Skeletal Muscle Channelopathies Research Articles

Quality of life in hypokalemic periodic paralysis - a survey

Primary hypokalemic periodic paralysis (HypoPP) is a skeletal muscle channelopathy most commonly caused by pathogenic variants in the calcium channel gene, CACNA1S. HypoPP can present with attacks of paralysis and/or permanent muscle weakness. Previous studies have shown that patients with HypoPP can have impaired quality of life (QoL). In this cross-sectional study, we aimed to describe the QoL in patients with HypoPP caused by pathogenic variants in CACNA1S using The Individualized Neuromuscular Quality of Life (INQoL) questionnaire, a validated tool to measure the QoL of patients with neuromuscular diseases (higher score, worse QoL). We showed that muscle weakness and fatigue were the symptoms with the greatest impact on participants' lives and that “activities”, in the life domain of the INQoL, was most affected by HypoPP. Furthermore, we showed that the total INQoL score increased with age. Low QoL was primarily driven by progressive permanent muscle weakness and not attacks of paralysis, although half of the participants reported that attacks of paralysis challenged their daily life. The results suggest that special attention should be given to muscle weakness and fatigue in patients with HypoPP.

Idiopathic generalized epilepsy in a family with SCN4A-related myotonia.

Myotonia is a clinical sign typical of a group of skeletal muscle channelopathies, the non-dystrophic myotonias. These disorders are electrophysiologically characterized by altered membrane excitability, due to specific genetic variants in known causative genes (CLCN1 and SCN4A). Juvenile Myoclonic Epilepsy (JME) is an epileptic syndrome identified as idiopathic generalized epilepsy, its genetics is complex and still unclarified. The co-occurrence of these two phenotypes is rare and the causes likely have a genetic background. In this study, we have genetically investigated an Italian family in which co-segregates myotonia, JME, or abnormal EEG without seizures was observed. All six individuals of the family, 4 affected and 2 unaffected, were clinically evaluated; EMG and EEG examinations were performed. For genetic testing, Exome Sequencing was performed for the six family members and Sanger sequencing was used to confirm the candidate variant. Four family members, the mother and three siblings, were affected by myotonia. Moreover, EEG recordings revealed interictal generalized sharp-wave discharges in all affected individuals, and two siblings were affected by JME. All four affected members share the same identified variant, c.644 T > C, p.Ile215Thr, in SCN4A gene. Variants that could account for the epileptic phenotype alone, separately from the myotonic one, were not identified. These results provide supporting evidence that both myotonic and epileptic phenotypes could share a common genetic background, due to variants in SCN4A gene. SCN4A pathogenic variants, already known to be causative of myotonia, likely increase the susceptibility to epilepsy in our family. This study analyzed all members of an Italian family, in which the mother and three siblings had myotonia and epilepsy. Genetic analysis allowed to identify a variant in the SCN4A gene, which appears to be the cause of both clinical signs in this family.

A retrospective study of accuracy and usefulness of electrophysiological exercise tests

ObjectivesThis study aimed to determine the usefulness of electrophysiological exercise tests. The significance of slightly abnormal exercise tests was also examined.MethodsWe identified all the patients who had undergone exercise testing between February 2007 to June 2022 in Tampere University Hospital, Finland. Their medical records after diagnostic workup and exercise test reports were reviewed. A binary logistic regression was performed to evaluate the association between positive test result in short exercise test, long exercise test, or short exercise test with cooling and genetically confirmed skeletal muscle channelopathy or myotonic disorder.ResultsWe identified 256 patients. 27 patients were diagnosed with nondystrophic myotonia, periodic paralysis, myotonic dystrophy type 1, myotonic dystrophy type 2, or other specified myopathy. 14 patients were suspected to have a skeletal muscle channelopathy, but pathogenic variants could not be identified. The remaining 215 patients were diagnosed with other conditions than skeletal muscle channelopathy or myotonic disorder. The combined sensitivity of exercise tests was 59.3% and specificity 99.1%. Abnormal exercise test result was associated with increased risk of skeletal muscle channelopathy or myotonic disorder (OR 164.3, 95% CI 28.3–954.6, p < 0.001).ConclusionsElectrophysiological exercise test is not optimal to exclude skeletal muscle channelopathy. It may be useful if a skeletal muscle channelopathy is suspected and genetic testing is negative or indeterminate and further evidence is required. Slightly abnormal exercise test results are possible in various conditions and result from different aetiologies. There is a demand for neurophysiological studies with higher sensitivity to detect skeletal muscle channelopathies.

P186 Spectrum of skeletal muscle channelopathies in a cohort of Inherited neuromuscular disorders

P186 Spectrum of skeletal muscle channelopathies in a cohort of Inherited neuromuscular disorders

Prevalence of genetically confirmed skeletal muscle channelopathies in the era of next generation sequencing

We provide an up-to-date and accurate minimum point prevalence of genetically defined skeletal muscle channelopathies which is important for understanding the population impact, planning for treatment needs and future clinical trials. Skeletal muscle channelopathies include myotonia congenita (MC), sodium channel myotonia (SCM), paramyotonia congenita (PMC), hyperkalemic periodic paralysis (hyperPP), hypokalemic periodic paralysis (hypoPP) and Andersen- Tawil Syndrome (ATS). Patients referred to the UK national referral centre for skeletal muscle channelopathies and living in UK were included to calculate the minimum point prevalence using the latest data from the Office for National Statistics population estimate. We calculated a minimum point prevalence of all skeletal muscle channelopathies of 1.99/100 000 (95% CI 1.981–1.999). The minimum point prevalence of MC due to CLCN1 variants is 1.13/100 000 (95% CI 1.123–1.137), SCN4A variants which encode for PMC and SCM is 0.35/100 000 (95% CI 0.346 – 0.354) and for periodic paralysis (HyperPP and HypoPP) 0.41/100 000 (95% CI 0.406–0.414). The minimum point prevalence for ATS is 0.1/100 000 (95% CI 0.098–0.102). There has been an overall increase in point prevalence in skeletal muscle channelopathies compared to previous reports, with the biggest increase found to be in MC. This can be attributed to next generation sequencing and advances in clinical, electrophysiological and genetic characterisation of skeletal muscle channelopathies.

Functional analysis of the mutant channels associated with skeletal muscle channelopathies

Functional analysis of the mutant channels associated with skeletal muscle channelopathies

Update on Muscle Channelopathy

Skeletal muscle channelopathy are rare heterogeneous episodic disorders with marked genotypic and phenotypic variability resulting in periodic paralysis, falls in young people, which often misdiagnosed or undiagnosed due to its rarity often. The symptoms are miscommunicated to the treating physician due to its episodic nature and not uncommonly physical examination by the time patient attend the clinic or hospital will be unremarkable apart from periodic muscle paralysis where patient will presented to ED with flaccid weakness. In this review we will present two patients where the diagnosis was initially missed and it took some time to reach the correct diagnosis.

Muscle Channelopathies

This article describes the clinical features, diagnosis, pathophysiology, and management of nondystrophic myotonia and periodic paralysis. An increasing awareness exists about the genotype-phenotype overlap in skeletal muscle channelopathies, and thus genetic testing is needed to make a definitive diagnosis. Electrodiagnostic testing in channelopathies is highly specialized with significant overlap in various mutation subtypes. Randomized clinical trials have now been conducted in these disorders with expanded treatment options for patients with muscle channelopathies. Skeletal muscle channelopathies are rare heterogeneous conditions characterized by lifelong symptoms that require a comprehensive management plan that includes pharmacologic and nonpharmacologic interventions. The significant variability in biophysical features of various mutations, coupled with the difficulties of performing clinical trials in rare diseases, makes it challenging to design and implement treatment trials for muscle channelopathies.

In silico versus functional characterization of genetic variants: lessons from muscle channelopathies.

Accurate determination of the pathogenicity of missense genetic variants of uncertain significance is a huge challenge for implementing genetic data in clinical practice. In silico predictive tools are used to score variants' pathogenicity. However, their value in clinical settings is often unclear, as they have not usually been validated against robust functional assays. We compared nine widely used in silico predictive tools, including more recently developed tools (EVE and REVEL) with detailed cell-based electrophysiology, for 126 CLCN1 variants discovered in patients with the skeletal muscle channelopathy myotonia congenita. We found poor accuracy for most tools. The highest accuracy was obtained with MutationTaster (84.58%) and REVEL (82.54%). Both of these scores showed poor specificity, although specificity was better using EVE. Combining methods based on concordance improved performance overall but still lacked specificity. Our calculated statistics for the predictive tools were different to reported values for other genes in the literature, suggesting that the utility of the tools varies between genes. Overall, current predictive tools for this chloride channel are not reliable for clinical use, and tools with better specificity are urgently required. Improving the accuracy of predictive tools is a wider issue and a huge challenge for effective clinical implementation of genetic data.

COVID-19 infection and vaccination in patients with skeletal muscle channelopathies.

Introduction/AimsAlthough we have gained insight into coronavirus disease‐2019 (COVID‐19) caused by severe acute respiratory syndrome–coronavirus 2 since the beginning of the pandemic, our understanding of the consequences for patients with neuromuscular disorders is evolving. In this study we aimed to study the impact of COVID‐19 and COVID‐19 vaccination on skeletal muscle channelopathies.MethodsWe conducted a survey of patients with genetically confirmed skeletal muscle channelopathies seen at the UK Nationally Commissioned Channelopathy Service.ResultsThirty‐eight patient responses were received. Six patients had COVID‐19 infection leading to exacerbation of their underlying muscle channelopathy. No major complications were reported. Thirty‐six patients had received one or two COVID‐19 vaccinations and the majority (68%) had no worsening of their underlying channelopathy. Thirty‐two percent reported worsening of their usual symptoms of their muscle channelopathy, but all reported recovery to baseline levels. No serious adverse events were reported.DiscussionThe overall rates of COVID‐19 infection were low in our study and COVID‐19 vaccine uptake rates were high. Our results have been useful to inform patients that a subset of patients have reversible worsening of their channelopathy post–COVID‐19 vaccination. Our study provides information for giving advice to patients with skeletal muscle channelopathies regarding COVID‐19 infection and vaccination.

Abstract P1099: Evidence Of Sialylation Pathway Alterations In Peripheral Blood Of Brugada Syndrome Patients

Brugada syndrome (BrS) is a cardiac arrhythmia associated with a higher risk of SCD. BrS is considered a genetic disorder, and the most commonly mutated gene is SCN5A, encoding the alpha subunit of the voltage-gated cardiac sodium channel (NaV1.5). Mutations of SCN5A usually lead to impairment of NaV1.5 functionality, which is considered the main mechanism of the disease. However, SCN5A mutations are responsible for only 30% of BrS cases. Therefore, it is conceivable that other mechanisms such as post-translational modifications (PTMs) affect NaV1.5 activity. Among others, sialylation may alter ion channel activity by carrying a sugar with a negative charge. Alterations in sialylation have been described in several cardiovascular diseases, as myocardial infarction, Chagas disease, and congenital disorders of glycosylation. For these reasons, the aim was to investigate changes in sialylation in BrS patients to obtain new information on the pathogenesis of BrS. Peripheral blood mononuclear cells (PBMCs) were collected from BrS patients and healthy controls. SNA lectin, a sialic acid-binding protein, was used to characterize the protein sialylation status of PBMCs by Western blot and flow cytometry. Gene expression of enzymes involved in the sialic acid pathway was also examined. The results showed a significant decrease in intracellular and extracellular protein sialylation levels in BrS PBMCs compared with controls. In addition, changes in gene expression of enzymes involved in the sialic acid pathway were detected. Moreover, these changes correlated with clinical parameters associated with phenotypic expression of the disease, such as arrhythmogenic BrS substrate area and potential duration. These findings support that BrS should be considered a systemic disease and are consistent with the presence of overlapping non-cardiac pathologies, as epilepsy, cancer, diabetes, skeletal muscle channelopathies, and laminopathies in BrS patients. Moreover, the discovery that molecular alterations can be found in the peripheral blood of BrS patients supports the existence of a biomarker of the disease. It is a challenge to develop an effective diagnostic test to screen broadly for BrS. In this direction, a multiomic study is ongoing in our center.

Genetic spectrum and founder effect of non-dystrophic myotonia: a Japanese case series study.

Non-dystrophic myotonias (NDM) are rare skeletal muscle channelopathies, mainly linked to two voltage-gated ion channel genes, CLCN1 and SCN4A. The aim of this study is to identify the clinical and genetic features of patients with NDM in Japan. We collected a Japanese nationwide case series of patients with clinical diagnosis of NDM (1999-2021). Among 71 out of 88 pedigrees, using Sanger and next-generation sequencing targeting both CLCN1 and SCN4A genes, variants classified as pathogenic/likely pathogenic/unknown significance were detected from CLCN1 (31 probands), SCN4A (36 probands), or both genes (4 probands), and 11 of them were novel. Pedigrees carrying mono-allelic CLCN1 variants were more commonly seen than that with bi-allelic/double variants (24:7). Compared to patients with CLCN1 variants, patients harboring SCN4A variants showed younger onset age (5.64 ± 4.70years vs. 9.23 ± 5.21years), fewer warm-up phenomenon, but more paramyotonia, hyperCKemia, transient muscle weakness, and cold-induced myotonia. Haplotype analysis verified founder effects of the hot spot variants in both CLCN1 (p.T539A) and SCN4A (p.T1313M). This study reveals variants in CLCN1 and SCN4A from 80.7% of our case series, extending genetic spectrum of NDM, and would further our understanding of clinical similarity/diversity between CLCN1- and SCN4A-related NDM, as well as the genetic racial differences.

Phenotypical variability and atypical presentations in a French cohort of Andersen-Tawil syndrome.

Andersen-Tawil syndrome (ATS) is a skeletal muscle channelopathy caused by KCNJ2 mutations, characterized by a clinical triad of periodic paralysis, cardiac arrhythmias and dysmorphism. The muscle phenotype, particularly the atypical forms with prominent permanent weakness or predominantly painful symptoms, remains incompletely characterized. A retrospective clinical, histological, electroneuromyography (ENMG) and genetic analysis of molecularly confirmed ATS patients, diagnosed and followed up at neuromuscular reference centers in France, was conducted. Thirty-five patients from 27 unrelated families carrying 17 different missense KCNJ2 mutations (four novel mutations) and a heterozygous KCNJ2 duplication are reported. The typical triad was observed in 42.9% of patients. Cardiac abnormalities were observed in 65.7%: 56.5% asymptomatic and 39.1% requiring antiarrhythmic drugs. 71.4% of patients exhibited dysmorphic features. Muscle symptoms were reported in 85.7%, amongst whom 13.3% had no cardiopathy and 33.3% no dysmorphic features. Periodic paralysis was present in 80% and was significantly more frequent in men. Common triggers were exercise, immobility and carbohydrate-rich diet. Ictal serum potassium concentrations were low in 53.6%. Of the 35 patients, 45.7% had permanent weakness affecting proximal muscles, which was mild and stable or slowly progressive over several decades. Four patients presented with exercise-induced pain and myalgia attacks. Diagnostic delay was 14.4±9.5years. ENMG long-exercise test performed in 25 patients (71.4%) showed in all a decremental response up to 40%. Muscle biopsy performed in 12 patients revealed tubular aggregates in six patients (associated in two of them with vacuolar lesions), dystrophic features in one patient and non-specific myopathic features in one patient; it was normal in four patients. Recognition of atypical features (exercise-induced pain or myalgia and permanent weakness) along with any of the elements of the triad should arouse suspicion. The ENMG long-exercise test has a high diagnostic yield and should be performed. Early diagnosis is of utmost importance to improve disease prognosis.

Excitability properties of mouse and human skeletal muscle fibres compared by muscle velocity recovery cycles

Mouse models of skeletal muscle channelopathies are not phenocopies of human disease. In some cases (e.g. Myotonia Congenita) the phenotype is much more severe, whilst in others (e.g. Hypokalaemic periodic paralysis) rodent physiology is protective. This suggests a species’ difference in muscle excitability properties. In humans these can be measured indirectly by the post-impulse changes in conduction velocity, using Muscle Velocity Recovery Cycles (MVRCs). We performed MVRCs in mice and compared their muscle excitability properties with humans. Mouse Tibialis Anterior MVRCs (n = 70) have only one phase of supernormality (increased conduction velocity), which is smaller in magnitude (p = 9 × 10−21), and shorter in duration (p = 3 × 10−24) than human (n = 26). This abbreviated supernormality is followed by a period of late subnormality (reduced velocity) in mice, which overlaps in time with the late supernormality seen in human MVRCs. The period of late subnormality suggests increased t-tubule Na+/K+-pump activity. The subnormal phase in mice was converted to supernormality by blocking ClC-1 chloride channels, suggesting relatively higher chloride conductance in skeletal muscle. Our findings help explain discrepancies in phenotype between mice and humans with skeletal muscle channelopathies and potentially other neuromuscular disorders. MVRCs are a valuable new tool to compare in vivo muscle membrane properties between species and will allow further dissection of the molecular mechanisms regulating muscle excitability.

Targeted pharmacotherapy for trafficking defective ClC-1 mutations in myotonia congenita

A delayed muscle relaxation after a voluntary contraction is the main clinical feature of myotonia congenita (MC). MC is a skeletal muscle channelopathy caused by loss-of-function mutations in the ClC-1 chloride channel, which impair channel function or plasma membrane expression. To date, no direct activator of ClC-1 channel is known. A potential strategy may be to restore surface expression of trafficking-defective ClC-1 mutants by using pharmacological chaperones. Thus, we tested the ability of niflumic acid (NFA), a reversible inhibitor of ClC-1, to restore membrane expression of MC ClC-1 mutants (A531V, V947E, G411C).

CHANNELOPATHIES AND RELATED DISORDERS: EP.221 Diagnostic delay and atypical phenotypes in a French cohort of Andersen-Tawil syndrome

Andersen- Tawil syndrome (ATS) is a skeletal muscle channelopathy caused by KCNJ2 mutations and characterized by a triad of periodic paralysis, cardiac arrhythmias and dysmorphism. The absence of the complete triad and/or the presence of atypical presentations can delay diagnosis. The muscle phenotype, particularly the atypical forms with permanent weakness or exercise intolerance, remains incompletely characterized. We report 33 patients from 26 unrelated families carrying 17 different missense KCJN2 mutations, 4 of them previously unreported. Only 14 patients (42.4%) present with the typical triad. 30 (90.9%) patients had muscle symptoms, 5 of them (15.2%) without cardiac or dysmorphic features. Periodic paralysis was noticed in 26 (78%), in most of them mirroring hypokalemic periodic paralysis. Ictal serum potassium concentrations were low in 13 (65%) and normal in 7 (35%). Sixteen (48.5%) had permanent weakness, 12 (36.4%) of them also had periodic paralysis. Seven (21.2%) patients presented with atypical muscle features. 4 of the latter had permanent weakness without periodic paralysis, 2 presented with mild proximal weakness and exercise intolerance and 1 with myalgia and raised CK levels. Weakness which usually affected scapular and pelvic girdles, was mild and slowly progressive. Only 1 patient needed walking aids (cane). Diagnostic delay was increased in patients presenting with atypical neuromuscular features or permanent weakness: 18.3 ±12.1 years vs 12±9.7 for those presenting with the classical triad. EMG with long exercise test was performed in 21 (63.6%), showing in all cases a decremental response up to >40%. Muscle biopsy was performed in 13, revealing no abnormalities in 5, dystrophic features in 5 and tubular aggregates in 3. No genotype-phenotype correlation was found. This work provides novel insights into the phenotypical characterization of muscular involvement in ATS and highlights the high diagnostic yield of the long exercise test.

The practice of active patient involvement in rare disease research using ICT: experiences and lessons from the RUDY JAPAN project

BackgroundThe role of patients in medical research is changing, as more emphasis is being placed on patient involvement, and patient reported outcomes are increasingly contributing to clinical decision-making. Information and communication technology provides new opportunities for patients to actively become involved in research. These trends are particularly noticeable in Europe and the US, but less obvious in Japan. The aim of this study was to investigate the practice of active involvement of patients in medical research in Japan by utilizing a digital platform, and to analyze the outcomes to clarify what specific approaches could be put into practice.MethodsWe developed the RUDY JAPAN system, an ongoing rare disease medical research platform, in collaboration with the Rare and Undiagnosed Diseases Study (RUDY) project in the UK. After 2 years of preparation, RUDY JAPAN was launched in December 2017. Skeletal muscle channelopathies were initially selected as target diseases, and hereditary angioedema was subsequently added. Several approaches for active patient involvement were designed through patient-researcher collaboration, namely the Steering Committee, questionnaire development, dynamic consent, and other communication strategies. We analyzed our practices and experiences focusing on how each approach affected and contributed to the research project.ResultsRUDY JAPAN has successfully involved patients in this research project in various ways. While not a part of the initial decision-making phase to launch the project, patients have increasingly been involved since then. A high level of patient involvement was achieved through the Steering Committee, a governance body that has made a major contribution to RUDY JAPAN, and the process of the questionnaire development. The creation of the Patient Network Forum, website and newsletter cultivated dialogue between patients and researchers. The registry itself allowed patient participation through data input and control of data usage through dynamic consent.ConclusionsWe believe the initial outcomes demonstrate the feasibility and utility of active patient involvement in Japan. The collaboration realized through RUDY JAPAN was enabled by digital technologies. It allowed busy patients and researchers to find the space to meet and work together for the Steering Committee, questionnaire development and various communication activities. While the practice of active patient involvement in Japan is still in its early stages, this research confirms its viability if the right conditions are in place. (331 words).

Presunta parálisis periódica hipercalémica con miotonía en perros Shi-Tzú y Pastor Alemán

En los humanos, las canalopatías del músculo esquelético son entidades de base genética y de incidencia muy baja. En relación al canal de sodio en particular, las diferentes mutaciones SCN4A pueden dar lugar a diversos fenotipos que van desde la parálisis muscular (parálisis periódica hipercalémica, parálisis periódica hipocalémica) a la hiperexcitabilidad muscular (paramiotonía congénita, miotonías agravadas por potasio), pasando por fenotipos combinados de las dos anteriores. Existe una estrecha interrelación entre la parálisis periódica hipercalémica y las miotonías; ambas características pueden coexistir en el mismo paciente.Este trabajo describe un cuadro clínico que afectó a una camada de 7 cachorros Pastor Alemán y a una camada de 4 cachorros Shi-Tzú, con episodios caracterizados por un fenotipo clínico de parálisis muscular combinado con hiperexcitabilidad muscular.De acuerdo a los hallazgos clínicos (fenomenología clínica, edad de aparición, recurrencia de los episodios, ausencia de signos clínicos entre los episodios) y de laboratorio (hipercalemia), y a los resultados del tratamiento con acetazolamida, el diagnóstico presuntivo fue parálisis periódica hipercalémica con miotonía/paramiotonía por una canalopatía SCN4A.Destacamos la importancia del diagnóstico clínico para estas raras patologías. En ausencia de diagnóstico genético, los métodos de diagnóstico complementario tiene valor limitado debido al carácter funcional de estos trastornos.Proponemos esta comunicación como la primera de este tipo en perros de raza Pastor Alemán y Shi-Tzú. In humans, skeletal muscle channelopathies are genetic disease with very low incidence. Regarding the sodium channel in particular, the different SCN4A mutations can result in various phenotypes ranging from muscle paralysis (hyperkalemic periodic paralysis, hypokalemic periodic paralysis) to muscle hyperexcitability (congenit paramyotonia, myotonias aggravated by potassium), through combined phenotypes of the previous two. There is a close interrelation between hyperkalemic periodic paralysis and myotonias; both characteristics can coexist in the same patient.This work describes a clinical picture affecting a litter of 7 German Shepherd puppies and a litter of 4 Shih-Tzu puppies with episodes characterized by a clinical phenotype of muscular paralysis combined with muscular hyperexcitability.Based on clinical findings (clinical phenomenology, age of onset, episodic recurrence, absense of clinical signs in the period between episodes) and laboratory (hyperkalemia), and the results of treatment with acetazolamide, the presumptive diagnosis was hyperkalemic periodic paralysis with myotonia/paramyotonia due to SCN4A channelopathy.We emphasize the importance of clinical diagnosis for these rare pathologies. In absence of genetic diagnosis, complementary diagnostic methods have limited value due to the functional nature of these disorders.We proposed this description as the first communication of this type in German Shepherd and Shi-Tzu dogs.

Next-generation sequencing application to investigate skeletal muscle channelopathies in a large cohort of Italian patients

Non-dystrophic myotonias and periodic paralyses are a heterogeneous group of disabling diseases classified as skeletal muscle channelopathies. Their genetic characterization is essential for prognostic and therapeutic purposes; however, several genes are involved. Sanger-based sequencing of a single gene is time-consuming, often expensive; thus, we designed a next-generation sequencing panel of 56 putative candidate genes for skeletal muscle channelopathies, codifying for proteins involved in excitability, excitation-contraction coupling, and metabolism of muscle fibres. We analyzed a large cohort of 109 Italian patients with a suspect of NDM or PP by next-generation sequencing. We identified 24 patients mutated in CLCN1 gene, 15 in SCN4A, 3 in both CLCN1 and SCN4A, 1 in ATP2A1, 1 in KCNA1 and 1 in CASQ1. Eight were novel mutations: p.G395Cfs*32, p.L843P, p.V829M, p.E258E and c.1471+4delTCAAGAC in CLCN1, p.K1302R in SCN4A, p.L208P in ATP2A1 and c.280–1G>C in CASQ1 genes. This study demonstrated the utility of targeted next generation sequencing approach in molecular diagnosis of skeletal muscle channelopathies and the importance of the collaboration between clinicians and molecular geneticists and additional methods for unclear variants to make a conclusive diagnosis.

MYASTHENIA & RELATED DISORDERS: P.36 Novel SCN4A mutations causing mild myotonic phenotype

Mutations in the sodium channel gene, SCN4A, encoding the Nav1.4 voltage-gated sodium channel, are well known causes of the skeletal muscle channelopathies: periodic paralyses (hyperkalemic, normokalemic, and hypokalemic periodic paralysis), paramyotonia congenita and other forms of myotonia. In addition, rare recessive SCN4A mutations have been associated with congenital myasthenic myopathy and congenital myopathy with hypotonia. To date over 40 dominant mutations in SCN4A have been reported to cause variable phenotypes depending on the type and location of the mutations.