Characterization of a new rabbit model of X-linked adrenoleukodystrophy and developing adeno-associated virus-based gene therapy

Abstract

Adrenoleukodystrophy (X-ALD) is an X-linked metabolic disorder caused by mutations in the ABCD1 gene. ABCD1 encodes a peroxisomal transmembrane protein called ATP-binding cassette subfamily D member 1. Defective ABCD1 proteins result in abnormal accumulation of very long chain fatty acids (VLCFAs) in the body. The most severe type of X-ALD is cerebral ALD (cALD). cALD patients present with demyelination in the brain. Unfortunately, there are no satisfactory therapies to date to cure X-ALD, except bone marrow transplantation at the very early stage of the disease.

Currently, mouse models with the absence of ABCD1 display limited phenotypes and cannot faithfully recapitulate X-ALD in human. To better understand the pathological features of X-ALD for the development of effective treatments, we characterized here, for the first time, an ABCD1-mutated rabbit model. We observed that blood plasma VLCFA levels (C26:0/C22:0) in the rabbits were significantly increased up to 4.5-fold compared with the wild-type rabbits. However, we were unable to observe obvious neural degeneration in all ABCD1-mutated rabbits despite the elevation of plasma VLCFA levels. Magnetic resonance imaging (MRI) showed no obvious lesions in the central nervous system of the rabbit model during 21 months of observation.

Gene therapy using viruses as delivery vehicles can efficiently transfer a therapeutic gene to human cells. Adeno-associated virus (AAV)-based gene therapy is of great interest in recent research due to its safety profile. Different serotypes of AAV are available, each of which selectively targets different cell types. AAV9 has a high virus infection rate to the heart, liver and brain. Therefore, AAV9 would be a promising vector to treat X-ALD.

In this project, AAV9 with a functional ABCD1 gene was designed and constructed. Its therapeutic effects were examined in vivo for treating X-ALD. The novel ABCD1-mutated rabbit model was used to mimic the conditions of X-ALD patients. Our gene therapy successfully decreased the blood plasma VLCFA levels in the rabbit model, but the therapeutic effects could only last for 2-7 weeks. Two out of three rabbits were dead within two months after the therapy. Immuno-rejection is suspected in the AAV-treated rabbits, eventually causing death.

This study suggests that the ABCD1-mutated rabbit model is not yet an ideal cALD model, but it shows a higher degree of similarity on human X-ALD compared to the existing X-ALD mouse models. The safety consideration on rAAV9-hABCD1 gene therapy is also provided in our study.