Current understanding of congenital myasthenic syndromes

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Investigation of congenital myasthenic syndromes (CMSs) disclosed a diverse array of molecular targets at the motor endplate. Clinical, electrophysiologic and morphologic studies paved the way for detecting CMS-related mutations in proteins such as the acetylcholine receptor, acetylcholinesterase, choline acetyltransferase, rapsyn, MuSK and Nav1.4. Analysis of electrophysiologic and biochemical properties of mutant proteins expressed in heterologous systems contributed crucially to defining the molecular consequences of the observed mutations and resulted in improved therapy of different CMSs. Recent crystallography studies of choline acetyltransferase and homology structural models of the acetylcholine receptor are providing further clues to how point mutations alter protein function.

Introduction

Congenital myasthenic syndromes (CMSs) are a heterogeneous group of disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. At the normal neuromuscular junction, activation of acetylcholine receptors (AChRs) by acetylcholine (ACh) triggers an endplate potential (EPP) that activates voltage-dependent sodium channels of Nav1.4 type, giving rise to a propagated action potential. A high concentration of AChRs on crests of the synaptic folds [1] and of Nav1.4 in the depth of the folds [2, 3] ensures that excitation is propagated beyond the endplate (EP) [4]. The safety margin of neuromuscular transmission is a function of the difference between the depolarization caused by the EPP and the depolarization required to activate Nav1.4 channels. All CMSs identified so far have been traced to one or more factors that render the EPP subthreshold for activating Nav1.4 channels or a mutation involving Nav1.4 itself [5].

Factors determining the safety margin can be resolved into those that govern the number of ACh molecules per synaptic vesicle, those that affect quantal release of ACh, and those that determine the efficacy of individual quanta. Quantal efficacy, in turn, depends upon geometry of the synaptic space, density of acetylcholinesterase (AChE) in the synaptic basal lamina, density and distribution of AChRs on the postsynaptic junctional folds, and properties of the AChR ion channel. Combined electromyographic, morphologic and in vitro electrophysiologic studies of the neuromuscular junction identified factors that compromise the safety margin in different CMSs and pointed to candidate gene products. This candidate gene approach proved to be a powerful means of discovering diverse molecular targets in identified CMSs.

Section snippets

Current classification of CMSs

CMSs are conveniently classified according to their target as presynaptic, synaptic basal lamina-associated or postsynaptic (Table 1). The frequencies of the different types of CMS shown in Table 1 are based on 205 index patients investigated at the Mayo Clinic, US. Intercostal muscle specimens, intact from origin to insertion, were obtained from 108 patients for correlative studies of in vitro parameters of neuromuscular transmission, ultrastructural and cytochemical studies of EPs, and

CMS caused by defects in choline acetyltransferase

Combined clinical and electrophysiologic clues led to discovery of this type of CMS [6]. Affected patients have sudden episodes of respiratory embarrassment and bulbar paralysis, culminating in apnea against a background of variable or no myasthenic symptoms. Between crises, the electromyographic hallmark of impaired neuromuscular transmission — a decremental response of the compound muscle action potential (CMAP) on 2 Hz stimulation — is usually absent but appears after a conditioning train of

CMS caused by defects in the endplate species of AChE

With few exceptions, EP AChE deficiency causes a severe CMS. The clinical clues are a repetitive CMAP evoked by single nerve stimulus and absence of AChE from the EPs. Electron microscopy shows small nerve terminals that are often isolated from the synaptic cleft by an intruding Schwann cell process, and degeneration of the junctional folds. Encasement of the nerve terminals restricts evoked release of ACh. The absence of AChE prolongs the lifetime of ACh in the synaptic space; this increases

CMS caused by defects in AChR

Mutations have now been discovered in all AChR subunits and in different domains of the subunits. Mutations are divided into two major classes: those that reduce expression of AChR and those that alter its kinetic properties. The kinetic mutations divide further into slow-channel mutations that increase, and fast-channel mutations that decrease, the synaptic response to ACh. The slow- and fast-channel mutations represent physiologic opposites in both their phenotypic consequences and

CMS caused by defects in rapsyn

Rapsyn, under the influence of agrin and MuSK, concentrates AChR in the postsynaptic membrane and links it to the subsynaptic cytoskeleton through dystroglycan (Figure 8). The discovery of mutations in RAPSN came from CMS patients with demonstrated EP AChR deficiency who carried no mutation in any subunit of the AChR [58]. No fewer than 21 mutations in rapsyn have been identified (see Figure 8a,b and [57]), and all patients with mutations in the translated region of RAPSN carry the N88K

CMS caused by mutations in MuSK

MuSK (a muscle-specific receptor tyrosine kinase) plays a crucial role in the agrin–MuSK–rapsyn pathway in organizing the postsynaptic scaffold and in inducing the high concentration of AChR and tyrosine kinases of the ErB family in the postsynaptic membrane. Daniel Hantai's group now report a frameshift and a missense mutation (220insC and V789M) in MuSK in a CMS patient whose endplates are deficient in AChR and MuSK [64••]. In a series of elegant expression studies, they also show that the

Sodium-channel myasthenia

Only one patient with this type of CMS has been observed to date [65••]. This was the case of a 20-year-old normokalemic woman with eyelid ptosis, marked generalized fatigable weakness, and recurrent attacks of respiratory and bulbar paralysis since birth that caused anoxic brain injury. The clue that the defect resides in NaV1.4, the skeletal muscle sodium channel encoded by SCN4A, came from the observation that EPPs depolarizing the muscle fiber resting potential to −40 mV failed to excite

Therapy of CMS

CMSs that decrease the synaptic response to ACh are treated with drugs that augment cholinergic stimulation, namely inhibitors of AChE and, in some cases, with 3,4-diaminopyridine (3,4-DAP). 3,4-DAP increases the number of quanta released by nerve impulses, and AChE inhibitors increase the number of AChRs activated by each quantum. By contrast CMSs that increase the synaptic response are treated with long-lived open-channel blockers of AChR. However, there is no satisfactory therapy for

Conclusions

The road to discovery of the molecular basis of diverse types of CMS was through the candidate gene approach. The road was paved by accessibility of the neuromuscular synapse to structural and electrophysiologic studies, the pioneering discoveries of the past that illuminated quantal release mechanisms, the fate of ACh in the synaptic space, the structure of EP associated proteins, the ion channel properties of AChR, and the recent and rapid development of the tools of molecular genetics.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

Work in the authors’ laboratories was supported by grants from the NIH to AG Engel (NS-2677) and to SM Sine (NS-31744) and by a Muscular Dystrophy Association Grant to AG Engel. We thank Dr H-L Wang for providing the receptor homology model.

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