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

SLX4: multitasking to maintain genome stability

The SLX4/FANCP tumor suppressor has emerged as a key player in the maintenance of genome stability, making pivotal contributions to the repair of interstrand cross-links, homologous recombination, and in response to replication stress genome-wide as well as at specific loci such as common fragile sites and telomeres. SLX4 does so in part by acting as a scaffold that controls and coordinates the XPF-ERCC1, MUS81-EME1, and SLX1 structure-specific endonucleases in different DNA repair and recombination mechanisms. It also interacts with other important DNA repair and cell cycle control factors including MSH2, PLK1, TRF2, and TOPBP1 as well as with ubiquitin and SUMO. This review aims at providing an up-to-date and comprehensive view on the key functions that SLX4 fulfills to maintain genome stability as well as to highlight and discuss areas of uncertainty and emerging concepts.

Loss of Cohesin Subunit Rec8 Switches Rad51 Mediator Dependence in Resistance to Formaldehyde Toxicity in Ustilago maydis

DNA-protein cross-links (DPCs) are frequently occurring lesions that provoke continual threats to the integrity of the genome by interference with replication and transcription. Reactive aldehydes generated from endogenous metabolic processes or produced in the environment are sources that trigger cross-linking of DNA with associated proteins. DNA repair pathways in place for removing DPCs, or for bypassing them to enable completion of replication, include homologous recombination (HR) and replication fork remodeling (FR) systems. Here, we surveyed a set of mutants defective in known HR and FR components to determine their contribution toward maintaining resistance to chronic formaldehyde (FA) exposure in Ustilago maydis, a fungus that relies on the BRCA2-family member Brh2 as the principal Rad51 mediator in repair of DNA strand breaks. We found that, in addition to Brh2, Rad52 was also vital for resistance to FA. Deleting the gene for Rec8, a kleisin subunit of cohesin, eliminated the requirement for Brh2, but not Rad52, in FA resistance. The Rad51K133R mutant variant that is able to bind DNA but unable to dissociate from it was able to support resistance to FA. These findings suggest a model for DPC repair and tolerance that features a specialized role for Rad52, enabling Rad51 to access DNA in its noncanonical capacity of replication fork protection rather than DNA strand transfer.

A senataxin-associated exonuclease SAN1 is required for resistance to DNA interstrand cross-links

Interstrand DNA cross-links (ICLs) block both replication and transcription, and are commonly repaired by the Fanconi anemia (FA) pathway. However, FA-independent repair mechanisms of ICLs remain poorly understood. Here we report a previously uncharacterized protein, SAN1, as a 5′ exonuclease that acts independently of the FA pathway in response to ICLs. Deletion of SAN1 in HeLa cells and mouse embryonic fibroblasts causes sensitivity to ICLs, which is prevented by re-expression of wild type but not nuclease-dead SAN1. SAN1 deletion causes DNA damage and radial chromosome formation following treatment with Mitomycin C, phenocopying defects in the FA pathway. However, SAN1 deletion is not epistatic with FANCD2, a core FA pathway component. Unexpectedly, SAN1 binds to Senataxin (SETX), an RNA/DNA helicase that resolves R-loops. SAN1-SETX binding is increased by ICLs, and is required to prevent cross-link sensitivity. We propose that SAN1 functions with SETX in a pathway necessary for resistance to ICLs.

When DNA interstrand cross-links damage occurs, it causes disruption of replication and transcription. Here the authors identify FAM120B/SAN1, a 5′ exonuclease involved in the repair process of Interstrand Crosslinks independently of the Fanconi Anemia pathway.

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.

Structural mechanism of DNA interstrand cross-link unhooking by the bacterial FAN1 nuclease

DNA interstrand cross-links (ICLs) block the progress of the replication and transcription machineries and can weaken chromosomal stability, resulting in various diseases. FANCD2-FANCI-associated nuclease (FAN1) is a conserved structure-specific nuclease that unhooks DNA ICLs independently of the Fanconi anemia pathway. Recent structural studies have proposed two different mechanistic features for ICL unhooking by human FAN1: a specific basic pocket that recognizes the terminal phosphate of a 1-nucleotide (nt) 5' flap or FAN1 dimerization. Herein, we show that despite lacking these features, Pseudomonas aeruginosa FAN1 (PaFAN1) cleaves substrates at ∼3-nt intervals and resolves ICLs. Crystal structures of PaFAN1 bound to various DNA substrates revealed that its conserved basic Arg/Lys patch comprising Arg-228 and Lys-260 recognizes phosphate groups near the 5' terminus of a DNA substrate with a 1-nt flap or a nick. Substitution of Lys-260 did not affect PaFAN1's initial endonuclease activity but significantly decreased its subsequent exonuclease activity and ICL unhooking. The Arg/Lys patch also interacted with phosphates at a 3-nt gap, and this interaction could drive movement of the scissile phosphates into the PaFAN1-active site. In human FAN1, the ICL-resolving activity was not affected by individual disruption of the Arg/Lys patch or basic pocket. However, simultaneous substitution of both FAN1 regions significantly reduced its ICL-resolving activity, suggesting that these two basic regions play a complementary role in ICL repair. On the basis of these findings, we propose a conserved role for two basic regions in FAN1 to guide ICL unhooking and to maintain genomic stability.

Genome-Wide Association Study for Susceptibility to and Recoverability From Mastitis in Danish Holstein Cows

Because mastitis is very frequent and unavoidable, adding recovery information into the analysis for genetic evaluation of mastitis is of great interest from economical and animal welfare point of view. Here we have performed genome-wide association studies (GWAS) to identify associated single nucleotide polymorphisms (SNPs) and investigate the genetic background not only for susceptibility to – but also for recoverability from mastitis. Somatic cell count records from 993 Danish Holstein cows genotyped for a total of 39378 autosomal SNP markers were used for the association analysis. Single SNP regression analysis was performed using the statistical software package DMU. Substitution effect of each SNP was tested with a t-test and a genome-wide significance level of P-value < 10^-4 was used to declare significant SNP-trait association. A number of significant SNP variants were identified for both traits. Many of the SNP variants associated either with susceptibility to – or recoverability from mastitis were located in or very near to genes that have been reported for their role in the immune system. Genes involved in lymphocyte developments (e.g., MAST3 and STAB2) and genes involved in macrophage recruitment and regulation of inflammations (PDGFD and PTX3) were suggested as possible causal genes for susceptibility to – and recoverability from mastitis, respectively. However, this is the first GWAS study for recoverability from mastitis and our results need to be validated. The findings in the current study are, therefore, a starting point for further investigations in identifying causal genetic variants or chromosomal regions for both susceptibility to – and recoverability from mastitis.

Unresolved recombination intermediates lead to ultra-fine anaphase bridges, chromosome breaks and aberrations

The resolution of joint molecules that link recombining sister chromatids is essential for chromosome segregation. Here, we determine the fate of unresolved recombination intermediates arising in cells lacking two nucleases required for resolution (GEN1 -/- knockout cells depleted of MUS81). We find that intermediates persist until mitosis and form a distinct class of anaphase bridges, which we term homologous recombination ultra-fine bridges (HR-UFBs). HR-UFBs are distinct from replication stress-associated UFBs, which arise at common fragile sites, and from centromeric UFBs. HR-UFBs are processed by BLM helicase to generate single-stranded RPA-coated bridges that are broken during mitosis. In the next cell cycle, DNA breaks activate the DNA damage checkpoint response, and chromosome fusions arise by non-homologous end joining. Consequently, the cells undergo cell cycle delay and massive cell death. These results lead us to present a model detailing how unresolved recombination intermediates can promote DNA damage and chromosomal instability.

The Human DNA glycosylases NEIL1 and NEIL3 Excise Psoralen-Induced DNA-DNA Cross-Links in a Four-Stranded DNA Structure

Interstrand cross-links (ICLs) are highly cytotoxic DNA lesions that block DNA replication and transcription by preventing strand separation. Previously, we demonstrated that the bacterial and human DNA glycosylases Nei and NEIL1 excise unhooked psoralen-derived ICLs in three-stranded DNA via hydrolysis of the glycosidic bond between the crosslinked base and deoxyribose sugar. Furthermore, NEIL3 from Xenopus laevis has been shown to cleave psoralen- and abasic site-induced ICLs in Xenopus egg extracts. Here we report that human NEIL3 cleaves psoralen-induced DNA-DNA cross-links in three-stranded and four-stranded DNA substrates to generate unhooked DNA fragments containing either an abasic site or a psoralen-thymine monoadduct. Furthermore, while Nei and NEIL1 also cleave a psoralen-induced four-stranded DNA substrate to generate two unhooked DNA duplexes with a nick, NEIL3 targets both DNA strands in the ICL without generating single-strand breaks. The DNA substrate specificities of these Nei-like enzymes imply the occurrence of long uninterrupted three- and four-stranded crosslinked DNA-DNA structures that may originate in vivo from DNA replication fork bypass of an ICL. In conclusion, the Nei-like DNA glycosylases unhook psoralen-derived ICLs in various DNA structures via a genuine repair mechanism in which complex DNA lesions can be removed without generation of highly toxic double-strand breaks.

Systematic analysis of DNA crosslink repair pathways during development and aging in Caenorhabditis elegans

DNA interstrand crosslinks (ICLs) are generated by endogenous sources and chemotherapeutics, and pose a threat to genome stability and cell survival. Using Caenorhabditis elegans mutants, we identify DNA repair factors that protect against the genotoxicity of ICLs generated by trioxsalen/ultraviolet A (TMP/UVA) during development and aging. Mutations in nucleotide excision repair (NER) components (e.g. XPA-1 and XPF-1) imparted extreme sensitivity to TMP/UVA relative to wild-type animals, manifested as developmental arrest, defects in adult tissue morphology and functionality, and shortened lifespan. Compensatory roles for global-genome (XPC-1) and transcription-coupled (CSB-1) NER in ICL sensing were exposed. The analysis also revealed contributions of homologous recombination (BRC-1/BRCA1), the MUS-81, EXO-1, SLX-1 and FAN-1 nucleases, and the DOG-1 (FANCJ) helicase in ICL resolution, influenced by the replicative-status of the cell/tissue. No obvious or critical role in ICL repair was seen for non-homologous end-joining (cku-80) or base excision repair (nth-1, exo-3), the Fanconi-related proteins BRC-2 (BRCA2/FANCD1) and FCD-2 (FANCD2), the WRN-1 or HIM-6 (BLM) helicases, or the GEN-1 or MRT-1 (SNM1) nucleases. Our efforts uncover replication-dependent and -independent ICL repair networks, and establish nematodes as a model for investigating the repair and consequences of DNA crosslinks in metazoan development and in adult post-mitotic and proliferative germ cells.

DNA damage response and cancer therapeutics through the lens of the Fanconi Anemia DNA repair pathway

Fanconi Anemia (FA) is a rare, inherited genomic instability disorder, caused by mutations in genes involved in the repair of interstrand DNA crosslinks (ICLs). The FA signaling network contains a unique nuclear protein complex that mediates the monoubiquitylation of the FANCD2 and FANCI heterodimer, and coordinates activities of the downstream DNA repair pathway including nucleotide excision repair, translesion synthesis, and homologous recombination. FA proteins act at different steps of ICL repair in sensing, recognition and processing of DNA lesions. The multi-protein network is tightly regulated by complex mechanisms, such as ubiquitination, phosphorylation, and degradation signals that are critical for the maintenance of genome integrity and suppressing tumorigenesis. Here, we discuss recent advances in our understanding of how the FA proteins participate in ICL repair and regulation of the FA signaling network that assures the safeguard of the genome. We further discuss the potential application of designing small molecule inhibitors that inhibit the FA pathway and are synthetic lethal with DNA repair enzymes that can be used for cancer therapeutics.

The Role of PALB2 in the DNA Damage Response and Cancer Predisposition

The deoxyribonucleic acid (DNA) damage response (DDR) is a major feature in the maintenance of genome integrity and in the suppression of tumorigenesis. PALB2 (Partner and Localizer of Breast Cancer 2 (BRCA2)) plays an important role in maintaining genome integrity through its role in the Fanconi anemia (FA) and homologous recombination (HR) DNA repair pathways. Since its identification as a BRCA2 interacting partner, PALB2 has emerged as a pivotal tumor suppressor protein associated to hereditary cancer susceptibility to breast and pancreatic cancers. In this review, we discuss how other DDR proteins (such as the kinases Ataxia Telangiectasia Mutated (ATM) and ATM- and Rad3-Related (ATR), mediators BRCA1 (Breast Cancer 1)/BRCA2 and effectors RAD51/DNA Polymerase η (Polη) interact with PALB2 to orchestrate DNA repair. We also examine the involvement of PALB2 mutations in the predisposition to cancer and the role of PALB2 in stimulating error-free DNA repair through the FA/HR pathway.

Potential Strategies to Target Protein-Protein Interactions in the DNA Damage Response and Repair Pathways

This review article discusses some insights about generating novel mechanistic inhibitors of the DNA damage response and repair (DDR) pathways by focusing on protein-protein interactions (PPIs) of the key DDR components. General requirements for PPI strategies, such as selecting the target PPI site on the basis of its functionality, are discussed first. Next, on the basis of functional rationale and biochemical feasibility to identify a PPI inhibitor, 26 PPIs in DDR pathways (BER, MMR, NER, NHEJ, HR, TLS, and ICL repair) are specifically discussed for inhibitor discovery to benefit cancer therapies using a DNA-damaging agent.

Ubiquitin-like modifications in the DNA damage response

Genomic DNA is damaged at an extremely high frequency by both endogenous and environmental factors. An improper response to DNA damage can lead to genome instability, accelerate the aging process and ultimately cause various human diseases, including cancers and neurodegenerative disorders. The mechanisms that underlie the cellular DNA damage response (DDR) are complex and are regulated at many levels, including at the level of post-translational modification (PTM). Since the discovery of ubiquitin in 1975 and ubiquitylation as a form of PTM in the early 1980s, a number of ubiquitin-like modifiers (UBLs) have been identified, including small ubiquitin-like modifiers (SUMOs), neural precursor cell expressed, developmentally down-regulated 8 (NEDD8), interferon-stimulated gene 15 (ISG15), human leukocyte antigen (HLA)-F adjacent transcript 10 (FAT10), ubiquitin-fold modifier 1 (UFRM1), URM1 ubiquitin-related modifier-1 (URM1), autophagy-related protein 12 (ATG12), autophagy-related protein 8 (ATG8), fan ubiquitin-like protein 1 (FUB1) and histone mono-ubiquitylation 1 (HUB1). All of these modifiers have known roles in the cellular response to various forms of stress, and delineating their underlying molecular mechanisms and functions is fundamental in enhancing our understanding of human disease and longevity. To date, however, the molecular mechanisms and functions of these UBLs in the DDR remain largely unknown. This review summarizes the current status of PTMs by UBLs in the DDR and their implication in cancer diagnosis, therapy and drug discovery.

Chronic treatment with cisplatin induces chemoresistance through the TIP60-mediated Fanconi anemia and homologous recombination repair pathways

The Fanconi anemia pathway in coordination with homologous recombination is essential to repair interstrand crosslinks (ICLs) caused by cisplatin. TIP60 belongs to the MYST family of acetyltransferases and is involved in DNA repair and regulation of gene transcription. Although the physical interaction between the TIP60 and FANCD2 proteins has been identified that is critical for ICL repair, it is still elusive whether TIP60 regulates the expression of FA and HR genes. In this study, we found that the chemoresistant nasopharyngeal carcinoma cells, derived from chronic treatment of cisplatin, show elevated expression of TIP60. Furthermore, TIP60 binds to the promoters of FANCD2 and BRCA1 by using the chromatin immunoprecipitation experiments and promote the expression of FANCD2 and BRCA1. Importantly, the depletion of TIP60 significantly reduces sister chromatid exchange, a measurement of HR efficiency. The similar results were also shown in the FNACD2-, and BRCA1-deficient cells. Additionally, these TIP60-deficient cells encounter more frequent stalled forks, as well as more DNA double-strand breaks resulting from the collapse of stalled forks. Taken together, our results suggest that TIP60 promotes the expression of FA and HR genes that are important for ICL repair and the chemoresistant phenotype under chronic treatment with cisplatin.

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.

XPF knockout via CRISPR/Cas9 reveals that ERCC1 is retained in the cytoplasm without its heterodimer partner XPF

The XPF/ERCC1 heterodimeric complex is essentially involved in nucleotide excision repair (NER), interstrand crosslink (ICL), and double-strand break repair. Defects in XPF lead to severe diseases like xeroderma pigmentosum (XP). Up until now, XP-F patient cells have been utilized for functional analyses. Due to the multiple roles of the XPF/ERCC1 complex, these patient cells retain at least one full-length allele and residual repair capabilities. Despite the essential function of the XPF/ERCC1 complex for the human organism, we successfully generated a viable immortalised human XPF knockout cell line with complete loss of XPF using the CRISPR/Cas9 technique in fetal lung fibroblasts (MRC5Vi cells). These cells showed a markedly increased sensitivity to UVC, cisplatin, and psoralen activated by UVA as well as reduced repair capabilities for NER and ICL repair as assessed by reporter gene assays. Using the newly generated knockout cells, we could show that human XPF is markedly involved in homologous recombination repair (HRR) but dispensable for non-homologous end-joining (NHEJ). Notably, ERCC1 was not detectable in the nucleus of the XPF knockout cells indicating the necessity of a functional XPF/ERCC1 heterodimer to allow ERCC1 to enter the nucleus. Overexpression of wild-type XPF could reverse this effect as well as the repair deficiencies.

Substrate preference of Gen endonucleases highlights the importance of branched structures as DNA damage repair intermediates

Human GEN1 and yeast Yen1 are endonucleases with the ability to cleave Holliday junctions (HJs), which are proposed intermediates in recombination. In vivo, GEN1 and Yen1 function secondarily to Mus81, which has weak activity on intact HJs. We show that the genetic relationship is reversed in Drosophila, with Gen mutants having more severe defects than mus81 mutants. In vitro, DmGen, like HsGEN1, efficiently cleaves HJs, 5΄ flaps, splayed arms, and replication fork structures. We find that the cleavage rates for 5΄ flaps are significantly higher than those for HJs for both DmGen and HsGEN1, even in vast excess of enzyme over substrate. Kinetic studies suggest that the difference in cleavage rates results from a slow, rate-limiting conformational change prior to HJ cleavage: formation of a productive dimer on the HJ. Despite the stark difference in vivo that Drosophila uses Gen over Mus81 and humans use MUS81 over GEN1, we find the in vitro activities of DmGen and HsGEN1 to be strikingly similar. These findings suggest that simpler branched structures may be more important substrates for Gen orthologs in vivo, and highlight the utility of using the Drosophila model system to further understand these enzymes.

Preparation of Stable Nitrogen Mustard DNA Interstrand Cross-Link Analogs for Biochemical and Cell Biological Studies

Nitrogen mustards (NMs) react with two bases on opposite strands of a DNA duplex to form a covalent linkage, yielding adducts called DNA interstrand cross-links (ICLs). This prevents helix unwinding, blocking essential processes such as replication and transcription. Accumulation of ICLs causes cell death in rapidly dividing cells, especially cancer cells, making ICL-forming agents like NMs valuable in chemotherapy. However, the repair of ICLs can contribute to chemoresistance through a number of pathways that remain poorly understood. One of the impediments in studying NM ICL repair mechanisms has been the difficulty of generating site-specific and stable NM ICLs. Here, we describe two methods to synthesize stable NM ICL analogs that make it possible to study DNA ICL repair. As a proof of principle of the suitability of these NM ICLs for biochemical and cell biological studies, we use them in primer extension assays with Klenow polymerase. We show that the NM ICL analogs block the polymerase activity and remain intact under our experimental conditions.

A role for the base excision repair enzyme NEIL3 in replication-dependent repair of interstrand DNA cross-links derived from psoralen and abasic sites

Interstrand DNA-DNA cross-links are highly toxic lesions that are important in medicinal chemistry, toxicology, and endogenous biology. In current models of replication-dependent repair, stalling of a replication fork activates the Fanconi anemia pathway and cross-links are "unhooked" by the action of structure-specific endonucleases such as XPF-ERCC1 that make incisions flanking the cross-link. This process generates a double-strand break, which must be subsequently repaired by homologous recombination. Recent work provided evidence for a new, incision-independent unhooking mechanism involving intrusion of a base excision repair (BER) enzyme, NEIL3, into the world of cross-link repair. The evidence suggests that the glycosylase action of NEIL3 unhooks interstrand cross-links derived from an abasic site or the psoralen derivative trioxsalen. If the incision-independent NEIL3 pathway is blocked, repair reverts to the incision-dependent route. In light of the new model invoking participation of NEIL3 in cross-link repair, we consider the possibility that various BER glycosylases or other DNA-processing enzymes might participate in the unhooking of chemically diverse interstrand DNA cross-links.

Control of structure-specific endonucleases to maintain genome stability

Structure-specific endonucleases (SSEs) have key roles in DNA replication, recombination and repair, and emerging roles in transcription. These enzymes have specificity for DNA secondary structure rather than for sequence, and therefore their activity must be precisely controlled to ensure genome stability. In this Review, we discuss how SSEs are controlled as part of genome maintenance pathways in eukaryotes, with an emphasis on the elaborate mechanisms that regulate the members of the major SSE families - including the xeroderma pigmentosum group F-complementing protein (XPF) and MMS and UV-sensitive protein 81 (MUS81)-dependent nucleases, and the flap endonuclease 1 (FEN1), XPG and XPG-like endonuclease 1 (GEN1) enzymes - during processes such as DNA adduct repair, Holliday junction processing and replication stress. We also discuss newly characterized connections between SSEs and other classes of DNA-remodelling enzymes and cell cycle control machineries, which reveal the importance of SSE scaffolds such as the synthetic lethal of unknown function 4 (SLX4) tumour suppressor for the maintenance of genome stability.

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.

DNA damage repair and survival outcomes in advanced gastric cancer patients treated with first-line chemotherapy

The DNA damage response (DDR) network is exploited by cancer cells to withstand chemotherapy. Gastric cancer (GC) carries deregulation of the DDR and harbors genetic defects that fuel its activation. The ATM-Chk2 and ATR-Chk1-Wee1 axes are deputed to initiate DNA repair. Overactivation of these pathways in cancer cells may represent an adaptive response for compensating genetic defects deregulating G₁ -S transition (e.g., TP53) and ATM/ATR-initiated DNA repair (e.g., ARID1A). We hypothesized that DDR-linked biomarkers may predict clinical outcomes in GC patients treated with chemotherapy. Immunohistochemical assessment of DDR kinases (pATM, pChk2, pChk1 and pWee1) and DNA damage markers (γ-H2AX and pRPA32) was performed in biological samples from 110 advanced GC patients treated with first-line chemotherapy, either in phase II trials or in routine clinical practice. In 90 patients, this characterization was integrated with targeted ultra-deep sequencing for evaluating the mutational status of TP53 and ARID1A. We recorded a positive association between the investigated biomarkers. The combination of two biomarkers (γ-H2AX^high /pATM^high ) was an adverse factor for both progression-free survival (multivariate Cox: HR 2.23, 95%CI: 1.47-3.40) and overall survival (multivariate Cox: HR: 2.07, 95%CI: 1.20-3.58). The relationship between the γ-H2AX^high /pATM^high model and progression-free survival was consistent across the different TP53 backgrounds and was maintained in the ARID1A wild-type setting. Conversely, this association was no longer observed in an ARID1A-mutated subgroup. The γ-H2AX^high /pATM^high model negatively impacted survival outcomes in GC patients treated with chemotherapy. The mutational status of ARID1A, but apparently not TP53 mutations, affects its predictive significance.

The SMX DNA Repair Tri-nuclease

The efficient removal of replication and recombination intermediates is essential for the maintenance of genome stability. Resolution of these potentially toxic structures requires the MUS81-EME1 endonuclease, which is activated at prometaphase by formation of the SMX tri-nuclease containing three DNA repair structure-selective endonucleases: SLX1-SLX4, MUS81-EME1, and XPF-ERCC1. Here we show that SMX tri-nuclease is more active than the three individual nucleases, efficiently cleaving replication forks and recombination intermediates. Within SMX, SLX4 co-ordinates the SLX1 and MUS81-EME1 nucleases for Holliday junction resolution, in a reaction stimulated by XPF-ERCC1. SMX formation activates MUS81-EME1 for replication fork and flap structure cleavage by relaxing substrate specificity. Activation involves MUS81's conserved N-terminal HhH domain, which mediates incision site selection and SLX4 binding. Cell cycle-dependent formation and activation of this tri-nuclease complex provides a unique mechanism by which cells ensure chromosome segregation and preserve genome integrity.

Maintenance of genome stability by Fanconi anemia proteins

Persistent dysregulation of the DNA damage response and repair in cells causes genomic instability. The resulting genetic changes permit alterations in growth and proliferation observed in virtually all cancers. However, an unstable genome can serve as a double-edged sword by providing survival advantages in the ability to evade checkpoint signaling, but also creating vulnerabilities through dependency on alternative genomic maintenance factors. The Fanconi anemia pathway comprises an intricate network of DNA damage signaling and repair that are critical for protection against genomic instability. The importance of this pathway is underlined by the severity of the cancer predisposing syndrome Fanconi anemia which can be caused by biallelic mutations in any one of the 21 genes known thus far. This review delineates the roles of the Fanconi anemia pathway and the molecular actions of Fanconi anemia proteins in confronting replicative, oxidative, and mitotic stress.

Mutations in DONSON disrupt replication fork stability and cause microcephalic dwarfism

To ensure efficient genome duplication, cells have evolved numerous factors that promote unperturbed DNA replication and protect, repair and restart damaged forks. Here we identify downstream neighbor of SON (DONSON) as a novel fork protection factor and report biallelic DONSON mutations in 29 individuals with microcephalic dwarfism. We demonstrate that DONSON is a replisome component that stabilizes forks during genome replication. Loss of DONSON leads to severe replication-associated DNA damage arising from nucleolytic cleavage of stalled replication forks. Furthermore, ATM- and Rad3-related (ATR)-dependent signaling in response to replication stress is impaired in DONSON-deficient cells, resulting in decreased checkpoint activity and the potentiation of chromosomal instability. Hypomorphic mutations in DONSON substantially reduce DONSON protein levels and impair fork stability in cells from patients, consistent with defective DNA replication underlying the disease phenotype. In summary, we have identified mutations in DONSON as a common cause of microcephalic dwarfism and established DONSON as a critical replication fork protein required for mammalian DNA replication and genome stability.

Defects in homologous recombination repair behind the human diseases: FA and HBOC

Hereditary breast and ovarian cancer (HBOC) syndrome and a rare childhood disorder Fanconi anemia (FA) are caused by homologous recombination (HR) defects, and some of the causative genes overlap. Recent studies in this field have led to the exciting development of PARP inhibitors as novel cancer therapeutics and have clarified important mechanisms underlying genome instability and tumor suppression in HR-defective disorders. In this review, we provide an overview of the basic molecular mechanisms governing HR and DNA crosslink repair, highlighting BRCA2, and the intriguing relationship between HBOC and FA.

The immune receptor Trem1 cooperates with diminished DNA damage response to induce preleukemic stem cell expansion

Fanconi anemia (FA) is an inherited bone marrow failure syndrome with extremely high risk of leukemic transformation. Here we investigate the relationship between DNA damage response (DDR) and leukemogenesis using the Fanca knockout mouse model. We found that chronic exposure of the Fanca^-/- hematopoietic stem cells to DNA crosslinking agent mitomycin C in vivo leads to diminished DDR, and the emergence/expansion of pre-leukemia stem cells (pre-LSCs). Surprisingly, although genetic correction of Fanca deficiency in the pre-LSCs restores DDR and reduces genomic instability, but fails to prevent pre-LSC expansion or delay leukemia development in irradiated recipients. Furthermore, we identified transcription program underlying dysregulated DDR and cell migration, myeloid proliferation, and immune response in the Fanca^-/- pre-LSCs. Forced expression of the downregulated DNA repair genes, Rad51c or Trp53i13, in the Fanca^-/- pre-LSCs partially rescues DDR but has no effect on leukemia, whereas shRNA knockdown of the upregulated immune receptor genes Trem1 or Pilrb improves leukemia-related survival, but not DDR or genomic instability. Furthermore, Trem1 cooperates with diminished DDR in vivo to promote Fanca^-/- pre-LSC expansion and leukemia development. Our study implicates diminishing DDR as a root cause of FA leukemogenesis, which subsequently collaborates with other signaling pathways for leukemogenic transformation.

The functional status of DNA repair pathways determines the sensitization effect to cisplatin in non-small cell lung cancer cells

PURPOSE

Cisplatin can cause a variety of DNA crosslink lesions including intra-strand and inter-strand crosslinks (ICLs), which are associated with the sensitivity of cancer cells to cisplatin. Here, we aimed to assess the contribution of the Fanconi anemia (FA), homologous recombination (HR) and nucleotide excision repair (NER) pathways to cisplatin resistance in non-small cell lung cancer (NSCLC)-derived cells.

METHODS

The expression of FA, HR and NER pathway-associated genes was assessed by RT-qPCR and Western blotting. siRNAs were used to knock down the expression of these genes. CCK-8 and flow cytometry assays were used to assess the viability and apoptotic rate of NSCLC-derived cells, respectively. Immunofluorescence and alkaline comet assays were used to assess the repair of ICLs.

RESULTS

We found that acquired cisplatin-resistant NSCLC-derived A549/DR cells exhibited markedly enhanced FA and HR repair pathway capacities compared to its parental A549 cells and another independent NSCLC-derived cell line, Calu-1, which possesses a moderate innate resistance to cisplatin. siRNA-mediated silencing of the FA-associated genes FANCL and RAD18 and the HR-associated genes BRCA1 and BRCA2 significantly potentiated the sensitivity of A549/DR cells to cisplatin compared to A549 and Calu-1 cells, suggesting that the acquired cisplatin resistance in A549/DR cells may be attributed to enhanced FA and HR pathway capacities responsible for ICL repair. Although we found that expression knockdown of the NER-associated genes XPA and ERCC1 sensitized the three NSCLC-derived cell lines to cisplatin, the sensitization effect was more significant in Calu-1 cells than in A549 and A549/DR cells, implying that the innate cisplatin resistance in Calu-1 cells may result from an increased NER activity.

CONCLUSIONS

Our results indicate that the functional status of DNA repair pathways determine the sensitivity of NSCLC cells to cisplatin. Direct targeting of the pathway that is involved in cisplatin resistance may be an effective strategy to surmount cisplatin resistance in NSCLC.

Fan1 deficiency results in DNA interstrand cross-link repair defects, enhanced tissue karyomegaly, and organ dysfunction

Thongthip et al. describe a FANCD2/FANCI-associated nuclease 1 (Fan1)-deficient mouse and show that FAN1 is required for cellular and organismal resistance to DNA interstrand cross-links. Karyomegaly becomes prominent in the kidneys and livers of Fan1-deficient mice with age, and mice develop liver dysfunction.

Deficiency of FANCD2/FANCI-associated nuclease 1 (FAN1) in humans leads to karyomegalic interstitial nephritis (KIN), a rare hereditary kidney disease characterized by chronic renal fibrosis, tubular degeneration, and characteristic polyploid nuclei in multiple tissues. The mechanism of how FAN1 protects cells is largely unknown but is thought to involve FAN1's function in DNA interstrand cross-link (ICL) repair. Here, we describe a Fan1-deficient mouse and show that FAN1 is required for cellular and organismal resistance to ICLs. We show that the ubiquitin-binding zinc finger (UBZ) domain of FAN1, which is needed for interaction with FANCD2, is not required for the initial rapid recruitment of FAN1 to ICLs or for its role in DNA ICL resistance. Epistasis analyses reveal that FAN1 has cross-link repair activities that are independent of the Fanconi anemia proteins and that this activity is redundant with the 5′–3′ exonuclease SNM1A. Karyomegaly becomes prominent in kidneys and livers of Fan1-deficient mice with age, and mice develop liver dysfunction. Treatment of Fan1-deficient mice with ICL-inducing agents results in pronounced thymic and bone marrow hypocellularity and the disappearance of c-kit⁺ cells. Our results provide insight into the mechanism of FAN1 in ICL repair and demonstrate that the Fan1 mouse model effectively recapitulates the pathological features of human FAN1 deficiency.

The FANCD2-FANCI complex is recruited to DNA interstrand crosslinks before monoubiquitination of FANCD2

The Fanconi anaemia (FA) pathway is important for the repair of DNA interstrand crosslinks (ICL). The FANCD2–FANCI complex is central to the pathway, and localizes to ICLs dependent on its monoubiquitination. It has remained elusive whether the complex is recruited before or after the critical monoubiquitination. Here, we report the first structural insight into the human FANCD2–FANCI complex by obtaining the cryo-EM structure. The complex contains an inner cavity, large enough to accommodate a double-stranded DNA helix, as well as a protruding Tower domain. Disease-causing mutations in the Tower domain are observed in several FA patients. Our work reveals that recruitment of the complex to a stalled replication fork serves as the trigger for the activating monoubiquitination event. Taken together, our results uncover the mechanism of how the FANCD2–FANCI complex activates the FA pathway, and explains the underlying molecular defect in FA patients with mutations in the Tower domain.

FANCD2 and FANCI are essential components of the Fanconi anaemia DNA damage repair pathway. Here the authors present the cryo-EM structure of the FANCD2-FANCI complex, providing insight into how the complex is recruited to stalled replication forks.

The ubiquitin family meets the Fanconi anemia proteins

Fanconi anaemia (FA) is a hereditary disorder characterized by bone marrow failure, developmental defects, predisposition to cancer and chromosomal abnormalities. FA is caused by biallelic mutations that inactivate genes encoding proteins involved in replication stress-associated DNA damage responses. The 20 FANC proteins identified to date constitute the FANC pathway. A key event in this pathway involves the monoubiquitination of the FANCD2-FANCI heterodimer by the collective action of at least 10 different proteins assembled in the FANC core complex. The FANC core complex-mediated monoubiquitination of FANCD2-FANCI is essential to assemble the heterodimer in subnuclear, chromatin-associated, foci and to regulate the process of DNA repair as well as the rescue of stalled replication forks. Several recent works have demonstrated that the activity of the FANC pathway is linked to several other protein post-translational modifications from the ubiquitin-like family, including SUMO and NEDD8. These modifications are related to DNA damage responses but may also affect other cellular functions potentially related to the clinical phenotypes of the syndrome. This review summarizes the interplay between the ubiquitin and ubiquitin-like proteins and the FANC proteins that constitute a major pathway for the surveillance of the genomic integrity and addresses the implications of their interactions in maintaining genome stability.

Involvement of translesion synthesis DNA polymerases in DNA interstrand crosslink repair

DNA interstrand crosslinks (ICLs) covalently join the two strands of a DNA duplex and block essential processes such as DNA replication and transcription. Several important anti-tumor drugs such as cisplatin and nitrogen mustards exert their cytotoxicity by forming ICLs. However, multiple complex pathways repair ICLs and these are thought to contribute to the development of resistance towards ICL-inducing agents. While the understanding of many aspects of ICL repair is still rudimentary, studies in recent years have provided significant insights into the pathways of ICL repair. In this perspective we review the recent advances made in elucidating the mechanisms of ICL repair with a focus on the role of TLS polymerases. We describe the emerging models for how these enzymes contribute to and are regulated in ICL repair, discuss the key open questions and examine the implications for this pathway in anti-cancer therapy.

The Fanconi anaemia pathway: new players and new functions

The Fanconi anaemia pathway repairs DNA interstrand crosslinks (ICLs) in the genome. Our understanding of this complex pathway is still evolving, as new components continue to be identified and new biochemical systems are used to elucidate the molecular steps of repair. The Fanconi anaemia pathway uses components of other known DNA repair processes to achieve proper repair of ICLs. Moreover, Fanconi anaemia proteins have functions in genome maintenance beyond their canonical roles of repairing ICLs. Such functions include the stabilization of replication forks and the regulation of cytokinesis. Thus, Fanconi anaemia proteins are emerging as master regulators of genomic integrity that coordinate several repair processes. Here, we summarize our current understanding of the functions of the Fanconi anaemia pathway in ICL repair, together with an overview of its connections with other repair pathways and its emerging roles in genome maintenance.

Hyper-active non-homologous end joining selects for synthetic lethality resistant and pathological Fanconi anemia hematopoietic stem and progenitor cells

The prominent role of Fanconi anemia (FA) proteins involves homologous recombination (HR) repair. Poly[ADP-ribose] polymerase1 (PARP1) functions in multiple cellular processes including DNA repair and PARP inhibition is an emerging targeted therapy for cancer patients deficient in HR. Here we show that PARP1 activation in hematopoietic stem and progenitor cells (HSPCs) in response to genotoxic or oxidative stress attenuates HSPC exhaustion. Mechanistically, PARP1 controls the balance between HR and non-homologous end joining (NHEJ) in double strand break (DSB) repair by preventing excessive NHEJ. Disruption of the FA core complex skews PARP1 function in DSB repair and led to hyper-active NHEJ in Fanca^−/− or Fancc^−/− HSPCs. Re-expression of PARP1 rescues the hyper-active NHEJ phenotype in Brca1^−/−Parp1^−/− but less effective in Fanca^−/−Parp1^−/− cells. Inhibition of NHEJ prevents myeloid/erythroid pathologies associated with synthetic lethality. Our results suggest that hyper-active NHEJ may select for “synthetic lethality” resistant and pathological HSPCs.

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.

Crystal structure and SUMO binding of Slx1-Slx4 complex

The SLX1-SLX4 complex is a structure-specific endonuclease that cleaves branched DNA structures and plays significant roles in DNA recombination and repair in eukaryotic cells. The heterodimeric interaction between SLX1 and SLX4 is essential for the endonuclease activity of SLX1. Here, we present the crystal structure of Slx1 C-terminal zinc finger domain in complex with the C-terminal helix-turn-helix domain of Slx4 from Schizosaccharomyces pombe at 2.0 Å resolution. The structure reveals a conserved binding mechanism underling the Slx1-Slx4 interaction. Structural and sequence analyses indicate Slx1 C-terminal domain is actually an atypical C4HC3-type RING finger which normally possesses E3 ubiquitin ligase activity, but here is absolutely required for Slx1 interaction with Slx4. Furthermore, we found the C-terminal tail of S. pombe Slx1 contains a SUMO-interacting motif and can recognize Pmt3 (S. pombe SUMO), suggesting that Slx1-Slx4 complex could be recruited by SUMOylated protein targets to take part in replication associated DNA repair processes.

Chronic treatment with cisplatin induces replication-dependent sister chromatid recombination to confer cisplatin-resistant phenotype in nasopharyngeal carcinoma

Cisplatin can cause intrastrand and interstrand crosslinks between purine bases and is a chemotherapeutic drug widely used to treat cancer. However, the major barrier to the efficacy of the treatment is drug resistance. Homologous recombination (HR) plays a central role in restoring stalled forks caused by DNA lesions. Here, we report that chronic treatment with cisplatin induces HR to confer cisplatin resistance in nasopharyngeal carcinoma (NPC) cells. A high frequency of sister chromatid exchanges (SCE) occurs in the cisplatin-resistant NPC cells. In addition, several genes in the Fanconi anemia (FA) and template switching (TS) pathways show elevated expression. Significantly, depletion of HR gene BRCA1, TS gene UBC13, or FA gene FANCD2 suppresses SCE and causes cells to accumulate in the S phase, concomitantly with high γH2AX foci formation in the presence of low-dose cisplatin. Consistent with this result, depletion of several genes in the HR, TS, or FA pathway sensitizes the cisplatin-resistant NPC cells to cisplatin. Our results suggest that the enhanced HR, in coordination with the FA and TS pathways, underlies the cisplatin resistance. Targeting the HR, TS, or FA pathways could be a potential therapeutic strategy for treating cisplatin-resistant cancer.

Role of Deubiquitinating Enzymes in DNA Repair

Both proteolytic and nonproteolytic functions of ubiquitination are essential regulatory mechanisms for promoting DNA repair and the DNA damage response in mammalian cells. Deubiquitinating enzymes (DUBs) have emerged as key players in the maintenance of genome stability. In this minireview, we discuss the recent findings on human DUBs that participate in genome maintenance, with a focus on the role of DUBs in the modulation of DNA repair and DNA damage signaling.

What is the DNA repair defect underlying Fanconi anemia?

Fanconi anemia (FA) is a rare human genetic disease characterized by bone marrow failure, cancer predisposition, and genomic instability. It has been known for many years that FA patient-derived cells are exquisitely sensitive to DNA interstrand cross-linking agents such as cisplatin and mitomycin C. On this basis, it was widely assumed that failure to repair endogenous interstrand cross-links (ICLs) causes FA, although the endogenous mutagen that generates these lesions remained elusive. Recent genetic evidence now suggests that endogenous aldehydes are the driving force behind FA. Importantly, aldehydes cause a variety of DNA lesions, including ICLs and DNA protein cross-links (DPCs), re-kindling the debate about which DNA lesions cause FA. In this review, we discuss new developments in our understanding of DPC and ICL repair, and how these findings bear on the question of which DNA lesion underlies FA.

Physical interaction between SLX4 (FANCP) and XPF (FANCQ) proteins and biological consequences of interaction-defective missense mutations

SLX4 (FANCP) and XPF (FANCQ) proteins interact with each other and play a vital role in the Fanconi anemia (FA) DNA repair pathway. We have identified a SLX4 region and several amino acid residues that are responsible for this interaction. The study has revealed that the global minor allele, SLX4(Y546C), is defective in this interaction and cannot complement Fancp knockout mouse cells in mitomycin C-induced cytotoxicity or chromosomal aberrations. These results highly suggest this allele, as well as SLX4(L530Q), to be pathogenic. The interacting partner XPF is involved in various DNA repair pathways, and certain XPF mutations cause progeria, Cockayne syndrome (CS), and/or FA phenotypes. Because several atypical xeroderma pigmentosum (XP) phenotype-causing XPF missense mutations are located in the SLX4-interacting region, we suspected the disruption of the interaction with SLX4 in these XPF mutants, thereby causing severer phenotypes. The immunoprecipitation assay of cell extracts revealed that those XPF mutations, except XPF(C236R), located in the SLX4-interacting region cause instability of XPF protein, which could be the reason for the FA, progeria and/or CS phenotypes.

The Fanconi anemia ID2 complex: dueling saxes at the crossroads

Fanconi anemia (FA) is a rare recessive genetic disease characterized by congenital abnormalities, bone marrow failure and heightened cancer susceptibility in early adulthood. FA is caused by biallelic germ-line mutation of any one of 16 genes. While several functions for the FA proteins have been ascribed, the prevailing hypothesis is that the FA proteins function cooperatively in the FA-BRCA pathway to repair damaged DNA. A pivotal step in the activation of the FA-BRCA pathway is the monoubiquitination of the FANCD2 and FANCI proteins. Despite their importance for DNA repair, the domain structure, regulation, and function of FANCD2 and FANCI remain poorly understood. In this review, we provide an overview of our current understanding of FANCD2 and FANCI, with an emphasis on their posttranslational modification and common and unique functions.

Fanconi anemia: a model disease for studies on human genetics and advanced therapeutics

Fanconi anemia (FA) is characterized by bone marrow failure, malformations, and chromosome fragility. We review the recent discovery of FA genes and efforts to develop genetic therapies for FA in the last five years. Because current data exclude FANCM as an FA gene, 15 genes remain bona fide FA genes and three (FANCO, FANCR and FANCS) cause an FA like syndrome. Monoallelic mutations in 6 FA associated genes (FANCD1, FANCJ, FANCM, FANCN, FANCO and FANCS) predispose to breast and ovarian cancer. The products of all these genes are involved in the repair of stalled DNA replication forks by unhooking DNA interstrand cross-links and promoting homologous recombination. The genetic characterization of patients with FA is essential for developing therapies, including hematopoietic stem cell transplantation from a savior sibling donor after embryo selection, gene therapy, or genome editing using genetic recombination or engineered nucleases. Newly acquired knowledge about FA promises to provide therapeutic strategies in the near future.

Exploiting the Fanconi Anemia Pathway for Targeted Anti-Cancer Therapy

Genome instability, primarily caused by faulty DNA repair mechanisms, drives tumorigenesis. Therapeutic interventions that exploit deregulated DNA repair in cancer have made considerable progress by targeting tumor-specific alterations of DNA repair factors, which either induces synthetic lethality or augments the efficacy of conventional chemotherapy and radiotherapy. The study of Fanconi anemia (FA), a rare inherited blood disorder and cancer predisposition syndrome, has been instrumental in understanding the extent to which DNA repair defects contribute to tumorigenesis. The FA pathway functions to resolve blocked replication forks in response to DNA interstrand cross-links (ICLs), and accumulating knowledge of its activation by the ubiquitin-mediated signaling pathway has provided promising therapeutic opportunities for cancer treatment. Here, we discuss recent advances in our understanding of FA pathway regulation and its potential application for designing tailored therapeutics that take advantage of deregulated DNA ICL repair in cancer.

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.

Replication stress and cancer

Genome instability is a hallmark of cancer, and DNA replication is the most vulnerable cellular process that can lead to it. Any condition leading to high levels of DNA damage will result in replication stress, which is a source of genome instability and a feature of pre-cancerous and cancerous cells. Therefore, understanding the molecular basis of replication stress is crucial to the understanding of tumorigenesis. Although a negative aspect of replication stress is its prominent role in tumorigenesis, a positive aspect is that it provides a potential target for cancer therapy. In this Review, we discuss the link between persistent replication stress and tumorigenesis, with the goal of shedding light on the mechanisms underlying the initiation of an oncogenic process, which should open up new possibilities for cancer diagnostics and treatment.

SLX4: not SIMply a nuclease scaffold?

In this issue, Guervilly et al. (2015) and Ouyang et al. (2015) identify SUMO-interacting motifs (SIMs) in the SLX4 DNA repair nuclease scaffold protein that promote its functions in genome stability maintenance pathways independently of its ubiquitin-binding properties.

UHRF1 contributes to DNA damage repair as a lesion recognition factor and nuclease scaffold

We identified ubiquitin-like with PHD and RING finger domain 1 (UHRF1) as a binding factor for DNA interstrand crosslink (ICL) lesions through affinity purification of ICL-recognition activities. UHRF1 is recruited to DNA lesions in vivo and binds directly to ICL-containing DNA. UHRF1-deficient cells display increased sensitivity to a variety of DNA damages. We found that loss of UHRF1 led to retarded lesion processing and reduced recruitment of ICL repair nucleases to the site of DNA damage. UHRF1 interacts physically with both ERCC1 and MUS81, two nucleases involved in the repair of ICL lesions. Depletion of both UHRF1 and components of the Fanconi anemia (FA) pathway resulted in increased DNA damage sensitivity compared to defect of each mechanism alone. These results suggest that UHRF1 promotes recruitment of lesion-processing activities via its affinity to recognize DNA damage and functions as a nuclease recruitment scaffold in parallel to the FA pathway.

UHRF1 is a sensor for DNA interstrand crosslinks and recruits FANCD2 to initiate the Fanconi anemia pathway

The Fanconi anemia (FA) pathway is critical for the cellular response to toxic DNA interstrand crosslinks (ICLs). Using a biochemical purification strategy, we identified UHRF1 as a protein that specifically interacts with ICLs in vitro and in vivo. Reduction of cellular levels of UHRF1 by RNAi attenuates the FA pathway and sensitizes cells to mitomycin C. Knockdown cells display a drastic reduction in FANCD2 foci formation. Using live-cell imaging, we observe that UHRF1 is rapidly recruited to chromatin in response to DNA crosslinking agents and that this recruitment both precedes and is required for the recruitment of FANCD2 to ICLs. Based on these results, we describe a mechanism of ICL sensing and propose that UHRF1 is a critical factor that binds to ICLs. In turn, this binding is necessary for the subsequent recruitment of FANCD2, which allows the DNA repair process to initiate.

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UHRF1 is a sensor for DNA interstrand crosslinks (ICLs)

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UHRF1 is recruited to ICLs within seconds of their appearance in the genome

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Recruitment of UHRF1 is required for proper recruitment of FANCD2 to ICLs

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UHRF1 is an integral part of the Fanconi anemia DNA repair pathway