title: Disease severity and thin filament regulation in M9R TPM3 nemaline myopathy.
abstract: The mechanism of muscle weakness was investigated in an Australian family with an M9R mutation in TPM3 (alpha-tropomyosin(slow)). 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 alpha-tropomyosin(slow) 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 alpha-tropomyosin(slow) dimers, composed of equal ratios of wild-type and M9R-alpha-tropomyosin(slow), 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 alpha-tropomyosin(slow) compared with beta-tropomyosin; the M9R substitution within alpha-tropomyosin(slow) greatly reduced this interaction. Finally, transfection of the M9R mutated and wild-type alpha-tropomyosin(slow) 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.
authors: Institute for Neuromuscular Research, The Children's Hospital at Westmead, Faculty of Medicine, University of Sydney, Sydney, New South Wales, Australia.
authors: Biljana
authors: B
authors: Ilkovski
authors: Nancy
authors: N
authors: Mokbel
authors: Raymond A
authors: RA
authors: Lewis
authors: Kendall
authors: K
authors: Walker
authors: Kristen J
authors: KJ
authors: Nowak
authors: Ana
authors: A
authors: Domazetovska
authors: Nigel G
authors: NG
authors: Laing
authors: Velia M
authors: VM
authors: Fowler
authors: Kathryn N
authors: KN
authors: North
authors: Sandra T
authors: ST
authors: Cooper
mesh_terms: D000328:Adult; D015153:Blotting, Western; D002675:Child, Preschool; D005260:Female; D005333:Fetus; D006801:Humans; D007150:Immunohistochemistry; D007223:Infant; D007231:Infant, Newborn; D007525:Isoelectric Focusing; D008875:Middle Aged; D018657:Muscle Fibers, Slow-Twitch / Q000473:pathology; D018482:Muscle, Skeletal / Q000473:pathology*; D009154:Mutation; D017696:Myopathies, Nemaline / Q000235:genetics* / Q000378:metabolism* / Q000473:pathology*; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D051348:Tropomodulin / Q000378:metabolism; D014335:Tropomyosin / Q000235:genetics* / Q000378:metabolism
chemical_list: D020033:Protein Isoforms; C498783:TPM3 protein, human; D051348:Tropomodulin; D014335:Tropomyosin