Genetic advances in central nervous system malformations in the fetus and neonate

Genetic Advances in Central Nervous System Malformations in the Fetus and Neonate Linda Bone Jeng, Rocio Tarvin, and Nathaniel H. Robin Malformations of the central nervous system (CNS) are commonly encountered by the pediatric neurologist when called to evaluate a fetus or newborn. Such malformations may be isolated or appear as part of a genetic syndrome. In the past few years there have been great advances in identifying the genes and genetic alterations for many isolated CNS malformations and syndromes with CNS malformations. Therefore, it is important to look for associated anomalies in any infant with a CNS malformation, as well as consideration of the rest of the family. We have chosen four malformations (holoprosencephaly, hydrocephalus, lissencephaly, and schizencephaly) to serve as a paradigm for genetic malformations of the CNS. Understanding the underlying genetic etiology of a disorder allows us to give more accurate recurrence risk counseling, to better estimate potential complications, and to better manage the patient's care. As research continues, additional malformations and syndromes will be understood on the genetic level, and combining this genetic information with neurologic understanding will translate into better medical care for the patient.

Copyright 9 2001 by W.B. Saunders Company

HE DEVELOPMENT of the central nervous system (CNS) is very complex. Neurogenesis, apoptosis, neurulation, neural crest separation, cellular migrations, axonal pathfinding, dendritic sprouting, synaptogenesis, neurotransmitter biosynthesis, and myelination all must occur properly for a normal brain to develop, l Furthermore, all the supporting structures must also develop normally, including the vascular supply, immune system, glial cells, and meninges.1 Each of these processes is under genetic control, often with multiple genes being crucial to a given step in normal CNS development. It is no wonder, then, that CNS anomalies are the most common finding in genetic syndromes, and hundreds of genetic syndromes have some defined abnormality of CNS structure or function. Therefore, one must recognize the high likelihood that there are contributing genetic factors for such patients. This is obvious for the child with multiple congenital anomalies, as such children are often diagnosed with a defined genetic syndrome. However, it is also very important to consider genetic factors when evaluating the fetus or newborn with an apparently isolated CNS abnormality, such as holoprosencephaly or hydrocephalus. Recognizing the possible genetic basis for such anomalies is important for many reasons. By identifying a genetic basis, accurate recurrence risk can be identified. This can be the difference between a 50% recurrence risk for an autosomal-dominant condition in which one parent is mildly affected, to 25% for an autosomal-recessive condition, to under 1% for a new mutation. Another benefit to making an accurate genetic diagnosis is assessing

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Seminars in Pediatric Neurology, Vol 8, No 2 (June), 2001 : pp 89-99

the risk for future complications, like learning disabilities and mental retardation. For example, an infant with congenital hydrocephalus has a reasonably favorable prognosis for normal intelligence if corrected early, but if the hydrocephalus is caused by a mutation in L1CAM, the risk for mental retardation approaches 100%. 2 When evaluating the fetus/neonate with a CNS anomaly (Table 1), one needs to look not only for obvious major anomalies, like a congenital heart defect or limb anomaly, but it is important to look for subtle findings, like wide-spaced eyes, altered skin pigment, and abnormal palmar creases. Each of these may provide clues to the presence of an underlying genetic cause for the patient's CNS anomaly. Of course, such anomalies can only be detected if they are apparent at the time of the evaluation. This is impossible when evaluating a fetus by ultrasonography and often difficult when evaluating a newborn. Key manifestations may not yet be apparent (eg, poor growth, unusually fair complexion) or may be too subtle to detect at this point (eg, wide-set eyes, short fingers). It is for these reasons that one must conduct a careful evaluation of the entire family.

From the Departments of Genetics, Pediatrics, Reproductive Biology, and Otolaryngology, Case Western Reserve University School of Medicine, University Hospitals of Cleveland, Cleveland, OH. Address reprint requests to Nathaniel H. Robin, MD, Center for Human Genetics, 11100 Euclid Ave, Lakeside 1500, Cleveland, OH 44106-6055. Copyright 9 2001 by W.B. Saunders Company 1071-9091/01/0802-0004535.00/0 do#lO.1053/spen.2001.24836 89

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JENG, TARVIN, AND ROBIN Table 1. Approach to the Evaluation of the Fetus/Neonate With CNS Evaluation

Neonate

Fetus

Personal medical history (eg, prenatal and birth history, other associated medical problems) Family history (eg, similar abnormalities, other congenital/medical problems, recurrent miscarriages, early deaths) Detailed dysmorphic examination (eg, intercanthal and outercanthal distances, ear length, palm creases) Appropriate testing (eg, chromosomes, CT/MRI, hearing)

Therefore, when evaluating the newborn and especially the fetus, one must look to the family evaluation for diagnostic clues. This certainly includes a detailed family history, looking for not only similar and related problems, but also for any other medical issues that may on the surface seem unrelated. For example, a family history of partial adontia or anosmia may be related to holoprosencephaly. In some instances it may be appropriate to examine the patient's parents, as the wide clinical variability of some conditions such as tuberous sclerosis 3 can cause a parent to be so subtlety affected that they are not diagnosed. This would have important implications, as the recurrence risk for unaffected parents would be significantly lower than the 50% recurrence risk for an affected parent. Genetic research is progressing at a rapid rate. Thanks in large part to the Human Genome Project, new disease-related genes are being identified on what seems like a daily basis. As we would expect, many of these genes are involved in CNS development. Although these discoveries have given us insight into both normal and abnormal CNS development, it is the hope and expectation that new and better therapies will emerge. Although that is a hope for the future, today these discoveries have provided new and more exact diagnostic tests, providing families with additional information. However, as these advances have occurred in a relatively short period of time, pediatric neurologists have been forced to keep up with a new science and a new body of literature. This article reviews a series of CNS malformations that present in the fetal and neonatal periods. We have chosen these disorders not only because they are among the more common malformations that present in the fetal and neonatal periods, but also because they represent the paradigm by which genetics will impact on the field of pediatric neurology.

Same

Same Encourage the detailed physical examination after birth Level II ultrasound, amniocentesis

GENETICALLY DETERMINED CNS MALFORMATIONS

Holoprosencephaly Perhaps no CNS malformation better represents the etiologic complexity than holoprosencephaly (HPE). This is because the HPE phenotype is the common endpoint of a nearly endless number of perturbations of CNS midline development. HPE is a developmental field defect manifested by a spectrnm of abnormalities of the forebrain and midface (Fig 1). It results from incomplete cleavage of the

Fig 1. A male patient with holoprosencephaly demonstrating characteristic facial features including microcephaly, midface hypoplasia, hypotelorism, cleft lip/palate, and a single nasal opening,

GENETIC ADVANCES IN CNS MALFORMATIONS

Table 2. Etiology of Holoprosencephaly Teratogenic Maternal diabetes Alcohol Retinoic acid Salicylates Anticonvulsants Genetic Single gene defect Monogenic genetic syndrome Chromosomal anomaly

embryonic forebrain before the fourth week of gestation. Although the occurrence of HPE in live births is uncommon (1/16,000 births), 4 it is one of the most common defects of the developing brain occurring in approximately 1 in 250 conceptuses. 5 Etiology and genetics. Holoprosencephlay is extremely heterogeneous with both teratogenic and genetic causes (Table 2). The most studied teratogen in humans is maternal diabetes. Infants of diabetic mothers have a 1% risk (a 200-fold increase) for HPE. Other teratogens including alcohol, retinoic acid, salicylates, and anitconvulsants have been associated with HPE, although their biologic significance is not yet known. 6 HPE can be due to a single gene defect, it can occur as part of a monogenic genetic syndrome, or it can be due to a chromosomal anomaly. It is estimated that approximately 18% to 25% of HPE cases have a recognized monogenic syndrome. 7 Estimates of the frequency of chromosomal aberrations in live births with HPE range from 24% to 45%; trisomy 13 accounting for more than 50% of HPE cases with chromosomal aberrations. Autosomai-dominant forms with 70% to 80% penetrance, autosomal-recessive forms, and X-linked inheritance have all been observed. However, most cases of HPE are sporadic, s Holoprosencephaly is seen in a number of genetic syndromes including Meckel-Gruber syndrome and Smith-Lemli-Opitz syndrome. It is important to remember that, when HPE is present, the facial characteristics of any syndrome will be masked by the HPE facies. Therefore, it is important to focus on other findings in various syndromes. When HPE is accompanied by extracranial malformations, a genetic syndrome is highly probable. For example, with Meckel-Gruber syndrome and associated HPE, one would see the characteristic encephalocele, polydactyly, and cys-

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tic dysplasia of the kidneys. Inheritance is autosomal recessive, and heterozygotes do not have any clinical features. The Meckel-Gruber locus maps to 17q21-q24. 9 Similarly, when HPE is associated with genital abnormalities and two-three toe syndactyly, Smith Lemi-Opitz syndrome (SLOS) must be considered. SLOS is an autosomal-recessive disorder characterized by anteverted nostrils, ptosis of eyelids, syndactyly of second and third toes, hypospadias, and cryptorchidism. Infants have feeding difficulties and vomiting. Patients have been found to have abnormal cholesterol biosynthesis with low plasma cholesterol. 1~ Prenatally, this is evident by low maternal serum estriol on the triple check. Genes associated with HPE. At least 12 genetic loci on 11 different chromosomes have been identified that are likely to contain genes critical for normal brain development. A minimal critical region has been identified in five loci designated HPE1-HPE5. A gene has been identified in four of these critical regions (Table 3). Sonic Hedgehog (SHH) maps to 7q36 near the HPE3 locus making it an excellent candidate gene for HPE. ~1 SHH is a secreted factor expressed early in development of the ventral forebrain that is critical for ventral patterning of the developing neural tube. 12 SHH's role in the pathogenesis of HPE has been demonstrated in knockout mice with homozygous null mutations for SHH that show cyclopia and CNS features of the holoprosencephaly sequence. ~3 In addition, recent studies of patients with HPE demonstrate heterozygous mutations in the SHH gene with unique mutations located throughout its coding region. 14 No genotype-phenotype correlation has been found with respect to the type or location of mutation in the SHH gene. Furthermore, a single SHH mutation can manifest any part of the full spectrum of HPE phenotypes including microforms and clinically normal individuals. This emphasizes the need for genotype analysis in clinically normal individuals with a family history of

Table 3. Holoprosencephaly Genes Critical Region Loci HPE1 HPE2 HPE3 HPE4 HPE5

Map Site 21q22.3 2p21 7q36 18pl 1.3 13q32

Gene -SIX3 SHH TGIF ZIC2

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HPE. Given this intrafamilial clinical variability, it is speculated that other genes acting in the same or different developmental pathways may modify the expression of HPE phenotypes. 14 ZIC2 was the second gene associated with HPE, which maps to 13q32 (HPE5). 15 ZIC2 is a transcription factor that is expressed in the dorsal neural tube, neural retina, and the distal limb bud during mouse embryonic development. 16 This chromosomal region was deleted in a series of patients with major congenital malformations, including brain anomalies such as HPE. In humans, ZIC2 showed expression in fetal brain, in midline stripes adjacent to the expression of GLI genes, mediators of the SHH pathway. 17 SIX3 maps to 2p21, the HPE2 l o c u s . Is Vertebrae SIX3 genes participate in midline forebrain and eye formation in several organisms. It is homologous to the Drosophila sine oculis/optix family of genes, which play a crucial role for the patterning of the visual system. 19 TGIF maps to 18p11.3 in the HPE4 minimal

Fig 2. MRI of patient with holoprosencephaly. Single "monoventricle" with lack of midline separation of the cerebral hemispheres anteriorly. No interhemispheric fissure or other midline structures are identified. MRI scan is T2 weighted, and CSF in the ventricle and subarachnoid space is white.

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Table 4. Holoprosencephaly Recurrence Risk for Siblings of Affected Offspring of the Same Biologic Parents Nonsyndromic, Autosomal Dominant, and Negative for SHH Mutation Obligate carrier Incomplete or microform Overall Sporadic, nonchromosomal, nonsyndromic

16% to 21% 13% to 14% 29% to 35% 6%

critical region. 2~ It is expressed during early brain development in mice. TGIF acts as a repressor of retinoic acid regulated transcription. 22 However, only 10% of patients carrying a TGIF deletion show HPE in contrast to the high concordance of deletions at the HPE2, HPE3, and HPE5 loci. Diagnosis and recurrence risk. Diagnosis is based on the overall pattern of abnormalities (Fig 2), chromosome analysis, and molecular studies in selected cases and family history. Because the HPE sequence can occur in the setting of other wellrecognized syndromes, the evaluation of a patient with HPE must include a complete investigation for other systemic anomalies. Chromosome studies and detailed family history of all children diagnosed with holoprosencephaly should be performed to verify cytogenetic status and identify possible monogenic cases so that effective counseling can be given. Specific chromosome findings in an affected patient should lead to further family studies facilitating the identification of a higher recurrence risk and the potential for prenatal karyotyping. Careful examination of the proband's family for microforms of HPE is essential. The examiner should look for microcephaly, hypotelorism, cleft lip and palate, absent frenulum, anosmia, and single central incisor. 12 For nonsyndromic cases, if the findings are consistent with autosomal-dominant HPE and the family is negative for SHH mutations, the risk for an obligate carrier is on the order of 16% to 21%; the risk for an incomplete form or microform is 13% to 14%; and the overall risk is 29% to 35%. For sporadic, nonchromosomal, nonsyndromic HPE, a recurrence risk of approximately 6% may be given (Table 4). 23

Hydrocephalus Hydrocephalus is among the most common CNS abnormalities, and it has many causes. By definition, hydrocephalus refers to an increased volume

GENETIC ADVANCES IN CNS MALFORMATIONS

of cerebral spinal fluid (CSF) relative to cerebral parenchyma, usually in the lateral ventricles. 24 The resorbtive capacity of the brain (parenchymal resorption, foramina, subarachnoid space, granulations, and other resorptive sites) under normal conditions is greater than the production of CSF, and the resorptive capacity changes with the amount of CSF production. 24 Hydrocephalus develops when there is excessive CSF production, decreased CSF resorption, or obstruction of CSF flow. In some cases, CSF pressure increases and the ventricles enlarge. This causes an increased surface area for resorption, but it is not enough to prevent development of hydrocephalus. Hydrocephalus secondary to overproduction is rare, and the most common cause of congenital hydrocephalus is aqueductal stenosis. 24 Hydrocephalus may be categorized as obstructive (abnormal adsorption or circulation of CSF) or nonobstructive (loss of brain tissue leading to enlargement of CSF space). Obstructive hydrocephalus is divided into communicating (obstruction outside ventricles) and noncommunicating (obstruction within the ventricles), z4 Communicating hydrocephalus may be secondary to hemorrhage or infection. Aqueductal stenosis accounts for most noncommunicating hydrocephalus. When looking at causes of obstructive hydrocephalus, aqueductal stenosis accounts for 39% to 43%, communicating hydrocephalus is 36% to 38%, Dandy-Walker malformation is 9% to 13%, and 7% to 16% are other causes. Other anomalies associated with hydrocephalus include neurofibromatosis, cardiac defects, and skeletal dysplasias. 25 Aqueductal stenosis may be acquired (secondary to inflammation or obstruction from tumors) or genetic with X-linked or autosomal-recessive inheritance. 24 Aqueductal stenosis is of unknown cause in 75% of patients. However, X-linked inheritance of aqueductal stenosis is the most common cause of hydrocephalus. 24'26 Hydrocephalus due to aqueductal stenosis presents prenatally after 20 weeks of gestation. Progression may vary from intrauterine fetal demise to long-term survival complicated by developmental delay and mental retardation. X-linked hydrocephalus has variable expressivity, but associated findings include macrocephaly, adducted thumbs, spasticity, mental retardation, agenesis of the corpus callosum, fused thalami, and hypoplastic corticospinal tract. This form of hydrocephalus tends to be very severe with

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an extremely high occurrence of stillbirths. 25 Incidence of X-linked aqueductal stenosis is 17 to 36 in 1,000,000 males. 24 Etiology and genetics. Hydrocephalus may be isolated (acquired or congenital) or associated with a syndrome. Development of hydrocephalus may be secondary to mechanical obstruction from tumors, cysts, inflammation, hemorrhage, aplasia or stenosis, and external pressure from vascular malformations. The overall incidence of hydrocephalus is 0.1 to 3.5 in 1,000, whereas the incidence of isolated congenital hydrocephalus is 0.5 to 0.8 in 1,000. 24 When looking at patients with hydrocephalus, 20% to 40% have extracranial malformations and 25% have cytogenetic anomalies. X-linked hydrocephalus accounts for 2% to 15% of males with primary idiopathic hydrocephalus. More than 250 syndromes have associated hydrocephalus and 70% to 80% of hydrocephalus cases have associated anomalies. 25 Many of these are single gene defects. The most common is X-linked hydrocephalus, which has mutations in the L1CAM gene. Genes associated with hydrocephalus. L1CAM (L1) is a cell adhesion molecule that maps to Xq28. The protein has six Ig-like extracellular domains, five fibronectinlike domains, one transmembrane domain, and one short intracytoplasmic domain. The protein is involved in axon outgrowth and pathfinding, cell-cell adhesion, signaling, and learning/long-term memory. 27 Single amino acid changes occurring throughout the protein may result in different phenotypes, anything from hydrocephalus to mental retardation. 28 Four neurologic conditions are associated with L1 mutations: Xlinked hydrocephalus, MASA syndrome, complicated spastic paraplegia, and X-linked agenesis of the corpus callosum. Together these diseased are grouped under the name CRASH syndrome. Mutations in the different region of the protein result in reasonably predictable severity with some intrafamilial variability, suggesting other influencing f a c t o r s . 27

Diagnosis and recurrence risk. Diagnosis of hydrocephalus is dependent on ascertainment. Children of different ages present with very different signs and symptoms. Increased intracranial pressure may present with classic "sun-downing" (loss of upward gaze) or with vomiting, irritability, or lethargy. Hydrocephalus in the fetus/neonate is often recognized on routine prenatal ultrasound

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Fig 3. Ultrasound study of patient with hydrocephalus. Prenatal ultrasound shows ventriculomegaly. CSF in the ventricle is black.

examination or by presentation with macrocrania, tense and bulging fontanel, widened sutures, sparse hair, rapidly increasing occipitofrontal circumference (OFC), or seizures. 25 Subtle signs of hydrocephalus in older infants may include developmental delay, personality changes, and gait disturbances. 25'29 The head has a characteristic shape with untreated hydrocephalus; the frontal region is prominent, and the parietal areas become rounded. Imaging with routine skull films maybe be helpful, but ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI) (Fig 3) allows final diagnosis, categorization of hydrocephalus type, and detection of associate malformations. 25 The ratio of lateral ventricle to hemispheric width has been widely used to diagnosis hydrocephalus. 24 Other findings on imaging suggestive of hydrocephalus include shrunken choroid plexus and shifted midline. The thickness of the cerebral mantle may provide some prognostic information. However, a normal mantle thickness does not guarantee that neurologic outcome will be normal. 24 Isolated hydrocephalus has a good prognosis, whereas presence of associated anomalies usually suggests a poorer prognosis. Unfortunately, this does not always hold true, making prognostic determinations very difficult. Once a diagnosis of hydrocephalus is made, it is important to look carefully for associated malformations because this information changes prognosis and ultimate diagnosis. This evaluation should include an echocardiogram because cardiac find-

JENG, TARVIN, AND ROBIN

ings may be present in up to 15% of cases, and an infectious workup in both mother and baby (particularly for cytomegalovirus and toxoplasmosis). 24 Finally, a karyotype should be performed on all cases; abnormal karyotypes are detected in about 25% of cases. 25 Recurrence risk is dependent on the type of hydrocephalus: 12% for males of male probands with aqueductal stenosis, 1 in 154 for communicating hydrocephalus, 0 in 54 for Dandy-Walker, 0 in 22 for other types. However, there is a 6% recurrence risk for siblings of a patient with hydrocephalus. For parents with a male child with X-linked hydrocephalus, the empiric recurrence risk is 4%, as calculated by Halliday et al (Table 5). 24,30

Lissencephaly Lissencephaly is the classic neuronal migration disorder resulting in a smooth cortex. Like HPE, lissencephaly represents the common phenotypic expression of a number of possible genetic alterations. Clinically, lissencephaly is a devastating genetic disease of children and is often associated with mental retardation and intractable epilepsy. It is a disorder of neuronal cell migration that occurs between 12 and 16 weeks of gestation. 31 The neuronal precursor cells of the paraventricular zone normally migrate to the cortical folds on the outer portion of the brain, but in lissencephaly, these cells fail to proceed to the correct region. 32'33 Neuronal cell migration disorders are a spectrum including agyria, pachygyria, polymicrogyria, heterotopia, and ectopia. Lissencephaly is the absence of any gyral formation, or agyria, and the word lissencephaly actually means "smooth brain. ''34 Pachygyria may also be seen in association with lissencephaly in some cases. Cross-sectional analysis of the brain shows loss of the normal cerebral white matter to cortex ratio of 10:1, instead this ratio is 1:4 to 5. 34

Table 5. Hydrocephalus Recurrence Risk Aqueductal stenosis in male probands Any sibling Male sibling Aqueductal stenosis in female probands Any siblings Communicating hydrocephalus Dandy-Walker malformation Other X-linked

6% 12% 1% 1/154 0/54 0/22 4% empiric risk

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Lissencephaly is subdivided into at least two types. Type 1, or classical lissencephaly, is characterized by an entirely smooth cortex, but with relative sparing of the cerebellum. 31 The cortex lacks both normal gyri and sulci and has only four layers, instead of the normal six layers. 3~'36 These layers are the marginal, superficial cellular, cell sparse, and deep cellular. 31 When studied by MRI, the cortical gray matter is significantly thicker than 0.5 cm as seen in normal brainsY -37 Type 1 lissencephaly associated with facial dysmorphism suggests Miller-Dieker syndrome (MDS). MDS is a contiguous gene syndrome with multiple genes deleted characterized by type 1 lissencephaly, microcephaly, high forehead with vertical furrowing, bitemporal narrowing, small nose, upslanting palpebral fissures, thin upper lip, sacral dimples, joint contractures, and abnormal genitalia in males. 31 Patients typically suffer with low birth weight, feeding problems, hypotonia, recurrent chest infections, profound mental retardation, and seizures that are difficult to control. 31'38 The cortex is typically agyric with some areas of pachygyria.31 In most cases, the lissencephaly seen in MDS is based on the deletion of the LIS 1 gene located at 17p13.3. Deletion of other genes in this region are likely responsible for the other associated findings of the syndrome. Type 2 lissencephaly, or cobblestone lissencephaly, shows acellular zone pockets on microscopic examination. The cortex does not have the normal six layers. Instead, there are two layers of neurons, and there is disorganization. 34 Widespread leptomeningeal proliferation with both neuronal and glial ectopia in the leptomeninges is present. The pia may also be disrupted by migrating neurons, 35'36 and the cerebellum is not spared, unlike type 1 lissencephaly. 31 On MRI, the cortex is normal or mildly thickened, and areas of pachygyria and polymicrogyria a r e s e e n . 31'36'39 The subarachnoid space may be eliminated by thickened meninges adhered to the cortex; this then progresses to development of hydrocephalus. 31 Dandy-Walker malformation, poor myelination, fused cerebral hemispheres, absent or hypoplastic corpus callosum and septum pellucidum, and occipital cephaloceles may be seen in association with type 2 lissencephaly. 31'34 Often, there is associated congenital muscular dystrophy and eye abnormalities. The underlying pathogenesis of type

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2 lissencephaly is believed to be overmigration, cells migrating past the normal cortex into the leptomeningies. 34 Type 2 lissencephaly is most commonly seen as Walker-Warburg syndrome, but is also present in Muscle-Eye-Brain and Fukuyama congenital muscular dystrophy.31 Walker-Warburg syndrome, also known as cerebro-ocular dysplasia-muscular dystrophy syndrome, is an autosomal-recessive disorder characterized by hydrocephalus, type 2 lissencephaly, and retinal dysplasia. The brain is smooth with nodular areas, somewhat like a cobblestone street. Other associated findings include encephalocele, Dandy-Walker malformation, microphthalmia, colobomata, cataracts, corniai clouding, congenital muscular dystrophy, genital anomalies in males, congenital contractures, and cleft lip and palate. Clinical findings associated with lissencephaly vary depending on the degree of cortical malformation, but may give hints to diagnosing the type of lissencephaly. In classical lissencephaly (type 1), there are often subtle seizures, severe mental retardation, and sometimes intractable epilepsy. Clinical features found in type II, but not type I, include severe cerebellar hypoplasia, hydrocephalus, muscular dystrophy, and eye anomalies. 36'4~ Etiology and genetics. Lissencephaly may be isolated or associated with genetic syndromes, such as MDS and Walker-Warburg syndrome. The inheritance pattern may be autosomal dominant, autosomal recessive, or X-linked. Cytogeneticly detectable chromosome 17 alterations are frequently found in patients with lissencephaly, both classical lissencephaly and MDS, but some alterations are not detected without fluorescence in situ hybridization (FISH) analysis or direct sequencing. Males with x-linked lissencephaly (XLIS) have classical lissencephaly, whereas heterozygous females demonstrate the double cortex anomaly. Double cortex is characterized by a second layer of heterotopic cortical neurons underlying the normal c o r t e x . 36 This is suggested to be the result of random X-inactivation. Two populations of cells exist, cells expressing the normal gene and those expressing the mutant gene. The normal cells migrate normally, and the mutant cells have slowed migration, resulting in two layers of cortex, the double cortex. 34 Genes associated with lissencephaly. Genes involved with Type 1 lissencephaly include LIS1

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located at 17p13.3 and doublecortin (DCX, XLIS) located at Xq22.3 (Table 6). Mutations in these genes are associated with classical lissencephaly, double cortex, and MDS and account for 76% of isolated lissencephaly. Mutations in LIS 1 and doublecortin result in different patterns of lissencephaly. LIS 1 mutations result in posterior-biased lissencephaly (with sparing of the frontal region), whereas doublecortin mutations result in anteriorbiased lissencephaly (with sparing of the occipital region). 34,36,41 Mutations in the LIS 1 gene account for 60% of classical lissencephaly cases and >90% of MDS cases. 36'42 Some mutations in LIS1 act in a dominant manner, showing that a 50% decrease in the protein results in abnormal neuronal migration. 36 LIS 1 is a conserved protein, which resembles the typical beta subunit of a heterotrimeric G protein. 3%43 The gene, originally positionally cloned from patients with MDS, 34'43'44 encodes a 45 kDa protein that is the beta subunit of platelet-activating factor acetyl hydrolase (PAFAH), which inactivates platelet-activating factor (PAF). 34"45 Thus, lissencephaly may be secondary to a defect in PAF metabolism. 33'44 However, data in simple organisms show a loss of LIS1 gene product results in the same phenotype as a loss of dynein or dynactin. 33 Both LIS 1 and dynein are found at the kinetochores and in the cell cortex. 33 Dynein is involved in retrograde axonal transport, microtuble movement to the periphery, vesicular transport on microtubules, mitosis, and is required for nuclear translocation. 34 Expression of LIS l is highest in the central nervous system, particularly in neurons. 33 Therefore, if LIS1 acts on dynein, then LIS1 mutations may results in aberrant division of neuronal progenitor cells, causing a decrease in cortical neurons by interfering with the nomlal timing of cell division that is required for neuronal cell migration. 33 LIS 1-knockout mice have brain abnormalities similar to that seen in human lissencephaly. 33'46 Doublecortin (DCX) associates with microtubules and plays a role in regulating their stability.

Table 6. Lissencephaly Genes Type of Lissencephaly Type I, classical X-linked Miller-Dieker syndrome Type II, cobblestone Walker-Warburg syndrome

Gene

Locus

DCX/XLIS LIS1

Xq22.3 17p13.3

--

Fig 4. MRI of patient with lissencephaly. Axial MRI scan shows the complete lack of any gyri or sulci on the surface of the brain, which is smooth. Mild dilation of the lateral ventricles is present posteriorly (colpocephaly).

Mutations in doublecortin result in either classical lissencephaly or double cortex depending on the sex of the individual, as mentioned above. Diagnosis and recurrence risk. Depending on the overall pattern of clinical findings, whether there are associated findings or not, and the family history, a preliminary diagnosis can be made. Lissencephaly can be easily diagnosed on imaging studies, such as CT or MRI (Fig 4). The pattern seen on MRI can distinguish between type 1 and type 2 lissencephaly. Chromosome studies should be performed, and analysis can be directed based on the inheritance pattern or any positive family history. FISH may be required in classical lissencephaly and MDS cases that do not show any cytogeneticly detectable abnormality. Further mutation analysis of the LIS1 gene is indicated in cases of classical lissencephaly, if other testing is negative, because it may not only give diagnostic information, but aid in counseling about recurrence risk. Isolated lissencephaly has a recurrence risk of 5% to 12%, but may be as high as 25% in recessive cases. Most cases of lissencephaly are secondary to a deletion of 17p13, some of which are detectable

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cytogeneticly, whereas others require FISH analysis. Classical lissencephaly with a known LIS1 deletion or mutation has a very remote recurrence risk, 36 unless there is a balanced translocation in one parent. Therefore, LIS1 analysis in cases of classical lissencephaly may be very helpful in counseling families. Cases not caused by the 17p 13 deletion show an even higher recurrence risk. Thus, it is essential to determine which gene or region is involved for each family to accurately predict recurrence risk.

Schizencephaly Porencephaly includes any cerebral cortex cavity or cleft. 46 There are two types of porencephaly, developmental and encephaloclastic. Developmental porencephaly is due to abnormal neuronal development or migration. Schizencephaly is another term for developmental porencephaly. Additional terms include true, prenatal, basket handle, embryonic, and congenital porencephaly. Encephaloclastic porencephaly is the result of cortical destruction and demonstrates gliosis or inflammatory changes. Glial scarring must be absent for a diagnosis of schizencephaly to be made. 47 Schizencephaly is defined as a cleft of the cerebral mantle in the region of the primary cerebral fissure. 47'48 There is in-folding of the cerebral cortex with intact pia present, creating a pialependymal seam. The cortex is often thickened. 46 The cleft extends across the cerebral hemispheres from the pial surface to the ventricular surface (ependyma). 48 These finding are usually bilateral, but may be unilateral with contralateral heterotopia or gyral anomalies. Associated finding include absence of the septum pellucidum, septo-optic dysplasia, heterotopia, and microgyria. 46 Some investigators suggest that schizencephaly and septooptic dysplasia are part of a spectrum. Schizencephaly has been subdivided into type I and type II. In type I schizencephaly, the two sides of the cleft are apposed, and there is no hydrocephalus associated. Type II has associated hydrocephalus with a completely open cleft that connects the subarachnoid space to the ventricles. However, there have been cases reported of type I schizencephaly progressing to type II in utero. 46 Etiology and genetics. Schizencephaly may be either acquired or developmental in etiology. Vascular disturbances, maternal trauma, fetal cytomegalovirus infection, or prenatal exposure to

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drugs may play a role in development of schizencephaly. 48 Acquired schizencephaly is often due to a vascular insult around 12 weeks of gestation, whereas developmental schizencephaly is either sporadic or familial. Developmental schizencephaly is due to a primary failure of neuronal development or migration in the early gestational period. 48 The presence of bilateral involvement suggests a genetic basis for schizencephaly. Inheritance may be autosomal recessive or autosomal dominant with incomplete penetrance, but maternal (nongenetic) factors may be involved also. 48 Genes associated with schizencephaly. Mutations in the human homeobox gene, EMX2 (empty spiracles 2) are responsible for familial schizencephaly, 1'49'5~ and the inheritance may be autosomal dominant with variability. 51 Two brothers with the same mutation have very different clinical severity and brain malformation. This suggests that there are other factors that play a role in determining the effect of a mutation in the EMX2 gene. 5~ Diagnosis and recurrence risk. Typical presenting features of schizencephaly include seizures

Fig 5. MRI of a patient with open lip schizencephaly. Axial MRI scan shows a communication between the subarachnoid fluid and the ventricular system on the right side (white arrowhead). CSF in the subarachnoid space and the ventricles is black on this Tl-weighted image.

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(tonic-clonic, partial motor, sensory and infantile spasms), hemiparesis, quadriparesis, or developmental delay. 46'52 Diagnosis is made by imaging, ultrasonography, CT, or MRI (Fig 5). However, there are arguments that MRI is needed to not miss the diagnosis. The distinguishing features on imaging are cerebral gray matter continuing into the cleft and the pial-ependymal connection. Recurrence risk for sporadic schizencephaly is very low, but familial schizencephaly may have 25% to 50% recurrence risk, depending on the mode of inheritance. CONCLUSIONS Here we have presented the current state of genetic knowledge on a few, representative CNS abnormalities. However, this illustrates how genet-

ics has added insight into their development and, more importantly, into their management. Similarly, other CNS abnormalities will soon be understood at the genetic level, such as epilepsy, mental retardation, and attention deficit disorder. Our understanding of CNS abnormalities is steadily increasing. As research continues, neurology and genetics together will be able to better define normal and abnormal brain development. Advances in these areas will allow for easier, more accurate diagnosis and will direct proper treatment and management for patients. In the end, patients will benefit optimally from these advances. Acknowledgment The authors thank Dr. Stuart Morrison for his assistance in preparing MR images of central nervous system malformations.

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