Periventricular heterotopia: an X-linked dominant epilepsy locus causing aberrant cerebral cortical development

Reduction of spontaneous inhibitory synaptic activity in experimental heterotopic gray matter

Neuronal heterotopia has a strong association with epilepsy, but the mechanisms that underlie this relationship are largely unknown. We have utilized the in utero irradiated rat model to study circuit abnormalities in experimentally induced subcortical heterotopic gray matter. Spontaneous and miniature inhibitory (IPSCs) and excitatory (EPSCs) postsynaptic currents were recorded from visualized heterotopic pyramidal neurons in in vitro hemispheric slices and compared with control neocortical pyramidal neurons using the whole cell patch-clamp technique. The frequency of spontaneous and miniature IPSCs was significantly reduced in pyramidal neurons from heterotopic cortex. Amplitude and kinetics of IPSCs were not different between the two groups. Spontaneous and miniature EPSCs were not different between the two groups. Short-term synaptic plasticity of stimulus-evoked EPSCs showed depression in heterotopic neurons and facilitation in control pyramidal neurons. This study shows a selective impairment of the GABAergic circuitry in experimental heterotopic gray matter. We have reported similar findings in normotopic dysplastic cortex from this model. Taken together, these studies demonstrate a pervasive defect in inhibition throughout the cortex of irradiated rats with cortical dysplasia and neuronal heterotopia. This may have important implications regarding cortical development and function following in utero injuries.

Molecular genetic and morphologic integration in malformations of the nervous system for etiologic classification

Molecular genetics has brought new insight into the etiology and pathogenesis of nervous system malformations, and provided a means of precise genetic diagnosis including the prenatal detection of many cerebral dysgeneses. Many cerebral malformations previously thought to be a single disorder are now known to be common end results of many independent genetic mutations. Examples are holoprosencephaly and lissencephaly. Gradients of genetic expression along the axes of the neural tube established at the time of gastrulation may explain many varieties and clinical expressions of cerebral malformations, including the involvement of non-neural tissues, such as midfacial hypoplasia from defective neural crest migration. A new classification of CNS malformations is proposed that integrates, but does not discard traditional morphologic criteria, but integrates them with new molecular genetic criteria.

The Drosophila slamdance gene: a mutation in an aminopeptidase can cause seizure, paralysis and neuronal failure

We report here the characterization of slamdance (sda), a Drosophila melanogaster "bang-sensitive" (BS) paralytic mutant. This mutant exhibits hyperactive behavior and paralysis following a mechanical "bang" or electrical shock. Electrophysiological analyses have shown that this mutant is much more prone to seizure episodes than normal flies because it has a drastically lowered seizure threshold. Through genetic mapping, molecular cloning, and RNA interference, we have demonstrated that the sda phenotype can be attributed to a mutation in the Drosophila homolog of the human aminopeptidase N (APN) gene. Furthermore, using mRNA in situ hybridization and LacZ staining, we have found that the sda gene is expressed specifically in the central nervous system at particular developmental stages. Together, these results suggest that the bang sensitivity in sda mutants is caused by a defective APN gene that somehow increases seizure susceptibility. Finally, by using the sda mutation as a sensitized background, we have been able to identify a rich variety of sda enhancers and other independent BS mutations.

Central nervous system embryogenesis and its failures

The well-orchestrated development of the central nervous system (CNS) requires highly integrated regulatory processes to ensure its precise spatial organization that provides the foundation for proper function. As emphasized in this review, the type, timing, and location of regulatory molecules influence the different stages of development from neuronal induction, regional specification, neuronal specification, and neuronal migration to axonal growth and guidance, neuronal survival, and synapse formation. The known molecular mechanisms are summarized from studies of invertebrates and lower vertebrates, in which we have learned more about the different ligands, receptors, transcription factors, and the intracellular signaling pathways that play specific roles in the different stages of development. Despite known molecular mechanisms of some disturbances, most of the clinical entities that arise from failures of CNS embryogenesis remain unexplained. As more novel genes and their functions are discovered, existing mechanisms will be refined and tenable explanations will be made. With these limitations, two specific clinical entities that have been relatively well studied, holoprosencephaly and neuronal migration defects, are discussed in more detail to illustrate the complexity of regulatory mechanisms that govern well-defined stages of CNS development.

Hereditary subependymal heterotopia associated with mega cisterna magna: antenatal diagnosis with magnetic resonance imaging

Bilateral nodular subependymal heterotopia has recently been identified as a hereditary disease linked to the X-chromosome. The sonographic findings are very subtle and difficult to observe during the second trimester when the germinal matrix is at its largest. Fetal magnetic resonance imaging facilitates visualization of the periventricular area. We report a case of bilateral nodular heterotopia associated with mega cisterna magna diagnosed by ultrasound and magnetic resonance imaging at 29 weeks' gestation. Magnetic resonance imaging of the brain of the mother revealed similar findings to those observed in the fetus and neonate. This case confirms the association between mega cisterna magna and bilateral periventricular nodular heterotopia and demonstrates that neuroimaging studies of the mother can contribute to the fetal diagnosis.

Filamin A-interacting protein (FILIP) regulates cortical cell migration out of the ventricular zone

Precisely regulated radial migration out of the ventricular zone is essential for corticogenesis. Here, we identify a mechanism that can tether ventricular zone cells in situ. FILIP interacts with Filamin A, an indispensable actin-binding protein that is required for cell motility, and induces its degradation in COS-7 cells. Degradation of Filamin A is identified in the cortical ventricular zone, where filip mRNA is localized. Furthermore, most ventricular zone cells that overexpress FILIP fail to migrate in explants. These results demonstrate that FILIP functions through a Filamin A F-actin axis to control the start of neocortical cell migration from the ventricular zone.

Periventricular nodular heterotopia: report of a pediatric series

Periventricular nodular heterotopia is a malformation that occurs in both males and females and is associated with a variety of clinical and neuroradiologic signs. A gene called filamin-1 (FLN-1) has recently been identified. We review the clinical and imaging findings from a series of pediatric patients with periventricular nodular heterotopia. Five patients (three males and two females; age range = 4-18 years) were investigated. In our series, periventricular nodular heterotopia can be the common denominator in different conditions. Periventricular nodular heterotopia can occur alone or be associated with cortical malformations. Epilepsy was present in three of the five patients and was resistant to drugs in one female. Mental retardation was present in three of the five patients. Two male patients had normal intelligence, with no cortical anomalies; patient 3 had unilateral periventricular nodular heterotopia. The associated malformations were more severe in the female patients and slight only in patient 1. The two females showed anomalies rarely reported in association with bilateral periventricular nodular heterotopia. We believe that other genes can be involved in children with atypical neuroradiologic periventricular nodular heterotopia. No mutations were detected in 6 of the 48 exons of the FLN-1 gene, although this does not allow any definitive conclusions to be reached. We conclude that our series of patients with periventricular nodular heterotopia clearly highlights the complexity of the clinical, neurologic, and neuroradiologic characteristics associated with this malformation.

Normotopic and heterotopic cortical representations of mystacial vibrissae in rats with subcortical band heterotopia

The tish rat is a neurological mutant exhibiting bilateral cortical heterotopia similar to those found in certain epileptic patients. Previous work has shown that thalamocortical fibers originating in the ventroposteromedial nucleus, which in normal animals segregate as 'barrel' representations for individual whiskers, terminate in both normotopic and heterotopic areas of the tish cortex (Schottler et al., 1998). Thalamocortical innervation terminates as barrels in layer IV and diffusely in layer VI of the normotopic area. Discrete patches of terminals are also observed in the underlying heterotopic area suggesting that representations of individual vibrissa may be present in the heterotopic somatosensory areas. The present study examines this issue by investigating the organization of the vibrissal somatosensory system in the tish cortex. Staining for cytochrome oxidase or Nissl substance reveals a normal complement of vibrissal barrels in the normotopic area of the tish cortex. Dense patches of cytochrome oxidase staining are also found in the underlying lateral portions of the heterotopic area (i.e. the same area that is innervated by the ventroposteromedial nucleus). Injections of retrograde tracers into vibrissal areas of either the normotopic or heterotopic area produce topographically organized labeling of neurons restricted to one or a small number of barreloids within the ventroposteromedial nucleus of the thalamus. Physical stimulation of a single whisker (D3 or E3) elicits enhanced uptake of [(14)C]2-deoxyglucose in restricted zones of both the normotopic and heterotopic areas, demonstrating that single whisker stimulation can increase functional activity in both normotopic and heterotopic neurons. These findings indicate that the barrels are intact in the normotopic area and are most consistent with the hypothesis that at least some of the individual vibrissae are 'dually' represented in normotopic and heterotopic positions in the primary somatosensory areas of the tish cortex.

Neuronal migration disorders

Neuronal migration disorders are a category of developmental brain disorders leading to cortical dysplasia. This group of disorders is characterized by defective movement of neurons from the place of origin along the lining of the lateral ventricle, to the eventual place of residence in the correct laminar position within the cerebral cortex. As a result of defective migration, affected individuals typically display mental retardation and epilepsy. Although patients with the more severe forms of these disorders often present during infancy, patients may present at any age from newborn to adulthood. The migration defect may be generalized or focal, and may be disturbed at any of several stages, leading to several distinct radiographical and clinical presentations. The human phenotypes suggests that there are at least four distinct and clinically-important steps in cortical neuronal migration, and the identification of the responsible genes suggests that multiple cellular processes are critical for correct neuronal positioning.

Genetics of epilepsy

Understanding the molecular biology of epilepsy is a challenge for modern science. Epilepsy results from alternations in fundamental mechanisms of brain and membrane function. Although an understanding of the mode of inheritance and the etiology of genetic epilepsy syndromes forms the basis for genetic counseling, the development of specific therapies will come from knowing the basic mechanisms of epilepsy. Defining the genes causing epilepsy requires an unambiguous definition of seizure phenotype, along with the stability of that trait, an unremitting clinical course, and an abundance of clinical material. This article reviews the task of defining the genetics of epilepsy and discusses genetic methodology, idiopathic generalized and localization-related partial epilepsies, neuronal migration disorders, progressive myoclonus epilepsies, molecular biology of epileptogenesis, and future research.

Pathologic characteristics of the cortical dysplasias

The gross and microscopic features of cortical dysplasia (malformations caused by abnormalities of cortical development) are reviewed and illustrated in this article. The pathologic associations of neurocutaneous disorders, neoplasms, and hippocampal sclerosis with cortical dysplasia also are discussed.

Disorders of cortical formation: radiologic-pathologic correlation

The advent of newer imaging techniques, such as high-resolution MR imaging and surface reconstructions of three-dimensional data sets, has led to a greater in vivo understanding of cortical malformations of the brain. Disorders of cortical formation are illustrated with routine imaging, surface reconstruction, and pathogenic specimens.

Neuronal migration, cerebral cortical development, and cerebral cortical anomalies

Cerebral cortical malformations are relatively common anomalies identified by neuroimaging and pathologically in patients with epilepsy and mental retardation. A disruption in neuronal migration during central nervous system development has been postulated as the pathogenesis for many of these disorders. Recently, the cell migration hypothesis has been proven accurate for lissencephaly, subcortical band heterotopia, and periventricular nodular heterotopia. Furthermore, advances in cellular and molecular biology have begun elucidating the fundamental mechanisms underlying these migration disorders. These data have resulted in redefining and recategorizing specific malformations based on their molecular genetic abnormality. In this review we shall discuss the current understanding of neuronal migration in the developing cerebral cortex, the evaluation of these patients, and attempt to describe the pathogenesis for several well-characterized human disorders of cell migration.

Magnetic resonance imaging: role in the understanding of cerebral malformations

Magnetic resonance imaging (MRI) has had an enormous impact on the practice of medicine, and especially, on the clinical neurosciences. One area in which MRI has had a particularly large impact has been on the analysis and understanding of cerebral malformations. This manuscript describes the manner in which MRI in conjunction with modern molecular biology has helped to shed new light on our understanding and classification of cerebral malformations.

X-linked malformations of cortical development

Disorders of the development of the human cortex are recognized as significant causes of mental retardation, epilepsy, and congenital neurologic deficits. These malformations may be restricted to the brain or may be one component of a generalized malformation syndrome. Through the efforts of several groups, a large number of human cortical malformations have been identified and classified. Studies of informative families and sporadic patients with specific chromosomal rearrangements or deletions have demonstrated a genetic basis for many of these disorders. Subsequent work has facilitated a precise genetic diagnosis and provided insight into the molecular basis of some of these malformations. This review will discuss four cortical malformation syndromes, which are known or likely to have an X-linked inheritance pattern: bilateral periventricular nodular heterotopia, X-linked lissencephaly/subcortical band heterotopia, X-linked lissencephaly with abnormal genitalia, and X-linked bilateral perisylvian polymicrogyria.

Human brain malformations and their lessons for neuronal migration

The developmental steps required to build a brain have been recognized as a distinctive sequence since the turn of the twentieth century. As marking tools for experimental embryology emerged, the cellular events of cortical histogenesis have been intensively scrutinized. On this rich backdrop, molecular genetics provides the opportunity to play out the molecular programs that orchestrate these cellular events. Genetic studies of human brain malformation have proven a surprising source for finding the molecules that regulate CNS neuronal migration. These studies also serve to relate the significance of genes first identified in murine species to the more complex human brain. The known genetic repertoire that is special to neuronal migration in brain has rapidly expanded over the past five years, making this an appropriate time to take stock of the emerging picture. We do this from the perspective of human brain malformation syndromes, noting both what is now known of their genetic bases and what remains to be discovered.

Periventricular heterotopia may result from radial glial fiber disruption

Periventricular heterotopia (PVH) are collections of neurons and glia heterotopically located adjacent to the ventricles. The pathogenesis of periventricular heterotopia is believed to be a failure of cells to migrate from the ventricular zone. Mutations in filamin-1 (FLN1) have recently been identified as a genetic defect that results in an X-linked dominant form of PVH. In addition to this X-linked form, PVH may be found sporadically or occasionally as part of other syndromes. The pathogenesis(es) of PVH has not been entirely elucidated for patients with or without FLN1 mutation. In an attempt to better understand the pathogenesis of PVH, we examined 5 fetuses (gestational ages 21 to 34 wk), 3 females and 2 males, with PVH. Neuropathologic examination of these 5 fetuses revealed several to multiple periventricular nodules. No case showed the extensive periventricular heterotopia most commonly found in females with FLN1 mutations. By immunohistochemistry, neurofilament-positive cells were identified within the PVH in 3 of 5 cases and glial fibrillary acidic protein-positive cells surrounded the nodules in all 5 cases, but positive cells were only found within the nodules of 3 cases. Surprisingly, small collections of CD68-positive macrophages were found at the base of the nodules in 4 of the 5 cases. Moreover, in all cases, the radial glia highlighted with vimentin, showed disorganization specifically around the nodules. These data suggest that at least one pathogenesis for PVH is a disruption of the radial glial organization, resulting in a failure of cells to migrate from the ventricular zone.

Structural and functional aspects of filamins

Filamins are a family of high molecular mass cytoskeletal proteins that organize filamentous actin in networks and stress fibers. Over the past few years it has become clear that filamins anchor various transmembrane proteins to the actin cytoskeleton and provide a scaffold for a wide range of cytoplasmic signaling proteins. The recent cloning of three human filamins and studies on filamin orthologues from chicken and Drosophila revealed unexpected complexity of the filamin family, the biological implications of which have just started to be addressed. Expression of dysfunctional filamin-A leads to the genetic disorder of ventricular heterotopia and gives reason to expect that abnormalities in the other isogenes may also be connected with human disease. In this review aspects of filamin structure, its splice variants, binding partners and biological function will be discussed.

Malformations of cortical development and epilepsy

Although once thought to be rare, malformations of cortical development are being increasingly recognized as the underlying cause of developmental delay in children and of epilepsy in children and young adults. Advances in neuroimaging and developmental neurobiology have created the tools by which these important malformations have been investigated. Through a symbiotic type of relationship, these investigations, and the search for a better understanding of these malformations, have led to advances in neuroimaging techniques and better understanding of both normal and abnormal brain development. In this review, the most common malformations or cortical development associated with epilepsy are discussed in regard to their clinical manifestations, classification, imaging appearance and basic neurobiology.

Cortical malformations and epilepsy

Brain malformations, resulting from aberrant patterns of brain development, are highly correlated with childhood seizure syndromes, as well as with cognitive disabilities and other neurological disorders. The structural malformations, often referred to as cortical dysplasia, are extremely varied, reflecting diverse underlying processes and critical timing of the developmental aberration. Recent studies have revealed a genetic basis for many forms of dysplasia. Gene mutations responsible for such common forms of dysplasia as lissencephaly and tuberous sclerosis have been identified, and investigators are beginning to understand how these gene mutations interrupt and/or misdirect the normal developmental pattern. Laboratory investigations, using animal models of cortical dysplasia, are beginning to elucidate how these structural malformations give rise to epilepsy and other functional pathologies.

Cell migration and cerebral cortical development

This annotation describes the clinical and pathological features of several conditions believed to result from a primary defect in cell migration which include the lissencephalies, pachygria, polymicrogyrias, and focal cortical dysplasia. A variety of factors must be considered in pathogeneses, including cellular proliferation, cell death, post-migrational intracortical growth and development, axonogenesis and dendritogenesis. At least two distinct types of lissencephaly exist. Classic (also known as Type I) lissencephaly is the prototypic pattern being seen in autosomal dominant Miller-Dieker syndrome, in addition to autosomal recessive and X-linked forms. The Miller-Dieker syndrome locus (LIS-1) encodes the platelet activating factor acetylhydrolase-1, beta1 subunit. The gene for an X-linked form of lissencephaly (XLIS) encodes a protein called doublecortin. Cobblestone (type II) lissencephaly is most commonly seen in patients with the Walker-Warburg syndrome, and also occurs in a group of disorders associated with congenital muscular dystrophy, including Finnish 'muscle-eye-brain' disease and Fukuyama muscular dystrophy. Controversy exits as to whether polymicrogyria is a malformation or a disruption of development. The answer is likely both. Polymicrogyria is believed to arise from defects occurring between 17 and 25 or 26 weeks gestation. Heterotopia can be sporadic, inherited as a simple Mendelian trait, or may be part of a more complex syndrome being characterized by collections of disorganized grey matter in inappropriate places. X-linked periventricular heterotopia syndrome is caused by mutations in filamin-1. In addition to those described above, many other syndromes show lissencephaly, pachygyria and polymicrogyria and many cases are not easily classified into any particular syndrome.

Causes of epilepsies: insights from discordant monozygous twins

Whereas some patients with epilepsy have known acquired or genetic causes, in many the cause is unknown. By analyzing monozygotic twins, discordant for epilepsy, subtle etiological factors may be detected. We analyzed 12 monozygotic, discordant twins for factors explaining discordancy. These factors were presence of major clinical risk factors, presence of possibly epileptogenic lesions on brain magnetic resonance imaging (MRI), and quantitative brain volume abnormalities. Major risk factors, with associated acquired lesions were found in 4 of 12 twins. An MRI lesion without a major risk factor was found in a further 4 of 12 twins. Two of these had unilateral malformations of cortical development, 1 had bilateral periventricular heterotopia, and 1 had focal atrophy. Significant twin-twin differences in MRI volumes without obvious MRI lesions or major risk factors were found in 2 of 12 twins. Both had larger volumes than their co-twins, and idiopathic generalized epilepsy. No clinical or MRI findings accounting for discordance for epilepsy were found in 2 of 12 twins. In 10 of 12 pairs a clinical or MR correlate of epilepsy was found; some of those were subtle and only apparent by twin-twin comparison. They may be due to occult acquired factors, such as prenatal insults, or to genetic abnormalities resulting from postfertilization genetic processes.

Genetics of brain development and malformation syndromes

The identification of the specific genes responsible for several childhood neurologic disorders has provided a framework with which to understand key development stages in human brain development. Common genetic disorders of brain development include septo-optic dysplasia, schizencephaly, holoprosencephaly, periventricular heterotopia, lissencephaly, and Joubert syndrome. For each of these disorders, a critical step in brain development is interrupted. The identification of the responsible genes is providing scientists a window into the key modulators of brain development, and providing clinicians the opportunity to offer genetic testing to individual patients and their families.

Neocortical malformation as consequence of nonadaptive regulation of neuronogenetic sequence

Variations in the structure of the neocortex induced by single gene mutations may be extreme or subtle. They differ from variations in neocortical structure encountered across and within species in that these "normal" structural variations are adaptive (both structurally and behaviorally), whereas those associated with disorders of development are not. Here we propose that they also differ in principle in that they represent disruptions of molecular mechanisms that are not normally regulatory to variations in the histogenetic sequence. We propose an algorithm for the operation of the neuronogenetic sequence in relation to the overall neocortical histogenetic sequence and highlight the restriction point of the G1 phase of the cell cycle as the master regulatory control point for normal coordinate structural variation across species and importantly within species. From considerations based on the anatomic evidence from neocortical malformation in humans, we illustrate in principle how this overall sequence appears to be disrupted by molecular biological linkages operating principally outside the control mechanisms responsible for the normal structural variation of the neocortex. MRDD Research Reviews 6:22-33, 2000.

Genetics of neuronal migration in the cerebral cortex

The development of the cerebral cortex requires large-scale movement of neurons from areas of proliferation to areas of differentiation and adult function in the cortex proper, and the patterns of this neuronal migration are surprisingly complex. The migration of neurons is affected by several naturally occurring genetic defects in humans and mice; identification of the genes responsible for some of these conditions has recently yielded new insights into the mechanisms that regulate migration. Other key genes have been identified via the creation of induced mutations that can also cause dramatic disorders of neuronal migration. However, our understanding of the physiological and biochemical links between these genes is still relatively spotty. A number of molecules have also been studied in mice (Reelin, mDab1, and the VLDL and ApoE2 receptors) that appear to represent part of a coherent signaling pathway that regulates migration, because multiple genes cause an indistinguishable phenotype when mutated. On the other hand, two human genes that cause lissencephaly (LIS1, DCX) encode proteins that have recently been implicated as regulators or microtubule dynamics. This article reviews some of the mutant phenotypes in light of the mechanisms of neuronal migration. MRDD Research Reviews 6:34-40, 2000.

Neuronal migration disorders: from genetic diseases to developmental mechanisms

Neurons that constitute the cerebral cortex must migrate hundreds of cell-body distances from their place of birth, and through several anatomical boundaries, to reach their final position within the correct cortical layer. Human neurological conditions associated with abnormal neuronal migration, together with spontaneous and engineered mouse mutants, define at least four distinct steps in cortical neuronal migration. Many of the genes that control neuronal migration have strong genetic or biochemical links to the cytoskeleton, suggesting that the field of neuronal migration might be closing in on the underlying cytoskeletal events.

Neuronal migration defects of the cerebral cortex: a destination debacle

Disruptions in neuronal migration have been postulated as the basis for many cerebral malformations including lissencephaly, cortical heterotopia, and double cortex. Recently, the genetic basis for some of these disorders has been identified. In this review, we highlight recent advances in our understanding of the molecular mechanisms of neuronal migration and its relationship to cerebral cortical development and neuronal migration disorders. This has allowed us to begin categorizing specific malformations based on their molecular etiology.

Distribution and initiation of seizure activity in a rat brain with subcortical band heterotopia

PURPOSE

Misplaced (heterotopic) cortical neurons are a common feature of developmental epilepsies. To better understand seizure disorders associated with cortical heterotopia, the sites of aberrant discharge activity were investigated in vivo and in vitro in a seizure-prone mutant rat (tish) exhibiting subcortical band heterotopia.

METHODS

Depth electrode recordings and postmortem assessment of regional c-fos mRNA levels were used to characterize the distribution of aberrant discharge activity during spontaneous seizures in vivo. Electrophysiologic recordings of spontaneous and evoked activity also were performed by using in vitro brain slices from the tish rat treated with proconvulsant drugs (penicillin and 4-aminopyridine).

RESULTS

Depth electrode recordings demonstrate that seizure activity begins almost simultaneously in the normotopic and heterotopic areas of the tish neocortex. Spontaneous seizures induce c-fos mRNA in normotopic and heterotopic neocortical areas, and limbic regions. The threshold concentrations of proconvulsant drugs for inducing epileptiform spiking were similar in the normotopic and heterotopic areas of tish brain slices. Manipulations that blocked communication between the normotopic and heterotopic areas of the cortex inhibited spiking in the heterotopic, but not the normotopic, area of the cortex.

CONCLUSIONS

These findings indicate that aberrant discharge activity occurs in normotopic and heterotopic areas of the neocortex, and in certain limbic regions during spontaneous seizures in the tish rat. Normotopic neurons are more prone to exhibit epileptiform activity than are heterotopic neurons in the tish cortex, and heterotopic neurons are recruited into spiking by activity initiated in normotopic neurons. The findings indicate that seizures in the tish brain primarily involve telencephalic structures, and suggest that normotopic neurons are responsible for initiating seizures in the dysplastic neocortex.

Clinical spectrum of cortical dysplasia in childhood: diagnosis and treatment issues

Disorders of cortical development form a spectrum of lesions produced by insults to the developing neocortex. These conditions typically first manifest in childhood with epilepsy, developmental delay, and focal neurologic signs. Although the clinical and electrophysiologic findings are often nonspecific, high-resolution magnetic resonance imaging facilitates diagnosis during life, and assists in delineating specific clinical syndromes. While many patients are dysmorphic and severely affected by mental retardation and epilepsy, some have normal or near-normal cognitive function and no seizures. Molecular studies of dysplastic cortex are providing new insights into the basic mechanisms of brain function and development, while pathologic analysis of tissue removed at surgery is helping to define epileptic circuitry. Treatment of the epilepsy associated with cortical dysplasia is often frustrating, but surgical approaches based on accurately defining epileptogenic regions are proving increasingly successful. Genetic diagnosis is important for accurate counseling of families.

Current concepts of cerebral malformation syndromes

In the past, children with many brain malformations were classified as having static encephalopathies (cerebral palsy), often attributed to perinatal or prenatal distress. Understanding of the frequency and clinical manifestations of brain malformations, however, has increased dramatically in the past 10 to 15 years. During this time, it has become apparent that many static encephalopathies in children have a brain malformation as their substrate. Most of the increase in our knowledge can be attributed to advances in neuroimaging and in molecular biology. In general, radiologic analysis of the brain allows similar malformations to be classified together. Subsequent genetic analysis of the affected children often reveals the affected gene, leading to identification of the gene product and, ideally, an ultimate understanding of the molecular mechanism of malformation. Currently, many genes involved in the complicated process of neuronal proliferation, migration, and organization are being identified. Knowledge of these genes and a better radiologic classification system enable the referring physician to give better care, more sophisticated genetic counseling, and a more precise prognosis for the child. To illustrate this mechanism of classification, three groups of malformations are discussed, in which a combination of neuroimaging analysis and molecular biologic analysis have led to a new understanding of the malformation syndromes.

Recent advances in the genetics of epilepsy: insights from human and animal studies

Progress in understanding the genetics of epilepsy is proceeding at a dizzying pace. Due in large part to rapid progress in molecular genetics, gene defects underlying many of the inherited epilepsies have been mapped, and several more are likely to be added each year. In this review, we summarize the available information on the genetic basis of human epilepsies and epilepsy syndromes, and correlate these advances with rapidly expanding information about the mechanisms of epilepsy gained from both spontaneous and transgenic animal models. We also provide practical suggestions for clinicians confronted with families in which multiple members are afflicted with epilepsy.

Clinical and imaging features of cortical malformations in childhood

OBJECTIVE

To determine the types, relative frequencies, clinical features, and MRI characteristics of malformations of cortical development (MCD) occurring in a cohort of children referred to a tertiary pediatric center.

METHODS

Original MR images were reviewed by two investigators, who were blinded to clinical details, to determine the elemental imaging features of each malformation and to label these malformations according to an existing system of classification. Clinical information was collected by a review of hospital records.

RESULTS

A total of 109 children with MCD were identified. There were 58 boys and 51 girls, age 8 days to 18 years at initial imaging (mean age, 5 years). Seizures were present in 75%, developmental delay or intellectual disability in 68%, abnormal neurologic findings in 48%, and congenital anomalies apart from the CNS malformation in 18%. The main malformations identified were heterotopic gray matter (19%), cortical tubers (17%), focal cortical dysplasia (16%), polymicrogyria (16%), agyria/pachygyria (15%), schizencephaly/cleft (5%), transmantle dysplasia (5%), and hemimegalencephaly (4%). Eight patients had features of more than one malformation. Most lesions were multilobar (47%), with the frontal lobe being the most common lobe involved (78%). A total of 68% of patients had other cerebral malformations including ventricular dilatation or dysmorphism (46%) and abnormalities of the corpus callosum (29%).

CONCLUSIONS

This study illustrates the spectrum of MCD in a pediatric cohort and highlights some of the differences between pediatric and adult patients. Patients with MCD presenting in childhood have a wider spectrum of malformations and more varied, often more severe, clinical manifestations. The lesions are frequently multifocal or generalized and many are associated with noncortical developmental brain anomalies.

Neuronal migration disorders in humans and in mouse models--an overview

The spectrum of neuronal migration disorders (NMD) in humans encompasses developmental brain defects with a range of clinical and pathological features. A simple classification distinguishes agyria/pachygyria, heterotopia, polymicrogyria and cortical dysplasia as distinct clinico-pathological entities. Many of these conditions are associated with intractable epilepsy. When considering the pathogenesis of NMD, a critical developmental process is the migration of neuroblasts along the processes of radial glia during the formation of the layered structure of the cerebral cortex. In addition, faulty cytodifferentiation and programmed cell death play important roles in the generation of dysplasias and heterotopias respectively. A number of genes have been identified that participate in the regulation of neuronal migration. Mouse models, in which these genes are mutated, provide insight into the developmental pathways that underlie normal and abnormal neuronal migration.

Thyroid hormone regulates reelin and dab1 expression during brain development

The reelin and dab1 genes are necessary for appropriate neuronal migration and lamination during brain development. Since these processes are controlled by thyroid hormone, we studied the effect of thyroid hormone deprivation and administration on the expression of reelin and dab1. As shown by Northern analysis, in situ hybridization, and immunohistochemistry studies, hypothyroid rats expressed decreased levels of reelin RNA and protein during the perinatal period [embryonic day 18 (E18) and postnatal day 0 (P0)]. The effect was evident in Cajal-Retzius cells of cortex layer I, as well as in layers V/VI, hippocampus, and granular neurons of the cerebellum. At later ages, however, Reelin was more abundant in the cortex, hippocampus, cerebellum, and olfactory bulb of hypothyroid rats (P5), and no differences were detected at P15. Conversely, Dab1 levels were higher at P0, and lower at P5 in hypothyroid animals. In line with these results, reelin RNA and protein levels were higher in cultured hippocampal slices from P0 control rats compared to those from hypothyroid animals. Significantly, thyroid-dependent regulation of reelin and dab1 was confirmed in vivo and in vitro by hormone treatment of hypothyroid rats and organotypic cultures, respectively. In both cases, thyroid hormone led to an increase in reelin expression. Our data suggest that the effects of thyroid hormone on neuronal migration may be in part mediated through the control of reelin and dab1 expression during brain ontogenesis.

Cortical malformations and epilepsy: new insights from animal models

In the last decade, the recognition of the high frequency of cortical malformations among patients with epilepsy especially children, has led to a renewed interest in the study of the pathophysiology of cortical development. This field has also been spurred by the recent development of several experimental genetic and non-genetic, primarily rodent, models of cortical malformations. Epileptiform activity in these animals can appear as spontaneous seizure activity in vivo, in vitro hyperexcitability, or reduced seizure susceptibility in vitro and in vivo. In the neonatal freeze lesion model, that mimics human microgyria, hyperexcitability is caused by a reorganization of the network in the borders of the malformation. In the prenatal methylazoxymethanol model, that causes a diffuse cortical malformation, hyperexcitability is associated with alteration of firing properties of discrete neuronal subpopulations together with the formation of bridges between normally unconnected structures. In agreement with clinical evidence, these experimental data suggest that cortical malformations can both form epileptogenic foci and alter brain development in a manner that causes a diffuse hyperexcitability of the cortical network.

Central nervous system neuronal migration

Widespread cell migrations are the hallmark of vertebrate brain development. In the early embryo, morphogenetic movements of precursor cells establish the rhombomeres of the hindbrain, the external germinal layer of the cerebellum, and the regional boundaries of the forebrain. In midgestation, after primary neurogenesis in compact ventricular zones has commenced, individual postmitotic cells undergo directed migrations along the glial fiber system. Radial migrations establish the neuronal layers. Three molecules have been shown to function in glial guided migration--astrotactin, glial growth factor, and erbB. In the postnatal period, a wave of secondary neurogenesis produces huge numbers of interneurons destined for the cerebellar cortex, the hippocampal formation, and the olfactory bulb. Molecular analysis of the genes that mark stages of secondary neurogenesis show similar expression patterns of a number of genes. Thus these three regions may have genetic pathways in common. Finally, we consider emerging studies on neurological mutant mice, such as reeler, and human brain malformations. Positional cloning and identification of mutated genes has led to new insights on laminar patterning in brain.

Periventricular nodular heterotopia and childhood absence epilepsy

A young female presented with an epileptic syndrome resembling childhood absence epilepsy, a normal neurologic examination, generalized 3-Hz spike-and-wave discharges, and clinical absences. Her seizures responded to treatment with valproic acid. Other abnormalities in her electroencephalogram prompted neuroimaging studies, which demonstrated periventricular nodular heterotopia. Review of published reports confirmed this presentation to be atypical of this developmental lesion. The authors describe their patient and discuss this unexpected association and the relevant reports briefly.

Characterization of nodular neuronal heterotopia in children

Neuronal heterotopia are seen in various pathologies and are associated with intractable epilepsy. We examined brain tissue from four children with subcortical or periventricular nodular heterotopia of different aetiologies: one with severe epilepsy following focal brain trauma at 17 weeks gestation, one with hemimegalencephaly and intractable epilepsy, one with focal cortical dysplasia and intractable epilepsy, and one dysmorphic term infant with associated hydrocephalus and polymicrogyria. The connectivity of nodules was investigated using histological and carbocyanine dye (DiI) tracing techniques. DiI crystal placement adjacent to heterotopic nodules revealed numerous DiI-labelled fibres within a 2-3 mm radius of the crystals. Although we observed labelled fibres closely surrounding nodules, the majority did not penetrate them. Placement of DiI crystals within nodules also identified a limited number of projections out of the nodules and in one case there was evidence for connectivity between adjacent nodules. The cellular and neurochemical composition of nodules was also examined using immunohistochemistry for calretinin and neuropeptide Y (NPY), which are normally expressed in GABAergic cortical interneurons. Within heterotopic nodules from all cases, numerous calretinin-positive neurons were identified, along with a few cell bodies and many processes positive for NPY. Calretinin-positive neurons within nodules were less morphologically complex than those in the cortex, which may reflect incomplete differentiation into an inhibitory neuronal phenotype. There were also abnormal clusters of calretinin-positive cells in the overlying cortical plate, indicating that the migratory defect which produces heterotopic nodules also affects development of the cortex itself. Thus, heterotopic nodules consisting of multiple neuronal cell types are associated with malformation in the overlying cortical plate, and have limited connectivity with other brain regions. This abnormal development of connectivity may affect neuronal maturation and consequently the balance of excitation and inhibition in neuronal circuits, leading to their epileptogenic potential.

Male monozygotic twins discordant for periventricular nodular heterotopia and epilepsy

PURPOSE

To determine zygosity and study cerebral structure in apparently identical twins with discordant manifestation of focal epilepsy.

METHODS

Male twins in their fifth decade were scanned by using magnetic resonance imaging (MRI) to detect structural abnormalities. Zygosity was determined by using 10 microsatellite markers.

RESULTS

DNA analysis showed that the twins were >99.99% likely to be monozygous; they were discordant for bilateral symmetric periventricular nodular heterotopia (PNH) and epilepsy.

CONCLUSIONS

The discordant occurrence of PNH and epilepsy in monozygotic male twins carries implications with respect to somatic mosaicism, currently held to be responsible for PNH in affected male subjects.

Familial epilepsy with unilateral and bilateral malformations of cortical development

PURPOSE

To describe a family in whom two sisters with epilepsy, mental retardation, and microcephaly had different malformations of cortical development detected by magnetic resonance imaging (MRI).

METHODS

Clinical investigation of the patients and their family. High-resolution MRI, cognitive testing, and repeated EEG recording in both patients.

RESULTS

In one patient, the malformation was bilateral and diffuse but much more pronounced in the parietal and occipital regions, with MRI characteristics indicating pachygyria-polymicrogyria. In the other patient, the abnormality involved the right hemisphere, predominating around the perisylvian region, with MRI more clearly indicative of polymicrogyria. A brother also had severe epilepsy, diffuse EEG abnormalities, mental retardation, and microcephaly, but could not be studied neuroradiologically.

CONCLUSIONS

Lack of MRI studies in the parents and brother does not allow a precise hypothesis on the mode of transmission. However, findings from this family indicate that unilateral malformations of cortical development detected during investigations after seizure onset may be genetically based, suggesting that a single genetic abnormality could be responsible for bilateral or unilateral malformations.

Heterotopic neurogenesis in a rat with cortical heterotopia

Early cellular development was studied in the neocortex of the tish rat. This neurological mutant is seizure-prone and displays cortical heterotopia similar to those observed in certain epileptic patients. The present study demonstrates that a single cortical preplate is formed in a typical superficial position of the developing tish neocortex. In contrast, two cortical plates are formed: one in a normotopic position and a second in a heterotopic position in the intermediate zone. As the normotopic cortical plate is formed, it characteristically separates the subplate cells from the superficial Cajal-Retzius cells. In contrast, the heterotopic cortical plate is not intercalated between the preplate cells because of its deeper position in the developing cortex. Cellular proliferation occurs in two zones of the developing tish cortex. One proliferative zone is located in a typical position in the ventricular/subventricular zone. A second proliferative zone is located in a heterotopic position in the superficial intermediate zone, i.e., between the two cortical plates. This misplaced proliferative zone may contribute cells to both the normotopic and heterotopic cortical plates. Taken together, these findings indicate that misplaced cortical plate cells, but not preplate cells, comprise the heterotopia of the tish cortex. Heterotopic neurogenesis is an early developmental event that is initiated before the migration of most cortical plate cells. It is concluded that misplaced cellular proliferation, in addition to disturbed neuronal migration, can play a key role in the formation of large cortical heterotopia.

Mutations in filamin 1 prevent migration of cerebral cortical neurons in human periventricular heterotopia

Long-range, directed migration is particularly dramatic in the cerebral cortex, where postmitotic neurons generated deep in the brain migrate to form layers with distinct form and function. In the X-linked dominant human disorder periventricular heterotopia (PH), many neurons fail to migrate and persist as nodules lining the ventricular surface. Females with PH present with epilepsy and other signs, including patent ductus arteriosus and coagulopathy, while hemizygous males die embryonically. We have identified the PH gene as filamin 1 (FLN1), which encodes an actin-cross-linking phosphoprotein that transduces ligand-receptor binding into actin reorganization, and which is required for locomotion of many cell types. FLN1 shows previously unrecognized, high-level expression in the developing cortex, is required for neuronal migration to the cortex, and is essential for embryogenesis.

Regional and cellular patterns of reelin mRNA expression in the forebrain of the developing and adult mouse

The reelin gene encodes an extracellular protein that is crucial for neuronal migration in laminated brain regions. To gain insights into the functions of Reelin, we performed high-resolution in situ hybridization analyses to determine the pattern of reelin expression in the developing forebrain of the mouse. We also performed double-labeling studies with several markers, including calcium-binding proteins, GAD65/67, and neuropeptides, to characterize the neuronal subsets that express reelin transcripts. reelin expression was detected at embryonic day 10 and later in the forebrain, with a distribution that is consistent with the prosomeric model of forebrain regionalization. In the diencephalon, expression was restricted to transverse and longitudinal domains that delineated boundaries between neuromeres. During embryogenesis, reelin was detected in the cerebral cortex in Cajal-Retzius cells but not in the GABAergic neurons of layer I. At prenatal stages, reelin was also expressed in the olfactory bulb, and striatum and in restricted nuclei in the ventral telencephalon, hypothalamus, thalamus, and pretectum. At postnatal stages, reelin transcripts gradually disappeared from Cajal-Retzius cells, at the same time as they appeared in subsets of GABAergic neurons distributed throughout neocortical and hippocampal layers. In other telencephalic and diencephalic regions, reelin expression decreased steadily during the postnatal period. In the adult, there was prominent expression in the olfactory bulb and cerebral cortex, where it was restricted to subsets of GABAergic interneurons that co-expressed calbindin, calretinin, neuropeptide Y, and somatostatin. This complex pattern of cellular and regional expression is consistent with Reelin having multiple roles in brain development and adult brain function.