ABCD1 gene mutation in an Italian family with X-linkedadrenoleukodystrophy: case series

Structural insights into substrate recognition and translocation of human peroxisomal ABC transporter ALDP

Dysfunctions of ATP-binding cassette, subfamily D, member 1 (ABCD1) cause X-linked adrenoleukodystrophy, a rare neurodegenerative disease that affects all human tissues. Residing in the peroxisome membrane, ABCD1 plays a role in the translocation of very long-chain fatty acids for their β-oxidation. Here, the six cryo-electron microscopy structures of ABCD1 in four distinct conformational states were presented. In the transporter dimer, two transmembrane domains form the substrate translocation pathway, and two nucleotide-binding domains form the ATP-binding site that binds and hydrolyzes ATP. The ABCD1 structures provide a starting point for elucidating the substrate recognition and translocation mechanism of ABCD1. Each of the four inward-facing structures of ABCD1 has a vestibule that opens to the cytosol with variable sizes. Hexacosanoic acid (C26:0)-CoA substrate binds to the transmembrane domains (TMDs) and stimulates the ATPase activity of the nucleotide-binding domains (NBDs). W339 from the transmembrane helix 5 (TM5) is essential for binding substrate and stimulating ATP hydrolysis by substrate. ABCD1 has a unique C-terminal coiled-coil domain that negatively modulates the ATPase activity of the NBDs. Furthermore, the structure of ABCD1 in the outward-facing state indicates that ATP molecules pull the two NBDs together and open the TMDs to the peroxisomal lumen for substrate release. The five structures provide a view of the substrate transport cycle and mechanistic implication for disease-causing mutations.

X‑linked adrenoleukodystrophy caused by maternal ABCD1 mutation and paternal X chromosome inactivation

X-linked adrenoleukodystrophy (X-ALD) is the most common peroxisomal disorder. It is caused by defects in the ATP-binding cassette subfamily D member 1 (ABCD1) gene, resulting in impaired peroxisomal β-oxidation of very-long-chain fatty acids (VLCFAs). As an X-linked recessive disease, female X-ALD carriers are typically asymptomatic. In the present study, a 7-year-old girl was diagnosed with cerebral ALD. Brain magnetic resonance imaging revealed asymmetric demyelination of bilateral white matter. Plasma VLCFAs level showed a substantial increase. Whole exome and Sanger sequencing revealed an ABCD1 c.919C>T (p.Q307X) heterozygous pathogenic mutation, which was inherited from the asymptomatic mother. X chromosome inactivation (XCI) analysis revealed that the normal paternal X chromosome was almost completely inactivated. Thus, the maternal ABCD1 mutation and paternal XCI were responsible for causing the disease in the patient. XCI may be one reason female X-ALD carriers can be symptomatic.