SCN1A mutation mosaicism in a family with severe myoclonic epilepsy in infancy.

Purpose: To investigate the genetic background of familial severe myoclonic epilepsy in infancy (SMEI) cases.
Methods: We performed mutation analyses of the sodium-channel gene SCN1A in 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 of SCN1A that 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 identical SCN1A mutations in the two brothers. This highlights the importance of investigating parental mosaicism even in sporadic SMEI cases.