Dominant-negative effects of KCNQ2 mutations are associated with epileptic encephalopathy.

OBJECTIVE: Mutations in KCNQ2 and KCNQ3, encoding the voltage-gated potassium channels KV 7.2 and KV 7.3, are known to cause benign familial neonatal seizures mainly by haploinsufficiency. Here, we set out to determine the disease mechanism of 7 de novo missense KCNQ2 mutations that were recently described in patients with a severe epileptic encephalopathy including pharmacoresistant seizures and pronounced intellectual disability. METHODS: Mutations were inserted into the KCNQ2 cDNA. Potassium currents were recorded using 2-microelectrode voltage clamping, and surface expression was analyzed by a biotinylation assay in cRNA-injected Xenopus laevis oocytes. RESULTS: We observed a clear loss of function for all mutations. Strikingly, 5 of 7 mutations exhibited a drastic dominant-negative effect on wild-type KV 7.2 or KV 7.3 subunits, either by globally reducing current amplitudes (3 pore mutations) or by a depolarizing shift of the activation curve (2 voltage sensor mutations) decreasing potassium currents at the subthreshold level at which these channels are known to critically influence neuronal firing. One mutation significantly reduced surface expression. Application of retigabine, a recently marketed KV 7 channel opener, partially reversed these effects for the majority of analyzed mutations. INTERPRETATION: The development of severe epilepsy and cognitive decline in children carrying 5 of the 7 studied KCNQ2 mutations can be related to a dominant-negative reduction of the resulting potassium current at subthreshold membrane potentials. Other factors such as genetic modifiers have to be postulated for the remaining 2 mutations. Retigabine or similar drugs may be used as a personalized therapy for this severe disease.

Anion- and proton-dependent gating of ClC-4 anion/proton transporter under uncoupling conditions.

ClC-4 is a secondary active transporter that exchanges Cl(-) ions and H(+) with a 2:1 stoichiometry. In external SCN(-), ClC-4 becomes uncoupled and transports anions with high unitary transport rate. Upon voltage steps, the number of active transporters varies in a time-dependent manner, resembling voltage-dependent gating of ion channels. We here investigated modification of the voltage dependence of uncoupled ClC-4 by protons and anions to quantify association of substrates with the transporter. External acidification shifts voltage dependence of ClC-4 transport to more positive potentials and leads to reduced transport currents. Internal pH changes had less pronounced effects. Uncoupled ClC-4 transport is facilitated by elevated external [SCN(-)] but impaired by internal Cl(-) and I(-). Block by internal anions indicates the existence of an internal anion-binding site with high affinity that is not present in ClC channels. The voltage dependence of ClC-4 coupled transport is modulated by external protons and internal Cl(-) in a manner similar to what is observed under uncoupling conditions. Our data illustrate functional differences but also similarities between ClC channels and transporters.

Retigabine/Ezogabine, a KCNQ/K(V)7 channel opener: pharmacological and clinical data.

INTRODUCTION: Epilepsy is a serious and common chronic neurological disease with an urgent need for novel treatment options, because 30% of all epilepsy patients do not respond to currently available drugs. Retigabine/Ezogabine (RTG) is a third-generation antiepileptic drug (AED) with a novel mechanism of action. It enhances the activity of voltage-gated K(V)7 potassium channels. AREAS COVERED: The mechanism of action of RTG is reported in this paper, along with its pharmacodynamics and pharmacokinetics, based on a literature search from 1995 to 2011. Assessment of clinical efficacy and safety was performed using the published data of one Phase II and two Phase III clinical trials (RESTORE 1 and 2). EXPERT OPINION: RTG is an efficacious AED with a unique mechanism of action. It offers a new treatment option which could be particularly interesting for patients who are resistant to currently available AEDs. However, future investigations will show if such a "rational drug therapy" will be truly advantageous. RTG seems to have a low interaction profile, but its interactions with lamotrigine in particular should be further explored. Side effects are common and mainly related to the central nervous system, but also affect peripheral organs, such as the bladder, due its relaxing effect on smooth muscle. Slow titration could be an option to reduce such side effects.