Dok-7 mutations underlie a neuromuscular junction synaptopathy

MuSK myasthenia gravis IgG4 disrupts the interaction of LRP4 with MuSK but both IgG4 and IgG1-3 can disperse preformed agrin-independent AChR clusters

A variable proportion of patients with generalized myasthenia gravis (MG) have autoantibodies to muscle specific tyrosine kinase (MuSK). During development agrin, released from the motor nerve, interacts with low density lipoprotein receptor-related protein-4 (LRP4), which then binds to MuSK; MuSK interaction with the intracellular protein Dok7 results in clustering of the acetylcholine receptors (AChRs) on the postsynaptic membrane. In mature muscle, MuSK helps maintain the high density of AChRs at the neuromuscular junction. MuSK antibodies are mainly IgG4 subclass, which does not activate complement and can be monovalent, thus it is not clear how the antibodies cause disruption of AChR numbers or function to cause MG. We hypothesised that MuSK antibodies either reduce surface MuSK expression and/or inhibit the interaction with LRP4. We prepared MuSK IgG, monovalent Fab fragments, IgG1-3 and IgG4 fractions from MuSK-MG plasmas. We asked whether the antibodies caused endocytosis of MuSK in MuSK-transfected cells or if they inhibited binding of LRP4 to MuSK in co-immunoprecipitation experiments. In parallel, we investigated their ability to reduce AChR clusters in C2C12 myotubes induced by a) agrin, reflecting neuromuscular development, and b) by Dok7- overexpression, producing AChR clusters that more closely resemble the adult neuromuscular synapse. Total IgG, IgG4 or IgG1-3 MuSK antibodies were not endocytosed unless cross-linked by divalent anti-human IgG. MuSK IgG, Fab fragments and IgG4 inhibited the binding of LRP4 to MuSK and reduced agrin-induced AChR clustering in C2C12 cells. By contrast, IgG1-3 antibodies did not inhibit LRP4-MuSK binding but, surprisingly, did inhibit agrin-induced clustering. Moreover, both IgG4 and IgG1-3 preparations dispersed agrin-independent AChR clusters in Dok7-overexpressing C2C12 cells. Thus interference by IgG4 antibodies of the LRP4-MuSK interaction will be one pathogenic mechanism of MuSK antibodies, but IgG1-3 MuSK antibodies will also contribute to the reduced AChR density and neuromuscular dysfunction in myasthenia patients with MuSK antibodies.

[Congenital myasthenic syndromes: difficulties in the diagnosis, course and prognosis, and therapy--The French National Congenital Myasthenic Syndrome Network experience]

Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders caused by genetic defects affecting neuromuscular transmission and leading to muscle weakness accentuated by exertion. Three different aspects have been investigated by members of the national French CMS Network: the difficulties in making a proper diagnosis; the course and long-term prognosis; and the response to therapy, especially for CMS that do not respond to cholinesterase inhibitors. CMS diagnosis is late in most cases because of confusion with other entities such as: congenital myopathies, due to the frequent presentation in patients of myopathies such as permanent muscle weakness, atrophy and scoliosis, and the abnormalities of internal structure, diameter and distribution of fibers (type I predominance, type II atrophy) seen on biopsy; seronegative autoimmune myasthenia gravis, when CMS is of late onset; and metabolic myopathy, with the presence of lipidosis in muscle. The long-term prognosis of CMS was studied in a series of 79 patients recruited with the following gene mutations: CHRNA; CHRNE; DOK7; COLQ; RAPSN; AGRN; and MUSK. Disease-course patterns (progressive worsening, exacerbation, stability, improvement) could be variable throughout life in a given patient. DOK7 patients had the most severe disease course with progressive worsening: of the eight wheelchair-bound and ventilated patients, six had mutations of this gene. Pregnancy was a frequent cause of exacerbation. Anticholinesterase agents are the first-line therapy for CMS patients, except for cases of slow-channel CMS, COLQ and DOK7. In our experience, 3,4-DAP was a useful complement for several patients harboring CMS with AChR loss or RAPSN gene mutations. Ephedrine was given to 18 patients (eight DOK7, five COLQ, four AGRN and one RAPSN). Tolerability was good. Therapeutic responses were encouraging even in the most severely affected patients, particularly with DOK7 and COLQ. Salbutamol was a good alternative in one patient who was allergic to ephedrine.

A mouse model of the slow channel myasthenic syndrome: Neuromuscular physiology and effects of ephedrine treatment

In the slow channel congenital myasthenic syndrome mutations in genes encoding the muscle acetylcholine receptor give rise to prolonged ion channel activations. The resulting cation overload in the postsynaptic region leads to damage of synaptic structures, impaired neuromuscular transmission and fatigable muscle weakness. Previously we identified and characterised in detail the properties of the slow channel syndrome mutation εL221F. Here, using this mutation, we generate a transgenic mouse model for the slow channel syndrome that expresses mutant human ε-subunits harbouring an EGFP tag within the M3-M4 cytoplasmic region, driven by a ~1500 bp region of the CHRNB promoter. Fluorescent mutant acetylcholine receptors are assembled, cluster at the motor endplates and give rise to a disease model that mirrors the human condition. Mice demonstrate mild fatigable muscle weakness, prolonged endplate and miniature endplate potentials, and variable degeneration of the postsynaptic membrane. We use our model to investigate ephedrine as a potential treatment. Mice were assessed before and after six weeks on oral ephedrine (serum ephedrine concentration 89 ± 3 ng/ml) using an inverted screen test and in vivo electromyography. Treated mice demonstrated modest benefit for screen hang time, and in measures of compound muscle action potentials and mean jitter that did not reach statistical significance. Ephedrine and salbutamol show clear benefit when used in the treatment of DOK7 or COLQ congenital myasthenic syndromes. Our results highlight only a modest potential benefit of these β2-adrenergic receptor agonists for the treatment of the slow channel syndrome.

Clinical features in a large Iranian family with a limb-girdle congenital myasthenic syndrome due to a mutation in DPAGT1

Mutations in DPAGT1 are a newly recognised cause of congenital myasthenic syndrome. DPAGT1 encodes an early component of the N-linked glycosylation pathway. Initially mutations in DPAGT1 have been associated with the onset of the severe multisystem disorder - congenital disorder of glycosylation type 1J. However, recently it was established that certain mutations in this gene can cause symptoms restricted to muscle weakness resulting from defective neuromuscular transmission. We report four cases from a large Iranian pedigree with prominent limb-girdle weakness and minimal craniobulbar symptoms who harbour a novel mutation in DPAGT1, c.652C>T, p.Arg218Trp. This myasthenic syndrome may mimic myopathic disorders and is likely under-diagnosed.

The role of MuSK in synapse formation and neuromuscular disease

Muscle-specific kinase (MuSK) is essential for each step in neuromuscular synapse formation. Before innervation, MuSK initiates postsynaptic differentiation, priming the muscle for synapse formation. Approaching motor axons recognize the primed, or prepatterned, region of muscle, causing motor axons to stop growing and differentiate into specialized nerve terminals. MuSK controls presynaptic differentiation by causing the clustering of Lrp4, which functions as a direct retrograde signal for presynaptic differentiation. Developing synapses are stabilized by neuronal Agrin, which is released by motor nerve terminals and binds to Lrp4, a member of the low-density lipoprotein receptor family, stimulating further association between Lrp4 and MuSK and increasing MuSK kinase activity. In addition, MuSK phosphorylation is stimulated by an inside-out ligand, docking protein-7 (Dok-7), which is recruited to tyrosine-phosphorylated MuSK and increases MuSK kinase activity. Mutations in MuSK and in genes that function in the MuSK signaling pathway, including Dok-7, cause congenital myasthenia, and autoantibodies to MuSK, Lrp4, and acetylcholine receptors are responsible for myasthenia gravis.

Regulation of muscle acetylcholine receptor turnover by β subunit tyrosine phosphorylation

At the neuromuscular junction (NMJ), the postsynaptic localization of muscle acetylcholine receptor (AChR) is regulated by neural signals and occurs via several processes including metabolic stabilization of the receptor. However, the molecular mechanisms that influence receptor stability remain poorly defined. Here, we show that neural agrin and the tyrosine phosphatase inhibitor, pervanadate slow the degradation of surface receptor in cultured muscle cells. Their action is mediated by tyrosine phosphorylation of the AChR β subunit, as agrin and pervandate had no effect on receptor half-life in AChR-β(3F/3F) muscle cells, which have targeted mutations of the β subunit cytoplasmic tyrosines. Moreover, in wild type AChR-β(3Y) muscle cells, we found a linear relationship between average receptor half-life and the percentage of AChR with phosphorylated β subunit, with half-lives of 12.7 and 23 h for nonphosphorylated and phosphorylated receptor, respectively. Surprisingly, pervanadate increased receptor half-life in AChR-β(3Y) myotubes in the absence of clustering, and agrin failed to increase receptor half-life in AChR-β(3F/3F) myotubes even in the presence of clustering. The metabolic stabilization of the AChR was mediated specifically by phosphorylation of βY390 as mutation of this residue abolished β subunit phosphorylation but did not affect δ subunit phosphorylation. Receptor stabilization also led to higher receptor levels, as agrin increased surface AChR by 30% in AChR-β(3Y) but not AChR-β(3F/3F) myotubes. Together, these findings identify an unexpected role for agrin-induced phosphorylation of β(Y390) in downregulating AChR turnover. This likely stabilizes AChR at developing synapses, and contributes to the extended half-life of AChR at adult NMJs.

Exome sequencing detection of two untranslated GFPT1 mutations in a family with limb-girdle myasthenia

The term 'limb-girdle myasthenia' (LGM) was first used to describe three siblings with proximal limb weakness without oculobulbar involvement, but with EMG decrement and responsiveness to anticholinesterase medication. We report here that exome sequencing in the proband of this family revealed several sequence variations in genes linked to proximal limb weakness. However, the only mutations that cosegregated with disease were an intronic IVS7-8A>G mutation and the previously reported 3'-UTR c.*22C>A mutation in GFPT1, a gene linked to LGM. A minigene assay showed that IVS7-8A>G activates an alternative splice acceptor that results in retention of the last seven nucleotides of intron 7 and a frameshift leading to a termination codon 13 nucleotides downstream from the new splice site. An anconeus muscle biopsy revealed mild reduction of the axon terminal size and postsynaptic fold simplification. The amplitudes of miniature endplate potentials and quantal release were also diminished. The DNA of the mildly affected father of the proband showed only the intronic mutation along with sequence variations in other genes potentially relevant to LGM. Thus, this study performed in the family originally described with LGM showed two GFPT1 untranslated mutations, which may cause disease by reducing GFPT1 expression and ultimately impairing protein glycosylation.

Synaptic dysfunction in congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are hereditary disorders of neuromuscular transmission characterized by fatigable muscle weakness. The number of cases recognized is increasing with improved diagnosis. To date we have identified over 300 different mutations present in over 350 unrelated kinships. The underlying genetic defects are diverse, involving a series of different genes with a variety of different phenotypes. The type of treatment and its effectiveness will depend on the underlying pathogenic mechanism. We aim to define the molecular mechanism for each mutation identified and feed this information back to the clinic as a basis to tailor patient treatment. Here, we describe some of the methods that can be used to define if a DNA sequence variant is pathogenic with reference to variants in DOK7. We highlight a new mechanism for disruption of AChR function, where a mutation in the AChR ɛ-subunit gene causes reduced ion channel conductance and discuss new methods for identifying gene mutations. The study of these disorders is proving highly informative for understanding the diverse molecular mechanisms that can underlie synaptic dysfunction.

A mutation causes MuSK reduced sensitivity to agrin and congenital myasthenia

Congenital myasthenic syndromes (CMSs) are a heterogeneous group of genetic disorders affecting neuromuscular transmission. The agrin/muscle-specific kinase (MuSK) pathway is critical for proper development and maintenance of the neuromuscular junction (NMJ). We report here an Iranian patient in whom CMS was diagnosed since he presented with congenital and fluctuating bilateral symmetric ptosis, upward gaze palsy and slowly progressive muscle weakness leading to loss of ambulation. Genetic analysis of the patient revealed a homozygous missense mutation c.2503A>G in the coding sequence of MUSK leading to the p.Met835Val substitution. The mutation was inherited from the two parents who were heterozygous according to the notion of consanguinity. Immunocytochemical and electron microscopy studies of biopsied deltoid muscle showed dramatic changes in pre- and post-synaptic elements of the NMJs. These changes induced a process of denervation/reinnervation in native NMJs and the formation, by an adaptive mechanism, of newly formed and ectopic NMJs. Aberrant axonal outgrowth, decreased nerve terminal ramification and nodal axonal sprouting were also noted. In vivo electroporation of the mutated MuSK in a mouse model showed disorganized NMJs and aberrant axonal growth reproducing a phenotype similar to that observed in the patient's biopsy specimen. In vitro experiments showed that the mutation alters agrin-dependent acetylcholine receptor aggregation, causes a constitutive activation of MuSK and a decrease in its agrin- and Dok-7-dependent phosphorylation.

Congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders caused by genetic defects affecting neuromuscular transmission and leading to muscle weakness accentuated by exertion. The characterization of CMS comprises two complementary steps: establishing the diagnosis and identifying the pathophysiological type of CMS. The combination of clinical, electrophysiological, and morphological studies allows the physician to refer a given CMS to mutation(s) in one of the 18 causative genes discovered to date and, in turn, to classify the CMS according to the location of the mutated proteins at the neuromuscular junction into presynaptic compartment, synaptic basal lamina, and postsynaptic compartment CMS. This complete characterization is essential for counseling and therapy of the patient, depending on the molecular background of the respective CMS. Despite comprehensive characterization, the phenotypic expression of one given gene involved is variable, and the etiology of many CMS remains to be discovered.

Defective presynaptic choline transport underlies hereditary motor neuropathy

The neuromuscular junction (NMJ) is a specialized synapse with a complex molecular architecture that provides for reliable transmission between the nerve terminal and muscle fiber. Using linkage analysis and whole-exome sequencing of DNA samples from subjects with distal hereditary motor neuropathy type VII, we identified a mutation in SLC5A7, which encodes the presynaptic choline transporter (CHT), a critical determinant of synaptic acetylcholine synthesis and release at the NMJ. This dominantly segregating SLC5A7 mutation truncates the encoded product just beyond the final transmembrane domain, eliminating cytosolic-C-terminus sequences known to regulate surface transporter trafficking. Choline-transport assays in both transfected cells and monocytes from affected individuals revealed significant reductions in hemicholinium-3-sensitive choline uptake, a finding consistent with a dominant-negative mode of action. The discovery of CHT dysfunction underlying motor neuropathy identifies a biological basis for this group of conditions and widens the spectrum of disorders that derive from impaired NMJ transmission. Our findings compel consideration of mutations in SLC5A7 or its functional partners in relation to unexplained motor neuronopathies.

Salbutamol benefits children with congenital myasthenic syndrome due to DOK7 mutations

Congenital myasthenic syndromes due to DOK7 mutations cause fatigable limb girdle weakness. Treatment with ephedrine improves muscle strength. Salbutamol, a β(2)-adrenergic receptor agonist with fewer side effects and more readily available, has been effective in adult and anecdotal childhood cases. This study reports the effects of salbutamol on motor function in childhood DOK7 congenital myasthenic syndrome. Nine children (age range 5.9-15.1years) were treated with oral salbutamol, 2-4mg TDS. The effect on timed tests of motor function, pre- and up to 28months post-treatment, was audited retrospectively. All 9 reported functional benefit within 1month, with progressive improvement to a plateau at 12-18months. Within the first month, all 3 non-ambulant children resumed walking with assistance. Although improvements were seen in some timed tests (timed 10m, arm raise time, 6min walk time) this did not fully reflect the observed functional benefits in daily living activities. No major side effects were reported. We conclude that oral salbutamol treatment significantly improves strength in children with DOK7 congenital myasthenic syndrome and is well tolerated. Outcome measures need to be refined further, both to accurately reflect functional abilities in children and to document progress and treatment response.

Structural mechanisms of the agrin-LRP4-MuSK signaling pathway in neuromuscular junction differentiation

The neuromuscular junction (NMJ) is the most extensively studied model of neuronal synaptogenesis. Acetylcholine receptor (AChR) clustering on the postsynaptic membrane is a cardinal event in the differentiation of NMJs. AChR clustering and postsynaptic differentiation is orchestrated by sophisticated interactions among three proteins: the neuron-secreted proteoglycan agrin, the co-receptor LRP4, and the muscle-specific receptor tyrosine kinase MuSK. LRP4 and MuSK act as scaffolds for multiple binding partners, resulting in a complex and dynamic network of interacting proteins that is required for AChR clustering. In this review, we discuss the structural basis for NMJ postsynaptic differentiation mediated by the agrin-LRP4-MuSK signaling pathway.

Somatic complex I disruptive mitochondrial DNA mutations are modifiers of tumorigenesis that correlate with low genomic instability in pituitary adenomas

Mitochondrial DNA (mtDNA) mutations leading to the disruption of respiratory complex I (CI) have been shown to exhibit anti-tumorigenic effects, at variance with those impairing only the function but not the assembly of the complex, which appear to contribute positively to cancer development. Owing to the challenges in the analysis of the multi-copy mitochondrial genome, it is yet to be determined whether tumour-associated mtDNA lesions occur as somatic modifying factors or as germ-line predisposing elements. Here we investigated the whole mitochondrial genome sequence of 20 pituitary adenomas with oncocytic phenotype and identified pathogenic and/or novel mtDNA mutations in 60% of the cases. Using highly sensitive techniques, namely fluorescent PCR and allele-specific locked nucleic acid quantitative PCR, we identified the most likely somatic nature of these mutations in our sample set, since none of the mutations was detected in the corresponding blood tissue of the patients analysed. Furthermore, we have subjected a series of 48 pituitary adenomas to a high-resolution array comparative genomic hybridization analysis, which revealed that CI disruptive mutations, and the oncocytic phenotype, significantly correlate with low number of chromosomal aberrations in the nuclear genome. We conclude that CI disruptive mutations in pituitary adenomas are somatic modifiers of tumorigenesis most likely contributing not only to the development of oncocytic change, but also to a less aggressive tumour phenotype, as indicated by a stable karyotype.

New insights into Dok-4 PTB domain structure and function

The seven members of the Dok adapter protein family share a highly conserved phosphotyrosine-binding (PTB) domain. In the case of Dok-1, 2 and 3, the PTB domain binds to the lipid phosphatase Ship1, a key component of their inhibitory signaling mechanisms in immune cells. In contrast to most other Dok family members, Dok-4 is expressed widely but is poorly understood, largely because of limited knowledge of its partner molecules. We previously showed that, in contrast to the Dok-1 PTB domain (defined as aa 107-260), the homologous sequence in Dok-4 (aa 100-233) bound very poorly to Ret, a known Dok-4 partner. In the current study, we show that binding of Dok-4 to Ret requires residues C-terminal to the previously defined PTB domain boundaries (up to aa 246). These residues are predicted to form an extension in a critical C-terminal α-helix. We show that the Dok-4 PTB domain also binds the phosphorylated NPXY motifs in Ship1 but not Ship2. Finally, we found that a rare human single nucleotide polymorphism causing a R186H substitution in the PTB domain abolishes tyrosine phosphorylation of Dok-4 by Ret. In addition to providing a clearer understanding of Dok-4 PTB domain structure and function, our findings point to a potential mechanism for Dok-4 inhibitory signaling in T-cells and to the possibility of a rare Dok-4-related phenotype in humans.

Beware of moving targets: reference proteome content fluctuates substantially over the years

Reference proteomes are generated by increasingly sophisticated annotation pipelines as part of regular genome build releases; yet, the corresponding changes in reference proteomes' content are dramatic. In the history of the NCBI-curated human proteome, the total number of entries has remained roughly constant but approximately half of the proteins from the 2003 build 33 are no longer represented by entries in current releases, while about the same number of new proteins have been added (for sequence identity thresholds 50-90%). Although mostly hypothetical proteins are affected, there are also spectacular cases of entry removal/addition of well studied proteins. The changes between the 2003 and recent human proteomes are in a similar order of magnitude as the differences between recent human and chimpanzee proteome releases. As an application example, we show that the proteome fluctuations affect the interpretation (about 74% of hits) of organelle-specific mass-spectrometry data. Although proteome quality tends to improve with more recent releases as, for example, the fraction of proteins with functional annotation has increased over time, existing evidence implies that, apparently, the proteome content still remains incomplete, not just pertaining to isoforms/sequence variants but also to proteins and their families that are clearly distinct.

Mutations in DPAGT1 cause a limb-girdle congenital myasthenic syndrome with tubular aggregates

Congenital myasthenic syndromes are a heterogeneous group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. We performed whole-exome sequencing to determine the underlying defect in a group of individuals with an inherited limb-girdle pattern of myasthenic weakness. We identify DPAGT1 as a gene in which mutations cause a congenital myasthenic syndrome. We describe seven different mutations found in five individuals with DPAGT1 mutations. The affected individuals share a number of common clinical features, including involvement of proximal limb muscles, response to treatment with cholinesterase inhibitors and 3,4-diaminopyridine, and the presence of tubular aggregates in muscle biopsies. Analyses of motor endplates from two of the individuals demonstrate a severe reduction of endplate acetylcholine receptors. DPAGT1 is an essential enzyme catalyzing the first committed step of N-linked protein glycosylation. Our findings underscore the importance of N-linked protein glycosylation for proper functioning of the neuromuscular junction. Using the DPAGT1-specific inhibitor tunicamycin, we show that DPAGT1 is required for efficient glycosylation of acetylcholine-receptor subunits and for efficient export of acetylcholine receptors to the cell surface. We suggest that the primary pathogenic mechanism of DPAGT1 mutations is reduced levels of acetylcholine receptors at the endplate region. These individuals share clinical features similar to those of congenital myasthenic syndrome due to GFPT1 mutations, and their disorder might be part of a larger subgroup comprising the congenital myasthenic syndromes that result from defects in the N-linked glycosylation pathway and that manifest through impaired neuromuscular transmission.

Myasthenic syndrome AChRα C-loop mutant disrupts initiation of channel gating

Congenital myasthenic syndromes (CMSs) are neuromuscular disorders that can be caused by defects in ace-tylcholine receptor (AChR) function. Disease-associated point mutants can reveal the unsuspected functional significance of mutated residues. We identified two pathogenic mutations in the extracellular domain of the AChR α subunit (AChRα) in a patient with myasthenic symptoms since birth: a V188M mutation in the C-loop and a heteroallelic G74C mutation in the main immunogenic region. The G74C mutation markedly reduced surface AChR expression in cultured cells, whereas the V188M mutant was expressed robustly but had severely impaired kinetics. Single-channel patch-clamp analysis indicated that V188M markedly decreased the apparent AChR channel opening rate and gating efficiency. Mutant cycle analysis of energetic coupling among conserved residues within or dispersed around the AChRα C-loop revealed that V188 is functionally linked to Y190 in the C-loop and to D200 in β-strand 10, which connects to the M1 transmembrane domain. Furthermore, V188M weakens inter-residue coupling of K145 in β-strand 7 with Y190 and with D200. Cumulatively, these results indicate that V188 of AChRα is part of an interdependent tetrad that contributes to rearrangement of the C-loop during the initial coupling of agonist binding to channel gating.

The spectrum of mutations that underlie the neuromuscular junction synaptopathy in DOK7 congenital myasthenic syndrome

Congenital myasthenic syndromes (CMS) are a group of inherited diseases that affect synaptic transmission at the neuromuscular junction and result in fatiguable muscle weakness. A subgroup of CMS patients have a recessively inherited limb-girdle pattern of weakness caused by mutations in DOK7. DOK7 encodes DOK7, an adaptor protein that is expressed in the skeletal muscle and heart and that is essential for the development and maintenance of the neuromuscular junction. We have screened the DOK7 gene for mutations by polymerase chain reaction amplification and bi-directional sequencing of exonic and promoter regions and performed acetylcholine receptor (AChR) clustering assays and used exon trapping to determine the pathogenicity of detected variants. Approximately 18% of genetically diagnosed CMSs in the UK have mutations in DOK7, with mutations in this gene identified in more than 60 kinships to date. Thirty-four different pathogenic mutations were identified as well as 27 variants likely to be non-pathogenic. An exon 7 frameshift duplication c.1124_1127dupTGCC is commonly found in at least one allele. We analyse the effect of the common frameshift c.1124_1127dupTGCC and show that 10/11 suspected missense mutations have a deleterious effect on AChR clustering. We identify for the first time homozygous or compound heterozygous mutations that are localized 5' to exon 7. In addition, three silent variants in the N-terminal half of DOK7 are predicted to alter the splicing of the DOK7 RNA transcript. The DOK7 gene is highly polymorphic, and within these many variants, we define a spectrum of mutations that can underlie DOK7 CMS that will inform in managing this disorder.

Molecular mechanisms underlying maturation and maintenance of the vertebrate neuromuscular junction

The vertebrate neuromuscular junction (NMJ), a peripheral synapse formed between motoneuron and skeletal muscle, is characterized by a protracted postnatal period of maturation and life-long maintenance. In neuromuscular disorders such as congenital myasthenic syndromes (CMSs), disruptions of NMJ maturation and/or maintenance are frequently observed. In particular, defective neuromuscular transmission associated with structural and molecular abnormalities at the pre- and postsynaptic membranes, as well as at the synaptic cleft, has been reported in these patients. Here, we review recent advances in the understanding of molecular and cellular events that mediate NMJ maturation and maintenance. The underlying regulatory mechanisms, including key molecular regulators at the presynaptic nerve terminal, synaptic cleft, and postsynaptic muscle membrane, are discussed.

Molecular control of neuromuscular junction development

Skeletal muscle innervation is a multi-step process leading to the neuromuscular junction (NMJ) apparatus formation. The transmission of the signal from nerve to muscle occurs at the NMJ level. The molecular mechanism that orchestrates the organization and functioning of synapses is highly complex, and it has not been completely elucidated so far. Neuromuscular junctions are assembled on the muscle fibers at very precise locations called end plates (EP). Acetylcholine receptor (AChR) clusterization at the end plates is required for an accurate synaptic transmission. This review will focus on some mechanisms responsible for accomplishing the correct distribution of AChRs at the synapses. Recent evidences support the concept that a dual transcriptional control of AChR genes in subsynaptic and extrasynaptic nuclei is crucial for AChR clusterization. Moreover, new players have been discovered in the agrin-MuSK pathway, the master organizer of postsynaptical differentiation. Mutations in this pathway cause neuromuscular congenital disorders. Alterations of the postynaptic apparatus are also present in physiological conditions characterized by skeletal muscle wasting. Indeed, recent evidences demonstrate how NMJ misfunctioning has a crucial role at the onset of age-associated sarcopenia.

Signaling and aging at the neuromuscular synapse: lessons learnt from neuromuscular diseases

The neuromuscular junction (NMJ) is a specialized synapse between motor neurons and skeletal muscle with a complex signaling network that assures highly reliable neuromuscular transmission. Diseases of the NMJ cause skeletal muscle fatigue and include inherited and acquired disorders that affect presynaptic, intrasynaptic or postsynaptic components. Moreover, fragmentation of the NMJ contributes to sarcopenia, the loss of muscle mass during aging. Studies from recent years indicate that the formation and stabilization of NMJs differs between various muscles and that this difference affects their response under pathological conditions. This review summarizes the most important mechanisms involved in the development, maintenance and dysfunction of the NMJ and it discusses their significance in myasthenic disorders and aging and as targets for possible future treatment of NMJ dysfunction.

Activation of receptor protein-tyrosine kinases from the cytoplasmic compartment

It is widely accepted that receptor protein-tyrosine kinases (RTKs) are activated upon dimerization by binding to their extracellular ligands. However, EGF receptor (EGFR) dimerization per se does not require ligand binding. Instead, its cytoplasmic kinase domains have to form characteristic head-to-tail asymmetric dimers to become active, where one 'activator' domain activates the other 'receiver' domain. The non-catalytic, cytoplasmic regions of RTKs, namely the juxtamembrane and carboxy terminal portions, also regulate kinase activity. For instance, the juxtamembrane region of the RTK MuSK inhibits the kinase domain probably together with a cellular factor(s). These findings suggest that RTKs could be activated by cytoplasmic proteins. Indeed, Dok-7 and cytohesin have recently been identified as such activators of MuSK and EGFR, respectively. Given that failure of Dok-7 signaling causes myasthenia, and inhibition of cytohesin signaling reduces the proliferation of EGFR-dependent cancer cells, cytoplasmic activators of RTKs may provide new therapeutic targets.

Congenital myasthenic syndromes in 2012

Congenital myasthenic syndromes (CMS) represent a heterogeneous group of disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. Clinical, electrophysiologic, and morphologic studies have paved the way for detecting CMS-related mutations in proteins residing in the nerve terminal, the synaptic basal lamina, or in the postsynaptic region of the motor endplate. The disease proteins identified to date include the acetylcholine receptor, acetylcholinesterase, choline acetyltransferase, rapsyn, and Na(v)1.4, muscle-specific kinase, agrin, β2-laminin, downstream of tyrosine kinase 7, and glutamine-fructose-6-phosphate transaminase 1. Analysis of electrophysiologic and biochemical properties of mutant proteins expressed in heterologous systems have contributed crucially to defining the molecular consequences of the observed mutations and have resulted in improved therapy of most CMS.

Current status of the congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are heterogeneous disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. Clinical, electrophysiologic, and morphologic studies have paved the way for detecting CMS-related mutations in proteins residing in the nerve terminal, the synaptic basal lamina, and in the postsynaptic region of the motor endplate. The disease proteins identified to date include choline acetyltransferase (ChAT), the endplate species of acetylcholinesterase (AChE), β2-laminin, the acetylcholine receptor (AChR), rapsyn, plectin, Na(v)1.4, the muscle specific protein kinase (MuSK), agrin, downstream of tyrosine kinase 7 (Dok-7), and glutamine-fructose-6-phosphate transaminase 1 (GFPT1). Myasthenic syndromes associated with centronuclear myopathies were recently recognized. Analysis of properties of expressed mutant proteins contributed to finding improved therapy for most CMS. Despite these advances, the molecular basis of some phenotypically characterized CMS remains elusive. Moreover, other types of CMS and disease genes likely exist and await discovery.

Functional consequences and structural interpretation of mutations of human choline acetyltransferase

Choline acetyltransferase (ChAT; EC 2.3.1.6) catalyzes synthesis of acetylcholine from acetyl-CoA (AcCoA) and choline in cholinergic neurons. Mutations in CHAT cause potentially lethal congenital myasthenic syndromes associated with episodic apnea (ChAT-CMS). Here, we analyze the functional consequences of 12 missense and one nonsense mutations of CHAT in 11 patients. Nine of the mutations are novel. We examine expression of the recombinant missense mutants in Bosc 23 cells, determine their kinetic properties and thermal stability, and interpret the functional effects of 11 mutations in the context of the atomic structural model of human ChAT. Five mutations (p.Trp421Ser, p.Ser498Pro, p.Thr553Asn, p.Ala557Thr, and p.Ser572Trp) reduce enzyme expression to less than 50% of wild-type. Mutations with severe kinetic effects are located in the active-site tunnel (p.Met202Arg, p.Thr553Asn, and p.Ala557Thr) or adjacent to the substrate binding site (p.Ser572Trp), or exert their effect allosterically (p.Trp421Ser and p.Ile689Ser). Two mutations with milder kinetic effects (p.Val136Met and p.Ala235Thr) are also predicted to act allosterically. One mutation (p.Thr608Asn) below the nucleotide binding site of CoA enhances dissociation of AcCoA from the enzyme-substrate complex. Two mutations introducing a proline residue into an α-helix (p.Ser498Pro and p.Ser704Pro) impair the thermal stability of ChAT.

The extracellular region of Lrp4 is sufficient to mediate neuromuscular synapse formation

Neuromuscular synapse formation requires an exchange of signals between motor neurons and muscle. Agrin, supplied by motor neurons, binds to Lrp4 in muscle, stimulating phosphorylation of MuSK and recruitment of a signaling complex essential for synapse-specific transcription and anchoring of key proteins in the postsynaptic membrane. Lrp4, like the LDLR and other Lrp-family members, contains an intracellular region with motifs that can regulate receptor trafficking, as well as assembly of an intracellular signaling complex. Here, we show that the intracellular region of Lrp4 is dispensable for Agrin to stimulate MuSK phosphorylation and clustering of acetylcholine receptors in cultured myotubes. Moreover, muscle-selective expression of a Lrp4-CD4 chimera, composed of the extracellular and transmembrane regions of Lrp4 and the intracellular region of CD4, rescues neuromuscular synapse formation and the neonatal lethality of lrp4 mutant mice, demonstrating that Lrp4, lacking the Lrp4 intracellular region, is sufficient for presynaptic and postsynaptic differentiation.

Agrin binds to the N-terminal region of Lrp4 protein and stimulates association between Lrp4 and the first immunoglobulin-like domain in muscle-specific kinase (MuSK)

Neuromuscular synapse formation depends upon coordinated interactions between motor neurons and muscle fibers, leading to the formation of a highly specialized postsynaptic membrane and a highly differentiated nerve terminal. Synapse formation begins as motor axons approach muscles that are prepatterned in the prospective synaptic region in a manner that depends upon Lrp4, a member of the LDL receptor family, and muscle-specific kinase (MuSK), a receptor tyrosine kinase. Motor axons supply Agrin, which binds Lrp4 and stimulates further MuSK phosphorylation, stabilizing nascent synapses. How Agrin binds Lrp4 and stimulates MuSK kinase activity is poorly understood. Here, we demonstrate that Agrin binds to the N-terminal region of Lrp4, including a subset of the LDLa repeats and the first of four β-propeller domains, which promotes association between Lrp4 and MuSK and stimulates MuSK kinase activity. In addition, we show that Agrin stimulates the formation of a functional complex between Lrp4 and MuSK on the surface of myotubes in the absence of the transmembrane and intracellular domains of Lrp4. Further, we demonstrate that the first Ig-like domain in MuSK, which shares homology with the NGF-binding region in Tropomyosin Receptor Kinase (TrKA), is required for MuSK to bind Lrp4. These findings suggest that Lrp4 is a cis-acting ligand for MuSK, whereas Agrin functions as an allosteric and paracrine regulator to promote association between Lrp4 and MuSK.

Clinical features in a series of fast channel congenital myasthenia syndrome

Fast channel congenital myasthenic syndromes are rare, but frequently result in severe weakness. We report a case of 12 fast channel patients to highlight clinical features and management difficulties. Patients were diagnosed through genetic screening and identification of mutations shown to cause fast channel syndrome. Data was obtained from clinical notes, history, examination and follow up. Patterns of muscle weakness involved limb, trunk, bulbar, respiratory, facial and extraocular muscles. Patients responded to treatment with anticholinesterase medication and 3,4-diaminopyridine. Fast channel syndrome contrasted with AChR deficiency in the occurrence of severe respiratory crises in infancy and childhood. The death of two children even when on treatment and the family histories of sibling deaths re-inforces the need for accurate genetic diagnosis, optimised pharmacological treatment and additional supportive measures to manage acute respiratory crises. Referral to a specialist paediatric respiratory centre and regular resuscitation training for parents are recommended.

A valid mouse model of AGRIN-associated congenital myasthenic syndrome

Congenital myasthenic syndromes (CMS) are inherited diseases affecting the neuromuscular junction (NMJ). Mutations in AGRIN (AGRN) and other genes in the AGRIN signaling pathway cause CMS, and gene targeting studies in mice confirm the importance of this pathway for NMJ formation. However, these mouse mutations are complete loss-of-function alleles that result in an embryonic failure of NMJ formation, and homozygous mice do not survive postpartum. Therefore, mouse models of AGRIN-related CMS that would allow preclinical testing or studies of postnatal disease progression are lacking. Using chemical mutagenesis in mice, we identified a point mutation in Agrn that results in a partial loss-of-function allele, creating a valid model of CMS. The mutation changes phenylalanine 1061 to serine in the SEA domain of AGRIN, a poorly characterized motif shared by other extracellular proteoglycans. NMJs in homozygous mice progressively degrade postnataly. Severity differs with genetic background, in different muscles, and in different regions within a muscle in a pattern matching mouse models of motor neuron disease. Mutant NMJs have decreased acetylcholine receptor density and an increased subsynaptic reticulum, evident by electron microscopy. Synapses eventually denervate and the muscles atrophy. Molecularly, several factors contribute to the partial loss of AGRIN's function. The mutant protein is found at NMJs, but is processed differently than wild-type, with decreased glycosylation, changes in sensitivity to the protease neurotrypsin and other proteolysis, and less efficient externalization and secretion. Therefore, the Agrn point mutation is a model for CMS caused by Agrn mutations and potentially other related neuromuscular diseases.

Genetic heterogeneity and pathophysiological mechanisms in congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a rare heterogeneous group of inherited neuromuscular disorders associated with distinctive clinical, electrophysiological, ultrastructural and genetic abnormalities. These genetic defects either impair neuromuscular transmission directly or result in secondary impairments, which eventually compromise the safety margin of neuromuscular transmission. In this report we will explore the significant progress made in understanding the molecular pathogenesis of CMS, which is important for both patients and clinicians in terms of reaching a definite diagnosis and selecting the most appropriate treatment.

Paediatric electromyography in the modern world: a personal view

Paediatric electromyography (EMG) is an invaluable diagnostic test for the investigation of neuromuscular disease, but its use is inconsistent between and within different countries. One perception is that the procedure is painful; however, in comparison with common investigations performed routinely in children, EMG is better tolerated. While some developments, such as those within clinical genetics, would appear to mark its demise, paradoxically the more genetic abnormalities that are discovered in conditions such as hereditary neuropathy, the more precise a delineation of the phenotype is required. EMG has particular strengths in the diagnosis of neuropathies, motor neuronopathy and neuromuscular transmission disorders such as myasthenia. Also, it can supplement the investigation of myopathies. Areas of development include the diagnosis of myasthenia, delineation of bulbar palsy as a cause of dysphagia, more accurate and earlier prediction of prognosis in neonatal brachial palsy and investigation of channelopathies. It is a valuable diagnostic tool in developed countries and those with limited resources.

Congenital myasthenic syndrome associated with epidermolysis bullosa caused by homozygous mutations in PLEC1 and CHRNE

Mutations in the plectin gene (PLEC1) cause epidermolysis bullosa simplex (EBS), which may associate with muscular dystrophy (EBS-MD) or pyloric atresia (EBS-PA). The association of EBS with congenital myasthenic syndrome (CMS) is also suspected to result from PLEC1 mutations. We report here a consanguineous patient with EBS and CMS for whom mutational analysis of PLEC1 revealed a homozygous 36 nucleotide insertion (1506_1507ins36) that results in a reduced expression of PLEC1 mRNA and plectin in the patient muscle. In addition, mutational analysis of CHRNE revealed a homozygous 1293insG, which is a well-known low-expressor receptor mutation. A skin biopsy revealed signs of EBS, and an anconeus muscle biopsy showed signs of a mild myopathy. Endplate studies showed fragmentation of endplates, postsynaptic simplification, and large collections of thread-like mitochondria. Amplitudes of miniature endplate potentials were diminished, but the endplate quantal content was actually increased. The complex phenotype presented here results from mutations in two separate genes. While the skin manifestations are because of the PLEC1 mutation, footprints of mutations in PLEC1 and CHRNE are present at the neuromuscular junction of the patient indicating that abnormalities in both genes contribute to the CMS phenotype.

Therapy update in nerve, neuromuscular junction and myopathic disorders

PURPOSE OF REVIEW

The objective of this review is to summarize recent advances in the treatment of various neuromuscular disorders including neuropathies, neuromuscular junction disorders, and myopathies.

RECENT FINDINGS

Immunotherapy with sophisticated agents for myasthenia gravis and inflammatory myopathies, neuroprotection with vitamin E for chemotherapy-induced neuropathy, and promising gene transfer and exon-skipping therapies for muscular dystrophy are among the most exciting recent developments in the treatment of neuromuscular disorders.

SUMMARY

In spite of significant advances, therapy in many neuromuscular diseases remains far from satisfactory. Better understanding of the underlying molecular and pathophysiologic processes for both hereditary and acquired disorders should lead to more refined and successful therapeutic approaches, reducing physical and other types of disability while posing fewer side effects.

Antibody effector mechanisms in myasthenia gravis-pathogenesis at the neuromuscular junction

Myasthenia gravis (MG) is an autoimmune disorder caused by autoantibodies that are either directed to the muscle nicotinic acetylcholine receptor (AChR) or to the muscle-specific tyrosine kinase (MuSK). These autoantibodies define two distinct subforms of the disease-AChR-MG and MuSK-MG. Both AChR and MuSK are expressed on the postsynaptic membrane of the neuromuscular junction (NMJ), which is a highly specialized region of the muscle dedicated to receive and process signals from the motor nerve. Autoantibody binding to proteins of the postsynaptic membrane leads to impaired neuromuscular transmission and muscle weakness. Pro-inflammatory antibodies of the human IgG1 and IgG3 subclass modulate the AChR, cause complement activation, and attract lymphocytes; together acting to decrease levels of the AChR and AChR-associated proteins and to reduce postsynaptic folding. In patients with anti-MuSK antibodies, there is no evidence of loss of junctional folds and no apparent loss of AChR density. Anti-MuSK antibodies are predominantly of the IgG4 isotype, which functionally differs from other IgG subclasses in its anti-inflammatory activity. Moreover, IgG4 undergoes a posttranslational modification termed Fab arm exchange that prevents cross-linking of antigens. These findings suggest that MuSK-MG may be different in etiological and pathological mechanisms from AChR-MG. The effector functions of IgG subclasses on synapse structure and function are discussed in this review.

Dok-7 regulates neuromuscular synapse formation by recruiting Crk and Crk-L

Agrin, released by motor neurons, promotes neuromuscular synapse formation by stimulating MuSK, a receptor tyrosine kinase expressed in skeletal muscle. Phosphorylated MuSK recruits docking protein-7 (Dok-7), an adaptor protein that is expressed selectively in muscle. In the absence of Dok-7, neuromuscular synapses fail to form, and mutations that impair Dok-7 are a major cause of congenital myasthenia in humans. How Dok-7 stimulates synaptic differentiation is poorly understood. Once recruited to MuSK, Dok-7 directly stimulates MuSK kinase activity. This unusual activity of an adapter protein is mediated by the N-terminal region of Dok-7, whereas most mutations that cause congenital myasthenia truncate the C-terminal domain. Here, we demonstrate that Dok-7 also functions downstream from MuSK, and we identify the proteins that are recruited to the C-terminal domain of Dok-7. We show that Agrin stimulates phosphorylation of two tyrosine residues in the C-terminal domain of Dok-7, which leads to recruitment of two adapter proteins: Crk and Crk-L. Furthermore, we show that selective inactivation of Crk and Crk-L in skeletal muscle leads to severe defects in neuromuscular synapses in vivo, revealing a critical role for Crk and Crk-L downstream from Dok-7 in presynaptic and postsynaptic differentiation.

Docking proteins

Docking proteins comprise a distinct category of intracellular, noncatalytic signalling protein, that function downstream of a variety of receptor and receptor-associated tyrosine kinases and regulate diverse physiological and pathological processes. The growth factor receptor bound 2-associated binder/Daughter of Sevenless, insulin receptor substrate, fibroblast growth factor receptor substrate 2 and downstream of tyrosine kinases protein families fall into this category. This minireview focuses on the structure, function and regulation of these proteins.

Congenital stridor with feeding difficulty as a presenting symptom of Dok7 congenital myasthenic syndrome

OBJECTIVE

The congenital myasthenic syndromes (CMS) are a group of genetic disorders of neuromuscular transmission causing fatigable weakness. Symptoms may be present from birth, but diagnosis is often delayed for several years, notably in post-synaptic CMS due to mutations in the DOK7 gene. Recently, we noted a subgroup of children with CMS in whom congenital stridor and bilateral vocal cord palsy predated other symptoms. All had mutations in the DOK7 gene. The purpose of this study was to review our population of DOK7 CMS patients with congenital stridor and assess whether there were other phenotypic features which might raise suspicion of a diagnosis of CMS in the neonatal period, in the absence of limb weakness and ptosis and prompt earlier referral for neurophysiological investigation, genetic diagnosis and appropriate treatment.

METHODS

A retrospective case review of 11 DOK7 CMS patients at a tertiary referral centre.

RESULTS

Six patients were identified with DOK7 mutations and congenital stridor, four requiring intubation soon after birth. Four patients had a diagnosis of bilateral vocal cord palsy and three required tracheostomy, successfully decannulated in one after 3 years. All six patients had difficulty with feeding, with weak suck and swallow necessitating nasogastric feeding in five, two of whom required gastrostomy. Despite all six children having had neonatal symptoms, the mean age at CMS diagnosis was 5 years and 9 months.

CONCLUSION

CMS, particularly caused by mutations in the DOK7 gene, is a rare but treatable cause of congenital stridor in the neonate. A combination of congenital stridor, especially with an apparently idiopathic bilateral vocal cord palsy and weak suck and swallow should alert the clinician to the possibility of CMS and prompt early referral for neurophysiology and genetic investigations. Confirmation of a CMS diagnosis enables treatment to be initiated, informed management of the VCP and anticipation of myasthenic symptoms, particularly life-threatening respiratory decompensation. Treatment may allow early decannulation or possible avoidance of tracheostomy. At least 12 genes are known to cause CMS; the presence of congenital stridor may help target genetic diagnosis.