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BACKGROUNDFXLEARN, the first-ever large multisite trial of effects of disease-targeted pharmacotherapy on learning, was designed to explore a paradigm for measuring effects of mechanism-targeted treatment in fragile X syndrome (FXS). In FXLEARN, the effects of metabotropic glutamate receptor type 5 (mGluR5) negative allosteric modulator (NAM) AFQ056 on language learning were evaluated in 3- to 6-year-old children with FXS, expected to have more learning plasticity than adults, for whom prior trials of mGluR5 NAMs have failed.METHODSAfter a 4-month single-blind placebo lead-in, participants were randomized 1:1 to AFQ056 or placebo, with 2 months of dose optimization to the maximum tolerated dose, then 6 months of treatment during which a language-learning intervention was implemented for both groups. The primary outcome was a centrally scored videotaped communication measure, the Weighted Communication Scale (WCS). Secondary outcomes were objective performance-based and parent-reported cognitive and language measures.RESULTSFXLEARN enrolled 110 participants, randomized 99, and had 91 who completed the placebo-controlled period. Although both groups made language progress and there were no safety issues, the change in WCS score during the placebo-controlled period was not significantly different between the AFQ056 and placebo-treated groups, nor were there any significant between-group differences in change in any secondary measures.CONCLUSIONDespite the large body of evidence supporting use of mGluR5 NAMs in animal models of FXS, this study suggests that this mechanism of action does not translate into benefit for the human FXS population and that better strategies are needed to determine which mechanisms will translate from preclinical models to humans in genetic neurodevelopmental disorders.TRIAL REGISTRATIONClincalTrials.gov NCT02920892.FUNDING SOURCESNeuroNEXT network NIH grants U01NS096767, U24NS107200, U24NS107209, U01NS077323, U24NS107183, U24NS107168, U24NS107128, U24NS107199, U24NS107198, U24NS107166, U10NS077368, U01NS077366, U24NS107205, U01NS077179, and U01NS077352; NIH grant P50HD103526; and Novartis IIT grant AFQ056X2201T for provision of AFQ056.

Patients with myotonia congenita suffer from slowed muscle relaxation caused by hyperexcitability. The diaphragm is only mildly affected in myotonia congenita; discovery of the mechanism underlying its resistance to myotonia could identify novel therapeutic targets. Intracellular recordings from two mouse models of myotonia congenita revealed the diaphragm had less myotonia than either the extensor digitorum longus (EDL) or the soleus muscles. A mechanism contributing to resistance of the diaphragm to myotonia was reduced depolarization of the interspike membrane potential during repetitive firing of action potentials, a process driven by build‐up of K+ in small invaginations of muscle membrane known as t‐tubules. We explored differences between diaphragm and EDL that might underlie reduction of K+ build‐up in diaphragm t‐tubules. Smaller size of diaphragm fibres, which promotes diffusion of K+ out of t‐tubules, was identified as a contributor. Intracellular recording revealed slower repolarization of action potentials in diaphragm suggesting reduced Kv conductance. Higher resting membrane conductance was identified suggesting increased Kir conductance. Computer simulation found that a reduction of Kv conductance had little effect on K+ build‐up whereas increased Kir conductance lessened build‐up, although the effect was modest. Our data and computer simulation suggest opening of K+ channels during action potentials has little effect on K+ build‐up whereas opening of K+ channels during the interspike interval slightly lessens K+ build‐up. We conclude that activation of K+ channels may lessen myotonia by opposing depolarization to action potential threshold without worsening K+ build‐up in t‐tubules. Key points: In mouse models of the muscle disease myotonia congenita, the diaphragm has much less myotonia (muscle stiffness) than the extensor digitorum longus or soleus muscles.Identifying why the diaphragm is resistant to myotonia may help in developing novel therapy.We found the reason the diaphragm has less myotonia is that it is less prone to depolarization caused by K+ build‐up in t‐tubules during repetitive firing of action potentials.Smaller fibre size contributes to resistance to K+ build‐up with differences in K+ currents playing little role.Our data suggest drugs that open K+ channels may be effective in treating myotonia as they may lessen excitability without worsening K+ build‐up in t‐tubules. [ABSTRACT FROM AUTHOR]

Patients with myotonia congenita suffer from slowed relaxation of muscle (myotonia), due to hyperexcitability caused by loss-of-function mutations in the ClC-1 chloride channel. A recent study suggested that block of large-conductance voltage- and Ca2+- activated K+ channels (BK) may be effective as therapy. The mechanism underlying efficacy was suggested to be lessening of the depolarizing effect of build-up of K+ in t-tubules of muscle during repetitive firing. BK channels are widely expressed in the nervous system and have been shown to play a central role in regulation of excitability, but their contribution to muscle excitability has not been determined. We performed intracellular recordings as well as force measurements in both wild type and BK−/− mouse extensor digitorum longus muscles. Action potential width was increased in BK−/− muscle due to slowing of repolarization, consistent with the possibility K+ build-up in t-tubules is lessened by block of BK channels in myotonic muscle. However, there was no difference in the severity of myotonia triggered by block of muscle Cl− channels with 9-anthracenecarboxylic acid (9AC) in wild type and BK−/− muscle fibers. Further study revealed no difference in the interspike membrane potential during repetitive firing suggesting there was no reduction in K+ build-up in t-tubules of BK−/− muscle. Force recordings following block of muscle Cl− channels demonstrated little reduction in myotonia in BK−/− muscle. In contrast, the current standard of care, mexiletine, significantly reduced myotonia. Our data suggest BK channels regulate muscle excitability, but are not an attractive target for therapy of myotonia. [ABSTRACT FROM AUTHOR]

Introduction/Aims: Myotonia congenita (MC) is the most common hereditary channelopathy in humans. Characterized by muscle stiffness, MC may be transmitted as either an autosomal dominant (Thomsen) or a recessive (Becker) disorder. MC is caused by variants in the voltage‐gated chloride channel 1 (CLCN1) gene, important for the normal repolarization of the muscle action potential. More than 250 disease‐causing variants in the CLCN1 gene have been reported. This study provides an MC genotype–phenotype spectrum in a large cohort of Greek patients and focuses on novel variants and disease epidemiology, including additional insights for the variant CLCN1:c.501C > G. Methods: Sanger sequencing for the entire coding region of the CLCN1 gene was performed. Targeted segregation analysis of likely candidate variants in additional family members was performed. Variant classification was based on American College of Medical Genetics (ACMG) guidelines. Results: Sixty‐one patients from 47 unrelated families were identified, consisting of 51 probands with Becker MC (84%) and 10 with Thomsen MC (16%). Among the different variants detected, 11 were novel and 16 were previously reported. The three most prevalent variants were c.501C > G, c.2680C > T, and c.1649C > G. Additionally, c.501C > G was detected in seven Becker cases in‐cis with the c.1649C > G. Discussion: The large number of patients in whom a diagnosis was established allowed the characterization of genotype–phenotype correlations with respect to both previously reported and novel findings. For the c.501C > G (p.Phe167Leu) variant a likely nonpathogenic property is suggested, as it only seems to act as an aggravating modifying factor in cases in which a pathogenic variant triggers phenotypic expression. [ABSTRACT FROM AUTHOR]

Objectives Becker muscular dystrophy (BMD) is a relatively less investigated neuromuscular disease, partially overlapping the phenotype of Duchenne dystrophy (DMD). Physiopathological and anatomical patterns are still not comprehensively known, despite recent effort in the search of early biomarkers. Aim of this study was to selectively compare normal appearing muscles of BMD with healthy controls. Methods Among a pool of 40 BMD patients and 20 healthy controls, Sartorius and gracilis muscles were selected on the basis of a blinded clinical quantitative/qualitative evaluation, if classified as normal (0 or 1 on Mercuri scale) and subsequently segmented on diffusion tensor MRI scans with a tractographic approach. Diffusion derived parameters were extracted. Results Non-parametric testing revealed significant differences between normal and normal appearing BMD derived parameters in both muscles, the difference being more evident in sartorius. Bonferroni-corrected P -values (<.05) of Mann-Whitney test could discriminate between BMD and controls for standard deviation of all diffusion parameters (mean diffusivity, fractional anisotropy, axial and radial diffusivity) in both sartorius and gracilis, while in sartorius the significant difference was found also in the average values of the same parameters (with exception of RD). Conclusions This method could identify microstructural alterations in BMD normal appearing sartorius and gracilis. Advances in knowledge Diffusion based MRI could be able to identify possible early or subclinical microstructural alterations in dystrophic patients with BMD. [ABSTRACT FROM AUTHOR]

Background and purpose: Myotonia congenita (MC) is a muscle channelopathy in which pathogenic variants in a key sarcolemmal chloride channel Gene (CLCN1) cause myotonia. This study used muscle magnetic resonance imaging (MRI) to quantify contractile properties and fat replacement of muscles in a Danish cohort of MC patients. Methods: Individuals with the Thomsen (dominant) and Becker (recessive) variants of MC were studied. Isometric muscle strength, whole‐body MRI, and clinical data were collected. The degree of muscle fat replacement of thigh, calf, and forearm muscles was quantitively calculated on Dixon MRI as fat fractions (FFs). Contractility was evaluated as the muscle strength per contractile muscle cross‐sectional area (PT/CCSA). Muscle contractility was compared with clinical data. Results: Intramuscular FF was increased and contractility reduced in calf and in forearm muscles compared with controls (FF = 7.0–14.3% vs. 5.3–9.6%, PT/CCSA = 1.1–4.9 Nm/cm2 vs. 1.9–5.8 Nm/cm2 [p < 0.05]). Becker individuals also showed increased intramuscular FF and reduced contractility of thigh muscles (FF = 11.9% vs. 9.2%, PT/CCSA = 1.9 Nm/cm2 vs. 3.2 Nm/cm2 [p < 0.05]). Individual muscle analysis showed that increased FF was limited to seven of 18 examined muscles (p < 0.05). There was a weak correlation between reduced contractility and severity of symptoms. Conclusions: Individuals with MC have increased fat replacement and reduced contractile properties of muscles. Nonetheless, changes were small and likely did not impact clinically on their myotonic symptoms. [ABSTRACT FROM AUTHOR]

Myotonia congenita is a hereditary muscle disease mainly characterized by muscle hyperexcitability, which leads to a sustained burst of discharges that correlates with the magnitude and duration of involuntary aftercontractions, muscle stiffness, and hypertrophy. Mutations in the chloride voltage-gated channel 1 (CLCN1) gene that encodes the skeletal muscle chloride channel (ClC-1) are responsible for this disease, which is commonly known as myotonic chloride channelopathy. The biophysical properties of the mutated channel have been explored and analyzed through in vitro approaches, providing important clues to the general function/dysfunction of the wild-type and mutated channels. After an exhaustive search for CLCN1 mutations, we report in this review more than 350 different mutations identified in the literature. We start discussing the physiological role of the ClC-1 channel in skeletal muscle functioning. Then, using the reported functional effects of the naturally occurring mutations, we describe the biophysical and structural characteristics of the ClC-1 channel to update the knowledge of the function of each of the ClC-1 helices, and finally, we attempt to point out some patterns regarding the effects of mutations in the different helices and loops of the protein. [ABSTRACT FROM AUTHOR]

Voltage-gated sodium channels (Nav) are key players in excitable tissues with the capability to generate and propagate action potentials. Mutations in the genes encoding Navs can lead to severe inherited diseases, and some of these so-called channelopathies show temperature-sensitive phenotypes, for example, paramyotonia congenita, Brugada syndrome, febrile seizure syndromes, and inherited pain syndromes like erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD). Nevertheless, most investigations of mutation-induced gating effects have been conducted at room temperature, and thus the role of cooling or warming in channelopathies remains poorly understood. Here, we investigated the temperature sensitivity of four Nav subtypes: Nav1.3, Nav1.5, Nav1.6, and Nav1.7, and two mutations in Nav1.7 causing IEM (Nav1.7/L823R) and PEPD (Nav1.7/I1461T) expressed in cells of the human embryonic kidney cell line using an automated patch clamp system. Our experiments at 15°C, 25°C, and 35°C revealed a shift of the voltage dependence of activation to more hyperpolarized potentials with increasing temperature for all investigated subtypes. Nav1.3 exhibited strongly slowed inactivation kinetics compared with the other subtypes that resulted in enhanced persistent current, especially at 15°C, indicating a possible role in cold-induced hyperexcitability. Impaired fast inactivation of Nav1.7/I1461T was significantly enhanced by a cooling temperature of 15°C. The subtype-specific modulation as well as the intensified mutation-induced gating changes stress the importance to consider temperature as a regulator for channel gating and its impact on cellular excitability as well as disease phenotypes. [ABSTRACT FROM AUTHOR]

Paradoxical pseudomyotonia has previously been described in the English Cocker Spaniel (ECS) and English Springer Spaniel (ESS) breeds, without the identification of potentially causative variants. This disease is characterised by episodes of exercise‐induced generalised myotonic‐like muscle stiffness, phenotypically similar to congenital pseudomyotonia in cattle, and paramyotonia congenita and Brody disease in people. Four additional affected ESS dogs with paradoxical pseudomyotonia are described in this report, together with the identification of the autosomal recessive c.126C>A(p.(Cys42Ter)) SLC7A10 nonsense variant as candidate disease‐causing variant in both ECS and ESS. The variant has an estimated prevalence of 2.5% in both breeds in the British study samples, but was not identified in the Belgian study samples. Genetic testing‐based breeding should be a useful tool to eliminate this disease in the future, although an effective treatment option is available for severely affected dogs. [ABSTRACT FROM AUTHOR]

Myotonia is a finding caused by muscle hyperexcitability and occurs as a result of delayed relaxation. The two disease groups in which myotonia is seen are myotonic dystrophy from muscular dystrophies and non-dystrophic myotonias from muscle ion channel diseases. Congenital myotonia is a rare inherited disease of skeletal muscle characterized by diffuse muscle hypertrophy, non-progressive myotonia, and difficulty in relaxation after sudden muscle contraction. In this article, we present a 59-year-old patient, who applied to the neurosurgery clinic due to increased muscle weakness, pain, and muscle stiffness in the legs for the past 1 year, was diagnosed with spinal stenosis and was referred to us before surgery and was diagnosed with myotonia congenita as a result of clinical evaluation and EMG. Congenital myotonia may have a subclinical course in some patients and may be difficult to diagnose in the presence of concomitant symptomatic spinal cord pathology. It is very important to inform anesthesiologists before surgery to prevent perioperative complications when the diagnosis is made.

Myotonia congenita (MC) is a rare genetic disease caused by mutations in the skeletal muscle chloride channel gene (CLCN1), encoding the voltage-gated chloride channel ClC-1 in skeletal muscle. Our study reported the clinical and molecular characteristics of six patients with MC and systematically review the literature on Chinese people. We retrospectively analyzed demographics, clinical features, family history, creatine kinase (CK), electromyography (EMG), treatment, and genotype data of our patients and reviewed the clinical data and CLCN1 mutations in literature. The median ages at examination and onset were 26.5 years (range 11–50 years) and 6.5 years (range 1.5–11 years), respectively, in our patients, and 21 years (range 3.5–65 years, n = 45) and 9 years (range 0.5–26 years, n = 50), respectively, in literature. Similar to previous reports, myotonia involved limb, lids, masticatory, and trunk muscles to varying degrees. Warm-up phenomenon (5/6), percussion myotonia (3/5), and grip myotonia (6/6) were common. Menstruation triggered myotonia in females, not observed in Chinese patients before. The proportion of abnormal CK levels (4/5) was higher than data from literature. Electromyography performed in six patients revealed myotonic changes (100%). Five novel CLCN1 mutations, including a splicing mutation (c.853 + 4A>G), a deletion mutation (c.2010_2014del), and three missense mutations (c.2527C>T, c.1727C>T, c.2017 G > C), were identified. The c.892 G > A (p.A298T) mutation was the most frequent mutation in the Chinese population. Our study expanded the clinical and genetic spectrum of patients with MC in the China. The MC phenotype in Chinese people is not different from that found in the West, while the genotype is different.

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 since they have usually not been validated against robust functional assays. We compare 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 with Mutation Taster (84.58%) and REVEL (82.54%). However, both scores have poor specificity. EVE has better specificity. Combined methods based on concordance, improved performance overall but still lacked specificity. Our calculated statistics for the predictive tools are different to reported values for other genes in the literature suggesting that 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. [ABSTRACT FROM AUTHOR]

• The current minimum point prevalence for all skeletal muscle channelopathies is 1.99/100 000. • The minimum point prevalence of myotonia congenita is 1.13/100 000; for paramyotonia congenita/sodium channel myotonia is 0.35/100 000, for hypoPP/hyperPP is 0.41/100 000 and for Andersen Tawil Syndrome is 0.1/100 000. • Next generation sequencing as well as increased referral rates have been primary contributors to the almost doubled point prevalence rate from 1.12/100 000 to 1.99/100 000. 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. [ABSTRACT FROM AUTHOR]

Myotonia congenita (MC) is a rare neuromuscular disease caused by mutations within the CLCN1 gene encoding skeletal muscle chloride channels. MC is characterized by delayed muscle relaxation during contraction, resulting in muscle stiffness. There is a lack of MC case reports and data on the prevalence among Malaysians. We report a clinical case of a 50-year-old woman presents with muscle stiffness and cramp episodes that started in early childhood. She had difficulty initiating muscle movement and presented with transient muscle weakness after rest, which usually improved after repeated contraction (warmup phenomenon). She was diagnosed with MC after myotonic discharge on electromyography (EMG). Her brother had similar symptoms; however, no additional family members showed MC symptoms. Serum creatine kinase levels were elevated in both the proband and her brother with 447 U/L and 228 U/L recorded, respectively. Genetic analysis by whole-exome sequencing (WES) revealed a previously reported pathogenic CLCN1 gene variant c.1667T>A (p.I556N). Genetic screening of all family members revealed that the same variant was observed in the children of both the proband and her brother; however, the children did not present with either clinical or electrophysiological MC symptoms. The multiplex ligation-dependent probe amplification (MLPA) analysis conducted identified neither exon deletion nor duplication in CLCN1. In conclusion, this report describes the first case of MC in Malaysia in which incomplete penetrance observed in this family is caused by a known pathogenic CLCN1 variant. [ABSTRACT FROM AUTHOR]

Myotonia congenita is a genetic muscle disorders characterized by muscle stiffness, pain, and non-dystrophic changes on muscle biopsy. It results from pathogenic variants in CLCN1. The condition manifests in early childhood, but symptoms can be variable. Most children are two or three years old when parents first notice their muscle stiffness, particularly in the legs, often triggered by sudden activity after rest. We investigated the clinical and genetic features of Korean patients with myotonia congenita. We conducted a retrospective review of medical records within the myopathy database from August 2002 to March 2024. Among the records examined, we identified 19 patients with pathogenic variants in CLCN1. Then, we analyzed the patient's clinical, laboratory, electrodiagnostic, and genetic findings. This study analyzed 19 patients from 16 unrelated families. A family history was positive in 14 (87.5%) unrelated families. Genetic analysis revealed that 11 unrelated families exhibited an autosomal dominant pattern, while three exhibited a recessive pattern, and two were sporadic cases. Among 19 patients, 14 (73.7%) were male and 5 (26.3%) were female. The median age of myotonia onset was 7 years [interquartile range: 4 – 13.5 years]. Myotonia was presented in all 19 patients. Muscle stiffness at the onset of exercise was present in 17 (89.5%) patients. Facial muscle involvement was noted in five (26.3%) patients. Grip myotonia was present in 15 (78.9%) patients, and warm-up phenomenon in 12 (63.2%) patients. Muscle hypertrophy in the lower leg was evident in three patients. The most common exacerbating factor was exertion (15/19, 89.5%). Creatine kinase levels were measured in 16 patients, and the median value was 159, with seven (43.8%) patients showing elevation above this value. Needle electromyography was performed in 17 patients, and myotonic discharges were observed in all cases. The most prevalent pathogenic variant of CLCN1 was c.892G>A, which was observed in six unrelated families (37.5%), followed by c.826G>T in two (12.5%). Additionally, the following variants were identified: c.566C>G, c.1297T>C, c.1063G>A, c.950G>A, c.1438C>T, c.2509-1G>A, c.949C>T, and c.962T>A, each in one family. Among them, c.826G>T and c.2509-1G>A were novel variants. Our study showed the clinical and genetic features of Korean patients diagnosed with myotonia congenita, contributing to the understanding of the manifestation and genetic basis of this disease. [ABSTRACT FROM AUTHOR]

The DMD Hub, a collaboration between Duchenne UK and the John Walton Muscular Dystrophy Research Centre at Newcastle University, has successfully established a UK clinical trial accelerator network (CTAN) able and ready to deliver trials in DMD (www.dmdhub.org). Here we present the DMD Hub CTAN model which includes building workforce capacity, establishing an effective communication infrastructure, harmonising costings and trial set up, enabling fair and equitable recruitment, providing training and education opportunities to staff, adapting to new challenges and evolving to ensure utility and sustainability. The DMD Hub has developed a suite of tools and services for patients, clinical trial and clinical sites, contract research organisations and industry, spanning the clinical trial landscape. We are now delivering these services to industry taking a national, central coordinating role in the set-up and delivery of DMD trials using the established site network and infrastructure. The offer to industry covers early engagement with sites, study and site feasibility, support with study set up including costing and reviewing documentation for the national ethic approval, and recruitment to clinical research studies. The aim is to provide a collaborative, harmonised and streamlined approach to clinical trial set up and delivery that benefits all stakeholders. For sponsors this means quicker set up and delivery timelines, enabled through site engagement, patient insight into trial implementation and recruitment, and robust site feasibility; for sites, it will lead to improved processes, less duplication of effort and increased number of clinical trials; for patients, it will ensure effective recruitment and increased opportunity to access clinical trials. The DMD Hub network model has been successfully replicated in other neuromuscular diseases; the Becker Muscular Dystrophy Hub (www.bmd-hub.org) was launched in early 2024 and several of the DMD Hub tools have also been used as models for other rare disease areas in recent successful grant applications. A nationally coordinated, inclusive approach based on specific needs of the different stakeholders can successfully improve clinical trial delivery and increase capacity. [ABSTRACT FROM AUTHOR]

Mutations in SCN4C are associated with several neuromuscular disorders including hyper/hypokaliemic periodic paralysis, paramyotonia congenita and sodium channel myotonia. Nevertheless, also there are described in some cases with phenotype of non-congenital myopathy. This is a 64-year-old woman who complained to postural alteration in her back for approximately 6 months, diagnosed with kyphoscoliosis by traumatology, with obvious difficulty climbing stairs for approximately 2 years. She does not presented weakness of the shoulder girdle, nor neuro-motor symptoms during childhood. Regarding family history, she reported that her mother had very pronounced hyperkyphosis, but she did not report a family history of loss of strength. She did not report orthopnea or bulbar symptoms. In the neurological examination, I did not detect winged scapula, negative Gowers maneuver, normal heel and toe gait, positive Beevor's sign, anserine gait, inability to acquire standing from sitting in a low seat. Interosseous atrophy. Whole-body muscle MRI: At the level of the pelvic girdle and both extremities, chronic changes predominated, showing atrophy in the muscle bellies of the gluteus minimus, tensor fasciae latae, and internal gastrocnemius on both sides, with a predominance on the left. Marked atrophy with fatty infiltration of lumbar paravertebral muscles. The gluteus maximus muscles in their distal aspect, lateralis and medialis vastus, semimembranosus and soleus on both sides presented partial atrophy with edema. The left tibialis anterior muscle, and the right lateral gastrocnemius muscle, as well as the anteromedial aspect of the middle third of the soleus muscle bilaterally presented subtle edema without atrophy, as a sign of more acute involvement. At the level of the shoulder girdle and both arms, acute and chronic changes were not detected. Normal level of cks. Echocardiogram and MRI of the spine were normal. Dry gout study for Pompe disease was negative. Neurophysiological study was normal. Massive sequencing study of the human exome, in order to identify genomic variants in the 136 selected genes associated with myopathies and muscular dystrophies: 3 variants were detected in the TTN and SCN4A genes: TTN (c.59113C>T): VUS, TTN: c.23378-10C>A: probably benign, SCN4A: c.3502C>T: probably pathogenic. Her healthy biological brother is not a carrier of the SCN4A c.3502C>T;p.(Leu1168Phe) variant. The patient was asked about the phenomenon of myotonia, which she denied. After this diagnostic study, we can conclude that the patient's diagnosis is Myopathy secondary to variant SCN4A c.3502C>T (predominance of the Pelvian Girdle with important impact of the abdominal musculature). [ABSTRACT FROM AUTHOR]

Myotonia congenita (MC) is a rare neuromuscular disease caused by mutations within the CLCN1 gene encoding skeletal muscle chloride channels. MC is characterized by delayed muscle relaxation during contraction, resulting in muscle stiffness. There is a lack of MC case reports and data on the prevalence among Malaysians. We report a clinical case of a 50-year-old woman presents with muscle stiffness and cramp episodes that started in early childhood. She had difficulty initiating muscle movement and presented with transient muscle weakness after rest, which usually improved after repeated contraction (warm-up phenomenon). She was diagnosed with MC after myotonic discharge on electromyography (EMG). Her brother had similar symptoms; however, no additional family members showed MC symptoms. Serum creatine kinase levels were elevated in both the proband and her brother with 447 U/L and 228 U/L recorded, respectively. Genetic analysis by whole-exome sequencing (WES) revealed a previously reported pathogenic CLCN1 gene variant c.1667T>A (p.I556N). Genetic screening of all family members revealed that the same variant was observed in the children of both the proband and her brother; however, the children did not present with either clinical or electrophysiological MC symptoms. The multiplex ligation-dependent probe amplification (MLPA) analysis conducted identified neither exon deletion nor duplication in CLCN1. In conclusion, this report describes the first case of MC in Malaysia in which incomplete penetrance observed in this family is caused by a known pathogenic CLCN1 variant.

Optically pumped magnetometers (OPM) are quantum sensors that enable the contactless, non-invasive measurement of biomagnetic muscle signals, i.e., magnetomyography (MMG). Due to the contactless recording, OPM-MMG might be preferable to standard electromyography (EMG) for patients with neuromuscular diseases, particularly when repetitive recordings for diagnostic and therapeutic monitoring are mandatory. OPM-MMG studies have focused on recording physiological muscle activity in healthy individuals, whereas research on neuromuscular patients with pathological altered muscle activity is non-existent. Here, we report a proof-of-principle study on the application of OPM-MMG in patients with neuromuscular diseases. Specifically, we compare the muscular activity during maximal isometric contraction of the left rectus femoris muscle in three neuromuscular patients with severe (Transthyretin Amyloidosis in combination with Pompe’s disease), mild (Charcot-Marie-Tooth disease, type 2), and without neurogenic, but myogenic, damage (Myotonia Congenita). Seven healthy young participants served as the control group. As expected, and confirmed by using simultaneous surface electromyography (sEMG), a time-series analysis revealed a dispersed interference pattern during maximal contraction with high amplitudes. Furthermore, both patients with neurogenic damage (ATTR and CMT2) showed a reduced variability of the MMG signal, quantified as the signal standard deviation of the main component of the frequency spectrum, highlighting the reduced possibility of motor unit recruitment due to the loss of motor neurons. Our results show that recording pathologically altered voluntary muscle activity with OPM-MMG is possible, paving the way for the potential use of OPM-MMG in larger studies to explore the potential benefits in clinical neurophysiology. [ABSTRACT FROM AUTHOR]

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• We identified a SCN4A mutation (p.L703P) in a Chinese family with non-dystrophic myotonias (NDM). • Characterization of mutant channel revealed both gain-of-function and loss-of-function changes in gating properties. • Literature search results in two independent case reports of NDM patients carrying L703P mutation in Nav1.4 channels. • The L703P is a recurrent mutation associated with NDM. The non-dystrophic myotonias are inherited skeletal muscle disorders characterized by skeletal muscle stiffness after voluntary contraction, without muscle atrophy. Based on their clinical features, non-dystrophic myotonias are classified into myotonia congenita, paramyotonia congenita, and sodium channel myotonia. Using whole-exome next-generation sequencing, we identified a L703P mutation (c.2108T>C, p.L703P) in SCN4A in a Chinese family diagnosed with non-dystrophic myotonias. The clinical findings of patients in this family included muscle stiffness and hypertrophy. The biophysical properties of wildtype and mutant channels were investigated using whole-cell patch clamp. L703P causes both gain-of-function and loss-of-function changes in Nav1.4 properties, including decreased current density, impaired recovery, enhanced activation and slow inactivation. Our study demonstrates that L703P is a pathogenic variant for myotonia, and provides additional electrophysiological information for understanding the pathogenic mechanism of SCN4A -associated channelopathies. [ABSTRACT FROM AUTHOR]

Jorge Andres Olave-Rodriguez,1 Francisco Javier Bonilla-Escobar,2– 4 Estephania Candelo,5,6 Lisa Ximena Rodriguez-Rojas1,7 1Universidad Icesi, Faculty of Health Sciences, Cali, Colombia; 2Somos Ciencia al Servicio de la Comunidad, Fundación SCISCO/Science to Serve the Community, SCISCO Foundation, Cali, Colombia; 3Universidad del Valle, Cali, Colombia; 4Institute for Clinical Research Education, University of Pittsburgh, Pittsburgh, PA, USA; 5Centro de Investigaciones Clínicas, Fundación Valle del Lili, Cali, Colombia; 6Centro enfermedades raras y malformaciones congenitas (CIACER), Universidad Icesi, Cali, Colombia; 7Human Genetics Department, Fundación Valle del Lili, Cali, ColombiaCorrespondence: Lisa Ximena Rodriguez-Rojas Cra. 98 ## 18-49, Cali, Valle del Cauca, ColombiaEmail lixiro@gmail.com; lisa.rodriguez@fvl.org.coBackground: Becker’s type myotonia congenita is an autosomal recessive nondystrophic skeletal muscle disorder characterized by muscle stiffness and the inability of muscle relaxation after voluntary contraction. It is caused by mutations in the CLCN1 gene, which encodes for a chloride channel mainly expressed in the striated muscle. Most cases have been reported in the European population, and only mexiletine has demonstrated a randomized placebo-controlled, double-blinded effectiveness.Case Presentation: We present two male siblings from Colombia with Latino ancestry, without parental consanguinity, with myotonia during voluntary movements, muscle hypertrophy of lower extremities, transient weakness, and severe muscle fatigue after exercise from three years of age. A genetic panel for dystrophic muscle disorders and a muscle biopsy were both negative. Genetic testing was performed in their second decade of life. Both patients’ exomic sequencing test reported the mutation c.1129C >T (p.Arg377&ast;) affecting exon 10 of the CLCN1, generating a premature stop codon. This mutation was described as pathogenic and observed in only one other patient in the United Kingdom.Conclusion: To our knowledge, these are the first cases of Becker’s type myotonia congenita reported in Colombia. Increasing awareness of healthcare providers for this type of disease in the region could lead to the identification of undiagnosed patients. Limited availability of medical geneticists as well as genetic testing may be the cause of the lack of previous description of cases, in addition to the delay in the diagnosis of the patients. Further epidemiological studies can reveal underdiagnosed myotonias in the country and in the Latin-American region.Keywords: Becker type myotonia congenita, myotonia congenita, Colombia, muscular diseases, siblings case reports

Myotonia congenita is a rare neuromuscular disorder caused by CLCN1 mutations resulting in delayedmuscle relaxation. Extramuscularmanifestations are not considered to be present in chloride skeletal channelopathies, although recently some cardiac manifestations have been described. We report a family with autosomal dominant myotonia congenita and Brugada syndrome. Bearing in mind the previously reported cases of cardiac arrhythmias in myotonia congenita patients, we discuss the possible involvement of the CLCN1-gene mutations in primary cardiac arrhythmia. [ABSTRACT FROM AUTHOR]

Introduction: In myotonia congenita (MC), activation with exercise or cooling can induce transient changes in compound motor action potential (CMAP) parameters, thus providing a guide to genetic analysis. Material and methods: We performed the short exercise test (SET) and the short exercise test with cooling (SETC) in 30 patients with genetically confirmed Becker disease (BMC) to estimate their utility in the diagnosis of BMC. Results: Although we observed a significant decrease in CMAP amplitude immediately after maximal voluntary effort in both tests in the whole BMC group, in men this decline was significantly smaller than in women, especially in SET. Clinical implications/future directions: In men with a clinical suspicion of BMC, a small decrease in CMAP amplitude in SET together with a typical decline in SETC does not exclude the diagnosis of BMC. Our results show a sex-specific difference in chloride channel function in BMC, which needs further investigation. [ABSTRACT FROM AUTHOR]

Non-dystrophic myotonias (NDM) encompass chloride and sodium channelopathy. Mutations in CLCN1 lead to either the autosomal dominant form or the recessive form of myotonia congenita (MC). The main symptom is sti􀀀ness worsening after rest and improving by physical exercise. Patients with recessive mutations often show muscle hypertrophy, and transient weakness mostly in their lower limbs. Mutations in SCN4A can lead to Hyper-, Hypo- or Normo-kalemic Periodic Paralysis or to di􀀀erent forms of myotonia (Paramyotonia Congenita-PMC and Sodium Channel Myotonia-SCM and severe neonatal episodic laryngospasm-SNEL). SCM often presents facial muscle sti􀀀ness, cold sensitivity, and muscle pain, whereas myotonia worsens in PMC patients with the repetition of the muscle activity and cold. Patients a􀀀ected by chloride or sodium channelopathies may show similar phenotypes and symptoms, making the diagnosis more diffcult to reach. Herein we present a woman in whom sodium and chloride channelopathies coexist yielding a complex phenotype with features typical of both MC and PMC. Disease onset was in the second decade with asthenia, weakness, warm up and limb sti􀀀ness, and her symptoms had been worsening through the years leading to frequent heavy retrosternal compression, tachycardia, sti􀀀ness, and symmetrical pain in her lower limbs. She presented severe lid lag myotonia, a hypertrophic appearance at four limbs and myotonic discharges at EMG. Her symptoms have been triggered by exposure to cold and her daily life was impaired. All together, clinical signs and instrumental data led to the hypothesis of PMC and to the administration of mexiletine, then replaced by acetazolamide because of gastrointestinal side e􀀀ects. Analysis of SCN4A revealed a new variant, p.Glu1607del. Nonetheless the severity of myotonia in the lower limbs and her general sti􀀀ness led to hypothesize that the impairment of sodium channel, Nav1.4, alone could not satisfactorily explain the phenotype and a second genetic "factor" was hypothesized. CLCN1 was targeted, and p.Met485Val was detected in homozygosity. This case highlights that proper identification of signs and symptoms by an expert neurologist is crucial to target a successful genetic diagnosis and appropriate therapy. [ABSTRACT FROM AUTHOR]

Myotonia congenita (MC) is an inherited rare disease characterized by impaired muscle relaxation after contraction, resulting in muscle stiffness. It is caused by loss-of-function mutations in the skeletal muscle chloride channel ClC-1, important for the stabilization of restingmembrane potential and for the repolarization phase of action potentials. Thanks to in vitro functional studies, the molecular mechanisms by which ClC-1 mutations alter chloride ion influx into the cell have been in part clarified, classifying them in "gating-defective" or "expression-defective" mutations. To date, the treatment of MC is only palliative because no direct ClC-1 activator is available. An ideal drug should be one which is able to correct biophysical defects of ClC-1 in the case of gating-defective mutations or a drug capable to recover ClC-1 protein expression on the plasma membrane for trafficking-defective ones. In this study, we tested the ability of niflumic acid (NFA), a commercial nonsteroidal antiinflammatory drug, to act as a pharmacological chaperone on traffickingdefective MC mutants (A531V, V947E). Wild-type (WT) or MC mutant ClC-1 channels were expressed in HEK293 cells and whole-cell chloride currents were recorded with the patch-clamp technique before and after NFA incubation. Membrane biotinylation assays and western blot were performed to support electrophysiological results. A531V and V947E mutations caused a decrease in chloride current density due to a reduction of ClC-1 total protein level and channel expression on the plasma membrane. The treatment of A531V and V947E-transfected cells with 50 µM NFA restored chloride currents, reaching levels similar to those of WT. Furthermore, no significant difference was observed in voltage dependence, suggesting that NFA increased protein membrane expression without altering the function of ClC-1. Indeed, biochemical experiments confirmed that V947E total protein expression and its plasma membrane distribution were recovered after NFA incubation, reaching protein levels similar to WT. Thus, the use of NFA as a pharmacological chaperone in trafficking defective ClC-1 channel mutations could represent a good strategy in the treatment of MC. Because of the favorable safety profile of this drug, our study may easily open the way for confirmatory human pilot studies aimed at verifying the antimyotonic activity of NFA in selected patients carrying specific ClC-1 channel mutations. [ABSTRACT FROM AUTHOR]

Introduction/Aims: Consistency of differences between non‐dystrophic myotonias over time measured by standardized clinical/patient‐reported outcomes is lacking. Evaluation of longitudinal data could establish clinically relevant endpoints for future research. Methods: Data from prospective observational study of 95 definite/clinically suspected non‐dystrophic myotonia participants (six sites in the United States, United Kingdom, and Canada) between March 2006 and March 2009 were analyzed. Outcomes included: standardized symptom interview/exam, Short Form‐36, Individualized Neuromuscular Quality of Life (INQoL), electrophysiological short/prolonged exercise tests, manual muscle testing, quantitative grip strength, modified get‐up‐and‐go test. Patterns were assigned as described by Fournier et al. Comparisons were restricted to confirmed sodium channelopathies (SCN4A, baseline, year 1, year 2: n = 34, 19, 13), chloride channelopathies (CLCN1, n = 32, 26, 18), and myotonic dystrophy type 2 (DM2, n = 9, 6, 2). Results: Muscle stiffness was the most frequent symptom over time (54.7%–64.7%). Eyelid myotonia and paradoxical handgrip/eyelid myotonia were more frequent in SCN4A. Grip strength and combined manual muscle testing remained stable. Modified get‐up‐and‐go showed less warm up in SCN4A but remained stable. Median post short exercise decrement was stable, except for SCN4A (baseline to year 2 decrement difference 16.6% [Q1, Q3: 9.5, 39.2]). Fournier patterns type 2 (CLCN1) and 1 (SCN4A) were most specific; 40.4% of participants had a change in pattern over time. INQoL showed higher impact for SCN4A and DM2 with scores stable over time. Discussion: Symptom frequency and clinical outcome assessments were stable with defined variability in myotonia measures supporting trial designs like cross over or combined n‐of‐1 as important for rare disorders. [ABSTRACT FROM AUTHOR]

Case Description: A 10‐month‐old castrated male domestic longhair cat was evaluated for increasing frequency of episodic limb rigidity. Clinical Findings: The cat presented for falling over and lying recumbent with its limbs in extension for several seconds when startled or excited. Upon examination, the cat had hypertrophied musculature, episodes of facial spasm, and a short‐strided, stiff gait. Diagnostics: Electromyography (EMG) identified spontaneous discharges that waxed and waned in amplitude and frequency, consistent with myotonic discharges. A high impact 8‐base pair (bp) deletion across the end of exon 3 and intron 3 of the chloride voltage‐gated channel 1 (CLCN1) gene was identified using whole genome sequencing. Treatment and Outcome: Phenytoin treatment was initiated at 3 mg/kg po q24 h and resulted in long‐term improvement. Clinical Relevance: This novel mutation within the CLCN1 gene is a cause of myotonia congenita in cats and we report for the first time its successful treatment. [ABSTRACT FROM AUTHOR]

In Becker muscular dystrophy (BMD), muscle weakness progresses relatively slowly, with a highly variable rate among patients. This complicates clinical trials, as clinically relevant changes are difficult to capture within the typical duration of a trial. Therefore, predictors for disease progression are needed. We assessed if temporal increase of fat fraction (FF) in BMD follows a sigmoidal trajectory and whether fat fraction at baseline (FFbase) could therefore predict FF increase after 2 years (ΔFF). Thereafter, for two different MR‐based parameters, we tested the additional predictive value to FFbase. We used 3‐T Dixon data from the upper and lower leg, and multiecho spin‐echo MRI and 7‐T 31P MRS datasets from the lower leg, acquired in 24 BMD patients (age: 41.4 [SD 12.8] years). We assessed the pattern of increase in FF using mixed‐effects modelling. Subsequently, we tested if indicators of muscle damage like standard deviation in water T2 (stdT2) and the phosphodiester (PDE) over ATP ratio at baseline had additional value to FFbase for predicting ∆FF. The association between FFbase and ΔFF was described by the derivative of a sigmoid function and resulted in a peak ΔFF around 0.45 FFbase (fourth‐order polynomial term: t = 3.7, p <.001). StdT2 and PDE/ATP were not significantly associated with ∆FF if FFbase was included in the model. The relationship between FFbase and ∆FF suggests a sigmoidal trajectory of the increase in FF over time in BMD, similar to that described for Duchenne muscular dystrophy. Our results can be used to identify muscles (or patients) that are in the fast progressing stage of the disease, thereby facilitating the conduct of clinical trials. [ABSTRACT FROM AUTHOR]

Copyright of Kranion is the property of Publicidad Permanyer SLU and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Myotonia congenita is a rare neuromuscular disorder caused by CLCN1 mutations resulting in delayed muscle relaxation. Extramuscular manifestations are not considered to be present in chloride skeletal channelopathies, although recently some cardiac manifestations have been described. We report a family with autosomal dominant myotonia congenita and Brugada syndrome. Bearing in mind the previously reported cases of cardiac arrhythmias in myotonia congenita patients, we discuss the possible involvement of the CLCN1-gene mutations in primary cardiac arrhythmia.

Myotonia congenita (MC) is an inherited rare disease characterized by impaired muscle relaxation after contraction, resulting in muscle stiffness. It is caused by loss-of-function mutations in the skeletal muscle chloride channel ClC-1, important for the stabilization of resting membrane potential and for the repolarization phase of action potentials. Thanks to in vitro functional studies, the molecular mechanisms by which ClC-1 mutations alter chloride ion influx into the cell have been in part clarified, classifying them in “gating-defective” or “expression-defective” mutations. To date, the treatment of MC is only palliative because no direct ClC-1 activator is available. An ideal drug should be one which is able to correct biophysical defects of ClC-1 in the case of gating-defective mutations or a drug capable to recover ClC-1 protein expression on the plasma membrane for trafficking-defective ones. In this study, we tested the ability of niflumic acid (NFA), a commercial nonsteroidal anti-inflammatory drug, to act as a pharmacological chaperone on trafficking-defective MC mutants (A531V, V947E). Wild-type (WT) or MC mutant ClC-1 channels were expressed in HEK293 cells and whole-cell chloride currents were recorded with the patch-clamp technique before and after NFA incubation. Membrane biotinylation assays and western blot were performed to support electrophysiological results. A531V and V947E mutations caused a decrease in chloride current density due to a reduction of ClC-1 total protein level and channel expression on the plasma membrane. The treatment of A531V and V947E-transfected cells with 50 µM NFA restored chloride currents, reaching levels similar to those of WT. Furthermore, no significant difference was observed in voltage dependence, suggesting that NFA increased protein membrane expression without altering the function of ClC-1. Indeed, biochemical experiments confirmed that V947E total protein expression and its plasma membrane distribution were recovered after NFA incubation, reaching protein levels similar to WT. Thus, the use of NFA as a pharmacological chaperone in trafficking defective ClC-1 channel mutations could represent a good strategy in the treatment of MC. Because of the favorable safety profile of this drug, our study may easily open the way for confirmatory human pilot studies aimed at verifying the antimyotonic activity of NFA in selected patients carrying specific ClC-1 channel mutations.

Background: Congenital myotonia is a congenital disorder that affects skeletal muscles with myotonia. Affected muscles show stiffness and pain sometimes. The two major types of myotonia congenita are known as Thomsen disease and Becker disease. These conditions are distinguished by the severity of their symptoms and their patterns of inheritance. The causative factor is mutations in CLCN1 gene. Myotonia congenita is rarely reported in black especially in black African. Case presentation: This is a case report of Becker Congenital Myotonia in a 36-year-old male from Benin. The symptoms arose at the age of 7 years with regular and progressive course and muscles pains. Electromyogram, blood sampling, laboratory investigations and muscles biopsy confirm the diagnostic with molecular finding. Conclusion: The authors report a case of Becker congenital myotonia in a black African with molecular confirmation. Mexiletine was used as symptomatic agent with good results. [ABSTRACT FROM AUTHOR]

• Mouse TA MVRCs show late sub normality instead of late super normality. • Extra pulses augment mouse subnormality suggesting higher Na+/K+-ATPase activity. • ClC-1 block uncovers activity-dependant late supernormality as seen in human MVRCs. • Data suggest relatively higher t-tubule Na+/K+-ATPase and ClC-1 activity in mouse TA. • Our data help explain mouse-human phenotype difference in muscle channelopathies. 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. [ABSTRACT FROM AUTHOR]

Simple Summary: Paramyotonia congenita (PMC) is a rare muscle disorder that causes myotonia and weakness of facial and limb muscles. The electromyography in PMC shows continual spontaneous, high-frequency spike potentials in skeletal muscles. Genetic mutations in the Nav1.4 channel that cause hyperexcitability of muscle fibers are responsible for PMC. However, the genotype–phenotype relationship is highly diversified, and the molecular pathology remains unclear. Here, we investigated the electrophysiology in the Nav1.4 channel with mutations, p.V781I and p.A1737T, which were found in two Taiwanese patients. We identified the distinct changes in gating mechanisms altered by mutations which may underlie the clinical phenotype. Paramyotonia congenita (PMC) is a rare skeletal muscle disorder characterized by muscle stiffness upon repetitive exercise and cold exposure. PMC was reported to be caused by dominant mutations in the SCN4A gene encoding the α subunit of the Nav1.4 channel. Recently, we identified two missense mutations of the SCN4A gene, p.V781I and p.A1737T, in two PMC families. To evaluate the changes in electrophysiological properties caused by the mutations, both mutant and wild-type (WT) SCN4A genes were expressed in CHO-K1 and HEK-293T cells. Then, whole-cell patch-clamp recording was employed to study the altered gating of mutant channels. The activation curve of transient current showed a hyperpolarizing shift in both mutant Nav1.4 channels as compared to the WT channel, whereas there was a depolarizing shift in the fast inactivation curve. These changes confer to an increase in window current in the mutant channels. Further investigations demonstrated that the mutated channel proteins generate significantly larger resurgent currents as compared to the WT channel and take longer to attain the peak of resurgent current than the WT channel. In conclusion, the current study demonstrates that p.V781I and p.A1737T mutations in the Nav1.4 channel increase both the sustained and the resurgent Na+ current, leading to membrane hyperexcitability with a lower firing threshold, which may influence the clinical phenotype. [ABSTRACT FROM AUTHOR]

Mutations in human CLC-1 chloride channel are associated with the skeletal muscle disorder myotonia congenita. The disease-causing mutant A531V manifests enhanced proteasomal degradation of CLC-1. We recently found that CLC-1 degradation is mediated by cullin 4 ubiquitin ligase complex. It is currently unclear how quality control and protein degradation systems coordinate with each other to process the biosynthesis of CLC-1. Herein we aim to ascertain the molecular nature of the protein quality control system for CLC-1. We identified three CLC-1-interacting proteins that are well-known heat shock protein 90 (Hsp90)-associated co-chaperones: FK506-binding protein 8 (FKBP8), activator of Hsp90 ATPase homolog 1 (Aha1), and Hsp70/Hsp90 organizing protein (HOP). These co-chaperones promote both the protein level and the functional expression of CLC-1 wild-type and A531V mutant. CLC-1 biosynthesis is also facilitated by the molecular chaperones Hsc70 and Hsp90β. The protein stability of CLC-1 is notably increased by FKBP8 and the Hsp90β inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) that substantially suppresses cullin 4 expression. We further confirmed that cullin 4 may interact with Hsp90β and FKBP8. Our data are consistent with the idea that FKBP8 and Hsp90β play an essential role in the late phase of CLC-1 quality control by dynamically coordinating protein folding and degradation.

Electrical membrane properties of skeletal muscle fibers have been thoroughly studied over the last five to six decades. This has shown that muscle fibers from a wide range of species, including fish, amphibians, reptiles, birds, and mammals, are all characterized by high resting membrane permeability for Cl(-) ions. Thus, in resting human muscle, ClC-1 Cl(-) ion channels account for ∼80% of the membrane conductance, and because active Cl(-) transport is limited in muscle fibers, the equilibrium potential for Cl(-) lies close to the resting membrane potential. These conditions-high membrane conductance and passive distribution-enable ClC-1 to conduct membrane current that inhibits muscle excitability. This depressing effect of ClC-1 current on muscle excitability has mostly been associated with skeletal muscle hyperexcitability in myotonia congenita, which arises from loss-of-function mutations in the CLCN1 gene. However, given that ClC-1 must be drastically inhibited (∼80%) before myotonia develops, more recent studies have explored whether acute and more subtle ClC-1 regulation contributes to controlling the excitability of working muscle. Methods were developed to measure ClC-1 function with subsecond temporal resolution in action potential firing muscle fibers. These and other techniques have revealed that ClC-1 function is controlled by multiple cellular signals during muscle activity. Thus, onset of muscle activity triggers ClC-1 inhibition via protein kinase C, intracellular acidosis, and lactate ions. This inhibition is important for preserving excitability of working muscle in the face of activity-induced elevation of extracellular K(+) and accumulating inactivation of voltage-gated sodium channels. Furthermore, during prolonged activity, a marked ClC-1 activation can develop that compromises muscle excitability. Data from ClC-1 expression systems suggest that this ClC-1 activation may arise from loss of regulation by adenosine nucleotides and/or oxidation. The present review summarizes the current knowledge of the physiological factors that control ClC-1 function in active muscle.

Introduction: Non-dystrophic myotonias (NDMs) are skeletal muscle ion channelopathies caused by CLCN1 or SCN4A mutations. This study aimed to describe the clinical, myopathological, and genetic analysis of NDM in a large Chinese cohort. Methods: We reviewed the clinical manifestations, laboratory results, electrocardiogram, electromyography, muscle biopsy, genetic analysis, treatment, and follow-up of 20 patients (from 18 families) with NDM. Results: Cases included myotonia congenita (MC, 17/20) and paramyotonia congenita (PMC, 3/20). Muscle stiffness and hypertrophy, grip and percussion myotonia, and the warm-up phenomenon were frequently observed in MC and PMC patients. Facial stiffness, eye closure myotonia, and cold sensitivity were more common in PMC patients and could be accompanied by permanent weakness. Nine MC patients and two PMC patients had cardiac abnormalities, mainly manifested as cardiac arrhythmia, and the father of one patient died of sudden cardiac arrest. Myotonic runs in electromyography were found in all patients, and seven MC patients had mild myopathic changes. There was no difference in muscle pathology between MC and PMC patients, most of whom had abnormal muscle fiber type distribution or selective muscle fiber atrophy. Nineteen CLCN1 variants were found in 17 MC patients, among which c.795T>G (p.D265E) was a new variant, and two SCN4A variants were found in three PMC patients. The patients were treated with mexiletine and/or carbamazepine, and the symptoms of myotonia were partially improved. Conclusions: MC and PMC have considerable phenotypic overlap. Genetic investigation contributes to identifying the subtype of NDM. The muscle pathology of NDM lacks specific changes. [ABSTRACT FROM AUTHOR]

High-throughput DNA sequencing is increasingly employed to diagnose single gene neurological and neuromuscular disorders. Large volumes of data present new challenges in data interpretation and its useful translation into clinical and genetic counselling for families. Even when a plausible gene is identified with confidence, interpretation of the clinical significance and inheritance pattern of variants can be challenging. We report our approach to evaluating variants in the skeletal muscle chloride channel ClC-1 identified in 223 probands with myotonia congenita as an example of these challenges. Sequencing of CLCN1, the gene that encodes CLC-1, is central to the diagnosis of myotonia congenita. However, interpreting the pathogenicity and inheritance pattern of novel variants is notoriously difficult as both dominant and recessive mutations are reported throughout the channel sequence, ClC-1 structure-function is poorly understood and significant intra- and interfamilial variability in phenotype is reported. Heterologous expression systems to study functional consequences of CIC-1 variants are widely reported to aid the assessment of pathogenicity and inheritance pattern. However, heterogeneity of reported analyses does not allow for the systematic correlation of available functional and genetic data. We report the systematic evaluation of 95 CIC-1 variants in 223 probands, the largest reported patient cohort, in which we apply standardized functional analyses and correlate this with clinical assessment and inheritance pattern. Such correlation is important to determine whether functional data improves the accuracy of variant interpretation and likely mode of inheritance. Our data provide an evidence-based approach that functional characterization of ClC-1 variants improves clinical interpretation of their pathogenicity and inheritance pattern, and serve as reference for 34 previously unreported and 28 previously uncharacterized CLCN1 variants. In addition, we identify novel pathogenic mechanisms and find that variants that alter voltage dependence of activation cluster in the first half of the transmembrane domains and variants that yield no currents cluster in the second half of the transmembrane domain. None of the variants in the intracellular domains were associated with dominant functional features or dominant inheritance pattern of myotonia congenita. Our data help provide an initial estimate of the anticipated inheritance pattern based on the location of a novel variant and shows that systematic functional characterization can significantly refine the assessment of risk of an associated inheritance pattern and consequently the clinical and genetic counselling. [ABSTRACT FROM AUTHOR]

Abstract Background Myotonia congenita is a rare neuromuscular disease, which is characterized by a delay in muscle relaxation after evoked or voluntary contraction. Myotonia congenita can be inherited in a dominant (Thomsen disease) and recessive form (Becker disease) and both are caused by pathogenic variants in the CLCN1 gene. Noncanonical splice site variants are often classified as variants of uncertain significance, due to insufficient accuracy of splice-predicting tools. Functional analysis using minigene plasmids is widely used in such cases. Moreover, functional analysis is very useful in investigation of the disease pathogenesis, which is necessary for development of future therapeutic approaches. To our knowledge only one noncanonical splice site variant in the CLCN1 gene was functionally characterized to date. We further contribute to this field by evaluation the molecular mechanism of splicing alteration caused by the c.1582 + 5G > A in a homozygous state. Case presentation We report a clinical case of an affected 6-y.o boy with athletic appearance due to muscle hypertrophy, calf muscle stiffness, cramping and various myotonic signs in a consanguineous family with no history of neuromuscular disorders. The neurological examination showed percussion-activated myotonia in the hands and legs. Plasma creatine kinase enzyme and transaminases levels were normal. Electromyography at the time of examination shows myotonic runs in the upper and lower extremities. Conclusions Functional analysis of the variant in a minigene system showed alteration of splicing leading to loss of function, thereby confirming that the variant is pathogenic.

IntroductionNon-dystrophic myotonias (NDMs) are skeletal muscle ion channelopathies caused by CLCN1 or SCN4A mutations. This study aimed to describe the clinical, myopathological, and genetic analysis of NDM in a large Chinese cohort.MethodsWe reviewed the clinical manifestations, laboratory results, electrocardiogram, electromyography, muscle biopsy, genetic analysis, treatment, and follow-up of 20 patients (from 18 families) with NDM.ResultsCases included myotonia congenita (MC, 17/20) and paramyotonia congenita (PMC, 3/20). Muscle stiffness and hypertrophy, grip and percussion myotonia, and the warm-up phenomenon were frequently observed in MC and PMC patients. Facial stiffness, eye closure myotonia, and cold sensitivity were more common in PMC patients and could be accompanied by permanent weakness. Nine MC patients and two PMC patients had cardiac abnormalities, mainly manifested as cardiac arrhythmia, and the father of one patient died of sudden cardiac arrest. Myotonic runs in electromyography were found in all patients, and seven MC patients had mild myopathic changes. There was no difference in muscle pathology between MC and PMC patients, most of whom had abnormal muscle fiber type distribution or selective muscle fiber atrophy. Nineteen CLCN1 variants were found in 17 MC patients, among which c.795T>G (p.D265E) was a new variant, and two SCN4A variants were found in three PMC patients. The patients were treated with mexiletine and/or carbamazepine, and the symptoms of myotonia were partially improved.ConclusionsMC and PMC have considerable phenotypic overlap. Genetic investigation contributes to identifying the subtype of NDM. The muscle pathology of NDM lacks specific changes.

Introduction: Myotonic disorders are a group of diseases affecting the muscle, in different ways. Myotonic dystrophy type 1 (DM1) is related to (CTG)n expansion in the 3-untranslated region of the dystrophia myotonica protein kinase (DMPK) gene and is the most frequent and disabling form, causing muscular, visibility, respiratory, and cardiac impairment. Non-dystrophic myotonias (NDMs) affect the skeletal muscle alone. In particular, mutations in the chloride channel (CLCN1) gene cause myotonia congenita (MC), which can have autosomal dominant or recessive inheritance. Case report: We describe a patient with a family history of asymptomatic or paucisymptomatic myotonia, who presented handgrip myotonia which sharply reduced after mexiletine administration. Molecular analysis showed both a paternally inherited DMPK expansion and a maternally inherited CLCN1 mutation. Conclusions: Only one other similar case was reported so far; however, the segregation of the two mutations and the characteristics of the muscle were not studied. Since our patient lacked the classical phenotypical and muscle histopathological characteristics of DM1 and showed mild splicing alterations despite a pathogenic DMPK expansion and the nuclear accumulation of toxic RNA, we may speculate that the co-occurrence of a CLCN1 mutation could have attenuated the severity of DM1 phenotype. [ABSTRACT FROM AUTHOR]

• Myomex is a randomised, double-blind, placebo-controlled, cross-over study. • It assessed the efficacy and safety of mexiletine in nondystrophic myotonias. • Mexiletine significantly improved stiffness over the 18-day treatment period. • Quality of life was also significantly improved. • Mexiletine is efficient and well-tolerated in nondystrophic myotonias. The MYOMEX study was a multicentre, randomised, double-blind, placebo-controlled, cross-over study aimed to compare the effects of mexiletine vs. placebo in patients with myotonia congenita (MC) and paramyotonia congenita (PC). The primary endpoint was the self-reported score of stiffness severity on a 100 mm visual analogic scale (VAS). Mexiletine treatment started at 200 mg/day and was up-titrated by 200 mg increment each three days to reach a maximum dose of 600 mg/day for total treatment duration of 18 days for each cross-over period. The modified intent-to-treat population included 25 patients (13 with MC and 12 with PC; mean age, 43.0 years; male, 68.0%). The median VAS score for mexiletine was 71.0 at baseline and decreased to 16.0 at the end of the treatment while the score did not change for placebo (81.0 at baseline vs. 78.0 at end of treatment). A mixed effects linear model analysis on ranked absolute changes showed a significant effect of treatment (p < 0.001). The overall score of the Individualized Neuromuscular Quality of Life questionnaire (INQoL) was significantly improved (p < 0.001). No clinically significant adverse events were reported. In conclusion, mexiletine improved stiffness and quality of life in patients with nondystrophic myotonia and was well tolerated. [ABSTRACT FROM AUTHOR]