Idiopathic inflammatory myopathies: inclusion-body myositis, polymyositis, and dermatomyositis.

In this review, the main emphasis is on new advances concerning sporadic inclusion-body myositis and hereditary inclusion-body myopathy. Polymyositis and dermatomyositis are reviewed briefly. Hypotheses are presented regarding the possible cause and significance of abnormally accumulated beta-amyloid protein, two other epitopes of beta-amyloid precursor protein, hyperphosphorylated tau, alpha 1-antichymotrypsin, ubiquitin, and prion protein in sporadic inclusion-body myositis and hereditary inclusion-body myopathy. Because most of those proteins are also accumulated at the neuromuscular junction, "junctionalization" of other muscle fiber nuclei is a possibility. Attention is given to the fact that vacuolated muscle fibers in hereditary inclusion-body myopathy may represent early changes because they are virtually free of congophilic amyloid deposit but, like sporadic inclusion-body myositis, contain large accumulations of beta-amyloid protein and prion.