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Congenital myasthenic syndromes are a heterogeneous group of conditions in which muscle weakness resulting from impaired neuromuscular transmission is often present from infancy. One form of congenital myasthenic syndrome is due to a reduction of the number of acetylcholine receptors (AChRs) at the neuromuscular junction. We describe four new cases of AChR deficiency, characterized by a reduction in both miniature endplate potential amplitude and AChR abundance accompanied by elongation of the neuromuscular junction and some decrease in postsynaptic folding. A number of cytoplasmic proteins are normally associated with the postsynaptic membrane and may contribute to the clustering of AChRs at the neuromuscular junction. We therefore investigated the expression of several of these proteins in these AChR-deficiency patients. In each patient, immunolabelling of the neuromuscular junction for rapsyn, dystrophin, beta-dystroglycan and a form of beta-spectrin was strong but that for utrophin was markedly reduced or absent. This suggested that a defect in utrophin expression might underlie the congenital AChR deficiency. However, a reduction in utrophin labelling was also seen in three patients with adult acquired autoimmune myasthenia gravis in whom AChR loss results directly from the extracellular binding of autoantibodies. We conclude that the loss of AChRs in AChR deficiency does not result from the absence of rapsyn or beta-dystroglycan and that reduction of utrophin is probably secondary to the loss of AChRs. The possible role of AChRs and/or utrophin in determining the extent of postsynaptic folding is discussed.

Utrophin abundance is reduced at neuromuscular junctions of patients with both inherited and acquired acetylcholine receptor deficiencies