Human RECQ1 promotes restart of replication forks reversed by DNA topoisomerase I inhibition

Functions of ADP-ribose transferases in the maintenance of telomere integrity

The ADP-ribose transferase (ART) family comprises 17 enzymes that catalyze mono- or poly-ADP-ribosylation, a post-translational modification of proteins. Present in all subcellular compartments, ARTs are implicated in a growing number of biological processes including DNA repair, replication, transcription regulation, intra- and extra-cellular signaling, viral infection and cell death. Five members of the family, PARP1, PARP2, PARP3, tankyrase 1 and tankyrase 2 are mainly described for their crucial functions in the maintenance of genome stability. It is well established that the most describedrole of PARP1, 2 and 3 is the repair of DNA lesions while tankyrases 1 and 2 are crucial for maintaining the integrity of telomeres. Telomeres, nucleoprotein complexes located at the ends of eukaryotic chromosomes, utilize their unique structure and associated set of proteins to orchestrate the mechanisms necessary for their own protection and replication. While the functions of tankyrases 1 and 2 at telomeres are well known, several studies have also brought PARP1, 2 and 3 to the forefront of telomere protection. The singular quality of the telomeric environment has highlighted protein interactions and molecular pathways distinct from those described throughout the genome. The aim of this review is to provide an overview of the current knowledge on the multiple roles of PARP1, PARP2, PARP3, tankyrase 1 and tankyrase 2 in the maintenance and preservation of telomere integrity.

The expanding universe of PARP1-mediated molecular and therapeutic mechanisms

ADP-ribosylation (ADPRylation) is a post-translational modification of proteins catalyzed by ADP-ribosyl transferase (ART) enzymes, including nuclear PARPs (e.g., PARP1 and PARP2). Historically, studies of ADPRylation and PARPs have focused on DNA damage responses in cancers, but more recent studies elucidate diverse roles in a broader array of biological processes. Here, we summarize the expanding array of molecular mechanisms underlying the biological functions of nuclear PARPs with a focus on PARP1, the founding member of the family. This includes roles in DNA repair, chromatin regulation, gene expression, ribosome biogenesis, and RNA biology. We also present new concepts in PARP1-dependent regulation, including PAR-dependent post-translational modifications, "ADPR spray," and PAR-mediated biomolecular condensate formation. Moreover, we review advances in the therapeutic mechanisms of PARP inhibitors (PARPi) as well as the progress on the mechanisms of PARPi resistance. Collectively, the recent progress in the field has yielded new insights into the expanding universe of PARP1-mediated molecular and therapeutic mechanisms in a variety of biological processes.

DNA repair defects in cancer and therapeutic opportunities

DNA repair and DNA damage signaling pathways are critical for the maintenance of genomic stability. Defects of DNA repair and damage signaling contribute to tumorigenesis, but also render cancer cells vulnerable to DNA damage and reliant on remaining repair and signaling activities. Here, we review the major classes of DNA repair and damage signaling defects in cancer, the genomic instability that they give rise to, and therapeutic strategies to exploit the resulting vulnerabilities. Furthermore, we discuss the impacts of DNA repair defects on both targeted therapy and immunotherapy, and highlight emerging principles for targeting DNA repair defects in cancer therapy.

Human topoisomerases and their roles in genome stability and organization

Human topoisomerases comprise a family of six enzymes: two type IB (TOP1 and mitochondrial TOP1 (TOP1MT), two type IIA (TOP2A and TOP2B) and two type IA (TOP3A and TOP3B) topoisomerases. In this Review, we discuss their biochemistry and their roles in transcription, DNA replication and chromatin remodelling, and highlight the recent progress made in understanding TOP3A and TOP3B. Because of recent advances in elucidating the high-order organization of the genome through chromatin loops and topologically associating domains (TADs), we integrate the functions of topoisomerases with genome organization. We also discuss the physiological and pathological formation of irreversible topoisomerase cleavage complexes (TOPccs) as they generate topoisomerase DNA–protein crosslinks (TOP-DPCs) coupled with DNA breaks. We discuss the expanding number of redundant pathways that repair TOP-DPCs, and the defects in those pathways, which are increasingly recognized as source of genomic damage leading to neurological diseases and cancer.

Topoisomerases have essential roles in transcription, DNA replication, chromatin remodelling and, as recently revealed, 3D genome organization. However, topoisomerases also generate DNA–protein crosslinks coupled with DNA breaks, which are increasingly recognized as a source of disease-causing genomic damage.

DNA Damage Tolerance Pathways in Human Cells: A Potential Therapeutic Target

DNA lesions arising from both exogenous and endogenous sources occur frequently in DNA. During DNA replication, the presence of unrepaired DNA damage in the template can arrest replication fork progression, leading to fork collapse, double-strand break formation, and to genome instability. To facilitate completion of replication and prevent the generation of strand breaks, DNA damage tolerance (DDT) pathways play a key role in allowing replication to proceed in the presence of lesions in the template. The two main DDT pathways are translesion synthesis (TLS), which involves the recruitment of specialized TLS polymerases to the site of replication arrest to bypass lesions, and homology-directed damage tolerance, which includes the template switching and fork reversal pathways. With some exceptions, lesion bypass by TLS polymerases is a source of mutagenesis, potentially contributing to the development of cancer. The capacity of TLS polymerases to bypass replication-blocking lesions induced by anti-cancer drugs such as cisplatin can also contribute to tumor chemoresistance. On the other hand, during homology-directed DDT the nascent sister strand is transiently utilised as a template for replication, allowing for error-free lesion bypass. Given the role of DNA damage tolerance pathways in replication, mutagenesis and chemoresistance, a more complete understanding of these pathways can provide avenues for therapeutic exploitation. A number of small molecule inhibitors of TLS polymerase activity have been identified that show synergy with conventional chemotherapeutic agents in killing cancer cells. In this review, we will summarize the major DDT pathways, explore the relationship between damage tolerance and carcinogenesis, and discuss the potential of targeting TLS polymerases as a therapeutic approach.

Genome-wide investigations on regulatory functions of RECQ1 helicase

DNA helicase RECQ1 (also known as RECQL or RECQL1) is a candidate breast cancer susceptibility gene significantly correlated with clinical outcomes of sporadic breast cancer patients. Prior studies have suggested that RECQ1 maintains genomic stability by regulating a wide variety of core cellular functions including DNA replication, DNA damage response, and transcription. However, it is unclear which, if any, of these are the primary functions of RECQ1 as related to its role in suppressing breast cancer. We describe here an unbiased integrative genomics approach that enabled us to discover a previously unknown regulatory role of RECQ1 in promoting Estrogen Receptor alpha (ERα) expression and the expression of specific ERα target genes in ER positive breast cancer cells. We discuss potential future applications of similar experimental strategies in advancing the mechanistic understanding and elucidating specific new details of genome-wide functions of RECQ1 and other RecQ helicases in maintaining genomic stability and preventing cancer.

Beyond PARP1: The Potential of Other Members of the Poly (ADP-Ribose) Polymerase Family in DNA Repair and Cancer Therapeutics

The proteins within the Poly-ADP Ribose Polymerase (PARP) family encompass a diverse and integral set of cellular functions. PARP1 and PARP2 have been extensively studied for their roles in DNA repair and as targets for cancer therapeutics. Several PARP inhibitors (PARPi) have been approved for clinical use, however, while their efficacy is promising, tumours readily develop PARPi resistance. Many other members of the PARP protein family share catalytic domain homology with PARP1/2, however, these proteins are comparatively understudied, particularly in the context of DNA damage repair and tumourigenesis. This review explores the functions of PARP4,6-16 and discusses the current knowledge of the potential roles these proteins may play in DNA damage repair and as targets for cancer therapeutics.

Linking DNA repair and cell cycle progression through serine ADP-ribosylation of histones

Although serine ADP-ribosylation (Ser-ADPr) by Poly(ADP-ribose)-polymerases is a cornerstone of the DNA damage response, how this regulates DNA repair and genome stability is unknown. Here, we exploit the ability to manipulate histone genes in Dictyostelium to identify that ADPr of the histone variant H3b at S10 and S28 maintains genome stability by integrating double strand break (DSB) repair with mitotic entry. Given the critical requirement for mitotic H3S10/28 phosphorylation, we develop separation of function mutations that maintain S10 phosphorylation whilst disrupting ADPr. Mechanistically, this reveals a requirement for H3bS10/28 ADPr in non-homologous end-joining by recruiting Ku to DSBs. Moreover, this also identifies H3bS10/S28 ADPr is critical to prevent premature mitotic entry with unresolved DNA damage, thus maintaining genome stability. Together, these data demonstrate how serine ADPr of histones coordinates DNA repair with cell cycle progression to maintain genome stability.

RECON syndrome is a genome instability disorder caused by mutations in the DNA helicase RECQL1

Despite being the first homolog of the bacterial RecQ helicase to be identified in humans, the function of RECQL1 remains poorly characterized. Furthermore, unlike other members of the human RECQ family of helicases, mutations in RECQL1 have not been associated with a genetic disease. Here, we identify 2 families with a genome instability disorder that we have named RECON (RECql ONe) syndrome, caused by biallelic mutations in the RECQL gene. The affected individuals had short stature, progeroid facial features, a hypoplastic nose, xeroderma, and skin photosensitivity and were homozygous for the same missense mutation in RECQL1 (p.Ala459Ser), located within its zinc binding domain. Biochemical analysis of the mutant RECQL1 protein revealed that the p.A459S missense mutation compromised its ATPase, helicase, and fork restoration activity, while its capacity to promote single-strand DNA annealing was largely unaffected. At the cellular level, this mutation in RECQL1 gave rise to a defect in the ability to repair DNA damage induced by exposure to topoisomerase poisons and a failure of DNA replication to progress efficiently in the presence of abortive topoisomerase lesions. Taken together, RECQL1 is the fourth member of the RecQ family of helicases to be associated with a human genome instability disorder.

Transcriptional regulation by a RecQ helicase

RecQ helicases participate in a variety of DNA metabolic processes through their multiple biochemical activities. In vitro characterization and cellular studies have suggested that RECQ1 (also known as RECQL or RECQL1) performs its diverse functions through specific interactions with DNA and protein partners. We have taken an unbiased approach to determine the contribution of RECQ1 in genome maintenance and as a putative susceptibility factor in breast cancer. Here, we provide methodology to map the genome-wide binding sites of RECQ1 together with the profiling of RECQ1-dependent transcriptome to investigate its role in gene regulation. The described approach will be helpful to develop a mechanistic framework for elucidating critical functions of RECQ1 and other RecQ homologs in distinct chromatin and biological contexts.

PARP Inhibitors and Myeloid Neoplasms: A Double-Edged Sword

Simple Summary

Poly(ADP-ribose) polymerase (PARP) inhibitors, which are medications approved to treat various solid tumors, including breast, prostate, ovarian, and prostate cancers, are being examined in hematological malignancies. This review summarizes the potential role of PARP inhibitors in the treatment of myeloid diseases, particularly acute myeloid leukemia (AML). We review ongoing clinical studies investigating the safety and efficacy of PARP inhibitors in the treatment of AML, focusing on specific molecular and genetic AML subgroups that could be particularly sensitive to PARP inhibitor treatment. We also discuss reports describing an increased risk of treatment-related myeloid neoplasms in patients receiving PARP inhibitors for solid tumors.

Abstract

Despite recent discoveries and therapeutic advances in aggressive myeloid neoplasms, there remains a pressing need for improved therapies. For instance, in acute myeloid leukemia (AML), while most patients achieve a complete remission with conventional chemotherapy or the combination of a hypomethylating agent and venetoclax, de novo or acquired drug resistance often presents an insurmountable challenge, especially in older patients. Poly(ADP-ribose) polymerase (PARP) enzymes, PARP1 and PARP2, are involved in detecting DNA damage and repairing it through multiple pathways, including base excision repair, single-strand break repair, and double-strand break repair. In the context of AML, PARP inhibitors (PARPi) could potentially exploit the frequently dysfunctional DNA repair pathways that, similar to deficiencies in homologous recombination in BRCA-mutant disease, set the stage for cell killing. PARPi appear to be especially effective in AML with certain gene rearrangements and molecular characteristics (RUNX1-RUNX1T1 and PML-RARA fusions, FLT3- and IDH1-mutated). In addition, PARPi can enhance the efficacy of other agents, particularly alkylating agents, TOP1 poisons, and hypomethylating agents, that induce lesions ordinarily repaired via PARP1-dependent mechanisms. Conversely, emerging reports suggest that long-term treatment with PARPi for solid tumors is associated with an increased incidence of myelodysplastic syndrome (MDS) and AML. Here, we (i) review the pre-clinical and clinical data on the role of PARPi, specifically olaparib, talazoparib, and veliparib, in aggressive myeloid neoplasms and (ii) discuss the reported risk of MDS/AML with PARPi, especially as the indications for PARPi use expand to include patients with potentially curable cancer.

ID3 promotes homologous recombination via non-transcriptional and transcriptional mechanisms and its loss confers sensitivity to PARP inhibition

The inhibitor of DNA-binding 3 (ID3) is a transcriptional regulator that limits interaction of basic helix-loop-helix transcription factors with their target DNA sequences. We previously reported that ID3 loss is associated with mutational signatures linked to DNA repair defects. Here we demonstrate that ID3 exhibits a dual role to promote DNA double-strand break (DSB) repair, particularly homologous recombination (HR). ID3 interacts with the MRN complex and RECQL helicase to activate DSB repair and it facilitates RAD51 loading and downstream steps of HR. In addition, ID3 promotes the expression of HR genes in response to ionizing radiation by regulating both chromatin accessibility and activity of the transcription factor E2F1. Consistently, analyses of TCGA cancer patient data demonstrate that low ID3 expression is associated with impaired HR. The loss of ID3 leads to sensitivity of tumor cells to PARP inhibition, offering new therapeutic opportunities in ID3-deficient tumors.

Residues at the interface between zinc binding and winged helix domains of human RECQ1 play a significant role in DNA strand annealing activity

RECQ1 is the shortest among the five human RecQ helicases comprising of two RecA like domains, a zinc-binding domain and a RecQ C-terminal domain containing the winged-helix (WH). Mutations or deletions on the tip of a β-hairpin located in the WH domain are known to abolish the unwinding activity. Interestingly, the same mutations on the β-hairpin of annealing incompetent RECQ1 mutant (RECQ1^T1) have been reported to restore its annealing activity. In an attempt to unravel the strand annealing mechanism, we have crystallized a fragment of RECQ1 encompassing D2–Zn–WH domains harbouring mutations on the β-hairpin. From our crystal structure data and interface analysis, we have demonstrated that an α-helix located in zinc-binding domain potentially interacts with residues of WH domain, which plays a significant role in strand annealing activity. We have shown that deletion of the α-helix or mutation of specific residues on it restores strand annealing activity of annealing deficient constructs of RECQ1. Our results also demonstrate that mutations on the α-helix induce conformational changes and affects DNA stimulated ATP hydrolysis and unwinding activity of RECQ1. Our study, for the first time, provides insight into the conformational requirements of the WH domain for efficient strand annealing by human RECQ1.

NAD+ bioavailability mediates PARG inhibition-induced replication arrest, intra S-phase checkpoint and apoptosis in glioma stem cells

Elevated expression of the DNA damage response proteins PARP1 and poly(ADP-ribose) glycohydrolase (PARG) in glioma stem cells (GSCs) suggests that glioma may be a unique target for PARG inhibitors (PARGi). While PARGi-induced cell death is achieved when combined with ionizing radiation, as a single agent PARG inhibitors appear to be mostly cytostatic. Supplementation with the NAD⁺ precursor dihydronicotinamide riboside (NRH) rapidly increased NAD⁺ levels in GSCs and glioma cells, inducing PARP1 activation and mild suppression of replication fork progression. Administration of NRH+PARGi triggers hyperaccumulation of poly(ADP-ribose) (PAR), intra S-phase arrest and apoptosis in GSCs but minimal PAR induction or cytotoxicity in normal astrocytes. PAR accumulation is regulated by select PARP1- and PAR-interacting proteins. The involvement of XRCC1 highlights the base excision repair pathway in responding to replication stress while enhanced interaction of PARP1 with PCNA, RPA and ORC2 upon PAR accumulation implicates replication associated PARP1 activation and assembly with pre-replication complex proteins upon initiation of replication arrest, the intra S-phase checkpoint and the onset of apoptosis.

WRN helicase safeguards deprotected replication forks in BRCA2-mutated cancer cells

The tumor suppressor BRCA2 protects stalled forks from degradation to maintain genome stability. However, the molecular mechanism(s) whereby unprotected forks are stabilized remains to be fully characterized. Here, we demonstrate that WRN helicase ensures efficient restart and limits excessive degradation of stalled forks in BRCA2-deficient cancer cells. In vitro, WRN ATPase/helicase catalyzes fork restoration and curtails MRE11 nuclease activity on regressed forks. We show that WRN helicase inhibitor traps WRN on chromatin leading to rapid fork stalling and nucleolytic degradation of unprotected forks by MRE11, resulting in MUS81-dependent double-strand breaks, elevated non-homologous end-joining and chromosomal instability. WRN helicase inhibition reduces viability of BRCA2-deficient cells and potentiates cytotoxicity of a poly (ADP)ribose polymerase (PARP) inhibitor. Furthermore, BRCA2-deficient xenograft tumors in mice exhibited increased DNA damage and growth inhibition when treated with WRN helicase inhibitor. This work provides mechanistic insight into stalled fork stabilization by WRN helicase when BRCA2 is deficient.

The role of patient-derived ovarian cancer organoids in the study of PARP inhibitors sensitivity and resistance: from genomic analysis to functional testing

Epithelial ovarian cancer (EOC) harbors distinct genetic features such as homologous recombination repair (HRR) deficiency, and therefore may respond to poly ADP-ribose polymerase inhibitors (PARPi). Over the past few years, PARPi have been added to the standard of care for EOC patients in both front-line and recurrent settings. Next-generation sequencing (NGS) genomic analysis provides key information, allowing for the prediction of PARPi response in patients who are PARPi naïve. However, there are indeed some limitations in NGS analyses. A subset of patients can benefit from PARPi, despite the failed detection of the predictive biomarkers such as BRCA1/2 mutations or HRR deficiency. Moreover, in the recurrent setting, the sequencing of initial tumor does not allow for the detection of reversions or secondary mutations restoring proficient HRR and thus leading to PARPi resistance. Therefore, it becomes crucial to better screen patients who will likely benefit from PARPi treatment, especially those with prior receipt of maintenance PARPi therapy. Recently, patient-derived organoids (PDOs) have been regarded as a reliable preclinical platform with clonal heterogeneity and genetic features of original tumors. PDOs are found feasible for functional testing and interrogation of biomarkers for predicting response to PARPi in EOC. Hence, we review the strengths and limitations of various predictive biomarkers and highlight the role of patient-derived ovarian cancer organoids as functional assays in the study of PARPi response. It was found that a combination of NGS and functional assays using PDOs could enhance the efficient screening of EOC patients suitable for PARPi, thus prolonging their survival time.

GAS6/AXL Inhibition Enhances Ovarian Cancer Sensitivity to Chemotherapy and PARP Inhibition through Increased DNA Damage and Enhanced Replication Stress

Over 80% of women with high-grade serous ovarian cancer (HGSOC) develop tumor resistance to chemotherapy and die of their disease. There are currently no FDA-approved agents to improve sensitivity to first-line platinum- and taxane-based chemotherapy or to PARP inhibitors. Here, we tested the hypothesis that expression of growth arrest-specific 6 (GAS6), the ligand of receptor tyrosine kinase AXL, is associated with chemotherapy response and that sequestration of GAS6 with AVB-S6-500 (AVB-500) could improve tumor response to chemotherapy and PARP inhibitors. We found that GAS6 levels in patient tumor and serum samples collected before chemotherapy correlated with ovarian cancer chemoresponse and patient survival. Compared with chemotherapy alone, AVB-500 plus carboplatin and/or paclitaxel led to decreased ovarian cancer-cell survival in vitro and tumor burden in vivo. Cells treated with AVB-500 plus carboplatin had more DNA damage, slower DNA replication fork progression, and fewer RAD51 foci than cells treated with carboplatin alone, indicating AVB-500 impaired homologous recombination (HR). Finally, treatment with the PARP inhibitor olaparib plus AVB-500 led to decreased ovarian cancer-cell survival in vitro and less tumor burden in vivo. Importantly, this effect was seen in HR-proficient and HR-deficient ovarian cancer cells. Collectively, our findings suggest that GAS6 levels could be used to predict response to carboplatin and AVB-500 could be used to treat platinum-resistant, HR-proficient HGSOC. IMPLICATIONS: GAS6/AXL is a novel target to sensitize ovarian cancers to carboplatin and olaparib. Additionally, GAS6 levels can be associated with response to carboplatin treatment.

Replication-dependent cytotoxicity and Spartan-mediated repair of trapped PARP1-DNA complexes

The antitumor activity of poly(ADP-ribose) polymerase inhibitors (PARPis) has been ascribed to PARP trapping, which consists in tight DNA-protein complexes. Here we demonstrate that the cytotoxicity of talazoparib and olaparib results from DNA replication. To elucidate the repair of PARP1-DNA complexes associated with replication in human TK6 and chicken DT40 lymphoblastoid cells, we explored the role of Spartan (SPRTN), a metalloprotease associated with DNA replication, which removes proteins forming DPCs. We find that SPRTN-deficient cells are hypersensitive to talazoparib and olaparib, but not to veliparib, a weak PARP trapper. SPRTN-deficient cells exhibit delayed clearance of trapped PARP1 and increased replication fork stalling upon talazoparib and olaparib treatment. We also show that SPRTN interacts with PARP1 and forms nuclear foci that colocalize with the replicative cell division cycle 45 protein (CDC45) in response to talazoparib. Additionally, SPRTN is deubiquitinated and epistatic with translesion synthesis (TLS) in response to talazoparib. Our results demonstrate that SPRTN is recruited to trapped PARP1 in S-phase to assist in the excision and replication bypass of PARP1-DNA complexes.

Lamin A/C recruits ssDNA protective proteins RPA and RAD51 to stalled replication forks to maintain fork stability

Lamin A/C provides a nuclear scaffold for compartmentalization of genome function that is important for genome integrity. Lamin A/C dysfunction is associated with cancer, aging, and degenerative diseases. The mechanisms whereby lamin A/C regulates genome stability remain poorly understood. We demonstrate a crucial role for lamin A/C in DNA replication. Lamin A/C binds to nascent DNA, especially during replication stress (RS), ensuring the recruitment of replication fork protective factors RPA and RAD51. These ssDNA-binding proteins, considered the first and second responders to RS respectively, function in the stabilization, remodeling, and repair of the stalled fork to ensure proper restart and genome stability. Reduced recruitment of RPA and RAD51 upon lamin A/C depletion elicits replication fork instability (RFI) characterized by MRE11 nuclease–mediated degradation of nascent DNA, RS-induced DNA damage, and sensitivity to replication inhibitors. Importantly, unlike homologous recombination–deficient cells, RFI in lamin A/C-depleted cells is not linked to replication fork reversal. Thus, the point of entry of nucleases is not the reversed fork but regions of ssDNA generated during RS that are not protected by RPA and RAD51. Consistently, RFI in lamin A/C-depleted cells is rescued by exogenous overexpression of RPA or RAD51. These data unveil involvement of structural nuclear proteins in the protection of ssDNA from nucleases during RS by promoting recruitment of RPA and RAD51 to stalled forks. Supporting this model, we show physical interaction between RPA and lamin A/C. We suggest that RS is a major source of genomic instability in laminopathies and lamin A/C-deficient tumors.

Temporally distinct post-replicative repair mechanisms fill PRIMPOL-dependent ssDNA gaps in human cells

PRIMPOL repriming allows DNA replication to skip DNA lesions, leading to ssDNA gaps. These gaps must be filled to preserve genome stability. Using a DNA fiber approach to directly monitor gap filling, we studied the post-replicative mechanisms that fill the ssDNA gaps generated in cisplatin-treated cells upon increased PRIMPOL expression or when replication fork reversal is defective because of SMARCAL1 inactivation or PARP inhibition. We found that a mechanism dependent on the E3 ubiquitin ligase RAD18, PCNA monoubiquitination, and the REV1 and POLζ translesion synthesis polymerases promotes gap filling in G2. The E2-conjugating enzyme UBC13, the RAD51 recombinase, and REV1-POLζ are instead responsible for gap filling in S, suggesting that temporally distinct pathways of gap filling operate throughout the cell cycle. Furthermore, we found that BRCA1 and BRCA2 promote gap filling by limiting MRE11 activity and that simultaneously targeting fork reversal and gap filling enhances chemosensitivity in BRCA-deficient cells.

The trans cell cycle effects of PARP inhibitors underlie their selectivity toward BRCA1/2-deficient cells

In this study, Simoneau et al. investigated why PARPi is more effective than other DNA-damaging drugs when used to treat BRCA1/2-deficient tumors. They show that PARPi induces DSBs progressively through trans-cell-cycle ssDNA gaps, and BRCA1/2-deficient cells fail to slow down and repair DSBs over multiple cell cycles, explaining the unique efficacy of PARPi in BRCA1/2-deficient cells.

PARP inhibitor (PARPi) is widely used to treat BRCA1/2-deficient tumors, but why PARPi is more effective than other DNA-damaging drugs is unclear. Here, we show that PARPi generates DNA double-strand breaks (DSBs) predominantly in a trans cell cycle manner. During the first S phase after PARPi exposure, PARPi induces single-stranded DNA (ssDNA) gaps behind DNA replication forks. By trapping PARP on DNA, PARPi prevents the completion of gap repair until the next S phase, leading to collisions of replication forks with ssDNA gaps and a surge of DSBs. In the second S phase, BRCA1/2-deficient cells are unable to suppress origin firing through ATR, resulting in continuous DNA synthesis and more DSBs. Furthermore, BRCA1/2-deficient cells cannot recruit RAD51 to repair collapsed forks. Thus, PARPi induces DSBs progressively through trans cell cycle ssDNA gaps, and BRCA1/2-deficient cells fail to slow down and repair DSBs over multiple cell cycles, explaining the unique efficacy of PARPi in BRCA1/2-deficient cells.

Replication gaps are a key determinant of PARP inhibitor synthetic lethality with BRCA deficiency

Mutations in BRCA1 or BRCA2 (BRCA) is synthetic lethal with poly(ADP-ribose) polymerase inhibitors (PARPi). Lethality is thought to derive from DNA double-stranded breaks (DSBs) necessitating BRCA function in homologous recombination (HR) and/or fork protection (FP). Here, we report instead that toxicity derives from replication gaps. BRCA1- or FANCJ-deficient cells, with common repair defects but distinct PARPi responses, reveal gaps as a distinguishing factor. We further uncouple HR, FP, and fork speed from PARPi response. Instead, gaps characterize BRCA-deficient cells, are diminished upon resistance, restored upon resensitization, and, when exposed, augment PARPi toxicity. Unchallenged BRCA1-deficient cells have elevated poly(ADP-ribose) and chromatin-associated PARP1, but aberrantly low XRCC1 consistent with defects in backup Okazaki fragment processing (OFP). 53BP1 loss resuscitates OFP by restoring XRCC1-LIG3 that suppresses the sensitivity of BRCA1-deficient cells to drugs targeting OFP or generating gaps. We highlight gaps as a determinant of PARPi toxicity changing the paradigm for synthetic lethal interactions.

Pharmacological Basis of Breast Cancer Resistance to Therapies - An Overview

Breast cancer (BC) is a molecular heterogeneous disease and often patients with similar clinico-pathological characteristics may display different response to treatment. Cellular processes, including uncontrolled cell-cycle, constitutive activation of signalling pathways parallel to or downstream of HER2 and alterations in DNA-repair mechanisms are the main features altered in the tumor. These cellular processes play significant roles in the emergence of therapy resistance. The introduction of target therapies as well as immunotherapies has improved the management of breast cancer. Furthermore, several therapeutic options are available to overcome resistance and physicians could overcome the challenge of resistant BC using combinatorial drug strategies and incorporating novel biomarkers. Molecular profiling promises to help in refine personalized treatment decisions and catalyse the development of further strategies when resistances inevitably occur. The search for biological explanations for treatment failure helps to clarify the phenomenon and allows to incorporate new biomarkers into clinical practice that can lead to adequate solutions to overcome it. This review provides a summary of genetic and molecular aspects of resistance mechanisms to available treatments for BC patients, and its clinical implications.

DNA replication stress and emerging prospects for PARG inhibitors in ovarian cancer therapy

Poly (ADP-ribosyl)ation has central functions in maintaining genome stability, including facilitating DNA replication and repair. In cancer cells these processes are frequently disrupted, and thus interfering with poly (ADP-ribosyl)ation can exacerbate inherent genome instability and induce selective cytotoxicity. Indeed, inhibitors of poly (ADP-ribose) polymerase (PARP) are having a major clinical impact in treating women with BRCA-mutant ovarian cancer, based on a defect in homologous recombination. However, only around half of ovarian cancers harbour defects in homologous recombination, and most sensitive tumours eventually acquire PARP inhibitor resistance with treatment. Thus, there is a pressing need to develop alternative treatment strategies to target tumours with both inherent and acquired resistance to PARP inhibition. Several novel inhibitors of poly (ADP-ribose)glycohydrolase (PARG) have been described, with promising anti-cancer activity in vitro that is distinct from PARP inhibitors. Here we discuss, the role of poly (ADP-ribosyl)ation in genome stability, and the potential for PARG inhibitors as a complementary strategy to PARP inhibitors in the treatment of ovarian cancer.

RECQ1 Promotes Stress Resistance and DNA Replication Progression Through PARP1 Signaling Pathway in Glioblastoma

Glioblastoma (GBM) is the most common aggressive primary malignant brain tumor, and patients with GBM have a median survival of 20 months. Clinical therapy resistance is a challenging barrier to overcome. Tumor genome stability maintenance during DNA replication, especially the ability to respond to replication stress, is highly correlated with drug resistance. Recently, we identified a protective role for RECQ1 under replication stress conditions. RECQ1 acts at replication forks, binds PCNA, inhibits single-strand DNA formation and nascent strand degradation in GBM cells. It is associated with the function of the PARP1 protein, promoting PARP1 recruitment to replication sites. RECQ1 is essential for DNA replication fork protection and tumor cell proliferation under replication stress conditions, and as a target of RECQ1, PARP1 effectively protects and restarts stalled replication forks, providing new insights into genomic stability maintenance and replication stress resistance. These findings indicate that tumor genome stability targeting RECQ1-PARP1 signaling may be a promising therapeutic intervention to overcome therapy resistance in GBM.

The emerging determinants of replication fork stability

A universal response to replication stress is replication fork reversal, where the nascent complementary DNA strands are annealed to form a protective four-way junction allowing forks to avert DNA damage while replication stress is resolved. However, reversed forks are in turn susceptible to nucleolytic digestion of the regressed nascent DNA arms and rely on dedicated mechanisms to protect their integrity. The most well studied fork protection mechanism involves the BRCA pathway and its ability to catalyze RAD51 nucleofilament formation on the reversed arms of stalled replication forks. Importantly, the inability to prevent the degradation of reversed forks has emerged as a hallmark of BRCA deficiency and underlies genome instability and chemosensitivity in BRCA-deficient cells. In the past decade, multiple factors underlying fork stability have been discovered. These factors either cooperate with the BRCA pathway, operate independently from it to augment fork stability in its absence, or act as enablers of fork degradation. In this review, we examine these novel determinants of fork stability, explore the emergent conceptual underpinnings underlying fork protection, as well as the impact of fork protection on cellular viability and cancer therapy.

Mechanisms for Maintaining Eukaryotic Replisome Progression in the Presence of DNA Damage

The eukaryotic replisome coordinates template unwinding and nascent-strand synthesis to drive DNA replication fork progression and complete efficient genome duplication. During its advancement along the parental template, each replisome may encounter an array of obstacles including damaged and structured DNA that impede its progression and threaten genome stability. A number of mechanisms exist to permit replisomes to overcome such obstacles, maintain their progression, and prevent fork collapse. A combination of recent advances in structural, biochemical, and single-molecule approaches have illuminated the architecture of the replisome during unperturbed replication, rationalised the impact of impediments to fork progression, and enhanced our understanding of DNA damage tolerance mechanisms and their regulation. This review focusses on these studies to provide an updated overview of the mechanisms that support replisomes to maintain their progression on an imperfect template.

Dual PARP and RAD51 Inhibitory Drug Conjugates Show Synergistic and Selective Effects on Breast Cancer Cells

The genetic principle of synthetic lethality has most successfully been exploited in therapies engaging Poly-ADP-ribose-polymerase (PARP) inhibitors to treat patients with homologous recombination (HR)-defective tumors. In this work, we went a step further following the idea of a local molecular cooperation and designed hybrid compounds M1–M3. The drug conjugates M1–M3 combine Olaparib, the first PARP inhibitor approved for clinical use, with Cpd 1, an inhibitor of RAD51 that blocks its HR functions and yet permits RAD51 nucleoprotein filament formation on single-stranded DNA. While in M2 and M3, the parental drugs are linked by -CO-(CH₂)_n-CO-spacers (n = 2 and 4, respectively), they are directly merged omitting the piperazine ring of Olaparib in M1. Monitoring anti-survival effects of M1–M3 in six breast cancer cell lines of different molecular subtypes showed that in each cell line, at least one of the drug conjugates decreased viability by one to two orders of magnitude compared with parental drugs. While triple-negative breast cancer (TNBC) cells with frequent BRCA1 pathway dysfunction were sensitive to spacer-linked hybrid compounds M1 and M2 regardless of their HR capacities, non-TNBC cells were responsive to the merged drug conjugate M1 only, suggesting different spatial requirements for dual inhibition in these two groups of cell lines. These results demonstrate that, depending on chemical linkage, dual PARP1-RAD51 inhibitory drugs can either sensitize non-TNBC and re-sensitize TNBC cells, or discriminate between these groups of cells.

The Replication Stress Response on a Narrow Path Between Genomic Instability and Inflammation

The genome of eukaryotic cells is particularly at risk during the S phase of the cell cycle, when megabases of chromosomal DNA are unwound to generate two identical copies of the genome. This daunting task is executed by thousands of micro-machines called replisomes, acting at fragile structures called replication forks. The correct execution of this replication program depends on the coordinated action of hundreds of different enzymes, from the licensing of replication origins to the termination of DNA replication. This review focuses on the mechanisms that ensure the completion of DNA replication under challenging conditions of endogenous or exogenous origin. It also covers new findings connecting the processing of stalled forks to the release of small DNA fragments into the cytoplasm, activating the cGAS-STING pathway. DNA damage and fork repair comes therefore at a price, which is the activation of an inflammatory response that has both positive and negative impacts on the fate of stressed cells. These new findings have broad implications for the etiology of interferonopathies and for cancer treatment.

RADX prevents genome instability by confining replication fork reversal to stalled forks

RAD51 facilitates replication fork reversal and protects reversed forks from nuclease degradation. Although potentially a useful replication stress response mechanism, unregulated fork reversal can cause genome instability. Here we show that RADX, a single-strand DNA binding protein that binds to and destabilizes RAD51 nucleofilaments, can either inhibit or promote fork reversal depending on replication stress levels. RADX inhibits fork reversal at elongating forks, thereby preventing fork slowing and collapse. Paradoxically, in the presence of persistent replication stress, RADX localizes to stalled forks to generate reversed fork structures. Consequently, inactivating RADX prevents fork-reversal-dependent telomere dysfunction in the absence of RTEL1 and blocks nascent strand degradation when fork protection factors are inactivated. Addition of RADX increases SMARCAL1-dependent fork reversal in conditions in which pre-binding RAD51 to a model fork substrate is inhibitory. Thus, RADX directly interacts with RAD51 and single-strand DNA to confine fork reversal to persistently stalled forks.

RecQ helicases in DNA repair and cancer targets

Helicases are enzymes that use the energy derived from ATP hydrolysis to catalyze the unwinding of DNA or RNA. The RecQ family of helicases is conserved through evolution from prokaryotes to higher eukaryotes and plays important roles in various DNA repair pathways, contributing to the maintenance of genome integrity. Despite their roles as general tumor suppressors, there is now considerable interest in exploiting RecQ helicases as synthetic lethal targets for the development of new cancer therapeutics. In this review, we summarize the latest developments in the structural and mechanistic study of RecQ helicases and discuss their roles in various DNA repair pathways. Finally, we consider the potential to exploit RecQ helicases as therapeutic targets and review the recent progress towards the development of small molecules targeting RecQ helicases as cancer therapeutics.

PARP Activity Fine-tunes the DNA Replication Choreography of Chk1-depleted Cells

Checkpoint Kinase 1 (Chk1) prevents DNA damage by adjusting the replication choreography in the face of replication stress. Chk1 depletion provokes slow and asymmetrical fork movement, yet the signals governing such changes remain unclear. We sought to investigate whether poly(ADP-ribose) polymerases (PARPs), key players of the DNA damage response, intervene in the DNA replication of Chk1-depleted cells. We demonstrate that PARP inhibition selectively alleviates the reduced fork elongation rates, without relieving fork asymmetry in Chk1-depleted cells. While the contribution of PARPs to fork elongation is not unprecedented, we found that their role in Chk1-depleted cells extends beyond fork movement. PARP-dependent fork deceleration induced mild dormant origin firing upon Chk1 depletion, augmenting the global rates of DNA synthesis. Thus, we have identified PARPs as novel regulators of replication fork dynamics in Chk1-depleted cells.

Mechanisms of damage tolerance and repair during DNA replication

Accurate duplication of chromosomal DNA is essential for the transmission of genetic information. The DNA replication fork encounters template lesions, physical barriers, transcriptional machinery, and topological barriers that challenge the faithful completion of the replication process. The flexibility of replisomes coupled with tolerance and repair mechanisms counteract these replication fork obstacles. The cell possesses several universal mechanisms that may be activated in response to various replication fork impediments, but it has also evolved ways to counter specific obstacles. In this review, we will discuss these general and specific strategies to counteract different forms of replication associated damage to maintain genomic stability.

A fork in the road: Where homologous recombination and stalled replication fork protection part ways

In response to replication hindrances, DNA replication forks frequently stall and are remodelled into a four-way junction. In such a structure the annealed nascent strand is thought to resemble a DNA double-strand break and remodelled forks are vulnerable to nuclease attack by MRE11 and DNA2. Proteins that promote the recruitment, loading and stabilisation of RAD51 onto single-stranded DNA for homology search and strand exchange in homologous recombination (HR) repair and inter-strand cross-link repair also act to set up RAD51-mediated protection of nascent DNA at stalled replication forks. However, despite the similarities of these pathways, several lines of evidence indicate that fork protection is not simply analogous to the RAD51 loading step of HR. Protection of stalled forks not only requires separate functions of a number of recombination proteins, but also utilises nucleases important for the resection steps of HR in alternative ways. Here we discuss how fork protection arises and how its differences with HR give insights into the differing contexts of these two pathways.

The PARP Enzyme Family and the Hallmarks of Cancer Part 1. Cell Intrinsic Hallmarks

The 17-member poly (ADP-ribose) polymerase enzyme family, also known as the ADP-ribosyl transferase diphtheria toxin-like (ARTD) enzyme family, contains DNA damage-responsive and nonresponsive members. Only PARP1, 2, 5a, and 5b are capable of modifying their targets with poly ADP-ribose (PAR) polymers; the other PARP family members function as mono-ADP-ribosyl transferases. In the last decade, PARP1 has taken center stage in oncology treatments. New PARP inhibitors (PARPi) have been introduced for the targeted treatment of breast cancer 1 or 2 (BRCA1/2)-deficient ovarian and breast cancers, and this novel therapy represents the prototype of the synthetic lethality paradigm. Much less attention has been paid to other PARPs and their potential roles in cancer biology. In this review, we summarize the roles played by all PARP enzyme family members in six intrinsic hallmarks of cancer: uncontrolled proliferation, evasion of growth suppressors, cell death resistance, genome instability, reprogrammed energy metabolism, and escape from replicative senescence. In a companion paper, we will discuss the roles of PARP enzymes in cancer hallmarks related to cancer-host interactions, including angiogenesis, invasion and metastasis, evasion of the anticancer immune response, and tumor-promoting inflammation. While PARP1 is clearly involved in all ten cancer hallmarks, an increasing body of evidence supports the role of other PARPs in modifying these cancer hallmarks (e.g., PARP5a and 5b in replicative immortality and PARP2 in cancer metabolism). We also highlight controversies, open questions, and discuss prospects of recent developments related to the wide range of roles played by PARPs in cancer biology. Some of the summarized findings may explain resistance to PARPi therapy or highlight novel biological roles of PARPs that can be therapeutically exploited in novel anticancer treatment paradigms.

Mechanisms of Vertebrate DNA Interstrand Cross-Link Repair

DNA interstrand cross-links (ICLs) covalently connect the two strands of the double helix and are extremely cytotoxic. Defective ICL repair causes the bone marrow failure and cancer predisposition syndrome, Fanconi anemia, and upregulation of repair causes chemotherapy resistance in cancer. The central event in ICL repair involves resolving the cross-link (unhooking). In this review, we discuss the chemical diversity of ICLs generated by exogenous and endogenous agents. We then describe how proliferating and nonproliferating vertebrate cells unhook ICLs. We emphasize fundamentally new unhooking strategies, dramatic progress in the structural analysis of the Fanconi anemia pathway, and insights into how cells govern the choice between different ICL repair pathways. Throughout, we highlight the many gaps that remain in our knowledge of these fascinating DNA repair pathways.

Entinostat, a selective HDAC1/2 inhibitor, potentiates the effects of olaparib in homologous recombination proficient ovarian cancer

OBJECTIVE

Poly ADP ribose polymerase inhibitors (PARPi) are most effective in BRCA1/2 mutated ovarian tumors. Better treatments are needed for homologous recombination HR-proficient cancer, including CCNE1 amplified subtypes. We have shown that histone deacetylase inhibitors (HDACi) sensitize HR-proficient ovarian cancer to PARPi. In this study, we provide complementary preclinical data for an investigator-initiated phase 1/2 clinical trial of the combination of olaparib and entinostat in recurrent, HR-proficient ovarian cancer.

METHODS

We assessed the in vitro effects of the combination of olaparib and entinostat in SKOV-3, OVCAR-3 and primary cells derived from CCNE1 amplified high grade serous ovarian cancer (HGSOC) patients. We then tested the combination in a SKOV-3 xenograft model and in a patient-derived xenograft (PDX) model.

RESULTS

Entinostat potentiates the effect of olaparib in reducing cell viability and clonogenicity of HR-proficient ovarian cancer cells. The combination reduces peritoneal metastases in a SKOV-3 xenograft model and prolongs survival in a CCNE1 amplified HR-proficient PDX model. Entinostat also enhances olaparib-induced DNA damage. Further, entinostat decreases BRCA1, a key HR repair protein, associated with decreased Ki-67, a proliferation marker, and increased cleaved PARP, a marker of apoptosis. Finally, entinostat perturbs replication fork progression, which increases genome instability.

CONCLUSION

Entinostat inhibits HR repair by reducing BRCA1 expression and stalling replication fork progression, leading to irreparable DNA damage and ultimate cell death. This work provides preclinical support for the clinical trial of the combination of olaparib and entinostat in HR-proficient ovarian cancer and suggests potential benefit even for CCNE1 amplified subtypes.

DNA2 in Chromosome Stability and Cell Survival-Is It All about Replication Forks?

The conserved nuclease-helicase DNA2 has been linked to mitochondrial myopathy, Seckel syndrome, and cancer. Across species, the protein is indispensable for cell proliferation. On the molecular level, DNA2 has been implicated in DNA double-strand break (DSB) repair, checkpoint activation, Okazaki fragment processing (OFP), and telomere homeostasis. More recently, a critical contribution of DNA2 to the replication stress response and recovery of stalled DNA replication forks (RFs) has emerged. Here, we review the available functional and phenotypic data and propose that the major cellular defects associated with DNA2 dysfunction, and the links that exist with human disease, can be rationalized through the fundamental importance of DNA2-dependent RF recovery to genome duplication. Being a crucial player at stalled RFs, DNA2 is a promising target for anti-cancer therapy aimed at eliminating cancer cells by replication-stress overload.

Targeting Replicative Stress and DNA Repair by Combining PARP and Wee1 Kinase Inhibitors Is Synergistic in Triple Negative Breast Cancers with Cyclin E or BRCA1 Alteration

Simple Summary

Triple-negative breast cancer (TNBC) is a subtype of invasive breast cancer with an aggressive phenotype that has decreased survival compared with other types of breast cancers, due in part to the lack of biomarker driven targeted therapies. Here, we show that breast cancer patients whose tumors show high levels of cyclin E expression have a higher prevalence of BRCA1/2 alterations and have the worst clinical outcomes. In vitro and in vivo studies revealed that combination therapies with poly (ADP-ribose) polymerase (PARP) and Wee1 kinase inhibitors in TNBC cells with either BRCA1 mutations or high levels of cyclin E results in synergistic cell death due to induction of replicative stress and downregulation of DNA repair. These studies suggest that by preselecting patients whose tumors have high cyclin E levels or harbor mutations in BRCA1, only those cases with the highest replicative stress properties will be subjected to combination treatment and likely result in synergistic activity of the two agents.

Abstract

The identification of biomarker-driven targeted therapies for patients with triple negative breast cancer (TNBC) remains a major clinical challenge, due to a lack of specific targets. Here, we show that cyclin E, a major regulator of G1 to S transition, is deregulated in TNBC and is associated with mutations in DNA repair genes (e.g., BRCA1/2). Breast cancers with high levels of cyclin E not only have a higher prevalence of BRCA1/2 mutations, but also are associated with the worst outcomes. Using several in vitro and in vivo model systems, we show that TNBCs that harbor either mutations in BRCA1/2 or overexpression of cyclin E are very sensitive to the growth inhibitory effects of AZD-1775 (Wee 1 kinase inhibitor) when used in combination with MK-4837 (PARP inhibitor). Combination treatment of TNBC cell lines with these two agents results in synergistic cell killing due to induction of replicative stress, downregulation of DNA repair and cytokinesis failure that results in increased apoptosis. These findings highlight the potential clinical application of using cyclin E and BRCA mutations as biomarkers to select only those patients with the highest replicative stress properties that may benefit from combination treatment with Wee 1 kinase and PARP inhibitors.

Genome-Wide Analysis Unveils DNA Helicase RECQ1 as a Regulator of Estrogen Response Pathway in Breast Cancer Cells

Susceptibility to breast cancer is significantly increased in individuals with germ line mutations in RECQ1 (also known as RECQL or RECQL1), a gene encoding a DNA helicase essential for genome maintenance. We previously reported that RECQ1 expression predicts clinical outcomes for sporadic breast cancer patients stratified by estrogen receptor (ER) status. Here, we utilized an unbiased integrative genomics approach to delineate a cross talk between RECQ1 and ERα, a known master regulatory transcription factor in breast cancer. We found that expression of ESR1, the gene encoding ERα, is directly activated by RECQ1. More than 35% of RECQ1 binding sites were cobound by ERα genome-wide. Mechanistically, RECQ1 cooperates with FOXA1, the pioneer transcription factor for ERα, to enhance chromatin accessibility at the ESR1 regulatory regions in a helicase activity-dependent manner. In clinical ERα-positive breast cancers treated with endocrine therapy, high RECQ1 and high FOXA1 coexpressing tumors were associated with better survival. Collectively, these results identify RECQ1 as a novel cofactor for ERα and uncover a previously unknown mechanism by which RECQ1 regulates disease-driving gene expression in ER-positive breast cancer cells.

Interferon-stimulated gene 15 accelerates replication fork progression inducing chromosomal breakage

DNA replication is highly regulated by the ubiquitin system, which plays key roles upon stress. The ubiquitin-like modifier ISG15 (interferon-stimulated gene 15) is induced by interferons, bacterial and viral infection, and DNA damage, but it is also constitutively expressed in many types of cancer, although its role in tumorigenesis is still largely elusive. Here, we show that ISG15 localizes at the replication forks, in complex with PCNA and the nascent DNA, where it regulates DNA synthesis. Indeed, high levels of ISG15, intrinsic or induced by interferon-β, accelerate DNA replication fork progression, resulting in extensive DNA damage and chromosomal aberrations. This effect is largely independent of ISG15 conjugation and relies on ISG15 functional interaction with the DNA helicase RECQ1, which promotes restart of stalled replication forks. Additionally, elevated ISG15 levels sensitize cells to cancer chemotherapeutic treatments. We propose that ISG15 up-regulation exposes cells to replication stress, impacting genome stability and response to genotoxic drugs.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Trex2 responds to damaged replication forks in diverse ways

Three prime Repair Exonuclease 2 (Trex2) alters replication fork (RF) stability and mutation levels in cells defective for homologous recombination (HR). Trex2 has multiple functions that can either cause or supress RF instability in cells with different HR-defects. Why does Trex2 have such diverse effects on RF maintenance?

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.