This Article  • Abstract  • PDF  • Reply to article  • Send to a friend  • Save in My Folder  • Save to citation manager  • Permissions  Citing Articles  • Citation map  • Citing articles on HighWire  • Citing articles on Web of Science (53)  • Contact me when this article is cited  Related Content  • Similar articles in this journal  Topic Collections  • Neurology  • Neurology, Other  • Alert me on articles by topic  Social Bookmarking   What's this?

Recent Advances

Andrew G. Engel, MD; Kinji Ohno, MD, PhD; Steven M. Sine, PhD

Arch Neurol. 1999;56:163-167.

ABSTRACT
Congenital myasthenic syndromes (CMS) can arise from presynaptic, synaptic, or postsynaptic defects. Mutations of the acetylcholine receptor (AChR) that increase or decrease the synaptic response to acetylcholine (ACh) are a common cause of the postsynaptic CMS. An increased response occurs in the slow-channel syndromes. Here, dominant mutations in different AChR subunits and in different domains of the subunits prolong the activation episodes of AChR by either delaying channel closure or increasing the affinity of AChR for ACh. A decreased synaptic response to ACh occurs with recessive, loss-of-function mutations. Missense mutations in the low-affinity, fast-channel syndrome and in a disorder associated with mode-switching kinetics of AChR result in brief activation episodes and reduce the probability of channel opening. Mutations causing premature termination of the translational chain or missense mutations preventing the assembly or glycosylation of AChR curtail the expression of AChR. These mutations are concentrated in the subunit, probably because substitution of the fetal for the adult subunit can rescue humans from fatal null mutations in . Recent molecular genetic studies have also elucidated the pathogenesis of the CMS caused by absence of the asymmetric form of acetylcholinesterase from the synaptic basal lamina. Endplate acetylcholinesterase deficiency is now known to be caused by mutations in the collagenic tail subunit of the asymmetric enzyme that prevents the association of the collagenic tail subunit with the catalytic subunit or its insertion into the basal lamina.

INTRODUCTION

Jump to Section  • Top  • Introduction  • Mutations in achr subunits...  • Endplate ache deficiency arises...  • Future prospects  • Author information  • References

Congenital myasthenic syndromes (CMS) are heterogeneous disorders arising from presynaptic, synaptic, or postsynaptic defects. In each CMS, the specific defect compromises the safety margin of neuromuscular transmission by one or more mechanisms. The clinical phenotypes of CMS are often similar; therefore, precise diagnosis requires correlation of clinical, in vitro electrophysiological, morphological, and, whenever possible, molecular genetic studies.¹

Prior to 1990, the investigations involving patients with CMS were based on clinical, morphologic, and conventional microelectrode studies. Since then, four developments have paved the way for molecular analysis of CMS. First, by 1993, the complementary DNA sequences of the , , , and subunits of adult and of the subunit of fetal human acetylcholine receptor (AChR) were known, allowing molecular genetic analysis. Second, in the early 1990s, Milone et al² succeeded in patch-clamping endplates in human intercostal muscles, permitting analysis of the activity of single AChR channels. Third, the use of mammalian expression systems facilitated detailed analysis of how human AChR mutants alter the kinetics of the AChR channel. Coincident with this, we¹ hypothesized that a kinetic abnormality of AChR at the single-channel level predicts, and that severe endplate AChR deficiency may predict, one or more mutations in the subunits of AChR. This hypothesis was subsequently confirmed by the discovery of mutations in different subunits of AChR that either increase^3-7 or decrease^7-14 the synaptic response to acetylcholine (ACh). Fourth, the recent cloning of the catalytic^15-16 and collagenic tail (ColQ)¹⁷ subunits of asymmetric acetylcholinesterase (AChE) opened the door to identifying the genetic causes of endplate AChE deficiency.¹

MUTATIONS IN AChR SUBUNITS CAUSE POSTSYNAPTIC CMS

Jump to Section  • Top  • Introduction  • Mutations in achr subunits...  • Endplate ache deficiency arises...  • Future prospects  • Author information  • References

Since 1994, we and other investigators have identified 56 AChR subunit gene mutations in 69 CMS kinships. Table 1 indicates the identified mutations according to their functional consequences and subunit locations. It includes 34 published and 17 unpublished CMS mutations observed in our laboratories, three slow-channel mutations in the subunit¹⁸ and a frame-shifting rearrangement in the subunit described by Croxen et al,¹⁹ and a slow-channel mutation in the subunit detected by Gomez et al.²⁰ Interestingly, 38 of the 56 mutations and all 27 null mutations occur in the subunit of AChR, highlighting the susceptibility of the subunit gene to mutation.

View this table: [in this window] [in a new window] Fifty-six Acetylcholine Receptor Subunit Gene Mutations in 69 Kinships

Increased Response to ACh: Slow-Channel Mutations

The clues for the diagnosis of a slow-channel CMS consist of selectively severe weakness of the forearm extensor muscles, a repetitive compound muscle action potential response to single-nerve stimuli that is accentuated by edrophonium, and a prolonged and biexponentially decaying miniature endplate current. Eleven slow-channel CMS mutations have been reported to date.^{3-6,18, 20-23} The different mutations occur in different AChR subunits and in different functional domains of the subunits ( Figure 1, A). Each is dominant, causing a pathologic gain of function.

View larger version (61K): [in this window] [in a new window] A, Schematic diagrams of slow-channel congenital myasthenic syndrome mutations reported to date. The drawing on the left shows a section through the acetylcholine receptor lodged in a lipid bilayer. The extracellular part of the acetylcholine receptor forms a funnel-shaped channel that narrows to a gate formed by the helical M2 domain of each subunit. Two mutations in the extracellular domain near the acetylcholine binding site are indicated. The drawing on the right shows slow-channel mutations detected between the M2 and M3 domains of the subunit, in the M2 domains of the , , and subunits, and in the M1 domain of the subunit. B, Schematic diagram and list of 24 low-expressor or null mutations in the subunit.

The phenotypic consequences of the slow-channel CMS mutations stem from prolonged opening episodes of the AChR channel. These cause (1) cationic overloading of the junctional sarcoplasm and an endplate myopathy with loss of AChR from degenerating junctional folds and (2) a depolarization block due to staircase summation of prolonged endplate potentials.^3-5 Patch-clamp studies at the endplate, mutation analysis, and expression studies in human embryonic kidney fibroblast (HEK) cells dissected three types of slow-channel CMS. Those residing in the M2 domain, which lines the channel pore, act predominantly by slowing channel closure.^{3, 5-6} A mutation near the ACh binding site on the subunit increases affinity of AChR for ACh, causing repeated reopenings of the channel during the prolonged ACh occupancy.⁴ Another type of CMS has features of the two preceding types and the mutations reside in the M1 or M2 domain.^5-6,22

Recent studies indicate that quinidine, a long-lived open-channel blocker of AChR, is beneficial in the slow-channel CMS. Fukudome et al²⁴ demonstrated that drug levels attainable in clinical practice shorten and even normalize the prolonged opening episodes of mutant slow-channel CMS acetylcholine receptors expressed in HEK cells, and Harper and Engel²⁵ found that doses of the drug producing serum levels of 0.7 to 2.5 µg/mL (2.1-7.7 µmol/L) benefit patients with slow-channel CMS by clinical and electromyographic criteria.

Decreased Response to ACh: The Low-Affinity, Fast-Channel Mutations

Mutation analysis in two patients revealed two mutations in different alleles of the subunit gene: a common P121L mutation plus a null mutation in the second allele, so that P121L defined the clinical phenotype. In these patients, the postsynaptic response to ACh is markedly diminished, although the number of AChR per endplate is normal.^{8, 26} Patch-clamp studies show infrequent AChR channel events, abnormally brief activation episodes due to diminished channel reopenings during ACh occupancy, and increased resistance to desensitization by ACh.⁸ Genetically engineered P121L-AChR expressed in HEK cells has a markedly decreased rate of channel opening and shows greatly reduced affinity for ACh in the open-channel and desensitized states.⁸

It is interesting to note that the P121L mutation and the slow-channel CMS mutations have opposite effects: slow-channel CMS mutations increase the duration of activation episodes, enhance ACh binding affinity, increase desensitization by ACh, and cause an endplate myopathy; P121L shortens the duration of activation episodes, reduces ACh binding affinity, decreases desensitization by ACh, and leaves no anatomical footprint.

Recently we encountered a second low-affinity, fast-channel mutation, V285I, combined with a low-expressor mutation, F233V, in the other allele.²⁷ Detailed kinetic studies of V285I are in progress.

Decreased Response to ACh: A Mutation Causing Mode-Switching Kinetics

In this disorder, an in-frame duplication in the long cytoplasmic loop of , 1254ins18, appears in combination with a cysteine-loop null mutation, C128S.¹³ The 1254ins18 mutation, which determines the phenotype, causes mode switching in the kinetics of receptor activation in which the normal high efficiency of gating is accompanied by two new modes that gate inefficiently. In the two abnormal modes the channel opens more slowly and closes more rapidly than normal. The 1245ins18 AChR at the endplate shows abnormally brief activation episodes during steady-state agonist application, and appears electrically silent during the synaptic response to ACh. The phenotypic consequences are endplate AChR deficiency, simplification of the postsynaptic region, and compensatory expression of fetal AChR that restores electrical activity at the endplate and rescues the phenotype.

AChR Deficiency Caused by Recessive Mutations in AChR Subunits

Severe endplate AChR deficiency can result from different types of recessive mutations in AChR subunit genes. The mutations are either homozygous or, more frequently, heterozygous. Morphologic studies show an increased number of endplate regions distributed over an increased span of the muscle fiber. The integrity of the junctional folds is preserved, but some endplate regions are simplified and smaller than normal. The distribution of AChR on the junctional folds is patchy and the density of the reaction for AChR is attenuated. Conventional microelectrode studies reveal a decreased amplitude of the miniature endplate potentials and currents, and frequently high or higher than normal quantal release by nerve impulse. Single-channel recordings at the endplate^{7, 10, 13} or immunocytochemical studies¹² often reveal the presence of fetal AChR that harbors the (-AChR) instead of the adult subunit at the endplate.

Different types of recessive mutations causing severe endplate AChR deficiency have now been identified ( Figure 1, B): (1) Mutations causing premature termination of the translational chain—these mutations are frame shifting,^{7, 11-12,14, 28} occur at a splice site,^{9, 14, 28} or produce a stop codon directly.⁷ (2) Missense mutation in a signal peptide region (G-8R).⁸ (3) Missense mutations in residues essential for assembly of the pentameric receptor—mutations of this type were observed in the subunit at an N-glycosylation site (S143L),⁸ in cysteine 128 (C128S), a residue that is an essential part of the C128-C142 disulfide loop in the extracellular domain,¹³ and in arginine 147 (R147L) in the extracellular domain, which lies between isoleucine 145 and threonine 150, residues that contribute to subunit assembly.⁷ (4) Missense mutations affecting both AChR expression and kinetics. For example, R311W⁷ and 1254ins18¹³ in the long cytoplasmic loop between M3 and M4 decrease, whereas P245L in the M1 domain⁷ increases the open duration of channel events. In the case of R311W and P245L, the kinetic consequences are modest and are likely overshadowed by the reduced expression of the mutant gene.

There are two possible reasons that recessive mutations causing AChR deficiency are concentrated in the subunit. First, expression of the fetal type subunit, although at a low level, may compensate for absence of the subunit,^{7, 12-13} whereas patients harboring null mutations in subunits other than might not survive for lack of a substituting subunit. Second, the gene encoding the subunit, and especially the exons coding for the long cytoplasmic loop, have a high GC content, which likely predisposes to DNA rearrangements.

ENDPLATE AChE DEFICIENCY ARISES FROM MUTATIONS IN THE ColQ SUBUNIT OF ASYMMETRIC AChE

Jump to Section  • Top  • Introduction  • Mutations in achr subunits...  • Endplate ache deficiency arises...  • Future prospects  • Author information  • References

In skeletal muscle, AChE exists in homomeric globular forms of type T catalytic subunits (ACHE_T) and heteromeric asymmetric forms composed of 1, 2, or 3 tetrameric ACHE_T attached to a collagenic tail (ColQ). Asymmetric AChE is concentrated at the endplate where its ColQ anchors it into the basal lamina. The ACHET gene has been cloned in humans^15-16; COLQ complementary DNA has been cloned in Torpedo²⁹ and rodents,³⁰ but not in humans. In endplate AChE deficiency (endplate AD), the normal asymmetric species of AChE are absent from muscle.³¹ Endplate AD could stem from a defect that prevents binding of ColQ to ACHE_T or the insertion of ColQ into the basal lamina. In recent studies of six patients with endplate AD, Ohno and coworkers¹⁷ found no mutations in ACHET. They therefore cloned human COLQ complementary DNA, determined the genomic structure and chromosomal localization of COLQ, and then searched for mutations in this gene. Their search revealed six recessive truncation mutations of COLQ in the six patients. Coexpression of each COLQ mutant with wild-type ACHE_T in fibroblasts showed that a mutation proximal to the ColQ attachment domain for ACHET prevents association of ColQ with ACHE_T; mutations distal to the attachment domain generate a mutant species of AChE composed of one ACHE_T tetramer and a truncated ColQ strand. The mutant species lack part of the collagen domain and the entire C-terminal domain of ColQ, or only the C terminal domain of ColQ that is required for formation of the triple collagen helix, and this likely prevents their insertion into the basal lamina. Additional observations³² indicate that endplate AD can also arise from missense mutations in COLQ. Finally, rare cases of endplate AD could stem from defects in the basal lamina that prevent the binding of ColQ.

FUTURE PROSPECTS

Jump to Section  • Top  • Introduction  • Mutations in achr subunits...  • Endplate ache deficiency arises...  • Future prospects  • Author information  • References

Since 1994, molecular analysis of the CMS has provided clear insights into disease mechanisms and highlighted functionally significant domains of AChR and AChE. In the coming years molecular studies will undoubtedly be applied to presynaptic CMS, like those that alter the release of ACh quanta by nerve impulse or those that prevent the filling of synaptic vesicles with ACh. It is also likely that the molecular studies will provide clues for conventional and gene therapy and lead to the identification of novel CMS.

AUTHOR INFORMATION

Jump to Section  • Top  • Introduction  • Mutations in achr subunits...  • Endplate ache deficiency arises...  • Future prospects  • Author information  • References

Accepted for publication April 24, 1998.

Work in the authors' laboratories was supported by grants NS6277 (Dr Engel) and NS31744 (Dr Sine) from the National Institutes of Health, Bethesda, Md, and a research grant from the Muscular Dystrophy Association, Tucson, Ariz (Dr Engel).

Reprints: A. G. Engel, MD, Mayo Clinic, 200 First St SW, Rochester, MN 55905.

From the Department of Neurology and Neuromuscular Disease Laboratory (Drs Engel and Ohno) and the Receptor Biology Laboratory, Department of Physiology and Biophysics (Dr Sine), Mayo Clinic and Mayo Foundation, Rochester, Minn.

REFERENCES

Jump to Section  • Top  • Introduction  • Mutations in achr subunits...  • Endplate ache deficiency arises...  • Future prospects  • Author information  • References

1. Engel AG. Myasthenic syndromes. In: Engel AG, Franzini-Armstrong C, eds. Myology: Basic and Clinical. 2nd ed. New York, NY: McGraw-Hill; 1994:1798-1835. 2. Milone M, Hutchinson DO, Engel AG. Patch-clamp analysis of the properties of acetylcholine receptor channels at the normal human endplate. Muscle Nerve. 1994;17:1364-1369. FULL TEXT | WEB OF SCIENCE | PUBMED 3. Ohno K, Hutchinson DO, Milone M, et al. Congenital myasthenic syndrome caused by prolonged acetylcholine receptor channel openings due to a mutation in the M2 domain of the subunit. Proc Natl Acad Sci U S A. 1995;92:758-762. FREE FULL TEXT 4. Sine SM, Ohno K, Bouzat C, et al. Mutation of the acetylcholine receptor subunit causes a slow-channel myasthenic syndrome by enhancing agonist binding affinity. Neuron. 1995;15:229-239. FULL TEXT | WEB OF SCIENCE | PUBMED 5. Engel AG, Ohno K, Milone M, et al. New mutations in acetylcholine receptor subunit genes reveal heterogeneity in the slow-channel congenital myasthenic syndrome. Hum Mol Genet. 1996;5:1217-1227. FREE FULL TEXT 6. Milone M, Wang H-L, Ohno K, et al. Slow-channel syndrome caused by enhanced activation, desensitization, and agonist binding affinity due to mutation in the M2 domain of the acetylcholine receptor alpha subunit. J Neurosci. 1997;17:5651-5665. FREE FULL TEXT 7. Ohno K, Quiram P, Milone M, et al. Congenital myasthenic syndromes due to heteroallelic nonsense/missense mutations in the acetylcholine receptor subunit gene: identification and functional characterization of six new mutations. Hum Mol Genet. 1997;6:753-766. FREE FULL TEXT 8. Ohno K, Wang H-L, Milone M, et al. Congenital myasthenic syndrome caused by decreased agonist binding affinity due to a mutation in the acetylcholine receptor subunit. Neuron. 1996;17:157-170. FULL TEXT | WEB OF SCIENCE | PUBMED 9. Ohno K, Engel AG, Milone M, et al. A congenital myasthenic syndrome with severe acetylcholine receptor deficiency caused by heteroallelic frameshifting mutations in the epsilon subunit [abstract]. Neurology. 1995;45(suppl 4):A283. 10. Milone M, Ohno K, Pruitt JN, et al. Congenital myasthenic syndrome due to frameshifting acetylcholine receptor epsilon subunit mutation [abstract]. Soc Neurosci Abstr. 1996;22:1942. 11. Ohno K, Fukudome T, Nakano S, et al. Mutational analysis in a congenital myasthenic syndrome reveals a novel acetylcholine receptor epsilon subunit mutation [abstract]. Soc Neurosci Abstr. 1996;22:234. 12. Engel AG, Ohno K, Bouzat C, Sine SM, Griggs RG. End-plate acetylcholine receptor deficiency due to nonsense mutations in the subunit. Ann Neurol. 1996;40:810-817. FULL TEXT | WEB OF SCIENCE | PUBMED 13. Milone M, Wang H-L, Ohno K, et al. Mode switching kinetics produced by a naturally occurring mutation in the cytoplasmic loop of the human acetylcholine receptor subunit. Neuron. 1998;20:575-588. FULL TEXT | WEB OF SCIENCE | PUBMED 14. Ohno K, Anlar B, Özdirim E, et al. Myasthenic syndromes in Turkish kinships due to mutations in the acetylcholine receptor. Ann Neurol. 1998;44:234-241. FULL TEXT | WEB OF SCIENCE | PUBMED 15. Soreq H, Ben-Azis R, Prody CA, et al. Molecular cloning and construction of the coding region for human acetylcholinesterase reveals a G+C-rich attenuating structure. Proc Natl Acad Sci U S A. 1990;87:9688-9692. FREE FULL TEXT 16. Li Y, Camp S, Rachinsky TL, Getman D, Taylor P. Gene structure of mammalian acetylcholinesterase: alternative exons dictate tissue specific expression. J Biol Chem. 1991;266:23083-23090. FREE FULL TEXT 17. Ohno K, Brengman JM, Tsujino A, Engel AG. Human endplate acetylcholinesterase deficiency caused by mutations in the collagen-like tail subunit (ColQ) of the asymmetric enzyme. Proc Natl Acad Sci U S A. 1998;95:9654-9659. FREE FULL TEXT 18. Croxen R, Newland C, Beeson D, et al. Mutations in different functional domains of the human muscle acetylcholine receptor subunit in patients with the slow-channel congenital myasthenic syndrome. Hum Mol Genet. 1997;6:767-773. FREE FULL TEXT 19. Croxen R, Beeson D, Vincent A, Newsom-Davis J. Congenital myasthenic syndrome with a single nucleotide deletion at the intron/exon boundary in exon 12 of the gene encoding the acetylcholine receptor subunit [abstract]. Ann Neurol. 1996;40:513. 20. Gomez CM, Maselli R, Gammack J, et al. A beta-subunit mutation in the acetylcholine receptor gate causes severe slow-channel syndrome. Ann Neurol. 1996;39:712-723. FULL TEXT | WEB OF SCIENCE | PUBMED 21. Ohno K, Hutchinson DO, Milone M, et al. Molecular genetic basis of a slow channel syndrome [abstract]. Muscle Nerve. 1995;18:463. FULL TEXT 22. Wang H-L, Auerbach A, Bren N, et al. Mutation in the M1 domain of the acetylcholine receptor alpha subunit decreases the rate of agonist dissociation. J Gen Physiol. 1997;109:757-766. FREE FULL TEXT 23. Ohno K, Milone M, Brengman JM, et al. Slow-channel congenital myasthenic syndrome caused by a novel mutation in the acetylcholine receptor subunit [abstract]. Neurology. 1998;50:A432. 24. Fukudome T, Ohno K, Brengman JM, Engel AG. Quinidine normalizes the open duration of slow-channel mutants of the acetylcholine receptor. Neuroreport. 1998;9:1907-1911. WEB OF SCIENCE | PUBMED 25. Harper CM, Engel AG. Quinidine sulfate therapy for the slow-channel congenital myasthenic syndrome. Ann Neurol. 1998;43:480-484. FULL TEXT | WEB OF SCIENCE | PUBMED 26. Uchitel O, Engel AG, Walls TJ, et al. Congenital myasthenic syndromes, II: a syndrome attributed to abnormal interaction of acetylcholine with its receptor. Muscle Nerve. 1993;16:1293-1301. FULL TEXT | WEB OF SCIENCE | PUBMED 27. Milone M, Ohno K, Brengman JM, et al. Low-affinity fast-channel congenital myasthenic syndrome caused by new missense mutations in the acetylcholine receptor subunit [abstract]. Neurology. 1998;50:A432-A433. 28. Middleton LF, Ohno K, Christodoulou K, et al. Congenital myasthenic syndromes linked to chromosome 17p are caused by defects in acetylcholine receptor subunit gene [abstract]. Neurology. 1998;50:A432. 29. Krejci E, Coussen F, Duval N, et al. Primary structure of a collagenic tail peptide of Torpedo acetylcholinesterase: co-expression with catalytic subunit induces the production of collagen-tailed forms in transfected cells. EMBO J. 1991;10:1285-1293. WEB OF SCIENCE | PUBMED 30. Krejci E, Thomine S, Boschetti N, et al. The mammalian gene of acetylcholinesterase-associated collagen. J Biol Chem. 1997;272:22840-22847. FREE FULL TEXT 31. Engel AG, Lambert EH, Gomez MR. A new myasthenic syndrome with end-plate acetylcholinesterase deficiency, small nerve terminals, and reduced acetylcholine release. Ann Neurol. 1977;1:315-330. FULL TEXT | WEB OF SCIENCE | PUBMED 32. Ohno K, Brengman JM, Milone M, et al. Congenital endplate acetylcholinesterase deficiency: novel missense and null mutations in the collagen-like tail subunit of the asymmetric enzyme [abstract]. Am J Hum Genet. 1998;63:A377.

SECTION EDITOR: DAVID E. PLEASURE, MD

CiteULike    Connotea    Del.icio.us    Digg    Reddit    Technorati    Twitter     What's this?

THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES

The RNA-binding Protein HuR Binds to Acetylcholinesterase Transcripts and Regulates Their Expression in Differentiating Skeletal Muscle Cells
Deschenes-Furry et al.
J. Biol. Chem. 2005;280:25361-25368.
ABSTRACT | FULL TEXT  

Diagnosis of Congenital Myasthenic Syndrome
Millichap
AAP Grand Rounds 2004;11:54-54.
FULL TEXT  

Properties of the human muscle nicotinic receptor, and of the slow-channel myasthenic syndrome mutant {varepsilon}L221F, inferred from maximum likelihood fits
Hatton et al.
J. Physiol. 2003;547:729-760.
ABSTRACT | FULL TEXT  

Expression of foetal type acetylcholine receptor is restricted to type 1 muscle fibres in human neuromuscular disorders
Gattenlohner et al.
Brain 2002;125:1309-1319.
ABSTRACT | FULL TEXT  

A newly identified chromosomal microdeletion and an N-box mutation of the AChR{epsilon} gene cause a congenital myasthenic syndrome
Abicht et al.
Brain 2002;125:1005-1013.
ABSTRACT | FULL TEXT  

A common mutation ({epsilon}1267delG) in congenital myasthenic patients of Gypsy ethnic origin
Abicht et al.
Neurology 1999;53:1564-1564.
ABSTRACT | FULL TEXT  

Chromosome 17p-linked myasthenias stem from defects in the acetylcholine receptor {epsilon}-subunit gene
Middleton et al.
Neurology 1999;53:1076-1076.
ABSTRACT | FULL TEXT  

Recent Advances in Understanding Congenital Myasthenic Syndromes
JWatch Neurology 1999;1999:7-7.
FULL TEXT