X-Linked Adrenoleukodystrophy (X-ALD)

X-Linked Adrenoleukodystrophy (X-ALD)

Introduction

X Linked Adrenoleukodystrophy (X Ald) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Pathophysiology Flowchart

Overview

X-linked adrenoleukodystrophy (X-ALD)[^1] is a progressive peroxisomal disorder caused by pathogenic variants in the ABCD1 gene, which encodes the adrenoleukodystrophy protein (ALDP), a peroxisomal membrane transporter . ALDP is responsible for importing very long-chain fatty acids (VLCFAs; carbon chain length >= 22) into peroxisomes for beta-oxidation. When ALDP is dysfunctional, VLCFAs accumulate in all tissues, most critically in the adrenal cortex, the myelin-forming cells of the central and peripheral nervous system, and Leydig cells of the tes ([Dubey et al., 2005](https://pubmed.ncbi.nlm.nih.gov/15812459/)). [^2]

X-Linked Adrenoleukodystrophy (X-ALD)

Introduction

X Linked Adrenoleukodystrophy (X Ald) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Pathophysiology Flowchart

Overview

X-linked adrenoleukodystrophy (X-ALD)[^1] is a progressive peroxisomal disorder caused by pathogenic variants in the ABCD1 gene, which encodes the adrenoleukodystrophy protein (ALDP), a peroxisomal membrane transporter . ALDP is responsible for importing very long-chain fatty acids (VLCFAs; carbon chain length >= 22) into peroxisomes for beta-oxidation. When ALDP is dysfunctional, VLCFAs accumulate in all tissues, most critically in the adrenal cortex, the myelin-forming cells of the central and peripheral nervous system, and Leydig cells of the tes ([Dubey et al., 2005](https://pubmed.ncbi.nlm.nih.gov/15812459/)). [^2]

X-ALD is the most common peroxisomal disorder and the most common inherited leukodystrophy, with an estimated incidence of approximately 1 in 17,000-21,000 males 3(https://pmc.ncbi.nlm.nih.gov href="#references">[1]</a></sup>ncbi.nlm.nih.gov/articles/PMC11552335/). The disease encompasses a spectrum of phenotypes ranging from the devastating childhood cerebral form (CALD) to the slowly progressive adult adrenomyeloneuropathy (AMN) and isolated adrenal insufficiency. The discovery of gene therapy for CALD represents a landmark achievement in neurodegenerative disease treatment ([Eichler et al., 2017](https://www.nejm.org/doi/full/10.1056/NEJMoa1700554)). [^3]

Genetics

ABCD1 Gene

The ABCD1 gene is located on chromosome Xq28 and encodes a 745-amino acid protein belonging to the ATP-binding cassette (ABC) transporter superfamily. ALDP functions as a homodimer in the peroxisomal membrane, using ATP hydrolysis to transport coenzyme A-activated VLCFAs (VLCFA-CoA) from the cytosol into the peroxisomal matrix for degradation via beta-oxidation . [^4]

Mutation Spectrum

Over 900 different mutations have been identified in ABCD1 ([Kemp et al., 2012](https://pubmed.ncbi.nlm.nih.gov/22139506/)): [^5]

• Missense mutations: 53.3% of all variants; often lead to protein instability and reduced ALDP abundance
• Frameshift mutations: ~23% of variants; generally cause complete loss of ALDP function
• Nonsense mutations: ~10%; result in premature termination and non-functional protein
• Splice-site mutations: ~7%; disrupt mRNA processing
• Large deletions: ~4%; may remove single or multiple exons
• In-frame deletions/insertions: ~3%

Inheritance

X-ALD follows X-linked inheritance: [^7]

• Males: Hemizygous males (one X chromosome) are affected; all males with a pathogenic ABCD1 variant will develop some phenotype by adulthood
• Female carriers: Heterozygous females were historically considered unaffected, but it is now recognized that approximately 80% of female carriers develop symptoms by age 60, typically an AMN-like myelopathy. Female carriers virtually never develop the cerebral form or adrenal insufficiency
• De novo mutations: Account for approximately 4-8% of cases

Clinical Phenotypes

Childhood Cerebral ALD (CALD)

The most severe phenotype, CALD occurs in approximately 35-40% of affected males ([Engelen et al., 2012](https://pubmed.ncbi.nlm.nih.gov/22607948/)): [^8]

• Age of onset: Typically 4-8 years, after a period of normal development
• Early symptoms: Behavioral changes, declining school performance, visual and auditory processing difficulties
• Progressive phase: Rapid demyelination with inflammatory infiltrates, leading to progressive cognitive and motor decline
• Terminal phase: Vegetative state within 2-5 years of symptom onset
• Neuroimaging: Characteristic posterior-predominant white matter changes on MRI, best tracked using the Loes scoring system ([Loes et al., 2003](https://pubmed.ncbi.nlm.nih.gov/14504243/))
• Inflammatory cascade: [microglia
• Cerebral involvement: ~20% of AMN patients develop cerebral inflammatory demyelination over time, converting to a CALD-like phenotype
• Adrenal insufficiency: Present in most patients, may precede neurological symptoms by years
• Progression: Slowly progressive over decades; most patients require mobility aids within 10-15 years
• Female carriers: ~65% of heterozygous women develop a milder AMN-like phenotype, typically after age 40 ([Engelen et al., 2014](https://pubmed.ncbi.nlm.nih.gov/24781416/))

Adrenal Insufficiency Only (Addison's Disease)

• Presents as primary adrenocortical insufficiency without neurological symptoms
• May be the initial manifestation in boys or young men
• Approximately 10% of males with X-ALD present initially with isolated Addison's disease
• These patients are at risk of developing AMN or cerebral disease later in life
• Every male with unexplained Addison's disease should be tested for X-ALD

Female Carrier Phenotype

• Approximately 80% develop neurological symptoms by age 60
• Typically an AMN-like myelopathy with spastic paraparesis
• Peripheral neuropathy occurs in ~65% of symptomatic women
• Adrenal insufficiency is extremely rare in women
• Cerebral disease virtually never occurs
• Symptom onset is typically later (40s-50s) and progression is slower than in males

Pathophysiology

VLCFA Accumulation and Toxicity

The central pathological process in X-ALD is the accumulation of saturated VLCFAs (particularly C26:0 and C24:0) due to impaired peroxisomal beta-oxidation: [^9]

Inflammatory Demyelination in CALD

The cerebral form involves a catastrophic inflammatory cascade that distinguishes it from the non-inflammatory AMN phenotype ([Eichler et al., 2008](https://pubmed.ncbi.nlm.nih.gov/18493952/)): [^10]

• blood-brain-barrier breakdown: Disruption of the blood-brain-barrier with perivascular contrast enhancement on MRI, preceding clinical symptoms
• Inflammatory infiltrates: Dense perivascular cuffs of T lymphocytes and activated microglia/cell-types/microglia at the active demyelination edge
• Oxidative damage: Accumulated very long-chain fatty acids (VLCFA) induce oxidative stress and lipid peroxidation in [oligodendrocytes and astrocytes
• Complement activation: Deposition of complement components C3d and C5b-9 in demyelinating lesions
• Cytokine cascade: Elevated TNF-alpha, IL-1beta, and MCP-1 in cerebrospinal fluid, driving progressive neuroinflammation ([Lund et al., 2012](https://pubmed.ncbi.nlm.nih.gov/22705907/))

Axonal Degeneration in AMN

In contrast to CALD, AMN involves primarily a non-inflammatory, length-dependent axonopathy: [^11]

• Distal axonal degeneration of the longest tracts in the spinal cord (corticospinal tracts and dorsal columns)
• mitochondrial-dysfunction in axons
• Oxidative damage and energy failure
• Secondary demyelination rather than primary inflammatory demyelination
• This phenotype more closely resembles a metabolic neurodegenerative process

Diagnosis

Newborn Screening

X-ALD was added to the U.S. Recommended Uniform Screening Panel (RUSP) in February 2016. As of 2025, 46 states and Washington D.C. screen newborns for ALD: [^12]

• Screening measures C26:0-lysophosphatidylcholine (C26:0-LPC) levels in dried blood spots
• Elevated C26:0-LPC triggers confirmatory testing with plasma VLCFA analysis and ABCD1 gene sequencing
• Early identification allows monitoring for cerebral disease and timely intervention

Plasma VLCFA Analysis

The gold standard biochemical test for X-ALD diagnosis: [^13]

• Elevated plasma levels of C26:0, C24:0/C22:0 ratio, and C26:0/C22:0 ratio
• Sensitivity in males is >99%; approximately 85% of female carriers have elevated VLCFAs
• False negatives can occur in female carriers due to X-inactivation

Genetic Testing

ABCD1 gene sequencing confirms the diagnosis and enables carrier testing and prenatal diagnosis ([Moser et al., 2007](https://pubmed.ncbi.nlm.nih.gov/17440070/)): [^14]

• Sanger sequencing: Complete coding region analysis of all 10 ABCD1 exons; detects >95% of pathogenic variants
• Deletion/duplication analysis: MLPA (multiplex ligation-dependent probe amplification) identifies large deletions or duplications not detected by sequencing
• Carrier testing: Essential for at-risk female relatives; VLCFA levels may be normal in up to 20% of heterozygous carriers, making genetic testing the definitive method
• Prenatal diagnosis: Available via chorionic villus sampling (CVS) or amniocentesis when the familial mutation is known
• Newborn screening: Tandem mass spectrometry measurement of C26:0-lysophosphatidylcholine (C26:0-LPC) in dried blood spots is now included in newborn screening programs in many U.S. states and several countries ([Vogel et al., 2015](https://pubmed.ncbi.nlm.nih.gov/26202811/))
• Cascade testing: Family screening following an index case diagnosis is critical to identify pre-symptomatic males who may benefit from early HSCT

MRI Monitoring

All males with known ABCD1 mutations require regular MRI surveillance (typically annually from age 1-2 through adolescence): [^15]

• The Loes scoring system quantifies the extent and severity of white matter lesions (0-34 scale)
• Early Loes score (1-3) indicates nascent cerebral disease that may be amenable to treatment
• Gadolinium enhancement indicates active inflammation and a narrow treatment window

Treatment

Hematopoietic Stem Cell Transplantation (HSCT)

Allogeneic HSCT has been the standard of care for early cerebral ALD since the 1990s: [^16]

• Effective only when performed early, before significant neurological progression (Loes score < 9, ideally < 3)
• Donor-derived macrophages/[microglia repopulate the brain, providing functional ALDP and reducing inflammation
• Matched sibling donors preferred; matched unrelated donors and cord blood also used
• Significant transplant-related mortality (historically 10-20%) and morbidity (graft-versus-host disease)
• Stabilizes or improves cerebral disease in 60-70% of patients treated early

Gene Therapy: Skysona (Elivaldogene Autotemcel)

In September 2022, the FDA granted accelerated approval to SKYSONA (elivaldogene autotemcel, eli-cel), a lentiviral vector-based gene therapy for CALD [Accelerated et al., 2022](https://www.businesswire.com/news/home/20220916005595/en/): [^17]

• Mechanism: Autologous CD34+ hematopoietic stem cells are transduced ex vivo with the Lenti-D lentiviral vector carrying a functional ABCD1 cDNA, then reinfused after myeloablative conditioning
• STARBEAM trial (ALD-102): 88% of treated patients (15/17) were alive and free of major functional disability at 24 months, compared to an estimated 43% MFD-free survival in untreated historical controls
• Indication: Boys aged 4-17 with early, active CALD who lack a matched sibling donor
• Safety concern: Reports of hematologic malignancies (myelodysplastic syndrome, acute myeloid leukemia) related to insertional mutagenesis from the lentiviral vector
• Long-term monitoring required for all treated patients

AAV-Based Gene Therapy (Investigational)

Next-generation gene therapy approaches using adeno-associated virus (AAV) vectors are in development ([Zierfuss et al., 2025](https://www.mdpi.com/1422-0067/26/23/11645)): [^18]

• AAV9 vectors carrying ABCD1 can be delivered intravenously or intrathecally
• Potential advantages: broader CNS biodistribution, lower risk of insertional mutagenesis compared to lentiviral vectors
• Preclinical studies in the Abcd1-knockout mouse model have shown promise
• Multiple clinical trials are in early phases

Adrenal Hormone Replacement

All males with documented adrenal insufficiency require lifelong corticosteroid replacement:

• Hydrocortisone and fludrocortisone supplementation
• Stress-dose protocols for illness, surgery, or trauma
• Regular monitoring of adrenal function in all affected males, even if initially normal

Lorenzo's Oil

A 4:1 mixture of glyceryl trioleate and glyceryl trierucate:

• Normalizes plasma VLCFA levels by competitive inhibition of VLCFA elongation
• Does not halt or reverse neurological progression in CALD or AMN
• May have a role in delaying the onset of cerebral disease in asymptomatic boys (controversial; based on observational data)
• Not FDA-approved; considered experimental
• Currently not widely recommended as monotherapy

AMN Management

No disease-modifying therapy exists for AMN:

• Symptom management: antispasticity medications (baclofen, tizanidine), pain management
• Physical therapy and rehabilitation
• Bladder management (anticholinergics, intermittent catheterization)
• BDNF-related research and neuroprotective strategies are being explored

Prognosis

• CALD (untreated): Fatal within 2-5 years of symptom onset; complete disability before death
• CALD (HSCT/gene therapy, treated early): 60-88% stabilization of cerebral disease
• AMN: Slowly progressive; most patients require ambulatory aids within 5-15 years; lifespan may be near-normal but with significant disability
• Addison's only: Good prognosis with hormone replacement; requires surveillance for neurological progression
• Female carriers: Variable; many maintain function with mild-moderate symptoms

Epidemiology

• Combined incidence (males + female carriers): approximately 1 in 17,000 births
• Affects all ethnic groups and races worldwide
• No significant geographic variation in incidence
• With universal newborn screening, all affected males can now be identified before symptom onset in most US states

Background

The study of X Linked Adrenoleukodystrophy (X Ald) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.

External Links

• [ALD Connect Patient Organization](https://www.aldsconnect.org/)
• [GeneReviews: X-Linked Adrenoleukodystrophy](https://www.ncbi.nlm.nih.gov/books/NBK1315/)
• [NORD: X-Linked Adrenoleukodystrophy](https://rarediseases.org/rare-diseases/adrenoleukodystrophy/)
• [ClinicalTrials.gov: Adrenoleukodystrophy](https://clinicaltrials.gov/search?cond=Adrenoleukodystrophy)

See Also

• [astrocytes](/cell-types/astrocytes)
• [microglia](/cell-types/microglia)
• [Oligodendrocytes](/cell-types/oligodendrocytes)

Allen Brain Atlas Resources

• [Allen Brain Atlas - Gene Expression](https://human.brain-map.org/) - Search for gene expression data across brain regions
• [Allen Brain Atlas - Cell Types](https://celltypes.brain-map.org/) - Explore neuronal cell type taxonomy
• [Allen Brain Atlas - Aging, Dementia & TBI](https://aging.brain-map.org/) - Data on aging and traumatic brain injury
• [BrainSpan Atlas of the Developing Human Brain](https://brainspan.org/) - Developmental gene expression data

References

[^18]: [Reference missing - citation needed]

[^17]: [Reference missing - citation needed]

[^16]: [Reference missing - citation needed]

[^15]: [Reference missing - citation needed]

[^14]: [Reference missing - citation needed]

[^13]: [Reference missing - citation needed]

[^12]: [Reference missing - citation needed]

[^11]: [Reference missing - citation needed]

[^10]: [Reference missing - citation needed]

[^9]: [Reference missing - citation needed]

[^8]: [Reference missing - citation needed]

[^7]: [Reference missing - citation needed]

[^6]: [Reference missing - citation needed]

[^5]: [Reference missing - citation needed]

[^4]: [Reference missing - citation needed]

[^3]: [Reference missing - citation needed]

[^2]: [Reference missing - citation needed]

[^1]: [Reference missing - citation needed]

Recent advances in X-Linked