Disease Severity and Thin Filament Regulation in M9R TPM3 Nemaline Myopathy

• Published in: Journal of neuropathology and experimental neurology Vol. 67; no. 9; pp. 867 - 877
• Main Authors: Ilkovski, Biljana, Mokbel, Nancy, Lewis, Raymond A, Walker, Kendall, Nowak, Kristen J, Domazetovska, Ana, Laing, Nigel G, Fowler, Velia M, North, Kathryn N, Cooper, Sandra T
• Format: Journal Article
• Language: English
• Published: Hagerstown, MD American Association of Neuropathologists, Inc 01.09.2008; Lippincott Williams & Wilkins; Oxford University Press
• Subjects: Adult; Biological and medical sciences; Blotting, Western; Child, Preschool; Degenerative and inherited degenerative diseases of the nervous system. Leukodystrophies. Prion diseases; Female; Fetus; Humans; Immunohistochemistry; Infant; Infant, Newborn; Isoelectric Focusing; Medical sciences; Middle Aged; Muscle Fibers, Slow-Twitch - pathology; Muscle, Skeletal - pathology; Mutation; Myopathies, Nemaline - genetics; Myopathies, Nemaline - metabolism; Myopathies, Nemaline - pathology; Neurology; Protein Isoforms - genetics; Protein Isoforms - metabolism; Tropomodulin - metabolism; Tropomyosin - genetics; Tropomyosin - metabolism; Disease severity; Striated muscle disease; Nervous system diseases; Nemaline myopathy; Tropomyosin; Tropomodulin; Striated muscle; Congenital disease; Skeletal muscle; Genetic disease
• ISSN: 0022-3069; 1554-6578
• DOI: 10.1097/NEN.0b013e318183a44f

The mechanism of muscle weakness was investigated in an Australian family with an M9R mutation in TPM3 (α-tropomyosinslow). Detailed protein analyses of 5 muscle samples from 2 patients showed that nemaline bodies are restricted to atrophied Type 1 (slow) fibers in which the TPM3 gene is expressed. Developmental expression studies showed that α-tropomyosinslow is not expressed at significant levels until after birth, thereby likely explaining the childhood (rather than congenital) disease onset in TPM3 nemaline myopathy. Isoelectric focusing demonstrated that α-tropomyosinslow dimers, composed of equal ratios of wild-type and M9R-α-tropomyosinslow, are the dominant tropomyosin species in 3 separate muscle groups from an affected patient. These findings suggest that myopathy-related slow fiber predominance likely contributes to the severity of weakness in TPM3 nemaline myopathy because of increased proportions of fibers that express the mutant protein. Using recombinant proteins and far Western blot, we demonstrated a higher affinity of tropomodulin for α-tropomyosinslow compared with β-tropomyosin; the M9R substitution within α-tropomyosinslow greatly reduced this interaction. Finally, transfection of the M9R mutated and wild-type α-tropomyosinslow into myoblasts revealed reduced incorporation into stress fibers and disruption of the filamentous actin network by the mutant protein. Collectively, these results provide insights into the clinical features and pathogenesis of M9R-TPM3 nemaline myopathy.