Computed tomography in the diagnosis of Canavan's disease

Molecular basis of Canavan's disease: from human to mouse

Canavan's disease is an autosomal recessive disorder caused by aspartoacylase deficiency. The deficiency of aspartoacylase leads to increased concentration of N-acetylaspartic acid in brain and body fluids. The failure to hydrolyze N-acetylaspartic acid causes disruption of myelin, resulting in spongy degeneration of the white matter of the brain. The clinical features of the disease are hypotonia in early life, which changes to spasticity, macrocephaly, head lag, and progressive severe mental retardation. Although Canavan's disease is panethnic, it is most prevalent in the Ashkenazi Jewish population. Research at the molecular level led to the cloning of the gene for aspartoacylase and development of a knockout mouse for Canavan's disease. These developments have afforded new tools for research in the attempts to understand the pathophysiology of Canavan's disease, design new therapies, and explore methods for gene transfer to the central nervous system.

Canavan disease: a monogenic trait with complex genomic interaction

Canavan disease (CD) is an inherited leukodystrophy, caused by aspartoacylase (ASPA) deficiency, and accumulation of N-acetylaspartic acid (NAA) in the brain. The gene for ASPA has been cloned and more than 40 mutations have been described, with two founder mutations among Ashkenazi Jewish patients. Screening of Ashkenazi Jews for these two common mutations revealed a high carrier frequency, approximately 1/40, so that programs for carrier testing are currently in practice. The enzyme deficiency in CD interferes with the normal hydrolysis of NAA, which results in disruption of myelin and spongy degeneration of the white matter of the brain. The clinical features of the disease are macrocephaly, head lag, progressive severe mental retardation, and hypotonia in early life, which later changes to spasticity. A knockout mouse for CD has been generated, and used to study the pathophysiological basis for CD. Findings from the knockout mouse indicate that this monogenic trait leads to a series of genomic interaction in the brain. Changes include low levels of glutamate and GABA. Microarray expression analysis showed low level of expression of GABA-A receptor (GABRA6) and glutamate transporter (EAAT4). The gene Spi2, a gene involved in apoptosis and cell death, showed high level of expression. Such complexity of gene interaction results in the phenotype, the proteome, with spongy degeneration of the brain and neurological impairment of the mouse, similar to the human counterpart. Aspartoacylase gene transfer trial in the mouse brain using adenoassociated virus (AAV) as a vector are encouraging showing improved myelination and decrease in spongy degeneration in the area of the injection and also beyond that site.

Canavan disease: diagnosis and molecular analysis

Canavan disease, spongy degeneration of the brain, is an autosomal recessive disorder with increased prevalence among Ashkenazi Jews. The biochemical marker for this disease is increased levels of N-acetylaspartic acid, due to the defective enzyme, aspartoacylase. This discovery allowed for accurate diagnosis of the disease. The gene for aspartoacylase has been cloned and two mutations have been found to be responsible for Canavan disease among Ashkenazi Jewish patients in 98% of the cases. Molecular analysis of healthy Jewish individuals for these mutations has resulted in an unexpectedly high carrier frequency for Canavan disease among Jews. Therefore, carrier testing of the Jewish population is possible and indicated.

Biochemistry and molecular biology of Canavan disease

Canavan in 1931 described spongy degeneration of the brain in a child who was thought to have had Schilder's disease. Since that classic histological description, Canavan disease has become a distinct clinical entity, with the recognition by Van Bogaert and Bertrand that this is an autosomal recessive disease prevalant among children of Jewish extraction. Recent advances in the understanding of the biochemical defect led to an increase in awareness and ease in diagnosis, and indeed the disease is not as rare as initially thought. Exploring the molecular aspects of Canavan disease has led to exciting new developments in carrier detection and prevention of Canavan disease. Work is underway in our laboratory to develop a knock-out mouse for Canavan disease for understanding of the pathophysiology of this disease and formulating gene therapy.

Canavan disease: from spongy degeneration to molecular analysis

Establishing the basic defect in Canavan disease has led to reliable biochemical methods for the diagnosis of this disease. The isolation of the gene and identification of mutations causing Canavan disease have led to the possibility of using DNA methods for the diagnosis of Canavan disease and for carrier detection. A surprising finding is the high carrier frequency of this gene defect among Ashkenazi Jewish people. Analysis for two mutations leads to the identification of 97% of Jewish patients with Canavan disease, and screening of Ashkenazi Jews is possible. N-Acetylaspartic acid has been considered to be an inert compound. The pathophysiology of Canavan disease links lack of NAA hydrolysis to a severe, debilitating white matter disease. Currently, NAA is being studied in many other brain disorders, such as Alzheimer disease, Huntington disease, and stroke. However, the only disease with a specific defect in the metabolism of NAA is Canavan disease. An animal model for Canavan disease is needed to study some of the questions regarding the role of NAA in brain tissue, and for the study of therapeutic modalities, including gene therapy.

Canavan disease: biochemical and molecular studies

Deficiency of the enzyme aspartoacylase and the accumulation of N-acetylaspartic acid lead to a severe leukodystrophy and spongy degeneration of the brain, Canavan disease (McKusick 271900). Since our discovery in 1988 of the defect in Canavan disease, 144 patients with Canavan disease have been diagnosed in our laboratory. Most of these children are of Ashkenazi Jewish extraction. The level of enzyme activity can be used for carrier testing. Prenatal diagnosis has been difficult using the enzyme assay owing to the low activity of aspartoacylase in cultured chorionic villus samples or amniocytes. The determination of N-acetylaspartic acid in the amniotic fluid is another parameter for diagnosis; however, the levels may not always be elevated. Bovine and human aspartoacylase have been purified in our laboratory. Bovine and human cDNA and genomic clones have been isolated and six exons have been localized. This information is being used for the study of Canavan disease at the molecular level.

Variable course of Canavan disease in two boys with early infantile aspartoacylase deficiency

This is a report of two patients with Canavan disease from the Federal Republic of Germany. One is a severely retarded, macrocephalic boy, who had the characteristic laboratory findings of Canavan disease and progressive leucodystrophy on neuro-imaging. The other is retarded, with signs of a cerebral movement disorder showing no deterioration during the first 15 months. The significance of aspartoacylase deficiency in Canavan disease for differential diagnosis, genetic counselling and prenatal diagnosis of leucodystrophy is discussed.

Use of computed tomography, magnetic resonance imaging, and localized 1H magnetic resonance spectroscopy in Canavan's disease: a case report

The neuroradiological evaluation of Canavan's disease in a 38-month-old girl is discussed. Computed tomography showed diffuse symmetrical low attenuation values of the subcortical and deep cerebral white matter. Magnetic resonance imaging demonstrated symmetrical diffuse low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. With the use of 1H magnetic resonance spectroscopy, we were able to show elevated levels of N-acetylaspartic acid in the occipital lobe of our patient. The in vivo measurement of N-acetylaspartic acid in the brain by 1H magnetic resonance spectroscopy offers an additional noninvasive diagnostic test for establishing the diagnosis of Canavan's disease. With the increasing availability of magnetic resonance spectroscopy, clinicians may be able to confirm the diagnosis of Canavan's disease immediately after magnetic resonance imaging reveals the typical abnormalities of the white matter.

Megalencephaly: definition and classification

The various definitions and classifications of megalencephaly are reviewed, and numerous diseases and syndromes associated with megalencephaly are listed. A new definition of megalencephaly based on quantitative radiographic features is proposed. We define megalencephaly as a brain volume which exceeds the mean by more than twice the standard deviation. Furthermore, a modified etiopathogenic classification of megalencephaly results in three main groups, viz anatomic, metabolic and dynamic megalencephaly. The clinical pictures in these main groups of megalencephaly, and the largest subgroup of anatomic megalencephaly, familial anatomic megalencephaly, appear to be quite different.

Similarity of brain CT appearance in spongy degeneration to that of subacute necrotizing encephalomyelopathy

Reports of brain computed tomography (CT) findings in spongy degeneration describe radiolucent changes of the cerebral white matter, but have not described changes in the posterior fossa. We describe an infant with spongy degeneration in whom CT scans detected brainstem, cerebellar, and cerebral white matter radiolucencies before the diagnosis was established. The posterior fossa CT findings resembled the periventricular changes described in subacute necrotizing encephalomyelopathy (SNE). As the patient's initial clinical findings were consistent with SNE, the similarity of the posterior fossa radiolucencies was misleading. Without basal ganglia or thalamic involvement, or without variability in their appearance over time, posterior fossa periventricular radiolucencies are not diagnostic of a specific degenerative disorder.

Computed tomography in Alexander's disease

Computed tomography demonstrated contrast-enhancing lesions in the periventricular frontal regions, caudate nuclei, and thalami in an infant with Alexander's disease. The distribution of the enhancing lesions corresponded to the areas in which Rosenthal fibers were most prominent. These radiological findings have not been described in other white matter diseases; thus, they may help to distinguish Alexander's disease from Canavan's disease and decrease the necessity for diagnostic brain biopsy.