A second pedigree with autosomal dominant nemaline myopathy caused by TPM3 mutation: a clinical and pathological study.

International audienceThe slow alpha-tropomyosin (TPM3) gene has to date been associated with few cases of both dominant and recessive nemaline myopathies. We report the identification of a p.Arg167His mutation in a four-generation family presenting with a mild classical form of the disease. Clinically, there was no correlation between the age at presentation and the severity of the disease. The dominant-negative p.Arg167His mutation is a recurrent mutation, previously reported in one sporadic case. Histological studies showed discrepancy between the two reports. While a type II fibre predominance was described in the sporadic case, we observed an almost complete type I fibre predominance. This study emphasizes the variability in histopathological phenotypes seen with TPM3 mutations

Congenital myopathy-causing tropomyosin mutations induce thin filament dysfunction via distinct physiological mechanisms

In humans, congenital myopathy-linked tropomyosin mutations lead to skeletal muscle dysfunction, but the cellular and molecular mechanisms underlying such dysfunction remain obscure. Recent studies have suggested a unifying mechanism by which tropomyosin mutations partially inhibit thin filament activation and prevent proper formation and cycling of myosin cross-bridges, inducing force deficits at the fiber and whole-muscle levels. Here, we aimed to verify this mechanism using single membrane-permeabilized fibers from patients with three tropomyosin mutations (TPM2-null, TPM3-R167H and TPM2-E181K) and measuring a broad range of parameters. Interestingly, we identified two divergent, mutation-specific pathophysiological mechanisms. (i) The TPM2-null and TPM3-R167H mutations both decreased cooperative thin filament activation in combination with reductions in the myosin cross-bridge number and force production. The TPM3-R167H mutation also induced a concomitant reduction in thin filament length. (ii) In contrast, the TPM2-E181K mutation increased thin filament activation, cross-bridge binding and force generation. In the former mechanism, modulating thin filament activation by administering troponin activators (CK-1909178 and EMD 57033) to single membrane-permeabilized fibers carrying tropomyosin mutations rescued the thin filament activation defect associated with the pathophysiology. Therefore, administration of troponin activators may constitute a promising therapeutic approach in the future

Disrupted myosin cross-bridge cycling kinetics triggers muscle weakness in nebulin-related myopathy

Nebulin is a giant protein expressed at high levels in skeletal muscle. Mutations in the nebulin gene (NEB) lead to muscle weakness and various congenital myopathies. Despite increasing clinical and scientific interest, the pathogenesis of weakness remains unknown. The present study, therefore, aims at unraveling the underlying molecular mechanisms. Hence, we recorded and analyzed the mechanics as well as the X-ray diffraction patterns of human membrane-permeabilized single muscle fibers expressing nebulin mutations. Results demonstrated that, during contraction, the cycling rate of myosin heads attaching to actin is dramatically perturbed, causing a reduction in the fraction of myosin-actin interactions in the strong binding state. This phenomenon prevents complete thin-filament activation, more especially proper and full tropomyosin movement, further limiting additional binding of myosin cross-bridges. At the cell level, this reduces the force-generating capacity and, overall, provokes muscle weakness. To reverse such a negative cascade of events, future potential therapeutic interventions should, therefore, focus on the triggering component, the altered myosin cross-bridge cycling kinetics.—Ochala, J., Lehtokari, V.-L., Iwamoto, H., Li, M., Feng, H.-Z., Jin, J. P., Yagi, N., Wallgren-Pettersson, C. Pénisson-Besnier, I., Larsson, L. Disrupted myosin cross-bridge cycling kinetics triggers muscle weakness in nebulin-related myopathy

Multiexon deletions account for 15% of Congenital Myasthenic Syndrome with RAPSN mutations after negative DNA Sequencing

International audienceIntroduction: Post-synaptic congenital myasthenic syndromes (CMSs) (OMIM_ #608931) is a group of genetic disorders affecting neuromuscular transmission and due to acetylcholine receptor (AChR) deficiency in 80% of cases.[1] These autosomal recessive CMSs may be caused by mutations in genes encoding the AChR or one of the AChR-clustering or anchoring proteins, rapsyn, Dok-7 or MuSK.[1-4] Spectra of rapsyn mutations show allelic heterogeneity and suggest that the common substitution p.Asn88Lys (N88K) (variant_021217 in Q13702) results in less stable AChR clusters.[5] Until recently, all patients harbouring mutations in RAPSN are either homozygous for the p.Asn88Lys substitution or heteroallelic for p.Asn88Lys and a mutation which is in most of cases an amino acid substitution but can be also a null allele.[6] Analysis of disease severity in patients suggested that the second mutant allele may largely determine severity of the phenotype.[7] Recently, a patient with two non p.Asn88Lys in RAPSN has been described and the first chromosomal deletion event was described by Müller and colleagues.[8,9

Clinical outcome in 19 French and Spanish patients with valosin-containing protein myopathy associated with Paget's disease of bone and frontotemporal dementia

We report the clinical, histological and genetic findings in 10 families (19 patients) presenting mutations in the valosin-containing protein (VCP). The mean age at onset was 42 years. The clinical pattern was characterized by an early involvement of the proximal upper limbs with scapular winging. Axial and lower limb muscles were often affected, whereas facial, oculobulbar muscles were spared. Ten patients were wheelchair bound after a mean disease course of 9 years and six patients required canes for walking. Two patients required mechanically assisted ventilation and seven patients had reduced vital capacity. There was no cardiac involvement. Paget's disease of bone was observed in eight patients and cognitive impairment in nine patients. Seven patients died as a consequence of weakness and respiratory distress. Muscle biopsy showed rimmed vacuolar myopathy. Genetic analysis revealed missense heterozygous mutations mostly located in exon 5 of the VCP gene, four of which were not previously reported. We observed intrafamilial and interfamilial variability in terms of severity, distribution of weakness and presence or not of Paget's disease or cognitive impairment.status: publishe

Clinical, histological and genetic characterisation of patients with tubular aggregate myopathy caused by mutations in STIM1

Background: Tubular aggregate myopathies (TAMs) are muscle disorders characterised by abnormal accumulations of densely packed single-walled or double-walled membrane tubules in muscle fibres. Recently, STIM1, encoding a major calcium sensor of the endoplasmic reticulum, was identified as a TAM gene. Methods: The present study aims to define the clinical, histological and ultrastructural phenotype of tubular aggregate myopathy and to assess the STIM1 mutation spectrum. Results: We describe six new TAM families harbouring one known and four novel STIM1 mutations. All identified mutations are heterozygous missense mutations affecting highly conserved amino acids in the calcium-binding EF-hand domains, demonstrating the presence of a mutation hot spot for TAM. We show that the mutations induce constitutive STIM1 clustering, strongly suggesting that calcium sensing and consequently calcium homoeostasis is impaired. Histological and ultrastructural analyses define a common picture with tubular aggregates labelled with Gomori trichrome and Nicotinamide adenine dinucleotide (NADH) tetrazolium reductase, substantiating their endoplasmic reticulum origin. The aggregates were observed in both fibre types and were often accompanied by nuclear internalisation and fibre size variability. The phenotypical spectrum ranged from childhood onset progressive muscle weakness and elevated creatine kinase levels to adult-onset myalgia without muscle weakness and normal CK levels. Conclusions: The present study expands the phenotypical spectrum of STIM1-related tubular aggregate myopathy. STIM1 should therefore be considered for patients with tubular aggregate myopathies involving either muscle weakness or myalgia as the first and predominant clinical sign

Clinical, histological and genetic characterisation of patients with tubular aggregate myopathy caused by mutations in STIM1

Background: Tubular aggregate myopathies (TAMs) are muscle disorders characterised by abnormal accumulations of densely packed single-walled or double-walled membrane tubules in muscle fibres. Recently, STIM1, encoding a major calcium sensor of the endoplasmic reticulum, was identified as a TAM gene. Methods: The present study aims to define the clinical, histological and ultrastructural phenotype of tubular aggregate myopathy and to assess the STIM1 mutation spectrum. Results: We describe six new TAM families harbouring one known and four novel STIM1 mutations. All identified mutations are heterozygous missense mutations affecting highly conserved amino acids in the calcium-binding EF-hand domains, demonstrating the presence of a mutation hot spot for TAM. We show that the mutations induce constitutive STIM1 clustering, strongly suggesting that calcium sensing and consequently calcium homoeostasis is impaired. Histological and ultrastructural analyses define a common picture with tubular aggregates labelled with Gomori trichrome and Nicotinamide adenine dinucleotide (NADH) tetrazolium reductase, substantiating their endoplasmic reticulum origin. The aggregates were observed in both fibre types and were often accompanied by nuclear internalisation and fibre size variability. The phenotypical spectrum ranged from childhood onset progressive muscle weakness and elevated creatine kinase levels to adult-onset myalgia without muscle weakness and normal CK levels. Conclusions: The present study expands the phenotypical spectrum of STIM1-related tubular aggregate myopathy. STIM1 should therefore be considered for patients with tubular aggregate myopathies involving either muscle weakness or myalgia as the first and predominant clinical sign