PLK1 regulates CtIP and DNA2 interplay in long-range DNA end resection.

DNA double-strand break (DSB) repair is initiated by DNA end resection. CtIP acts in short-range resection to stimulate MRE11-RAD50-NBS1 (MRN) to endonucleolytically cleave 5'-terminated DNA to bypass protein blocks. CtIP also promotes the DNA2 helicase-nuclease to accelerate long-range resection downstream from MRN. Here, using AlphaFold2, we identified CtIP-F728E-Y736E as a separation-of-function mutant that is still proficient in conjunction with MRN but is not able to stimulate ssDNA degradation by DNA2. Accordingly, CtIP-F728E-Y736E impairs physical interaction with DNA2. Cellular assays revealed that CtIP-F728E-Y736E cells exhibit reduced DSB-dependent chromatin-bound RPA, impaired long-range resection, and increased sensitivity to DSB-inducing drugs. Previously, CtIP was shown to be targeted by PLK1 to inhibit long-range resection, yet the underlying mechanism was unclear. We show that the DNA2-interacting region in CtIP includes the PLK1 target site at S723. The integrity of S723 in CtIP is necessary for the stimulation of DNA2, and phosphorylation of CtIP by PLK1 in vitro is consequently inhibitory, explaining why PLK1 restricts long-range resection. Our data support a model in which CDK-dependent phosphorylation of CtIP activates resection by MRN in S phase, and PLK1-mediated phosphorylation of CtIP disrupts CtIP stimulation of DNA2 to attenuate long-range resection later at G2/M.

Context Matters: RNF168 Connects with PALB2 to Rewire Homologous Recombination in BRCA1 Haploinsufficiency.

In this issue of Molecular Cell, Zong et al. (2019) reveal RNF168-driven chromatin ubiquitylation as a key back-up mechanism to sustain homologous recombination (HR) independently of BRCA1. These findings provide new clues to carcinogenesis and cancer therapy in BRCA1 heterozygous mutation carriers.

CtIP-Mediated Fork Protection Synergizes with BRCA1 to Suppress Genomic Instability upon DNA Replication Stress.

Protecting stalled DNA replication forks from degradation by promiscuous nucleases is essential to prevent genomic instability, a major driving force of tumorigenesis. Several proteins commonly associated with the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) have been implicated in the stabilization of stalled forks. Human CtIP, in conjunction with the MRE11 nuclease complex, plays an important role in HR by promoting DSB resection. Here, we report an unanticipated function for CtIP in protecting reversed forks from degradation. Unlike BRCA proteins, which defend nascent DNA strands from nucleolytic attack by MRE11, we find that CtIP protects perturbed forks from erroneous over-resection by DNA2. Finally, we uncover functionally synergistic effects between CtIP and BRCA1 in mitigating replication-stress-induced genomic instability. Collectively, our findings reveal a DSB-resection- and MRE11-independent role for CtIP in preserving fork integrity that contributes to the survival of BRCA1-deficient cells.

Deficiency of FANCD2-associated nuclease KIAA1018/FAN1 sensitizes cells to interstrand crosslinking agents.

Cytotoxicity of cisplatin and mitomycin C (MMC) is ascribed largely to their ability to generate interstrand crosslinks (ICLs) in DNA, which block the progression of replication forks. The processing of ICLs requires the Fanconi anemia (FA) pathway, excision repair, and translesion DNA synthesis (TLS). It also requires homologous recombination (HR), which repairs double-strand breaks (DSBs) generated by cleavage of the blocked replication forks. Here we describe KIAA1018, an evolutionarily conserved protein that has an N-terminal ubiquitin-binding zinc finger (UBZ) and a C-terminal nuclease domain. KIAA1018 is a 5'-->3' exonuclease and a structure-specific endonuclease that preferentially incises 5' flaps. Like cells from FA patients, human cells depleted of KIAA1018 are sensitized to ICL-inducing agents and display chromosomal instability. The link of KIAA1018 to the FA pathway is further strengthened by its recruitment to DNA damage through interaction of its UBZ domain with monoubiquitylated FANCD2. We therefore propose to name KIAA1018 FANCD2-associated nuclease, FAN1.

Human CtIP: A 'double agent' in DNA repair and tumorigenesis.

Human CtIP was originally identified as an interactor of the retinoblastoma protein and BRCA1, two bona fide tumour suppressors frequently mutated in cancer. CtIP is renowned for its role in the resection of DNA double-strand breaks (DSBs) during homologous recombination, a largely error-free DNA repair pathway crucial in maintaining genome integrity. However, CtIP-dependent DNA end resection is equally accountable for alternative end-joining, a mutagenic DSB repair mechanism implicated in oncogenic chromosomal translocations. In addition, CtIP contributes to transcriptional regulation of G1/S transition, DNA damage checkpoint signalling, and replication fork protection pathways. In this review, we present a perspective on the current state of knowledge regarding the tumour-suppressive and oncogenic properties of CtIP and provide an overview of their relevance for cancer development, progression, and therapy.

RIF1 is essential for 53BP1-dependent nonhomologous end joining and suppression of DNA double-strand break resection.

The appropriate execution of DNA double-strand break (DSB) repair is critical for genome stability and tumor avoidance. 53BP1 and BRCA1 directly influence DSB repair pathway choice by regulating 5' end resection, but how this is achieved remains uncertain. Here we report that Rif1(-/-) mice are severely compromised for 53BP1-dependent class switch recombination (CSR) and fusion of dysfunctional telomeres. The inappropriate accumulation of RIF1 at DSBs in S phase is antagonized by BRCA1, and deletion of Rif1 suppresses toxic nonhomologous end joining (NHEJ) induced by PARP inhibition in Brca1-deficient cells. Mechanistically, RIF1 is recruited to DSBs via the N-terminal phospho-SQ/TQ domain of 53BP1, and DSBs generated by ionizing radiation or during CSR are hyperresected in the absence of RIF1. Thus, RIF1 and 53BP1 cooperate to block DSB resection to promote NHEJ in G1, which is antagonized by BRCA1 in S phase to ensure a switch of DSB repair mode to homologous recombination.

Prolyl isomerase PIN1 regulates DNA double-strand break repair by counteracting DNA end resection.

The regulation of DNA double-strand break (DSB) repair by phosphorylation-dependent signaling pathways is crucial for the maintenance of genome stability; however, remarkably little is known about the molecular mechanisms by which phosphorylation controls DSB repair. Here, we show that PIN1, a phosphorylation-specific prolyl isomerase, interacts with key DSB repair factors and affects the relative contributions of homologous recombination (HR) and nonhomologous end-joining (NHEJ) to DSB repair. We find that PIN1-deficient cells display reduced NHEJ due to increased DNA end resection, whereas resection and HR are compromised in PIN1-overexpressing cells. Moreover, we identify CtIP as a substrate of PIN1 and show that DSBs become hyperresected in cells expressing a CtIP mutant refractory to PIN1 recognition. Mechanistically, we provide evidence that PIN1 impinges on CtIP stability by promoting its ubiquitylation and subsequent proteasomal degradation. Collectively, these data uncover PIN1-mediated isomerization as a regulatory mechanism coordinating DSB repair.

Noncanonical mismatch repair as a source of genomic instability in human cells.

Mismatch repair (MMR) is a key antimutagenic process that increases the fidelity of DNA replication and recombination. Yet genetic experiments showed that MMR is required for antibody maturation, a process during which the immunoglobulin loci of antigen-stimulated B cells undergo extensive mutagenesis and rearrangements. In an attempt to elucidate the mechanism underlying the latter events, we set out to search for conditions that compromise MMR fidelity. Here, we describe noncanonical MMR (ncMMR), a process in which the MMR pathway is activated by various DNA lesions rather than by mispairs. ncMMR is largely independent of DNA replication, lacks strand directionality, triggers PCNA monoubiquitylation, and promotes recruitment of the error-prone polymerase-η to chromatin. Importantly, ncMMR is not limited to B cells but occurs also in other cell types. Moreover, it contributes to mutagenesis induced by alkylating agents. Activation of ncMMR may therefore play a role in genomic instability and cancer.

HELQ promotes RAD51 paralogue-dependent repair to avert germ cell loss and tumorigenesis.

Repair of interstrand crosslinks (ICLs) requires the coordinated action of the intra-S-phase checkpoint and the Fanconi anaemia pathway, which promote ICL incision, translesion synthesis and homologous recombination (reviewed in refs 1, 2). Previous studies have implicated the 3'-5' superfamily 2 helicase HELQ in ICL repair in Drosophila melanogaster (MUS301 (ref. 3)) and Caenorhabditis elegans (HELQ-1 (ref. 4)). Although in vitro analysis suggests that HELQ preferentially unwinds synthetic replication fork substrates with 3' single-stranded DNA overhangs and also disrupts protein-DNA interactions while translocating along DNA, little is known regarding its functions in mammalian organisms. Here we report that HELQ helicase-deficient mice exhibit subfertility, germ cell attrition, ICL sensitivity and tumour predisposition, with Helq heterozygous mice exhibiting a similar, albeit less severe, phenotype than the null, indicative of haploinsufficiency. We establish that HELQ interacts directly with the RAD51 paralogue complex BCDX2 and functions in parallel to the Fanconi anaemia pathway to promote efficient homologous recombination at damaged replication forks. Thus, our results reveal a critical role for HELQ in replication-coupled DNA repair, germ cell maintenance and tumour suppression in mammals.

APC/C(Cdh1) controls CtIP stability during the cell cycle and in response to DNA damage.

Human cells have evolved elaborate mechanisms for responding to DNA damage to maintain genome stability and prevent carcinogenesis. For instance, the cell cycle can be arrested at different stages to allow time for DNA repair. The APC/C(C) (dh1) ubiquitin ligase mainly regulates mitotic exit but is also implicated in the DNA damage-induced G2 arrest. However, it is currently unknown whether APC/C(C) (dh1) also contributes to DNA repair. Here, we show that Cdh1 depletion causes increased levels of genomic instability and enhanced sensitivity to DNA-damaging agents. Using an integrated proteomics and bioinformatics approach, we identify CtIP, a DNA-end resection factor, as a novel APC/C(C) (dh1) target. CtIP interacts with Cdh1 through a conserved KEN box, mutation of which impedes ubiquitylation and downregulation of CtIP both during G1 and after DNA damage in G2. Finally, we find that abrogating the CtIP-Cdh1 interaction results in delayed CtIP clearance from DNA damage foci, increased DNA-end resection, and reduced homologous recombination efficiency. Combined, our results highlight the impact of APC/C(C) (dh1) on the maintenance of genome integrity and show that this is, at least partially, achieved by controlling CtIP stability in a cell cycle- and DNA damage-dependent manner.

CDK targets Sae2 to control DNA-end resection and homologous recombination.

DNA double-strand breaks (DSBs) are repaired by two principal mechanisms: non-homologous end-joining (NHEJ) and homologous recombination (HR). HR is the most accurate DSB repair mechanism but is generally restricted to the S and G2 phases of the cell cycle, when DNA has been replicated and a sister chromatid is available as a repair template. By contrast, NHEJ operates throughout the cell cycle but assumes most importance in G1 (refs 4, 6). The choice between repair pathways is governed by cyclin-dependent protein kinases (CDKs), with a major site of control being at the level of DSB resection, an event that is necessary for HR but not NHEJ, and which takes place most effectively in S and G2 (refs 2, 5). Here we establish that cell-cycle control of DSB resection in Saccharomyces cerevisiae results from the phosphorylation by CDK of an evolutionarily conserved motif in the Sae2 protein. We show that mutating Ser 267 of Sae2 to a non-phosphorylatable residue causes phenotypes comparable to those of a sae2Delta null mutant, including hypersensitivity to camptothecin, defective sporulation, reduced hairpin-induced recombination, severely impaired DNA-end processing and faulty assembly and disassembly of HR factors. Furthermore, a Sae2 mutation that mimics constitutive Ser 267 phosphorylation complements these phenotypes and overcomes the necessity of CDK activity for DSB resection. The Sae2 mutations also cause cell-cycle-stage specific hypersensitivity to DNA damage and affect the balance between HR and NHEJ. These findings therefore provide a mechanistic basis for cell-cycle control of DSB repair and highlight the importance of regulating DSB resection.

Carcinogenic bacterial pathogen Helicobacter pylori triggers DNA double-strand breaks and a DNA damage response in its host cells.

The bacterial pathogen Helicobacter pylori chronically infects the human gastric mucosa and is the leading risk factor for the development of gastric cancer. The molecular mechanisms of H. pylori-associated gastric carcinogenesis remain ill defined. In this study, we examined the possibility that H. pylori directly compromises the genomic integrity of its host cells. We provide evidence that the infection introduces DNA double-strand breaks (DSBs) in primary and transformed murine and human epithelial and mesenchymal cells. The induction of DSBs depends on the direct contact of live bacteria with mammalian cells. The infection-associated DNA damage is evident upon separation of nuclear DNA by pulse field gel electrophoresis and by high-magnification microscopy of metaphase chromosomes. Bacterial adhesion (e.g., via blood group antigen-binding adhesin) is required to induce DSBs; in contrast, the H. pylori virulence factors vacuolating cytotoxin A, γ-glutamyl transpeptidase, and the cytotoxin-associated gene (Cag) pathogenicity island are dispensable for DSB induction. The DNA discontinuities trigger a damage-signaling and repair response involving the sequential ataxia telangiectasia mutated (ATM)-dependent recruitment of repair factors--p53-binding protein (53BP1) and mediator of DNA damage checkpoint protein 1 (MDC1)--and histone H2A variant X (H2AX) phosphorylation. Although most breaks are repaired efficiently upon termination of the infection, we observe that prolonged active infection leads to saturation of cellular repair capabilities. In summary, we conclude that DNA damage followed by potentially imprecise repair is consistent with the carcinogenic properties of H. pylori and with its mutagenic properties in vitro and in vivo and may contribute to the genetic instability and frequent chromosomal aberrations that are a hallmark of gastric cancer.

H2AX promotes replication fork degradation and chemosensitivity in BRCA-deficient tumours.

Histone H2AX plays a key role in DNA damage signalling in the surrounding regions of DNA double-strand breaks (DSBs). In response to DNA damage, H2AX becomes phosphorylated on serine residue 139 (known as γH2AX), resulting in the recruitment of the DNA repair effectors 53BP1 and BRCA1. Here, by studying resistance to poly(ADP-ribose) polymerase (PARP) inhibitors in BRCA1/2-deficient mammary tumours, we identify a function for γH2AX in orchestrating drug-induced replication fork degradation. Mechanistically, γH2AX-driven replication fork degradation is elicited by suppressing CtIP-mediated fork protection. As a result, H2AX loss restores replication fork stability and increases chemoresistance in BRCA1/2-deficient tumour cells without restoring homology-directed DNA repair, as highlighted by the lack of DNA damage-induced RAD51 foci. Furthermore, in the attempt to discover acquired genetic vulnerabilities, we find that ATM but not ATR inhibition overcomes PARP inhibitor (PARPi) resistance in H2AX-deficient tumours by interfering with CtIP-mediated fork protection. In summary, our results demonstrate a role for H2AX in replication fork biology in BRCA-deficient tumours and establish a function of H2AX separable from its classical role in DNA damage signalling and DSB repair.

Human CtIP promotes DNA end resection.

In the S and G2 phases of the cell cycle, DNA double-strand breaks (DSBs) are processed into single-stranded DNA, triggering ATR-dependent checkpoint signalling and DSB repair by homologous recombination. Previous work has implicated the MRE11 complex in such DSB-processing events. Here, we show that the human CtIP (RBBP8) protein confers resistance to DSB-inducing agents and is recruited to DSBs exclusively in the S and G2 cell-cycle phases. Moreover, we reveal that CtIP is required for DSB resection, and thereby for recruitment of replication protein A (RPA) and the protein kinase ATR to DSBs, and for the ensuing ATR activation. Furthermore, we establish that CtIP physically and functionally interacts with the MRE11 complex, and that both CtIP and MRE11 are required for efficient homologous recombination. Finally, we reveal that CtIP has sequence homology with Sae2, which is involved in MRE11-dependent DSB processing in yeast. These findings establish evolutionarily conserved roles for CtIP-like proteins in controlling DSB resection, checkpoint signalling and homologous recombination.

DNA end resection by CtIP and exonuclease 1 prevents genomic instability.

End resection of DNA-which is essential for the repair of DNA double-strand breaks (DSBs) by homologous recombination-relies first on the partnership between MRE11-RAD50-NBS1 (MRN) and CtIP, followed by a processive step involving helicases and exonucleases such as exonuclease 1 (EXO1). In this study, we show that the localization of EXO1 to DSBs depends on both CtIP and MRN. We also establish that CtIP interacts with EXO1 and restrains its exonucleolytic activity in vitro. Finally, we show that on exposure to camptothecin, depletion of EXO1 in CtIP-deficient cells increases the frequency of DNA-PK-dependent radial chromosome formation. Thus, our study identifies new functions of CtIP and EXO1 in DNA end resection and provides new information on the regulation of DSB repair pathways, which is a key factor in the maintenance of genome integrity.

RNAi Screening Uncovers a Synthetic Sick Interaction between CtIP and the BARD1 Tumor Suppressor.

Human CtIP is best known for its role in DNA end resection to initiate DNA double-strand break repair by homologous recombination. Recently, CtIP has also been shown to protect reversed replication forks from nucleolytic degradation upon DNA replication stress. However, still little is known about the DNA damage response (DDR) networks that preserve genome integrity and sustain cell survival in the context of CtIP insufficiency. Here, to reveal such potential buffering relationships, we screened a DDR siRNA library in CtIP-deficient cells to identify candidate genes that induce synthetic sickness/lethality (SSL). Our analyses unveil a negative genetic interaction between CtIP and BARD1, the heterodimeric binding partner of BRCA1. We found that simultaneous disruption of CtIP and BARD1 triggers enhanced apoptosis due to persistent replication stress-induced DNA lesions giving rise to chromosomal abnormalities. Moreover, we observed that the genetic interaction between CtIP and BARD1 occurs independently of the BRCA1-BARD1 complex formation and might be, therefore, therapeutical relevant for the treatment of BRCA-defective tumors.

MRE11 complex links RECQ5 helicase to sites of DNA damage.

RECQ5 DNA helicase suppresses homologous recombination (HR) possibly through disruption of RAD51 filaments. Here, we show that RECQ5 is constitutively associated with the MRE11-RAD50-NBS1 (MRN) complex, a primary sensor of DNA double-strand breaks (DSBs) that promotes DSB repair and regulates DNA damage signaling via activation of the ATM kinase. Experiments with purified proteins indicated that RECQ5 interacts with the MRN complex through both MRE11 and NBS1. Functional assays revealed that RECQ5 specifically inhibited the 3'-->5' exonuclease activity of MRE11, while MRN had no effect on the helicase activity of RECQ5. At the cellular level, we observed that the MRN complex was required for the recruitment of RECQ5 to sites of DNA damage. Accumulation of RECQ5 at DSBs was neither dependent on MDC1 that mediates binding of MRN to DSB-flanking chromatin nor on CtIP that acts in conjunction with MRN to promote resection of DSBs for repair by HR. Collectively, these data suggest that the MRN complex recruits RECQ5 to sites of DNA damage to regulate DNA repair.

A stapled peptide mimetic of the CtIP tetramerization motif interferes with double-strand break repair and replication fork protection.

Cancer cells display high levels of DNA damage and replication stress, vulnerabilities that could be exploited by drugs targeting DNA repair proteins. Human CtIP promotes homology-mediated repair of DNA double-strand breaks (DSBs) and protects stalled replication forks from nucleolytic degradation, thus representing an attractive candidate for targeted cancer therapy. Here, we establish a peptide mimetic of the CtIP tetramerization motif that inhibits CtIP activity. The hydrocarbon-stapled peptide encompassing amino acid residues 18 to 28 of CtIP (SP18-28) stably binds to CtIP tetramers in vitro and facilitates their aggregation into higher-order structures. Efficient intracellular uptake of SP18-28 abrogates CtIP localization to damaged chromatin, impairs DSB repair, and triggers extensive fork degradation. Moreover, prolonged SP18-28 treatment causes hypersensitivity to DNA-damaging agents and selectively reduces the viability of BRCA1-mutated cancer cell lines. Together, our data provide a basis for the future development of CtIP-targeting compounds with the potential to treat patients with cancer.