Increasing recognition of malformations of cortical development and continuing improvements in imaging techniques, molecular biologic techniques, and knowledge of mechanisms of brain development have resulted in continual improvement of the understanding of these disorders. The authors propose a revised classification based on the stage of development (cell proliferation, neuronal migration, cortical organization) at which cortical development was first affected. The categories are based on known developmental steps, known pathologic features, known genetics (when possible), and, when necessary, neuroimaging features. In those cases in which the precise developmental and genetic features are uncertain, classification is based on known relationships among the genetics, pathologic features, and neuroimaging features. The major change since the prior classification has been a shift to using genotype, rather than phenotype, as the basis for classifying disorders wherever the genotype-phenotype relationship is adequately understood. Other substantial changes include more detailed classification of congenital microcephalies, particularly those in which the genes have been mapped or identified, and revised classification of congenital muscular dystrophies and polymicrogyrias. Information on genetic testing is also included. This classification allows a better conceptual understanding of the disorders, and the use of neuroimaging characteristics allows it to be applied to all patients without necessitating brain biopsy, as in pathology-based classifications.

A developmental and genetic classification for malformations of cortical development

Walker-Warburg syndrome (WWS) is an autosomal recessive developmental disorder characterized by congenital muscular dystrophy and complex brain and eye abnormalities. A similar combination of symptoms is presented by two other human diseases, muscle-eye-brain disease (MEB) and Fukuyama congenital muscular dystrophy (FCMD). Although the genes underlying FCMD (Fukutin) and MEB (POMGnT1) have been cloned, loci for WWS have remained elusive. The protein products of POMGnT1 and Fukutin have both been implicated in protein glycosylation. To unravel the genetic basis of WWS, we first performed a genomewide linkage analysis in 10 consanguineous families with WWS. The results indicated the existence of at least three WWS loci. Subsequently, we adopted a candidate-gene approach in combination with homozygosity mapping in 15 consanguineous families with WWS. Candidate genes were selected on the basis of the role of the FCMD and MEB genes. Since POMGnT1 encodes an O-mannoside N-acetylglucosaminyltransferase, we analyzed the possible implication of O-mannosyl glycan synthesis in WWS. Analysis of the locus for O-mannosyltransferase 1 (POMT1) revealed homozygosity in 5 of 15 families. Sequencing of the POMT1 gene revealed mutations in 6 of the 30 unrelated patients with WWS. Of the five mutations identified, two are nonsense mutations, two are frameshift mutations, and one is a missense mutation. Immunohistochemical analysis of muscle from patients with POMT1 mutations corroborated the O-mannosylation defect, as judged by the absence of glycosylation of α-dystroglycan. The implication of O-mannosylation in MEB and WWS suggests new lines of study in understanding the molecular basis of neuronal migration.

Mutations in the O-Mannosyltransferase Gene POMT1 Give Rise to the Severe Neuronal Migration Disorder Walker-Warburg Syndrome

Fukuyama congenital muscular dystrophy (FCMD), muscle-eye-brain disease (MEB), and Walker-Warburg syndrome are congenital muscular dystrophies (CMDs) with associated developmental brain defects. Mutations reported in genes of FCMD and MEB patients suggest that the genes may be involved in protein glycosylation. Dystroglycan is a highly glycosylated component of the muscle dystrophin-glycoprotein complex that is also expressed in brain, where its function is unknown. Here we show that brain-selective deletion of dystroglycan in mice is sufficient to cause CMD-like brain malformations, including disarray of cerebral cortical layering, fusion of cerebral hemispheres and cerebellar folia, and aberrant migration of granule cells. Dystroglycan-null brain loses its high-affinity binding to the extracellular matrix protein laminin, and shows discontinuities in the pial surface basal lamina (glia limitans) that probably underlie the neuronal migration errors. Furthermore, mutant mice have severely blunted hippocampal long-term potentiation with electrophysiologic characterization indicating that dystroglycan might have a postsynaptic role in learning and memory. Our data strongly support the hypothesis that defects in dystroglycan are central to the pathogenesis of structural and functional brain abnormalities seen in CMD.

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Deletion of brain dystroglycan recapitulates aspects of congenital muscular dystrophy

The many recent discoveries concerning the molecular biologic bases of malformations of cortical development and the discovery of new such malformations have rendered previous classifications out of date. A revised classification of malformations of cortical development is proposed, based on the stage of development (cell proliferation, neuronal migration, cortical organization) at which cortical development was first affected. The categories have been created based on known developmental steps, known pathologic features, known genetics (when possible), and, when necessary, neuroimaging features. In many cases, the precise developmental and genetic features are uncertain, so classification was made based on known relationships among the genetics, pathologic features, and neuroimaging features. A major change since the prior classification has been the elimination of the separation between diffuse and focal/multifocal malformations, based on the recognition that the processes involved in these processes are not fundamentally different; the difference may merely reflect mosaicism, X inactivation, the influence of modifying genes, or suboptimal imaging. Another change is the listing of fewer specific disorders to reduce the need for revisions; more detail is added in other smaller tables that list specific malformations and malformation syndromes. This classification is useful to the practicing physician in that its framework allows a better conceptual understanding of the disorders, while the component of neuroimaging characteristics allows it to be applied to all patients without necessitating brain biopsy, as in pathology-based classifications.

Classification system for malformations of cortical development: Update 2001

http://www.physiology.uiowa.edu/campbell/Publications/PDF/296Durbeej.pdf

Muscular dystrophies involving the dystrophin-glycoprotein complex: an overview of current mouse models

Background: Three rare autosomal recessive disorders share the combination of congenital muscular dystrophy and brain malformations including a neuronal migration defect: muscle–eye-brain disease (MEB), Walker–Warburg syndrome (WWS), and Fukuyama congenital muscular dystrophy (FCMD). In addition, ocular abnormalities are a constant feature in MEB and WWS. Lack of consistent ocular abnormalities in FCMD has allowed a clear clinical demarcation of this syndrome, whereas the phenotypic distinction between MEB and WWS has remained controversial. The MEB gene is located on chromosome 1p32-p34. Objectives: To establish distinguishing diagnostic criteria for MEB and WWS and to determine whether MEB and WWS are allelic disorders. Methods: The authors undertook clinical characterization followed by linkage analysis in 19 MEB/WWS families with 29 affected individuals. With use of clinical diagnostic criteria based on Finnish patients with MEB, each patient was categorized as having either MEB or WWS. A linkage and haplotype analysis using 10 markers spanning the MEB locus was performed on the entire family resource. Results: Patients in 11 families were classified as having MEB and in 8 families as WWS. Strong evidence in favor of genetic heterogeneity was obtained in the 19 families. There was evidence for linkage to 1p32-p34 in all but 1 of the 11 pedigrees segregating the MEB phenotype. In contrast, linkage to the MEB locus was excluded in seven of eight of the WWS families. Conclusion: These results allow the classification of MEB and WWS as distinct disorders on both clinical and genetic grounds and provide a basis for the mapping of the WWS gene(s).

http://www.ub.edu/geneticaclasses/brucormand/pdfs/18_1.pdf

A. Ahmad

Papers

Clinical and genetic distinction between Walker–Warburg syndrome and muscle–eye–brain disease