Pleomorphic mitochondrial and different filamentous inclusions in inflammatory myopathies associated with mtDNA deletions

Mitochondrial changes are frequently observed in muscle fibers of patients with inclusion body myositis (IBM) and polymyositis (PM), suggesting that mitochondrial function may be especially impaired in these forms of inflammatory myopathies. Intranuclear and cytoplasmic tubulofilamentous inclusions are characteristic, although not totally specific for IBM. In the present cases, the inclusions were strikingly pleomorphic when chloroquine had been given for long periods. The nuclear inclusions were always tubular, whereas the cytoplasmic filaments had either a tubular, a helical, or a cross-striated structure with different diameters and arrangements in association with myelin-like figures, and vacuoles. Abnormal mitochondria containing paracrystalline, globoid, and other inclusions, noted in IBM, were occasionally also seen in PM or vasculitis. By contrast, in the latter, no intranuclear or cytoplasmic tubulofilamentous inclusions were apparent in muscle fibers. This study reports for the first time the presence of membrane-bound crystalloid inclusions in a muscle fiber with numerous abnormal mitochondria; similar structures have thus far only been observed in macrophages. The identity and function of these inclusions remains unknown. Using PCR analysis we detected different mtDNA deletions not only in IBM, but also in PM and vasculitis, indicating at least some degree of association between the structural mitochondrial abnormalities and the mtDNA mutations. There was no topographical correlation between the presence of tubular or helical filaments and the mitochondrial abnormalities. As already noted by others, the mitochondrial changes in IBM were more frequent than expected in this age group. It is suggested that the presence of the mtDNA deletions in IBM and PM are not primary, but rather the result of the underlying, presumably immunological disorder causing nuclear and secondary or simultaneous mitochondrial changes.