Inclusion body myositis: Abnormal protein accumulation does not trigger apoptosis

To examine whether apoptosis may contribute to muscle fiber loss in inclusi on body myositis, we used the terminal deoxynucleotidyl transferase-mediate d X-dUTP nick-end labeling (TUNEL) assay to compare the occurrence of DNA f ragmentation in muscle samples from patients with inclusion body myositis a nd polymyositis. TUNEL-positive nuclei in nonnecrotic muscle fibers were ra re even in the vicinity of amyloid-like material; significantly more freque nt in polymyositis than inclusion body myositis; and several times less fre quent than necrotic muscle fibers or mononuclear cell myocytotoxicity in bo th patient groups. Apoptosis is unlikely to play a significant role in the pathogenesis of inclusion body myositis.