Mutations in K v 7.2 ( KCNQ2 ) and K v 7.3 ( KCNQ3 ) genes, encoding for voltage-gated K + channel subunits underlying the neuronal M-current, have been associated with a wide spectrum of early-onset epileptic disorders ranging from benign familial neonatal seizures to severe epileptic encephalopathies. The aim of the present work has been to investigate the molecular mechanisms of channel dysfunction caused by voltage-sensing domain mutations in K v 7.2 (R144Q, R201C, and R201H) or K v 7.3 (R230C) recently found in patients with epileptic encephalopathies and/or intellectual disability. Electrophysiological studies in mammalian cells transfected with human K v 7.2 and/or K v 7.3 cDNAs revealed that each of these four mutations stabilized the activated state of the channel, thereby producing gain-of-function effects, which are opposite to the loss-of-function effects produced by previously found mutations. Multistate structural modeling revealed that the R201 residue in K v 7.2, corresponding to R230 in K v 7.3, stabilized the resting and nearby voltage-sensing domain states by forming an intricate network of electrostatic interactions with neighboring negatively charged residues, a result also confirmed by disulfide trapping experiments. Using a realistic model of a feedforward inhibitory microcircuit in the hippocampal CA1 region, an increased excitability of pyramidal neurons was found upon incorporation of the experimentally defined parameters for mutant M-current, suggesting that changes in network interactions rather than in intrinsic cell properties may be responsible for the neuronal hyperexcitability by these gain-of-function mutations. Together, the present results suggest that gain-of-function mutations in K v 7.2/3 currents may cause human epilepsy with a severe clinical course, thus revealing a previously unexplored level of complexity in disease pathogenetic mechanisms.