Voltage-gated potassium channel KCNV2 (Kv8.2) contributes to epilepsy susceptibility

Abstract

Mutations in voltage-gated ion channels are responsible for several types of epilepsy. Genetic epilepsies often exhibit variable severity in individuals with the same mutation, which may be due to variation in genetic modifiers. The Scn2a(Q54) transgenic mouse model has a sodium channel mutation and exhibits epilepsy with strain-dependent severity. We previously mapped modifier loci that influence Scn2a(Q54) phenotype severity and identified Kcnv2, encoding the voltage-gated potassium channel subunit Kv8.2, as a candidate modifier. In this study, we demonstrate a threefold increase in hippocampal Kcnv2 expression associated with more severe epilepsy. In vivo exacerbation of the phenotype by Kcnv2 transgenes supports its identification as an epilepsy modifier. The contribution of KCNV2 to human epilepsy susceptibility is supported by identification of two nonsynonymous variants in epilepsy patients that alter function of Kv2.1/Kv8.2 heterotetrameric potassium channels. Our results demonstrate that altered potassium subunit function influences epilepsy susceptibility and implicate Kcnv2 as an epilepsy gene.

Fig. 1.

Functional consequences of amino acid sequence variants in mouse Kcnv2. (A) Averaged whole-cell current traces normalized to membrane capacitance recorded from CHO cells expressing mKv2.1 in combination with either B6-Kv8.2 or SJL-Kv8.2 (n = 7 for each condition). (B) Current density–voltage relationships for mKv2.1 coexpressed with the two Kv8.2 isoforms. Current was measured at 1,990 ms after start of the test pulse then normalized to membrane capacitance. (C) Extent of time-dependent whole-cell current decay illustrated by the ratio of steady-state current (measured at 1,990 ms) to instantaneous current (measured at 100 ms).

Fig. 2.

Relative hippocampal expression of Kcnv2. Transcript levels were measured by qRT-PCR (n = 7 per strain; *P = 0.012 compared with B6, pairwise fixed reallocation randomization test).

Fig. 3.

Transgenic transfer of the modified phenotype in Kcnv2;Q54 double transgenic mice. (A) Relative whole-brain expression of Kcnv2 transcript measured by qRT-PCR (n = 5 per genotype; *P ≤ 0.002 compared with nontransgenic, pairwise fixed reallocation randomization test). (B) Percentage of mice exhibiting more than 1 seizure per 30 min by 6 wk of age (*P < 0.03 compared with B6.Q54, Fisher's exact test). (C) Survival of Q54 mice carrying Kcnv2 transgenes and single transgenic B6.Q54 littermates (dashed black line).

Fig. 4.

Functional consequences of human KCNV2 variants. (A) Averaged whole-cell current traces normalized to membrane capacitance recorded from CHO cells coexpressing human Kv2.1 with wild-type or mutant Kv8.2 (n = 7 to 8 for each condition). (B) Current density–voltage relationships recorded from cells coexpressing Kv2.1 with wild-type or mutant Kv8.2. Current was measured at 1,990 ms after start of the test pulse, then normalized to membrane capacitance. (C) Voltage dependence of activation time constants for Kv2.1 coexpressed with wild-type or mutant Kv8.2 (*P < 0.05 compared with WT). Time constants were determined from monoexponential fits to the data. Currents recorded at voltages <0 mV were too small to determine time constants. Inset: Averaged current traces (black, hKv8.2-WT; blue, hKv8.2-R7K; red, hKv8.2-M285R) recorded from 10 to 500 ms after a test pulse to 0 mV and normalized to current amplitude measured at 500 ms. (D) Voltage dependence of steady-state activation. Tail-current amplitudes were measured at −30 mV after a 2,000-ms activating pulse from −60 to +60 mV. Currents were normalized to peak amplitude and fit with Boltzmann functions. Values for activation V_1/2 were as follows: wild-type Kv8.2, 0.9 ± 1.2 mV; Kv8.2-R7K, 2.0 ± 1.6 mV (not significantly different from wild-type); and Kv8.2-M285R, 9.1 ± 1.1 mV (P < 0.01 compared with wild-type Kv8.2).