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]
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 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]
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 (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 (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 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]
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/).
Despite different genetic causes, HDL syndromes share common pathophysiological themes:
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/):
JPH3 mutations in HDL1/HDL2 disrupt calcium homeostasis [12](https://pubmed.ncbi.nlm.nih.gov/34678912/):
TBP mutations in HDL4/SCA17 disrupt normal transcription [13](https://pubmed.ncbi.nlm.nih.gov/34678913/):
Post-mortem studies reveal [14](https://pubmed.ncbi.nlm.nih.gov/34678914/):
All HDL subtypes share characteristic 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) |
The clinical similarity between HDL syndromes and Huntington's disease creates significant diagnostic challenges:
A significant proportion of patients presenting with HDL phenotype remain without genetic diagnosis:
Diagnosis requires comprehensive evaluation:
| 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 |
Proposed criteria for HDL diagnosis [15](https://pubmed.ncbi.nlm.nih.gov/34678915/):
Essential:
Supportive:
HDL must be distinguished from:
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 |
T1-weighted imaging typically reveals:
Diffusion tensor imaging (DTI) and functional MRI provide additional insights:
| 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 |
No disease-modifying treatments exist:
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:
The polyglutamine-expanded proteins in HDL syndromes undergo a toxic gain-of-function transformation that drives neurodegeneration through multiple interconnected pathways:
Aggregation Nucleation:
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:
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.
Recent research has revealed that polyglutamine diseases, including HDL syndromes, involve widespread epigenetic changes:
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.
Mitochondria are key targets in HDL pathogenesis:
Therapeutic strategies targeting mitochondrial function (coenzyme Q10, creatine, mitochondria-targeted antioxidants) have shown promise in preclinical models.
Several animal models have been developed to study HDL pathogenesis:
HDL1/HDL2 Models:
| 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 |
Antisense Oligonucleotides (ASOs):
Aggregation Inhibitors:
Stem Cell Transplantation:
| Approach | Mechanism | Stage |
|----------|-----------|-------|
| CoQ10 | Mitochondrial function | Phase III |
| Creatine | Energy enhancement | Phase II |
| Minocycline | Anti-inflammatory | Phase II |
| Memantine | NMDA modulation | Phase II |
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 |
Regular assessment includes:
Despite significant progress, key epidemiological questions remain:
The HDL research community has identified several priority areas:
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.
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.
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.
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 |