Huntington disease-like 2 (HDL2) is a rare trinucleotide repeat expansion disorder that closely resembles Huntington disease. It is considered as one of the core neuroacanthocytosis syndromes (NAS).
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Epidemiology
HDL2 occurs at a median age of 41 years, range 12 to 66 years, and typically affects those of Southern African ethnicity 1. The highest reported number of affected individuals is 69, making HDL2 incredibly rare 1.
Clinical presentation
Presentation is similar to Huntington disease. Chorea and dystonia are present in the majority of cases 1,2. Bradykinesia, tremor and rigidity may also occur 2. Dementia and psychiatric disturbance, namely depression and aggression, are considered universal 1. Acanthocytosis is present in a small proportion patients affected by HDL2; due to the low prevalence of acanthocytosis, there is some debate as to whether HDL2 should be considered a neuroacanthocytosis syndrome 1,3.
Pathology
Etiology
A diagnosis of HLD2 is based on genetic profiling. Specifically, the absence of genetic alteration in the huntingtin gene and the presence of repeat expansions in the junctophilin 3 (JPH3) gene. JPH3 is a structural membrane protein that is highly expressed in the brain and has a role in cross-talk between ion channels and the cell surface 4. The exact pathogenesis of JPH3 alteration is unclear, but preliminary data suggest that loss of JPH3 causes a movement-disorder-like phenotype 5.
Radiographic features
Magnetic resonance imaging demonstrates bilateral atrophy of the caudate, putamen and globus pallidus 6. This is particularly apparent in the caudate nucleus, which is most frequently affected. HDL2 differs from the other neuroacanthocytosis syndromes due to the occurrence of generalized cortical atrophy 6.
Treatment and prognosis
Supportive treatment is at the center of patient care. HDL2 is often aggressive, rapidly progressing to death in 10-20 years 7.
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