Periodic paralysis.

Periodic paralysis is a rare, dominantly inherited disorder of skeletal muscle in which episodic attacks of weakness are caused by a transient impairment of fiber excitability. Attacks of weakness are often elicited by characteristic environmental triggers, which were the basis for clinically delineating subtypes of periodic paralysis and are an important distinction for optimal disease management. All forms of familial periodic paralysis are caused by mutations of ion channels, often selectively expressed in skeletal muscle, that destabilize the resting potential. The missense mutations usually alter channel function through gain-of-function changes rather than producing a complete loss-of-function null. The knowledge of which channel gene harbors a variant, whether that variant is expected to (or known to) alter function, and how altered function impairs fiber excitability aides in the interpretation of patient signs and symptoms, the interpretation of gene test results, and how to optimize therapeutic intervention for symptom management and improve quality of life.

Clinical and molecular characterization of myotonia congenita using whole-exome sequencing in Egyptian patients.

BACKGROUND: Myotonia Congenita (MC) is a rare disease classified into two major forms; Thomsen and Becker disease caused by mutations in the CLCN1 gene, which affects muscle excitability and encodes voltage-gated chloride channels (CLC-1). While, there are no data regarding the clinical and molecular characterization of myotonia in Egyptian patients. METHODS: Herein, we report seven Egyptian MC patients from six unrelated families. Following the clinical diagnosis, whole-exome sequencing (WES) was performed for genetic diagnosis. Various in silico prediction tools were utilized to interpret variant pathogenicity. The candidate variants were then validated using Sanger sequencing technique. RESULTS: In total, seven cases were recruited. The ages at the examination were ranged from eight months to nineteen years. Clinical manifestations included warm-up phenomenon, hand grip, and percussion myotonia. Electromyography was performed in all patients and revealed myotonic discharges. Molecular genetic analysis revealed five different variants. Of them, we identified two novel variants in the CLCN1 gene ( c.1583G > C; p.Gly528Ala and c.2203_2216del;p.Thr735ValfsTer57) and three known variants in the CLCN1 and SCN4A gene. According to in silico tools, the identified novel variants were predicted to have deleterious effects. CONCLUSIONS: As the first study to apply WES among Egyptian MC patients, our findings reported two novel heterozygous variants that expand the CLCN1 mutational spectrum for MC diagnosis. These results further confirm that genetic testing is essential for early diagnosis of MC, which affects follow-up treatment and prognostic assessment in clinical practice.

Construction of a Prognostic Model for Hepatocellular Carcinoma Based on Macrophage Polarization-Related Genes.

Background: The progression of hepatocellular carcinoma (HCC) is related to macrophage polarization (MP). Our aim was to identify genes associated with MP in HCC patients and develop a prognostic model based on these genes. Results: We successfully developed a prognostic model consisting of six MP-related genes (SCN4A, EBF3, ADGRB2, HOXD9, CLEC1B, and MSC) to calculate the risk score for each patient. Patients were then classified into high- and low-risk groups based on their median risk score. The performance of the MP-related prognostic model was evaluated using Kaplan-Meier and ROC curves, which yielded favorable results. Additionally, the nomogram demonstrated good clinical effectiveness and displayed consistent survival predictions with actual observations. Gene Set Enrichment Analysis (GSEA) revealed enrichment of pathways related to KRAS signaling downregulation, the G2M checkpoint, and E2F targets in the high-risk group. Conversely, pathways associated with fatty acid metabolism, xenobiotic metabolism, bile acid metabolism, and adipogenesis were enriched in the low-risk group. The risk score positively correlated with the number of invasion-related genes. Immune checkpoint expression differed significantly between the two groups. Patients in the high-risk group exhibited increased sensitivity to mitomycin C, cisplatin, gemcitabine, rapamycin, and paclitaxel, while those in the low-risk group showed heightened sensitivity to doxorubicin. These findings suggest that the high-risk group may have more invasive HCC with greater susceptibility to specific drugs. IHC staining revealed higher expression levels of SCN4A in HCC tissues. Furthermore, experiments conducted on HepG2 cells demonstrated that supernatants from cells with reduced SCN4A expression promoted M2 macrophage polarization marker, CD163 in THP-1 cells. Reduced SCN4A expression induced HCC-related genes, while increased SCN4A expression reduced their expression in HepG2 cells. Conclusion: The MP-related prognostic model comprising six MPRGs can effectively predict HCC prognosis, infer invasiveness, and guide drug therapy. SCN4A is identified as a suppressor gene in HCC.

Genetic analysis of 37 cases with primary periodic paralysis in Chinese patients.

BACKGROUND: Primary periodic paralysis (PPP) is an inherited disorders of ion channel dysfunction characterized by recurrent episodes of flaccid muscle weakness, which can classified as hypokalemic (HypoPP), normokalemic (NormoPP), or hyperkalemic (HyperPP) according to the potassium level during the paralytic attacks. However, PPP is charactered by remarkable clinical and genetic heterogeneity, and the diagnosis of suspected patients is based on the characteristic clinical presentation then confirmed by genetic testing. At present, there are only limited cohort studies on PPP in the Chinese population. RESULTS: We included 37 patients with a clinical diagnosis of PPP. Eleven (29.7%) patients were tested using a specific gene panel and 26 (70.3%) by the whole-exome sequencing (WES). Twenty-two cases had a genetic variant identified, representing a diagnostic rate of 59.5% (22/37). All the identified mutations were either in the SCN4A or the CACNA1S gene. The overall detection rate was comparable between the panel (54.5%: 6/11) and WES (61.5%: 16/26). The remaining patients unresolved through panel sequencing were further analyzed by WES, without the detection of any mutation. The novel atypical splicing variant c.2020-5G > A affects the normal splicing of the SCN4A mRNA, which was confirmed by minigene splicing assay. Among 21 patients with HypoPP, 15 patients were classified as HypoPP-2 with SCN4A variants, and 6 HypoPP-1 patients had CACNA1S variants. CONCLUSIONS: Our results suggest that SCN4A alleles are the main cause in our cohort, with the remainder caused by CACNA1S alleles, which are the predominant cause in Europe and the United States. Additionally, this study identified 3 novel SCN4A and 2 novel CACNA1S variants, broadening the mutation spectrum of genes associated with PPP.

Novel compound heterozygous mutations in SCN4A as a potential genetic cause contributing to myopathic manifestations: A case report and literature review.

Background: SCN4A mutations account for a diverse array of clinical manifestations, encompassing periodic paralysis, myotonia, and newly recognized symptoms like classical congenital myopathy or congenital myasthenic syndromes. We describe the initial occurrence of myopathic features mimic with recessive classical CM in a Korean infant presenting with novel compound heterozygous SCN4A mutations. The infant exhibited profound hypotonia after birth, thereby expanding the spectrum of SCN4A-related channelopathy. Methods: The genetic analyses comprised targeted exome sequencing, employing a Celemics G-Mendeliome DES Panel, along with Sanger sequencing. Results: Considering the clinical manifestations observed in the proband, SCN4A variants emerged as the primary contenders for autosomal recessive (AR) congenital myopathy 22a, classic (#620351). Sanger sequencing validated the association of SCN4A variants with the phenotype, affirming the AR nature of the compound heterozygous variants in both the carrier mother (c.3533G > T/p.Gly1178Val) and the father (c.4216G > A/p.Ala1406Thr). Conclusion: Our report emphasizes the association of novel compound heterozygous mutations in SCN4A with myopathic features resembling CM, as supporting by muscle biopsy. It is essential to note that pathogenic SCN4A LoF mutations are exceedingly rare. This study contributes to our understanding of SCN4A mutations and their role in myopathic features mimic with classical CM.

A c.1775C > T Point Mutation of Sodium Channel Alfa Subunit Gene (SCN4A) in a Three-Generation Sardinian Family with Sodium Channel Myotonia.

Background: The nondystrophic myotonias are rare muscle hyperexcitability disorders caused by gain-of-function mutations in the SCN4A gene or loss-of-function mutations in the CLCN1 gene. Clinically, they are characterized by myotonia, defined as delayed muscle relaxation after voluntary contraction, which leads to symptoms of muscle stiffness, pain, fatigue, and weakness. Diagnosis is based on history and examination findings, the presence of electrical myotonia on electromyography, and genetic confirmation. Methods: Next-generation sequencing including the CLCN1 and SCN4A genes was performed in patients with clinical neuromuscular disorders. Electromyography, Short Exercise Test, in vivo and in vitro electrophysiology, site-directed mutagenesis and heterologous expression were collected. Results: A heterozygous point mutation (c.1775C > T, p.Thr592Ile) of muscle voltage-gated sodium channel α subunit gene (SCN4A) has been identified in five female patients over three generations, in a family with non-dystrophic myotonia. The muscle stiffness and myotonia involve mainly the face and hands, but also affect walking and running, appearing early after birth and presenting a clear cold sensitivity. Very hot temperatures, menstruation and pregnancy also exacerbate the symptoms; muscle pain and a warm-up phenomenon are variable features. Neither paralytic attacks nor post-exercise weakness has been reported. Muscle hypertrophy with cramp-like pain and increased stiffness developed during pregnancy. The symptoms were controlled with both mexiletine and acetazolamide. The Short Exercise Test after muscle cooling revealed two different patterns, with moderate absolute changes of compound muscle action potential amplitude. Conclusions: The p.Thr592Ile mutation in the SCN4A gene identified in this Sardinian family was responsible of clinical phenotype of myotonia.

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

OBJECTIVES: 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. METHODS: 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. RESULTS: 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. SIGNIFICANCE: 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. PLAIN LANGUAGE SUMMARY: 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.

Acetazolamide-responsive myotonia with a novel Ile239Thr mutation in SCN4A gene: a case report.

Introduction and importance: Sodium channel myotonia (SCM) belongs to the group of sodium channelopathies with mutations involving SCN4A gene. The main feature of sodium channel myotonia is pure myotonia without episodes of weakness or paralysis. One of the sodium channel myotonia has been classified as acetazolamide-responsive myotonia because of the effectiveness of acetazolamide as an antimyotonic drug. Case presentation: The child presented with generalized muscle hypertrophy and stiffness involving arms, thighs, calves, chest, and back muscles with unusually prominent trapezius muscle. The parents described the warm-up phenomenon as an improvement in stiffness as the day passes and with repetitive action. Percussion myotonia was illustrated in the thenar eminence and trapezius muscle. Characteristic 'dive-bomber' sound was present in electromyography, and whole-exome sequencing revealed a novel Ile239Thr mutation in the SCN4A gene. Acetazolamide was prescribed for the condition, and regular follow-up shows an excellent clinical response. Clinical discussion: This case presents a pure myotonic phenotype without episodes of weakness or paralysis. Generalized myotonia with muscle hypertrophy and demonstrating warm-up phenomenon resembles myotonia congenita (a chloride channelopathy). However, genetic analysis revealed a novel Ile239Thr mutation involving SCN4A gene indicating this case to be a sodium channelopathy. Conclusion: This case limelight sodium channel myotonia with a novel Ile239Thr mutation in SCN4A gene that phenotypically resembles myotonia congenita but genetically belongs to sodium channelopathy highlighting the poor correlation between genotypes and phenotypes in non-dystrophic myotonia. Acetazolamide can be a safe and cost-effective antimyotonic drug in sodium channel myotonia.

Hypokalemic Periodic Paralysis Type 2 Due to SCN4A Val1105Met Mutation: A Case Study.

Hypokalemic Periodic Paralysis Type 2 (HOKPP2) is a rare autosomal dominant disorder characterized by recurrent episodes of muscle weakness, paralysis, and hypokalemia. In this case report, we present the clinical details of a 49-year-old female diagnosed with HOKPP2. Genetic testing revealed a heterozygous mutation in the Sodium Voltage-Gated Channel Alpha Subunit 4 (SCN4A) gene, confirming the diagnosis of HOKPP2. Management strategies, including potassium supplementation and lifestyle modifications, were implemented, resulting in a significant decrease in the frequency of symptomatic episodes. This case highlights the importance of considering HOKPP2 in patients with recurrent muscle weakness, particularly those with a familial history of similar symptoms. Furthermore, it underscores the crucial role of genetic testing in guiding patient management and facilitating genetic counseling.

SCN4A-related congenital myopathy in a Han Chinese patient: A case report and literature review.

SCN4A mutations have been shown to be associated with myotonia, paramyotonia congenita, and periodic paralyses. More recently, loss-of-function variants in the SCN4A gene were also noted to be associated with rarer, autosomal recessive forms of congenital myasthenic syndrome and congenital myopathy. Diagnosis is challenging as the initial clinical presentation and histological features on muscle biopsies are non-specific. We report a Han Chinese patient presented with congenital myopathy with two missense SCN4A variants. The patient had an antenatal history of reduced fetal movements, polyhydramnios and a very preterm birth. At birth, she was noted to have low Apgar score, respiratory distress syndrome and hypotonia. Delayed motor development was noted in early childhood. Dysmorphic features such as an elongated face, dolichocephaly and high arched palate were present. At 16 years of age, the patient developed progressive muscle weakness and was wheelchair-bound by age 20. Muscle biopsy revealed non-specific changes only. Targeted hereditary myopathy panel testing by next generation sequencing revealed two previously unreported missense variants c.1841A > T p.(Asn614Ile) and c.4420G > A p.(Ala1474Thr) in the SCN4A gene. The clinical features of SCN4A-related congenital myopathy and myasthenic syndrome were reviewed. This case exemplifies the utility of next generation sequencing in the diagnosis of undifferentiated muscle disease.

Sodium Channel Gene Variants in Fetuses with Abnormal Sonographic Findings: Expanding the Prenatal Phenotypic Spectrum of Sodium Channelopathies.

Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of action potentials in the brain and muscle. Pathogenic variants in genes encoding VGSCs have been associated with severe disorders including epileptic encephalopathies and congenital myopathies. In this study, we identified pathogenic variants in genes encoding the α subunit of VGSCs in the fetuses of two unrelated families with the use of trio-based whole exome sequencing, as part of a larger cohort study. Sanger sequencing was performed for variant confirmation as well as parental phasing. The fetus of the first family carried a known de novo heterozygous missense variant in the SCN2A gene (NM_001040143.2:c.751G>A p.(Val251Ile)) and presented intrauterine growth retardation, hand clenching and ventriculomegaly. Neonatally, the proband also exhibited refractory epilepsy, spasms and MRI abnormalities. The fetus of the second family was a compound heterozygote for two parentally inherited novel missense variants in the SCN4A gene (NM_000334.4:c.4340T>C, p.(Phe1447Ser), NM_000334.4:c.3798G>C, p.(Glu1266Asp)) and presented a severe prenatal phenotype including talipes, fetal hypokinesia, hypoplastic lungs, polyhydramnios, ear abnormalities and others. Both probands died soon after birth. In a subsequent pregnancy of the latter family, the fetus was also a compound heterozygote for the same parentally inherited variants. This pregnancy was terminated due to multiple ultrasound abnormalities similar to the first pregnancy. Our results suggest a potentially crucial role of the VGSC gene family in fetal development and early lethality.

Diagnostics in skeletal muscle channelopathies.

INTRODUCTION: Skeletal muscle channelopathies (SMCs) are a heterogenous group of disorders, caused by mutations in skeletal ion channels leading to abnormal muscle excitability, resulting in either delayed muscle relaxation (myotonia) which characterizes non-dystrophic myotonias (NDMs), or membrane transient inactivation, causing episodic weakness, typical of periodic paralyses (PPs). AREAS COVERED: SMCs include myotonia congenita, paramyotonia congenita, and sodium-channel myotonia among NDMs, and hyper-normokalemic, hypokalemic, or late-onset periodic paralyses among PPs. When suspecting an SMC, a structured diagnostic approach is required. Detailed personal and family history and clinical examination are essential, while neurophysiological tests should confirm myotonia and rule out alternative diagnosis. Moreover, specific electrodiagnostic studies are important to further define the phenotype of de novo cases and drive molecular analyses together with clinical data. Definite diagnosis is achieved through genetic testing, either with Sanger sequencing or multigene next-generation sequencing panel. In still unsolved patients, more advanced techniques, as exome-variant sequencing or whole-genome sequencing, may be considered in expert centers. EXPERT OPINION: The diagnostic approach to SMC is still mainly based on clinical data; moreover, definite diagnosis is sometimes complicated by the difficulty to establish a proper genotype-phenotype correlation. Lastly, further studies are needed to allow the genetic characterization of unsolved patients.

An Instance of Hypokalemic Periodic Paralysis in Adolescent Brothers: A Case Report.

Hypokalemic periodic paralysis (HypoPP) is a rare autosomal dominant disease caused by mutations in either calcium or sodium transmembrane voltage-gated ion channels of skeletal muscle or endoplasmic reticulum. Most cases of HypoPP are associated with a mutation in the gene encoding a calcium channel, the CACNA1S gene. Mutations in the channels create leakage currents that disrupt resting potential and depolarize the muscle fiber resulting in transient flaccid paralysis and low extracellular potassium (K+). Patients experience episodes of muscle paralysis typically provoked by exertion and diet. Treatment focuses on the prevention of such episodes with carbonic-anhydrase inhibitors or potassium-sparing diuretics as well as to treatment of acute episodes with oral K+ supplementation. Due to the rarity of the disease, the literature surrounding the disease and pharmacological management is limited. We present a case of two adolescent brothers who present with a confirmed diagnosis of periodic episodes of paralysis and are seeking treatment. Both brothers experience paralytic episodes provoked by acute changes in diet and exercise. However, the lack of literature and treatment guidelines surrounding the disease emphasizes the importance of documenting cases and the effectiveness of treatment outcomes. Additionally, it reminds providers to keep HypoPP on the differential when faced with a young patient experiencing paralytic episodes.

Gene panel analysis of 119 index patients with suspected periodic paralysis in Japan.

Introduction: Genetic factors are recognized as the major reason for patients with periodic paralysis. The goal of this study was to determine the genetic causes of periodic paralysis in Japan. Methods: We obtained a Japanese nationwide case series of 119 index patients (108 men and 11 women) clinically suspected of periodic paralysis, and a gene panel analysis, targeting CACNA1S, SCN4A, and KCNJ2 genes, was conducted. Results: From 34 cases, 25 pathogenic/likely pathogenic/unknown significance variants were detected in CACNA1S (nine cases), SCN4A (19 cases), or KCNJ2 (six cases), generating a molecular diagnostic rate of 28.6%. In total, seven variants have yet been found linked to periodic paralysis previously. The diagnostic yield of patients with hypokalemic and hyperkalemic periodic paralyzes was 26.2 (17/65) and 32.7% (17/52), respectively. A considerably higher yield was procured from patients with than without positive family history (18/25 vs. 16/94), onset age ≤20 years (24/57 vs. 9/59), or recurrent paralytic attacks (31/94 vs. 3/25). Discussion: The low molecular diagnostic rate and specific genetic proportion of the present study highlight the etiological complexity of patients with periodic paralysis in Japan.

Hyperkalemic periodic paralysis associated with a novel missense variant located in the inner pore of Nav1.4.

BACKGROUND: Hyperkalemic periodic paralysis (HyperPP) is an autosomal dominantly inherited disease characterized by episodic paralytic attacks with hyperkalemia, and is caused by mutations of the SCN4A gene encoding the skeletal muscle type voltage-gated sodium channel Nav1.4. The pathological mechanism of HyperPP was suggested to be associated with gain-of-function changes for Nav1.4 gating, some of which are defects of slow inactivation. CASE PRESENTATION & METHODS: We identified a HyperPP family consisting of the proband and his mother, who showed a novel heterozygous SCN4A variant, p.V792G, in an inner pore lesion of segment 6 in Domain II of Nav1.4. Clinical and neurophysiological evaluations were conducted for the proband and his mother. We explored the pathogenesis of the variant by whole-cell patch clamp technique using HEK293T cells expressing the mutant Nav1.4 channel. RESULTS: Functional analysis of Nav1.4 with the V792G mutation revealed a hyperpolarized shift of voltage-dependent activation and fast inactivation. Moreover, steady-state slow inactivation in V792G was impaired with larger residual currents in comparison with wild-type Nav1.4. CONCLUSION: V792G in SCN4A is a pathogenic variant associated with the HyperPP phenotype and the inner pore lesion of Nav1.4 plays a crucial role in slow inactivation.

Coexistence of SCN4A and CLCN1 mutations in a family with atypical myotonic features: A clinical and functional study.

Non-dystrophic myotonias include several entities with possible clinical overlap, i.e. myotonia congenita caused by CLCN1 gene mutations, as well as paramyotonia congenita and sodium channel myotonia caused by SCN4A gene mutations. Herein, we describe the clinical features of five relatives affected by clinical and neurophysiological myotonia, with an aspecific and mixed phenotype. Next-generation sequencing identified the novel p.K1302R variant in SCN4A and the p.H838P variant in CLCN1. Segregation of the two mutations with the disease was confirmed by genotyping affected and non-affected family members. Patch-clamp experiments showed that sodium currents generated by p.K1302R and WT hNav1.4 were very similar. Mutant channel showed a small negative shift (5 mV) in the voltage-dependence of activation, which increased the likelihood of the channel to open at more negative voltages. The p.H838P mutation caused a reduction in chloride current density and a small voltage-dependence shift towards less negative potentials, in agreement with its position into the CBS2 domain of the C-terminus. Our results demonstrated that the mild functional alterations induced by p.K1302R and p.H838P in combination may be responsible for the mixed myotonic phenotypes. The K1302R mutant was sensitive to mexiletine and lamotrigine, suggesting that both drugs might be useful for the K1302R carriers.

Case report: Sodium and chloride muscle channelopathy coexistence: A complicated phenotype and a challenging diagnosis.

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 stiffness 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 different forms of myotonia (Paramyotonia Congenita-PMC and Sodium Channel Myotonia-SCM and severe neonatal episodic laryngospasm-SNEL). SCM often presents facial muscle stiffness, cold sensitivity, and muscle pain, whereas myotonia worsens in PMC patients with the repetition of the muscle activity and cold. Patients affected by chloride or sodium channelopathies may show similar phenotypes and symptoms, making the diagnosis more difficult 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 stiffness, and her symptoms had been worsening through the years leading to frequent heavy retrosternal compression, tachycardia, stiffness, 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 effects. Analysis of SCN4A revealed a new variant, p.Glu1607del. Nonetheless the severity of myotonia in the lower limbs and her general stiffness 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.

Periodic paralysis due to cumulative effects of rare variants in SCN4A with small functional alterations.

INTRODUCTION/AIMS: Mutations in the SCN4A gene encoding a voltage-gated sodium channel (Nav1.4) cause hyperkalemic periodic paralysis (HyperPP) and hypokalemic periodic paralysis (HypoPP). Typically, both HyperPP and HypoPP are considered as monogenic disorders caused by a missense mutation with a large functional effect. However, a few cases with atypical periodic paralysis phenotype have been caused by multiple mutations in ion-channel genes expressed in skeletal muscles. In this study we investigated the underlying pathogenic mechanisms in such cases. METHODS: We clinically assessed two families: proband 1 with HyperPP and proband 2 with atypical periodic paralysis with hypokalemia. Genetic analyses were performed by next-generation sequencing and conventional Sanger sequencing, followed by electrophysiological analyses of the mutant Nav1.4 channels expressed in human embryonic kidney 293T (HEK293T) cells using the whole-cell patch-clamp technique. RESULTS: In proband 1, K880del was identified in the SCN4A gene. In proband 2, K880del and a novel mutation, R1639H, were identified in the same allele of the SCN4A gene. Functional analyses revealed that the K880del in SCN4A has a weak functional effect on hNav1.4, increasing the excitability of the sarcolemma, which could represent a potential pathogenic factor. Although R1639H alone did not reveal functional changes strong enough to be pathogenic, Nav1.4 with both K880del and R1639H showed enhanced activation compared with K880del alone, indicating that R1639H may modify the hNav1.4 channel function. DISCUSSION: A cumulative effect of variants with small functional alterations may be considered as the underpinning oligogenic pathogenic mechanisms for the unusual phenotype of periodic paralysis.

Clinical comparison and functional study of the L703P: a recurrent mutation in human SCN4A that causes sodium channel myotonia.

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.

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 ± 4.70 years vs. 9.23 ± 5.21 years), 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.