Huntington's Disease

🧠 Standard of Care & Symptoms

Huntington's Disease (HD) is an autosomal dominant, progressive, neurodegenerative disorder characterized by a triad of motor, cognitive, and psychiatric symptoms.

• Presentation: Key clinical features.
  • Chorea: The hallmark motor sign, presenting as involuntary, brief, random, jerky movements that flow from one muscle group to another, affecting the limbs, face, and trunk.
  • *Cognitive Decline:* Progressive executive dysfunction (impaired planning, organizing, and cognitive flexibility) that develops into subcortical dementia, with memory recall deficits appearing later in the course.
  • *Psychiatric Disturbance:* Depression, irritability, and anxiety are highly prevalent. Obsessive-compulsive behaviors, apathy, and personality changes often precede the onset of motor symptoms by years.
  • *Motor Impairment & Rigidity:* As the disease progresses, chorea is replaced by dystonia (sustained muscle contractions), bradykinesia (slow movements), dysarthria (slurred speech), and dysphagia (difficulty swallowing).

🧬 Diagnostics & CAG Repeat Expansion

Diagnosis is confirmed through genetic testing showing an expanded CAG trinucleotide repeat in the *HTT* gene. Neuroimaging (MRI or CT) shows selective atrophy of the caudate nucleus and putamen.

• Genetic Analysis: CAG repeat sizing.
  • Normal Alleles: 10 to 35 CAG repeats. Individuals do not develop the disease.
  • Intermediate Alleles: 36 to 39 CAG repeats. Associated with reduced penetrance; individuals may or may not develop symptoms during their lifespan.
  • Fully Penetrant Alleles: 40 or more CAG repeats. Associated with complete penetrance; individuals will develop the disease if they live a normal lifespan. Larger repeat numbers are linked to earlier onset.

Pathogenesis & Mutant Huntingtin Excitotoxicity

The degeneration of striatal GABAergic medium spiny neurons is driven by the toxic gain-of-function of the mutant huntingtin protein:

• Polyglutamine (PolyQ) Expansion: The **CAG repeat expansion** on chromosome 4p16.3 translates into an abnormally long polyglutamine tract at the N-terminus of the huntingtin (HTT) protein.
• Protein Misfolding & Aggregation: The expanded polyQ tract causes the mutant huntingtin (mHTT) protein to misfold. It forms intranuclear and cytoplasmic aggregates that disrupt normal cellular function.
• Striatal Neuronal Loss: mHTT aggregates impair mitochondrial function, disrupt axonal transport, and inhibit transcription factors (like BDNF expression). This leads to excitotoxicity and apoptosis of medium spiny GABAergic neurons in the **caudate and putamen (striatum)**, leading to loss of motor inhibition.

💊 VMAT2 Inhibitors & Symptom Management

Therapy is supportive and symptomatic, focusing on controlling chorea and managing psychiatric agitation.

Chorea Management

• VMAT2 Inhibitors: **Deutetrabenazine** and **Tetrabenazine** are the first-line, FDA-approved medications for moderate-to-severe chorea. They block Vesicular Monoamine Transporter 2 (VMAT2), depleting presynaptic dopamine to reduce hyperkinetic movements. Valbenazine is also used.
• Neuroleptics (Atypical Antipsychotics): Atypical neuroleptics such as **Olanzapine**, Risperidone, or Aripiprazole serve as second-line agents. They block postsynaptic dopamine receptors, managing both chorea and psychiatric agitation.

Psychiatric & Multidisciplinary Care

• Antidepressants: SSRIs (e.g., Sertraline, Escitalopram) are standard of care for depression, obsessive-compulsive symptoms, and irritability.
• Supportive Services: Speech therapy is critical to manage dysphagia and reduce aspiration risk. Physical and occupational therapy assist with mobility and fall prevention.

🔬 Active Clinical Trials

Clinical trials are currently evaluating huntingtin-lowering antisense oligonucleotides (ASOs), gene silencing microRNAs, and small molecules targeting somatic expansion.

🗺️ Next Steps After Diagnosis

If you or a family member has recently been diagnosed with Huntington's Disease, establish these clinical care pathways:

1. Confirm CAG Repeat Size: Verify your genetic test results with a board-certified genetic counselor to understand the repeat length.
2. Consult a Movement Disorder Specialist: Establish care with a neurologist specializing in movement disorders to manage chorea.
3. Initiate Symptomatic Therapies: Discuss Deutetrabenazine or Tetrabenazine if chorea interferes with daily activities.
4. Build a Multidisciplinary Support Team: Engage physical, occupational, and speech therapists early to preserve function.

❓ Patient FAQ

Q: What is genetic counseling, and why is it required before testing?
A: Genetic counseling is a process of education and psychological support. Because Huntington's Disease is an autosomal dominant condition with no current cure, learning one's genetic status has profound emotional, social, and financial implications. Counseling ensures patients fully understand the implications of the test for themselves and their children before making a decision.

Q: Can the CAG repeat size change when passed to children?
A: Yes. During the formation of sperm or egg cells (gametes), the CAG repeat sequence can expand or contract. This instability is more pronounced during paternal transmission (when inherited from the father), a phenomenon known as **anticipation**, which can lead to an earlier onset of symptoms in the offspring.