Progressive myoclonus epilepsy KCNC1 variant causes a developmental dendritopathy

Mutations in KCNC1 can cause severe neurological dysfunction, including intellectual disability, epilepsy, and ataxia. The Arg320His variant, which occurs in the voltage-sensing domain of the channel, causes a highly penetrant and specific form of progressive myoclonus epilepsy with severe ataxia, designated myoclonus epilepsy and ataxia due to potassium channel mutation (MEAK). KCNC1 encodes the voltage-gated potassium channel K_V 3.1, a channel that is important for enabling high-frequency firing in interneurons, raising the possibility that MEAK is associated with reduced interneuronal function.

To determine how this variant triggers MEAK, we expressed K_V 3.1b^R320H in cortical interneurons in vitro and investigated the effects on neuronal function and morphology. We also performed electrophysiological recordings of oocytes expressing K_V 3.1b to determine whether the mutation introduces gating pore currents.

Expression of the K_V 3.1b^R320H variant profoundly reduced excitability of mature cortical interneurons, and cells expressing these channels were unable to support high-frequency firing. The mutant channel also had an unexpected effect on morphology, severely impairing neurite development and interneuron viability, an effect that could not be rescued by blocking K_V 3 channels. Oocyte recordings confirmed that in the adult K_V 3.1b isoform, R320H confers a dominant negative loss-of-function effect by slowing channel activation, but does not introduce potentially toxic gating pore currents.

Overall, our data suggest that, in addition to the regulation of high-frequency firing, K_V 3.1 channels play a hitherto unrecognized role in neuronal development. MEAK may be described as a developmental dendritopathy.