Nemaline Myopathy with Exclusively Intranuclear Rods and a Novel Mutation in ACTA1 (Q139H)

Abstract

Nemaline myopathies (NM) are a rare group of muscle disorders, but represent one of the most common forms of congenital myopathy. The clinical picture ranges from severe muscular hypotonia often leading to death during childhood to mild forms with long life expectancy. Diagnosis is made by muscle biopsy showing characteristic sarcoplasmic and sometimes intranuclear rod bodies. So far, disease-associated mutations have been detected in six genes without any simple correlation between genotype and phenotype or histological findings. We report a patient with a phenotype typical of congenital onset nemaline myopathy and exclusively intranuclear rods. Mutation analysis revealed a new heterozygous missense mutation in exon 3 of the ACTA1 gene (Q139H). Molecular modelling predicts that substitution of Q139 for H139 alters the amino acid side chains and hydrogen bonding which may alter the nucleotide binding cleft by adding ‘bulk’ to the mutated molecule. Two-dimensional gel electrophoresis demonstrates that mutant actin Q139H is expressed at approximately half the level of wild-type actin in the patient's muscle. We speculate that these alterations, although not directly affecting the nuclear export signal, negatively interfere with the nuclear export of the mutated protein and thereby cause retention of mutant actin and intranuclear rod formation.

Key words

nemaline myopathy - intranuclear rod bodies - ACTA1 mutation

References

• 1 Agrawal PB, Greenleaf RS, Tomczak KK, Lehtokari VL, Wallgren-Pettersson C, Wallefeld W. et al . Nemaline myopathy with minicores caused by mutation of the CLF2 gene encoding the skeletal muscle actin-binding protein, cofilin-2.  Am J Hum Genet. 2007;  80 162-167
• 2 Clarkson E, Costa CF, Machesky LM. Congenital myopathies: diseases of the actin cytoskeleton.  J Pathol. 2004;  204 407-417
• 3 Cooper ST, Lo HP, North KN. Single section Western blot: improving the molecular diagnosis of the muscular dystrophies.  Neurology. 2003;  61 93-97
• 4 Costa CF, Rommelaere H, Watershoot D, Sethi KK, Nowak KJ, Laing NG. et al . Myopathy mutations in a skeletal muscle actin cause a range of molecular defects.  J Cell Sci. 2004;  117 3367-3377
• 5 Domazetovska A, Ilkovski B, Cooper ST, Ghoddusi M, Hardeman EC, Minamide LS. et al . Mechanisms underlying intranuclear rod formation.  Brain. 2007;  130 3275-3284
• 6 Domazetovska A, Ilkovski B, Kumar V, Valova VA, Vandebrouck A, Hutchinson DO. et al . Intranuclear rod myopathy - molecular pathogenesis and mechanisms of weakness.  Ann Neurol. 2007;  62 597-608
• 7 Goebel HH, Piirsoo A, Warlo I, Schofer O, Kehr S, Gaude M. Infantile intranuclear rod myopathy.  J Child Neurol. 1997;  12 22-30
• 8 Hennessey ES, Harrison A, Drummond DR, Sparrow JC. Mutant actin: a dead end?.  J Muscle Res Cell Motil. 1992;  13 127-131
• 9 Hutchinson DO, Charlton A, Graham L, Laing N, Ilkovski B, North KN. Autosomal dominant nemaline myopathy with intranuclear rods due to a mutation of the skeletal muscle ACTA1 gene: clinical and pathological variability within a kindred.  Neuromuscul Disord. 2006;  16 113-121
• 10 Ilkovski B, Cooper ST, Nowak K, Ryan MM, Yang N, Schnell C. et al . Nemaline myopathy caused mutations in the muscle alpha-skeletal-actin gene.  Am J Hum Genet. 2001;  68 1333-1343
• 11 Ilkovski B, Nowak KJ, Domazetovska A, Maxwell AL, Clement S, Davies KE. et al . Evidence for a dominant-negative effect in ACTA1 nemaline myopathy caused by abnormal folding, aggregation and altered polymerisation of mutant actin isoforms.  Hum Mol Genet. 2004;  13 1727-1743
• 12 Kaimaktchiev V, Goebel H, Laing N, Narus M, Weeks D, Nixon R. Intranuclear nemaline rod myopathy.  Muscle Nerve. 2006;  34 369-372
• 13 Morris JP, Nneji G, Squire JM. The three-dimensional structure of the nemaline rod.  J Cell Biol. 1990;  6 2961-2978
• 14 Nowak KJ, Wattanasirichaigoon D, Goebel HH, Wilce M, Pelin K, Donner K. et al . Mutations in the skeletal muscle alpha-actin gene in patients with actin myopathy and nemaline myopathy.  Nat Genet. 1999;  23 208-212
• 15 Ryan MM, Ilkovski B, Strickland CD, Schnell C, Sanoudou D, Midgett C. et al . Clinical course correlates poorly with muscle pathology in nemaline myopathy.  Neurology. 2003;  60 665-673
• 16 Sparrow JC, Nowak KJ, Durling HJ, Beggs AH, Wallgren-Pettersson C, Romero N. et al . Muscle disease caused by mutations in the skeletal muscle alpha-actin gene (ACTA1).  Neuromuscul Disord. 2003;  13 519-531
• 17 Wada A, Fukuda M, Mishima M, Nishida E. Nuclear export of actin: a novel mechanism regulating the subcellular localization of a major cytoskeletal protein.  EMBO J. 1998;  17 1635-1641
• 18 Wallgren-Pettersson C, Laing NG. Report of the 70^th ENMC International Workshop: Nemaline myopathy 11-13 June 1999, Naarden, The Netherlands.  Neuromuscul Disord. 2000;  10 299-306
• 19 Wallgren-Pettersson C, Pelin K, Nowak KJ, Muntoni F, Romero NB, Goebel HH. et al . Genotype- Phenotype correlations in nemaline myopathy caused by mutations in the genes for nebulin and skeletal muscle alpha-actin.  Neuromuscul Disord. 2004;  14 461-470

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