title: Physiological and genetic analysis of multiple sodium channel variants in a model of genetic absence epilepsy.
abstract: In excitatory neurons, SCN2A (NaV1.2) and SCN8A (NaV1.6) sodium channels are enriched at the axon initial segment. NaV1.6 is implicated in several mouse models of absence epilepsy, including a missense mutation identified in a chemical mutagenesis screen (Scn8a(V929F)). Here, we confirmed the prior suggestion that Scn8a(V929F) exhibits a striking genetic background-dependent difference in phenotypic severity, observing that spike-wave discharge (SWD) incidence and severity are significantly diminished when Scn8a(V929F) is fully placed onto the C57BL/6J strain compared with C3H. Examination of sequence differences in NaV subunits between these two inbred strains suggested NaV1.2(V752F) as a potential source of this modifier effect. Recognising that the spatial co-localisation of the NaV channels at the axon initial segment (AIS) provides a plausible mechanism for functional interaction, we tested this idea by undertaking biophysical characterisation of the variant NaV channels and by computer modelling. NaV1.2(V752F) functional analysis revealed an overall gain-of-function and for NaV1.6(V929F) revealed an overall loss-of-function. A biophysically realistic computer model was used to test the idea that interaction between these variant channels at the AIS contributes to the strain background effect. Surprisingly this modelling showed that neuronal excitability is dominated by the properties of NaV1.2(V752F) due to "functional silencing" of NaV1.6(V929F) suggesting that these variants do not directly interact. Consequent genetic mapping of the major strain modifier to Chr 7, and not Chr 2 where Scn2a maps, supported this biophysical prediction. While a NaV1.6(V929F) loss of function clearly underlies absence seizures in this mouse model, the strain background effect is apparently not due to an otherwise tempting Scn2a variant, highlighting the value of combining physiology and genetics to inform and direct each other when interrogating genetic complex traits such as absence epilepsy.
authors.affiliation: The Florey Institute for Neuroscience and Mental Health, The University of Melbourne, Australia.
authors.forename: M K
authors.initials: MK
authors.lastname: Oliva
authors.affiliation: The Jackson Laboratory, Bar Harbor, ME, USA.
authors.forename: T C
authors.initials: TC
authors.lastname: McGarr
authors.affiliation: The Jackson Laboratory, Bar Harbor, ME, USA.
authors.forename: B J
authors.initials: BJ
authors.lastname: Beyer
authors.affiliation: The Florey Institute for Neuroscience and Mental Health, The University of Melbourne, Australia.
authors.forename: E
authors.initials: E
authors.lastname: Gazina
authors.affiliation: The Florey Institute for Neuroscience and Mental Health, The University of Melbourne, Australia.
authors.forename: D I
authors.initials: DI
authors.lastname: Kaplan
authors.affiliation: The Florey Institute for Neuroscience and Mental Health, The University of Melbourne, Australia.
authors.forename: L
authors.initials: L
authors.lastname: Cordeiro
authors.affiliation: The Florey Institute for Neuroscience and Mental Health, The University of Melbourne, Australia.
authors.forename: E
authors.initials: E
authors.lastname: Thomas
authors.affiliation: Department for Neurology, Center for Neuroscience and Regeneration Research, Yale University, New Haven, USA.
authors.forename: S D
authors.initials: SD
authors.lastname: Dib-Hajj
authors.affiliation: Department for Neurology, Center for Neuroscience and Regeneration Research, Yale University, New Haven, USA.
authors.forename: S G
authors.initials: SG
authors.lastname: Waxman
authors.affiliation: The Jackson Laboratory, Bar Harbor, ME, USA.
authors.forename: W N
authors.initials: WN
authors.lastname: Frankel
authors.affiliation: The Florey Institute for Neuroscience and Mental Health, The University of Melbourne, Australia; The Centre for Neural Engineering, The University of Melbourne, Australia. Electronic address: spetrou@unimelb.edu.au.
authors.forename: S
authors.initials: S
authors.lastname: Petrou
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology; D001921:Brain / Q000503:physiopathology*; D004195:Disease Models, Animal; D004832:Epilepsy, Absence / Q000235:genetics* / Q000503:physiopathology*; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D008959:Models, Neurological; D062551:NAV1.2 Voltage-Gated Sodium Channel / Q000235:genetics* / Q000378:metabolism*; D062557:NAV1.6 Voltage-Gated Sodium Channel / Q000235:genetics* / Q000378:metabolism*
chemical_list: D062551:NAV1.2 Voltage-Gated Sodium Channel; D062557:NAV1.6 Voltage-Gated Sodium Channel; C491046:Scn8a protein, mouse
keywords: Absence seizures; Computational analysis; Genetic epilepsy; Murine AE; Sodium channels