Clinical and Pathologic Features of Congenital Myasthenic Syndromes Caused by 35 Genes-A Comprehensive Review.

Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders characterized by impaired neuromuscular signal transmission due to germline pathogenic variants in genes expressed at the neuromuscular junction (NMJ). A total of 35 genes have been reported in CMS (AGRN, ALG14, ALG2, CHAT, CHD8, CHRNA1, CHRNB1, CHRND, CHRNE, CHRNG, COL13A1, COLQ, DOK7, DPAGT1, GFPT1, GMPPB, LAMA5, LAMB2, LRP4, MUSK, MYO9A, PLEC, PREPL, PURA, RAPSN, RPH3A, SCN4A, SLC18A3, SLC25A1, SLC5A7, SNAP25, SYT2, TOR1AIP1, UNC13A, VAMP1). The 35 genes can be classified into 14 groups according to the pathomechanical, clinical, and therapeutic features of CMS patients. Measurement of compound muscle action potentials elicited by repetitive nerve stimulation is required to diagnose CMS. Clinical and electrophysiological features are not sufficient to identify a defective molecule, and genetic studies are always required for accurate diagnosis. From a pharmacological point of view, cholinesterase inhibitors are effective in most groups of CMS, but are contraindicated in some groups of CMS. Similarly, ephedrine, salbutamol (albuterol), amifampridine are effective in most but not all groups of CMS. This review extensively covers pathomechanical and clinical features of CMS by citing 442 relevant articles.

Eosinophilic fasciitis with subjacent myositis.

INTRODUCTION: Eosinophilic fasciitis (EF) is a rare disorder that can present with muscle symptoms that mimic other neuromuscular diseases. METHODS: We report the case of a 43-year-old woman with chronic muscle aches, tightness, and stiffness with hypertrophied, well-defined muscles despite physical inactivity, and thickened skin with reduced elasticity and discoloration. RESULTS: Except for mild peripheral eosinophilia, laboratory studies, including blood count, electrolytes, paraneoplastic panel, muscle enzymes, thyroid function, and serum protein electrophoresis, were normal. Nerve conduction studies and needle electromyography were normal. Magnetic resonance imaging of the thighs demonstrated superficial and deep fascial thickening with T2 hyperintensity and post-gadolinium enhancement. Fascial and muscle biopsy demonstrated an inflammatory exudate in the perimysium and endomysium with fragmented perimysial connective tissue and thickened, inflamed fascia. EF was diagnosed. The patient was treated with methotrexate and prednisone followed by improvement of muscle stiffness and tightness. CONCLUSION: EF should be considered when patients present with muscle pain or enlarged muscles. Muscle Nerve 56: 525-529, 2017.

Microvascular alterations and the role of complement in dermatomyositis.

Different mechanisms have been proposed to explain the pathological basis of perifascicular muscle fibre atrophy in dermatomyositis. These include ischaemia due to immune-mediated microvascular injury, enhanced expression of type 1 interferon-induced gene transcripts in perifascicular capillaries and muscle fibres, and occlusion of larger perimysial blood vessels. Microvascular complement deposition is a feature of dermatomyositis pathology but the trigger for complement activation, the predominant complement pathway involved, or its role in the pathogenesis of the disease, has not been clearly defined. In the first step of this study we examined the density of capillaries and transverse vessels and searched for occlusion or depletion of larger perimysial blood vessels in 10 patients with dermatomyositis. This revealed an invariable association of perifascicular atrophy with capillary and transverse vessel depletion. The capillary and transverse vessel densities in non-atrophic fibre regions were not significantly different from those in muscle specimens of 10 age-matched controls. Next, in the same 10, as well as in 40 additional dermatomyositis patients, we searched for vascular deposits of IgG, IgM, and the C5b-9 complement membrane attack complex. Thirty-one of 50 dermatomyositis specimens contained C5b-9 reactive endomysial microvessels but none of these or other vessels reacted for IgG. Ten of 50 specimens harboured IgM-positive capillaries but only a few of these reacted for C5b-9. Finally, we analysed and compared different pathways of complement activation in dermatomyositis, lupus nephritis, and necrotic muscle fibres in Duchenne dystrophy. In lupus nephritis, C5-b9 deposits co-localized with IgG, IgM, C1q, and C4d, consistent with immune complex dependent activation of the classical complement pathway. In both dermatomyositis and Duchenne dystrophy, C5-b9 deposits co-localized with C1q and C4d and rarely with IgM indicating activation of the classical complement pathway. We conclude that: perifascicular atrophy in dermatomyositis is consistently associated with focal microvascular depletion, and that microvascular membrane attack complex deposits in dermatomyositis result from activation of the classical complement pathway triggered by direct binding of C1q to injured endothelial cells.

IntSplice: prediction of the splicing consequences of intronic single-nucleotide variations in the human genome.

Precise spatiotemporal regulation of splicing is mediated by splicing cis-elements on pre-mRNA. Single-nucleotide variations (SNVs) affecting intronic cis-elements possibly compromise splicing, but no efficient tool has been available to identify them. Following an effect-size analysis of each intronic nucleotide on annotated alternative splicing, we extracted 105 parameters that could affect the strength of the splicing signals. However, we could not generate reliable support vector regression models to predict the percent-splice-in (PSI) scores for normal human tissues. Next, we generated support vector machine (SVM) models using 110 parameters to directly differentiate pathogenic SNVs in the Human Gene Mutation Database and normal SNVs in the dbSNP database, and we obtained models with a sensitivity of 0.800±0.041 (mean and s.d.) and a specificity of 0.849±0.021. Our IntSplice models were more discriminating than SVM models that we generated with Shapiro-Senapathy score and MaxEntScan::score3ss. We applied IntSplice to a naturally occurring and nine artificial intronic mutations in RAPSN causing congenital myasthenic syndrome. IntSplice correctly predicted the splicing consequences for nine of the ten mutants. We created a web service program, IntSplice (http://www.med.nagoya-u.ac.jp/neurogenetics/IntSplice) to predict splicing-affecting SNVs at intronic positions from -50 to -3.

A novel desmin mutation leading to autosomal recessive limb-girdle muscular dystrophy: distinct histopathological outcomes compared with desminopathies.

BACKGROUND: Autosomal recessive limb girdle muscular dystrophy (LGMD2) is a heterogeneous group of myopathies characterised by progressive muscle weakness involving proximal muscles of the shoulder and pelvic girdles including at least 17 different genetic entities. Additional loci have yet to be identified as there are families which are unlinked to any of the known loci. Here we have investigated a consanguineous family with LGMD2 with two affected individuals in order to identify the causative gene defect. METHODS AND RESULTS: We performed genome wide homozygosity mapping and mapped the LGMD2 phenotype to chromosome 2q35-q36.3. DNA sequence analysis of the highly relevant candidate gene DES revealed a homozygous splice site mutation c.1289-2A>G in the two affected family members. Immunofluorescent staining and western blot analysis showed that the expression and the cytoskeletal network formation of mutant desmin were well preserved in skeletal muscle fibres. Unlike autosomal dominant desminopathies, ultrastructural alterations such as disruption of myofibrillar organisation, formation of myofibrillar degradation products and dislocation/aggregation of membranous organelles were not present. This novel splice site mutation results in addition of 16 amino acids within the tail domain of desmin, which has been suggested to interact with lamin B protein. We also detected a specific disruption of desmin-lamin B interaction in the skeletal muscle of the patient by confocal laser scanning microscopy. CONCLUSIONS: Our study reveals that autosomal recessive mutations in DES cause LGMD2 phenotype without features of myofibrillar myopathy.

Advancing the Understanding of Vesicle-Associated Membrane Protein 1-Related Congenital Myasthenic Syndrome: Phenotypic Insights, Favorable Response to 3,4-Diaminopyridine, and Clinical Characterization of Five New Cases.

BACKGROUND: Congenital myasthenic syndromes (CMS) are a group of inherited neuromuscular junction (NMJ) disorders arising from gene variants encoding diverse NMJ proteins. Recently, the VAMP1 gene, responsible for encoding the vesicle-associated membrane protein 1 (VAMP1), has been associated with CMS. METHODS: This study presents a characterization of five new individuals with VAMP1-related CMS, providing insights into the phenotype. RESULTS: The individuals with VAMP1-related CMS exhibited early disease onset, presenting symptoms prenatally or during the neonatal period, alongside severe respiratory involvement and feeding difficulties. Generalized weakness at birth was a common feature, and none of the individuals achieved independent walking ability. Notably, all cases exhibited scoliosis. The clinical course remained stable, without typical exacerbations seen in other CMS types. The response to anticholinesterase inhibitors and salbutamol was only partial, but the addition of 3,4-diaminopyridine (3,4-DAP) led to significant and substantial improvements, suggesting therapeutic benefits of 3,4-DAP for managing VAMP1-related CMS symptoms. Noteworthy is the identification of the VAMP1 (NM_014231.5): c.340delA; p.Ile114SerfsTer72 as a founder variant in the Iberian Peninsula and Latin America. CONCLUSIONS: This study contributes valuable insights into VAMP1-related CMS, emphasizing their early onset, arthrogryposis, facial and generalized weakness, respiratory involvement, and feeding difficulties. Furthermore, the potential efficacy of 3,4-DAP as a useful therapeutic option warrants further exploration. The findings have implications for clinical management and genetic counseling in affected individuals. Additional research is necessary to elucidate the long-term outcomes of VAMP1-related CMS.

Anesthesia and Duchenne or Becker muscular dystrophy: review of 117 anesthetic exposures.

BACKGROUND: Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are associated with life-threatening perioperative complications, including rhabdomyolysis, hyperkalemia, and hyperthermia. Current recommendations contraindicate use of succinylcholine and volatile anesthetics; however, the latter recommendation remains controversial. OBJECTIVE: To review the perioperative outcomes of patients with DMD and BMD. METHODS: We reviewed records of patients with DMD or BMD who underwent anesthetic management at our institution from January 1990 through December 2011. RESULTS: We identified 47 patients (DMD, 37; BMD, 10) who underwent 117 anesthetic exposures (DMD, 101; BMD, 16). Volatile anesthetic agents were used 66 times (DMD, 59; BMD, 7). One patient with undiagnosed BMD received succinylcholine and developed acute rhabdomyolysis and hyperkalemic cardiac arrest. All other major complications were attributed to the procedure (i.e., large bleeding), to preexisting comorbidities (i.e., respiratory failure, cardiac disease), or to both. CONCLUSIONS: Use of succinylcholine in children with dystrophinopathy is contraindicated. These patients have significant comorbidities and are frequently undergoing extensive operations; complications related to these factors can develop, as evidenced by our series. These complications may occur with use of volatile and nonvolatile anesthetics. However, because most of our patients were older than 8 years at the time of surgery, our observation cannot be generalized to younger dystrophin-deficient children.

Impaired gating of γ- and ε-AChR respectively causes Escobar syndrome and fast-channel myasthenia.

OBJECTIVE: To dissect the kinetic defects of acetylcholine receptor (AChR) γ subunit variant in an incomplete form of the Escobar syndrome without pterygium and compare it with those of a variant of corresponding residue in the AChR ε subunit in a congenital myasthenic syndrome (CMS). METHODS: Whole exome sequencing, α-bungarotoxin binding assay, single channel patch-clamp recordings, and maximum likelihood analysis of channel kinetics. RESULTS: We identified compound heterozygous variants in AChR γ and ε subunits in three Escobar syndrome (1-3) and three CMS patients (4-6), respectively. Each Escobar syndrome patient carries γP121R along with γV221Afs*44 in patients 1 and 2, and γY63* in patient 3. Three CMS patients share εP121T along with εR20W, εG-8R, and εY15H in patients 4, 5, and 6, respectively. Surface expressions of γP121R- and εP121T-AChR were 80% and 138% of the corresponding wild-type AChR, whereas εR20W, εG-8R, and εY15H reduced receptor expression to 27%, 35%, and 30% of wild-type εAChR, respectively. γV221Afs*44 and γY63* are null variants. Thus, γP121R and εP121T determine the phenotype. γP121R and εP121T shorten channel opening burst duration to 28% and 18% of corresponding wild-type AChR by reducing the channel gating equilibrium constant 44- and 63-fold, respectively. INTERPRETATION: Similar impairment of channel gating efficiency of a corresponding P121 residue in the acetylcholine-binding site of the AChR γ and ε subunits causes Escobar syndrome without pterygium and fast-channel CMS, respectively, suggesting that therapy for the fast-channel CMS will benefit Escobar syndrome.

Infantile muscular dystrophy in Canadian aboriginals is an αB-crystallinopathy.

OBJECTIVE: A recessively transmitted fatal hypertonic infantile muscular dystrophy has been described in Canadian aboriginals. The affected infants present with progressive limb and axial muscle stiffness and develop severe respiratory insufficiency, and most die in the first year of life. We sought to determine the genetic basis of this disease. METHODS: We performed histochemical, immunocytochemical, electron microscopy, and molecular genetic studies in a cohort of 12 patients affected by this disease. RESULTS: Conventional histochemical and electron microscopy studies suggested myofibrillar myopathy (MFM). Therefore, we searched for ectopic expression of multiple proteins typical of MFM. Alpha B-crystallin (αBC) expression was absent from all fibers using a monoclonal antibody raised against the entire protein. However, a monoclonal antibody directed against the first 10 residues of αBC immunostained portions of abnormal fibers. Pursuing this clue, we searched for mutations in the gene for αBC (CRYAB) in available DNA samples of 8 patients. All harbored a homozygous deletion, c.60C, predicting a Ser to Ala change at codon 21 and a stop codon after 23 missense residues (p.Ser21AlafsX24). Clinically unaffected parents were heterozygous for this mutation. INTERPRETATION: The homozygous c.60delC in CRYAB pinpoints the genetic basis of the fatal infantile hypertonic muscular dystrophy of Canadian aboriginals. MFMs are typically transmitted by dominant inheritance, but in this disease the parental phenotype is rescued by limited expression of the highly truncated nonfunctional mutant gene product. The severe patient phenotype is due to homozygosity for the markedly hypomorphic allele. Ann Neurol, 2011.

BAG3 mutations: another cause of giant axonal neuropathy.

Mutations in Bcl-2 associated athanogene-3 (BAG3) are a rare cause of myofibrillar myopathy, characterised by rapidly progressive proximal and axial myopathy, cardiomyopathy and respiratory compromise. Neuropathy has been documented neurophysiologically in previously reported cases of BAG3-associated myofibrillar myopathy and in some cases giant axons were observed on nerve biopsies; however, neuropathy was not thought to be a dominant feature of the disease. In the context of inherited neuropathy, giant axons are typically associated with autosomal recessive giant axonal neuropathy caused by gigaxonin mutations but have also been reported in association with NEFL- and SH3TC2-associated Charcot-Marie-Tooth disease. Here, we describe four patients with heterozygous BAG3 mutations with clinical evidence of a sensorimotor neuropathy, with predominantly axonal features on neurophysiology. Three patients presented with a significant neuropathy. Muscle magnetic resonance imaging (MRI) in one patient revealed mild to moderate atrophy without prominent selectivity. Examination of sural nerve biopsies in two patients demonstrated giant axons. This report confirms the association of giant axonal neuropathy with BAG3-associated myofibrillar myopathy, and highlights that neuropathy may be a significant feature.

Proteomic profiling of sporadic late-onset nemaline myopathy.

OBJECTIVE: To define the proteomic profile of sporadic late-onset nemaline myopathy (SLONM) and explore its pathogenesis. METHODS: We performed mass spectrometry on laser-dissected frozen muscle samples from five patients with SLONM, three of whom with an associated monoclonal protein (MP), and four controls, to determine the proteomic profile of SLONM. Furthermore, we assessed the role of the MP by evaluating the expression of the immunoglobulin light chain variable regions (IGVL). RESULTS: There were 294 differentially expressed proteins: 272 upregulated and 22 downregulated. Among the top 100 upregulated proteins, the most common categories were: nuclear or nucleic acid metabolism (24%), extracellular matrix and basal lamina (17%), immune response (13%), and actin dynamics (8%). Downregulated proteins consisted mostly of contractile proteins. Among upregulated proteins, there were 65 with a role related to the immune system, including eight proteins involved in major histocompatibility complex 1 (MHC1) and antigen processing, 15 in MHCII complex and phagocytosis, and 23 in B and/or T-cell function. Among nine upregulated immunoglobulin proteins, there were two IGVL genes. However, these were also detected in SLONM cases without an MP, with no evidence of clonally dominant immunoglobulin deposition. In muscle sections from SLONM patients, nemaline rods tended to accumulate in atrophic fibers with marked rarefaction of the myofibrils. Increased MHC1 reactivity was present in fibers containing nemaline rods as well as adjacent nonatrophic fibers. CONCLUSION: Our findings suggest that aberrant immune activation is present in SLONM, but do not support a direct causal relationship between the MP and SLONM.

Beneficial effects of albuterol in congenital endplate acetylcholinesterase deficiency and Dok-7 myasthenia.

INTRODUCTION: Congenital myasthenic syndromes (CMS) are disabling but treatable disorders. Anticholinesterase therapy is effective in most of them, but is contraindicated in endplate (EP) acetylcholinesterase (AChE) deficiency, the slow-channel syndrome, Dok-7 myasthenia, and ß(2) -laminin deficiency, and is not useful in CMS due to defects in muscle-specific kinase (MuSK), agrin, and plectin. EP AChE, Dok-7, and ß(2)-laminin deficiencies respond favorably to ephedrine, but ephedrine can no longer be prescribed in the USA. METHODS: We used albuterol, another sympathomimetic agent, to treat 3 patients with EP AChE deficiency and 15 with Dok-7 myasthenia. Response to therapy was evaluated by a 9-point questionnaire pertaining to activities of daily life. RESULTS: Comparison of the pre- and posttreatment responses indicated a beneficial response to albuterol (P < 0.001) in both patient groups. The adverse effects of therapy were like those of ephedrine. CONCLUSION: Our observations should spur controlled, prospective clinical trials of albuterol in these as well as other CMS.

Expanding Spectrum of Desmin-Related Myopathy, Long-term Follow-up, and Cardiac Transplantation.

BACKGROUND AND OBJECTIVES: We aimed to determine the genetic and clinical phenotypes of patients with desmin-related myopathy and long-term outcomes after cardiac transplantation. METHODS: We performed a retrospective review of cardiac and neurologic manifestations of patients with genetically confirmed desmin-related myopathy (January 1, 1999-January 1, 2020). RESULTS: Twenty-five patients in 20 different families were recognized. Median age at onset of symptoms was 20 (range 4-50) years; median follow-up time was 36 (range 1-156) months. Twelve patients initially presented with skeletal muscle involvement, and 13 presented with cardiac disease. Sixteen patients had both cardiac and skeletal muscle involvement. Clinically muscle weakness distribution was distal (n = 11), proximal (n = 4), or both (n = 7) in 22 patients. Skeletal muscle biopsy from patients with missense and splice site variants (n = 12) showed abnormal fibers containing amorphous material in Gomori trichrome-stained sections. Patients with cardiac involvement had atrioventricular conduction abnormalities or cardiomyopathy. The most common ECG abnormality was complete atrioventricular block in 11 patients, all of whom required a permanent pacemaker at a median age of 25 (range 16-48) years. Sudden cardiac death resulting in implantable cardioverter-defibrillator (ICD) shocks or resuscitation was reported in 3 patients; a total of 5 patients had ICDs. Orthotopic cardiac transplantation was performed in 3 patients at 20, 35, and 39 years of age. DISCUSSION: Pathogenic variants in desmin can lead to varied neurologic and cardiac phenotypes beginning at a young age. Two-thirds of the patients have both neurologic and cardiac symptoms, usually starting in the third decade. Heart transplantation was tolerated with improved cardiac function and quality of life.

Myofibrillar myopathies.

PURPOSE OF REVIEW: The aim of this communication is to provide an up-to-date overview of myofibrillar myopathies (MFMs). RECENT FINDINGS: The most important recent advance in the MFMs has been the identification of mutation in Bag3 (Bcl-2-associated athanogene-3) as a new cause of MFM. Although, the typical clinical manifestations of MFMs are slowly progressive weakness, the patients with Bag3opathy may have had a rapidly progressive and more severe phenotype. SUMMARY: Several MFM disease genes have recently been recognized. The identified disease proteins (desmin, alphaB-crystallin, myotilin, Zasp, filamin C, and Bag3) interact with components or with chaperones of the Z-disk. In each case the molecular defect leads to a largely stereotyped cascade of structural perturbation of the muscle fiber architecture.

Mutation in BAG3 causes severe dominant childhood muscular dystrophy.

OBJECTIVE: Myofibrillar myopathies (MFMs) are morphologically distinct but genetically heterogeneous muscular dystrophies in which disintegration of Z disks and then of myofibrils is followed by ectopic accumulation of multiple proteins. Cardiomyopathy, neuropathy, and dominant inheritance are frequent associated features. Mutations in alphaB-crystallin, desmin, myotilin, Zasp, or filamin-C can cause MFMs and were detected in 32 of 85 patients of the Mayo MFM cohort. Bag3, another Z-disk-associated protein, has antiapoptotic properties, and its targeted deletion in mice causes fulminant myopathy with early lethality. We therefore searched for mutations in BAG3 in 53 unrelated MFM patients. METHODS: We searched for mutations in BAG3 by direct sequencing. We analyzed structural changes in muscle by histochemistry, immunocytochemistry, and electron microscopy, examined mobility of the mutant Bag3 by nondenaturing electrophoresis, and searched for abnormal aggregation of the mutant protein in COS-7 (SV-40 transformed monkey kidney fibroblast-7) cells. RESULTS: We identified a heterozygous p.Pro209Leu mutation in three patients. All presented in childhood, had progressive limb and axial muscle weakness, and experienced development of cardiomyopathy and severe respiratory insufficiency in their teens; two had rigid spines, and one a peripheral neuropathy. Electron microscopy showed disintegration of Z disks, extensive accumulation of granular debris and larger inclusions, and apoptosis of 8% of the nuclei. On nondenaturing electrophoresis of muscle extracts, the Bag3 complex migrated faster in patient than control extracts, and expression of FLAG-labeled mutant and wild-type Bag3 in COS cells showed abnormal aggregation of the mutant protein. INTERPRETATION: We conclude mutation in Bag3 defines a novel severe autosomal dominant childhood muscular dystrophy.

Sleep and Breathing After Nusinersen Therapy in a Child With Spinal Muscular Atrophy.

BACKGROUND: Spinal muscular atrophy (SMA) type 3 is an autosomal recessive neurological disorder associated with a deletion/mutation in the survival motor neuron gene, with gradually progressive degeneration of the motor neurons of the spinal cord and brainstem, which causes muscle weakness responsible for impairment of swallowing, breathing, and mobility. REPORT OF CASE: We report an 11-year-old girl with SMA type 3 with moderate to severe obstructive sleep apnea (OSA) syndrome refractory to adenotonsillectomy and noninvasive ventilatory support. She was started on nusinersen, which is a novel disease modifying therapy for SMA. This new treatment led to improvement of the OSA in a short period, likely from better respiratory muscle function. CONCLUSIONS: The improvement in OSA supports the role of nusinersen in sleep-related upper respiratory muscle function in SMA type 3.

Determinants of the repetitive-CMAP occurrence and therapy efficacy in slow-channel myasthenia.

OBJECTIVE: To find determinants of the occurrence of repetitive compound muscle action potential (R-CMAP) and to assess the efficacy of channel blocker therapy in slow-channel congenital myasthenic syndrome (SCCMS). METHODS: Neurologic examination, EMG study, laboratory test, muscle biopsy, and next-generation and Sanger sequencing; literature review of reported patients with SCCMS, including EMG, kinetics of mutant acetylcholine receptors (AChRs), and response to therapy; and simulation of the decay phase of endplate potential (EPP) were performed. RESULTS: Three newly characterized and 57 reported patients with SCCMS with mutations of AChR subunits were included. In patients with R-CMAP, the length of channel opening bursts of mutant AChR was increased 8.68 ± 2.82 (mean ± SD)-fold compared to wild-type; in patients without R-CMAP, the length was increased 3.84 ± 0.65-fold (95% confidence interval 3.18-6.50, p = 0.000014). The EPP amplitude after refractory period of action potential in muscle fiber is above the threshold in patients with R-CMAP but below the threshold in patients without R-CMAP. In patients with good results from channel blocker therapy, treatment was initiated 11.60 ± 5.17 years after onset of symptoms; in patients with no to moderate benefit from channel blocker therapy, treatment was initiated 30.70 ± 12.72 years after onset (95% confidence interval -28.57 to -9.63, p = 0.00089). CONCLUSIONS: In SCCMS, the R-CMAP occurrence is related to the extent of prolongation of the opening episodes of mutant AChR channel. Channel blocker treatment is more effective the sooner it is started after the onset of symptoms. CLASSIFICATION OF EVIDENCE: This study provides Class IV evidence that channel blocker therapy in patients with SCCMS improves symptoms.

A novel fast-channel myasthenia caused by mutation in ß subunit of AChR reveals subunit-specific contribution of the intracellular M1-M2 linker to channel gating.

Genetic variants causing the fast-channel congenital myasthenic syndrome (CMS) have been identified in the α, δ, and ε but not the ß subunit of acetylcholine receptor (AChR). A 16-year-old girl with severe myasthenia had low-amplitude and fast-decaying miniature endplate potentials. Mutation analysis revealed two heteroallelic variants in CHRNB1 encoding the AChR ß subunit: a novel c.812C>T (p.P248L) variant in M1-M2 linker (p.P271L in HGVS nomenclature), and a ~430 bp deletion causing loss of exon 8 leading to frame-shift and a premature stop codon (p.G251Dfs*21). P248 is conserved in all ß subunits of different species, but not in other AChR subunits. Measurements of radio-labeled α-bungarotoxin binding show that ßP248L reduces AChR expression to 60% of wild-type. Patch clamp recordings of ACh-elicited single channel currents demonstrate that ßP248L shortens channel opening bursts from 3.3 ms to 1.2 ms, and kinetic analyses predict that the decay of the synaptic response is accelerated 2.4-fold due to reduced probability of channel reopening. Substituting ßP248 with threonine, alanine or glycine reduces the burst duration to 2.3, 1.7, and 1.5 ms, respectively. In non-ß subunits, substituting leucine for residues corresponding to ßP248 prolongs the burst duration to 4.5 ms in the α subunit, shortens it to 2.2 ms in the δ subunit, and has no effect in the ε subunit. Conversely, substituting proline for residues corresponding to ßP248 prolongs the burst duration to 8.7 ms in the α subunit, to 4.6 ms in the δ subunit, but has no effect in the ε subunit. Thus, this fast channel CMS is caused by the dual defects of ßP248L in reducing expression of the mutant receptor and accelerating the decay of the synaptic response. The results also reveal subunit-specific contributions of the M1-M2 linker to the durations of channel opening bursts.

Developmental brain abnormalities and acute encephalopathy in a patient with myopathy with extrapyramidal signs secondary to pathogenic variants in MICU1.

Mitochondria play a variety of roles in the cell, far beyond their widely recognized role in ATP generation. One such role is the regulation and sequestration of calcium, which is done with the help of the mitochondrial calcium uniporter (MCU) and its regulators, MICU1 and MICU2. Genetic variations in MICU1 and MICU2 have been reported to cause myopathy, developmental disability and neurological symptoms typical of mitochondrial disorders. The symptoms of MICU1/2 deficiency have generally been attributed to calcium regulation in the metabolic and biochemical roles of mitochondria. Here, we report a female child with heterozygous MICU1 variants and multiple congenital brain malformations on MRI. Specifically, she shows anterior perisylvian polymicrogyria, dysmorphic basal ganglia, and cerebellar dysplasia in addition to white matter abnormalities. These novel findings suggest that MICU1 is necessary for proper neurodevelopment through a variety of potential mechanisms, including calcium-mediated regulation of the neuronal cytoskeleton, Miro1-MCU complex-mediated mitochondrial movement, or enhancing ATP production. This case provides new insight into the molecular pathogenesis of MCU dysfunction and may represent a novel diagnostic feature of calcium-based mitochondrial disease.