Mutant SNAP25B causes myasthenia, cortical hyperexcitability, ataxia, and intellectual disability.

Neurology 2014 Dec 7;83(24):2247-55. Epub 2014 Nov 7.

From the Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN.

Objective: To identify and characterize the molecular basis of a syndrome associated with myasthenia, cortical hyperexcitability, cerebellar ataxia, and intellectual disability.

Methods: We performed in vitro microelectrode studies of neuromuscular transmission, performed exome and Sanger sequencing, and analyzed functional consequences of the identified mutation in expression studies.

Results: Neuromuscular transmission at patient endplates was compromised by reduced evoked quantal release. Exome sequencing identified a dominant de novo variant, p.Ile67Asn, in SNAP25B, a SNARE protein essential for exocytosis of synaptic vesicles from nerve terminals and of dense-core vesicles from endocrine cells. Ca(2+)-triggered exocytosis is initiated when synaptobrevin attached to synaptic vesicles (v-SNARE) assembles with SNAP25B and syntaxin anchored in the presynaptic membrane (t-SNAREs) into an α-helical coiled-coil held together by hydrophobic interactions. Pathogenicity of the Ile67Asn mutation was confirmed by 2 measures. First, the Ca(2+) triggered fusion of liposomes incorporating v-SNARE with liposomes containing t-SNAREs was hindered when t-SNAREs harbored the mutant SNAP25B moiety. Second, depolarization of bovine chromaffin cells transfected with mutant SNAP25B or with mutant plus wild-type SNAP25B markedly reduced depolarization-evoked exocytosis compared with wild-type transfected cells.

Conclusion: Ile67Asn variant in SNAP25B is pathogenic because it inhibits synaptic vesicle exocytosis. We attribute the deleterious effects of the mutation to disruption of the hydrophobic α-helical coiled-coil structure of the SNARE complex by replacement of a highly hydrophobic isoleucine by a strongly hydrophilic asparagine.

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Source
http://dx.doi.org/10.1212/WNL.0000000000001079DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4277673PMC
December 2014
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