One Gene, Two Faces: The Subtypes of Alexander Disease

Alexander disease is a rare astrocytopathy characterized by the progressive degeneration of white matter in the brain. It is caused by mutations in the gene encoding glial fibrillary acidic protein (GFAP), a key structural component of astrocytes. The resulting dysfunctional protein accumulates into aggregates known as Rosenthal fibers, disrupting astrocyte function and leading to widespread neurological symptoms.

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Frequently asked questions

What are Rosenthal fibers?

Rosenthal fibers are abnormal clumps of protein that accumulate inside astrocytes and are the pathological hallmark of Alexander disease. They are primarily composed of the mutant GFAP protein along with stress proteins, and their presence is central to the disease process.

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The Central Role of the GFAP Gene

The GFAP gene provides instructions for making glial fibrillary acidic protein (GFAP), which forms the intermediate filaments that give astrocytes their structure and strength. Alexander disease arises from heterozygous, gain-of-function variants in this gene—most of them de novo rather than inherited. These variants cause a toxic buildup of GFAP protein that triggers a cellular stress response, progressively impairing astrocyte function and ultimately leading to demyelination and neuronal death.

Evolving Classification Systems

The clinical presentation of Alexander disease is highly variable, which has led to evolving classification systems over time. Initially, the disorder was categorized based on the age of onset.

• Infantile-onset: Birth to 2 years
• Juvenile-onset: 2 to 14 years
• Adult-onset: Over 14 years

While simple, this system did not fully capture the distinct clinical and radiological patterns observed. In 2011, a new classification was proposed based on statistical analysis of 215 cases, dividing the disease into two more distinct subtypes. This latent class analysis identified Type I and Type II, a framework that has gained wide acceptance for its clinical utility. Other systems, such as a three-group classification (cerebral, bulbospinal, intermediate), have also been suggested, highlighting the ongoing effort to best categorize this complex disease.

Clinical Presentations: Type I vs. Type II

The two-type system helps clinicians anticipate the disease course and primary symptoms. Each type accounts for approximately half of all cases, but they present with markedly different features.

Type I Alexander Disease

Type I is the classic, severe form of the disease, with symptom onset typically before the age of four. It is characterized by significant cerebral involvement. Key clinical features include:

• Macrocephaly (an enlarged head)
• Seizures that are often difficult to control
• Developmental delay and subsequent loss of milestones (psychomotor regression)
• Spasticity and other signs of pyramidal tract dysfunction
• Failure to thrive

Neuroimaging is critical for diagnosis. MRI scans in Type I patients characteristically show extensive white matter abnormalities with a frontal predominance. Swelling of the basal ganglia and thalamus may also be present.

Type II Alexander Disease

Type II can have an onset at any age but most commonly appears in late childhood, adolescence, or adulthood. This form primarily affects the brainstem and spinal cord, leading to a different set of symptoms. Hallmarks of Type II include bulbar and cerebellar signs such as:

• Difficulty with speech (dysarthria) and swallowing (dysphagia)
• Problems with coordination and balance (ataxia)
• Abnormal eye movements
• Symptoms of autonomic dysfunction

MRI findings in Type II are distinct from Type I. They often show abnormalities in the posterior fossa (cerebellum and brainstem) and cervical spinal cord. A highly specific finding is a “tadpole-like” appearance of the brainstem, caused by marked atrophy of the medulla oblongata and upper spinal cord.

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When Alexander disease is strongly suspected, single-gene testing targeting only the GFAP gene is generally performed as a primary test. However, when it is difficult to differentiate it from other leukodystrophies based solely on symptoms or MRI findings, or when no variant is identified in the single-gene test, broader next-generation sequencing (NGS)—such as gene panel testing, whole exome sequencing (WES), or whole genome sequencing (WGS)—can be utilized to investigate the causative gene from multiple angles.

* This article is educational and does not replace individualized medical advice. Diagnostic and management decisions should be made with a qualified clinician.

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References

1. Prust M, et al. GFAP mutations, age at onset, and clinical subtypes in Alexander disease. 2011. https://doi.org/10.1212/WNL.0b013e3182309f72
2. Kang YR, et al. Older adult-onset Alexander disease with atypical clinicoradiological features: a case report. 2023. https://doi.org/10.3389/fneur.2023.1139047
3. Sosunov A, et al. Alexander disease: an astrocytopathy that produces a leukodystrophy. 2018. https://doi.org/10.1111/bpa.12601