Huntington's Disease-Like 2 (HDL2), caused by a CTG/CAG expansion in JPH3 on chromosome 16q24, is the most common Huntington's Disease (HD) phenocopy in populations with African ancestry.
Neuroacanthocytosis (NA) syndromes are a group of rare diseases characterized by neurological disorders and misshaped spiky red blood cells (acanthocytes) including Chorea-Acanthocytosis (ChAc), McLeod syndrome (MLS), Huntington disease-like 2 (HDL 2), pantothenate kinase-associated neurodegeneration (PKAN), abeta- and hypobetalipoproteinemia and aceruloplasminemia.
Available data from cell and animal models and human brain suggest that HDL2 is a complex disease in which transcripts and proteins expressed bidirectionally from the junctophilin-3 locus contribute to pathogenesis through both gain-and loss-of-function mechanisms.
Recent evidence indicates that the HDL2 expansion may give rise to a toxic polyQ protein translated from an antisense mRNA derived from the JPH3 locus.
However, 15% (20/130) of black South African patients and no white patients (0/171) had an expansion in JPH3, confirming the diagnosis of Huntington disease like 2 (HDL2).
Molecular analyses reveal a promoter at the transgene locus driving the expression of a CAG repeat transcript (HDL2-CAG) from the strand antisense to JPH3, which encodes an expanded polyglutamine (polyQ) protein.
The aim of this study was to perform DNA analysis in patients with clinical diagnosis of Huntington's disease (HD) after molecular exclusion of HD and further molecular examinations for other neurodegenerative diseases such as Huntington's disease-like 2 (HDL-2; gene JPH3), dentatorubral pallidoluysian atrophy (DRPLA; gene ATN1) and spinocerebellar ataxia type 17 (SCA17; gene TBP).
The existence of a JPH3 splice variant with the CTG repeat in 3' untranslated region suggested that transcripts containing an expanded CUG repeat could play a role in the pathogenesis of HDL2, similar to the proposed pathogenic role of expanded CUG repeats in myotonic dystrophy type 1 (DM1).
Three patients from a previously described family with autosomal dominant chorea-acanthocytosis were found to have the CTG trinucleotide repeat expansion mutation of the junctophilin-3 gene associated with Huntington's disease-like 2 (HDL2).