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Neurobiol Dis. 2015 Aug;80:80-92. doi: 10.1016/j.nbd.2015.04.017. Epub 2015 May 22.

A recurrent KCNQ2 pore mutation causing early onset epileptic encephalopathy has a moderate effect on M current but alters subcellular localization of Kv7 channels.

Author information

1
Aix-Marseille Université, GMGF, Marseille, France; INSERM, UMR_S 910, Marseille, France.
2
Aix-Marseille Université, CNRS, CRN2M-UMR7286, Marseille, France.
3
Aix-Marseille Université, Institut de Neurobiologie de la Méditerranée (INMED), Marseille, France; INSERM, UMR_S 901, Marseille, France.
4
Aix-Marseille Université, GMGF, Marseille, France; INSERM, UMR_S 910, Marseille, France; APHM, Hôpital d'Enfants de la Timone, Service de neurologie pédiatrique, Marseille, France.
5
Aix-Marseille Université, GMGF, Marseille, France; INSERM, UMR_S 910, Marseille, France; APHM, Hôpital d'enfants de la Timone, Département de génétique médicale et de biologie cellulaire, Marseille France.
6
CHU Besançon, Service de génétique et neuropédiatrie, Besançon, France.
7
Université Pierre et Marie Curie, Groupe de Recherche Clinique « Déficiences Intellectuelles de Causes Rares », Paris, France; APHP, service de neurologie pédiatrique, Hôpital Trousseau, Paris, France.
8
APHP, Service de Génétique Médicale et Centre de Références « Déficiences Intellectuelles de Causes Rares », Groupe Hospitalier Pitié-Salpêtrière, Paris, France; Université Pierre et Marie Curie, Groupe de Recherche Clinique « Déficiences Intellectuelles de Causes Rares », Paris, France.
9
APHP, service de neurologie pédiatrique, Hôpital Trousseau, Paris, France.
10
CHU de Nantes, Service de génétique médicale, Nantes, France.
11
CHU d'Angers, Service de neurologie pédiatrique, Angers, France.
12
CHU d'Angers, Département de Biochimie et Génétique, Angers, France.
13
CHU de Tours, Service de neurologie pédiatrique, Tours, France.
14
CHU de Tours, Service de génétique, Tours, France.
15
CHU de Lille, Service de neurologie pédiatrique, Lille, France.
16
CHU de Saint Etienne, Service de génétique médicale, France.
17
Hospices Civils de Lyon, Service de génétique, Lyon, France.
18
Aix-Marseille Université, GMGF, Marseille, France; INSERM, UMR_S 910, Marseille, France; APHM, Hôpital d'Enfants de la Timone, Service de neurologie pédiatrique, Marseille, France. Electronic address: mathieu.milh@ap-hm.fr.
19
Aix-Marseille Université, Institut de Neurobiologie de la Méditerranée (INMED), Marseille, France; INSERM, UMR_S 901, Marseille, France. Electronic address: laurent.aniksztejn@inserm.fr.

Abstract

Mutations in the KCNQ2 gene encoding the voltage-dependent potassium M channel Kv7.2 subunit cause either benign epilepsy or early onset epileptic encephalopathy (EOEE). It has been proposed that the disease severity rests on the inhibitory impact of mutations on M current density. Here, we have analyzed the phenotype of 7 patients carrying the p.A294V mutation located on the S6 segment of the Kv7.2 pore domain (Kv7.2(A294V)). We investigated the functional and subcellular consequences of this mutation and compared it to another mutation (Kv7.2(A294G)) associated with a benign epilepsy and affecting the same residue. We report that all the patients carrying the p.A294V mutation presented the clinical and EEG characteristics of EOEE. In CHO cells, the total expression of Kv7.2(A294V) alone, assessed by western blotting, was only 20% compared to wild-type. No measurable current was recorded in CHO cells expressing Kv7.2(A294V) channel alone. Although the total Kv7.2(A294V) expression was rescued to wild-type levels in cells co-expressing the Kv7.3 subunit, the global current density was still reduced by 83% compared to wild-type heteromeric channel. In a configuration mimicking the patients' heterozygous genotype i.e., Kv7.2(A294V)/Kv7.2/Kv7.3, the global current density was reduced by 30%. In contrast to Kv7.2(A294V), the current density of homomeric Kv7.2(A294G) was not significantly changed compared to wild-type Kv7.2. However, the current density of Kv7.2(A294G)/Kv7.2/Kv7.3 and Kv7.2(A294G)/Kv7.3 channels were reduced by 30% and 50% respectively, compared to wild-type Kv7.2/Kv7.3. In neurons, the p.A294V mutation induced a mislocalization of heteromeric mutant channels to the somato-dendritic compartment, while the p.A294G mutation did not affect the localization of the heteromeric channels to the axon initial segment. We conclude that this position is a hotspot of mutation that can give rise to a severe or a benign epilepsy. The p.A294V mutation does not exert a dominant-negative effect on wild-type subunits but alters the preferential axonal targeting of heteromeric Kv7 channels. Our data suggest that the disease severity is not necessarily a consequence of a strong inhibition of M current and that additional mechanisms such as abnormal subcellular distribution of Kv7 channels could be determinant.

KEYWORDS:

Early epileptic encephalopathy; Kv7 channels; M-current; Subcellular channel expression; p.A294G mutation; p.A294V mutation

PMID:
26007637
DOI:
10.1016/j.nbd.2015.04.017
[Indexed for MEDLINE]

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