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

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.