Epilepsies are characterized by recurrent seizures, which can cause motor, sensory, cognitive, psychic or autonomic disturbances. The seizures are the clinical manifestation of an underlying transient abnormality of cortical neuronal activity, and their phenotypic expression is determined by the point of origin of the hyperexcitability and its degree of spread in the brain.
Causes of sporadic or recurrent seizures include acquired structural brain damage, altered metabolic states and inborn brain malformations. However, about 1% of people develop recurrent unprovoked seizures for no obvious reason. These 'idiopathic' epilepsies are assumed to be mainly genetic in origin.
Idiopathic epilepsies are caused predominantly by mutations in genes that code for ion channels or their accessory subunits. The channels that are involved belong to either the class of voltage-gated ion channels, which are important for action potential generation and control, or the ligand-gated ion channels, which are mainly involved in synaptic transmission. Epilepsy syndromes that have been attributed to ion channel defects include familial nocturnal frontal lobe epilepsy, benign familial neonatal convulsions, and generalized epilepsy with febrile seizures plus (GEFS+).
Although ion channels undoubtedly have an important role in idiopathic epilepsies, other pathways can also lead to neuronal hyperexcitability. Mutant forms of any gene that is involved in neuronal plasticity, development of neuronal networks or neuronal metabolism are potential candidates for causing epileptogenesis.
In childhood and adolescence, about 30–40% of all epilepsies belong to the group of idiopathic generalized epilepsies (IGE). Little is known about the genes that underlie epileptogenesis in IGE, although linkage studies and association approaches have highlighted numerous candidate regions in the genome, including chromosomal region 3q26, which contains the voltage-gated chloride channel gene CLCN2.
More than 200 inherited syndromes are known in which epileptic seizures are a prominent clinical feature. The epilepsy is often accompanied by other neurological symptoms, such as mental retardation, dementia or ataxia. The genes that underlie these syndromes are involved in tasks as different as glycogen metabolism, respiratory chain activity and brain development.
It is difficult to predict how many more genes that are associated with epilepsy are waiting to be discovered in the human genome. Learning about the gene families that can cause epilepsy will not only help us to understand the complex pathways that underlie neuronal hyperexcitability, but should also lead to the development of more powerful and precise treatment strategies.
Genetic factors can cause recurrent abnormal synchronization and episodic hyperexcitability of neuronal networks through various mechanisms. Many of the genes that have been implicated in idiopathic epilepsies code for ion channels, whereas syndromes with epilepsy as a main feature are caused by genes that are involved in functions as diverse as cortical development, mitochondrial function and cell metabolism. Each 'epilepsy gene' that is identified provides new and fascinating insights into the molecular basis of neuronal excitability and brain function.
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The author declares no competing financial interests.
Encyclopedia of Life Sciences
- ABSENCE EPILEPSY
A non-convulsive form of epilepsy that is characterized by a sudden, brief impairment of consciousness.
Brief, involuntary twitching of a muscle or a group of muscles. Familiar examples of normal myoclonus include hiccups and jerks experienced when drifting off to sleep.
- FRAGILE X SYNDROME
A genetic condition, commonly transmitted from mother to son, that is associated with mental retardation, abnormal facial features and enlarged testicles.
- ANGELMAN'S SYNDROME
A genetic disorder that is caused by deletion or disruption of UBE3A (E6-AP). The symptoms of Angelman's syndrome include hyperactivity, ataxia, problems with speech and language, and an unusually happy demeanour.
A term that refers to any chromosome in a cell that is not a sex chromosome.
- PARAMYOTONIA CONGENITA
A rare autosomal dominant disorder in which muscle fibres are slow to relax after contraction.
- PATCH CLAMP
Technique whereby a very small electrode tip is sealed onto a patch of cell membrane, making it possible to record the flow of current through individual ion channels or pores within the patch.
- MISSENSE MUTATION
A mutation that results in the substitution of an amino acid in a protein.
A protein domain that consists of an array of β-sheet motifs, which are configured in a ring to resemble the blades of a propeller.
- G PROTEIN
A heterotrimeric GTP-binding and -hydrolysing protein that interacts with cell-surface receptors, often stimulating or inhibiting the activity of a downstream enzyme. G proteins consist of three subunits: the α-subunit, which contains the guanine-nucleotide-binding site; and the β- and γ-subunits, which function as a heterodimer.
- FRINGS MOUSE MODEL
An inbred mouse strain with a seizure phenotype that is characterized by wild running, loss of righting reflex, tonic flexion and tonic extension in response to high-intensity sound stimulation.
- AUDIOGENIC EPILEPSY
A form of epilepsy in which the seizures are provoked by auditory stimuli.
- INTENTION TREMOR
A tremor that is exacerbated by voluntary goal-directed movements; for example, trying to put a key in a lock.
A speech impairment that is caused by damage to the nerves or muscles that control speech articulation. Although the speech is difficult to understand, it is usually linguistically normal, thereby distinguishing this condition from language disorders.
- RNA INTERFERENCE
(RNAi). A method by which double-stranded RNA that is encoded on an exogenous vector can be used to interfere with normal RNA processing, causing rapid degradation of the endogenous RNA and thereby precluding translation. This provides a simple way of studying the effects of the absence of a gene product in simple organisms and in cells.
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