Polymorphisms in the Voltage-gated Sodium Channel Genes and Antiepileptic Efficacy of Carbamazepine

BACKGROUND: Voltage-gated sodium channels are responsible for the initial-depolarization component of action potentials in brain neurons, and hence they are the target for widely used antiepileptic drugs such as carbamazepine (CBZ). With the working hypothesis that genetic defect in voltage-gated sodium channels can alter the response to CBZ, this study was performed to elucidate the relationship between single-nucleotide polymorphisms (SNPs) of the SCN1A, SCN1B, and SCN2A genes and CBZ resistance in Korean epileptics. METHODS: Candidate SNPs of SCN1A, SCN1B, and SCN2A were developed using the pooled DNA from healthy controls (n=200), of which representative SNPs of each of SCN1A, SCN1B, and SCN2A were determined based on theoretical functional values. Each representative SNP was genotyped for a CBZ-resistant group (CRE, n=168) and a CBZ- responsive group (CSE, n=154), and the frequencies of alleles and genotypes of each SNP were compared between the two groups. RESULTS: Eighteen SNPs were developed in SCN1A, SCN1B, and SCN2A. SCN1A-PM in exon 16 of SCN1A, SCN1B-PM in exon 3 of a splicing variant of SCN1B, and SCN2A-PM in the 7th intronic sequence of SCN2A were selected as the representative SNPs for these genes. The distributions of alleles and genotypes of each representative SNP did not differ between the CRE and CSE groups. CONCLUSIONS: In Korean epileptics, there appears to be no significant relationship between representative SNPs of SCN1A, SCN1B, and SCN2A and CBZ resistance.

Gene-to-Gene Interaction between Sodium Channel-Related Genes in Determining the Risk of Antiepileptic Drug Resistance

The pathogenesis of antiepileptic drug (AED) resistance is multifactorial. However, most candidate gene association studies typically assess the effects of candidate genes independently of each other, which is partly because of the limitations of the parametric-statistical methods for detecting the gene-to-gene interactions. A total of 200 patients with drug-resistant epilepsy and 200 patients with drug-responsive epilepsy were genotyped for 3 representative the single nucleotide polymorphisms (SNPs) of the voltage-gated sodium channel genes (SCN1A, SCN1B, and SCN2A) by polymerase chain reaction and direct sequencing analysis. Besides the typical parametric statistical method, a new statistical method (multifactor dimensionality reduction [MDR]) was used to determine whether gene-to-gene interactions increase the risk of AED resistance. None of the individual genotypes or alleles tested in the present study showed a significant association with AED resistance, regardless of their theoretical functional value. With the MDR method, of three possible 2-locus genotype combinations, the combination of SCN2A-PM with SCN1B-PM was the best model for predicting susceptibility to AED resistance, with a p value of 0.0547. MDR, as an analysis paradigm for investigating multi-locus effects in complex disorders, may be a useful statistical method for determining the role of gene-to-gene interactions in the pathogenesis of AED resistance.