KCNQ2 and KCNQ3 are two homologous K ؉ channel subunits that can combine to form heterotetrameric channels with properties of neuronal M channels. Loss-of-function mutations in either subunit can lead to benign familial neonatal convulsions (BFNC), a generalized, idiopathic epilepsy of the newborn. We now describe a syndrome in which BFNC is followed later in life by myokymia, involuntary contractions of skeletal muscles. All affected members of the myokymia͞BFNC family carried a mutation (R207W) that neutralized a charged amino acid in the S4 voltage-sensor segment of KCNQ2. This substitution led to a shift of voltage-dependent activation of KCNQ2 and a dramatic slowing of activation upon depolarization. Myokymia is thought to result from hyperexcitability of the lower motoneuron, and indeed both KCNQ2 and KCNQ3 mRNAs were detected in the anterior horn of the spinal cord where the cells of the lower motoneurons arise. We propose that a difference in firing patterns between motoneurons and central neurons, combined with the drastically slowed voltage activation of the R207W mutant, explains why this particular KCNQ2 mutant causes myokymia in addition to BFNC.
Our study identified a splice-donor-site mutation that was probably causing a nonfunctional GABRG2 subunit. This mutation occurred in heterozygosity in the affected members of a single nuclear family, exhibiting a phenotypic spectrum of childhood absence epilepsy and febrile convulsions. The GABRG2 gene seems to confer a rare rather than a frequent major susceptibility effect to common idiopathic absence epilepsy syndromes.
Summary:Purpose: To identify and characterize the mutation(s) causing nocturnal frontal lobe epilepsy in a German extended family.Methods: Neuronal nicotinic acetylcholine receptor (nAChR) subunit genes were screened by direct sequencing. Once a CHRNA4 mutation was identified, its biophysical and pharmacologic properties were characterized by expression experiments in Xenopus oocytes.Results: We report a new CHRNA4 mutation, causing a α4-T265I amino acid exchange at the extracellular end of the second transmembrane domain (TM). Functional studies of α4-T265I revealed an increased ACh sensitivity of the mutated receptors.α4-T265I is associated with an unusual low penetrance of the epilepsy phenotype. Sequencing of the TM1-TM3 parts of the 1 known nAChR subunits did not support a two-locus model involving a second nAChR sequence variation.Conclusions: nAChR mutations found in familial epilepsy are not always associated with an autosomal dominant mode of inheritance. α4-T265I is the first nAChR allele showing a markedly reduced penetrance consistent with a major gene effect. The low penetrance of the mutation is probably caused by unknown genetic or environmental factors or both.
In 1998, mutations in the voltage gated potassium channel gene KCNQ2 were found to be the main cause underlying the autosomal dominant inherited syndrome of benign familial neonatal convulsions (BFNC). In one BFNC family a mutation was found in an homologous gene, KCNQ3. We have now identified another brain-expressed member of this ion channel subfamily, KCNQ5, which maps to chromosome 6q14. On the genomic level KCNQ5 is composed of 14 exons, which are coding for 897 amino acid residues. Mutation analysis made KCNQ5 unlikely as a candidate gene for benign neonatal convulsions in patients with a positive family history for neonatal or early infantile seizures, but without mutations in the KCNQ2 or KCNQ3 genes.
Recently, the gene coding for the tandem pore domain K(+)-channel TASK-3 (KCNK9) has been localized to the chromosomal region 8q24. Because mutations in ion channel genes have been recognized as an important factor in the etiology of abnormal neuronal excitability, TASK-3 is an interesting candidate gene for epilepsies linked to 8q24. We therefore performed a mutation analysis of the TASK-3 gene in 65 patients with childhood and juvenile absence epilepsy. Only one silent nucleotide exchange (636C/T) was detected in exon 2 of the TASK-3 coding region. No evidence for an allelic association was found between the exon 2 polymorphism and absence epilepsy. Accordingly, genetic variation of the TASK-3 coding region does not play a major role in the etiology of idiopathic absence epilepsies.
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