Adult-Onset Leukoencephalopathy with Axonal Spheroids and Pigmented Glia (ALSP)

Adult-Onset Leukoencephalopathy with Axonal Spheroids and Pigmented Glia (ALSP)

Introduction

ALSP is a progressive neurodegenerative disease caused by CSF1R mutations, representing a prototypical primary brain microgliopathy. The disease typically presents in middle adulthood with cognitive decline, motor symptoms, and psychiatric manifestations. While no disease-modifying therapy is currently approved, emerging treatments targeting TREM2 and hematopoietic stem cell transplantation offer potential therapeutic strategies. Early diagnosis through genetic testing and characteristic MRI findings is essential for appropriate management and family counseling. Ongoing research continues to elucidate disease mechanisms and develop targeted therapies for this rare but devastating condition.

Adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) is a rare, progressive neurodegenerative disorder primarily affecting the cerebral white matter. ALSP is caused by heterozygous loss-of-function mutations in the CSF1R (colony stimulating factor 1 receptor) gene, making it the prototypical primary brain microgliopathy—a disease driven by microglial dysfunction.

Adult-Onset Leukoencephalopathy with Axonal Spheroids and Pigmented Glia (ALSP)

Introduction

ALSP is a progressive neurodegenerative disease caused by CSF1R mutations, representing a prototypical primary brain microgliopathy. The disease typically presents in middle adulthood with cognitive decline, motor symptoms, and psychiatric manifestations. While no disease-modifying therapy is currently approved, emerging treatments targeting TREM2 and hematopoietic stem cell transplantation offer potential therapeutic strategies. Early diagnosis through genetic testing and characteristic MRI findings is essential for appropriate management and family counseling. Ongoing research continues to elucidate disease mechanisms and develop targeted therapies for this rare but devastating condition.

Adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) is a rare, progressive neurodegenerative disorder primarily affecting the cerebral white matter. ALSP is caused by heterozygous loss-of-function mutations in the CSF1R (colony stimulating factor 1 receptor) gene, making it the prototypical primary brain microgliopathy—a disease driven by microglial dysfunction.

Previously described as two separate entities — hereditary diffuse leukoencephalopathy with spheroids (HDLS) and pigmentary orthochromatic leukodystrophy (POLD) — these conditions were unified under the ALSP designation when CSF1R mutations were identified as the shared genetic cause in 2012.[@r2017] ALSP typically manifests in the fourth to fifth decade of life (mean onset ~43 years) and progresses to severe disability and death within 5-10 years of symptom onset. Women appear to have a significantly younger age of onset than men (40 vs. 47 years).

The estimated prevalence is approximately 1 in 100,000, though ALSP is likely underdiagnosed due to clinical overlap with more common conditions including Alzheimer's disease, frontotemporal dementia, multiple sclerosis, and vascular dementia.

Overview

ALSP represents a unique category of neurodegenerative disease characterized by:

• Primary involvement of cerebral white matter
• Autosomal dominant inheritance via CSF1R mutations
• Pathology featuring axonal spheroids and pigmented microglia
• Progressive cognitive, motor, and psychiatric decline

Epidemiology

ALSP is a rare disease with an estimated prevalence of approximately 1 in 100,000 to 1 in 1,000,000, though it is likely underdiagnosed due to clinical overlap with more common conditions including [Alzheimer's disease](/diseases/alzheimers-disease), [frontotemporal dementia](/diseases/frontotemporal-dementia), [multiple sclerosis](/diseases/multiple-sclerosis), and vascular dementia[^1].

The disease typically manifests in the fourth to fifth decade of life, with a mean age of onset around 43 years. Women tend to present significantly earlier than men (mean age 40 vs. 47 years)[^2]. The disease shows autosomal dominant inheritance with high penetrance, though de novo mutations account for approximately 20-40% of cases. There is no clear gender predilection, and cases have been reported across multiple ethnic backgrounds.

Genetics and Molecular Basis

CSF1R Gene and Protein

The CSF1R gene, located on chromosome 5q32, encodes a 108 kDa type III receptor tyrosine kinase (RTK) that belongs to the platelet-derived growth factor receptor family. CSF1R is the receptor for two ligands: colony stimulating factor 1 (CSF-1/M-CSF) and interleukin-34 (IL-34). The receptor plays a pivotal role in regulating the survival, differentiation, proliferation, and activation of mononuclear phagocytic cells, including microglia in the central nervous system[^3].[@v2020]

Mutation Spectrum

Over 100 pathogenic variants in CSF1R have been identified in ALSP patients, with the majority affecting the tyrosine kinase domain. The mutation spectrum includes:

• Missense mutations in the tyrosine kinase domain (TKD): Account for approximately 90% of all pathogenic variants, predominantly affecting exons 12-22. These mutations disrupt kinase autophosphorylation and downstream signaling[^4].
• Frameshift and nonsense mutations: Result in truncated, non-functional proteins, typically causing haploinsufficiency.
• Splice-site mutations: Disrupt normal mRNA processing.

CSF1R Signaling Pathway

Under normal conditions, ligand binding triggers CSF1R dimerization and autophosphorylation, activating downstream pathways including:

• PI3K/AKT pathway: Promotes microglial survival and anti-inflammatory functions.
• MAPK/ERK pathway: Regulates proliferation and differentiation.
• PLCγ signaling: Mediates calcium-dependent cellular functions.
• TREM2 interactions: CSF1R and TREM2 share downstream signaling pathways. TREM2 activation can partially compensate for CSF1R loss of function, providing a rationale for therapeutic TREM2 agonism[^5].

Pathophysiology

Microglial Dysfunction

ALSP is fundamentally a disease of microglial homeostasis. CSF1R haploinsufficiency leads to:

• Impaired microglial survival and renewal: Reduced CSF1R signaling compromises the ability of microglia to self-renew and respond to injury.[@v2015]
• Dysfunctional phagocytosis: Microglial phagocytic capacity is impaired, leading to accumulation of cellular debris.
• Pigmented glia: Characteristic pigmented macrophages accumulate in affected white matter.
• White matter degeneration: Progressive demyelination with relative preservation of subcortical U-fibers. Loss of axons and myelin proceeds in a frontoparietal-to-occipital gradient.
• Corpus callosum thinning: Atrophy of the corpus callosum, particularly the body and splenium, is a characteristic feature.

Clinical Presentation

The most common initial presentation (approximately 55% of cases) involves cognitive decline resembling Frontotemporal Dementia or early-onset Alzheimer's Disease[^6]:

• Executive dysfunction: Impaired planning, judgment, and abstract thinking — often the earliest and most prominent cognitive deficit.
• Behavioral changes: Apathy, disinhibition, personality changes, and social conduct problems.
• Memory impairment: Typically less prominent than executive dysfunction initially, though may become pronounced as disease progresses.
• Progressive aphasia: Word-finding difficulties, reduced verbal fluency, and eventual motor speech impairment.

• Pyramidal signs: Spasticity, hyperreflexia, Babinski sign, and progressive weakness. Upper motor neuron signs often develop early and may dominate the clinical picture.
• Parkinsonism: Bradykinesia, rigidity, and resting tremor — may mimic [Parkinson's disease](/diseases/parkinsons-disease) and lead to diagnostic confusion.
• Gait ataxia: Unsteady gait, frequent falls, and impaired coordination.
• Seizures: Occur in approximately 30-50% of patients and may be the presenting symptom in some cases[^8].

Psychiatric Manifestations

Depression and anxiety are frequently reported (30-40% of patients), often preceding the recognition of cognitive decline by months to years[^9]. Psychotic symptoms including visual or auditory hallucinations may also occur, particularly in later disease stages.

Diagnosis

Neuroimaging Findings

Characteristic neuroimaging findings on MRI are essential for diagnosis and help distinguish ALSP from other white matter disorders[^10]:

• White matter hyperintensities (WMH): Bilateral, asymmetric, confluent T2/FLAIR hyperintensities in the frontoparietal periventricular white matter, often extending to involve the deep white matter. The lesions typically spare the subcortical U-fibers initially.
• Corpus callosum thinning: Progressive atrophy of the corpus callosum, particularly the body and splenium — a characteristic finding that helps distinguish ALSP from other leukoencephalopathies.
• Diffusion-weighted imaging (DWI): Persistent hyperintense lesions (restricted diffusion) within the white matter lesions — a distinctive feature that helps differentiate ALSP from multiple sclerosis and vascular disease. These represent areas of ongoing axonal degeneration.
• CT calcifications: Punctate calcifications in the frontal periventricular white matter are present in approximately 50% of patients and are highly suggestive of ALSP when present.
• Cortical atrophy: Frontoparietal cortical atrophy develops as the disease progresses, reflecting secondary neuronal loss.
• Progression pattern: White matter involvement typically progresses in a frontoparietal-to-occipital gradient.

Genetic Testing

Definitive diagnosis requires identification of a pathogenic variant in CSF1R through sequencing of exons 12-22 (tyrosine kinase domain) or whole-gene analysis. Genetic testing should be considered in any adult presenting with progressive white matter disease and cognitive decline, particularly when MRI features are suggestive. Testing should include both targeted CSF1R sequencing and comprehensive panel testing for leukodystrophies.

Brain Biopsy

In the pre-genetic testing era, brain biopsy was often required for diagnosis. Histopathological findings include axonal spheroids (on neurofilament immunostaining), pigmented macrophages (PAS-positive, autofluorescent), and white matter loss with gliosis. Biopsy is now rarely necessary when genetic testing is available, but may still be considered in atypical cases.

Differential Diagnosis

ALSP must be distinguished from several more common conditions[^11]:

| Condition | Key Distinguishing Features |
|-----------|----------------------------|
| Multiple Sclerosis | Presence of oligoclonal bands in CSF, enhancing lesions, spinal cord involvement, and periventricular lesions perpendicular to the lateral ventricles |
| Frontotemporal Dementia | Predominant frontotemporal atrophy on MRI without significant white matter changes favors FTD; white matter involvement on MRI suggests ALSP |
| CADASIL | Notch3 mutation testing and characteristic temporal pole involvement (white matter hyperintensities) distinguish CADASIL |
| Vascular Dementia | Vascular risk factors, lacunar infarcts, and stepwise progression favor vascular disease; ALSP shows more confluent, symmetric white matter changes |
| Metachromatic Leukodystrophy | ARSA enzyme activity deficiency and childhood/young adult onset distinguish MLD; adult-onset MLD is rare |
| Adult Polyglucosan Body Disease | GBE1 mutation, characteristic periodic acid-Schiff-positive polyglucosan bodies in nerve/brain biopsy |
| Hereditary Cerebral Amyloid Angiopathy | CAA-related hemorrhages and cortical/subcortical microbleeds on MRI |

Treatment and Management

Current Management

No disease-modifying therapy is currently approved for ALSP. Management is symptomatic:

• Antiepileptic drugs: For seizure management (levetiracetam, valproate)
• Antidepressants: SSRIs for depression and anxiety
• Antispasticity agents: Baclofen or tizanidine for spasticity
• Supportive care: Physical therapy, occupational therapy, speech therapy, and eventual palliative care

Hematopoietic Stem Cell Transplantation (HSCT)

Case reports have described stabilization or improvement following allogeneic HSCT, particularly when performed early in the disease course. Donor-derived microglia can repopulate the brain, potentially restoring CSF1R-dependent microglial functions. However, HSCT carries significant morbidity and mortality risks, and prospective controlled trial data are lacking.

Iluzanebart (VGL101) — Emerging Therapy

Iluzanebart is a human monoclonal IgG1 agonist antibody targeting TREM2, developed by Vigil Neuroscience for ALSP treatment. The rationale is that trem2 activation can compensate for CSF1R dysfunction by activating overlapping downstream signaling pathways.

• IGNITE Phase 2 trial: Iluzanebart demonstrated a favorable safety and tolerability profile across 20 mg/kg and 40 mg/kg dose cohorts. However, the trial showed no significant beneficial effects on biomarker or clinical efficacy endpoints.
• Ongoing development: Vigil Neuroscience has pursued a potential accelerated approval pathway based on biomarker endpoints.

Prognosis

ALSP is invariably progressive, with median disease duration from symptom onset to death of approximately 6.8 years (range 2-29 years). Rapid cognitive decline, early motor involvement, and younger age at onset are associated with faster progression. The cause of death is most commonly aspiration pneumonia, followed by status epilepticus and general debilitation.

Conclusion

ALSP is a progressive neurodegenerative disease caused by CSF1R mutations, representing a prototypical primary brain microgliopathy. The disease typically presents in middle adulthood with cognitive decline, motor symptoms, and psychiatric manifestations. While no disease-modifying therapy is currently approved, emerging treatments targeting TREM2 and hematopoietic stem cell transplantation offer potential therapeutic strategies. Early diagnosis through genetic testing and characteristic MRI findings is essential for appropriate management and family counseling. Ongoing research continues to elucidate disease mechanisms and develop targeted therapies for this rare but devastating condition.

Background

The study of Adult Onset Leukoencephalopathy With Axonal Spheroids And Pigmented Glia (Alsp) 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.

Recent Research (2024-2026)

Recent advances in Adult-Onset Leukoencephalopathy with Axonal Spheroids and Pigmented Glia (ALSP) have focused on understanding disease mechanisms, identifying biomarkers, and developing novel therapeutic approaches. Key developments include:

• Genetic studies: Identification of new genetic risk factors and mechanistic insights
• Biomarker research: Development of diagnostic and prognostic biomarkers
• Therapeutic approaches: Investigation of novel treatment strategies
• Clinical trials: Ongoing Phase I-III trials for new therapies

Pathogenesis Pathway

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

See Also

• White Matter Disorders
• Leukodystrophies
• [Adult-Onset Demyelinating Diseases](/diseases)
• Myelin Disorders
• Neurogenetics
• [Demyelination in Neurodegeneration](/diseases/neurodegeneration)

References

External Links

• [ALSP Foundation](https://alspinfo.org/) - Patient organization and information resource