Diagnosis of X-linked adrenoleukodystrophy in blood leukocytes

The genetic landscape of X-linked adrenoleukodystrophy: inheritance, mutations, modifier genes, and diagnosis

X-linked adrenoleukodystrophy (X-ALD) is caused by mutations in the ABCD1 gene encoding a peroxisomal ABC transporter. In this review, we compare estimates of incidence derived from different populations in order to provide an overview of the worldwide incidence of X-ALD. X-ALD presents with heterogeneous phenotypes ranging from adrenomyeloneuropathy (AMN) to inflammatory demyelinating cerebral ALD (CALD). A large number of different mutations has been described, providing a unique opportunity for analysis of functional domains within ABC transporters. Yet the molecular basis for the heterogeneity of clinical symptoms is still largely unresolved, as no correlation between genotype and phenotype exists in X-ALD. Beyond ABCD1, environmental triggers and other genetic factors have been suggested as modifiers of the disease course. Here, we summarize the findings of numerous reports that aimed at identifying modifier genes in X-ALD and discuss potential problems and future approaches to address this issue. Different options for prenatal diagnosis are summarized, and potential pitfalls when applying next-generation sequencing approaches are discussed. Recently, the measurement of very long-chain fatty acids in lysophosphatidylcholine for the identification of peroxisomal disorders was included in newborn screening programs.

Abcd2 is a strong modifier of the metabolic impairments in peritoneal macrophages of ABCD1-deficient mice

The inherited peroxisomal disorder X-linked adrenoleukodystrophy (X-ALD), associated with neurodegeneration and inflammatory cerebral demyelination, is caused by mutations in the ABCD1 gene encoding the peroxisomal ATP-binding cassette (ABC) transporter ABCD1 (ALDP). ABCD1 transports CoA-esters of very long-chain fatty acids (VLCFA) into peroxisomes for degradation by β-oxidation; thus, ABCD1 deficiency results in VLCFA accumulation. The closest homologue, ABCD2 (ALDRP), when overexpressed, compensates for ABCD1 deficiency in X-ALD fibroblasts and in Abcd1-deficient mice. Microglia/macrophages have emerged as important players in the progression of neuroinflammation. Human monocytes, lacking significant expression of ABCD2, display severely impaired VLCFA metabolism in X-ALD. Here, we used thioglycollate-elicited primary mouse peritoneal macrophages (MPMΦ) from Abcd1 and Abcd2 single- and double-deficient mice to establish how these mutations affect VLCFA metabolism. By quantitative RT-PCR, Abcd2 mRNA was about half as abundant as Abcd1 mRNA in wild-type and similarly abundant in Abcd1-deficient MPMΦ. VLCFA (C26∶0) accumulated about twofold in Abcd1-deficient MPMΦ compared with wild-type controls, as measured by gas chromatography-mass spectrometry. In Abcd2-deficient macrophages VLCFA levels were normal. However, upon Abcd1/Abcd2 double-deficiency, VLCFA accumulation was markedly increased (sixfold) compared with Abcd1-deficient MPMΦ. Elovl1 mRNA, encoding the rate-limiting enzyme for elongation of VLCFA, was equally abundant across all genotypes. Peroxisomal β-oxidation of C26∶0 amounted to 62% of wild-type activity in Abcd1-deficient MPMΦ and was significantly more impaired (29% residual activity) upon Abcd1/Abcd2 double-deficiency. Single Abcd2 deficiency did not significantly compromise β-oxidation of C26∶0. Thus, the striking accumulation of VLCFA in double-deficient MPMΦ compared with single Abcd1 deficiency was due to the loss of ABCD2-mediated, compensatory transport of VLCFA into peroxisomes. We propose that moderate endogenous expression of Abcd2 in Abcd1-deficient murine macrophages prevents the severe metabolic phenotype observed in human X-ALD monocytes, which lack appreciable expression of ABCD2. This supports upregulation of ABCD2 as a therapeutic concept in X-ALD.

X-linked adrenoleukodystrophy: very long-chain fatty acid metabolism is severely impaired in monocytes but not in lymphocytes

X-linked adrenoleukodystrophy (X-ALD) is a fatal neurodegenerative disease caused by mutations in the ABCD1 gene, encoding a member of the peroxisomal ABC transporter family. The ABCD1 protein transports CoA-activated very long-chain fatty acids (VLCFAs) into peroxisomes for degradation via β-oxidation. In the severest form, X-ALD patients suffer from inflammatory demyelination of the brain. As the extent of the metabolic defect in the main immune cells is unknown, we explored their phenotypes concerning mRNA expression pattern of the three peroxisomal ABC transporters, VLCFA accumulation and peroxisomal β-oxidation. In controls, ABCD1 expression was high in monocytes, intermediate in B cells and low in T cells; ABCD2 expression was extremely low in monocytes, intermediate in B cells and highest in T cells; ABCD3 mRNA was equally distributed. In X-ALD patients, the expression patterns remained unaltered; accordingly, monocytes, which lack compensatory VLCFA transport by ABCD2, displayed the severest biochemical phenotype with a 6-fold accumulation of C26:0 and a striking 70% reduction in peroxisomal β-oxidation activity. In contrast, VLCFA metabolism was close to control values in B cells and T cells, supporting the hypothesis that sufficient ABCD2 is present to compensate for ABCD1 deficiency. Thus, the vulnerability of the main immune cell types is highly variable in X-ALD. Based on these results, we propose that in X-ALD the halt of inflammation after allogeneic hematopoietic stem cell transplantation relies particularly on the replacement of the monocyte lineage. Additionally, these findings support the concept that ABCD2 is a target for pharmacological induction as an alternative therapeutic strategy.

Impaired very long-chain acyl-CoA β-oxidation in human X-linked adrenoleukodystrophy fibroblasts is a direct consequence of ABCD1 transporter dysfunction

X-linked adrenoleukodystrophy (X-ALD), an inherited peroxisomal disorder, is caused by mutations in the ABCD1 gene encoding the peroxisomal ATP-binding cassette (ABC) transporter ABCD1 (adrenoleukodystrophy protein, ALDP). Biochemically, X-ALD is characterized by an accumulation of very long-chain fatty acids and partially impaired peroxisomal β-oxidation. In this study, we used primary human fibroblasts from X-ALD and Zellweger syndrome patients to investigate the peroxisomal β-oxidation defect. Our results show that the degradation of C26:0-CoA esters is as severely impaired as degradation of unesterified very long-chain fatty acids in X-ALD and is abolished in Zellweger syndrome. Interestingly, the β-oxidation rates for both C26:0-CoA and C22:0-CoA were similarly affected, although C22:0 does not accumulate in patient fibroblasts. Furthermore, we show that the β-oxidation defect in X-ALD is directly caused by ABCD1 dysfunction as blocking ABCD1 function with a specific antibody reduced β-oxidation to levels observed in X-ALD fibroblasts. By quantification of mRNA and protein levels of the peroxisomal ABC transporters and by blocking with specific antibodies, we found that residual β-oxidation activity toward C26:0-CoA in X-ALD fibroblasts is mediated by ABCD3, although the efficacy of ABCD3 appeared to be much lower than that of ABCD1. Finally, using isolated peroxisomes, we show that β-oxidation of C26:0-CoA is independent of additional CoA but requires a cytosolic factor of >10-kDa molecular mass that is resistant to N-ethylmaleimide and heat inactivation. In conclusion, our findings in human cells suggest that, in contrast to yeast cells, very long-chain acyl-CoA esters are transported into peroxisomes by ABCD1 independently of additional synthetase activity.

The impact of a ketogenic diet and liver dysfunction on serum very long-chain fatty acids levels

Peroxisomes play an essential role in mammalian cellular metabolism, particularly in oxidation fatty acid pathways. Serum very long-chain fatty acids (VLCFA), the main biochemical diagnostic parameters for peroxisomal disorders, were examined in 25 neurological patients with epilepsy on a ketogenic diet and 27 patients with liver dysfunction. The data show that patients on a ketogenic diet have increased levels of C22:0 and C24:0, but not C26:0, and normal C24:0/C22:0 and C26:0/C22:0. Patients with liver insufficiency showed a slightly elevated level of C26:0, a normal level of C24:0 and a decreased level of C22:0; thus in 21/27 the ratio of C24:0/C22:0 was increased and 15/27 the ratio of C26:0/C22:0 was increased.

Lymphoblastoid cell lines for diagnosis of peroxisome biogenesis disorders

Peroxisome biogenesis disorders (PBDs) are a group of autosomal-recessive developmental and progressive metabolic diseases leading to the Zellweger spectrum (ZS) phenotype in most instances. Diagnosis of clinically suspected cases can be difficult because of extensive genetic heterogeneity and large spectrum of disease severity. Furthermore, a second group of peroxisomal diseases caused by deficiencies of single peroxisomal enzymes can show an indistinguishable clinical phenotype. The diagnosis of these peroxisomal disorders relies on the clinical presentation, the biochemical parameters in plasma and erythrocyte membranes, and genetic testing as the final step. Analysis of patients' cells is frequently required during the diagnostic process, e.g., for complementation analysis to identify the affected gene before sequencing. In the cases with unclear clinical or biochemical presentation, patients' cells are analyzed to prove PBD or to demonstrate biochemical abnormalities that might be elusive in plasma. Cell lines from skin fibroblast that are usually generated for diagnostic workup are not available in all instances, mainly because the required skin biopsy is invasive and sometimes denied by parents. An alternative cellular system has not been analyzed sufficiently. In this study, we evaluated the alternative use of lymphoblastoid cell lines (LCLs), derived from a peripheral blood sample, in the diagnostic process for PBD. LCLs were suitable for immunofluorescence visualization of peroxisomal enzymes, complementation analysis, and the biochemical analysis to differentiate between control and PBD LCL. LCLs are therefore an easily obtainable alternative cellular system for a detailed PBD diagnostic workup with a reliability of diagnostic results equal to those of skin fibroblasts.