SCN1AMutation Mosaicism in a Family with Severe Myoclonic Epilepsy in Infancy

Abstract
Summary: Purpose:To investigate the genetic background of familial severe myoclonic epilepsy in infancy (SMEI) cases.Methods:We performed mutation analyses of the sodium‐channel geneSCN1Ain two Japanese brothers with clinical features of SMEI and their parents, who had no history of febrile and epileptic seizures.Results:Each patient showed nucleotide changes (c.[730G>T; 735G>T; 736A>T]) in the coding exon 6 ofSCN1Athat led to a truncation of the channel protein. Their father showed no mutations, but their mother showed the same mutation in a subpopulation of lymphocytes.Conclusions:The maternal mosaicism explains the identicalSCN1Amutations in the two brothers. This highlights the importance of investigating parental mosaicism even in sporadic SMEI cases.