**Epileptic Channelopathies and Dysfunctional Excitability - From Gene Mutations to Novel Treatments**

Sigrid Marie Blom1 and Henrik Sindal Jensen2 *1Department of Physiology, University of Bern 2Neuroscience Drug Discovery, H. Lundbeck A/S 1Switzerland 2Denmark* 

#### **1. Introduction**

192 Novel Aspects on Epilepsy

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Epilepsy is not a single disorder, but a collection of disorders that all are characterized by episodic abnormal synchronous electrical activity in the brain. This abnormal activity represents a disturbance of the balance between excitatory and inhibitory neurotransmission. The majority (50%) of epilepsies are cryptogenic, meaning there is a presumptive but no identifiable underlying etiology. Approximately 20% of epilepsies have an identifiable cause (i.e. they are symptomatic) and are usually a result of trauma to the head, stroke, brain tumours, or infections. The remaining 30% are idiopathic, meaning there is no apparent underlying cause (Berg et al., 1999). However, as they are usually associated with a family history of similar seizures, they are mostly considered to be genetic. Mutations in over 70 genes have been found to cause epilepsy (Noebels, 2003). Given the dependence of seizures on synaptic transmission and neuronal excitability, it is not surprising that many of these mutations affect the function of ion channels. Since the identification of the first epilepsycausing ion channel mutation, scientists have come a long way in the understanding of the pathogenesis of the disease. This chapter deals with some of the main questions that have been asked, and looks at some of the proposed answers to the questions. How do mutations in certain ion channels lead to hyperexcitability and seizures? Why do mutations in one ion channel cause a particular epilepsy syndrome? Why are the seizures often initiated during specific physiological events? And why do most of the childhood epilepsies remit with age? Furthermore, ion channels as targets for antiepileptic drugs will be discussed.

## **2. Idiopathic epilepsies**

In most cases genetic epilepsy syndromes have a complex rather than a simple inheritance pattern. Although the epilepsies described here are thought to be monogenic, not even those considered inherited in a dominant fashion have a penetrance of 100%. Mutations within the same gene can result in clinically distinct phenotypes. Variable expressivity is also a common feature of inherited epilepsy demonstrated by family members with the same mutation that exhibit differences in the clinical severity of the disease (Hayman et al., 1997).

Epileptic Channelopathies and Dysfunctional

**3.2 GEFS+ and SMEI** 

abundantly expressed in the brain (Yu and Catterall, 2003).

Excitability - From Gene Mutations to Novel Treatments 195

molecules (Patino and Isom, 2010). Nine α subunits (NaV1.1 – NaV1.9 encoded by SCN1A-SCN11A) and four β subunits (encoded by SCN1B-SCN4B) have been characterized so far. In addition, the enigmatic NaX channel, which appears not to be gated by voltage but rather by sodium, is encoded by the SCN7A gene (previously assigned as SCN6A) (Hiyama et al., 2002). NaV1.1, NaV1.2, NaV1.3 and NaV1.6 are the sodium channel α subunits most

Febrile seizures, i. e. seizures induced by elevated body temperature, affect approximately 3% of children under 6 years of age and are by far the most common seizure disorder. Generalized Epilepsy with Febrile Seizures Plus (GEFS+) is an autosomal dominant epileptic syndrome where the febrile seizures may persist beyond 6 years of age and which may be associated with afebrile generalized seizures (Scheffer and Berkovic, 1997). The disease has a penetrance of approximately 60%. In 1998, GEFS+ was linked to mutation in SCN1B, the voltage-gated sodium channel β1 subunit gene (Wallace et al., 1998). GEFS+ can also result from mutations in the sodium channel α subunit genes SCN1A (Escayg et al., 2000) and SCN2A (Sugawara et al., 2001), and from mutations in the GABRG2 gene which encodes the γ2 subunit of the GABAA receptor (Baulac et al., 2001). Heterozygous mutations in SCN1A can also result in Severe Myoclonic Epilepsy of Infancy (SMEI), also known as Dravet syndrome (Claes et al., 2001). This rare form of epilepsy is characterized by generalized tonic, clonic, and tonic-clonic seizures that are initially induced by fever, light, sound, or physical activity and typically begin around 6-9 months of age. Later, SMEI patients also manifest other seizure types including absence, myoclonic, and simple and complex partial seizures. Psychomotor development stagnates around the second year of life and the patients often respond poorly to antiepileptic drugs. The disorder usually occurs in isolated

patients as a result of *de novo* mutations (Claes et al., 2003; Ohmori et al., 2002).

As sodium channels are responsible for the upstroke of the action potential one might expect that epilepsy-causing mutations in sodium channel genes increase the activity of the channel, thereby allowing increased influx of sodium ions and consequently neuronal hyperexcitability. Indeed, biophysical analyses of the mutant channels have shown that several of the mutations are gain-of-function mutations that increase sodium currents, e. g. by impairing inactivation or by causing a hyperpolarizing shift in the voltage-dependence of the channel (Lossin et al., 2002; Spampanato et al., 2003; Spampanato et al., 2004). The first identified GEFS+ mutation, a C121W missense mutation that disrupts a conserved disulphide bridge in the extracellular Ig domain of the β1 subunit, causes subtle changes in modulation of sodium channel function and alter the ability of β1 to mediate protein-protein interactions that are critical for channel localization (Meadows et al., 2002; Wallace et al., 1998). Electrophysiological and biochemical studies on the mutant C121W β1 subunit coexpressed with NaV1.2 or NaV1.3 have shown that the C121W mutation causes a reduction in current rundown during high-frequency channel activation and increases the fraction of sodium channels that are available to open at subthreshold membrane potentials (Meadows et al., 2002). The mutation is therefore thought to enhance sodium channel function, thereby

On the other hand, many of the characterized sodium channel mutations are found to cause attenuation of sodium current (Barela et al., 2006; Lossin et al., 2003; Sugawara et al., 2001).

**3.3 How mutations in sodium channels can cause seizures** 

increasing neuronal excitability and predisposing to seizures.

On the other hand, some of the disorders display locus heterogeneity where mutations in distinct genes result in the same syndrome. This indicates that other factors beside the primary mutation influence the clinical manifestation of the epilepsy, e.g. environmental factors, developmental events, or differences in inheritance of genetic susceptibility alleles. The latter is supported by mouse models where differences between the genetic backgrounds of two mouse strains influence the severity of a disease caused by the same sodium channel mutation (Bergren et al., 2005).

Unfortunately, discovery of the responsible gene for an epilepsy syndrome have not led to a prompt understanding of the pathogenesis of the disease. Many of the mutated channels have been characterized in expression systems, but only in some cases have this led to a better understanding of the disease. In other cases this have led to more confusion, as some mutations in a particular channel are found to enhance channel function while others appear to cause a loss of function, even though the clinical manifestation are similar. There are also large discrepancies between results depending on the expression system used to characterize the channels. The mutated channel can e.g. show enhanced function when expressed in *Xenopus laevis* oocytes, while the opposite is shown when expressed in mammalian cells (Meadows et al., 2002). To make it even more difficult, it has been demonstrated that depending on the type of neuron in which a mutated channel is expressed, it can have strikingly different effects on the excitability of the cell (Waxman, 2007). While a mutation can make one type of neuron hyperexcitable, the same mutation can make another neuron hypoexcitable. So changes in neuronal fuction are not necessarily predictable solely from the change in the behaviour of the mutated channel itself, but have to be considered in the cell background in which the mutated channel is expressed. Further, depending on whether a mutated channel mainly is expressed in excitatory or inhibitory neurons, it can have completely opposite effects on the excitability status of the neuronal network (Yu et al., 2006).

The ion channel mutations are bound to cause relative subtle changes in neuronal function. Mutations that cause dramatic changes would likely result in a more severe phenotypes or lethality. The mutations apparently allow normal behaviour under most circumstances, but disturb the equilibrium between excitatory and inhibitory neuronal networks, so that small external perturbations such as fever are sufficient to break the homeostasis and induce seizures.
