[Show abstract][Hide abstract] ABSTRACT: X-linked myotubular myopathy (XLMTM) is a devastating, rare, congenital myopathy caused by mutations in the MTM1 gene, resulting in a lack of or dysfunction of the enzyme myotubularin. This leads to severe perinatal weakness and distinctive muscle pathology. It was originally thought that XLMTM was related to developmental arrest in myotube maturation; however, the generation and characterization of several animal models have significantly improved our understanding of clinical and pathological aspects of this disorder. Myotubularin is now known to participate in numerous cellular processes including endosomal trafficking, excitation-contraction coupling, cytoskeletal organization, neuromuscular junction structure, autophagy, and satellite cell proliferation and survival. The available vertebrate models of XLMTM, which vary in severity from complete absence to reduced functional levels of myotubularin, recapitulate features of the human disease to a variable extent. Understanding how pathological endpoints in animals with XLMTM translate to human patients will be essential to interpret preclinical treatment trials and translate therapies into human clinical studies. This review summarizes the published animal models of XLMTM, including those of zebrafish, mice, and dogs, with a focus on their pathological features as compared to those seen in human XLMTM patients.
Preview · Article · Jan 2016 · Journal of Neuropathology and Experimental Neurology
[Show abstract][Hide abstract] ABSTRACT: The congenital myopathies—central core disease (CCD), multi-minicore disease (MmD), myotubular/centronuclear myopathy (MTM/CNM), nemaline myopathy (NM), and congenital fiber type disproportion (CFTD)—are a heterogeneous group of inherited early-onset neuromuscular disorders defined by and named after characteristic histopathological features. Although presentation is usually nonspecific with hypotonia and prominent axial and proximal weakness, additional clinical features may suggest a specific diagnosis. Disproportionate respiratory involvement is common and determines morbidity and mortality in these patients. Causative genes encode proteins involved in sarcomeric assembly, membrane trafficking, autophagy, calcium homeostasis and excitation–contraction coupling. Different mutations in the same gene may give rise to different histopathological features, whilst the same histopathological feature may be caused by mutations in different genes, complicating the diagnostic pathway. Management is currently mainly supportive but more specific pharmacological and genetic therapies are under investigation.
[Show abstract][Hide abstract] ABSTRACT: Congenital myopathies are a clinically and genetically heterogeneous group of muscle disorders characterized by congenital or early-onset hypotonia and muscle weakness, and specific pathological features on muscle biopsy. The phenotype ranges from foetal akinesia resulting in in utero or neonatal mortality, to milder disorders that are not life-limiting. Over the past decade, more than 20 new congenital myopathy genes have been identified. Most encode proteins involved in muscle contraction; however, mutations in ion channel-encoding genes are increasingly being recognized as a cause of this group of disorders. SCN4A encodes the α-subunit of the skeletal muscle voltage-gated sodium channel (Nav1.4). This channel is essential for the generation and propagation of the muscle action potential crucial to muscle contraction. Dominant SCN4A gain-of-function mutations are a well-established cause of myotonia and periodic paralysis. Using whole exome sequencing, we identified homozygous or compound heterozygous SCN4A mutations in a cohort of 11 individuals from six unrelated kindreds with congenital myopathy. Affected members developed in utero- or neonatal-onset muscle weakness of variable severity. In seven cases, severe muscle weakness resulted in death during the third trimester or shortly after birth. The remaining four cases had marked congenital or neonatal-onset hypotonia and weakness associated with mild-to-moderate facial and neck weakness, significant neonatal-onset respiratory and swallowing difficulties and childhood-onset spinal deformities. All four surviving cohort members experienced clinical improvement in the first decade of life. Muscle biopsies showed myopathic features including fibre size variability, presence of fibrofatty tissue of varying severity, without specific structural abnormalities. Electrophysiology suggested a myopathic process, without myotonia. In vitro functional assessment in HEK293 cells of the impact of the identified SCN4A mutations showed loss-of-function of the mutant Nav1.4 channels. All, apart from one, of the mutations either caused fully non-functional channels, or resulted in a reduced channel activity. Each of the affected cases carried at least one full loss-of-function mutation. In five out of six families, a second loss-of-function mutation was present on the trans allele. These functional results provide convincing evidence for the pathogenicity of the identified mutations and suggest that different degrees of loss-of-function in mutant Nav1.4 channels are associated with attenuation of the skeletal muscle action potential amplitude to a level insufficient to support normal muscle function. The results demonstrate that recessive loss-of-function SCN4A mutations should be considered in patients with a congenital myopathy.
[Show abstract][Hide abstract] ABSTRACT: Background:
Fetal akinesia/hypokinesia, arthrogryposis and severe congenital myopathies are heterogeneous conditions usually presenting before or at birth. Although numerous causative genes have been identified for each of these disease groups, in many cases a specific genetic diagnosis remains elusive. Due to the emergence of next generation sequencing, virtually the entire coding region of an individual's DNA can now be analysed through "whole" exome sequencing, enabling almost all known and novel disease genes to be investigated for disorders such as these.
Genomic DNA samples from 45 patients with fetal akinesia/hypokinesia, arthrogryposis or severe congenital myopathies from 38 unrelated families were subjected to next generation sequencing. Clinical features and diagnoses for each patient were supplied by referring clinicians. Genomic DNA was used for either whole exome sequencing or a custom-designed neuromuscular sub-exomic supercapture array containing 277 genes responsible for various neuromuscular diseases. Candidate disease-causing variants were investigated and confirmed using Sanger sequencing. Some of the cases within this cohort study have been published previously as separate studies.
A conclusive genetic diagnosis was achieved for 18 of the 38 families. Within this cohort, mutations were found in eight previously known neuromuscular disease genes (CHRND, CHNRG, ECEL1, GBE1, MTM1, MYH3, NEB and RYR1) and four novel neuromuscular disease genes were identified and have been published as separate reports (GPR126, KLHL40, KLHL41 and SPEG). In addition, novel mutations were identified in CHRND, KLHL40, NEB and RYR1. Autosomal dominant, autosomal recessive, X-linked, and de novo modes of inheritance were observed.
By using next generation sequencing on a cohort of 38 unrelated families with fetal akinesia/hypokinesia, arthrogryposis, or severe congenital myopathy we therefore obtained a genetic diagnosis for 47 % of families. This study highlights the power and capacity of next generation sequencing (i) to determine the aetiology of genetically heterogeneous neuromuscular diseases, (ii) to identify novel disease genes in small pedigrees or isolated cases and (iii) to refine the interplay between genetic diagnosis and clinical evaluation and management.
Full-text · Article · Nov 2015 · Orphanet Journal of Rare Diseases
[Show abstract][Hide abstract] ABSTRACT: The neuromuscular junction (NMJ) consists of a tripartite synapse with a presynaptic nerve terminal, Schwann cells that ensheathe the terminal bouton, and a highly specialized postsynaptic membrane. Synaptic structural integrity is crucial for efficient signal transmission. Congenital myasthenic syndromes (CMSs) are a heterogeneous group of inherited disorders that result from impaired neuromuscular transmission, caused by mutations in genes encoding proteins that are involved in synaptic transmission and in forming and maintaining the structural integrity of NMJs. To identify further causes of CMSs, we performed whole-exome sequencing (WES) in families without an identified mutation in known CMS-associated genes. In two families affected by a previously undefined CMS, we identified homozygous loss-of-function mutations in COL13A1, which encodes the alpha chain of an atypical non-fibrillar collagen with a single transmembrane domain. COL13A1 localized to the human muscle motor endplate. Using CRISPR-Cas9 genome editing, modeling of the COL13A1 c.1171delG (p.Leu392Sfs∗71) frameshift mutation in the C2C12 cell line reduced acetylcholine receptor (AChR) clustering during myotube differentiation. This highlights the crucial role of collagen XIII in the formation and maintenance of the NMJ. Our results therefore delineate a myasthenic disorder that is caused by loss-of-function mutations in COL13A1, encoding a protein involved in organization of the NMJ, and emphasize the importance of appropriate symptomatic treatment for these individuals.
Preview · Article · Nov 2015 · The American Journal of Human Genetics
[Show abstract][Hide abstract] ABSTRACT: Background:
Clinical presentation with motor delay, proximal weakness, and learning difficulties will raise the possibility of a dystrophinopathy, dystroglycanopathy, or myotonic dystrophy. This differential should also include the more recently described choline kinase beta-related muscular dystrophy. This condition is typically characterized by large and abnormally distributed mitochondria on muscle biopsy, which can distinguish this condition from the other muscle conditions in the differential.
We present a case of a boy with choline kinase beta mutations with relatively mild clinical presentation, including proximal weakness, learning difficulties and raised creatine kinase. Investigations included muscle magnetic resonance imaging (MRI) with T1 axial sequences through thigh and calves, and needle muscle biopsy of the left vastus lateralis muscle.
MRI showed involvement mainly of the quadriceps femoris, sartorius, and adductor magnus, with selective sparing of the gracilis, hamstrings, and adductor longus and brevis. Muscle biopsy revealed chronic dystrophic features. Oxidative stains demonstrated enlarged mitochondria accentuated peripherally or present diffusely in a few fibres giving a coarsely stippled appearance. A homozygous C.722A>G (p.Asn241Ser) mutation was detected in exon 6 of the CHKB gene.
This selective pattern of skeletal muscle involvement might be helpful for identifying other patients with this condition, even in the absence of diagnostic muscle pathology.
No preview · Article · Oct 2015 · Pediatric Neurology
[Show abstract][Hide abstract] ABSTRACT: Background:
Currently, there is no satisfactory treatment for McArdle disease. Sodium valproate is one of a group of drugs known as histone deacetylase inhibitors (HDACIs) that can affect gene expression by acetylating lysine residues, which in turn has a direct effect on chromatin. A recent clinical trial of the drug in McArdle sheep that were given sodium valproate showed the presence of phosphorylase positive muscle fibres.
Aims: The aim of this pilot study is to determine the feasibility of performing a clinical trial of sodium valproate in people with McArdle disease.
Methods: 15 subjects will receive sodium valproate modified release 20mg/kg/day (maximum dose 2.0g/day) administered orally once daily for six months. Outcome measurements include cycle ergometry (rating of perceived exertion, oxygen consumption and the respiratory quotient, serum lactate and ammonia levels, maximum heart rate and workload), the number of phosphorylase positive fibres on muscle biopsy pre and post treatment, maximum walking distance measured by the 12 minute walk test, forearm exercise test, blood laboratory parameters, quality of life questionnaire, symptom diary and side effect diary.
Results: We expected an improvement in functional capacity in patients treated with sodium valproate in association with an increase in phosphorylase expression in muscle fibres.
Conclusion: This pilot study might be the initial step in exploring a novel treatment option for patients with McArdle disease.
Funder: Muscular Dystrophy Campaign
Full-text · Article · Mar 2015 · Neuromuscular Disorders