A novel mouse model for ataxia-telangiectasia with a N-terminal mutation displays a behavioral defect and a low incidence of lymphoma but no increased oxidative burden

A-T neurodegeneration and DNA damage-induced transcriptional stress

Loss of the ATM protein kinase in humans results in Ataxia-telangiectasia, a disorder characterized by childhood-onset neurodegeneration of the cerebellum as well as cancer predisposition and immunodeficiency. Although many aspects of ATM function are well-understood, the mechanistic basis of the progressive cerebellar ataxia that occurs in patients is not. Here we review recent progress related to the role of ATM in neurons and the cerebellum that comes from many sources: animal models, post-mortem brain tissue samples, and human neurons in culture. These observations have revealed new insights into the consequences of ATM loss on DNA damage, gene expression, and immune signaling in the brain. Many results point to the importance of reactive oxygen species as well as single-strand DNA breaks in the progression of molecular events leading to neuronal dysfunction. In addition, innate immunity signaling pathways appear to play a critical role in ATM functions in microglia, responding to various forms of nucleic acid sensors and regulating survival of neurons and other cell types. Overall, the results lead to an updated view of transcriptional stress and DNA damage resulting from ATM loss that results in changes in gene expression as well as neuroinflammation that contribute to the cerebellar neurodegeneration observed in patients.

Malignancy-associated immune responses: Lessons from human inborn errors of immunity

It is widely understood that cancer is a significant cause of morbidity and mortality worldwide. Despite numerous available treatments, prognosis for many remains poor, thus, the development of novel therapies remains essential. Given the incredible success of many immunotherapies in this field, the important contribution of the immune system to the control, and elimination, of malignancy is clear. While many immunotherapies target higher-order pathways, for example, through promoting T-cell activation via immune checkpoint blockade, the potential to target specific immunological pathways is largely not well researched. Precisely understanding how immunity can be tailored to respond to specific challenges is an exciting idea with great potential, and may trigger the development of new therapies for cancer. Inborn Errors of Immunity (IEI) are a group of rare congenital disorders caused by gene mutations that result in immune dysregulation. This heterogeneous group, spanning widespread, multisystem immunopathology to specific immune cell defects, primarily manifest in immunodeficiency symptoms. Thus, these patients are particularly susceptible to life-threatening infection, autoimmunity and malignancy, making IEI an especially complex group of diseases. While precise mechanisms of IEI-induced malignancy have not yet been fully elucidated, analysis of these conditions can highlight the importance of particular genes, and downstream immune responses, in carcinogenesis and may help inform mechanisms which can be utilised in novel immunotherapies. In this review, we examine the links between IEIs and cancer, establishing potential connections between immune dysfunction and malignancy and suggesting roles for specific immunological mechanisms involved in preventing carcinogenesis, thus, guiding essential future research focused on cancer immunotherapy and providing valuable insight into the workings of the immune system in both health and disease.

Genome-Wide Screening in Human Embryonic Stem Cells Highlights the Hippo Signaling Pathway as Granting Synthetic Viability in ATM Deficiency

ATM depletion is associated with the multisystemic neurodegenerative syndrome ataxia-telangiectasia (A-T). The exact linkage between neurodegeneration and ATM deficiency has not been established yet, and no treatment is currently available. In this study, we aimed to identify synthetic viable genes in ATM deficiency to highlight potential targets for the treatment of neurodegeneration in A-T. We inhibited ATM kinase activity using the background of a genome-wide haploid pluripotent CRISPR/Cas9 loss-of-function library and examined which mutations confer a growth advantage on ATM-deficient cells specifically. Pathway enrichment analysis of the results revealed the Hippo signaling pathway as a major negative regulator of cellular growth upon ATM inhibition. Indeed, genetic perturbation of the Hippo pathway genes SAV1 and NF2, as well as chemical inhibition of this pathway, specifically promoted the growth of ATM-knockout cells. This effect was demonstrated in both human embryonic stem cells and neural progenitor cells. Therefore, we suggest the Hippo pathway as a candidate target for the treatment of the devastating cerebellar atrophy associated with A-T. In addition to the Hippo pathway, our work points out additional genes, such as the apoptotic regulator BAG6, as synthetic viable with ATM-deficiency. These genes may help to develop drugs for the treatment of A-T patients as well as to define biomarkers for resistance to ATM inhibition-based chemotherapies and to gain new insights into the ATM genetic network.

Ataxia telangiectasia mutated is required for efficient proximal airway epithelial cell regeneration following influenza A virus infection

Children and young adults with mutant forms of ataxia telangiectasia mutated (ATM), a kinase involved in DNA damage signaling and mitochondrial homeostasis, suffer from recurrent respiratory infections, immune deficiencies, and obstructive airways disease associated with disorganized airway epithelium. We previously showed in mice how Atm was required to mount a protective immune memory response to influenza A virus [IAV; Hong Kong/X31 (HKx31), H3N2]. Here, Atm wildtype (WT) and knockout (Atm-null) mice were used to investigate how Atm is required to regenerate the injured airway epithelium following IAV infection. When compared with WT mice, naive Atm-null mice had increased airway resistance and reduced lung compliance that worsened during infection before returning to naïve levels by 56 days postinfection (dpi). Although Atm-null lungs appeared pathologically normal before infection by histology, they developed an abnormal proximal airway epithelium after infection that contained E-cadherin+, Sox2+, and Cyp2f2+ cells lacking secretoglobin family 1 A member 1 (Scgb1a1) protein expression. Patchy and low expression of Scgb1a1 were eventually observed by 56 dpi. Genetic lineage tracing in HKx31-infected mice revealed club cells require Atm to rapidly and efficiently restore Scgb1a1 expression in proximal airways. Since Scgb1a1 is an immunomodulatory protein that protects the lung against a multitude of respiratory challenges, failure to efficiently restore its expression may contribute to the respiratory diseases seen in individuals with ataxia telangiectasia.

A novel, ataxic mouse model of ataxia telangiectasia caused by a clinically relevant nonsense mutation

Ataxia Telangiectasia (A-T) and Ataxia with Ocular Apraxia Type 1 (AOA1) are devastating neurological disorders caused by null mutations in the genome stability genes, A-T mutated (ATM) and Aprataxin (APTX), respectively. Our mechanistic understanding and therapeutic repertoire for treating these disorders are severely lacking, in large part due to the failure of prior animal models with similar null mutations to recapitulate the characteristic loss of motor coordination (i.e., ataxia) and associated cerebellar defects. By increasing genotoxic stress through the insertion of null mutations in both the Atm (nonsense) and Aptx (knockout) genes in the same animal, we have generated a novel mouse model that for the first time develops a progressively severe ataxic phenotype associated with atrophy of the cerebellar molecular layer. We find biophysical properties of cerebellar Purkinje neurons (PNs) are significantly perturbed (e.g., reduced membrane capacitance, lower action potential [AP] thresholds, etc.), while properties of synaptic inputs remain largely unchanged. These perturbations significantly alter PN neural activity, including a progressive reduction in spontaneous AP firing frequency that correlates with both cerebellar atrophy and ataxia over the animal's first year of life. Double mutant mice also exhibit a high predisposition to developing cancer (thymomas) and immune abnormalities (impaired early thymocyte development and T-cell maturation), symptoms characteristic of A-T. Finally, by inserting a clinically relevant nonsense-type null mutation in Atm, we demonstrate that Small Molecule Read-Through (SMRT) compounds can restore ATM production, indicating their potential as a future A-T therapeutic.

SIRT3, a metabolic target linked to ataxia-telangiectasia mutated (ATM) gene deficiency in diffuse large B-cell lymphoma

Inactivation of Ataxia-telangiectasia mutated (ATM) gene results in an increased risk to develop cancer. We show that ATM deficiency in diffuse large B-cell lymphoma (DLBCL) significantly induce mitochondrial deacetylase sirtuin-3 (SIRT3) activity, disrupted mitochondrial structure, decreased mitochondrial respiration, and compromised TCA flux compared with DLBCL cells expressing wild type (WT)-ATM. This corresponded to enrichment of glutamate receptor and glutamine pathways in ATM deficient background compared to WT-ATM DLBCL cells. ATM^−/− DLBCL cells have decreased apoptosis in contrast to radiosensitive non-cancerous A-T cells. In vivo studies using gain and loss of SIRT3 expression showed that SIRT3 promotes growth of ATM CRISPR knockout DLBCL xenografts compared to wild-type ATM control xenografts. Importantly, screening of DLBCL patient samples identified SIRT3 as a putative therapeutic target, and validated an inverse relationship between ATM and SIRT3 expression. Our data predicts SIRT3 as an important therapeutic target for DLBCL patients with ATM null phenotype.

The recent advances in non-homologous end-joining through the lens of lymphocyte development

Lymphocyte development requires ordered assembly and subsequent modifications of the antigen receptor genes through V(D)J recombination and Immunoglobulin class switch recombination (CSR), respectively. While the programmed DNA cleavage events are initiated by lymphocyte-specific factors, the resulting DNA double-strand break (DSB) intermediates activate the ATM kinase-mediated DNA damage response (DDR) and rely on the ubiquitously expressed classical non-homologous end-joining (cNHEJ) pathway including the DNA-dependent protein kinase (DNA-PK), and, in the case of CSR, also the alternative end-joining (Alt-EJ) pathway, for repair. Correspondingly, patients and animal models with cNHEJ or DDR defects develop distinct types of immunodeficiency reflecting their specific DNA repair deficiency. The unique end-structure, sequence context, and cell cycle regulation of V(D)J recombination and CSR also provide a valuable platform to study the mechanisms of, and the interplay between, cNHEJ and DDR. Here, we compare and contrast the genetic consequences of DNA repair defects in V(D)J recombination and CSR with a focus on the newly discovered cNHEJ factors and the kinase-dependent structural roles of ATM and DNA-PK in animal models. Throughout, we try to highlight the pending questions and emerging differences that will extend our understanding of cNHEJ and DDR in the context of primary immunodeficiency and lymphoid malignancies.

ATM, ATR and DNA-PKcs kinases-the lessons from the mouse models: inhibition ≠ deletion

DNA damage, especially DNA double strand breaks (DSBs) and replication stress, activates a complex post-translational network termed DNA damage response (DDR). Our review focuses on three PI3-kinase related protein kinases-ATM, ATR and DNA-PKcs, which situate at the apex of the mammalian DDR. They are recruited to and activated at the DNA damage sites by their respective sensor protein complexes-MRE11/RAD50/NBS1 for ATM, RPA/ATRIP for ATR and KU70-KU80/86 (XRCC6/XRCC5) for DNA-PKcs. Upon activation, ATM, ATR and DNA-PKcs phosphorylate a large number of partially overlapping substrates to promote efficient and accurate DNA repair and to coordinate DNA repair with other DNA metabolic events (e.g., transcription, replication and mitosis). At the organism level, robust DDR is critical for normal development, aging, stem cell maintenance and regeneration, and physiological genomic rearrangements in lymphocytes and germ cells. In addition to endogenous damage, oncogene-induced replication stresses and genotoxic chemotherapies also activate DDR. On one hand, DDR factors suppress genomic instability to prevent malignant transformation. On the other hand, targeting DDR enhances the therapeutic effects of anti-cancer chemotherapy, which led to the development of specific kinase inhibitors for ATM, ATR and DNA-PKcs. Using mouse models expressing kinase dead ATM, ATR and DNA-PKcs, an unexpected structural function of these kinases was revealed, where the expression of catalytically inactive kinases causes more genomic instability than the loss of the proteins themselves. The spectrum of genomic instabilities and physiological consequences are unique for each kinase and depends on their activating complexes, suggesting a model in which the catalysis is coupled with DNA/chromatin release and catalytic inhibition leads to the persistence of the kinases at the DNA lesion, which in turn affects repair pathway choice and outcomes. Here we discuss the experimental evidences supporting this mode of action and their implications in the design and use of specific kinase inhibitors for ATM, ATR and DNA-PKcs for cancer therapy.

Hematopoietic Stem Cell Transplantation Restores Naïve T-Cell Populations in Atm-Deficient Mice and in Preemptively Treated Patients With Ataxia-Telangiectasia

Background: Ataxia-telangiectasia (A-T) is a multisystem disorder with progressive cerebellar ataxia, immunodeficiency, chromosomal instability, and increased cancer susceptibility. Cellular immunodeficiency is based on naïve CD4⁺ and CD8⁺ T-cell lymphopenia. Hematopoietic stem cell transplantation (HSCT) offers a potential to cure immunodeficiency and cancer due to restoration of the lymphopoietic system. The aim of this investigation was to analyze the effect of HSCT on naïve CD4⁺ as well as CD8⁺ T-cell numbers in A-T.

Methods: We analyzed total numbers of peripheral naïve (CD45RA⁺CD62L⁺) and memory (CD45RO⁺CD62L⁻) CD4⁺ and CD8⁺ T-cells of 32 A-T patients. Naïve (CD62L^highCD44^low) and memory (CD62L^lowCD44^high) T-cells were also measured in Atm-deficient mice before and after HSCT with GFP-expressing bone marrow derived hematopoietic stem cells. In addition, we analyzed T-cells in the peripheral blood of two A-T patients after HLA-identic allogeneic HSCT.

Results: Like in humans, naïve CD4⁺ as well as naïve CD8⁺ lymphocytes were decreased in Atm-deficient mice. HSCT significantly inhibited thymic lymphomas and increased survival time in these animals. Donor cell chimerism increased up to more than 50% 6 months after HSCT accompanied by a significant increase of naïve CD4 and CD8 T-cell subpopulations, but not of memory T-cells. This finding was also identified in the blood of the A-T patients after HSCT.

Conclusion: HSCT seems to be a feasible strategy to overcome immunodeficiency and might be a conceivable strategy to avoid T-cell driven cancer in A-T at higher risk for malignancy. Naïve CD4 and CD8 T-cells counts are suitable markers for monitoring immune reconstitution post-HSCT. However, risks and benefits of HSCT in A-T have to be properly weighted.

Ataxia-telangiectasia mutated is required for the development of protective immune memory after influenza A virus infection

Ataxia-telangiectasia (A-T), caused by mutations in the A-T mutated (ATM) gene, is a neurodegenerative disorder affecting ∼1 in 40,000-100,000 children. Recurrent respiratory infections are a common and challenging comorbidity, often leading to the development of bronchiectasis in individuals with A-T. The role of ATM in development of immune memory in response to recurrent respiratory viral infections is not well understood. Here, we infect wild-type (WT) and Atm-null mice with influenza A virus (IAV; HKx31, H3N2) and interrogate the immune memory with secondary infections designed to challenge the B cell memory response with homologous infection (HKx31) and the T cell memory response with heterologous infection (PR8, H1N1). Although Atm-null mice survived primary and secondary infections, they lost more weight than WT mice during secondary infections. This enhanced morbidity to secondary infections was not attributed to failure to effectively clear virus during the primary IAV infection. Instead, Atm-null mice developed persistent peribronchial inflammation, characterized in part by clusters of B220⁺ B cells. Additionally, levels of select serum antibodies to hemagglutinin-specific IAV were significantly lower in Atm-null than WT mice. These findings reveal that Atm is required to mount a proper memory response to a primary IAV infection, implying that vaccination of children with A-T by itself may not be sufficiently protective against respiratory viral infections.

Inflammation, a significant player of Ataxia-Telangiectasia pathogenesis?

INTRODUCTION

Ataxia-Telangiectasia (A-T) syndrome is an autosomal recessive neurodegenerative disorder characterized by cerebellar ataxia, oculocutaneous telangiectasia, immunodeficiency, chromosome instability, radiosensitivity, and predisposition to malignancy. There is growing evidence that A-T patients suffer from pathologic inflammation that is responsible for many symptoms of this syndrome, including neurodegeneration, autoimmunity, cardiovascular disease, accelerated aging, and insulin resistance. In addition, epidemiological studies have shown A-T heterozygotes, somewhat like deficient patients, are susceptible to ionizing irradiation and have a higher risk of cancers and metabolic disorders.

AREA COVERED

This review summarizes clinical and molecular findings of inflammation in A-T syndrome.

CONCLUSION

Ataxia-Telangiectasia Mutated (ATM), a master regulator of the DNA damage response is the protein known to be associated with A-T and has a complex nuclear and cytoplasmic role. Loss of ATM function may induce immune deregulation and systemic inflammation.

Ataxia telangiectasia syndrome: moonlighting ATM

INTRODUCTION

Ataxia-telangiectasia (A-T) a multisystem disorder primarily characterized by cerebellar degeneration, telangiectasia, immunodeficiency, cancer susceptibility and radiation sensitivity. Identification of the gene defective in this syndrome, ataxia-telangiectasia mutated gene (ATM), and further characterization of the disorder together with a greater insight into the function of the ATM protein have expanded our knowledge about the molecular pathogenesis of this disease. Area covered: In this review, we have attempted to summarize the different roles of ATM signaling that have provided new insights into the diverse clinical phenotypes exhibited by A-T patients. Expert commentary: ATM, in addition to DNA repair response, is involved in many cytoplasmic roles that explain diverse phenotypes of A-T patients. It seems accumulation of DNA damage, persistent DNA damage response signaling, and chronic oxidative stress are the main players in the pathogenesis of this disease.

Neurodegeneration in ataxia-telangiectasia: Multiple roles of ATM kinase in cellular homeostasis

Ataxia-telangiectasia (A-T) is characterized by neuronal degeneration, cancer, diabetes, immune deficiency, and increased sensitivity to ionizing radiation. A-T is attributed to the deficiency of the protein kinase coded by the ATM (ataxia-telangiectasia mutated) gene. ATM is a sensor of DNA double-strand breaks (DSBs) and signals to cell cycle checkpoints and the DNA repair machinery. ATM phosphorylates numerous substrates and activates many cell-signaling pathways. There has been considerable debate about whether a defective DNA damage response is causative of the neurological aspects of the disease. In proliferating cells, ATM is localized mainly in the nucleus; however, in postmitotic cells such as neurons, ATM is mostly cytoplasmic. Recent studies reveal an increasing number of roles for ATM in the cytoplasm, including activation by oxidative stress. ATM associates with organelles including mitochondria and peroxisomes, both sources of reactive oxygen species (ROS), which have been implicated in neurodegenerative diseases and aging. ATM is also associated with synaptic vesicles and has a role in regulating cellular homeostasis and autophagy. The cytoplasmic roles of ATM provide a new perspective on the neurodegenerative process in A-T. This review will examine the expanding roles of ATM in cellular homeostasis and relate these functions to the complex A-T phenotype. Developmental Dynamics 247:33-46, 2018. © 2017 Wiley Periodicals, Inc.