A novel epilepsy mutation in the sodium channel SCN1A identifies a cytoplasmic domain for beta subunit interaction.
A mutation in the sodium channel SCN1A was identified in a small Italian family with dominantly inherited generalized epilepsy with febrile seizures plus (GEFS+). The mutation, D1866Y, alters an evolutionarily conserved aspartate residue in the C-terminal cytoplasmic domain of the sodium channel alpha subunit. The mutation decreased modulation of the alpha subunit by beta1, which normally causes a negative shift in the voltage dependence of inactivation in oocytes. There was less of a shift with the mutant channel, resulting in a 10 mV difference between the wild-type and mutant channels in the presence of beta1. This shift increased the magnitude of the window current, which resulted in more persistent current during a voltage ramp. Computational analysis suggests that neurons expressing the mutant channels will fire an action potential with a shorter onset delay in response to a threshold current injection, and that they will fire multiple action potentials with a shorter interspike interval at a higher input stimulus. These results suggest a causal relationship between a positive shift in the voltage dependence of sodium channel inactivation and spontaneous seizure activity. Direct interaction between the cytoplasmic C-terminal domain of the wild-type alpha subunit with the beta1 or beta3 subunit was first demonstrated by yeast two-hybrid analysis. The SCN1A peptide K1846-R1886 is sufficient for beta subunit interaction. Coimmunoprecipitation from transfected mammalian cells confirmed the interaction between the C-terminal domains of the alpha and beta1 subunits. The D1866Y mutation weakens this interaction, demonstrating a novel molecular mechanism leading to seizure susceptibility.
Spampanato
J
J
Department of Microbiology and Molecular Genetics, University of California, Irvine, Irvine, California 92697-4025, USA.
Kearney
J A
JA
de Haan
G
G
McEwen
D P
DP
Escayg
A
A
Aradi
I
I
MacDonald
B T
BT
Levin
S I
SI
Soltesz
I
I
Benna
P
P
Montalenti
E
E
Isom
L L
LL
Goldin
A L
AL
Meisler
M H
MH
D000200:Action Potentials / Q000235:genetics / Q000502:physiology; D000595:Amino Acid Sequence; D000818:Animals; D006224:Cricetinae; D003412:Cricetulus; D003593:Cytoplasm; D004829:Epilepsy, Generalized / Q000150:complications / Q000235:genetics* / Q000503:physiopathology; D005260:Female; D006801:Humans; D015640:Ion Channel Gating / Q000235:genetics / Q000502:physiology; D007700:Kinetics; D008297:Male; D008959:Models, Neurological; D008969:Molecular Sequence Data; D009154:Mutation; D062550:NAV1.1 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins / Q000235:genetics* / Q000502:physiology*; D009474:Neurons / Q000502:physiology; D009865:Oocytes; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins; D012441:Saccharomyces cerevisiae; D003294:Seizures, Febrile / Q000150:complications / Q000235:genetics / Q000503:physiopathology; D015222:Sodium Channels / Q000235:genetics* / Q000502:physiology*; D062547:Voltage-Gated Sodium Channel beta-1 Subunit; D014982:Xenopus laevis
D062550:NAV1.1 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins; D011994:Recombinant Proteins; C568242:SCN1A protein, human; C470379:SCN1B protein, human; D015222:Sodium Channels; D062547:Voltage-Gated Sodium Channel beta-1 Subunit