Rapsyn mutations in hereditary myasthenia: distinct early- and late-onset phenotypes

Mutation analysis of CHRNA1, CHRNB1, CHRND, and RAPSN genes in multiple pterygium syndrome/fetal akinesia patients

Multiple pterygium syndromes (MPS) comprise a group of multiple congenital anomaly disorders characterized by webbing (pterygia) of the neck, elbows, and/or knees and joint contractures (arthrogryposis). MPS are phenotypically and genetically heterogeneous but are traditionally divided into prenatally lethal and nonlethal (Escobar) types. Previously, we and others reported that recessive mutations in the embryonal acetylcholine receptor g subunit (CHRNG) can cause both lethal and nonlethal MPS, thus demonstrating that pterygia resulted from fetal akinesia. We hypothesized that mutations in acetylcholine receptor-related genes might also result in a MPS/fetal akinesia phenotype and so we analyzed 15 cases of lethal MPS/fetal akinesia without CHRNG mutations for mutations in the CHRNA1, CHRNB1, CHRND, and rapsyn (RAPSN) genes. No CHRNA1, CHRNB1, or CHRND mutations were detected, but a homozygous RAPSN frameshift mutation, c.1177-1178delAA, was identified in a family with three children affected with lethal fetal akinesia sequence. Previously, RAPSN mutations have been reported in congenital myasthenia. Functional studies were consistent with the hypothesis that whereas incomplete loss of rapsyn function may cause congenital myasthenia, more severe loss of function can result in a lethal fetal akinesia phenotype.

Congenital myasthenic syndromes: spotlight on genetic defects of neuromuscular transmission

The neuromuscular junction (NMJ) is a complex structure that efficiently communicates the electrical impulse from the motor neuron to the skeletal muscle to induce muscle contraction. Genetic and autoimmune disorders known to compromise neuromuscular transmission are providing further insights into the complexities of NMJ function. Congenital myasthenic syndromes (CMSs) are a genetically and phenotypically heterogeneous group of rare hereditary disorders affecting neuromuscular transmission. The understanding of the molecular basis of the different types of CMSs has evolved rapidly in recent years. Mutations were first identified in the subunits of the nicotinic acetylcholine receptor (AChR), but now mutations in ten different genes - encoding post-, pre- or synaptic proteins - are known to cause CMSs. Pathogenic mechanisms leading to an impaired neuromuscular transmission modify AChRs or endplate structure or lead to decreased acetylcholine synthesis and release. However, the genetic background of many CMS forms is still unresolved. A precise molecular classification of CMS type is of paramount importance for the diagnosis, counselling and therapy of a patient, as different drugs may be beneficial or deleterious depending on the molecular background of the particular CMS.

Childhood myasthenia: clinical subtypes and practical management

In recent years, understanding of the pathogenesis and clinical presentation of distinct myasthenia subtypes has increased significantly. This article reviews the clinical manifestations of autoimmune myasthenia gravis (including myasthenia associated with anti-muscle-specific kinase antibodies), ocular myasthenia, and antibody negative myasthenia. The following treatments are examined: cholinesterase inhibitors, immunosuppressants, and thymectomy. Inherited congenital myasthenic syndromes (CMS) are now increasingly recognized, and most commonly present during childhood. This article outlines the presynaptic, synaptic basal lamina-associated, and postsynaptic classification of CMS and the clinical presentation and aetiology of individual syndromes. Relevant investigations and treatment options (including the role of pyridostigmine, 3,4-diaminopyridine, fluoxetine, and ephedrine) are discussed.

The therapy of congenital myasthenic syndromes

Congenital myasthenic syndromes (CMSs) are heterogeneous disorders in which the safety margin of neuromuscular transmission is compromised by one or more mechanisms. Specific diagnosis of a CMS is important as some medications that benefit one type of CMS can be detrimental in another type. In some CMSs, strong clinical clues point to a specific diagnosis. In other CMSs, morphologic and in vitro electrophysiologic studies of the neuromuscular junction, determination of the number of acetylcholine receptors (AchRs) per junction, and molecular genetic studies may be required for a specific diagnosis. Strategies for therapy are based on whether a given CMS decreases or increases the synaptic response to acetylcholine (ACh). Cholinesterase inhibitors that increase the synaptic response to ACh and 3,4-diaminopyridine, which increases ACh release, are useful when the synaptic response to ACh is attenuated. Long-lived open-channel blockers of the AChR, quinidine, and fluoxetine, are useful when the synaptic response is increased by abnormally prolonged opening episodes of the AChR channel. Ephedrine has beneficial effects in some CMSs but its mechanism of action is not understood.

Mutations in the embryonal subunit of the acetylcholine receptor (CHRNG) cause lethal and Escobar variants of multiple pterygium syndrome

Multiple pterygium syndromes (MPSs) comprise a group of multiple-congenital-anomaly disorders characterized by webbing (pterygia) of the neck, elbows, and/or knees and joint contractures (arthrogryposis). In addition, a variety of developmental defects (e.g., vertebral anomalies) may occur. MPSs are phenotypically and genetically heterogeneous but are traditionally divided into prenatally lethal and nonlethal (Escobar) types. To elucidate the pathogenesis of MPS, we undertook a genomewide linkage scan of a large consanguineous family and mapped a locus to 2q36-37. We then identified germline-inactivating mutations in the embryonal acetylcholine receptor gamma subunit (CHRNG) in families with both lethal and nonlethal MPSs. These findings extend the role of acetylcholine receptor dysfunction in human disease and provide new insights into the pathogenesis and management of fetal akinesia syndromes.

Molecular architecture of the neuromuscular junction

The neuromuscular junction (NMJ) is a complex structure that serves to efficiently communicate the electrical impulse from the motor neuron to the skeletal muscle to signal contraction. Over the last 200 years, technological advances in microscopy allowed visualization of the existence of a gap between the motor neuron and skeletal muscle that necessitated the existence of a messenger, which proved to be acetylcholine. Ultrastructural analysis identified vesicles in the presynaptic nerve terminal, which provided a beautiful structural correlate for the quantal nature of neuromuscular transmission, and the imaging of synaptic folds on the muscle surface demonstrated that specializations of the underlying protein scaffold were required. Molecular analysis in the last 20 years has confirmed the preferential expression of synaptic proteins, which is guided by a precise developmental program and maintained by signals from nerve. Although often overlooked, the Schwann cell that caps the NMJ and the basal lamina is proving to be critical in maintenance of the junction. Genetic and autoimmune disorders are known that compromise neuromuscular transmission and provide further insights into the complexities of NMJ function as well as the subtle differences that exist among NMJ that may underlie the differential susceptibility of muscle groups to neuromuscular transmission diseases. In this review we summarize the synaptic physiology, architecture, and variations in synaptic structure among muscle types. The important roles of specific signaling pathways involved in NMJ development and acetylcholine receptor (AChR) clustering are reviewed. Finally, genetic and autoimmune disorders and their effects on NMJ architecture and neuromuscular transmission are examined.

Increased expression of rapsyn in muscles prevents acetylcholine receptor loss in experimental autoimmune myasthenia gravis

Myasthenia gravis is usually caused by autoantibodies to the acetylcholine receptor (AChR). The AChR is clustered and anchored in the postsynaptic membrane of the neuromuscular junction (NMJ) by a cytoplasmic protein called rapsyn. We previously showed that resistance to experimental autoimmune myasthenia gravis (EAMG) in aged rats correlates with increased rapsyn concentration at the NMJ. It is possible, therefore, that endogenous rapsyn expression may be an important determinant of AChR loss and neuromuscular transmission failure in the human disease, and that upregulation of rapsyn expression could be used therapeutically. To examine first a potential therapeutic application of rapsyn upregulation, we induced acute EAMG in young rats by passive transfer of AChR antibody, mAb 35, and used in vivo electroporation to over-express rapsyn unilaterally in one tibialis anterior. We looked at the compound muscle action potentials (CMAPs) in the tibialis anterior, at rapsyn and AChR expression by quantitative radioimmunoassay and immunofluorescence, and at the morphology of the NMJs, comparing the electroporated and untreated muscles, as well as the control and EAMG rats. In control rats, transfected muscle fibres had extrasynaptic rapsyn aggregates, as well as slightly increased rapsyn and AChR concentrations at the NMJ. In EAMG rats, despite deposits of the membrane attack complex, the rapsyn-overexpressing muscles showed no decrement in the CMAPs, no loss of AChR, and the majority had normal postsynaptic folds, whereas endplates of untreated muscles showed typical AChR loss and morphological damage. These data suggest not only that increasing rapsyn expression could be a potential treatment for selected muscles of myasthenia gravis patients, but also lend support to the hypothesis that individual differences in innate rapsyn expression could be a factor in determining disease severity.

Congenital myasthenic syndromes

PURPOSE OF REVIEW

Congenital myasthenic syndromes are a heterogeneous group of diseases caused by genetic defects affecting neuromuscular transmission. In this article, a strategy that leads to the diagnosis of congenital myasthenic syndromes is presented, and recent advances in the clinical, genetic and molecular aspects of congenital myasthenic syndrome are outlined.

RECENT FINDINGS

Besides the identification of new mutations in genes already known to be implicated in congenital myasthenic syndromes (genes for the acetylcholine receptor subunits and the collagen tail of acetylcholinesterase), mutations in other genes have more recently been discovered and characterized (genes for choline acetyltransferase, rapsyn, and the muscle sodium channel SCN4A). Fluoxetine has recently been proposed as an alternative treatment for 'slow channel' congenital myasthenic syndrome.

SUMMARY

The characterization of congenital myasthenic syndromes comprises two complementary steps: establishing the diagnosis and identifying the pathophysiological type of congenital myasthenic syndrome. Characterization of the type of congenital myasthenic syndrome has allowed it to be classified as caused by presynaptic, synaptic and postsynaptic defects. A clinically and muscle histopathologically oriented genetic study has identified several genes in which mutations cause the disease. Despite comprehensive characterization, the phenotypic expression of one given gene involved is variable, and the aetiology of many congenital myasthenic syndromes remains to be discovered.

Deletion of N-terminal rapsyn domains disrupts clustering and has dominant negative effects on clustering of full-length rapsyn

The peripheral muscle membrane protein rapsyn is essential for the formation and maintenance of high density acetylcholine receptor aggregates at the neuromuscular synapse. Rapsyn is concentrated at synaptic sites and is colocalized with acetylcholine receptors from the earliest stages of synaptogenesis. Previous studies have shown that recombinant rapsyn expressed in heterologous cells forms clusters, and acetylcholine receptors coexpressed with rapsyn are colocalized with rapsyn clusters. However, the molecular interactions involved in clustering of rapsyn are not well defined. To analyze the process of cluster formation by rapsyn we examined the formation of rapsyn clusters and complexes using mutant constructs specifically deleted for individual domains of rapsyn in the presence and absence of tagged, full-length rapsyn. Specific deletions of the tetratricopeptide repeat (TPR) domains 1 and 3 of rapsyn abrogated not only clustering of mutant rapsyns, but also, in a dominant negative fashion, the clustering of tagged, full-length rapsyn. We also analyzed rapsyn protein complexes isolated from cells transfected with tagged and untagged rapsyn. Our results show that both tagged and untagged rapsyn are present in immunoprecipitates of rapsyn from cotransfected cells, demonstrating that rapsyn molecules interact directly or indirectly to form oligomers. Mutants that were dominant negatives were also present in complexes containing tagged, full-length rapsyn. Together these results indicate that rapsyn forms clusters at the synapse by oligomerization, and suggest models for the mechanistic bases of this oligomerization via interactions mediated by TPRs 1 and 3.

A newly identified chromosomal microdeletion of the rapsyn gene causes a congenital myasthenic syndrome

The objective is mutation analysis of the RAPSN gene in a patient with sporadic congenital myasthenic syndrome (CMS). Mutations in various genes encoding proteins expressed at the neuromuscular junction may cause CMS. Most mutations affect the epsilon subunit gene of the acetylcholine receptor (AChR) leading to endplate AChR deficiency. Recently, mutations in the RAPSN gene have been identified in several CMS patients with AChR deficiency. In most patients, RAPSN N88K was identified, either homozygously or heteroallelic to a second missense mutation. A sporadic CMS patient from Germany was analyzed for RAPSN mutations by RFLP, long-range PCR and sequence analysis. Clinically, the patient presents with an early onset CMS, associated with arthrogryposis multiplex congenita, recurrent episodes of respiratory insufficiency provoked by infections, and a moderate general weakness, responsive to anticholinesterase treatment. The mutation RAPSN N88K was found heterozygously to a large deletion of about 4.5 kb disrupting the RAPSN gene. Interestingly, an Alu-mediated unequal homologous recombination may have caused the deletion. We hypothesize that numerous interspersed Alu elements may predispose the RAPSN locus for genetic rearrangements.

Distinct phenotypes of congenital acetylcholine receptor deficiency

We contrast the phenotypes associated with hereditary acetylcholine receptor deficiency arising from mutations in either the acetylcholine receptor epsilon subunit or the endplate acetylcholine receptor clustering protein rapsyn. Mutational screening was performed by amplification of promoter and coding regions by PCR and direct DNA sequencing. We identified mutations in 37 acetylcholine receptor deficiency patients; 18 had acetylcholine receptor-epsilon mutations, 19 had rapsyn mutations. Mutated acetylcholine receptor-epsilon associated with bulbar symptoms, ptosis and ophthalmoplegia at birth, and generalized weakness. Mutated rapsyn caused either an early onset (rapsyn-EO) or late onset (rapsyn-LO) phenotype. Rapsyn-EO associated with arthrogryposis and life-threatening exacerbations during early childhood. Rapsyn-LO presented with limb weakness in adolescence or adulthood resembling seronegative myasthenia gravis. Awareness of distinct phenotypic features of acetylcholine receptor deficiency resulting from acetylcholine receptor-epsilon or rapsyn mutations should facilitate targeted genetic diagnosis, avoid inappropriate immunological therapy and, in some infants, prompt the rapid introduction of treatment that could be life saving.

Common founder effect of rapsyn N88K studied using intragenic markers

Mutations in the human gene encoding rapsyn have been linked to a recessive form of postsynaptic congenital myasthenic syndrome due to deficient clustering of acetylcholine receptors at the endplate. All patients reported to date carry the N88K mutation, suggesting a possible common founder effect. To decrease the likelihood of a recombination event occurring within the span of neighboring microsatellite markers, we used seven intragenic single nucleotide polymorphisms (SNPs) spanning 8 kb to characterize the haplotype associated with N88K. In three affected N88K homozygous individuals, we identified a common haplotype present in all heterozygous carriers of N88K. Of note, in two asymptomatic N88K homozygous individuals, a second haplotype was present that differed at three SNP sites downstream from the N88K mutation. Our findings of a common haplotype associated with the N88K mutation support a founder effect. The discordant haplotype found in homozygous individuals suggests that recombination events may have occurred within the rapsyn gene and that this may have implications in the phenotypic expression of the disease.

Mutation in the AChR ion channel gate underlies a fast channel congenital myasthenic syndrome

BACKGROUND

Most congenital myasthenic syndromes (CMS) have postsynaptic defects from mutations within the muscle acetylcholine receptor (AChR). Mutations underlying the slow channel syndrome cause a "gain of function" and usually show dominant inheritance, whereas mutations underlying AChR deficiency or the fast channel syndrome cause a "loss of function" and show recessive inheritance.

OBJECTIVE

To characterize the disease mechanism underlying an apparently dominantly inherited CMS that responds to IV edrophonium.

METHODS

DNA from CMS patients was analyzed for mutations by single-strand conformation polymorphism analysis, DNA sequence analysis, and restriction endonuclease digestion. Functional analysis of mutations was by alpha-bungarotoxin binding studies and by patch clamp analysis of mutant AChR expressed in human embryonic kidney cells.

RESULTS

Analysis of muscle biopsies from father and son in an affected kinship showed normal endplate morphology and AChR number but severely reduced miniature endplate potentials. DNA analysis revealed that each harbors a single missense mutation in the AChR alpha-subunit gene, alphaF256L. Expression studies demonstrate this mutation underlies a fast channel phenotype with fewer and shorter ion channel activations. The major effect of alphaF256L, located within the M2 transmembrane domain, is on channel gating, both reducing the opening and increasing the closure rate.

CONCLUSIONS

Mutation alphaF256L results in fast channel kinetics. Expression studies suggest a dominant-negative effect within the AChR pentamer, severely compromising receptor function.