Malignant migrating partial seizures of infancy (MMPSI) is a rare epileptic encephalopathy of infancy that combines pharmacoresistant seizures with developmental delay1. We performed exome sequencing in 3 probands with MMPSI and identified de novo gain-of-function mutations in the C-terminal domain of the KCNT1 potassium channel. We sequenced KCNT1 in 9 additional patients with MMPSI and identified mutations in 4 of them, in total identifying mutations in 6 out of 12 unrelated patients. Functional studies showed that the mutations led to constitutive activation of the channel, mimicking the effects of phosphorylation of the C-terminal domain by protein kinase C. In addition to regulating ion flux, KCNT1 has a non conducting function as its C terminus interacts with cytoplasmic proteins involved in developmental signaling pathways. These results provide a target for future diagnostic approaches and research in this devastating condition.

In humans, absence of Fragile X mental retardation protein (FMRP), an RNA-binding protein, results in Fragile X syndrome (FXS), the most common inherited form of intellectual disability. Here we report through biochemical and electrophysiological studies that FMRP binds the C-terminus of the Slack sodium-activated potassium channel to activate the channel. The findings suggest that Slack activity may provide a link between patterns of neuronal firing and changes in protein translation.

In severe early-onset epilepsy, precise clinical and molecular genetic diagnosis is complex, as many metabolic and electro-physiological processes have been implicated in disease causation. The clinical phenotypes share many features such as complex seizure types and developmental delay. Molecular diagnosis has historically been confined to sequential testing of candidate genes known to be associated with specific sub-phenotypes, but the diagnostic yield of this approach can be low. We conducted whole-genome sequencing (WGS) on six patients with severe early-onset epilepsy who had previously been refractory to molecular diagnosis, and their parents. Four of these patients had a clinical diagnosis of Ohtahara Syndrome (OS) and two patients had severe non-syndromic early-onset epilepsy (NSEOE). In two OS cases, we found de novo non-synonymous mutations in the genes KCNQ2 and SCN2A. In a third OS case, WGS revealed paternal isodisomy for chromosome 9, leading to identification of the causal homozygous missense variant in KCNT1, which produced a substantial increase in potassium channel current. The fourth OS patient had a recessive mutation in PIGQ that led to exon skipping and defective glycophosphatidyl inositol biosynthesis. The two patients with NSEOE had likely pathogenic de novo mutations in CBL and CSNK1G1, respectively. Mutations in these genes were not found among 500 additional individuals with epilepsy. This work reveals two novel genes for OS, KCNT1 and PIGQ. It also uncovers unexpected genetic mechanisms and emphasizes the power of WGS as a clinical tool for making molecular diagnoses, particularly for highly heterogeneous disorders.

Summary Disease-causing mutations in ion channels generally alter intrinsic gating properties such as activation, inactivation or voltage-dependence. We examined nine different mutations of the KCNT1 (Slack) Na+-activated K+ channel that give rise to three distinct forms of epilepsy. All produced many fold-increases in current amplitude over that of the wild type channel. This could not be accounted for by increases in the intrinsic open probability of individual channels. Rather, greatly increased opening was a consequence of cooperative interactions between multiple channels in a patch. The degree of cooperative gating was much greater for all of the mutant channels than for the wild type channel, and could explain increases in current even in a mutant with reduced unitary conductance. We also found that the same mutation gives rise to different forms of epilepsy in different individuals. Our findings indicate that a major consequence of the mutations is to alter channel-channel interactions.

Fragile X Mental Retardation Protein (FMRP) is an RNA-binding protein that regulates synaptic plasticity by repressing translation of specific mRNAs. We found that FMRP binds mRNA encoding the voltage-gated potassium channel Kv3.1b in brainstem synaptosomes. To explore the regulation of Kv3.1b by FMRP, we investigated Kv3.1b immunoreactivity and potassium currents in the auditory brainstem sound localization circuit of male mice. The unique features of this circuit allowed us to control neuronal activity in vivo by exposing animals to high-frequency amplitude modulated (AM) stimuli, which elicit predictable and stereotyped patterns of input to the anterior ventral cochlear nucleus (AVCN) and medial nucleus of the trapezoid body (MNTB). In wild type (WT) animals, Kv3.1b is expressed along a tonotopic gradient in the MNTB, with highest levels in neurons at the medial, high-frequency end. At baseline, Fmr1−/− mice, which lack FMRP, displayed dramatically flattened tonotopicity in Kv3.1b immunoreactivity and K+ currents relative to WT controls. Moreover, following 30 minutes of acoustic stimulation, levels of Kv3.1b immunoreactivity were significantly elevated in both the MNTB and AVCN of WT, but not Fmr1−/−, mice. These results suggest that FMRP is necessary for maintenance of the gradient in Kv3.1b protein levels across the tonotopic axis of the MNTB, and are consistent with a role for FMRP as a repressor of protein translation. Using numerical simulations, we demonstrate that Kv3.1b tonotopicity may be required for accurate encoding of stimulus features such as modulation rate, and that disruption of this gradient, as occurs in Fmr1−/− animals, degrades processing of this information.