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.

Variable phenotype of del45-55 Becker patients correlated with nNOSµ mislocalization and RYR1 hypernitrosylation.

Duchenne and Becker muscular dystrophies (DMD and BMD) are muscle-wasting diseases caused by mutations in the DMD gene-encoding dystrophin. Usually, out-of-frame deletions give rise to DMD, whereas in-frame deletions result in BMD. BMD patients exhibit a less severe disease because an abnormal but functional dystrophin is produced. This is the rationale for attempts to correct the reading frame by using an exon-skipping strategy. In order to apply this approach to a larger number of patients, a multi-exon skipping strategy of exons 45-55 has been proposed, because it should correct the mRNA reading frame in almost 75% of DMD patients with a deletion. The resulting dystrophin lacks part of the binding site for the neuronal nitric oxide synthase (nNOSµ), which normally binds to spectrin-like repeats 16 and 17 of the dystrophin. Since these domains are encoded by exons 42-45, we investigated the nNOSµ status in muscle biopsies from 12 BMD patients carrying spontaneous deletions spaning exons 45-55. We found a wide spectrum of nNOSµ expression and localization. The strictly cytosolic mislocalization of nNOSµ was associated with the more severe phenotypes. Cytosolic NO production correlated with both hypernitrosylation of the sarcoplasmic reticulum calcium-release-channel ryanodine receptor type-1 (RyR1) and release of calstabin-1, a central hub of Ca(2+) signaling and contraction in muscle. Finally, this study shows that the terminal truncation of the nNOS-binding domain in the 'therapeutic' del45-55 dystrophin is not innocuous, since it can perturb the nNOS-dependent stability of the RyR1/calstabin-1 complex.

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.

ANO5 gene analysis in a large cohort of patients with anoctaminopathy: confirmation of male prevalence and high occurrence of the common exon 5 gene mutation.

Limb girdle muscular dystrophy type 2L or anoctaminopathy is a condition mainly characterized by adult onset proximal lower limb muscular weakness and raised CK values, due to recessive ANO5 gene mutations. An exon 5 founder mutation (c.191dupA) has been identified in most of the British and German LGMD2L patients so far reported. We aimed to further investigate the prevalence and spectrum of ANO5 gene mutations and related clinical phenotypes, by screening 205 undiagnosed patients referred to our molecular service with a clinical suspicion of anoctaminopathy. A total of 42 unrelated patients had two ANO5 mutations (21%), whereas 14 carried a single change. We identified 34 pathogenic changes, 15 of which are novel. The c.191dupA mutation represents 61% of mutated alleles and appears to be less prevalent in non-Northern European populations. Retrospective clinical analysis corroborates the prevalently proximal lower limb phenotype, the male predominance and absence of major cardiac or respiratory involvement. Identification of cases with isolated hyperCKaemia and very late symptomatic male and female subjects confirms the extension of the phenotypic spectrum of the disease. Anoctaminopathy appears to be one of the most common adult muscular dystrophies in Northern Europe, with a prevalence of about 20%-25% in unselected undiagnosed cases.

A myopathy-related actin mutation increases contractile function.

Nemaline myopathy (NM) is the most common congenital myopathy and is caused by mutations in various genes including NEB (nebulin), TPM2 (beta-tropomyosin), TPM3 (gamma-tropomyosin), and ACTA1 (skeletal alpha-actin). 20-25% of NM cases carry ACTA1 defects and these particular mutations usually induce substitutions of single residues in the actin protein. Despite increasing clinical and scientific interest, the contractile consequences of these subtle amino acid substitutions remain obscure. To decipher them, in the present study, we originally recorded and analysed the mechanics as well as the X-ray diffraction patterns of human membrane-permeabilized single muscle fibres with a particular peptide substitution in actin, i.e. p.Phe352Ser. Results unravelled an unexpected cascade of molecular and cellular events. During contraction, p.Phe352Ser greatly enhances the strain of individual cross-bridges. Paradoxically, p.Phe352Ser also slightly lowers the number of cross-bridges by altering the rate of myosin head attachment to actin monomers. Overall, at the cell level, these divergent mechanisms conduct to an improved steady-state force production. Such results provide new surprising scientific insights and crucial information for future therapeutic strategies.

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.

Early onset collagen VI myopathies: Genetic and clinical correlations.

OBJECTIVE: Mutations in the genes encoding the extracellular matrix protein collagen VI (ColVI) cause a spectrum of disorders with variable inheritance including Ullrich congenital muscular dystrophy, Bethlem myopathy, and intermediate phenotypes. We extensively characterized, at the clinical, cellular, and molecular levels, 49 patients with onset in the first 2 years of life to investigate genotype-phenotype correlations. METHODS: Patients were classified into 3 groups: early-severe (18%), moderate-progressive (53%), and mild (29%). ColVI secretion was analyzed in patient-derived skin fibroblasts. Chain-specific transcript levels were quantified by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), and mutation identification was performed by sequencing of complementary DNA. RESULTS: ColVI secretion was altered in all fibroblast cultures studied. We identified 56 mutations, mostly novel and private. Dominant de novo mutations were detected in 61% of the cases. Importantly, mutations causing premature termination codons (PTCs) or in-frame insertions strikingly destabilized the corresponding transcripts. Homozygous PTC-causing mutations in the triple helix domains led to the most severe phenotypes (ambulation never achieved), whereas dominant de novo in-frame exon skipping and glycine missense mutations were identified in patients of the moderate-progressive group (loss of ambulation). INTERPRETATION: This work emphasizes that the diagnosis of early onset ColVI myopathies is arduous and time-consuming, and demonstrates that quantitative RT-PCR is a helpful tool for the identification of some mutation-bearing genes. Moreover, the clinical classification proposed allowed genotype-phenotype relationships to be explored, and may be useful in the design of future clinical trials.

Multiexon deletions account for 15% of congenital myasthenic syndromes with RAPSN mutations after negative DNA sequencing.

Congenital myasthenic syndromes (CMS) are a heterogeneous group of genetic disorders that give rise to a defect in neuromuscular transmission. We described here three patients with a characteristic phenotype of recessive CMS and presenting mutation in the gene encoding rapsyn (RAPSN). Familial analysis showed that one allelic mutation failed to be detected by direct sequencing. An allelic quantification on patient's DNA identified three novel multi-exon deletions of RAPSN. These three genomic rearrangements in RAPSN represent 15% of our CMS patients with RAPSN mutations and we emphasize that single-nucleotide polymorphism markers and a gene dosage method should be performed in addition to DNA direct sequencing analysis particularly when there is a genetic counselling issue.

Truncating mutations in C-terminal titin may cause more severe tibial muscular dystrophy (TMD).

Mutations in C-terminal titin cause autosomal dominant tibial muscular dystrophy (TMD) as reported previously. Samples from 25 new families and 25 sporadic new distal myopathy cases were screened for titin mutations. Three novel mutations were discovered in two families from Spain and two families from France. Two mutations, g.292998delT and g.293376delA lead to frameshift and premature stop codons in the second last and the last titin gene (TTN) exons, Mex5 and Mex6, respectively. The third was a nonsense mutation g.293379C>T (p.Q33396X) in Mex6. Patients with the upstream Mex5 mutation showed a more severe phenotype with earlier onset implying a genotype-phenotype correlation.

Carotid artery dissection in an adult with the Simpson-Golabi-Behmel syndrome.

We report on the case of a 44-year-old man affected with the Simpson-Golabi-Behmel syndrome (SGBS) (OMIM 312870) presenting with ischemic stroke due to a dissection of the right internal carotid. Molecular genetic analysis revealed the p.Gly556Arg mutation in exon 8 of the gene encoding glypican 3 (GPC3). This is the second case of a GPC3 missense mutation to be reported. The only risk factor found in this patient was carotid redundancy, a deformation that is significantly associated with spontaneous carotid dissection. The natural history of SGBS in adults is poorly known, and this case raises the question of a possible vascular risk associated with the disease.

A sensitive assay for measuring SMN mRNA levels in peripheral blood and in muscle samples of patients affected with spinal muscular atrophy.

Different therapeutic strategies are currently evaluated in spinal muscular atrophy (SMA) that are aimed at increasing full-length (FL) mRNA levels produced from the SMN2 gene. Assays measuring SMN mRNA levels are needed. We have developed a sensitive, comparative assay based on multiplex fluorescent reverse-transcription polymerase chain reaction (RT-PCR) that can measure, in the same reaction, the levels of SMN mRNA with and without exon 7 sequences as well as those of total SMN mRNA. This assay allows to calculate directly the ratios of FL SMN mRNA to SMN mRNA without exon 7 (Delta7). We have used this assay to compare the levels of SMN transcripts in the blood of 75 unrelated normal subjects and of 48 SMA patients, and in muscle samples of 8 SMA patients. The SMN1 and the SMN2 genes produced very similar levels of total mRNA. Levels of transcripts lacking exon 7 were linearly dependent on the number of SMN2 copies, both in SMA patients and in controls. In patients, FL mRNA levels correlated with SMN2 copy number. A significant but weaker inverse correlation was also observed between FL or Delta7 mRNA levels and disease severity, but patients with three SMN2 copies and different SMA types displayed similar mRNA levels. A significantly higher FL to Delta7 ratio was measured in blood cells than in skeletal muscle (0.80+/-0.18 versus 0.47+/-0.11). This assay can be used as a sensitive biomarker for monitoring the effects of various drugs in forthcoming clinical trials of SMA.

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

The 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.

Diagnosis of myotubular myopathy in the oldest known manifesting female carrier: a clinical and genetic study.

X-linked myotubular myopathy is a congenital myopathy due to mutation in the MTM1 gene, encoding myotubularin. Most of the affected male neonates die early of respiratory failure. The female carriers are usually asymptomatic. The authors report a novel MTM1 mutation in a 77-year-old woman. She presented with progressive ptosis since childhood, proximal limb weakness, and a severe restrictive respiratory dysfunction with a hemidiaphragmatic paresis, leading to death at 84 years of age. The muscle biopsy showed centrally nucleated fibers and mitochondrial abnormalities. A stop mutation Leu498X in MTM1 gene was identified in the proband and in her two healthy daughters. The X-inactivation pattern was random in the proband's blood and muscle DNA, and in blood DNA from her two unaffected MTM1 mutation carrier daughters. Two large heteroplasmic deletions were also detected in the muscle mitochondrial DNA of the propositus, raising the question of their putative impact on the phenotype.

Myotilinopathy in a family with late onset myopathy.

Mutations in titin are well known cause of late onset autosomal dominant distal myopathy. Mutations in another sarcomeric protein, myotilin, were first identified in two families with dominant limb girdle muscular phenotype. Recently, however, myotilin mutations have been associated with more distal phenotypes in patients with late onset myofibrillar myopathy. We report here a multigenerational French family in which gene sequencing identified a S60F myotilin mutation in all patients with full penetrance despite very late onset. The family was originally reported as a distal myopathy but intrafamilial variability was remarkable with proximal or distal muscle weakness or both. Extended morphological characteristics of muscle biopsy findings in myotilinopathy indicate that immunohistochemistry may be important for selection of molecular genetic approach in myofibrillar myopathy.

Detection of TRIM32 deletions in LGMD patients analyzed by a combined strategy of CGH array and massively parallel sequencing.

Defects in TRIM32 were reported in limb-girdle muscular dystrophy type 2H (LGMD2H), sarcotubular myopathies (STM) and in Bardet-Biedl syndrome. Few cases have been described to date in LGMD2H/STM, but this gene is not systematically analysed because of the absence of specific signs and difficulties in protein analysis. By using high-throughput variants screening techniques, we identified variants in TRIM32 in two patients presenting nonspecific LGMD. We report the first case of total inactivation by homozygous deletion of the entire TRIM32 gene. Of interest, the deletion removes part of the ASTN2 gene, a large gene in which TRIM32 is nested. Despite the total TRIM32 gene inactivation, the patient does not present a more severe phenotype. However, he developed a mild progressive cognitive impairment that may be related to the loss of function of ASTN2 because association between ASTN2 heterozygous deletions and neurobehavioral disorders was previously reported. Regarding genomic characteristics at breakpoint of the deleted regions of TRIM32, we found a high density of repeated elements, suggesting a possible hotspot. These observations illustrate the importance of high-throughput technologies for identifying molecular defects in LGMD, confirm that total loss of function of TRIM32 is not associated with a specific phenotype and that TRIM32/ASTN2 inactivation could be associated with cognitive impairment.

Compound heterozygous mutations of the TNXB gene cause primary myopathy.

Complete deficiency of the extracellular matrix glycoprotein tenascin-X (TNX) leads to recessive forms of Ehlers-Danlos syndrome, clinically characterized by hyperextensible skin, easy bruising and joint hypermobility. Clinical and pathological studies, immunoassay, and molecular analyses were combined to study a patient suffering from progressive muscle weakness. Clinical features included axial and proximal limb muscle weakness, subclinical heart involvement, minimal skin hyperextensibility, no joint abnormalities, and a history of easy bruising. Skeletal muscle biopsy disclosed striking muscle consistency and the abnormal presence of myotendinous junctions in the muscle belly. TNX immunostaining was markedly reduced in muscle and skin, and serum TNX levels were undetectable. Compound heterozygous mutations were identified: a previously reported 30kb deletion and a non-synonymous novel missense mutation in the TNXB gene. This study identifies a TNX-deficient patient presenting with a primary muscle disorder, thus expanding the phenotypic spectrum of TNX-related abnormalities. Biopsy findings provide evidence that TNX deficiency leads to muscle softness and to mislocalization of myotendinous junctions.

HTLV-1-associated inflammatory myopathies: low proviral load and moderate inflammation in 13 patients from West Indies and West Africa.

BACKGROUND: The Human T-cell Leukemia Virus type 1 (HTLV-1) is the causative agent of several inflammatory diseases, including HTLV-1-associated inflammatory myopathies (HAIM). Little is known about the virological and immunological characteristics of this viral disease. OBJECTIVES: To characterize the histological and virological features of HAIM patients, in order to better understand the pathogenetic mechanisms and unravel new biological markers of this disease. STUDY DESIGN: We conducted a retrospective study on 13 patients with HAIM, based on blood and muscle samples. We included blood samples from HTLV-1-infected individuals without myopathy as controls. Muscle biopsies were used for a broad immunohistological evaluation of tissue damage and inflammation, as well as identification of infected cells through in situ hybridization. DNA extracted from patients' PBMC was used to identify the virus genotype by sequencing and to assess the proviral load by quantitative PCR. Anti-viral antibodies in plasma samples were titrated by indirect immunofluorescence. RESULTS: Patients originate from HTLV-1 endemic areas, the West Indies and West Africa. Histological alterations and inflammation in patients muscles were mostly moderate, with classical features of idiopathic myositis and rare HTLV-1-infected infiltrating cells. In all patients, HTLV-1 belonged to the A subtype, transcontinental subgroup. Anti-HTLV-1 antibodies titers were high, but the proviral load was not elevated compared to asymptomatic HTLV-1 carriers. CONCLUSION: We show here that muscle inflammation is moderate in HAIM, and accompanied by a low HTLV-1 proviral load, suggesting that the pathogenetic events do not exactly mirror those of other HTLV-1-associated inflammatory diseases.

Diagnostic assessment and long-term follow-up of 13 patients with Very Long-Chain Acyl-Coenzyme A dehydrogenase (VLCAD) deficiency.

Very Long-Chain Acyl-CoA dehydrogenase (VLCAD) deficiency is an inborn error of mitochondrial long-chain fatty acid oxidation (FAO) most often occurring in childhood with cardiac or liver involvement, but rhabdomyolysis attacks have also been reported in adults. We report in this study the clinical, biochemical and molecular studies in 13 adult patients from 10 different families with VLCAD deficiency. The enzyme defect was demonstrated in cultured skin fibroblasts or lymphocytes. All patients exhibited exercise intolerance and recurrent rhabdomyolysis episodes, which were generally triggered by strenuous exercise, fasting, cold or fever (mean age at onset: 10 years). Inaugural life-threatening general manifestations also occurred before the age of 3 years in four patients. Increased levels of long-chain acylcarnitines with tetradecenoylcarnitine (C14:1) as the most prominent species were observed in all patients. Muscle biopsies showed a mild lipidosis in four patients. For all patients but two, molecular analysis showed homozygous (4 patients) or compound heterozygous genotype (7 patients). For the two remaining patients, only one mutation in a heterozygous state was detected. This study confirms that VLCAD deficiency, although being less frequent than CPT II deficiency, should be systematically considered in the differential diagnosis of exercise-induced rhabdomyolysis. Measurement of fasting blood acylcarnitines by tandem mass spectrometry allows accurate biochemical diagnosis and should therefore be performed in all patients presenting with unexplained muscle exercise intolerance or rhabdomyolysis.

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.