A mutation in a rare type of intron in a sodium-channel gene results in aberrant splicing and causes myotonia.

Many mutations in the skeletal-muscle sodium-channel gene SCN4A have been associated with myotonia and/or periodic paralysis, but so far all of these mutations are located in exons. We found a patient with myotonia caused by a deletion/insertion located in intron 21 of SCN4A, which is an AT-AC type II intron. This is a rare class of introns that, despite having AT-AC boundaries, are spliced by the major or U2-type spliceosome. The patient's skeletal muscle expressed aberrantly spliced SCN4A mRNA isoforms generated by activation of cryptic splice sites. In addition, genetic suppression experiments using an SCN4A minigene showed that the mutant 5' splice site has impaired binding to the U1 and U6 snRNPs, which are the cognate factors for recognition of U2-type 5' splice sites. One of the aberrantly spliced isoforms encodes a channel with a 35-amino acid insertion in the cytoplasmic loop between domains III and IV of Nav1.4. The mutant channel exhibited a marked disruption of fast inactivation, and a simulation in silico showed that the channel defect is consistent with the patient's myotonic symptoms. This is the first report of a disease-associated mutation in an AT-AC type II intron, and also the first intronic mutation in a voltage-gated ion channel gene showing a gain-of-function defect.

[Encephalitis associated with positive anti-GluR antibodies showing abnormal appearance in basal ganglia, pulvinar and gray matter on MRI--case report].

We treated a 25-year-old woman with encephalitis. Following delivery, the patient developed fever, consciousness disturbance, cognitive dysfunction, and progressive motor dysfunction. In addition, mycobacterium tuberculosis was found in the lung, though there was no evidence of such infection in the central nervous system. Cerebrospinal fluid analysis revealed a slight elevation of mononuclear cells with a normal protein level indicating a possible viral infection. We could not find the origin of the infection, though the serum anti-glutamate epsilon2 receptor antibody was positive. Intravenous administration of methylprednisolone (1000 mg/day for 3 days) was temporarily effective for improvement of the clinical signs and symptoms. However, she finally demonstrated rapid deterioration resulting in death. Diffusion-weighted brain magnetic resonance imaging demonstrated abnormal high intensity lesions in the bilateral pulvinar and gray matter, with an abnormal appearance mimicking pulvinar sign.

[A survey of cardiologists, diabetologists, gynecologists and ophthalmologists practicing in Osaka on the medical consultation behaviors of myotonic dystrophy patients].

An anonymous postal survey of cardiologists, diabetologists, gynecologists, and ophthalmologists in Osaka was performed to assess the medical care-seeking behaviors of and problems associated with the medical management of patients with myotonic dystrophy (DM). The questionnaires were sent to 927 cardiologists, 357 diabetologists, 882 gynecologists, and 915 ophthalmologists. Of these, 172 cardiologists, 85 diabetologists, 220 gynecologists, and 154 ophthalmologists responded. More than 30% of responders had provided care to DM patients, and approximately 10% had experience diagnosing DM patients. These facts suggest that DM patients receive medical care from various specialists due to complications involving multiple systems and some of them visit other specialists prior to neurologists. Some patients were diagnosed after perinatal or perioperative difficulties. Therefore, it seems important to improve the ability of physicians to identify DM patients. Because specialists with experience diagnosing DM paid more attention to the characteristic features of DM, such as grip myotonia and hatchet face, a simple screening test may be useful for detecting DM. Some responders pointed out the negative attitude of DM patients toward medical care and the lack of neurologists for consultation as problems in the medical management of DM patients. Cooperation among neurologists and other specialists and education of DM patients are important to improve the medical management of DM patients.

Hypokalaemic periodic paralysis with a charge-retaining substitution in the voltage sensor.

Familial hypokalaemic periodic paralysis is a rare skeletal muscle disease caused by the dysregulation of sarcolemmal excitability. Hypokalaemic periodic paralysis is characterized by repeated episodes of paralytic attacks with hypokalaemia, and several variants in CACNA1S coding for CaV1.1 and SCN4A coding for NaV1.4 have been established as causative mutations. Most of the mutations are substitutions to a non-charged residue, from the positively charged arginine (R) in transmembrane segment 4 (S4) of a voltage sensor in either CaV1.1 or NaV1.4. Mutant channels have aberrant leak currents called 'gating pore currents', and the widely accepted consensus is that this current is the essential pathological mechanism that produces susceptibility to anomalous depolarization and failure of muscle excitability during a paralytic attack. Here, we have identified five hypokalaemic periodic paralysis cases from two different ethnic backgrounds, Japanese and French, with charge-preserving substitutions in S4 from arginine, R, to lysine, K. An R to K substitution has not previously been reported for any other hypokalaemic periodic paralysis families. One case is R219K in NaV1.4, which is located at the first charge in S4 of Domain I. The other four cases all have R897K in CaV1.1, which is located at the first charge in S4 of Domain III. Gating pore currents were not detected in expression studies of CaV1.1-R897K. NaV1.4-R219K mutant channels revealed a distinct, but small, gating pore current. Simulation studies indicated that the small-amplitude gating pore current conducted by NaV1.4-R219K is not likely to be sufficient to be a risk factor for depolarization-induced paralytic attacks. Our rare cases with typical hypokalaemic periodic paralysis phenotypes do not fit the canonical view that the essential defect in hypokalaemic periodic paralysis mutant channels is the gating pore current and raise the possibility that hypokalaemic periodic paralysis pathogenesis might be heterogeneous and diverse.

A204E mutation in Nav1.4 DIS3 exerts gain- and loss-of-function effects that lead to periodic paralysis combining hyper- with hypo-kalaemic signs.

Periodic paralyses (PP) are characterized by episodic muscle weakness and are classified into the distinct hyperkalaemic (hyperPP) and hypokalaemic (hypoPP) forms. The dominantly-inherited form of hyperPP is caused by overactivity of Nav1.4 - the skeletal muscle voltage-gated sodium channel. Familial hypoPP results from a leaking gating pore current induced by dominant mutations in Nav1.4 or Cav1.1, the skeletal muscle voltage-gated calcium channel. Here, we report an individual with clinical signs of hyperPP and hypokalaemic episodes of muscle paralysis who was heterozygous for the novel p.Ala204Glu (A204E) substitution located in one region of Nav1.4 poor in disease-related variations. A204E induced a significant decrease of sodium current density, increased the window current, enhanced fast and slow inactivation of Nav1.4, and did not cause gating pore current in functional analyses. Interestingly, the negative impact of A204E on Nav1.4 activation was strengthened in low concentration of extracellular K+. Our data prove the existence of a phenotype combining signs of hyperPP and hypoPP due to dominant Nav1.4 mutations. The hyperPP component would result from gain-of-function effects on Nav1.4 and the hypokalemic episodes of paralysis from loss-of-function effects strengthened by low K+. Our data argue for a non-negligible role of Nav1.4 loss-of-function in familial hypoPP.

A case of non-dystrophic myotonia with concomitant mutations in the SCN4A and CLCN1 genes.

Non-dystrophic myotonias are caused by mutations of either the skeletal muscle chloride (CLCN1) or sodium channel (SCN4A) gene. They exhibit several distinct phenotypes, including myotonia congenita, paramyotonia congenita and sodium channel myotonia, and a genotype-phenotype correlation has been established. However, there are atypical cases that do not fit with the standard classification. We report a case of 27-year-old male who had non-dystrophic myotonia with periodic paralysis and two heterozygous mutations, E950K in CLCN1 and F1290L in SCN4A. His mother, who exhibited myotonia without paralytic attack, only harbored E950K, and no mutations were identified in his asymptomatic father. Therefore, the E950K mutation was presumed to be pathogenic, although it was reported as an extremely rare genetic variant. The proband experienced paralytic attacks that lasted for weeks and were less likely to be caused by CLCN1 mutation alone. Functional analysis of the F1290L mutant channel heterologously expressed in cultured cells revealed enhanced activation inducing membrane hyperexcitability. We therefore propose that the two mutations had additive effects on membrane excitability that resulted in more prominent myotonia in the proband. Our case stresses the value of performing genetic analysis of both CLCN1 and SCN4A genes for myotonic patients with an atypical phenotype.

Splicing misregulation of SCN5A contributes to cardiac-conduction delay and heart arrhythmia in myotonic dystrophy.

Myotonic dystrophy (DM) is caused by the expression of mutant RNAs containing expanded CUG repeats that sequester muscleblind-like (MBNL) proteins, leading to alternative splicing changes. Cardiac alterations, characterized by conduction delays and arrhythmia, are the second most common cause of death in DM. Using RNA sequencing, here we identify novel splicing alterations in DM heart samples, including a switch from adult exon 6B towards fetal exon 6A in the cardiac sodium channel, SCN5A. We find that MBNL1 regulates alternative splicing of SCN5A mRNA and that the splicing variant of SCN5A produced in DM presents a reduced excitability compared with the control adult isoform. Importantly, reproducing splicing alteration of Scn5a in mice is sufficient to promote heart arrhythmia and cardiac-conduction delay, two predominant features of myotonic dystrophy. In conclusion, misregulation of the alternative splicing of SCN5A may contribute to a subset of the cardiac dysfunctions observed in myotonic dystrophy.

A Kir3.4 mutation causes Andersen-Tawil syndrome by an inhibitory effect on Kir2.1.

OBJECTIVE: To identify other causative genes for Andersen-Tawil syndrome, which is characterized by a triad of periodic paralysis, cardiac arrhythmia, and dysmorphic features. Andersen-Tawil syndrome is caused in a majority of cases by mutations in KCNJ2, which encodes the Kir2.1 subunit of the inwardly rectifying potassium channel. METHODS: The proband exhibited episodic flaccid weakness and a characteristic TU-wave pattern, both suggestive of Andersen-Tawil syndrome, but did not harbor KCNJ2 mutations. We performed exome capture resequencing by restricting the analysis to genes that encode ion channels/associated proteins. The expression of gene products in heart and skeletal muscle tissues was examined by immunoblotting. The functional consequences of the mutation were investigated using a heterologous expression system in Xenopus oocytes, focusing on the interaction with the Kir2.1 subunit. RESULTS: We identified a mutation in the KCNJ5 gene, which encodes the G-protein-activated inwardly rectifying potassium channel 4 (Kir3.4). Immunoblotting demonstrated significant expression of the Kir3.4 protein in human heart and skeletal muscles. The coexpression of Kir2.1 and mutant Kir3.4 in Xenopus oocytes reduced the inwardly rectifying current significantly compared with that observed in the presence of wild-type Kir3.4. CONCLUSIONS: We propose that KCNJ5 is a second gene causing Andersen-Tawil syndrome. The inhibitory effects of mutant Kir3.4 on inwardly rectifying potassium channels may account for the clinical presentation in both skeletal and heart muscles.

[Compound heterozygous mutations in the muscle chloride channel gene (CLCN1) in a Japanese family with Thomsen's disease].

Autosomal-dominant type of myotonia (Thomsen's disease) and autosomal-recessive one (Becker's disease) are caused by mutations in the skeletal muscle voltage-gated chloride channel gene (CLCN1). Clinical manifestation of the diseases ranges from minimum to severely disabling myotonia. We report a Japanese family with Thomsen's disease, featuring an index female young patient who possesses two dominantly-inherited mutated CLCN1 alleles. She showed severe myotonic symptoms from 18 months of age, associated with moderate muscle hypertrophy. Her mother had mild myotonic signs without muscle hypertrophy. Her father was quite normal by both clinical and electromyographic examinations. With genomic DNA extracted from blood leukocytes, all 23 exons of the CLCN1 gene were analyzed by direct sequencing of PCR products. The analysis revealed compound heterozygous mutations of T539A and M560T in the index patient, a heterozygous mutation of T539A in her mother, and a heterozygous mutation of M560T in her father. Since both mutations were previously described in families of Thomsen's disease, her father was regarded as a non-symptomatic carrier. The family reveals that compound heterozygosity of two dominantly inheritable disease mutations exacerbates the myotonia, suggesting the dosage effect of CLCN1 mutation responsible for myotonia congenita of Thomsen type.

A sodium channel myotonia due to a novel SCN4A mutation accompanied by acquired autoimmune myasthenia gravis.

Mutations of the voltage gated sodium channel gene (SCN4A) are responsible for non-dystrophic myotonia including hyperkalemic periodic paralysis, paramyotonia congenita, and sodium channel myotonia, as well as congenital myasthenic syndrome. In vitro functional analyses have demonstrated the non-dystrophic mutants to show a gain-of-function defect of the channel; a disruption of fast inactivation, an enhancement of activation, or both, while the myasthenic mutation presents a loss-of function defect. This report presents a case of non-dystrophic myotonia that is incidentally accompanied with acquired myasthenia. The patient presented a marked warm-up phenomenon of myotonia but the repeated short exercise test suggested mutations of the sodium channel. The genetic analysis identified a novel mutation, G1292D, of SCN4A. A functional study of the mutant channel revealed marked enhancement of activation and slight impairment of fast inactivation, which should induce muscle hyperexcitability. The effects of the alteration of channel function to the myasthenic symptoms were explored by using stimulation of repetitive depolarization pulses. A use-dependent channel inactivation was reduced in the mutant in comparison to normal channel, thus suggesting an opposing effect to myasthenia.

A novel mutation in the calcium channel gene in a family with hypokalemic periodic paralysis.

Hypokalemic periodic paralysis (HypoPP) type 1 is an autosomal dominant disease caused by mutations in the Ca(V)1.1 calcium channel encoded by the CACNA1S gene. Only seven mutations have been found since the discovery of the causative gene in 1994. We describe a patient with HypoPP who had a high serum potassium concentration after recovery from a recent paralysis, which complicated the correct diagnosis. This patient and other affected family members had a novel mutation, p.Arg900Gly, in the CACNA1S gene.

Misregulated alternative splicing of BIN1 is associated with T tubule alterations and muscle weakness in myotonic dystrophy.

Myotonic dystrophy is the most common muscular dystrophy in adults and the first recognized example of an RNA-mediated disease. Congenital myotonic dystrophy (CDM1) and myotonic dystrophy of type 1 (DM1) or of type 2 (DM2) are caused by the expression of mutant RNAs containing expanded CUG or CCUG repeats, respectively. These mutant RNAs sequester the splicing regulator Muscleblind-like-1 (MBNL1), resulting in specific misregulation of the alternative splicing of other pre-mRNAs. We found that alternative splicing of the bridging integrator-1 (BIN1) pre-mRNA is altered in skeletal muscle samples of people with CDM1, DM1 and DM2. BIN1 is involved in tubular invaginations of membranes and is required for the biogenesis of muscle T tubules, which are specialized skeletal muscle membrane structures essential for excitation-contraction coupling. Mutations in the BIN1 gene cause centronuclear myopathy, which shares some histopathological features with myotonic dystrophy. We found that MBNL1 binds the BIN1 pre-mRNA and regulates its alternative splicing. BIN1 missplicing results in expression of an inactive form of BIN1 lacking phosphatidylinositol 5-phosphate-binding and membrane-tubulating activities. Consistent with a defect of BIN1, muscle T tubules are altered in people with myotonic dystrophy, and membrane structures are restored upon expression of the normal splicing form of BIN1 in muscle cells of such individuals. Finally, reproducing BIN1 splicing alteration in mice is sufficient to promote T tubule alterations and muscle weakness, a predominant feature of myotonic dystrophy.

Use of the muscle volume analyzer to evaluate enzyme replacement therapy in late-onset Pompe disease.

The muscle volume analyzer (MVA) can predict limb muscle weight based on bioelectric impedance analysis, whereas the conventional handheld dynamometer (HHD) measures muscle strength. In this study, a 26-year-old female on invasive ventilation due to late-onset Pompe disease was treated with enzyme replacement therapy (ERT) for 12 months. MVA measurements demonstrated time-dependent improvement from the baseline compared to HHD measurements, showing remarkably fluctuating muscle strength. Thus, the MVA can be used as an alternative, particularly for patients suffering from severe limb muscle weakness.