Functional Analysis of the mutated ion-channel gene in severe myoclonic epilepsy in infancy

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Severe myoclonic epilepsy in infancy(SMEI) is a malignant infant-onset epileptic syndrome with febrile seizures. We analyzed the voltage-gated sodium channel α1-subunit (SCN1A) gene, β1-subunit (SCN1B) gene and γ-aminobutyric acid _A receptor γ2-subunit (GABRG2) gene in DNAs from peripheral blood cells of patients with SMEI and patients with other types of epilepsy. Mutations of the SCN1A gene were detected in 83% of the patients with SMEI, although none with other types of epilepsy. The mutations included deletion, insertion, missense and nonsense mutations. We could not find any mutations of the SCN1B and GABRG2 genes in all patients. Our data suggested that the SCN1A mutations were significantly correlated with SME (p<0.0001).We cloned the wild type SCN1A cDNA, made the mutant type cDNAs by PCR-based mutagenesis, and constructed the SCN1A cDNA expression plasmids with Lumio-tag at the C-terminal region. When the expression plasmids were transfected into human embryonic kidney 293 cells, we found that some kinds of mutant forms are localized on cell membrane. The result suggests that the mutant forms of SCN1A may function dominant-negatively in the presence of the wild type SCN1A on the cell membrane.We found two alternative isoforms of SCN1A mRNA in human brain tissues, when we were cloning the cDNA for the expression system. Our analyses revealed that the isoforms loss the gate region of ion-channel, and that many mutations we detected had occurred in the region of SCN1A. These results suggest that the alternative isoforms also may negatively function for wild type of sodium ion-channel, like as a kind of mutant form.