Neutralization of a negative charge in the S1-S2 region of the KV7.2 (KCNQ2) channel affects voltage-dependent activation in neonatal epilepsy

The voltage-gated potassium channels K(V)7.2 and K(V)7.3 (genes KCNQ2 and KCNQ3) constitute a major component of the M-current controlling the firing rate in many neurons. Mutations within these two channel subunits cause benign familial neonatal convulsions (BFNC). Here we identified a novel BFNC-causing mutation (E119G) in the S1-S2 region of K(V)7.2. Electrophysiological investigations in Xenopus oocytes using two-microelectrode voltage clamping revealed that the steady-state activation curves for E119G alone and its coexpressions with K(V)7.2 and/or K(V)7.3 wild-type (WT) channels were significantly shifted in the depolarizing direction compared to K(V)7.2 or K(V)7.2/K(V)7.3. These shifts reduced the relative current amplitudes for mutant channels particularly in the subthreshold range of an action potential (about 45% reduction at -50mV for E119G compared to K(V)7.2, and 33% for E119G/K(V)7.3 compared to K(V)7.2/K(V)7.3 channels). Activation kinetics were significantly slowed for mutant channels. Our results indicate that small changes in channel gating at subthreshold voltages are sufficient to cause neonatal seizures and demonstrate the importance of the M-current for this voltage range. This was confirmed by a computer model predicting an increased burst duration for the mutation. On a molecular level, these results reveal a critical role in voltage sensing of the negatively charged E119 in SI-S2 of K(V)7.2, a region that - according to molecular modelling might interact with a positive charge in the S4 segment.