Mutations and polymorphisms of the CLCN2 gene in idiopathic epilepsy

B. de Paiva A, Souza KS, de Souza JLB, G. S. B. Lima PL, da Silva DJ, Pitombeira MS, Borges VK, Dias DA, Bispo LM, Santos CF, Freua F, Silva PDS, Alves IS, Portella LB, Cunha PR, Salomao RPA, Pedroso JL, Miyajima VP, Miyajima F, Cali E, Wade C, Sudarsanam A, O’Driscoll M, Hayton T, Barsottini OGP, Klebe S, Kok F, Lucato LT, Houlden H, Depienne C, Lynch DS, Braga-Neto P. Expanding the phenotypic spectrum of CLCN2-related leucoencephalopathy and ataxia

Mutations in CLCN2 are a rare cause of autosomal recessive leucoencephalopathy with ataxia and specific imaging abnormalities. Very few cases have been reported to date. Here, we describe the clinical and imaging phenotype of 12 additional CLCN2 patients and expand the known phenotypic spectrum of this disorder. Informed consent was obtained for all patients. Patients underwent either whole-exome sequencing or focused/panel-based sequencing to identify variants. Twelve patients with biallelic CLCN2 variants are described. This includes three novel likely pathogenic missense variants. All patients demonstrated typical MRI changes, including hyperintensity on T2-weighted images in the posterior limbs of the internal capsules, midbrain cerebral peduncles, middle cerebellar peduncles and cerebral white matter. Clinical features included a variable combination of ataxia, headache, spasticity, seizures and other symptoms with a broad range of age of onset. This report is now the largest case series of patients with CLCN2-related leucoencephalopathy and reinforces the finding that, although the imaging appearance is uniform, the phenotypic expression of this disorder is highly heterogeneous. Our findings expand the phenotypic spectrum of CLCN2-related leucoencephalopathy by adding prominent seizures, severe spastic paraplegia and developmental delay.

Pharmacological modulation of chloride channels as a therapeutic strategy for neurological disorders

Chloride homeostasis is critical in the physiological functions of the central nervous system (CNS). Its concentration is precisely regulated by multiple ion-transporting proteins such as chloride channels and transporters that are widely distributed in the brain cells, including neurons and glia. Unlike ion transporters, chloride channels provide rapid responses to efficiently regulate ion flux. Some of chloride channels are also permeable to selected organic anions such as glutamate and γ-aminobutyric acid, suggesting neuroexcitatory and neuroinhibitory functions while gating. Dysregulated chloride channels are implicated in neurological disorders, e.g., ischemia and neuroinflammation. Modulation of chloride homeostasis through chloride channels has been suggested as a potential therapeutic approach for neurological disorders. The drug design for CNS diseases is challenging because it requires the therapeutics to traverse the blood-brain-barrier. Small molecules are a well-established modality with better cell permeability due to their lower molecular weight and flexibility for structure optimization compared to biologics. In this article, we describe the important roles of chloride homeostasis in each type of brain cells and introduce selected chloride channels identified in the CNS. We then discuss the contribution of their dysregulations towards the pathogenesis of neurological disorders, emphasizing the potential of targeting chloride channels as a therapeutic strategy for CNS disease treatment. Along with this literature survey, we summarize the small molecules that modulate chloride channels and propose the potential strategy of optimizing existing drugs to brain-penetrants to support future CNS drug discovery.

Genetics in Epilepsy

The presence of unprovoked, recurrent seizures, particularly when drug resistant and associated with cognitive and behavioral deficits, warrants investigation for an underlying genetic cause. This article provides an overview of the major classes of genes associated with epilepsy phenotypes divided into functional categories along with the recommended work-up and therapeutic considerations. Gene discovery in epilepsy supports counseling and anticipatory guidance but also opens the door for precision medicine guiding therapy with a focus on those with disease-modifying effects.

Natural Medicines for the Treatment of Epilepsy: Bioactive Components, Pharmacology and Mechanism

Epilepsy is a chronic disease that can cause temporary brain dysfunction as a result of sudden abnormal discharge of the brain neurons. The seizure mechanism of epilepsy is closely related to the neurotransmitter imbalance, synaptic recombination, and glial cell proliferation. In addition, epileptic seizures can lead to mitochondrial damage, oxidative stress, and the disorder of sugar degradation. Although the mechanism of epilepsy research has reached up to the genetic level, the presently available treatment and recovery records of epilepsy does not seem promising. Recently, natural medicines have attracted more researches owing to their low toxicity and side-effects as well as the excellent efficacy, especially in chronic diseases. In this study, the antiepileptic mechanism of the bioactive components of natural drugs was reviewed so as to provide a reference for the development of potential antiepileptic drugs. Based on the different treatment mechanisms of natural drugs considered in this review, it is possible to select drugs clinically. Improving the accuracy of medication and the cure rate is expected to compensate for the shortage of the conventional epilepsy treatment drugs.

West Syndrome Caused By a Chloride/Proton Exchange-Uncoupling CLCN6 Mutation Related to Autophagic-Lysosomal Dysfunction

Vesicular chloride/proton exchangers of the CLC family are critically involved in the function of the endosomal-lysosomal pathway. Their dysfunction leads to severe disorders including intellectual disability and epilepsy for ClC-4, Dent's disease for ClC-5, and lysosomal storage disease and osteopetrosis for ClC-7. Here, we report a de novo variant p.Glu200Ala (p.E200A; c.599A>C) of the late endosomal ClC-6, encoded by CLCN6, in a patient with West syndrome (WS), severe developmental delay, autism, movement disorder, microcephaly, facial dysmorphism, and visual impairment. Mutation of this conserved glutamate uncouples chloride transport from proton antiport by ClC-6. This affects organellar ion homeostasis and was shown to be deleterious for other CLCs. In this study, we found that upon heterologous expression, the ClC-6 E200A variant caused autophagosome accumulation and impaired the clearance of autophagosomes by blocking autophagosome-lysosome fusion. Our study provides clinical and functional support for an association between CLCN6 variants and WS. Our findings also provide novel insights into the molecular mechanisms underlying the pathogenesis of WS, suggesting an involvement of autophagic-lysosomal dysfunction.

Development and validation of a potent and specific inhibitor for the CLC-2 chloride channel

CLC-2 is a voltage-gated chloride channel that is widely expressed in mammalian tissues. In the central nervous system, CLC-2 appears in neurons and glia. Studies to define how this channel contributes to normal and pathophysiological function in the central nervous system raise questions that remain unresolved, in part due to the absence of precise pharmacological tools for modulating CLC-2 activity. Herein, we describe the development and optimization of AK-42, a specific small-molecule inhibitor of CLC-2 with nanomolar potency (IC50 = 17 ± 1 nM). AK-42 displays unprecedented selectivity (>1,000-fold) over CLC-1, the closest CLC-2 homolog, and exhibits no off-target engagement against a panel of 61 common channels, receptors, and transporters expressed in brain tissue. Computational docking, validated by mutagenesis and kinetic studies, indicates that AK-42 binds to an extracellular vestibule above the channel pore. In electrophysiological recordings of mouse CA1 hippocampal pyramidal neurons, AK-42 acutely and reversibly inhibits CLC-2 currents; no effect on current is observed on brain slices taken from CLC-2 knockout mice. These results establish AK-42 as a powerful tool for investigating CLC-2 neurophysiology.

Dynamical model of the CLC-2 ion channel reveals conformational changes associated with selectivity-filter gating

This work reports a dynamical Markov state model of CLC-2 "fast" (pore) gating, based on 600 microseconds of molecular dynamics (MD) simulation. In the starting conformation of our CLC-2 model, both outer and inner channel gates are closed. The first conformational change in our dataset involves rotation of the inner-gate backbone along residues S168-G169-I170. This change is strikingly similar to that observed in the cryo-EM structure of the bovine CLC-K channel, though the volume of the intracellular (inner) region of the ion conduction pathway is further expanded in our model. From this state (inner gate open and outer gate closed), two additional states are observed, each involving a unique rotameric flip of the outer-gate residue GLUex. Both additional states involve conformational changes that orient GLUex away from the extracellular (outer) region of the ion conduction pathway. In the first additional state, the rotameric flip of GLUex results in an open, or near-open, channel pore. The equilibrium population of this state is low (∼1%), consistent with the low open probability of CLC-2 observed experimentally in the absence of a membrane potential stimulus (0 mV). In the second additional state, GLUex rotates to occlude the channel pore. This state, which has a low equilibrium population (∼1%), is only accessible when GLUex is protonated. Together, these pathways model the opening of both an inner and outer gate within the CLC-2 selectivity filter, as a function of GLUex protonation. Collectively, our findings are consistent with published experimental analyses of CLC-2 gating and provide a high-resolution structural model to guide future investigations.

Genetic and molecular basis of epilepsy-related cognitive dysfunction

Epilepsy is a common neurological disease characterized by recurrent seizures. About 70 million people were affected by epilepsy or epileptic seizures. Epilepsy is a complicated complex or symptomatic syndromes induced by structural, functional, and genetic causes. Meanwhile, several comorbidities are accompanied by epileptic seizures. Cognitive dysfunction is a long-standing complication associated with epileptic seizures, which severely impairs quality of life. Although the definitive pathogenic mechanisms underlying epilepsy-related cognitive dysfunction remain unclear, accumulating evidence indicates that multiple risk factors are probably involved in the development and progression of cognitive dysfunction in patients with epilepsy. These factors include the underlying etiology, recurrent seizures or status epilepticus, structural damage that induced secondary epilepsy, genetic variants, and molecular alterations. In this review, we summarize several theories that may explain the genetic and molecular basis of epilepsy-related cognitive dysfunction.

De novo SCN1A, SCN8A, and CLCN2 mutations in childhood absence epilepsy

This study aimed to identify monogenic mutations from Chinese patients with childhood absence epilepsy (CAE) and summarize their characteristics. A total of 100 patients with CAE were recruited in Peking University First Hospital from 2005 to 2016 and underwent telephone and outpatient follow-up review. We used targeted disease-specific gene capture sequencing (involving 300 genes) to identify pathogenic variations for these patients. We identified three de novo epilepsy-related gene mutations, including missense mutations of SCN1A (c. 5399 T > A; p. Val1800Asp), SCN8A (c. 2371 G > T; p. Val791Phe), and CLCN2 (c. 481 G > A; p. Gly161Ser), from three patients, separately. All recruited patients presented typical CAE features and good prognosis. To date, CAE has been considered a complex disease caused by multiple susceptibility genes. In this study, we observed that 3% of typical CAE patients had a de novo mutation of a known monogenic epilepsy-related gene. Our study suggests that a significant proportion of typical CAE cases may be monogenic forms of epilepsy. For genetic generalized epilepsies, such as CAE, further studies are needed to clarify the contributions of de novo or inherited rare monogenic coding, noncoding and copy number variants.

Chloride Channels in Astrocytes: Structure, Roles in Brain Homeostasis and Implications in Disease

Astrocytes are the most abundant cell type in the CNS (central nervous system). They exert multiple functions during development and in the adult CNS that are essential for brain homeostasis. Both cation and anion channel activities have been identified in astrocytes and it is believed that they play key roles in astrocyte function. Whereas the proteins and the physiological roles assigned to cation channels are becoming very clear, the study of astrocytic chloride channels is in its early stages. In recent years, we have moved from the identification of chloride channel activities present in astrocyte primary culture to the identification of the proteins involved in these activities, the determination of their 3D structure and attempts to gain insights about their physiological role. Here, we review the recent findings related to the main chloride channels identified in astrocytes: the voltage-dependent ClC-2, the calcium-activated bestrophin, the volume-activated VRAC (volume-regulated anion channel) and the stress-activated Maxi-Cl⁻. We discuss key aspects of channel biophysics and structure with a focus on their role in glial physiology and human disease.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

The genetics of the epilepsies

While genetic causes of epilepsy have been hypothesized from the time of Hippocrates, the advent of new genetic technologies has played a tremendous role in elucidating a growing number of specific genetic causes for the epilepsies. This progress has contributed vastly to our recognition of the epilepsies as a diverse group of disorders, the genetic mechanisms of which are heterogeneous. Genotype-phenotype correlation, however, is not always clear. Nonetheless, the developments in genetic diagnosis raise the promise of a future of personalized medicine. Multiple genetic tests are now available, but there is no one test for all possible genetic mutations, and the balance between cost and benefit must be weighed. A genetic diagnosis, however, can provide valuable information regarding comorbidities, prognosis, and even treatment, as well as allow for genetic counseling. In this review, we will discuss the genetic mechanisms of the epilepsies as well as the specifics of particular genetic epilepsy syndromes. We will include an overview of the available genetic testing methods, the application of clinical knowledge into the selection of genetic testing, genotype-phenotype correlations of epileptic disorders, and therapeutic advances as well as a discussion of the importance of genetic counseling.

Retraction. Clc-2 knockout attenuated experimental temporal lobe epilepsy in mice by tonic inhibition mediated by GABAA receptors

Temporal lobe epilepsy (TLE), the most prevalent form of epilepsy, is often associated with drug-resistant seizures. In TLE, altered function of γ-aminobutyric acid (GABA)A receptors (GABAARs) results in potentiation of excitatory and/or failure of inhibitory neurotransmission, which contributes to seizure induction and propagation. Our previous study suggested that chloride channel-2 (Clc-2) contributed to chronically elevated tonic inhibition mediated by GABAARs in a rat model of TLE. In the present study, we used Clc-2 knockout mice to investigate further the role of Clc-2 and its interaction with tonic GABAergic inhibition in a model of TLE. The results revealed that knockout of Clc-2 decreased tonic seizure protection, latency of clonic seizure, seizure threshold and mortality protection in mice. Clc-2 knockout decreased the action potential (AP)peak and APthreshold, Clc-2 currents and GABAAR-mediated tonic inhibition in CA1 pyramidal neurons. Thus, the voltage-gated chloride channel Clc-2, which was functionally upregulated in CA1 pyramidal cells after seizures, may provide protection against TLE by its regulation of action potentials, Clc-2 currents and GABAARs in the CA1 region of the hippocampus.

CLC channel function and dysfunction in health and disease

CLC channels and transporters are expressed in most tissues and fulfill diverse functions. There are four human CLC channels, ClC-1, ClC-2, ClC-Ka, and ClC-Kb, and five CLC transporters, ClC-3 through −7. Some of the CLC channels additionally associate with accessory subunits. Whereas barttin is mandatory for the functional expression of ClC-K, GlialCam is a facultative subunit of ClC-2 which modifies gating and thus increases the functional variability within the CLC family. Isoform-specific ion conduction and gating properties optimize distinct CLC channels for their cellular tasks. ClC-1 preferentially conducts at negative voltages, and the resulting inward rectification provides a large resting chloride conductance without interference with the muscle action potential. Exclusive opening at voltages negative to the chloride reversal potential allows for ClC-2 to regulate intracellular chloride concentrations. ClC-Ka and ClC-Kb are equally suited for inward and outward currents to support transcellular chloride fluxes. Every human CLC channel gene has been linked to a genetic disease, and studying these mutations has provided much information about the physiological roles and the molecular basis of CLC channel function. Mutations in the gene encoding ClC-1 cause myotonia congenita, a disease characterized by sarcolemmal hyperexcitability and muscle stiffness. Loss-of-function of ClC-Kb/barttin channels impairs NaCl resorption in the limb of Henle and causes hyponatriaemia, hypovolemia and hypotension in patients suffering from Bartter syndrome. Mutations in CLCN2 were found in patients with CNS disorders but the functional role of this isoform is still not understood. Recent links between ClC-1 and epilepsy and ClC-Ka and heart failure suggested novel cellular functions of these proteins. This review aims to survey the knowledge about physiological and pathophysiological functions of human CLC channels in the light of recent discoveries from biophysical, physiological, and genetic studies.

Epilepsy, energy deficiency and new therapeutic approaches including diet

Metabolic dysfunction leading to epilepsy is well recognised. Dietary therapy, in particular the ketogenic diet, is now considered an effective option. Recent genetic studies have highlighted the central role that metabolism can play in setting seizure susceptibility. Here we discuss various metabolic disorders implicated in epilepsy focusing on energy deficiency due to genetic and environmental causes. We argue that low, uncompensated brain glucose levels can precipitate seizures. We will also explore mechanisms of disease and therapy in an attempt to identify common metabolic pathways involved in modulating seizure susceptibility. Finally, newer therapeutic approaches based on diet manipulation in the context of energy deficiency are discussed.

Genetic epilepsy syndromes without structural brain abnormalities: clinical features and experimental models

Research in genetics of epilepsy represents an area of great interest both for clinical purposes and for understanding the basic mechanisms of epilepsy. Most mutations in epilepsies without structural brain abnormalities have been identified in ion channel genes, but an increasing number of genes involved in a diversity of functional and developmental processes are being recognized through whole exome or genome sequencing. Targeted molecular diagnosis is now available for different forms of epilepsy. The identification of epileptogenic mutations in patients before epilepsy onset and the possibility of developing therapeutic strategies tested in experimental models may facilitate experimental approaches that prevent epilepsy or decrease its severity. Functional analysis is essential for better understanding pathogenic mechanisms and gene interactions. In vitro experimental systems are either cells that usually do not express the protein of interest or neurons in primary cultures. In vivo/ex vivo systems are organisms or preparations obtained from them (e.g., brain slices), which should better model the complexity of brain circuits and actual pathophysiological conditions. Neurons differentiated from induced pluripotent stem cells generated from the skin fibroblasts of patients have recently allowed the study of mutations in human neurons having the genetic background of a given patient. However, there is remarkable complexity underlying epileptogenesis in the clinical dimension, as reflected by the fact that experimental models have not provided yet results having clinical translation and that, with a few exceptions concerning rare conditions, no new curative treatment has emerged from any genetic finding in epilepsy.

Chloride channelopathies of ClC-2

Chloride channels (ClCs) have gained worldwide interest because of their molecular diversity, widespread distribution in mammalian tissues and organs, and their link to various human diseases. Nine different ClCs have been molecularly identified and functionally characterized in mammals. ClC-2 is one of nine mammalian members of the ClC family. It possesses unique biophysical characteristics, pharmacological properties, and molecular features that distinguish it from other ClC family members. ClC-2 has wide organ/tissue distribution and is ubiquitously expressed. Published studies consistently point to a high degree of conservation of ClC-2 function and regulation across various species from nematodes to humans over vast evolutionary time spans. ClC-2 has been intensively and extensively studied over the past two decades, leading to the accumulation of a plethora of information to advance our understanding of its pathophysiological functions; however, many controversies still exist. It is necessary to analyze the research findings, and integrate different views to have a better understanding of ClC-2. This review focuses on ClC-2 only, providing an analytical overview of the available literature. Nearly every aspect of ClC-2 is discussed in the review: molecular features, biophysical characteristics, pharmacological properties, cellular function, regulation of expression and function, and channelopathies.

Brain white matter oedema due to ClC-2 chloride channel deficiency: an observational analytical study

BACKGROUND

Mutant mouse models suggest that the chloride channel ClC-2 has functions in ion and water homoeostasis, but this has not been confirmed in human beings. We aimed to define novel disorders characterised by distinct patterns of MRI abnormalities in patients with leukoencephalopathies of unknown origin, and to identify the genes mutated in these disorders. We were specifically interested in leukoencephalopathies characterised by white matter oedema, suggesting a defect in ion and water homoeostasis.

METHODS

In this observational analytical study, we recruited patients with leukoencephalopathies characterised by MRI signal abnormalities in the posterior limbs of the internal capsules, midbrain cerebral peduncles, and middle cerebellar peduncles from our databases of patients with leukoencephalopathies of unknown origin. We used exome sequencing to identify the gene involved. We screened the candidate gene in additional patients by Sanger sequencing and mRNA analysis, and investigated the functional effects of the mutations. We assessed the localisation of ClC-2 with immunohistochemistry and electron microscopy in post-mortem human brains of individuals without neurological disorders.

FINDINGS

Seven patients met our inclusion criteria, three with adult-onset disease and four with childhood-onset disease. We identified homozygous or compound-heterozygous mutations in CLCN2 in three adult and three paediatric patients. We found evidence that the CLCN2 mutations result in loss of function of ClC-2. The remaining paediatric patient had an X-linked family history and a mutation in GJB1, encoding connexin 32. Clinical features were variable and included cerebellar ataxia, spasticity, chorioretinopathy with visual field defects, optic neuropathy, cognitive defects, and headaches. MRI showed restricted diffusion suggesting myelin vacuolation that was confined to the specified white matter structures in adult patients, and more diffusely involved the brain white matter in paediatric patients. We detected ClC-2 in all components of the panglial syncytium, enriched in astrocytic endfeet at the perivascular basal lamina, in the glia limitans, and in ependymal cells.

INTERPRETATION

Our observations substantiate the concept that ClC-2 is involved in brain ion and water homoeostasis. Autosomal-recessive CLCN2 mutations cause a leukoencephalopathy that belongs to an emerging group of disorders affecting brain ion and water homoeostasis and characterised by intramyelinic oedema.

FUNDING

European Leukodystrophies Association, INSERM and Assistance Publique-Hôpitaux de Paris, Dutch Organisation for Scientific Research (ZonMw), E-Rare, Hersenstichting, Optimix Foundation for Scientific Research, Myelin Disorders Bioregistry Project, National Institute of Neurological Disorders and Stroke, and Genetic and Epigenetic Networks in Cognitive Dysfunction (GENCODYS) Project (funded by the European Union Framework Programme 7).

Regulation of ClC-2 gating by intracellular ATP

ClC-2 is a voltage-dependent chloride channel that activates slowly at voltages negative to the chloride reversal potential. Adenosine triphosphate (ATP) and other nucleotides have been shown to bind to carboxy-terminal cystathionine-ß-synthase (CBS) domains of ClC-2, but the functional consequences of binding are not sufficiently understood. We here studied the effect of nucleotides on channel gating using single-channel and whole-cell patch clamp recordings on transfected mammalian cells. ATP slowed down macroscopic activation and deactivation time courses in a dose-dependent manner. Removal of the complete carboxy-terminus abolishes the effect of ATP, suggesting that CBS domains are necessary for ATP regulation of ClC-2 gating. Single-channel recordings identified long-lasting closed states of ATP-bound channels as basis of this gating deceleration. ClC-2 channel dimers exhibit two largely independent protopores that are opened and closed individually as well as by a common gating process. A seven-state model of common gating with altered voltage dependencies of opening and closing transitions for ATP-bound states correctly describes the effects of ATP on macroscopic and microscopic ClC-2 currents. To test for a potential pathophysiological impact of ClC-2 regulation by ATP, we studied ClC-2 channels carrying naturally occurring sequence variants found in patients with idiopathic generalized epilepsy, G715E, R577Q, and R653T. All naturally occurring sequence variants accelerate common gating in the presence but not in the absence of ATP. We propose that ClC-2 uses ATP as a co-factor to slow down common gating for sufficient electrical stability of neurons under physiological conditions.

Clinical review of genetic epileptic encephalopathies

Seizures are a frequently encountered finding in patients seen for clinical genetics evaluations. The differential diagnosis for the cause of seizures is quite diverse and complex, and more than half of all epilepsies have been attributed to a genetic cause. Given the complexity of such evaluations, we highlight the more common causes of genetic epileptic encephalopathies and emphasize the usefulness of recent technological advances. The purpose of this review is to serve as a practical guide for clinical geneticists in the evaluation and counseling of patients with genetic epileptic encephalopathies. Common syndromes will be discussed, in addition to specific seizure phenotypes, many of which are refractory to anti-epileptic agents. Divided by etiology, we overview the more common causes of infantile epileptic encephalopathies, channelopathies, syndromic, metabolic, and chromosomal entities. For each condition, we will outline the diagnostic evaluation and discuss effective treatment strategies that should be considered.

The genetics of monogenic idiopathic epilepsies and epileptic encephalopathies

The group of idiopathic epilepsies encompasses numerous syndromes without known organic substrate. Genetic anomalies are thought to be responsible for pathogenesis, with a monogenic or polygenic model of inheritance. Over the last two decades, a number of genetic anomalies and encoded proteins have been related to particular idiopathic epilepsies and epileptic encephalopathies. Most of these mutations involve subunits of neuronal ion channels (e.g. potassium, sodium, and chloride channels), and may result in abnormal neuronal hyperexcitability manifesting with seizures. However non-ion channel proteins may also be affected. Correlations between genotype and phenotype are not easy to establish, since genetic and non-genetic factors are likely to play a role in determining the severity of clinical features. The growing number of discoveries on this topic are improving classification, prognosis and counseling of patients and families with these forms of epilepsy, and may lead to targeted therapeutic approaches in the near future. In this article the authors have reviewed the main genetic discoveries in the field of the monogenic idiopathic epilepsies and epileptic encephalopathies, in order to provide epileptologists with a concise and comprehensive summary of clinical and genetic features of these seizure disorders.

ClC-2 voltage-gated channels constitute part of the background conductance and assist chloride extrusion

The function of voltage-gated chloride channels in neurons is essentially unknown. The voltage-gated chloride channel ClC-2 mediates a chloride current in pyramidal cells of the hippocampus. We directly show that ClC-2 assists chloride extrusion after high chloride load. Furthermore, the loss of this chloride channel leads to a dramatic increase of the input resistance of CA1 pyramidal cells, making these cells more excitable. Surprisingly, basal synaptic transmission, as judged from recordings of field EPSPs, was decreased. This difference was eliminated when GABAergic inhibition was blocked. Recordings from hippocampal interneurons revealed ClC-2-mediated currents in a subset of these cells. An observed increase in GABAergic inhibition could thus be explained by an increase in the excitability of interneurons, caused by the loss of ClC-2. Together, we suggest a dual role for ClC-2 in neurons, providing an additional efflux pathway for chloride and constituting a substantial part of the background conductance, which regulates excitability. In ClC-2 knock-out mice, an increased inhibition seemingly balances the hyperexcitability of the network and thereby prevents epilepsy.

Mutations affecting GABAergic signaling in seizures and epilepsy

The causes of epilepsies and epileptic seizures are multifactorial. Genetic predisposition may contribute in certain types of epilepsies and seizures, whether idiopathic or symptomatic of genetic origin. Although these are not very common, they have offered a unique opportunity to investigate the molecular mechanisms underlying epileptogenesis and ictogenesis. Among the implicated gene mutations, a number of GABAA receptor subunit mutations have been recently identified that contribute to several idiopathic epilepsies, febrile seizures, and rarely to certain types of symptomatic epilepsies, like the severe myoclonic epilepsy of infancy. Deletion of GABAA receptor genes has also been linked to Angelman syndrome. Furthermore, mutations of proteins controlling chloride homeostasis, which indirectly defines the functional consequences of GABAA signaling, have been identified. These include the chloride channel 2 (CLCN2) and the potassium chloride cotransporter KCC3. The pathogenic role of CLCN2 mutations has not been clearly demonstrated and may represent either susceptibility genes or, in certain cases, innocuous polymorphisms. KCC3 mutations have been associated with hereditary motor and sensory polyneuropathy with corpus callosum agenesis (Andermann syndrome) that often manifests with epileptic seizures. This review summarizes the recent progress in the genetic linkages of epilepsies and seizures to the above genes and discusses potential pathogenic mechanisms that contribute to the age, sex, and conditional expression of these seizures in carriers of these mutations.

Genetic testing in the epilepsies--report of the ILAE Genetics Commission

In this report, the International League Against Epilepsy (ILAE) Genetics Commission discusses essential issues to be considered with regard to clinical genetic testing in the epilepsies. Genetic research on the epilepsies has led to the identification of more than 20 genes with a major effect on susceptibility to idiopathic epilepsies. The most important potential clinical application of these discoveries is genetic testing: the use of genetic information, either to clarify the diagnosis in people already known or suspected to have epilepsy (diagnostic testing), or to predict onset of epilepsy in people at risk because of a family history (predictive testing). Although genetic testing has many potential benefits, it also has potential harms, and assessment of these potential benefits and harms in particular situations is complex. Moreover, many treating clinicians are unfamiliar with the types of tests available, how to access them, how to decide whether they should be offered, and what measures should be used to maximize benefit and minimize harm to their patients. Because the field is moving rapidly, with new information emerging practically every day, we present a framework for considering the clinical utility of genetic testing that can be applied to many different syndromes and clinical contexts. Given the current state of knowledge, genetic testing has high clinical utility in few clinical contexts, but in some of these it carries implications for daily clinical practice.

Sexually dimorphic expression of KCC2 and GABA function

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate depolarizing effects, which result in activation of calcium-sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors acquire their classical hyperpolarizing signaling. The switch from depolarizing to hyperpolarizing GABA(A)-ergic signaling is triggered through the developmental shift in the balance of chloride cotransporters that either increase (i.e. NKCC1) or decrease (i.e. KCC2) intracellular chloride. The maturation of GABA(A) signaling follows sex-specific patterns, which correlate with the developmental expression profiles of chloride cotransporters. This has first been demonstrated in the substantia nigra, where the switch occurs earlier in females than in males. As a result, there are sensitive periods during development when drugs or conditions that activate GABA(A) receptors mediate different transcriptional effects in males and females. Furthermore, neurons with depolarizing or hyperpolarizing GABA(A)-ergic signaling respond differently to neurotrophic factors like estrogens. Consequently, during sensitive developmental periods, GABA(A) receptors may act as broadcasters of sexually differentiating signals, promoting gender-appropriate brain development. This has particular implications in epilepsy, where both the pathophysiology and treatment of epileptic seizures involve GABA(A) receptor activation. It is important therefore to study separately the effects of these factors not only on the course of epilepsy but also design new treatments that may not necessarily disturb the gender-appropriate brain development.

CLC chloride channels and transporters: from genes to protein structure, pathology and physiology

CLC genes are expressed in species from bacteria to human and encode Cl(-)-channels or Cl(-)/H(+)-exchangers. CLC proteins assemble to dimers, with each monomer containing an ion translocation pathway. Some mammalian isoforms need essential beta -subunits (barttin and Ostm1). Crystal structures of bacterial CLC Cl(-)/H(+)-exchangers, combined with transport analysis of mammalian and bacterial CLCs, yielded surprising insights into their structure and function. The large cytosolic carboxy-termini of eukaryotic CLCs contain CBS domains, which may modulate transport activity. Some of these have been crystallized. Mammals express nine CLC isoforms that differ in tissue distribution and subcellular localization. Some of these are plasma membrane Cl(-) channels, which play important roles in transepithelial transport and in dampening muscle excitability. Other CLC proteins localize mainly to the endosomal-lysosomal system where they may facilitate luminal acidification or regulate luminal chloride concentration. All vesicular CLCs may be Cl(-)/H(+)-exchangers, as shown for the endosomal ClC-4 and -5 proteins. Human diseases and knockout mouse models have yielded important insights into their physiology and pathology. Phenotypes and diseases include myotonia, renal salt wasting, kidney stones, deafness, blindness, male infertility, leukodystrophy, osteopetrosis, lysosomal storage disease and defective endocytosis, demonstrating the broad physiological role of CLC-mediated anion transport.

GABA(A) receptors in normal development and seizures: friends or foes?

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate excitatory effects resulting in activation of calcium sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors transmit inhibitory signals. The maturation of GABA(A) signaling follows sex-specific patterns, which appear to also be important for the sexual differentiation of the brain. The inhibitory effects of GABA(A) receptor activation have been widely exploited in the treatment of conditions where neuronal silencing is necessary. For instance, drugs that target GABA(A) receptors are the mainstay of treatment of seizures. Recent evidence suggests however that the physiology and function of GABA(A) receptors changes in the brain of a subject that has epilepsy or status epilepticus.This review will summarize the physiology of and the developmental factors regulating the signaling and function of GABA(A) receptors; how these may change in the brain that has experienced prior seizures; what are the implications for the age and sex specific treatment of seizures and status epilepticus. Finally, the implications of these changes for the treatment of certain forms of medically refractory epilepsies and status epilepticus will be discussed.

A polygenic heterogeneity model for common epilepsies with complex genetics

Approximately 40% of epilepsy has a complex genetic basis with an unknown number of susceptibility genes. The effect of each susceptibility gene acting alone is insufficient to account for seizure phenotypes, but certain numbers or combinations of variations in susceptibility genes are predicted to raise the level of neuronal hyperexcitability above a seizure threshold for a given individual in a given environment. Identities of susceptibility genes are beginning to be determined, initially by translation of knowledge gained from gene discovery in the monogenic epilepsies. This entrée into idiopathic epilepsies with complex genetics has led to the experimental validation of susceptibility variants in the first few susceptibility genes. The genetic architecture so far emerging from these results is consistent with what we have designated as a polygenic heterogeneity model for the epilepsies with complex genetics.

Leukoencephalopathy upon disruption of the chloride channel ClC-2

ClC-2 is a broadly expressed plasma membrane chloride channel that is modulated by voltage, cell swelling, and pH. A human mutation leading to a heterozygous loss of ClC-2 has previously been reported to be associated with epilepsy, whereas the disruption of Clcn2 in mice led to testicular and retinal degeneration. We now show that the white matter of the brain and spinal cord of ClC-2 knock-out mice developed widespread vacuolation that progressed with age. Fluid-filled spaces appeared between myelin sheaths of the central but not the peripheral nervous system. Neuronal morphology, in contrast, seemed normal. Except for the previously reported blindness, neurological deficits were mild and included a decreased conduction velocity in neurons of the central auditory pathway. The heterozygous loss of ClC-2 had no detectable functional or morphological consequences. Neither heterozygous nor homozygous ClC-2 knock-out mice had lowered seizure thresholds. Sequencing of a large collection of human DNA and electrophysiological analysis showed that several ClC-2 sequence abnormalities previously found in patients with epilepsy most likely represent innocuous polymorphisms.

Linkage and mutational analysis of CLCN2 in childhood absence epilepsy

In order to assess the chloride channel gene CLCN2 as a candidate susceptibility gene for childhood absence epilepsy, parametric and non-parametric linkage analysis was performed in 65 nuclear pedigrees. This provided suggestive evidence for linkage with heterogeneity: NPL score=2.3, p<0.009; HLOD=1.5, alpha=0.44. Mutational analysis of the entire genomic sequence of CLCN2 was performed in 24 unrelated patients from pedigrees consistent with linkage, identifying 45 sequence variants including the known non-synonymous polymorphism rs2228292 (G2154C, Glu718Asp) and a novel variant IVS4+12G>A. Intra-familial association analysis using the pedigrees and a further 308 parent-child trios showed suggestive evidence for transmission disequilibrium of the G2154C minor allele: AVE-PDT chi(1)2 = 5.17, p<0.03. Case-control analysis provided evidence for a protective effect of the IVS4+12G>A minor allele: chi(1)2 = 7.27, p<0.008. The 65 nuclear pedigrees were screened for three previously identified mutations shown to segregate with a variety of idiopathic generalised epilepsy phenotypes (597insG, IVS2-14del11 and G2144A) but none were found. We conclude that CLCN2 may be a susceptibility locus in a subset of cases of childhood absence epilepsy.

Advances in genetics of juvenile myoclonic epilepsies

One by one, mutation-containing mendelian genes that cause monogenic juvenile myoclonic epilepsies (JME) and single nucleotide polymorphisms (SNP)-susceptibility alleles that increase risks for nonmendelian complex JME should fall to the power of molecular genetics. Of 15 chromosome loci, 3 mendelian genes (alpha1-subunit of the GABA(A) receptor [GABRA1], chloride channel 2 gene [CLCN2], and Myoclonin1/EFHC1) and 2 SNP-susceptibility alleles of putative JME genes in epistases (bromodomain-containing protein 2 [BRD2] and connexin [Cx]-36) have been identified, so far. Antiepileptic drugs now can be designed against the specific molecular defects of JME.

Channeling studies in yeast: yeast as a model for channelopathies?

Regulation of the concentration of ions within a cell is mediated by their specific transport and sequestration across cellular membranes. This regulation constitutes a major factor in the maintenance of correct cellular homeostasis, with the transport occurring through the action of a large number of different channel proteins localized to the plasma membrane as well as to various organelles. These ion channels vary in specificity from broad (cationic vs anionic) to highly selective (chloride vs sodium). Mutations in many of these channels result in a large number of human diseases, collectively termed channelopathies. Characterization of many of these channels has been undertaken in a variety of both prokaryotic and eukaryotic organisms. Among these organisms is the budding yeast Saccharomyces cerevisiae. Possessing a fully annotated genome, S. cerevisiae would appear to be an ideal organism in which to study this class of proteins associated to diseases. We have compiled and reviewed a list of yeast ion channels, each possessing a human homolog implicated in a channelopathy. Although yeast has been used for the study of other human disease, it has been under utilized for channelopathy research. The utility of using yeast as a model system for studying ion channels associated to human disease is illustrated using yeast lacking the GEF1 gene product that encodes the human homolog to the chloride channel CLC-3.

Allelic association of a truncation mutation of the KCNMB3 gene with idiopathic generalized epilepsy

The gene encoding the beta3-subunit regulatory subunit (KCNMB3) of large conductance calcium-sensitive potassium (BK) channels represents a positional and functional candidate gene for idiopathic generalized epilepsy (IGE). A single base pair deletion in exon 4 of KCNMB3 (delA750) alters/truncates the terminal 21 amino acids of the 3-subunit and affects channel inactivation of the beta3b-isoform. The present association study tested whether the KCNMB3 delA750 mutation confers susceptibility to common IGE syndromes. In total, 592 unrelated German IGE patients and 462 healthy population controls were genotyped for the delA750 truncation mutation. The frequency of the delA750 mutation was significantly increased in the IGE patients (7.9%) compared to that in the controls (5.5%; P = 0.016, one-sided; OR = 1.52; 95%-CI: 1.05-2.21). The increase of the delA750 frequency was accentuated in 312 patients exhibiting typical absence seizures (8.8%, P = 0.005, one-sided; OR = 1.72; 95%-CI: 1.13-2.62) relatively to that observed in the 237 patients with myoclonic seizures on awakening (7.2%; P = 0.11, one-sided; OR = 1.36; 95%-CI: 0.85-2.19), when compared with controls. The present results suggest that the functional KCNMB3 beta3b-truncation confers a common epileptogenic effect preferentially to the ictogenesis of typical absence seizures.

Quantification of chloride channel 2 (CLCN2) gene isoforms in normal versus lesion- and epilepsy-associated brain tissue

The chloride channel 2 (CLCN2) gene codes for a protein organized in N- and C-terminal regions with regulatory functions and a transmembrane region which forms the ring of the pore. Mutations in the gene have previously been described in patients with idiopathic familial epilepsy. In this study we looked for new isoforms of CLCN2 and we estimated expression levels by real time PCR in brain tissue containing epileptic foci. Samples used in this study were first analyzed and selected to exclude mutations in the coding region of the gene. Four isoforms (skipping exons 3, 16, 22 and 6/7) were identified and quantified by Real Time PCR and compared with total expression of the gene. Expression of the region common to all CLCN2 isoforms was 50% less in epilepsy-associated brain tissue than in controls. The ratio of the various isoforms was slightly greater in epileptic than control tissue. The greatest difference was recorded in the temporal lobe for the isoform with skipped exon 22. Analysis of these isoforms in brain tissue containing epileptic foci suggests that CLCN2 could be implicated in epilepsy, even in the absence of mutations.

Exploration of the Genetic Architecture of Idiopathic Generalized Epilepsies

PURPOSE

Idiopathic generalized epilepsy (IGE) accounts for approximately 20% of all epilepsies and affects about 0.2% of the general population. The etiology of IGE is genetically determined, but the complex pattern of inheritance suggests an involvement of a large number of susceptibility genes. The objective of the present study was to explore the genetic architecture of common IGE syndromes and to dissect out susceptibility loci predisposing to absence or myoclonic seizures.

METHODS

Genome-wide linkage scans were performed in 126 IGE-multiplex families of European origin ascertained through a proband with idiopathic absence epilepsy or juvenile myoclonic epilepsy. Each family had at least two siblings affected by IGE. To search for seizure type-related susceptibility loci, linkage analyses were carried out in family subgroups segregating either typical absence seizures or myoclonic and generalized tonic-clonic seizures on awakening.

RESULTS

Nonparametric linkage scans revealed evidence for complex and heterogeneous genetic architectures involving linkage signals at 5q34, 6p12, 11q13, 13q22-q31, and 19q13. The signal patterns differed in their composition, depending on the predominant seizure type in the families.

CONCLUSIONS

Our results are consistent with heterogeneous configurations of susceptibility loci associated with different IGE subtypes. Genetic determinants on 11q13 and 13q22-q31 seem to predispose preferentially to absence seizures, whereas loci on 5q34, 6p12, and 19q13 confer susceptibility to myoclonic and generalized tonic-clonic seizures on awakening.

Mutations in the K+/Cl- cotransporter gene kazachoc (kcc) increase seizure susceptibility in Drosophila

During a critical period in the developing mammalian brain, there is a major switch in the nature of GABAergic transmission from depolarizing and excitatory, the pattern of the neonatal brain, to hyperpolarizing and inhibitory, the pattern of the mature brain. This switch is believed to play a major role in determining neuronal connectivity via activity-dependent mechanisms. The GABAergic developmental switch may also be particularly vulnerable to dysfunction leading to seizure disorders. The developmental GABA switch is mediated primarily by KCC2, a neuronal K+/Cl- cotransporter that determines the intracellular concentration of Cl- and, hence, the reversal potential for GABA. Here, we report that kazachoc (kcc) mutations that reduce the level of the sole K+/Cl- cotransporter in the fruitfly Drosophila melanogaster render flies susceptible to epileptic-like seizures. Drosophila kcc protein is widely expressed in brain neuropil, and its level rises with developmental age. Young kcc mutant flies with low kcc levels display behavioral seizures and demonstrate a reduced threshold for seizures induced by electroconvulsive shock. The kcc mutation enhances a series of other Drosophila epilepsy mutations indicating functional interactions leading to seizure disorder. Both genetic and pharmacological experiments suggest that the increased seizure susceptibility of kcc flies occurs via excitatory GABAergic signaling. The kcc mutants provide an excellent model system in which to investigate how modulation of GABAergic signaling influences neuronal excitability and epileptogenesis.

Mutations in the CLCN2 gene are a rare cause of idiopathic generalized epilepsy syndromes

Mutations in the chloride channel gene CLCN2 have been reported in families with generalized and focal epilepsy syndromes. To evaluate the contribution of mutations in the CLCN2 gene to the etiology of epilepsies in our population, we screened 96 patients with different epilepsy syndromes and a putative genetic background. No definite mutations were found in our study population. We conclude that mutations in the CLCN2 gene are only a rare cause of idiopathic generalized epilepsy.

Mutations in the GABRA1 and EFHC1 genes are rare in familial juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME), accounting for approximately 25% of idiopathic generalized epilepsies, is genetically heterogeneous. Mutations in the alpha-1 subunit of the GABAA receptor (GABRA1) and EFHC1 genes have been reported in a few families with autosomal dominant (AD) JME. We have investigated the contribution of these two genes to familial JME in our cohort of 54 JME Caucasian families. Syndromic classification of JME was based on previously published criteria. We considered kindreds with at least one affected first-degree relative and the evidence of a vertical transmission as definite AD JME, and families with at least one affected second-degree relative as probable AD JME. We included 33 families meeting criteria for definitive AD JME and 21 that were classified as probable AD JME. None of these families were considered informative enough to analyze candidate loci for JME using linkage analysis. We have systematically screened coding exons of these two genes using temperature gradient capillary electrophoresis. Every heteroduplex with an abnormal mobility was sequenced. No disease-causing mutations in the GABRA1 gene were identified. Analysis of EFHC1 gene found one putative disease-causing mutation R221H that was previously reported as a tandem mutation. Several synonymous and non-synonymous coding polymorphisms were identified but the allelic frequency did not differ between controls and affected individuals. Our data suggests that the majority of familial AD JME is not caused by mutations in the GABRA1 and EFHC1 genes.

CLC chloride channels and transporters

CLC proteins are found in cells from prokaryotes to mammals and perform functions in plasma membranes and intracellular vesicles. Several genetic human diseases and mouse models underscore their broad physiological functions in mammals. These functions range from the control of excitability to transepithelial transport, endocytotic trafficking and acidification of synaptic vesicles. The recent crystallization of bacterial CLC proteins gave surprising insights into CLC Cl(-)-channel permeation and gating and provides an excellent basis for structure-function studies. Surprisingly, the CLC from Escherichia coli functions as a Cl-/H+ exchanger, thus demonstrating the thin line separating transporters and channels.

Sacred disease secrets revealed: the genetics of human epilepsy

Neurons throughout the brain suddenly discharging synchronously and recurrently cause primarily generalized seizures. Discharges localized awhile in one part of the brain cause focal-onset seizures. A genetically determined generalized hyperexcitability had been predicted in primarily generalized seizures, but surprisingly the first epilepsy gene discovered, CHRNA4, was in a focal (frontal lobe)-onset syndrome. Another surprise with CHRNA4 was its encoding of an ion channel present throughout the brain. The reason why CHRNA4 causes focal-onset seizures is unknown. Recently, the second focal (temporal lobe)-onset epilepsy gene, LGI1 (unknown function), was discovered. CHRNA4 led the way to mutation identifications in 15 ion channel genes, most causing primarily generalized epilepsies. Potassium channel mutations cause benign familial neonatal convulsions. Sodium channel mutations cause generalized epilepsy with febrile seizures plus or, if more severe, severe myoclonic epilepsy of infancy. Chloride and calcium channel mutations are found in rare families with the common syndromes childhood absence epilepsy and juvenile myoclonic epilepsy (JME). Mutations in the EFHC1 gene (unknown function) occur in other rare JME families, and yet in other families, associations are present between JME (or other generalized epilepsies) and single nucleotide polymorphisms in the BRD2 gene (unknown function) and the malic enzyme 2 (ME2) gene. Hippocrates predicted the genetic nature of the 'sacred' disease. Genes underlying the 'malevolent' forces seizing 1% of humans have now been revealed. These, however, still account for a mere fraction of the genetic contribution to epilepsy. Exciting years are ahead, in which the genetics of this extremely common, and debilitating, neurological disorder will be solved.

Mechanisms, genetics, and pathogenesis of juvenile myoclonic epilepsy: review

PURPOSE OF REVIEW

This review addresses the mechanisms, genetics and pathogenesis of juvenile myoclonic epilepsy (JME, Janz syndrome).

RECENT FINDINGS

Although JME is a well defined clinical syndrome among the idiopathic generalized epilepsies (IGEs), recent studies suggest that JME is distinct from other IGE syndromes and must be considered separately for the purposes of genetic studies. Clinical, morphological and metabolic data suggest a preferential role for frontal regions in this syndrome. However, JME is clinically and genetically heterogeneous. Although several major genes for JME have been identified and pathogenetic mechanisms suggested based on these findings, these genes account for only a small proportion of JME cases, suggesting multifactorial or complex inheritance in most. The roles played by other major genes, susceptibility genes and environmental factors in the pathogenesis of JME remain to be defined.

SUMMARY

JME is clinically and genetically heterogeneous and should be considered separately from other IGE syndromes. Proposed mechanisms, such as those involving microdysgenesis or altered neuronal inhibition, may be related to different genetic abnormalities in different patients. Major genes account for relatively few cases, and most cases appear to involve multifactorial or complex inheritance.