Short Communication Light and Electron Microscopic Localization of ,B-amyloid Protein in Muscle Biopsies of Patients with Inclusion-body Myositis

In 11 of 11 inclusion-body myositis (IBM) patients, including one hereditary case, vacuolated muscle fibers contained large and multiple small inclusions immunoreactive for 0-amyloid protein (PAP). All IBM muscle biopsies had characteristic cytoplasmic tubulo-filaments (CTFs) by electron microscopy. None of 14 control muscle biopsies contained the PAP immunoreactive (IR) inclusions characteristic of IBM. On the light microscopy level rAP-IR inclusions colocalized with ubiquitin immunoreactivity. By immunogold electronmicroscopy, f3AP immunoreactivity was localized to a) amorphous, poorly defined structures, b) dense floccular material c) clusters of loosely packed amyloidlike fibrils 6-8 nm in diameter, and d) poorly defined loose fibrillar structures 6-8 nm in diameter. PAP immunoreactive structures were often in proximity to CTFs, but CTFs themselves never contained f3AP-IR Our study pro-vides the first demonstration of f3AP accumulations in abnormal human muscle. This finding suggests that in addition to Alzbeimer's disease, Down syndrome, and Dutch-type hereditary cerebrovascular amyloidosis, P3AP may play an important role in the pathogenesis of other diseases, including ones outside the central nervous system, for example, IBM. (Am JPathol 1992, 141:31-36) arms, male predominance, and often a poor or no to immunosuppression