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Juvenile Huntington's Disease
Overview
Juvenile Huntington's Disease (JHD), also called Huntington's disease with childhood onset, is a rare, rapidly progressive neurodegenerative disorder affecting individuals under 20 years of age. It accounts for approximately 1-5% of all Huntington's disease cases. JHD is caused by the same CAG trinucleotide repeat expansion in the [HTT](/proteins/huntingtin) gene as adult-onset HD, but presents with distinct clinical features, more severe progression, and is more frequently inherited from the paternal allele. [@storrie2007][@fischer2007]
The defining feature of JHD is an exceptionally long CAG repeat expansion, typically exceeding 60 repeats. The longer the repeat, the earlier the onset and the more likely the presentation will include parkinsonian features, cognitive decline, and seizures rather than the classic chorea seen in adult HD. [@roland2013]
Pathway / Mechanism Diagram
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Juvenile Huntington's Disease
Overview
Juvenile Huntington's Disease (JHD), also called Huntington's disease with childhood onset, is a rare, rapidly progressive neurodegenerative disorder affecting individuals under 20 years of age. It accounts for approximately 1-5% of all Huntington's disease cases. JHD is caused by the same CAG trinucleotide repeat expansion in the [HTT](/proteins/huntingtin) gene as adult-onset HD, but presents with distinct clinical features, more severe progression, and is more frequently inherited from the paternal allele. [@storrie2007][@fischer2007]
The defining feature of JHD is an exceptionally long CAG repeat expansion, typically exceeding 60 repeats. The longer the repeat, the earlier the onset and the more likely the presentation will include parkinsonian features, cognitive decline, and seizures rather than the classic chorea seen in adult HD. [@roland2013]
Pathway / Mechanism Diagram
Mermaid diagram (expand to render)
Genetics
| Property | Value | |----------|-------| | CAG Repeat Threshold | >60 repeats (classic JHD definition); sometimes >55 in broader definitions | | Parental Origin | ~80% paternal (vs ~50% in adult HD) | | Anticipation | Strong; longer repeats expand more during spermatogenesis | | Inheritance | Autosomal dominant | | Chromosome | 4p16.3 (HTT gene) | | Protein | Huntingtin (3,142 amino acids) |
The mechanism of paternal predominance in JHD relates to germline CAG repeat instability during spermatogenesis. Longer CAG tracts are more prone to expansion when transmitted through sperm, explaining why JHD disproportionately inherits from fathers. [@keogen2019]
Clinical Features
Motor Symptoms
JHD motor presentation differs markedly from adult HD:
Bradykinesia and rigidity: Hypokinetic-parkinsonian features dominate, especially with very long CAG repeats (>80)
Dystonia: Prominent early involvement, often affecting axial musculature
Chorea: May be absent or less prominent than in adults; when present, typically less severe
Seizures: Present in 30-50% of JHD cases (rare in adult HD), particularly with early onset
Myoclonus: Observed in some patients
Ataxia: Cerebellar signs can emerge as disease progresses [@nance2018]
Cognitive Symptoms
Rapid cognitive decline: More accelerated than in adult HD
Developmental regression: Loss of previously acquired milestones
Learning difficulties: Problems with working memory, executive function, attention