Category: Canavan disease

Leukodystrophy called Canavan disease (CD, Canavan Disease) is a neurodegenerative disease. Patients with Canavan disease develop mental retardation, suffer from epilepsy, and die prematurely. Brain damage in patients with CD is characterised by progressive central nervous system (CNS) vacuolation, edema and loss of oligodendrocytes, cells responsible for myelin formation in the CNS.

In 1996, CD was the first neurogenic brain disease to be treated with gene therapy. CD is caused by a loss-of-function mutation in the gene encoding aspartoacylase (ASPA), an enzyme in oligodendrocytes.

Under normal conditions, ASPA breaks down N-acetyl-Laspartate (NAA) into aspartate and acetate.

The biochemical consequence of ASPA deficiency is an accumulation of NAA and its derivative NAAG in the brain, blood and urine. NAA is the most abundant free amino acid in the CNS and its role in the brain is not fully understood. The metabolism of NAA is extremely segmented: production of NAA, catalysed by the 8-like N acetyltransferase (NAT8l) enzyme, takes place in neurons, while its degradation is restricted to oligodendrocytes.

The etiology of CD is believed to be related to the cytotoxic and osmotic effects of excess NAA, but also to potential hypomyelination due to lack of NAA-derived acetate. However, no experimental data has come to support these hypotheses.

To assess the contribution of NAA to the complex pathology of CD, we created and characterised mouse models with varying levels of NAA. Here we show that, overall, loss of function of ASA or increase of NAA, by themselves, is well tolerated. However, oligodendrocytes suffering from ASPA deficiency are extremely vulnerable to NAA toxicity in vivo.

Our data as well as several publications concerning the correction of ASPA gene expression in neurons show the need to improve the means of gene expression for targeted reintroduction of ASPA into oligodendrocytes.

We therefore modified drug vectors to direct ASPA gene therapy specifically to oligodendrocytes from symptomatic ASPA-deficient mice.

We observe that this treatment stops and reverses the progression of the disease, but in an incomplete way. Our hypothesis is that future approaches to gene therapy for CD should include strategies aimed primarily at restoring the expression of ASPA by oligodendrocytes associated with further decrease in NAA.

Deficiency in the enzyme aspartoacylase leads to a severe leukodystrophy, called Canavan disease, which is characterized by the accumulation of the brain metabolite N-acetylaspartate (NAA). NAA is synthesized and released mainly by neurons. Many neurons use NAA also to synthesize the small neuropeptide N-acetylaspartylglutamate (NAAG). Extracellular NAAG secreted by neurons can be degraded to NAA and glutamate by an enzyme expressed by astrocytes. Thus, degradation of NAAG is another source of NAA. The current project will further investigate the role of NAAG in the NAA accumulation observed in Canavan disease. This will be done using different transgenic mouse models, including a mouse model of Canavan disease, that differ in their capability to synthesize NAAG. In the project, histological and biochemical methods will be used to evaluate the influence of the genetic changes in these mouse models on the accumulation of NAA and the development of brain spongiform degeneration and myelin loss. The aim of this project is to examine whether reduced NAAG synthesis leads to reduced NAA accumulation and reduced pathological changes in the Canavan disease mouse model. If this would be the case, inhibition of NAAG synthesizing enzymes would potentially be an alternative or additional option for a substrate reduction therapy of Canavan disease.

Project financed by ELA up to: 9 700 €