely by disrupting a second, transient interaction with SLX4. Finally, mutation of residues in the nuclease domain did not affect localization of XPF‐ERCC1 to the ICL but did prevent incisions on the ICL substrate. Our data support a model in which the ICL repair‐specific function of XPF‐ERCC1 is dependent on recruitment, positioning and substrate recognition.

Single-stranded DNA Binding by the Helix-Hairpin-Helix Domain of XPF Protein Contributes to the Substrate Specificity of the ERCC1-XPF Protein Complex

The nucleotide excision repair protein complex ERCC1-XPF is required for incision of DNA upstream of DNA damage. Functional studies have provided insights into the binding of ERCC1-XPF to various DNA substrates. However, because no structure for the ERCC1-XPF-DNA complex has been determined, the mechanism of substrate recognition remains elusive. Here we biochemically characterize the substrate preferences of the helix-hairpin-helix (HhH) domains of XPF and ERCC-XPF and show that the binding to single-stranded DNA (ssDNA)/dsDNA junctions is dependent on joint binding to the DNA binding domain of ERCC1 and XPF. We reveal that the homodimeric XPF is able to bind various ssDNA sequences but with a clear preference for guanine-containing substrates. NMR titration experiments and in vitro DNA binding assays also show that, within the heterodimeric ERCC1-XPF complex, XPF specifically recognizes ssDNA. On the other hand, the HhH domain of ERCC1 preferentially binds dsDNA through the hairpin region. The two separate non-overlapping DNA binding domains in the ERCC1-XPF heterodimer jointly bind to an ssDNA/dsDNA substrate and, thereby, at least partially dictate the incision position during damage removal. Based on structural models, NMR titrations, DNA-binding studies, site-directed mutagenesis, charge distribution, and sequence conservation, we propose that the HhH domain of ERCC1 binds to dsDNA upstream of the damage, and XPF binds to the non-damaged strand within a repair bubble.

A rapid method for detecting protein-nucleic acid interactions by protein induced fluorescence enhancement

Many fundamental biological processes depend on intricate networks of interactions between proteins and nucleic acids and a quantitative description of these interactions is important for understanding cellular mechanisms governing DNA replication, transcription, or translation. Here we present a versatile method for rapid and quantitative assessment of protein/nucleic acid (NA) interactions. This method is based on protein induced fluorescence enhancement (PIFE), a phenomenon whereby protein binding increases the fluorescence of Cy3-like dyes. PIFE has mainly been used in single molecule studies to detect protein association with DNA or RNA. Here we applied PIFE for steady state quantification of protein/NA interactions by using microwell plate fluorescence readers (mwPIFE). We demonstrate the general applicability of mwPIFE for examining various aspects of protein/DNA interactions with examples from the restriction enzyme BamHI, and the DNA repair complexes Ku and XPF/ERCC1. These include determination of sequence and structure binding specificities, dissociation constants, detection of weak interactions, and the ability of a protein to translocate along DNA. mwPIFE represents an easy and high throughput method that does not require protein labeling and can be applied to a wide range of applications involving protein/NA interactions.

The Nucleotide Excision Repair Pathway Limits L1 Retrotransposition

Long interspersed elements 1 (L1) are active mobile elements that constitute almost 17% of the human genome. They amplify through a “copy-and-paste” mechanism termed retrotransposition, and de novo insertions related to these elements have been reported to cause 0.2% of genetic diseases. Our previous data demonstrated that the endonuclease complex ERCC1-XPF, which cleaves a 3′ DNA flap structure, limits L1 retrotransposition. Although the ERCC1-XPF endonuclease participates in several different DNA repair pathways, such as single-strand annealing, or in telomere maintenance, its recruitment to DNA lesions is best characterized in the nucleotide excision repair (NER) pathway. To determine if the NER pathway prevents the insertion of retroelements in the genome, we monitored the retrotransposition efficiencies of engineered L1 elements in NER-deficient cells and in their complemented versions. Core proteins of the NER pathway, XPD and XPA, and the lesion binding protein, XPC, are involved in limiting L1 retrotransposition. In addition, sequence analysis of recovered de novo L1 inserts and their genomic locations in NER-deficient cells demonstrated the presence of abnormally large duplications at the site of insertion, suggesting that NER proteins may also play a role in the normal L1 insertion process. Here, we propose new functions for the NER pathway in the maintenance of genome integrity: limitation of insertional mutations caused by retrotransposons and the prevention of potentially mutagenic large genomic duplications at the site of retrotransposon insertion events.

Chemical Incorporation of Chain-Terminating Nucleoside Analogs as 3'-Blocking DNA Damage and Their Removal by Human ERCC1-XPF Endonuclease

Nucleoside/nucleotide analogs that lack the 3′-hydroxy group are widely utilized for HIV therapy. These chain-terminating nucleoside analogs (CTNAs) block DNA synthesis after their incorporation into growing DNA, leading to the antiviral effects. However, they are also considered to be DNA damaging agents, and tyrosyl-DNA phosphodiesterase 1, a DNA repair enzyme, is reportedly able to remove such CTNA-modifications of DNA. Here, we have synthesized phosphoramidite building blocks of representative CTNAs, such as acyclovir, abacavir, carbovir, and lamivudine, and oligonucleotides with the 3′-CTNAs were successfully synthesized on solid supports. Using the chemically synthesized oligonucleotides, we investigated the excision of the 3′-CTNAs in DNA by the human excision repair cross complementing protein 1-xeroderma pigmentosum group F (ERCC1-XPF) endonuclease, which is one of the main components of the nucleotide excision repair pathway. A biochemical analysis demonstrated that the ERCC1-XPF endonuclease cleaved 2–7 nt upstream from the 3′-blocking CTNAs, and that DNA synthesis by the Klenow fragment was resumed after the removal of the CTNAs, suggesting that ERCC1-XPF participates in the repair of the CTNA-induced DNA damage.

Structure and mechanism of nucleases regulated by SLX4

SLX4 is a multidomain platform that regulates various proteins that are involved in genome maintenance and stability. Among these proteins are three structure-selective nucleases (SSEs). XPF-ERCC1 and MUS81-EME1 are structurally similar and function as heterodimers of highly similar subunits, in which only one is active. Two independent modules are formed from subunits of the heterodimers - a dimer of nuclease and nuclease-like domains and a dimer of tandem helix-hairpin-helix HhH2 domains. Both modules are responsible for substrate recognition. The third SSE, SLX1, contains GIY-YIG and RING domains and is a promiscuous nuclease. Structural data imply that SLX1 exists in free form as an autoinhibited homodimer. Association with SLX4 platform disrupts the homodimer and activates SLX1. This review discusses the available structural and mechanistic information on SLX4-regulated SSEs.

Factors affecting platinum sensitivity in cervical cancer

The present study aimed to investigate the association between nedaplatin (NDP) sensitivity and the expression of biological factors in cervical cancer. A total of 45 cervical cancer specimens, including 18 pretreatment biopsies and 27 surgical specimens, were used in histoculture drug response assays to determine the chemosensitivity of cervical cancer specimens to NDP. Each specimen was assessed for immunohistochemical expression of Ki-67, p53, B-cell lymphoma-2 (Bcl-2), Bcl-2-associated X protein (Bax), cleaved caspase-3, cyclooxygenase-2 (COX-2), and excision repair cross-complementation group 1 (ERCC1). The results revealed that low or negative expression of p53, Bcl-2 and COX-2, and high or positive expression of cleaved caspase-3 were significantly correlated with high sensitivity to NDP. However, there were no significant differences in Ki-67, Bax or ERCC1 expression between the low and high sensitivity groups. These findings indicate that sensitivity to platinum may be easily predicted by immunostaining for the detection of these specific factors in pretreatment biopsies or surgical specimens. The expression profiles of these targets may therefore provide additional information for planning individualized chemotherapy in the treatment of cervical cancer.

Inhibition of Topoisomerase (DNA) I (TOP1): DNA Damage Repair and Anticancer Therapy

Most chemotherapy regimens contain at least one DNA-damaging agent that preferentially affects the growth of cancer cells. This strategy takes advantage of the differences in cell proliferation between normal and cancer cells. Chemotherapeutic drugs are usually designed to target rapid-dividing cells because sustained proliferation is a common feature of cancer [1,2]. Rapid DNA replication is essential for highly proliferative cells, thus blocking of DNA replication will create numerous mutations and/or chromosome rearrangements—ultimately triggering cell death [3]. Along these lines, DNA topoisomerase inhibitors are of great interest because they help to maintain strand breaks generated by topoisomerases during replication. In this article, we discuss the characteristics of topoisomerase (DNA) I (TOP1) and its inhibitors, as well as the underlying DNA repair pathways and the use of TOP1 inhibitors in cancer therapy.

The ERCC1 and ERCC4 (XPF) genes and gene products

The ERCC1 and ERCC4 genes encode the two subunits of the ERCC1-XPF nuclease. This enzyme plays an important role in repair of DNA damage and in maintaining genomic stability. ERCC1-XPF nuclease nicks DNA specifically at junctions between double-stranded and single-stranded DNA, when the single-strand is oriented 5' to 3' away from a junction. ERCC1-XPF is a core component of nucleotide excision repair and also plays a role in interstrand crosslink repair, some pathways of double-strand break repair by homologous recombination and end-joining, as a backup enzyme in base excision repair, and in telomere length regulation. In many of these activities, ERCC1-XPF complex cleaves the 3' tails of DNA intermediates in preparation for further processing. ERCC1-XPF interacts with other proteins including XPA, RPA, SLX4 and TRF2 to perform its functions. Disruption of these interactions or direct targeting of ERCC1-XPF to decrease its DNA repair function might be a useful strategy to increase the sensitivity of cancer cells to some DNA damaging agents. Complete deletion of either ERCC1 or ERCC4 is not compatible with viability in mice or humans. However, mutations in the ERCC1 or ERCC4 genes cause a remarkable array of rare inherited human disorders. These include specific forms of xeroderma pigmentosum, Cockayne syndrome, Fanconi anemia, XFE progeria and cerebro-oculo-facio-skeletal syndrome.

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.

Mechanism and regulation of incisions during DNA interstrand cross-link repair

A critical step in DNA interstrand cross-link repair is the programmed collapse of replication forks that have stalled at an ICL. This event is regulated by the Fanconi anemia pathway, which suppresses bone marrow failure and cancer. In this perspective, we focus on the structure of forks that have stalled at ICLs, how these structures might be incised by endonucleases, and how incision is regulated by the Fanconi anemia pathway.

Mouse SLX4 is a tumor suppressor that stimulates the activity of the nuclease XPF-ERCC1 in DNA crosslink repair

SLX4 binds to three nucleases (XPF-ERCC1, MUS81-EME1, and SLX1), and its deficiency leads to genomic instability, sensitivity to DNA crosslinking agents, and Fanconi anemia. However, it is not understood how SLX4 and its associated nucleases act in DNA crosslink repair. Here, we uncover consequences of mouse Slx4 deficiency and reveal its function in DNA crosslink repair. Slx4-deficient mice develop epithelial cancers and have a contracted hematopoietic stem cell pool. The N-terminal domain of SLX4 (mini-SLX4) that only binds to XPF-ERCC1 is sufficient to confer resistance to DNA crosslinking agents. Recombinant mini-SLX4 enhances XPF-ERCC1 nuclease activity up to 100-fold, directing specificity toward DNA forks. Mini-SLX4-XPF-ERCC1 also vigorously stimulates dual incisions around a DNA crosslink embedded in a synthetic replication fork, an essential step in the repair of this lesion. These observations define vertebrate SLX4 as a tumor suppressor, which activates XPF-ERCC1 nuclease specificity in DNA crosslink repair.

The cutting edges in DNA repair, licensing, and fidelity: DNA and RNA repair nucleases sculpt DNA to measure twice, cut once

To avoid genome instability, DNA repair nucleases must precisely target the correct damaged substrate before they are licensed to incise. Damage identification is a challenge for all DNA damage response proteins, but especially for nucleases that cut the DNA and necessarily create a cleaved DNA repair intermediate, likely more toxic than the initial damage. How do these enzymes achieve exquisite specificity without specific sequence recognition or, in some cases, without a non-canonical DNA nucleotide? Combined structural, biochemical, and biological analyses of repair nucleases are revealing their molecular tools for damage verification and safeguarding against inadvertent incision. Surprisingly, these enzymes also often act on RNA, which deserves more attention. Here, we review protein-DNA structures for nucleases involved in replication, base excision repair, mismatch repair, double strand break repair (DSBR), and telomere maintenance: apurinic/apyrimidinic endonuclease 1 (APE1), Endonuclease IV (Nfo), tyrosyl DNA phosphodiesterase (TDP2), UV Damage endonuclease (UVDE), very short patch repair endonuclease (Vsr), Endonuclease V (Nfi), Flap endonuclease 1 (FEN1), exonuclease 1 (Exo1), RNase T and Meiotic recombination 11 (Mre11). DNA and RNA structure-sensing nucleases are essential to life with roles in DNA replication, repair, and transcription. Increasingly these enzymes are employed as advanced tools for synthetic biology and as targets for cancer prognosis and interventions. Currently their structural biology is most fully illuminated for DNA repair, which is also essential to life. How DNA repair enzymes maintain genome fidelity is one of the DNA double helix secrets missed by James Watson and Francis Crick, that is only now being illuminated though structural biology and mutational analyses. Structures reveal motifs for repair nucleases and mechanisms whereby these enzymes follow the old carpenter adage: measure twice, cut once. Furthermore, to measure twice these nucleases act as molecular level transformers that typically reshape the DNA and sometimes themselves to achieve extraordinary specificity and efficiency.

Molecular docking and molecular dynamics study on the effect of ERCC1 deleterious polymorphisms in ERCC1-XPF heterodimer

Excision repair cross complementation group 1 (ERCC1) is an important protein in the nucleotide excision repair (NER) pathway, which is responsible for removing DNA adducts induced by platinum based compounds. The heterodimer ERCC1-XPF is one of two endonucleases required for NER. Genetic variations or polymorphisms in ERCC1 gene alter DNA repair capacity. Reduced DNA repair (NER) capacity may result in tumors and enhances cisplatin chemotherapy in cancer patients, which functions by causing DNA damage. Therefore, ERCC1 variants have the potential to be used as a strong candidate biomarker in cancer treatments. In this study we identified five variants V116M, R156Q, A199T, S267P, and R322C of ERCC1 gene as highly deleterious. Further structural and functional analysis has been conducted for ERCC1 protein in the presence of three variants V116M, R156Q, and A199T. Occurrence of theses variations adversely affected the regular interaction between ERCC1 and XPF protein. Analysis of 20 ns molecular dynamics simulation trajectories reveals that the predicted deleterious variants altered the ERCC1-XPF complex stability, flexibility, and surface area. Notably, the number of hydrogen bonds in ERCC1-XPF mutant complexes decreased in the molecular dynamic simulation periods. Overall, this study explores the link between the ERCC1 deleterious variants and cisplatin chemotherapy for various cancers with the help of molecular docking and molecular dynamic approaches.

Induction of apoptosis and suppression of ERCC1 expression by the potent amonafide analogue 8-c in human colorectal carcinoma cells

Previous studies have reported that 8-c [6-(2-(2-(dimethylamino)ethylamino)ethylamino)-2-octyl-1H-benzo[de]isoquinoline-1,3(2H)-dione], a novel amonafide analogue, was generated as a new anticancer candidate. However, little is known about its activity in chemoresistant cells. In this study, the antitumor effects of 8-c on the multi-drug-resistant human colorectal carcinoma cancer cell lines HCT-116/L-OHP and HCT-8/VCR have been investigated for the first time. 8-c showed similar concentration-dependent inhibitory activities against multi-drug-resistant cells and corresponding parental cell lines by the MTT assay after 48 h of treatment. 8-c treatment resulted in the induction of apoptosis, as evidenced by fluorescent staining analysis, comet assay data, and the increase in the number of apoptotic cells as detected by flow cytometry. Western blot, qPCR, and siRNA techniques were used to elucidate the molecular mechanism. Our study suggested that the apoptotic effect of 8-c can be attributed to the upregulation of p53, caspase-3, and cleaved poly(ADP-ribose) polymerase (PARP) and the downregulation of Bcl-2. Furthermore, ERCC1 is essential for nucleotide excision repair. ERCC1 expression was correlated with sensitivity to chemotherapy in various colon cancer cell lines. It is intriguing that decreases in ERCC1 protein and mRNA levels were also observed in the HCT-116/L-OHP and HCT-8/VCR cells after exposure to 8-c. Further transient transfection of multi-drug-resistant cells with ERCC1 siRNA enhanced 8-c-induced cytotoxicity. In contrast, epidermal growth factor-induced increase in ERCC1 protein levels was shown to rescue cell viability upon 8-c treatment. These findings suggest that 8-c has a strong potential to be developed as a new antitumor agent for the treatment of multi-drug-resistant colon cancer cells, and is worthy of further studies.

Oxidative DNA damage and nucleotide excision repair

SIGNIFICANCE

Oxidative DNA damage is repaired by multiple, overlapping DNA repair pathways. Accumulating evidence supports the hypothesis that nucleotide excision repair (NER), besides base excision repair (BER), is also involved in neutralizing oxidative DNA damage.

RECENT ADVANCES

NER includes two distinct sub-pathways: transcription-coupled NER (TC-NER) and global genome repair (GG-NER). The CSA and CSB proteins initiate the onset of TC-NER. Recent findings show that not only CSB, but also CSA is involved in the repair of oxidative DNA lesions, in the nucleus as well as in mitochondria. The XPG protein is also of importance for the removal of oxidative DNA lesions, as it may enhance the initial step of BER. Substantial evidence exists that support a role for XPC in NER and BER. XPC deficiency not only results in decreased repair of oxidative lesions, but has also been linked to disturbed redox homeostasis.

CRITICAL ISSUES

The role of NER proteins in the regulation of the cellular response to oxidative (mitochondrial and nuclear) DNA damage may be the underlying mechanism of the pathology of accelerated aging in Cockayne syndrome patients, a driving force for internal cancer development in XP-A and XP-C patients, and a contributor to the mixed exhibited phenotypes of XP-G patients.

FUTURE DIRECTIONS

Accumulating evidence indicates that DNA repair factors can be involved in multiple DNA repair pathways. However, the distinct detailed mechanism and consequences of these additional functions remain to be elucidated and can possibly shine a light on clinically related issues.

Malfunction of nuclease ERCC1-XPF results in diverse clinical manifestations and causes Cockayne syndrome, xeroderma pigmentosum, and Fanconi anemia

Cockayne syndrome (CS) is a genetic disorder characterized by developmental abnormalities and photodermatosis resulting from the lack of transcription-coupled nucleotide excision repair, which is responsible for the removal of photodamage from actively transcribed genes. To date, all identified causative mutations for CS have been in the two known CS-associated genes, ERCC8 (CSA) and ERCC6 (CSB). For the rare combined xeroderma pigmentosum (XP) and CS phenotype, all identified mutations are in three of the XP-associated genes, ERCC3 (XPB), ERCC2 (XPD), and ERCC5 (XPG). In a previous report, we identified several CS cases who did not have mutations in any of these genes. In this paper, we describe three CS individuals deficient in ERCC1 or ERCC4 (XPF). Remarkably, one of these individuals with XP complementation group F (XP-F) had clinical features of three different DNA-repair disorders--CS, XP, and Fanconi anemia (FA). Our results, together with those from Bogliolo et al., who describe XPF alterations resulting in FA alone, indicate a multifunctional role for XPF.

DNA repair endonuclease ERCC1-XPF as a novel therapeutic target to overcome chemoresistance in cancer therapy

The ERCC1–XPF complex is a structure-specific endonuclease essential for the repair of DNA damage by the nucleotide excision repair pathway. It is also involved in other key cellular processes, including DNA interstrand crosslink (ICL) repair and DNA double-strand break (DSB) repair. New evidence has recently emerged, increasing our understanding of its requirement in these additional roles. In this review, we focus on the protein–protein and protein–DNA interactions made by the ERCC1 and XPF proteins and discuss how these coordinate ERCC1–XPF in its various roles. In a number of different cancers, high expression of ERCC1 has been linked to a poor response to platinum-based chemotherapy. We discuss prospects for the development of DNA repair inhibitors that target the activity, stability or protein interactions of the ERCC1–XPF complex as a novel therapeutic strategy to overcome chemoresistance.

Xeroderma pigmentosum group F protein binds to Eg5 and is required for proper mitosis: implications for XP-F and XFE

The xeroderma pigmentosum group F-cross-complementing rodent repair deficiency group 1 (XPF-ERCC1) complex is a structure-specific endonuclease involved in nucleotide excision repair (NER) and interstrand cross-link (ICL) repair. Patients with XPF mutations may suffer from two forms of xeroderma pigmentosum (XP): XP-F patients show mild photosensitivity and proneness to skin cancer but rarely show any neurological abnormalities, whereas XFE patients display symptoms of severe XP symptoms, growth retardation and accelerated aging. Xpf knockout mice display accelerated aging and die before weaning. These results suggest that the XPF-ERCC1 complex has additional functions besides NER and ICL repair and is essential for development and growth. In this study, we show a partial colocalization of XPF with mitotic spindles and Eg5. XPF knockdown in cells led to an increase in the frequency of abnormal nuclear morphology and mitosis. Similarly, the frequency of abnormal nuclei and mitosis was increased in XP-F and XFE cells. In addition, we showed that Eg5 enhances the action of XPF-ERCC1 nuclease activity. Taken together, these results suggest that the interaction between XPF and Eg5 plays a role in mitosis and DNA repair and offer new insights into the pathogenesis of XP-F and XFE.