Kv7.2 and Kv7.3 are neuronal voltage-gated potassium channel subunits encoded by KCNQ2 and KCNQ3 genes, underlying the M-current, a slowly activating and non-inactivating current involved in neuronal excitability control. Kv7.2 mutations lead to neonatal-onset epilepsies including developmental and epileptic encephalopathies (DEE), characterized by seizures, electroencephalographic abnormalities, and developmental delay. When expressed in vitro, most Kv7.2 variants cause loss-of-function (LoF) effects. In this study, we aimed to functionally characterize a de novo KCNQ2 mutation in the pore of Kv7.2 (A265V) found in a patient with DEE. Functional properties of the currents from wt and/or mutant channels heterologously-expressed in CHO cells were recorded using the whole-cell configuration of the patch clamp technique. Currents from Kv7.2 A265V channels revealed a strong decrease in the peak current, both in homomeric configuration and when expressed together with Kv7.2 or Kv7.3 subunits (1:1 ratio). Interestingly, incorporation of Kv7.2 A265V subunits into homomeric or heteromeric channels with Kv7.2 or Kv7.3 subunits led to the appearance of an unusual current inactivation process. In fact, in homomeric Kv7.2 A265V channels, the ratio between the peak and steady-state current during a depolarizing pulse at +20mV was 0.5±0.02 (n=30), while it was close to unit in Kv7.2 channels. This effect was less intense upon co-expression with Kv7.2 or Kv7.2 and Kv7.3 subunits (ratio of 0.73±0.03 and 0.91±0.01, respectively). The time constant for inactivation of Kv7.2 A265V was 297.5±36.6ms. Thus, these data reveal, a unique novel mechanism responsible for LoF effects, namely, the mutation induced occurrence of inactivation.