Force-Sarcomere Length Relations in Patients with Thin Filament Myopathy Caused by Mutations in NEB, ACTA1, TPM2 and TPM3

Abstract

Mutations in the nebulin gene (NEB), skeletal muscle alpha-actin1 gene (ACTA1), beta-tropomyosin 2 gene (TPM2) and alpha-tropomyosin 3 gene (TPM3) lead to thin filament myopathies, such as nemaline myopathy (NM), congenital fiber type disproportion (CFTD) and cap disease (CAP). A hallmark feature of these myopathies is muscle weakness. Here, we aimed to elucidate the effect of NEB, ACTA1, TPM2 and TPM3 mutations on thin filament length by determining the sarcomere length-dependence of force.Quadriceps biopsies from NM, CFTD, and CAP patients (n=18) with mutations in the NEB, ACTA1, TPM2 or TPM3 were compared to biopsies from controls (n=3). Using permeabilized muscle fibers, maximal active tension was determined at incremental sarcomere lengths (range 2.0-3.5 μm) to obtain the force-sarcomere length relationship.The maximal active tension (Fmax (in mN/mm2, mean±SD)) was significantly lower in biopsies from severe NEB (18±5), mild NEB (76±5), severe ACTA1 (54±13) and severe TPM3 (95±14) patients compared to biopsies of controls (164±17), whereas no significant changes in Fmax were observed in biopsies from mild ACTA1 (139±27), mild TPM2 (1201±8) and mild TPM3 (156±13) patients. The classification of severity is based on the age of onset.No shift in the force-sarcomere length relationship was observed in mild ACTA1, TPM3 and TPM2 patients. Interestingly, in contrast to patients with ACTA1, TPM2 and TPM3 mutations, fiber preparations from both mildly and severly affected NEB patients showed a leftward shift of the force-sarcomere length relationship (a leftward shift of the force-sarcomere length relationship indicates shorter thin filaments).Our data suggest that mutations in NEB result in the most pronounced changes in thin filament length. Insights in the mechanisms underlying weakness in patients with thin filament mutations are necessary to improve specific treatment strategies.