[Pathophysiological characterization of congenital myasthenic syndromes: the example of mutations in the MUSK gene]

A novel ECEL1 mutation expands the phenotype of distal arthrogryposis multiplex congenita type 5D to include pretibial vertical skin creases

Arthrogryposis multiplex congenita (AMC) is a heterogeneous disorder characterized by multiple joint contractures often in association with other congenital abnormalities. Pretibial linear vertical creases are a rare finding associated with arthrogryposis, and the etiology of the specific condition is unknown. We aimed to genetically and clinically characterize a boy from a consanguineous family, presenting with AMC and pretibial vertical linear creases on the shins. Whole exome sequencing and variant analysis revealed homozygous novel missense variants of ECEL1 (c.1163T > C, p.Leu388Pro, NM_004826) and MUSK (c.2572C > T, p.Arg858Cys, NM_005592). Both variants are predicted to have deleterious effects on the protein function, with amino acid positions highly conserved among species. The variants segregated in the family, with healthy mother, father, and sister being heterozygous carriers and the index patient being homozygous for both mutations. We report on a unique patient with a novel ECEL1 homozygous mutation, expanding the phenotypic spectrum of Distal AMC Type 5D to include vertical linear skin creases. The homozygous mutation in MUSK is of unknown clinical significance. MUSK mutations have previously shown to cause congenital myasthenic syndrome, a neuromuscular disorder with defects in the neuromuscular junction.

Inherited disorders of the neuromuscular junction: an update

Congenital myasthenic syndromes (CMSs) are a group of heterogeneous inherited disorders caused by mutations in genes affecting the function and structure of the neuromuscular junction. This review updates the reader on established and novel subtypes of congenital myasthenia, and the treatment strategies for these increasingly heterogeneous disorders. The discovery of mutations associated with the N-glycosylation pathway and in the family of serine peptidases has shown that causative genes encoding ubiquitously expressed molecules can produce defects at the human neuromuscular junction. By contrast, mutations in lipoprotein-like receptor 4 (LRP4), a long-time candidate gene for congenital myasthenia, and a novel phenotype of myasthenia with distal weakness and atrophy due to mutations in AGRN have now been described. In addition, a pathogenic splicing mutation in a nonfunctional exon of CHRNA1 has been reported emphasizing the importance of analysing nonfunctional exons in genetic analysis. The benefit of salbutamol and ephedrine alone or combined with pyridostigmine or 3,4-DAP is increasingly being reported for particular subtypes of CMS.

Effect of complement and its regulation on myasthenia gravis pathogenesis

Myasthenia gravis (MG) is primarily caused by antibodies directed towards the skeletal muscle acetylcholine receptor, leading to muscle weakness. Although these antibodies may induce compromise of neuromuscular transmission by blocking acetylcholine receptor function or antigenic modulation, the predominant mechanism of injury to the neuromuscular junction is complement-mediated lysis of the postsynaptic membrane. The vast majority of data to support the role of complement derives from experimentally acquired MG (EAMG). In this article, we review studies that demonstrate the central role of complement in EAMG and MG pathogenesis along with the emerging role of complement in T- and B-cell function, as well as the potential for complement inhibitor-based therapy to treat human MG.