Lack of potassium current in W309R mutant KCNQ3 channel causing benign familial neonatal convulsions (BFNC)

Phenotypic Spectrum in a Family Sharing a Heterozygous KCNQ3 Variant

BACKGROUND AND PURPOSE

Mutations in KCNQ3 have classically been associated with benign familial neonatal and infantile seizures and more recently identified in patients with neurodevelopmental disorders and abnormal electroencephalogram (EEG) findings. We present 4 affected patients from a family with a pathogenic mutation in KCNQ3 with a unique constellation of clinical findings.

METHODS

A family of 3 affected siblings and mother sharing a KCNQ3 pathogenic variant are described, including clinical history, genetic results, and EEG and magnetic resonance imaging (MRI) findings.

RESULTS

This family shows a variety of clinical manifestations, including neonatal seizures, developmental delays, autism spectrum disorder, and anxiety. One child developed absence epilepsy, 2 children have infrequent convulsive seizures that have persisted into childhood, and their parent developed adult-onset epilepsy. An underlying c.1091G>A (R364H) variant in KCNQ3 was found in all affected individuals.

CONCLUSIONS

The phenotypic variability of KCNQ3 channelopathies continues to expand as more individuals and families are described, and the variant identified in this family adds to the understanding of the manifestations of KCNQ3-related disorders.

Ion Channel Genes and Epilepsy: Functional Alteration, Pathogenic Potential, and Mechanism of Epilepsy

Ion channels are crucial in the generation and modulation of excitability in the nervous system and have been implicated in human epilepsy. Forty-one epilepsy-associated ion channel genes and their mutations are systematically reviewed. In this paper, we analyzed the genotypes, functional alterations (funotypes), and phenotypes of these mutations. Eleven genes featured loss-of-function mutations and six had gain-of-function mutations. Nine genes displayed diversified funotypes, among which a distinct funotype-phenotype correlation was found in SCN1A. These data suggest that the funotype is an essential consideration in evaluating the pathogenicity of mutations and a distinct funotype or funotype-phenotype correlation helps to define the pathogenic potential of a gene.

Epilepsy and ion channels

Many mutations of genes for ion channels result in some epilepsies. Their electrophysiological studies reveal pathophysiological mechanisms underlining epilepsy and also mechanism of action of several antiepileptic drugs. In this review, We briefly summarize pathophysiology of epilepsy and the mechanisms of antiepileptic drugs.

Novel KCNQ3 Mutation in a Large Family with Benign Familial Neonatal Epilepsy: A Rare Cause of Neonatal Seizures

Benign familial neonatal seizures (BFNS) present a rare familial epilepsy syndrome caused by genetic alterations in the voltage-gated potassium channels Kv7.2 and Kv7.3, encoded by KCNQ2 and KCNQ3. While most BFNS families carry alterations in KCNQ2, mutations in KCNQ3 appear to be less common. Here, we describe a family with 6 individuals presenting with neonatal focal and generalized seizures. Genetic testing revealed a novel KCNQ3 variant, c.835G>T, cosegregating with seizures in 4 tested individuals. This variant results in a substitution of the highly conserved amino acid valine localized within the pore-forming transmembrane segment S5 (p.V279F). Functional investigations in Xenopus laevis oocytes revealed a loss of function, which supports p.V279F as a pathogenic mutation. When p.V279F was coexpressed with the wild-type (WT) Kv7.2 subunits, the resulting potassium currents were about 10-fold reduced compared to the WT Kv7.3 and Kv7.2 coexpression. Genotype-phenotype correlation shows an incomplete penetrance of p.V279F. Response to antiepileptic treatment was variable, but evaluation of treatment response remained challenging due to the self-limiting character of the disease. The identification of the pathogenic variant helped to avoid unnecessary investigations in affected family members and allowed guided therapy.

Functional analysis of potassium channels in Kv7.2 G271V mutant causing early onset familial epilepsy

Kv7 (KCNQ) channels underlying a class of voltage-gated K+ current are best known for regulating neuronal excitability. The first glycine (G) residue in the pore helix of Kv7.2 (KCNQ2) subunit is highly conserved among different classes of Kv7 channel family. A missense mutation causing the replacement of the corresponding G residues with a valine (p.G271V) in Kv7.2 was found in a large, four-generation pedigree. Here, we set out to examine the molecular pathomechanism of G271V mutants using patch clamp technology combined with biochemical and immunocytochemical techniques in transiently transfected human embryonic kidney (HEK) 293 cells. The expression of Kv7.2 protein had the same intensity for both wild type (WT) and G271V. In transfected HEK cells, G271V mutants induced large depolarizing shifts of the conductance-voltage relationships and marked slowing of current activation kinetics compared to WT. In addition, G271V mutants abolished currents in homomeric channels, and resulted in about 50% reduction of current in Kv7.2/G271V/Kv7.3 heteromultimeric condition, indicating a more severe functional defect. To test for G271V mutant channel expression in surface membrane, we performed fluorescence confocal microscopy imaging, which revealed no differences between the mutant and WT, suggesting that G271V channels fail to open in response to depolarization even though they are present in the membrane. Furthermore, pharmacologic intervention experiments revealed that upon specific incubation of transfected HEK 293 cells expressing G271V heteromultimeric channels in presence of Kv7 channel enhancer retigabine (ezogabine), the potassium currents increased significantly, suggesting the potential of retigabine as gene-specific therapy.

A novel KCNQ3 mutation in familial epilepsy with focal seizures and intellectual disability

Mutations in the KCNQ2 gene encoding for voltage-gated potassium channel subunits have been found in patients affected with early onset epilepsies with wide phenotypic heterogeneity, ranging from benign familial neonatal seizures (BFNS) to epileptic encephalopathy with cognitive impairment, drug resistance, and characteristic electroencephalography (EEG) and neuroradiologic features. By contrast, only few KCNQ3 mutations have been rarely described, mostly in patients with typical BFNS. We report clinical, genetic, and functional data from a family in which early onset epilepsy and neurocognitive deficits segregated with a novel mutation in KCNQ3 (c.989G>T; p.R330L). Electrophysiological studies in mammalian cells revealed that incorporation of KCNQ3 R330L mutant subunits impaired channel function, suggesting a pathogenetic role for such mutation. The degree of functional impairment of channels incorporating KCNQ3 R330L subunits was larger than that of channels carrying another KCNQ3 mutation affecting the same codon but leading to a different amino acid substitution (p.R330C), previously identified in two families with typical BFNS. These data suggest that mutations in KCNQ3, similarly to KCNQ2, can be found in patients with more severe phenotypes including intellectual disability, and that the degree of the functional impairment caused by mutations at position 330 in KCNQ3 may contribute to clinical disease severity.

Site-directed mutagenesis of neonatal convulsions associated KCNQ2 gene and its protein expression

OBJECTIVE

To study the protocol of construction of a KCNQ2-c.812G>T mutant and its eukaryotic expression vector, the c.812G>T (p.G271V) mutation, which was detected in a Chinese pedigree of benign familial infantile convulsions (BFIC), and to examine the expression of mutant protein in human embyonic kidney (HEK) 293 cells.

METHODS

A KCNQ2 mutation c.812G>T was engineered on KCNQ2 cDNAs cloned into pcDNA3.0 by sequence overlap extension PCR and restriction enzymes. HEK293 cells were co-transfected with pRK5-GFP and KCNQ2 plasmid (the wild type or mutant) using lipofectamine and then subjected to confocal microscopy. The transfected cells were immunostained to visualize the intracellular expression of the mutant molecules.

RESULTS

Direct sequence analysis revealed a G to T transition at position 812. The c.812G>T mutation was correctly combined to eukaryotic expressive vector pcDNA3.0 and expressed in HEK293 cells. Immunostaining of transfected cells showed the expression of both the wild type and mutant molecules on the plasma membrane, which suggested that the c.812G>T mutation at the pore forming region of KCNQ2 channel did not impair normal protein expression in HEK293 cells.

CONCLUSIONS

Successful construction of mutant KCNQ2 eukaryotic expression vector and expression of KCNQ2 protein in HEK293 cells provide a basis for further study on the functional effects of convulsion-causing KCNQ2 mutations and for understanding the molecular pathogenesis of epilepsy.