Pure haploinsufficiency for Dravet syndrome Na V 1.1 ( SCN1A ) sodium channel truncating mutations

• Published in: Epilepsia (Copenhagen) Vol. 53; no. 1; pp. 87 - 100
• Main Authors: Bechi, Giulia, Scalmani, Paolo, Schiavon, Emanuele, Rusconi, Raffaella, Franceschetti, Silvana, Mantegazza, Massimo
• Format: Journal Article
• Language: English
• Published: 01.01.2012
• ISSN: 0013-9580; 1528-1167
• DOI: 10.1111/j.1528-1167.2011.03346.x

Summary 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 V 1.1 α subunit. About 50% of SCN1A DS mutations truncate Na V 1.1, possibly causing complete loss of its function. However, it has not been investigated yet if Na V 1.1 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 V 1.1 mutants, R222* and R1234*, on coexpressed wild‐type Na + channels. Methods:  We engineered R222* or R1234* in the human cDNA of Na V 1.1 (hNa V 1.1) and studied their effect on coexpressed wild‐type hNa V 1.1, hNa V 1.2 or hNa V 1.3 cotransfecting tsA‐201 cells, and on hNa V 1.6 transfecting an human embryonic kidney (HEK) cell line stably expressing this channel. We also studied hippocampal neurons dissociated from Na V 1.1 knockout (KO) mice, an animal model of DS expressing a truncated Na V 1.1 channel. Key Findings:  We found no modifications of current amplitude coexpressing the truncated mutants with hNa V 1.1, hNa V 1.2, or hNa V 1.3, but a 30% reduction coexpressing them with hNa V 1.6. However, we showed that also coexpression of functional full‐length hNa V 1.1 caused a similar reduction. Therefore, this effect should not be involved in the pathomechanism of DS. Some gating properties of hNa V 1.1, hNa V 1.3, and hNa V 1.6 were modified, but recordings of hippocampal neurons dissociated from Na V 1.1 KO mice did not show any significant modifications of these properties. Therefore, Na V 1.1 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 V 2.1 mutants and hNa V 1.1 ones, and shown that hNa V 1.6 expression can be reduced in physiologic conditions by coexpression of hNa V 1.1. Moreover, our data may provide useful information for the development of therapeutic approaches.