Congenital Hyaline Body Myopathy

Abstract

Hyaline body myopathy is a rare congenital myopathy characterized by subsarcolemmal eosinophilic formations called hyaline bodies and observed in type 1 fibers. The name myosin storage myopathy was based on the findings in hyaline bodies of large inclusions consisting of slow heavy chain myosin (MyHC 1). Causative mutations were identified in the MYH7 gene, encoding for beta cardiac myosin heavy chain 7 (Table 29.1). Furthermore, a scapuloperoneal muscular dystrophy with autosomal dominant inheritance was also recognized to be due to mutations in the MYH7 gene. The onset ranges between the first to the fifth decade, and the weakness is typically scapuloperoneal, involving neck flexion, shoulder abduction, knee flexion, and ankle dorsiflexion; the progression is slow. MYH7 mutations decreased the extent of self-assembly of the light meromyosin rod (less than 50–60 %) compared to the wild-type myosin protein. The assembly of skeletal muscle filaments is a multistep process that involves both the proper folding of alpha-helices into coiled coils and the assembly of these coiled coils, in proper register, into filaments. Defects in any one of these steps can result in improper filament formation leading to muscle weakness.