A De Novo Mutation in the Sodium-Activated Potassium Channel KCNT2 Alters Ion Selectivity and Causes Epileptic Encephalopathy

Abstract

Summary Early onset epileptic encephalopathies (EOEE) are a debilitating spectrum of disorders associated with cognitive impairments. We present a clinical report of a KCNT2 mutation in an EOEE patient. The de novo heterozygous variant Phe240Leu SLICK was identified by exome sequencing and confirmed by Sanger sequencing. Phe240Leu rSlick and hSLICK channels were electrophysiologically heterologously characterized to reveal three significant alterations to channel function. First, [Cl−]i-sensitivity was reversed in Phe240Leu channels. Second, predominantly K+-selective WT channels were made to favor Na+ over K+ by Phe240Leu. Third and consequent to altered ion selectivity, Phe240Leu channels had larger inward conductance. Further, rSlick channels induced membrane hyperexcitability when expressed in primary neurons, resembling the cellular seizure phenotype. Taken together, our results confirm that Phe240Leu is a ‘change-of-function’ KCNT2 mutation, demonstrating unusual altered selectivity in KNa channels. These findings establish pathogenicity of the Phe240Leu KCNT2 mutation in the reported EOEE patient.

De novo gain‐of‐function variants in KCNT2 as a novel cause of developmental and epileptic encephalopathy

“…The functional changes herein described for KCNT2 R190P and KCNT2 R190H channels are rather different from those reported for the KCNT2 F240L variant recently associated to DEE . In fact, currents carried by KCNT2 F240L homomeric channels were inhibited (rather than enhanced as in wild‐type KCNT2) by an increase in intracellular Cl – concentration and showed a marked loss of selectivity for K + over Na + ions, a result consistent with the location of the F240 residue in the pore helix immediately before the selectivity filter. At this position, it has been proposed that the F240 residue interacts with residues in both S 5 and S 6 helices to modulate open probability and selectivity of KCNT2 channels …”

“…The KCNT2 ‐associated phenotype comprises West syndrome (individual #1 and published case) subsequently followed by Lennox‐Gastaut syndrome (individual #1) as well as EIMFS (individual #2). It therefore significantly parallels the spectrum of KCNT1 ‐related DEE, which predominantly presents with EIMFS, but may also evolve into West syndrome .…”

“…KCNT1 and KCNT2 subunits can form functional homo‐ or heteromeric channels . Whereas KCNT1 has been associated with DEE (ie, epilepsy of infancy with migrating focal seizures [EIMFS]) as well as a spectrum of focal epilepsy disorders, KCNT2 has been suspected of being a promising candidate gene for epilepsy and only very recently been linked to West syndrome in a single individual …”

Sorting nexin-27 regulates AMPA receptor trafficking through the synaptic adhesion protein LRFN2

“…While known SNX27-associated neuronal integral proteins, such as NMDA receptors (26,39), 5-HT4 receptor (4), metabotropic glutamate receptor 5 (mGluR5) (45), neuroligin 2 (44,46), and Kir3 channels (27), have provided some insight into these complex phenotypes our unbiased quantitative identification of the neuronal SNX27 interactome has revealed an additional cohort of integral neuronal proteins that associate with SNX27. Besides LRFN2 (see discussion below), many of these proteins function in an array of neuronal activities: the high affinity glutamate transporter, SLC1A3, and the sodium bicarbonate cotransporter, SLC4A7, are associated with controlling glutamate neurotoxicity (47)(48)(49)(50); SLC6A11, a sodium-dependent transporter, uptakes GABA and modulates GABAergic tone (32); KCNT2, an outward rectifying potassium channel, is associated with early infantile epileptic encephalopathies (33,51); KIDINS220, a scaffold in neurotrophin signalling, is associated with spastic paraplegia and intellectual disability (35,52); and ADAM22 is a receptor for the neuronal secreted protein LGI1 (36), the product of the causative gene for autosomal dominant partial epilepsy with auditory features (53). This significant expansion in the neuronal targets for SNX27-mediated endosomal sorting has therefore broadened our understanding of those integral proteins whose perturbed cell surface expression may underly the complex neurological phenotypes observed in SNX27-associated pathologies.…”

Cardiac metabolic effects of KNa1.2 channel deletion and evidence for its mitochondrial localization

“…Mammalian sodium-activated potassium (K Na ) channels are encoded by 2 genes: Kcnt1 (K Na 1.1; Slack/SLO2.2) (14) and Kcnt2 (K Na 1.2; Slick/SLO2.1) (15). Both K Na 1.1 and K Na 1.2 channels play important neurologic roles in the termination of seizure progression in epilepsy (16,17). Although K Na 1.2 expression has been reported in cardiac tissue (15,18) and a generic K Na channel activity has been demonstrated in the cardiac cell membrane (19), notably the Kcnt2 2/2 mice have no cardiac phenotype (3,18).…”