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

Abstract

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 21 -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 a-helical coiled-coil held together by hydrophobic interactions. Pathogenicity of the Ile67Asn mutation was confirmed by 2 measures. First, the Ca 21 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 wildtype 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 a-helical coiled-coil structure of the SNARE complex by replacement of a highly hydrophobic isoleucine by a strongly hydrophilic asparagine. Neurology ® 2014;83:2247-2255 GLOSSARY AChR 5 acetylcholine receptor; cDNA 5 complementary DNA; CMS 5 congenital myasthenic syndrome; EP 5 endplate; MEPP 5 miniature endplate potential; SNAP25B 5 synaptosomal-associated protein, 25B; SNARE 5 soluble N-ethylmaleimide-sensitive factor attachment protein receptor; t-SNAREs 5 target membrane-attached SNAP25B and syntaxin; v-SNARE 5 synaptic vesicle-attached synaptobrevin.The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are core components of the synaptic vesicle fusion machinery. Synaptobrevin attached to the synaptic vesicles is referred to as a v-SNARE, and SNAP25 (synaptosomal-associated protein of 25 kD) and syntaxin attached to the target plasma membrane are designated as t-SNAREs.1 The assembled complex is a coiled-coil in which a-helices are held together by strong hydrophobic interactions. The complex is stabilized by complexin, a small soluble neuronal protein.2 In the resting state, Munc18 binds to a closed form of syntaxin and blocks formation of the SNARE complex but assists the SNARE complex to effect exocytosis in the active state. 3 SNAP25 also participates in endocytosis at hippocampal synapses 4 and negatively modulates the neuronal voltage-gated calcium channel during intense activity.5 Also, ...

Homozygous mutations in VAMP1 cause a presynaptic congenital myasthenic syndrome

“…In conclusion, the identification of biallelic variants in VAMP1 as a novel cause of CMS, in addition to other genes (eg, SNAP25B, SYT2 ) previously associated with similar presynaptic abnormalities of neuromuscular transmission,5, 6 highlights the crucial role of different SNAREs in NMJ physiology. Intriguingly, the relatively mild phenotype showed by our patients compared to the mouse model, which dies prematurely, suggests the possible existence of species‐specific compensation of vesicle fusion and release at the nerve terminal, perhaps through genetic modifiers in humans but not in mice or fruit flies.…”

“…Several molecular causes can be implicated in CMS pathophysiology, including mutations in genes encoding proteins associated with the muscle nicotinic acetylcholine receptor and the synaptic basal lamina, or (more rarely) involved in the NMJ presynaptic transmission 2, 3, 4, 5, 6…”

Electrophysiologic features of SYT2 mutations causing a treatable neuromuscular syndrome

“…Of note, a patient with SNAP25 mutation also benefited from 3,4-DAP treatment but not from pyridostigmine, emphasizing the similarity in presynaptic pathophysiology. 7 We describe the clinical and neurophysiologic features of a novel human neuromuscular syndrome caused by autosomal-dominant mutations in the synaptic vesicle calcium sensor SYT2. The phenotype of this mutation is suggestive of a distal hereditary neuropathy.…”

Clinical and genetic basis of congenital myasthenic syndromes

“…Deficiency of SNAP25B was reported with an autosomal dominant pattern of CMS presenting in an 11-year-old African-American female patient with a history of intrauterine hypotonia, cyanosis at birth, delayed psychomotor development, multiple joint contractures, muscle weakness, fatigable ptosis, associated with ataxia and early childhood epilepsy, whose biomolecular studies revealed a decrease in the amount of acetylcholine released from the presynaptic nerve terminal from each motor nerve impulse 4,18 .…”