Functional characterization of the adrenoleukodystrophy protein (ALDP) and disease pathogenesis

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.

Structure and Function of the ABCD1 Variant Database: 20 Years, 940 Pathogenic Variants, and 3400 Cases of Adrenoleukodystrophy

The progressive neurometabolic disorder X-linked adrenoleukodystrophy (ALD) is caused by pathogenic variants in the ABCD1 gene, which encodes the peroxisomal ATP-binding transporter for very-long-chain fatty acids. The clinical spectrum of ALD includes adrenal insufficiency, myelopathy, and/or leukodystrophy. A complicating factor in disease management is the absence of a genotype–phenotype correlation in ALD. Since 1999, most ABCD1 (likely) pathogenic and benign variants have been reported in the ABCD1 Variant Database. In 2017, following the expansion of ALD newborn screening, the database was rebuilt. To add an additional level of confidence with respect to pathogenicity, for each variant, it now also reports the number of cases identified and, where available, experimental data supporting the pathogenicity of the variant. The website also provides information on a number of ALD-related topics in several languages. Here, we provide an updated analysis of the known variants in ABCD1. The order of pathogenic variant frequency, overall clustering of disease-causing variants in exons 1–2 (transmembrane domain spanning region) and 6–9 (ATP-binding domain), and the most commonly reported pathogenic variant p.Gln472Argfs*83 in exon 5 are consistent with the initial reports of the mutation database. Novel insights include nonrandom clustering of high-density missense variant hotspots within exons 1, 2, 6, 8, and 9. Perhaps more importantly, we illustrate the importance of collaboration and utility of the database as a scientific, clinical, and ALD-community-wide resource.

Gene expression profiling of adenosine triphosphate-binding cassette transporters in response to K-ras activation and hypoxia in human pancreatic cancer cell cultures

OBJECTIVES

Pancreatic cancer (PC) is hypoxic and highly resistant to conventional chemotherapy. We sought to determine whether K-ras oncogene and/or hypoxia can induce expression of drug resistance-promoting adenosine triphosphate-binding cassette (ABC) transporters in human PC cell lines.

METHODS

Immortalized near-normal human pancreatic ductal epithelial(HPDE) cells, HPDE cells expressing K-rasG12V oncogene, and PCcell lines (MIA PaCa-2, PANC-1, BxPC-3) were subjected to hypoxia and examined for messenger RNA expression of 48 ABC transporters.

RESULTS

Mutant K-ras activation and/or hypoxia of HPDE cells led to induction of various ABC transporters. In the case of PC cell lines, no clear correlation was found between expression of constitutively active K-ras and global ABC transporter expression. Moreover, hypoxic treatment of PC cell lines had different effects on ABC transporter expression.Importantly, PC cell lines did not express the multidrug resistance 1 ABC transporter, a major mechanism of drug resistance. However, multi drug resistance 1 expression in the cells was up-regulated in response to continuous exposure to low doses of vincristine, indicating that drug resistance could be induced.

CONCLUSIONS

Expression of K-ras oncogene and hypoxia, as well as exposure to drugs, can contribute to drug resistance in PC cells.

Adrenal steroids in adrenomyeloneuropathy. Dehydroepiandrosterone sulfate, androstenedione and 17alpha-hydroxyprogesterone

Adrenoleukodystrophy (ALD) and its adult variant adrenomyeloneuropathy (AMN) are X-linked diseases associated with a deficiency in the peroxisomal degradation of saturated very long chain fatty acids (VLCFA) resulting in an accumulation of VLCFA in the central and peripheral myelin, the adrenal cortex and the testis. Adrenal insufficiency with clinical hypocortisolism occurs in approximately two thirds of the patients with AMN. We studied the circulating adrenal hormones 17alpha-hydroxyprogesterone (17alpha-OHP), androstenedione and dehydroepiandrosterone sulphate (DHEAS) in 63 male AMN patients (age 17-65 years) and the DHEAS serum levels in 95 healthy male controls (age 30-65 years). 34 of the patients presented with the phenotype of only spinal cord and peripheral nerve disability without hypocortisolism, 29 of the patients presented with the phenotype of either additional hypocortisolism or Addison's syndrome only. Normal 17alpha-OHP concentrations were found in all patients with no significant difference between patients without and with hypocortisolism (6.07 +/- 0.61 nmol/l and 4.76 +/- 0.37 nmol/l). Androstenedione concentration was significantly (p < 0.01) lower in patients with hypocortisolism (2.99 +/- 0.65 pmol/l versus 5.71 +/- 0.68 pmol/l). As serum levels of DHEAS are agedependent we divided the two groups into two subgroups each (subgroup one: age 17-40 years, subgroup two: age 41-65 years). The DHEAS concentration of patients without and with hypocortisolism was significantly (p < 0.01) lower in both subgroups (1. 4.35 +/- 0.84 micromol/l, n = 15, 2. 15 +/- 0.28 micromol/l, n = 19; 1. 1.90 +/- 0.57 micromol/, n = 21, 2. 0.96 +/- 0.29 micromol/l, n = 8) compared to controls (1. 9.0 +/- 0.96 micromol/l; 2. 5.21 +/- 0.25 micromol/l). In conclusion, androstenedione and DHEAS serum concentrations are subnormal in all AMN patients and may therefore serve as sensitive markers of the adrenal function in adrenomyeloneuropathy.

Progress in X-linked adrenoleukodystrophy

PURPOSE OF REVIEW

The purpose of this article is to review and evaluate the new information about X-linked adrenoleukodystrophy that has been reported in 2002 and 2003.

RECENT FINDINGS

X-linked adrenoleukodystrophy has two distinct neurological phenotypes: adrenomyeloneuropathy, a non-inflammatory axonopathy mostly in adults, and an intensely inflammatory cerebral myelinopathy mostly in children. The two forms often co-occur in the same family. Heterozygous women and the X-linked adrenoleukodystrophy mouse model often have the adrenomyeloneuropathy phenotype. More than 500 distinct mutations in the defective gene (ABCD1) have been identified, and except in one unique family, do not correlate with the phenotype. Bone marrow transplantation is beneficial in patients with early cerebral involvement. A panel of brain neuroimaging studies aids the selection of patients for bone marrow transplantation. Lorenzo's oil administered to neurologically asymptomatic boys who are less than 6 years old and have a normal magnetic resonance imaging scan appears to reduce the probability of developing neurological abnormalities later in life.

SUMMARY

Progress has been achieved in the delineation of the phenotypes, pathogenesis, diagnosis and prevention of X-linked adrenoleukodystrophy, and therapies are emerging.