M.I.2 Congenital myasthenic syndromes and the formation of the neuromuscular junction

Abstract

The congenital myasthenic syndromes (CMS) are rare inherited disorders of neuromuscular transmission, characterised by fatigable muscle weakness. CMS account for only around 4% of all myasthenias, but this number is increasing with improved diagnosis. Except for the autosomal dominant slow channel syndrome, CMS show recessive inheritance and most are caused by acetylcholine receptor (AChR) loss. Typically patients present soon after birth with feeding difficulties, a weak cry, and more seriously, difficulties in breathing requiring resuscitation. Remarkable differences in severity occur even within families harbouring the same mutation, implying other modifying influences. However, not all CMS present at birth; some syndromes do not manifest until childhood (e.g. with delays in reaching motor milestones), or even adulthood when they may be mistaken for seronegative myasthenia gravis. Initial studies on the underlying basis of these disorders focused on the genes encoding the AChR, and the mutations identified were found to effect channel kinetics and/or AChR numbers in the postsynaptic membrane. However, genes encoding proteins responsible for AChR clustering and the maintenance of synaptic structure may also give rise to CMS, the latest of which is Dok-7. Dok-7 is essential for neuromuscular synaptogenesis and its interaction with MuSK appears to be crucial for this process. We have identified a series of differing DOK7 mutations underlying a CMS that is characterised by small, simplified neuromuscular junctions. The study of these mutations is helping elucidate the underlying pathogenic mechanism of DOK7 mutations and the role of Dok-7 in the AChR clustering pathway. The diversity of affected genes in CMS and the different pathogenic mechanisms of mutations within these genes is proving highly informative for understanding synaptic dysfunction. This is true not only for the AChR itself but also for molecules involved in forming and maintaining the neuromuscular synapse.