Ataxia-telangiectasia: mild neurological presentation despite null ATM mutation and severe cellular phenotype

Ataxia-telangiectasia in Latin America: clinical features, immunodeficiency, and mortality in a multicenter study

Ataxia-telangiectasia (AT) is a rare genetic disorder leading to neurological defects, telangiectasias, and immunodeficiency. We aimed to study the clinical and immunological features of Latin American patients with AT and analyze factors associated with mortality. Referral centers from 9 Latin American countries participated in this retrospective cohort study, and 218 patients were included.  Median (IQR) ages at symptom onset and diagnosis were 1.0 (1.0-2.0)  and 5.0 (3.0-8.0) years, respectively. Most patients presented recurrent airway infections, which was significantly associated with IgA deficiency. IgA deficiency was observed in 60.8% of patients and IgG deficiency in 28.6%. T- and B-lymphopenias were also present in most cases. Mean survival was 24.2 years, and Kaplan-Meier 20-year-survival rate was 52.6%, with higher mortality associated with female gender and low IgG levels. These findings suggest that immunologic status should be investigated in all patients with AT.

Effectiveness of Person-Environment-Occupation Model on a Pediatric Neurodegenerative Disease: A Case Report of a Child with Ataxia-Telangiectasia

Ataxia-telangiectasia is a rare neurodegenerative disorder characterized by progressive cerebellar ataxia, oculomotor apraxia, and choreoathetosis. Ataxia-telangiectasia is a devastating disease that negatively affects the participation of patients in daily occupations and consequently has adverse impacts on the quality of life of them and their families. This study aimed to investigate the effects of an occupational therapy intervention based on the Person-Environment-Occupation Model (PEO) on a 9-year old boy with ataxia-telangiectasia. Following a ten-session intervention, the client experienced significant improvement in occupational performance as well as participation in daily occupations as measured by Canadian Occupational Performance Measure (COPM) and the Pediatric Evaluation of Disability Inventory (PEDI), respectively.

The natural history of ataxia-telangiectasia (A-T): A systematic review

BACKGROUND

Ataxia-telangiectasia is an autosomal recessive, multi-system, and life-shortening disease caused by mutations in the ataxia-telangiectasia mutated gene. Although widely reported, there are no studies that give a comprehensive picture of this intriguing condition.

OBJECTIVES

Understand the natural history of ataxia-telangiectasia (A-T), as reported in scientific literature.

SEARCH METHODS

107 search terms were identified and divided into 17 searches. Each search was performed in PubMed, Ovid SP (MEDLINE) 1946-present, OVID EMBASE 1980 -present, Web of Science core collection, Elsevier Scopus, and Cochrane Library.

SELECTION CRITERIA

All human studies that report any aspect of A-T.

DATA COLLECTION AND ANALYSIS

Search results were de-duplicated, data extracted (including author, publication year, country of origin, study design, population, participant characteristics, and clinical features). Quality of case-control and cohort studies was assessed by the Newcastle-Ottawa tool. Findings are reported descriptively and where possible data collated to report median (interquartile range, range) of outcomes of interest.

MAIN RESULTS

1314 cases reported 2134 presenting symptoms. The most common presenting symptom was abnormal gait (1160 cases; 188 studies) followed by recurrent infections in classical ataxia-telangiectasia and movement disorders in variant ataxia-telangiectasia. 687 cases reported 752 causes of death among which malignancy was the most frequently reported cause. Median (IQR, range) age of death (n = 294) was 14 years 0 months (10 years 0 months to 23 years 3 months, 1 year 3 months to 76 years 0 months).

CONCLUSIONS

This review demonstrates the multi-system involvement in A-T, confirms that neurological symptoms are the most frequent presenting features in classical A-T but variants have diverse manifestations. We found that most individuals with A-T have life limited to teenage or early adulthood. Predominance of case reports, and case series demonstrate the lack of robust evidence to determine the natural history of A-T. We recommend population-based studies to fill this evidence gap.

Progressive Depletion of B and T Lymphocytes in Patients with Ataxia Telangiectasia: Results of the Italian Primary Immunodeficiency Network

Ataxia telangiectasia (AT) is a rare neurodegenerative genetic disorder due to bi-allelic mutations in the Ataxia Telangiectasia Mutated (ATM) gene. The aim of this paper is to better define the immunological profile over time, the clinical immune-related manifestations at diagnosis and during follow-up, and to attempt a genotype–phenotype correlation of an Italian cohort of AT patients. Retrospective data of 69 AT patients diagnosed between December 1984 and November 2019 were collected from the database of the Italian Primary Immunodeficiency Network. Patients were classified at diagnosis as lymphopenic (Group A) or non-lymphopenic (Group B). Fifty eight out of 69 AT patients (84%) were genetically characterized and distinguished according to the type of mutations in truncating/truncating (TT; 27 patients), non-truncating (NT)/T (28 patients), and NT/NT (5 patients). In 3 patients, only one mutation was detected. Data on age at onset and at diagnosis, cellular and humoral compartment at diagnosis and follow-up, infectious diseases, signs of immune dysregulation, cancer, and survival were analyzed and compared to the genotype. Lymphopenia at diagnosis was related per se to earlier age at onset. Progressive reduction of cellular compartment occurred during the follow-up with a gradual reduction of T and B cell number. Most patients of Group A carried bi-allelic truncating mutations, had a more severe B cell lymphopenia, and a reduced life expectancy. A trend to higher frequency of interstitial lung disease, immune dysregulation, and malignancy was noted in Group B patients. Lymphopenia at the onset and the T/T genotype are associated with a worst clinical course. Several mechanisms may underlie the premature and progressive immune decline in AT subjects.

Atypical Ataxia Presentation in Variant Ataxia Telangiectasia: Iranian Case-Series and Review of the Literature

Ataxia-telangiectasia (AT) is a rare autosomal recessive neurodegenerative multisystem disorder. A minority of AT patients can present late-onset atypical presentations due to unknown mechanisms. The demographic, clinical, immunological and genetic data were collected by direct interview and examining the Iranian AT patients with late-onset manifestations. We also conducted a systematic literature review for reported atypical AT patients. We identified three Iranian AT patients (3/249, 1.2% of total registry) with later age at ataxia onset and slower neurologic progression despite elevated alpha-fetoprotein levels, history of respiratory infections, and immunological features of the syndrome. Of note, all patients developed autoimmunity in which a decrease of naïve T cells and regulatory T cells were observed. The literature searches also summarized data from 73 variant AT patients with atypical presentation indicating biallelic mild mutations mainly lead to an atypical phenotype with an increased risk of cancer. Variant AT patients present with milder phenotype or atypical form of classical symptoms causing under- or mis- diagnosis. Although missense mutations are more frequent, an atypical presentation can be associated with deleterious mutations due to unknown modifying factors.

Dysfunction of cerebellar microglia in Ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is a multisystem autosomal recessive disease caused by mutations in the ATM gene and characterized by cerebellar atrophy, progressive ataxia, immunodeficiency, male and female sterility, radiosensitivity, cancer predisposition, growth retardation, insulin-resistant diabetes, and premature aging. ATM phosphorylates more than 1500 target proteins, which are involved in cell cycle control, DNA repair, apoptosis, modulation of chromatin structure, and other cytoplasmic as well as mitochondrial processes. In our quest to better understand the mechanisms by which ATM deficiency causes cerebellar degeneration, we hypothesized that specific vulnerabilities of cerebellar microglia underlie the etiology of A-T. Our hypothesis is based on the recent finding that dysfunction of glial cells affect a variety of process leading to impaired neuronal functionality (Song et al., 2019). Whereas astrocytes and neurons descend from the neural tube, microglia originate from the hematopoietic system, invade the brain at early embryonic stage, and become the innate immune cells of the central nervous system and important participants in development of synaptic plasticity. Here we demonstrate that microglia derived from Atm^-/- mouse cerebellum display accelerated cell migration and are severely impaired in phagocytosis, secretion of neurotrophic factors, and mitochondrial activity, suggestive of apoptotic processes. Interestingly, no microglial impairment was detected in Atm-deficient cerebral cortex, and Atm deficiency had less impact on astroglia than microglia. Collectively, our findings validate the roles of glial cells in cerebellar attrition in A-T.

Autosomal recessive adult onset ataxia

Autosomal recessive ataxias (ARCA) represent a complex group of diseases ranging from primary ataxias to rare and complex metabolic disorders in which ataxia is a part of the clinical picture. Small number of ARCA manifest exclusively in adulthood, while majority of typical childhood onset ARCA may also start later with atypical clinical presentation. We have systematically searched the literature for ARCA with adult onset, both in the group of primary ataxias including those that are less frequently described in isolated or in a small number of families, and also in the group of complex and metabolic diseases in which ataxia is only part of the clinical picture. We propose an algorithm that could be used when encountering a patient with adult onset sporadic or recessive ataxia in whom the acquired causes are excluded. ARCA are frequently neglected in the differential diagnosis of adult-onset ataxias. Rising awareness of their clinical significance is important, not only because some of these disorders may be potentially treatable, but also for prognostic implications and inclusion of patients to future clinical trials with disease modifying agents.

Compound heterozygous variants including a novel copy number variation in a child with atypical ataxia-telangiectasia: a case report

BACKGROUND

Ataxia-telangiectasia is a rare autosomal recessive, neurodegenerative disorder caused by alterations in the ATM gene. The majority of ATM pathogenic variants are frameshift or nonsense variants which are predicted to truncate the whole ATM protein. Herein, we report on an ataxia telangiectasia child with atypical phenotype who was identified as compound heterozygous for two ATM variants involving a previously described pathogenic single nucleotide variation (SNV) and a novel copy number variation (CNV).

CASE PRESENTATION

A 6-year-old boy presented with delayed development and oculomotor apraxia. Brain magnetic resonance imaging showed interval development of mild atrophy in the cerebellum. Serum alpha fetoprotein level was in normal range. Next-generation sequencing and single-nucleotide polymorphism array tests were performed. Next-generation sequencing revealed a heterozygous nonsense pathogenic variant in ATM, c.742C > T (p.Arg248Ter) inherited from the father. Single-nucleotide polymorphism array revealed a compound heterozygous CNV, arr[GRCh37] 11q22.3(10851766-108183226) × 1, 31460 bp (exons 24-40 deletion of ATM) inherited from the mother, which was validated by reverse transcription-polymerase chain reaction analysis (RT-PCR). We demonstrated that this variant (NM_000051.4:c.3403_6006del) generated a product of in-frame deletion of exon 24-40 of ATM (p.Ser1135_Gln2002del).

CONCLUSIONS

The compound heterozygosity for ATM variants involving a previously described pathogenic SNV and a novel CNV may be associated with the atypical clinical manifestations. This clinical report extends the genetic and phenotypic spectrum of ATM pathogenic variants in atypical ataxia-telangiectasia, thus making implementation of advanced analysis beyond the routine next-generation sequencing an important consideration in diagnosis and rehabilitation services for children with ataxia-telangiectasia.

Multidisciplinary Management of Ataxia Telangiectasia: Current Perspectives

Ataxia telangiectasia (A-T) is a rare autosomal recessive disease caused by mutations in the ataxia telangiectasia mutated (ATM) gene. In the absence of a family history, the diagnosis of A-T is usually not made until the child is older and symptomatic. Classic A-T is characterized by a constellation of clinical symptoms including progressive ataxia, oculocutaneous telangiectasias and sinopulmonary disease and is usually associated with absence of ATM protein. Other laboratory features associated with A-T include elevated serum levels of alpha-fetoprotein (AFP) and increased chromosomal breakage with in vitro exposure to ionizing radiation. Sinopulmonary symptoms can occur to varying degrees across the lifespan. Some children will also have hypogammaglobulinemia and impaired antibody responses requiring supplemental gamma globulin. People with hypomorphic ATM mutations are often considered to have mild A-T with onset of ataxia and neurological progression occurring later in life with less impairment of the immune system. The risk of malignancy, however, is significantly increased in people with either classic or mild A-T. While hematological malignancies are most common in the first two decades of life, solid organ malignancies become increasingly common during young adulthood. Deterioration of neurologic function with age is associated with dysphagia with aspiration, growth faltering, loss of ambulation and decline in pulmonary function, morbidities that contribute to shortened life expectancy and decreased quality of life. Premature death is often due to malignancies or chronic respiratory insufficiency. A-T is currently managed with supportive care and symptomatic treatment. Current clinical trials, however, represent progress and hope towards disease-modifying therapies for A-T.

Chromosome instability syndromes

Fanconi anaemia (FA), ataxia telangiectasia (A-T), Nijmegen breakage syndrome (NBS) and Bloom syndrome (BS) are clinically distinct, chromosome instability (or breakage) disorders. Each disorder has its own pattern of chromosomal damage, with cells from these patients being hypersensitive to particular genotoxic drugs, indicating that the underlying defect in each case is likely to be different. In addition, each syndrome shows a predisposition to cancer. Study of the molecular and genetic basis of these disorders has revealed mechanisms of recognition and repair of DNA double-strand breaks, DNA interstrand crosslinks and DNA damage during DNA replication. Specialist clinics for each disorder have provided the concentration of expertise needed to tackle their characteristic clinical problems and improve outcomes. Although some treatments of the consequences of a disorder may be possible, for example, haematopoietic stem cell transplantation in FA and NBS, future early intervention to prevent complications of disease will depend on a greater understanding of the roles of the affected DNA repair pathways in development. An important realization has been the predisposition to cancer in carriers of some of these gene mutations.

[Ataxia-telangiectasia with rare phenotype and unusual pedigree]

The authors present an unique familial case of ataxia-telangiectasia (AT) mimicking autosomal dominant inheritance with different phenotypes in a 3-year-old boy (ataxia and moderate dyskinesia since 1.5 years) and his 31-year-old mother (mild dystonia, predominantly torticollis, since 10 years). Exome sequencing of the boy detected two heterozygous ATM mutations c.1564_1565delGA (p.Glu522fs) and c.6154G>A (p.Glu2052Lys) reported earlier. Sanger sequencing found both mutations in the child, the father was heterozygous for c.1564_1565delGA, the mother for 6154G>A earlier reported in the rare A-T phenotype of 'pure' local dystonia. Exome sequencing of the mother, who considered herself healthy, detected the allelic ATM mutation c.7630-2A>C in intron 51.

The Cerebellar Cognitive Affective Syndrome in Ataxia-Telangiectasia

Ataxia-telangiectasia (AT) is an autosomal recessive, multisystem disease causing cerebellar ataxia, mucocutaneous telangiectasias, immunodeficiency, and malignancies. A pilot study reported cognitive and behavioral manifestations characteristic of the cerebellar cognitive affective / Schmahmann syndrome (CCAS). We set out to test and further define these observations because a more comprehensive understanding of the spectrum of impairments in AT is essential for optimal management. Twenty patients (12 males; 9.86 ± 5.5 years, range 4.3 to 23.2) were grouped by age: AT-I (toddlers and preschoolers, n = 7, 4.3-5.9 years), AT-II (school children, n = 7, 5.9-9.8 years), AT-III (adolescents/young adults, n = 6, 12.6-23.2 years). Standard and experimental tests investigated executive, linguistic, visual-spatial, and affective/social-cognitive domains. Results were compared to standard norms and healthy controls. Cognitive changes in AT-I were limited to mild visual-spatial disorganization. Spatial deficits were greater in AT-II, with low average scores on executive function (auditory working memory), expressive language (vocabulary), academic abilities (math, spelling, reading), social cognition (affect recognition from faces), and emotional/psychological processing. Full Scale IQ scores were low average to borderline impaired. AT-III patients had the greatest level of deficits which were evident particularly in spatial skills, executive function (auditory working memory, sequencing, word/color interference, set-shifting, categorization errors, perseveration), academic achievement, social cognition (affect recognition from faces), and behavioral control. Full Scale IQ scores in this group fell in the impaired range, while language was borderline impaired for comprehension, and low average for expression. Cognitive deficits in AT at a young age are mild and limited to visual-spatial functions. More widespread cognitive difficulties emerge with age and disease progression, impacting executive function, spatial skills, affect, and social cognition. Linguistic processing remains mildly affected. Recognition of the CCAS in children with AT may facilitate therapeutic interventions to improve quality of life.

Trajectories of motor abnormalities in milder phenotypes of ataxia telangiectasia

OBJECTIVE

To describe and classify the neurologic trajectories in patients with mild neurologic forms of ataxia telangiectasia (A-T) from the Dutch A-T cohort, combined with patients reported in the literature.

METHODS

Clinical, genetic, and laboratory data of 14 patients with mild neurologic phenotypes of A-T from the Dutch cohort were analyzed and combined with corresponding data from the literature. A mild neurologic phenotype was defined by a later onset, nonataxia presenting or dominant feature, or slower progression compared to the classic A-T phenotype. Neurologic trajectories were classified based on age at onset, presenting feature, and follow-up data.

RESULTS

One hundred five patients were included in the study. Neurologic trajectories were categorized into 6 groups: patients with childhood-onset extrapyramidal (EP) features with cerebellar symptoms developing later (group 1; 18 patients), childhood-onset cerebellar symptoms, with EP features developing later (group 2; 35 patients), childhood- to adolescence-onset dystonia, without cerebellar symptoms (group 3; 23 patients), childhood- to adolescence-onset isolated cerebellar symptoms (group 4; 22 patients), childhood- to adult-onset prominent muscle weakness (group 5; 2 patients), and patients with adult-onset EP features, with anterior horn cell disease arising subsequently (group 6; 5 patients).

CONCLUSIONS

This systematic study of the different motor abnormalities and their course over time in patients with mild phenotypes of A-T, enabled us to recognize 6 essentially different phenotypic patterns. Awareness of these different trajectories of motor abnormalities in milder forms of A-T will contribute to a reduction of diagnostic delay in this severe multisystem disorder.

Diagnosis and treatment of pediatric onset isolated dystonia

Isolated dystonia refers to a genetic heterogeneous group of progressive conditions with onset of symptoms during childhood or adolescence, progressive course with frequent generalization and marked functional impairment. There are well-known monogenic forms of isolated dystonia with pediatric onset such as DYT1 and DYT6 transmitted with autosomal dominant inheritance and low penetrance. Genetic findings of the past years have widened the etiological spectrum and the phenotype. The recently discovered genes (GNAL, ANO-3, KTM2B) or variant of already known diseases, such as Ataxia-Teleangectasia, are emerging as another causes of pediatric onset dystonia, sometimes with a more complex phenotype, but their incidence is unknown and still a considerable number of cases remains genetically undetermined. Due to the severe disability of pediatric onset dystonia treatment remains unsatisfactory and still mainly based upon oral pharmacological agents. However, deep brain stimulation is now extensively applied with good to excellent results especially when patients are treated early during the course of the disease.

Ataxia-telangiectasia: A review of movement disorders, clinical features, and genotype correlations

Ataxia-telangiectasia is an autosomal recessive neurodegenerative disorder that was initially thought to present exclusively in childhood. With the discovery of the ATM gene, the phenotypic spectrum of the condition has expanded. This review elaborates the expanded phenomenology, including oculomotor apraxia and immunodeficiency, and estimates the presence of each movement disorder feature from previously reported literature. Initial manifestations of Ataxia-telangiectasia include cerebellar symptoms (67%), dystonia (18%), choreoathetosis (10%), and tremor (4%), with parkinsonism and myoclonus not reported as initial features. The prevalence of movement disorders during the course of the disease includes cerebellar symptoms (96%), dystonia (89%), parkinsonism (41%), choreoathetosis (89%), myoclonus (92%), and tremor (74%). Phenomenology and age of onset is modulated by presence of residual ATM kinase activity, with genotypes heavily truncating the ATM protein associated with the most severe phenotypes. Ataxia-telangiectasia commonly results in a spectrum of movement disorders beyond ataxia and telangiectasias. © 2018 International Parkinson and Movement Disorder Society.

Ataxia-telangiectasia (A-T): An emerging dimension of premature ageing

A-T is a prototype genome instability syndrome and a multifaceted disease. A-T leads to neurodegeneration - primarily cerebellar atrophy, immunodeficiency, oculocutaneous telangiectasia (dilated blood vessels), vestigial thymus and gonads, endocrine abnormalities, cancer predisposition and varying sensitivity to DNA damaging agents, particularly those that induce DNA double-strand breaks. With the recent increase in life expectancy of A-T patients, the premature ageing component of this disease is gaining greater awareness. The complex A-T phenotype reflects the ever growing number of functions assigned to the protein encoded by the responsible gene - the homeostatic protein kinase, ATM. The quest to thoroughly understand the complex A-T phenotype may reveal yet elusive ATM functions.

Ataxia telangiectasia: a review

DEFINITION OF THE DISEASE

Ataxia telangiectasia (A-T) is an autosomal recessive disorder primarily characterized by cerebellar degeneration, telangiectasia, immunodeficiency, cancer susceptibility and radiation sensitivity. A-T is often referred to as a genome instability or DNA damage response syndrome.

EPIDEMIOLOGY

The world-wide prevalence of A-T is estimated to be between 1 in 40,000 and 1 in 100,000 live births.

CLINICAL DESCRIPTION

A-T is a complex disorder with substantial variability in the severity of features between affected individuals, and at different ages. Neurological symptoms most often first appear in early childhood when children begin to sit or walk. They have immunological abnormalities including immunoglobulin and antibody deficiencies and lymphopenia. People with A-T have an increased predisposition for cancers, particularly of lymphoid origin. Pulmonary disease and problems with feeding, swallowing and nutrition are common, and there also may be dermatological and endocrine manifestations.

ETIOLOGY

A-T is caused by mutations in the ATM (Ataxia Telangiectasia, Mutated) gene which encodes a protein of the same name. The primary role of the ATM protein is coordination of cellular signaling pathways in response to DNA double strand breaks, oxidative stress and other genotoxic stress.

DIAGNOSIS

The diagnosis of A-T is usually suspected by the combination of neurologic clinical features (ataxia, abnormal control of eye movement, and postural instability) with one or more of the following which may vary in their appearance: telangiectasia, frequent sinopulmonary infections and specific laboratory abnormalities (e.g. IgA deficiency, lymphopenia especially affecting T lymphocytes and increased alpha-fetoprotein levels). Because certain neurological features may arise later, a diagnosis of A-T should be carefully considered for any ataxic child with an otherwise elusive diagnosis. A diagnosis of A-T can be confirmed by the finding of an absence or deficiency of the ATM protein or its kinase activity in cultured cell lines, and/or identification of the pathological mutations in the ATM gene.

DIFFERENTIAL DIAGNOSIS

There are several other neurologic and rare disorders that physicians must consider when diagnosing A-T and that can be confused with A-T. Differentiation of these various disorders is often possible with clinical features and selected laboratory tests, including gene sequencing.

ANTENATAL DIAGNOSIS

Antenatal diagnosis can be performed if the pathological ATM mutations in that family have been identified in an affected child. In the absence of identifying mutations, antenatal diagnosis can be made by haplotype analysis if an unambiguous diagnosis of the affected child has been made through clinical and laboratory findings and/or ATM protein analysis.

GENETIC COUNSELING

Genetic counseling can help family members of a patient with A-T understand when genetic testing for A-T is feasible, and how the test results should be interpreted.

MANAGEMENT AND PROGNOSIS

Treatment of the neurologic problems associated with A-T is symptomatic and supportive, as there are no treatments known to slow or stop the neurodegeneration. However, other manifestations of A-T, e.g. immunodeficiency, pulmonary disease, failure to thrive and diabetes can be treated effectively.

Individual Cytokines Modulate the Neurological Symptoms of ATM Deficiency in a Region Specific Manner

Ataxia-telangiectasia (A-T) is a multisystemic neurodegenerative disease of childhood caused by the absence of functional ATM (A-T mutated) protein. The cerebellar cortex has the most obvious neuropathology, yet cells in other brain regions are also abnormal. A-T mouse models have been produced that replicate much, though not all, of the complex A-T phenotype. Nongenetic factors, including modulations of the immune status of the animal, have also recently been found to play a role in the disease phenotype. Here we report that these modulations show both cytokine and brain region specificity. The CNS changes induced by broad-spectrum immune challenges, such as lipopolysaccharide (LPS) injections are a complex mixture of neuroprotective (TNFα) and neurodegenerative (IL1β) cytokine responses that change over time. For example, LPS first induces a protective response in A-T neurons through activation of tissue repair genes through infiltration of monocytes with M2 phenotype, followed over time by a set of more degenerative responses. Additional phenotypic complexity arises because the neuronal response to an immune challenge is regionally variable; cerebellum and cortex differ in important ways in their patterns of cellular and biochemical changes. Tracking these changes reveals an important though not exclusive role for the MAP kinase pathway. Our findings suggest brain responses to cytokine challenges are temporally and regionally specific and that both features are altered by the absence of ATM. This implies that management of the immune status of A-T patients might have significant clinical benefit.

Ataxia-telangiectasia - A historical review and a proposal for a new designation: ATM syndrome

The authors review ataxia telangiectasia, emphasizing historical aspects, genetic discoveries, and the clinical presentations of the classical and atypical forms. In fact, ataxia telangiectasia represents a multisystem entity with pleomorphic neurological and systemic manifestations. ATM syndrome is proposed as a more adequate designation for this entity.

Ataxia telangiectasia: more variation at clinical and cellular levels

Ataxia telangiectasia (A-T) is a rare recessively inherited disorder resulting in a progressive neurological decline. It is caused by biallelic mutation of the ATM gene that encodes a 370 kDa serine/threonine protein kinase responsible for phosphorylating many target proteins. ATM is activated by auto(trans)phosphorylation in response to DNA double strand breaks and leads to the activation of cell cycle checkpoints and either DNA repair or apoptosis as part of the cellular response to DNA damage. The allelic heterogeneity in A-T is striking. While the majority of mutations are truncating, leading to instability and loss of the ATM protein from the allele, a significant proportion of patients carry one of a small number of mutations that are either missense or leaky splice site mutations resulting in retention of some ATM with activity. The allelic heterogeneity in ATM, therefore, results in an equally striking clinical heterogeneity. There is also locus heterogeneity because mutation of the MRE11 gene can cause an obvious A-T like disorder both clinically and also at the cellular level and mutation of the RNF168 gene results in a much milder clinical phenotype, neurologically, with the major clinical feature being an immunological defect.

Acute lymphoblastic leukemia in early childhood as the presenting sign of ataxia-telangiectasia variant

Ataxia-telangiectasia (A-T), an autosomal recessive disorder is characterized by progressive neurodegeneration, immunodeficiency, sensitivity to ionizing radiation, and predisposition to cancer, especially to lymphoid malignancies. A-T variant is characterized by a milder clinical phenotype and is caused by missense or leaky splice site mutations that produce residual ataxia telangiectasia mutated (ATM) kinase activity. Lymphoid malignancy can precede the diagnosis of A-T, particularly in young children with mild neurological symptoms. We studied a consanguineous family with four A-T variant patients, three of them developed T-ALL at a young age before the diagnosis of A-T was established. ATM mutation analysis detected two new missense mutations both within exon 12: c.1514T>C and c.1547T>C. All four patients are homozygous for the two mutations, while their parents are heterozygous for the mutations. ATM protein level was low in all patients and the response to the radiomimetic agent, neocarzinostatin, was reduced. Leukemic presentation in a young age in three members of consanguineous family led to the identification of a new missense mutation in the ATM gene. The diagnosis of A-T or A-T variant should be considered in children with neurological abnormalities who develop T-ALL at a young age.

A nervous predisposition to unrepaired DNA double strand breaks

Ataxia-telangiectasia (A-T) has for a long time stood apart from most other human neurodegenerative syndromes by the characteristic failure of cells derived from these patients to properly repair DNA damage-induced by ionizing radiation. The discovery of mutations in the ATM gene as being the underlying cause for A-T and the demonstration that the ATM protein functions as a DNA damage-responsive kinase has defined current research focusing on decoding how the cell responds to genotoxic stress. Yet, despite significant advances in delineating the cellular DNA damage response pathways coordinated by ATM, very little headway has been made toward understanding how loss of ATM leads to progressive cerebellar ataxia and whether this can be attributed to an underlying defect in DNA double strand break repair (DSBR). Since its identification, A-T has been used as the archetypal model for how a deficiency in DNA repair affects both the development and maintenance of the nervous and immune systems in humans as well as contributing to the process of tumourigenesis. However, following the growing availability and cost effectiveness of next generation sequencing technologies, the increasing recognition of novel human disorders associated with abnormal DNA repair has demonstrated that the neuropathology typified by A-T is an 'exception' rather than the 'rule'. As a consequence, this throws into doubt the longstanding hypothesis that the neurodegeneration seen in A-T is due to the progressive loss of damaged neurons that have acquired toxic levels of unrepaired DNA lesions over time. Therefore, this review aims to address the question: Is defective DNA double strand break repair an underlying cause of neurodegeneration?

Very mild presentation in adult with classical cellular phenotype of ataxia telangiectasia

BACKGROUND

The major clinical feature of ataxia telangiectasia (A-T) is severe progressive neurodegeneration with onset in infancy. This classical A-T phenotype is caused by biallelic null mutations in the ATM gene, leading to the absence of ATM protein and increased cellular radiosensitivity. We report an unusual case of A-T in a 41-year-old mother, A-T210, who had very mild neurological symptoms despite complete loss of ATM protein.

METHODS

A neurological examination was performed, cellular radiosensitivity was assessed, and the ATM gene was sequenced. Skin fibroblasts and a lymphoblastoid cell line (LCL) were assayed for ATM protein expression and kinase activity.

RESULTS

Patient A-T210 showed mild chorea, dystonia, and gait ataxia, walked independently, and drove a car. LCL and skin fibroblasts were radiosensitive and did not express ATM protein. Two ATM-null mutations were identified.

CONCLUSIONS

The severe neurodegeneration resulting from loss of ATM can be mitigated in some circumstances.

Variant ataxia-telangiectasia presenting as primary-appearing dystonia in Canadian Mennonites

OBJECTIVE

To compare the phenotype of primary-appearing dystonia due to variant ataxia-telangiectasia (A-T) with that of other dystonia ascertained for genetics research.

METHODS

Movement disorder specialists examined 20 Canadian Mennonite adult probands with primary-appearing dystonia, as well as relatives in 4 families with parent-child transmission of dystonia. We screened for the exon 43 c.6200 C>A (p. A2067D) ATM mutation and mutations in DYT1 and DYT6. Clinical features of the individuals with dystonia who were harboring ATM mutations were compared with those of individuals without mutations.

RESULT

Genetic analysis revealed a homozygous founder mutation in ATM in 13 members from 3 of the families, and no one harbored DYT6 or DYT1 mutations. Dystonia in ATM families mimicked other forms of early-onset primary torsion dystonia, especially DYT6, with prominent cervical, cranial, and brachial involvement. Mean age at onset was markedly younger in the patients with variant A-T (n = 12) than in patients with other dystonia (n = 23), (12 years vs 40 years, p < 0.05). The patients with A-T were remarkable for the absence of notable cerebellar atrophy on MRI, lack of frank ataxia on examination, and absence of ocular telangiectasias at original presentation, as well as the presence of prominent myoclonus-dystonia in 2 patients. Many also developed malignancies.

CONCLUSION

Ataxia and telangiectasias may not be prominent features of patients with variant A-T treated for dystonia in adulthood, and variant A-T may mimic primary torsion dystonia and myoclonus-dystonia.

Stable brain ATM message and residual kinase-active ATM protein in ataxia-telangiectasia

The gene that is mutated in ataxia-telangiectasia (A-T), ATM, is catalytically activated in response to DNA damage. Yet a full accounting for the CNS deficits in human A-T or its mouse models remains elusive. We have analyzed the CNS phenotypes of two mouse Atm alleles--Atm(tm1Bal) (Bal) and Atm(tm1Awb) (Awb). Neither mutant has detectable mRNA or protein in peripheral tissues. In brain, although Bal/Bal mice have no ATM protein, they have nearly normal amounts of Atm mRNA. Bal/Bal neurons exhibit extensive cell cycle reentry and degeneration in both cortex and cerebellum. Unexpectedly, in Awb/Awb mice a novel mRNA is found in which the engineered mutation is excised. This mRNA is apparently translated and produces a catalytically active ATM protein that responds to DNA damage by phosphorylating p53 and Chk2. Prompted by these results, we examined eight cases of human A-T and found evidence for residual ATM protein in seven of them. These findings offer important new insights into the human disease and the role of brain ATM activity in the severity of the neurological symptoms of A-T.

The ZFHX3 (ATBF1) transcription factor induces PDGFRB, which activates ATM in the cytoplasm to protect cerebellar neurons from oxidative stress

Ataxia telangiectasia (A-T) is a neurodegenerative disease caused by mutations in the large serine-threonine kinase ATM. A-T patients suffer from degeneration of the cerebellum and show abnormal elevation of serum alpha-fetoprotein. Here, we report a novel signaling pathway that links ATM via cAMP-responsive-element-binding protein (CREB) to the transcription factor ZFHX3 (also known as ATBF1), which in turn promotes survival of neurons by inducing expression of platelet-derived growth factor receptor β (PDGFRB). Notably, AG1433, an inhibitor of PDGFRB, suppressed the activation of ATM under oxidative stress, whereas AG1433 did not inhibit the response of ATM to genotoxic stress by X-ray irradiation. Thus, the activity of a membrane-bound tyrosine kinase is required to trigger the activation of ATM in oxidative stress, independent of the response to genotoxic stress. Kainic acid stimulation induced activation of ATM in the cerebral cortex, hippocampus and deep cerebellar nuclei (DCN), predominately in the cytoplasm in the absence of induction of γ-H2AX (a marker of DNA double-strand breaks). The activation of ATM in the cytoplasm might play a role in autophagy in protection of neurons against oxidative stress. It is important to consider DCN of the cerebellum in the etiology of A-T, because these neurons are directly innervated by Purkinje cells, which are progressively lost in A-T.

Cytoplasmic ATM in neurons modulates synaptic function

ATM is a PI 3-kinase involved in DNA double-strand break repair. ATM deficiency leads to ataxia-telangiectasia (A-T), a syndrome of cancer susceptibility, hypersensitivity to ionizing radiation, immune deficiency, and sterility [1, 2]-phenotypes that can straightforwardly be attributed to a defective response to DNA damage. Yet patients with A-T also suffer from ataxia, speech defects, and abnormal body movements [3-5]-neurological phenotypes whose origins remain largely unexplained. Compounding the discordance, Atm mutations in mouse interfere with DNA repair but have only mild neurological symptoms [6-9], suggesting that the link between DNA damage and the death of neurons can be broken [10-12]. We find that in neurons, ATM protein has a substantial cytoplasmic distribution. We show that in Atm(tm1Awb) mice, hippocampal long-term potentiation is significantly reduced, as is the rate of spontaneous vesicular dye release, suggesting a functional importance of cytoplasmic ATM. In the cytoplasm, ATM forms a complex with two synaptic vesicle proteins, VAMP2 and synapsin-I, both of which must be phosphorylated to bind ATM. Also, cytoplasmic ATM physically associates with the homologous PI 3-kinase, ATR. The neurological symptoms of ataxia-telangiectasia may thus result from defective nonnuclear functions of ATM not associated with DNA repair.

Ataxia-telangiectasia: atypical presentation and toxicity of cancer treatment

BACKGROUND

The onset of progressive cerebellar ataxia in early childhood is considered a key feature of ataxia-telangiectasia (A-T), accompanied by ocular apraxia, telangiectasias, immunodeficiency, cancer susceptibility and hypersensitivity to ionizing radiation.

METHODS

We describe the clinical features and course of three Mennonite children who were diagnosed with A-T following the completion of therapy for lymphoid malignancies.

RESULTS

Prior to cancer therapy, all had non-progressive atypical neurological abnormalities, with onset by age 30 months, including dysarthria, dyskinesia, hypotonia and/or dystonia, without telangiectasias. Cerebellar ataxia was noted in only one of the children and was mild until his death at age eight years. None had severe infections. All three children were "cured" of their lymphoid malignancies, but experienced severe adverse effects from the treatments administered. The two children who received cranial irradiation developed supratentorial primitive neuroectodermal tumors of the brain, an association not previously described, with fatal outcomes.

CONCLUSIONS

The range of neurological presentations of A-T is broad. Ataxia and telangiectasias may be minimal or absent and the course seemingly non-progressive. The diagnosis of A-T should be considered in all children with neuromotor dysfunction or peripheral neuropathy, particularly those who develop lymphoid malignancies. The consequences of missing the diagnosis may be dire. Radiation therapy and radiomimetic drugs should be avoided in individuals with A-T.

An update on inherited ataxias

This article provides an overview of recent advances in the field of inherited ataxias. In the past few years, new causative mutations that broaden the diagnostic spectrum of ataxias have been described. In addition, important advances have unveiled the molecular pathology of these disorders, resulting in a classification based on the pathogenetic pathways rather than clinical or genetic features. As concepts of treatment principles emerge, debate continues as to whether such concepts might be applicable to more than one genetically defined disorder or whether each ataxia disorder requires its own unique therapeutic approach. New clinical assessment instruments have been developed that will facilitate future interventional trials. A recent phase 2 clinical trial suggested a positive effect of high-dose idebenone in Friedreich's ataxia, raising hopes that a treatment option will soon be available for this disorder.

The neurological phenotype of ataxia-telangiectasia: solving a persistent puzzle

Human genomic instability syndromes affect the nervous system to different degrees of severity, attesting to the vulnerability of the CNS to perturbations of genomic integrity and the DNA damage response (DDR). Ataxia-telangiectasia (A-T) is a typical genomic instability syndrome whose major characteristic is progressive neuronal degeneration but is also associated with immunodeficiency, cancer predisposition and acute sensitivity to ionizing radiation and radiomimetic chemicals. A-T is caused by loss or inactivation of the ATM protein kinase, which mobilizes the complex, multi-branched cellular response to double strand breaks in the DNA by phosphorylating numerous DDR players. The link between ATM's function in the DDR and the neuronal demise in A-T has been questioned in the past. However, recent studies of the ATM-mediated DDR in neurons suggest that the neurological phenotype in A-T is indeed caused by deficiency in this function, similar to other features of the disease. Still, major issues concerning this phenotype remain open, including the presumed differences between the DDR in post-mitotic neurons and proliferating cells, the nature of the damage that accumulates in the DNA of ATM-deficient neurons under normal life conditions, the mode of death of ATM-deficient neurons, and the lack of a major neuronal phenotype in the mouse model of A-T. A-T remains a prototype disease for the study of the DDR's role in CNS development and maintenance.

Mutations in String/CDC25 inhibit cell cycle re-entry and neurodegeneration in a Drosophila model of Ataxia telangiectasia

Mutations in ATM (Ataxia telangiectasia mutated) result in Ataxia telangiectasia (A-T), a disorder characterized by progressive neurodegeneration. Despite advances in understanding how ATM signals cell cycle arrest, DNA repair, and apoptosis in response to DNA damage, it remains unclear why loss of ATM causes degeneration of post-mitotic neurons and why the neurological phenotype of ATM-null individuals varies in severity. To address these issues, we generated a Drosophila model of A-T. RNAi knockdown of ATM in the eye caused progressive degeneration of adult neurons in the absence of exogenously induced DNA damage. Heterozygous mutations in select genes modified the neurodegeneration phenotype, suggesting that genetic background underlies variable neurodegeneration in A-T. The neuroprotective activity of ATM may be negatively regulated by deacetylation since mutations in a protein deacetylase gene, RPD3, suppressed neurodegeneration, and a human homolog of RPD3, histone deacetylase 2, bound ATM and abrogated ATM activation in cell culture. Moreover, knockdown of ATM in post-mitotic neurons caused cell cycle re-entry, and heterozygous mutations in the cell cycle activator gene String/CDC25 inhibited cell cycle re-entry and neurodegeneration. Thus, we hypothesize that ATM performs a cell cycle checkpoint function to protect post-mitotic neurons from degeneration and that cell cycle re-entry causes neurodegeneration in A-T.