X-linked adrenoleukodystrophy
In X-linked adrenoleukodystrophy (X-ALD), a mutation occurs in the peroxisomal ATP-binding cassette (ABC transporter). This leads to cerebral inflammatory demyelination caused by the myelin destabilization that occurs in these patients. The inflammatory demyelination begins in the corpus callosum and it slowly progresses outwards towards both hemispheres. In X-ALD patients, abnormally high levels of very long chain fatty acid (VLCFA) accumulate in various body tissues and fluids. This increased concentration then incorporates into various complex lipids where they are not normally found. This has been found to be directly involved in the cerebral inflammation. The accumulated and embedded VLCFA in the complex lipids could lead to the destabilization of myelin sheath and eventually to demyelination. Alexander disease
Alexander disease is unique from the leukodystrophies mentioned above in that it is the result of spontaneous mutation, that is it is not inherited. This means that the mutation found in the affected individual is not found in either of his or her parents. It is due to the accumulation of Glial fibrillary acidic protein (GFAP) as the result of a mutation in the GFAP gene; which, rather than being found in association with lysosomes or peroxisomes, is an intermediate filament linked to the nuclear envelope. Intermediate filaments are proteins responsible for the makeup of the cellular cytoskeleton, and thus this type of mutation is involved in malfunctioning structural development of the cells. In fact, cytoskeletal and transporter molecule defects have been observed in the astrocytes (type of glial cell) of affected individuals.
Oligodendrocyte precursor cells and neural stem cells have been transplanted successfully and have shown to be healthy a year later. Fractional anisotropy and radial diffusivity maps showed possible myelination in the region of the transplant. Induced pluripotent stem cells, oligodendrocyte precursor cells, gene correction, and transplantation to promote the maturation, survival, and myelination of oligodendrocytes seem to be the primary routes for possible treatments.For three types of leukodystrophies (X-linked adrenoleukodystrophy (X-ALD), metachromatic leukodystrophy (MLD) and Krabbe Disease (globoid cell leukodystrophy - GLD), gene therapy using autologous hematopoietic stem cells to transfer the disease gene with lentiviral vectors have shown to be successful and are currently being used in clinical trials for X-ALD and MLD. The progression of X-ALD has shown to be disrupted with hematopoietic stem cell gene therapy but the exact reason why demyelination stops and the amount of stem cells needed is unclear. While there is an accumulation of very long chain fatty acids in the brain, it does not seem to be the reason behind the disease as gene therapy does not correct it.For those leukodystrophies that result from a deficiency of lysozyme enzymes, such as Krabbe disease, enzyme replacement therapy seems hopeful. However, enzyme delivery proves difficult, because the blood-brain barrier severely limits what can pass into the central nervous system. Current gene therapy research for metachromatic leukodystrophy has been reviewed with an emphasis on ex vivo transplantation of genetically modified hematopoietic stem cells.