X-linked adrenoleukodystrophy: very long-chain fatty acid metabolism, ABC half-transporters and the complicated route to treatment

X-linked adrenoleukodystrophy (X-ALD) is caused by mutations in the ABCD1 gene that encodes a peroxisomal membrane located ABC half-transporter named ALDP. Mutations in ALDP result in elevated levels of very long-chain fatty acids (VLCFA) and reduced VLCFA beta-oxidation in peroxisomes. The peroxisomal membrane harbors three additional closely related ABC half-transporters, ALDRP, PMP70 and PMP69 (PMP70R). ABC half-transporters must dimerize to form a functional full-transporter. Whether ALDP forms a homodimer or a heterodimer has not yet been resolved, but most indirect evidence favors homodimerization. The peroxisomal ABC half-transporters are functionally related. Over-expression of ALDRP can correct the biochemical defect both in X-ALD patients cells and the Abcd1 knockout mouse, providing an exciting new possibility for treatment of X-ALD patients. This paper provides an overview of current knowledge and the problems that have been encountered.