Myoclonus dystonia and muscular dystrophy: ɛ‐sarcoglycan is part of the dystrophin‐associated protein complex in brain

BACKGROUND Myoclonus-dystonia is a neurogenic movement disorder caused by mutations in the gene encoding ɛ-sarcoglycan. By contrast, mutations in the α-, β-, γ-, and δ-sarcoglycan genes cause limb girdle muscular dystrophies. The sarcoglycans are part of the dystrophin-associated protein complex in muscle that is disrupted in several types of muscular dystrophy. Intriguingly, patients with myoclonus-dystonia have no muscle pathology; conversely, limb-girdle muscular dystrophy patients have not been reported to have dystonia-associated features. To gain further insight into the molecular mechanisms underlying these differences, we searched for evidence of a sarcoglycan complex in the brain. METHODS Immunoaffinity chromatography and mass spectrometry were used to purify ubiquitous and brain-specific ɛ-sarcoglycan directly from tissue. Cell models were used to determine the effect of mutations on the trafficking and assembly of the brain sarcoglycan complex. RESULTS Ubiquitous and brain-specific ɛ-sarcoglycan isoforms copurify with β-, δ-, and ζ-sarcoglycan, β-dystroglycan, and dystrophin Dp71 from brain. Incorporation of a muscular dystrophy-associated β-sarcoglycan mutant into the brain sarcoglycan complex impairs the formation of the βδ-sarcoglycan core but fails to abrogate the association and membrane trafficking of ɛ- and ζ-sarcoglycan. CONCLUSIONS ɛ-Sarcoglycan is part of the dystrophin-associated protein complex in brain. Partial preservation of ɛ- and ζ-sarcoglycan in brain may explain the absence of myoclonus dystonia-like features in muscular dystrophy patients. © 2016 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.