Adrenoleukodystrophy pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:
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Overview
Adrenoleukodystrophy is an X-linked disease characterized by excessive accumulation of very long chain fatty acids (VLCFA), originally described by Moser et al in 1981. The gene, ABCD1, transfers fatty acids into peroxisomes to undergo β-oxidation. However, VLCFA accumulation is necessary but not sufficient for the pathogenesis. The CNS pathology suggests the role of cellular and humoral immune processes.
Pathophysiology
Physiology
The most common form of ALD is X-linked (the defective gene is on the X chromosome, location Xq28), and is characterized by excessive accumulation of very long chain fatty acids (VLCFA) — fatty acids chains with 24–30 carbon atoms (particularly hexacosanoate, C26) in length. This was originally described by Moser et al in 1981.[1] So, when the ALD gene was discovered in 1993, it was a surprise that the corresponding protein was in fact a member of a family of transporter proteins, not an enzyme. It is still a mystery as to how the transporter affects the function of the fatty acid enzyme and, for that matter, how high levels of very long chain fatty acids cause the loss of myelin on nerve fibers.
Pathogenesis
Pathology in CNS shows distinct immunological features. A predominant Th1 cytokine response suggests the role of cell mediated immunity. Immunocytochemical analysis done on the CSF from the ALD patients showed IgG, B cells and the presence of cytoplasmic IgA in postmortem brain suggesting the role of the humoral immune process. There was increased expression of the antigen-presenting molecule, which presents foreign and self-lipid to MHC-unrestricted T cells, CD1 in cerebral lesions clearly showing that both VLCFA and a myelin component such as proteolipid protein serves as the antigen for the immune response. [2] TNF-alpha also has an important role in the pathogenesis of ALD.
Genetics
The gene (ABCD1 or "ATP-binding cassette, subfamily D, member 1") codes for a protein that transfers fatty acids into peroxisomes, the cellular organelles where the fatty acids undergo β-oxidation.[3] A dysfunctional gene leads to the accumulation of very long chain fatty acids (VLCFA).
Gross pathology
- Adrenal glands show atrophy.
- Brain shows visible demyelination involving both cerebellar hemispheres, parts of the medulla oblongata, the pons and the midbrain.
- Spinal cord shows grayish white matter area.
Microscopic Pathology
References
- ↑ Moser HW, Moser AB, Frayer KK, Chen W, Schulman JD, O'Neill BP, Kishimoto Y. Adrenoleukodystrophy: increased plasma content of saturated very long chain fatty acids. Neurology 1981;31:1241-9. PMID 7202134.
- ↑ Hudspeth, Michelle P.; Raymond, Gerald V. (2007). "Immunopathogenesis of adrenoleukodystrophy: Current understanding". Journal of Neuroimmunology. 182 (1–2): 5–12. doi:10.1016/j.jneuroim.2006.10.009. ISSN 0165-5728.
- ↑ Mosser J, Douar AM, Sarde CO, Kioschis P, Feil R, Moser H, Poustka AM, Mandel JL, Aubourg P. Putative X-linked adrenoleukodystrophy gene shares unexpected homology with ABC transporters. Nature 1993;361:726-30. PMID 8441467.