Background: We describe a severe form of congenital myasthenic syndrome (CMS) associated with congenital nephrosis and ocular malformations caused by two truncating mutations in the gene encoding the laminin β2 subunit ( LAMB2 ). Methods and results: Mutational analysis in the affected patient, who has a history of a serious untoward reaction to treatment with acetylcholinesterase inhibition, revealed two frame-shifting heteroallelic mutations, a maternally inherited 1478delG and a paternally inherited 4804delC . An anconeus muscle biopsy demonstrated a profound distortion of the architecture and function of the neuromuscular junction, which was strikingly similar to that seen in mice lacking laminin β2 subunit. The findings included: pronounced reduction of the axon terminal size with encasement of the nerve endings by Schwann cells, severe widening of the primary synaptic cleft and invasion of the synaptic space by the processes of Schwann cells, and moderate simplification of postsynaptic folds and intact expression of the endplate acetylcholinesterase. The endplate potential quantal content was notably reduced, while the frequencies and amplitudes of miniature endplate potentials were only moderately diminished and the decay phases of miniature endplate potentials were normal. Western blot analysis of muscle and kidney tissue and immunohistochemistry of kidney tissue showed no laminin β2 expression. Conclusion: This case, which represents a new type of synaptic CMS, exemplifies the wide variability of phenotypes associated with LAMB2 mutations and underscores the fundamental role that laminin β2 plays in the development of the human neuromuscular junction.

We describe a severe congenital myasthenic syndrome (CMS) caused by two missense mutations in the gene encoding the muscle specific receptor tyrosine kinase (MUSK). The identified MUSK mutations M605I and A727V are both located in the kinase domain of MuSK. Intracellular microelectrode recordings and microscopy studies of the neuromuscular junction conducted in an anconeus muscle biopsy revealed decreased miniature endplate potential amplitudes, reduced endplate size and simplification of secondary synaptic folds, which were consistent with postsynaptic deficit. The study also showed a striking reduction of the endplate potential quantal content, consistent with additional presynaptic failure. Expression studies in MuSK deficient myotubes revealed that A727V, which is located within the catalytic loop of the enzyme, caused severe impairment of agrin-dependent MuSK phosphorylation, aggregation of acetylcholine receptors (AChRs) and interaction of MuSK with Dok-7, an essential intracellular binding protein of MuSK. In contrast, M605I, resulted in only moderate impairment of agrin-dependent MuSK phosphorylation, aggregation of AChRs and interaction of MuSK with Dok-7. There was no impairment of interaction of mutants with either the low-density lipoprotein receptor-related protein, Lrp4 (a co-receptor of agrin) or with the mammalian homolog of the Drosophila tumorous imaginal discs (Tid1). Our findings demonstrate that missense mutations in MUSK can result in a severe form of CMS and indicate that the inability of MuSK mutants to interact with Dok-7, but not with Lrp4 or Tid1, is a major determinant of the pathogenesis of the CMS caused by MUSK mutations.

Many patients with the limb-girdle variant of congenital myasthenic syndrome (CMS) possess mutations in the human Dok-7 gene (DOK7). We identified six unrelated CMS patients with DOK7 mutations. Two patients, one mildly and the other moderately affected, were homozygous for the previously described 1263insC mutation. The common 1124_1127dupTGCC mutation was detected in the other four patients, whose clinical phenotypes range from mildly to severely affected. This striking phenotypic heterogeneity found both within and between mutational classes is made more compelling by data from our electrophysiological studies and electron microscopy of the neuromuscular junction (NMJ). Indeed, several aspects of the physiological and morphometric data do not correlate with genotype or severity of clinical phenotype. Overall, our study corroborates the findings of others and provides an additional demonstration of the considerable phenotypic variability associated with CMS due to DOK7 mutations.

In vitro pretreatment of human muscles with PB protected up to 20% of muscle AChE and ameliorated some deleterious effects on endplate physiology induced by soman.