EZH2 promotes degradation of stalled replication forks by recruiting MUS81 through histone H3 trimethylation

PARP Trapping Beyond Homologous Recombination and Platinum Sensitivity in Cancers

Poly(ADP-ribose) polymerase inhibitors (PARPis) have recently been approved for the treatment of ovarian and breast cancers with BRCA mutations, as well as for maintenance therapies regardless of BRCA mutation for ovarian and primary peritoneal cancers that previously responded to platinum-based chemotherapy. The rationale of these indications is derived from the facts that cancer cells with BRCA mutations are defective in homologous recombination (HR), which confers synthetic lethality with PARPis, and that some of the sensitivity-determining factors for PARPis are shared with platinums. Although BRCA1 and BRCA2 are central for HR, more players within and beyond HR are emerging as response determinants to PARPis. Furthermore, there are similarities as well as differences in the DNA lesions and repair pathways induced by PARPis, platinums, and camptothecin topoisomerase 1 (TOP1) inhibitors. Here we review the sensitivity-determining factors for PARPis and the rationale for using PARPis as single agents and in combination therapy for cancers.

Resistance to PARP Inhibitors: Lessons from Preclinical Models of BRCA-Associated Cancer

Inhibitors of poly(ADP-ribose) polymerase (PARP) have recently entered the clinic for the treatment of homologous recombination–deficient cancers. Despite the success of this approach, resistance to PARP inhibitors (PARPis) is a clinical hurdle, and it is poorly understood how cancer cells escape the deadly effects of PARPis without restoring BRCA1/2 function. By synergizing the advantages of next-generation sequencing with functional genetic screens in tractable model systems, novel mechanisms providing useful insights into DNA damage response (DDR) have been identified. BRCA1/2 models not only are tools to explore therapy escape mechanisms but also yield basic knowledge about DDR pathways and PARPis’ mechanism of action. Moreover, alterations that render cells resistant to targeted therapies may cause new synthetic dependencies that can be exploited to combat resistant disease.

Exploiting DNA repair defects in colorectal cancer

Colorectal cancer (CRC) is the third leading cause of cancer‐related deaths worldwide. Therapies that take advantage of defects in DNA repair pathways have been explored in the context of breast, ovarian, and other tumor types, but not yet systematically in CRC. At present, only immune checkpoint blockade therapies have been FDA approved for use in mismatch repair‐deficient colorectal tumors. Here, we discuss how systematic identification of alterations in DNA repair genes could provide new therapeutic opportunities for CRCs. Analysis of The Cancer Genome Atlas Colon Adenocarcinoma (TCGA‐COAD) and Rectal Adenocarcinoma (TCGA‐READ) PanCancer Atlas datasets identified 141 (out of 528) cases with putative driver mutations in 29 genes associated with DNA damage response and repair, including the mismatch repair and homologous recombination pathways. Genetic defects in these pathways might confer repair‐deficient characteristics, such as genomic instability in the absence of homologous recombination, which can be exploited. For example, inhibitors of poly(ADP)‐ribose polymerase are effectively used to treat cancers that carry mutations in BRCA1 and/or BRCA2 and have shown promising results in CRC preclinical studies. HR deficiency can also occur in cells with no detectable BRCA1/BRCA2 mutations but exhibiting BRCA‐like phenotypes. DNA repair‐targeting therapies, such as ATR and CHK1 inhibitors (which are most effective against cancers carrying ATM mutations), can be used in combination with current genotoxic chemotherapies in CRCs to further improve therapy response. Finally, therapies that target alternative DNA repair mechanisms, such as thiopurines, also have the potential to confer increased sensitivity to current chemotherapy regimens, thus expanding the spectrum of therapy options and potentially improving clinical outcomes for CRC patients.

The Fanconi Anemia Pathway in Cancer

Fanconi anemia (FA) is a complex genetic disorder characterized by bone marrow failure (BMF), congenital defects, inability to repair DNA interstrand cross-links (ICLs), and cancer predisposition. FA presents two seemingly opposite characteristics: (a) massive cell death of the hematopoietic stem and progenitor cell (HSPC) compartment due to extensive genomic instability, leading to BMF, and (b) uncontrolled cell proliferation leading to FA-associated malignancies. The canonical function of the FA proteins is to collaborate with several other DNA repair proteins to eliminate clastogenic (chromosome-breaking) effects of DNA ICLs. Recent discoveries reveal that the FA pathway functions in a critical tumor-suppressor network to preserve genomic integrity by stabilizing replication forks, mitigating replication stress, and regulating cytokinesis. Homozygous germline mutations (biallelic) in 22 FANC genes cause FA, whereas heterozygous germline mutations in some of the FANC genes (monoallelic), such as BRCA1 and BRCA2, do not cause FA but significantly increase cancer susceptibility sporadically in the general population. In this review, we discuss our current understanding of the functions of the FA pathway in the maintenance of genomic stability, and we present an overview of the prevalence and clinical relevance of somatic mutations in FA genes.

Perturbing cohesin dynamics drives MRE11 nuclease-dependent replication fork slowing

Pds5 is required for sister chromatid cohesion, and somewhat paradoxically, to remove cohesin from chromosomes. We found that Pds5 plays a critical role during DNA replication that is distinct from its previously known functions. Loss of Pds5 hinders replication fork progression in unperturbed human and mouse cells. Inhibition of MRE11 nuclease activity restores fork progression, suggesting that Pds5 protects forks from MRE11-activity. Loss of Pds5 also leads to double-strand breaks, which are again reduced by MRE11 inhibition. The replication function of Pds5 is independent of its previously reported interaction with BRCA2. Unlike Pds5, BRCA2 protects forks from nucleolytic degradation only in the presence of genotoxic stress. Moreover, our iPOND analysis shows that the loading of Pds5 and other cohesion factors on replication forks is not affected by the BRCA2 status. Pds5 role in DNA replication is shared by the other cohesin-removal factor Wapl, but not by the cohesin complex component Rad21. Interestingly, depletion of Rad21 in a Pds5-deficient background rescues the phenotype observed upon Pds5 depletion alone. These findings support a model where loss of either component of the cohesin releasin complex perturbs cohesin dynamics on replication forks, hindering fork progression and promoting MRE11-dependent fork slowing.

Maternal Embryonic Leucine Zipper Kinase (MELK), a Potential Therapeutic Target for Neuroblastoma

Maternal embryonic leucine zipper kinase (MELK) activates pathways that mediate aggressive tumor growth and therapy resistance in many types of adult cancers. Pharmacologic and genomic inhibition of MELK impairs tumor growth and increases sensitivity to radiation and chemotherapy. On the basis of these promising preclinical studies, early-phase adult clinical trials testing the MELK inhibitor OTS167 are ongoing. To investigate whether MELK is also a therapeutic target in neuroblastoma, we analyzed MELK expression in primary tumors and cell lines, and examined the effects of OTS167 on neuroblastoma growth. In primary tumors, high levels of MELK were associated with advanced stage disease and inferior survival. Higher levels of MELK were also detected in tumorigenic versus nontumorigenic neuroblastoma cell lines, and cells with higher levels of MELK expression were more sensitive to OTS167 than low-MELK expressing cells. OTS167 suppressed the growth of neuroblastoma xenografts, and in a preclinical model of minimal residual disease, survival was prolonged with MELK inhibition. OTS167 treatment downregulated MELK and its target enhancer of zeste homolog 2 (EZH2), a component of the polycomb repressive complex 2 (PRC2) that is known to modulate the DNA damage response. We also show that OTS167 reduced the formation of collapsed replication forks induced by camptothecin or radiation. Taken together, our results indicate that MELK indirectly mediates efficient processing of replication-associated DNA lesions in neuroblastoma, and that OTS167 sensitizes cells to DNA-damaging agents by abrogating this process. Further studies evaluating the activity of combination treatment regimens with OTS167 in neuroblastoma are warranted.

Advances in understanding DNA processing and protection at stalled replication forks

The replisome, the molecular machine dedicated to copying DNA, encounters a variety of obstacles during S phase. Without a proper response to this replication stress, the genome becomes unstable, leading to disease, including cancer. The immediate response is localized to the stalled replisome and includes protection of the nascent DNA. A number of recent studies have provided insight into the factors recruited to and responsible for protecting stalled replication forks. In response to replication stress, the SNF2 family of DNA translocases has emerged as being responsible for remodeling replication forks in vivo. The protection of stalled replication forks requires the cooperation of RAD51, BRCA1, BRCA2, and many other DNA damage response proteins. In the absence of these fork protection factors, fork remodeling renders them vulnerable to degradation by nucleases and helicases, ultimately compromising genome integrity. In this review, we focus on the recent progress in understanding the protection, processing, and remodeling of stalled replication forks in mammalian cells.

Replication stress: Driver and therapeutic target in genomically instable cancers

Genomically instable cancers are characterized by progressive loss and gain of chromosomal fragments, and the acquisition of complex genomic rearrangements. Such cancers, including triple-negative breast cancers and high-grade serous ovarian cancers, typically show aggressive behavior and lack actionable driver oncogenes. Increasingly, oncogene-induced replication stress or defective replication fork maintenance is considered an important driver of genomic instability. Paradoxically, while replication stress causes chromosomal instability and thereby promotes cancer development, it intrinsically poses a threat to cellular viability. Apparently, tumor cells harboring high levels of replication stress have evolved ways to cope with replication stress. As a consequence, therapeutic targeting of such compensatory mechanisms is likely to preferentially target cancers with high levels of replication stress and may prove useful in potentiating chemotherapeutic approaches that exert their effects by interfering with DNA replication. Here, we discuss how replication stress drives chromosomal instability, and the cell cycle-regulated mechanisms that cancer cells employ to deal with replication stress. Importantly, we discuss how mechanisms involving DNA structure-specific resolvases, cell cycle checkpoint kinases and mitotic processing of replication intermediates offer possibilities in developing treatments for difficult-to-treat genomically instable cancers.

USP1 Is Required for Replication Fork Protection in BRCA1-Deficient Tumors

BRCA1-deficient tumor cells have defects in homologous-recombination repair and replication fork stability, resulting in PARP inhibitor sensitivity. Here, we demonstrate that a deubiquitinase, USP1, is upregulated in tumors with mutations in BRCA1. Knockdown or inhibition of USP1 resulted in replication fork destabilization and decreased viability of BRCA1-deficient cells, revealing a synthetic lethal relationship. USP1 binds to and is stimulated by fork DNA. A truncated form of USP1, lacking its DNA-binding region, was not stimulated by DNA and failed to localize and protect replication forks. Persistence of monoubiquitinated PCNA at the replication fork was the mechanism of cell death in the absence of USP1. Taken together, USP1 exhibits DNA-mediated activation at the replication fork, protects the fork, and promotes survival in BRCA1-deficient cells. Inhibition of USP1 may be a useful treatment for a subset of PARP-inhibitor-resistant BRCA1-deficient tumors with acquired replication fork stabilization.

PARPs in genome stability and signal transduction: implications for cancer therapy

The poly(ADP-ribose) polymerase (PARP) superfamily of enzymes catalyses the ADP-ribosylation (ADPr) of target proteins by using nicotinamide adenine dinucleotide (NAD+) as a donor. ADPr reactions occur either in the form of attachment of a single ADP-ribose nucleotide unit on target proteins or in the form of ADP-ribose chains, with the latter called poly(ADP-ribosyl)ation. PARPs regulate many cellular processes, including the maintenance of genome stability and signal transduction. In this review, we focus on the PARP family members that possess the ability to modify proteins by poly(ADP-ribosyl)ation, namely PARP1, PARP2, Tankyrase-1, and Tankyrase-2. Here, we detail the cellular functions of PARP1 and PARP2 in the regulation of DNA damage response and describe the function of Tankyrases in Wnt-mediated signal transduction. Furthermore, we discuss how the understanding of these pathways has provided some major breakthroughs in the treatment of human cancer.

Regulation of Structure-Specific Endonucleases in Replication Stress

Replication stress results in various forms of aberrant replication intermediates that need to be resolved for faithful chromosome segregation. Structure-specific endonucleases (SSEs) recognize DNA secondary structures rather than primary sequences and play key roles during DNA repair and replication stress. Holliday junction resolvase MUS81 (methyl methane sulfonate (MMS), and UV-sensitive protein 81) and XPF (xeroderma pigmentosum group F-complementing protein) are a subset of SSEs that resolve aberrant replication structures. To ensure genome stability and prevent unnecessary DNA breakage, these SSEs are tightly regulated by the cell cycle and replication checkpoints. We discuss the regulatory network that control activities of MUS81 and XPF and briefly mention other SSEs involved in the resolution of replication intermediates.

Chromatin as a Platform for Modulating the Replication Stress Response

Eukaryotic DNA replication occurs in the context of chromatin. Recent years have seen major advances in our understanding of histone supply, histone recycling and nascent histone incorporation during replication. Furthermore, much is now known about the roles of histone remodellers and post-translational modifications in replication. It has also become clear that nucleosome dynamics during replication play critical roles in genome maintenance and that chromatin modifiers are important for preventing DNA replication stress. An understanding of how cells deploy specific nucleosome modifiers, chaperones and remodellers directly at sites of replication fork stalling has been building more slowly. Here we will specifically discuss recent advances in understanding how chromatin composition contribute to replication fork stability and restart.

A tough row to hoe: when replication forks encounter DNA damage

Eukaryotic cells continuously experience DNA damage that can perturb key molecular processes like DNA replication. DNA replication forks that encounter DNA lesions typically slow and may stall, which can lead to highly detrimental fork collapse if appropriate protective measures are not executed. Stabilization and protection of stalled replication forks ensures the possibility of effective fork restart and prevents genomic instability. Recent efforts from multiple laboratories have highlighted several proteins involved in replication fork remodeling and DNA damage response pathways as key regulators of fork stability. Homologous recombination factors such as RAD51, BRCA1, and BRCA2, along with components of the Fanconi Anemia pathway, are now known to be crucial for stabilizing stalled replication forks and preventing nascent strand degradation. Several checkpoint proteins have additionally been implicated in fork protection. Ongoing work in this area continues to shed light on a sophisticated molecular pathway that balances the action of DNA resection and fork protection to maintain genomic integrity, with important implications for the fate of both normal and malignant cells following replication stress.

Chromatin modifiers Mdm2 and RNF2 prevent RNA:DNA hybrids that impair DNA replication

The p53-Mdm2 system is key to tumor suppression. We have recently reported that p53 as well as Mdm2 are capable of supporting DNA replication fork progression. On the other hand, we found that Mdm2 is a modifier of chromatin, modulating polycomb repressor complex (PRC)-driven histone modifications. Here we show that, similar to Mdm2 knockdown, the depletion of PRC members impairs DNA synthesis, as determined in fiber assays. In particular, the ubiquitin ligase and PRC1 component RNF2/Ring1B is required to support DNA replication, similar to Mdm2. Moreover, the Ring finger domain of Mdm2 is not only essential for its ubiquitin ligase activity, but also for proper DNA replication. Strikingly, Mdm2 overexpression can rescue RNF2 depletion with regard to DNA replication fork progression, and vice versa, strongly suggesting that the two ubiquitin ligases perform overlapping functions in this context. H2A overexpression also rescues fork progression upon depletion of Mdm2 or RNF2, but only when the ubiquitination sites K118/K119 are present. Depleting the H2A deubiquitinating enzyme BAP1 reduces the fork rate, suggesting that both ubiquitination and deubiquitination of H2A are required to support fork progression. The depletion of Mdm2 elicits the accumulation of RNA/DNA hybrids, suggesting R-loop formation as a mechanism of impaired DNA replication. Accordingly, RNase H overexpression or the inhibition of the transcription elongation kinase CDK9 each rescues DNA replication upon depletion of Mdm2 or RNF2. Taken together, our results suggest that chromatin modification by Mdm2 and PRC1 ensures smooth DNA replication through the avoidance of R-loop formation.

Prediction of DNA Repair Inhibitor Response in Short-Term Patient-Derived Ovarian Cancer Organoids

Based on genomic analysis, 50% of high-grade serous ovarian cancers (HGSC) are predicted to have DNA repair defects. Whether this substantial subset of HGSCs actually have functional repair defects remains unknown. Here, we devise a platform for functional profiling of DNA repair in short-term patient-derived HGSC organoids. We tested 33 organoid cultures derived from 22 patients with HGSC for defects in homologous recombination (HR) and replication fork protection. Regardless of DNA repair gene mutational status, a functional defect in HR in the organoids correlated with PARP inhibitor sensitivity. A functional defect in replication fork protection correlated with carboplatin and CHK1 and ATR inhibitor sensitivity. Our results indicate that a combination of genomic analysis and functional testing of organoids allows for the identification of targetable DNA damage repair defects. Larger numbers of patient-derived organoids must be analyzed to determine whether these assays can reproducibly predict patient response in the clinic.Significance: Patient-derived ovarian tumor organoids grow rapidly and match the tumors from which they are derived, both genetically and functionally. These organoids can be used for DNA repair profiling and therapeutic sensitivity testing and provide a rapid means of assessing targetable defects in the parent tumor, offering more suitable treatment options. Cancer Discov; 8(11); 1404-21. ©2018 AACR. This article is highlighted in the In This Issue feature, p. 1333.

DYNLL1 binds to MRE11 to limit DNA end resection in BRCA1-deficient cells

Limited DNA end resection is the key to impaired homologous recombination in BRCA1-mutant cancer cells. Here, using a loss-of-function CRISPR screen, we identify DYNLL1 as an inhibitor of DNA end resection. The loss of DYNLL1 enables DNA end resection and restores homologous recombination in BRCA1-mutant cells, thereby inducing resistance to platinum drugs and inhibitors of poly(ADP-ribose) polymerase. Low BRCA1 expression correlates with increased chromosomal aberrations in primary ovarian carcinomas, and the junction sequences of somatic structural variants indicate diminished homologous recombination. Concurrent decreases in DYNLL1 expression in carcinomas with low BRCA1 expression reduced genomic alterations and increased homology at lesions. In cells, DYNLL1 limits nucleolytic degradation of DNA ends by associating with the DNA end-resection machinery (MRN complex, BLM helicase and DNA2 endonuclease). In vitro, DYNLL1 binds directly to MRE11 to limit its end-resection activity. Therefore, we infer that DYNLL1 is an important anti-resection factor that influences genomic stability and responses to DNA-damaging chemotherapy.

On your marks, get SET(D1A): the race to protect stalled replication forks

We recently identified that methylation of lysine 4 of histone H3 (H3K4) by SETD1A (SET domain containing 1A) maintains genome stability by protecting newly-replicated DNA from degradation. Mechanistically, SETD1A-dependent histone methylation regulates nucleosome mobilisation by FANCD2 (FA complementation group D2), a crucial step in maintaining genome integrity with important implications in chemo-sensitivity.

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.

Opposing Roles of FANCJ and HLTF Protect Forks and Restrain Replication during Stress

The DNA helicase FANCJ is mutated in hereditary breast and ovarian cancer and Fanconi anemia (FA). Nevertheless, how loss of FANCJ translates to disease pathogenesis remains unclear. We addressed this question by analyzing proteins associated with replication forks in cells with or without FANCJ. We demonstrate that FANCJ-knockout (FANCJ-KO) cells have alterations in the replisome that are consistent with enhanced replication stress, including an aberrant accumulation of the fork remodeling factor helicase-like transcription factor (HLTF). Correspondingly, HLTF contributes to fork degradation in FANCJ-KO cells. Unexpectedly, the unrestrained DNA synthesis that characterizes HLTF-deficient cells is FANCJ dependent and correlates with S1 nuclease sensitivity and fork degradation. These results suggest that FANCJ and HLTF promote replication fork integrity, in part by counteracting each other to keep fork remodeling and elongation in check. Indicating one protein compensates for loss of the other, loss of both HLTF and FANCJ causes a more severe replication stress response.

Restored replication fork stabilization, a mechanism of PARP inhibitor resistance, can be overcome by cell cycle checkpoint inhibition

Poly(ADP-ribose) polymerase (PARP) inhibition serves as a potent therapeutic option eliciting synthetic lethality in cancers harboring homologous recombination (HR) repair defects, such as BRCA mutations. However, the development of resistance to PARP inhibitors (PARPis) poses a clinical challenge. Restoration of HR competency is one of the many molecular factors contributing to PARPi resistance. Combination therapy with cell cycle checkpoint (ATR, CHK1, and WEE1) inhibitors is being investigated clinically in many cancers, particularly in ovarian cancer, to enhance the efficacy and circumvent resistance to PARPis. Ideally, inhibition of ATR, CHK1 and WEE1 proteins will abrogate G2 arrest and subsequent DNA repair via restored HR in PARPi-treated cells. Replication fork stabilization has recently been identified as a potential compensatory PARPi resistance mechanism, found in the absence of restored HR. ATR, CHK1, and WEE1 each possess different roles in replication fork stabilization, providing different mechanisms to consider when developing combination therapies to avoid continued development of drug resistance. This review examines the impact of ATR, CHK1, and WEE1 on replication fork stabilization. We also address the therapeutic potential for combining PARPis with cell cycle inhibitors and the possible consequence of combination therapies which do not adequately address both restored HR and replication fork stabilization as PARPi resistance mechanisms.

Preserving replication fork integrity and competence via the homologous recombination pathway

Flaws in the DNA replication process have emerged as a leading driver of genome instability in human diseases. Alteration to replication fork progression is a defining feature of replication stress and the consequent failure to maintain fork integrity and complete genome duplication within a single round of S-phase compromises genetic integrity. This includes increased mutation rates, small and large scale genomic rearrangement and deleterious consequences for the subsequent mitosis that result in the transmission of additional DNA damage to the daughter cells. Therefore, preserving fork integrity and replication competence is an important aspect of how cells respond to replication stress and avoid genetic change. Homologous recombination is a pivotal pathway in the maintenance of genome integrity in the face of replication stress. Here we review our recent understanding of the mechanisms by which homologous recombination acts to protect, restart and repair replication forks. We discuss the dynamics of these genetically distinct functions and their contribution to faithful mitoticsegregation.

Mechanisms for stalled replication fork stabilization: new targets for synthetic lethality strategies in cancer treatments

Timely and faithful duplication of the entire genome depends on completion of replication. Replication forks frequently encounter obstacles that may cause genotoxic fork stalling. Nevertheless, failure to complete replication rarely occurs under normal conditions, which is attributed to an intricate network of proteins that serves to stabilize, repair and restart stalled forks. Indeed, many of the components in this network are encoded by tumour suppressor genes, and their loss of function by mutation or deletion generates genomic instability, a hallmark of cancer. Paradoxically, the same fork-protective network also confers resistance of cancer cells to chemotherapeutic drugs that induce high-level replication stress. Here, we review the mechanisms and major pathways rescuing stalled replication forks, with a focus on fork stabilization preventing fork collapse. A coherent understanding of how cells protect their replication forks will not only provide insight into how cells maintain genome stability, but also unravel potential therapeutic targets for cancers refractory to conventional chemotherapies.

EZH2 promotes DNA replication by stabilizing interaction of POLδ and PCNA via methylation-mediated PCNA trimerization

Background

Proliferating cell nuclear antigen (PCNA), a ring-shaped homotrimer complex, promotes DNA replication via binding to DNA polymerase. Trimerized PCNA is critical for DNA replication. Enhancer of zeste homologue 2 (EZH2), which primarily acts as a histone methyltransferase, is essential for proliferation. However, how EZH2 promotes proliferation by controlling DNA replication through PCNA remains elusive.

Results

Here, we showed that low EZH2 levels repressed the proliferation of human dental pulp cells (hDPCs). The EZH2 protein level was dramatically upregulated in hDPCs at S phase in the absence of H3K27 trimethylation. Molecularly, EZH2 interacted with PCNA via the PIP box and dimethylated PCNA at lysine 110. Dimethylation of PCNA is essential for stabilization of the PCNA trimer and the binding of DNA polymerase δ to PCNA.

Conclusions

Our data reveal the direct interaction between PCNA and EZH2 and a novel mechanism by which EZH2 orchestrates genome duplication.

Electronic supplementary material

The online version of this article (10.1186/s13072-018-0213-1) contains supplementary material, which is available to authorized users.

Overexpression of PP1-NIPP1 limits the capacity of cells to repair DNA double-strand breaks

The ubiquitously expressed nuclear protein NIPP1 (also known as PPP1R8) recruits phosphoproteins for regulated dephosphorylation by the associated protein phosphatase PP1. To bypass the PP1 titration artifacts seen upon NIPP1 overexpression, we have engineered covalently linked fusions of PP1 and NIPP1, and demonstrate their potential to selectively explore the function of the PP1:NIPP1 holoenzyme. By using inducible stable cell lines, we show that PP1-NIPP1 fusions cause replication stress in a manner that requires both PP1 activity and substrate recruitment via the ForkHead Associated domain of NIPP1. More specifically, PP1-NIPP1 expression resulted in the build up of RNA-DNA hybrids (R-loops), enhanced chromatin compaction and a diminished repair of DNA double-strand breaks (DSBs), culminating in the accumulation of DSBs. These effects were associated with a reduced expression of DNA damage signaling and repair proteins. Our data disclose a key role for dephosphorylation of PP1:NIPP1 substrates in setting the threshold for DNA repair, and indicate that activators of this phosphatase hold therapeutic potential as sensitizers for DNA-damaging agents.

Histone Methylation by SETD1A Protects Nascent DNA through the Nucleosome Chaperone Activity of FANCD2

Components of the Fanconi anemia and homologous recombination pathways play a vital role in protecting newly replicated DNA from uncontrolled nucleolytic degradation, safeguarding genome stability. Here we report that histone methylation by the lysine methyltransferase SETD1A is crucial for protecting stalled replication forks from deleterious resection. Depletion of SETD1A sensitizes cells to replication stress and leads to uncontrolled DNA2-dependent resection of damaged replication forks. The ability of SETD1A to prevent degradation of these structures is mediated by its ability to catalyze methylation on Lys4 of histone H3 (H3K4) at replication forks, which enhances FANCD2-dependent histone chaperone activity. Suppressing H3K4 methylation or expression of a chaperone-defective FANCD2 mutant leads to loss of RAD51 nucleofilament stability and severe nucleolytic degradation of replication forks. Our work identifies epigenetic modification and histone mobility as critical regulatory mechanisms in maintaining genome stability by restraining nucleases from irreparably damaging stalled replication forks.

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Methylation of H3K4 by SETD1A maintains genome stability during replication stress

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SETD1A and H3K4 methylation stabilize RAD51 nucleofilaments to protect nascent DNA

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SETD1A-dependent H3K4 methylation enhances FANCD2-dependent histone remodeling

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Histone mobility stabilizes RAD51 nucleofilaments to inhibit fork degradation

RPA and RAD51: fork reversal, fork protection, and genome stability

Replication protein A (RPA) and RAD51 are DNA-binding proteins that help maintain genome stability during DNA replication. These proteins regulate nucleases, helicases, DNA translocases, and signaling proteins to control replication, repair, recombination, and the DNA damage response. Their different DNA-binding mechanisms, enzymatic activities, and binding partners provide unique functionalities that cooperate to ensure that the appropriate activities are deployed at the right time to overcome replication challenges. Here we review and discuss the latest discoveries of the mechanisms by which these proteins work to preserve genome stability, with a focus on their actions in fork reversal and fork protection.

Histone methylation in DNA repair and clinical practice: new findings during the past 5-years

DNA double-strand breaks (DSBs) are highly toxic lesions that can impair cellular homeostasis and genome stability to result in tumorigenesis for inappropriate repair. Although DSBs are repaired by homologous recombination (HR) or non-homologous end-joining (NHEJ), the related mechanisms are still incompletely unclear. Indeed, more and more evidences indicate that the methylation of histone lysine has an important role in choosing the pathways of DNA repair. For example, tri-methylated H3K36 is required for HR repair, while di-methylated H4K20 can recruit 53BP1 for NHEJ repair. Here, we reviewed the recent progress in the molecular mechanisms by which histone methylation functions in DNA double-strand breaks repair (DSBR). The insight into the mechanisms of histone methylation repairing DNA damage will supply important cues for clinical cancer treatment.

Homologous Recombination and Replication Fork Protection: BRCA2 and More!

BRCA2 is a breast and ovarian tumor suppressor that guards against genome instability, a hallmark of cancer. Significant progress has been made in improving our understanding of BRCA2 function from biochemical, cellular, and mouse studies. The knowledge gained has been actively exploited to develop therapeutic strategies, including PARP inhibition, which has shown promising clinical outcomes. Recently, tremendous excitement has been generated by the findings of the roles of BRCA2 and other proteins in suppressing replication stress through homologous recombination and in the protection of stalled replication forks. Processes such as mitotic DNA synthesis and fork reversal have taken center stage in these studies. Here, we discuss our recent findings in the context of these advances.

The evolving landscape of predictive biomarkers of response to PARP inhibitors

Poly(ADP-ribose) polymerase inhibitors (PARPis) are DNA-damaging agents that trap PARP-DNA complexes and interfere with DNA replication. Three PARPis - olaparib, niraparib, and rucaparib - were recently approved by the FDA for the treatment of breast and ovarian cancers. These PARPis, along with 2 others (talazoparib and veliparib), are being evaluated for their potential to treat additional malignancies, including prostate cancers. While lack of PARP-1 confers high resistance to PARPis, it has not been established whether or not the levels of PARP-1 directly correlate with tumor response. In this issue of the JCI, Makvandi and coworkers describe an approach to address this question using [18F]FluorThanatrace, an [18F]-labeled PARP-1 inhibitor, for PET. The tracer was taken up by patient tumor tissue and appeared to differentiate levels of PARP-1 expression; however, future studies should be aimed at determining if this tracer can be used to stratify patient response to PARPi therapy.

Genomic and Molecular Landscape of DNA Damage Repair Deficiency across The Cancer Genome Atlas

DNA damage repair (DDR) pathways modulate cancer risk, progression, and therapeutic response. We systematically analyzed somatic alterations to provide a comprehensive view of DDR deficiency across 33 cancer types. Mutations with accompanying loss of heterozygosity were observed in over 1/3 of DDR genes, including TP53 and BRCA1/2. Other prevalent alterations included epigenetic silencing of the direct repair genes EXO5, MGMT, and ALKBH3 in ∼20% of samples. Homologous recombination deficiency (HRD) was present at varying frequency in many cancer types, most notably ovarian cancer. However, in contrast to ovarian cancer, HRD was associated with worse outcomes in several other cancers. Protein structure-based analyses allowed us to predict functional consequences of rare, recurrent DDR mutations. A new machine-learning-based classifier developed from gene expression data allowed us to identify alterations that phenocopy deleterious TP53 mutations. These frequent DDR gene alterations in many human cancers have functional consequences that may determine cancer progression and guide therapy.

Rucaparib: a novel PARP inhibitor for BRCA advanced ovarian cancer

Rucaparib is a potent small-molecule inhibitor of poly (ADP-ribose) polymerase (PARP) proteins (PARP-1, PARP-2 and PARP-3) that play an important role in repairing DNA damage and maintaining genomic stability. Tumors with mutations in BRCA1/2 or other homologous recombination deficiency (HRD) genes are particularly sensitive to PARP inhibitors because of “synthetic lethality”, whereby a therapeutic agent can take advantage of an intrinsic weakness in DNA repair. Rucaparib has been investigated in several preclinical and clinical studies showing promising activity in BRCA-mutant and BRCA–wild-type epithelial ovarian cancers (EOCs). Dose-escalation Phase I studies have established the recommended Phase II dose to be 600 mg twice a day for oral rucaparib. Phase II and III studies have defined its role as treatment for BRCA-mutant recurrent high-grade EOC and as maintenance treatment for platinum-sensitive relapsed EOC following response to platinum-based chemotherapy. Genomic loss of heterozygosity has also been investigated as a potential signature of HRD and as a potential predictive biomarker of response. Treatment-induced adverse events (AEs) have been observed in almost all patients treated with rucaparib, but mainly lower grade; with the most common being nausea, vomiting, asthenia/fatigue, anemia and transient transaminitis. The majority of AEs occurred early in treatment, were transient and have been easily managed with supportive treatment, dose interruption or discontinuation. This review will analyze the results of clinical trials investigating efficacy and safety of rucaparib in patients with ovarian cancer.

From checkpoint to checkpoint: DNA damage ATR/Chk1 checkpoint signalling elicits PD-L1 immune checkpoint activation

Multiple clinical studies have revealed a link between genomic instability and response to anti-PD-1/PD-L1 therapy in cancer management. A recent study has revealed an important role for the ATR/Chk1 DNA damage checkpoint in regulating PD-L1 expression, raising important clinical and translational questions for therapy selection and study design.

Crosstalk of DNA double-strand break repair pathways in poly(ADP-ribose) polymerase inhibitor treatment of breast cancer susceptibility gene 1/2-mutated cancer

Germline mutations in breast cancer susceptibility gene 1 or 2 (BRCA1 or BRCA2) significantly increase cancer risk in hereditary breast and ovarian cancer syndrome (HBOC). Both genes function in the homologous recombination (HR) pathway of the DNA double‐strand break (DSB) repair process. Therefore, the DNA‐repair defect characteristic of cancer cells brings about a therapeutic advantage for poly(ADP‐ribose) polymerase (PARP) inhibitor‐induced synthetic lethality. PARP inhibitor‐based therapeutics initially cause cancer lethality but acquired resistance mechanisms have been found and need to be elucidated. In particular, it is essential to understand in detail the mechanism of DNA damage and repair to PARP inhibitor treatment. Further investigations have shown the roles of BRCA1/2 and its associations to other molecules in the DSB repair system. Notably, the repair pathway chosen in BRCA1‐deficient cells could be entirely different from that in BRCA2‐deficient cells after PARP inhibitor treatment. The present review describes synthetic lethality and acquired resistance mechanisms to PARP inhibitor through the DSB repair pathway and subsequent repair process. In addition, recent knowledge of resistance mechanisms is discussed. Our model should contribute to the development of novel therapeutic strategies.

Fork Protection and Therapy Resistance in Hereditary Breast Cancer

The BRCA-Fanconi anemia (FA) pathway preserves the genome and suppresses cancer and is a main determinant of chemotherapeutic efficacy. The hereditary breast cancer genes BRCA1 and BRCA2 function in DNA double-strand break repair mediating distinct steps of homologous recombination (HR). More recently, independent of DNA repair, functions in the replication stress response have come to light, providing insight as to how the BRCA-FA pathway also balances genome preservation with proliferation. The BRCA-FA proteins associate with the replisome and contribute to the efficiency and recovery of replication following perturbations that slow or arrest DNA replication. Although the full repertoire of functions in the replication stress response remains to be elucidated, the function of BRCA1 and BRCA2 in protecting stalled replication forks contributes along with HR to the sensitivity of BRCA-associated tumors to chemotherapy. Moreover, chemoresistance evolves from restoration of either HR and/or fork protection. Although mechanisms underlying the restoration of HR have been characterized, it remains less clear how restoration of fork protection is achieved. Here, we outline mechanisms of “rewired” fork protection and chemotherapy resistance in BRCA cancer. We propose that mechanisms are linked to permissive replication that limits fork remodeling and therefore opportunities for fork degradation. Combating this chemoresistance mechanism will require drugs that inactivate replication bypass mechanisms.

A game of substrates: replication fork remodeling and its roles in genome stability and chemo-resistance

During the hours that human cells spend in the DNA synthesis (S) phase of the cell cycle, they may encounter adversities such as DNA damage or shortage of nucleotides. Under these stresses, replication forks in DNA may experience slowing, stalling, and breakage. Fork remodeling mechanisms, which stabilize slow or stalled replication forks and ensure their ability to continue or resume replication, protect cells from genomic instability and carcinogenesis. Fork remodeling includes DNA strand exchanges that result in annealing of newly synthesized strands (fork reversal), controlled DNA resection, and cleavage of DNA strands. Defects in major tumor suppressor genes BRCA1 and BRCA2, and a subset of the Fanconi Anemia genes have been shown to result in deregulation in fork remodeling, and most prominently, loss of kilobases of nascent DNA from stalled replication forks. This phenomenon has recently gained spotlight as a potential marker and mediator of chemo-sensitivity in cancer cells and, conversely, its suppression - as a hallmark of acquired chemo-resistance. Moreover, nascent strand degradation at forks is now known to also trigger innate immune response to self-DNA. An increasingly sophisticated molecular description of these events now points at a combination of unbalanced fork reversal and end-resection as a root cause, yet also reveals the multi-layered complexity and heterogeneity of the underlying processes in normal and cancer cells.

PARPi focus the spotlight on replication fork protection in cancer

PARP inhibitors (PARPi) kill BRCA1/2-mutated cancers, which become resistant when DNA repair functions are restored. Now, MUS81 nuclease inhibition due to EZH2 downregulation is found to restore DNA replication fork protection but not repair, leading to PARPi-resistance in mutant BRCA2 cells and patients. This challenges the DNA repair dominance in synthetic lethality.