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.