Fluorescence in situ hybridization analysis with LIS1 specific probes reveals a high deletion mutation rate in isolated lissencephaly sequence

Responsible Genes for Neuronal Migration in the Chromosome 17p13.3: Beyond Pafah1b1(Lis1), Crk and Ywhae(14-3-3ε)

The 17p13.3 chromosome region is often deleted or duplicated in humans, resulting in severe neurodevelopmental disorders such as Miller–Dieker syndrome (MDS) and 17p13.3 duplication syndrome. Lissencephaly can also be caused by gene mutations or deletions of a small piece of the 17p13.3 region, including a single gene or a few genes. PAFAH1B1 gene, coding for LIS1 protein, is a responsible gene for lissencephaly and MDS and regulates neuronal migration by controlling microtubules (MTs) and cargo transport along MTs via dynein. CRK is a downstream regulator of the reelin signaling pathways and regulates neuronal migration. YWHAE, coding for 14-3-3ε, is also responsible for MDS and regulates neuronal migration by binding to LIS1-interacting protein, NDEL1. Although these three proteins are known to be responsible for neuronal migration defects in MDS, there are 23 other genes in the MDS critical region on chromosome 17p13.3, and little is known about their functions in neurodevelopment, especially in neuronal migration. This review will summarize the recent progress on the functions of LIS1, CRK, and 14-3-3ε and describe the recent findings of other molecules in the MDS critical regions in neuronal migration.

Interneuron Heterotopia in the Lis1 Mutant Mouse Cortex Underlies a Structural and Functional Schizophrenia-Like Phenotype

LIS1 is one of the principal genes related to Type I lissencephaly, a severe human brain malformation characterized by an abnormal neuronal migration in the cortex during embryonic development. This is clinically associated with epilepsy and cerebral palsy in severe cases, as well as a predisposition to developing mental disorders, in cases with a mild phenotype. Although genetic variations in the LIS1 gene have been associated with the development of schizophrenia, little is known about the underlying neurobiological mechanisms. We have studied how the Lis1 gene might cause deficits associated with the pathophysiology of schizophrenia using the Lis1/sLis1 murine model, which involves the deletion of the first coding exon of the Lis1 gene. Homozygous mice are not viable, but heterozygous animals present abnormal neuronal morphology, cortical dysplasia, and enhanced cortical excitability. We have observed reduced number of cells expressing GABA-synthesizing enzyme glutamic acid decarboxylase 67 (GAD67) in the hippocampus and the anterior cingulate area, as well as fewer parvalbumin-expressing cells in the anterior cingulate cortex in Lis1/sLis1 mutants compared to control mice. The cFOS protein expression (indicative of neuronal activity) in Lis1/sLis1 mice was higher in the medial prefrontal (mPFC), perirhinal (PERI), entorhinal (ENT), ectorhinal (ECT) cortices, and hippocampus compared to control mice. Our results suggest that deleting the first coding exon of the Lis1 gene might cause cortical anomalies associated with the pathophysiology of schizophrenia.

Human iPSC-Derived Cerebral Organoids Model Cellular Features of Lissencephaly and Reveal Prolonged Mitosis of Outer Radial Glia

Classical lissencephaly is a genetic neurological disorder associated with mental retardation and intractable epilepsy, and Miller-Dieker syndrome (MDS) is the most severe form of the disease. In this study, to investigate the effects of MDS on human progenitor subtypes that control neuronal output and influence brain topology, we analyzed cerebral organoids derived from control and MDS-induced pluripotent stem cells (iPSCs) using time-lapse imaging, immunostaining, and single-cell RNA sequencing. We saw a cell migration defect that was rescued when we corrected the MDS causative chromosomal deletion and severe apoptosis of the founder neuroepithelial stem cells, accompanied by increased horizontal cell divisions. We also identified a mitotic defect in outer radial glia, a progenitor subtype that is largely absent from lissencephalic rodents but critical for human neocortical expansion. Our study, therefore, deepens our understanding of MDS cellular pathogenesis and highlights the broad utility of cerebral organoids for modeling human neurodevelopmental disorders.

Genes and brain malformations associated with abnormal neuron positioning

Neuronal positioning is a fundamental process during brain development. Abnormalities in this process cause several types of brain malformations and are linked to neurodevelopmental disorders such as autism, intellectual disability, epilepsy, and schizophrenia. Little is known about the pathogenesis of developmental brain malformations associated with abnormal neuron positioning, which has hindered research into potential treatments. However, recent advances in neurogenetics provide clues to the pathogenesis of aberrant neuronal positioning by identifying causative genes. This may help us form a foundation upon which therapeutic tools can be developed. In this review, we first provide a brief overview of neural development and migration, as they relate to defects in neuronal positioning. We then discuss recent progress in identifying genes and brain malformations associated with aberrant neuronal positioning during human brain development.

TUBA1A mutations cause wide spectrum lissencephaly (smooth brain) and suggest that multiple neuronal migration pathways converge on alpha tubulins

We previously showed that mutations in LIS1 and DCX account for approximately 85% of patients with the classic form of lissencephaly (LIS). Some rare forms of LIS are associated with a disproportionately small cerebellum, referred to as lissencephaly with cerebellar hypoplasia (LCH). Tubulin alpha1A (TUBA1A), encoding a critical structural subunit of microtubules, has recently been implicated in LIS. Here, we screen the largest cohort of unexplained LIS patients examined to date to determine: (i) the frequency of TUBA1A mutations in patients with lissencephaly, (ii) the spectrum of phenotypes associated with TUBA1A mutations and (iii) the functional consequences of different TUBA1A mutations on microtubule function. We identified novel and recurrent TUBA1A mutations in approximately 1% of children with classic LIS and in approximately 30% of children with LCH, making this the first major gene associated with the rare LCH phenotype. We also unexpectedly found a TUBA1A mutation in one child with agenesis of the corpus callosum and cerebellar hypoplasia without LIS. Thus, our data demonstrate a wider spectrum of phenotypes than previously reported and allow us to propose new recommendations for clinical testing. We also provide cellular and structural data suggesting that LIS-associated mutations of TUBA1A operate via diverse mechanisms that include disruption of binding sites for microtubule-associated proteins (MAPs).

New trends in neuronal migration disorders

Neuronal migration disorders are an heterogeneous group of disorders of nervous system development and they are considered to be one of the most significant causes of neurological and developmental disabilities and epileptic seizures in childhood. In the last ten years, molecular biologic and genetic investigations have widely increased our knowledge about the regulation of neuronal migration during development. One of the most frequent disorders is lissencephaly. It is characterized by a paucity of normal gyri and sulci resulting in a "smooth brain". There are two pathologic subtypes: classical and cobblestone. Classical lissencephaly is caused by an arrest of neuronal migration whereas cobblestone lissencephaly caused by overmigration. Heterotopia is another important neuronal migration disorder. It is characterized by a cluster of disorganized neurons in abnormal locations and it is divided into three main groups: periventricular nodular heterotopia, subcortical heterotopia and marginal glioneural heterotopia. Polymicrogyria develops at the final stages of neuronal migration, in the earliest phases of cortical organization; bilateral frontoparietal form is characterized by bilateral, symmetric polymicrogyria in the frontoparietal regions. Bilateral perisylvian polymicrogyria causes a clinical syndrome which manifests itself in the form of mild mental retardation, epilepsy and pseudobulbar palsy. Schizencephaly is another important neuronal migration disorder whose clinical characteristics are extremely variable. This review reports the main clinical and pathophysiological aspects of these disorders paying particular attention to the recent advances in molecular genetics.

Intragenic deletions and duplications of the LIS1 and DCX genes: a major disease-causing mechanism in lissencephaly and subcortical band heterotopia

Classical lissencephaly, or isolated lissencephaly sequence (ILS), and subcortical band heterotopia (SBH) are neuronal migration disorders associated with severe mental retardation and epilepsy. Abnormalities of the LIS1 and DCX genes are implicated in the majority of patients with these disorders and account for approximately 75% of patients with ILS, whereas mutations of DCX account for 85% of patients with SBH. The molecular basis of disease in patients with ILS and SBH, in whom no abnormalities have been identified, has been questioned. We studied a series of 83 patients with ILS, SBH or pachygyria, in whom no abnormalities of the LIS1 or DCX genes had been identified, for intragenic deletions and duplications by multiplex ligation-dependent probe amplification (MLPA). In 52 patients with ILS, we identified 12 deletions and 6 duplications involving the LIS1 gene (35%), with the majority resulting in grade 3 lissencephaly. Three deletions of the DCX gene were identified in the group of nine female patients with SBH (out of 31 patients with DCX-suggestive brain anomalies), ie 33%. We estimate an overall mutation detection rate of approximately 85% by LIS1 and DCX sequencing and MLPA in ILS, and 90% by DCX sequencing and MLPA in SBH. Our results show that intragenic deletions and duplications of the LIS1 and DCX genes account for a significant number of patients with ILS and SBH, where no molecular defect had previously been identified. Incorporation of deletion/duplication analysis of the LIS1 and DCX genes will be important for the molecular diagnosis of patients with ILS and SBH.

Fluorescence in situ hybridization (FISH) in diagnostic and investigative neuropathology

Over the last decade, fluorescence in situ hybridization (FISH) has emerged as a powerful clinical and research tool for the assessment of target DNA dosages within interphase nuclei. Detectable alterations include aneusomies, deletions, gene amplifications, and translocations, with primary advantages to the pathologist including its basis in morphology, its applicability to archival, formalin-fixed paraffin-embedded (FFPE) material, and its similarities to immunohistochemistry. Recent technical advances such as improved hybridization protocols, markedly expanded probe availability resulting from the human genome sequencing initiative, and the advent of high-throughput assays such as gene chip and tissue microarrays have greatly enhanced the applicability of FISH. In our lab, we currently utilize only a limited battery of DNA probes for routine diagnostic purposes, with determination of chromosome 1p and 19q dosage in oligodendroglial neoplasms representing the most common application. However, research applications are numerous and will likely translate into a growing list of clinically useful markers in the near future. In this review, we highlight the advantages and disadvantages of FISH and familiarize the reader with current applications in diagnostic and investigative neuropathology.

Variations in genes regulating neuronal migration predict reduced prefrontal cognition in schizophrenia and bipolar subjects from mediterranean Spain: A preliminary study

Both neural development and prefrontal cortex function are known to be abnormal in schizophrenia and bipolar disorder. In order to test the hypothesis that these features may be related with genes that regulate neuronal migration, we analyzed two genomic regions: the lissencephaly critical region (chromosome 17p) encompassing the LIS1 gene and which is involved in human lissencephaly; and the genes related to the platelet-activating-factor, functionally related to LIS1, in 52 schizophrenic patients, 36 bipolar I patients and 65 normal control subjects. In addition, all patients and the 25 control subjects completed a neuropsychological battery. Thirteen (14.8%) patients showed genetic variations in either two markers related with lissencephaly or in the platelet-activating-factor receptor gene. These patients performed significantly worse in the Wisconsin Card Sorting Test-Perseverative Errors in comparison with patients with no lissencephaly critical region/platelet-activating-factor receptor variations. The presence of lissencephaly critical region/platelet-activating-factor receptor variations was parametrically related to perseverative errors, and this accounted for 17% of the variance (P = 0.0001). Finally, logistic regression showed that poor Wisconsin Card Sorting Test-Perseverative Errors performance was the only predictor of belonging to the positive lissencephaly critical region/platelet-activating-factor receptor group. These preliminary findings suggest that the variations in genes involved in neuronal migration predict the severity of the prefrontal cognitive deficits in both disorders.

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.

Epileptic-like convulsions associated with LIS-1 in the cytoskeletal control of neurotransmitter signaling in Caenorhabditis elegans

Cortical malformations are a collection of disorders affecting brain development. Mutations in the LIS1 gene lead to a disorganized and smooth cerebral cortex caused by failure in neuronal migration. Among the clinical consequences of lissencephaly are mental retardation and intractable epilepsy. It remains unclear whether the seizures result from aberrant neuronal placement, disruption of intrinsic properties of neurons, or both. The nematode Caenorhabditis elegans offers an opportunity to study such convulsions in a simple animal with a defined nervous system. Here we show that convulsions mimicking epilepsy can be induced by a mutation in a C. elegans lis-1 allele (pnm-1), in combination with a chemical antagonist of gamma-aminobutyric acid (GABA) neurotransmitter signaling. Identical convulsions were obtained using C. elegans mutants defective in GABA transmission, whereas none of these mutants or the antagonist alone caused convulsions, indicating a threshold was exceeded in response to this combination. Crosses between pnm-1 and fluorescent marker strains designed to exclusively illuminate either the processes of GABAergic neurons or synaptic vesicles surprisingly showed no deviations in neuronal architecture. Instead, presynaptic defects in GABAergic vesicle distribution were clearly evident and could be phenocopied by RNAi directed against cytoplasmic dynein, a known LIS1 interactor. Furthermore, mutations in UNC-104, a neuronal-specific kinesin, and SNB-1, a synaptic vesicle-associated protein termed synaptobrevin, exhibit similar convulsion phenotypes following chemical induction. Taken together, these studies establish C. elegans as a system to investigate subtle cytoskeletal mechanisms regulating intrinsic neuronal activity and suggest that it may be possible to dissociate the epileptic consequences of lissencephaly from the more phenotypically overt cortical defects associated with neuronal migration.

Cortical dysplasia and epilepsy: animal models

Cortical dysplasia syndromes--those conditions of abnormal brain structure/organization that arise during aberrant brain development--frequently involve epileptic seizures. Neuropathological and neuroradiological analyses have provided descriptions and categorizations based on gross anatomical and cellular histological features (e.g., lissencephaly, heterotopia, giant cells), as well as on the developmental mechanisms likely to be involved in the abnormality (e.g., cell proliferation, migration). Recently, the genes responsible for several cortical dysplastic conditions have been identified and the underlying molecular processes investigated. However, it is still unclear how the various structural abnormalities associated with cortical dysplasia are related to (i.e., "cause") chronic seizures. To elucidate these relationships, a number of animal models of cortical dysplasia have been developed in rats and mice. Some models are based on laboratory manipulations that injure the brain (e.g., freeze, undercut, irradiation, teratogen exposure) of immature animals; others are based on spontaneous genetic mutations or on gene manipulations (knockouts/transgenics) that give rise to abnormal cortical structures. Such models of cortical dysplasia provide a means by which investigators can not only study the developmental mechanisms that give rise to these brain lesions, but also examine the cause-effect relationships between structural abnormalities and epileptogenesis.

LIS1 missense mutations: variable phenotypes result from unpredictable alterations in biochemical and cellular properties

Mutations in one allele of the human LIS1 gene cause a severe brain malformation, lissencephaly. Although most LIS1 mutations involve deletions, several point mutations with a single amino acid alteration were described. Patients carrying these mutations reveal variable phenotypic manifestations. We have analyzed the functional importance of these point mutations by examining protein stability, folding, intracellular localization, and protein-protein interactions. Our data suggest that the mutated proteins were affected at different levels, and no single assay could be used to predict the lissencephaly phenotype. Most interesting are those mutant proteins that retain partial folding and interactions. In the case of LIS1 mutated in F31S, the cellular phenotype may be modified by overexpression of specific interacting proteins. Overexpression of the PAF-AH alpha1 subunit dissolved aggregates induced by this mutant protein and increased its half-life. Overexpression of NudE or NudEL localized this mutant protein to spindle poles and kinetochores but had no effect on protein stability. Our results implicate that there are probably different biochemical and cellular mechanisms obstructed in each patient yielding the varied lissencephaly phenotypes.

Frequent translocations occur between low copy repeats on chromosome 22q11.2 (LCR22s) and telomeric bands of partner chromosomes

The chromosome 22q11.2 region is susceptible to rearrangements, mediated by low copy repeats (LCR22s). Deletions and duplications are mediated by homologous recombination events between LCR22s. The recurrent balanced constitutional translocation t(11;22)(q23;q11) breakpoint occurs in an LCR22 and is mediated by double strand breaks in AT-rich palindromes on both chromosomes 11 and 22. Recently, two cases of a t(17;22)(q11;q11) were reported, mediated by a similar mechanism (21). Except for these constitutional translocations, the molecular basis for non-recurrent, reciprocal 22q11.2 translocations is not known. To determine whether there are specific mechanisms that could mediate translocations, we analyzed cell lines derived from 14 different individuals by genotyping and FISH mapping. Somatic cell hybrid analysis was carried out for four cell lines. In five cell lines, the translocation breakpoints occurred in the same LCR22 as for the t(11;22) translocation, suggesting that similar molecular mechanisms are responsible. An additional three occurred in other LCR22s, and six were in non-LCR22 regions, mostly in the proximal half of the 22q11.2 region. The translocation breakpoints on the partner chromosomes were all located in the telomeric bands, proximal to the most telomeric unique sequence probe, in eight cell lines and distal to those loci in six. Therefore, several of the breakpoints were found to occur in the vicinity of highly dynamic regions of the genome, 22q11.2 and telomeric bands. We hypothesize that these regions are more susceptible to breakage and repair, resulting in translocations.

New syndrome of simplified gyral pattern, micromelia, dysmorphic features and early death

We report two sisters with a new syndrome of simplified gyral pattern, normal head circumference at birth but with subsequent development of microcephaly, intractable seizures, and early death. Dysmorphic features included coarse face, hypertrichosis, short nose, paranasal widening, long philtrum, short neck, upper limb micromelia, single transverse palmar lines, and clasp thumbs. The proband had repeated convulsions from shortly after birth and she required continuous artificial ventilation. Neurological examination showed absent sucking, rooting, Moro and grasping reflexes. MRI revealed a diffuse simplified gyral pattern with apparent agyria over the frontal lobes. Biochemical screening gave normal results. Her older sister had bilateral renal pelvic dilatation on prenatal ultrasound. She also developed severe convulsions on the first day of life, and she had to be artificially ventilated for 38 days. She had severe developmental retardation and neurological examination showed absence of spontaneous movements and Moro reflex, weak sucking reflex, and hypertonicity. CT scan of the brain showed a simplified gyral pattern. At 3 months, she developed hypocalcemia and hyperphosphatemia with normal levels of vitamin D and alkaline phosphatase, and parathyroid hormone level was low. Other biochemical tests gave normal results. She died at 5 months due to a massive aspiration event. Based on the unique clinical and radiological features found in our patients, we propose that this is a new syndrome.

Refinement of a 400-kb critical region allows genotypic differentiation between isolated lissencephaly, Miller-Dieker syndrome, and other phenotypes secondary to deletions of 17p13.3

Deletions of 17p13.3, including the LIS1 gene, result in the brain malformation lissencephaly, which is characterized by reduced gyration and cortical thickening; however, the phenotype can vary from isolated lissencephaly sequence (ILS) to Miller-Dieker syndrome (MDS). At the clinical level, these two phenotypes can be differentiated by the presence of significant dysmorphic facial features and a more severe grade of lissencephaly in MDS. Previous work has suggested that children with MDS have a larger deletion than those with ILS, but the precise boundaries of the MDS critical region and causative genes other than LIS1 have never been fully determined. We have completed a physical and transcriptional map of the 17p13.3 region from LIS1 to the telomere. Using fluorescence in situ hybridization, we have mapped the deletion size in 19 children with ILS, 11 children with MDS, and 4 children with 17p13.3 deletions not involving LIS1. We show that the critical region that differentiates ILS from MDS at the molecular level can be reduced to 400 kb. Using somatic cell hybrids from selected patients, we have identified eight genes that are consistently deleted in patients classified as having MDS. In addition, deletion of the genes CRK and 14-3-3 epsilon delineates patients with the most severe lissencephaly grade. On the basis of recent functional data and the creation of a mouse model suggesting a role for 14-3-3 epsilon in cortical development, we suggest that deletion of one or both of these genes in combination with deletion of LIS1 may contribute to the more severe form of lissencephaly seen only in patients with MDS.

Cortical dysgenesis and epilepsy

Cortical dysgenesis (CD) describes a wide spectrum of brain anomalies that involve abnormal development of the cerebral cortex. There is a strong association between CD and epilepsy, and it comprises a significant proportion of children and adults whose epilepsy cannot be controlled with medications. There has been intense effort to define the relationship between CD and epilepsy so that more effective therapies can be devised. These efforts have ranged from detailed study of people with CD and epilepsy from a clinical standpoint to single-cell analysis of mRNA expression and postsynaptic receptor function. Animal models have also been developed to mimic certain aspects of CD in a situation when quantitative, controlled, and interventional experiments can be performed that would not be possible in a clinical setting. This review will give an overview of human CD syndromes and their causes, when possible, and describe some specific abnormalities in dysplastic cortex that may underlie its epileptogenic potential. It will also review several animal models of CD that have been studied mechanistically from the standpoint of epileptogenesis. In conclusion, some general trends will be proposed based on human and animal studies to encapsulate our current understanding of CD and how it causes epilepsy.

Chromosome imbalances associated with epilepsy

Epilepsy is among the most frequent findings in many, especially autosomal, chromosome aberrations. Its incidence, however, is very variable, and there are very few aberrations in which epilepsy is a constant finding. Even siblings and monozygotic twins with the same aberration are often discordant for seizure disorders. Similar observations can be made for congenital (major) malformations in chromosome aberrations. The common explanation is that in these instances epilepsy is not caused by the action of a single gene in single or triple dose, but is influenced by the combined action of a number of genes within and outside of the aneuploid segment. The situation is comparable to a polygenic model of inheritance. Gene mutations associated with epilepsy are known, to date, only for two disorders: the lissencephaly 1 gene in Miller-Dieker syndrome and mutations in the UBE3A gene in Angelman syndrome. Chromosome aberrations in which epilepsy is a major and consistent finding include Angelman syndrome due to loss of the maternal 15q11.2-q12 segment, tetrasomy of the maternal segment 15pter-q13 due to an additional inv dup chromosome, Miller-Dieker syndrome due to deletion of the 17p13.3 segment including the lissencephaly1 gene, ring chromosome 20, and Wolf-Hirschhorn syndrome due to deletion of at least the 4p16.3 segment.

Epilepsy and genetic malformations of the cerebral cortex

Malformations of the cerebral cortex are an important cause of developmental disabilities and epilepsy. Here we review those malformations for which a genetic basis has been elucidated or is suspected and the types of associated epilepsy. Schizencephaly (cleft brain) has a wide anatomo-clinical spectrum, including partial epilepsy in most patients. Familial occurrence is rare. Heterozygous mutations in the EMX2 gene were reported in 13 patients. X-linked bilateral periventricular nodular heterotopia (BPNH) consists of typical BPNH with epilepsy in females and prenatal lethality in males. About 88% of patients have partial epilepsy. Filamin A mutations, all leading to a truncated protein, have been reported in three families and in sporadic patients. The most frequent forms of lissencephaly (agyria-pachygyria) are caused by mutations of LIS1. XLIS mutations cause classical lissencephaly in hemizygous males and subcortical band heterotopia (SBH) in heterozygous females. The thickness of the heterotopic band and the degree of pachygyria correlate with the likelihood of developing Lennox-Gastaut syndrome. Mutations of the coding region of XLIS were found in all reported pedigrees and in 38-91% of sporadic female patients with SBH. With few exceptions, children with LIS1 mutations have isolated lissencephaly, with severe developmental delay and infantile spasms. Autosomal recessive lissencephaly with cerebellar hypoplasia, accompanied by severe developmental delay, seizures, and hypotonia has been associated with mutations of the reelin gene. Fukuyama congenital muscular dystrophy is due to mutations of the fukutin gene and is accompanied by polymicrogyria. Febrile seizures and epilepsy with generalized tonic-convulsions appear in about 50% of children but are usually not severe. Tuberous sclerosis (TS) is caused by mutations in at least two genes, TSC1 and TSC2; 75% of cases are sporadic; 60% of patients have epilepsy, manifested in 50% of them as infantile spasms. TSC1 mutations seem to cause a milder disease with fewer cortical tubers and lower frequency of seizures. Among several syndromes featuring polymicrogyria, bilateral perisylvian polymicrogyria had familial occurrence on several occasions. Genetic heterogeneity is likely, including autosomal recessive, X-linked dominant, X-linked recessive inheritance, and association with 22q11.2 deletions. About 65% of patients have severe epilepsy, often Lennox-Gastaut syndrome.

Clinical and molecular basis of classical lissencephaly: Mutations in the LIS1 gene (PAFAH1B1)

Classical lissencephaly (LIS) and subcortical band heterotopia (SBH) are related cortical malformations secondary to abnormal migration of neurons during early brain development. Approximately 60% of patients with classical LIS, and one patient with atypical SBH have been found to have deletions or mutations of the LIS1 gene, located on 17p13.3. This gene encodes the LIS1 or PAFAH1B1 protein with a coiled-coil domain at the N-terminus and seven WD40 repeats at the C-terminus. It is highly conserved between species and has been shown to interact with multiple proteins involved with cytoskeletal dynamics, playing a role in both cellular division and motility, as well as the regulation of brain levels of platelet activating factor. Here we report 65 large deletions of the LIS1 gene detected by FISH and 41 intragenic mutations, including four not previously reported, the majority of which have been found as a consequence of the investigation of 220 children with LIS or SBH by our group. All intragenic mutations are de novo, and there have been no familial recurrences. Eight-eight percent (36/41) of the mutations result in a truncated or internally deleted protein-with missense mutations found in only 12% (5/41) thus far. Mutations occurred throughout the gene except for exon 7, with clustering of three of the five missense mutations in exon 6. Only five intragenic mutations were recurrent. In general, the most severe LIS phenotype was seen in patients with large deletions of 17p13.3, with milder phenotypes seen with intragenic mutations. Of these, the mildest phenotypes were seen in patients with missense mutations.

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.

Epileptogenic brain malformations: clinical presentation, malformative patterns and indications for genetic testing

We review here those malformations of the cerebral cortex which are most often observed in epilepsy patients, for which a genetic basis has been elucidated or is suspected and give indications for genetic testing. There are three forms of lissencephaly (agyria-pachygyria) resulting from mutations of known genes, which can be distinguished because of their distinctive imaging features. They account for about 85% of all lissencephalies. Lissencephaly with posteriorly predominant gyral abnormality is caused by mutations of the LIS1 gene on chromosome 17. Anteriorly predominant lissencephaly in hemizygous males and subcortical band heterotopia (SBH) in heterozygous females are caused by mutations of the XLIS(or DCX) gene. Mutations of the coding region of XLIS were found in all reported pedigrees, and in most sporadic female patients with SBH. Missense mutations of both LIS1 and XLIS genes have been observed in some of the rare male patients with SBH. Autosomal recessive lissencephaly with cerebellar hypoplasia has been associated with mutations of the reelin gene. With few exceptions, children with lissencephaly have severe developmental delay and infantile spasms early in life. Patients with SBH have a mild to severe mental retardation with epilepsy of variable severity and type. X-linked bilateral periventricular nodular heterotopia (BPNH) consists of typical BPNH with focal epilepsy in females and prenatal lethality in males. About 88% of patients have focal epilepsy. Filamin A (FLNA) mutations have been reported in some families and in sporadic patients. Additional, possibly autosomal recessive gene(s) are likely to be involved in causing BPNH non-linked to FLN1. Tuberous sclerosis (TS) is a dominant disorder caused by mutations in at lest two genes, TSC1 and TSC2. 75% of cases are sporadic. Most patients with TS have epilepsy. Infantile spasms are a frequent early manifestation of TS. Schizencephaly (cleft brain) has a wide anatomo-clinical spectrum, including focal epilepsy in most patients. Familial occurrence is rare. Heterozygous mutations in the EMX2 gene have been reported in some patients. However, at present, there is no clear indication on the possible pattern of inheritance and on the practical usefulness that mutation detection in an individual with schizencephaly would carry in terms of genetic counselling. Amongst several syndromes featuring polymicrogyria, bilateral perisylvian polymicrogyria had familial occurrence on several occasions. Genetic heterogeneity is likely, including autosomal recessive, X-linked dominant, X-linked recessive inheritance and association to 22q11.2 deletions. FISH analysis for 22q11.2 is advisable in all patients with perisylvian polymicrogyria. Parents of an affected child with normal karyotype should be given up to a 25% recurrence risk.

Molecular cytogenetics

In the past decade, clinical cytogenetics has undergone remarkable advancement as molecular biology techniques have been applied to conventional chromosome analysis. The limitations of conventional banding analysis in the accurate diagnosis and interpretation of certain chromosome abnormalities have largely been overcome by these new technologies, which include fluorescence in situ hybridization (FISH), comparative genomic hybridization (CGH), and multicolor FISH (M-FISH, SKY, and Rx-FISH). Clinical applications include diagnosis of microdeletion and microduplication syndromes, detection of subtelomeric rearrangements in idiopathic mental retardation, identification of marker and derivative chromosomes, prenatal diagnosis of trisomy syndromes, and gene rearrangements and gene amplification in tumors. Molecular cytogenetic methods have expanded the possibilities for precise genetic diagnoses, which are extremely important for clinical management of patients and appropriate counseling of their families.

Lissencephaly and subcortical band heterotopia: molecular basis and diagnosis

Magnetic resonance imaging is now used routinely in the evaluation of developmental and neurological disorders and provides exquisite images of the living human brain. Consequently, it is evident that cortical malformations are more common than previously thought. Among the most severe is classical lissencephaly, in which the cortex lacks the complex folding that characterizes the normal human brain. Lissencephaly includes agyria and pachygyria, and merges with subcortical band heterotopia. Current molecular genetic techniques combined with the identification of affected patients have enabled the detection of two of the genes responsible: LIS1 (PAFAH1B1) on chromosome 17 and DCX (doublecortin) on the X chromosome. This review highlights the discovery of these genes and discusses the advances made in understanding the molecular basis of cortical development and improvements in diagnosis and genetic counseling.

Subcortical band heterotopia in rare affected males can be caused by missense mutations in DCX (XLIS) or LIS1

Subcortical band heterotopia (SBH) are bilateral and symmetric ribbons of gray matter found in the central white matter between the cortex and the ventricular surface, which comprises the less severe end of the lissencephaly (agyria-pachygyria-band) spectrum of malformations. Mutations in DCX (also known as XLIS ) have previously been described in females with SBH. We have now identified mutations in either the DCX or LIS1 gene in three of 11 boys studied, demonstrating for the first time that mutations of either DCX or LIS1 can cause SBH or mixed pachygyria-SBH (PCH-SBH) in males. All three changes detected are missense mutations, predicted to be of germline origin. They include a missense mutation in exon 4 of DCX in a boy with PCH-SBH (R78H), a different missense mutation in exon 4 of DCX in a boy with mild SBH and in his mildly affected mother (R89G) and a missense mutation in exon 6 of LIS1 in a boy with SBH (S169P). The missense mutations probably account for the less severe brain malformations, although other patients with missense mutations in the same exons have had diffuse lissencephaly. Therefore, it appears likely that the effect of the specific amino acid change on the protein determines the severity of the phenotype, with some mutations enabling residual protein function and allowing normal migration in a larger proportion of neurons. However, we expect that somatic mosaic mutations of both LIS1 and DCX will also prove to be an important mechanism in causing SBH in males.