Mutations in Kv7.5 Channels Associated with Intellectual Disability or Epileptic Encephalopathy

The M-current is a slowly activating and deactivating K+ current that plays a crucial role in regulating neuronal excitability by impeding repetitive action potential firing during sustained depolarizing inputs. Inhibition of the M-current leads to enhanced neuronal excitability associated with neurological disorders. Although their role is poorly understood, Kv7.5 channels, which are highly expressed in the brain, co-assemble with Kv7.3 subunits and are thought to contribute to M-current heterogeneity. Here we show the functional consequences of de novo missense Kv7.5 mutations identified by whole exome sequencing in four children affected by intellectual disability and/or epileptic encephalopathy. We also mapped the position of the mutants in a homology model of Kv7.5 based on the cryo-EM structure of Kv7.1 channels. Electrophysiological characterization showed that 3 of the mutations (V145G in S1, L341I in S6, and S448I in the unstructured C-terminus) resulted in loss-of-function, causing a depolarizing shift of the voltage-dependence of activation and slowing of activation kinetics. The positions of V145 and L341 in the Kv7.5 protein lead us to hypothesize that mutations at these sites influence voltage sensor movement or its coupling to the pore, in the case of V145, and the stability of the open state of the pore, in the case of L341. On the other hand, P369R (in helix-A of the C-terminus) causes a hyperpolarizing shift of the voltage-dependence of activation, acceleration of activation kinetics and slowing of deactivation kinetics. These findings suggest that P369R is a gain-of-function mutation, increasing the stability of the open state of Kv7.5 channels. In summary, these data characterize the first mutations in Kv7.5 channels to be associated with human neurological disease, and the findings suggest that Kv7.5 channels present a potential therapeutic target for the treatment of neuronal excitability disorders.