H-ABC
WHAT IS H-ABC
H-ABC stands for “Hypomyelination with atrophy of the basal ganglia and cerebellum,” because of the key injury to the myelin of the brain, the basal ganglia and the cerebellum.
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Myelin is a part of the brain that may also be called “white matter.” When an individual does not have enough myelin, they are said to have “hypomyelination,” where “hypo” means less than expected. Myelin is to neurons in the brain like the insulation around the electrical cords in your house. It enhances the efficiency of communications between cells in the brain, and supports the health of cells in the brain. Myelin is made by cells called oligodendrocytes. In H-ABC, researchers think these special cells do not do their job properly, leading to insufficient development of myelin
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The basal ganglia, also called deep gray nuclei, are collections of neurons deep in the brain. They help coordinate how smoothly movements are executed, their coordination and their speed. In H-ABC, researchers think these special cells get sick and die, leading to atrophy of these structures.
The cerebellum is a structure at the base of the brain, that helps regulate information coming from the brain as it exits to the spinal cord. It helps regulate balance and the smoothness of movements. In H-ABC, researchers think these special cells get sick and die, leading to atrophy of this structure.
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H-ABC is due to mutations in a gene called TUBB4A, and is part of a larger disease group called TUBB4A related leukoencephalopathy. TUBB4A makes a special structural protein called tubulin, and tubulins come together to build microtubules. Microtubules serve to provide structure to cells and to move cellular components in the cell.
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Individuals with deleterious changes to one of the copies of the TUBB4A gene are thought to make an abnormal copy of the normal tubulin protein, interfering with normal microtubule function.
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People with TUBB4A related leukoencephalopathy and H-ABC frequently have problems with motor skills, which can extend to walking, sitting, using their hands, but also speech and swallowing.
SIGNS & SYMPTOMS
The symptoms and the progress of H-ABC differ depending on when the disease first appears.
When symptoms begin in the first few months of life, the effect of the disease tends to be severe and its progress more rapid. In these cases, symptoms may include:
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A small head (microcephaly)
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Uncontrolled, jerky eye movements (nystagmus) and poor vision
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Problems with swallowing and speech
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Delays in reaching developmental milestones for movement
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Low muscle tone (hypotonia)
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Poor coordination or clumsiness (ataxia)
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Muscle and limb stiffness (spasticity)
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Involuntary movements, including twisting, writhing and abnormal postures (dystonia and choreoathetosis)
When symptoms begin later, in early childhood, the effect of the disease tends to be milder and its progress slower. In these cases, the primary symptom is progressively greater trouble with movement, including:
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Stiffness of the arms and legs (spasticity)
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Gait problems, and other problems with coordination (ataxia)
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Involuntary movements, including twisting, writhing and abnormal postures (eg. dystonia)
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Rigidity
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Learning difficulties and attention problems
In some cases, the symptoms can be very mild and begin in later in childhood or adolescence, or even in adulthood.
TREATMENT
Currently there is no cure for H-ABC, but treatment is available to manage symptoms and improve the quality of life for individuals affected by the disease. There are neurologists specialising in hypomyelination disorders and centres of excellence across Europe and US that offer treatment.
