Sodium channel mutations causing generalized epilepsy with febrile seizures + (Barela et al. 2006)

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A novel mutation, R859C, in the Nav1.1 sodium channel was identified in a 4-generation, 33-member Caucasian family with a clinical presentation consistent with GEFS+. The mutation neutralizes a positively charged arginine in the domain 2 S4 voltage sensor of the Nav1.1 channel Ą subunit. When the mutation was placed in the rat Nav1.1 channel and expressed in Xenopus oocytes, the mutant channel displayed a positive shift in the voltage-dependence of sodium channel activation, slower recovery from slow inactivation, and lower levels of current compared to the wild-type channel. Computational analysis suggests that neurons expressing the mutant channel have higher thresholds for firing a single action potential and for firing multiple action potentials, along with decreased repetitive firing. Therefore, this mutation should lead to decreased neuronal excitability, in contrast to most previous GEFS+ sodium channel mutations that have changes predicted to increase neuronal firing.
1 . Barela AJ, Waddy SP, Lickfett JG, Hunter J, Anido A, Helmers SL, Goldin AL, Escayg A (2006) An epilepsy mutation in the sodium channel SCN1A that decreases channel excitability. J Neurosci 26:2714-23 [PubMed]
2 . Spampanato J, Aradi I, Soltesz I, Goldin AL (2004) Increased neuronal firing in computer simulations of sodium channel mutations that cause generalized epilepsy with febrile seizures plus. J Neurophysiol 91:2040-50 [PubMed]
Model Information (Click on a link to find other models with that property)
Model Type: Neuron or other electrically excitable cell;
Brain Region(s)/Organism:
Cell Type(s):
Channel(s): I Na,t; I K; I Sodium;
Gap Junctions:
Gene(s): Nav1.1 SCN1A;
Simulation Environment: NEURON;
Model Concept(s): Action Potentials; Epilepsy;
Implementer(s): Lickfett, Jay ; Goldin, Al [agoldin at];
Search NeuronDB for information about:  I Na,t; I K; I Sodium;
:  ichanR859C1.mod 
:   Alan Goldin Lab, University of California, Irvine
:   Jay Lickfett - Last Modified: 6 July 2005

:  This file defines the R859C mutation (w/ B1) described in:
:		Barela et al. An Epilepsy Mutation in the Sodium Channel SCN1A That Decreases
:	    Channel Excitability.  J. Neurosci. 26(10): p. 2714-2723 
:   The model is derived from the one described in:
:    	Spampanato et al. (2004a) Increased Neuronal Firing in Computer Simulations 
:		of Sodium Channel Mutations that Cause Generalized Epilepsy with Febrile Seizures Plus.
:		Journal of Neurophysiology 91:2040-2050
:	and
:	 	Spampanato et al. (2004b) A Novel Epilepsy Mutation 
:   	in the Sodium Channel SCN1A Identifies a Cytoplasmic Domain for 
:		Beta Subunit Interaction. J. Neurosci. 24(44):10022-10034

    (mA) = (milliamp)
    (mV) = (millivolt)
    (uF) = (microfarad)
    (molar) = (1/liter)
    (nA) = (nanoamp)
    (mM) = (millimolar)
    (um) = (micron)
    (S) = (siemens)
    FARADAY = 96520 (coul)
    R = 8.3134  (joule/degC)


    SUFFIX ichanR859C1 
    USEION nat READ enat WRITE inat VALENCE 1
    RANGE gnat, gkf
    RANGE gnatbar, gkfbar
    RANGE gl, el
    RANGE minf, mtau, hinf, htau, sinf, stau, nfinf, nftau, inat, m, h, s

INDEPENDENT {t FROM 0 TO 100 WITH 100 (ms)}

    celsius = 6.3 (degC)
    dt (ms) 
    enat  (mV)
    gnatbar (mho/cm2)   
    ekf  (mV)
    gkfbar (mho/cm2)
    gl (mho/cm2)    
    el (mV)

    v (mV) 
    gnat (mho/cm2) 
    gkf (mho/cm2)
    inat (mA/cm2)
    ikf (mA/cm2)
    il (mA/cm2)
    minf hinf sinf nfinf
    mtau (ms) htau (ms) stau (ms) nftau (ms)
    mexp hexp sexp nfexp

    m h s nf

    SOLVE states
    gnat = gnatbar*m*m*m*h*s  
    inat = gnat*(v - enat)
    gkf = gkfbar*nf*nf*nf*nf
    ikf = gkf*(v-ekf)
    il = gl*(v-el)



    m = minf
    h = hinf
    s = sinf

    nf = nfinf

PROCEDURE states() {        : Computes state variables m, h, s and n 
                            : at the current v and dt.        

    m = m + mexp*(minf-m)
    h = h + hexp*(hinf-h)
    s = s + sexp*(sinf-s)
    nf = nf + nfexp*(nfinf-nf)
    return 0;


PROCEDURE rates(v (mV)) {   :Computes rate and other constants at current v.
                            :Call once from HOC to initialize inf at resting v.

    LOCAL  alpha, beta, sum
    q10 = 3^((celsius - 6.3)/10)
    :"m" sodium activation system
    minf = 1/(1+exp(-(v+21.3)*3.5*0.03937))   	
    mtau = 0.15								 

    :"h" sodium fast inactivation system
    hinf = 1/(1+exp((v+41.9)/6.7))				
    htau = 23.12*exp(-0.5*((v+77.58)/43.92)^2) 
    :"s" sodium slow inactivation system
    sinf = 1/(1+exp((v+46.0)/6.6))				
    stau = 1000*(190.2*exp(-0.5*((v+90.4)/38.9)^2))

    :"nf" fKDR activation system			
    alpha = -0.07*vtrap((v+65-47),-6)
    beta = 0.264/exp((v+65-22)/40)
    sum = alpha+beta        
    nftau = 1/sum      
    nfinf = alpha/sum

PROCEDURE trates(v (mV)) {  :Build table with rate and other constants at current v.
                            :Call once from HOC to initialize inf at resting v.
    LOCAL tinc
    TABLE minf, mexp, hinf, hexp, sinf, sexp, nfinf, nfexp, mtau, htau, stau, nftau
        DEPEND dt, celsius FROM -100 TO 100 WITH 200
    rates(v)    : not consistently executed from here if usetable_hh == 1
                : so don't expect the tau values to be tracking along with
                : the inf values in hoc

    tinc = -dt * q10
    mexp = 1 - exp(tinc/mtau)
    hexp = 1 - exp(tinc/htau)
    sexp = 1 - exp(tinc/stau)
    nfexp = 1 - exp(tinc/nftau)

FUNCTION vtrap(x,y) {  :Traps for 0 in denominator of rate eqns.

    if (fabs(x/y) < 1e-6) {
        vtrap = y*(1 - x/y/2)
        vtrap = x/(exp(x/y) - 1)


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