Abstracts

The majority of pediatric epilepsy patients have cortical seizures although little is known at the cellular level regarding underlying mechanisms in humans. Most mutations in familial epilepsy syndromes are in ion channel and transporter genes, and in model systems, subunit expression levels critically impact channel and neuronal network function. Because electrophysiologic analysis has identified aberrant sodium currents within surgically resected seizure foci, we hypothesize that altered expression of ion channel subunit genes may be accountable, and thus may be critical for the expression of epileptogenesis in cortical tissue.
Neocortical tissue classified as [lsquo]focal[rsquo] or [lsquo]control[rsquo] by source localization techniques as appropriate, was recovered from patients with medically intractable epilepsy. Expression analysis was performed using commercial cDNA arrays containing voltage-gated ion channel and ion transporter subunit genes, and/or using Real-Time RT-PCR amplification to confirm array results and to assay genes with expression below background 32P hybridization levels or not on the array. Total RNA was purified and mRNA amplified using gene-specific primers, or random-hexamer primers, respectively. Relative gene expression was determined by taking the ratio of [lsquo]focal[rsquo] to [lsquo]control[rsquo] signal, following normalization of expression between a patient[apos]s samples (internal control) and to non-epileptic post-mortem cortex (external control).
Over 25% of genes with expression levels above background on a cDNA array exhibited greater than 2-fold expression differences between [lsquo]focal[rsquo] and internal [lsquo]control[rsquo] tissue, with a further 40% showing changes over 1.4-fold. Real-Time RT-PCR analysis on additional genes identified similar amplitudes of expression changes. Almost twice as many high-expressing genes showed a decrease in expression in [lsquo]focal[rsquo] tissue as showed increased expression by cDNA array. The direction (but not amplitude) of expression change of most genes was similar between patients with ganglioma, encephalopathy or idiopathic epilepsy. Less than 15% of genes showed patient specific expression changes, with the majority being increases in expression. Expression differences were also noted between separate foci removed from a single patient, suggesting different expression profiles in primary and secondary foci.
These results suggest that small changes in expression of voltage-gated ion channel subunits may contribute to the abnormal behavior of seizure foci, and that there may be a common mechanism behind cortical epilepsies with differing etiologies. Further studies are ongoing to determine whether correlations exist between amplitude of expression changes, patient phenotype and abnormal electrophysiologic behavior, and whether isolated affected genes are specific to patient, etiology or seizure localization.
[Supported by: Falk Foundation (LBKH, WvD, KEH, CJM), International Rett Syndrome Association (LBKH), Children[apos]s Research Foundation (LBKH, CJM)]