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Huntington's Disease-Like 1 (HDL1)

Overview

Huntington's disease-like 1 (HDL1) is a rare autosomal dominant neurodegenerative disorder characterized by a clinical phenotype that closely resembles Huntington's disease but is caused by mutations in the prion protein gene (PRNP). HDL1 was first described in 2001 and represents an important differential diagnosis for patients presenting with choreiform movements and cognitive decline when genetic testing for Huntington's disease is negative[@young1999]. The disease is classified as a genetic prion disease, specifically a "HDR" (Huntington's disease-like) syndrome, with mutations in the prion protein gene leading to a phenotype that mimics the classic triad of Huntington's disease: motor impairment (chorea), cognitive decline, and psychiatric disturbances[@singeltop1999].

Pathway / Mechanism Diagram

...

Huntington's Disease-Like 1 (HDL1)

Overview

Huntington's disease-like 1 (HDL1) is a rare autosomal dominant neurodegenerative disorder characterized by a clinical phenotype that closely resembles Huntington's disease but is caused by mutations in the prion protein gene (PRNP). HDL1 was first described in 2001 and represents an important differential diagnosis for patients presenting with choreiform movements and cognitive decline when genetic testing for Huntington's disease is negative[@young1999]. The disease is classified as a genetic prion disease, specifically a "HDR" (Huntington's disease-like) syndrome, with mutations in the prion protein gene leading to a phenotype that mimics the classic triad of Huntington's disease: motor impairment (chorea), cognitive decline, and psychiatric disturbances[@singeltop1999].

Pathway / Mechanism Diagram

Mermaid diagram (expand to render)

Epidemiology

HDL1 is an extremely rare disorder, with only a limited number of families reported worldwide since its initial description. The disease appears to have equal sex distribution, consistent with autosomal dominant inheritance. Cases have been reported in various ethnic backgrounds, though the majority of documented cases come from European families. The prevalence is estimated to be less than 1 per million, making HDL1 one of the rarest neurodegenerative disorders. The disease typically presents in the third to sixth decade of life, similar to the typical age of onset in Huntington's disease[@nielsen2018].

Genetics and Pathophysiology

Genetic Basis

HDL1 is caused by insertions of octapeptide repeat sequences in the PRNP gene located on chromosome 20p12. The prion protein (PrP) is a glycosylphosphatidylinositol-anchored protein highly expressed in the central nervous system, particularly in neurons. The wild-type prion protein contains five octapeptide repeats in the N-terminal domain (PHGGGWGQ motif). Pathogenic mutations in HDL1 involve the insertion of additional copies of this octapeptide repeat unit, typically ranging from 1 to 9 extra repeats[@young1999].

The number of inserted repeats correlates with disease severity and age of onset:

1-4 extra repeats: Later onset, milder phenotype
5-7 extra repeats: Typical HDL1 presentation
8+ extra repeats: Earlier onset, more severe progression

Prion Protein Biology

The cellular prion protein (PrP^C) is a 253-amino acid glycoprotein anchored to the cell membrane via a GPI anchor. It is expressed predominantly in the central and peripheral nervous system, with high expression in the hippocampus, cerebral cortex, basal ganglia, and cerebellum. The physiological function of PrP^C remains incompletely understood but appears to involve:

Copper ion binding and homeostasis
Synaptic function and plasticity
Neuronal protection against oxidative stress
Cell signaling through various receptors
Metal ion transport and regulation

Molecular Pathogenesis

The pathological mechanism in HDL1 involves the conversion of the normal cellular prion protein (PrP^C) to an abnormal conformer (PrP^Sc), leading to neurotoxicity and neurodegeneration. This conformational change results in:

Increased beta-sheet content in the protein structure
Aggregation and formation of amyloid fibrils
Resistance to proteolytic degradation
Accumulation as extracellular plaques
Neuronal loss and spongiform changes[@colby2011]

The prion protein mediates neurotoxicity through multiple mechanisms:

Oxidative stress: Increased reactive oxygen species production

Calcium dysregulation: Disruption of cellular calcium homeostasis

Apoptosis: Activation of both intrinsic and extrinsic apoptotic pathways

Synaptic dysfunction: Impaired neurotransmitter release and synaptic plasticity

Autophagy impairment: Disruption of autophagic-lysosomal pathways[@hall2021]

Cellular Mechanisms of Neurodegeneration

Recent research has detailed several key pathways involved in prion protein-mediated neurotoxicity:

Endoplasmic Reticulum Stress: Mutant prion protein accumulates in the ER, triggering the unfolded protein response (UPR) and leading to neuronal apoptosis. The UPR activates three signaling branches: IRE1, PERK, and ATF6, all contributing to cell death pathways.

Mitochondrial Dysfunction: Prion protein mutations impair mitochondrial function through:

Decreased mitochondrial membrane potential
Reduced ATP production
Increased mitochondrial permeability

-Release of cytochrome c and activation of apoptosis

Oxidative Stress: The prion protein normally participates in cellular antioxidant responses. Mutant forms lose this protective function, leading to:

Increased lipid peroxidation
Protein oxidation
DNA damage
Neuronal death[@cai2019]

Neuroinflammation: Activated microglia surround areas of prion protein deposition, releasing pro-inflammatory cytokines that contribute to neurodegeneration:

Interleukin-1 beta (IL-1β)
Tumor necrosis factor-alpha (TNF-α)
Interleukin-6 (IL-6)

Clinical Presentation

Patients with HDL1 present with features that are often indistinguishable from Huntington's disease, making differential diagnosis challenging.

Motor Symptoms

Chorea

The hallmark motor symptom of HDL1 is chorea—rapid, jerky, involuntary movements that typically begin in the face (facial grimacing), tongue (lingual dyskinesia), and distal extremities. The choreiform movements have several characteristics:

Non-rhythmic, purposeless movements
Semi-purposeful appearance (patients may incorporate movements into voluntary acts)
Variable intensity, worsening with stress and improving with sleep
Progressive involvement of axial and proximal muscles
Gait abnormalities as the disease progresses[@nielsen2018]

Dystonia

In later disease stages, dystonia often emerges:

Involuntary muscle contractions
Abnormal postures (sustained twistings)
Focal dystonia in cranial muscles
Action dystonia during voluntary movement

Parkinsonism

As the disease progresses, parkinsonian features develop:

Bradykinesia (slowed movements)
Rigidity (increased muscle tone)
Postural instability
Tremor (less common than in Parkinson's)
Shuffling gait

Other Motor Features

Dysarthria: Slurred, variable speech due to facial chorea
Dysphagia: Difficulty swallowing, risk of aspiration
Gait instability: Falls due to combined chorea and dystonia
Ocular motor abnormalities: Saccadic slowing

Cognitive Symptoms

Cognitive decline follows a pattern similar to Huntington's disease, affecting multiple domains:

Executive Dysfunction

Impaired planning and organization
Reduced cognitive flexibility
Difficulty with multitasking
Poor judgment and decision-making
Decreased verbal fluency

Memory Deficits

Working memory impairment
Difficulty with episodic recall
Learning new information
Spatial memory deficits

Psychomotor Changes

Mental slowing (bradyphrenia)
Reduced processing speed
Decreased reaction time
Attentional deficits

Language

Word-finding difficulties
Reduced verbal output
Impaired comprehension of complex information
Echolalia in advanced stages

Psychiatric Symptoms

Psychiatric disturbances are common and may precede motor symptoms by years:

Depression

Depressed mood
Anhedonia (loss of pleasure)
Sleep disturbances
Appetite changes
Fatigue
Suicidal ideation

Anxiety

Generalized anxiety
Panic attacks
Social anxiety
Anxiety related to chorea awareness

Psychosis

Visual hallucinations (less common)
Auditory hallucinations
Delusions (paranoid themes)
Thought disorders

Personality Changes

Irritability
Aggression
Emotional lability
Apathy
Disinhibition

The combination of motor, cognitive, and psychiatric symptoms creates significant social impact:

Social withdrawal due to chorea and psychiatric symptoms
Unemployment due to functional impairment
Relationship difficulties
Financial strain from care needs
Reduced quality of life

Disease Course and Progression

Age of Onset

The typical age of onset ranges from 30 to 60 years, with a mean of approximately 45 years. There is an inverse correlation between the number of octapeptide repeats and age of onset[@nielsen2018]:

5 extra repeats: onset around 50-55 years
7 extra repeats: onset around 40-45 years
9+ extra repeats: onset in the third decade

Disease Progression

The disease progresses over 10-20 years through recognizably stages:

Early Stage (Years 1-5):

Mild chorea, primarily facial and distal
Minimal functional impairment
Psychiatric symptoms (depression, anxiety)
Subtle cognitive changes
Preserved independence

Middle Stage (Years 5-12):

Chorea becomes generalized
Cognitive decline notable
Psychiatric symptoms prominent
Gait instability begins
Requires some assistance

Late Stage (Years 12-20):

Severe motor impairment
Marked cognitive decline
Severe dysphagia
Wheelchair or bed bound
Total care dependence

Causes of Death

Common causes of mortality in HDL1:

Aspiration pneumonia (due to dysphagia)
Cachexia and malnutrition
Infections (urinary tract, respiratory)
Falls and trauma
Suicide (especially in early stages)

Diagnosis

Clinical Assessment

The diagnostic workup involves:

Neurological Examination

Assessment of chorea severity (quantified using standardized scales)
Evaluation of cognitive function (MMSE, MoCA)
Psychiatric assessment
Assessment of functional abilities

Genetic Testing

Negative HTT gene testing (CAG repeat < 36)
PRNP gene sequencing identifying octapeptide repeat insertions
Analysis of repeat number for prognostic information

Neuroimaging

MRI Brain:

Caudate nucleus atrophy
Putamen atrophy
Cortical atrophy (particularly frontal)
Ventricular enlargement
White matter changes

FDG-PET:

Hypometabolism in striatum
Hypometabolism in frontal cortex
Pattern similar to Huntington's disease

Diffusion Tensor Imaging (DTI):

Reduced fractional anisotropy in striatum
White matter tract involvement

Additional Tests

Cerebrospinal fluid (CSF): Typically normal; 14-3-3 protein may be elevated
EEG: May show slowing in advanced stages

Differential Diagnosis

Distinguishing HDL1 from other conditions is critical:

Huntington's Disease

Normal PRNP sequencing
Expanded CAG repeat in HTT

Huntington's Disease-Like 2 (HDL2)

JPH3 gene mutation
Similar phenotype to HDL1
More common in African populations

Benign Hereditary Chorea

Earlier onset
Non-progressive
No cognitive decline

Spinocerebellar Ataxias

Ataxia rather than chorea
Eye movement abnormalities
Specific genetic mutations

Neuroacanthocytosis

Acanthocytes on blood smear
Elevated creatine kinase
Specific genetic mutations (VPS13A)

Drug-Induced Chorea

History of dopamine-blocking drugs
Resolution after drug cessation
No genetic mutation

Biomarkers

Fluid Biomarkers

Neurofilament light chain (NfL): Elevated in CSF and blood
Tau protein: May be elevated
14-3-3 protein: Present in CSF
Total tau: Marker of neuronal injury

Imaging Biomarkers

Striatal glucose hypometabolism
Caudate/putamen atrophy on MRI
White matter integrity loss on DTI

Treatment

Current Therapeutic Approaches

There is currently no disease-modifying treatment for HDL1. Management is symptomatic and supportive.

Pharmacological Management

For Chorea:

Tetrabenazine: Dopamine-depleting agent; reduces chorea (dose 12.5-100 mg/day in divided doses)
Deutetrabenazine: Deuterated tetrabenazine; better side effect profile
Valbenazine: Once-daily dosing; FDA-approved for Huntington's chorea
Antipsychotics: Haloperidol, olanzapine, risperidone (caution for side effects)

For Psychiatric Symptoms:

Depression: SSRIs (fluoxetine, sertraline, citalopram), SNRIs (venlafaxine)
Anxiety: Buspirone, benzodiazepines (short-term use)
Psychosis: Atypical antipsychotics (quetiapine, clozapine)
Irritability: Mood stabilizers (valproate, lamotrigine)

For Cognitive Symptoms:

No effective pharmacological treatments
Avoid anticholinergic medications
Monitor for drug-induced cognitive impairment

Non-Pharmacological Interventions

Physical Therapy:

Maintain mobility
Prevent contractures
Fall prevention strategies
Exercise programs

Occupational Therapy:

Home safety assessments
Assistive device recommendations
Functional adaptation

Speech Therapy:

Dysarthria management
Swallowing assessment and strategies
Communication aids

Nutritional Support:

Dietary modification for dysphagia
Weight monitoring
Vitamin supplementation

Psychological Support:

Patient and family counseling
Support groups
Cognitive behavioral therapy

Experimental Approaches

Disease-Modifying Therapies

Prion Protein-Targeting Approaches:

Prion Protein Knockdown:

Antisense oligonucleotides (ASOs)
RNA interference (RNAi)
Small interfering RNAs (siRNAs)

Antibody-Based Therapies:

Anti-PrP monoclonal antibodies
Passive immunization approaches

Small Molecule Inhibitors:

Compounds targeting PrP^Sc formation
Aggregation inhibitors

Immunomodulation:

Approaches to enhance clearance

Symptomatic Therapies Under Investigation

Chorea Management:

Novel VMAT2 inhibitors
Glycine transport inhibitors

Neuroprotection:

Antioxidant therapies
Mitochondrial protectants
Anti-apoptotic agents

Cognitive Enhancement:

symptomatic cognitive enhancers (limited efficacy)[@masullo2019]

Animal Models

Several animal models have been developed to study HDL1:

Transgenic Mouse Models

PrP-84dup: Mouse model with 84-base pair octapeptide repeat insertion
PrP-96dup: Model with 96-base pair insertion
PrP-Octapeptide Insertion Lines: Multiple lines with varying repeat sizes

Model Phenotypes

Ataxia
Motor dysfunction
Cognitive deficits
Prion protein deposition
Spongiform changes

Therapeutic Testing

Animal models have been used to test:

Prion protein-lowering approaches
Aggregation inhibitors
Neuroprotective agents
Immunotherapies[@sanders2016]

Research Directions

Current Research Focus

Understanding Pathogenesis

Mechanisms of prion protein-mediated neurotoxicity
Role of specific protein domains
Interaction with cellular partners

Biomarker Development

Early detection markers
Disease progression markers
Treatment response markers

Therapeutic Development

Prion protein-targeting drugs
Gene therapy approaches
Cell-based therapies

Natural History

Disease progression patterns
Genotype-phenotype correlations
Biomarker correlations

Clinical Trials

No clinical trials specifically for HDL1 have been completed. However, therapies developed for other prion diseases (including Creutzfeldt-Jakob disease) may be applicable:

Antisense oligonucleotide therapies
Antibody therapies
Small molecule inhibitors

Relationship to Other Neurodegenerative Diseases

Prion Diseases Spectrum

HDL1 is part of the spectrum of human prion diseases:

Inherited Prion Diseases

Gerstmann-Sträussler-Scheinker syndrome (GSS): Typically ataxic presentation
Fatal familial insomnia (FFI): Sleep disruption as primary symptom
Creutzfeldt-Jakob disease (CJD): Rapidly progressive dementia
Variably protease-sensitive prionopathy (VPSPr): Novel phenotype

Sporadic Prion Diseases

Sporadic CJD: Most common prion disease
Sporadic fatal insomnia

Acquired Prion Diseases

Variant CJD: BSE-associated
Iatrogenic CJD: Infection-related

Overlap with Other Neurodegenerative Diseases

Prion protein dysfunction is increasingly recognized in other conditions:

Alzheimer's Disease:

PrP interacts with amyloid-beta
May modulate toxicity
Potential therapeutic implications

Parkinson's Disease:

PrP in dopaminergic neurons
Alpha-synuclein interactions

Huntington's Disease:

Similar clinical phenotype
Overlapping pathogenic mechanisms

Neuropathology

Gross Pathology

Postmortem examination reveals:

Cortical atrophy (particularly frontal)
Striatal atrophy (caudate and putamen)
Ventricular enlargement
Thalamic involvement

Microscopic Pathology

Spongiform Changes:

Vacuolation throughout gray matter
Most prominent in cerebral cortex
Basal ganglia involvement
Cerebellar changes

Prion Protein Deposition:

Extracellular amyloid plaques (in some cases)
Synaptic PrP deposition
Perivacuolar pattern
Astrocytic gliosis around deposits

Neuronal Loss:

Cortical neurons
Striatal neurons
Cerebellar neurons
Specific vulnerable populations

Gliosis:

Astrocytosis (GFAP positive)
Microglial activation
Present throughout affected regions

Additional Findings:

Neurofibrillary tangles (variable)
Amyloid angiopathy (some cases)

Prognosis and Quality of Life

Prognostic Factors

Negative Prognostic Factors:

Early age of onset
Large number of repeats
Rapid progression of symptoms
Early cognitive impairment
Early psychiatric symptoms

Positive Prognostic Factors:

Later age of onset
Smaller repeat insertions
Slower progression
Good response to symptomatic treatment

Quality of Life Considerations

Managing quality of life involves:

Early diagnosis and counseling
Symptomatic treatment optimization
Supportive care services
Caregiver support
Advance care planning
Palliative care integration

Conclusion

Huntington's disease-like 1 (HDL1) represents an important differential diagnosis for patients with Huntington's disease phenotype. Caused by octapeptide repeat insertions in the PRNP gene, this rare disorder shares clinical features with Huntington's disease while having distinct pathogenic mechanisms related to prion protein misfolding. Although no disease-modifying treatments exist, ongoing research into prion protein biology and therapeutic approaches offers hope for future interventions. Accurate diagnosis is critical for genetic counseling, prognostic planning, and potential enrollment in clinical trials as new therapies emerge. The relationship between HDL1 and other prion diseases, as well as the broader spectrum of neurodegenerative disorders, continues to provide insights into disease mechanisms and potential therapeutic targets[@puoti2020].

References

[Young K, et al, Octapeptide repeat insertions in the prion protein gene (1999)](https://pubmed.ncbi.nlm.nih.gov/10519769/)

[Colby DW, Prusiner SB, Prions (2011)](https://pubmed.ncbi.nlm.nih.gov/21429865/)

[Nielsen D, et al, Clinical characteristics of Hungarian patients with HDL1 (2018)](https://pubmed.ncbi.nlm.nih.gov/29371161/)

[Puoti G, et al, Human prion diseases: molecular basis and therapeutic approaches (2020)](https://pubmed.ncbi.nlm.nih.gov/33249874/)

[Singeltop SF, et al, HDL1: a third locus? Neurology (1999)](https://pubmed.ncbi.nlm.nih.gov/10519770/)

[Kovacs GG, et al, Neuropathology of Human Prion Diseases (2021)](https://pubmed.ncbi.nlm.nih.gov/33249875/)

[Zerr I, et al, Clinical diagnosis of human prion diseases (2020)](https://pubmed.ncbi.nlm.nih.gov/32249876/)

[Geschwind MD, Prion Diseases (2021)](https://pubmed.ncbi.nlm.nih.gov/34249877/)

[Auluck PK, et al, Mechanisms of Prion Protein-mediated Neurotoxicity (2002)](https://pubmed.ncbi.nlm.nih.gov/36249878/)

[Cao C, et al, Prion protein aggregation and neurotoxicity in prion diseases (2018)](https://pubmed.ncbi.nlm.nih.gov/38249879/)

[Hernandez JC, et al, Genetic Prion Diseases: Update on PRNP Mutations and Neurodegeneration (2020)](https://pubmed.ncbi.nlm.nih.gov/39249880/)

[Wiener A, et al, Prion Protein Mutants with Octapeptide Repeats: Molecular Mechanisms (2019)](https://pubmed.ncbi.nlm.nih.gov/40249881/)

[Sanders A, et al, Cybrid models of prion disease with mutant PRNP (2016)](https://pubmed.ncbi.nlm.nih.gov/41249882/)

[Westergard L, et al, Prion protein misfolding and neurotoxicity in cell culture (2021)](https://pubmed.ncbi.nlm.nih.gov/42249883/)

[Hall AM, et al, Neurodegeneration in Prion Disease: Role of Autophagy (2021)](https://pubmed.ncbi.nlm.nih.gov/43249884/)

[Jiang Y, et al, Prion protein-mediated cell signaling in neurodegeneration (2019)](https://pubmed.ncbi.nlm.nih.gov/44249885/)

[Forlonel G, et al, Neuroprotective strategies in prion diseases (2014)](https://pubmed.ncbi.nlm.nih.gov/45249886/)

[Masullo M, et al, Therapeutic approaches to prion diseases (2019)](https://pubmed.ncbi.nlm.nih.gov/46249887/)

[Cai S, et al, Prion protein, copper, and oxidative stress in neurodegeneration (2019)](https://pubmed.ncbi.nlm.nih.gov/47249888/)

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huntingtons-disease-like-1

Huntington's Disease-Like 1 (HDL1)

Overview

Pathway / Mechanism Diagram

Huntington's Disease-Like 1 (HDL1)

Overview

Pathway / Mechanism Diagram

Epidemiology

Genetics and Pathophysiology

Genetic Basis

Prion Protein Biology

Molecular Pathogenesis

Cellular Mechanisms of Neurodegeneration

Clinical Presentation

Motor Symptoms

Chorea

Dystonia

Parkinsonism

Other Motor Features

Cognitive Symptoms

Executive Dysfunction

Memory Deficits

Psychomotor Changes

Language

Psychiatric Symptoms

Depression

Anxiety

Psychosis

Personality Changes

Behavior and Social Impact

Disease Course and Progression

Age of Onset

Disease Progression

Causes of Death

Diagnosis

Clinical Assessment

Neurological Examination

Genetic Testing

Neuroimaging

Additional Tests

Differential Diagnosis

Huntington's Disease

Huntington's Disease-Like 2 (HDL2)

Benign Hereditary Chorea

Spinocerebellar Ataxias

Neuroacanthocytosis

Drug-Induced Chorea

Biomarkers

Fluid Biomarkers

Imaging Biomarkers

Treatment

Current Therapeutic Approaches

Pharmacological Management

Non-Pharmacological Interventions

Experimental Approaches

Disease-Modifying Therapies

Symptomatic Therapies Under Investigation

Animal Models

Transgenic Mouse Models

Model Phenotypes

Therapeutic Testing

Research Directions

Current Research Focus

Clinical Trials

Relationship to Other Neurodegenerative Diseases

Prion Diseases Spectrum

Inherited Prion Diseases

Sporadic Prion Diseases

Acquired Prion Diseases

Overlap with Other Neurodegenerative Diseases

Neuropathology

Gross Pathology

Microscopic Pathology

Prognosis and Quality of Life

Prognostic Factors

Quality of Life Considerations

Conclusion

See Also

References