[Show abstract][Hide abstract] ABSTRACT: Mutations in the skeletal muscle channel (SCN4A), encoding the Nav1.4 voltage-gated sodium channel, are causative of a variety of muscle channelopathies, including non-dystrophic myotonias and periodic paralysis. The effects of many of these mutations on channel function have been characterized both in vitro and in vivo. However, little is known about the consequences of SCN4A mutations downstream from their impact on the electrophysiology of the Nav1.4 channel. Here we report the discovery of a novel SCN4A mutation (c.1762A>G; p.I588V) in a patient with myotonia and periodic paralysis, located within the S1 segment of the second domain of the Nav1.4 channel. Using N-ethyl-N-nitrosourea mutagenesis, we generated and characterized a mouse model (named draggen), carrying the equivalent point mutation (c.1744A>G; p.I582V) to that found in the patient with periodic paralysis and myotonia. Draggen mice have myotonia and suffer from intermittent hind-limb immobility attacks. In-depth characterization of draggen mice uncovered novel systemic metabolic abnormalities in Scn4a mouse models and provided novel insights into disease mechanisms. We discovered metabolic alterations leading to lean mice, as well as abnormal AMP-activated protein kinase activation, which were associated with the immobility attacks and may provide a novel potential therapeutic target.
[Show abstract][Hide abstract] ABSTRACT: Recurrent rhabdomyolysis complicates a number of inherited muscle and metabolic disorders and represents a serious, potentially life-threatening condition which frequently requires critical care. Identification of the underlying genetic cause has traditionally relied upon detailed history and examination findings which subsequently guide the investigative work-up. However, in many cases the causative molecular defect remains undetermined. This study aims to investigate whether utilising next-generation sequencing (NGS) technology early in the diagnostic pathway might offer a rapid, cost-effective tool for the diagnosis of patients with recurrent attacks of rhabdomyolysis when a genetic aetiology is suspected. We have designed a “rhabdomyolysis gene panel” comprised of 48 genes known or predicted to cause rhabdomyolysis using NGS technology. Over 200 patients have been recruited. In addition, array CGH and whole exome sequencing may be used. A pilot study of 53 patients with a panel of sequenced 35 rhabdomyolysis genes using an amplicon based sequencing panel on an Illumina MiSeq was performed. 52 of the first 53 first evaluated patients have a variant in at least 1 gene. 49 patients have heterozygous variants in at least two different genes. We identified 15 cases out of 52 with probable pathogenic mutations using this approach. The pilot study showed that the rhabdomyolysis genetic panel is a potentially useful way to identify genetic alterations in patients with rhabdomyolysis. The high number of symptomatic patients with mutations identified in more than one gene associated with rhabdomyolysis suggests that gene–gene interaction(s) may play an important role. We are currently preparing a new extended panel of 48 genes.
[Show abstract][Hide abstract] ABSTRACT: Myotonia is often a painful and disabling symptom which can interfere with daily motor function resulting in significant morbidity. Since myotonic disorders are rare it has generally proved difficult to obtain class I level evidence for anti-myotonic drug efficacy by performing randomized placebo controlled trials. Current treatment guidance is therefore largely based on anecdotal reports and physician experience. Despite the genetic channel heterogeneity of the myotonic disorders the sodium channel antagonists have become the main focus of pharmacological interest. Mexiletine is currently regarded as the first choice sodium channel blocker based on a recent placebo controlled randomized trial. However, some patients do not respond to mexiletine or have significant side effects limiting its use. There is a clinical need to develop additional antimyotonic agents. The study of Desaphy et al. is therefore important and provides in vitro evidence that a number of existing drugs with sodium channel blocking capability could potentially be repurposed as anti-myotonic drugs. Translation of these potentially important in vitro findings into clinical practice requires carefully designed randomized controlled trials. Here we discuss Desaphy's findings in the wider context of attempts to develop additional therapies for patients with clinically significant myotonia.
[Show abstract][Hide abstract] ABSTRACT: Sporadic inclusion body myositis (IBM) is the most common acquired myopathy occurring in adults aged over 50years. The aim of the study was to assess prospectively the clinical features and functional impact of sporadic inclusion body myositis (IBM). Clinical data, manual muscle testing (MMT), quantitative muscle testing (QMT) of quadriceps muscle and IBM functional rating scale (IBM-FRS) were collected according to a standardised protocol at baseline (n=51) and one-year follow-up (n=23). MMT, quadriceps QMT and IBM-FRS significantly declined after one year (by 5.2%, 27.9%, and 13.8%, respectively). QMT of the quadriceps muscle and IBM-FRS were the most sensitive measures of disease progression. After a median time of seven years of disease duration, 63% of patients had lost independent walking. Disease onset after 55years of age, but not sex or treatment, is predictive of a shorter time to requirement of a walking stick. We detected no differences in disease presentation and progression between clinically and pathologically defined IBM patients. The study provides evidence that quadriceps QMT and IBM-FRS could prove helpful as outcome measures in future therapeutic trials in IBM.
[Show abstract][Hide abstract] ABSTRACT: To assess whether exon deletions or duplications in CLCN1 are associated with recessive myotonia congenita (MC).
We performed detailed clinical and electrophysiologic characterization in 60 patients with phenotypes consistent with MC. DNA sequencing of CLCN1 followed by multiplex ligation-dependent probe amplification to screen for exon copy number variation was undertaken in all patients.
Exon deletions or duplications in CLCN1 were identified in 6% of patients with MC. Half had heterozygous exonic rearrangements. The other 2 patients (50%), with severe disabling infantile onset myotonia, were identified with both a homozygous mutation, Pro744Thr, which functional electrophysiology studies suggested was nonpathogenic, and a triplication/homozygous duplication involving exons 8-14, suggesting an explanation for the severe phenotype.
These data indicate that copy number variation in CLCN1 may be an important cause of recessive MC. Our observations suggest that it is important to check for exon deletions and duplications as part of the genetic analysis of patients with recessive MC, especially in patients in whom sequencing identifies no mutations or only a single recessive mutation. These results also indicate that additional, as yet unidentified, genetic mechanisms account for cases not currently explained by either CLCN1 point mutations or exonic deletions or duplications.
[Show abstract][Hide abstract] ABSTRACT: Over 20 years ago single clonal deletions were the first mitochondrial DNA (mtDNA) genetic defects described in association with human disease. Since then very large numbers of children and adults harbouring such deletions have been described and it is clear they are an important cause of human mitochondrial disease. However, there still remain many important challenges in relation to our understanding of mechanisms leading to deletion formation and propagation and in relation to the factors determining the complex and varying relationship between genotype and clinical phenotype. Although multidisciplinary team care is essential and can improve quality of life and outcomes for patients, a definitive molecular treatment for single mtDNA deletions remains an important translational research goal. Patients with mtDNA deletions exhibit a very wide range of different clinical phenotypes with marked variation in age at onset and disease severity. Single mtDNA deletions may enter into the differential diagnosis of many different paediatric and adult presentations across a wide range of medical specialties, although neurological presentations are amongst the most common. In this review, we examine the molecular mechanisms underpinning mtDNA replication and we consider the hypotheses proposed to explain the formation and propagation of single large-scale mtDNA deletions. We also describe the range of clinical features associated with single mtDNA deletions, outline a molecular diagnostic approach and discuss current management including the role of aerobic and resistance exercise training programmes.
[Show abstract][Hide abstract] ABSTRACT: Acetazolamide has been the most commonly used treatment for hypokalemic periodic paralysis since 1968. However, its mechanism of efficacy is not fully understood, and it is not known whether therapy response relates to genotype. We undertook a clinical and genetic study to evaluate the response rate of patients treated with acetazolamide and to investigate possible correlations between response and genotype.
We identified a total of 74 genotyped patients for this study. These included patients who were referred over a 15-year period to the only U.K. referral center or to a Chinese center and who underwent extensive clinical evaluation. For all genotyped patients, the response to acetazolamide therapy in terms of attack frequency and severity was documented. Direct DNA sequencing of CACNA1S and SCN4A was performed.
Only 46% of the total patient cohort (34 of 74) reported benefit from acetazolamide. There was a greater chance of benefit in patients with mutations in CACNA1S (31 responded of 55 total) than in those with mutations in SCN4A (3 responded of 19 total). Patients with mutations that resulted in amino acids being substituted by glycine in either gene were the least likely to report benefit.
This retrospective study indicates that only approximately 50% of genotyped patients with hypokalemic periodic paralysis respond to acetazolamide. We found evidence supporting a relationship between genotype and treatment response. Prospective randomized controlled trials are required to further evaluate this relationship. Development of alternative therapies is required.
[Show abstract][Hide abstract] ABSTRACT: The potential of MRI to provide quantitative measures of neuromuscular pathology for use in therapeutic trials is being increasingly recognised. Magnetisation transfer (MT) imaging shows particular promise in this context, being sensitive to pathological changes, particularly in skeletal muscle, where measurements correlate with clinically measured muscle strength. Radiofrequency (RF) transmit field (B(1)) inhomogeneities can be particularly problematic in measurements of the MT ratio (MTR) and may obscure genuine muscle MTR changes caused by disease. In this work, we evaluate, for muscle imaging applications, a scheme previously proposed for the correction of RF inhomogeneity artefacts in cerebral MTR maps using B(1) information acquired in the same session. We demonstrate the theoretical applicability of this scheme to skeletal muscle using a two-pool model of pulsed quantitative MT. The correction scheme is evaluated practically in MTR imaging of the lower limbs of 28 healthy individuals and in two groups of patients with representative neuromuscular diseases: Charcot-Marie-Tooth disease type 1A and inclusion body myositis. The correction scheme was observed to reduce both the within-subject and between-subject variability in the calf and thigh muscles of healthy subjects and patient groups in histogram- and region-of-interest-based approaches. This method of correcting for RF inhomogeneity effects in MTR maps using B(1) data may markedly improve the sensitivity of MTR mapping indices as measures of pathology in skeletal muscle.
NMR in Biomedicine 07/2011; 25(2):262-70. · 3.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To assess the clinical course and genotype-phenotype correlations in patients with selenoprotein-related myopathy (SEPN1-RM) due to selenoprotein N1 gene (SEPN1) mutations for a retrospective cross-sectional study.
Forty-one patients aged 1-60 years were included. Clinical data including scoliosis, respiratory function, and growth measurements were collected by case note review.
Mean age at onset was 2.7 years, ranging from birth to the second decade of life. All but 2 remained independently ambulant: one lost ambulation at age 5 years and another in his late 50s. The mean age of starting nocturnal noninvasive ventilation (NIV) was 13.9 years. One child required full-time NIV at the age of 1 year while in 2 cases NIV was started at 33 years. Two patients died from respiratory failure at the age of 10 and 22 years, respectively. The mean age at scoliosis onset was 10 years, in most cases preceded by rigidity of the spine. Fourteen patients had successful spinal surgery (mean age 13.9 years). Twenty-one were underweight; however, overt feeding difficulties were not a feature.
This study describes the largest population affected by SEPN1-RM reported so far. Our findings show that the spectrum of severity is wider than previously reported. Respiratory insufficiency generally develops by 14 years but may occur as early as in infancy or not until the fourth decade. Motor abilities remain essentially static over time even in patients with early presentation. Most adult patients remain ambulant and fully employed.
[Show abstract][Hide abstract] ABSTRACT: MRI may provide treatment outcome measures in neuromuscular conditions. The authors assessed MRI magnetisation transfer ratios (MTRs) in lower-limb musculature as markers of pathology in peripheral neuropathies and compared the findings with associated clinical data. Ten patients with Charcot-Marie-Tooth disease type 1A (CMT1A) and nine patients with chronic inflammatory demyelinating polyneuropathy (CIDP) were compared with 10 healthy subjects. The MTR in the calf muscles was significantly lower than controls in the two patient groups (both p<0.001). The median MTRs (IQR) were 50.5(1.6) percentage units (p.u.) (control), 41.5(10.6) p.u. (CMT1A) and 39.3(8.7) p.u. (CIDP). Moreover, anterior lower leg MTR correlated strongly with strength of ankle dorsiflexion, measured with the Medical Research Council scale, in CIDP (ρ=0.88, p<0.001) and also in CMT1A (ρ=0.50, p<0.05), where MTR also showed an association with disease duration (ρ=-0.86, p<0.001). Short tau inversion recovery MRI of the same muscles showed abnormalities associated with regions of reduced MTR (p<0.001), and MTR was also reduced in other muscles otherwise deemed normal appearing (p<0.001), indicating that MTR may be more sensitive to muscle damaged by denervation than conventional MRI. The significant reductions in muscle MTR in peripheral neuropathies and the associated correlations with clinical measures indicate that MTR has potential as an imaging outcome measure in future therapeutic trials.
Journal of neurology, neurosurgery, and psychiatry 05/2011; 83(1):29-32. · 4.87 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background / Purpose:
Skeletal muscle channelopathies are a group of neuromuscular disorders that are caused by mutations of voltage gated ion channels resulting in altered membrane excitability. They include the non dystrophic myotonias and the periodic paralyses. There are two main types of periodic paralysis: hyperkalemic periodic paralysis (HyperPP) and hypokalemic periodic paralysis (HypoPP) which are the result of mutations in the genes coding for the skeletal muscle voltage gated calcium channel (CACNA1S) or sodium channel (SCN4A). The causative mutations of HypoPP have been found to localise in the voltage sensing segments (S4) of SCN4A and CACNA1S. However, the causative mutations for other skeletal muscle channelopathies have not been found to cluster in such a localized area of the causative gene.To investigate genetic heterogeneity in the skeletal muscle channelopathies and potential mechanisms for phenotypic variability including differential allelic expression and abnormal protein trafficking.Exonic sequencing of causative genes will be used to find additional mutations within these genes that cause skeletal muscle channelopathies. Phenotypic variation will be looked at using qPCR and immunohistochemistry of muscle samples.
SCN4A and CACNA1S S4 segments in an initial group of patients were sequenced and two novel mutations of SCN4A were found; p.S653G in a patient with HyperPP and p.R222Q in a patient with Myotonia Congenita. This is only the second SCN4A S4 arginine mutation to be reported in a phenotype other than HypoPP. R222W has previously been described, in HypoPP (Matthews et al 2009). Functional studies of the variability in voltage sensor function and resultant phenotype with differing residues may lead to better understanding of the pathomechansims of disease.
UK Neuromuscular Translational Research Conference 2011; 05/2011