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FP6

X-ALD Résumé de rapport

Project ID: 502987
Financé au titre de: FP6-LIFESCIHEALTH
Pays: Austria

Final Report Summary - X-ALD (X-linked Adrenoleukodystrophy (X-ALD): pathogenesis, animal models and therapy)

X-ALD is the most common (1:18 000) monogenetic disorder affecting the myelin within the Central nervous system (CNS). It affects boys (between 5-12 years), adult males (20-50 years), and women (> 40 years). All forms of X-ALD are characterised by neurological abnormalities, which in the severe variant result in early death. X-ALD is characterised biochemically by the accumulation of fatty acids with more than 22 carbon atoms (Very long-chain fatty acids (VLCFA)); but the role of VLCFA accumulation in the pathophysiology of the disease remains unknown.

The gene affected in X-ALD, ALD (ABCD1, current nomenclature), codes for a protein that is probably involved in the transport of VLCFA into a specific intracellular organelle, called the peroxisome. The ALD protein (ALDP) is an ATP-binding-cassette (ABC) half-transporter, which forms homodimers or possibly heterodimers with either of two other peroxisomal ABC half-transporters, ALD-related protein (ALDRP, ABCD2) and PMP70 (ABCD3). Like other ABC transporters, ALDP binds and hydrolyses ATP to drive transport. VLCFA are considered good candidate substrates for transport across the peroxisomal membrane by the ALD protein, but proof of such transport has yet to be demonstrated. To this end, the group of Prof. Ron Wanders at the Academic Medical Center in Amsterdam has developed a new system in which the properties of ALDP can be studied under in vivo conditions making use of baker's yeast (S. cerevisiae) as well as an in vitro system in which the properties of ALDP can be studied in artificial proteoliposomes.

Using a new strategy based on the notion that the preferred codon usage of S. cerevisiae is different from that of higher eukaryotes like H. sapiens, we were finally successful in expressing human ALDP in S. cerevisiae. Expression of the HsABCD1 cDNA in pxa1 / pxa2 cells, which are disturbed in oleate ß-oxidation, led to correction of both oleate ß-oxidation per se as well as growth on oleate-containing medium. These data imply that homodimeric (ALDP-ALDP) ALDP is able to catalyse the transport of oleoyl-CoA across the peroxisomal membrane, just like the Pxa1p / Pxa2p heterodimer.

The finding that the pxa1 / pxa2-HsABCD1 strain is not only able to oxidise oleate but also a range of other fatty acids, implies that the ALDP homodimer accepts a broad range of acyl-CoA esters. In order to generate definitive evidence for the postulation that ALDP transports acyl-CoA esters, liposome experiments have been performed using peroxisomes, either isolated from different yeast mutants or from tissues taken from different mutant mice including the Abcd1(-/-) mouse. The experiments have shown that proteoliposomes prepared from peroxisomes prepared from wild type S. cerevisiae are able to transport acyl-CoA esters as shown for lauroylCoA (C12:CoA).

Importantly, transport was deficient when proteoliposomes were prepared from peroxisomes isolated from the pxa1 / pxa2 strain. Taken together, the data, generated by Prof. Wander's group, give credit to the idea that ALDP in its homodimeric form is able to transport acyl-CoA esters across the peroxisomal membrane, although the precise substrate specificity remains to be clarified.

To summarise, the project has made substantial progress in the understanding of the pathogenic mechanisms leading to X-ALD, in the identification of candidates for modifier genes that could account for the phenotypic variability, in the expansion and characterisation of the X-ALD mouse models, and most importantly, in the further development of novel therapeutic approaches for X-ALD. With respect to gene therapy, this project has moved even beyond the original scope to the first clinical application of gene therapy in X-ALD. A major impact of the work carried out in this project is expected with regard to future therapeutic treatment of X-ALD patients within the next 10 years.

The EU-project X-ALD was coordinated by Prof. Dr Johannes Berger, Medical University Vienna, Center for Brain Research. More information on X-ALD can be found at the internet site http://www.x-ald.nl maintained by Stephan Kemp at the Academic Medical Centre in Amsterdam, the Netherlands.

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Johannes BERGER, (Workgroup Leader)
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