Molecular characterization of congenital myasthenic syndromes in Spain

Congenital myasthenic syndromes (CMS) are a heterogeneous group of genetic disorders, all of which impair neuromuscular transmission. Epidemiological data and frequencies of gene mutations are scarce in the literature. Here we describe the molecular genetic and clinical findings of sixty-four genetically confirmed CMS patients from Spain. Thirty-six mutations in the CHRNE, RAPSN, COLQ, GFPT1, DOK7, CHRNG, GMPPB, CHAT, CHRNA1, and CHRNB1 genes were identified in our patients, with five of them not reported so far. These data provide an overview on the relative frequencies of the different CMS subtypes in a large Spanish population. CHRNE mutations are the most common cause of CMS in Spain, accounting for 27% of the total. The second most common are RAPSN mutations. We found a higher rate of GFPT1 mutations in comparison with other populations. Remarkably, several founder mutations made a large contribution to CMS in Spain: RAPSN c.264C > A (p.Asn88Lys), CHRNE c.130insG (Glu44Glyfs*3), CHRNE c.1353insG (p.Asn542Gluf*4), DOK7 c.1124_1127dup (p.Ala378Serfs*30), and particularly frequent in Spain in comparison with other populations, COLQ c.1289A > C (p.Tyr430Ser). Furthermore, we describe phenotypes and distinguishing clinical signs associated with the various CMS genes which might help to identify specific CMS subtypes to guide diagnosis and management.

RYR1-related congenital myopathy with fatigable weakness, responding to pyridostigimine

The spectrum of RYR1 mutation associated disease encompasses congenital myopathies, exercise induced rhabdomyolysis, malignant hyperthermia susceptibility and King-Denborough syndrome. We report the clinical phenotype of two siblings who presented in infancy with hypotonia and striking fatigable ptosis. Their response to pyridostigimine was striking, but genetic screening for congenital myasthenic syndromes was negative, prompting further evaluation. Muscle MRI was abnormal with a selective pattern of involvement evocative of RYR1-related myopathy. This directed sequencing of the RYR1 gene, which revealed two heterozygous c.6721C>T (p.Arg2241X) nonsense mutations and novel c.8888T>C (p.Leu2963Pro) mutations in both siblings. These cases broaden the RYR1-related disease spectrum to include a myasthenic-like phenotype, including partial response to pyridostigimine. RYR1-related myopathy should be considered in the presence of fatigable weakness especially if muscle imaging demonstrates structural abnormalities. Single fibre electromyography can also be helpful in cases like this.

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.

Ephedrine treatment in congenital myasthenic syndrome due to mutations in DOK7

BACKGROUND

Mutations in the postsynaptic adaptor protein Dok-7 underlie congenital myasthenic syndrome (CMS) with a characteristic limb girdle pattern of muscle weakness. Patients usually do not respond to or worsen with the standard CMS treatments: cholinesterase inhibitors and 3,4-diaminopyridine. However, anecdotal reports suggest they may improve with ephedrine.

METHODS

This was an open prospective follow-up study to determine muscle strength in response to ephedrine in Dok-7 CMS. Patients were first evaluated as inpatients for suitability for a trial of treatment with ephedrine. The response was assessed at 2 and 6 to 8 months follow-up clinic visits using a quantitative myasthenia gravis (severity) score (QMG) and mobility measures.

RESULTS

Ten out of 12 of the cohort with DOK7 mutations tolerated ephedrine. We noted a progressive response to treatment over the 6 to 8 months assessment period with a significant improvement at the final QMG score (p = 0.009). Mobility scores also improved (p = 0.0006). Improvements in the subcomponents of the QMG score that measured proximal muscle function (those muscle groups most severely affected) were most marked, and in some cases were dramatic. All patients reported enhanced activities of daily living at 6-8 months.

CONCLUSION

Ephedrine appears to be an effective treatment for Dok-7 CMS. It is well-tolerated by most patients and improvement in strength can be profound. Determining the long-term response and the most effective dosing regimen will require further research.

CLASSIFICATION OF EVIDENCE

This study provides Class IV evidence that ephedrine given at doses between 15 and 90 mg/day improves muscle strength in patients with documented mutations in DOK7.

A human recombinant autoantibody-based immunotoxin specific for the fetal acetylcholine receptor inhibits rhabdomyosarcoma growth in vitro and in a murine transplantation model

Rhabdomyosarcoma (RMS) is the most common malignant soft tissue tumor in children and is highly resistant to all forms of treatment currently available once metastasis or relapse has commenced. As it has recently been determined that the acetylcholine receptor (AChR) gamma-subunit, which defines the fetal AChR (fAChR) isoform, is almost exclusively expressed in RMS post partum, we recombinantly fused a single chain variable fragment (scFv) derived from a fully human anti-fAChR Fab-fragment to Pseudomonas exotoxin A to generate an anti-fAChR immunotoxin (scFv35-ETA). While scFv35-ETA had no damaging effect on fAChR-negative control cell lines, it killed human embryonic and alveolar RMS cell lines in vitro and delayed RMS development in a murine transplantation model. These results indicate that scFv35-ETA may be a valuable new therapeutic tool as well as a relevant step towards the development of a fully human immunotoxin directed against RMS. Moreover, as approximately 20% of metastatic malignant melanomas (MMs) display rhabdoid features including the expression of fAChR, the immunotoxin we developed may also prove to be of significant use in the treatment of these more common and most often fatal neoplasms.

Germline mutation in DOK7 associated with fetal akinesia deformation sequence

BACKGROUND

Fetal akinesia deformation sequence syndrome (FADS) is a heterogeneous disorder characterised by fetal akinesia and developmental defects including, in some case, pterygia. Multiple pterygium syndromes (MPS) are traditionally divided into prenatally lethal and non-lethal (such as Escobar) types. Previously, we and others reported that homozygous mutations in the fetal acetylcholine receptor gamma subunit (CHRNG) can cause both lethal and non-lethal MPS, demonstrating that pterygia resulted from fetal akinesia, and that mutations in the acetylcholine receptor subunits CHRNA1, CHRND, and Rapsyn (RAPSN) can also result in a MPS/FADS phenotype.

METHODS

We hypothesised that mutations in other acetylcholine receptor related genes may interfere with neurotransmission at the neuromuscular junction and so we analysed 14 cases of lethal MPS/FADS without CHRNG, CHRNA1, CHRNB1, CHRND, or RAPSN mutations for mutations in DOK7.

RESULTS

A homozygous DOK7 splice site mutation, c.331+1G>T, was identified in a family with three children affected with lethal FADS. Previously DOK7 mutations have been reported to underlie a congenital myaesthenic syndrome with a characteristic "limb girdle" pattern of muscle weakness.

CONCLUSION

This finding is consistent with the hypothesis that whereas incomplete loss of DOK7 function may cause congenital myasthenia, more severe loss of function can result in a lethal fetal akinesia phenotype.

Dok-7/MuSK signaling and a congenital myasthenic syndrome

Skeletal muscle contraction is controlled by motor neurons, which contact the muscle at the neuromuscular junction (NMJ). The formation and maintenance of the NMJ, which includes the aggregation of densely packed clusters of acetylcholine receptor (AChR) opposite the motor nerve terminal, is orchestrated by muscle-specific receptor tyrosine kinase, MuSK. Recently, a MuSK-interacting cytoplasmic adaptor-like protein Dok-7 was identified and its localization at the postsynaptic region of the NMJ was revealed. Mice lacking Dok-7 have a phenotype indistinguishable from MuSK-deficient mice, and fail to form both AChR clusters and NMJs. In cultured myotubes, Dok-7 is required for MuSK activation and AChR clustering. Thus, Dok-7 is essential for neuromuscular synaptogenesis and it appears that the regulatory interaction of Dok-7 with MuSK is integrally involved in this process. In humans there are both autoimmune and genetic causes of defective neuromuscular transmission that gives rise to the fatigable muscle weakness known as myasthenia. DOK7 has been found to be a major locus for mutations that underlie a genetic form of myasthenia with a characteristic 'limb girdle' pattern of muscle weakness (DOK7 CMS). Patients with DOK7 CMS have small, simplified NMJs but normal AChR function. The most common mutation causes a COOH-terminal truncation, which greatly impairs Dok-7's ability to activate MuSK. Recently, a series of differing DOK7 mutations have been identified, which affect not only the COOH-terminal region but also the NH2-terminal moiety. The study of these mutations may help understand the underlying pathogenic mechanism of DOK7 CMS.

Pre- and post-synaptic abnormalities associated with impaired neuromuscular transmission in a group of patients with 'limb-girdle myasthenia'

The properties of neuromuscular junctions (NMJs) were studied in motor-point biopsy samples from eight patients with congenital myasthenic syndromes affecting primarily proximal limb muscles ['limb-girdle myasthenia' (LGM)]. All had moderate to severe weakness of the proximal muscles, without short-term clinical fatigability but with marked variation in strength over periods of weeks or months, with little or no facial weakness or ptosis and no ophthalmoplegia. Most had a characteristic gait and stance. All patients showed decrement of the compound muscle action potential (CMAP) on repetitive stimulation at 3 Hz, and increased jitter and blocking was detected by SFEMG, confirming the presence of impaired neuromuscular transmission. None of the patients had serum antibodies against acetylcholine receptors (AChRs). Two of the patients had similarly affected siblings. Intracellular recording from isolated nerve-muscle preparations revealed that the quantal content (the number of ACh quanta released per nerve impulse) was only approximately 50% of that in controls. However, the quantal size (amplitude of miniature end-plate currents) and the kinetic properties of synaptic potentials and currents were similar to control values. The area of synaptic contact and extent of post-synaptic folding were approximately 50% of control values. Thus, the quantal content per unit area of synaptic contact was normal. The number of AChRs per NMJ was also reduced to approximately 50% of normal, so the local AChR density was normal. Immunolabelling studies revealed qualitatively normal distributions and abundance of each of 14 proteins normally concentrated at the NMJ, including components of the basal lamina, post-synaptic membrane and post-synaptic cytoskeleton. DNA analysis failed to detect mutations in the genes encoding any of the following proteins: AChR subunits, rapsyn, ColQ, ChAT or muscle-specific kinase. Response of these patients to treatment was varied: few showed long-term improvement with pyridostigmine and some even deteriorated with treatments, while others had intolerable side-effects. Several patients showed improvement with 3,4-diaminopyridine, but this was generally only transient. Ephedrine was helpful in half of the patients. We conclude that impaired neuromuscular transmission in these LGM patients results from structural abnormalities of the NMJ, including reduced size and post-synaptic folding, rather from any abnormality in the immediate events of neuromuscular transmission.

Alpha7-acetylcholine receptor antibodies in two patients with Rasmussen encephalitis

Rasmussen encephalitis (RE) sera were screened for antibodies to human alpha7 nicotinic acetylcholine receptors (nAChRs) using electrophysiology, calcium imaging, and ligand binding assays. Sera from two of nine patients with RE blocked ACh-induced currents through alpha7 nAChRs and the ACh-induced rise in intracellular free calcium ([Ca2+]i) and inhibited (125)I-alpha-bungarotoxin binding in cells expressing alpha7 nAChRs. Thus, the alpha7 nAChR is a potential target for pathogenic antibodies in patients with RE.

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.

Absence of antibodies to glutamate receptor type 3 (GluR3) in Rasmussen encephalitis

OBJECTIVE

To determine the prevalence of serum antibodies to the ionotropic glutamate receptor 3 (GluR3) in patients with Rasmussen encephalitis (RE), a severe epileptic disorder, and to compare with serum from control subjects and patients with intractable epilepsy (IE).

METHODS

The authors looked for serum immunoglobulin (Ig) G antibodies to GluR3 in 30 patients with RE, including two patients who had plasma exchange and 12 who had been treated with IV Igs with varying results, and 49 patients with IE and 23 healthy individuals, using ELISA with GluR3B peptide, Western blot analysis of recombinant full-length GluR3, immunoprecipitation of [35S]- and [125I]-labeled GluR3 extracellular domains, immunohistochemistry on rat brain sections, and electrophysiology of GluR3 expressed in Xenopus oocytes.

RESULTS

Low levels of antibodies to the GluR3B peptide were detected using ELISA in only 4 of the 79 patients with epilepsy (2 with RE and 2 with IE); binding to GluR3B in other sera was shown to be nonspecific. One other patient with IE had antibodies to recombinant GluR3 on Western blot analysis. However, none of the sera tested precipitated either the [35S]- or the [125I]-labeled GluR3 domains; none bound to rat brain sections in a manner similar to rabbit antibodies to GluR3; and none of the nine sera tested affected the electrophysiologic function of GluR3.

CONCLUSIONS

GluR3 antibodies were only infrequently found in Rasmussen encephalitis or intractable epilepsy.

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

Rapsyn mutations in 16 unrelated patients with a congenital/hereditary myasthenic syndrome were identified, and a mutation (N88K) common to each of them was found. Two distinct phenotypes were noted: early and late onset. The former is frequently associated with arthrogryposis multiplex congenita and life-threatening crises. The late-onset phenotype developed in adolescence or adulthood and was initially mistaken for seronegative myasthenia gravis. Recognition of this late-onset phenotype should prevent inappropriate immunotherapy.

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.

Scenarios for autoimmunization of T and B cells in myasthenia gravis

We have studied responses in thymoma patients to interferon-alpha and to the acetylcholine receptor (AChR) in early-onset myasthenia gravis (EOMG), seeking clues to autoimmunizing mechanisms. Our new evidence implicates a two-step process: (step 1) professional antigen-presenting cells and thymic epithelial cells prime AChR-specific T cells; then (step 2) thymic myoid cells subsequently provoke germinal center formation in EOMG. Our unifying hypothesis proposes that AChR epitopes expressed by neoplastic or hyperplastic thymic epithelial cells aberrantly prime helper T cells, whether generated locally or infiltrating from the circulation. These helper T cells then induce antibody responses against linear epitopes that cross-react with whole AChR and attack myoid cells in the EOMG thymus. The resulting antigen-antibody complexes and the recruitment of professional antigen-presenting cells increase the exposure of thymic cells to the infiltrates and provoke local germinal center formation and determinant spreading. Both these and the consequently enhanced heterogeneity and pathogenicity of the autoantibodies should be minimized by early thymectomy.

Properties of the human muscle nicotinic receptor, and of the slow-channel myasthenic syndrome mutant epsilonL221F, inferred from maximum likelihood fits

The mechanisms that underlie activation of nicotinic receptors are investigated using human recombinant receptors, both wild type and receptors that contain the slow channel myasthenic syndrome mutation, epsilonL221F. The method uses the program HJCFIT, which fits the rate constants in a specified mechanism directly to a sequence of observed open and shut times by maximising the likelihood of the sequence with exact correction for missed events. A mechanism with two different binding sites was used. The rate constants that apply to the diliganded receptor (opening, shutting and total dissociation rates) were estimated robustly, being insensitive to the exact assumptions made during fitting, as expected from simulation studies. They are sufficient to predict the main physiological properties of the receptors. The epsilonL221F mutation causes an approximately 4-fold reduction in dissociation rate from diliganded receptors, and a smaller increase in opening rate and mean open time. These are sufficient to explain the approximately 6-fold slowing of decay of miniature synaptic currents seen in patients. The distinction between the two binding sites was less robust, the estimates of rate constants being dependent to some extent on assumptions, e.g. whether an extra short-lived shut state was included or whether the EC50 was constrained. The results suggest that the two binding sites differ by roughly 10-fold in the affinity of the shut receptor for ACh in the wild type, and that in the epsilonL221F mutation the lower affinity is increased so the sites become more similar.

Recessive inheritance and variable penetrance of slow-channel congenital myasthenic syndromes

BACKGROUND

Slow-channel congenital myasthenic syndromes (SCCMS) typically show dominant inheritance. They are caused by missense mutations within the subunits of muscle nicotinic acetylcholine receptors (AChR) that result in prolonged ion channel activations. SCCMS mutations within the AChR subunit are located in various functional domains, whereas fully described mutations in AChR non- subunits have, thus far, been located only in the M2 channel-lining domain. The authors identified and characterized two -subunit mutations, located outside M2, that underlie SCCMS in three kinships. In two of the three kinships, the syndrome showed an atypical inheritance pattern.

METHODS

These methods included clinical diagnosis, mutation detection, haplotype analysis, and functional expression studies using single-channel recordings of mutant AChR transiently transfected into HEK293 cells.

RESULTS

The authors identified two SCCMS mutations in the AChR subunit, L78P and L221F. Both mutations prolonged ACh-induced ion channel activations. L78P is present in a consanguineous family and appears to be pathogenic only when present on both alleles, and L221F shows variable penetrance in one of the two families that were identified harboring this mutation.

CONCLUSION

SCCMS mutations may show a recessive inheritance pattern and variable penetrance. A diagnosis of SCCMS should not be ruled out in cases of CMS with an apparent recessive inheritance pattern.

Myasthenia gravis: diagnostic and management dilemmas

This review focuses on the diagnostic tests that may help to confirm myasthenia in patients without acetylcholine receptor antibodies, including the newly discovered anti-muscle-specific receptor tyrosine kinase antibody and other investigations. We discuss management dilemmas, particularly those that revolve around treatments whose efficacy is questionable or unproved, such as thymectomy. We review the important issue of how treatments are assessed, and examine future treatment trial designs.

Human mitochondrial DNA deletions associated with mutations in the gene encoding Twinkle, a phage T7 gene 4-like protein localized in mitochondria

The gene products involved in mammalian mitochondrial DNA (mtDNA) maintenance and organization remain largely unknown. We report here a novel mitochondrial protein, Twinkle, with structural similarity to phage T7 gene 4 primase/helicase and other hexameric ring helicases. Twinkle colocalizes with mtDNA in mitochondrial nucleoids. Screening of the gene encoding Twinkle in individuals with autosomal dominant progressive external ophthalmoplegia (adPEO), associated with multiple mtDNA deletions, identified 11 different coding-region mutations co-segregating with the disorder in 12 adPEO pedigrees of various ethnic origins. The mutations cluster in a region of the protein proposed to be involved in subunit interactions. The function of Twinkle is inferred to be critical for lifetime maintenance of human mtDNA integrity.

End-plate gamma- and epsilon-subunit mRNA levels in AChR deficiency syndrome due to epsilon-subunit null mutations

Acetylcholine receptor (AChR) deficiency is the most common of the congenital myasthenic syndromes (CMS). Typically, the number of AChRs, measured by alpha-bungarotoxin binding, is reduced to 10-30% of normal levels, the miniature end-plate potentials are correspondingly reduced, and there are morphological changes at the motor end-plates. The majority of these syndromes are due to either missense or frameshift mutations within the gene encoding the adult-specific epsilon-subunit. These are often null mutations, but some mutant epsilon-subunits can be incorporated, at low levels, into functional AChRs in transfected cell lines. It is not clear, therefore, whether upregulation of the mutant epsilon-subunit mRNA could generate sufficient AChR to support neuromuscular transmission, albeit at a reduced level. Conversely, it might be that the mutant epsilon-subunit transcripts are subject to mRNA surveillance and 'nonsense-mediated' loss, leading to reduced epsilon-subunit mRNA expression. In either case, it is thought that neuromuscular transmission may be provided partly or entirely by incorporation of the foetal-specific gamma-subunit into end-plate AChR. gamma-Subunit mRNA is expressed at low levels in normal human muscle, but might be upregulated in CMS. The study of mRNA levels for AChR subunits should improve our understanding of genotype-phenotype relationships in CMS. Here we have defined homozygous epsilon-subunit mutations in four unrelated families with AChR deficiency and studied the steady-state levels of mRNA for AChR subunits at the motor end-plates by in situ hybridization. Although we demonstrated that each mutation would lead to almost complete absence of surface adult AChR expression, we detected similar robust expression of alpha- and epsilon-subunit mRNAs at end-plates of patient and control muscles, suggesting that mRNA transcripts for the epsilon-subunit are neither upregulated nor degraded preferentially. Interestingly, we were unable to detect any increase in gamma-subunit mRNA expression at CMS end-plates. Transgenic mice lacking the epsilon-subunit die 2-3 months after birth, suggesting that alpha(2)betadelta(2) pentamers cannot sustain neuromuscular transmission. Therefore, we tentatively conclude that the persistent low level expression of the gamma-subunit, which is present in normal human muscles as well as in AChR deficiency syndromes, is sufficient to enable patients with epsilon-subunit null alleles to survive.

Acetylcholine receptor delta subunit mutations underlie a fast-channel myasthenic syndrome and arthrogryposis multiplex congenita

Limitation of movement during fetal development may lead to multiple joint contractures in the neonate, termed arthrogryposis multiplex congenita. Neuromuscular disorders are among the many different causes of reduced fetal movement. Many congenital myasthenic syndromes (CMSs) are due to mutations of the adult-specific epsilon subunit of the acetylcholine receptor (AChR), and, thus, functional deficits do not arise until late in gestation. However, an earlier effect on the fetus might be predicted with some defects of other AChR subunits. We studied a child who presented at birth with joint contractures and was subsequently found to have a CMS. Mutational screening revealed heteroallelic mutation within the AChR delta subunit gene, delta 756ins2 and delta E59K. Expression studies demonstrate that delta 756ins2 is a null mutation. By contrast, both fetal and adult AChR containing delta E59K have shorter than normal channel activations that predict fast decay of endplate currents. Thus, delta E59K causes dysfunction of fetal as well as the adult AChR and would explain the presence of joint contractures on the basis of reduced fetal movement. This is the first report of the association of AChR gene mutations with arthrogryposis multiplex congenita. It is probable that mutations that severely disrupt function of fetal AChR will underlie additional cases.

Dominantly inherited familial myasthenia gravis as a separate genetic entity without involvement of defined candidate gene loci

Myasthenia gravis (MG) is a sporadic autoimmune disorder affecting neuromuscular transmission. Very rarely autoimmune myasthenia gravis may be inherited within a family. We present here the genetic analysis of a Hungarian family where nine members from two generations are affected by myasthenia gravis. Genetic characterisation of this unique Hungarian family using linkage analysis and mutation screening excludes the involvement of defined candidate gene loci. These findings point to familial MG as a separate genetic entity. Identification of the underlying genetic defect in this family may greatly enhance our understanding of the pathogenesis of myasthenia gravis.

Presynaptic neuronal antigens expressed by a small cell lung carcinoma cell line

Small cell lung carcinoma (SCLC) is a tumour of neuroendocrine origin often found in association with autoimmune paraneoplastic neurological disorders. We established a SCLC cell line from a woman with Lambert-Eaton myasthenic syndrome (LEMS) who developed antibodies to both the P/Q-type voltage-gated calcium channels (VGCC) and the muscle acetycholine receptor (AChR). We used a range of techniques to establish which neuronal antigens were expressed in her tumour cell line. The results show that many proteins involved in exocytosis are present in the SCLC cells, and that depolarisation-dependent release of [(3)H]-serotonin is linked to calcium influx through P/Q-type VGCCs. In addition, some of the subunits encoding the AChR and both agrin and ARIA, molecules released from the motor nerve during development, were expressed. These results suggest that many potential antigenic targets are present in SCLC, and indicate a surprising 'motor nerve terminal'-like characteristic of this line.

Molecular targets for autoimmune and genetic disorders of neuromuscular transmission

The neuromuscular junction is the target of a variety of autoimmune, neurotoxic and genetic disorders, most of which result in muscle weakness. Most of the diseases, and many neurotoxins, target the ion channels that are essential for neuromuscular transmission. Myasthenia gravis is an acquired autoimmune disease caused in the majority of patients by antibodies to the acetylcholine receptor, a ligand-gated ion channel. The antibodies lead to loss of acetylcholine receptor, reduced efficiency of neuromuscular transmission and muscle weakness and fatigue. Placental transfer of these antibodies in women with myasthenia can cause fetal or neonatal weakness and occasionally severe deformities. Lambert Eaton myasthenic syndrome and acquired neuromyotonia are caused by antibodies to voltage-gated calcium or potassium channels, respectively. In the rare acquired neuromyotonia, reduced repolarization of the nerve terminal leads to spontaneous and repetitive muscle activity. In each of these disorders, the antibodies are detected by immunoprecipitation of the relevant ion channel labelled with radioactive neurotoxins. Genetic disorders of neuromuscular transmission are due mainly to mutations in the genes for the acetylcholine receptor. These conditions show recessive or dominant inheritance and result in either loss of receptors or altered kinetics of acetylcholine receptor channel properties. Study of these conditions has greatly increased our understanding of synaptic function and of disease aetiology.

The conformation of the main immunogenic region on the alpha-subunit of muscle acetylcholine receptor is affected by neighboring receptor subunits

Myasthenia gravis (MG) is caused by autoantibodies to the acetylcholine receptor (AChR). Experiments with fetal (alpha(2)betagammadelta) and adult (alpha(2)betaepsilondelta) AChR and with recombinant subunit dimers showed that some monoclonal antibodies (mAbs) against the main immunogenic region (MIR), located on the alpha-subunit of the AChR, bind better to fetal AChR and to alphagamma subunit dimer than to adult AChR and alphaepsilon dimer and equally to both alphabeta and alphadelta. However, other anti-MIR mAbs prefer adult AChR and alphaepsilon dimer, bind well to alphabeta but weakly to alphadelta. These results suggest that the MIR conformation is affected by the neighboring gamma/epsilon- and delta-subunits and may contribute to understanding the antibody specificities in MG.

IgG from "seronegative" myasthenia gravis patients binds to a muscle cell line, TE671, but not to human acetylcholine receptor

Antibodies to acetylcholine receptor (AChR) are found in 85% of patients with myasthenia gravis (seropositive MG [SPMG]) and are thought to be pathogenic; but in 15% of MG patients, the standard immunoprecipitation test for anti-AChR is negative (seronegative MG [SNMG]). Here, we used a novel approach, fluorescence-activated cell sorting analysis, to measure binding of SPMG and SNMG IgG antibodies to rhabdomyosarcoma cell lines that express human adult (TE671-epsilon) or fetal (TE671-gamma) AChR, and to human embryonic kidney (HEK) fibroblasts that express adult human AChR (HEK-AChR). We found that whereas most SPMG antibodies bind to all three cell lines, IgG from 8 of 15 SNMG sera/plasmas bind to the surface of both TE671 cell lines but not to HEK-AChR cells. These results indicate that SNMG antibodies bind to a muscle surface antigen that is not the AChR, which strongly supports previous studies that suggest that SNMG should be considered a distinct subtype of MG.

Mapping of autosomal dominant progressive external ophthalmoplegia to a 7-cM critical region on 10q24

OBJECTIVE

To map the gene responsible for autosomal dominant progressive external opthalmoplegia.

BACKGROUND

The pathogenesis of progressive external ophthalmoplegia (PEO) can be associated with multiple deletions of mitochondrial DNA (mtDNA). PEO may show autosomal dominant (adPEO) or autosomal recessive (arPEO) patterns of inheritance, indicating that the genetic defect has a Mendelian basis and most likely involves a nuclear gene encoding a protein that interacts with the mitochondrial genome. adPEO is heterogeneous genetically, and thus far disease loci have been identified on chromosomes 3 and 10. The locus on chromosome 10q23-q25 was assigned by linkage analysis in a single Finnish family.

METHODS

Samples from a large Pakistani family with adPEO, in which clinical symptoms are bilateral ptosis, limitations of eye movements, and varying degrees of proximal muscle weakness, were collected. Muscle biopsy and mtDNA rearrangement analysis was used to confirm the diagnosis. Genomewide linkage analysis was set up using a set of 391 microsatellite markers.

RESULTS

The muscle biopsy from an affected member showed ragged red fibers, increased succinic dehydrogenase staining, lack of cytochrome oxidase activity, and multiple deletions of mtDNA. The disease locus was mapped to 10q23.31-q25.1 by linkage analysis, and a maximum lod score of 5.72 was obtained with D10S1267.

CONCLUSION

By analysis of meiotic recombinations in affected individuals, the critical region was restricted to the 7-cM interval between D10S198 and D10S1795.

Novel functional epsilon-subunit polypeptide generated by a single nucleotide deletion in acetylcholine receptor deficiency congenital myasthenic syndrome

Acetylcholine receptor (AChR) deficiency is a recessively inherited congenital myasthenic syndrome in which fatigable muscle weakness results from impaired neuromuscular transmission caused by reduced AChR numbers. In mature muscle, AChRs consist of alpha2 betadelta together with the adult-specific epsilon subunit. We have identified a deletion of the first nucleotide in exon 12 of the AChR epsilon-subunit gene (epsilon1267delG) and demonstrate its recessive inheritance segregates with disease in 6 unrelated cases of AChR deficiency. In addition, we found that both healthy and AChR-deficient muscle contain a population of AChR epsilon-subunit mRNA transcripts that retain intron 11. We investigated the possible consequences of combining this mutation with the alternative mRNA species through AChR expression studies in human embryonic kidney cells and Xenopus oocytes. Epsilon1267delG generates a polypeptide that lacks M4 and is not detected in surface AChR, whereas retention of intron 11 in the RNA transcript restores the reading frame, conserves M4, and generates a polypeptide that is incorporated into functional surface AChR, although at a reduced level, consistent with the disease phenotype. Our results indicate that for some AChR deficiency mutations located between M3 and M4, the retention of intron 11 in the epsilon-subunit mRNA transcripts may rescue adult AChR function.