SummaryHeterozygous mutations in proline-rich transmembrane protein 2 (PRRT2) underlie a group of paroxysmal disorders, including epilepsy, kinesigenic dyskinesia, and migraine. Most of the mutations lead to impaired PRRT2 expression, suggesting that loss of PRRT2 function may contribute to pathogenesis. We show that PRRT2 is enriched in presynaptic terminals and that its silencing decreases the number of synapses and increases the number of docked synaptic vesicles at rest. PRRT2-silenced neurons exhibit a severe impairment of synchronous release, attributable to a sharp decrease in release probability and Ca2+ sensitivity and associated with a marked increase of the asynchronous/synchronous release ratio. PRRT2 interacts with the synaptic proteins SNAP-25 and synaptotagmin 1/2. The results indicate that PRRT2 is intimately connected with the Ca2+-sensing machinery and that it plays an important role in the final steps of neurotransmitter release.

Several genes predisposing to autism spectrum disorders (ASDs) with or without epilepsy have been identified, many of which are implicated in synaptic function. Here we report a Q555X mutation in synapsin 1 (SYN1), an X-linked gene encoding for a neuron-specific phosphoprotein implicated in the regulation of neurotransmitter release and synaptogenesis. This nonsense mutation was found in all affected individuals from a large French-Canadian family segregating epilepsy and ASDs. Additional mutations in SYN1 (A51G, A550T and T567A) were found in 1.0 and 3.5% of French-Canadian individuals with autism and epilepsy, respectively. The majority of these SYN1 mutations were clustered in the proline-rich D-domain which is substrate of multiple protein kinases. When expressed in synapsin I (SynI) knockout (KO) neurons, all the D-domain mutants failed in rescuing the impairment in the size and trafficking of synaptic vesicle pools, whereas the wild-type human SynI fully reverted the KO phenotype. Moreover, the nonsense Q555X mutation had a dramatic impact on phosphorylation by MAPK/Erk and neurite outgrowth, whereas the missense A550T and T567A mutants displayed impaired targeting to nerve terminals. These results demonstrate that SYN1 is a novel predisposing gene to ASDs, in addition to epilepsy, and strengthen the hypothesis that a disturbance of synaptic homeostasis underlies the pathogenesis of both diseases.

See Lerche (doi:) for a scientific commentary on this article. PRRT2 mutations cause heterogeneous paroxysmal neurological disorders. Using iPSC-derived neurons from patients homozygous for a nonsense PRRT2 mutation and cortical neurons from PRRT2-knockout mice, Fruscione et al. show that PRRT2 is a negative modulator of voltage-dependent NaV1.2/1.6 channels. Increased neuronal excitability may contribute to the paroxysmal nature of PRRT2-linked diseases.

Early B-cell factor 2 (EBF2) is one of four mammalian members of an atypical helix-loop-helix transcription factor family (COE). COE proteins have been implicated in various aspects of nervous and immune system development. We and others have generated and described mice carrying a null mutation of Ebf2, a gene previously characterized in the context of Xenopus laevis primary neurogenesis and neuronal differentiation. In addition to deficits in neuroendocrine and olfactory development, and peripheral nerve maturation, Ebf2 null mice feature an ataxic gait and obvious motor deficits associated with clear-cut abnormalities of cerebellar development. The number of Purkinje cells (PCs) in the Ebf2 null is markedly decreased, resulting in a small cerebellum with notable foliation defects, particularly in the anterior vermis. We show that this stems from the defective migration of a molecularly defined PC subset that subsequently dies by apoptosis. Part of the striped cerebellar topography is disrupted due to cell death and, in addition, many of the surviving PCs, that would normally adopt a zebrin II-negative phenotype, transdifferentiate to Zebrin II-positive, an unprecedented finding suggesting that Ebf2 is required for the establishment of a proper cerebellar cortical map.