ATR activation is regulated by dimerization of ATR activating proteins

Targeting ATR Pathway in Solid Tumors: Evidence of Improving Therapeutic Outcomes

The DNA damage response (DDR) system is a complicated network of signaling pathways that detects and repairs DNA damage or induces apoptosis. Critical regulators of the DDR network include the DNA damage kinases ataxia telangiectasia mutated Rad3-related kinase (ATR) and ataxia-telangiectasia mutated (ATM). The ATR pathway coordinates processes such as replication stress response, stabilization of replication forks, cell cycle arrest, and DNA repair. ATR inhibition disrupts these functions, causing a reduction of DNA repair, accumulation of DNA damage, replication fork collapse, inappropriate mitotic entry, and mitotic catastrophe. Recent data have shown that the inhibition of ATR can lead to synthetic lethality in ATM-deficient malignancies. In addition, ATR inhibition plays a significant role in the activation of the immune system by increasing the tumor mutational burden and neoantigen load as well as by triggering the accumulation of cytosolic DNA and subsequently inducing the cGAS-STING pathway and the type I IFN response. Taken together, we review stimulating data showing that ATR kinase inhibition can alter the DDR network, the immune system, and their interplay and, therefore, potentially provide a novel strategy to improve the efficacy of antitumor therapy, using ATR inhibitors as monotherapy or in combination with genotoxic drugs and/or immunomodulators.

NRF2 promotes radiation resistance by cooperating with TOPBP1 to activate the ATR-CHK1 signaling pathway

Background: Radiation resistance is the main limitation of the application of radiotherapy. Ionizing radiation (IR) kills cancer cells mainly by causing DNA damage, particularly double-strand breaks (DSBs). Radioresistant cancer cells have developed the robust capability of DNA damage repair to survive IR. Nuclear factor erythroid 2-related factor 2 (NRF2) has been correlated with radiation resistance. We previously reported a novel function of NRF2 as an ATR activator in response to DSBs. However, little is known about the mechanism that how NRF2 regulates DNA damage repair and radiation resistance. Methods: The TCGA database and tissue microarray were used to analyze the correlation between NRF2 and the prognosis of lung cancer patients. The radioresistant lung cancer cells were constructed, and the role of NRF2 in radiation resistance was explored by in vivo and in vitro experiments. Immunoprecipitation, immunofluorescence and extraction of chromatin fractions were used to explore the underlying mechanisms. Results: In this study, the TCGA database and clinical lung cancer samples showed that high expression of NRF2 was associated with poor prognosis in lung cancer patients. We established radioresistant lung cancer cells expressing NRF2 at high levels, which showed increased antioxidant and DNA repair abilities. In addition, we found that NRF2 can be involved in the DNA damage response independently of its antioxidant function. Mechanistically, we demonstrated that NRF2 promoted the phosphorylation of replication protein A 32 (RPA32), and DNA topoisomerase 2-binding protein 1 (TOPBP1) was recruited to DSB sites in an NRF2-dependent manner. Conclusion: This study explored the novel role of NRF2 in promoting radiation resistance by cooperating with RPA32 and TOPBP1 to activate the ATR-CHK1 signaling pathway. In addition, the findings of this study not only provide novel insights into the molecular mechanisms underlying the radiation resistance of lung cancer cells but also validate NRF2 as a potential target for radiotherapy.

Structural characterization of human RPA70N association with DNA damage response proteins

The heterotrimeric Replication protein A (RPA) is the ubiquitous eukaryotic single-stranded DNA (ssDNA) binding protein and participates in nearly all aspects of DNA metabolism, especially DNA damage response. The N-terminal OB domain of the RPA70 subunit (RPA70N) is a major protein-protein interaction element for RPA and binds to more than 20 partner proteins. Previous crystallography studies of RPA70N with p53, DNA2 and PrimPol fragments revealed that RPA70N binds to amphipathic peptides that mimic ssDNA. NMR chemical-shift studies also provided valuable information on the interaction of RPA70N residues with target sequences. However, it is still unclear how RPA70N recognizes and distinguishes such a diverse group of target proteins. Here, we present high-resolution crystal structures of RPA70N in complex with peptides from eight DNA damage response proteins. The structures show that, in addition to the ssDNA mimicry mode of interaction, RPA70N employs multiple ways to bind its partners. Our results advance the mechanistic understanding of RPA70N-mediated recruitment of DNA damage response proteins.

Phosphoregulation of the checkpoint kinase Mec1ATR

Yeast Mec1, and its mammalian ortholog, Ataxia-Telangiectasia and Rad3-related, are giant protein kinases central to replication stress and double strand DNA break repair. Mec1ATR, in complex with Ddc2ATRIP, is a 'sensor' of single stranded DNA, and phosphorylates numerous cell cycle and DNA repair factors to enforce cell cycle arrest and facilitate repair. Over the last several years, new techniques - particularly in structural biology - have provided molecular mechanisms for Mec1ATR function. It is becoming increasingly clear how post-translational modification of Mec1ATR and its interaction partners modulates the DNA damage checkpoint. In this review, we summarise the most recent work unravelling Mec1ATR function in the DNA damage checkpoint and provide a molecular context for its regulation by phosphorylation.

DNA replication: Mechanisms and therapeutic interventions for diseases

Accurate and integral cellular DNA replication is modulated by multiple replication-associated proteins, which is fundamental to preserve genome stability. Furthermore, replication proteins cooperate with multiple DNA damage factors to deal with replication stress through mechanisms beyond their role in replication. Cancer cells with chronic replication stress exhibit aberrant DNA replication and DNA damage response, providing an exploitable therapeutic target in tumors. Numerous evidence has indicated that posttranslational modifications (PTMs) of replication proteins present distinct functions in DNA replication and respond to replication stress. In addition, abundant replication proteins are involved in tumorigenesis and development, which act as diagnostic and prognostic biomarkers in some tumors, implying these proteins act as therapeutic targets in clinical. Replication-target cancer therapy emerges as the times require. In this context, we outline the current investigation of the DNA replication mechanism, and simultaneously enumerate the aberrant expression of replication proteins as hallmark for various diseases, revealing their therapeutic potential for target therapy. Meanwhile, we also discuss current observations that the novel PTM of replication proteins in response to replication stress, which seems to be a promising strategy to eliminate diseases.

MRN-dependent and independent pathways for recruitment of TOPBP1 to DNA double-strand breaks

Ataxia Telangiectasia mutated and RAD3-related (ATR) kinase is activated by DNA replication stress and also by various forms of DNA damage, including DNA double-strand breaks (DSBs). Recruitment to sites of damage is insufficient for ATR activation as one of two known ATR activators, either topoisomerase II-binding protein (TOPBP1) or Ewing’s tumor-associated antigen 1, must also be present for signaling to initiate. Here, we employ our recently established DSB-mediated ATR activation in Xenopus egg extract (DMAX) system to examine how TOPBP1 is recruited to DSBs, so that it may activate ATR. We report that TOPBP1 is only transiently present at DSBs, with a half-life of less than 10 minutes. We also examined the relationship between TOPBP1 and the MRE11-RAD50-NBS1 (MRN), CtBP interacting protein (CtIP), and Ataxia Telangiectasia mutated (ATM) network of proteins. Loss of MRN prevents CtIP recruitment to DSBs, and partially inhibits TOPBP1 recruitment. Loss of CtIP has no impact on either MRN or TOPBP1 recruitment. Loss of ATM kinase activity prevents CtIP recruitment and enhances MRN and TOPBP1 recruitment. These findings demonstrate that there are MRN-dependent and independent pathways that recruit TOPBP1 to DSBs for ATR activation. Lastly, we find that both the 9-1-1 complex and MDC1 are dispensable for TOPBP1 recruitment to DSBs.

Targeting Replication Stress Response Pathways to Enhance Genotoxic Chemo- and Radiotherapy

Proliferating cells regularly experience replication stress caused by spontaneous DNA damage that results from endogenous reactive oxygen species (ROS), DNA sequences that can assume secondary and tertiary structures, and collisions between opposing transcription and replication machineries. Cancer cells face additional replication stress, including oncogenic stress that results from the dysregulation of fork progression and origin firing, and from DNA damage induced by radiotherapy and most cancer chemotherapeutic agents. Cells respond to such stress by activating a complex network of sensor, signaling and effector pathways that protect genome integrity. These responses include slowing or stopping active replication forks, protecting stalled replication forks from collapse, preventing late origin replication firing, stimulating DNA repair pathways that promote the repair and restart of stalled or collapsed replication forks, and activating dormant origins to rescue adjacent stressed forks. Currently, most cancer patients are treated with genotoxic chemotherapeutics and/or ionizing radiation, and cancer cells can gain resistance to the resulting replication stress by activating pro-survival replication stress pathways. Thus, there has been substantial effort to develop small molecule inhibitors of key replication stress proteins to enhance tumor cell killing by these agents. Replication stress targets include ATR, the master kinase that regulates both normal replication and replication stress responses; the downstream signaling kinase Chk1; nucleases that process stressed replication forks (MUS81, EEPD1, Metnase); the homologous recombination catalyst RAD51; and other factors including ATM, DNA-PKcs, and PARP1. This review provides an overview of replication stress response pathways and discusses recent pre-clinical studies and clinical trials aimed at improving cancer therapy by targeting replication stress response factors.

Genetic Polymorphisms Associated with Vincristine Pharmacokinetics and Vincristine-Induced Peripheral Neuropathy in Pediatric Oncology Patients

Simple Summary

Vincristine is a type of chemotherapy that is often used in the treatment of children with cancer. The main side effect of vincristine is nerve damage. Patients experience symptoms such as tingling, pain or muscle weakness. Some children are more sensitive to vincristine than others, which may depend on variations in genes and in the breakdown of vincristine by the body. In this study, we investigated the effect of variations in genes on nerve damage due to vincristine and breakdown of vincristine by the body. We found that nine variations in seven genes were associated with nerve damage due to vincristine, whereas three variations in three genes were associated with the breakdown of vincristine by the body. It is important that future studies try to replicate these findings. Our findings help us towards the goal of tailoring vincristine treatment to each child, with optimal therapeutic effect while limiting nerve damage.

Abstract

Vincristine (VCR) is an important component of curative chemotherapy for many childhood cancers. Its main side effect is VCR-induced peripheral neuropathy (VIPN), a dose limiting toxicity. Some children are more susceptible to VIPN, which is at least partially dependent on genetic factors and pharmacokinetics (PK). In this study, we identify and replicate genetic variants associated with VCR PK and VIPN. Patient samples from a randomized clinical trial studying the effect of administration duration of VCR on VIPN in 90 patients were used. PK sampling was conducted on between one and five occasions at multiple time points. A linear two-compartment model with first-order elimination was used, and targeted next-generation DNA sequencing was performed. Genotype–trait associations were analyzed using mixed-effect models or logistic regression analysis for repeated measures, or Poisson regression analysis in which the highest VIPN score per patient was included. Nine single-nucleotide polymorphisms (SNPs) in seven genes (NDRG1, GARS, FIG4, FGD4, SEPTIN9, CEP72, and ETAA1) were associated with VIPN. Furthermore, three SNPs in three genes (MTNR1B, RAB7A and SNU13) were associated with PK of VCR. In conclusion, PK of VCR and VIPN are influenced by SNPs; upfront identification of those that lead to an altered susceptibility to VIPN or VCR exposure could help individualize VCR treatment.

Delineation of a minimal topoisomerase II binding protein 1 (TOPBP1) for regulated activation of ATR at DNA double-strand breaks

Topoisomerase II Binding Protein 1 (TOPBP1) is an important activator of the DNA damage response kinase Ataxia Telangiectasia and Rad3-related (ATR), although the mechanism by which this activation occurs is not yet known. TOPBP1 contains nine copies of the BRCA1 C-terminal repeat (BRCT) motif, which allows protein–protein and protein–DNA interactions. TOPBP1 also contains an ATR activation domain (AAD), which physically interacts with ATR and its partner ATR-interacting protein (ATRIP) in a manner that stimulates ATR kinase activity. It is unclear which of TOPBP1’s nine BRCT domains participate in the reaction, as well as the individual roles played by these relevant BRCT domains. To address this knowledge gap, here, we delineated a minimal TOPBP1 that can activate ATR at DNA double-strand breaks in a regulated manner. We named this minimal TOPBP1 “Junior” and we show that Junior is composed of just three regions: BRCT0-2, the AAD, and BRCT7&8. We further defined the individual functions of these three regions by showing that BRCT0-2 is required for recruitment to DNA double-strand breaks and is dispensable thereafter, and that BRCT7&8 is dispensable for recruitment but essential to allow the AAD to multimerize and activate ATR. The delineation of TOPBP1 Junior creates a leaner, simplified, and better understood TOPBP1 and provides insight into the mechanism of ATR activation.

The Potential Important Role of Mitochondrial Rieske Iron–Sulfur Protein as a Novel Therapeutic Target for Pulmonary Hypertension in Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is the third leading cause of death worldwide, which is often due to pulmonary hypertension (PH). The underlying molecular mechanisms are poorly understood, and current medications are neither specific nor always effective. In this review, we highlight the recent findings on the roles of altered mitochondrial bioenergetics in PH in COPD. We also discuss the central role of mitochondrial reactive oxygen species (ROS) generation mediated by Rieske iron–sulfur protein (RISP) and review the contributions of RISP-dependent DNA damage and NF-κB-associated inflammatory signaling. Finally, the potential importance of mitochondrial RISP and its associated molecules as novel therapeutic targets for PH in COPD are meticulously discussed.

DNA damage responses that enhance resilience to replication stress

During duplication of the genome, eukaryotic cells may experience various exogenous and endogenous replication stresses that impede progression of DNA replication along chromosomes. Chemical alterations in template DNA, imbalances of deoxynucleotide pools, repetitive sequences, tight DNA-protein complexes, and conflict with transcription can negatively affect the replication machineries. If not properly resolved, stalled replication forks can cause chromosome breaks leading to genomic instability and tumor development. Replication stress is enhanced in cancer cells due, for example, to the loss of DNA repair genes or replication-transcription conflict caused by activation of oncogenic pathways. To prevent these serious consequences, cells are equipped with diverse mechanisms that enhance the resilience of replication machineries to replication stresses. This review describes DNA damage responses activated at stressed replication forks and summarizes current knowledge on the pathways that promote faithful chromosome replication and protect chromosome integrity, including ATR-dependent replication checkpoint signaling, DNA cross-link repair, and SLX4-mediated responses to tight DNA-protein complexes that act as barriers. This review also focuses on the relevance of replication stress responses to selective cancer chemotherapies.

An extending ATR-CHK1 circuitry: the replication stress response and beyond

The maintenance of genomic integrity relies on the coordination of a wide range of cellular processes and efficient repair of DNA damage. Since its discovery over two decades ago, the ATR kinase has been recognized as the master regulator of the circuitry orchestrating the cellular responses to DNA damage and replication stress. Recent studies reveal that ATR additionally functions in the unperturbed cell cycle through its control of replication fork speed and stability, replication origin firing, completion of genome duplication, and chromosome segregation. Here, we discuss several recently discovered mechanisms through which ATR safeguards genomic integrity during the cell cycle, from S phase to mitosis.