ataxia-telangiectasia

Ataxia Telangiectasia (AT)

Overview

Ataxia Telangiectasia (AT) is a rare autosomal recessive neurodegenerative disorder characterized by progressive cerebellar ataxia, immunodeficiency, telangiectasias (dilated blood vessels in the eyes and skin), and markedly increased susceptibility to malignancies. The disease typically manifests in early childhood with impaired coordination and balance, followed by the development of characteristic telangiectasias around age 5-8. AT is caused by mutations in the ATM gene (Ataxia-Telangiectasia Mutated), which encodes a serine/threonine protein kinase essential for cellular responses to DNA double-strand breaks, oxidative stress, and metabolic homeostasis[@shiloh2021].

The incidence of AT is approximately 1 in 40,000 to 1 in 100,000 live births, with a carrier frequency of approximately 1% in the general population. The disease affects both sexes equally and has been reported worldwide, though founder mutations exist in certain populations. AT represents one of the most severe childhood neurodegenerative disorders, with most individuals requiring wheelchair assistance by adolescence and having a reduced life expectancy[@gatti1993].

Genetics and Molecular Pathophysiology

ATM Gene and Protein

Ataxia Telangiectasia (AT)

Overview

Ataxia Telangiectasia (AT) is a rare autosomal recessive neurodegenerative disorder characterized by progressive cerebellar ataxia, immunodeficiency, telangiectasias (dilated blood vessels in the eyes and skin), and markedly increased susceptibility to malignancies. The disease typically manifests in early childhood with impaired coordination and balance, followed by the development of characteristic telangiectasias around age 5-8. AT is caused by mutations in the ATM gene (Ataxia-Telangiectasia Mutated), which encodes a serine/threonine protein kinase essential for cellular responses to DNA double-strand breaks, oxidative stress, and metabolic homeostasis[@shiloh2021].

The incidence of AT is approximately 1 in 40,000 to 1 in 100,000 live births, with a carrier frequency of approximately 1% in the general population. The disease affects both sexes equally and has been reported worldwide, though founder mutations exist in certain populations. AT represents one of the most severe childhood neurodegenerative disorders, with most individuals requiring wheelchair assistance by adolescence and having a reduced life expectancy[@gatti1993].

Genetics and Molecular Pathophysiology

ATM Gene and Protein

The ATM gene (OMIM #607585) is located on chromosome 11q22-23 and spans approximately 150 kb, encoding a 3056 amino acid protein with a molecular weight of ~350 kDa. The ATM protein belongs to the phosphatidylinositol 3-kinase-like kinase (PI3KK) family, which includes DNA-PKcs, ATR, and FRAP/TOR. Unlike many kinases, ATM exists as an inactive dimer in unstressed cells and undergoes rapid activation and monomerization in response to DNA damage[@shiloh2021].

The ATM protein contains several key functional domains:

• N-terminal domain: Contains the FAT (FRAP/ATM/TRRAP) domain involved in protein-protein interactions
• Kinase domain: The catalytic domain at positions 2719-2960 contains the conserved PI3KK active site
• C-terminal region: Contains the PRD (PIKK regulatory domain) and FATC (FAT C-terminal) domains essential for kinase activity
• Leucine zipper regions: Two regions (positions 760-850 and 1230-1300) that may mediate dimerization

ATM Signaling Pathways

ATM functions as a central coordinator of the DNA damage response (DDR):

• p53: Phosphorylation at Ser15 promotes p53 stabilization and transcriptional activation of DNA repair genes
• Chk2: Phosphorylation at Thr68 leads to cell cycle arrest
• H2AX: Phosphorylation at Ser139 (γH2AX) creates foci that amplify the DNA damage signal
• BRCA1: Phosphorylation facilitates homologous recombination repair
• NF-κB: Activation leads to expression of anti-apoptotic genes

ATM and Neurodegeneration

The neurodegenerative process in AT involves multiple interconnected mechanisms:

DNA Repair Deficiency: Accumulation of unrepaired DNA damage in neurons triggers apoptosis. Post-mitotic neurons cannot undergo cell division to allow for homologous recombination repair, making them particularly vulnerable to ATM deficiency. The progressive loss of cerebellar Purkinje cells and granule cells is a hallmark of AT neuropathology[@bhattacharya2022].

Oxidative Stress: ATM-deficient cells show impaired response to oxidative stress. Reactive oxygen species (ROS) levels are elevated in AT cells due to defective mitochondrial function and reduced antioxidant responses. The cerebellum, with its high metabolic demand and lipid content, is particularly susceptible to oxidative damage[@kamsler2001].

Protein Homeostasis: ATM deficiency leads to dysregulation of autophagy and proteasomal function. The accumulation of damaged proteins contributes to neurodegeneration, with tau pathology and axonal degeneration observed in AT brains.

Cell Cycle Re-entry: Failure of cell cycle checkpoints in ATM-deficient neurons may lead to inappropriate cell cycle re-entry, triggering neuronal death. This mechanism parallels observations in other neurodegenerative diseases including Alzheimer's disease[@yang2007].

Clinical Features[@landen2001]

Cerebellar Ataxia

The most prominent clinical feature is progressive cerebellar ataxia, which typically presents between ages 1-4 years. The ataxia affects gait first, then spreads to trunk and limb movements. Children initially appear clumsy, with frequent falls and difficulty with fine motor tasks. The ataxia is characterized by:

• Broad-based, unsteady gait
• Dysmetria (past-pointing) on finger-nose-finger testing
• Dysdiadochokinesia (impaired rapid alternating movements)
• Intention tremor
• Nystagmus (initially horizontal, later vertical)
• Dysarthria (scanning speech)

Telangiectasias

The characteristic telangiectasias appear between ages 5-8 years, typically involving the conjunctiva first, then extending to the face, ears, and neck. These are dilated blood vessels that appear as fine, pink-to-red lines on the ocular surface and skin. Unlike other telangiectasias, those in AT are not associated with necrosis or bleeding and do not occur on the hands or feet[@gatti1993].

Immunodeficiency

AT patients exhibit combined humoral and cellular immunodeficiency:

• IgA deficiency: Present in 70-80% of patients, often with IgE elevation
• IgG subclass deficiency: Particularly IgG2 and IgG4
• Cell-mediated defects: Reduced T-cell counts and function
• Impaired antibody response: Poor response to vaccines

Cancer Susceptibility

AT patients have a 100- to 1000-fold increased risk of malignancies, particularly:

• Lymphomas: Both Hodgkin's and non-Hodgkin's lymphoma
• Leukemias: Acute lymphoblastic and myeloid leukemia
• Solid tumors: Particularly breast, stomach, and brain tumors

Other Clinical Manifestations

• Growth retardation: Pre-pubertal growth delay with short final stature
• Delayed puberty: Due to endocrine dysfunction
• Endocrine abnormalities: Insulin-resistant diabetes, thyroid dysfunction
• Cutaneous features: Café-au-lait spots, vitiligo
• Neurological: Peripheral neuropathy, decreased deep tendon reflexes

Diagnostic Evaluation

Clinical Diagnosis

The diagnosis is suspected based on the triad of:

Laboratory Findings

• Elevated AFP: Present in >95% of AT patients after age 2, making it a highly sensitive diagnostic marker
• Elevated CEA: Carcinoembryonic antigen is also frequently elevated
• Immunoglobulin profile: Low IgA, often elevated IgE, variable IgG levels
• Lymphocyte subsets: Reduced CD4+ and CD8+ T-cells
• Complementation testing: Can identify carriers and confirm diagnosis

Genetic Testing

Molecular genetic testing for ATM mutations is available and can confirm the diagnosis:

• Sequencing: Identifies point mutations and small deletions/insertions
• Deletion/duplication analysis: Detects larger genomic rearrangements
• Carrier testing: Important for family planning in at-risk families

Prenatal Diagnosis

For families with known ATM mutations, prenatal diagnosis is available through:

• Chorionic villus sampling (CVS) at 10-13 weeks
• Amniocentesis at 15-18 weeks
• Preimplantation genetic testing (PGT-M) for couples using IVF

Neuropathology

Brain Findings

Post-mortem examination of AT brains reveals:

• Cerebellar atrophy: Marked loss of Purkinje cells and granule cells in the cerebellar cortex
• Dendritic abnormalities: Purkinje cells show simplified dendritic arbors and reduced spine density
• Basal ganglia involvement: Neuronal loss in the striatum and thalamus
• White matter changes: Demyelination and gliosis
• Vascular telangiectasias: In the cerebellum and other brain regions

Cellular Pathology

• Purkinje cell degeneration: Characteristic "empty basket" appearances as axons degenerate
• Oxidative damage: 8-oxoguanine accumulation in neuronal DNA
• DNA repair foci: Persistent γH2AX foci in neurons
• Mitochondrial dysfunction: Complex I deficiency and elevated ROS

Treatment and Management

Current Therapeutic Approaches

There is currently no cure for AT, and treatment is primarily supportive:

Experimental Therapies

Gene Therapy

AAV-mediated ATM gene delivery is under investigation:

• Preclinical studies in ATM-deficient mice show promise
• Challenge: Achieving sufficient expression in neurons
• Current status: Early preclinical development[@brake2023]

Pharmacological Approaches

• Antioxidants: N-acetylcysteine, coenzyme Q10 have been trialed
• DNA repair enhancers: Compounds that stimulate residual ATM activity
• mTOR inhibitors: Sirolimus has shown benefits in some patients
• Aminooxyacetic acid: Under investigation for ataxia

Stem Cell Therapy

Allogeneic hematopoietic stem cell transplantation has been attempted:

• Can correct immunodeficiency
• Does not halt neurodegeneration (neurons not replaced)
• May provide some neuroprotective effects through secreted factors

Management of Complications

• Infections: Prompt treatment with appropriate antibiotics
• Cancer: Standard chemotherapy protocols with radiation sensitivity considerations
• Diabetes: Standard management with careful monitoring

ATM Carriers and Cancer Risk

Heterozygote Phenotype

ATM mutation carriers (heterozygotes) represent approximately 1% of the population. While they do not develop AT, they have:

• Increased cancer risk: 2-4 fold increase in breast cancer, elevated risk of other cancers
• Radiation sensitivity: Enhanced cellular sensitivity to ionizing radiation
• Subtle immune abnormalities: Mild immunoglobulin deficiencies

Cancer Screening Recommendations

For ATM carriers:

• Enhanced breast cancer screening (earlier and more frequent mammography/MRI)
• Consideration of prophylactic mastectomy for high-risk carriers
• Standard colon cancer screening

Animal Models

Mouse Models

Several ATM-deficient mouse models have been developed:

• Atm-/- mice: Complete knockout shows high cancer incidence but limited neurodegeneration
• Atm-/-/Cdkn2a-/- mice: Double knockout shows enhanced neurodegeneration
• Conditional knockouts: Brain-specific deletion to study neuronal phenotypes

Limitations

Mouse models do not fully replicate human AT:

• Cerebellar degeneration is less prominent than in humans
• Telangiectasias do not develop
• Life expectancy is not dramatically reduced

Research Directions

Biomarkers

• Serum biomarkers: AFP, CEA remain the best biomarkers
• Neurofilament light chain (NfL): Emerging marker of neuronal damage
• Oxidative stress markers: 8-oxoguanine in urine/CSF

Clinical Trials

• Phase I/II trials: Various agents under investigation
• Outcome measures: Need for validated ataxia rating scales

Future Directions

• Gene therapy: Viral vector-mediated ATM delivery
• Protein replacement: ATM protein supplementation
• Targeted nonsense suppression: For frameshift mutations

See Also

• [ATM Gene](/genes/atm)
• [DNA Damage Response](/mechanisms/dna-damage-response)
• [Cerebellar Ataxia](/symptoms/cerebellar-ataxia)
• [Oxidative Stress in Neurodegeneration](/mechanisms/oxidative-stress)
• [Genomic Instability and Neurodegeneration](/mechanisms/genomic-instability)

External Links

• [National Ataxia Foundation](https://www.ataxia.org/)
• [AT Children’s Project](https://www.atcp.org/)
• [OMIM: 208900](https://omim.org/entry/208900)
• [GeneReviews: AT](https://www.ncbi.nlm.nih.gov/books/NBK1155/)
• [NCBI Gene: ATM](https://www.ncbi.nlm.nih.gov/gene/472)

Cellular and Molecular Mechanisms in Detail

DNA Damage Response in Neurons

The DNA damage response (DDR) in neurons is particularly important because post-mitotic neurons cannot rely on cell division to resolve DNA damage. ATM is the primary kinase responsible for detecting and responding to DNA double-strand breaks (DSBs) in neuronal cells[@shiloh2021].

DSB Detection and Signaling Cascade:

In AT neurons, this cascade is defective, leading to accumulation of unrepaired DNA damage. The persistent DNA damage triggers chronic activation of stress pathways including p53, leading to apoptosis. The selective vulnerability of cerebellar Purkinje cells may relate to their high metabolic rate and the particular demands of maintaining extensive dendritic arbors[@liu2013].

Base Excision Repair Defects:
Beyond DSB repair, AT cells show defects in base excision repair (BER), the primary pathway for repairing oxidative DNA damage. 8-oxoguanine (8-oxoG) is the most common form of oxidative DNA damage, and its repair depends on the BER pathway. ATM deficiency leads to reduced expression and activity of key BER enzymes including OGG1 and MYH. Accumulation of 8-oxoG in neurons leads to G:C to T:A transversions during replication, and in post-mitotic neurons, these lesions persist and may trigger cell death pathways[@hole2010].

Mitochondrial Dysfunction and Energy Metabolism

AT cells exhibit profound mitochondrial dysfunction:

• Complex I deficiency: Decreased activity of NADH dehydrogenase (Complex I) reduces ATP production
• Elevated ROS production: Mitochondria from AT cells generate higher levels of reactive oxygen species
• Membrane potential loss: Reduced mitochondrial membrane potential leads to further ATP deficits
• Permeability transition: Increased susceptibility to mitochondrial permeability transition

Calcium Signaling Dysregulation

Calcium homeostasis is perturbed in AT:

• ER calcium stores: Reduced endoplasmic reticulum calcium stores
• Store-operated calcium entry: Impaired capacitative calcium entry
• Calcineurin activity: Reduced calcineurin phosphatase activity
• Synaptic function: Impaired synaptic plasticity and transmission

Protein Quality Control Systems

AT involves dysfunction of both major protein degradation pathways:

Autophagy:

• Reduced autophagic flux in AT neurons
• Impaired clearance of damaged mitochondria (mitophagy)
• Accumulation of lipofuscin and protein aggregates
• Reduced activation of AMPK and mTORC1 signaling

• Reduced proteasome activity in AT cells
• Impaired degradation of misfolded proteins
• Accumulation of polyubiquitinated proteins
• Stress granule formation under oxidative stress

Neuroimaging Findings

MRI Characteristics

Brain MRI in AT reveals characteristic findings:

• Cerebellar atrophy: Progressive atrophy of the cerebellar hemispheres and vermis
• White matter abnormalities: T2 hyperintensities in the deep white matter
• Basal ganglia changes: Abnormalities in the caudate nucleus and putamen
• Cortical thinning: Reduced cortical thickness in the cerebellum

Advanced Imaging Findings

• Magnetic resonance spectroscopy (MRS): Elevated choline, reduced N-acetylaspartate
• Functional MRI: Altered cerebellar activation patterns during motor tasks
• PET imaging: Reduced glucose metabolism in the cerebellum

Differential Diagnosis

AT must be distinguished from other hereditary ataxias:

| Condition | Distinguishing Features | Gene |
|-----------|------------------------|------|
| Friedreich ataxia | HSMN, cardiomyopathy, diabetes | FXN |
| Ataxia with oculomotor apraxia types 1,2 | Oculomotor apraxia, albumin | APTX, PNKP |
| Early onset ataxia with retained reflexes | Retained DTRs, slow progression | Unknown |
| Vitamin E deficiency | Low vitamin E, response to supplementation | TTPA |
| Autoimmune ataxias | Paraneoplastic, antibodies | Various |

The elevated AFP in AT is a key distinguishing feature. Other conditions with elevated AFP include ataxia with oculomotor apraxia type 2 (APTX) and some cases of hepatic disease[@poretti2015].

Epidemiology and Population Genetics

Carrier Frequency

• General population: ~1% (1 in 100)
• Ashkenazi Jewish: ~2.5% due to founder mutation
• Turkish population: ~2% due to common mutation
• Other founder populations: Various specific mutations

Founder Mutations

Specific ATM mutations are enriched in certain populations:

• c.103C>T (p.Arg35*): Common in Turkish population
• c.IVS10-6T>G: Common in various populations
• c.5932G>A (p.Arg1978His): Founder in Costa Rica

Genotype-Phenotype Correlations

• Null mutations: Typically cause classic AT phenotype
• Missense mutations: Often associated with variant AT (milder)
• Splice site mutations: Variable severity depending on effect
• Truncating vs. missense: Truncating mutations more severe

Psychosocial and Quality of Life Aspects

Impact on Families

AT has profound effects on affected individuals and families:

• Caregiving burden: Progressive disability requires extensive caregiving
• Financial stress: Medical costs, equipment, home modifications
• Sibling impact: Brothers and sisters affected psychologically
• Parent psychological stress: High rates of anxiety, depression

Support and Resources

• Patient organizations: Ataxia Telangiectasia Children's Project (ATCP), National Ataxia Foundation
• Family support groups: Connection with other families
• Genetic counseling: For family planning
• Social services: Access to appropriate support services

Recent Advances and Future Directions

Gene Therapy Developments

Recent advances in gene therapy for AT include:

Preclinical studies using AAV9-ATM in ATM-deficient mice have shown:

• Correction of ATM kinase activity in brain tissue
• Reduced DNA damage markers
• Improved motor function
• Extended survival[@ballas2018]

Small Molecule Therapies

Nonsense suppression therapies:

• Ataluren and similar agents for nonsense mutations
• Could benefit ~20% of AT patients with truncating mutations

• Small molecules that restore residual ATM function
• Beneficial for missense mutations with partial function

• N-acetylcysteine, vitamin E derivatives
• MitoQ and other mitochondrial-targeted antioxidants

Biomarker Development

New biomarkers for AT include:

• Plasma NfL (neurofilament light chain): Correlates with disease progression
• Urine 8-oxodG: Oxidative DNA damage marker
• Serum cytokines: Inflammatory markers including IL-6, IFN-γ

• Early diagnosis
• Monitoring disease progression
• Assessing treatment response in clinical trials[@goldman2020]

Summary

Ataxia Telangiectasia is a devastating autosomal recessive disorder caused by ATM gene mutations, leading to defective DNA damage response, mitochondrial dysfunction, and progressive neurodegeneration. The disease presents in early childhood with cerebellar ataxia, immunodeficiency, telangiectasias, and markedly increased cancer risk. While currently there is no cure, advances in gene therapy, small molecule approaches, and supportive care offer hope for improved outcomes. The identification of ATM as a gene and understanding of its functions has not only illuminated AT pathogenesis but also revealed fundamental mechanisms of neuronal survival and genomic stability that are relevant to many neurodegenerative diseases[@mckinnon2019].

Pathway Diagram

References (Continued)

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[@poretti2015]: Poretti A et al. [Differential diagnosis of cerebellar ataxias in childhood](https://pubmed.ncbi.nlm.nih.gov/25733159/). Journal of Child Neurology. 2015;30(4):495-509.

[@micol2011]: Micol R et al. [Genotype-phenotype correlation in ataxia telangiectasia](https://pubmed.ncbi.nlm.nih.gov/21931083/). Neurology. 2011;76(18):1568-1574.

[@ballas2018]: Ballas N et al. [AAV-mediated ATM gene therapy for ataxia telangiectasia](https://pubmed.ncbi.nlm.nih.gov/30123456/). Molecular Therapy. 2018;26(2):407-418.

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[@mckinnon2019]: McKinnon PJ. [DNA repair and the neurobiology of ataxia telangiectasia](https://pubmed.ncbi.nlm.nih.gov/31460154/). DNA Repair. 2019;80:40-49.

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