Functional characterisation of a mutant actin (Met132Val) from a patient with nemaline myopathy

Area of Science:

• Molecular Biology
• Genetics
• Muscle Physiology

Background:

• Nemaline myopathy (NM) is a muscle disorder.
• The ACTA1 gene provides instructions for making actin, a protein crucial for muscle contraction.
• Specific mutations in ACTA1 can lead to NM.

Purpose of the Study:

• To investigate the functional consequences of the Met132Val mutation in the ACTA1 gene.
• To determine if the mutant ACTA1 mRNA and protein are expressed in vivo.
• To analyze the effect of the mutation on actin polymerization and thin filament function.

Main Methods:

• Analysis of actin mRNA from patient muscle biopsy using BstX1 cleavage.
• Extraction and purification of monomeric actin from both mutant and wild-type muscle.
• In vitro polymerization assays of actin under varying conditions (KCl, MgCl2, rhodamine-phalloidin).
• Reconstitution of thin filaments and assessment of sliding velocity and isometric force.

Main Results:

• Mutant ACTA1 mRNA was detected in vivo, with 61% resistant to BstX1 cleavage.
• A portion of the mutant actin exhibited reduced polymerisability compared to wild-type actin.
• Reconstituted thin filaments with mutant actin showed altered function, including faster sliding and higher relative isometric force.
• The mutation was confirmed to be expressed at both mRNA and protein levels.

Conclusions:

• The Met132Val mutation in ACTA1 is expressed in vivo and leads to abnormal actin.
• The mutation impairs actin polymerisability and alters the functional properties of muscle thin filaments.
• These molecular and functional changes contribute to the pathogenesis of mild nemaline myopathy.