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Bechi G.,Besta Foundation Neurological Institute | Scalmani P.,Besta Foundation Neurological Institute | Schiavon E.,University of Nice Sophia Antipolis | Rusconi R.,University of Nice Sophia Antipolis | And 3 more authors.
Epilepsia | Year: 2012

Purpose: Dravet syndrome (DS), a devastating epileptic encephalopathy, is mostly caused by mutations of the SCN1A gene, coding for the voltage-gated Na + channel Na V1.1 α subunit. About 50% of SCN1A DS mutations truncate Na V1.1, possibly causing complete loss of its function. However, it has not been investigated yet if Na V1.1 truncated mutants are dominant negative, if they impair expression or function of wild-type channels, as it has been shown for truncated mutants of other proteins (e.g., Ca V channels). We studied the effect of two DS truncated Na V1.1 mutants, R222* and R1234*, on coexpressed wild-type Na + channels. Methods: We engineered R222* or R1234* in the human cDNA of Na V1.1 (hNa V1.1) and studied their effect on coexpressed wild-type hNa V1.1, hNa V1.2 or hNa V1.3 cotransfecting tsA-201 cells, and on hNa V1.6 transfecting an human embryonic kidney (HEK) cell line stably expressing this channel. We also studied hippocampal neurons dissociated from Na V1.1 knockout (KO) mice, an animal model of DS expressing a truncated Na V1.1 channel. Key Findings: We found no modifications of current amplitude coexpressing the truncated mutants with hNa V1.1, hNa V1.2, or hNa V1.3, but a 30% reduction coexpressing them with hNa V1.6. However, we showed that also coexpression of functional full-length hNa V1.1 caused a similar reduction. Therefore, this effect should not be involved in the pathomechanism of DS. Some gating properties of hNa V1.1, hNa V1.3, and hNa V1.6 were modified, but recordings of hippocampal neurons dissociated from Na V1.1 KO mice did not show any significant modifications of these properties. Therefore, Na V1.1 truncated mutants are not dominant negative, consistent with haploinsufficiency as the cause of DS. Significance: We have better clarified the pathomechanism of DS, pointed out an important difference between pathogenic truncated Ca V2.1 mutants and hNa V1.1 ones, and shown that hNa V1.6 expression can be reduced in physiologic conditions by coexpression of hNa V1.1. Moreover, our data may provide useful information for the development of therapeutic approaches. © 2011 International League Against Epilepsy.

Cestele S.,University of Nice Sophia Antipolis | Labate A.,University of Catanzaro | Rusconi R.,University of Nice Sophia Antipolis | Tarantino P.,National Research Council Italy | And 6 more authors.
Epilepsia | Year: 2013

Purpose To report the identification of the T1174S SCN1A (Na V1.1) mutation in a three-generation family with both epileptic and familial hemiplegic migraine (FHM) phenotypes and clarify the pathomechanism. Methods The five affected individuals underwent detailed clinical analyses. Mutation analyses was performed by direct sequencing of SCN1A; functional studies by expression in tsA-201 cells. A computational model was used to compare the effects of T1174S with those of a typical FHM mutation (Q1489K). Key Findings The proband had benign occipital epilepsy (BOE); two relatives had simple febrile seizures (FS) and later developed BOE. Two additional relatives had FHM without epilepsy or FS. All affected members and one obliged carrier carried the T1174S mutation. Functional effects were divergent: positive shift of the activation curve and deceleration of recovery from fast inactivation, consistent with loss of function, and increase of persistent current (I NaP), consistent with gain of function. The INaP increase was inhibited by dialysis of the cytoplasm, consistent with a modulation. Therefore, as shown by the computational model, T1174S could in some conditions induce overall loss of function, and in others gain of function; Q1489K induced gain of function in all the conditions. Significance Modulation of the properties of T1174S can lead to a switch between overall gain and loss of function, consistent with a switch between promigraine end epileptogenic effect and, thus, with coexistence of epileptic and FHM phenotypes in the same family. These findings may help to shed light on the complex genotype-phenotype relationship of SCN1A mutations. © Wiley Periodicals, Inc. © 2013 International League Against Epilepsy.

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