Huntington Disease

Huntington Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by CAG trinucleotide repeat expansion in the HTT gene, resulting in a mutant [huntingtin protein](/proteins/huntingtin) (mHTT) with toxic gain-of-function. The disease manifests with progressive motor, cognitive, and psychiatric symptoms, typically onsetting in middle age. HD affects approximately 5-10 per 100,000 people worldwide, with higher prevalence in populations of European ancestry.

Overview

Huntington's disease is an autosomal dominant neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the HTT gene on chromosome 4p16.3[@the1993]. The mutation results in an expanded polyglutamine tract in the huntingtin protein, leading to toxic gain-of-function that disrupts neuronal function and survival. The disease is characterized by progressive motor, cognitive, and psychiatric disturbances, with an insidious onset typically in the third to fifth decade of life. Neuropathologically, HD is characterized by selective degeneration of striatal GABAergic medium spiny [neurons](/entities/neurons) and cortical pyramidal neurons, with prominent atrophy of the caudate nucleus and putamen[@vonsattel2011].

Epidemiology

Huntington Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by CAG trinucleotide repeat expansion in the HTT gene, resulting in a mutant [huntingtin protein](/proteins/huntingtin) (mHTT) with toxic gain-of-function. The disease manifests with progressive motor, cognitive, and psychiatric symptoms, typically onsetting in middle age. HD affects approximately 5-10 per 100,000 people worldwide, with higher prevalence in populations of European ancestry.

Overview

Huntington's disease is an autosomal dominant neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the HTT gene on chromosome 4p16.3[@the1993]. The mutation results in an expanded polyglutamine tract in the huntingtin protein, leading to toxic gain-of-function that disrupts neuronal function and survival. The disease is characterized by progressive motor, cognitive, and psychiatric disturbances, with an insidious onset typically in the third to fifth decade of life. Neuropathologically, HD is characterized by selective degeneration of striatal GABAergic medium spiny [neurons](/entities/neurons) and cortical pyramidal neurons, with prominent atrophy of the caudate nucleus and putamen[@vonsattel2011].

Epidemiology

• Prevalence: Approximately 5-10 per 100,000 people worldwide
• Population: Higher prevalence in populations of European ancestry (5-10/100,000) compared to Asian populations (0.5-1/100,000)
• Age of Onset: Typically 30-50 years, but can present at any age
• Sex: Equal prevalence in males and females
• Disease Duration: 15-20 years from onset to death

Genetics

Causative Gene

• HTT (Huntingtin): Located on chromosome 4p16.3; CAG repeat expansion causes HD[@the1993]
• Normal: < 26 CAG repeats
• Intermediate (reduced penetrance): 27-35 repeats
• Full penetrance: ≥ 36 repeats
• Juvenile onset (Westphal variant): > 60 repeats, usually paternal inheritance

Genetic Modifiers

• CAG Repeat Length: Strong inverse correlation between repeat length and age of onset[@gusella2023]
• DNA Repair Genes: MSH3, FAN1, and other DNA repair pathway genes modify age of onset[@hong2023]
• HTT haplotypes: Different haplotypes affect disease progression

Pathophysiology

Mutant Huntingtin Toxicity

The mutant huntingtin protein (mHTT) acquires toxic functions through:[@ross2014]

• Transcriptional dysregulation: mHTT interacts with transcription factors (REST, NCoR, SP1), disrupting gene expression
• Protein aggregation: mHTT forms insoluble aggregates in neurons
• Loss of normal function: Wild-type huntingtin is essential for neuronal health

Striatal Degeneration

The striatum (caudate nucleus and putamen) is most vulnerable in HD:[@vonsattel2011]

• Medium spiny neurons (MSNs): Most severely affected GABAergic projection neurons
• Loss of dopaminergic signaling: Disruption of direct and indirect pathways
• Early neurochemical changes: Reduced GABA, substance P, and enkephalin

Cortical Involvement

• Cortical thinning: Progressive loss of cortical volume
• Pyramidal neuron dysfunction: Disruption of corticostriatal connections
• Executive dysfunction: Correlates with prefrontal cortical pathology

Molecular Mechanisms

• Mitochondrial dysfunction: Impaired energy metabolism, increased oxidative stress[@huntington2001]
• Neuroinflammation: Microglial activation, elevated cytokines
• Synaptic dysfunction: Loss of synaptic markers, impaired neurotransmission
• [Autophagy](/entities/autophagy) impairment: Defective clearance of mHTT aggregates

Clinical Features

Motor Symptoms

• Chorea: Involuntary dance-like movements (writhing, fidgeting, grimacing)
• Dystonia: Involuntary muscle contractions
• Bradykinesia: Slowed movements (especially in juvenile HD)
• Parkinsonism: Rigidity, postural instability
• Impaired coordination: Ataxia, dysarthria, dysphagia
• Eye movement abnormalities: Slow saccades, gaze palsy

Cognitive Symptoms

• Executive dysfunction: Impaired planning, flexibility, working memory
• Memory deficits: Particularly for procedural learning
• Psychomotor slowing: Reduced information processing speed
• Language problems: Word-finding difficulties, reduced verbal fluency

Psychiatric Symptoms

• Depression: Most common psychiatric manifestation
• Irritability and aggression: Can be challenging to manage
• Apathy: Loss of motivation and initiative
• Psychosis: Hallucinations and delusions (less common)
• Obsessive-compulsive symptoms: Repetitive behaviors

Diagnosis

Clinical Diagnosis

• Unified Huntington's Disease Rating Scale (UHDRS): Standardized assessment[@huntington1996]
• Motor examination: Quantifies chorea, dystonia, bradykinesia
• Cognitive testing: Executive function, memory, language
• Psychiatric evaluation: Depression, irritability, psychosis

Genetic Testing

• CAG repeat analysis: Confirms diagnosis, predicts onset[@the1993]
• Predictive testing: Available for at-risk individuals
• Prenatal testing: For families with known mutation

Neuroimaging

• MRI/CT: Striatal atrophy, ventricular enlargement
• PET: Reduced glucose metabolism in striatum
• Diffusion tensor imaging: White matter tract changes

Treatment

Symptomatic Treatment

Motor Symptoms (Chorea)

• Tetrabenazine: First FDA-approved VMAT2 inhibitor[@huntington2006]
• Deutetrabenazine: Improved tolerability, FDA-approved for HD[@huntington2017]
• Valbenazine: Once-daily dosing, under investigation[@factor2020]

Psychiatric Symptoms

• Depression: SSRIs (citalopram, sertraline, escitalopram)
• Irritability: Mood stabilizers (valproate, lamotrigine), atypical antipsychotics
• Psychosis: Atypical antipsychotics (risperidone, olanzapine, quetiapine)

Disease-Modifying Therapies

Gene Silencing

• Antisense oligonucleotides (ASOs): Tominersen (discontinued), WVE-003 (ongoing)[@tabrizi2019]
• RNA interference: AAV-delivered microRNA approaches in development

Neuroprotective Strategies

• CoQ10: Mixed results in clinical trials[@huntington2001]
• Creatine: Shown safe, potential benefit in early disease
• [HDAC](/entities/hdac-enzymes) inhibitors: Preclinical promise, clinical translation ongoing

Non-Pharmacological

• Physical therapy: Maintain mobility, reduce fall risk
• Speech therapy: Address dysarthria and swallowing difficulties
• Occupational therapy: Home modifications, adaptive equipment
• Nutritional support: Prevent weight loss, maintain caloric intake

Current Research

Clinical Trials

• SELECT-HD (Wave Life Sciences): WVE-003 SNP3-selective ASO[@tabrizi2019]
• Gene editing approaches: CRISPR/Cas9, base editing in preclinical development
• Immunotherapy: Anti-mHTT antibodies in development

Biomarkers

• CSF [neurofilament light](/biomarkers/neurofilament-light-chain-nfl) chain (NfL): Marker of neurodegeneration[@weir2024]
• MRI volumetric measures: Striatal and cortical atrophy rates
• Digital health measures: Wearable sensors for motor assessment

Emerging Targets

• HTT lowering: ASOs, RNAi, gene editing
• Protein aggregation inhibitors: Small molecule modulators
• Cell replacement therapy: iPSC-derived striatal neurons[@zhang2017]

Open Questions

See Also

• [Mitochondrial Dysfunction in Neurodegenerative Diseases Comparison](/diseases/mitochondrial-dysfunction-neurodegeneration-comparison)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Amyotrophic Lateral Sclerosis](/diseases/amyotrophic-lateral-sclerosis)
• [Frontotemporal Dementia](/diseases/frontotemporal-dementia)
• [Huntington's Disease Treatment](/therapeutics/huntington-disease-treatment)

External Links

• [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
• [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)
• [Huntington's Disease Society of America](https://hdsa.org/)
• [CHDI Foundation](https://chdifoundation.org/)

References