AXONAL ABNORMALITIES IN THE BRAIN OF THE ABCD1 KNOCKOUT MOUSE, AN ANIMAL MODEL OF X-LINKED ADRENOLEUKODYSTROPHY

Objectives: X-linked adrenoleukodystrophy (X-ALD) is caused by mutations in the ABCD1 gene encoding a peroxisomal ABC transporter. Mice that carry the ABCD1 gene defect develop a late onset phenotype that appears comparable to adrenomyeloneuropathy (AMN), the adult form of X-ALD. We set out to examine the brain of the ABCD1 knockout mouse by diffusion tensor imaging (DTI) and ultrastructural analysis.

Methods: Four ABCD1 knockout mice were studied at the age of 11 months and compared to four age matched wild type mice. In vivo DTI was performed at 4.7 T with 300 micron spatial resolution. After perfusion fixation in vitro 3-D DTI with 100 micron isotropic spatial resolution was performed in a 14 T vertical bore. Following imaging half the brain was used for biochemical analysis. From the other half, three blocks were prepared corresponding to the genu, splenium and middle of the corpus callosum and assessed by electron microscopy.

Results: Our measurements revealed 12% higher fractional diffusion anisotropy (p=0.02) in the genu of the corpus callosum in the ABCD1 knockout mouse compared to the wild type. Electron microscopy in the corresponding region showed smaller more densely packed axons. Our ABCD1 knockout mice showed a 4-fold increase of very long chain fatty acids compared to the wild type.

Conclusion: We report imaging abnormalities and supportive histopathology in the AMN mouse model. In human AMN patients a distal-proximal axonopathy has been demonstrated in the spinal cord dorsal columns and the lateral corticospinal tracts. High-field, high resolution DTI and electron microscopy now provide evidence for axonal degeneration in the brain of the ABCD1 knockout model.