EXCITATION-CONTRACTION COUPLING AND MUSCLE HOMEOSTASIS EC.O.1 Genetic and histological characterisation of excitation-contraction coupling related structural myopathy cohort S. Beecroft 1 , R. Choi 2 , C. McLean 3 , M. Olive 4 , M. Ryan 5 , M. Davis 6 , N. Laing 1 , B. Launikonis 2 , G. Ravenscroft 1 1 Harry Perkins Institute of Medical Research- UWA, Perth, Australia; 2 The University of Queensland, St Lucia, Australia; 3 Alfred Health, Melbourne, Australia; 4 Institute of Neuropathology, Barcelona, Spain; 5 Royal Children’s Hospital, Melbourne, Australia; 6 PathWest Laboratory Medicine, Perth, Australia Tubular aggregates and cylindrical spirals appear to be related entities, both arising from the sarcoplasmic reticulum, and sometimes appearing in the same biopsy. Tubular aggregates are inclusions of regular arrays of membrane tubules, while cylindrical spirals are accumulations of spiral lamellae around a central core. Although these have been reported as secondary findings in a range of conditions, some pure genetic forms of myopathy are characterised by the presence of tubular aggregates or cylindrical spirals. Tubular aggregate myopathy typically presents in childhood, causing slowly progressive weakness and cramps. Cylindrical spiral myopathy variably causes weakness, myalgia and exertional cramps, with onset in adulthood. Both diseases are dominant. The scarcity of cases impedes genetic and functional investigation. We have a cohort of two families and 10 isolated probands with tubular aggregate myopathy or cylindrical spirals. They underwent histological investigation, electron microscopy and next-generation DNA sequencing.Tubular aggregate myopathy genetic findings include known ORAI1 mutation c.G325A, p.V109M, and a novel putative STIM1 mutation c.A356G:p.D119G, and other candidate genes. Compound heterozygous TTN variants c.28654C>T, p.Q9552* and c.A15925G, p.I5309V were found in a cylindrical spiral myopathy case. The TTN missense change is predicted to affect splicing. Functional characterisation is ongoing, including mutant cell lines, cDNA analysis and fluorescence-based calcium handling assays. Here, we describe the work to date on characterising this cohort, including the histological and genetic features. http://dx.doi.org/10.1016/j.nmd.2017.06.023 EC.O.2 The blurred scenario of the new Calcium-related myopathies: clinical, radiological and molecular characterization of CASQ1, STIM1 and ORAI1 myopathies diagnosed in Padova neuromuscular center C. Semplicini 1 , C. Bertolin 1 , B. Pantic 1 , L. Bello 1 , S. Vianello 1 , F. Catapano 1 , I. Colombo 2 , M. Moggio 2 , G. Sorarù 1 , G. Cenacchi 3 , C. Calore 1 , R. Stramare 1 , E. Pegoraro 1 1 University of Padova, Padova, Italy; 2 University of Milano, Milano, Italy; 3 Alma Mater Studiorum University of Bologna, Bologna, Italy The triad is the skeletal muscle substructure responsible for excitation– contraction coupling by regulation of intracellular calcium homeostasis. A rapidly growing list of skeletal myopathies are caused by gene mutations in components of the triad (CASQ1, STIM1 and ORAI1), but the clinical and molecular features of these diseases are still largely unknown. We collected the clinical, radiological, molecular and histopathological features of patients with mutations in CASQ1, STIM1 and ORAI1 genes diagnosed at Padova neuromuscular center. Thirty patients were identified: 18 CASQ1-mutated, all but one sharing the same mutation; 10 STIM1 from a single family; 2 unrelated ORAI1. CASQ1 patients presented in the 5th decade with exercise intolerance, myalgias, rhabdomyolysis and late moderate lower limb proximal weakness. STIM1 patients presented early onset, proximal greater than distal muscle weakness and progression over time. ORAI1 patients presented exercise intolerance and myalgias. Muscle MRI of CASQ1 patients (n = 10) showed a peculiar pattern of fibro-fatty substitution (asymmetric involvement of anterior > posterior thigh and posterior leg). STIM1 patients’ MRI (n = 7) showed a homogeneous pattern of substitution with a posterior-to-anterior gradient. Muscle histopathology showed in all p.Asp244Gly CASQ1 patients large optically empty vacuoles, positive to sarcoplasmic reticulum protein. STIM1 and ORAI1 patients showed typical tubular aggregates (TAs) defined histopathological. The patient presenting the novel CASQ1 mutation presented histopathological features resembling more to TA than to vascular CASQ1 pattern (NADH-positive debris). We present a large series of patients with calcium-related myopathy, caused by mutation of CASQ1, STIM1 and ORAI1. The diseases differ for clinical features (exercise intolerance vs. weakness; early vs. late onset; slow vs. rapid evolution) and histopathological pattern (TAs vs. vascular myopathy), but overlaps are frequent. http://dx.doi.org/10.1016/j.nmd.2017.06.024 EC.O.3 Dysferlin reduces calcium leak and stabilizes excitation-contraction coupling in mature muscle R. Bloch, V. Lukyanenko, J. Muriel University of Maryland School of Medicine, Baltimore, MD, USA Dysferlin normally concentrates in the transverse tubules of skeletal muscle. When it is absent in dysferlin-null (A/J) fibers, osmotic shock injury (OSI) decreases the amplitude of voltage-induced Ca 2+ -transients (VICTs) and also markedly increases sarcoplasmic Ca 2+ (Kerr et al, PNAS, 2013). Neither occurs in control A/WySnJ fibers. To dissect the molecular pathways involved in these changes, we studied drugs that target L-type Ca 2+ -channels (LTCC: diltiazem, nifedipine, verapamil) and ryanodine receptors (RyR1: dantrolene, tetracaine, S107) on A/WySnJ and A/J FDB myofibers in culture to assess their effect on VICTs following OSI. We also examined A/J fibers transfected to express N-terminal Venus chimaeras of dysferlin (V-Dysf). All Ca 2+ antagonists inhibited VICTs in A/J and A/WySnJ fibers at high concentrations, but 1–10 µM diltiazem specifically increased VICT amplitudes by ~15% in A/J fibers, restoring them to control values. All the inhibitors at low concentrations improved recovery of VICTs in A/J fibers after OSI. These results strongly suggest that the decrease in VICTs after OSI is mediated by Ca 2+ -induced SR Ca 2+ leak through the RyR1. Consistent with this, injured A/J fibers produced Ca 2+ sparks, bursts and waves. Treatment of A/J fibers with 10 µM S107 (stabilizer of RyR1-FKBP coupling that reduces Ca 2+ leak) or expression of V-Dysf each protected A/J fibers against the decrease in VICTs following OSI and prevented OSI-induced Ca 2+ waves. A mutant dysferlin, Dysf-V67D, was not protective and, unlike wild type dysferlin, failed to traffic to the t-tubules. Our data suggest that, in the absence of active dysferlin in the t-tubules, OSI causes increased leak of SR Ca 2+ through the RyR1 that leads to muscle damage. We conclude that an important function of dysferlin in mature myofibers is to stabilize excitation-contraction coupling by reducing Ca 2+ leak when muscle is mechanically stressed. Supported by the Jain Foundation, MDA and NIH (RO1 AR064268). http://dx.doi.org/10.1016/j.nmd.2017.06.025 EC.O.4 Reversible endogenous downregulation of myostatin pathway in wasting neuromuscular diseases explains challenges of anti-myostatin therapeutic approaches V. Mariot 1 , R. Joubert 2 , C. Hourdé 3 , L. Féasson 4 , M. Hanna 1 , F. Muntoni 1 , T. Maisonobe 5 , L. Servais 6 , R. Le Panse 7 , O. Benveniste 7 , T. Stojkovic 5 , P. Machado 1 , T. Voit 1 , A. Buj-Bello 2 , J. Dumonceaux 1 1 University College of London, London, UK; 2 Genethon, Evry, France; 3 Université de Savoie – Mont Blanc, Le Bourget du Lac, France; 4 Inter-University Laboratory of Human Movement Biology, Le Bourget du Lac, France; 5 Groupe Hospitalier Pitié-Salpêtrière, Paris, France; 6 I-Motion, Paris, France; 7 Sorbonne Universités, Paris, France S97 Abstracts 2017/Neuromuscular Disorders 27 (2017) S96–S249