The link between a mutation and the corresponding phenotype in genetic epilepsies is sometimes not trivial. Mutations in the same gene can lead to different phenotypes (phenotypic heterogeneity) and different genes can lead to the same phenotype (genetic heterogeneity). These issues appear to be particularly prominent in some forms of seizure disorders. One of the many active research fields in genetics is studying whether environmental factors or other mutations lead to the development of a given syndrome.
Generalized epilepsy with febrile seizures plus (GEFS+) and severe myoclonic epilepsy of infancy (SMEI) can be due to the mutations in the same residue of the alpha-subunit of a voltage gated sodium channel encoded by SCN1A. Japanese researches now report in Epilepsia that the interaction with the beta-subunit of the channel rescues the GEFS+ associated mutant A1685V but not A1685D considered to be responsible for SMEI.
It’s a small step up on our understanding of such mutants. Computational analysis suggests that both variants have strong effects. E.g. Polyphen-2 predicts both to be probably damaging. It will still require further research on interactions to assess the differences. Part of this research is carried out in EuroEPINOMICS projects.