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.

Expression of glutamate transporter, GABRA6, serine proteinase inhibitor 2 and low levels of glutamate and GABA in the brain of knock-out mouse for Canavan disease.

Canavan disease (CD) is an autosomal recessive leukodystrophy characterized by spongy degeneration of the brain. The clinical features of CD are hypotonia, megalencephaly, and mental retardation leading to early death. While aspartoacylase (ASPA) activity increases with age in the wild type mouse brain, there is no ASPA activity in the CD mouse brain. So far ASPA deficiency and elevated NAA have been ascribed with the CD. Other factors affecting the brain that result from ASPA deficiency may lead pathophysiology of CD. The NMR spectra and amino acid analysis showed lower levels of glutamate and gamma-aminobutyric acid in the CD mouse brain compared to the wild type. Microarray gene expression on CD mouse brain showed glutamate transporter-EAAT4 and gamma-aminobutyric acid-A receptor, subunit alpha6 (GABRA6) were lower 9.7- and 119.1-fold, respectively. Serine proteinase inhibitor 2 (Spi2) was 29.9-fold higher in the CD mouse brain compared to the wild type. The decrease of GABRA6 and high expression of Spi2 in CD mouse brain were also confirmed by real-time RT-PCR. This first report showing abnormal expression of EAAT4, GABRA6, Spi2 combined with lower levels of glutamate and GABA are likely to be associated with the pathophysiology of CD.