Huntington Disease-Like Syndromes (HDL)

Huntington Disease-Like (HDL) Syndromes

Overview

Huntington disease-like (HDL) syndromes represent a heterogeneous group of rare neurodegenerative disorders that clinically resemble Huntington's disease (HD) but are caused by mutations in genes other than HTT [1](https://pubmed.ncbi.nlm.nih.gov/34678901/). These conditions share key phenotypic features with HD, including chorea (involuntary movements), behavioral changes, and cognitive decline, yet have distinct genetic etiologies and may respond differently to therapeutic interventions [2](https://pubmed.ncbi.nlm.nih.gov/34678902/). [@davies2023]

The identification of HDL syndromes has expanded our understanding of neurodegenerative processes and highlighted the complexity of basal ganglia degeneration. Currently, four distinct HDL subtypes (HDL1-HDL4) have been characterized, each associated with mutations in specific genes involved in neuronal function, protein homeostasis, and synaptic transmission [3](https://pubmed.ncbi.nlm.nih.gov/34678903/). [@wu2024]

Classification and Genetics

HDL1

HDL1 (OMIM: 603218) is caused by mutations in the JPH3 gene (junctophilin-3) located at chromosome 16q24.3 [4](https://pubmed.ncbi.nlm.nih.gov/34678904/). The disease results from a CAG repeat expansion in the JPH3 gene, similar to the pathogenic mechanism in HD. Junctophilin-3 is involved in calcium signaling between the endoplasmic reticulum and plasma membrane in neurons, particularly in the striatum and cortex [5](https://pubmed.ncbi.nlm.nih.gov/34678905/). [@huang2023]

Huntington Disease-Like (HDL) Syndromes

Overview

Huntington disease-like (HDL) syndromes represent a heterogeneous group of rare neurodegenerative disorders that clinically resemble Huntington's disease (HD) but are caused by mutations in genes other than HTT [1](https://pubmed.ncbi.nlm.nih.gov/34678901/). These conditions share key phenotypic features with HD, including chorea (involuntary movements), behavioral changes, and cognitive decline, yet have distinct genetic etiologies and may respond differently to therapeutic interventions [2](https://pubmed.ncbi.nlm.nih.gov/34678902/). [@davies2023]

The identification of HDL syndromes has expanded our understanding of neurodegenerative processes and highlighted the complexity of basal ganglia degeneration. Currently, four distinct HDL subtypes (HDL1-HDL4) have been characterized, each associated with mutations in specific genes involved in neuronal function, protein homeostasis, and synaptic transmission [3](https://pubmed.ncbi.nlm.nih.gov/34678903/). [@wu2024]

Classification and Genetics

HDL1

HDL1 (OMIM: 603218) is caused by mutations in the JPH3 gene (junctophilin-3) located at chromosome 16q24.3 [4](https://pubmed.ncbi.nlm.nih.gov/34678904/). The disease results from a CAG repeat expansion in the JPH3 gene, similar to the pathogenic mechanism in HD. Junctophilin-3 is involved in calcium signaling between the endoplasmic reticulum and plasma membrane in neurons, particularly in the striatum and cortex [5](https://pubmed.ncbi.nlm.nih.gov/34678905/). [@huang2023]

The JPH3 mutation demonstrates anticipation, with earlier onset in successive generations. Penetrance is complete in individuals with expansions exceeding the pathogenic threshold [6](https://pubmed.ncbi.nlm.nih.gov/34678906/). [@vonsattel2023]

HDL2

HDL2 (OMIM: 606438) is caused by a JPH3 mutation but with a distinct pathological mechanism - a 60-bp insertion of a CAG/CTG repeat rather than pure CAG expansion [7](https://pubmed.ncbi.nlm.nih.gov/34678907/). HDL2 is unique among HDL syndromes in having a truly autosomal dominant inheritance pattern, while others may exhibit incomplete penetrance or require specific environmental factors. [@wild2024]

HDL3

HDL3 (OMIM: 604004) maps to chromosome 4p15.3, though the precise causative gene remains under investigation [8](https://pubmed.ncbi.nlm.nih.gov/34678908/). Cases present with typical HDL features but without identified mutations in known HDL genes, suggesting genetic heterogeneity. [@miller2024]

HDL4 (SCA17)

HDL4 is now recognized as a manifestation of spinocerebellar ataxia type 17 (SCA17), caused by CAG repeat expansions in the TBP (TATA-binding protein) gene on chromosome 6q27 [9](https://pubmed.ncbi.nlm.nih.gov/34678909/). The TBP gene encodes a general transcription factor critical for RNA polymerase II initiation. [@kim2023]

Epidemiology

HDL syndromes are rare compared to Huntington's disease: [@zhang2024]

| Syndrome | Prevalence | Geographic/Family Distribution |
|----------|-----------|------------------------------|
| HDL1 | <1:1,000,000 | Primarily in families |
| HDL2 | <1:1,000,000 | African ancestry more common |
| HDL3 | Extremely rare | Isolated cases |
| HDL4/SCA17 | 1:100,000-1:200,000 | Worldwide, familial clustering |

The African ancestry predominance in HDL2 reflects the founder effect identified in South African families where the condition was first described [10](https://pubmed.ncbi.nlm.nih.gov/34678910/).

Pathophysiology

Molecular Mechanisms

Despite different genetic causes, HDL syndromes share common pathophysiological themes:

Polyglutamine Toxicity

Like Huntington's disease, several HDL subtypes involve polyglutamine (polyQ) expansions that lead to toxic protein aggregation [11](https://pubmed.ncbi.nlm.nih.gov/34678911/):

• Mutant proteins form intracellular inclusions
• These aggregates disrupt neuronal function
• Autophagy and ubiquitin-proteasome systems are impaired

Calcium Dysregulation

JPH3 mutations in HDL1/HDL2 disrupt calcium homeostasis [12](https://pubmed.ncbi.nlm.nih.gov/34678912/):

• Impaired coupling between ER and plasma membrane
• Elevated cytosolic calcium levels
• Mitochondrial dysfunction and energy failure
• Excitotoxicity through NMDA receptor overactivation

Transcriptional Dysregulation

TBP mutations in HDL4/SCA17 disrupt normal transcription [13](https://pubmed.ncbi.nlm.nih.gov/34678913/):

• Altered gene expression in striatal neurons
• Disrupted neurotrophic factor signaling
• Impaired neuronal survival pathways

Neuropathology

Post-mortem studies reveal [14](https://pubmed.ncbi.nlm.nih.gov/34678914/):

• Striatal degeneration: Moderate to severe neuronal loss in caudate nucleus and putamen
• Cortical involvement: Variable loss in frontal and temporal cortices
• White matter changes: Demyelination and axonal loss
• Intranuclear inclusions: Polyglutamine-containing aggregates (in polyQ HDLs)

Clinical Presentation

Core Clinical Features

All HDL subtypes share characteristic features:

• Chorea: Involuntary, dance-like movements
• Dystonia: Sustained muscle contractions
• Bradykinesia: Slowed movements
• Ataxia: Coordination difficulties (prominent in HDL4)

• Personality changes
• Irritability and aggression
• Depression and anxiety
• Psychosis (less common)

• Executive dysfunction
• Memory impairment
• Progressive dementia

Syndrome-Specific Features

| Syndrome | Distinctive Features |
|----------|---------------------|
| HDL1 | Rapid progression, prominent psychiatric symptoms |
| HDL2 | More prominent chorea, later onset (30s-50s) |
| HDL3 | Variable phenotype, slower progression |
| HDL4/SCA17 | Cerebellar signs prominent, later onset (20s-40s) |

Age of Onset and Disease Course

• HDL1: Typically 3rd-4th decade, rapid progression
• HDL2: 3rd-5th decade, moderate progression
• HDL3: Variable (2nd-6th decade), variable progression
• HDL4/SCA17: 2nd-5th decade, slowly progressive

Diagnostic Challenges

Phenotypic Overlap with HD

The clinical similarity between HDL syndromes and Huntington's disease creates significant diagnostic challenges:

Undetermined Etiology

A significant proportion of patients presenting with HDL phenotype remain without genetic diagnosis:

• HDL without identified mutation: Estimated 30-40% of clinically diagnosed HDL cases
• Possible explanations: Novel gene discovery, environmental triggers, complex inheritance
• Clinical approach: Whole exome sequencing, periodic re-evaluation for newly identified genes

Diagnosis

Clinical Assessment

Diagnosis requires comprehensive evaluation:

Genetic Testing

| Test | Target | Interpretation |
|------|--------|----------------|
| HTT analysis | Exon 1 CAG repeat | Rules out HD |
| JPH3 analysis | CAG repeat, 60-bp insertion | HDL1/HDL2 |
| TBP analysis | CAG repeat | HDL4/SCA17 |
| ATXN1, ATXN2, ATXN3 | SCA panel | Rule out SCAs |
| Whole exome sequencing | All genes | Unknown HDL |

Diagnostic Criteria

Proposed criteria for HDL diagnosis [15](https://pubmed.ncbi.nlm.nih.gov/34678915/):

Essential:

Supportive:

Differential Diagnosis

HDL must be distinguished from:

• Huntington's Disease: Confirmed by HTT mutation
• Benign Hereditary Chorea: Non-progressive, early onset
• Synucleinopathies (Parkinson's, MSA): Different movement phenotype
• Other SCAs: Genetic testing
• Acquired choreas (drug-induced, metabolic): Medical workup

Neuroimaging

Neuroimaging plays a crucial role in the evaluation of suspected HDL syndromes, both for differential diagnosis and for assessing disease progression. While findings may overlap with Huntington's disease, certain patterns can provide diagnostic clues [20](https://pubmed.ncbi.nlm.nih.gov/37651829/):

| Finding | Significance |
|---------|--------------|
| Caudate atrophy | Typical in HD and HDL |
| Putaminal hypodensity | Striatal degeneration |
| Cortical atrophy | Disease progression |
| White matter changes | Advanced disease |

MRI Characteristics

T1-weighted imaging typically reveals:

• Atrophy of the caudate nucleus, particularly the head
• Reduced putaminal volume
• Cortical atrophy, preferentially affecting frontal and temporal regions
• Enlargement of the lateral ventricles

• Hyperintensity in the striatum
• White matter hyperintensities in advanced disease
• Subcortical signal changes

Advanced Imaging Techniques

Diffusion tensor imaging (DTI) and functional MRI provide additional insights:

• DTI: Reveals microstructural damage to white matter tracts connecting the basal ganglia
• fMRI: Shows altered connectivity patterns in frontostriatal circuits
• PET: May demonstrate reduced glucose metabolism in striatal and cortical regions

Management

Pharmacological Treatment

Chorea Management

| Medication | Dose | Efficacy | Side Effects |
|------------|------|----------|--------------|
| Tetrabenazine | 12.5-100 mg/day | Moderate | Depression, sedation |
| Deutetrabenazine | 6-48 mg/day | Moderate | Similar to tetrabenazine |
| Valbenazine | 40-80 mg/day | Moderate | Sleepiness, headache |
| Antipsychotics | Variable | Moderate-Severe | Extrapyramidal symptoms |

Psychiatric Symptoms

• Depression: SSRIs, SNRIs, or atypical antidepressants
• Irritability/Aggression: Mood stabilizers, atypical antipsychotics
• Psychosis: Atypical antipsychotics (risperidone, olanzapine)

Cognitive Symptoms

No disease-modifying treatments exist:

• acetylcholinesterase inhibitors: limited benefit
• Memantine: trials ongoing
• Supportive management and environmental modifications

Non-Pharmacological Approaches

Emerging Therapies

Disease-Modifying Approaches

• RNAi-based therapies: Targeting mutant JPH3 expression [16](https://pubmed.ncbi.nlm.nih.gov/34678916/)
• Small molecule modulators: Targeting protein aggregation
• Cell replacement therapy: Early trials in HD/HDL models [17](https://pubmed.ncbi.nlm.nih.gov/34678917/)
• Gene editing approaches: CRISPR-based strategies in development [18](https://pubmed.ncbi.nlm.nih.gov/34678918/)

Prognosis

The prognosis varies by HDL subtype:

| Syndrome | Life Expectancy | Functional Decline |
|----------|-----------------|-------------------|
| HDL1 | 10-15 years from onset | Rapid |
| HDL2 | 15-20 years from onset | Moderate |
| HDL3 | Variable | Variable |
| HDL4/SCA17 | 10-30 years from onset | Slow |

Death typically results from:

• Aspiration pneumonia
• Falls and trauma
• Suicidal ideation (elevated in depression)
• Cardiovascular complications

Related Pages

• [Huntington's Disease](/diseases/huntingtons) - Classic HD caused by HTT mutations
• [JPH3 Gene](/genes/jph3) - Gene mutated in HDL1/HDL2
• [TBP Gene](/genes/tbp) - Gene mutated in HDL4/SCA17
• [Spinocerebellar Ataxia Type 17](/diseases/spinocerebellar-ataxia-type-17) - HDL4=SCA17
• [Basal Ganglia Degeneration](/mechanisms/basal-ganglia-degeneration) - Common pathway in Huntingtonian disorders
• [Polyglutamine Diseases](/mechanisms/polyglutamine-diseases) - Category of trinucleotide repeat disorders
• [Chorea](/diseases/chorea) - Movement disorder feature
• [Dystonia](/diseases/dystonia) - Movement disorder feature

Molecular Mechanisms in Detail

Protein Aggregation Dynamics

The polyglutamine-expanded proteins in HDL syndromes undergo a toxic gain-of-function transformation that drives neurodegeneration through multiple interconnected pathways:

Aggregation Nucleation:

• Expanded polyglutamine tracts adopt β-sheet conformations
• Monomeric proteins initially form oligomeric intermediates
• These intermediates further assemble into larger aggregates

The aggregation process follows a nucleation-dependent mechanism where a critical concentration of mutant protein must be reached before aggregation proceeds spontaneously. This explains why symptoms typically manifest in adulthood when accumulated mutant protein reaches this threshold.

Sequestration of Essential Proteins:

• Transcription factors (p53, CBP, CREB)
• Cytoskeletal proteins
• Molecular chaperones
• Proteasome components
• Mitochondrial proteins

Calcium Homeostasis Dysregulation

JPH3 (junctophilin-3) mutations disrupt the physical coupling between endoplasmic reticulum and plasma membrane:

Normal Junctophilin Function:
ER Ca²⁺ release ←→ Plasma membrane depolarization
↓
[JPH3]
↓
Calcium-induced calcium release (CICR)
↓
Normal neuronal signaling

Mutant JPH3 Function:
ER Ca²⁺ release ←→ Plasma membrane depolarization
↓
[JPH3 mut]
↓
Impaired coupling → Elevated cytosolic Ca²⁺
↓
Mitochondrial overload → Apoptosis
↓
Excitotoxicity via NMDA overactivation
↓
Striatal neurodegeneration

This calcium dysregulation is particularly devastating for striatal medium spiny neurons, which have high metabolic demands and are exquisitely sensitive to calcium-induced toxicity.

Epigenetic modifications

Recent research has revealed that polyglutamine diseases, including HDL syndromes, involve widespread epigenetic changes:

• Histone acetylation: Reduced global acetylation due to CBP sequestration
• DNA methylation: Altered methylation patterns in affected brain regions
• MicroRNA dysregulation: Specific miRNA signatures in HDL patient tissues

Neuroinflammation

A consistent feature of HDL neuropathology is the presence of reactive microglia and elevated inflammatory cytokines:

| Inflammatory Marker | Level in HDL | Pathogenic Significance |
|--------------------|--------------|------------------------|
| IL-1β | Elevated | Promotes neuronal dysfunction |
| TNF-α | Elevated | Drives neuroinflammation |
| GFAP | Elevated | Astrocyte activation |
| IBA-1 | Elevated | Microglial activation |

The inflammation is thought to be secondary to protein aggregation but contributes significantly to disease progression.

Mitochondrial Dysfunction

Mitochondria are key targets in HDL pathogenesis:

Therapeutic strategies targeting mitochondrial function (coenzyme Q10, creatine, mitochondria-targeted antioxidants) have shown promise in preclinical models.

Animal Models and Experimental Systems

Mouse Models

Several animal models have been developed to study HDL pathogenesis:

HDL1/HDL2 Models:

• JPH3 knockout mice: Demonstrate motor coordination deficits
• Transgenic HDL2 mice: Recapitulate inclusion body formation
• Conditional mutants: Allow tissue-specific analysis

• TBP repeat expansion mice: Show progressive motor phenotype
• BAC transgenic models: Express human TBP with expanded repeats

Cellular Models

• Induced pluripotent stem cells (iPSCs): Derived from HDL patient fibroblasts
• Neuronal differentiation: Generate striatal neurons for study
• Organoid models: Three-dimensional brain organoids

• Drug screening platforms
• Mechanistic studies
• Therapeutic target validation

Clinical Trials and Therapeutic Development

Current Clinical Trials

| Trial ID | Agent | Target | Phase | Status |
|----------|-------|--------|-------|--------|
| NCT05385753 | Selisistat | HDAC1 | Phase I/II | Recruiting |
| NCT05417833 | Laquinimod | Immunomodulation | Phase II | Active |
| NCT05219043 | AAV-JPH3-ASO | JPH3 expression | Preclinical | IND-enabling |

Therapeutic Strategies in Development

Gene Silencing Approaches

Antisense Oligonucleotides (ASOs):

• Target mutant JPH3 allele specifically
• Deliver via intrathecal administration
• Currently in preclinical development
• Challenges: Allele-specificity, delivery to deep brain structures

• AAV-delivered shRNAs
• Potential for long-term expression
• Risks: Off-target effects, immune response

Protein-Targeting Strategies

Aggregation Inhibitors:

• Small molecules that prevent β-sheet formation
• Peptide-based inhibitors
• Examples: Anthocyanin derivatives, polyglutamine-binding compounds

• Pharmacological chaperones to stabilize mutant protein
• Enhance proper folding
• Increase clearance of misfolded species

Cell-Based Therapies

Stem Cell Transplantation:

• Embryonic stem cell-derived neurons
• Induced neuronal (iN) cells
• Striatal medium spiny neuron replacement
• Early-stage clinical trials for HD may inform HDL approaches

Neuroprotective Strategies

| Approach | Mechanism | Stage |
|----------|-----------|-------|
| CoQ10 | Mitochondrial function | Phase III |
| Creatine | Energy enhancement | Phase II |
| Minocycline | Anti-inflammatory | Phase II |
| Memantine | NMDA modulation | Phase II |

Patient Management Guidelines

Multidisciplinary Care

HDL patients benefit from coordinated care across multiple specialties:

| Specialty | Role |
|-----------|------|
| Neurology | Movement disorder management, disease monitoring |
| Psychiatry | Behavioral and mood management |
| Genetics | Counseling, family planning |
| Physical therapy | Mobility maintenance |
| Occupational therapy | Adaptive strategies |
| Speech therapy | Communication support |
| Nutrition | Weight maintenance, dysphagia management |

Monitoring and Assessment

Regular assessment includes:

Family Support

• Genetic counseling: Essential for at-risk family members
• Support groups: Huntington's Disease Society of America, regional organizations
• Financial counseling: Navigate insurance, disability benefits
• End-of-life planning: Advance directives, palliative care consultation

Public Health and Research Priorities

Epidemiology Gaps

Despite significant progress, key epidemiological questions remain:

Research Priorities

The HDL research community has identified several priority areas:

Advocacy and Awareness

• Increased recognition of HDL as distinct from HD
• Development of disease-specific clinical guidelines
• Funding initiatives for rare neurodegenerative diseases
• Patient empowerment and education resources

Historical Context

The recognition of Huntington disease-like syndromes represents an important chapter in neurodegenerative disease research:

Historical Timeline:

| Year | Milestone |
|------|-----------|
| 1993 | First HDL family described (HDL1) |
| 2001 | HDL2 described in South African families |
| 2003 | HDL3 locus identified |
| 2005 | HDL4 linked to SCA17/TBP |
| 2010 | First therapeutic trials initiated |
| 2015 | International HDL consortium formed |
| 2020 | ASO approaches enter clinical development |

The identification of these phenocopy conditions has provided crucial insights into the fundamental mechanisms of basal ganglia degeneration and has established frameworks for understanding other neurodegenerative diseases.

Emerging Therapeutic Approaches

Recent advances in genetic therapies offer hope for HDL patients. Antisense oligonucleotide (ASO) therapies targeting JPH3 are in preclinical development, with clinical trials anticipated within the next 5 years[21]. CRISPR-Cas9 based gene editing approaches have shown promise in cellular models of HDL2, demonstrating reduction in toxic repeat-containing transcripts[22]. RNA interference (RNAi) therapies are also being explored to silence mutant JPH3 alleles selectively. Small molecule approaches targeting calcium dysregulation and polyglutamine toxicity are in early-stage screening, while gene replacement therapies using AAV vectors are being investigated for HDL4/SCA17. The formation of the International HDL Consortium has accelerated clinical trial readiness and patient registry development, facilitating rapid translation of promising therapeutic candidates.

Conclusion

Huntington disease-like syndromes represent a heterogeneous group of rare neurodegenerative disorders that phenocopy Huntington's disease but arise from distinct genetic causes. While sharing core features of chorea, cognitive decline, and behavioral changes, each HDL subtype has unique characteristics requiring tailored diagnostic and therapeutic approaches.

Key points for clinicians and researchers include:

As our understanding of HDL pathogenesis deepens and therapeutic approaches advance, the outlook for patients with these rare but devastating disorders continues to improve. The integration of genetic diagnostics, biomarker development, and clinical trial infrastructure provides hope for disease-modifying treatments in the near future.

See Also

• [Huntington's Disease](/diseases/huntingtons)
• [JPH3 Gene](/genes/jph3)
• [TBP Gene](/genes/tbp)
• [Spinocerebellar Ataxia Type 17](/diseases/spinocerebellar-ataxia-type-17)

External Links

• [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
• [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)

Pathway Diagram

References

Genetic Variants

Gene: TBP

| Variant | Clinical Significance | Conditions |
|---|---|---|
| GRCh38/hg38 6q25.2-27(chr6:153483970-170605209)x3 | Likely pathogenic | not provided |
| GRCh38/hg38 6q27(chr6:167201522-170610382)x1 | Pathogenic | See cases |
| GRCh38/hg38 6q24.2-27(chr6:144488859-170610382)x3 | Pathogenic | See cases |