The majority of myoclonus dystonia cases are the result of a mutation in the epsilon sarcoglycan gene (SGCE). This gene is found on chromosome 7, with its specific cytogenic location being 7q21.3. The 70,985 bp SGCE gene encodes the protein epsilon (ε)-sarcoglycan. The five proteins that make up the sarcoglycan family function as integral membrane proteins that anchor the cytoskeleton of cells to the extracellular matrix. Epsilon sarcoglycan is a membrane protein that can be found in the liver, lungs, kidney, and spleen, but is most prevalent in muscle and neuronal cells. Its prevalence in both muscle fibers and the synapses of neurons suggest why symptoms of both myoclonus and dystonia appear from the improperly functioning protein. Recessive mutations in the other sarcoglycans also result in muscular disorders, further supporting that mutations in the SGCE gene cause myoclonus dystonia. Epsilon sarcoglycan itself is part of the dystrophin-associated protein (DAP) complex that binds the sarcolemma of muscle cells to the extracellular connective tissue. The purpose is to reduce the mechanical force on the sarcolemma as a result of muscle contraction. In addition to myoclonus dystonia, problems associated with a dysfunctional DAP complex include Duchenne muscular dystrophy. Upwards of 65 mutations of the SGCE gene are thought to cause myoclonus dystonia. The majority of the mutations lead to a truncated protein product that results in the loss-of-function of the epsilon sarcoglycan protein. The dysfunctional protein is ultimately recycled by the cell by degradation mediated by the proteasome, resulting in significant shortages of the integral membrane protein in both neurons and muscle fibers. The mutant allele is inherited in a dominant fashion—that is the mutation can be inherited if one parent has that allele. However, genomic imprinting occurs on the mother's allele, so only the father's allele is expressed. Therefore, inheriting a mutated, paternal allele of the SGCE gene will result in the expression of the dysfunctional epsilon sarcoglycan protein. Offspring will not produce a mutant protein product in 95% of cases where the mother passes on a mutation in the SGCE gene. While SGCE gene mutations are the central cause of myoclonus dystonia, there have been separate cases where individuals and families present symptoms akin to myoclonus dystonia but lack the mutations at this locus. Base-pair deletions of the DYT1 gene, missense mutations in the DRD2 gene, maternal uniparental disomy, and chromosome 18 linkage have all been associated in rare cases myoclonus dystonia where the SGCE gene is unaffected.
Myoclonic dystonia
Focal dystonia
Torsion dystonia
Holmes tremor
Dopamine-responsive dystonia
Pseudoathetosis
Tardive dyskinesia
Paroxysmal kinesigenic choreoathetosis