Germline mutations in the homeobox gene EMX2 in patients with severe schizencephaly

Unilateral schizencephaly and contralateral polymicrogyria associated with umbilical cord mass

We report a 6-month-old boy with diffuse hypertonia and developmental delay who had unilateral separated-lip schizencephaly and contralateral polymicrogyria. The contralateral polymicrogyria was associated with an incomplete clefting in that hemisphere. An umbilical cord hamartoma is presumed to have caused hypoperfusion to the early developing brain, resulting in bilateral lesions.

Cryptic t(1;12)(q44;p13.3) translocation in a previously described syndrome with polymicrogyria, segregating as an apparently X-linked trait

We report on the multistep progression to the correct genetic diagnosis in an apparently new syndrome of mental retardation and multiple congenital anomalies, including hypogenitalism and polymicrogyria. We had previously reported it as an X-linked condition affecting four members (three males and one female) of a family [Zollino et al., 1992: Am J Med Genet 43:452-457]. Two of the four patients, both males, presented with a brain abnormality that was initially described as pachygyria, while the remaining two (one male and one female) did not. Our present study includes a clinical follow-up on the patients, neuroradiological reexamination of one patient, X linkage studies and X inactivation analyses, and finally molecular cytogenetics, which allowed us to establish definitely the genetic causes of the condition. After the detection of a subtle t(1;12)(q44;p13.3) balanced translocation in healthy carriers, two unbalanced segregation products were observed in different patients, resulting in 1q44qter monosomy and 12p13.3pter trisomy in patients with polymicrogyria and severe psychomotor delay, 12p13.3pter monosomy and 1q44qter trisomy in the other two patients without polymicrogyria, with less severe mental retardation and less distinctive physical anomalies. Thus, this condition is no longer to be considered X-linked, but the result of cryptic autosomal imbalance. Furthermore, this study identified an approximately 14 Mb interval in 1q44qter pathogenetically related to polymicrogyria.

Antisense transcripts at the EMX2 locus in human and mouse

The homeodomain transcription factor EMX2 is critical for central nervous system and urogenital development. In addition, EMX2 maps to a region of allelic deletion corresponding to a putative endometrial tumor suppressor at 10q26. We now report another polyadenylated transcript that is transcribed on the strand opposite to EMX2 and overlaps with the EMX2 transcript. This transcript was designated EMX2OS (OS, opposite strand), and an orthologous transcript present at the murine Emx2 locus was designated Emx2os. Alternative splicing to generate transcripts with varying 5' sequences was detected in the human but not the mouse. Neither ortholog contains a significant open reading frame, nor is primary sequence conserved between the two species. The sense and antisense transcripts display coordinate expression in that EMX2 and EMX2OS are abundant in normal postmenopausal endometrium, reduced in premenopausal endometrium, and absent or reduced in a majority of primary endometrial tumors. EMX2, EMX2OS, Emx2, and Emx2os are abundant in the uterine endometrium, with sense and antisense transcripts exhibiting identical expression patterns. Conservation of functional human and murine EMX2 antisense genes, of overlap between the sense and the antisense transcripts, and of identical cellular expression patterns suggests a biological function for EMX2OS, presumably to regulate EMX2.

Sox1-deficient mice suffer from epilepsy associated with abnormal ventral forebrain development and olfactory cortex hyperexcitability

Mutations in several classes of embryonically-expressed transcription factor genes are associated with behavioral disorders and epilepsies. However, there is little known about how such genetic and neurodevelopmental defects lead to brain dysfunction. Here we present the characterization of an epilepsy syndrome caused by the absence of the transcription factor SOX1 in mice. In vivo electroencephalographic recordings from SOX1 mutants established a correlation between behavioral changes and cortical output that was consistent with a seizure origin in the limbic forebrain. In vitro intracellular recordings from three major forebrain regions, neocortex, hippocampus and olfactory (piriform) cortex (OC) showed that only the OC exhibits abnormal enhanced synaptic excitability and spontaneous epileptiform discharges. Furthermore, the hyperexcitability of the OC neurons was present in mutants prior to the onset of seizures but was completely absent from both the hippocampus and neocortex of the same animals. The local inhibitory GABAergic neurotransmission remained normal in the OC of SOX1-deficient brains, but there was a severe developmental deficit of OC postsynaptic target neurons, mainly GABAergic projection neurons within the olfactory tubercle and the nucleus accumbens shell. Our data show that SOX1 is essential for ventral telencephalic development and suggest that the neurodevelopmental defect disrupts local neuronal circuits leading to epilepsy in the SOX1-deficient mice.

Neurodevelopmental liabilities in epilepsy

The epilepsies are a heterogenous group of cortical disorders characterized by recurrent excessive neuronal activity. In the pathophysiology of these disorders are included mechanisms of abnormal neuronal potential generation secondary to channelopathies, excitotoxic mechanisms secondary to imbalance in neuronal excitation and inhibition, and abnormalities of neuronal organization. In this article the mechanisms of the epilepsies are reviewed, in the setting of three examples: autosomal dominant nocturnal frontal lobe epilepsy, mesial temporal sclerosis and malformations of cortical development.

Brain development and the genetics of brain development

The progress made in the understanding of the genetics of human brain malformations has lead to insight into the formation of brain and into mechanisms of disease affecting brain. It bears upon neurologists and geneticists to recognize the patterns of diseases of brain formation, to properly diagnose such disorders, to assess the recurrence risk of these malformations, and to guide families with appropriate expectations for outcomes. This article may serve as a guide to neurologists in their approach to these disorders. Because this area is one of rapid progress, the clinician is advised to seek more current information that may be available through on-line databases and other sources.

Non-allelic heterogeneity in familial unilateral renal adysplasia

We report three families with dominant unilateral renal adysplasia without vesico-ureteral reflux. No dysmorphia or anomalies were evident in the reproductive system. Ophthalmological examination excluded the presence of optic nerve coloboma or other ocular anomalies. No mutations were detected in the EMX(2) and in PAX(2) genes of affected members. Other homeobox genes could be responsible for this anomaly in these three families.

Reduced anxiety-- and depression-like behaviors in Emx1 homozygous mutant mice

Emx1 is a mammalian homolog of the Drosophila gap gene empty spiracles (ems). Although it has been implicated in the formation of the mouse forebrain, the neuronal functions of this homeobox gene remain unknown. The restricted expression of Emx1 to the cerebral cortex and hippocampus suggests that it might play a role in emotional and other behavioral processes. The present study examined the phenotypes of Emx1-deficient mice generated by gene targeting technology in a battery of behavioral tests with a fixed inter-trial interval of 7 days. Compared with their wild-type littermates, the Emx1 homozygous mutant mice displayed markedly lowered anxiety-like behaviors in the elevated plus maze and dark/light exploration tests. Moreover, they exhibited less depressive-like response as indicated by the reduced duration of immobility in the forced swimming paradigm. There was a trend toward reduction in prepulse inhibition of acoustic startle in the homozygotes. No significant alterations in locomotor activity and susceptibility to pentylenetetrazol-induced seizure were found. This behavioral profile indicates an involvement of Emx1 in the emotional responses of mice.

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.

Interaction of gap genes in the Drosophila head: tailless regulates expression of empty spiracles in early embryonic patterning and brain development

Unlike gap genes in the trunk region of Drosophila embryos, gap genes in the head were presumed not to regulate each other's transcription. Here, we show that in tailless (tll) loss-of-function mutants the empty spiracles (ems) expression domain in the head expands, whereas it retracts in tll gain-of-function embryos. We have identified a 304bp element in the ems-enhancer which is sufficient to drive expression in the head and brain and which contains two TLL and two BCD binding sites. Transgenic reporter gene lines containing mutations of the TLL binding sites demonstrate that tll directly inhibits the expression of ems in the early embryonic head and the protocerebral brain anlage. These results are the first demonstration of direct transcriptional regulation between gap genes in the head.

Emx2 in adult neural precursor cells

Emx2 is a vertebrate homeobox gene involved in the control of the central nervous system development. In the formation of cerebral cortex, Emx2 expression is restricted mainly to the germinal ventricular zone fading away in the first postmitotic neurons. This expression pattern, the severe impairment of cortex organization and the size in mutant mice suggest a role of Emx2 in the control of proliferation and migration of neural precursor cells. The observed persistence of Emx2 expression in adult neurogenic areas in vivo is here confirmed at later stages. We also find that Emx2 is expressed at high levels in adult neural stem cells (ANSCs) in vitro and is down modulated upon differentiation. Overexpression of Emx2 gene in ANSCs has an anti-proliferative effect but it does not influence a particular differentiation pathway. Our results suggest that Emx2 may act promoting an asymmetric mode of cell division thereby increasing the size of a transit amplifying population.

Mechanisms of cerebral cortical patterning in mice and humans

All the higher mental and cognitive functions unique to humans depend on the neocortex ('new' cortex, referring to its relatively recent appearance in evolution), which is divided into discrete areas that subserve distinct functions, such as language, movement and sensation. With a few notable exceptions, all neocortical areas have six layers of neurons and a remarkably similar thickness and overall cell density, despite subtle differences in their cellular architecture. Furthermore, all neocortical areas are formed over roughly the same time period during development and provide little hint at early developmental stages of the rich functional diversity that becomes apparent as development comes to an end. How these areas are formed has long fascinated developmental neuroscientists, because the formation of new cortical areas, with the attendant appearance of new cortical functions, is what must have driven the evolution of mammalian behavior.

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.

Characterization of the homeodomain gene EMX2: sequence conservation, expression analysis, and a search for mutations in endometrial cancers

Previous loss-of-heterozygosity studies in endometrial carcinoma mapped a putative tumor suppressor gene to 10q25.3-26.1. An analysis of genomic sequences for the deletion interval showed several expressed sequence tags and the homeodomain gene EMX2, a homologue of Drosophila melanogaster empty spiracles. Expression studies showed that EMX2 transcripts are abundant in the adult uterus and that message levels seem to be inversely correlated with endometrial proliferation. EMX2 RNA was more abundant in quiescent postmenopausal endometrium than in premenopausal endometrium. We found decreased EMX2 expression in a subset of primary endometrial tumors, and four of six endometrial cancer cell lines investigated failed to express EMX2. The predicted protein showed extensive amino acid conservation with EMX2 sequences from several vertebrates. There was also considerable evolutionary conservation in the 3' untranslated region. To examine the potential function of EMX2 in endometrial tumorigenesis, we investigated 20 primary tumors and 6 endometrial cancer cell lines for mutations. Two primary tumors had mutations. Inactivation or reduced expression of EMX2 in cancers, coupled with increased expression in the quiescent endometrium, indicate that this homeodomain gene is involved in maintenance of the differentiated state.

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.

Human HOX gene mutations

HOX genes play a fundamental role in the development of the vertebrate central nervous system, axial skeleton, limbs, gut, urogenital tract and external genitalia, but it is only in the last 4 years that mutations in two of the 39 human HOX genes have been shown to cause congenital malformations; HOXD13, which is mutated in synpolydactyly, and HOXA13, which is mutated in Hand-Foot-Genital syndrome. Here we review the mutations already identified in these two genes, consider how these mutations may act, and discuss the possibility that further mutations remain to be discovered both in developmental disorders and in cancer.

Genetics of mental retardation

Aim of this review is to present the latest advances in the identification of the genetic determinants of intellectual deficiency. Mental retardation (MR) is often associated with other neurologic symptoms, metabolic disorders, or malformation syndromes. The purpose of the review is to subdivide the large field of MR into categories that may help professionals in making a diagnosis. Nonspecific MR can also segregate in families and the mapping and cloning of corresponding mutant genes will eventually advance our understanding of normal and abnormal brain functioning. Several genes responsible for nonspecific X-linked mental retardation have been identified in the last 12 to 24 months and are being intensively investigated. This will hopefully lead to new possibilities of either genetic or pharmacological therapy.

Genetic evidence for a novel gene(s) involved in urogenital development on 10q26

BACKGROUND

Although the frequent association between distal 10q monosomy and urogenital anomalies suggests the presence of a gene(s) for urogenital development on distal 10q, molecular deletion mapping has not been performed for the putative gene(s). In this study, we examined genotype-phenotype correlations in patients with distal 10q monosomy.

METHODS

This study consisted of six karyotypic males (cases 1 through 6) and four karyotypic females (cases 7 through 10) with 10q26 monosomy. Cases 3 through 5 and 7 through 10 had urinary anomalies such as vesicoureteral reflux and hypoplastic kidney, and cases 1 through 6, 8, and 9 exhibited genital anomalies such as micropenis, hypospadias, cryptorchidism, and hypoplastic labia majora. Fluorescence in situ hybridization (FISH) for 10q telomere, whole chromosome 10 painting, and microsatellite analysis for 35 loci on distal 10q were performed in cases 1 through 8.

RESULTS

FISH and whole chromosome painting confirmed distal 10q monosomy in cases 1 through 8. Microsatellite analysis revealed that hemizygosity for the region distal to D10S186 was shared by cases with urinary anomalies and that for the region distal to D10S1248 was common to cases with genital anomalies. Furthermore, it was indicated that PAX2, GFRA1, and EMX2 on distal 10q, in which the deletions could affect urinary and/or genital development, were present in two copies in cases 1 through 8.

CONCLUSIONS

The results suggest that a novel gene(s) for urinary development and that for genital development reside in the approximately 20 cM region distal to D10S186 and in the approximately 10 cM region distal to D10S1248, respectively, although it remains to be determined whether the two types of genes are identical or different.

Schizencephaly: clinical and imaging features in 30 infantile cases

Schizencephaly is an uncommon structural disorder of cerebral cortical development, characterized by congenital clefts spanning the cerebral hemispheres from the pial surface to the lateral ventricles and lined by cortical gray matter. Either an antenatal environmental incident or a genetic origin could be responsible for this lesion which occurs between the third and fourth month of gestation. We report the clinical and cranial imaging features of 30 children, of whom 15 had unilateral and 15 had bilateral lesions. Their ages at the time of the first presentation ranged from 1 month to 10 years. They were thoroughly studied from clinical, epileptical, imaging and electroencephalographic (EEG) viewpoints. Five patients were investigated by cranial computed tomography (CT), eight by cranial magnetic resonance (MR) imaging, and 17 by both methods. The clinical features consisted of mild hemiparesis in 17 cases (57%), 12/17 were related to a unilateral phenotype (80% of all unilateral forms) and 5/17 to a bilateral phenotype. A tetraparesis was present in nine cases, all of which were due to a bilateral cleft. Bilateral forms were significantly associated with tetraparesis, whereas unilateral forms were associated with hemiparesis. Mental retardation was observed in 17 cases (57%), and was observed significantly more often in bilateral clefts (80%). When both hemispheres are involved, an absence of reorganization of the brain function between the two hemispheres leads to severe mental deficits, in addition to the cerebral anomaly itself. Eleven patients had seizures (seven from unilateral and three from bilateral forms). The degree of malformation was not related to the severity of epilepsy. Migration disorders, such as dysplasia or heterotopia, were observed in 30% of cases and are also important etiopathogenetic factors. The septum pellucidum was absent in 13 cases (43%), with septo-optical dysplasia in two cases. Corpus callosum dysgenesis was noted in 30% of cases. Four cases of mega cisterna magna were noted. Although familial cases and environmental factors have been previously reported, schizencephaly appears to be, in the majority of cases, sporadic.

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.

Focal polymicrogyria in mother and son

This 9-year-old boy was admitted at the age of 2 with a diagnosis of congenital hemiparesis while the rest of physical and neurological examination was normal. His score in the Wechsler intelligence scale was 80. Right fronto-parietal cortical dysplasia with hemisphere atrophy was evident by computerized tomography scanning and magnetic resonance imaging. The latter, also disclosed abnormal thick cortex which was interpreted as polymicrogyria or pachygyria. Karyotype was normal. He had a hemifacial motor seizure at the age of 7. At the age of 8 frequent atonic or inhibitory seizures were presented. Asymmetric bilateral spike discharges with high voltage in the right hemisphere during the EEG recording were found. His mother, a 35-year-old woman (Full scale; Adult intelligence scale: 85) also had congenital hemiparesis. She never had seizures and her EEG was normal. Magnetic resonance imaging disclosed right fronto-parietal cortical dysplasia with ipsilateral hemisphere atrophy. Karyotype was normal. Our cases should be interpreted as a familial presentation of the anomaly, probably with autosomal-dominant transmission.

Agenesis of corpus callosum: clinical description and etiology

In 135 children (aged 3 months to 15 years) with structural defects of the central nervous system found on magnetic resonance imaging, agenesis of the corpus callosum was evident in 7. The etiology of agenesis of the corpus callosum has been established in four children: partial trisomy of chromosome 13, partial duplication of the long arm of chromosome 10, Aicardi's syndrome, and intracranial bleeding during the fetal period as a result of injury. Agenesis of the corpus callosum coexisted with a Dandy-Walker malformation in one other patient, which suggests a genetic etiology. In spite of these variable etiologies, dysmorphic features were identified in all seven patients, as was psychomotor retardation. Epileptic seizures had occurred in six patients, and all manifested abnormalities on neurologic examination.

Surgery for malformations of cortical development causing epilepsy

Malformations of cortical development (MCD) are responsible for many cases of refractory epilepsy in adults and children. The results of surgical treatment are difficult to assess from the published literature. Judging from the limited number of adequately reported cases, approximately 40% of all cases of MCD treated surgically may be rendered seizure-free over a minimum 2-year follow-up period. This figure is the same for focal cortical dysplasia (FCD), the most common variety of MCD in surgical reports. In comparison with outcome for epilepsy associated with hippocampal sclerosis, this figure is low. Part of the difference may be artificial and related to limited reporting. Much of the difference is likely to relate to the complex underlying biology of MCD. Analysis of epileptogenesis in MCD has been undertaken. Different types of MCD have different sequelae. Some varieties are intrinsically epileptogenic; these include FCD and heterotopia. Although in most cases, the visualized MCD lies within the region of brain responsible for generating seizures (the epileptogenic zone), it may not constitute the entire epileptogenic zone in all cases. For polymicrogyria and schizencephaly in particular, the visualized abnormalities are probably not the most important component of the epileptogenic zone. There is evidence that the epileptogenic zone is spatially distributed and also, in some cases, temporally distributed. These findings may explain poor surgical outcome and the inadequacy of current presurgical evaluative methods. New preoperative techniques offer the opportunity of improved presurgical planning and selection of cases more likely to be rendered seizure-free by current surgical techniques. Of paramount importance is improved reporting. The establishment of a central registry may facilitate this aim. Specific recommendations are made for surgical strategies based on current experience and understanding.

Syndromes associated with Homo sapiens pol II regulatory genes

The molecular basis of human characteristics is an intriguing but an unresolved problem. Human characteristics cover a broad spectrum, from the obvious to the abstract. Obvious characteristics may include morphological features such as height, shape, and facial form. Abstract characteristics may be hidden in processes that are controlled by hormones and the human brain. In this review we examine exaggerated characteristics presented as syndromes. Specifically, we focus on human genes that encode transcription factors to examine morphological, immunological, and hormonal anomalies that result from deletion, insertion, or mutation of genes that regulate transcription by RNA polymerase II (the Pol II genes). A close analysis of abnormal phenotypes can give clues into how sequence variations in regulatory genes and changes in transcriptional control may give rise to characteristics defined as complex traits.

Fate mapping of the mouse prosencephalic neural plate

Little is known about the behavior of cells within the anterior neural plate or tube in developing mammalian embryos in utero due to technical limitations. Here we labeled neuroepithelial cells with vital dye and traced their siblings for 1 or 2 days using the whole-embryo culture system. The results demonstrated that rostral cell movement from the midbrain to the forebrain in the mouse neural plate was restricted at the boundary by the five-somite stage. Coincident with restriction of cell intermingling, expression of a transcription factor, Pax6, and a cell adhesion molecule, cadherin-6, commmenced to demarcate the forebrain compartment. Within this compartment, we also mapped several prospective regions of the telencephalon and diencephalon to the eyes. The fate map of the mouse prosencephalic neural plate was very similar to those of other vertebrates, providing evidence that mammalian-specific brain structures, represented in the cerebral neocortex, could evenly develop along the conserved framework of neuromeres.

Bilateral frontoparietal polymicrogyria and epilepsy

Two patients with bilateral frontoparietal polymicrogyria are reported. Severe developmental delay, mental retardation, spastic tetraplegia, and seizures were the main clinical features. Magnetic resonance imaging revealed a bilateral thick cortex with irregular gyri and a festoonlike gray-white matter junction. Bilateral frontoparietal polymicrogyria may represent a further form of the bilateral polymicrogyria syndromes in addition to perisylvian and parasagittal parieto-occipital polymicrogyria.

The role of homeobox genes in kidney development

Homeodomain-containing transcription factors are essential for a variety of processes in vertebrate development, including organogenesis. They have been shown to regulate cell proliferation, pattern segmental identity and determine cell fate decisions during embryogenesis. During nephrogenesis, homeobox genes play an important role at multiple developmental stages, from the early events in intermediate mesoderm to terminal differentiation of glomerular and tubular epithelia. Increasingly sophisticated genetic approaches will probably reveal additional functions for this class of transcription factors in the developing kidney and lead to the identification of critical downstream target genes.

Mutation analysis of the EMX2 gene in Kallmann's syndrome

OBJECTIVE

To investigate the possibility that a mutation in the human EMX2 gene may be involved in Kallmann's syndrome.

DESIGN

In vitro experiment.

SETTING

Academic Medical Center.

PATIENTS

One hundred and twenty patients with Kallman's syndrome or idiopathic hypogonadotrophic hypogonadism (IHH).

INTERVENTION

Peripheral blood leukocytes were used to obtain DNA.

MAIN OUTCOMES MEASURES

Single-stranded conformational polymorphism (SSCP) analysis was used to identify possible mutations of the EMX2 gene.

RESULTS

One hundred and twenty patients with Kallmann's syndrome or IHH, had no mutations noted in this gene.

CONCLUSION

It is unlikely that EMX2 mutations are a clinically significant cause of IHH or Kallman's syndrome.

Cerebral cortical dysplasia and digital constriction rings in Adams-Oliver syndrome

Adams-Oliver syndrome (AOS) is characterised by aplasia cutis congenita of the scalp and variable degrees of terminal transverse limb defects. Short fingers and hypoplastic nails also occur in this predominantly autosomal dominant syndrome which displays marked variability of expression and lack of penetrance in some cases. We describe a boy with AOS whose sister is also mildly affected. Their mother has hypoplastic fifth toenails which may represent very mild expression of the syndrome. Brain (computed tomography) imaging to investigate mild left hemiparesis in the boy demonstrated severe cortical dysplasia of central, occipital and anterior regions of the right cerebral hemisphere. A variety of brain and cranial malformations has been reported in AOS but dysplasia of the cerebral cortex has not been noted previously. In addition, the boy and his sister have apparent constriction rings present on the toes which are uncommon in AOS.

Genetic basis of the human epilepsies

Genetic factors contribute to aetiology in up to 40% of patients with epilepsy. Over 100 single gene Mendelian disorders include epilepsy as one component of what is usually a complex neurological phenotype, but the majority of idiopathic or primary epilepsies display a 'complex' non-Mendelian pattern of inheritance. There have been significant recent advances in understanding the genetic basis of inherited epilepsies at a molecular level. Epilepsy genes fall into several distinct categories including those in which mutations cause abnormal brain development, progressive neurodegeneration, disturbed energy metabolism and abnormal function of ion channels. Ion channel genes involved include those encoding neuronal nicotinic acetylcholine receptor subunits and voltage-gated potassium and sodium channels.

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.

Focal cortical dysplasia: a neuropathological and developmental perspective

Focal cortical dysplasia (FCD) is a rare, sporadic disorder which is a recognised cause of chronic epilepsy. It is proposed to result from disordered neuronal migration and differentiation and has characteristic histological features which include disturbed cortical lamination, large abnormal neurons and the presence of large balloon cells with glassy eosinophilic cytoplasm and pleomorphic eccentric nuclei. These latter express both glial and neuronal markers indicative of abnormal neuroglial differentiation. In this paper we review the current literature on the neuropathology of FCD and discuss potential mechanisms. We focus on growth factors, signalling pathways and candidate genes with known roles in Drosophila and vertebrate brain development that could be responsible for the developmental brain changes seen in FCD. At issue are the factors that influence cell fate and differentiation and which regulate neural migration. Some of the molecular pathways, such as those involving the Notch and the Wnt pathways have particularly important roles in neuroglial differentiation in vertebrates, and these are proposed as potential candidates.

Bilateral perisylvian polymicrogyria in three generations

A family is described in which bilateral perisylvian polymicrogyria was present in 6 members of 3 consecutive generations. Typical anatomic and clinical findings of the syndrome, with a mild phenotype, were present in the 5 affected women from all 3 generations. More severe impairment was observed in the only affected male individual, a boy, in the third generation. Analysis of the pedigree and severity of the phenotype in the affected boy are consistent with transmission of an X-linked dominant trait, although other patterns of inheritance cannot be ruled out with certainty.

Syntelencephaly associated with connected transhemispheric cleft of focal cortical dysplasia

The authors report a female with syntelencephaly associated with a connected transhemispheric cleft of focal cortical dysplasia. Syntelencephaly is a rare anomaly characterized by fusion of the hemispheres in the posterior frontal and parietal regions and is considered a new variant of holoprosencephaly. Cranial magnetic resonance imaging of the patient revealed syntelencephaly associated with bilateral fused clefts of focal cortical dysplasia without the pial-ependymal seam, which was regarded as an incomplete type of schizencephaly. The underlying mechanism is discussed.

Review : Otx and Emx Homeobox Genes in Brain Development

A number of gene families have recently been identified that play a role in the control of the development of the central nervous system of vertebrates. Many of these genes are homeobox genes. The most well- known and best-studied among them are the Hox genes. Collectively, these control regionalization and cell identity in the developing hindbrain and spinal cord. Other homeobox gene families, including the Otx and Emx genes, control brain development. In particular, Otx2 seems to play a crucial role in the early estab lishment of the rostral brain; Otx1 and Otx2 cooperate to define the posterior boundary of midbrain; and Emx1 and Emx2 play a major role in the developing cerebral cortex. Some of these results may be relevant for the deeper understanding of congenital brain defects and multifactorial brain disorders. NEURO SCIENTIST 5:164-172, 1999

Homeobox genes and cancer

Homeobox-containing genes are a family of regulatory genes encoding transcription factors that primarily play a crucial role during development. Several indications suggest their involvement in the control of cell growth and, when dysregulated, in oncogenesis. We will describe the implications, in tumor origin and evolution, of members of the homeobox gene families HOX, EMX, PAX, and MSX as well as of other divergent homeobox genes. We will also propose a model for the function of the HOX gene network in controlling cell identity to account for the involvement of some HOX genes in both normal development and oncogenesis.

Joubert's syndrome: new cases and review of clinicopathologic correlation

The authors report on seven patients, six males and one female, with Joubert's syndrome who underwent developmental evaluation, neurologic and ophthalmologic examinations, and magnetic resonance imaging of the brain. All patients had severe developmental delay, hypotonia, impairment of smooth visual pursuit, and saccadic eye movements. Six had jerky eye movements and ptosis was observed in two patients and retinal dystrophy in one. The posterior lobe of the vermis was absent in all patients and the small rudimentary anterior lobe lacked fusion in the midline, with cleft formation in five patients. Malformation of the pontomesencephalic junction, with prominent superior cerebellar peduncles and deep interpeduncular fossa, was observed in all patients. Abnormal cerebellar-brainstem and cerebellocortical connections because of the lack of the posterior vermis and dysplasia of the deep cerebellar nuclei might be responsible for the abnormal eye movements and retarded development in Joubert's syndrome. Correlation between radiologic findings and clinical symptoms and the possible role of abnormal patterning of the midbrain-hindbrain by homeotic genes during embryonic development are reviewed.

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.