Huntington's Disease Overview
Huntington's disease is an autosomal-dominant neurodegenerative disorder caused by CAG trinucleotide repeat expansion in the HTT gene on chromosome 4p16.3, characterized by progressive choreic movements, cognitive decline, and psychiatric symptoms, with no current disease-modifying treatment available. 1
Genetic Basis and Inheritance
The disease results from expansion of a polymorphic CAG repeat in exon 1 of the huntingtin (HTT) gene. The genetic architecture follows clear patterns 1:
- ≥40 CAG repeats: Fully penetrant disease (100% specific for diagnosis)
- 36-39 CAG repeats: Variable penetrance - affected and unaffected individuals reported
- 27-35 CAG repeats: Rare, not convincingly associated with HD phenotype but potentially mutable
- ≤26 CAG repeats: Never associated with HD phenotype
The expanded CAG repeat translates into an abnormally long polyglutamine tract in the huntingtin protein, which causes neuronal dysfunction through multiple mechanisms including disruption of proteostasis, transcriptional dysregulation, mitochondrial dysfunction, and direct protein toxicity 2.
Clinical Presentation
Classic Adult-Onset Form
The typical presentation occurs in mid-life and includes 1:
- Motor symptoms: Choreic movements (involuntary, dance-like movements) are the hallmark
- Cognitive decline: Progressive impairment affecting executive function, memory, and processing speed
- Psychiatric manifestations: Depression, anxiety, irritability, and behavioral changes often precede motor symptoms
The disease progresses over 15-20 years with selective neuronal loss primarily in the caudate nucleus and putamen 3.
Juvenile-Onset Variant
Approximately 5% of patients present before age 20 with a distinct phenotype 1:
- Rigidity and spasticity (rather than chorea)
- Rapid progression
- Intellectual decline
- Typically associated with larger CAG repeat expansions, often inherited paternally
Epidemiology
The prevalence is approximately 3-5 per 100,000 individuals globally, though this exceeds 15 per 100,000 in populations of western European origin 1. Recent data suggest increasing prevalence in some regions due to improved diagnostic capabilities and expanded genetic testing 4.
Pathophysiology
The mutant huntingtin protein causes widespread cellular dysfunction through 5, 2:
- Excitotoxicity and dopaminergic imbalance
- Mitochondrial dysfunction and metabolic defects
- Impaired autophagy and lysosomal function
- Transcriptional dysregulation
- Neuroinflammation
- Oxidative stress
- Disrupted axonal transport
Macroscopic changes begin in the striatum with cortical involvement as disease progresses 2.
Diagnosis
Diagnosis requires clinical features consistent with HD confirmed by genetic testing demonstrating CAG repeat expansion ≥36 in the HTT gene 6.
Key Diagnostic Considerations:
- Genetic testing is definitive when CAG repeats ≥40 (100% specific) 1
- Family history is typically positive given autosomal dominant inheritance
- Predictive testing in asymptomatic at-risk individuals requires specialized protocols with genetic counseling 7
- Differential diagnosis must be considered with negative genetic testing, as HD-phenocopies exist 6
Important Caveats:
The 36-39 CAG repeat range presents diagnostic uncertainty - individuals may or may not develop clinical disease, making specificity indeterminate in this range 1. Careful genetic counseling is essential for these patients.
Management
Currently, no disease-modifying therapies are clinically approved; management is entirely symptomatic and supportive 1.
Symptomatic Treatment Approach:
- Chorea management: Tetrabenazine or deutetrabenazine (though trial evidence is limited) 6
- Psychiatric symptoms: Antidepressants, antipsychotics, mood stabilizers as clinically indicated
- Cognitive support: Environmental modifications, occupational therapy
- Multidisciplinary care: Neurology, psychiatry, genetic counseling, physical therapy, occupational therapy, speech therapy, nutrition 6, 3
Emerging Therapies:
Huntingtin-lowering strategies, particularly antisense oligonucleotide therapy, represent the most promising disease-modifying approach currently in clinical trials 2. These therapies aim to reduce levels of mutant huntingtin protein before irreversible neuronal loss occurs.
Clinical Pitfalls
- Underrecognition of nonmotor symptoms: Psychiatric and cognitive symptoms frequently precede motor manifestations and may be the primary source of functional impairment, yet are often overlooked when focusing solely on chorea 8
- Delayed diagnosis: The variable presentation and potential for psychiatric symptoms to dominate early disease can lead to misdiagnosis
- Genetic counseling complexity: The 36-39 repeat range requires nuanced counseling about uncertain penetrance
- Anticipation phenomenon: CAG repeats can expand when transmitted, particularly through paternal inheritance, leading to earlier onset in subsequent generations