Prevalence and mutation spectrum of skeletal muscle channelopathies in the Netherlands

Few reliable data exist on the prevalence of skeletal muscle channelopathies. We determined the minimum point prevalence of genetically-defined skeletal muscle channelopathies in the Netherlands and report their mutation spectrum. Minimum point prevalence rates were calculated as number of genetically-confirmed skeletal muscle channelopathy patients (CLCN1, SCN4A, CACNA1S and KCNJ2 gene mutations) in the Netherlands (1990–2015) divided by the total number of at-risk individuals. Rates were expressed as cases/100.000 and 95% confidence intervals were calculated based on Poisson distribution. Results of standardized genetic diagnostic procedures were used to analyze mutation spectra. We identified 405 patients from 234 unrelated pedigrees, resulting in a minimum point prevalence of 2.38/100.000 (95% CI 2.16–2.63) for skeletal muscle channelopathies in the Netherlands. Minimum point prevalence rates for the disease groups, non-dystrophic myotonia and periodic paralysis, were 1.70/100.000 and 0.69/100.000 respectively.Sixty-one different CLCN1 mutations (including 12 novel mutations) were detected in myotonia congenita. Twenty-eight different SCN4A missense mutations (including three novel mutations) were identified in paramyotonia congenita/sodium channel myotonia, hypokalemic periodic paralysis and hyperkalemic periodic paralysis. Four different CACNA1S missense mutations were detected in hypokalemic periodic paralysis and five KCNJ2 missense mutations in Andersen–Tawil syndrome. The minimum point prevalence rates for genetically-defined skeletal muscle channelopathies confirm their rare disease status in the Netherlands. Rates are almost twice as high as in the UK and more in line with pre-genetic prevalence estimates in parts of Scandinavia. Future diagnostic and therapeutic studies may benefit from knowledge of the mutation spectrum of skeletal muscle channelopathies.     

A Sodium Channel Myotonia Presenting with Intermittent Dysphagia as a Manifestation of a Rare <Emphasis Type="Italic">SCN4A</Emphasis> Variant

The voltage gated sodium channel SCN4A mutations account for non-dystrophic myotonia and include a heterogeneous group of conditions that include hyperkalemic periodic paralysis, paramyotonica congenita, potassium-aggravated myotonia, and hypokalemic periodic paralysis type 2. This case report proposes that a rare variant p.Pro1629Leu in SCN4A can cause a skeletal muscle deficit with intermittent dysphagia.     

Mutation spectrum and health status in skeletal muscle channelopathies in Japan

Skeletal muscle channelopathies, including non-dystrophic myotonia and periodic paralysis, are rare hereditary disorders caused by mutations of various ion channel genes. To define the frequency of associated mutations of skeletal muscle channelopathies in Japan, clinical and genetic data of two academic institutions, which provides genetic analysis service, were reviewed. Of 105 unrelated pedigrees genetically confirmed, 66 pedigrees were non-dystrophic myotonias [CLCN1 (n = 30) and SCN4A (n = 36)], 11 were hyperkalemic periodic paralysis (SCN4A), and 28 were hypokalemic periodic paralysis [CACNA1S (n = 16) and SCN4A (n = 12)]. Of the 30 families with myotonia congenita, dominant form (Thomsen type) consisted 67%, and unique mutations, A298T, P480T, T539A, and M560T, not found in Western countries, were commonly identified in CLCN1. Hypokalemic periodic paralysis caused by SCN4A mutations consisted 43% in Japan, which was much higher than previous reports. Furthermore, the quality of life of the patients was assessed using the patient-reported outcome measures, SF-36 and INQoL, for 41 patients. This study indicated that the etiology of skeletal muscle channelopathies in Japan was not identical to previous reports from Western countries, and provided crucial information for genetics as well as future therapeutic interventions.     

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

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

Clinical Characteristics and Analysis of CLCN1 in Patients with "EMG Disease"

Background and Purpose

While the etiology and clinical features of "EMG disease" - which is characterized by diffusely increased insertional activity on needle electromyography (EMG) in the absence of neuromuscular disease - are not well known, some authorities believe it may be a form of myotonia congenita (MC). The aims of this study were to determine the clinical features of EMG disease and its relationship with CLCN1 mutations in patients.

Methods

The detailed clinical and electrophysiological features of EMG disease were evaluated in six patients. All 23 coding exons and exon-intron boundaries in CLCN1 gene were analyzed by direct sequencing to detect nucleotide changes.

Results

The common clinical symptoms of EMG disease were chronic muscle stiffness or generalized myalgia, which were aggravated in a cold environment. Four patients complained of action myotonia several times a year. Short trains of provoked positive sharp waves were documented on needle EMG, but myotonic discharges, fibrillation potentials, and fasciculations were not. Increased insertional activity was identified at the asymptomatic muscles studied. One novel heterozygous mutation was identified in one patient following genetic testing for CLCN1 mutations (c.1679T>C, p.Met560Thr).

Conclusions

The clinical features of EMG disease might be quite similar to those of MC, but CLCN1 mutation was found in only one subject. It is thus difficult to accept that EMG disease lies within the phenotypic spectrum of MC. Additional testing is needed to verify the pathogenetic cause of the diffusely increased insertional activity associated with this condition.     

Muscle channelopathies and electrophysiological approach

Myotonic syndromes and periodic paralyses are rare disorders of skeletal muscle characterized mainly by muscle stiffness or episodic attacks of weakness. Familial forms are caused by mutation in genes coding for skeletal muscle voltage ionic channels. Familial periodic paralysis and nondystrophic myotonias are disorders of skeletal muscle excitability caused by mutations in genes coding for voltage-gated ion channels. These diseases are characterized by episodic failure of motor activity due to muscle weakness (paralysis) or stiffness (myotonia). Clinical studies have identified two forms of periodic paralyses: hypokalemic periodic paralysis (hypoKPP) and hyperkalemic periodic paralysis (hyperKPP), based on changes in serum potassium levels during the attacks, and three distinct forms of myotonias: paramyotonia congenita (PC), potassium-aggravated myotonia (PAM), and myotonia congenita (MC). PC and PAM have been linked to missense mutations in the SCN4A gene, which encodes α subunit of the voltage-gated sodium channel, whereas MC is caused by mutations in the chloride channel gene (CLCN1). Exercise is known to trigger, aggravate, or relieve symptoms. Therefore, exercise can be used as a functional test in electromyography to improve the diagnosis of these muscle disorders. Abnormal changes in the compound muscle action potential can be disclosed using different exercise tests. Five electromyographic (EMG) patterns (I-V) that may be used in clinical practice as guides for molecular diagnosis are discussed.     

Heterozygous CLCN1 mutations can modulate phenotype in sodium channel myotonia

Nondystrophic myotonias are characterized by muscle stiffness triggered by voluntary movement. They are caused by mutations in either the CLCN1 gene in myotonia congenita or in the SCN4A gene in paramyotonia congenita and sodium channel myotonias. Clinical and electrophysiological phenotypes of these disorders have been well described. No concomitant mutations in both genes have been reported yet. We report five patients from three families showing myotonia with both chloride and sodium channel mutations. Their clinical and electrophysiological phenotypes did not fit with the phenotype known to be associated with the mutation initially found in SCN4A gene, which led us to screen and find an additional mutation in CLCN1 gene. Our electrophysiological and clinical observations suggest that heterozygous CLCN1 mutations can modify the clinical and electrophysiological expression of SCN4A mutation.     

Whole-Body Muscle MRI in Patients with Hyperkalemic Periodic Paralysis Carrying the SCN4A Mutation T704M: Evidence for Chronic Progressive Myopathy with Selective Muscle Involvement

Background and Purpose

Hyperkalemic periodic paralysis (hyperKPP) is a muscle sodium-ion channelopathy characterized by recurrent paralytic attacks. A proportion of affected individuals develop fixed or chronic progressive weakness that results in significant disability. However, little is known about the pathology of hyperKPP-induced fixed weakness, including the pattern of muscle involvement. The aim of this study was to characterize the patterns of muscle involvement in hyperKPP by whole-body magnetic resonance imaging (MRI).

Methods

We performed whole-body muscle MRI in seven hyperKPP patients carrying the T704M mutation in the SCN4A skeletal sodium-channel gene. Muscle fat infiltration, suggestive of chronic progressive myopathy, was analyzed qualitatively using a grading system and was quantified by the two-point Dixon technique.

Results

Whole-body muscle MRI analysis revealed muscle atrophy and fatty infiltration in hyperKPP patients, especially in older individuals. Muscle involvement followed a selective pattern, primarily affecting the posterior compartment of the lower leg and anterior thigh muscles. The muscle fat fraction increased with patient age in the anterior thigh (r=0.669, p=0.009), in the deep posterior compartment of the lower leg (r=0.617, p=0.019), and in the superficial posterior compartment of the lower leg (r=0.777, p=0.001).

Conclusions

Our whole-body muscle MRI findings provide evidence for chronic progressive myopathy in hyperKPP patients. The reported data suggest that a selective pattern of muscle involvement-affecting the posterior compartment of the lower leg and the anterior thigh-is characteristic of chronic progressive myopathy in hyperKPP.     

Measuring quality of life impairment in skeletal muscle channelopathies

Background and purpose

Fatigue and pain have been previously shown to be important determinants for decreasing quality of life (QoL) in one report in patients with non‐dystrophic myotonia. The aims of our study were to assess QoL in skeletal muscle channelopathies (SMC) using INQoL (individualized QoL) and SF‐36 questionnaires.

Methods

We administered INQoL and SF‐36 to 66 Italian patients with SMC (26: periodic paralysis, 36: myotonia congenita and 4: Andersen‐Tawil) and compared the results in 422 patients with myotonic dystrophies (DM1: 382; and DM2: 40).

Results

(i) INQoL index in SMC is similar to that in DMs (P = 0.79). (ii) Patients with myotonia congenita have the worst perception of QoL. (iii) Myotonia has the most detrimental effect on patients with myotonia congenita, followed by patients with DM2 and then by patients with DM1 and hyperkalemic periodic paralysis. (iv) Pain is a significant complaint in patients with myotonia congenita, hypokalemic periodic paralysis and DM2 but not in DM1. (v) Fatigue has a similar detrimental effect on all patient groups except for patients with hyperkalemic periodic paralysis in whom muscle weakness and myotonia more than fatigue affect QoL perception. (vi) Muscle symptoms considered in INQoL correlate with physical symptoms assessed by SF‐36 (R from ?0.34 to ?0.76).

Conclusions

QoL perception in patients with SMC is similar to that of patients with DMs, chronic multisystem disabling conditions. Our results provide information to target treatment and health care of these patients. The sensitivity of INQoL to changes in QoL in the SMC needs to be further explored in longitudinal studies.

     

Clinical and genetic spectrum of a Chinese cohort with SCN4A gene mutations

Skeletal muscle sodium channelopathies due to SCN4A gene mutations have a broad clinical spectrum. However, each phenotype has been reported in few cases of Chinese origin. We present detailed phenotype and genotype data from a cohort of 40 cases with SCN4A gene mutations seen in neuromuscular diagnostic service in Huashan hospital, Fudan University. Cases were referred from 6 independent provinces from 2010 to 2018. A questionnaire covering demographics, precipitating factors, episodes of paralysis and myotonia was designed to collect the clinical information. Electrodiagnostic studies and muscle MRI were retrospectively analyzed. The clinical spectrum of patients included: 6 Hyperkalemic periodic paralysis (15%), 18 Hypokalemic periodic paralysis (45%), 7 sodium channel myotonia (17.5%), 4 paramyotonia congenita (10%) and 5 heterozygous asymptomatic mutation carriers (12.5%). Review of clinical information highlights a significant delay to diagnosis (median 15 years), reports of pain and myalgia in the majority of patients, male predominance, circadian rhythm and common precipitating factors. Electrodiagnostic studies revealed subclinical myotonic discharges and a positive long exercise test in asymptomatic carriers. Muscle MRI identified edema and fatty infiltration in gastrocnemius and soleus. A total of 13 reported and 2 novel SCN4A mutations were identified with most variants distributed in the transmembrane helix S4 to S6, with a hotspot mutation p.Arg675Gln accounting for 32.5% (13/40) of the cohort. Our study revealed a higher proportion of periodic paralysis in SCN4A-mutated patients compared with cohorts from England and the Netherlands. It also highlights the importance of electrodiagnostic studies in diagnosis and segregation studies.     

Asymptomatic myotonia congenita unmasked by severe hypothyroidism

Myotonia congenita is an inherited muscle disorder sustained by mutations in the skeletal muscle chloride channel gene CLCN1. Symptoms vary from mild to severe and generalized myotonia and worsen with cold, stressful events and hormonal fluctuations. Here we report the case of a young woman who sought medical attention because of subacute onset of diffuse and severe limb myotonia. CLCN1 gene sequencing showed a heterozygous transversion (T550M), two polymorphisms and one silent mutation. Thyroid function screening revealed severe hypothyroidism. She was placed on l-thyroxine replacement therapy which dramatically improved myotonia. We conclude that hypothyroidism unmasked a genetically determined, clinically asymptomatic chloride channelopathy. Diagnostic work-up in patients with clinically isolated myotonia should not be limited to genetic screening of non-dystrophic or dystrophic myotonias. Considering the high prevalence of hypothyroidism in females, systematic thyroid function screening by looking for additional hypothyroid symptoms and serum TSH levels measurement is mandatory in these patients.     

Mutation in the S4 segment of the adult skeletal sodium channel gene in an Italian Paramyotonia Congenita (PC) family

The periodic paralyses are a group of autosomal dominant muscle diseases sharing the common feature of episodic stiffness and weakness, usually occurring with muscle cooling (as in the case of paramyotonia congenita, PC pheno-type) or changes in extracellular K+ levels resulting from various precipitating factors (hyperkalemic periodic paralysis, HYPP and hypokalemic periodic paralysis, Hypo PP). It is now known that HYPP maps to chromosome 17q, and that PC and a form of myotonia congenita without periodic paralysis also map to the 17q locus, thus indicating that they derive from allelic variants. So far, these disorders have been described in various ethnic groups but, to our knowledge, have never been reported in Italy. We describe a mutation in an S4 segment of the adult skeletal muscle sodium channel in a clinically-defined Italian family that leads to the paramyotonia congenita (PC) phenotype with dominant autosomal inheritance and temperature-related symptoms (regional weakness following cooling and exercise), present since childhood in all of the affected family members. This study was supported by a grant from MURST 60% given to Prof. G. Meola.     

Successful treatment of normokalemic periodic paralysis with hydrochlorothiazide

Background

Periodic paralysis (PP) is an autosomal dominant muscle disorder characterized by periodic muscle weakness attacks associated with serum potassium level variations. It is classified into hypokalemic (hypoKPP), hyperkalemic (hyperKPP), and normokalemic (normoKPP) forms based on the ictal serum potassium level. HyperKPP and normoKPP are caused by mutations of the same gene SCN4A, the gene encoding the skeletal muscle voltage-gated sodium channel. Prophylactic treatment with thiazide diuretics is highly effective in preventing attacks in hyperKPP. However, the efficacy and safety of such diuretics in normoKPP remain unclear.

Novel Insights into the Pathomechanisms of Skeletal Muscle Channelopathies

The nondystrophic myotonias and primary periodic paralyses are an important group of genetic muscle diseases characterized by dysfunction of ion channels that regulate membrane excitability. Clinical manifestations vary and include myotonia, hyperkalemic and hypokalemic periodic paralysis, progressive myopathy, and cardiac arrhythmias. The severity of myotonia ranges from severe neonatal presentation causing respiratory compromise through to mild later-onset disease. It remains unclear why the frequency of attacks of paralysis varies greatly or why many patients develop a severe permanent fixed myopathy. Recent detailed characterizations of human genetic mutations in voltage-gated muscle sodium (gene: SCN4A), chloride (gene: CLCN1), calcium (gene: CACNA1S), and inward rectifier potassium (genes: KCNJ2, KCNJ18) channels have resulted in new insights into disease mechanisms, clinical phenotypic variation, and therapeutic options     

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.     

Coexistence of <Emphasis Type="Italic">CLCN1</Emphasis> and <Emphasis Type="Italic">SCN4A</Emphasis> mutations in one family suffering from myotonia

Non-dystrophic myotonias are characterized by clinical overlap making it challenging to establish genotype-phenotype correlations. We report clinical and electrophysiological findings in a girl and her father concomitantly harbouring single heterozygous mutations in SCN4A and CLCN1 genes. Functional characterization of N1297S hNav1.4 mutant was performed by patch clamp. The patients displayed a mild phenotype, mostly resembling a sodium channel myotonia. The CLCN1 c.501C>G (p.F167L) mutation has been already described in recessive pedigrees, whereas the SCN4A c.3890A>G (p.N1297S) variation is novel. Patch clamp experiments showed impairment of fast and slow inactivation of the mutated Nav1.4 sodium channel. The present findings suggest that analysis of both SCN4A and CLCN1 genes should be considered in myotonic patients with atypical clinical and neurophysiological features.     

Association between RELN Gene Polymorphisms and Attention Deficit Hyperactivity Disorder in Korean Children

Objective

Attention deficit hyperactivity disorder (ADHD) is common disorder of the school-age population. ADHD is familial and genetic studies estimate heritability at 80–90%. The aim of the present study was to investigate the association between the genetic type and alleles for RELNgene (rs736707, rs2229864, rs362746, rs362726, rs362691, rs1062831, rs607755, and rs2072403) in Korean children with ADHD.

Methods

The sample consisted of 180 ADHD children and 159 control children. We diagnosed ADHD according to DSM-IV. ADHD symptoms were evaluated with Conners'' Parent Rating Scales and Dupaul Parent ADHD Rating Scales. Blood samples were taken from the 339 subjects, DNA was extracted from blood lymphocytes, and PCR was performed for RELN Polymorphism. Alleles and genotype frequencies were compared using the chi-square test. We compared the allele and genotype frequencies of RELN gene polymorphism in the ADHD and control groups.

Results

This study showed that there was a significant correlation among the frequencies of the rs736707 (OR=1.40, 95% CI=1.03–1.90, p=0.031) of alleles of RELN, but the final conclusions are not definite.

Conclusion

Follow up studies with larger patient or pure subgroups are expected. These results suggested that RELN might be related to ADHD symptoms.     

Becker’s myotonia: novel mutations and clinical variability in patients born to consanguineous parents

Myotonia congenita is an inherited muscle disease present from childhood that is characterized by impaired muscle relaxation after contraction resulting in muscle stiffness; moreover, skeletal striated muscle groups may be involved. Myotonia congenita occurs due to chloride (Cl) channel mutations that reduce the stabilizing Cl conductance, and it is caused by mutations in the CLCN1 gene. This paper describes four patients from two different healthy consanguineous Turkish families with muscle stiffness and easy fatigability. A genetic investigation was performed. Mutation analyses showed a homozygous p.Tyr150* (c.450C > A) mutation in patients 1, 2 and 3 and a homozygous p.Leu159Cysfs*11 (c.475delC) mutation in patient 4 in the CLCN1 gene. These mutations have never been reported before and in silico analyses showed that the mutations were disease causing. They may be predicted to cause nonsense-mediated mRNA decay. Our data expand the spectrum of CLCN1 mutations and provide insights for genotype–phenotype correlations of myotonia congenita.     

Whole‐body high‐field MRI shows no skeletal muscle degeneration in young patients with recessive myotonia congenita

Kornblum C, Lutterbey GG, Czermin B, Reimann J, von Kleist‐Retzow J‐C, Jurkat‐Rott K, Wattjes MP. Whole‐body high‐field MRI shows no skeletal muscle degeneration in young patients with recessive myotonia congenita.
Acta Neurol Scand: 2010: 121: 131–135.
© 2009 The Authors Journal compilation © 2009 Blackwell Munksgaard. Background – Muscle magnetic resonance imaging (MRI) is the most sensitive method in the detection of dystrophic and non‐dystrophic abnormalities within striated muscles. We hypothesized that in severe myotonia congenita type Becker muscle stiffness, prolonged transient weakness and muscle hypertrophy might finally result in morphologic skeletal muscle alterations reflected by MRI signal changes. Aim of the study – To assess dystrophic and/or non‐dystrophic alterations such as fatty or connective tissue replacement and muscle edema in patients with severe recessive myotonia congenita. Methods – We studied three seriously affected patients with myotonia congenita type Becker using multisequence whole‐body high‐field MRI. All patients had molecular genetic testing of the muscle chloride channel gene (CLCN1). Results – Molecular genetic analyses demonstrated recessive CLCN1 mutations in all patients. Two related patients were compound heterozygous for two novel CLCN1 mutations, Q160H and L657P. None of the patients showed skeletal muscle signal changes indicative of fatty muscle degeneration or edema. Two patients showed muscle bulk hypertrophy of thighs and calves in line with the clinical appearance. Conclusions – We conclude that (i) chloride channel dysfunction alone does not result in skeletal muscle morphologic changes even in advanced stages of myotonia congenita, and (ii) MRI skeletal muscle alterations in myotonic dystrophy must be clear consequences of the dystrophic disease process.     

Severe myoclonic epilepsy of infancy (Dravet syndrome): Clinical and genetic features of nine Turkish patients

Purpose:

Mutations of the α-1 subunit sodium channel gene (SCN1A) cause severe myoclonic epilepsy of infancy (SMEI). To date, over 300 mutations related to SMEI have been described. In the present study, we report new SCN1A mutations and the clinical features of SMEI cases.

Materials and Methods:

We studied the clinical and genetic features of nine patients diagnosed with SMEI at the Pediatric Neurology Department of Istanbul Medical Faculty.

Results:

Five patients had nonsense mutations, two had missense mutations, one had a splice site mutation and one had a deletion mutation of the SCN1A gene. Mutations at c.3705+5G splice site, p.trip153X nonsense mutation and deletion at c.2416_2946 have not been previously described. The seizures started following whole cell pertussis vaccination in all patients. The seizures ceased in one patient and continued in the other eight patients. Developmental regression was severe in three patients, with frequent status epilepticus. The type of mutation was not predictive for the severity of the disease. Two of the three patients with severe regression had nonsense and missense mutations.

Conclusions:

Dravet syndrome can be result of several different types of mutation in SCN1A gene. Onset of the seizures after pertussis vaccination is an important clue for the diagnosis and neuro- developmental delay should be expected in all patients.