The earliest pathologic alterations in dysferlinopathy

Background: Dysferlinopathies are associated with proximal or distal muscul ar dystrophy. Dysferlin immunolocalizes to the muscle fiber periphery but d oes not associate with the dystrophin-glycoprotein complex; its function in humans, and the mechanism by which it causes muscle fiber injury, are not known. The authors therefore searched for pathogenetic clues by examining e arly abnormalities in nonnecrotic muscle fibers in dysferlinopathy. Five dy sferlin-deficient patients were investigated. Weakness was distal in two, p roximal in one, and both proximal and distal in two. Patient 5 was only mil dly affected. Methods: Immunoblot analysis, membrane attack complex (MAC) i mmunolocalization, and quantitative electron microscopy. Results: In Patien ts 1 through 4, but not in 5, part or the entire surface of isolated nonnec rotic muscle fibers immunostained for MAC. Quantitative electron microscopy of 175 nonnecrotic muscle fibers revealed one or more of the following: 1) small (0.11 to 1.8 mum) plasmalemmal defects in 64% of fibers; 2) thickene d basal lamina over some defects; 3) replacement of the plasma membrane by one to multiple layers of small vesicles in 57% of fibers; 4) papillary pro jections, frequently disintegrating, in 24 to 36% of fibers in Patients 1 t hrough 4 but absent in fibers of Patient 5; 5) small subsarcolemmal vacuole s, some undergoing exocytosis, in 57% of fibers; and 6) infrequent subsarco lemmal regions containing rough endoplasmic reticulum and abundant fi ee ri bosomes. Conclusions: Dysferlin is likely required for maintaining the stru ctural integrity of the muscle fiber plasma membrane, and plasma membrane i njury is an early event in the pathogenesis of dysferlinopathy. MAC activat ion can participate in but is not an initial or primary event causing muscl e fiber injury.