Eight novel mutations in the ABCD1 gene and clinical characteristics of 25 Chinese patients with X-linked adrenoleukodystrophy

Central precocious puberty in a boy with X-linked adrenoleukodystrophy caused by a novel ABCD1 mutation

X-linked adrenoleukodystrophy (X-ALD) is a rare genetic disorder caused by pathogenic variants in the ABCD1 gene. The symptoms include primary adrenal insufficiency (PAI), progressive spinal cord disease, inflammatory demyelinating cerebral disease, and primary hypogonadism. It is exceptionally rare that pediatric PAI is accompanied by central precocious puberty (CPP). The purpose of this study was to better understand the diversity of clinical manifestations of X-ALD and to identify the ABCD1 gene mutation in a case of a boy with X-ALD accompanied by CPP. We collected clinical, laboratory and imaging data, and used whole-exome sequencing (WES) analysis to evaluate the pathogenicity of the variant. We also predicted the potential deleterious effects of the novel mutation using Mutation Taster and generated three-dimensional protein structures using Swiss-Model and PyMOL Viewer software. The patient presented with PAI accompanied by CPP. Adrenal gland CT revealed adrenal hypoplasia. Gonadotropin-releasing hormone stimulation tests revealed CPP. WES revealed a novel variant (c.1376dup) in the ABCD1 gene, which resulted in a reading frameshift and a premature termination codon (p.Leu461ProfsTer95). Sanger sequencing confirmed that the variant was inherited from his heterozygous mother. Mutation Taster predicted that the variant could be harmful. The overall three-dimensional structures of the mutant wild-type proteins were visually distinct. Our results shed light on additional aspects of X-ALD. The premature activation of the hypothalamic-pituitary-gonadal axis may possibly be related to the pathogenic ABCD1 gene mutation.

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.

Identification of Two Novel Mutations of ABCD1 Gene in Pedigrees with X-Linked Adrenoleukodystrophy and Review of the Literature

BACKGROUND

X-linked adrenoleukodystrophy (ALD) is an inherited peroxisomal metabolism disorder, resulting 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 the 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.

RESULTS

In this study, we reported two cases of X-ALD, which were first 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, NM_000033.3 (ABCD1): c.874_876delGAG (p.Glu292del) and NM_000033.3 (ABCD1): 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 heterozygous 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.

CONCLUSIONS

The 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.

Clinical characteristics and phenotype distribution in 10 Chinese patients with X-linked adrenoleukodystrophy

X-linked adrenoleukodystrophy (X-ALD) is the most frequent type of inherited demyelinating peroxisomal disease caused by mutations in the ATP binding cassette subfamily D member 1 (ABCD1) gene. The rate of early recognition and genetic diagnosis of X-ALD remains low due to its variable clinical manifestations. The present study summarized the clinical features Chinese X-ALD patients and performed a follow-up study to further precisely characterize this disease. A total of 10 patients diagnosed with X-ALD between 1994 and 2016 at Shandong Provincial Hospital Affiliated to Shandong University (Jinan, China) were included in the present study. Through reviewing their medical records and performing telephone follow-ups, the clinical features, biochemical laboratory data, brain images, treatments and long-term outcomes were retrospectively summarized. Mutation analysis of the ABCD1 gene was performed in certain patients. Most of the patients (8/10) had the childhood cerebral form of X-ALD. One patient presented with the olivo-ponto-cerebellar form, the rarest form of X-ALD. In all patients, brain magnetic resonance images revealed abnormalities with typical T2-weighted hyperintensity. Analysis of very long chain fatty acid revealed high plasma levels of hexacosanoic acid in all patients. Increased adrenocorticotropic hormone, decreased cortisol and neurophysiological manifestations were also observed. Three different mutations of the ABCD1 gene were identified in the 3 patients subjected to genotyping. During the follow-ups, most patients took neurotrophic drugs and received hydrocortisone replacement when required. One patient received a hematopoietic stem cell transplantation, but died 1 year following the transplantation. Chronic myelopathy and peripheral neuropathy progressed with time, gradually leading to a vegetative state or paralysis within several years of clinical symptom onset. In conclusion, male patients with adrenocortical insufficiency should be further investigated for X-ALD. Early detection is critical to prevent the progression of X-ALD with mutation analysis of ABCD1 the most accurate method to confirm diagnosis.

Long-term outcome of patients with X-linked adrenoleukodystrophy: A retrospective cohort study

BACKGROUND

X-linked adrenoleukodystrophy (X-ALD) is a peroxisomal disorder associated with leukodystrophy, myeloneuropathy and adrenocortical insufficiency. We performed a retrospective cohort study to evaluate long-term outcome of patients with X-ALD.

METHOD

All patients with X-ALD diagnosed between 1989 and 2012 were included. Electronic patient charts were reviewed for clinical features, biochemical investigations, molecular genetic testing, neuroimaging, long-term outcome and treatment.

RESULTS

Forty-eight patients from 18 unrelated families were included (15 females; 33 males). Seventeen patients were symptomatic at the time of the biochemical diagnosis including 14 with neurocognitive dysfunction and 3 with Addison disease only. Thirty-one asymptomatic individuals were identified by positive family history of X-ALD. During follow-up, eight individuals developed childhood cerebral X-ALD (CCALD), one individual developed adrenomyeloneuropathy (AMN), six individuals developed Addison disease only, and five individuals remained asymptomatic. Direct sequencing of ABCD1 confirmed the genetic diagnosis in 29 individuals. Seven patients with CCALD underwent hematopoietic stem cell transplantation (HSCT). Nine patients lost the follow-up. There was no correlation between clinical severity score, Loes score and elevated degree of elevated very long chain fatty acid (VLCFA) levels in CCALD.

CONCLUSION

Our study reports forty-eight new patients with X-ALD and their long-term outcome. Only 35% of the patients presented with neurological features or Addison disease. The remaining individuals were identified due to positive family history. Close monitoring of asymptomatic males resulted in early HSCT to prevent progressive lethal neurodegenerative disease. Identification of patients with X-ALD is important to improve neurodevelopmental outcome of asymptomatic males.