Paramyotonia Congenita with Persistent Distal and Facial Muscle Weakness: A Case Report with Literature Review.

BACKGROUND: Paramyotonia congenita (PC; OMIM 168300) is a non-dystrophic myotonia caused by mutations in the SCN4A gene. Transient muscle stiffness, usually induced by exposure to cold and aggravated by exercise, is the predominant clinical symptom, and interictal persistent weakness is uncommon. CASE REPORT: We report a family with a history of PC accompanied by persistent hand muscle weakness with masticatory muscle involvement. Persistent weakness was exacerbated with age, and MR analysis showed marked atrophy of temporal, masseter, and finger flexor muscles with fatty replacement. The PC causative mutation T1313M in the SCN4A gene was prevalent in the family. Administration of acetazolamide chloride improved clinical symptoms and the results of cold and short exercise tests. Phenotypic variation within the family was remarkable, as the two younger affected patients did not present with persistent weakness or muscle atrophy. CONCLUSIONS: PC associated with the T1313M mutation is a possible cause of persistent distal hand weakness.

Shear wave velocity is sensitive to changes in muscle stiffness that occur independently from changes in force.

Clinical assessments for many musculoskeletal disorders involve evaluation of muscle stiffness, although it is not yet possible to obtain quantitative estimates from individual muscles. Ultrasound elastography can be used to estimate the material properties of unstressed, homogeneous, and isotropic materials by tracking the speed of shear wave propagation; these waves propagate faster in stiffer materials. Although elastography has been applied to skeletal muscle, there is little evidence that shear wave velocity (SWV) can directly estimate muscle stiffness since this tissue violates many of the assumptions required for there to be a direct relationship between SWV and stiffness. The objective of this study was to evaluate the relationship between SWV and direct measurements of muscle force and stiffness in contracting muscle. Data were collected from six isoflurane-anesthetized cats. We measured the short-range stiffness in the soleus via direct mechanical testing in situ and SWV via ultrasound imaging. Measurements were taken during supramaximal activation at optimum muscle length, with muscle temperature varying between 26°C and 38°C. An increase in temperature causes a decrease in muscle stiffness at a given force, thus decoupling the tension-stiffness relationship normally present in muscle. We found that increasing muscle temperature decreased active stiffness from 4.0 ± 0.3 MPa to 3.3 ± 0.3 MPa and SWV from 16.9 ± 1.5 m/s to 15.9 ± 1.6 m/s while force remained unchanged (mean ± SD). These results demonstrate that SWV is sensitive to changes in muscle stiffness during active contractions. Future work is needed to determine how this relationship is influenced by changes in muscle structure and tension.NEW & NOTEWORTHY Shear wave ultrasound elastography is a noninvasive tool for characterizing the material properties of muscle. This study is the first to compare direct measurements of stiffness with ultrasound measurements of shear wave velocity (SWV) in a contracting muscle. We found that SWV is sensitive to changes in muscle stiffness, even when controlling for muscle tension, another factor that influences SWV. These results are an important step toward developing noninvasive tools for characterizing muscle structure and function.

Trouble at the junction: When myopathy and myasthenia overlap.

Although myopathies and neuromuscular junction disorders are typically distinct, their coexistence has been reported in several inherited and acquired conditions. Affected individuals have variable clinical phenotypes but typically display both a decrement on repetitive nerve stimulation and myopathic findings on muscle biopsy. Inherited causes include myopathies related to mutations in BIN1, DES, DNM2, GMPPB, MTM1, or PLEC and congenital myasthenic syndromes due to mutations in ALG2, ALG14, COL13A1, DOK7, DPAGT1, or GFPT1. Additionally, a decrement due to muscle fiber inexcitability is observed in certain myotonic disorders. The identification of a defect of neuromuscular transmission in an inherited myopathy may assist in establishing a molecular diagnosis and in selecting patients who would benefit from pharmacological correction of this defect. Acquired cases meanwhile stem from the co-occurrence of myasthenia gravis or Lambert-Eaton myasthenic syndrome with an immune-mediated myopathy, which may be due to paraneoplastic disorders or exposure to immune checkpoint inhibitors.

Overlap of periodic paralysis and paramyotonia congenita caused by SCN4A gene mutations two family reports and literature review.

OBJECTIVE: To verify the diagnosis of channelopathies in two families and explore the mechanism of the overlap between periodic paralysis (PP) and paramyotonia congenita (PMC). METHODS: We have studied two cases with overlapping symptoms of episodic weakness and stiffness in our clinical center using a series of assessment including detailed medical history, careful physical examination, laboratory analyses, muscle biopsy, electrophysiological evaluation, and genetic analysis. RESULTS: The first proband and part of his family with the overlap of PMC and hyperkalemic periodic paralysis (HyperPP) has been identified as c.2111C > T (T704M) substitution of the gene SCN4A. The second proband and part of his family with the overlap of PMC and hypokalemic periodic paralysis type 2 (HypoPP2) has been identified as c.4343G > A (R1448H) substitution of the gene SCN4A. In addition, one member of the second family with overlapping symptoms has been identified as a novel mutation c.2111C > T without the mutation c.4343G > A. CONCLUSIONS: SCN4A gene mutations can cause the overlap of PMC and PP (especially the HypoPP2). The clinical symptoms of episodic weakness and stiffness could happen at a different time or temperature. Based on diagnosis assessments such as medical history and muscle biopsy, further evaluations on long-time exercise test, genetic analysis, and patch clamp electrophysiology test need to be done in order to verify the specific subtype of channelopathies. Furthermore, the improvement of one member in the pregnancy period can be used as a reference for the other female in the child-bearing period with T704M.

N1366S mutation of human skeletal muscle sodium channel causes paramyotonia congenita.

KEY POINTS: Paramyotonia congenita is a hereditary channelopathy caused by missense mutations in the SCN4A gene, which encodes the α subunit of the human skeletal muscle voltage-gated sodium channel NaV1.4. Affected individuals suffered from myotonia and paralysis of muscles, which were aggravated by exposure to cold. We report a three-generation Chinese family with patients presenting paramyotonia congenita and identify a novel N1366S mutation of NaV1.4. Whole-cell electrophysiological recordings of the N1366S channel reveal a gain-of-function change of gating in response to cold. Modelling and molecular dynamic simulation data suggest that an arginine-to-serine substitution at position 1366 increases the distance from N1366 to R1454 and disrupts the hydrogen bond formed between them at low temperature. We demonstrate that N1366S is a disease-causing mutation and that the temperature-sensitive alteration of N1366S channel activity may be responsible for the pronounced paramyotonia congenita symptoms of these patients. ABSTRACT: Paramyotonia congenita is an autosomal dominant skeletal muscle channelopathy caused by missense mutations in SCN4A, the gene encoding the α subunit of the human skeletal muscle voltage-gated sodium channel NaV1.4. We report a three-generation family in which six members present clinical symptoms of paramyotonia congenita characterized by a marked worsening of myotonia by cold and by the presence of clear episodes of paralysis. We identified a novel mutation in SCN4A (Asn1366Ser, N1366S) in all patients in the family but not in healthy relatives or in 500 normal control subjects. Functional analysis of the channel protein expressed in HEK293 cells by whole-cell patch clamp recording revealed that the N1366S mutation led to significant alterations in the gating process of the NaV1.4 channel. The N1366S mutant displayed a cold-induced hyperpolarizing shift in the voltage dependence of activation and a depolarizing shift in fast inactivation, as well as a reduced rate of fast inactivation and accelerated recovery from fast inactivation. In addition, homology modelling and molecular dynamic simulation of N1366S and wild-type NaV1.4 channels indicated that the arginine-to-serine substitution disrupted the hydrogen bond formed between N1366 and R1454. Together, our results suggest that N1366S is a gain-of-function mutation of NaV1.4 at low temperature and the mutation may be responsible for the clinical symptoms of paramyotonia congenita in the affected family and constitute a basis for studies into its pathogenesis.

Splicing biomarkers of disease severity in myotonic dystrophy.

OBJECTIVE: To develop RNA splicing biomarkers of disease severity and therapeutic response in myotonic dystrophy type 1 (DM1) and type 2 (DM2). METHODS: In a discovery cohort, we used microarrays to perform global analysis of alternative splicing in DM1 and DM2. The newly identified splicing changes were combined with previous data to create a panel of 50 putative splicing defects. In a validation cohort of 50 DM1 subjects, we measured the strength of ankle dorsiflexion (ADF) and then obtained a needle biopsy of tibialis anterior (TA) to analyze splice events in muscle RNA. The specificity of DM-associated splicing defects was assessed in disease controls. The CTG expansion size in muscle tissue was determined by Southern blot. The reversibility of splicing defects was assessed in transgenic mice by using antisense oligonucleotides to reduce levels of toxic RNA. RESULTS: Forty-two splicing defects were confirmed in TA muscle in the validation cohort. Among these, 20 events showed graded changes that correlated with ADF weakness. Five other splice events were strongly affected in DM1 subjects with normal ADF strength. Comparison to disease controls and mouse models indicated that splicing changes were DM-specific, mainly attributable to MBNL1 sequestration, and reversible in mice by targeted knockdown of toxic RNA. Splicing defects and weakness were not correlated with CTG expansion size in muscle tissue. INTERPRETATION: Alternative splicing changes in skeletal muscle may serve as biomarkers of disease severity and therapeutic response in myotonic dystrophy.

Muscle MRI reveals distinct abnormalities in genetically proven non-dystrophic myotonias.

We assessed the presence, frequency and pattern of MRI abnormalities in non-dystrophic myotonia patients. We reviewed T1-weighted and STIR (short-tau-inversion-recovery) 3T MRI sequences of lower limb muscles at thigh and calf level in 21 patients with genetically confirmed non-dystrophic myotonia: 11 with CLCN1 mutations and 10 with SCN4A mutations, and 19 healthy volunteers. The MRI examinations of all patients showed hyperintensity within muscles on either T1-weighted or STIR images. Mild extensive or marked T1-weighted changes were noted in 10/21 patients and no volunteers. Muscles in the thigh were equally likely to be affected but in the calf there was sparing of tibialis posterior. Oedema was common in calf musculature especially in the medial gastrocnemius with STIR hyperintensity observed in 18/21 patients. In 10/11 CLCN1 patients this included a previously unreported "central stripe", also present in 3/10 SCN4A patients but no volunteers. Degree of fatty infiltration correlated with age (rho=0.46, p<0.05). Muscle MRI is frequently abnormal in non-dystrophic myotonia providing evidence of fatty infiltration and/or oedema. The pattern is distinct from other myotonic disorders; in particular the "central stripe" has not been reported in other conditions. Correlations with clinical parameters suggest a potential role for MRI as a biomarker.

Molecular mechanisms of muscle atrophy in myotonic dystrophies.

Myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2) are multisystemic diseases that primarily affect skeletal muscle, causing myotonia, muscle atrophy, and muscle weakness. DM1 and DM2 pathologies are caused by expansion of CTG and CCTG repeats in non-coding regions of the genes encoding myotonic dystrophy protein kinase (DMPK) and zinc finger protein 9 (ZNF9) respectively. These expansions cause DM pathologies through accumulation of mutant RNAs that alter RNA metabolism in patients' tissues by targeting RNA-binding proteins such as CUG-binding protein 1 (CUGBP1) and Muscle blind-like protein 1 (MBNL1). Despite overwhelming evidence showing the critical role of RNA-binding proteins in DM1 and DM2 pathologies, the downstream pathways by which these RNA-binding proteins cause muscle wasting and muscle weakness are not well understood. This review discusses the molecular pathways by which DM1 and DM2 mutations might cause muscle atrophy and describes progress toward the development of therapeutic interventions for muscle wasting and weakness in DM1 and DM2. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Alternative splicing of human insulin receptor gene (INSR) in type I and type II skeletal muscle fibers of patients with myotonic dystrophy type 1 and type 2.

INSR, one of those genes aberrantly expressed in myotonic dystrophy type 1 (DM1) and type 2 (DM2) due to a toxic RNA effect, encodes for the insulin receptor (IR). Its expression is regulated by alternative splicing generating two isoforms: IR-A, which predominates in embryonic tissue, and IR-B, which is highly expressed in adult, insulin-responsive tissues (skeletal muscle, liver, and adipose tissue). The aberrant INSR expression detected in DM1 and DM2 muscles tissues, characterized by a relative increase of IR-A versus IR-B, was pathogenically related to the insulin resistance occurring in DM patients. To assess if differences in the aberrant splicing of INSR could underlie the distinct fiber type involvement observed in DM1 and DM2 muscle tissues, we have used laser capture microdissection (LCM) and RT-PCR, comparing the alternative splicing of INSR in type I and type II muscle fibers isolated from muscle biopsies of DM1, DM2 patients and controls. In the controls, the relative amounts of IR-A and IR-B showed no obvious differences between type I and type II fibers, as in the whole muscle tissue. In DM1 and DM2 patients, both fiber types showed a similar, relative increase of IR-A versus IR-B, as also evident in the whole muscle tissue. Our data suggest that the distinct fiber type involvement in DM1 and DM2 muscle tissues would not be related to qualitative differences in the expression of INSR. LCM can represent a powerful tool to give a better understanding of the pathogenesis of myotonic dystrophies, as well as other myopathies.

Clinical aspects, molecular pathomechanisms and management of myotonic dystrophies.

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) was described more than 100 years ago and is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) was identified only 18 years ago and is caused by a (CCTG)n expansion in ZNF9/CNBP. When transcribed into CUG/CCUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in spliceopathy of downstream effector genes. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders requiring different diagnostic and management strategies. DM1 may present in four different forms: congenital, early childhood, adult onset and late-onset oligosymptomatic DM1. Congenital DM1 is the most severe form of DM characterized by extreme muscle weakness and mental retardation. In DM2 the clinical phenotype is extremely variable and there are no distinct clinical subgroups. Congenital and childhood-onset forms are not present in DM2 and, in contrast to DM1, myotonia may be absent even on EMG. Due to the lack of awareness of the disease among clinicians, DM2 remains largely underdiagnosed. The delay in receiving the correct diagnosis after onset of first symptoms is very long in DM: on average more than 5 years for DM1 and more than 14 years for DM2 patients. The long delay in the diagnosis of DM causes unnecessary problems for the patients to manage their lives and anguish with uncertainty of prognosis and treatment.

Nuclear ribonucleoprotein-containing foci increase in size in non-dividing cells from patients with myotonic dystrophy type 2.

Myotonic dystrophies (DM) are genetically based neuromuscular disorders characterized by the accumulation of mutant transcripts into peculiar intranuclear foci, where different splicing factors (among which the alternative splicing regulator muscleblind-like 1 protein, MBNL1) are ectopically sequestered. The aim of the present investigation was to describe the dynamics of the DM-specific intranuclear foci in interphase nuclei and during mitosis, as well as after the exit from the cell cycle. Primary cultures of skin fibroblasts from DM2 patients were used, as a model system to reproduce in vitro, as accurately as possible, the in vivo conditions. Cycling and resting fibroblasts were investigated by immunocytochemical and morphometric techniques, and the relative amounts of MBNL1 were also estimated by western blotting. MBNL1-containing foci were exclusively found in the nucleus during most of the interphase, while being observed in the cytoplasm during mitosis when they never associate with the chromosomes; the foci remained in the cytoplasm at cytodieresis, and underwent disassembly in early G1 to be reformed in the nucleus at each cell cycle. After fibroblasts had stopped dividing in late-passage cultures, the nuclear foci were observed to progressively increase in size. Interestingly, measurements on muscle biopsies taken from the same DM2 patients at different ages demonstrated that, in the nuclei of myofibers, the MBNL1-containing foci become larger with increasing patient's age. As a whole, these results suggest that in non-dividing cells of DM2 patients the sequestration in the nuclear foci of factors needed for RNA processing would be continuous and progressive, eventually leading to the onset (and the worsening with time) of the pathological traits. This is consistent with the evidence that in DM patients the most affected organs or tissues are those where non-renewing cells are mainly present, i.e., the central nervous system, heart and skeletal muscle.

Sequestration of MBNL1 in tissues of patients with myotonic dystrophy type 2.

The pathogenesis of myotonic dystrophy type 2 includes the sequestration of MBNL proteins by expanded CCUG transcripts, which leads to an abnormal splicing of their target pre-mRNAs. We have found CCUG(exp) RNA transcripts of the ZNF9 gene associated with the formation of ribonuclear foci in human skeletal muscle and some non-muscle tissues present in muscle biopsies and skin excisions from myotonic dystrophy type 2 patients. Using RNA-FISH and immunofluorescence-FISH methods in combination with a high-resolution confocal microscopy, we demonstrate a different frequency of nuclei containing the CCUG(exp) foci, a different expression pattern of MBNL1 protein and a different sequestration of MBNL1 by CCUG(exp) repeats in skeletal muscle, vascular smooth muscle and endothelia, Schwann cells, adipocytes, and ectodermal derivatives. The level of CCUG(exp) transcription in epidermal and hair sheath cells is lower compared with that in other tissues examined. We suppose that non-muscle tissues of myotonic dystrophy type 2 patients might be affected by a similar molecular mechanism as the skeletal muscle, as suggested by our observation of an aberrant insulin receptor splicing in myotonic dystrophy type 2 adipocytes.

Co-segregation of DM2 with a recessive CLCN1 mutation in juvenile onset of myotonic dystrophy type 2.

Myotonic dystrophy type 2 (DM2) is a common adult onset muscular dystrophy caused by a dominantly transmitted (CCTG)( n ) expansion in intron 1 of the CNBP gene. In DM2 there is no obvious evidence for an intergenerational increase of expansion size, and no congenital cases have been confirmed. We describe the clinical and histopathological features, and provide the genetic and molecular explanation for juvenile onset of myotonia in a 14-year-old female with DM2 and her affected mother presenting with a more severe phenotype despite a later onset of symptoms. Histological and immunohistochemical findings correlated with disease severity or age at onset in both patients. Southern blot on both muscle and blood samples revealed only a small increase in the CCTG repeat number through maternal transmission. Fluorescence in situ hybridization, in combination with MBNL1 immunofluorescence on muscle sections, showed the presence of mutant mRNA and MBNL1 in nuclear foci; the fluorescence intensity and its area appeared to be similar in the two patients. Splicing analysis of the INSR, CLCN1 and MBNL1 genes in muscle tissue demonstrates that the level of aberrant splicing isoforms was lower in the daughter than in the mother. However, in the CLCN1 gene, a heterozygous mutation c.501C>G p.F167L was present in the daughter's DNA and found to be maternally inherited. Biomolecular findings did not explain the unusual young onset in the daughter. The co-segregation of DM2 with a recessive CLCN1 mutation provided the explanation for the unusual clinical findings.

Cultured myoblasts from patients affected by myotonic dystrophy type 2 exhibit senescence-related features: ultrastructural evidence.

Myotonic dystrophy type 2 (DM2) is an autosomal dominant disorder caused by the expansion of the tetranucleotidic repeat (CCTG)n in the first intron of the Zinc Finger Protein-9 gene. In DM2 tissues, the expanded mutant transcripts accumulate in nuclear focal aggregates where splicing factors are sequestered, thus affecting mRNA processing. Interestingly, the ultrastructural alterations in the splicing machinery observed in the myonuclei of DM2 skeletal muscles are reminiscent of the nuclear changes occurring in age-related muscle atrophy. Here, we investigated in vitro structural and functional features of satellite cell-derived myoblasts from biceps brachii, in the attempt to investigate cell senescence indices in DM2 patients by ultrastructural cytochemistry. We observed that in satellite cell-derived DM2 myoblasts, cell-senescence alterations such as cytoplasmic vacuolization, reduction of the proteosynthetic apparatus, accumulation of heterochromatin and impairment of the pre-mRNA maturation pathways occur earlier than in myoblasts from healthy patients. These results, together with preliminary in vitro observations on the early onset of defective structural features in DM2 myoblast derived-myotubes, suggest that the regeneration capability of DM2 satellite cells may be impaired, thus contributing to the muscular dystrophy in DM2 patients.

3 Tesla sodium inversion recovery magnetic resonance imaging allows for improved visualization of intracellular sodium content changes in muscular channelopathies.

OBJECTIVES: To implement different sodium (²³Na)-magnetic resonance imaging (MRI) contrasts at 3 Tesla and to evaluate if a weighting toward intracellular sodium can be achieved, using 2 rare muscular channelopathies as model diseases. MATERIALS AND METHODS: Both lower legs of 6 patients with hypokalemic periodic paralysis (HypoPP), 5 patients with paramyotonia congenita (PC), and 5 healthy volunteers were examined on a 3 Tesla system with 3 different ²³Na-MRI pulse sequences. HypoPP and PC are rare muscle diseases, which are well characterized by elevated myoplasmic sodium at rest and after cooling, respectively. Intra- and interindividual comparisons were performed before and after provocation of one lower leg muscle. Three different ²³Na-MRI sequences were applied: (i) The total tissue sodium concentration was measured using a spin-density sequence (²³Na-TSC). (ii) A T1-contrast was applied to assess whether the known changes of the intracellular sodium concentration can be visualized by T1-weighting (²³Na-T1). (iii) An inversion recovery (²³Na-IR) sequence was used to utmost suppress the sodium signal from extracellular or vasogenic edema. Furthermore, a potential influence of the temperature dependency of the sodium relaxation times was considered. Additionally, H-MRI was performed to examine potential lipomatous or edematous changes. RESULTS: In HypoPP, all Na sequences showed significantly (P<0.05) higher signal intensities compared with PC patients and healthy subjects. In muscles of PC patients, provocation induced a significant (P=0.0007) increase (>20%) in the muscular ²³Na-IR signal and a corresponding decrease of muscle strength. Additionally, a tendency to higher ²³Na-T1 (P=0.07) and ²³Na-TSC (P=0.07) signal intensities was observed. Provocation revealed no significant changes in ¹H-MRI. In volunteers and in the contralateral, not cooled lower leg, there were no significant signal intensity changes after provocation. Furthermore, the ²³Na-IR sequence allows for a suppression of signal emanating from intravascular sodium and vasogenic edema. CONCLUSIONS: Our results indicate that the ²³Na-IR sequence allows for a weighting toward intracellular sodium. The combined application of the ²³Na-TSC and the ²³Na-IR sequence enables an improved analysis of pathophysiological changes that occur in muscles of patients with muscular channelopathies.

Muscle pathology in myotonic dystrophy: light and electron microscopic investigation in eighteen patients.

Myotonic dystrophy (DM) is the most common muscular dystrophy in adults. Two known genetic subtypes include DM1 (myotonic dystrophy type 1) and DM2 (myotonic dystrophy type 2). Genetic testing is considered as the only reliable diagnostic criterion in myotonic dystrophies. Relatively little is known about DM1 and DM2 myopathology. Thus, the aim of our study was to characterise light and electron microscopic features of DM1 and DM2 in patients with genetically proven types of the disease. We studied 3 DM1 cases and 15 DM2 cases from which muscle biopsies were taken for diagnostic purposes during the period from 1973 to 2006, before genetic testing became available at our hospital. The DM1 group included 3 males (age at biopsy 15-19). The DM2 group included 15 patients (5 men and 10 women, age at biopsy 26-60). The preferential type 1 fibre atrophy was seen in all three DM1 cases in light microscopy, and substantial central nucleation was present in two biopsies. Electron microscopy revealed central nuclei in all three examined muscle biopsies. No other structural or degenerative changes were detected, probably due to the young age of our patients. Central nucleation, prevalence of type 2 muscle fibres, and the presence of pyknotic nuclear clumps were observed in DM2 patients in light microscopy. Among the ultrastructural abnormalities observed in our DM2 group, the presence of internal nuclei, severely atrophied muscle fibres, and lipofuscin accumulation were consistent findings. In addition, a variety of ultrastructural abnormalities were identified by us in DM2. It appears that no single ultrastructural abnormality is characteristic for the DM2 muscle pathology. It seems, however, that certain constellations of morphological changes might be indicative of certain types of myotonic dystrophy.

Severe phenotypes of paralysis periodica paramyotonia are associated with the Met1592Val mutation in the voltage-gated sodium channel gene (SCN4A) in a Chinese family.

Paralysis periodica paramyotonia (PPP) is caused by mutation of the adult skeletal muscle sodium channel gene's alpha (α)-subunit (SCN4A). Here, we report four generations of a Chinese family affected by a remarkably severe form of PPP with progressive myopathy. Routine electromyograms (EMG) showed myotonic discharge and after a long exercise test, compound motor action potential amplitudes were markedly decreased by 40-55%. Muscle biopsy revealed obvious vacuolar changes. Moreover, genetic analysis revealed the Met1592Val mutation in the α-subunit, SCN4A. The patients showed a striking clinical and electrophysiological improvement during treatment with acetazolamide. Thus, our findings showed that mutation of Met1592Val in the SCN4A gene is associated with aggressive development of PPP characterized by severe vacuolar myopathy.

Myotonia congenita and myotonic dystrophy in the same family: coexistence of a CLCN1 mutation and expansion in the CNBP (ZNF9) gene.

Myotonia is characterized by hyperexcitability of the muscle cell membrane. Myotonic disorders are divided into two main categories: non-dystrophic and dystrophic myotonias. The non-dystrophic myotonias involve solely the muscle system, whereas the dystrophic myotonias are characterized by multisystem involvement and additional muscle weakness. Each category is further subdivided into different groups according to additional clinical features or/and underlying genetic defects. However, the phenotypes and the pathological mechanisms of these myotonic disorders are still not entirely understood. Currently, four genes are identified to be involved in myotonia: the muscle voltage-gated sodium and chloride channel genes SCN4A and CLCN1, the myotonic dystrophy protein kinase (DMPK) gene, and the CCHC-type zinc finger, nucleic acid binding protein gene CNBP. Additional gene(s) and/or modifying factor(s) remain to be identified. In this study, we investigated a large Norwegian family with clinically different presentations of myotonic disorders. Molecular analysis revealed CCTG repeat expansions in the CNBP gene in all affected members, confirming that they have myotonic dystrophy type 2. However, a CLCN1 mutation c.1238C>G, causing p.Phe413Cys, was also identified in several affected family members. Heterozygosity for p.Phe413Cys seems to exaggerate the severity of myotonia and thereby, to some degree, contributing to the pronounced variability in the myotonic phenotype in this family.

Dysregulation and cellular mislocalization of specific miRNAs in myotonic dystrophy type 1.

Myotonic Dystrophy Type-1 (DM1) is caused by the expansion of a CTG repeat with a peculiar pattern of multisystemic involvement affecting skeletal muscles, the heart, the eye, the central nervous system and the endocrine system. Since microRNA expression is disrupted in several myopathies, the expression of 24 candidate microRNAs was analyzed in skeletal muscle biopsies of 15 DM1 patients. Controls were constituted by biopsies without overt pathological features derived from 14 subjects with suspected neuromuscular disorder of undetermined nature. We found that miR-1 and miR-335 were up-regulated, whereas miR-29b and c, and miR-33 were down-regulated in DM1 biopsies compared to controls. We also found that the cellular distribution of muscle specific miR-1, miR-133b and miR-206 was severely altered in DM1 skeletal muscles. MicroRNA dysregulation was likely functionally relevant, since it impacted on the expression of the predicted miR-1, and miR-29 targets. The observed miRNA dysregulations and myslocalizations may contribute to DM1 pathogenetic mechanisms.