Nonsyndromic mental retardation and cryptogenic epilepsy in women with doublecortin gene mutations

DCX variants in two unrelated Chinese families with subcortical band heterotopia: Two case reports and review of literature

Introduction

Subcortical band heterotopia (SBH) is a rare brain developmental malformation caused by deficient neuronal migration during embryogenesis. Published literature on pediatric SBH cases caused by DCX mutations is limited.

Methods

The detailed clinical and genetic features of two pediatric SBH with DCX mutations were analyzed. The available literature on DCX mutations was reviewed.

Results

Both patients were girls with varying degrees of developmental delay. Patient 1 was short in stature with peculiar facial features. Patient 2 had an early seizure onset and developed drug-resistant epilepsy. Whole-exome sequencing (WES) revealed two de novo heterozygous variants of DCX (NM_178153.3), including a novel missense variant of c.568A > G (p.K190E) in P1 and a reported nonsense variant of c.814C > T (p.R272*) in P2. We reviewed all the available literature regarding DCX mutations. A total of 153 different mutations have been reported, with the majority of 99 (64.7 %) being missense mutations.

Conclusion

Our study expanded the mutational spectrum of DCX, which has important implications for the study of genotype-phenotype correlations. Furthermore, it provided insights to better understand SBH and genetic counseling.

Lissencephaly: Update on diagnostics and clinical management

Lissencephaly represents a spectrum of rare malformations of cortical development including agyria, pachygyria and subcortical band heterotopia. The progress in molecular genetics has led to identification of 31 lissencephaly-associated genes with the overall diagnostic yield over 80%. In this review, we focus on clinical and molecular diagnosis of lissencephaly and summarize the current knowledge on histopathological changes and their correlation with the MRI imaging. Additionally we provide the overview of clinical follow-up recommendations and available data on epilepsy management in patients with lissencephaly.

The Impact of X-Chromosome Inactivation on Phenotypic Expression of X-Linked Neurodevelopmental Disorders

Nearly 20% of genes located on the X chromosome are associated with neurodevelopmental disorders (NDD) due to their expression and role in brain functioning. Given their location, several of these genes are either subject to or can escape X-chromosome inactivation (XCI). The degree to which genes are subject to XCI can influence the NDD phenotype between males and females. We provide a general review of X-linked NDD genes in the context of XCI and detailed discussion of the sex-based differences related to MECP2 and FMR1, two common X-linked causes of NDD that are subject to XCI. Understanding the effects of XCI on phenotypic expression of NDD genes may guide the development of stratification biomarkers in X-linked disorders.

Spared cognitive and behavioral functions prior to epilepsy onset in a rat model of subcortical band heterotopia

Subcortical band heterotopia (SBH), also known as doublecortex syndrome, is a malformation of cortical development resulting from mutations in the doublecortin gene (DCX). It is characterized by a lack of migration of cortical neurons that accumulate in the white matter forming a heterotopic band. Patients with SBH may present mild to moderate intellectual disability as well as epilepsy. The SBH condition can be modeled in rats by in utero knockdown (KD) of Dcx. The affected cells form an SBH reminiscent of that observed in human patients and the animals develop a chronic epileptic condition in adulthood. Here, we investigated if the presence of a SBH is sufficient to induce cognitive impairment in juvenile Dcx-KD rats, before the onset of epilepsy. Using a wide range of behavioral tests, we found that the presence of SBH did not appear to affect motor control or somatosensory processing. In addition, cognitive abilities such as learning, short-term and long-term memory, were normal in pre-epileptic Dcx-KD rats. We suggest that the SBH presence is not sufficient to impair these behavioral functions.

Genetic Basis of Brain Malformations

Malformations of cortical development (MCD) represent a major cause of developmental disabilities, severe epilepsy, and reproductive disadvantage. Genes that have been associated to MCD are mainly involved in cell proliferation and specification, neuronal migration, and late cortical organization. Lissencephaly-pachygyria-severe band heterotopia are diffuse neuronal migration disorders causing severe global neurological impairment. Abnormalities of the LIS1, DCX, ARX, RELN, VLDLR, ACTB, ACTG1, TUBG1, KIF5C, KIF2A, and CDK5 genes have been associated with these malformations. More recent studies have also established a relationship between lissencephaly, with or without associated microcephaly, corpus callosum dysgenesis as well as cerebellar hypoplasia, and at times, a morphological pattern consistent with polymicrogyria with mutations of several genes (TUBA1A, TUBA8, TUBB, TUBB2B, TUBB3, and DYNC1H1), regulating the synthesis and function of microtubule and centrosome key components and hence defined as tubulinopathies. MCD only affecting subsets of neurons, such as mild subcortical band heterotopia and periventricular heterotopia, have been associated with abnormalities of the DCX, FLN1A, and ARFGEF2 genes and cause neurological and cognitive impairment that vary from severe to mild deficits. Polymicrogyria results from abnormal late cortical organization and is inconstantly associated with abnormal neuronal migration. Localized polymicrogyria has been associated with anatomo-specific deficits, including disorders of language and higher cognition. Polymicrogyria is genetically heterogeneous, and only in a small minority of patients, a definite genetic cause has been identified. Megalencephaly with normal cortex or polymicrogyria by MRI imaging, hemimegalencephaly and focal cortical dysplasia can all result from mutations in genes of the PI3K-AKT-mTOR pathway. Postzygotic mutations have been described for most MCD and can be limited to the dysplastic tissue in the less diffuse forms.

Familial pachygyria in both genders related to a DCX mutation

Doublecortin (DCX) and tubulin play critical roles in neuronal migration. DCX mutations usually cause anterior dominant lissencephaly in males and subcortical band heterotopia (SBH) in females. We used whole-exome sequencing to investigate causative gene variants in a large family with late-childhood-onset focal epilepsy and anterior dominant pachygyria without SBH in both genders. Two potential variants were found for the genes encoding DCX and beta tubulin isotype 1 (TUBB1). The novel DCX mutation (p.D90G, NP_000546.2) appeared to be a major causative variant, whereas the novel mutation of TUBB1 (p.R62fsX, NP_110400.1) was found only in patients with more-severe intellectual disability after gender matching. We report an unusual DCX-related disorder exhibiting familial pachygyria without SBH in both genders.

The genetics of the epilepsies

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

Novel DCX mutation-caused lissencephaly in a boy and very mild heterotopia in his mother

We describe a novel mutation in DCX in a family in which a proband boy had classical lissencephaly and his mother had extremely mild subcortical band heterotopia. No factors that would make the mother's symptoms milder, such as somatic mosaicism or skewed X chromosome inactivation, were observed. From this family, we conclude that a DCX mutation causes a pleiotropic phenotype in the female even if X chromosome inactivation pattern is not skewed, and the novel missense mutation in DCX produced relatively mild dysfunction of the doublecortin protein.

Malformations of cortical development: clinical features and genetic causes

Malformations of cortical development are common causes of developmental delay and epilepsy. Some patients have early, severe neurological impairment, but others have epilepsy or unexpected deficits that are detectable only by screening. The rapid evolution of molecular biology, genetics, and imaging has resulted in a substantial increase in knowledge about the development of the cerebral cortex and the number and types of malformations reported. Genetic studies have identified several genes that might disrupt each of the main stages of cell proliferation and specification, neuronal migration, and late cortical organisation. Many of these malformations are caused by de-novo dominant or X-linked mutations occurring in sporadic cases. Genetic testing needs accurate assessment of imaging features, and familial distribution, if any, and can be straightforward in some disorders but requires a complex diagnostic algorithm in others. Because of substantial genotypic and phenotypic heterogeneity for most of these genes, a comprehensive analysis of clinical, imaging, and genetic data is needed to properly define these disorders. Exome sequencing and high-field MRI are rapidly modifying the classification of these disorders.

The diverse genetic landscape of neurodevelopmental disorders

Advances in genetic tools and sequencing technology in the past few years have vastly expanded our understanding of the genetics of neurodevelopmental disorders. Recent high-throughput sequencing analyses of structural brain malformations, cognitive and neuropsychiatric disorders, and localized cortical dysplasias have uncovered a diverse genetic landscape beyond classic Mendelian patterns of inheritance. The underlying genetic causes of neurodevelopmental disorders implicate numerous cell biological pathways critical for normal brain development.

XLMR protein related to neurite extension (Xpn/KIAA2022) regulates cell-cell and cell-matrix adhesion and migration

X-linked mental retardation (XLMR) is a common cause of moderate to severe intellectual disability in males. XLMR protein related to neurite extension (Xpn, also known as KIAA2022) has been implicated as a gene responsible for XLMR in humans. Although Xpn is highly expressed in the developing brain and is involved in neurite outgrowth in PC12 cells and neurons, little is known about the functional role of Xpn. Here, we show that Xpn regulates cell-cell and cell-matrix adhesion and migration in PC12 cells. Xpn knockdown enhanced cell-cell and cell-matrix adhesion mediated by N-cadherin and β1-integrin, respectively. N-Cadherin and β1-integrin expression at the mRNA and protein levels was significantly increased in Xpn knockdown PC12 cells. Furthermore, overexpressed Xpn protein was strongly expressed in the nuclei of PC12 and 293T cells. Finally, depletion of Xpn perturbed cellular migration by enhancing N-cadherin and β1-integrin expression in a PC12 cell wound healing assay. We conclude that Xpn regulates cell-cell and cell-matrix adhesion and cellular migration by regulating the expression of adhesion molecules.

New insights into genotype-phenotype correlations for the doublecortin-related lissencephaly spectrum

X-linked isolated lissencephaly sequence and subcortical band heterotopia are allelic human disorders associated with mutations of doublecortin (DCX), giving both familial and sporadic forms. DCX encodes a microtubule-associated protein involved in neuronal migration during brain development. Structural data show that mutations can fall either in surface residues, likely to impair partner interactions, or in buried residues, likely to impair protein stability. Despite the progress in understanding the molecular basis of these disorders, the prognosis value of the location and impact of individual DCX mutations has largely remained unclear. To clarify this point, we investigated a cohort of 180 patients who were referred with the agyria-pachygyria subcortical band heterotopia spectrum. DCX mutations were identified in 136 individuals. Analysis of the parents' DNA revealed the de novo occurrence of DCX mutations in 76 cases [62 of 70 females screened (88.5%) and 14 of 60 males screened (23%)], whereas in the remaining cases, mutations were inherited from asymptomatic (n = 14) or symptomatic mothers (n = 11). This represents 100% of families screened. Female patients with DCX mutation demonstrated three degrees of clinical-radiological severity: a severe form with a thick band (n = 54), a milder form (n = 24) with either an anterior thin or an intermediate thickness band and asymptomatic carrier females (n = 14) with normal magnetic resonance imaging results. A higher proportion of nonsense and frameshift mutations were identified in patients with de novo mutations. An analysis of predicted effects of missense mutations showed that those destabilizing the structure of the protein were often associated with more severe phenotypes. We identified several severe- and mild-effect mutations affecting surface residues and observed that the substituted amino acid is also critical in determining severity. Recurrent mutations representing 34.5% of all DCX mutations often lead to similar phenotypes, for example, either severe in sporadic subcortical band heterotopia owing to Arg186 mutations or milder in familial cases owing to Arg196 mutations. Taken as a whole, these observations demonstrate that DCX-related disorders are clinically heterogeneous, with severe sporadic and milder familial subcortical band heterotopia, each associated with specific DCX mutations. There is a clear influence of the individual mutated residue and the substituted amino acid in determining phenotype severity.

Diffuse malformations of cortical development

Malformations of cortical development (MCD) represent a major cause of developmental disabilities and severe epilepsy. Advances in imaging and genetics have improved the diagnosis and classification of these conditions. Up to now, eight genes have been involved in different types of MCD. Lissencephaly-pachygyria and subcortical band heterotopia (SBH) represent a malformative spectrum resulting from mutations of either LIS1 or DCX genes. LIS1 mutations cause a more severe malformation in the posterior brain regions. DCX mutations usually cause anteriorly predominant lissencephaly in males and SBH in female patients. Additional forms are X-linked lissencephaly with corpus callosum agenesis and ambiguous genitalia associated with mutations of the ARX gene. Lissencephaly with cerebellar hypoplasia (LCH) encompass heterogeneous disorders named LCH types a to d. LCHa is related to mutation in LIS1 or DCX, LCHb with mutation of the RELN gene, and LCHd could be related to the TUBA1A gene. Polymicrogyria encompasses a wide range of clinical, etiological, and histological findings. Among several syndromes, recessive bilateral fronto-parietal polymicrogyria has been associated with mutations of the GPR56 gene. Bilateral perisylvian polymicrogyria has been associated with mutations in the SRPX2 gene in a few individuals and with linkage to chromosome Xq28 in a some other families. X-linked bilateral periventricular nodular heterotopia (PNH) consists of PNH with focal epilepsy in females and prenatal lethality in males. Filamin A (FLNA) mutations have been reported in some families and in sporadic patients. It is possible to infer the most likely causative gene by brain imaging studies and other clinical findings.

Lissencephaly and band heterotopia: LIS1, TUBA1A, and DCX mutations in Hungary

The spectrum of lissencephaly ranges from absent (agyria) or decreased (pachygyria) convolutions to less severe malformation known as subcortical band heterotopia. Mutations involving LIS1 and TUBA1A result in the classic form of lissencephaly, whereas mutations of the DCX gene cause lissencephaly in males and subcortical band heterotopia in females. This report describes the clinical manifestations and imaging and genetic findings in 2 boys with lissencephaly and a girl with subcortical band heterotopia. An ovel mutation (c.83_84delAT, p.Tyr28Phefs*31) was identified in LIS1 in 1 of the boys with lissencephaly and another novel mutation (c.200delG, p.Ile68Leufs*87) was found in DCX in the girl with subcortical band heterotopia. The mutations appeared in the first half of the genes and are predicted to result in truncated proteins. A mutation was found in the TUBA1A gene (c.1205G>A, p.Arg402His) in the other boy. This mutation affects the folding of tubulin heterodimers, changing the interactions with proteins that bind microtubules.

Malformations of cortical development

Structural abnormalities of the brain are increasingly recognized in patients that suffer from pharmacoresistant focal epilepsies by applying high-resolution imaging techniques. In many of these patients, epilepsy surgery results in control of seizures. Neuropathologically, a broad spectrum of malformations of cortical development (MCD) is observed in respective surgical brain samples. These samples provide a unique basis to further understand underlying pathomechanisms by molecular approaches and develop improved diagnostics and entirely new therapeutic perspectives. Here we provide a comprehensive description of neuropathological findings, available classification systems as well as molecular mechanisms of MCDs. We emphasize the recently published ILEA classification system for focal cortical dysplasias (FCDs), which are now histopathologically distinguished as types I to III. However, this revised classification system represents a major challenge for molecular neuropathologists, as the underlying pathomechanisms in virtually all FCD entities will need to be specified in detail. The fact that only recently, the mammalian target of rapamycin (mTOR)-antagonist Everolimus has been introduced as a treatment of epilepsies in the context of tuberous sclerosis-associated brain lesions is a striking example of a successful translational "bedside to bench and back" approach. Hopefully, the exciting clinico-pathological developments in the field of MCDs will in short term foster further therapeutic breakthroughs for the frequently associated medically refractory epilepsies.

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.

Role of cytoskeletal abnormalities in the neuropathology and pathophysiology of type I lissencephaly

Type I lissencephaly or agyria-pachygyria is a rare developmental disorder which results from a defect of neuronal migration. It is characterized by the absence of gyri and a thickening of the cerebral cortex and can be associated with other brain and visceral anomalies. Since the discovery of the first genetic cause (deletion of chromosome 17p13.3), six additional genes have been found to be responsible for agyria–pachygyria. In this review, we summarize the current knowledge concerning these genetic disorders including clinical, neuropathological and molecular results. Genetic alterations of LIS1, DCX, ARX, TUBA1A, VLDLR, RELN and more recently WDR62 genes cause migrational abnormalities along with more complex and subtle anomalies affecting cell proliferation and differentiation, i.e., neurite outgrowth, axonal pathfinding, axonal transport, connectivity and even myelination. The number and heterogeneity of clinical, neuropathological and radiological defects suggest that type I lissencephaly now includes several forms of cerebral malformations. In vitro experiments and mutant animal studies, along with neuropathological abnormalities in humans are of invaluable interest for the understanding of pathophysiological mechanisms, highlighting the central role of cytoskeletal dynamics required for a proper achievement of cell proliferation, neuronal migration and differentiation.

Allelic diversity in human developmental neurogenetics: insights into biology and disease

One of the biggest challenges in neuroscience is illuminating the architecture of developmental brain disorders, which include structural malformations of the brain and nerves, intellectual disability, epilepsy, and some psychiatric conditions like autism and potentially schizophrenia. Ongoing gene identification reveals a great diversity of genetic causes underlying abnormal brain development, illuminating new biochemical pathways often not suspected based on genetic studies in other organisms. Our greater understanding of genetic disease also shows the complexity of allelic diversity, in which distinct mutations in a given gene can cause a wide range of distinct diseases or other phenotypes. These diverse alleles not only provide a platform for discovery of critical protein-protein interactions in a genetic fashion, but also illuminate the likely genetic architecture of as yet poorly characterized neurological disorders.

Neuropsychological profile of children with subcortical band heterotopia

AIM

Subcortical band heterotopia (SBH) or 'double cortex' is a malformation of cortical development resulting from impaired neuronal migration. So far, research has focused on the neurological, neuroimaging, and genetic correlates of SBH. More recently, clinical reports and small sample studies have documented neuropsychological dysfunction in patients with this malformation. This study aimed to characterize further the phenotype of patients with SBH by describing the neuropsychological profiles of children.

METHOD

Seven children (six females) aged 4 to 15 years were assessed for cognitive functioning (intellectual ability, processing speed, attention, working memory) and academic achievement (reading, spelling, arithmetic). Parents completed questionnaires examining their child's social skills and problem behaviours. Magnetic resonance images (MRI) conducted for routine clinical follow-up were coded by a paediatric neurologist. Genetic and seizure history were obtained from medical records.

RESULTS

There was variation in the neurological, neuroimaging, and genetic presentation of children in the sample. Impairments were observed in all areas of neuropsychological functioning examined. Intellectual ability was generally within the 'extremely low' range (full-scale IQ 44-74; performance IQ 45-72; verbal IQ 57-80). Generalized impairments in cognitive skills were typical, with severe impairments (scores greater than 2SD below the test mean) reported in processing speed, working memory, and arithmetic. Impairments in academic, social, and behavioural functioning were less generalized. No clear relationship between neuroimaging and neuropsychological impairments was found.

INTERPRETATION

Children with SBH demonstrate cognitive, academic, social, and behavioural problems, with the greatest difficulties in processing speed and complex cognitive skills.

Genetic basis in epilepsies caused by malformations of cortical development and in those with structurally normal brain

Epilepsy is the most common neurological disorder affecting young people. The etiologies are multiple and most cases are sporadic. However, some rare families with Mendelian inheritance have provided evidence of genes' important role in epilepsy. Two important but apparently different groups of disorders have been extensively studied: epilepsies associated with malformations of cortical development (MCDs) and epilepsies associated with a structurally normal brain (or with minimal abnormalities only). This review is focused on clinical and molecular aspects of focal cortical dysplasia, polymicrogyria, periventricular nodular heterotopia, subcortical band heterotopia, lissencephaly and schizencephaly as examples of MCDs. Juvenile myoclonic epilepsy, childhood absence epilepsy, some familial forms of focal epilepsy and epilepsies associated with febrile seizures are discussed as examples of epileptic conditions in (apparently) structurally normal brains.

Neuronal migration disorders

Lissencephaly-pachygyria-severe band heterotopia are diffuse neuronal migration disorders (NMDs) causing severe, global neurological impairment. Abnormalities of the LIS1, DCX, ARX, TUBA1A and RELN genes have been associated with these malformations. NMDs only affecting subsets of neurons, such as mild subcortical band heterotopia and periventricular heterotopia, cause neurological and cognitive impairment that vary from severe to mild deficits. They have been associated with abnormalities of the DCX, FLN1A, and ARFGEF2 genes. Polymicrogyria results from abnormal late cortical organization and is inconstantly associated with abnormal neuronal migration. Localized polymicrogyria has been associated with anatomo-specific deficits, including disorders of language and higher cognition. Polymicrogyria is genetically heterogeneous and only in a small minority of patients a definite genetic cause has been identified. Mutations of the GPR56 and SRPX2 genes have been related to isolated polymicrogyria. Focal migration abnormalities associated with abnormal cell types, such as focal cortical dysplasia, are highly epileptogenic and variably influence the functioning of the affected cortex. The functional consequences of abnormal neuronal migration are still poorly understood. Conservation of function in the malformed cortex, its atypical representation, and relocation outside the malformed area are all possible. Localization of function based on anatomic landmarks may not be reliable.

Refining the phenotype of alpha-1a Tubulin (TUBA1A) mutation in patients with classical lissencephaly

Mutations in the alpha-1a Tubulin (TUBA1A) gene have recently been found to cause cortical malformations resemblant of classical lissencephaly but with a specific combination of features. To date, TUBA1A mutations have been described in five patients and three foetuses. Our aims were to establish how common TUBA1A mutations are in patients with lissencephaly and to contribute to defining the phenotype associated with TUBA1A mutation. We performed mutation analysis in the TUBA1A gene in 46 patients with classical lissencephaly. In 44 of the patients, mutations in the LIS1 and/or DCX genes had previously been excluded; in 2 patients, mutation analysis was only performed in TUBA1A based on magnetic resonance imaging (MRI) findings. We identified three new mutations and one recurrent mutation in five patients with variable patterns of lissencephaly on brain MRI. Four of the five patients had congenital microcephaly, and all had dysgenesis of the corpus callosum and cerebellar hypoplasia, and variable cortical malformations, including subtle subcortical band heterotopia and absence or hypoplasia of the anterior limb of the internal capsule. We estimate the frequency of mutation in TUBA1A gene in patients with classical lissencephaly to be approximately 4%, and although not as common as mutations in the LIS1 or DCX genes, mutation analysis in TUBA1A should be included in the molecular genetic diagnosis of classical lissencephaly, particularly in patients with the combination of features highlighted in this paper.

Malformations of cortical development and epilepsy

Malformations of cortical development (MCDs) are macroscopic or microscopic abnormalities of the cerebral cortex that arise as a consequence of an interruption to the normal steps of formation of the cortical plate. The human cortex develops its basic structure during the first two trimesters of pregnancy as a series of overlapping steps, beginning with proliferation and differentiation of neurons, which then migrate before finally organizing themselves in the developing cortex. Abnormalities at any of these stages, be they environmental or genetic in origin, may cause disruption of neuronal circuitry and predispose to a variety of clinical consequences, the most common of which is epileptic seizures, A large number of MCDs have now been described, each with characteristic pathological, clinical, and imaging features. The causes of many of these MCDs have been determined through the study of affected individuals, with many MCDs now established as being secondary to mutations in cortical development genes. This review will highlight the best-known of the human cortical malformations associated with epilepsy. The pathological, clinical, imaging, and etioiogic features of each MCD will be summarized, with representative magnetic resonance imaging (MRI) images shown for each MCD, The malformations tuberous sclerosis, focal cortical dysplasia, hemimegalencephaiy, classical iissencephaly, subcortical band heterotopia, periventricular nodular heterotopia, polymicrogyria, and schizencephaly will be presented.

Genetic malformations of cortical development

The malformations of the cerebral cortex represent a major cause of developmental disabilities, severe epilepsy and reproductive disadvantage. The advent of high-resolution MRI techniques has facilitated the in vivo identification of a large group of cortical malformation phenotypes. Several malformation syndromes caused by abnormal cortical development have been recognised and specific causative gene defects have been identified. Periventricular nodular heterotopia (PNH) is a malformation of neuronal migration in which a subset of neurons fails to migrate into the developing cerebral cortex. X-linked PNH is mainly seen in females and is often associated with focal epilepsy. FLNA mutations have been reported in all familial cases and in about 25% of sporadic patients. A rare recessive form of PNH due ARGEF2 gene mutations has also been reported in children with microcephaly, severe delay and early seizures. Lissencephaly-pachygyria and subcortical band heterotopia (SBH) are disorders of neuronal migration and represent a malformative spectrum resulting from mutations of either LIS1 or DCX genes. LIS1 mutations cause a more severe malformation in the posterior brain regions. Most children have severe developmental delay and infantile spasms, but milder phenotypes are on record, including posterior SBH owing to mosaic mutations of LIS1. DCX mutations usually cause anteriorly predominant lissencephaly in males and SBH in female patients. Mutations of DCX have also been found in male patients with anterior SBH and in female relatives with normal brain magnetic resonance imaging. Autosomal recessive lissencephaly with cerebellar hypoplasia, accompanied by severe delay, hypotonia, and seizures, has been associated with mutations of the reelin (RELN) gene. X-linked lissencephaly with corpus callosum agenesis and ambiguous genitalia in genotypic males is associated with mutations of the ARX gene. Affected boys have severe delay and seizures with suppression-burst EEG. Early death is frequent. Carrier female patients can have isolated corpus callosum agenesis. Among several syndromes featuring polymicrogyria, bilateral perisylvian polymicrogyria shows genetic heterogeneity, including linkage to chromosome Xq28 in some pedigrees, autosomal dominant or recessive inheritance in others, and an association with chromosome 22q11.2 deletion in some patients. About 65% of patients have severe epilepsy. Recessive bilateral frontoparietal polymicrogyria has been associated with mutations of the GPR56 gene. Epilepsy is often present in patients with cortical malformations and tends to be severe, although its incidence and type vary in different malformations. It is estimated that up to 40% of children with drug-resistant epilepsy have a cortical malformation. However, the physiopathological mechanisms relating cortical malformations to epilepsy remain elusive.

Neuronal migration disorders, genetics, and epileptogenesis

Several malformation syndromes with abnormal cortical development have been recognized. Specific causative gene defects and characteristic electroclinical patterns have been identified for some. X-linked periventricular nodular heterotopia is mainly seen in female patients and is often associated with focal epilepsy. FLN1 mutations have been reported in all familial cases and in about 25% of sporadic patients. A rare recessive form of periventricular nodular heterotopia owing to ARGEF2 gene mutations has also been reported in children with microcephaly, severe delay, and early-onset seizures. Lissencephaly-pachygyria and subcortical band heterotopia represent a malformative spectrum resulting from mutations of either the LIS1 or the DCX (XLIS) gene. LIS1 mutations cause a more severe malformation posteriorly. Most children have severe developmental delay and infantile spasms, but milder phenotypes are on record, including posterior subcortical band heterotopia owing to mosaic mutations of LIS1. DCX mutations usually cause anteriorly predominant lissencephaly in male patients and subcortical band heterotopia in female patients. Mutations of the coding region of DCX were found in all reported pedigrees and in about 50% of sporadic female patients with subcortical band heterotopia. Mutations of XLIS have also been found in male patients with anterior subcortical band heterotopia and in female patients with normal brain magnetic resonance imaging. The thickness of the band and the severity of pachygyria correlate with the likelihood of developing severe epilepsy. Autosomal recessive lissencephaly with cerebellar hypoplasia, accompanied by severe delay, hypotonia, and seizures, has been associated with mutations of the reelin (RELN) gene. X-linked lissencephaly with corpus callosum agenesis and ambiguous genitalia in genotypic males is associated with mutations of the ARX gene. Affected boys have severe delay and infantile spasms with suppression-burst electroencephalograms. Early death is frequent. Carrier female patients can have isolated corpus callosum agenesis. Schizencephaly has a wide anatomoclinical spectrum, including focal epilepsy in most patients. Familial occurrence is rare. Initial reports of heterozygous mutations in the EMX2 gene have not been confirmed. Among several syndromes featuring polymicrogyria, bilateral perisylvian polymicrogyria shows genetic heterogeneity, including linkage to chromosome Xq28 in some pedigrees, autosomal dominant or recessive inheritance in others, and an association with chromosome 22q11.2 deletion in some patients. About 65% of patients have severe epilepsy. Recessive bilateral frontoparietal polymicrogyria has been associated with mutations of the GPR56 gene.

Genetic Malformations of the Cerebral Cortex and Epilepsy

We reviewed the epileptogenic cortical malformations for which a causative gene has been cloned or a linkage obtained. X-linked bilateral periventricular nodular heterotopia (BPNH) consists of typical BPNH with epilepsy in female patients and prenatal lethality in most males. About 90% of patients have focal epilepsy. Filamin A mutations have been reported in all families and in approximately 20% of sporadic patients. A rare recessive form of BPNH also has been reported. Most cases of lissencephaly-pachygyria are caused by mutations of LIS1 and XLIS genes. LIS1 mutations cause a more severe malformation posteriorly. Most children have isolated lissencephaly, with severe developmental delay and infantile spasms, but milder phenotypes have been recorded. XLIS usually causes anteriorly predominant lissencephaly in male patients and subcortical band heterotopia (SBH) in female patients. Thickness of the band and severity of pachygyria correlate with the likelihood of developing Lennox-Gastaut syndrome. Mutations of the coding region of XLIS are found in all reported pedigrees and in 50% of sporadic female patients with SBH. Autosomal recessive lissencephaly with cerebellar hypoplasia; accompanied by severe delay, hypotonia, and seizures, has been associated with mutations of the RELN gene. Schizencephaly has a wide anatomoclinical spectrum, including focal epilepsy in most patients. Familial occurrence is rare. Initial reports of heterozygous mutations in the EMX2 gene need confirmation. Among several syndromes featuring polymicrogyria, bilateral perisylvian polymicrogyria shows genetic heterogeneity, including linkage to Xq28 in some pedigrees, autosomal recessive inheritance in others, and association with 22q11.2 deletion in some patients. About 65% of patients have severe epilepsy, often Lennox-Gastaut syndrome. Recessive bilateral frontal polymicrogyria has been linked to chromosome 16q12.2-21.

Neuroimaging in disorders of cortical development

Malformations of cortical development are important causes of developmental delay and epilepsy. They are classified by the presumed stage during which normal development is interrupted: neuronal proliferation and differentiation, neuronal migration, and late migration/cortical organization. This article discusses the important malformations in each of these groups, how and why the malformations develop, and their imaging findings. A better understanding of these disorders helps in genetic counseling of the parents and may help in the treatment of associated epilepsy.

Genetics of the epilepsies

PURPOSE OF REVIEW

This article reviews the most significant advances in the field of genetics of the epilepsies during the past year, with emphasis on newly identified genes and functional studies leading to new insights into the pathophysiology of epilepsy.

RECENT FINDINGS

Mutations in the chloride channel gene CLCN2 have been associated with the most common forms of idiopathic generalized epilepsies. A mutation in the ATP1A2 sodium potassium ATPase pump gene has been described in a family in which familial hemiplegic migraine and benign familial infantile convulsions partly co-segregate. The leucine-rich, glioma-inactivated 1 gene (LGI1) (also known as epitempin) was found to be responsible for autosomal-dominant lateral temporal lobe epilepsy in additional families. The serine-threonine kinase 9 gene (STK9) was identified as the second gene associated with X-linked infantile spasms. Mutations in the Aristaless-related homeobox gene (ARX) have been recognized as a cause of X-linked infantile spasms and sporadic cryptogenic infantile spasms. A second gene underlying progressive myoclonus epilepsy of Lafora, NHLRC1, was shown to code for a putative E3 ubiquitin ligase.

SUMMARY

Genes associated with idiopathic generalized epilepsies remain within the ion channel family. Mutations in non-ion channel genes are responsible for autosomal-dominant lateral temporal lobe epilepsy, a form of idiopathic focal epilepsy, malformations of cortical development, and syndromes that combine X-linked mental retardation and epilepsy. Most genetic epilepsies have a complex mode of inheritance, and genes identified so far account only for a minority of families and sporadic cases. Functional studies are leading to a better understanding of the mechanisms underlying hyperexcitability and seizures.

Malformations of cortical development

Malformations of cortical development are an important cause of developmental delay and epilepsy. Proper identification of these malformations can greatly help in accurately counseling affected families and, in some cases, in the treatment of the epilepsy. Modem neuroimaging is an important tool in the diagnosis of these malformations.

Preserved episodic memory in subcortical band heterotopia

PURPOSE

Neuropsychological profiles of four patients with subcortical band heterotopia (SBH) are presented to delineate further the phenotype of this disorder.

METHODS

Standardized, norm-referenced measures of cognitive functioning, including intelligence, processing speed, attention, language, visuomotor skills, memory, and fine motor ability were administered to four patients with magnetic resonance imaging evidence of SBH.

RESULTS

Despite intellectual impairment and other severe cognitive deficits, all four patients displayed relatively intact episodic memory.

CONCLUSIONS

This selective sparing of memory functions has not been previously reported in individuals with SBH and suggests that doublecortin does not play a role in the development of memory systems in the mesial temporal region, which tend to be spared in SBH.

Malformations of cortical development: burdens and insights from important causes of human epilepsy

Malformations of cortical development (MCD) are important causes of chronic epilepsy in human beings. A blanket term, MCD encompasses many varied developmental disorders with diverse clinical manifestations in patients that neurologists, paediatricians, and learning disability psychiatrists will encounter. Advances in imaging and genetics have led to a significant increase in our understanding of MCD, which has in turn enriched our knowledge of human epileptogenesis and normal brain development and function. In this review, I discuss some of the most common or enlightening MCD: focal cortical dysplasia, periventricular heterotopia, polymicrogyria, band heterotopia and lissencephaly, dysembryoplastic neuroepithelial tumours, and microdysgenesis. Clinical and imaging features, genetic aetiologies, treatments, and the insights that have resulted from MCD study are covered. The burden of epilepsy due to MCD is significant and there is still much to learn about MCD.