Abstract
Congenital myasthenic syndromes (CMS) are caused by mutations in molecules expressed at the neuromuscular junction. We report clinical, structural, ultrastructural, and electrophysiologic features of 4 CMS patients with 6 heteroallelic variants in AGRN, encoding agrin. One was a 7.9-kb deletion involving the N-terminal laminin-binding domain. Another, c.4744G>A - at the last nucleotide of exon 26 - caused skipping of exon 26. Four missense mutations (p.S1180L, p.R1509W, p.G1675S, and p.Y1877D) expressed in conditioned media decreased AChR clusters in C2C12 myotubes. The agrin-enhanced phosphorylation of MuSK was markedly attenuated by p.Y1877D in the LG3 domain and moderately attenuated by p.R1509W in the LG1 domain but not by the other 2 mutations. The p.S1180L mutation in the SEA domain facilitated degradation of secreted agrin. The p.G1675S mutation in the LG2 domain attenuated anchoring of agrin to the sarcolemma by compromising its binding to heparin. Anchoring of agrin with p.R1509W in the LG1 domain was similarly attenuated. Mutations of agrin affect AChR clustering by enhancing agrin degradation or by suppressing MuSK phosphorylation and/or by compromising anchoring of agrin to the sarcolemma of the neuromuscular junction.
Highlights
Congenital myasthenic syndromes (CMS) are heterogeneous disorders in which neuromuscular transmission is impaired by defects in molecules expressed at the neuromuscular junction (NMJ)
We show that defective acetylcholine receptor (AChR) clustering is caused by defects in MuSK phosphorylation, and by enhanced agrin degradation and/or compromised anchoring of agrin to the sarcolemma and at the NMJ
We examined the effects of the 4 missense mutations, p.S1180L, p.R1509W, p.G1675S, and p.Y1877D, on AChR clustering in C2C12 myotubes by adding WT and mutant agrin fragments to the culture medium
Summary
Congenital myasthenic syndromes (CMS) are heterogeneous disorders in which neuromuscular transmission is impaired by defects in molecules expressed at the neuromuscular junction (NMJ). Defects in more than 30 genes are known to cause CMS. These genes encode subunits of the acetylcholine receptor (AChR), structural proteins for scaffolding AChR clusters, signaling molecules driving AChR clustering, presynaptic molecules involved in release and resynthesis of ACh, synaptic and postsynaptic molecules that facilitate the signal transduction at the NMJ, and glycosylation enzymes [2, 3]. Spinal motor neurons make neural agrin that includes exon 32, as well as exons 36 and/or 37, and transport it to motor nerve terminals. Neural agrin is released into the synaptic basal lamina, where it binds to the N-terminal domain of the low-density lipoprotein receptor-related protein 4 (LRP4) anchored in the postsynaptic membrane [5, 6].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
