Purpose: Generalized epilepsy with febrile seizures plus (GEFS+) and severe myoclonic epilepsy in infancy (SMEI) are associated with sodium channel α-subunit type-1 gene (SCN1A) mutations. Febrile seizures and partial seizures occur in both GEFS+ and SMEI; sporadic onset and seizure aggravation by antiepileptic drugs (AEDs) are features of SMEI. We thus searched gene mutations in isolated cases of partial epilepsy with antecedent FS (PEFS+) that showed seizure aggravations by AEDs.
Methods: Genomic DNA from four patients was screened for mutations in SCN1A, SCN2A, SCN1B, and GABRG2 using denaturing high-performance liquid chromatography (dHPLC) and sequencing. Whole-cell patch clamp analysis was used to characterize biophysical properties of two newly defined mutants of Na(v) 1.1 in tsA201 cells.
Results: Two heterozygous de novo mutations of SCN1A (R946H and F1765L) were detected, which were proven to cause loss of function of Na(v) 1.1. When the functional defects of mutants reported previously are compared, it is found that all mutants from PEFS+ have features of loss of function, whereas GEFS+ shows mild dysfunction excluding loss of function, coincident with mild clinical manifestations. PEFS+ is similar to SMEI clinically with possible AED-induced seizure aggravation and biophysiologically with features of loss of function, and different from SMEI by missense mutation without changes in hydrophobicity or polarity of the residues.
Conclusions: Isolated milder PEFS+ may associate with SCN1A mutations and loss of function of Na(v) 1.1, which may be the basis of seizure aggravation by sodium channel-blocking AEDs. This study characterized phenotypes biologically, which may be helpful in understanding the pathophysiologic basis, and further in management of the disease.