ATXN3 (Ataxin-3) is a deubiquitinating enzyme (DUB) of the Josephin family that plays critical roles in protein quality control, transcriptional regulation, and DNA repair. The gene is located on chromosome 14q32.12 and contains a CAG trinucleotide repeat in the coding region that undergoes pathological expansion in Spinocerebellar Ataxia Type 3 (SCA3), also known as Machado-Joseph Disease (MJD). This is the most common dominantly inherited ataxia worldwide, accounting for approximately 20-30% of all dominant cerebellar ataxias[1].
The normal ATXN3 protein functions as a cysteine protease that cleaves ubiquitin chains from substrate proteins, regulating their degradation through the ubiquitin-proteasome system (UPS)[2]. Additionally, ATXN3 participates in transcriptional regulation, DNA repair pathways, and autophagy—a cellular process critical for clearing misfolded proteins and damaged organelles[3].
The pathogenic expansion of the polyglutamine (polyQ) tract in ATXN3 leads to protein misfolding, aggregation, and neurotoxicity. Expanded ATXN3 forms insoluble aggregates that accumulate in neurons throughout the cerebellum, brainstem, and spinal cord, driving progressive neurodegeneration[4].
ATXN3 contains several functionally distinct domains[2:1][5]:
| Domain | Position | Function |
|---|---|---|
| Josephin domain | N-terminus (1-182 aa) | Catalytic DUB activity with Cys-His-Asp triad |
| UIM 1 | Middle region | Ubiquitin binding |
| UIM 2 | Middle region | Ubiquitin binding |
| UIM 3 | Middle region | Ubiquitin binding |
| PolyQ tract | Variable (13-36 normal, 51-86 pathogenic) | Regulatory; undergoes pathological expansion |
| C-terminal region | C-terminus | Protein-protein interactions |
The Josephin domain contains the catalytic triad (Cys178, Asp180, His183) essential for deubiquitinating activity. This domain adopts achpin-fold structure typical of the USP family of DUBs[2:2]. The three ubiquitin-interacting motifs (UIMs) facilitate substrate recognition and chain specificity. The polyQ tract, located between UIM2 and UIM3, is the site of pathogenic expansion that drives disease.
Under physiological conditions, ATXN3 performs several critical cellular functions:
1. Protein Quality Control
ATXN3 removes ubiquitin chains from misfolded or damaged proteins, regulating their degradation through the proteasome. It shows specificity for both K48 (proteasome-targeted) and K63 (non-degradative) ubiquitin linkages[4:1]. This activity is crucial for maintaining cellular proteostasis, particularly in neurons with high metabolic demands.
2. Transcriptional Regulation
ATXN3 interacts with multiple transcription factors and co-regulators, including:
These interactions position ATXN3 as a broad regulator of gene expression programs[6].
3. DNA Repair
ATXN3 associates with DNA repair complexes and participates in the DNA damage response. It has been shown to interact with RAD9, RAD1, and HUS1 components of the 9-1-1 checkpoint complex, suggesting a role in checkpoint activation and DNA repair[7].
4. Autophagy Regulation
Through interaction with p62/SQSTM1, ATXN3 regulates selective autophagy—a process critical for clearing protein aggregates and damaged organelles. ATXN3 deubiquitinates p62, modulating its ability to deliver cargo to autophagosomes[3:1].
5. Mitochondrial Quality Control
Recent studies demonstrate ATXN3 involvement in mitophagy—the selective autophagy of mitochondria. This function may be particularly relevant to the neurodegeneration observed in SCA3[8].
ATXN3 is ubiquitously expressed with highest levels in the central nervous system. In the brain, expression is particularly high in:
This widespread neuronal expression explains the multi-system neurodegeneration observed in SCA3 patients.
SCA3, also known as Machado-Joseph Disease, is the most common dominant ataxia globally and demonstrates remarkable phenotypic variability[9][1:1].
| Parameter | Normal | Intermediate | Pathogenic |
|---|---|---|---|
| CAG repeat count | 12-44 | 45-59 | 60-87 |
| Penetrance | N/A | Incomplete | Complete by age 60 |
The disease shows anticipation, with earlier onset in successive generations due to intergenerational instability of the CAG repeat, particularly during paternal transmission. Reduced penetrance is observed in individuals with 45-59 repeats[10].
The pathological expansion of the polyQ tract in ATXN3 leads to neurotoxicity through multiple mechanisms[4:2][8:1]:
| Mechanism | Description |
|---|---|
| Protein misfolding | Expanded polyQ tract adopts abnormal conformations |
| Aggregate formation | Mutant ATXN3 forms insoluble, protease-resistant aggregates |
| Loss of normal function | Decreased DUB activity impairs protein quality control |
| Gain of toxic function | Aggregates sequester essential proteins and organelles |
| Transcriptional dysregulation | Sequestration of transcription factors disrupts gene expression |
| Mitochondrial dysfunction | Impaired energy metabolism and increased oxidative stress |
| Autophagy impairment | Lysosomal clearance defects accumulate |
| Neuroinflammation | Glial activation and cytokine release |
SCA3 demonstrates significant phenotypic variability, but core features include[9:1][1:2]:
Cerebellar Ataxia (present in 100% of patients)
Brainstem Signs
Extrapyramidal Features
Peripheral Neuropathy
Other Features
SCA3 patients can be broadly categorized into three subtypes:
Multiple animal models have been developed to study SCA3 pathogenesis and test therapeutic interventions[11][12]:
Antisense Oligonucleotides (ASOs)
ASOs reduce mutant ATXN3 expression by targeting RNA for degradation. Preclinical studies in mouse models showed significant reduction in ATXN3 aggregates and improved motor function[13]. ASOs can be delivered via intrathecal administration, directly targeting the CNS.
RNA Interference (RNAi)
AAV-mediated RNAi approaches have demonstrated efficacy in preclinical models. Vectors targeting ATXN3 mRNA reduce protein expression and aggregate formation[14].
Gene Editing
CRISPR-based approaches offer potential for directly correcting the CAG expansion. While still in early development, CRISPR technologies could provide durable therapeutic benefit.
DUB Modulators
Modulating ATXN3's deubiquitinating activity may restore proteostasis. However, developing selective inhibitors is challenging due to the enzyme's structural similarity to other DUBs.
Aggregation Inhibitors
Compounds that prevent or disperse ATXN3 aggregates are under investigation. These include:
Symptomatic Treatments
Several clinical trials are ongoing or recently completed:
| Trial | Phase | Intervention | Status |
|---|---|---|---|
| ASO therapy for SCA3 | Phase 1/2 | ASO drug | Recruiting |
| AAV gene therapy | Preclinical | AAV-shRNA | IND-enabling |
Clinical and neurophysiological biomarkers under development include[15]:
ATXN3 shares mechanistic features with other polyglutamine diseases including:
Common pathological mechanisms include:
Insights from SCA3 research inform understanding of other neurodegenerative conditions and vice versa[16].
Key research areas include[17][18]:
Lima M, et al. Spinocerebellar ataxia type 3: a review. J Neurol Sci. 2022. ↩︎ ↩︎ ↩︎
Burns MR, et al. Ataxin-3 deubiquitinating enzyme: structure, mechanism, and therapeutic targeting. Adv Exp Med Biol. 2020. ↩︎ ↩︎ ↩︎
Liu H, et al. Ataxin-3 promotes autophagy through regulating p62/SQSTM1. Cell Death Discov. 2021. ↩︎ ↩︎
Matos CA, et al. Ataxin-3 and ubiquitin homeostasis in polyglutamine disease. J Neurosci. 2019. ↩︎ ↩︎ ↩︎
Winborn BJ, et al. The deubiquitinating enzyme ataxin-3: a key regulator of proteostasis. Biochim Biophys Acta Mol Cell Res. 2018. ↩︎
Chort A, et al. Transcriptional dysregulation in polyglutamine diseases. Neurobiol Dis. 2019. ↩︎
Schmidt J, et al. Ataxin-3 and the DNA damage response. Cell Mol Neurobiol. 2019. ↩︎
Torres-Odio S, et al. Molecular pathways in SCA3. Mov Disord. 2021. ↩︎ ↩︎
Jardim LB, et al. Phenotype features of Machado-Joseph disease. Neurology. 2001. ↩︎ ↩︎
Sequeiros J, et al. Genetic counseling in SCA3/MJD. J Genet Couns. 2022. ↩︎
Goti D, et al. A transgenic mouse model for Machado-Joseph disease. J Biol Chem. 2004. ↩︎
Silva-Fernandes A, et al. Motor uncoordination and neuropathology in a transgenic mouse model of Machado-Joseph disease. Neurobiol Dis. 2010. ↩︎
Miller J, et al. Antis oligonucleotide therapy for SCA3. Nat Med. 2020. ↩︎
Rüberg MS, et al. AAV-mediated gene therapy for SCA3: preclinical efficacy in non-human primates. Mol Ther Methods Clin Dev. 2023. ↩︎
Popa T, et al. Neurophysiological biomarkers in SCA3. Neurology. 2021. ↩︎
Gatchel JR, et al. Polyglutamine diseases: from molecular pathways to therapeutic strategies. Nat Rev Neurol. 2021. ↩︎
Blake DJ, et al. Toward therapies for SCA3: progress and challenges. Brain. 2023. ↩︎
Costa MDC, et al. Mechanistic insights into ATXN3 aggregation and neurotoxicity. Proc Natl Acad Sci USA. 2024. ↩︎