<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">Ataxin-3 (ATXN3)</th>
</tr>
<tr>
<td class="label">Gene</td>
<td>ATXN3</td>
</tr>
<tr>
<td class="label">UniProt</td>
<td><a href="https://www.uniprot.org/uniprot/P54252" target="_blank">P54252</a></td>
</tr>
<tr>
<td class="label">PDB</td>
<td>1YZB, 2KLZ, 4KXL</td>
</tr>
<tr>
<td class="label">Mol. Weight</td>
<td>~42 kDa (normal), expanded in disease</td>
</tr>
<tr>
<td class="label">Localization</td>
<td>Cytoplasm, Nucleus</td>
</tr>
<tr>
<td class="label">Family</td>
<td>Josephin family (deubiquitinating enzymes)</td>
</tr>
<tr>
<td class="label">Diseases</td>
<td><a href="/diseases/sca3">Spinocerebellar Ataxia Type 3</a></td>
</tr>
</table>
Ataxin-3 (ATXN3) is a deubiquitinating enzyme encoded by the ATXN3 gene. It belongs to the Josephin family of cysteine proteases and has a molecular weight of approximately 42 kDa in its normal form[@costa2023]. Ataxin-3 contains an expanded polyglutamine (polyQ) tract in patients with Spinocerebellar Ataxia Type 3 (SCA3/MJD), making it one of the classic polyQ repeat diseases[@kawaguchi2022]. The protein functions as a deubiquitinating enzyme (DUB) that removes ubiquitin chains from target proteins, regulating protein turnover, aggregate clearance, and cellular stress response[@jiang2023].
<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">Ataxin-3 (ATXN3)</th>
</tr>
<tr>
<td class="label">Gene</td>
<td>ATXN3</td>
</tr>
<tr>
<td class="label">UniProt</td>
<td><a href="https://www.uniprot.org/uniprot/P54252" target="_blank">P54252</a></td>
</tr>
<tr>
<td class="label">PDB</td>
<td>1YZB, 2KLZ, 4KXL</td>
</tr>
<tr>
<td class="label">Mol. Weight</td>
<td>~42 kDa (normal), expanded in disease</td>
</tr>
<tr>
<td class="label">Localization</td>
<td>Cytoplasm, Nucleus</td>
</tr>
<tr>
<td class="label">Family</td>
<td>Josephin family (deubiquitinating enzymes)</td>
</tr>
<tr>
<td class="label">Diseases</td>
<td><a href="/diseases/sca3">Spinocerebellar Ataxia Type 3</a></td>
</tr>
</table>
Ataxin-3 (ATXN3) is a deubiquitinating enzyme encoded by the ATXN3 gene. It belongs to the Josephin family of cysteine proteases and has a molecular weight of approximately 42 kDa in its normal form[@costa2023]. Ataxin-3 contains an expanded polyglutamine (polyQ) tract in patients with Spinocerebellar Ataxia Type 3 (SCA3/MJD), making it one of the classic polyQ repeat diseases[@kawaguchi2022]. The protein functions as a deubiquitinating enzyme (DUB) that removes ubiquitin chains from target proteins, regulating protein turnover, aggregate clearance, and cellular stress response[@jiang2023].
Ataxin-3 has a modular domain architecture consisting of an N-terminal Josephin domain (catalytic core) followed by a flexible interdomain, and a C-terminal polyglutamine (polyQ) tract whose length determines disease status[@albrecht2021]. The Josephin domain contains the catalytic triad (Cys178, His119, Asn119) required for protease activity and is structurally conserved among Josephin family members[@nicastro2022]. The polyQ tract, located in the C-terminal region, is polymorphic in the normal population (12-87 repeats) but expanded (>52 repeats) in SCA3 patients, leading to toxic gain-of-function[@lima2023]. Between the Josephin domain and polyQ tract, there are multiple binding motifs for proteins involved in transcription, [autophagy](/entities/autophagy), and DNA repair[@mueller2022]. Available PDB structures include [1YZB](https://www.rcsb.org/structure/1YZB), [2KLZ](https://www.rcsb.org/structure/2KLZ), and [4KXL](https://www.rcsb.org/structure/4KXL).
Ataxin-3 is a multitasking deubiquitinating enzyme with diverse cellular functions:
SCA3, also known as Machado-Joseph Disease (MJD), is caused by CAG trinucleotide repeat expansion in the ATXN3 gene, resulting in an expanded polyQ tract in the ataxin-3 protein[@takiyama2021]. The disease is characterized by progressive cerebellar ataxia (impaired coordination), spasticity, peripheral neuropathy, and often movement disorders including dystonia and tremor[@marinho2022]. Neuropathologically, SCA3 is characterized by neuronal loss in the cerebellar dentate nucleus, brainstem, and spinal cord, with widespread formation of neuronal intranuclear inclusions containing expanded ataxin-3[@schmidt2023].
The pathogenic mechanisms in SCA3 include:
Ataxin-3 pathology is observed in other neurodegenerative conditions. Ataxin-3 inclusions are found in the brains of patients with [Alzheimer's disease](/diseases/alzheimers-disease), [Parkinson's disease](/diseases/parkinsons-disease), and ALS, suggesting that ataxin-3 dysfunction may contribute to broader neurodegenerative processes[@seidel2020][@eriksson2021].
Ataxin-3 is an important therapeutic target for SCA3 and related disorders[@ashizawa2023]:
The expanded polyglutamine (polyQ) tract in ataxin-3 confers toxic properties through multiple mechanisms[@liu2024]. Beyond simple aggregation, the expanded polyQ tract alters the protein's conformation, making it more prone to misfolding and forming toxic oligomers. These oligomers can then seed the aggregation of other proteins, including wild-type ataxin-3, creating a feed-forward loop of aggregation and cellular dysfunction[@wang2023]. The polyQ-expanded ataxin-3 shows enhanced interaction with various cellular proteins, including transcription factors, autophagy receptors, and mitochondrial proteins, leading to broad dysregulation of cellular homeostasis[@zhang2023].
The toxicity of expanded ataxin-3 manifests at multiple cellular levels. At the molecular level, mutant ataxin-3 forms soluble oligomers and insoluble aggregates that sequester normal proteins and impair their function. At the cellular level, these aggregates cause endoplasmic reticulum stress, mitochondrial dysfunction, and impaired autophagy. At the network level, the loss of ataxin-3 function in protein quality control leads to the accumulation of damaged proteins and organelles, ultimately resulting in neuronal death[@liu2022a].
One of the central mechanisms in SCA3 pathogenesis is the disruption of cellular protein homeostasis (proteostasis)[@rodrigues2021]. Ataxin-3 normally functions as a deubiquitinating enzyme that removes ubiquitin from target proteins, facilitating their proper folding, recycling, or degradation. The mutant form of ataxin-3 not only loses its normal function but also gains toxic properties that actively disrupt proteostasis.
The impairment of proteasomal function is particularly significant in neurons, which are post-mitotic cells that cannot dilute out accumulated damaged proteins through cell division. Mutant ataxin-3 directly impairs the 26S proteasome by either inhibiting its activity or by sequestering it into aggregates, leading to the accumulation of polyubiquitinated proteins and cellular stress[@chen2023]. This proteasome impairment creates a vicious cycle where the cell cannot clear mutant ataxin-3, leading to further proteasome dysfunction.
Autophagy, particularly macroautophagy, is crucial for clearing large protein aggregates and damaged organelles. Ataxin-3 plays a normal role in regulating autophagic flux by deubiquitinating autophagy receptors and maintaining their function[@song2022]. Mutant ataxin-3 disrupts this process at multiple levels.
First, mutant ataxin-3 itself becomes a poor substrate for autophagy, resisting proper clearance even when autophagy is activated. Second, mutant ataxin-3 impairs the function of autophagy receptors, reducing the efficiency of aggregate clearance. Third, the expanded polyQ tract may directly interfere with the autophagy machinery, including the mTORC1 pathway and the autophagy initiation complex[@costa2021]. This combination of effects results in severe autophagic impairment, contributing to the accumulation of toxic aggregates and cellular dysfunction.
Ataxin-3 interacts with numerous transcription factors and chromatin-modifying enzymes, making it an important regulator of gene expression[@gao2023a]. The mutant form of ataxin-3 aberrantly interacts with these factors, leading to widespread transcriptional dysregulation.
Key transcription factors affected include histone deacetylases (HDACs), p53, and various nuclear receptors. Mutant ataxin-3 can sequester these factors into aggregates or alter their post-translational modification status, changing their activity and DNA binding[@nakamura2022]. Genome-wide studies have shown that SCA3 patient tissues and model systems display broad changes in gene expression, with particular effects on pathways related to neuronal survival, mitochondrial function, and stress response.
Mitochondria are critically affected in SCA3, with mutant ataxin-3 causing both functional and structural abnormalities[@liu2020]. Ataxin-3 is present in mitochondria, where it interacts with proteins involved in mitochondrial dynamics, quality control, and function.
Mutant ataxin-3 impairs mitochondrial fission-fusion dynamics by altering the localization and function of dynamin-related protein 1 (Drp1) and mitofusins[@nakao2019]. This leads to abnormal mitochondrial morphology and distribution. Additionally, mutant ataxin-3 reduces mitochondrial membrane potential, impairs oxidative phosphorylation, and increases reactive oxygen species (ROS) production. The combination of these defects creates a metabolic crisis in neurons, contributing to their vulnerability and death.
Several mouse models of SCA3 have been generated, providing critical insights into disease pathogenesis and therapeutic testing[@conditional2018]. Transgenic mouse models expressing human mutant ATXN3 recapitulate key features of the human disease, including progressive motor deficits, neuronal loss, and aggregate formation.
The most widely used models include the YAC-84Q mice (expressing full-length human ATXN3 with 84 glutamine repeats under the endogenous promoter) and various neuronal-specific transgenic lines. These models show age-dependent progression of symptoms, with early behavioral deficits appearing around 12 weeks of age and progressing over time. Neuropathological examination reveals the expected patterns of neuronal loss and inclusion formation[@kazemifard2021].
Conditional mouse models, where mutant ATXN3 expression can be turned on or off in adulthood, have demonstrated that continued expression of mutant protein is required for disease maintenance. Importantly, turning off mutant ATXN3 after disease onset can halt progression, suggesting that ongoing toxic gain-of-function drives the disease and that interventions targeting this process may be beneficial even after symptoms appear[@bilen2020].
Drosophila melanogaster models of SCA3 have been instrumental in identifying genetic modifiers and testing therapeutic compounds[@machadojoseph2022]. Flies expressing expanded polyQ tracts in ataxin-3 show progressive neurodegeneration, shortened lifespan, and movement deficits that can be quantitatively measured.
The advantages of Drosophila models include their rapid development, large numbers of offspring for genetic screening, and sophisticated genetic tools for manipulating gene expression. Screens using these models have identified numerous modifiers of polyQ toxicity, including components of the autophagy machinery, molecular chaperones, and histone modifiers[@sequeiros2024].
Zebrafish provide a vertebrate model system with advantages including transparency, rapid development, and amenable genetic manipulation. Zebrafish models of SCA3 show developmental defects when mutant ATXN3 is expressed during embryogenesis, providing a high-throughput system for testing therapeutic interventions[@ramos2023].
Genetic testing for CAG repeat expansion in the ATXN3 gene is the definitive diagnostic method for SCA3[@maia2022]. Individuals with greater than 52 CAG repeats are affected, while those with 51 or fewer repeats are considered unaffected. Intermediate alleles (44-51 repeats) show reduced penetrance and may cause milder disease or late-onset symptoms.
Pre-symptomatic testing is available for at-risk individuals from affected families, though the decision to undergo testing involves complex ethical considerations. Predictive testing programs typically include genetic counseling before and after testing to help individuals understand and cope with the results[@santos2021].
Cerebrospinal fluid (CSF) analysis in SCA3 patients reveals several potentially useful biomarkers[@wang2020]. Neurofilament light chain (NfL) levels are elevated in SCA3 patients compared to controls, reflecting ongoing neuroaxonal injury. The concentration of NfL in CSF correlates with disease severity and progression rate, making it useful for monitoring disease progression and treatment response.
Other CSF biomarkers under investigation include total tau, phosphorylated tau, and various inflammatory markers. The search for disease-specific biomarkers that can track ataxin-3 aggregation or function continues, as such markers would be invaluable for clinical trials and patient management[@fernandez2021].
Magnetic resonance imaging (MRI) shows characteristic patterns of atrophy in SCA3, with particular involvement of the brainstem, cerebellum, and spinal cord[@costa2020]. Quantitative MRI measurements can track disease progression over time and correlate with clinical measures. Advanced techniques such as diffusion tensor imaging (DTI) and volumetric analysis provide sensitive measures of neurodegeneration.
Positron emission tomography (PET) using various ligands has been explored to visualize aggregate burden in vivo, though specific ligands for ataxin-3 aggregates remain under development. Functional imaging studies have revealed altered connectivity patterns in SCA3 brains, providing insights into network-level dysfunction[@godinho2021].
Given the toxic gain-of-function nature of mutant ataxin-3, reducing its expression is a logical therapeutic strategy[@matsumoto2022]. Antisense oligonucleotides (ASOs) targeting ATXN3 mRNA have shown efficacy in mouse models, reducing mutant protein levels and improving behavioral outcomes. Several ASO-based approaches are in various stages of clinical development.
RNA interference (RNAi) approaches using viral vector-delivered shRNAs have shown promise in preclinical models. Adeno-associated virus (AAV) vectors can be engineered to target ATXN3 specifically, and ongoing studies are optimizing delivery to relevant brain regions[@tsuji2021].
Various small molecules have been tested for SCA3 treatment[@klockgether2021]. Compounds targeting aggregation (like methylene blue derivatives) can reduce aggregate formation in cellular and animal models. Autophagy enhancers, including rapamycin and its analogs, show benefit in models by boosting the cell's ability to clear mutant protein.
Histone deacetylase (HDAC) inhibitors have shown efficacy in SCA3 models, potentially by correcting transcriptional dysregulation. Several HDAC inhibitors have been tested in clinical trials, though results have been variable. Other approaches include mitochondrial protectants, antioxidant compounds, and compounds targeting specific signaling pathways implicated in pathogenesis[@jacobi2019].
While disease-modifying therapies are under development, symptomatic treatments remain important for SCA3 patient management[@nobrega2020]. Standard approaches include physical therapy to maintain strength and flexibility, occupational therapy to maximize functional independence, and speech therapy for dysarthria.
Pharmacological management includes medications for spasticity (like baclofen and tizanidine), tremor (like propranolol and gabapentin), and sleep disturbances. Botulinum toxin injections can help with severe dystonia. Regular monitoring for complications, including swallowing difficulties and respiratory dysfunction, is essential[@karmali2021].
Growing evidence suggests that polyQ aggregates can exist in distinct conformational "strains" with different biological properties. Understanding whether different SCA3 patient populations harbor distinct ataxin-3 strains could explain clinical variability and guide the development of strain-specific therapeutics[@chen2022].
Identifying individuals before symptom onset provides the best window for intervention. Combination of genetic testing, biomarker analysis, and neuroimaging may enable earlier identification of at-risk individuals and more effective intervention[@mordes2021].
Given the multiple pathogenic mechanisms in SCA3, combination therapies targeting different pathways may be more effective than single-target approaches. Future clinical trials may test combinations of gene silencing agents with small molecules targeting aggregation or autophagy[@martins2023].
Induced pluripotent stem cell (iPSC) technology has enabled the generation of patient-specific neuronal models for SCA3 research[@friedman2022]. Motor neurons differentiated from SCA3 patient-derived iPSCs recapitulate key disease features, including reduced cell survival, increased aggregate formation, and transcriptional dysregulation. These models provide human-relevant disease biology that can be leveraged for drug screening and mechanistic studies.
Comparative analysis of iPSC-derived neurons from different SCA3 patients has revealed interesting correlations between polyQ repeat length and phenotypic severity, though other genetic modifiers likely influence individual variation. The ability to generate large numbers of neurons from patients with defined repeat lengths enables detailed structure-function studies of the mutant protein[@gier2021].
SCA3 cells show altered responses to various cellular stresses, including oxidative stress, ER stress, and proteotoxic stress[^58]. Mutant ataxin-3-expressing cells are more vulnerable to these stresses, suggesting that the disease creates a "sensitized" state where additional insults can accelerate pathology. This has implications for understanding disease progression, as SCA3 patients may be particularly vulnerable to environmental stressors.
The integrated stress response (ISR), a cellular pathway activated by various stresses, is chronically activated in SCA3 models and patient tissues[^59]. This persistent activation may contribute to translational blockade and metabolic dysregulation. Therapeutic approaches to modulate the ISR, including ISR inhibitors, are under investigation.
Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease, is the most common dominant ataxia worldwide, accounting for approximately 20-50% of all dominant cerebellar ataxias depending on the population[^60]. The prevalence varies geographically, with higher rates reported in certain populations including those from Portugal (Azores islands), Brazil, and Japan. This distribution reflects both founder effects and historical migrations.
In the United States and Europe, SCA3 accounts for approximately 20-30% of autosomal dominant cerebellar ataxias. The disease typically presents in the third to fourth decade of life, though juvenile-onset cases occur with very large repeat expansions. Both genders are equally affected, reflecting autosomal dominant inheritance[^61].
SCA3 is characterized by progressive neurological decline over 15-30 years. The typical disease course begins with gait instability and clumsiness, followed by the development of other movement disorders including dystonia, tremor, and facial fasciculations. Progressive bulbar dysfunction leads to swallowing difficulties and speech impairment.
The rate of progression varies considerably among patients, even within the same family. Factors influencing progression include repeat length (longer repeats generally correlate with earlier onset and more rapid progression), age at onset (earlier onset typically predicts more severe disease), and potentially other genetic modifiers. Functional scales such as the Scale for the Assessment and Rating of Ataxia (SARA) are used to track disease progression in clinical settings[^62].