LRP4 third β-propeller domain mutations cause novel congenital myasthenia by compromising agrin-mediated MuSK signaling in a position-specific manner

Abstract

Congenital myasthenic syndromes (CMS) are heterogeneous disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. Using Sanger and exome sequencing in a CMS patient, we identified two heteroallelic mutations, p.Glu1233Lys and p.Arg1277His, in LRP4 coding for the postsynaptic low-density lipoprotein receptor-related protein 4. LRP4, expressed on the surface of the postsynaptic membrane of the neuromuscular junction, is a receptor for neurally secreted agrin, and LRP4 bound by agrin activates MuSK. Activated MuSK in concert with Dok-7 stimulates rapsyn to concentrate and anchor AChR on the postsynaptic membrane and interacts with other proteins implicated in the assembly and maintenance of the neuromuscular junction. LRP4 also functions as an inhibitor of Wnt/beta-catenin signaling. The identified mutations in LRP4 are located at the edge of its 3rd beta-propeller domain and decrease binding affinity of LRP4 for both MuSK and agrin. Mutations in the LRP4 3rd beta-propeller domain were previously reported to impair Wnt signaling and cause bone diseases including Cenani -Lenz syndactyly syndrome and sclerosteosis-2. By analyzing naturally occurring and artificially introduced mutations in the LRP4 3rd beta-propeller domain, we show that the edge of the domain regulates the MuSK signaling whereas its central cavity governs Wnt signaling. We conclude that LRP4 is a new CMS disease gene and that the 3rd beta propeller domain of LRP4 mediates the two signaling pathways in a position-specific manner.

Disruption of Lrp4 function by genetic deletion or pharmacological blockade increases bone mass and serum sclerostin levels

“…The same region was identified by a genome-wide association study (GWAS) (28). Moreover, a recent report proposed the central cavity of the third β-propeller domain of LRP4 to regulate WNT signaling whereas the edge of the domain is required for MusK signaling (47). This structural difference in binding allows LRP4 to interact with an array of binding partners to participate in multiple biological processes.…”

Mechanisms controlling neuromuscular junction stability

“…Some post-synaptic congenital myasthenic syndromes (CMS) are caused by mutations in agrin, musk and Rapsyn, all of them being part of a molecular pathway essential for AChR aggregation and positioning on the post-synaptic membrane [63]. LRP4 has been proposed to be a novel congenital myasthenic syndrome disease gene [64]. In myasthenia gravis (MG), a severely debilitating autoimmune disease that is due to a decrease in the efficiency of synaptic transmission at neuromuscular synapses, antibodies are generated against post-synaptic proteins, including acetylcholine receptors, MuSK, and (Lrp4), which prevent binding between MuSK and Lrp4, and inhibit agrin-stimulated MuSK phosphorylation [65].…”

Clinical and genetic basis of congenital myasthenic syndromes

“…The clinical phenotype results from missense mutations Glu1233Lys and Arg1277His. Allelic conditions include sclerosteosis type 2 and Cenani-Lenz syndactyly syndrome 2,32 .…”