Mutation analysis of the aspartoacylase gene in non-Jewish patients with Canavan disease

Cellular and molecular mechanisms of aspartoacylase and its role in Canavan disease

Canavan disease is an autosomal recessive and lethal neurological disorder, characterized by the spongy degeneration of the white matter in the brain. The disease is caused by a deficiency of the cytosolic aspartoacylase (ASPA) enzyme, which catalyzes the hydrolysis of N-acetyl-aspartate (NAA), an abundant brain metabolite, into aspartate and acetate. On the physiological level, the mechanism of pathogenicity remains somewhat obscure, with multiple, not mutually exclusive, suggested hypotheses. At the molecular level, recent studies have shown that most disease linked ASPA gene variants lead to a structural destabilization and subsequent proteasomal degradation of the ASPA protein variants, and accordingly Canavan disease should in general be considered a protein misfolding disorder. Here, we comprehensively summarize the molecular and cell biology of ASPA, with a particular focus on disease-linked gene variants and the pathophysiology of Canavan disease. We highlight the importance of high-throughput technologies and computational prediction tools for making genotype-phenotype predictions as we await the results of ongoing trials with gene therapy for Canavan disease.

Canavan's spongiform leukodystrophy (Aspartoacylase deficiency) with emphasis on sonographic features in infancy: description of a case report and review of the literature

Canavan disease (CD; MIM 271,900) or spongy degeneration of the central nervous system (CNS) is a lethal, rare autosomal recessive leukodystrophy, first described in 1931 (Canavan in Arch Neurol Psychiatry 25: 299-308, 1931). The clinical presentation includes severe neurologic impairment and macrocephaly with onset of symptoms at the age of 3-5 months. Biochemical and genetic fundamentals of the disease are elucidated. Imaging diagnosis is principally based on MRI with important role of MR spectroscopy. We report the cerebral sonographic findings in a severely affected infant with CD: Diffuse hyperechogenicity and small multicystic changes of white matter as well as an inverted pattern of echogenicity between cortical gray and subcortical white matter. These findings are compared to to the few cases found in literature and to normal ultrasound examples. Finally, ultrasound and MRI imaging findings are correlated.

Mapping the degradation pathway of a disease-linked aspartoacylase variant

Canavan disease is a severe progressive neurodegenerative disorder that is characterized by swelling and spongy degeneration of brain white matter. The disease is genetically linked to polymorphisms in the aspartoacylase (ASPA) gene, including the substitution C152W. ASPA C152W is associated with greatly reduced protein levels in cells, yet biophysical experiments suggest a wild-type like thermal stability. Here, we use ASPA C152W as a model to investigate the degradation pathway of a disease-causing protein variant. When we expressed ASPA C152W in Saccharomyces cerevisiae, we found a decreased steady state compared to wild-type ASPA as a result of increased proteasomal degradation. However, molecular dynamics simulations of ASPA C152W did not substantially deviate from wild-type ASPA, indicating that the native state is structurally preserved. Instead, we suggest that the C152W substitution interferes with the de novo folding pathway resulting in increased proteasomal degradation before reaching its stable conformation. Systematic mapping of the protein quality control components acting on misfolded and aggregation-prone species of C152W, revealed that the degradation is highly dependent on the molecular chaperone Hsp70, its co-chaperone Hsp110 as well as several quality control E3 ubiquitin-protein ligases, including Ubr1. In addition, the disaggregase Hsp104 facilitated refolding of aggregated ASPA C152W, while Cdc48 mediated degradation of insoluble ASPA protein. In human cells, ASPA C152W displayed increased proteasomal turnover that was similarly dependent on Hsp70 and Hsp110. Our findings underscore the use of yeast to determine the protein quality control components involved in the degradation of human pathogenic variants in order to identify potential therapeutic targets.

Canavan disease: clinical features and recent advances in research

Canavan disease (CD) is a genetic neurodegenerative leukodystrophy that results in the spongy degeneration of white matter in the brain. CD is characterized by mutations in the gene encoding aspartoacylase (ASPA), the substrate enzyme that hydrolyzes N-acetylaspartic acid (NAA) to acetate and aspartate. Elevated NAA and subsequent deficiency in acetate associated with this disease cause progressive neurological symptoms, such as macrocephaly, visuocognitive dysfunction, and psychomotor delay. The prevalence of CD is higher among Ashkenazi Jewish people, and several types of mutations have been reported in the gene coding ASPA. Highly elevated NAA is more specific to CD than other leukodystrophies, and an examination of urinary NAA concentration is useful for diagnosing CD. Many researchers are now examining the mechanisms responsible for white matter degeneration or dysmyelination in CD using mouse models, and several persuasive hypotheses have been suggested for the pathophysiology of CD. One is that NAA serves as a water pump; consequently, a disorder in NAA catabolism leads to astrocytic edema. Another hypothesis is that the hydrolyzation of NAA in oligodendrocytes is essential for myelin synthesis through the supply of acetate. Although there is currently no curative therapy for CD, dietary supplements are candidates that may retard the progression of the symptoms associated with CD. Furthermore, gene therapies using viral vectors have been investigated using rat models. These therapies have been found to be tolerable with no severe long-term adverse effects, reduce the elevated NAA in the brain, and may be applied to humans in the future.

A thematic review of scientific and family interests in Canavan Disease: where are the developmentalists?

BACKGROUND

Canavan Disease is a degenerative neurological condition resulting in a spongy deterioration of the brain. Much research has been conducted by the medical community regarding this condition, but little research can be found in the psychological literature.

METHOD

A review of the scientific literature related to Canavan Disease using the Psychinfo and PubMed databases was conducted covering a 5-year span from 2006 through 2011. Concurrently, a review of parent initiated topics found on the most popular Canavan Disease Internet discussion board was conducted for comparison purposes.

RESULTS

When comparing the topics discussed and information sought among parents with the themes noted in the extant scientific literature, researchers found an exceedingly small overlap between the two communities of interest. In the scientific literature, published research on Canavan Disease focused on three areas: the biochemistry of Canavan Disease, diagnosis and genetic counselling, and clinical therapeutic approaches in Canavan Disease. Of the 42 unique topics raised on a popular Internet discussion board, however, only three (7%) fell into the category of diagnosis and genetic counselling, none (0%) fell into the category of the biochemistry of Canavan Disease, and four fell into the category of clinical therapeutic approaches in Canavan Disease (10%). Of the four posts addressing clinical therapeutic approaches to Canavan Disease, only one post truly overlapped with the topics addressed by the scientific community. Worded differently, while these three categories comprise 100% of the extant scientific literature regarding Canavan Disease, they comprise only 17% of the parent-raised topics. The remaining 83% of parent-raised topics addressed concerns not currently being focusing upon by the scientific community, namely, non-medical practical issues, information regarding specific characteristics of Canavan Disease, non-medical developmental and quality of life issues, and day-to-day developmental and medical concerns.

CONCLUSION

By comparing the extant literature on Canavan Disease with the topics of interest raised by parents and caregivers, it seems clear that there is a significant 'underlap' of topics raised by these two communities of interest, one that may reflect a lack of sensitivity on the part of the scientific community to meet the needs of this population of knowledge seekers. It is the suggestion of these authors that developmental psychology may be the appropriate scientific field within which to address this need and fill this gap in the current literature.

Long-term follow-up after gene therapy for canavan disease

Canavan disease is a hereditary leukodystrophy caused by mutations in the aspartoacylase gene (ASPA), leading to loss of enzyme activity and increased concentrations of the substrate N-acetyl-aspartate (NAA) in the brain. Accumulation of NAA results in spongiform degeneration of white matter and severe impairment of psychomotor development. The goal of this prospective cohort study was to assess long-term safety and preliminary efficacy measures after gene therapy with an adeno-associated viral vector carrying the ASPA gene (AAV2-ASPA). Using noninvasive magnetic resonance imaging and standardized clinical rating scales, we observed Canavan disease in 28 patients, with a subset of 13 patients being treated with AAV2-ASPA. Each patient received 9 × 10(11) vector genomes via intraparenchymal delivery at six brain infusion sites. Safety data collected over a minimum 5-year follow-up period showed a lack of long-term adverse events related to the AAV2 vector. Posttreatment effects were analyzed using a generalized linear mixed model, which showed changes in predefined surrogate markers of disease progression and clinical assessment subscores. AAV2-ASPA gene therapy resulted in a decrease in elevated NAA in the brain and slowed progression of brain atrophy, with some improvement in seizure frequency and with stabilization of overall clinical status.

A novel aspartoacylase (ASPA) gene mutation in Canavan disease

Canavan disease is a severe autosomal recessive leukodystrophy characterized by macrocephaly, ataxia, severe motor and mental retardation, dysmyelination, and progressive spongial atrophy of the brain. The human aspartoacylase (ASPA) gene, which catalyzes the deacetylation of N-acetyl-L-aspartate, is mutated in Canavan disease. In the presented family sequencing analysis for the aspartoacylase gene was performed on the blood samples of the parents as the affected child had died due to Canavan disease. After the mutation was detected, prenatal diagnosis was also performed and heterozygous Y88X mutation was detected in the fetus. In this report, we present a novel mutation Y88X within the aspartoacylase gene in a consanguineous family with an affected child diagnosed as Canavan disease.

Two novel missense mutations in the aspartoacylase gene in a Chinese patient with congenital Canavan disease

We herein describe the first Chinese case of Canavan disease diagnosed by biochemical analysis and confirmed by DNA studies. We report two novel mutations: c.2T>C/M1T, an initiation codon mutation, and c.209A>G/N70S, which is located at the enzyme-substrate binding site. The combination of these two mutations resulted in a congenital form of Canavan disease.

Genome-wide gene expression profiling and mutation analysis of Saudi patients with Canavan disease

PURPOSE

Canavan disease, caused by a deficiency of aspartoacylase, is one of the most common cerebral degenerative diseases of infancy. The aims of this study were to identify the mutations associated with Canavan disease in Saudi Arabia and to identify differentially expressed genes likely to contribute to the development of this disease.

METHODS

Polymerase chain reaction, long polymerase chain reaction, multiplex ligation-dependent probe amplification, sequencing, array comparative genomic hybridization (aCGH), and global gene expression profiling were used to determine putative mutations and likely gene signatures in cultured fibroblasts of patients from Saudi Arabia.

RESULTS

One novel and one known large deletion and two previously known mutations (IVS4 + 1G>T and G27R) were identified. Compared with controls, 1440 genes were significantly modulated in Canavan patients (absolute fold change [FC] > or =4). Genome-wide gene expression profiling results indicated that some genes, involved in apoptosis, muscle contraction and development, mitochondrial oxidation, inflammation and glutamate, and aspartate metabolism, were significantly dysregulated.

CONCLUSIONS

Our findings indicate that the presence of muscle weakness and hypotonia in patients may be associated with the dysregulated gene activities of cell motility, muscle contraction and development, actin binding, and cytoskeletal-related activities. Overall, these observations are in accordance with previous studies performed in a knockout mouse model.

Mutational analysis of aspartoacylase: implications for Canavan disease

Mutations that result in near undetectable activity of aspartoacylase, which catalyzes the deacetylation of N-acetyl-l-aspartate, correlate with Canavan Disease, a neurodegenerative disorder usually fatal during childhood. The underlying biochemical mechanisms of how these mutations ablate activity are poorly understood. Therefore, we developed and tested a three-dimensional homology model of aspartoacylase based on zinc dependent carboxypeptidase A. Mutations of the putative zinc-binding residues (H21G, E24D/G, and H116G), the general proton donor (E178A), and mutants designed to switch the order of the zinc-binding residues (H21E/E24H and E24H/H116E) yielded wild-type aspartoacylase protein levels and undetectable ASPA activity. Mutations that affect substrate carboxyl binding (R71N) and transition state stabilization (R63N) also yielded wild-type aspartoacylase protein levels and undetectable aspartoacylase activity. Alanine substitutions of Cys124 and Cys152, residues indicated by homology modeling to be in close proximity and in the proper orientation for disulfide bonding, yielded reduced ASPA protein and activity levels. Finally, expression of several previously tested (E24G, D68A, C152W, E214X, D249V, E285A, and A305E) and untested (H21P, A57T, I143T, P183H, M195R, K213E/G274R, G274R, and F295S) Canavan Disease mutations resulted in undetectable enzyme activity, and only E285A and P183H showed wild-type aspartoacylase protein levels. These results show that aspartoacylase is a member of the caboxypeptidase A family and offer novel explanations for most loss-of-function aspartoacylase mutations associated with Canavan Disease.