Journal Article

Evidence for a dominant-negative effect in <i>ACTA1</i> nemaline myopathy caused by abnormal folding, aggregation and altered polymerization of mutant actin isoforms

Biljana Ilkovski, Kristen J. Nowak, Ana Domazetovska, Adam L. Maxwell, Sophie Clement, Kay E. Davies, Nigel G. Laing, Kathryn N. North and Sandra T. Cooper

in Human Molecular Genetics

Volume 13, issue 16, pages 1727-1743
Published in print August 2004 | ISSN: 0964-6906
Published online June 2004 | e-ISSN: 1460-2083 | DOI: http://dx.doi.org/10.1093/hmg/ddh185
Evidence for a dominant-negative effect in ACTA1 nemaline myopathy caused by abnormal folding, aggregation and altered polymerization of mutant actin isoforms

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We have studied a cohort of nemaline myopathy (NM) patients with mutations in the muscle α-skeletal actin gene (ACTA1). Immunoblot analysis of patient muscle demonstrates increased γ-filamin, myotilin, desmin and α-actinin in many NM patients, consistent with accumulation of Z line-derived nemaline bodies. We demonstrate that nebulin can appear abnormal secondary to a primary defect in actin, and show by isoelectric focusing that mutant actin isoforms are present within insoluble actin filaments isolated from muscle from two ACTA1 NM patients. Transfection of C2C12 myoblasts with mutant actinEGFP constructs resulted in abnormal cytoplasmic and intranuclear actin aggregates. Intranuclear aggregates were observed with V163L–, V163M– and R183G–actinEGFP constructs, and modeling shows these residues to be adjacent to the nuclear export signal of actin. V163L and V163M actin mutants are known to cause intranuclear rod myopathy, however, intranuclear bodies were not reported in patient R183G. Transfection studies in C2C12 myoblasts showed significant alterations in the ability of V136L and R183G actin mutants to polymerize and contribute to insoluble actin filaments. Thus, we provide direct evidence for a dominant-negative effect of mutant actin in NM. In vitro studies suggest that abnormal folding, altered polymerization and aggregation of mutant actin isoforms are common properties of NM ACTA1 mutants. Some of these effects are mutation-specific, and likely result in variations in the severity of muscle weakness seen in individual patients. A combination of these effects contributes to the common pathological hallmarks of NM, namely intranuclear and cytoplasmic rod formation, accumulation of thin filaments and myofibrillar disorganization.

Journal Article.  11038 words.  Illustrated.

Subjects: Genetics and Genomics

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