Inclusion body myositis: abnormal protein accumulation does not trigger apoptosis.

To examine whether apoptosis may contribute to muscle fiber loss in inclusion body myositis, we used the terminal deoxynucleotidyl transferase-mediated X-dUTP nick-end labeling (TUNEL) assay to compare the occurrence of DNA fragmentation in muscle samples from patients with inclusion body myositis and polymyositis. TUNEL-positive nuclei in nonnecrotic muscle fibers were rare even in the vicinity of amyloid-like material; significantly more frequent in polymyositis than inclusion body myositis; and several times less frequent than necrotic muscle fibers or mononuclear cell myocytotoxicity in both patient groups. Apoptosis is unlikely to play a significant role in the pathogenesis of inclusion body myositis.