Step 1 topics → Neurology and Special Senses → Huntington Disease

Huntington Disease

USMLE Step 1 trap: Mischaracterizes the Huntington mutation mechanism as loss-of-function rather than toxic gain-of-function. The CAG repeat in huntingtin causes a toxic gain-of-function via the expanded polyglutamine tract, not a simple loss of function.

Huntington disease is a relentlessly progressive autosomal dominant neurodegenerative disorder, and USMLE Step 1 tests it from multiple angles: pure genetics (inheritance pattern, repeat threshold, anticipation), pathology (which neurons die, what the brain looks like grossly), and clinical presentation (the triad of chorea, dementia, and psychiatric disturbance). The core mechanism is a CAG trinucleotide repeat expansion in the HTT gene on chromosome 4 that encodes a toxic polyglutamine tract in the huntingtin protein, selectively destroying GABAergic medium spiny neurons in the striatum via toxic gain-of-function. Expect to see it framed as a family history vignette where you need to predict inheritance risk or explain why an affected child has an earlier onset than their parent.

The tricky part is that Huntington sits at the intersection of several high-yield concepts, and each one has a classic wrong answer attached to it. Students frequently confuse the mechanism — assuming it's loss-of-function like most other genetic diseases — when it's actually toxic gain-of-function. They also confuse the anatomical site with Parkinson disease, conflating the caudate/striatal degeneration of Huntington with the substantia nigra dopaminergic loss of Parkinson. Both cause movement disorders, but the neuron types, locations, and movement phenomenology are completely different. USMLE Step 1 will absolutely exploit this confusion.

Anticipation adds another layer. Yes, Huntington demonstrates anticipation — offspring have earlier onset and more severe disease. But the exam may test whether you know the paternal bias: repeat expansion is more likely during spermatogenesis than oogenesis, so paternal transmission is associated with more pronounced anticipation. Most students learn anticipation as a generic concept and miss this directional asymmetry entirely.

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Common misconceptions

Common mistake
Wrong: Huntington disease involves a CAG repeat expansion on chromosome 4 encoding a polyglutamine tract in huntingtin that causes loss of function.
Right: The CAG repeat in huntingtin causes a toxic gain-of-function via the expanded polyglutamine tract, not a simple loss of function.
The CAG repeat expansion encodes a long polyglutamine tract that makes the huntingtin protein toxic — it misfolds, aggregates, and disrupts cellular function. This is a gain-of-function mutation: the mutant protein actively causes harm rather than simply failing to do its normal job. Loss-of-function would mean the protein is absent or nonfunctional and the cell suffers from that absence; here, the cell is being poisoned by the presence of the abnormal protein. This distinction matters because it explains why heterozygotes are affected — one toxic allele is enough.
Common mistake
Wrong: Huntington disease primarily destroys dopaminergic neurons in the substantia nigra.
Right: Huntington disease causes selective loss of GABAergic medium spiny neurons in the caudate nucleus and putamen (striatum), leading to caudate atrophy and ex vacuo hydrocephalus.
Huntington and Parkinson both cause movement disorders, but they kill completely different neurons in completely different places. Huntington destroys GABAergic medium spiny neurons in the caudate and putamen (striatum), which is why the gross brain shows caudate atrophy and the ventricles enlarge to fill the space (ex vacuo hydrocephalus). Parkinson destroys dopaminergic neurons in the substantia nigra pars compacta. The movements also differ: Huntington produces chorea (hyperkinetic, irregular, flowing), while Parkinson produces bradykinesia and resting tremor (hypokinetic).
Common mistake
Wrong: Anticipation in Huntington disease occurs equally with maternal and paternal transmission.
Right: Anticipation in Huntington disease is more pronounced with paternal transmission because repeat expansion is more likely during spermatogenesis.
Anticipation in trinucleotide repeat diseases happens because the repeat is unstable during DNA replication and tends to expand — but this expansion is not equally likely in all cell divisions. In Huntington, the repeat is more prone to expansion during spermatogenesis (which involves many more cell divisions) than during oogenesis. Practically, this means a father with Huntington is more likely to pass on a longer repeat to his children than a mother with the same repeat length, producing earlier onset in the offspring. This paternal bias is a specific testable fact that goes beyond just knowing 'anticipation exists.'
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What the exam tests

  1. Know the genetics: Huntington is autosomal dominant with full penetrance, caused by a CAG repeat expansion (>36 repeats) on chromosome 4 in the HTT gene — and understand why this produces anticipation (unstable repeat expands with each generation) and why anticipation is stronger with paternal inheritance.
  2. Know the pathology: Huntington selectively destroys GABAergic medium spiny neurons in the caudate nucleus and putamen (striatum), not dopaminergic neurons in the substantia nigra. Gross brain shows caudate atrophy and compensatory ex vacuo dilation of the lateral ventricles.
  3. Know the clinical triad and course: Huntington presents in mid-adulthood (onset ~30–50s) with chorea (involuntary dance-like movements), progressive dementia, and psychiatric symptoms (depression, personality change, psychosis). It is uniformly fatal, typically within 15–20 years of onset.

Can you avoid these mistakes?

A 45-year-old man is diagnosed with Huntington disease. His father had onset at age 50. What molecular mechanism explains why the son has earlier onset, and why is paternal transmission specifically relevant here?
On autopsy, a patient with a 20-year history of progressive chorea and dementia shows striking atrophy of a specific brain structure with compensatory ventricular enlargement. Which structure is atrophied, which neurons are lost, and what neurotransmitter did they use?
A classmate says Huntington disease is similar to other genetic diseases because the mutant protein simply doesn't work properly (loss-of-function). How would you correct this, and what is the actual mechanism by which the expanded polyglutamine tract causes disease?
A patient presents with involuntary writhing movements, personality change, and cognitive decline starting in his 40s. His mother had similar symptoms. What is the inheritance pattern, the causative gene location, and the minimum repeat length associated with disease?

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