Molecular Biomarkers for Adrenoleukodystrophy: An Unmet Need

X-linked adrenoleukodystrophy (ALD) is an inherited progressive neurometabolic disease caused by mutations in the ABCD1 gene and the accumulation of very long-chain fatty acids in plasma and tissues. Patients present with heterogeneous clinical manifestations which can include adrenal insufficiency, myelopathy, and/or cerebral demyelination. In the absence of a genotype-phenotype correlation, the clinical outcome of an individual cannot be predicted and currently there are no molecular markers available to quantify disease severity. Therefore, there is an unmet clinical need for sensitive biomarkers to monitor and/or predict disease progression and evaluate therapy efficacy. The increasing amount of biological sample repositories (‘biobanking’) as well as the introduction of newborn screening creates a unique opportunity for identification and evaluation of new or existing biomarkers. Here we summarize and review the many studies that have been performed to identify and improve knowledge surrounding candidate molecular biomarkers for ALD. We also highlight several shortcomings of ALD biomarker studies, which often include a limited sample size, no collection of longitudinal data, and no validation of findings in an external cohort. Nonetheless, these studies have generated a list of interesting biomarker candidates and this review aspires to direct future biomarker research.

Biochemical Studies in Fibroblasts to Interpret Variants of Unknown Significance in the ABCD1 Gene

Due to newborn screening for X-linked adrenoleukodystrophy (ALD), and the use of exome sequencing in clinical practice, the detection of variants of unknown significance (VUS) in the ABCD1 gene is increasing. In these cases, functional tests in fibroblasts may help to classify a variant as (likely) benign or pathogenic. We sought to establish reference ranges for these tests in ALD patients and control subjects with the aim of helping to determine the pathogenicity of VUS in ABCD1. Fibroblasts from 36 male patients with confirmed ALD, 26 healthy control subjects and 17 individuals without a family history of ALD, all with an uncertain clinical diagnosis and a VUS identified in ABCD1, were included. We performed a combination of tests: (i) a test for very-long-chain fatty acids (VLCFA) levels, (ii) a D3-C22:0 loading test to study the VLCFA metabolism and (iii) immunoblotting for ALD protein. All ALD patient fibroblasts had elevated VLCFA levels and a reduced peroxisomal ß-oxidation capacity (as measured by the D3-C16:0/D3-C22:0 ratio in the D3-C22:0 loading test) compared to the control subjects. Of the VUS cases, the VLCFA metabolism was not significantly impaired (most test results were within the reference range) in 6/17, the VLCFA metabolism was significantly impaired (most test results were within/near the ALD range) in 9/17 and a definite conclusion could not be drawn in 2/17 of the cases. Biochemical studies in fibroblasts provided clearly defined reference and disease ranges for the VLCFA metabolism. In 15/17 (88%) VUS we were able to classify the variant as being likely benign or pathogenic. This is of great clinical importance as new variants will be detected.

Identification of Two Novel Mutations of ABCD1 Gene in Pedigrees With X‐linked Adrenoleukodystrophy and Review of the Literatures

BackgroundX-linked adrenoleukodysrophy (ALD) is an inherited peroxisomal metabolism disorder, results from the loss-of-function mutation of ATP-binding cassette protein subfamily D1 ( ABCD1 ) gene. The dysfunction of ALD protein, a peroxisomal ATP-binding cassette transporter, results in the excessive saturated very long chain fatty acids (VLCFAs) accumulation in organs including brain, spine and adrenal cortex. X-ALD is characterized as the childhood, adolescent, adult cerebral ALD, adrenomyeloneuropathy (AMN), adrenal insufficiency, and asymptomatic phenotypes, exhibiting a high variety of clinical neurological manifestations with or without adrenocortical insufficiency. ResultsIn this study, we reported two cases of X-ALD, which were firstly diagnosed as adrenal insufficiency (Addison’s disease) and treated with adrenocortical supplement. However, both of the cases progressed as neurological symptoms and signs after decades. Elevated VLCFAs level, brain MRI scan and genetic analysis confirmed final diagnosis. In addition, we identified two novel mutations of ABCD1 gene, c.874_876delGAG (p.Glu292del) and c.96_97delCT (p.Tyr33Profs*161) in exon 1 of ABCD1 gene. Sanger sequencing confirmed that the proband’s mother of the first case was hemizygous carrying the same variant. Adrenal insufficiency-only type is very rare, however, it may be the starting performance of X-ALD. In addition, we summarized reported mutation sites and clinical manifestations to investigate the correlationship of phenotype-genotype of X-ALD. ConclusionsThe early warning manifestations should be noticed, and the probability of X-ALD should be considered. This report could be beneficial for the early diagnosis and genetic counseling for patients with X-ALD.

Atypical image presentation of X-linked adrenoleukodystrophy

Introduction: X-linked adrenoleukodystrophy (X-linked ADL-X) is the most common peroxisomal disorder affecting central nervous system, adrenal cortex and testicular functions. Central nervous system manifestations in X-linked adrenoleukodystrophy can be divided into 2 subcategories: cerebral forms that are associated with rapidly progressive inflammatory myelopathy and adrenomyeloneuropathy which is a non-inflammatory distal axonopathy. In the last quarter of the 20th century, the underlying mechanisms were described, including causal mutations of the ABCD1 gene in the disease. Clinical case: six year old male patient with adequate neurodevelopment, history of seizure at 2 years, origin unclear. At the age of 5, he develops motor symptoms of rapid progression. First consultation for hemiparesis of the left hemicuerpus, ataxic gait, behavioral alteration. During the following year he presents symptoms of rapid progression of motor involvement; dysarthria and quadriparesis, initially asymmetric neuroimages show a progression of the lesion finally compatible with ADL-X, which is confirmed with very long chain acids and spectroscopy. Conclusion: Radiological findings of asymmetric demyelination have rarely been described as a typical X-ALD presentation. A case with atypical radiological presentation is described to document other possibilities of radiological findings in this syndrome with metabolic involvement and to consider this type of presentation in the child population, avoiding delays in diagnosis and increased possibilities for treatment.

The Clinical Impact of CLIR Tools toward Rapid Resolution of Post-Newborn Screening Confirmatory Testing for X-Linked Adrenoleukodystrophy in California

Since the start of X-linked adrenoleukodystrophy (ALD) newborn screening in California, more than half of the diagnosed cases were found to have an ATP binding cassette subfamily D member 1 (ABCD1) gene variant of uncertain significance (VUS). To determine retrospectively the likelihood that these were true positive cases, we used a web-based post-analytical tool in Collaborative Laboratory Integrated Reports (CLIR). Confirmatory plasma very long-chain fatty-acids (VLCFA) profiles for ALD screen positive infant boys were run through the CLIR ALD tool. We compared the distribution by ABCD1 variant classification (pathogenic, likely pathogenic, VUS, and no variant) with the CLIR tool score interpretation (non-informative, possibly ALD, likely ALD, and very likely ALD) and the current case diagnosis. The study showed that CLIR tool positive interpretations were consistent with 100% of the pathogenic and likely pathogenic variants on the ABCD1 gene if a more conservative guideline was used. The tool interpretations were also consistent with screened cases that were determined to not have disease (our no-disorder group). The CLIR tool identified 19 diagnosed ALD cases with VUS to be potential false positives, representing a 40% reduction among all diagnosed ALD cases with VUS. The reduction could be extended to 65% if a more aggressive threshold was used. Identifying such preventable false positives could alleviate the follow-up burden for patients, their families, and California Special Care Centers.