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Transversal structural elements in cross-striated muscles, such as the M-band or the Z-disc, anchor and mechanically stabilize the contractile apparatus and its minimal unit—the sarcomere. The ability of proteins to target and interact with these structural sarcomeric elements is an inevitable necessity for the correct assembly and functionality of the myofibrillar apparatus. Specifically, the M-band is a well-recognized mechanical and signaling hub dealing with active forces during contraction, while impairment of its function leads to disease and death. Research on the M-band architecture is focusing on the assembly and interactions of the three major filamentous proteins in the region, mainly the three myomesin proteins including their embryonic heart (EH) isoform, titin and obscurin. These proteins form the basic filamentous network of the M-band, interacting with each other as also with additional proteins in the region that are involved in signaling, energetic or mechanosensitive processes. While myomesin-1, titin and obscurin are found in every muscle, the expression levels of myomesin-2 (also known as M-protein) and myomesin-3 are tissue specific: myomesin-2 is mainly expressed in the cardiac and fast skeletal muscles, while myomesin-3 is mainly expressed in intermediate muscles and specific regions of the cardiac muscle. Furthermore, EH-myomesin apart from its role during embryonic stages, is present in adults with specific cardiac diseases. The current work in structural, molecular, and cellular biology as well as in animal models, provides important details about the assembly of myomesin-1, obscurin and titin, the information however about the myomesin-2 and -3, such as their interactions, localization and structural details remain very limited. Remarkably, an increasing number of reports is linking all three myomesin proteins and particularly myomesin-2 to serious cardiovascular diseases suggesting that this protein family could be more important than originally thought. In this review we will focus on the myomesin protein family, the myomesin interactions and structural differences between isoforms and we will provide the most recent evidence why the structurally and biophysically unexplored myomesin-2 and myomesin-3 are emerging as hot targets for understanding muscle function and disease.

Idiopathic ventricular fibrillation (IVF) causes sudden death in young adult patients without structural or ischemic heart disease. Most IVF cases are sporadic and some patients present with short-coupled torsade de pointes, the genetics of which are poorly understood. A man who had a first syncope at the age of 35 presented with frequent short-coupled premature ventricular beats with bursts of polymorphic ventricular tachycardia and then died suddenly. By exome sequencing, we identified three rare variants: p.I784F in the SPRY1 of the ryanodine receptor 2 (RyR2), p.A96S in connexin 40 (Cx40), reported to affect electrical coupling and cardiac conduction, and a nonsense p.R244X in the cardiac-specific troponin I-interacting kinase (TNNI3K). We assessed intracellular Ca 2+ handling in WT and mutant human  RYR2 transfected HEK293 cells by fluorescent microscopy and an enhanced store overload-induced Ca 2+ release in response to cytosolic Ca 2+ was observed in RyR2-I784F cells. In addition, crystal structures and thermal melting temperatures revealed a conformational change in the I784F-SPRY1 domain compared to the WT-domain. The novel RyR2-I784F variant in SPRY1 domain causes a leaky channel under non-stress conditions. The presence of several variants affecting Ca 2+ handling and cardiac conduction suggests a possible oligogenic origin for the ectopies originating from Purkinje fibres.

Selenium is a trace element that is essential for human health and is incorporated into more than 25 human selenocysteine-containing (Sec-containing) proteins via unique Sec-insertion machinery that includes a specific, nuclear genome-encoded, transfer RNA (tRNA[Ser]Sec). Here, we have identified a human tRNA[Ser]Sec mutation in a proband who presented with a variety of symptoms, including abdominal pain, fatigue, muscle weakness, and low plasma levels of selenium. This mutation resulted in a marked reduction in expression of stress-related, but not housekeeping, selenoproteins. Evaluation of primary cells from the homozygous proband and a heterozygous parent indicated that the observed deficit in stress-related selenoprotein production is likely mediated by reduced expression and diminished 2'-O-methylribosylation at uridine 34 in mutant tRNA[Ser]Sec. Moreover, this methylribosylation defect was restored by cellular complementation with normal tRNA[Ser]Sec. This study identifies a tRNA mutation that selectively impairs synthesis of stress-related selenoproteins and demonstrates the importance of tRNA modification for normal selenoprotein synthesis.

Selenium, a trace element that is fundamental to human health, is incorporated into some proteins as selenocysteine (Sec), generating a family of selenoproteins. Sec incorporation is mediated by a multiprotein complex that includes Sec insertion sequence-binding protein 2 (SECISBP2; also known as SBP2). Here, we describe subjects with compound heterozygous defects in the SECISBP2 gene. These individuals have reduced synthesis of most of the 25 known human selenoproteins, resulting in a complex phenotype. Azoospermia, with failure of the latter stages of spermatogenesis, was associated with a lack of testis-enriched selenoproteins. An axial muscular dystrophy was also present, with features similar to myopathies caused by mutations in selenoprotein N (SEPN1). Cutaneous deficiencies of antioxidant selenoenzymes, increased cellular ROS, and susceptibility to ultraviolet radiation-induced oxidative damage may mediate the observed photosensitivity. Reduced levels of selenoproteins in peripheral blood cells were associated with impaired T lymphocyte proliferation, abnormal mononuclear cell cytokine secretion, and telomere shortening. Paradoxically, raised ROS in affected subjects was associated with enhanced systemic and cellular insulin sensitivity, similar to findings in mice lacking the antioxidant selenoenzyme glutathione peroxidase 1 (GPx1). Thus, mutation of SECISBP2 is associated with a multisystem disorder with defective biosynthesis of many selenoproteins, highlighting their role in diverse biological processes.

SCN5A Mutations and the Role of Genetic Background in the Pathophysiology of Brugada Syndrome Vincent Probst, MD, PhD ; Arthur A.M. Wilde, MD, PhD ; Julien Barc, MS ; Frederic Sacher, MD ; Dominique Babuty, MD ; Philippe Mabo, MD ; Jacques Mansourati, MD ; Solena Le Scouarnec, PhD ; Florence Kyndt, PharmD, PhD ; Cedric Le Caignec, MD, PhD ; Pascale Guicheney, PhD ; Laetitia Gouas, PhD ; Juliette Albuisson, MD ; Paola G. Meregalli, MD ; Hervé Le Marec, MD, PhD ; Hanno L. Tan, MD, PhD and Jean-Jacques Schott, PhD From the INSERM (V.P., J.B., S.L.S., F.K., H.L.M., J.J.S.), UMR915; CNRS (V.P., J.B., S.L.S., F.K., H.L.M., J.J.S.), ERL3147; Université de Nantes (V.P., J.B., S.L.S., F.K., H.L.M., J.J.S.), l’institut du thorax; CHU Nantes (V.P., H.L.M., J.J.S.), l’institut du thorax, Service de cardiologie, Nantes, France; Department of Cardiology (A.A.M.W., P.G.M., H.L.T.), Academic Medical Center, University of Amsterdam, The Netherlands; Service de rythmologie (F.S.), Hôpital cardiologique du Haut Leveque, Bordeaux, France; Service de cardiologie B (D.B.), Hôpital Trousseau, Tours, France; Departement de cardiologie (P.M.), Hôpital Pontchaillou, Rennes, France; Service de cardiologie (J.M.), centre hospitalo-universitaire de Brest, Brest, France; Service de Génétique Médicale (F.K., C.L.C., J.A.), Institut de Biologie, CHU de Nantes, France; INSERM (P.G., L.G.), U582, Institut de Myologie, Paris, France; and AP-HP (P.G., L.G.), Groupe Hospitalier Pitié-Salpêtrière, Paris, France. Correspondence to Vincent Probst, MD, PhD, Service de cardiologie du CHU de Nantes, CHU de Nantes, Hôpital Nord, Bd Jacques Monod, 44093 Nantes Cedex, France. E-mail vincent.probst{at}chu-nantes.fr Received January 22, 2009; accepted July 14, 2009. Background— Mutations in SCN5A are identified in 20% to 30% of probands affected by Brugada syndrome (BrS). However, in familial studies, the relationship between SCN5A mutations and BrS remains poorly understood. The aim of this study was to investigate the association of SCN5A mutations and BrS in a group of large genotyped families. Methods and Results— Families were included if at least 5 family members were carriers of the SCN5A mutation, which was identified in the proband. Thirteen large families composed of 115 mutation carriers were studied. The signature type I ECG was present in 54 mutation carriers (BrS-ECG+; 47%). In 5 families, we found 8 individuals affected by BrS but with a negative genotype (mutation-negative BrS-ECG+). Among these 8 mutation-negative BrS-ECG+ individuals, 3, belonging to 3 different families, had a spontaneous type I ECG, whereas 5 had a type I ECG only after the administration of sodium channel blockers. One of these 8 individuals had also experienced syncope. Mutation carriers had, on average, longer PR and QRS intervals than noncarriers, demonstrating that these mutations exerted functional effects. Conclusions— Our results suggest that SCN5A mutations are not directly causal to the occurrence of a BrS-ECG+ and that genetic background may play a powerful role in the pathophysiology of BrS. These findings add further complexity to concepts regarding the causes of BrS, and are consistent with the emerging notion that the pathophysiology of BrS includes various elements beyond mutant sodium channels. Key Words: death, sudden (if surviving, use heart arrest) • Brugada syndrome • SCN5A • genetics • tachyarrhythmias • arrhythmia   CLINICAL PERSPECTIVE Drs Probst and Wilde contributed equally to this work. Related Article Nature’s Genetic Gradients and the Clinical Phenotype Ali J. Marian Circ Cardiovasc Genet 2009 2: 537-539. [Extract] [Full Text] [PDF]

Rapid advances in allele‐specific silencing by RNA interference established a strategy of choice to cure dominant inherited diseases by targeting mutant alleles. We used this strategy for autosomal‐dominant centronuclear myopathy (CNM), a rare neuromuscular disorder without available treatment due to heterozygous mutations in the DNM2 gene encoding Dynamin 2. Allele‐specific siRNA sequences were developed in order to specifically knock down the human and murine DNM2 ‐mRNA harbouring the p.R465W mutation without affecting the wild‐type allele. Functional restoration was achieved in muscle from a knock‐in mouse model and in patient‐derived fibroblasts, both expressing the most frequently encountered mutation in patients. Restoring either muscle force in a CNM mouse model or DNM2 function in patient‐derived cells is an essential breakthrough towards future gene‐based therapy for dominant centronuclear myopathy. Synopsis Autosomal dominant centronuclear myopathy (AD‐CNM) is a rare congenital myopathy due to heterozygous mutations in the DNM2 gene encoding Dynamin 2. Allele‐specific silencing of the mutant allele alleviates the phenotype in a CNM knock‐in mouse model and patient‐derived fibroblasts. siRNA targeting the mutant allele leads to functional restoration of the impaired endocytosis in mutant human fibroblasts. Early intra‐muscular administration of AAV1 expressing allele‐specific shRNA prevents the disease in the mouse model. Late treatment in the disease's time course partially improve the phenotype due to a weaker transduction capacity of the muscle. Allele‐specific silencing is a promising therapeutic strategy for AD‐CNM. Graphical Abstract Autosomal dominant centronuclear myopathy (AD‐CNM) is a rare congenital myopathy due to heterozygous mutations in the DNM2 gene encoding Dynamin 2. Allele‐specific silencing of the mutant allele alleviates the phenotype in a CNM knock‐in mouse model and patient‐derived fibroblasts.

Autosomal dominant centronuclear myopathy is a rare congenital myopathy characterized by delayed motor milestones and muscular weakness. In 11 families affected by centronuclear myopathy, we identified recurrent and de novo missense mutations in the gene dynamin 2 ( DNM2 , 19p13.2), which encodes a protein involved in endocytosis and membrane trafficking, actin assembly and centrosome cohesion. The transfected mutants showed reduced labeling in the centrosome, suggesting that DNM2 mutations might cause centronuclear myopathy by interfering with centrosome function.

Dynamin-2 is a ubiquitously expressed GTP-ase that mediates membrane remodeling. Recent findings indicate that dynamin-2 also regulates actin dynamics. Mutations in dynamin-2 cause dominant centronuclear myopathy (CNM), a congenital myopathy characterized by progressive weakness and atrophy of skeletal muscles. However, the muscle-specific roles of dynamin-2 affected by these mutations remain elusive. Here we show that, in muscle cells, the GTP-ase activity of dynamin-2 is involved in de novo actin polymerization as well as in actin-mediated trafficking of the glucose transporter GLUT4. Expression of dynamin-2 constructs carrying CNM-linked mutations disrupted the formation of new actin filaments as well as the stimulus-induced translocation of GLUT4 to the plasma membrane. Similarly, mature muscle fibers isolated from heterozygous knock-in mice that harbor the dynamin-2 mutation p.R465W, an animal model of CNM, exhibited altered actin organization, reduced actin polymerization and impaired insulin-induced translocation of GLUT4 to the sarcolemma. Moreover, GLUT4 displayed aberrant perinuclear accumulation in biopsies from CNM patients carrying dynamin-2 mutations, further suggesting trafficking defects. These results suggest that dynamin-2 is a key regulator of actin dynamics and GLUT4 trafficking in muscle cells. Our findings also support a model in which impairment of actin-dependent trafficking contributes to the pathological mechanism in dynamin-2-associated CNM.

Selenium is an essential trace element and selenoprotein N (SelN) was the first selenium-containing protein shown to be directly involved in human inherited diseases. Mutations in the SEPN1 gene, encoding SelN, cause a group of muscular disorders characterized by predominant affection of axial muscles. SelN has been shown to participate in calcium and redox homeostasis, but its pathophysiological role in skeletal muscle remains largely unknown. To address SelN function in vivo, we generated a Sepn1-null mouse model by gene targeting. The Sepn1(-/-) mice had normal growth and lifespan, and were macroscopically indistinguishable from wild-type littermates. Only minor defects were observed in muscle morphology and contractile properties in SelN-deficient mice in basal conditions. However, when subjected to challenging physical exercise and stress conditions (forced swimming test), Sepn1(-/-) mice developed an obvious phenotype, characterized by limited motility and body rigidity during the swimming session, as well as a progressive curvature of the spine and predominant alteration of paravertebral muscles. This induced phenotype recapitulates the distribution of muscle involvement in patients with SEPN1-Related Myopathy, hence positioning this new animal model as a valuable tool to dissect the role of SelN in muscle function and to characterize the pathophysiological process.

Marked prolongation of the QT interval on the electrocardiogram associated with the polymorphic ventricular tachycardia Torsades de Pointes is a serious adverse event during treatment with antiarrhythmic drugs and other culprit medications, and is a common cause for drug relabeling and withdrawal. Although clinical risk factors have been identified, the syndrome remains unpredictable in an individual patient. Here we used genome-wide association analysis to search for common predisposing genetic variants. Cases of drug-induced Torsades de Pointes (diTdP), treatment tolerant controls, and general population controls were ascertained across multiple sites using common definitions, and genotyped on the Illumina 610k or 1M-Duo BeadChips. Principal Components Analysis was used to select 216 Northwestern European diTdP cases and 771 ancestry-matched controls, including treatment-tolerant and general population subjects. With these sample sizes, there is 80% power to detect a variant at genome-wide significance with minor allele frequency of 10% and conferring an odds ratio of ≥2.7. Tests of association were carried out for each single nucleotide polymorphism (SNP) by logistic regression adjusting for gender and population structure. No SNP reached genome wide-significance; the variant with the lowest P value was rs2276314, a non-synonymous coding variant in C18orf21 (p  =  3×10(-7), odds ratio = 2, 95% confidence intervals: 1.5-2.6). The haplotype formed by rs2276314 and a second SNP, rs767531, was significantly more frequent in controls than cases (p  =  3×10(-9)). Expanding the number of controls and a gene-based analysis did not yield significant associations. This study argues that common genomic variants do not contribute importantly to risk for drug-induced Torsades de Pointes across multiple drugs.