Nervous system KV7 disorders: breakdown of a subthreshold brake

Voltage-gated K+channels of the K(V)7 (KCNQ) family have been identified in the last 10-15 years by discovering the causative genes for three autosomal dominant diseases: cardiac arrhythmia (long QT syndrome) with or without congenital deafness (KCNQ1), a neonatal epilepsy (KCNQ2 and KCNQ3) and progressive deafness alone (KCNQ4). A fifth member of this gene family (KCNQ5) is not affected in a disease so far. Four genes (KCNQ2-5) are expressed in the nervous system. This review is focused on recent findings on the neuronal K(V)7 channelopathies, in particular on benign familial neonatal seizures (BFNS) and peripheral nerve hyperexcitability (PNH, neuromyotonia, myokymia) caused by KCNQ2 mutations. The phenotypic spectrum associated with KCNQ2 mutations is probably broader than initially thought, as patients with severe epilepsies and developmental delay, or with Rolando epilepsy have been described. With regard to the underlying molecular pathophysiology, it has been shown that mutations with very subtle changes restricted to subthreshold voltages can cause BFNS thereby proving in a human disease model that this is the relevant voltage range for these channels to modulate neuronal firing. The two mutations associated with PNH induce much more severe channel dysfunction with a dominant negative effect on wild type (WT) channels. Finally, K(V)7 channels present interesting targets for new therapeutic approaches to diseases caused by neuronal hyperexcitability, such as epilepsy, neuropathic pain, and migraine. The molecular mechanism of K(V)7 activation by retigabine, which is in phase III clinical testing to treat pharmacoresistant focal epilepsies, has been recently elucidated as a stabilization of the open conformation by binding to the pore region.