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Huntington's Disease (HD): Genetics, Symptoms, and Management

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the HTT gene (4p16.3). Onset typically 30–50 years; juvenile (<20) and late-onset (>60) forms exist.

Genetics & Pathogenesis

  • Normal: ≤26 CAG repeats.
  • Intermediate (27–35): Not affected; may expand in offspring.
  • Reduced penetrance (36–39): May or may not develop symptoms.
  • Full penetrance (≥40): Will develop HD; longer repeats → earlier onset, faster progression.
  • Anticipation: Paternal transmission → repeat expansion → earlier onset in successive generations.
  • Mutant huntingtin protein (mHTT): Toxic gain-of-function → transcriptional dysregulation, mitochondrial dysfunction, excitotoxicity, impaired axonal transport, proteostasis failure → striatal (caudate/putamen) then cortical neurodegeneration.

Clinical Triad

DomainManifestations
MotorChorea (involuntary dance-like movements), dystonia, rigidity, bradykinesia, gait impairment, dysarthria, dysphagia, oculomotor apraxia
CognitiveExecutive dysfunction (planning, flexibility), slowed processing, memory retrieval deficit, impaired insight, eventual dementia
PsychiatricDepression (most common, may precede motor), anxiety, irritability, apathy, OCD, psychosis, suicide risk ↑

Diagnosis

  • Clinical: Motor + cognitive/psychiatric + family history or genetic confirmation.
  • Genetic testing: CAG repeat sizing (predictive testing with counseling protocol).
  • Neuroimaging: Caudate atrophy (early), whole-brain atrophy (late).

Management (Symptomatic)

  • Chorea: VMAT2 inhibitors (deutetrabenazine, valbenazine); antipsychotics (risperidone, olanzapine) if chorea + psychosis.
  • Depression/Anxiety: SSRIs, SNRIs, mirtazapine.
  • Psychosis: Atypical antipsychotics (quetiapine, aripiprazole).
  • Rehabilitation: PT/OT/SLT for gait, ADLs, communication, swallowing.
  • Multidisciplinary clinic: Neurology, psychiatry, genetics, social work, palliative care.

Prognosis & Research

  • Median survival: 15–20 years post-motor onset.
  • Disease-modifying trials: HTT-lowering (ASOs, RNAi), selective mHTT degradation, CRISPR, neuroprotection.
  • Genetic counseling essential for at-risk relatives (predictive testing is a personal choice with profound implications).

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