Mutant torsinA, which causes early‐onset primary torsion dystonia, is redistributed to membranous structures enriched in vesicular monoamine transporter in cultured human SH‐SY5Y cells

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Abstract
A single GAG deletion in the DYT1 gene causes primary early‐onset, generalized torsion dystonia. The DYT1 protein product, torsinA, belongs to the AAA+ family of proteins. When overexpressed, wild‐type torsinA localizes mainly to the endoplasmic reticulum, whereas the mutant forms inclusions of unclear biogenetic origin. In this study, overexpressed wild‐type torsinA in human neuroblastoma (SH‐SY5Y) cell lines was distributed throughout the cell body and colocalized with a marker for the endoplasmic reticulum, confirming it is an endoplasmic reticulum protein. However, mutant torsinA showed perinuclear staining and formed distinct globular inclusions, which did not colocalize with endoplasmic reticulum markers. Immunoelectron microscopy of the mutant torsinA inclusions revealed membrane whorls staining for torsinA, as well as labeling of lamellae, isolated bilayers, and perinuclear membranes. This finding shows that mutant torsinA redistributes to specific membranous structures, which may represent different stages of maturation of the intracellular inclusions. The mutant torsinA‐containing bodies were immunoreactive for vesicular monoamine transporter 2 (VMAT2). VMAT2 expression is important for the exocytosis of bioactive monoamines in neurons. Abnormal processing, transport, or entrapment of VMAT2 within the mutant torsinA membranous inclusions, therefore, may affect cellular dopamine release, providing a potential pathogenic mechanism for the DYT1‐dependent dystonia. © 2004 Movement Disorder Society