To characterize a novel congenital myasthenic syndrome (CMS).

Neuromuscular junction formation is regulated by Wnt signaling pathways. Wnt3a binds to Frizzled and its coreceptor LRP5/LRP6; this increases the free form of beta-catenin; this suppresses rapsyn expression causing reduced AChR clustering. Variants in 32 genes causing CMS had been identified, but none were reported in Wnt signaling proteins. The microRNA (MIR) binding to the 3’ untranslated region (3’-UTR) regulates gene expression, but only one CMS-causing mutation was found in the GFPT1 3’-UTR.

Electron microscopy, intracellular recordings from patient’s endplates (EPs), whole exome sequencing, quantitative PCR, dual luciferase assay, expression of target gene in HEK cells, and immunoblotting. 

We identified a homozygous c.*226A>G mutation in the 3’-UTR of LRP6 in a 10-year-old boy born with CMS. He had a delayed motor development, fatigued abnormally on exertion, and his weakness was increased by fever. In intercostal muscle EPs, the mean amplitudes of miniature endplate potential and current were respectively reduced to 50% and 38% of control, but bungarotoxin binding sites per EP were normal. The mRNA of LRP6 in the patient’s muscle was 5-fold higher, but rapsyn expression was 3-fold lower than in controls. MIR-300 and MIR-let7a inhibited expression of reporter gene containing LRP6-3’-UTR mutation to a significantly lesser extent than that containing wild-type LRP6-3’-UTR. This was confirmed by immunoblotting of LRP6 in membranes of HEK cells cotransfected with LRP6 plasmids containing 3’-UTR and MIR plasmids. Furthermore, overexpression of LRP6 increased the cytosolic level of beta-catenin.  

We identify a new CMS due to a defect in Wnt signaling.  A 3’-UTR mutation of LRP6 gene decreases its bindings to MIR-300 and MIR-let7a resulting in increased expression of LRP6 and upregulates beta-catenin directly without Wnt-protein binding. The increase of beta-catenin entering nucleus downregulates rapsyn expression. This decreases AChR clustering at EP and hinders neuromuscular transmission.