SHORT COMMUNICATION Enhanced inactivation and acceleration of activation of the sodium channel associated with epilepsy in man Alexi K. Alekov, 1 MD. Masmudur Rahman, 1,2 Nenad Mitrovic, 1,2 Frank Lehmann-Horn 1 and Holger Lerche 1,2 Departments of 1 Applied Physiology and 2 Neurology, University of Ulm, Zentrum Klinische Forschung, Helmholtzstr. 8/1, D-89081, Ulm, Germany Keywords: febrile seizure, genetics, idiopathic generalized epilepsy, ion channel, patch clamp Abstract Generalized epilepsy with febrile seizures-plus (GEFS + ) is a benign Mendelian syndrome characterized by childhood-onset febrile and afebrile seizures. Three point mutations within two voltage-gated sodium channel genes have been identi®ed so far: in GEFS + type 1 a mutation in the b 1 -subunit gene SCN1B, and in GEFS + type 2 two mutations within the neuronal a-subunit gene SCN1A. Functional expression of the SCN1B and one of the SCN1A mutations revealed defects in fast channel inactivation which are in line with previous ®ndings on myotonia causing mutations in SCN4A, the skeletal muscle sodium channel a-subunit gene, all showing an impaired fast inactivation. We now studied the second GEFS + mutation (T875M in SCN1A), using the highly homologous SCN4A gene (mutation T685M). Unexpectedly, the experiments revealed a pronounced enhancement of both fast and slow inactivation and a defect of channel activation for T685M compared to wild-type channels. Steady-state fast and slow inactivation curves were shifted in the hyperpolarizing direction, entry into slow inactivation was threefold accelerated, recovery from slow inactivation was slowed by threefold and the time course of activation was slightly but signi®cantly accelerated. In contrast to other disease-causing mutations in SCN1A, SCN1B and SCN4A, the only mechanism that could explain hyperexcitability of the cell membrane would be the acceleration of activation. Because the enhancement of slow inactivation was the most obvious alteration in gating found for T685M, this might be the disease-causing mechanism for that mutation. In this case, the occurrence of epileptic seizures could be explained by a decrease of excitability of inhibitory neurons. Introduction Ion channel disorders are rare inherited diseases providing interesting models for studying dysfunction of excitability in vivo and in vitro. The ®rst so-called `channelopathies' identi®ed were the myotonias and hyperkalemic periodic paralysis, caused by mutations in the skeletal muscle sodium and chloride channels. More than 20 mutations have been described in SCN4A, the gene encoding the skeletal muscle voltage-gated sodium channel. They all lead to a disruption of fast channel inactivation that explain the pathological hyperexcitability via an increase of the sodium inward current causing depolarization of the sarcolemma. The same pathophysiolo- gical mechanism applies to one form of the long QT syndrome (LQT type 3, mutations in SCN5A), an inherited cardiac arrhythmia (reviewed by Lehmann-Horn & Jurkat-Rott, 1999). Recently, another form of idiopathic epilepsy called `generalized epilepsy with febrile seizures-plus' (GEFS + Scheffer & Berkovic, 1997), has been identi®ed as a sodium channel disorder (Wallace et al., 1998; Escayg et al., 2000a). In ®ve large families with autosomal dominant inheritance of GEFS + described so far, linkage has been found to either chromosome 19q13 (Wallace et al., 1998) or 2q21±33 (Baulac et al., 1999; Moulard et al., 1999; Pfeiffer et al., 1999; Lopes-Cendes et al., 2000). Up to now, one mutation has been found in SCN1B on chromosome 19 encoding the auxiliary b 1 - subunit (GEFS + type 1; Wallace et al., 1998) and two mutations have been identi®ed in the neuronal a-subunit gene, SCN1A, on chromo- some 2 (GEFS + type 2; Escayg et al., 2000a). Functional expression in heterologous systems revealed a loss of b 1 -subunit function for the SCN1B mutation resulting in a slight slowing of sodium channel fast inactivation (Wallace et al., 1998) and an acceleration of recovery from fast inactivation for one of the SCN1A mutations located in the voltage sensor of domain 4 (Alekov et al., 2000) as the main disease-causing mechanisms. Although the b 1 -subunit is also expressed in skeletal muscle, only symptoms of brain dysfunction, i.e. epileptic seizures, but no myotonia were reported for this family (Singh et al., 1999). In addition, the gating alterations found for the two epilepsy-causing mutations were much more subtle than those found for SCN4A mutations associated with myotonia, indicating that the brain shows a greater vulnerability to changes in excitability than does muscle tissue. We examined the disease-causing mechanism of the second GEFS + mutation described in SCN1A and found that the alterations in channel gating are different from those of the previously mentioned sodium channel mutations. Voltage-gated sodium channels are membrane-spanning proteins responsible for the initiation and propagation of action potentials in nerve and muscle cells. In response to membrane depolarization the channels open from the resting, closed, state, and then inactivate spontaneously. Upon repolarization they will recover from inactiva- Correspondence: Dr Holger Lerche, Departments of Applied Physiology and Neurology, as above. E-mail: holger.lerche@medizin.uni-ulm.de Received 12 January 2001, revised 3 April 2001, accepted 10 April 2001 European Journal of Neuroscience, Vol. 13, pp. 2171±2176, 2001 ã Federation of European Neuroscience Societies