Brain, Vol. 125, No. 11, 2507-2522,
November 2002
© 2002 Oxford University Press
Subcortical band heterotopia (SBH) in males: clinical, imaging and genetic findings in comparison with females
1 Department of Neurology and Neurosurgery, and the Montreal Neurological Institute and Hospital, Departments of 2 Human Genetics and 3 Pediatrics, McGill University, Montreal, 4 Health Sciences Centre, Winnipeg, Canada, 5 Department of Human Genetics, University of Chicago, Chicago, 6 Pediatric Neurology Service, Shodair Children’s Hospital, Helena, 7 Division of Pediatric Neurology, Department of Neurosciences, University of California, San Diego, 8 Epilepsy Center, Swedish Medical Center, Seattle, 9 Department of Neurology, Walter Reed Army Medical Center, Washington, USA, 10 Clinical Hospital, Sao Paulo University, Sao Paulo, 11 Porto Alegre Epilepsy Surgery Program, Hospital Sao Lucas, Pontificia Universidade Catolica do Rio Grande do Sul, Porto Alegre, 12 Department of Medical Genetics, University of Campinas, Campinas, Brazil, 13 Department of Neurology, Austin and Repatriation Medical Centre, University of Melbourne and Royal Children’s Hospital, Melbourne, Australia, 14 Neuroscience Unit, Institute of Child Health and Great Ormond Street Hospital for Children, University College London, London, UK, 15 Centre Saint-Paul, Marseille, France, 16 Division of Pediatrics, Toyonaka Municipal Hospital, Toyonaka, Japan, 17 Department of Neurosciences, ‘Santa Maria della Misericordia’ Hospital, Udine, and Associazione Anni Verdi, 18 Neurometabolic Unit, Institute of Neurological Sciences, University of Siena, Siena and Associazione Anni Verdi, Rome, 19 Ospedale Bellaria C.A. Pizzardi, Bologna and 20 Centre for Pediatric Neurology, Bologna University, Bologna, Italy
Correspondence to: Eva Andermann, MD, PhD, FCCMG, Neurogenetics Unit, Montreal Neurological Hospital and Institute, 3801 University Street, Montreal, Quebec H3A 2B4, Canada E-mail: mida{at}musica.mcgill.ca
Received August 13, 2001. Revised June 10, 2002. Accepted June 13, 2002.
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Summary |
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Subcortical band heterotopia (SBH) or double cortex syndrome is a neuronal migration disorder, which occurs very rarely in males: to date, at least 110 females but only 11 in males have been reported. The syndrome is usually associated with mutations in the doublecortin (DCX) (Xq22.3-q23) gene, and much less frequently in the LIS1 (17p13.3) gene. To determine whether the phenotypic spectrum, the genetic basis and genotype–phenotype correlations of SBH in males are similar to those in females, we compared the clinical, imaging and molecular features in 30 personally evaluated males and 60 previously reported females with SBH. Based on the MRI findings, we defined the following band subtypes: partial, involving one or two cerebral lobes; intermediate, involving two lobes and a portion of a third; diffuse, with substantial involvement of three or more lobes; and pachygyria-SBH, in which posterior SBH merges with anterior pachygyria. Karyo typing and mutation analysis of DCX and/or LIS1 were performed in 23 and 24 patients, respectively. The range of clinical phenotypes in males with SBH greatly overlapped that in females. MRI studies revealed that some anatomical subtypes of SBH, such as partial and intermediate posterior, pachygyria-SBH and diffuse bands with posterior predominance, were more frequently or exclusively present in males. Conversely, classical diffuse SBH and diffuse bands with anterior predominance were more frequent in females. Males had either mild or the most severe band subtypes, and these correlated with the over-representation of normal/borderline intelligence and severe mental retardation, respectively. Conversely, females who had predominantly diffuse bands exhibited mostly mild or moderate mental retardation. Seven patients (29%) had missense mutations in DCX; in four, these were germline mutations, whereas in three there was evidence for somatic mosaicism. A germline missense mutation of LIS1 and a partial trisomy of chromosome 9p were identified in one patient (4%) each. One male each had a possible pathogenic intronic base change in both DCX and LIS1 genes. Our study shows that SBH in males is a clinically heterogeneous syndrome, mostly occurring sporadically. The clinical spectrum is similar to that of females with SBH. However, the greater cognitive and neuroradiological heterogeneity and the small number of mutations identified to date in the coding sequences of the DCX and LIS1 genes in males differ from the findings in females. This suggests other genetic mechanisms such as mutations in the non-coding regions of the DCX or LIS1 genes, gonadal or somatic mosaicism, and finally mutations of other genes.
Keywords: DCX; double cortex; LIS1; male; subcortical band heterotopia
Abbreviations: AED = antiepileptic drugs; a–p = anterior–posterior; DCX = doublecortin gene; FISH = fluorescence in situ hybridization; FSIQ = full-scale intelligence quotient; IQ = intelligence quotient; LIS1 = lissencephaly gene 1; SBH = subcortical band heterotopia
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Introduction |
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Subcortical band heterotopia (SBH), also known as subcortical laminar heterotopia or double cortex syndrome, is a cortical malformation characterized by the presence of symmetrical and bilateral bands of heterotopic grey matter located between the ventricular wall and the cortical mantle, and clearly separated from both (Dobyns et al., 1996
; Harding, 1996).
Affected individuals typically present with epilepsy and variable degrees of mental retardation. Seizures often start in the first decade and vary from partial to generalized attacks. They may progress to multiple seizure types and are usually refractory to medication. Neurological examination may be normal, but dysarthria, hypotonia, poor fine motor control or, rarely, a pyramidal syndrome may be present (Palmini et al., 1991; Barkovich et al., 1994).
Diagnosis is based on MRI, which shows the characteristic isointensity of the heterotopic band with the cortex in all imaging sequences (Barkovich et al., 1989). The thickness and extent of the band can vary (Barkovich et al., 1994; Gleeson et al., 2000a), while the appearance of the overlying cortical mantle on MRI may be normal, show a simplified gyral pattern or, rarely, true pachygyria (Barkovich et al., 1994; Dobyns et al., 1996; Guerrini and Carrozzo, 2001).
Most patients with SBH are females (Andermann and Andermann, 1996; Dobyns et al., 1996): to date, at least 110 females with SBH (Matell, 1893; Jacob, 1936, 1938; Wiest and Hallervorden, 1958; Barkovich et al., 1989; Palmini et al., 1991; Gallucci et al., 1991; Ricci et al., 1992; Soucek et al., 1992; Hashimoto et al., 1993; Iannetti et al., 1993; Landy et al., 1993; Miura et al., 1993; Tohyama et al., 1993; Barkovich et al., 1994; De Volder et al., 1994; Parmeggiani et al., 1994; Scheffer et al., 1994; Harding, 1996; Berg et al., 1998; Vossler et al., 1999; Gleeson et al., 2000a), but only 13 males have been reported (Barkovich et al., 1994; Ketonen et al., 1994; Franzoni et al., 1995; Gigli et al. 1996; Ono et al., 1997; Federico et al., 1999; Pilz et al., 1999; Vossler et al., 1999; Pinard et al., 2000; Kato et al., 2001; Poolos et al., 2002).
Although most patients with SBH are sporadic, a syndrome of familial SBH with X-linked inheritance, in which the vast majority of carrier females have SBH and affected males usually have classical lissencephaly, has been described (Pinard et al., 1994; Scheffer et al., 1994; Andermann and Andermann, 1996; Dobyns et al., 1996). However, in at least one family, a mother and son were both found to have SBH (Pilz et al., 1999).
Two genes have been demonstrated to be involved in the aetiology of SBH: (i) DCX (also known as doublecortin or XLIS), located on chromosome Xq22.3-q23 (des Portes et al., 1997, 1998a, b; Ross et al., 1997; Gleeson et al., 1998, 1999; Sossey-Alaoui et al., 1998; Horesh et al., 1999) and (ii) LIS1 (also called PAFAH1B1 because it codes for the beta 1 subunit of brain platelet activating factor acetylhydrolase) on chromosome 17p13.3 (Ledbetter et al., 1992; Reiner et al., 1993; Hattori et al., 1994; Chong et al., 1997; Lo Nigro et al., 1997; Sapir et al., 1997; Pilz et al., 1999).
DCX mutations have been found in 
80% of sporadic females with SBH and in all multiplex families with SBH, both in SBH females and in males with lissencephaly (des Portes et al., 1998a, b; Gleeson et al., 1998, 1999, 2000a; Matsumoto et al., 2001). In sporadic males, mutations of this gene are usually associated with lissencephaly (agyria/pachygyria) (Pilz et al., 1998, 2002). These data suggest that SBH is a mild form of lissencephaly that usually results from the effects of random inactivation in heterozygous females (des Portes et al., 1998a, b; Gleeson et al., 1998, 1999). Palmini et al. (1993) previously postulated the developmental continuum among SBH, pachygyria and lissencephaly.
Only one sporadic male with SBH has been demonstrated to carry a missense mutation of the LIS1 gene located on chromosome 17p13.3 (Pilz et al., 1999). LIS1 mutations are responsible for 65% of classical lissencephaly (Pilz et al., 1998; Cardoso et al., 2002).
Several recent reports have demonstrated that LIS1 mutations are associated with more severe lissencephaly or SBH over the parietal and occipital regions, whereas DCX mutations are associated with more severe abnormalities over anterior brain regions (Pilz et al., 1998, 1999; Dobyns et al., 1999; Gleeson et al., 2000a).
Despite the progress in understanding the molecular basis of SBH in females, mutations in males have largely remained unidentified to date. It is also unclear whether the phenotypic spectrum of the rare males with SBH is the same as that in affected females, and whether similar genotype–phenotype correlations can be demonstrated in males with SBH. To clarify these issues, we present the clinical and imaging phenotypes and molecular genetic data for 30 males with SBH, and compare these with the corresponding features in 60 SBH females.
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Patients and methods |
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Patients
Five male patients (1, 4, 10, 20 and 24) were studied at the Montreal Neurological Hospital. The remaining 25 patients were from 21 centres on five continents. Eight patients have been reported previously: patients 2 and 25 (Pilz et al., 1999
); 7 (Franzoni et al., 1995); 15 (Ketonen et al., 1994; Gleeson et al., 2000b); 18 (Gigli et al. 1996; Federico et al., 1999); 19 (Barkovich et al., 1994; Pilz et al., 1999); 21 (Ono et al., 1997); and 27 (Vossler et al., 1999). All of these patients were known personally to at least one of the authors. For all males, detailed information regarding family history, abnormal pre- and peri-natal events, age at seizure onset, psychomotor development, cognitive function, neurological examination, EEG and neuro-imaging findings was available (Tables 1 and 2).
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For the meta-analysis, the 30 males were compared with 60 females with SBH whose detailed clinical information has been published (Palmini et al., 1991
; Gallucci et al., 1991; Ricci et al., 1992; Soucek et al., 1992; Hashimoto et al., 1993; Iannetti et al., 1993; Landy et al., 1993; Miura et al., 1993; Tohyama et al., 1993; Barkovich et al., 1994; De Volder et al., 1994; Parmeggiani et al., 1994; Scheffer et al., 1994; Harding, 1996; Berg et al., 1998; Vossler et al., 1999) (Table 3). The same electroclinical and neuroradiological classification criteria were used for males and females (Table 3).
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Methods
The International Classifications of Seizures [Commission on Classification and Terminology of the International League Against Epilepsy (CCTILAE, 1981
)] and of Epilepsies and Epileptic Syndromes (CCTILAE, 1989) were utilized for classification of seizures and epileptic syndromes, respectively. Both routine scalp EEG and prolonged video-EEG recordings were performed in all patients (Table 1). Seven patients in our series underwent surgery (Table 1). In patient 20, a frontal lobe biopsy was performed in association with callosotomy.
To stratify the patients according to cognitive function (Table 2), we defined six cohorts based on available clinical information and IQ: (i) normal, IQ 
80; (ii) borderline, IQ 70–80; (iii) mild delay, IQ 55–69; (iv) moderate delay, IQ 40–54; (v) severe delay, IQ 25–39; (vi) profound delay, untestable. Eight patients had a formal psychometric evaluation: four children were evaluated using the Wechsler Intelligence Scale—Revised (WISC-R), and in three patients the Wechsler Adult Intelligence Scale was used. Patient 20 underwent testing with the Vineland Adaptive Scale instead of the WISC-R. Eight patients were too young and four too severely retarded for formal psychometric evaluation. In the meta-analysis, the six categories of cognitive functioning were reduced to three: normal–borderline intelligence, mild–moderate mental retardation and severe–profound mental retardation (Table 3).
Cranial MRI studies were performed in all patients (Table 2). We termed SBH partial when it was highly localized involving one to two cerebral lobes, intermediate when it involved two lobes and a small portion of a third lobe, and diffuse when there was substantial involvement of at least three lobes. In practice, partial bands involved either the frontal lobe only or both parietal and occipital lobes. Intermediate bands involved part of the posterior frontal lobe plus the parietal and occipital lobes. Diffuse bands involved most of the frontal, parietal and occipital lobes, with anterior extension at least to the mid-frontal lobe and usually into the anterior frontal lobe. Extension into the temporal lobes was quite variable in all types. The term ‘simplified gyral pattern’ was used to denote an abnormal gyral pattern consisting of gyri of normal width (
1 cm) and cortical thickness (4 mm) that were separated by wide and shallow sulci. We used the term pachygyria to denote gyri with a width of 1.5 cm and abnormally thick cortex.
Routine or high-resolution karyotyping was performed in 23 patients, and fluorescence in situ hybridization (FISH) studies on 11 patients (3, 4, 8, 9, 11, 17, 19, 20, 22, 24 and 25) using cosmid probes (D17S370, D17S379, L132, 37E9 or 120A7) corresponding to the lissencephaly syndrome critical region of 17p13.3. In patient 18, FISH analysis was performed with a chromosome 9 ‘painting’ probe (ONCOR).
Clinical data and blood samples were obtained with informed consent from 24 patients, and DNA was extracted using a standard protocol. Mutation analysis was performed for both DCX and LIS1 in 16 patients, for DCX only in seven patients, and for LIS1 only in one patient. We did not analyse the second gene when a convincing mutation was detected in the first gene tested. Mutation detection was performed by direct sequencing of genomic DNA as described previously (Lo Nigro et al., 1997; Pilz et al., 1999; Cardoso et al., 2000). In most patients, the mutation was confirmed to be de novo by direct sequencing of both parents. The investigators were unaware of mutation data at the time of initial neuroimaging review. DNA samples from six patients were unavailable.
Differences in age at seizure onset between males and females were analysed using the Student’s t-test. Differences in seizure types at onset, clinical syndromes, cognitive function, neurological examination, dysmorphic features, imaging characteristics of the SBH and presence of additional brain abnormalities were analysed using Fisher’s exact test.
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Results |
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Prenatal and perinatal events
Prenatal events occurring within the first 4 months of pregnancy included: application of local heat to the lower abdomen, but with no skin burns (mother of patient 6, up to second month of pregnancy); moderate flu-like symptoms (mothers of patients 8 and 22, at 3 and 4 months of pregnancy, respectively, the former having been treated with paracetamol); exposure to bonding glue (mother of patient 8, at 4 months of pregnancy); and intake of doxylamine and an antihistaminic (H1-receptor)-decongestant preparation (mother of patient 19, at 2–3 months of pregnancy). Perinatal complications included cord around the neck (patient 8), foetal distress and marked meconium staining of the amniotic fluid (patient 19), and cephalo-pelvic disproportion (patient 22). The last two led to emergency Caesarian sections.
Clinical findings
The age range at the time of study was 1 month to 34 years. Patients 25 and 30 were the youngest, aged 2 and 9 months, respectively, at the time of the clinical evaluation.
Motor development and status were mildly delayed in eight patients (walking between 18 and 24 months: patients 2, 6, 10, 14, 16, 19, 20 and 22), moderately delayed in one (walking between 24 and 30 months: patient 18) and severely delayed in four (started walking only after 30 months: patients 17, 21, 23 and 26). Patient 11 did not walk at 16 months, and patient 24 was never able to walk, even with proper support. Patient 30 had a 3-month delay in gross motor development at the age of 9 months. Early motor development and function were normal in 14 males.
Language development and use were abnormal in 13 patients: three had mild delay (first words and sentences between 18 and 30 months of age: patients 10, 16 and 22), four had moderate delay (2, 7, 19 and 20: first words and sentences between 30 and 42 months), and three had severe delay (a few words, no sentences after 3.5 years in patients 21, 23 and 26). Patients 17, 24 and 27 were non-verbal at the ages of 5, 34 and 42 years, respectively. The remaining males had normal language development.
Cognitive function (Table 2) was normal in six patients and borderline in six. The rest had variable mental retardation, assessed to be mild in five, mild to moderate in one, moderate in one, severe in seven and profound in two. Given their very young age, no definitive statement could be made about the cognitive function of patients 25 and 30.
Abnormalities on examination
Eighteen patients had abnormal neurological findings (Tables 1 and 4). Of the associated malformations and dysmorphic features, microcephaly was the most frequently observed.
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Epilepsy
Seizure histories (Table 1)
Two patients (13 and 18) never had seizures. In one patient (18), the investigations that led to the diagnosis were initiated because of dysmorphic features and mental retardation. Ten of 28 patients (36%) had a single seizure type, mainly complex partial seizures. Eighteen patients (64%) had more than one seizure type. Age at onset of epilepsy ranged from 5 days to 16 years (mean 50.7 months, median 36 months) (Table 1). Eleven patients had tonic, atonic or myoclonic drop attacks. In one of them, this consisted of axial myoclonus precipitated by unexpected noise or when suddenly touched. Twelve patients had generalized tonic clonic seizures, and nine had generalized myoclonic jerks that often occurred in clusters after awakening. Atypical absences were present in five patients. Four had generalized tonic attacks; in one (26), these occurred as tonic spasms elicited by seeing food or eating. In patient 22, epilepsy began at the age of 18 months with generalized convulsive status epilepticus.
Sixteen patients had complex partial seizures. Simple motor and sensory attacks were noted in four and two patients, respectively; one patient had supplementary motor attacks occurring up to 30 times a night and several times during the day. One patient had daily minor attacks with subtle blinking or blurred vision, and occasionally appeared frightened or covered his eyes during these ‘blind spells’. Four patients had focal myoclonic attacks; in one (14), these were often followed by a drop attack.
Four patients had Lennox–Gastaut syndrome (19, 20, 24 and 27). Two had infantile spasms with developmental regression after their onset (11 and 17). The clinical picture in patients 16, 22 and 23 was also suggestive of Lennox–Gastaut syndrome, but the EEG findings were inconsistent with this diagnosis.
EEG results
Four patients had only focal spikes. In 14, bilateral independent or multifocal epileptic abnormalities were found, with side predominance noted in four (1, 6, 12 and 14) and secondary bilateral synchrony in two (1 and 16). Two patients had mostly generalized bilateral and synchronous epileptic activity; in one (2), this occurred during sleep. Generalized slow spike and waves in association with multifocal spikes or generalized polyspikes or polyspike and wave complexes were found in four patients. Hypsarrhythmia was the main finding in one patient (17). Background activity anomalies only and normal EEGs were found in two patients each. We had no information on the EEGs of patient 25.
Response to antiepileptic drugs
Seven patients (2, 3, 4, 7, 9, 15 and 27; 25%) responded satisfactorily to antiepileptic drugs (AEDs). Except for patient 27, these patients had normal or borderline intelligence or mild mental retardation and only one seizure type. Patient 9 had no more than two seizures per month with carbamazepine and vigabatrin, and patient 27 had a dramatic and sustained improvement upon addition of lamotrigine to valproic acid and phenytoin. The remaining five individuals received monotherapy: either valproic acid (2, 3 and 4) or carbamazepine (7 and 15). Patients 2 and 3 had no further seizures after starting treatment and the other three patients had two to five minor attacks per month, with no detectable effects on behaviour or cognition. Eight patients (8, 10, 11, 21, 22, 25, 29 and 30; 28%) responded poorly to AEDs, with slight reduction in seizure frequency, control of only some types of seizures, or persistence of multiple seizures at night. Thirteen patients (46%) were refractory to medical therapy, with up to 20–30 seizures daily despite appropriate treatment and trials of multiple drug regimens. Most developed severe behavioural problems, mainly aggression and decline in learning abilities.
Surgical therapy
The surgical procedures carried out in our patients are detailed in Table 1. Despite some early improvement, there was no sustained reduction in the frequency and severity of seizures in four out of seven patients who were surgically treated. Patient 14 had infrequent focal myoclonic jerks of the left hand 1 year after subpial transection, but no further progression to drop attacks. The marked reduction of drop attacks in patients 1 and 20 continued at 1 year after callosotomy.
MRI abnormalities (Table 2 and Fig. 1)
The subcortical band varied substantially in extent and thickness. Six major groups were seen: (1) thin partial frontal SBH with no involvement of the posterior regions (two patients); (2) thin partial posterior SBH with no involvement of the frontal regions (four patients); (3) medium or thick intermediate SBH that was always more prominent posteriorly (seven patients); (4) diffuse thin SBH (two patients); (5) diffuse medium or thick SBH (10 patients); and (6) anterior pachygyria that merged into posterior SBH (five patients). In intermediate posterior SBH, the band extended from the occipital pole to just reach the posterior frontal or temporal regions. In the two patients (12 and 29) with diffuse thin SBH (group 4), the band was asymmetric with a discontinuous appearance on the right side. Further heterogeneity was also found among patients with diffuse medium or thick SBH (group 5), three of whom had clear posterior predominance. Among the latter, one appeared thickest in the centro-parietal region (patient 17) and two had sparing of the anterior frontal regions (patients 19 and 20). The other patients in group 5 appeared thickest frontally (patient 14) or had no obvious differences in thickness between the front and the back. In this study, some of the scans did not permit precise localization of the central sulcus. These groups are different from and should not be confused with the lissencephaly-SBH grading system used in several other papers (Pilz et al., 1998; Dobyns et al., 1999). In this paper, group 6 with mixed pachygyria-SBH corresponds to lissencephaly-SBH grade 5, while the other groups are variants of lissencephaly-SBH grade 6.
When we considered the distribution of the malformations regardless of severity or thickness of the band, we were able to identify three main groups. In the first, SBH and the overlying cortical malformation were only present or more severe in anterior brain regions representing an anterior-to-posterior (a > p) gradient (eight patients). This cluster included partial frontal bands (group 1), diffuse bands that were thicker in the anterior head regions (patient 14 from group 5), and the anterior pachygyria-posterior SBH (group 6). In the next group, the posterior aspects of the cerebral hemispheres were the more severely involved comprising a posterior-to-anterior (p > a) gradient (14 patients). This category encompassed thin partial posterior (group 2) or intermediate posterior (group 3) bands and diffuse bands with clear posterior predominance (patients 17, 19 and 20 from group 5). Finally, we reviewed several patients with diffuse bands in whom the MRI showed no differences between anterior and posterior regions, or an a = p gradient (eight patients, comprising those from group 4 and patients 13, 15, 16, 18, 21 and 30 from group 5). Most bands were symmetric.
A simplified gyral pattern with short gyri and shallow sulci was observed in 10 patients. Five patients had true pachygyria. Eight had enlarged ventricles, two had a thin corpus callosum and one had cerebellar hypoplasia.
Neuropathology
In patient 20, a right frontal biopsy revealed a decreased number of neurones in the cortex, with some attempt at columnar alignment from white matter to the surface. The first two layers appeared well formed, but in the underlying layers, disorganization and neuronal clumps were seen. In all sections, the white matter contained an excessive number of neurones resembling those normally present in the lower layers of the cortex. In patient 12, examination of the resected tissue showed heterotopic grey matter arranged in a linear band and normal cortical thickness.
Family history
The mother of patient 1 has epilepsy and mental retardation (FSIQ 59), and lives in a foster home. She has two sisters and a mentally retarded brother with epilepsy. Her MRI as well as that of one of her affected sisters gave no evidence for SBH. Ten other siblings are healthy. The mother of patient 2 has had epilepsy since the age of 18 years with myoclonic seizures leading to secondary generalization and episodes of speech arrest. Her MRI showed bilateral asymmetric frontal SBH. Her family history was unremarkable. Patient 18 was the youngest child of probably consanguineous parents. A sister died on the 25th day of life with intractable seizures, and three brothers died at birth of unknown causes (Gigli et al. 1996; Federico et al., 1999). The remaining histories were unremarkable with respect to family history of seizures or mental retardation.
Genetic findings (Table 2)
A normal male karyotype was found in 22 of the 23 patients tested (96%). In patient 18, chromosome analysis revealed a de novo partial trisomy of the entire short arm of chromosome 9 in all cells analysed: 47,XY,+der(9)(q11-pter). In this male, mutation analysis of the coding sequences of both DCX and LIS1 was negative. FISH analysis for chromosome 17p13.3 was normal in all 11 males tested.
We detected mutations of either DCX or LIS1 in 10 of the 24 patients who underwent mutation analysis, which included the coding sequences as well as some flanking intronic sequences. Thus, the overall rate of mutation detection in males with SBH (42%) is much lower than in females with SBH (85%; Matsumoto et al., 2001). The mutations included seven missense and one intron mutations in DCX, and one missense and one intron mutation of LIS1. Genotype–phenotype comparison showed consistent differences between the groups defined by MRI appearance, as DCX mutations were detected in patients with a = p and a > p phenotypes, while LIS1 mutations were found in patients with p > a phenotypes. Specifically, DCX mutations were found in seven of 12 (58%) patients with a = p or a > p gradients, and LIS1 mutations were found in two of 12 (17%) patients with a p > a gradient.
The two patients with partial frontal bands (1 and 2) were found to have novel familial missense mutations of DCX inherited from their mothers. In the family of patient 1, a missense mutation of exon 5 of DCX was also detected in a maternal aunt with epilepsy and no evidence of SBH.
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Discussion |
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Our series demonstrates that the clinical spectrum of SBH in males overlaps with that in females in terms of representation of seizure types, epilepsy syndromes and response to antiepileptic therapy. However, there is increased heterogeneity with respect to cognitive function, neuroimaging and molecular genetic data in males compared with females (Tables 1–3).
Onset of symptoms and clinical course
In 65% of both males and females, the brain malformation was revealed by onset of seizures. In the remaining patients, the investigations that led to the diagnosis were prompted by the presence of developmental delay, sleep disorders, dysmorphic features, behavioural or learning problems alone or in association. Epilepsy was eventually diagnosed in 95% of patients of both sexes. Age at seizure onset was earlier in males than in females, but the difference was not statistically significant (Table 3).
No significant differences were found in the seizure types at onset or in types of habitual seizures (Table 3). Focal seizures predominated both at the onset of epilepsy and as habitual seizures, but in the latter they were usually combined with multiple seizure types. The Lennox–Gastaut syndrome and infantile spasms were equally represented in male and female patients. A similar percentage of patients from both groups had intractable epilepsy and underwent surgery (Table 3).
Neuropsychological investigations showed that the two extremes of the cognitive function levels: normal–borderline intelligence and severe–profound mental retardation were over-represented within the male group, whereas females exhibited mostly mild–moderate mental retardation (Table 3).
Analysis of SBH subtypes
The most striking differences among patients were revealed by the MRI findings. First, partial and intermediate SBH were significantly more frequent in males than in females (43% versus 10%, P = 0.0006) (Table 3). In addition, the SBH subtypes seen among males were more frequently confined to the posterior aspects of the cerebral hemispheres and highly localized when compared with females (patients 3–11, 27 and 28; Tables 2 and 3). Only two males (1 and 2) had partial frontal bands, while five out of six (83%) females with partial or intermediate SBH had frontal bands (Table 3). However, three other sporadic females with posterior bands out of a series of 30 females with SBH have been reported (Gleeson et al., 2000a). Thus, all the atypical band subtypes seen in males have been observed in females, although they are rare (Gleeson et al., 2000a; W.B.D., unpublished observations). Both male and female patients with partial frontal SBH were familial, whereas those with posterior SBH were sporadic.
The majority of the males with posterior partial or intermediate SBH had normal or borderline intelligence or mild mental retardation, normal early development and focal epilepsy. Only one male (11) with an intermediate posterior band had a moderate degree of mental retardation; his SBH was thick (1 cm) and he had infantile spasms. Patient 27 (Vossler et al., 1999) may also have had a posterior intermediate band. He had Lennox–Gastaut syndrome and had profound mental delay. Three of the four females with partial or intermediate posterior SBH (including those described by Gleeson et al., 2000a) had moderate or profound mental retardation: one had a thick intermediate posterior SBH and the remaining two had Lennox–Gastaut syndrome.
Secondly, diffuse bands were significantly over-represented among females (90% versus 40%, P = 0.000001) (Table 3). Within the diffuse SBH group, the malformation tended to predominate over the posterior brain regions in males and over the anterior brain regions in females. Similar to females, males with this type of SBH had mental development ranging from normal to severe delay depending on the thickness of the band, the degree of cortical pattern derangement, associated brain abnormalities and epileptic syndrome. For example, patient 12 with a thin, almost discontinuous band and only complex partial seizures had normal intelligence, whereas patient 17, whose SBH was thick and predominated over the posterior aspects, had had infantile spasms and showed severe mental retardation (Tables 1 and 2). Seizure type at presentation was mostly complex partial or myoclonic. Subsequently, myoclonic seizures and drop attacks in association predominated (Table 1). Attacks were intractable in all but one male with diffuse SBH, and four underwent surgery.
Finally, the pachygyria-SBH pattern was only observed in males, while no females have been reported (Table 3); however, we are aware of two such females (W.B.D., unpublished data). The males with this SBH subtype (22, 23, 24, 25 and 26) exhibited the most severe clinical phenotype (Tables 1–3). Early development and cognitive functioning ranged from normal to profound mental retardation. Patient 22 with mild delay and patient 25 with normal early development were the youngest, aged 3 years and 2 months, respectively, at the time of evaluation. The most severe degree of malformation in this group of patients is in the frontal cortex, an area that is relatively silent until 7 years of age, when its full maturation is achieved (Luria, 1973). In the other three patients, now aged 8, 20 and 34 years, interaction with the environment was virtually absent. In this group, generalized tonic clonic status epilepticus, atypical absences or myoclonic drop attacks marked the onset of habitual seizures, and epilepsy was intractable. Patient 24 is the most severely affected individual with SBH ever reported.
Mean age of onset of epilepsy was earlier in the pachygyria-SBH group (10.3 months) as compared with the partial/intermediate (4.4 years) and the diffuse (5.7 years) SBH groups. This finding contributes to the difference in age of onset of epilepsy between males and females (Table 3), as well as to the differences in developmental and cognitive functioning. The increased number of partial posterior bands as well as of pachygyria-SBH among males may explain, respectively, the significant over-representation of normal/borderline intelligence and severe/profound mental retardation within this group. Conversely, females who had predominantly diffuse bands, a paucity of partial bands and no pachygyria-SBH exhibited mostly mild/moderate mental retardation.
The genetic basis of SBH in males
Mutations of DCX are the major cause of SBH. In all but one or two SBH patients in whom a mutation has been demonstrated, this involves the DCX gene (Pilz et al., 1999; Gleeson et al., 2000b). In females, DCX mutations have been associated either with diffuse bands with no apparent gradient (a = p) or with bands compatible with an anterior-to-posterior gradient (a > p) of the malformation (Pilz et al., 1999; Gleeson et al., 2000a). However, because DCX is located on the X chromosome, males who are hemizygous for a DCX mutation usually have lissencephaly (Dobyns et al., 1996; Pilz et al., 1999). The molecular findings in patients 1, 2, 13 and 25 illustrate that some missense mutations in the DCX gene have sufficient residual function to result in SBH rather than lissencephaly (Pilz et al., 1999). Patients 1, 2 and 25 have band types that, although different in their severity, fit the anterior-to-posterior (a > p) gradient, which is mostly associated with DCX mutations (Pilz et al., 1999; Gleeson et al., 2000a), whereas patient 13 has an a = p gradient. Conversely, the specific amino acid alteration and its position in the protein may explain the differences in the band types.
An additional mechanism by which DCX mutations may be responsible for SBH in males is that of somatic mosaicism, which simulates the situation found in females due to random X-inactivation. In our series, three (12, 15 and 30) of the seven patients with missense mutations were found to have somatic mosaicism, and all three had an a = p gradient.
Another patient (27), with a p > a gradient, had a mutation in intron 5 of DCX, which may or may not be pathogenic. Based on the unexpected gradient for DCX, which was the only exception to the usual rule, we hypothesize that this mutation is not pathogenic.
Only seven out of 24 (29%) of the males tested had a mutation in the coding region of the DCX gene. This confirms the rare occurrence of DCX mutations in males. The over-representation of atypical bands (partial, intermediate and pachygyria-SBH), the paucity of classical diffuse and anteriorly predominating SBH, and the predominance of sporadic cases in males suggests that mutations of other genes responsible for corticogenesis (D’Arcangelo et al., 1995; Ogawa et al., 1995; Rakic and Caviness, 1995; Anton et al., 1997, 1999; Gonzales et al., 1997; Rio et al., 1997; Yoneshima et al., 1997; Fox et al., 1998; Rice et al., 1998) may operate to cause SBH in males.
An ideal candidate gene is LIS1. LIS1 mutations are usually associated with isolated lissencephaly sequence (ILS) or with Miller Dieker syndrome (MDS), diseases with agyria/pachygyria (Dobyns et al., 1991, 1993; Lo Nigro et al., 1997; Pilz et al., 1998). The malformations caused by mutations in LIS1 have a posterior predominance unless the deletion size is very large and involves a second neuronal migration gene located more distally in chromosome 17p. The children all have MDS and diffuse lissencephaly (W.B.D., unpublished results). The LIS1 point mutation detected in patient 19 confirms the role played by LIS1 mutations in some patients with SBH who exhibit a posterior-to-anterior gradient. This also suggests that a single amino acid change in a critical domain of the LIS1 protein, rather than a deletion, may explain a milder phenotype such as SBH (Pilz et al., 1999). The novel mutation in intron 10 of LIS1 found in patient 17 may or may not be pathogenic. We hypothesize that this mutation is pathogenic, as the MRI of this patient closely resembles the MRI of the patient who has a missense mutation of LIS1. If indeed it is pathogenic, this would represent only the second mutation of LIS1 associated with SBH. In general, the functional consequences of intron mutations are uncertain. While they may be pathogenic, functional studies are required to prove this conclusively.
These results recapitulate previous observations in patients with lissencephaly, in whom a = p and a > p gradients were seen in males with DCX mutations, and p > a gradients were seen in patients with LIS1 mutations (Pilz et al., 1998; Dobyns et al., 1999). A few a = p gradients were also seen among patients with LIS1 mutations, all of whom have deletions extending from LIS1 toward the telomere that deletes a second gene known to be involved in neuronal migration (W.B.D., unpublished data). No mutation was detected in 14 of the 24 patients studied. While sequencing of the coding region may miss some mutations of a gene, usually 10%, failure to detect mutations in more than half of the patients tested suggests additional loci.
One of these loci is likely to be located on chromosome 9p (Federico et al., 1999), based on observation of trisomy 9p in patient 18. Mutation analysis of DCX and LIS1 in this patient with a diffuse band was negative. Several other reported patients with this same duplication have had epilepsy, mental retardation and brain abnormalities, possibly due to a neuronal migration disorder (Stern, 1996; Saneto et al., 1998). The preponderance of females [14 out of 20 (70%) in the review by Wilson et al., 1985] with this syndrome would suggest that males are more severely affected with a higher likelihood of prenatal and perinatal death (Wilson et al., 1985).
It should be noted that DCX mutations can be observed in mothers and maternal relatives of patients with SBH who may or may not be symptomatic, and may or may not have detectable MRI evidence of SBH.
The small number of mutations identified to date in males with SBH and the preponderance of sporadic cases might suggest that additional mechanisms may be responsible for SBH in males. These include mutations in the non-coding regions of the LIS1 or DCX genes, as suggested by the findings in patients 17 and 27, alternative splicing or somatic mosaicism (patients 12, 15 and 30), gonadal mosaicism (Matsumoto et al., 2001) and finally mutations of other genes.
Conclusion
In conclusion, our report demonstrates that SBH in males is a clinically heterogeneous syndrome, mostly occurring sporadically. The clinical spectrum is similar to that of females with SBH, but there is increased heterogeneity with respect to the neuroradiological, cognitive and molecular genetic aspects. Males had a higher frequency of partial and intermediate SBH, diffuse SBH with posterior predominance and pachygyria-SBH, as well as a significantly lower frequency of classical diffuse band heterotopia compared with females. Seven patients had missense mutations in DCX, three of whom had somatic mosaicism. Four additional male patients with somatic mosaicism have recently been identified (Kato et al., 2001; Poolos et al., 2002), confirming the importance of this mechanism in the aetiology of SBH in males.
A missense mutation of LIS1 and partial trisomy of chromosome 9p have been identified in one patient each. The absence of mutations in the coding sequences of these genes in the remaining patients differs from the findings in females and suggests other genetic mechanisms.
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The authors wish to thank Ms Maria Teresa Bogdalek, Dr Neda Ladbon-Bernasconi, Ms Aman Badhwar, Mr Sridar Narayanan and Mr Nigel A. Goddard for their assistance and support with this project. M.D.D. was the recipient of fellowships from the Savoy Foundation for Epilepsy Research and from Parke-Davis Canada for epilepsy research training at the Montreal Neurological Institute. E.A. was funded by grants from the Medical Research Council of Canada (CIHR).
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