Novel truncating RAPSN mutations causing congenital myasthenic syndrome responsive to 3,4-diaminopyridine

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Abstract

Rapsyn is essential for clustering the acetylcholine receptor at the postsynaptic membrane of the neuromuscular junction. Direct sequencing of RAPSN in two children with congenital myasthenic syndromes with no mutation in any of the AChR subunits identified two heterozygous recessive mutations in each: a previously characterized N88K mutation in both, and a second frameshifting mutation in Patient (Pt) 1 and a nonsense mutation in Pt 2. An intercostal muscle biopsy in Pt 1 revealed decreased AChRs per endplate and decreased amplitude of the miniature endplate potential, predicted consequences of rapsyn deficiency. Clinically, both children manifested with hypomotility in utero, fatigable ocular and limb weakness since birth, decreased strength during viral illness, decremental response on electromyography, and absence of AChR antibodies. Pt 1, however, had a more severe clinical course with recurrent episodes of respiratory failure, contractures, and craniofacial malformations. In both patients, treatment with pyridostigmine was of some benefit, but the addition of 3,4-diaminopyridine led to significant clinical improvement. Thus, rapsyn deficiency predicting similar consequences at the cellular level can result in phenotypes with marked differences in severity of symptoms, risk of respiratory failure, and presence of contractures and craniofacial malformations.

Introduction

Congenital myasthenic syndromes (CMS) comprise a group of disorders in which the safety margin of neuromuscular transmission is compromised by mutations in genes encoding proteins of the presynaptic, synaptic, or postsynaptic regions [1]. CMS associated with deficiency of the acetylcholine receptor (AChR) at the endplate (EP) are due either to decreased synthesis or increased degradation of mutant receptors, or to failure of normal AChRs to localize and remain anchored at the postsynaptic membrane [1]. Rapsyn, a 43 kDa protein, functions to cluster AChRs at the postsynaptic membrane through interactions with the long cytoplasmic loop of subunits of the AChR [2] and with β-dystroglycan [3], [4], [5]. Mutations in the RAPSN gene encoding rapsyn have recently been recognized to cause CMS of variable clinical severity [6], [7], [8], [9], [10]. CMS due to mutations in the E-box consensus sequence of the RAPSN promoter lead to a relatively mild clinical phenotype, including a distinct facial malformation characterized by mandibular prognathism and jaw malocclusion in a group of Iranian and Iraqi Jews [7]. We here report two novel mutations in RAPSN and the associated clinical, morphologic, and electrophysiologic features.

Section snippets

Muscle specimens

Intercostal muscle specimens were obtained intact, from origin to insertion, from Pt 1 and from control subjects undergoing thoracic surgery. All human studies were in accord with the guidelines of the institutional review board of the Mayo Clinic.

Acetylcholinesterase (AChE) was visualized on glutaraldehyde-fixed teased single muscle fibers by Gautron's method [11]. AChR and AChE were colocalized in cryostat sections with rhodamine-labeled α-bungarotoxin (α-bgt) and a monoclonal anti-AChE

Patients

Patient (Pt) 1, a boy currently aged 14 years, had weak fetal movements in utero, and was delivered at term by caesarean section due to breech presentation. Hypotonia and limited limb movements were noted from birth. Ocular movements were normal and ptosis was absent at birth. Intermittent episodes of respiratory failure necessitated the use of mechanical ventilation for the first three months of life. At six months of age, during a severe episode of pneumonia, edrophonium was administered

Discussion

We describe two children with CMS with novel mutations in RAPSN. Each patient carries N88K; Pt 1 also carries 1177delAA, and Pt 2 also carries E333X. The N88K mutation has been previously characterized and shown to markedly impair co-clustering of rapsyn with the AChR [6]. The E333X mutation truncates rapsyn N-terminal to the RING-H2 domain and the 1177delAA mutation disrupts the RING-H2 domain; thus, both mutations likely interfere rapsyn binding to β-dystroglycan and, via dystroglycan, to the

Acknowledgements

This work was supported by NIH Grant NS6277 (to A.G.E.) and by a grant from the Muscular Dystrophy Association (to A.G.E.).

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