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PubMed This is a summary of 19 peer-reviewed journal articles Updated
Pediatric Neurology · Hypomyelination with Atrophy of the Basal Ganglia and Cerebellum

The Science of H-ABC: Genetics and Imaging

At a Glance

H-ABC is linked to a disease-causing change in the TUBB4A gene. Diagnosis combines a child's symptoms, MRI evidence of persistent low myelin and shrinking basal ganglia and cerebellum, and genetic testing that helps rule out similar conditions.

To understand H-ABC, it helps to look at the “blueprints” and “building blocks” of the brain. The diagnosis is reached by integrating the physical evidence seen on a brain scan (MRI), the clinical symptoms, and the genetic code that causes the condition [1][2].

The Biological “Scaffolding”

H-ABC is caused by a change (pathogenic variant) in the TUBB4A gene. This gene is responsible for making a protein called beta-tubulin 4A, which is a vital part of microtubules [3][4]. Think of microtubules as the internal “scaffolding” or “railroad tracks” inside your brain cells [5].

In H-ABC, it is thought that these tracks are structurally altered, which may impair the brain in two major ways:

  1. Oligodendrocytes: These are the cells that create myelin, the fatty insulation around nerves. When the microtubule scaffolding is disrupted, these cells likely cannot mature or produce enough myelin [6][7]. This inadequate formation of myelin is called hypomyelination [8].
  2. Neurons: The brain’s message-sending cells also rely on microtubules to maintain their shape and transport cellular cargo and organelles [9]. The genetic variant is associated with dysfunction in these neurons, particularly in the basal ganglia and the cerebellum, eventually leading to atrophy (shrinking) of those areas [3][1].

The MRI: A Diagnostic Snapshot

A doctor evaluating for H-ABC looks for a characteristic pattern on an MRI. This pattern is the hallmark of the disease [10]:

  • Persistent Hypomyelination: On an MRI, myelin appears in a certain color (signal). In H-ABC, this signal shows that the brain never adequately formed the insulation [8].
  • Basal Ganglia Atrophy: Specifically, two parts called the putamen and the caudate appear smaller than normal [2][1]. On later imaging, the putamen may become severely atrophic or difficult to distinguish [11].
  • Cerebellar Atrophy: The cerebellum, located at the back of the brain, also shows signs of shrinking, which often progresses over time [11][12].

Do You Always Need Two MRIs?

In infants, the brain is still actively building its myelin. A single early scan might simply show delayed myelination, which can happen for many reasons and may eventually catch up [8][13].

To explicitly confirm hypomyelination (a persistent lack of myelin) purely by imaging, guidelines often suggest comparing two MRIs spaced at least 6 months apart, with one performed after the child is 1 year old [13][8]. However, this is not a rigid universal rule. If a child has a clear clinical picture, a characteristic first MRI, and a confirmed pathogenic TUBB4A variant, a specialist may diagnose H-ABC without requiring a second scan immediately, especially to spare the child from repeated anesthesia or sedation.

Avoiding Misdiagnosis and Confirming Genetics

Because many rare conditions look similar on a brain scan, H-ABC can be confused with other disorders:

  • 4H Leukodystrophy: This also causes hypomyelination but typically spares the basal ganglia and is often accompanied by dental or hormonal issues [14][15].
  • Pelizaeus-Merzbacher Disease (PMD): This is a similar white-matter disease but is caused by a different gene (PLP1) [16][17].

Because of these similarities, molecular genetic testing is essential [18][19]. However, a genetic test is only definitive if the laboratory identifies a pathogenic or likely pathogenic variant in TUBB4A that matches the child’s symptoms. A “variant of uncertain significance” (VUS) is not a diagnosis. Genetic counseling is critical for interpreting these results and understanding the implications for parents and siblings.

Common questions in this guide

What causes H-ABC?
H-ABC is caused by a disease-causing change in the TUBB4A gene, which provides instructions for beta-tubulin 4A. This change can disrupt the cell structures that support myelin-forming cells and neurons, especially in the basal ganglia and cerebellum.
What can an MRI show in a child with H-ABC?
An H-ABC MRI may show persistent hypomyelination, meaning the brain has not formed enough of the insulating material around nerves. It may also show shrinkage of the putamen and caudate in the basal ganglia and atrophy of the cerebellum.
Does a child need two MRIs to diagnose H-ABC?
In infants, one early MRI may show delayed myelination that later catches up, so doctors often compare scans taken at least six months apart, with one after the first birthday. A second scan is not an absolute requirement when the clinical findings, MRI pattern, and disease-causing TUBB4A variant clearly support H-ABC, particularly when sedation or anesthesia would be difficult.
What does a pathogenic TUBB4A variant mean?
A pathogenic or likely pathogenic TUBB4A variant that matches a child's symptoms and MRI findings can support an H-ABC diagnosis. A variant of uncertain significance does not by itself confirm the condition, so a genetic counselor can help interpret the laboratory report.
How is H-ABC distinguished from 4H leukodystrophy or Pelizaeus-Merzbacher disease?
Doctors combine the MRI pattern with genetic testing because these disorders can look similar. 4H leukodystrophy typically spares the basal ganglia and may involve dental or hormonal problems, while Pelizaeus-Merzbacher disease is linked to changes in the PLP1 gene rather than TUBB4A.
What can genetic counseling explain about H-ABC and family planning?
A genetic counselor can explain the specific TUBB4A result, discuss what it may mean for parents and siblings, and help the family understand recurrence risk. Counseling can also clarify whether additional family testing or planning discussions are appropriate.

Get notified when new evidence is published on Hypomyelination with atrophy of basal ganglia and cerebellum.

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References

References (19)
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    Auditory impairment in H-ABC tubulinopathy.

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This page explains H-ABC MRI findings and TUBB4A testing for informational purposes only and does not constitute medical advice. Your child's pediatric neurologist, neuroradiologist, and genetic counselor should interpret the specific results.

Guide contents Hypomyelination with atrophy of basal ganglia and cerebellum
Research & Literature
02 Symptoms and the Path of Progression 04 Managing Symptoms and Supporting Daily Life