Titre : | Congenital myasthenic syndrome caused by decreased receptor channel openings due to a novel mutation in the nicotinic acetylcholine receptor epsilon-subunit : Communication 253 lors du Congrès international de myologie (International Congress of Myology) 9-13 mai 2005 Nantes, France |
contenu dans : | |
Auteurs : | Yu H, Auteur ; Ishigaki K ; Shimahara T ; Bournaud R ; Le Poupon C ; Prado de Carvalho L ; Corringer PJ ; Fardeau M ; Koenig J ; Eymard B ; Hantaï D |
Type de document : | Article |
Année de publication : | 2005 |
Pages : | p 255 |
Langues: | Anglais |
Mots-clés : | canal ionique ; colloque ; électromyographie ; étude de cas ; fauteuil roulant ; génétique moléculaire ; histochimie ; jonction neuromusculaire ; marche ; microscopie électronique ; muscle squelettique ; mutation génétique ; récepteur nicotinique ; sous-unité epsilon du récepteur nicotinique ; syndrome myasthénique congénital |
Résumé : |
Communication n° 253 :
Congenital myasthenic syndromes (CMS) are a group of rare genetic disorders that affect neuromuscular transmission. Most of the CMS are caused by mutations of the nicotinic acetylcholine receptor (nAChR) in skeletal muscle. These mutations lead to low expression and/or kinetic anomalies of the nAChR such as slow channel CMS and fast channel CMS. Here we report an early-onset and recessively inherited disabling case of CMS. with oculobulbar and limb involvement requiring wheel chair at 8 years. Cholinesterase inhibitors were efficient allowing walking. Electromyography revealed a decrement after repetitive stimulation. Deltoid muscle histological examination showed highly abnormal neuromuscular junctions with denervation and sprouting features. Electron microscopy showed a drastic reduction of the postsynaptic folds. Molecular genetic analysis identified a homozygous mutation within the nAChR epsilon-subunit (epsilon P282R). In order to assess the functional consequences of this mutation, we carried out site directed mutagenesis of the epsilon-subunit. Co-transfection of the alpha, beta, delta wild-type nAChR subunits with either the wild-type or mutant epsilon P282R subunits show the following: 1) upon transfection in HEK cells, the mutation causes a 2-fold decrease in the total amount of 125I-bungarotoxin binding sites, associated with a 10-fold reduction of the ACh apparent binding affinity. Whole cell electrophysiological recordings indicate that the mutation results in a 5-fold reduction of maximal ACh-evoked currents, a 10-fold decrease in ACh EC50, and a strong acceleration of desensitization. Mutational analysis at this position further shows that the positively charged arginine side chain causes the phenotype; 2) upon transfection in CHO cells, used to avoid the interference of endogen currents existing in HEK cells, single-channel recordings show that epsilon-P282R produces a shortened channel open time, a decreased open probability and decreased channel conductance. Altogether, these data show that the epsilonP282R mutation causes a loss of function phenotype, due to a destabilization of the active conformation of the receptor, a decreased expression, and a decreased channel conductance. On a 3D model, the mutation is located at the interface between the extracellular and transmembrane domains of the nAChR, suggesting that this position plays a key role in the long range allosteric coupling between the agonist binding site and the ion channel. This novel mutation may therefore lead to a fast channel CMS, the functional endpoint being reduced responsiveness of the postsynaptic membrane to acetylcholine. |