Force-Sarcomere Length Relations in Patients with Thin Filament Myopathy Caused by Mutations in NEB, ACTA1, TPM2, TPM3, KBTBD13, KLHL40 and KLHL41

Background: Mutations in NEB, ACTA1, TPM2, TPM3, KBTBD13, KLHL40 and −41 lead to thin filament myopathies, such as nemaline myopathy, congenital fiber type disproportion and cap disease. A hallmark feature of these myopathies is muscle weakness. Here, we aimed to elucidate whether mutations in NEB, ACTA1, TPM2, TPM3, KBTBD13, KLHL40 and KLHL41 affect the maximal force generating capacity of muscle fibers. Subsequently, by determining the sarcomere length-dependence of force, we investigated whether changes in thin filament length contributed to muscle weakness.Methods: Biopsies from NM, CFTD, and CAP patients (n=39) with mutations in NEB, ACTA1, TPM2, TPM3, KBTBD13, KLHL40 or KLHL41 were compared to biopsies from healthy controls (n=9). 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.Results: The maximal active tension (Fmax (in mN/mm2, mean±SEM)) was significantly lower in biopsies from NEB (46±5), ACTA1 (48±4), TPM2 (71±8), TPM3 (85±10), KBTBD13 (78±3), KLHL40 (2.8±0.2) and KLHL41 (63±4) patients compared to biopsies of controls (129±7).No shift in the force-sarcomere length relationship was observed in TPM3, TPM2, KBTBD 13, KLHL 40 and KLHL41 patients. In contrast, several patients with ACTA1 and NEB mutations showed a leftward shift of the force-sarcomere length relationship indicating shorter thin filaments. Furthermore, the slope of the descending limb of the force-sarcomere length relationship in these patients is less steep than the slope of the CTRL curve, suggesting heterogeneity of thin filaments.Conclusion: Our data suggest that mutations in NEB and ACTA1 result in changes in thin filament length. Insights in the mechanisms underlying weakness in patients with thin filament mutations are necessary to improve specific treatment strategies.