Functional Characterization of the Adrenoleukodystrophy Protein (Aldp) and Disease Pathogenesis

Abstract

X-linked adrenoleukodystrophy (X-ALD) is the most common peroxisomal disorder characterized by abnormal accumulation of saturated very long chain fatty acids in tissues and body fluids with predominance in brain white matter and adrenal cortex. The clinical phenotype is highly variable ranging from the severe childhood cerebral form to asymptomatic persons. The responsible ALD gene encodes the adrenoleukodystrophy protein (ALDP), a peroxisomal integral membrane protein that is a member of the ATP-binding cassette (ABC) transporter protein family. The patient gene mutations are heterogeneously distributed over the functional domains of ALDP. The extreme variability in clinical phenotype, even within one affected family, indicates that besides the ALD gene mutations other factors strongly influence the clinical phenotype. To understand the cell biology and function of mammalian peroxisomal ABC transporters and to determine their role in the pathogenesis of X-ALD we developed a system for expressing functional ABC protein domains in fusion with the maltose binding protein. Wild type and mutant fusion proteins of the nucleotide-binding fold were overexpressed, purified, and characterized by photoaffinity labeling with 8-azido ATP or 8-azido GTP and a coupled ATP regenerating enzyme assay for ATPase activity. Our studies provide evidence that peroxisomal ABC transporters utilize ATP to become a functional transporter and that ALD gene mutations alter peroxisomal transport function. The established disease model will be used further to study the influence of possible disease modifier proteins on ALDP function.

Retracted : Identification of seven novel mutations in ABCD1 by a DHPLC‐based assay in Italian patients with X‐linked adrenoleukodystrophy

“…We did not detect specific differences in severity of phenotype attributable to the site of the mutation. It has been speculated that some instances, if any, of genotype-phenotype correlation might require evaluation of residual ALDP expression (Gartner et al, 2002). In our case, however, Western blotting did not reveal differences in residual levels between the patient with a missense mutation and two cases with truncating mutations.…”

Adrenal steroids in adrenomyeloneuropathy

“…Missense mutations in the ATP-binding fold cause ATPase dysfunction and ALD gene mutations alter peroxisomal transport function and may cause X-ALD [16]. ALDP expression correlates with pathology in the adrenal gland, Leydig cells and oligodendrocytes in the central nervous system [14].…”

ATP and Substrate Binding Regulates Conformational Changes of Human Peroxisomal ABC Transporter ALDP

“…Mutation of R280 to Cystine may lock the protein in an outward-facing conformation (Coll et al, 2005;Lan et al, 2011), further structural studies of R280C will be needed to clarify its influence on the function of the protein. A large number of cases have been reported at the S606 and G512 position from signature motif and the Walker A motif respectively (Gartner et al, 2002;Kumar et al, 2011;Zhang et al, 2011), which interacts with the ATP molecule at the outward facing conformation. A large number of mutations occurred at the ATP binding site, which may affect the activity of ABCD1.…”