Targeted loss of Arx results in a developmental epilepsy mouse model and recapitulates the human phenotype in heterozygous females

Early-onset epileptic encephalopathy in a girl carrying a truncating mutation of the ARX gene: rethinking the ARX phenotype in females

Severe early-onset epilepsy is due to a number of known causes, although a clear etiology is not identifiable in up to a third of all the cases. Pathogenic sequence variations in the ARX gene have been described almost exclusively in males, whereas heterozygous female relatives, such as mothers, sisters and even grandmothers have been largely reported as asymptomatic or mildly affected. To investigate the pathogenic role of ARX in refractory epilepsy of early onset even in females, we have screened the ARX sequence in a population of 50 female subjects affected with unexplained epileptic encephalopathy with onset in the first year of life. We report the identification of a novel truncating mutation of the coding region of the ARX gene in a girl with a structurally normal brain. Our findings confirm the role of ARX in the pathogenesis of early epilepsy and underline the importance of screening of the ARX gene in both male and female subjects with otherwise unexplained early onset epileptic encephalopathy.

Mutations of EFHC1, linked to juvenile myoclonic epilepsy, disrupt radial and tangential migrations during brain development

Heterozygous mutations in Myoclonin1/EFHC1 cause juvenile myoclonic epilepsy (JME), the most common form of genetic generalized epilepsies, while homozygous F229L mutation is associated with primary intractable epilepsy in infancy. Heterozygous mutations in adolescent JME patients produce subtle malformations of cortical and subcortical architecture, whereas homozygous F229L mutation in infancy induces severe brain pathology and death. However, the underlying pathological mechanisms for these observations remain unknown. We had previously demonstrated that EFHC1 is a microtubule-associated protein (MAP) involved in cell division and radial migration during cerebral corticogenesis. Here, we show that JME mutations, including F229L, do not alter the ability of EFHC1 to colocalize with the centrosome and the mitotic spindle, but act in a dominant-negative manner to impair mitotic spindle organization. We also found that mutants EFHC1 expression disrupted radial and tangential migration by affecting the morphology of radial glia and migrating neurons. These results show how Myoclonin1/EFHC1 mutations disrupt brain development and potentially produce structural brain abnormalities on which epileptogenesis is established.

Neonatal seizures: controversies and challenges in translating new therapies from the lab to the isolette

Neonatal seizures have unique properties that have proved challenging for both clinicians and basic science researchers. Clinical therapies aimed at neonatal seizures have proven only partially effective and new therapies are slow to develop. This article will discuss neonatal seizures within the framework of the barriers that exist to the development of new therapies, and the challenges inherent in bringing new therapies from the bench to the bedside. With the European Union and USA creating national collaborative project infrastructure, improved collaborative resources should advance clinical research on urgently needed new therapies for this disorder.

A genetic diagnostic approach to infantile epileptic encephalopathies

Epileptic encephalopathies are characterized by frequent severe seizures, and/or prominent interictal epileptiform discharges on the electroencephalogram, developmental delay or deterioration, and usually a poor prognosis. The epileptiform abnormalities themselves are believed to contribute to the progressive disturbance in cerebral function. Determining the underlying aetiology responsible for infantile epileptic encephalopathy is a clinical challenge worth undertaking to facilitate advice on the recurrence risk and to allow for the option of prenatal testing, as often this category of epilepsy is associated with devastating hardship for families. This review takes advantage of recently published studies that have identified new genes associated with epilepsy and focuses on known monogenic causes where detection is useful for the process of genetic counselling. Based on the review, we present a diagnostic work-up in order to triage specific genetic testing for infants presenting with an epileptic encephalopathy.

Asymmetric polymicrogyria and periventricular nodular heterotopia due to mutation in ARX

Mutations in the ARX gene, at Xp22.3, cause several disorders, including infantile spasms, X-linked lissencephaly with abnormal genitalia (XLAG), callosal agenesis and isolated intellectual disability. Genotype/phenotype studies suggested that polyalanine tract expansion is associated with non-malformative phenotypes, while missense and nonsense mutations cause cerebral malformations, however, patients with structural normal brain and missense mutations have been reported. We report on a male patient born with cleft lip and palate who presented with infantile spasms and hemiplegia. MRI showed agenesis of corpus callosum (ACC), an interhemispheric cyst, periventricular nodular heterotopia (PVNH), and extensive left frontal polymicrogyria (PMG). Sequencing of the ARX gene in the patient identified a six basepair insertion (c.335ins6, exon 2). The insertion leads to a two-residue expansion of the first polyalanine tract and was described previously in a family with non-syndromic X-linked mental retardation. To our knowledge, ARX mutation causing PMG and PVNH is unique, but the spasms and ACC are common in ARX mutations. Clinicians should be aware of the broad clinical range of ARX mutations, and further studies are necessary to investigate the association with PMG and PVNH and to identify possible modifying factors.

Interneuron, interrupted: molecular pathogenesis of ARX mutations and X-linked infantile spasms

X-linked Infantile Spasms Syndrome (ISSX) is a catastrophic epilepsy of early childhood with intractable seizures, intellectual disability, and poor prognosis. A spectrum of mutations in the Aristaless-Related Homeobox gene (ARX) has been linked to ISSX, and downstream targets of this interneuron-expressed transcription factor are being defined. Recent advances combining in vitro and in vivo methods have unveiled complex interactions between Arx and its binding partners and their effects on cell migration and maturation that can help explain the diversity of ARX phenotypes. New mutant mouse models of Arx-induced pathology, including a recent human triplet-repeat expansion mutation with a phenotype of infantile spasms and electrographic seizures, provide valuable tools for exploring the pathophysiology of Arx and substrates for testing novel therapies.

Pediatric epilepsy: Five new things

Epilepsy in children can be very different from epilepsy in adults, both in seizure type and epilepsy syndrome. The goal in treating children is seizure freedom, no treatment side effects, and function that is no different from the general population. In a significant percentage of patients, this goal remains unachievable, but many aspects of epilepsy are becoming clearer. This review will highlight 5 areas where progress is being made to achieve these goals in pediatric epilepsy. Specific research animal models are being developed to reflect the unique features of different pediatric epilepsies. As genetic syndromes are better identified, for some patients this has led to improved treatment. New advances in drug therapy have led to 3 new medications approved for children. More effective drug choices can now be recommended due to comparative drug trials and better overall care of children can be provided due to awareness of the comorbidities of epilepsy.

Arx is required for normal enteroendocrine cell development in mice and humans

Enteroendocrine cells of the gastrointestinal (GI) tract play a central role in metabolism, digestion, satiety and lipid absorption, yet their development remains poorly understood. Here we show that Arx, a homeodomain-containing transcription factor, is required for the normal development of mouse and human enteroendocrine cells. Arx expression is detected in a subset of Neurogenin3 (Ngn3)-positive endocrine progenitors and is also found in a subset of hormone-producing cells. In mice, removal of Arx from the developing endoderm results in a decrease of enteroendocrine cell types including gastrin-, glucagon/GLP-1-, CCK-, secretin-producing cell populations and an increase of somatostatin-expressing cells. This phenotype is also observed in mice with endocrine-progenitor-specific Arx ablation suggesting that Arx is required in the progenitor for enteroendocrine cell development. In addition, depletion of human ARX in developing human intestinal tissue results in a profound deficit in expression of the enteroendocrine cell markers CCK, secretin and glucagon while expression of a pan-intestinal epithelial marker, CDX2, and other non-endocrine markers remained unchanged. Taken together, our findings uncover a novel and conserved role of Arx in mammalian endocrine cell development and provide a potential cause for the chronic diarrhea seen in both humans and mice carrying Arx mutations.

Identification of new epilepsy treatments: issues in preclinical methodology

Preclinical research has facilitated the discovery of valuable drugs for the symptomatic treatment of epilepsy. Yet, despite these therapies, seizures are not adequately controlled in a third of all affected individuals, and comorbidities still impose a major burden on quality of life. The introduction of multiple new therapies into clinical use over the past two decades has done little to change this. There is an urgent demand to address the unmet clinical needs for: (1) new symptomatic antiseizure treatments for drug-resistant seizures with improved efficacy/tolerability profiles, (2) disease-modifying treatments that prevent or ameliorate the process of epileptogenesis, and (3) treatments for the common comorbidities that contribute to disability in people with epilepsy. New therapies also need to address the special needs of certain subpopulations, that is, age- or gender-specific treatments. Preclinical development in these treatment areas is complex due to heterogeneity in presentation and etiology, and may need to be formulated with a specific seizure, epilepsy syndrome, or comorbidity in mind. The aim of this report is to provide a framework that will help define future guidelines that improve and standardize the design, reporting, and validation of data across preclinical antiepilepsy therapy development studies targeting drug-resistant seizures, epileptogenesis, and comorbidities.

Experimental models of seizures and epilepsies

Epilepsy is one of the most common neurological conditions that affect people of all ages. Epilepsy is characterized by occurrence of spontaneous recurrent seizures. Currently available drugs are ineffective in controlling seizures in approximately one-third of patients with epilepsy. Moreover, these drugs are associated with adverse effects, and none of them are effective in preventing development of epilepsy following an insult or injury. To develop an effective therapeutic strategy that can interfere with the process of development of epilepsy (epileptogenesis), it is crucial to study the changes that occur in the brain after an injury and before epilepsy develops. It is not possible to determine these changes in human tissue for obvious ethical reasons. Over the years, experimental models of epilepsies have contributed immensely in improving our understanding of mechanism of epileptogenesis as well as of seizure generation. There are many models that replicate at least some of the characteristics of human epilepsy. Each model has its advantages and disadvantages, and the investigator should be aware of this before selecting a specific model for his/her studies. Availability of a good animal model is a key to the development of an effective treatment. Unfortunately, there are many epilepsy syndromes, specifically pediatric, which still lack a valid animal model. It is vital that more research is done to develop animal models for such syndromes.

Epilepsy as a neurodevelopmental disorder

Epilepsy is characterized by spontaneous recurrent seizures and comprises a diverse group of syndromes with different etiologies. Epileptogenesis refers to the process whereby the brain becomes epileptic and can be related to several factors, such as acquired structural brain lesions, inborn brain malformations, alterations in neuronal signaling, and defects in maturation and plasticity of neuronal networks. In this review, we will focus on alterations of brain development that lead to an hyperexcitability phenotype in adulthood, providing examples from both animal and human studies. Malformations of cortical development (including focal cortical dysplasia, lissencephaly, heterotopia, and polymicrogyria) are frequently epileptogenic and result from defects in cell proliferation in the germinal zone and/or impaired neuronal migration and differentiation. Delayed or reduced arrival of inhibitory interneurons into the cortical plate is another possible cause of epileptogenesis. GABAergic neurons are generated during early development in the ganglionic eminences, and failure to pursue migration toward the cortex alters the excitatory/inhibitory balance resulting in aberrant network hyperexcitability. More subtle defects in the developmental assembly of excitatory and inhibitory synapses are also involved in epilepsy. For example, mutations in the presynaptic proteins synapsins and SNAP-25 cause derangements of synaptic transmission and plasticity which underlie appearance of an epileptic phenotype. Finally, there is evidence that defects in synapse elimination and remodeling during early "critical periods" can trigger hyperexcitability later in life. Further clarification of the developmental pathways to epilepsy has important implications for disease prevention and therapy.

Genes of early-onset epileptic encephalopathies: from genotype to phenotype

Early-onset epileptic encephalopathies are severe disorders in which cognitive, sensory, and motor development is impaired by recurrent clinical seizures or prominent interictal epileptiform discharges during the neonatal or early infantile periods. They include Ohtahara syndrome, early myoclonic epileptic encephalopathy, West syndrome, Dravet syndrome, and other diseases, e.g., X-linked myoclonic seizures, spasticity and intellectual disability syndrome, idiopathic infantile epileptic-dyskinetic encephalopathy, epilepsy and mental retardation limited to females, and severe infantile multifocal epilepsy. We summarize recent updates on the genes and related clinical syndromes involved in the pathogenesis of early-onset epileptic encephalopathies: Aristaless-related homeobox (ARX), cyclin-dependent kinase-like 5 (CDKL5), syntaxin-binding protein 1 (STXBP1), solute carrier family 25 member 22 (SLC25A22), nonerythrocytic α-spectrin-1 (SPTAN1), phospholipase Cβ1 (PLCβ1), membrane-associated guanylate kinase inverted-2 (MAGI2), polynucleotide kinase 3'-phosphatase (PNKP), sodium channel neuronal type 1α subunit (SCN1A), protocadherin 19 (PCDH19), and pyridoxamine 5-prime-phosphate oxidase (PNPO).

Animal models

Epilepsy accounts for a significant portion of the dis-ease burden worldwide. Research in this field is fundamental and mandatory. Animal models have played, and still play, a substantial role in understanding the patho-physiology and treatment of human epilepsies. A large number and variety of approaches are available, and they have been applied to many animals. In this chapter the in vitro and in vivo animal models are discussed,with major emphasis on the in vivo studies. Models have used phylogenetically different animals - from worms to monkeys. Our attention has been dedicated mainly to rodents.In clinical practice, developmental aspects of epilepsy often differ from those in adults. Animal models have often helped to clarify these differences. In this chapter, developmental aspects have been emphasized.Electrical stimulation and chemical-induced models of seizures have been described first, as they represent the oldest and most common models. Among these models, kindling raised great interest, especially for the study of the epileptogenesis. Acquired focal models mimic seizures and occasionally epilepsies secondary to abnormal cortical development, hypoxia, trauma, and hemorrhage.Better knowledge of epileptic syndromes will help to create new animal models. To date, absence epilepsy is one of the most common and (often) benign forms of epilepsy. There are several models, including acute pharmacological models (PTZ, penicillin, THIP, GBL) and chronic models (GAERS, WAG/Rij). Although atypical absence seizures are less benign, thus needing more investigation, only two models are so far available (AY-9944,MAM-AY). Infantile spasms are an early childhood encephalopathy that is usually associated with a poor out-come. The investigation of this syndrome in animal models is recent and fascinating. Different approaches have been used including genetic (Down syndrome,ARX mutation) and acquired (multiple hit, TTX, CRH,betamethasone-NMDA) models.An entire section has been dedicated to genetic models, from the older models obtained with spontaneous mutations (GEPRs) to the new engineered knockout, knocking, and transgenic models. Some of these models have been created based on recently recognized patho-genesis such as benign familial neonatal epilepsy, early infantile encephalopathy with suppression bursts, severe myoclonic epilepsy of infancy, the tuberous sclerosis model, and the progressive myoclonic epilepsy. The contribution of animal models to epilepsy re-search is unquestionable. The development of further strategies is necessary to find novel strategies to cure epileptic patients, and optimistically to allow scientists first and clinicians subsequently to prevent epilepsy and its consequences.

ARX homeodomain mutations abolish DNA binding and lead to a loss of transcriptional repression

Mutations in the Aristaless-related homeobox (ARX) gene are one of the most frequent causes of X-linked intellectual disability (ID). Several missense mutations, clustered in the paired-type homeodomain of ARX, have been identified. These mutations lead to a range of phenotypes from X-linked lissencephaly with abnormal genitalia to seizure disorders without brain malformations including X-linked infantile spasms with ID (ISSX-ID) and X-linked myoclonic epilepsy with spasticity and ID (XMESID). The effect of these mutations on the DNA-binding and transcriptional activity has been evaluated. Luciferase reporter assays showed altered repression activity of ARX by all mutations, causing brain malformations and ISSX-ID phenotypes, but not by the P353L mutation implicated in a milder phenotype of XMESID. Similarly, transient overexpression of wild-type ARX repressed endogenous expression of known ARX targets, LMO1 and SHOX2, when measured by real-time quantitative polymerase chain reaction. Overall, the molecular consequence of missense mutations correlated well with the severity of the clinical phenotype. In all mutations tested, except P353L, the DNA binding was abolished. Electrophoretic mobility shift assay results were validated using chromatin immunoprecipitation following overexpression of normal and selected missense mutations. Unlike wild-type ARX and clinically less severe mutations, the mutations leading to severe clinical phenotypes were not able to specifically bind to DNA upstream of known, endogenous ARX-regulated genes, LMO1 and SHOX2. In conclusion, the missense mutations in the ARX homeodomain represent loss-of-function mutations, which lead to a reduced or complete loss of DNA binding and as a consequence, a loss of transcriptional repression.

Nkx2.2 repressor complex regulates islet β-cell specification and prevents β-to-α-cell reprogramming

Regulation of cell differentiation programs requires complex interactions between transcriptional and epigenetic networks. Elucidating the principal molecular events responsible for the establishment and maintenance of cell fate identities will provide important insights into how cell lineages are specified and maintained and will improve our ability to recapitulate cell differentiation events in vitro. In this study, we demonstrate that Nkx2.2 is part of a large repression complex in pancreatic β cells that includes DNMT3a, Grg3, and HDAC1. Mutation of the endogenous Nkx2.2 tinman (TN) domain in mice abolishes the interaction between Nkx2.2 and Grg3 and disrupts β-cell specification. Furthermore, we demonstrate that Nkx2.2 preferentially recruits Grg3 and HDAC1 to the methylated Aristaless homeobox gene (Arx) promoter in β cells. The Nkx2.2 TN mutation results in ectopic expression of Arx in β cells, causing β-to-α-cell transdifferentiation. A corresponding β-cell-specific deletion of DNMT3a is also sufficient to cause Arx-dependent β-to-α-cell reprogramming. Notably, subsequent removal of Arx in the β cells of Nkx2.2(TNmut/TNmut) mutant mice reverts the β-to-α-cell conversion, indicating that the repressor activities of Nkx2.2 on the methylated Arx promoter in β cells are the primary regulatory events required for maintaining β-cell identity.

Genetic and biologic classification of infantile spasms

Infantile spasms constitute an age-dependent epilepsy, highly associated with cognitive impairment, autism, and movement disorders. Previous classification systems focused on a distinction between symptomatic and cryptogenic etiologies, and have not kept pace with recent discoveries of mutations in genes in key pathways of central nervous system development in patients with infantile spasms. Children with certain genetic syndromes are much likelier to manifest infantile spasms, and we review the literature to propose a genetic classification of these disorders. Children demonstrating genetic associations with infantile spasms also manifest phenotypes beyond epilepsy that may be explained by recent advances in the understanding of underlying biological mechanisms. Therefore we propose a biologic classification of genes highly associated with infantile spasms, and articulate models for infantile spasms pathogenesis based on those data. The two best described pathways of pathogenesis involve abnormalities in the gene regulatory network of gamma-aminobutyric acidergic forebrain development and abnormalities in molecules expressed at the synapse. These genetic and biologic classifications are flexible, and they should encourage much needed progress in syndrome recognition, clinical genetic testing, and the development of new therapies targeting specific pathways of pathogenesis.

The promise of an interneuron-based cell therapy for epilepsy

Of the nearly 3 million Americans diagnosed with epilepsy, approximately 30% are unresponsive to current medications. Recent data has shown that early postnatal transplantation of interneuronal precursor cells increases GABAergic inhibition in the host brain and dramatically suppresses seizure activity in epileptic mice. In this review, we will highlight findings from seizure-prone mice and humans that demonstrate the link between dysfunctional GABAergic inhibition and hyperexcitability. In particular, we will focus on rodent models of temporal lobe epilepsy, the most common and difficult to treat form of the disease, and interneuronopathies, an emerging classification. A wealth of literature showing a causal link between reduced GABA-mediated inhibition and seizures has directed our efforts to recover the loss of inhibition via transplantation of interneuronal precursors. Numerous related studies have explored the anticonvulsant potential of cell grafts derived from a variety of brain regions, yet the mechanism underlying the effect of such heterogeneous cell transplants is unknown. In discussing our recent findings and placing them in context with what is known about epilepsy, and how related transplant approaches have progressed, we hope to initiate a frank discussion of the best path toward the translation of this approach to patients with intractable forms of epilepsy.

Differential effects of a polyalanine tract expansion in Arx on neural development and gene expression

Polyalanine (poly-A) tracts exist in 494 annotated proteins; to date, expansions in these tracts have been associated with nine human diseases. The pathogenetic mechanism by which a poly-A tract results in these various human disorders remains uncertain. To understand the role of this mutation type, we investigated the change in functional properties of the transcription factor Arx when it has an expanded poly-A tract (Arx(E)), a mutation associated with infantile spasms and intellectual disabilities in humans. We found that although Arx(E) functions normally in the dorsal brain, its function in subpallial-derived populations of neurons is compromised. These contrasting functions are associated with the misregulation of Arx targets through the loss of the ability of Arx(E) to interact with the Arx cofactor Tle1. Our data demonstrate a novel mechanism for poly-A expansion diseases: the misregulation of a subset of target genes normally regulated by a transcription factor.

Treatment of infantile spasms: emerging insights from clinical and basic science perspectives

Infantile spasms is an epileptic encephalopathy of early infancy with specific clinical and electroencephalographic (EEG) features, limited treatment options, and a poor prognosis. Efforts to develop improved treatment options have been hindered by the lack of experimental models in which to test prospective therapies. The neuropeptide adrenocorticotropic hormone (ACTH) is effective in many cases of infantile spasms, although its mechanism(s) of action is unknown. This review describes the emerging candidate mechanisms that can underlie the therapeutic effects of ACTH in infantile spasms. These mechanisms can ultimately help to improve understanding and treatment of the disease. An overview of current treatments of infantile spasms, novel conceptual and experimental approaches to infantile spasms treatment, and a perspective on remaining clinical challenges and current research questions are presented here. This summary derives from a meeting of specialists in infantile spasms clinical care and research held in New York City on June 14, 2010.

In search of epilepsy biomarkers in the immature brain: goals, challenges and strategies

Epilepsy and seizures are very common in the early years of life and are often associated with significant morbidity and mortality. Identification of biomarkers for the early detection of epileptogenicity, epileptogenesis, comorbidities, disease progression and treatment implementation will be very important in implementing more effective therapies. This article summarizes the current needs in the search for new early life epilepsy-related biomarkers and discusses the candidate biomarkers that are under investigation, as well as the challenges associated with the identification and validation of these biomarkers.

The role of GABAergic system in neurodevelopmental disorders: a focus on autism and epilepsy

Autism spectrum disorders (ASD) and epilepsy are very common neurological disorders of childhood, with an estimated incidence of about 0.5 - 1 % in worldwide population. ASD and epilepsy are often associated, suggesting that common neurodevelopmental bases may exist for these two disorders. The neurodevelopmental bases of both ASD and epilepsy have been clearly showed by a number of genetic, neuroimaging and neuropathological studies. In recent years, dysfunction of inhibitory GABAergic circuits has been proposed as a cause for both disorders. Several studies performed on both animal models and postmortem human samples indicate that GABAergic neurons and circuits are altered in both ASD and epilepsy, suggesting that the excitation/inhibition imbalance resulting from neurodevelopmental defects in GABAergic circuitry might represent a common pathogenetic mechanism for these disorders. Here, we will review the most significant studies supporting this hypothesis.

Genetics and function of neocortical GABAergic interneurons in neurodevelopmental disorders

A dysfunction of cortical and limbic GABAergic circuits has been postulated to contribute to multiple neurodevelopmental disorders in humans, including schizophrenia, autism, and epilepsy. In the current paper, I summarize the characteristics that underlie the great diversity of cortical GABAergic interneurons and explore how the multiple roles of these cells in developing and mature circuits might contribute to the aforementioned disorders. Furthermore, I review the tightly controlled genetic cascades that determine the fate of cortical interneurons and summarize how the dysfunction of genes important for the generation, specification, maturation, and function of cortical interneurons might contribute to these disorders.

Altered GABA signaling in early life epilepsies

The incidence of seizures is particularly high in the early ages of life. The immaturity of inhibitory systems, such as GABA, during normal brain development and its further dysregulation under pathological conditions that predispose to seizures have been speculated to play a major role in facilitating seizures. Seizures can further impair or disrupt GABA_A signaling by reshuffling the subunit composition of its receptors or causing aberrant reappearance of depolarizing or hyperpolarizing GABA_A receptor currents. Such effects may not result in epileptogenesis as frequently as they do in adults. Given the central role of GABA_A signaling in brain function and development, perturbation of its physiological role may interfere with neuronal morphology, differentiation, and connectivity, manifesting as cognitive or neurodevelopmental deficits. The current GABAergic antiepileptic drugs, while often effective for adults, are not always capable of stopping seizures and preventing their sequelae in neonates. Recent studies have explored the therapeutic potential of chloride cotransporter inhibitors, such as bumetanide, as adjunctive therapies of neonatal seizures. However, more needs to be known so as to develop therapies capable of stopping seizures while preserving the age- and sex-appropriate development of the brain.

Carisbamate acutely suppresses spasms in a rat model of symptomatic infantile spasms

PURPOSE

Infantile spasms are the signature seizures of West syndrome. The conventional treatments for infantile spasms, such as adrenocorticotropic hormone (ACTH) and vigabatrin, are not always effective, especially in symptomatic infantile spasms (SIS). We tested the efficacy of carisbamate, a novel neurotherapeutic drug, to suppress spasms in the multiple-hit rat model of SIS, and compared it with phenytoin to determine if its effect is via sodium-channel blockade.

METHODS

Sprague-Dawley rats received right intracerebral infusions of doxorubicin and lipopolysaccharide at postnatal day 3 (PN3) and intraperitoneal p-chlorophenylalanine at PN5. A single intraperitoneal injection of carisbamate was administered at PN4, after the onset of spasms, at the following doses: 10 mg/kg (CRS-10), 30 mg/kg (CRS-30), and 60 mg/kg (CRS-60), and was compared to vehicle-injected group (VEH). Video-monitoring of PN6-7 CRS-60 or VEH-injected pups was also done.

KEY FINDINGS

Carisbamate acutely reduced both behavioral spasms (CRS-30 and CRS-60 groups only) and electroclinical spasms during the first 2-3 postinjection hours, without detectable toxicity or mortality. In contrast, phenytoin (20 or 50 mg/kg) failed to suppress spasms.

SIGNIFICANCE

Our findings provide preclinical evidence that carisbamate displays acute anticonvulsive effect on spasms through a sodium channel-independent mechanism. Because spasms in the multiple-hit rat model are refractory to ACTH and transiently sensitive to vigabatrin, carisbamate may constitute a candidate new therapy for SIS, including the ACTH-refractory spasms. Further confirmation with clinical studies is needed.

Copy number variants and infantile spasms: evidence for abnormalities in ventral forebrain development and pathways of synaptic function

Infantile spasms (ISS) are an epilepsy disorder frequently associated with severe developmental outcome and have diverse genetic etiologies. We ascertained 11 subjects with ISS and novel copy number variants (CNVs) and combined these with a new cohort with deletion 1p36 and ISS, and additional published patients with ISS and other chromosomal abnormalities. Using bioinformatics tools, we analyzed the gene content of these CNVs for enrichment in pathways of pathogenesis. Several important findings emerged. First, the gene content was enriched for the gene regulatory network involved in ventral forebrain development. Second, genes in pathways of synaptic function were overrepresented, significantly those involved in synaptic vesicle transport. Evidence also suggested roles for GABAergic synapses and the postsynaptic density. Third, we confirm the association of ISS with duplication of 14q12 and maternally inherited duplication of 15q11q13, and report the association with duplication of 21q21. We also present a patient with ISS and deletion 7q11.3 not involving MAGI2. Finally, we provide evidence that ISS in deletion 1p36 may be associated with deletion of KLHL17 and expand the epilepsy phenotype in that syndrome to include early infantile epileptic encephalopathy. Several of the identified pathways share functional links, and abnormalities of forebrain synaptic growth and function may form a common biologic mechanism underlying both ISS and autism. This study demonstrates a novel approach to the study of gene content in subjects with ISS and copy number variation, and contributes further evidence to support specific pathways of pathogenesis.

Molecular genetics of neuronal migration disorders

Cortical malformations associated with defects in neuronal migration result in severe developmental consequences including intractable epilepsy and intellectual disability. Genetic causes of migration defects have been identified with the advent and widespread use of high-resolution MRI and genetic techniques. Thus, the full phenotypic range of these genetic disorders is becoming apparent. Genes that cause lissencephaly, pachygyria, subcortical band heterotopia, and periventricular nodular heterotopias have been defined. Many of these genes are involved in cytoskeletal regulation including the function of microtubules (LIS1, TUBA1A,TUBB3, and DCX) and of actin (FilaminA). Thus, the molecular pathways regulating neuronal migration including the cytoskeletal pathways appear to be defined by human mutation syndromes. Basic science, including cell biology and animal models of these disorders, has informed our understanding of the pathogenesis of neuronal migration disorders and further progress depends on the continued integration of the clinical and basic sciences.

Screening and cell-based assessment of mutations in the Aristaless-related homeobox (ARX) gene

ARX mutations cause a diverse spectrum of human disorders, ranging from severe brain and genital malformations to non-syndromic intellectual disability (ID). ARX is a transcription factor with multiple domains that include four polyalanine (pA) tracts, the first two of which are frequently expanded by mutations. We progressively screened DNA samples from 613 individuals with ID initially for the most frequent ARX mutations (c.304ins(GCG)(7)'expansion' of pA1 and c.429_452dup 'dup24bp' of pA2). Five hundred samples without pA1 or pA2 mutations had the entire ARX ORF screened by single stranded polymorphism conformation (SSCP) and/or denaturing high pressure liquid chromatography (dHPLC) analysis. Overall, eight families with six mutations in ARX were identified (1.31%): five duplication mutations in pA2 (0.82%) with three new clinical reports of families with the dup24bp and two duplications larger than the dup24bp mutation discovered (dup27bp, dup33bp); and three point mutations (0.6%), including one novel mutation in the homeodomain (c.1074G>T). Four ultraconserved regions distal to ARX (uc466-469) were also screened in a subset of 94 patients, with three unique nucleotide changes identified in two (uc466, uc467). The subcellular localization of full length ARX proteins was assessed for 11 variants. Protein mislocalization increased as a function of pA2 tract length and phenotypic severity, as has been previously suggested for pA1. Similarly, protein mislocalization of the homeodomain mutations also correlated with clinical severity, suggesting an emerging genotype vs cellular phenotype correlation.

Deletion of Dlx1 results in reduced glutamatergic input to hippocampal interneurons

Dlx transcription factors are important in the differentiation of GABAergic interneurons. In mice lacking Dlx1, early steps in interneuron development appear normal. Beginning at ∼ 1 mo of age, primarily dendrite-innervating interneuron subtypes begin to undergo apoptosis in Dlx1(-/-) mice; this is accompanied by a reduction in GABAergic transmission and late-onset epilepsy. The reported reduction of synaptic inhibition is greater than might be expected given that interneuron loss is relatively modest in Dlx1(-/-) mice. Here we report that voltage-clamp recordings of CA1 interneurons in hippocampal slices prepared from Dlx1(-/-) animals older than postnatal day 30 (>P30) revealed a significant reduction in excitatory postsynaptic current (EPSC) amplitude. No changes in EPSCs onto interneurons were observed in cells recorded from younger animals (P9-12). Current-clamp recordings from interneurons at these early postnatal ages showed that interneurons in Dlx1(-/-) mutants were immature and more excitable, although membrane properties normalized by P30. Terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling, caspase-3, and NeuN staining did not reveal frank cell damage or loss in area CA3 of hippocampal sections from adult Dlx1(-/-) mice. Delayed interneuron maturation may lead to interneuron hyperexcitability, followed by a compensatory reduction in the strength of excitatory transmission onto interneurons. This reduced excitation onto surviving interneurons, coupled with the loss of a significant fraction of GABAergic inputs to excitatory neurons starting at P30, may underlie cortical dysrhythmia and seizures previously observed in adult Dlx1(-/-) mice.

Corpus callosum agenesis, severe mental retardation, epilepsy, and dyskinetic quadriparesis due to a novel mutation in the homeodomain of ARX

We report on a patient with agenesis of the corpus callosum (ACC), severe mental retardation, infantile spasms and subsequent intractable epilepsy, spastic/dyskinetic quadriparesis, severe limb contractures, and scoliosis. This complex, newly described phenotype, is due to a novel non-conservative missense mutation in the ARX homeodomain (c.1072A>T; p.R358W), inherited from the unaffected mother. Differently from previously reported non-conservative mutations falling within the same domain, p.R358W did not cause XLAG. It is therefore possible that differences in clinical manifestations between our patient and those with XLAG, are related to the different position of the amino acid substitution in the homeodomain, or to the different chemical properties introduced by the substitution itself. To test the hypothesis that the patient's mother was asymptomatic because of non-random X chromosome inactivation (XCI), we performed DNA methylation studies of the human androgen receptor gene, demonstrating skewing of the XCI ratio (85:15). The complex phenotype described here combines different traits that had previously been linked to various ARX mutations, including conservative missense mutations in the homeodomain and expansion in the first ARX polyalanine tract and contributes to the expanding pleiotropy associated with ARX mutations.

Molecular mechanisms of cognitive and behavioral comorbidities of epilepsy in children

Intellectual and developmental disabilities (IDDs) such as autistic spectrum disorders (ASDs) and epilepsies are heterogeneous disorders that have diverse etiologies and pathophysiologies. The high rate of co-occurrence of these disorders, however, suggests potentially shared underlying mechanisms. A number of well-known genetic disorders share epilepsy, intellectual disability, and autism as prominent phenotypic features, including tuberous sclerosis complex, Rett syndrome, and fragile X syndrome. In addition, mutations of several genes involved in neurodevelopment, including ARX, DCX, neuroligins, and neuropilin 2 have been identified in children with epilepsy, IDDs, ASDs, or a combination of thereof. Finally, in animal models, early life seizures can result in cellular and molecular changes that could contribute to learning and behavioral disabilities. Increased understanding of the common genetic, molecular, and cellular mechanisms of IDDs, ASDs, and epilepsy may provide insight into their underlying pathophysiology and elucidate new therapeutic approaches for these conditions.

Epilepsy genetics--past, present, and future

Human epilepsy is a common and heterogeneous condition in which genetics play an important etiological role. We begin by reviewing the past history of epilepsy genetics, a field that has traditionally included studies of pedigrees with epilepsy caused by defects in ion channels and neurotransmitters. We highlight important recent discoveries that have expanded the field beyond the realm of channels and neurotransmitters and that have challenged the notion that single genes produce single disorders. Finally, we project toward an exciting future for epilepsy genetics as large-scale collaborative phenotyping studies come face to face with new technologies in genomic medicine.

Contractions in the second polyA tract of ARX are rare, non-pathogenic polymorphisms

Aristaless related homeobox (ARX) is a transcription factor containing highly conserved octapeptide, homeobox, acidic, and aristaless domains, as well as four polyA tracts. The most frequent ARX mutation found to date in patients with X-linked infantile spasms, Partington syndrome or X-linked mental retardation, is a duplication of 24 bp in exon 2, resulting in the expansion of the second polyA tract. Although the pathogenic role of this expansion has been well characterized, the effect of contractions in the same polyA tract is still debated since different reports have associated contractions to either mental retardation or a normal phenotype. Here, we report two unrelated girls with epilepsy and mental retardation who inherited from their unaffected parents, of either sex, a deletion of 24 bp (c.441_464del), resulting in a contraction of eight alanines in the second polyA tract of ARX. Segregation studies revealed the c.441_464del also in two healthy relatives of one of the patients. This finding supports the hypothesis that this contraction represents a rare, benign polymorphism.

The maturation of cortical interneuron diversity: how multiple developmental journeys shape the emergence of proper network function

If the classical functional attribute of cortical GABAergic interneurons is to mediate synaptic inhibition in the adult cortex, it is becoming evident that their major task is instead to shape the spatio-temporal dynamics of the network oscillations that support most brain functions. This complex function involves a division of labour between morpho-physiologically diverse interneuron subtypes. Both the central network function and the bewildering heterogeneity of the interneuron population are especially emphasized during cortical development: at early postnatal stages, a single GABAergic neuron can efficiently pace the activity of hundreds of other cells, whereas some interneuron subtypes are still poorly developed. Given the role of coherent activity in brain development, this confers to GABAergic interneurons a major role in the proper maturation of cortical networks.

ARX spectrum disorders: making inroads into the molecular pathology

The Aristaless-related homeobox gene (ARX) is one of the most frequently mutated genes in a spectrum of X-chromosome phenotypes with intellectual disability (ID) as their cardinal feature. To date, close to 100 families and isolated cases have been reported to carry 44 different mutations, the majority of these (59%) being a result of polyalanine tract expansions. At least 10 well-defined clinical entities, including Ohtahara, Partington, and Proud syndromes, X-linked infantile spasms, X-linked lissencephaly with ambiguous genitalia, X-linked myoclonic epilepsy and nonsyndromic intellectual disability have been ascertained from among the patients with ARX mutations. The striking intra- and interfamilial pleiotropy together with genetic heterogeneity (same clinical entities associated with different ARX mutations) are becoming a hallmark of ARX mutations. Although males are predominantly affected, some mutations associated with malformation phenotypes in males also show a phenotype in carrier females. Recent progress in the study of the effect of ARX mutations through sophisticated animal (mice) and cellular models begins to provide crucial insights into the molecular function of ARX and associated molecular pathology, thus guiding future inquiries into therapeutic interventions.

Epilepsy and autism spectrum disorders: are there common developmental mechanisms?

Autistic spectrum disorders (ASD) and epilepsies are heterogeneous disorders that have diverse etiologies and pathophysiologies. The high rate of co-occurrence of these disorders suggest potentially shared underlying mechanisms. A number of well-known genetic disorders share epilepsy and autism as prominent phenotypic features, including tuberous sclerosis, Rett syndrome, and fragile X. In addition, mutations of several genes involved in neurodevelopment, including ARX, DCX, neuroligins and neuropilin2 have been identified in children with epilepsy, ASD or often both. Finally, in animal models, early-life seizures can result in cellular and molecular changes that could contribute to learning and behavioral disabilities as seen in ASD. Increased understanding of the common genetic, molecular and cellular mechanisms of ASD and epilepsy may provide insight into their underlying pathophysiology and elucidate new therapeutic approaches of both conditions.

Modeling new therapies for infantile spasms

Infantile spasms are the classical seizure type of West syndrome. Infantile spasms often herald a dismal prognosis, due to the high probability to evolve into intractable forms of epilepsies with significant cognitive deficits, especially if not adequately treated. The current therapies-high doses of adrenocorticotropic hormone, steroids, or the gamma-aminobutyric acid (GABA) transaminase inhibitor vigabatrin--are often toxic and may not always be effective. The need to identify new therapies for spasms has led to the generation of a number of rodent models of infantile spasms. These include acute and chronic models of infantile spasms, with cryptogenic or symptomatic origin, many of which are based on specific etiologies. In this review, we summarize the clinical experience with treating infantile spasms and the main features of the new animal models of infantile spasms and discuss their utility in the preclinical development of new therapies for infantile spasms.

Glucagon deficiency reduces hepatic glucose production and improves glucose tolerance in adult mice

The major role of glucagon is to promote hepatic gluconeogenesis and glycogenolysis to raise blood glucose levels during hypoglycemic conditions. Several animal models have been established to examine the in vivo function of glucagon in the liver through attenuation of glucagon via glucagon receptor knockout animals and pharmacological interventions. To investigate the consequences of glucagon loss to hepatic glucose production and glucose homeostasis, we derived mice with a pancreas specific ablation of the alpha-cell transcription factor, Arx, resulting in a complete loss of the glucagon-producing pancreatic alpha-cell. Using this model, we found that glucagon is not required for the general health of mice but is essential for total hepatic glucose production. Our data clarifies the importance of glucagon during the regulation of fasting and postprandial glucose homeostasis.

Dlx5 and Dlx6 regulate the development of parvalbumin-expressing cortical interneurons

Dlx5 and Dlx6 homeobox genes are expressed in developing and mature cortical interneurons. Simultaneous deletion of Dlx5 and 6 results in exencephaly of the anterior brain; despite this defect, prenatal basal ganglia differentiation appeared largely intact, while tangential migration of Lhx6(+) and Mafb(+) interneurons to the cortex was reduced and disordered. The migration deficits were associated with reduced CXCR4 expression. Transplantation of mutant immature interneurons into a wild-type brain demonstrated that loss of either Dlx5 or Dlx5&6 preferentially reduced the number of mature parvalbumin(+) interneurons; those parvalbumin(+) interneurons that were present had increased dendritic branching. Dlx5/6(+/-) mice, which appear normal histologically, show spontaneous electrographic seizures and reduced power of gamma oscillations. Thus, Dlx5&6 appeared to be required for development and function of somal innervating (parvalbumin(+)) neocortical interneurons. This contrasts with Dlx1, whose function is required for dendrite innervating (calretinin(+), somatostatin(+), and neuropeptide Y(+)) interneurons (Cobos et al., 2005).

The Rhox genes

Homeobox genes encode transcription factors that have crucial roles in embryogenesis. A recently discovered set of homeobox genes--the Rhox genes--are expressed during both embryogenesis and in adult reproductive tissues. The 33 known mouse Rhox genes are clustered together in a single region on the X chromosome, while likely descendents of the primodial Rhox cluster, Arx and Esx1, have moved to other positions on the X chromosome. Here, we summarize what is known about the regulation and function of Rhox cluster and Rhox-related genes during embryogenesis and gametogenesis. The founding member of the Rhox gene cluster--Rhox5 (previously known as Pem)--has been studied in the most depth and thus is the focus of this review. We also discuss the unusually rapid evolution of the Rhox gene cluster.

Mutations in ARX Result in Several Defects Involving GABAergic Neurons

Genetic investigations of X-linked mental retardation have demonstrated the implication of ARX in a wide spectrum of disorders extending from phenotypes with severe neuronal migration defects, such as lissencephaly, to mild or moderate forms of mental retardation without apparent brain abnormalities, but with associated features of dystonia and epilepsy. These investigations have in recent years directed attention to the role of this gene in brain development. Analysis of its spatio-temporal localization profile revealed expression in telencephalic structures at all stages of development, mainly restricted to populations of GABA-containing neurons. Furthermore, studies of the effects of ARX loss of function either in humans or in lines of mutant mice revealed varying defects, suggesting multiple roles of this gene during development. In particular, Arx has been shown to contribute to almost all fundamental processes of brain development: patterning, neuronal proliferation and migration, cell maturation and differentiation, as well as axonal outgrowth and connectivity. In this review, we will present and discuss recent findings concerning the role of ARX in brain development and how this information will be useful to better understand the pathophysiological mechanisms of mental retardation and epilepsy associated with ARX mutations.

Mutations in the nuclear localization sequence of the Aristaless related homeobox; sequestration of mutant ARX with IPO13 disrupts normal subcellular distribution of the transcription factor and retards cell division

Background

Aristaless related homeobox (ARX) is a paired-type homeobox gene. ARX function is frequently affected by naturally occurring mutations. Nonsense mutations, polyalanine tract expansions and missense mutations in ARX cause a range of intellectual disability and epilepsy phenotypes with or without additional features including hand dystonia, lissencephaly, autism or dysarthria. Severe malformation phenotypes, such as X-linked lissencephaly with ambiguous genitalia (XLAG), are frequently observed in individuals with protein truncating or missense mutations clustered in the highly conserved paired-type homeodomain.

Results

We have identified two novel point mutations in the R379 residue of the ARX homeodomain; c.1135C>A, p.R379S in a patient with infantile spasms and intellectual disability and c.1136G>T, p.R379L in a patient with XLAG. We investigated these and other missense mutations (R332P, R332H, R332C, T333N: associated with XLAG and Proud syndrome) predicted to affect the nuclear localisation sequences (NLS) flanking either end of the ARX homeodomain. The NLS regions are required for correct nuclear import facilitated by Importin 13 (IPO13). We demonstrate that missense mutations in either the N- or C-terminal NLS regions of the homeodomain cause significant disruption to nuclear localisation of the ARX protein in vitro. Surprisingly, none of these mutations abolished the binding of ARX to IPO13. This was confirmed by co-immunoprecipitation and immmuno fluorescence studies. Instead, tagged and endogenous IPO13 remained bound to the mutant ARX proteins, even in the RanGTP rich nuclear environment. We also identify the microtubule protein TUBA1A as a novel interacting protein for ARX and show cells expressing mutant ARX protein accumulate in mitosis, indicating normal cell division may be disrupted.

Conclusions

We show that the most likely, common pathogenic mechanism of the missense mutations in NLS regions of the ARX homeodomain is inadequate accumulation and distribution of the ARX transcription factor within the nucleus due to sequestration of ARX with IPO13.

A model of symptomatic infantile spasms syndrome

Infantile spasms are characterized by age-specific expression of epileptic spasms and hypsarrhythmia and often result in significant cognitive impairment. Other epilepsies or autism often ensue especially in symptomatic IS (SIS). Cortical or subcortical damage, including white matter, have been implicated in the pathogenesis of SIS. To generate a model of SIS, we recreated this pathology by injecting rats with lipopolysaccharide and doxorubicin intracerebrally at postnatal day (P) 3 and with p-chlorophenylalanine intraperitoneally at P5. Spasms occurred between P4 and 13 and were associated with ictal EEG correlates, interictal EEG abnormalities and neurodevelopmental decline. After P9 other seizures, deficits in learning and memory, and autistic-like behaviors (indifference to other rats, increased grooming) were observed. Adrenocorticotropic hormone (ACTH) did not affect spasms. Vigabatrin transiently suppressed spasms at P5. This new model of SIS will be useful to study the neurobiology and treatment of SIS, including those that are refractory to ACTH.

Developing an animal model for infantile spasms: pathogenesis, problems and progress

Infantile spasms (IS), the most common of the early epileptic encephalopathies, afflicts thousands of children each year and results in significant disability. Also known as West syndrome, IS is characterized by intractable stereotyped seizures, poor developmental outcome and a characteristic electroencephalogram (EEG) pattern. IS often progresses into another epileptic encephalopathy known as Lennox-Gastaut syndrome, and continues with the patient being burdened by lifelong epilepsy and varying degrees of mental retardation. Little is known about the biological basis of IS. As the etiologies of IS are diverse, the multiple causes must converge into a final common pathway that results in this specific epilepsy phenotype. Finding a model or models to test this final pathway is necessary both to understand why the greatest susceptibility to seizure development occurs during infancy and early childhood, and what underlies the decreased cognitive potential associated with IS. Furthermore, appropriate models would permit better testing of potential therapies directed specifically at IS. This review will describe the clinical features and etiologies of IS; the ideal features that IS models should contain; and the IS models that exist currently. Finally, we will discuss the limitations of these models and the potential avenues for future research on IS.

A triplet repeat expansion genetic mouse model of infantile spasms syndrome, Arx(GCG)10+7, with interneuronopathy, spasms in infancy, persistent seizures, and adult cognitive and behavioral impairment

Infantile spasms syndrome (ISS) is a catastrophic pediatric epilepsy with motor spasms, persistent seizures, mental retardation, and in some cases, autism. One of its monogenic causes is an insertion mutation [c.304ins (GCG)(7)] on the X chromosome, expanding the first polyalanine tract of the interneuron-specific transcription factor Aristaless-related homeobox (ARX) from 16 to 23 alanine codons. Null mutation of the Arx gene impairs GABA and cholinergic interneuronal migration but results in a neonatal lethal phenotype. We developed the first viable genetic mouse model of ISS that spontaneously recapitulates salient phenotypic features of the human triplet repeat expansion mutation. Arx((GCG)10+7) ("Arx plus 7") pups display abnormal spasm-like myoclonus and other key EEG features, including multifocal spikes, electrodecremental episodes, and spontaneous seizures persisting into maturity. The neurobehavioral profile of Arx mutants was remarkable for lowered anxiety, impaired associative learning, and abnormal social interaction. Laminar decreases of Arx+ cortical interneurons and a selective reduction of calbindin-, but not parvalbumin- or calretinin-expressing interneurons in neocortical layers and hippocampus indicate that specific classes of synaptic inhibition are missing from the adult forebrain, providing a basis for the seizures and cognitive disorder. A significant reduction of calbindin-, NPY (neuropeptide Y)-expressing, and cholinergic interneurons in the mutant striatum suggest that dysinhibition within this network may contribute to the dyskinetic motor spasms. This mouse model narrows the range of critical pathogenic elements within brain inhibitory networks essential to recreate this complex neurodevelopmental syndrome.