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Brain malformations, epilepsy, and infantile spasms
BRAIN MALFORMATIONS, EPILEPSY,AND INFANTILESPASMS
M. Elizabeth Ross Departmentof Neurology and Neuroscience Weill Medical College of Cornell University New York, New York 10021
I. I n t r o d u c t i o n II. Cortical M a l f o r m a t i o n : D i s o r d e r s o f N e u r o n a l Position
III. Neuronal Migration and Cortical Malformation A. Lissencephaly B. Chromosome 17-LinkedLissencephaly (LIS1) C. LIS1 Deficiencyin Mice Produces a Neuronal Migration Defect and Seizure Propensity D. X-LinkedLissencephaly E. LIS1 and DCX Gene Function and Possible Mechanisms of Epileptogenesis IV. Lissencephalywith Cerebellar Hypoplasia (LCH): reeler Mutant as Example V. Prenatal Chemical Exposure as a Model for Diffuse Heterotopia VI. Syndromesof Cortical Disorganization VII. DysplasiaAssociatedwith Altered Neurogenesis: The tish Rat Model VIII. MalformationsNot Yet Classified IX. Summary References
I. Introduction
Brain malformations encompass a broad spectrum of etiologies from defects in primary neurogenesis to neuronal migration and p r o g r a m m e d cell death. Many, t h o u g h certainly not all, of these malformations are associated with seizures. Some, like the neuronal migration defects, display an u n c o m m o n l y high incidence of infantile spasms. While this may simply reflect the severity of the malformation, it probably also speaks to the mechanisms generating this dramatic seizure type. Recent identification of several genes responsible for cortical malformations, and the promise of the isolation of other genes functioning in these pathways, will likely lead to better understanding of epileptogenic factors producing early onset seizures. These insights will undoubtedly contribute to improved strategies for treatment, even in children without developmental defects that are detectable by brain imaging. What then are the characteristics of out of place or heterotopic neurons that might make them p r o n e to seize? Pieces of the puzzle have been gleaned from the rapidly advancing area of cortical INTERNATIONALREVIEWOF NEUROBIOLOGY,VOL. 49
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malformations and are discussed from the perspective of their relationship to epilepsy.
Ih Cortical Malformation: Disorders of Neuronal Position
Clinical syndromes involving cerebral cortical malformation are the end results of gestation, recognized by the abnormal position of neural cells. This static picture can only suggest the mechanism by which the disorganized brain developed. Therefore, classification of a malformation as a neuronal migration syndrome must be tentative until the responsible gene defect is identified and the mechanism has been tested in animal models. T h e movement of cells from their origins in the ventricular zone may be impaired in a n u m b e r of ways. Primary neurogenesis or cell n u m b e r may be altered by virtue of disturbed cell proliferation, fate determination, and p r o g r a m m e d cell death. The failure of particular cells to differentiate or improper timing of the birth of a n e u r o n or glial cell may alter the fate and positional information of others in the region. The migration of cells from the ventricular zone may be curtailed by interference with the mechanical motors and cytoskeletal dynamics of the cells, as implicated in several lissencephaly syndromes. T h e r e may be a primary defect in the function of the radial glial scaffold on which neurons migrate, leading to altered cellular movements, as suspected in polymicrogyria. Alternatively, the molecular signals to initiate movement guide the cell in its migration and inform it that the final position has been reached may be altered. Once the position of the cell body is established, assembly and consolidation of n e u r i t e / a x o n a l projections and synapses are further established and refined, in part accounting for the survival of neurons and their associated glia. Thus, the events that organize brain structure include neurogenesis and early mid- and late migration, as well as axon projection and guidance, all of which are likely to overlap. Consideration will begin with the migration syndromes, as this area has seen the greatest progress in the last several years. It is becoming increasingly evident that more than the m o v e m e n t o f cells is hindered in the gene defects that impact cell movement.
IIh Neuronal Migration and Cortical Malformation
Thus far, over a dozen molecules that are peculiar to neuronal migration in brain have been reported (reviewed in Ross and Walsh,
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2001). At least half were identified first from clinical human studies. The rest were discovered either in mouse or in Drosophila and then mammalian relevance was confirmed in mouse. A partial classification of neuronal migration syndromes is outlined below and the reader is referred to a more extensive discussion for details (Ross and Walsh, 2001). The identified gene that is mutated in several of these syndromes appears in italics. By far the largest class of neuronal migration syndromes are the lissencephalies (from "lisos" smooth + "encephalos" brain). Meticulous phenotype examination, and, for some syndromes, genotype-phenotype correlation, has enabled grouping these malformations in a way that facilitates the discovery of molecular pathways governing neuronal migration. Lissencephaly Classical lissencephaly (LIS) Autosomal dominant (LIS1/PAFAH1BI), X-linked (DCX) Double cortex or subcortical band heterotopia (DC/SBH) X-linked (DCX) LIS with cerebellar hypoplasia (LCH) Autosomal dominant ( LIS1/PAFAH1B1), X-linked (DCX) Autosomal recessive (RELN) Cobblestone complex (cobblestone lissencephaly) Walker Warburg syndrome (WWS) Muscle eye brain disease (MEB) Fukuyama congenital muscular dystrophy Autosomal recessive (FCMD) Cobblestone complex only (CCO) Other migration disorders Bilateral periventricular nodular heterotopia (BPNH) X-linked (FLN1) Zellweger syndrome Autosomal (PEX2, PXRI ) Kallmann syndrome X-linked ( KAL1)
A. LISSENCEPHALY This term encompasses a range of simplified cortex from a total absence of cortical convolutions (agyria) to broadened gyri (pachygyria) with an abnormally thick cortex (typically 10-20 mm in classical LIS compared to 2.5-4 mm in normal) (Dobyns and Truwit, 1995, Dobyns et aL, 1996)
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FIG. 1. MRI appearance of lissencephaly (LIS) a n d subcortical b a n d heterotopia (SBH). (A) Classical LIS, grade 1 is the most severe form with complete loss of cortical convolutions a n d thickening of the cortical gray matter (here, ~ 2 0 m m c o m p a r e d to 2 - 4 m m in n o r m a l cortex). Grade 1 LIS reveals n o malformation gradient so that LIS1 or DCX mutations c a n n o t be distinguished in this most severe form. ( B ) A moderately thick subcortical b a n d (also known as double-cortex, DC) is indicated by the double-headed arrow. T h e circumferential band underlies a true cortex with relatively preserved gyri a n d is separated from the true cortex by a rim of white matter (dark on this T2-weighted image). (C a n d D). Images of an XLIS son (C) a n d his SBH m o t h e r (D). T h e mutation in DCX was confirmed by the sequence a n d predicts a single a m i n o acid substitution. Classical LIS, grade 4 in the son shows m a r k e d thickening of the cortical mantle (double-headed arrow in C) a n d pachygyria (a b r o a d e n e d gyrus is designated in the bracket). T h e SBH in the m o t h e r (D) is thin a n d presents only in the frontal pole (arrows). T h e pattern is consistent with a DCX mutation that partially inactivates the protein.
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(Fig. 1). This spectrum intersects with double cortex [DC, also known a subcortical band heterotopia (SBH)] in which neurons are partially h u n g up in their migrations to reside as a disorganized band of neurons in the white matter beneath a relatively normal cortex. Two genes, LIS1 and DCX, account for the majority of classical LIS (Pilz et aL, 1998). However, other nonclassical forms of LIS are becoming recognized for which L I S I and D C X mutations account for only a small proportion of cases (Ross et al., 2001; Dobyns, 2001). Clinical features c o m m o n to classical forms of LIS include p r o f o u n d mental retardation, mixed hypotonia and spasticity, and seizures in 80% within the first postnatal year and virtually all by 5 years (Guerrini et al., 1999).
B. CHROMOSOME17-LINKEDLISSENCEPHALY(IriS1) T h e first LIS syndrome delineated is Miller-Dieker syndrome (MDS), which is manifested by severe LIS and characteristic facial abnormalities (Dobyns et al., 1993). Chromosomal analysis shows visible deletions of 17p13.3 in over 90% of MDS patients, suggesting that this is a contiguous gene deletion syndrome in which the characteristic facial features of MDS are caused by the involvement of loci neighboring LISI. Isolated lissencephaly sequence (ILS) consists of LIS with a relatively normal facial appearance. Fluorescence in situ hybridization (FISH) studies show deletions of c h r o m o s o m e 17pl 3.3 in about 40% of ILS children, (Dobyns et al., 1994). T h e LIS1 gene encodes a noncatalytic subunit of platelet-activating factor (PAF) acetylhydrolase (Pafah), or P a f a h l b l , and is part of a Gprotein-like (0tl/0t2)l] trimer (Ho et al., 1997). Hereafter, this P a f a h l b l subunit is referred to as Lisl protein. T h e Pafah enzyme is known to regulate PAF, a potent lipid first messenger. Lisl is a soluble protein with seven WD40 repeats forming a seven-bladed propeller-like structure inw)lved in p r o t e i n - p r o t e i n interactions (Garcia-Higuera et al., 1996). Lisl binds tubulin and reduces microtubule catastrophe in vitro, suggesting that it may stabilize the microtubule cytoskeleton (Sapir et al., 1997, 1999). Significantly, Lisl is a highly conserved homologue of the NUDF protein in Aspergillus, where it is required for nuclear translocation through interaction with a dynein m o t o r (Xiang et al., 1995). When overexpressed in mammalian cells and neurons, Lisl has been shown to bind cytoplasmic dynein and to affect microtubule organization (Smith et al., 2000). In this model, Lisl promotes the peripheral, plus end-directed movement of microtubule segments by dynein motors that are attached to stable microtubules, oriented with minus ends a n c h o r e d at the centrosome and plus
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ends at the peripheral membrane skeleton. In conditions of low Lisl levels, dynein motor activity is reduced and microtubule segments accumulate near the nucleus. In addition, Lisl protein binds to homologues of the Aspergillus NUDE protein, which are also involved in translocation of the nucleus (Feng et al., 2000; Sasaki et al., 2000).
C. LIS1 DEFICIENCYIN MiCE PRODUCESA NEURONALMIGRATIONDEFECTAND SEIZUREPROPENSITY
Although homozygous null mice die in early embryogenesis, mice heterozygous for Lisl survive and reveal abnormalities primarily in the cerebral and olfactory cortex, but also in the hippocampus and cerebellum, due to the impaired, slower migration of neurons (Hirotsune et al., 1998, Clark etal., 2001). Interestingly, although these mice reveal migration defects, the malformation is distinct from the prototype neuronal migration mouse model reeler (see later) in that the L i s l + / - cortex is not inverted (e.g, layer V pyramidal cells are not displaced to the superficial layers), and the hippocampus and cerebellum are far less disorganized than in reel~.
While spontaneous seizures have been observed only rarely in L i s l - / + mice, anatomical and electrophysiological data demonstrate a number of hippocampal abnormalities (Fleck etal., 2000). There is potassiumdependent bursting activity in the hippocampus at 6.5 mM K+. Long-term potentiation is present, but its dynamic range of frequency facilitation of mossy fiber transmission is altered. This hyperexcitability is demonstrable at the level of Schaffer collaterals (CA1). There is a reduced transmission of mossy fibers in the CA3 region. In terms of hippocampal structure, there is moderate disorganization, particularly of the CAB region (Hirotsune et aL, 1998). Assessed by Golgi staining, there is stunting of the neurites of heterotopic pyramids, with normal appearing normotopic pyramidal neurons (Fleck et al., 2000).
D. X-LINKEDLISSENCEPHALY
XLIS refers to the syndrome of classical LIS in hemizygous males and double cortex in heterozygous females (Fig. 1). The clinical features and responsible gene have been delineated (Dobyns et al., 1996; Ross et al., 1997; Gleeson et al., 1998; des Portes et al., 1998). The clinical manifestations of XLIS are very similar to those of chromosome 17-associated LIS. DC/SBH
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is characterized by a symmetric band of gray matter that may be circumferential or frontal when seen with D C X mutations, located just beneath the cortex and separated from it by a thin band of white matter (Figs. 1B and 1D). The phenotype arises in D C X females because r a n d o m X inactivation will turn o f f t h e defective allele in, on average, 50% of neurons. Thus, only a fraction of cortical neurons will be hung up in their migrations (Ross et al., 1997). The thickness of the band heterotopia is determined by the severity of the D C X mutation, although unfavorable skewing of X inactivation may account for rare severe phenotypes in girls as well (Matsumoto et aL, 2001; Ross et al., 1997).
E. LIS1 ANDD C X GrNE FvNca-ion AnD POSSmLEMECHANISMSOF EPILEPTOGENESIS
Indicators thus far suggest an important role of Lisl and Dbcn proteins in the intrinsic motility of neurons. Lisl and Dbcn proteins have been implicated in the regulation of microtubule dynamics by virtue of binding tubulin and reducing microtubule catastrophe (Sapir et al., 1997, 1999; Gleeson et al., 1999; Taylor et al., 2000). Lisl may also have a role in signal transduction. Consistent with this notion, excess PAF, a potent first messenger and substrate for Pafah, collapses growth cones of neurons in dissociated cell culture (Bix and Clark, 1998). Clark and colleagues have demonstrated, in a yeast-two-hybrid system, that point mutations found in LIS patients interfere with the association of Lisl for the 29- and 30kD a subunits of Pafah (Sweeney et al., 2000). The fact that all patient mutations in L i s l examined in this system interfere with the binding to Lisl protein suggests that the heterotrimeric Pafah complex may be involved in regulating migration. Further evidence indicates that PAF interferes with the binding of Lisl to the 30-kD a subunit and that PAF rescues the migration phenotype of Lisl haploinsufficient neurons. This suggests a model in which the role of PAF is to mobilize the Lisl protein from Pafah to exert a downstream effect on migration. In such a model, PAF could rescue neurons expressing low levels of Lisl by displacing more Lisl protein from the complex (Clark etal., 2001). These downstream, non-Pafah-dependent effects of Lisl involve several additional p r o t e i n - p r o t e i n interactions. For example, a Lisl interaction with dynein heavy chain has been described that recapitulates the interaction of the homologous proteins previously demonstrated in Aspergillus (Smith et al., 2000). In addition, interactions have been identified in yeast two-hybrid screens between Lisl and NudE, thought to be involved in modulating dynein heavy chain interactions (Efimov and Morris, 2000; Kitagawa et al., 2OOO).
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FIG. 2. Aspects of Lisl and Dbcn function that could be associated with epileptogenesis. A scheme of the neuronal growth cone, with microtubules extending up to but not invading the leading edge of the growth cone that relies on an actin-based cytoskeleton. Lisl is known to be released from the Pafah complex upon binding to its substrate, PAF. Molecular partners for Lisl include binding of microtubules, dynein, and NudE, all thought to promote nuclear translocation and movement of organelles along the microtubule-based cytoskeleton. Interaction with microtubules a n d / o r protein motors such as dynein could position Lisl to influence protein transport into the leading edge and later established neurites. This could have effects on the turnover and composition of neurotransmitter receptors and other membrane components. Signal transduction aspects of Lisl function could influence dynamics of the actin-based cytoskeleton via small GTPases Rho, Rac, and Cdc42. Dbcn has been shown to bind microtubules and may modulate Lisl interactions with that cytoskeletal compartment. Multiple putative phosphorylation sites on Dbcn position the protein for a role in signal transduction as well.
These examples suggest several scenarios in which altered Lisl or Dbcn could affect neuronal excitability, which can be tested in genetically a l t e r e d m o u s e m o d e l s (Fig. 2). First, L i s l o r D b c n d y s f u n c t i o n c o u l d a l t e r t h e i n t r i n s i c f u n c t i o n o f t h e cell a n d its e x c i t a b i l i t y . T h i s c o u l d b e
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accomplished by disruption of the microtubule-based cytoskeleton a n d / o r dynein (or other) protein motor complexes to disturb protein trafficking within the cell. This could impact neurotransmitter availability (transport of synaptic vesicles) or receptor turnover in some neuronal types. Alternatively, disturbances in the relationship between the actin-based and the microtubule-based cytoskeleton could impact intracellular signaling, neurite extension, and synaptogenesis. Second, reduced neuronal migration might impair neuronal movement differentially according to the distance that must be covered. For example, the majority of GABAergic interneurons in the cerebral cortex derive from tangential migrations of young neurons from the ganglionic eminence (Anderson et al., 2001). Therefore, the Lisl-deficient neocortex may contain a relative paucity of inhibitory interneurons. There is some evidence against this possibility, however, as GABAergic interneurons of the hippocampus that also derive from the ganglionic eminence are not reduced in number but are displaced in the Lisl-deficient hippocampus (Pleasure et al., 2000; Fleck et al., 2000). Finally, because neurite outgrowth is slowed in cultured Lisl-deficient neurons, it is possible that longer projections, such as thalamocortical pathways, could be disturbed in classical LIS (Hirotsune et al., 1998). These deeper structures are not typically considered in the genesis of epilepsy. However, the importance of thalamocortical relays has been recognized in the generation of certain seizure types, including absence, spike, and slow wave epilepsy (Coulter and Lee, 1993; Zhang and Coulter, 1996). In these models, abnormal synchronization and rhythmicity seen in these electroencephalographic patterns are thought to arise from disturbances in a sort of "pacemaker" function of thalamoreticular and thalamocortical circuits (Buzsaki et al., 1990). Indeed, thalamocortical circuits have been implicated in the genesis of infantile spasms as well (see chapters by Olivier Dulac and colleagues, David A. McCormick, Csaba and Juhftsz and colleagues; Hayashi et al., 2000; Chugani et al., 1994; Tominaga et al., 1986). In addition to the role of these pathways in abnormal synchronous patterns, thalamoreticular and thalamocortical relays are responsible for setting up rhythmic circuits that control sleep (reviewed in McCormick and Bal, 1997). Alterations in this thalamocortical circuit could be consistent with the generation of interictal hypsarhythmia in West syndrome. Involvement of thalamocortical relays in West syndrome and the association of these relays to sleep could in part account for the frequent clinical observation of spasms precipitated upon waking from sleep (Mendez and Radtke, 2001). It might also explain the association of infantile spasms with an age of onset that precedes and overlaps the establishment of normal sleep architecture.
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IV. Ussencephaly with Cerebellar Hypoplasia (LCH): reeler Mutant as Example
Lissencephaly with cerebellar hypoplasia is a term used to describe a heterogeneous group of malformations in which a simplified, thickened cerebral cortex appears along with an underdeveloped cerebellum, as detected by magnetic resonance imaging (MRI). This group is only now evolving a nomenclature and comprises six broad classes, LCH a-f, that are grouped according to distinguishing features (Ross et al., 2001). Within these broader groups, three causative genes have been identified and a n o t h e r locus described. Syndromes of LCH have in c o m m o n a lissencephaly spectrum of agyria-pachygyria, with mildly ( 5 - 1 0 mm) or markedly (10-20 mm) thickened gray matter. Cerebellar involvement in LCH may be mild, with predominantly midline hypoplasia seen with LIS1 or D C X mutations (LCHa) to severe cerebellar defects with hypoplasia of the cerebellar hemispheres and abnormal or absent foliation (LCHb or d) (Ross et al., 2001; Kato et al., 1999). A mutation has been identified in the h u m a n reelin (RELN) gene that produces type LCHb. This distinctive pattern possesses a moderately thickened cortex ( 5 - 1 0 mm) and pachygyria, markedly abnormal hippocampal formation, and severe cerebellar hypoplasia with absent folia (Hong et al., 2000). This gene was first recognized in a spontaneously arising ataxic mouse mutant with a strikingly disorganized brain (Caviness, 1982). R E L N encodes a large 388-kDa protein containing eight EGF repeats and is secreted by Cajal-Retzius cells of the embryonic preplate, marginal zone, the cerebellar EGL, and pioneering cells of the hippocampus (D'Arcangelo et al., 1995; Schiffmann et al., 1997; Sheppard and Pearlman, 1997). In addition, R E L N mutations can lead to abnormalities of neurite outgrowth and synaptogenesis (Borrell et al., 1999a). Interestingly, while many malformations are associated with epilepsy, this is not universally the case and a spectrum of seizure severity is seen (Guerrini et al., 1999). For example, in nonclassical LCHb associated with a R E L N mutation, seizures are not a c o m m o n clinical feature, despite marked abnormalities in hippocampal formation (al Shahwan et al., 1995; Caviness and Rakic, 1978) and influences on neurite outgrowth and axon pathfinding (Borrell etal., 1999a). This suggests that the mechanism by which displaced neurons become seizure-prone may have more to do with intrinsic properties of the cells than position or connectivity.
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FJ{;. 3. MRI appearance of cobblestone LIS. P r o m i n e n t features include a r o u g h e n e d , " c o b b l e s t o n e " character of the cortical surface. This corresponds to neural cells that have broken past the b a s e m e n t m e m b r a n e to invade the arachnoid space. Stria-like bands of white inatter t h r o u g h the cortical gray are actually unmyelinated, glial tissue (arrow in A). T h e true white matter is markedly a b n o r m a l a n d is too bright on both Tl-weighted (A) a n d T2-weighted (B) images.
V. Prenatal Chemical Exposure as a Model for Diffuse Heterotopia
Prenatal treatment of rats with an alkylating, antimitotic agent, methazoxymethanol acetate (MAM), produces diffuse cortical heterotopia with a phenotype reminiscent of cobblestone LIS in humans (Mischel et al., 1995; Chevassus-Au-Louis et al., 1998). Cobblestone LIS is characterized by the migration of heterotopic young neurons beyond the marginal z o n e - - f u t u r e layer I - - i n t o the leptomeninges through gaps in the external basement m e m b r a n e (Fig. 3). The movement of neurons into the leptomeninges obliterates the subarachnoid space and gives rise to communicating hydrocephalus, or ventricular enlargement due to impaired reabsorption of cerebrospinal fluid (CSF) (Gelot et al., 1995; B o r n e m a n n et al., 1997). On first glance, the MRI features of cobblestone lissencephaly may be difficult to distinguish from those ofpolymicrogyria. T h e r e is a thickened cortical gray matter with a knobby, cobblestone surface and few or absent true sulci. Some areas will appear as pachygyria with a smooth surface. Bands o f " w h i t e matter" composed ofglial-fibrous tissue interrupt the gray matter. In the cortical dysplasia induced by MAM there are relatively normal projections as well as local fibers that do not normally innervate the
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cortex (Woo and Finlay, 1996; Chevassus-Au-Louis et al., 1999; Chevassusau-Louis and Represa, 1999). Thus, cortical hyperexcitability in this model is attributed to altered firing of discrete neuronal subpopulations together with the formation of bridges between normally u n c o n n e c t e d structures (Chevassus-au-Louis et al., 1999). The biochemical and electrophysiological characteristics of dysplastic brain in MAM-treated animals have been studied in some detail. Norepinephrine and serotonin concentrations are increased in brain while their receptors are reduced and the total cortical content of GAD and GABA is decreased (Johnston and Coyle, 1979). Data suggest not only selective loss of neuronal subsets in the MAM model, but also an alteration in transmitter release mechanisms. A translocation of protein kinase C from the cytosol to the presynaptic m e m b r a n e has been observed (Di Luca et al., 1997). This is accompanied by an e n h a n c e m e n t of glutamate release but not GABA from synaptosomes. While there are only rare spontaneous seizures in the MAM-treated, dysplastic cortex, there is hyperexcitability of the cortex in the presence of increased extracellular potassium, 4-aminopyridine, or bicuculline (Baraban and Schwartzkroin, 1995; Baraban etal., 2000). Changes in neuronal m e m b r a n e properties are indicated by increases in the frequency of action potential bursts to depolarization in over 80% of CA1 neurons in MAM-treated animals compared to 20% in normal controls (Baraban and Schwartzkroin, 1995). Indeed, recordings from the heterotopic clusters in the MAM model reveal that dysplastic clusters make connections locally with CA1 neurons and also with the neocortex (Baraban et al., 2000). These neurons have been shown to generate i n d e p e n d e n t epileptiform activity, which can be spread broadly to the neocortex, as well as the hippocampus. Severing the connections between the heterotopia in the CA1 region and cortex can eliminate the cortical spread of paroxysms (Chevassus-Au-Louis et al., 1998).
Vh Syndromes of Cortical Disorganization Although not necessarily a primary affector of neuronal migration, a n u m b e r of important processes may result in the heterotopic placement of neurons if disturbed. These syndromes may ultimately bear on the finetuning of neuronal migration and certainly pertain to the establishment of cortical structure. A partial list is outlined, where they are m e n t i o n e d because either clinical observation or animal models have revealed neuronal heterotopia and cortical dysplasia phenotypes (for additional discussion, see Ross and Walsh, 2001).
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Dysplasia associated with altered neurogenesis or survival Ectopic neurogenesis Autosomal recessive (tish) Schizencephaly Autosomal dominant ( E M X 2 ) Microcephaly Microcephaly vera Microcephaly with simplified gyri Microlissencephaly Malformations not yet classified Polymicrogyria (PMG) Diffuse Frontal Perisylvian Occipital Joubert syndrome
VII. Dysplasia Associated with Altered Neurogenesis: The tish Rat Model
Cortical dysplasia may arise from disturbances in the primary genesis of neural elements, both in cell number and in fate. An example is found in the tish rat, a mutant that first came to attention because of overt seizures. The brain reveals a subcortical heterotopia that is very similar in appearance to the human DC/SBH syndrome (Lee et al., 1997). Investigators have shown, however, that this band of disorganized neurons below the telencephalic cortex arises due to ectopic neurogenesis, forming a secondary ventricular germinal zone rather than an intrinsic neuronal migration defect (Lee et al., 1998). Interestingly, anterograde and retrograde tracing experiments indicated that both the heterotopia and the normal overlying cortex in the tish brain form appropriate projections to subcortical sites and bidirectional, topographic connections with the thalamus (Schottler et al., 1998). The genesis of seizures in the tish model has been studied extensively (Chen et al., 2000; Lee et al., 1997). Homozygous animals express spontaneous seizures at a rate of 1.5 to 15 spells per week. Both the normotopic cortex and the heterotopic cortex are hyperexcitable when exposed to penicillin or in conditions of reduced extracellular Mg2+, Exposure to proconvulsants induces cFos mRNA in the neocortex, hippocampus, and amygdala. Importantly, normotopic neurons appear to initiate seizures in
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the tish neocortex (Chen et al., 2000). In fact, when connections from the tish heterotopia to normotopic cortex are severed, the spiking threshold of the normotopic cortex is reduced dramatically, indicating release of inhibition of epileptogenic activity in the normotopic cortex (Chen et al., 2000). This has implications for the appropriate selection of patients with heterotopia for surgical intervention.
VIII. Malformations Not Yet Classified
These represent disorders in which neurons are observed to be out of place, but lack of gene identification or adequate genetic models prevents classification of the primary defect. Polymicrogyria (PMG) is important in this discussion both because epilepsy in children affected by PMG is p r o m i n e n t (Fig. 4) and because there are experimentally induced r o d e n t models having the appearance of PMG. Pathologically, two patterns of PMG are recognized: (1) a "four-layered" PMG in which the cortex comprises a molecular layer, an organized outer layer, a cell sparse layer, and a disorganized inner layer and (2) a completely disorganized " u n l a y e r e d " microgyrus. PMG can arise from several intrauterine insults, including hypoxemia with cortical laminar necrosis. This hypothesis of intrauterine insult is in part derived from experimental models in the rat p r o d u c e d by cortical lesions such as focal freezing injury (Jacobs etal., 1999). However, the detection of PMG in offspring of consanguineous parents or several affected family members indicates inherited causes as well (Sztriha and Nork, 2000; Guerrini et aL, 2000; Guerreiro et al., 2000; Borgatti et al., 1999). Lesion experiments suggest mechanisms that interfere with radial glial function and secondary disturbances of neuronal migration, although this is far from established as a general rule. A pathology that mimics PMG can be elicited by focal freezing o f the cortical surface in the early postnatal period. In this model, cortical hyperexcitability is caused by a reorganization of the network in the borders of the malformation (Chevassus-au-Louis et al., 1999). A striking observation in this PMG model in the rat neonatal cortex is disruption of the levels of specific GABAA receptor subtypes within the area o f the lesion, at the periphery, and at a distance (Redecker et al., 2000). Reductions in GABA o~5 and y2 subunits were p r o m i n e n t within the lesion, with even more substantial reductions in all subunits in the area medial to the lesion.
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A
F~c;. 4. MRI appearance of polymicrogyria (PMG). An example of perisylvian PMG in which the most severely affected cortex is centered on the sylvian fissure (arrow in B). White matter is of normal signal intensity. Disorganized microgyri with interdigitation of white matter give 1he cortex a thickened, rough appearance with loss of normal convolutions anteriorly and relative sparing of posterior cortex.
IX. Summary
The models of cortical dysplasia discussed earlier-- the L i s l knockout, the MAM-induced cobblestone LIS, the spontaneous tish mutant, and focal freeze injury-induced PMG--illustrate several important insights into epileptogenesis in malformed brain. First, the appearance of epilepsy varies according to the pathogenesis of the dysplasia and may well depend more on the intrinsic properties of the neurons in these models rather than on the disturbed position of the cells. This is supported by models such as the reeler mouse, in which the dysfunctional extracellular matrix molecule leads to a form of lissencephaly in mouse and human, but there is a far less impressive association with seizures than for LIS1 mutations. However, L i s l and Dcx mutations that appear to affect the cytoskeleton and perhaps intracellular protein trafficking are frequently associated with infantile spasms and epilepsy. Second, the possible mechanisms of epileptogenesis in these models include (a) a loss of subsets of neurons, (b) altered neurotransmitter release, (c) differences in neurotransmitter receptor levels and changes in receptor subnnit composition, (d) altered neurite density a n d / o r synaptogenesis, (e) changed membrane properties (e.g., altered voltage-gated channels), (f)altered cell morphology
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(neuronal differentiation), and (g) effects on cytoskeletal function. Finally, it is important to note that the "generator" of excitability in affected brain may be within the heterotopia or in the normotopic cortex. As additional genetic models come to light and the ability to distinguish their clinical counterparts improves, more individually tailored therapies, including standards for surgical interventions, will surely evolve.
References
al Shahwan, S. A., Bruyn, G. W., and al Deeb, S. M. (1995). Non-progressive familial congenital cerebellar hypoplasia.J. Neurol. Sci. 128, 71-77. Anderson, S.A., Marin, O., Horn, C., Jennings, K., and Rubenstein, J. L. (2001). Distinct cortical migrations from the medial and lateral ganglionic eminences. Development 128, 353-363. Baraban, S. C., and Schwartzkroin, P. A. (1995). Electrophysiology of CA1 pyramidal neurons in an animal model of neuronal migration disorders: Prenatal methylazoxymethanol treatment. Epilepsy Res. 22, 145-156. Baraban, S. C., Wenzel, H.J., Hochman, D. W., and Schwartzkroin, P. A. (2000). Characterization of heterotopic cell clusters in the hippocampus of rats exposed to methylazoxymethanol in utero. Epilepsy Res. 39, 87-102. Bix, G.J., and Clark, G. D. (1998). Platelet-activating factor receptor stimulation disrupts neuronal migration in vitro.9(. Neurosci. 18, 307-318. Borgatti, R., Triulzi, F., Zucca, C., Piccinelli, P., Balottin, U., Carrozzo, R., and Guerrini, R. (1999). Bilateral perisylvian polymicrogyria in three generations. Neurology 52, 1910-1913. Bornemann, A., Aigner, T., and Kirchner, T. (1997). Spatial and temporal development of the gliovascular tissue in type II lissencephaly. Acta Neuropathol. (Bed.) 93, 173-177. Borrell, V., Del Rio,J. A., Alcantara, S., Derer, M., Martinez, A., D'Arcangelo, G., Nakajima, K., Mikoshiba, K., Derer, P., Curran, T., and Soriano, E. (1999a). Reelin regulates the development and synaptogenesis of the layer-specific entorhino-hippocampal connections. J. Neurosci. 19, 1345-1358. Borrell, V., Ruiz, M., Del Rio@ A., and Soriano, E. (1999b). Development of commissural connections in the hippocampus of reeler mice: Evidence of an inhibitory influence of Cajal-Retzius cells. Exp. Neurol. 156, 268-282. Buzsaki, G., Smith, A., Berger, S., Fisher, L.J., and Gage, F. H. (1990). Petit mal epilepsy and parkinsonian tremor: Hypothesis of a common pacemaker. Neuroscience 36, 1-14. Caviness, V. (1982). Neocortical histogenesis in normal and reeler mice: A developmental study based on [3H]thymidine autoradiography. Dev. Brain Res. 4, 293-302. Caviness, V. S., and Rakic, P. (1978). Mechanisms of cortical development: A view from mutations in mice. Annu. Rev. Neurosci. 1,297-326. Chert, Z. F., Schottler, F., Bertram, E., Gall, C. M., Anzivino, M.J., and Lee, K. S. (2000). Distribution and initiation of seizure activity in a rat brain with subcortical band heterotopia. Epilepsia 41,493-501. Chevassus-au-Louis, N., and Represa, A. (1999). The right neuron at the wrong place: Biology of heterotopie neurons in cortical neuronal migration disorders, with special reference to associated pathologies. Cell Mol. Life Sci. 55, 1206-1215.
BRAIN MALFORMATIONS, EPILEPSY,AND IS
349
Chevassus-Au-Louis, N., Jorquera, I., Ben-Ari, Y., and Represa, A. (1999). Abnormal connections in the malformed cortex of rats with prenatal treatment with methylazoxymethanol may support hyperexcitability. Dev. Neurosci. 21,385-392. Chevassus-au-Louis, N., Baraban, S. C., Gaiarsa, J. L., and Ben-Ari, Y. (1999). Cortical malformations and epilepsy: New insights from animal models. Epilepsia 40, 811-821. Chevassus-Au-Louis, N., Rafiki, A.,Jorquera, I., Ben-Ari, Y., and Represa, A. (1998). Neocortex in the hippocampus: An anatomical and functional study of CA1 heterotopias after prenatal treatment with methylazoxymethanol in rats. J. Comp. Neurol 394, 520-536. Chugani, H. T., Rintahaka, P.J., Shewmon, D.A. (1994). Ictal patterns of cerebral glucose utilization in children with epilepsy. Epilepsia 35, 813-822. Clark, G. D., Bix, G.J., Shinoya, A., Hirotsune, S., Ross, M. E., Kholmanskih, P., Letourneau, P., McNeil, R. S., Abu-Issa, R., and Wynshaw-Boris, A. (2002). Platelet activating factor rescues neurite and migration defects in Pafahlbl(Lisl)-deficient neurons. Submitted. Coulter, D. A., and Lee, C.J. (1993). Thalamocortical rhythm generation in vitro: Extra- and intracellular recordings in mouse thalamocortical slices perfused with low Mg2+ medium. Brain Res. 631,137-142. D'Arcangelo, G., Miao, G. G., Chen, S. C., Soares, H. D., Morgan,J. I., and Curran, T. (1995). A protein related to extracellular matrix proteins deleted in the mouse mutant reeler. Nature 374, 719-723. des Portes, V., Pinard,J. M., Billuart, P., Vinet, M. C., Koulakoff, A., Carrie, A., Gelot, A., Dupuis, E., Motte, J., Berwald-Netter, Y., Catala, M., Kahn, A., Beldjord, C., and Chelly, J. (1998). A novel CNS gene required for neuronal migration and involved in X-linked subcortical laminar heterotopia and lissencephaly syndrome. Cell 92, 51-61. Di Luca, M., Caputi, A., Cattabeni, F., De Graan, P. N., Gispen, W. H., Raiteri, M., Fassio, A., Schmid, G., and Bonanno, G. (1997). Increased presynaptic protein kinase C activity and glutamate release in rats with a prenatally induced hippocampal lesion. Eur. J. Neurosci. 9, 472-479. Dobyns, W. B. (2001). Lissencephaly: The clinical and molecular genetic basis of diffuse malformations of neuronal migration. In "Neuronal Migration Disorders" (P. G. Barth, ed). McKeith, London. Dobyns, W.B., Andermann, E., Andermann, F., Czapansky-Beilman, D., Dubeau, F., Dulac, O., Guerrini, R., Hirsch, B., Ledbetter, D. H., Lee, N. S., Motte,J., Pinard,J. M., Radtke, R. A., Ross, M. E., Tampieri, D., Walsh, C. A., and Truwit, C. L. (1996). X-linked malformations of neuronal migration. Neurology 47, 331-339. Dobyns, W. B., Carrozzo, R., and Ledbetter, D. H. (1994). Frequent deletions of the LIS1 gene in classic lissencephaly. Annu. Neurol. 36, 489-490. Dobyns, W. B., Reiner, O., Carrozzo, R., and Ledbetter, D. H. (1993). Lissencephaly: A human brain malformation associated with deletion of the LIS1 gene located at chromosome 17p13. JAMA 270, 2838-2842. Dobyns, W. B., and Truwit, C. L. (1995). Lissencephaly and other malformations of cortical development: 1995 update. Neuropediatrics 26, 132-147. Efimov, V. P., and Morris, N. R. (2000). The LISl-related NUDF protein ofAspergiUus nidulans interacts with the coiled-coil domain of the N U D E / R O l l protein. J. Cell Biol. 150, 681 - 688. Feng, G., Olson, E. C., Stukenbuerg, P. T., Flanagan, L. A., Kirschner, M. W., and Walsh, C. A. (2000). LIS1 regulates CNS lamination by interacting with mNudE, a central component of the centrosome. Neuron 28, 653-664. Fleck, M.W., Hirotsune, S., Gambello, M.J., Phillips-Tansey, E., Suares, G., Mervis, R. F., Wynshaw-Boris, A., and McBain, C.J. (2000). Hippocampal abnormalities and enhanced excitability in a murine model of human lissencephaly. J. Neurosci. 20, 2439-2450.
350
~a. ELIZABETHROSS
Garcia-Higuera, I., Fenoglio,J., Li, Y., Lewis, C., Panchenko, M. P., Reiner, O., Smith, T. F., and Neer, E.J. (1996). Folding of proteins with WD-repeats: Comparison of six members of the WD-repeat superfamily to the G protein beta subunit. Biochemistry 35, 13985-13994. Gelot, A., Billette de Villemeur, T., Bordarier, C., Ruchoux, M. M., Moraine, C., and Ponsot, G. (1995). Developmental aspects of type II lissencephaly: Comparative study of dysplastic lesions in fetal and post-natal brains. Acta Neuropathol. (Berl.)89, 72-84. Gleeson,J. G., Allen, K. M., Fox,J. W., Lamperti, E. D., Berkovic, S., Scheffer, 1., Cooper, E., Dobyns, W. B., Minnerath, S., Ross, M. E., and Walsh, C. A. (1998). doublecortin, a brainspecific gene mutated in human X-linked lissencephaly and double cortex syndrome, encodes a putative signaling protein. Cell 92, 63-72. Gleeson,J. G., Minnerath, S., Allen, K.M., Fox, J. W., Hong, S., Berg, M., Kuzniecky, R., Reitnauer, P.J., Borgatti, R., Mira, A.P., Guerrini, R., Holmes, G., Rooney, C., Berkovic, S., Scheffer, I., Cooper, E. C., Ricci, S., Cusmai, R., Crawford, T. O., Brown, L., Anderman, E., Wheless, J., Dobyns, W.B., Ross, M.E., and Walsh, C.A. (1999). Characterization of mutations in the gene doublecortin in patients with double cortex syndrome. Ann. Neurol. 45, 146-153. Guerreiro, M.M., Andermnann, E., Guerrini, R., Dobyns, W.B., Kuzniecky, R., Silver, K., Van Bogaert, P., Gillain, C., David, P., kanbrosetto, G., Rosati, A., Bartolomei, F., Parmeggiani, A., Paetau, R., Salonen, O., Ignatius,J., Borgatti, R., Zucca, C., Bastos, A. C., Palmini, A., Fernandes, W., Montenegro, M. A., Cendes, F., and Andermann, F. (2000). Familial perisylvian polymicrogyria: A new familial syndrome of cortical maldevelopment. Ann. Neurol. 48, 39-48. Guerrini, R., Canapicchi, R., and Dobyns, W. B. (1999). Epilepsy and malformations of the cerebral cortex. Neurologia 14 (Suppl 3), 32-47. Guerrini, R., Barkovich, A.J., Sztriha, L., and Dobyns, W. B. (2000). Bilateral frontal polymicrogyria: A newly recognized brain malformation syndrome. Neurology 54, 909-913. Hayashi, M., Itoh, M., Araki, S., Kumada, S., Tanuma, N., Kohji, T., Kohyama, J., Iwakawa, Y., Satoh,J., and Morimatsu, Y. (2000). hnmunohistochemical analysis of brainstem lesions in infantile spasms. Neuropathology 20, 297-303. Hirotsune, S., Fleck, M. W., Gambello, M.J., Bix, G. J., Chen, A., Clark, G. D., Ledbetter, D. H., McBain, C.J., and Wynshaw-Boris, A. (1998). Graded reduction ofPafhhlbl (Lisl) activity results in neuronal migration defects and early embryonic lethality. Nature Genet. 19, 333-339. Ho, Y. S., Swenson, L., Derewenda, U., Serre, L., Wei, Y., Dauter, Z., Hattori, M., Adachi, T., Aoki,J., Arai, H., Inoue, K., and Derewenda, Z. (1997). Brain acetylhydrolase that inactivates platelet-activating factor is a G-protein-like trimner. Nature 385, 89-93. Hong, S.E., Shugart, Y.Y., Huang, D.T., Shahwan, S.A., Grant, P.E., Hourihane,J. O., Martin, N. D., and Walsh, C. A. (2000). Autosomal recessive lissencephaly with cerebellar hypoplasia is associated with human RELN mutations. Nature Genet. 26, 93-96. Jacobs, IL M., Hwang, B.J., and Prince, D. A. (1999). Focal epileptogenesis in a rat model of polymicrogyria.J. Neurophysiol. 81,159-173. Johnston, M.V., and Coyle,J. T. (1979). Histological and neurochemical effects of fetal treatment with methylazoxymethanol on rat neocortex in adulthood. Brain Res. 170, 135-155. Kato, M., Takizawa, N., Yamada, S., lto, A., Honma, T., Hashimoto, M., Saito, E., Ohta, T., Chikaoka, H., and Hayasaka, K. (1999). Diffuse pachygyria with cerebellar hypoplasia: A milder form of microlissencephaly or a new genetic syndrome? Ann. Neurol. 46, 660-663. Kitagawa, M., Umezu, M., Aoki, J., Koizumi, H., Arai, H., and Inoue, K. (2000). Direct association of LIS1, the lissencephaly gene product, with a mammalian homologue of a fungal nuclear distribution protein, rNUDE. FEBS Lett. 479, 57-62.
BRAIN MALFORMATIONS, EPILEPSY,AND IS
351
Lee, K. S., Collins,J. L., Anzivino, M.J., Frankel, E. A., and Schottler, F. (1998). Heterotopic neurogenesis in a rat with cortical heterotopia, jr Neurosci. 18, 9365-9375. Lee, K.S., Schotder, F., Collins,J. L., Lanzino, G., Couture, D., Rao, A., Hiramatsu, K., Goto, Y., Hong, S.C., Caner, H., Yamamoto, H., Chen, Z.F., Bertram, E., Berr, S., ()mary, R., Scrable, H.,Jackson, T., Goble,J., and Eisenman, L. (1997). A genetic animal model of human neocortical heterotopia associated with seizures. J. Neurosci. 17, 6236-6242. Matsumoto, N., Leventer, R.J, Kuc, J. A., Mewborn, S. K., Dudlicek, L. L., Ramocki, M. B., Pilz, D. T., Mills, P. L., Das, S., Ross, M. E., Ledbetter, D. H., and Dobyns, W. B. (2001). Mutation analysis of the DCX gene and genotype/phenotype correlation in subcortical band heterotopia. Eur.J. Hum. Genet. 9, 5-12. McCormick, D. A., and Bal, T. (1997). Sleep and arousal: Thalamocortical mechanisms. Annu. Rev. Neurosci. 20, 185-215. Mendez, M., and Radtke, R.A. (2001). Interactions between sleep and epilepsy. J. Clin. Neurophysiol. 18, 106-127. Mischel, P. S., Nguyen, L P., and Vinters, H. V. (1995). Cerebral cortical dysplasia associated with pediatric epilepsy. Review of neuropathologic features and proposal for a grading system.J. Neuropathol. Exp. Neurol. 54, 137-153. Pilz, D. T., Matsumoto, N., Minnerath, S., Mills, P., Gleeson,J. G., Walsh, C. A., Barkovich,J., Dobyns, W. B., Ledbetter, D. H., and Ross, M. E. (1998). LIS1 and XLIS/doublecortin mutations cause most human classical lissencephaly, but different patterns of malformation. Hum. Mol. Genet. 7, 2029-2037. Pleasure, S.J., Anderson, S., Hevner, R., Bagri, A., Marin, O., Lowenstein, D. H., and Rubenstein,J. L. (2000). Cell migration from the ganglionic eminences is required for the development of hippocampal GABAergic interneurons. Neuron 28, 727-740. Redecker, C., Luhmann, H.J., Hagemann, G., Fritschy, J. M., and Witte, O. W. (2000). Differential downregulation of GABAA receptor subunits in widespread brain regions in the freeze-lesion model of focal cortical malformations. J. Neurosci. 20, 5045-5053. Ross, M. E., and Walsh, C.A. (2001). Human brain malformations and their lessons for neuronal migration. Annu. Rev. Neu~vsci. 24, 1041-1070. Ross, M. E., Swanson, K., and Dobyns, W. B. (2001). Lissencephaly with cerebellar hypoplasia (LCH): A heterogeneous group of cortical malformations. Neuropediatrics 32, 256-263. Ross, M.E., Allen, K.M., Srivastava, A.K., Featherstone, T., Gleeson, J.G., Hirsch, B., Harding, B. N., Andermann, E., Abdullah, R., Berg, M., Bielman, D. C., Flanders, D., Guerrini, R., Motte,J., Puche Mira, A., Scheffer, I., Berkovic, S., Scaravilli, F., King, R. A., Ledbetter, D. H., Schlessinger, D., Dobyns, W. B., and Walsh, C. A. (1997). Linkage and physical mapping of X-linked lissencephaly/SBH (XLIS): A gene causing neuronal migration defects in human brain. Hum. Mo&c. Genet. 6, 555-562. Sapir, T., Cahana, A., Seger, R., Nekhai, S., and Reiner, O. (1999). LISI is a microtubuleassociated phosphoprotein. Enr. J. Biochem. 265, 181 - 188. Sapir, T., Elbaum, M., and Reiner, O. (1997). Reduction of microtubule catastrophe events by LIS1, platelet-activating factor acetylhydrolase subunit. EMBOJ. 16, 6977-6984. Sasaki, S., Shionoya, A., Ishida, M., Gambello, M.J., Yingling, J., Wynshaw-Boris, A., and Hirotsune, S. (2000). A LIS1/NUDEL/cytoplasmic dynien heavy chain complex in the developing and adult nervons system. Neuron 28, 681-696. Schiffmann, S. N., Bernier, B., and Goffinet, A. M. (1997). Reelin mRNA expression during mouse brain development. Eur.J. Neurosci. 9, 1055-1071. Schottler, F., Couture, D., Rao, A., Kahn, H., and Lee, K. S. (1998). Subcortical connections of normotopic and heterotopic neurons in sensory and motor cortices of the tish mutant rat.J. Comp, Neurol. 395, 29-42.
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Sheppard, A. M., and Pearlman, A. L. (1997). Abnormal reorganization of preplate neurons and their associated extracellular matrix: An early manifestation of altered neocortical development in the reeler mutant mouse.J. Comp. Neurol. 378, 173-179. Smith, D. S., Niethammer, M., Ayala, R., Zhou, Y., Gambello, M.J., Wynshaw-Boris, A., and Tsai, L. H. (2000). Regulation of cytoplasmic dynein behaviour and microtubule organization by mammalian Lisl. Nature Cell Biol. 2, 767-775. Sweeney, K.J., Clark, G. D., Prokscha, A., Dobyns, W. B., and Eichele, G. (2000). Lissencephaly associated mutations suggest a requirement for the PAFAH1B heterotrimeric complex in brain development. Mech. Dev. 92, 263-271. Sztriha, L., and Nork, M. (2000). Bilateral frontoparietal polymicrogyria and epilepsy. Pediatr. Nemvl. 22, 240-243. Taylor, K. R., Holzer, A. K., Bazan,J. F., Walsh, C. A., and Gleeson,J. G. (2000). Patient mutations in doublecortin define a repeated tubulin-binding domain. J. Biol. Chem. 275, 34442-34450. Tominaga, I., Yanai, K., Kashima, H., Kato, Y., Sekiyama, S., Yokochi, A., and Miura, I. (1986). An anatomo-clinical case of sequelae of acute encephalopathy: Infantile spasm with hypsarrhythmia. Rev. Neurol. (Paris) 142, 524-529. Woo, T. U., and Finlay, B. L. (1996). Cortical target depletion and ingrowth ofgeniculocortical axons: Implications for cortical specification. Cereb. Cortex. 6, 457-469. Xiang, X., Osmani, A. H., Osmani, S.A., Xin, M., and Morris, R. (1995). NudF, a nuclear migration gene in Aspergillus nidulans, is similar to the human LIS-1 gene required for neuronal migration. Mol. Biol. Cell 6, 297-310. Zhang, Y. F., and Coulter, D. A. (1996), Anticonvulsant drug effects on spontaneous thalamocortical rhythms in vitro: Phenytoin, carbamazepine, and phenobarbital. Epilepsy Res. 23, 55-70.
M. Elizabeth Ross Departmentof Neurology and Neuroscience Weill Medical College of Cornell University New York, New York 10021
I. I n t r o d u c t i o n II. Cortical M a l f o r m a t i o n : D i s o r d e r s o f N e u r o n a l Position
III. Neuronal Migration and Cortical Malformation A. Lissencephaly B. Chromosome 17-LinkedLissencephaly (LIS1) C. LIS1 Deficiencyin Mice Produces a Neuronal Migration Defect and Seizure Propensity D. X-LinkedLissencephaly E. LIS1 and DCX Gene Function and Possible Mechanisms of Epileptogenesis IV. Lissencephalywith Cerebellar Hypoplasia (LCH): reeler Mutant as Example V. Prenatal Chemical Exposure as a Model for Diffuse Heterotopia VI. Syndromesof Cortical Disorganization VII. DysplasiaAssociatedwith Altered Neurogenesis: The tish Rat Model VIII. MalformationsNot Yet Classified IX. Summary References
I. Introduction
Brain malformations encompass a broad spectrum of etiologies from defects in primary neurogenesis to neuronal migration and p r o g r a m m e d cell death. Many, t h o u g h certainly not all, of these malformations are associated with seizures. Some, like the neuronal migration defects, display an u n c o m m o n l y high incidence of infantile spasms. While this may simply reflect the severity of the malformation, it probably also speaks to the mechanisms generating this dramatic seizure type. Recent identification of several genes responsible for cortical malformations, and the promise of the isolation of other genes functioning in these pathways, will likely lead to better understanding of epileptogenic factors producing early onset seizures. These insights will undoubtedly contribute to improved strategies for treatment, even in children without developmental defects that are detectable by brain imaging. What then are the characteristics of out of place or heterotopic neurons that might make them p r o n e to seize? Pieces of the puzzle have been gleaned from the rapidly advancing area of cortical INTERNATIONALREVIEWOF NEUROBIOLOGY,VOL. 49
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malformations and are discussed from the perspective of their relationship to epilepsy.
Ih Cortical Malformation: Disorders of Neuronal Position
Clinical syndromes involving cerebral cortical malformation are the end results of gestation, recognized by the abnormal position of neural cells. This static picture can only suggest the mechanism by which the disorganized brain developed. Therefore, classification of a malformation as a neuronal migration syndrome must be tentative until the responsible gene defect is identified and the mechanism has been tested in animal models. T h e movement of cells from their origins in the ventricular zone may be impaired in a n u m b e r of ways. Primary neurogenesis or cell n u m b e r may be altered by virtue of disturbed cell proliferation, fate determination, and p r o g r a m m e d cell death. The failure of particular cells to differentiate or improper timing of the birth of a n e u r o n or glial cell may alter the fate and positional information of others in the region. The migration of cells from the ventricular zone may be curtailed by interference with the mechanical motors and cytoskeletal dynamics of the cells, as implicated in several lissencephaly syndromes. T h e r e may be a primary defect in the function of the radial glial scaffold on which neurons migrate, leading to altered cellular movements, as suspected in polymicrogyria. Alternatively, the molecular signals to initiate movement guide the cell in its migration and inform it that the final position has been reached may be altered. Once the position of the cell body is established, assembly and consolidation of n e u r i t e / a x o n a l projections and synapses are further established and refined, in part accounting for the survival of neurons and their associated glia. Thus, the events that organize brain structure include neurogenesis and early mid- and late migration, as well as axon projection and guidance, all of which are likely to overlap. Consideration will begin with the migration syndromes, as this area has seen the greatest progress in the last several years. It is becoming increasingly evident that more than the m o v e m e n t o f cells is hindered in the gene defects that impact cell movement.
IIh Neuronal Migration and Cortical Malformation
Thus far, over a dozen molecules that are peculiar to neuronal migration in brain have been reported (reviewed in Ross and Walsh,
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2001). At least half were identified first from clinical human studies. The rest were discovered either in mouse or in Drosophila and then mammalian relevance was confirmed in mouse. A partial classification of neuronal migration syndromes is outlined below and the reader is referred to a more extensive discussion for details (Ross and Walsh, 2001). The identified gene that is mutated in several of these syndromes appears in italics. By far the largest class of neuronal migration syndromes are the lissencephalies (from "lisos" smooth + "encephalos" brain). Meticulous phenotype examination, and, for some syndromes, genotype-phenotype correlation, has enabled grouping these malformations in a way that facilitates the discovery of molecular pathways governing neuronal migration. Lissencephaly Classical lissencephaly (LIS) Autosomal dominant (LIS1/PAFAH1BI), X-linked (DCX) Double cortex or subcortical band heterotopia (DC/SBH) X-linked (DCX) LIS with cerebellar hypoplasia (LCH) Autosomal dominant ( LIS1/PAFAH1B1), X-linked (DCX) Autosomal recessive (RELN) Cobblestone complex (cobblestone lissencephaly) Walker Warburg syndrome (WWS) Muscle eye brain disease (MEB) Fukuyama congenital muscular dystrophy Autosomal recessive (FCMD) Cobblestone complex only (CCO) Other migration disorders Bilateral periventricular nodular heterotopia (BPNH) X-linked (FLN1) Zellweger syndrome Autosomal (PEX2, PXRI ) Kallmann syndrome X-linked ( KAL1)
A. LISSENCEPHALY This term encompasses a range of simplified cortex from a total absence of cortical convolutions (agyria) to broadened gyri (pachygyria) with an abnormally thick cortex (typically 10-20 mm in classical LIS compared to 2.5-4 mm in normal) (Dobyns and Truwit, 1995, Dobyns et aL, 1996)
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FIG. 1. MRI appearance of lissencephaly (LIS) a n d subcortical b a n d heterotopia (SBH). (A) Classical LIS, grade 1 is the most severe form with complete loss of cortical convolutions a n d thickening of the cortical gray matter (here, ~ 2 0 m m c o m p a r e d to 2 - 4 m m in n o r m a l cortex). Grade 1 LIS reveals n o malformation gradient so that LIS1 or DCX mutations c a n n o t be distinguished in this most severe form. ( B ) A moderately thick subcortical b a n d (also known as double-cortex, DC) is indicated by the double-headed arrow. T h e circumferential band underlies a true cortex with relatively preserved gyri a n d is separated from the true cortex by a rim of white matter (dark on this T2-weighted image). (C a n d D). Images of an XLIS son (C) a n d his SBH m o t h e r (D). T h e mutation in DCX was confirmed by the sequence a n d predicts a single a m i n o acid substitution. Classical LIS, grade 4 in the son shows m a r k e d thickening of the cortical mantle (double-headed arrow in C) a n d pachygyria (a b r o a d e n e d gyrus is designated in the bracket). T h e SBH in the m o t h e r (D) is thin a n d presents only in the frontal pole (arrows). T h e pattern is consistent with a DCX mutation that partially inactivates the protein.
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(Fig. 1). This spectrum intersects with double cortex [DC, also known a subcortical band heterotopia (SBH)] in which neurons are partially h u n g up in their migrations to reside as a disorganized band of neurons in the white matter beneath a relatively normal cortex. Two genes, LIS1 and DCX, account for the majority of classical LIS (Pilz et aL, 1998). However, other nonclassical forms of LIS are becoming recognized for which L I S I and D C X mutations account for only a small proportion of cases (Ross et al., 2001; Dobyns, 2001). Clinical features c o m m o n to classical forms of LIS include p r o f o u n d mental retardation, mixed hypotonia and spasticity, and seizures in 80% within the first postnatal year and virtually all by 5 years (Guerrini et al., 1999).
B. CHROMOSOME17-LINKEDLISSENCEPHALY(IriS1) T h e first LIS syndrome delineated is Miller-Dieker syndrome (MDS), which is manifested by severe LIS and characteristic facial abnormalities (Dobyns et al., 1993). Chromosomal analysis shows visible deletions of 17p13.3 in over 90% of MDS patients, suggesting that this is a contiguous gene deletion syndrome in which the characteristic facial features of MDS are caused by the involvement of loci neighboring LISI. Isolated lissencephaly sequence (ILS) consists of LIS with a relatively normal facial appearance. Fluorescence in situ hybridization (FISH) studies show deletions of c h r o m o s o m e 17pl 3.3 in about 40% of ILS children, (Dobyns et al., 1994). T h e LIS1 gene encodes a noncatalytic subunit of platelet-activating factor (PAF) acetylhydrolase (Pafah), or P a f a h l b l , and is part of a Gprotein-like (0tl/0t2)l] trimer (Ho et al., 1997). Hereafter, this P a f a h l b l subunit is referred to as Lisl protein. T h e Pafah enzyme is known to regulate PAF, a potent lipid first messenger. Lisl is a soluble protein with seven WD40 repeats forming a seven-bladed propeller-like structure inw)lved in p r o t e i n - p r o t e i n interactions (Garcia-Higuera et al., 1996). Lisl binds tubulin and reduces microtubule catastrophe in vitro, suggesting that it may stabilize the microtubule cytoskeleton (Sapir et al., 1997, 1999). Significantly, Lisl is a highly conserved homologue of the NUDF protein in Aspergillus, where it is required for nuclear translocation through interaction with a dynein m o t o r (Xiang et al., 1995). When overexpressed in mammalian cells and neurons, Lisl has been shown to bind cytoplasmic dynein and to affect microtubule organization (Smith et al., 2000). In this model, Lisl promotes the peripheral, plus end-directed movement of microtubule segments by dynein motors that are attached to stable microtubules, oriented with minus ends a n c h o r e d at the centrosome and plus
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ends at the peripheral membrane skeleton. In conditions of low Lisl levels, dynein motor activity is reduced and microtubule segments accumulate near the nucleus. In addition, Lisl protein binds to homologues of the Aspergillus NUDE protein, which are also involved in translocation of the nucleus (Feng et al., 2000; Sasaki et al., 2000).
C. LIS1 DEFICIENCYIN MiCE PRODUCESA NEURONALMIGRATIONDEFECTAND SEIZUREPROPENSITY
Although homozygous null mice die in early embryogenesis, mice heterozygous for Lisl survive and reveal abnormalities primarily in the cerebral and olfactory cortex, but also in the hippocampus and cerebellum, due to the impaired, slower migration of neurons (Hirotsune et al., 1998, Clark etal., 2001). Interestingly, although these mice reveal migration defects, the malformation is distinct from the prototype neuronal migration mouse model reeler (see later) in that the L i s l + / - cortex is not inverted (e.g, layer V pyramidal cells are not displaced to the superficial layers), and the hippocampus and cerebellum are far less disorganized than in reel~.
While spontaneous seizures have been observed only rarely in L i s l - / + mice, anatomical and electrophysiological data demonstrate a number of hippocampal abnormalities (Fleck etal., 2000). There is potassiumdependent bursting activity in the hippocampus at 6.5 mM K+. Long-term potentiation is present, but its dynamic range of frequency facilitation of mossy fiber transmission is altered. This hyperexcitability is demonstrable at the level of Schaffer collaterals (CA1). There is a reduced transmission of mossy fibers in the CA3 region. In terms of hippocampal structure, there is moderate disorganization, particularly of the CAB region (Hirotsune et aL, 1998). Assessed by Golgi staining, there is stunting of the neurites of heterotopic pyramids, with normal appearing normotopic pyramidal neurons (Fleck et al., 2000).
D. X-LINKEDLISSENCEPHALY
XLIS refers to the syndrome of classical LIS in hemizygous males and double cortex in heterozygous females (Fig. 1). The clinical features and responsible gene have been delineated (Dobyns et al., 1996; Ross et al., 1997; Gleeson et al., 1998; des Portes et al., 1998). The clinical manifestations of XLIS are very similar to those of chromosome 17-associated LIS. DC/SBH
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is characterized by a symmetric band of gray matter that may be circumferential or frontal when seen with D C X mutations, located just beneath the cortex and separated from it by a thin band of white matter (Figs. 1B and 1D). The phenotype arises in D C X females because r a n d o m X inactivation will turn o f f t h e defective allele in, on average, 50% of neurons. Thus, only a fraction of cortical neurons will be hung up in their migrations (Ross et al., 1997). The thickness of the band heterotopia is determined by the severity of the D C X mutation, although unfavorable skewing of X inactivation may account for rare severe phenotypes in girls as well (Matsumoto et aL, 2001; Ross et al., 1997).
E. LIS1 ANDD C X GrNE FvNca-ion AnD POSSmLEMECHANISMSOF EPILEPTOGENESIS
Indicators thus far suggest an important role of Lisl and Dbcn proteins in the intrinsic motility of neurons. Lisl and Dbcn proteins have been implicated in the regulation of microtubule dynamics by virtue of binding tubulin and reducing microtubule catastrophe (Sapir et al., 1997, 1999; Gleeson et al., 1999; Taylor et al., 2000). Lisl may also have a role in signal transduction. Consistent with this notion, excess PAF, a potent first messenger and substrate for Pafah, collapses growth cones of neurons in dissociated cell culture (Bix and Clark, 1998). Clark and colleagues have demonstrated, in a yeast-two-hybrid system, that point mutations found in LIS patients interfere with the association of Lisl for the 29- and 30kD a subunits of Pafah (Sweeney et al., 2000). The fact that all patient mutations in L i s l examined in this system interfere with the binding to Lisl protein suggests that the heterotrimeric Pafah complex may be involved in regulating migration. Further evidence indicates that PAF interferes with the binding of Lisl to the 30-kD a subunit and that PAF rescues the migration phenotype of Lisl haploinsufficient neurons. This suggests a model in which the role of PAF is to mobilize the Lisl protein from Pafah to exert a downstream effect on migration. In such a model, PAF could rescue neurons expressing low levels of Lisl by displacing more Lisl protein from the complex (Clark etal., 2001). These downstream, non-Pafah-dependent effects of Lisl involve several additional p r o t e i n - p r o t e i n interactions. For example, a Lisl interaction with dynein heavy chain has been described that recapitulates the interaction of the homologous proteins previously demonstrated in Aspergillus (Smith et al., 2000). In addition, interactions have been identified in yeast two-hybrid screens between Lisl and NudE, thought to be involved in modulating dynein heavy chain interactions (Efimov and Morris, 2000; Kitagawa et al., 2OOO).
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FIG. 2. Aspects of Lisl and Dbcn function that could be associated with epileptogenesis. A scheme of the neuronal growth cone, with microtubules extending up to but not invading the leading edge of the growth cone that relies on an actin-based cytoskeleton. Lisl is known to be released from the Pafah complex upon binding to its substrate, PAF. Molecular partners for Lisl include binding of microtubules, dynein, and NudE, all thought to promote nuclear translocation and movement of organelles along the microtubule-based cytoskeleton. Interaction with microtubules a n d / o r protein motors such as dynein could position Lisl to influence protein transport into the leading edge and later established neurites. This could have effects on the turnover and composition of neurotransmitter receptors and other membrane components. Signal transduction aspects of Lisl function could influence dynamics of the actin-based cytoskeleton via small GTPases Rho, Rac, and Cdc42. Dbcn has been shown to bind microtubules and may modulate Lisl interactions with that cytoskeletal compartment. Multiple putative phosphorylation sites on Dbcn position the protein for a role in signal transduction as well.
These examples suggest several scenarios in which altered Lisl or Dbcn could affect neuronal excitability, which can be tested in genetically a l t e r e d m o u s e m o d e l s (Fig. 2). First, L i s l o r D b c n d y s f u n c t i o n c o u l d a l t e r t h e i n t r i n s i c f u n c t i o n o f t h e cell a n d its e x c i t a b i l i t y . T h i s c o u l d b e
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accomplished by disruption of the microtubule-based cytoskeleton a n d / o r dynein (or other) protein motor complexes to disturb protein trafficking within the cell. This could impact neurotransmitter availability (transport of synaptic vesicles) or receptor turnover in some neuronal types. Alternatively, disturbances in the relationship between the actin-based and the microtubule-based cytoskeleton could impact intracellular signaling, neurite extension, and synaptogenesis. Second, reduced neuronal migration might impair neuronal movement differentially according to the distance that must be covered. For example, the majority of GABAergic interneurons in the cerebral cortex derive from tangential migrations of young neurons from the ganglionic eminence (Anderson et al., 2001). Therefore, the Lisl-deficient neocortex may contain a relative paucity of inhibitory interneurons. There is some evidence against this possibility, however, as GABAergic interneurons of the hippocampus that also derive from the ganglionic eminence are not reduced in number but are displaced in the Lisl-deficient hippocampus (Pleasure et al., 2000; Fleck et al., 2000). Finally, because neurite outgrowth is slowed in cultured Lisl-deficient neurons, it is possible that longer projections, such as thalamocortical pathways, could be disturbed in classical LIS (Hirotsune et al., 1998). These deeper structures are not typically considered in the genesis of epilepsy. However, the importance of thalamocortical relays has been recognized in the generation of certain seizure types, including absence, spike, and slow wave epilepsy (Coulter and Lee, 1993; Zhang and Coulter, 1996). In these models, abnormal synchronization and rhythmicity seen in these electroencephalographic patterns are thought to arise from disturbances in a sort of "pacemaker" function of thalamoreticular and thalamocortical circuits (Buzsaki et al., 1990). Indeed, thalamocortical circuits have been implicated in the genesis of infantile spasms as well (see chapters by Olivier Dulac and colleagues, David A. McCormick, Csaba and Juhftsz and colleagues; Hayashi et al., 2000; Chugani et al., 1994; Tominaga et al., 1986). In addition to the role of these pathways in abnormal synchronous patterns, thalamoreticular and thalamocortical relays are responsible for setting up rhythmic circuits that control sleep (reviewed in McCormick and Bal, 1997). Alterations in this thalamocortical circuit could be consistent with the generation of interictal hypsarhythmia in West syndrome. Involvement of thalamocortical relays in West syndrome and the association of these relays to sleep could in part account for the frequent clinical observation of spasms precipitated upon waking from sleep (Mendez and Radtke, 2001). It might also explain the association of infantile spasms with an age of onset that precedes and overlaps the establishment of normal sleep architecture.
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IV. Ussencephaly with Cerebellar Hypoplasia (LCH): reeler Mutant as Example
Lissencephaly with cerebellar hypoplasia is a term used to describe a heterogeneous group of malformations in which a simplified, thickened cerebral cortex appears along with an underdeveloped cerebellum, as detected by magnetic resonance imaging (MRI). This group is only now evolving a nomenclature and comprises six broad classes, LCH a-f, that are grouped according to distinguishing features (Ross et al., 2001). Within these broader groups, three causative genes have been identified and a n o t h e r locus described. Syndromes of LCH have in c o m m o n a lissencephaly spectrum of agyria-pachygyria, with mildly ( 5 - 1 0 mm) or markedly (10-20 mm) thickened gray matter. Cerebellar involvement in LCH may be mild, with predominantly midline hypoplasia seen with LIS1 or D C X mutations (LCHa) to severe cerebellar defects with hypoplasia of the cerebellar hemispheres and abnormal or absent foliation (LCHb or d) (Ross et al., 2001; Kato et al., 1999). A mutation has been identified in the h u m a n reelin (RELN) gene that produces type LCHb. This distinctive pattern possesses a moderately thickened cortex ( 5 - 1 0 mm) and pachygyria, markedly abnormal hippocampal formation, and severe cerebellar hypoplasia with absent folia (Hong et al., 2000). This gene was first recognized in a spontaneously arising ataxic mouse mutant with a strikingly disorganized brain (Caviness, 1982). R E L N encodes a large 388-kDa protein containing eight EGF repeats and is secreted by Cajal-Retzius cells of the embryonic preplate, marginal zone, the cerebellar EGL, and pioneering cells of the hippocampus (D'Arcangelo et al., 1995; Schiffmann et al., 1997; Sheppard and Pearlman, 1997). In addition, R E L N mutations can lead to abnormalities of neurite outgrowth and synaptogenesis (Borrell et al., 1999a). Interestingly, while many malformations are associated with epilepsy, this is not universally the case and a spectrum of seizure severity is seen (Guerrini et al., 1999). For example, in nonclassical LCHb associated with a R E L N mutation, seizures are not a c o m m o n clinical feature, despite marked abnormalities in hippocampal formation (al Shahwan et al., 1995; Caviness and Rakic, 1978) and influences on neurite outgrowth and axon pathfinding (Borrell etal., 1999a). This suggests that the mechanism by which displaced neurons become seizure-prone may have more to do with intrinsic properties of the cells than position or connectivity.
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FJ{;. 3. MRI appearance of cobblestone LIS. P r o m i n e n t features include a r o u g h e n e d , " c o b b l e s t o n e " character of the cortical surface. This corresponds to neural cells that have broken past the b a s e m e n t m e m b r a n e to invade the arachnoid space. Stria-like bands of white inatter t h r o u g h the cortical gray are actually unmyelinated, glial tissue (arrow in A). T h e true white matter is markedly a b n o r m a l a n d is too bright on both Tl-weighted (A) a n d T2-weighted (B) images.
V. Prenatal Chemical Exposure as a Model for Diffuse Heterotopia
Prenatal treatment of rats with an alkylating, antimitotic agent, methazoxymethanol acetate (MAM), produces diffuse cortical heterotopia with a phenotype reminiscent of cobblestone LIS in humans (Mischel et al., 1995; Chevassus-Au-Louis et al., 1998). Cobblestone LIS is characterized by the migration of heterotopic young neurons beyond the marginal z o n e - - f u t u r e layer I - - i n t o the leptomeninges through gaps in the external basement m e m b r a n e (Fig. 3). The movement of neurons into the leptomeninges obliterates the subarachnoid space and gives rise to communicating hydrocephalus, or ventricular enlargement due to impaired reabsorption of cerebrospinal fluid (CSF) (Gelot et al., 1995; B o r n e m a n n et al., 1997). On first glance, the MRI features of cobblestone lissencephaly may be difficult to distinguish from those ofpolymicrogyria. T h e r e is a thickened cortical gray matter with a knobby, cobblestone surface and few or absent true sulci. Some areas will appear as pachygyria with a smooth surface. Bands o f " w h i t e matter" composed ofglial-fibrous tissue interrupt the gray matter. In the cortical dysplasia induced by MAM there are relatively normal projections as well as local fibers that do not normally innervate the
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cortex (Woo and Finlay, 1996; Chevassus-Au-Louis et al., 1999; Chevassusau-Louis and Represa, 1999). Thus, cortical hyperexcitability in this model is attributed to altered firing of discrete neuronal subpopulations together with the formation of bridges between normally u n c o n n e c t e d structures (Chevassus-au-Louis et al., 1999). The biochemical and electrophysiological characteristics of dysplastic brain in MAM-treated animals have been studied in some detail. Norepinephrine and serotonin concentrations are increased in brain while their receptors are reduced and the total cortical content of GAD and GABA is decreased (Johnston and Coyle, 1979). Data suggest not only selective loss of neuronal subsets in the MAM model, but also an alteration in transmitter release mechanisms. A translocation of protein kinase C from the cytosol to the presynaptic m e m b r a n e has been observed (Di Luca et al., 1997). This is accompanied by an e n h a n c e m e n t of glutamate release but not GABA from synaptosomes. While there are only rare spontaneous seizures in the MAM-treated, dysplastic cortex, there is hyperexcitability of the cortex in the presence of increased extracellular potassium, 4-aminopyridine, or bicuculline (Baraban and Schwartzkroin, 1995; Baraban etal., 2000). Changes in neuronal m e m b r a n e properties are indicated by increases in the frequency of action potential bursts to depolarization in over 80% of CA1 neurons in MAM-treated animals compared to 20% in normal controls (Baraban and Schwartzkroin, 1995). Indeed, recordings from the heterotopic clusters in the MAM model reveal that dysplastic clusters make connections locally with CA1 neurons and also with the neocortex (Baraban et al., 2000). These neurons have been shown to generate i n d e p e n d e n t epileptiform activity, which can be spread broadly to the neocortex, as well as the hippocampus. Severing the connections between the heterotopia in the CA1 region and cortex can eliminate the cortical spread of paroxysms (Chevassus-Au-Louis et al., 1998).
Vh Syndromes of Cortical Disorganization Although not necessarily a primary affector of neuronal migration, a n u m b e r of important processes may result in the heterotopic placement of neurons if disturbed. These syndromes may ultimately bear on the finetuning of neuronal migration and certainly pertain to the establishment of cortical structure. A partial list is outlined, where they are m e n t i o n e d because either clinical observation or animal models have revealed neuronal heterotopia and cortical dysplasia phenotypes (for additional discussion, see Ross and Walsh, 2001).
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Dysplasia associated with altered neurogenesis or survival Ectopic neurogenesis Autosomal recessive (tish) Schizencephaly Autosomal dominant ( E M X 2 ) Microcephaly Microcephaly vera Microcephaly with simplified gyri Microlissencephaly Malformations not yet classified Polymicrogyria (PMG) Diffuse Frontal Perisylvian Occipital Joubert syndrome
VII. Dysplasia Associated with Altered Neurogenesis: The tish Rat Model
Cortical dysplasia may arise from disturbances in the primary genesis of neural elements, both in cell number and in fate. An example is found in the tish rat, a mutant that first came to attention because of overt seizures. The brain reveals a subcortical heterotopia that is very similar in appearance to the human DC/SBH syndrome (Lee et al., 1997). Investigators have shown, however, that this band of disorganized neurons below the telencephalic cortex arises due to ectopic neurogenesis, forming a secondary ventricular germinal zone rather than an intrinsic neuronal migration defect (Lee et al., 1998). Interestingly, anterograde and retrograde tracing experiments indicated that both the heterotopia and the normal overlying cortex in the tish brain form appropriate projections to subcortical sites and bidirectional, topographic connections with the thalamus (Schottler et al., 1998). The genesis of seizures in the tish model has been studied extensively (Chen et al., 2000; Lee et al., 1997). Homozygous animals express spontaneous seizures at a rate of 1.5 to 15 spells per week. Both the normotopic cortex and the heterotopic cortex are hyperexcitable when exposed to penicillin or in conditions of reduced extracellular Mg2+, Exposure to proconvulsants induces cFos mRNA in the neocortex, hippocampus, and amygdala. Importantly, normotopic neurons appear to initiate seizures in
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the tish neocortex (Chen et al., 2000). In fact, when connections from the tish heterotopia to normotopic cortex are severed, the spiking threshold of the normotopic cortex is reduced dramatically, indicating release of inhibition of epileptogenic activity in the normotopic cortex (Chen et al., 2000). This has implications for the appropriate selection of patients with heterotopia for surgical intervention.
VIII. Malformations Not Yet Classified
These represent disorders in which neurons are observed to be out of place, but lack of gene identification or adequate genetic models prevents classification of the primary defect. Polymicrogyria (PMG) is important in this discussion both because epilepsy in children affected by PMG is p r o m i n e n t (Fig. 4) and because there are experimentally induced r o d e n t models having the appearance of PMG. Pathologically, two patterns of PMG are recognized: (1) a "four-layered" PMG in which the cortex comprises a molecular layer, an organized outer layer, a cell sparse layer, and a disorganized inner layer and (2) a completely disorganized " u n l a y e r e d " microgyrus. PMG can arise from several intrauterine insults, including hypoxemia with cortical laminar necrosis. This hypothesis of intrauterine insult is in part derived from experimental models in the rat p r o d u c e d by cortical lesions such as focal freezing injury (Jacobs etal., 1999). However, the detection of PMG in offspring of consanguineous parents or several affected family members indicates inherited causes as well (Sztriha and Nork, 2000; Guerrini et aL, 2000; Guerreiro et al., 2000; Borgatti et al., 1999). Lesion experiments suggest mechanisms that interfere with radial glial function and secondary disturbances of neuronal migration, although this is far from established as a general rule. A pathology that mimics PMG can be elicited by focal freezing o f the cortical surface in the early postnatal period. In this model, cortical hyperexcitability is caused by a reorganization of the network in the borders of the malformation (Chevassus-au-Louis et al., 1999). A striking observation in this PMG model in the rat neonatal cortex is disruption of the levels of specific GABAA receptor subtypes within the area o f the lesion, at the periphery, and at a distance (Redecker et al., 2000). Reductions in GABA o~5 and y2 subunits were p r o m i n e n t within the lesion, with even more substantial reductions in all subunits in the area medial to the lesion.
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A
F~c;. 4. MRI appearance of polymicrogyria (PMG). An example of perisylvian PMG in which the most severely affected cortex is centered on the sylvian fissure (arrow in B). White matter is of normal signal intensity. Disorganized microgyri with interdigitation of white matter give 1he cortex a thickened, rough appearance with loss of normal convolutions anteriorly and relative sparing of posterior cortex.
IX. Summary
The models of cortical dysplasia discussed earlier-- the L i s l knockout, the MAM-induced cobblestone LIS, the spontaneous tish mutant, and focal freeze injury-induced PMG--illustrate several important insights into epileptogenesis in malformed brain. First, the appearance of epilepsy varies according to the pathogenesis of the dysplasia and may well depend more on the intrinsic properties of the neurons in these models rather than on the disturbed position of the cells. This is supported by models such as the reeler mouse, in which the dysfunctional extracellular matrix molecule leads to a form of lissencephaly in mouse and human, but there is a far less impressive association with seizures than for LIS1 mutations. However, L i s l and Dcx mutations that appear to affect the cytoskeleton and perhaps intracellular protein trafficking are frequently associated with infantile spasms and epilepsy. Second, the possible mechanisms of epileptogenesis in these models include (a) a loss of subsets of neurons, (b) altered neurotransmitter release, (c) differences in neurotransmitter receptor levels and changes in receptor subnnit composition, (d) altered neurite density a n d / o r synaptogenesis, (e) changed membrane properties (e.g., altered voltage-gated channels), (f)altered cell morphology
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(neuronal differentiation), and (g) effects on cytoskeletal function. Finally, it is important to note that the "generator" of excitability in affected brain may be within the heterotopia or in the normotopic cortex. As additional genetic models come to light and the ability to distinguish their clinical counterparts improves, more individually tailored therapies, including standards for surgical interventions, will surely evolve.
References
al Shahwan, S. A., Bruyn, G. W., and al Deeb, S. M. (1995). Non-progressive familial congenital cerebellar hypoplasia.J. Neurol. Sci. 128, 71-77. Anderson, S.A., Marin, O., Horn, C., Jennings, K., and Rubenstein, J. L. (2001). Distinct cortical migrations from the medial and lateral ganglionic eminences. Development 128, 353-363. Baraban, S. C., and Schwartzkroin, P. A. (1995). Electrophysiology of CA1 pyramidal neurons in an animal model of neuronal migration disorders: Prenatal methylazoxymethanol treatment. Epilepsy Res. 22, 145-156. Baraban, S. C., Wenzel, H.J., Hochman, D. W., and Schwartzkroin, P. A. (2000). Characterization of heterotopic cell clusters in the hippocampus of rats exposed to methylazoxymethanol in utero. Epilepsy Res. 39, 87-102. Bix, G.J., and Clark, G. D. (1998). Platelet-activating factor receptor stimulation disrupts neuronal migration in vitro.9(. Neurosci. 18, 307-318. Borgatti, R., Triulzi, F., Zucca, C., Piccinelli, P., Balottin, U., Carrozzo, R., and Guerrini, R. (1999). Bilateral perisylvian polymicrogyria in three generations. Neurology 52, 1910-1913. Bornemann, A., Aigner, T., and Kirchner, T. (1997). Spatial and temporal development of the gliovascular tissue in type II lissencephaly. Acta Neuropathol. (Bed.) 93, 173-177. Borrell, V., Del Rio,J. A., Alcantara, S., Derer, M., Martinez, A., D'Arcangelo, G., Nakajima, K., Mikoshiba, K., Derer, P., Curran, T., and Soriano, E. (1999a). Reelin regulates the development and synaptogenesis of the layer-specific entorhino-hippocampal connections. J. Neurosci. 19, 1345-1358. Borrell, V., Ruiz, M., Del Rio@ A., and Soriano, E. (1999b). Development of commissural connections in the hippocampus of reeler mice: Evidence of an inhibitory influence of Cajal-Retzius cells. Exp. Neurol. 156, 268-282. Buzsaki, G., Smith, A., Berger, S., Fisher, L.J., and Gage, F. H. (1990). Petit mal epilepsy and parkinsonian tremor: Hypothesis of a common pacemaker. Neuroscience 36, 1-14. Caviness, V. (1982). Neocortical histogenesis in normal and reeler mice: A developmental study based on [3H]thymidine autoradiography. Dev. Brain Res. 4, 293-302. Caviness, V. S., and Rakic, P. (1978). Mechanisms of cortical development: A view from mutations in mice. Annu. Rev. Neurosci. 1,297-326. Chert, Z. F., Schottler, F., Bertram, E., Gall, C. M., Anzivino, M.J., and Lee, K. S. (2000). Distribution and initiation of seizure activity in a rat brain with subcortical band heterotopia. Epilepsia 41,493-501. Chevassus-au-Louis, N., and Represa, A. (1999). The right neuron at the wrong place: Biology of heterotopie neurons in cortical neuronal migration disorders, with special reference to associated pathologies. Cell Mol. Life Sci. 55, 1206-1215.
BRAIN MALFORMATIONS, EPILEPSY,AND IS
349
Chevassus-Au-Louis, N., Jorquera, I., Ben-Ari, Y., and Represa, A. (1999). Abnormal connections in the malformed cortex of rats with prenatal treatment with methylazoxymethanol may support hyperexcitability. Dev. Neurosci. 21,385-392. Chevassus-au-Louis, N., Baraban, S. C., Gaiarsa, J. L., and Ben-Ari, Y. (1999). Cortical malformations and epilepsy: New insights from animal models. Epilepsia 40, 811-821. Chevassus-Au-Louis, N., Rafiki, A.,Jorquera, I., Ben-Ari, Y., and Represa, A. (1998). Neocortex in the hippocampus: An anatomical and functional study of CA1 heterotopias after prenatal treatment with methylazoxymethanol in rats. J. Comp. Neurol 394, 520-536. Chugani, H. T., Rintahaka, P.J., Shewmon, D.A. (1994). Ictal patterns of cerebral glucose utilization in children with epilepsy. Epilepsia 35, 813-822. Clark, G. D., Bix, G.J., Shinoya, A., Hirotsune, S., Ross, M. E., Kholmanskih, P., Letourneau, P., McNeil, R. S., Abu-Issa, R., and Wynshaw-Boris, A. (2002). Platelet activating factor rescues neurite and migration defects in Pafahlbl(Lisl)-deficient neurons. Submitted. Coulter, D. A., and Lee, C.J. (1993). Thalamocortical rhythm generation in vitro: Extra- and intracellular recordings in mouse thalamocortical slices perfused with low Mg2+ medium. Brain Res. 631,137-142. D'Arcangelo, G., Miao, G. G., Chen, S. C., Soares, H. D., Morgan,J. I., and Curran, T. (1995). A protein related to extracellular matrix proteins deleted in the mouse mutant reeler. Nature 374, 719-723. des Portes, V., Pinard,J. M., Billuart, P., Vinet, M. C., Koulakoff, A., Carrie, A., Gelot, A., Dupuis, E., Motte, J., Berwald-Netter, Y., Catala, M., Kahn, A., Beldjord, C., and Chelly, J. (1998). A novel CNS gene required for neuronal migration and involved in X-linked subcortical laminar heterotopia and lissencephaly syndrome. Cell 92, 51-61. Di Luca, M., Caputi, A., Cattabeni, F., De Graan, P. N., Gispen, W. H., Raiteri, M., Fassio, A., Schmid, G., and Bonanno, G. (1997). Increased presynaptic protein kinase C activity and glutamate release in rats with a prenatally induced hippocampal lesion. Eur. J. Neurosci. 9, 472-479. Dobyns, W. B. (2001). Lissencephaly: The clinical and molecular genetic basis of diffuse malformations of neuronal migration. In "Neuronal Migration Disorders" (P. G. Barth, ed). McKeith, London. Dobyns, W.B., Andermann, E., Andermann, F., Czapansky-Beilman, D., Dubeau, F., Dulac, O., Guerrini, R., Hirsch, B., Ledbetter, D. H., Lee, N. S., Motte,J., Pinard,J. M., Radtke, R. A., Ross, M. E., Tampieri, D., Walsh, C. A., and Truwit, C. L. (1996). X-linked malformations of neuronal migration. Neurology 47, 331-339. Dobyns, W. B., Carrozzo, R., and Ledbetter, D. H. (1994). Frequent deletions of the LIS1 gene in classic lissencephaly. Annu. Neurol. 36, 489-490. Dobyns, W. B., Reiner, O., Carrozzo, R., and Ledbetter, D. H. (1993). Lissencephaly: A human brain malformation associated with deletion of the LIS1 gene located at chromosome 17p13. JAMA 270, 2838-2842. Dobyns, W. B., and Truwit, C. L. (1995). Lissencephaly and other malformations of cortical development: 1995 update. Neuropediatrics 26, 132-147. Efimov, V. P., and Morris, N. R. (2000). The LISl-related NUDF protein ofAspergiUus nidulans interacts with the coiled-coil domain of the N U D E / R O l l protein. J. Cell Biol. 150, 681 - 688. Feng, G., Olson, E. C., Stukenbuerg, P. T., Flanagan, L. A., Kirschner, M. W., and Walsh, C. A. (2000). LIS1 regulates CNS lamination by interacting with mNudE, a central component of the centrosome. Neuron 28, 653-664. Fleck, M.W., Hirotsune, S., Gambello, M.J., Phillips-Tansey, E., Suares, G., Mervis, R. F., Wynshaw-Boris, A., and McBain, C.J. (2000). Hippocampal abnormalities and enhanced excitability in a murine model of human lissencephaly. J. Neurosci. 20, 2439-2450.
350
~a. ELIZABETHROSS
Garcia-Higuera, I., Fenoglio,J., Li, Y., Lewis, C., Panchenko, M. P., Reiner, O., Smith, T. F., and Neer, E.J. (1996). Folding of proteins with WD-repeats: Comparison of six members of the WD-repeat superfamily to the G protein beta subunit. Biochemistry 35, 13985-13994. Gelot, A., Billette de Villemeur, T., Bordarier, C., Ruchoux, M. M., Moraine, C., and Ponsot, G. (1995). Developmental aspects of type II lissencephaly: Comparative study of dysplastic lesions in fetal and post-natal brains. Acta Neuropathol. (Berl.)89, 72-84. Gleeson,J. G., Allen, K. M., Fox,J. W., Lamperti, E. D., Berkovic, S., Scheffer, 1., Cooper, E., Dobyns, W. B., Minnerath, S., Ross, M. E., and Walsh, C. A. (1998). doublecortin, a brainspecific gene mutated in human X-linked lissencephaly and double cortex syndrome, encodes a putative signaling protein. Cell 92, 63-72. Gleeson,J. G., Minnerath, S., Allen, K.M., Fox, J. W., Hong, S., Berg, M., Kuzniecky, R., Reitnauer, P.J., Borgatti, R., Mira, A.P., Guerrini, R., Holmes, G., Rooney, C., Berkovic, S., Scheffer, I., Cooper, E. C., Ricci, S., Cusmai, R., Crawford, T. O., Brown, L., Anderman, E., Wheless, J., Dobyns, W.B., Ross, M.E., and Walsh, C.A. (1999). Characterization of mutations in the gene doublecortin in patients with double cortex syndrome. Ann. Neurol. 45, 146-153. Guerreiro, M.M., Andermnann, E., Guerrini, R., Dobyns, W.B., Kuzniecky, R., Silver, K., Van Bogaert, P., Gillain, C., David, P., kanbrosetto, G., Rosati, A., Bartolomei, F., Parmeggiani, A., Paetau, R., Salonen, O., Ignatius,J., Borgatti, R., Zucca, C., Bastos, A. C., Palmini, A., Fernandes, W., Montenegro, M. A., Cendes, F., and Andermann, F. (2000). Familial perisylvian polymicrogyria: A new familial syndrome of cortical maldevelopment. Ann. Neurol. 48, 39-48. Guerrini, R., Canapicchi, R., and Dobyns, W. B. (1999). Epilepsy and malformations of the cerebral cortex. Neurologia 14 (Suppl 3), 32-47. Guerrini, R., Barkovich, A.J., Sztriha, L., and Dobyns, W. B. (2000). Bilateral frontal polymicrogyria: A newly recognized brain malformation syndrome. Neurology 54, 909-913. Hayashi, M., Itoh, M., Araki, S., Kumada, S., Tanuma, N., Kohji, T., Kohyama, J., Iwakawa, Y., Satoh,J., and Morimatsu, Y. (2000). hnmunohistochemical analysis of brainstem lesions in infantile spasms. Neuropathology 20, 297-303. Hirotsune, S., Fleck, M. W., Gambello, M.J., Bix, G. J., Chen, A., Clark, G. D., Ledbetter, D. H., McBain, C.J., and Wynshaw-Boris, A. (1998). Graded reduction ofPafhhlbl (Lisl) activity results in neuronal migration defects and early embryonic lethality. Nature Genet. 19, 333-339. Ho, Y. S., Swenson, L., Derewenda, U., Serre, L., Wei, Y., Dauter, Z., Hattori, M., Adachi, T., Aoki,J., Arai, H., Inoue, K., and Derewenda, Z. (1997). Brain acetylhydrolase that inactivates platelet-activating factor is a G-protein-like trimner. Nature 385, 89-93. Hong, S.E., Shugart, Y.Y., Huang, D.T., Shahwan, S.A., Grant, P.E., Hourihane,J. O., Martin, N. D., and Walsh, C. A. (2000). Autosomal recessive lissencephaly with cerebellar hypoplasia is associated with human RELN mutations. Nature Genet. 26, 93-96. Jacobs, IL M., Hwang, B.J., and Prince, D. A. (1999). Focal epileptogenesis in a rat model of polymicrogyria.J. Neurophysiol. 81,159-173. Johnston, M.V., and Coyle,J. T. (1979). Histological and neurochemical effects of fetal treatment with methylazoxymethanol on rat neocortex in adulthood. Brain Res. 170, 135-155. Kato, M., Takizawa, N., Yamada, S., lto, A., Honma, T., Hashimoto, M., Saito, E., Ohta, T., Chikaoka, H., and Hayasaka, K. (1999). Diffuse pachygyria with cerebellar hypoplasia: A milder form of microlissencephaly or a new genetic syndrome? Ann. Neurol. 46, 660-663. Kitagawa, M., Umezu, M., Aoki, J., Koizumi, H., Arai, H., and Inoue, K. (2000). Direct association of LIS1, the lissencephaly gene product, with a mammalian homologue of a fungal nuclear distribution protein, rNUDE. FEBS Lett. 479, 57-62.
BRAIN MALFORMATIONS, EPILEPSY,AND IS
351
Lee, K. S., Collins,J. L., Anzivino, M.J., Frankel, E. A., and Schottler, F. (1998). Heterotopic neurogenesis in a rat with cortical heterotopia, jr Neurosci. 18, 9365-9375. Lee, K.S., Schotder, F., Collins,J. L., Lanzino, G., Couture, D., Rao, A., Hiramatsu, K., Goto, Y., Hong, S.C., Caner, H., Yamamoto, H., Chen, Z.F., Bertram, E., Berr, S., ()mary, R., Scrable, H.,Jackson, T., Goble,J., and Eisenman, L. (1997). A genetic animal model of human neocortical heterotopia associated with seizures. J. Neurosci. 17, 6236-6242. Matsumoto, N., Leventer, R.J, Kuc, J. A., Mewborn, S. K., Dudlicek, L. L., Ramocki, M. B., Pilz, D. T., Mills, P. L., Das, S., Ross, M. E., Ledbetter, D. H., and Dobyns, W. B. (2001). Mutation analysis of the DCX gene and genotype/phenotype correlation in subcortical band heterotopia. Eur.J. Hum. Genet. 9, 5-12. McCormick, D. A., and Bal, T. (1997). Sleep and arousal: Thalamocortical mechanisms. Annu. Rev. Neurosci. 20, 185-215. Mendez, M., and Radtke, R.A. (2001). Interactions between sleep and epilepsy. J. Clin. Neurophysiol. 18, 106-127. Mischel, P. S., Nguyen, L P., and Vinters, H. V. (1995). Cerebral cortical dysplasia associated with pediatric epilepsy. Review of neuropathologic features and proposal for a grading system.J. Neuropathol. Exp. Neurol. 54, 137-153. Pilz, D. T., Matsumoto, N., Minnerath, S., Mills, P., Gleeson,J. G., Walsh, C. A., Barkovich,J., Dobyns, W. B., Ledbetter, D. H., and Ross, M. E. (1998). LIS1 and XLIS/doublecortin mutations cause most human classical lissencephaly, but different patterns of malformation. Hum. Mol. Genet. 7, 2029-2037. Pleasure, S.J., Anderson, S., Hevner, R., Bagri, A., Marin, O., Lowenstein, D. H., and Rubenstein,J. L. (2000). Cell migration from the ganglionic eminences is required for the development of hippocampal GABAergic interneurons. Neuron 28, 727-740. Redecker, C., Luhmann, H.J., Hagemann, G., Fritschy, J. M., and Witte, O. W. (2000). Differential downregulation of GABAA receptor subunits in widespread brain regions in the freeze-lesion model of focal cortical malformations. J. Neurosci. 20, 5045-5053. Ross, M. E., and Walsh, C.A. (2001). Human brain malformations and their lessons for neuronal migration. Annu. Rev. Neu~vsci. 24, 1041-1070. Ross, M. E., Swanson, K., and Dobyns, W. B. (2001). Lissencephaly with cerebellar hypoplasia (LCH): A heterogeneous group of cortical malformations. Neuropediatrics 32, 256-263. Ross, M.E., Allen, K.M., Srivastava, A.K., Featherstone, T., Gleeson, J.G., Hirsch, B., Harding, B. N., Andermann, E., Abdullah, R., Berg, M., Bielman, D. C., Flanders, D., Guerrini, R., Motte,J., Puche Mira, A., Scheffer, I., Berkovic, S., Scaravilli, F., King, R. A., Ledbetter, D. H., Schlessinger, D., Dobyns, W. B., and Walsh, C. A. (1997). Linkage and physical mapping of X-linked lissencephaly/SBH (XLIS): A gene causing neuronal migration defects in human brain. Hum. Mo&c. Genet. 6, 555-562. Sapir, T., Cahana, A., Seger, R., Nekhai, S., and Reiner, O. (1999). LISI is a microtubuleassociated phosphoprotein. Enr. J. Biochem. 265, 181 - 188. Sapir, T., Elbaum, M., and Reiner, O. (1997). Reduction of microtubule catastrophe events by LIS1, platelet-activating factor acetylhydrolase subunit. EMBOJ. 16, 6977-6984. Sasaki, S., Shionoya, A., Ishida, M., Gambello, M.J., Yingling, J., Wynshaw-Boris, A., and Hirotsune, S. (2000). A LIS1/NUDEL/cytoplasmic dynien heavy chain complex in the developing and adult nervons system. Neuron 28, 681-696. Schiffmann, S. N., Bernier, B., and Goffinet, A. M. (1997). Reelin mRNA expression during mouse brain development. Eur.J. Neurosci. 9, 1055-1071. Schottler, F., Couture, D., Rao, A., Kahn, H., and Lee, K. S. (1998). Subcortical connections of normotopic and heterotopic neurons in sensory and motor cortices of the tish mutant rat.J. Comp, Neurol. 395, 29-42.
352
M. ELIZABETHROSS
Sheppard, A. M., and Pearlman, A. L. (1997). Abnormal reorganization of preplate neurons and their associated extracellular matrix: An early manifestation of altered neocortical development in the reeler mutant mouse.J. Comp. Neurol. 378, 173-179. Smith, D. S., Niethammer, M., Ayala, R., Zhou, Y., Gambello, M.J., Wynshaw-Boris, A., and Tsai, L. H. (2000). Regulation of cytoplasmic dynein behaviour and microtubule organization by mammalian Lisl. Nature Cell Biol. 2, 767-775. Sweeney, K.J., Clark, G. D., Prokscha, A., Dobyns, W. B., and Eichele, G. (2000). Lissencephaly associated mutations suggest a requirement for the PAFAH1B heterotrimeric complex in brain development. Mech. Dev. 92, 263-271. Sztriha, L., and Nork, M. (2000). Bilateral frontoparietal polymicrogyria and epilepsy. Pediatr. Nemvl. 22, 240-243. Taylor, K. R., Holzer, A. K., Bazan,J. F., Walsh, C. A., and Gleeson,J. G. (2000). Patient mutations in doublecortin define a repeated tubulin-binding domain. J. Biol. Chem. 275, 34442-34450. Tominaga, I., Yanai, K., Kashima, H., Kato, Y., Sekiyama, S., Yokochi, A., and Miura, I. (1986). An anatomo-clinical case of sequelae of acute encephalopathy: Infantile spasm with hypsarrhythmia. Rev. Neurol. (Paris) 142, 524-529. Woo, T. U., and Finlay, B. L. (1996). Cortical target depletion and ingrowth ofgeniculocortical axons: Implications for cortical specification. Cereb. Cortex. 6, 457-469. Xiang, X., Osmani, A. H., Osmani, S.A., Xin, M., and Morris, R. (1995). NudF, a nuclear migration gene in Aspergillus nidulans, is similar to the human LIS-1 gene required for neuronal migration. Mol. Biol. Cell 6, 297-310. Zhang, Y. F., and Coulter, D. A. (1996), Anticonvulsant drug effects on spontaneous thalamocortical rhythms in vitro: Phenytoin, carbamazepine, and phenobarbital. Epilepsy Res. 23, 55-70.