From OMIM
Hypomyelinating leukodystrophy-6 (HLD6), also known as hypomyelinating leukodystrophy with atrophy of the basal ganglia and cerebellum, is a neurologic disorder characterized by onset in infancy or early childhood of delayed motor development and gait instability, followed by extrapyramidal movement disorders such as dystonia, choreoathetosis, rigidity, opisthotonus, and oculogyric crises, progressive spastic tetraplegia, ataxia, and, more rarely, seizures. Most patients have cognitive decline and speech delay, but some can function normally. Brain MRI shows a combination of hypomyelination, cerebellar atrophy, and atrophy or disappearance of the putamen. The disorder usually shows sporadic occurrence, but sibs may be affected if a parent is somatic mosaic for the mutation (summary by Simons et al., 2013). Hypomyelinating leukodystrophies (HLD) comprise a genetically heterogeneous entity in which there is a substantial permanent deficit in myelin deposition within the brain, resulting in neurologic deficits (van der Knaap et al., 2002). For a general phenotypic description and a discussion of genetic heterogeneity of hypomyelinating leukodystrophy, see 312080.  http://www.omim.org/entry/612438

From MedlinePlus Genetics
TUBB4A-related leukodystrophy is a disorder that affects the nervous system. Leukodystrophies are conditions that involve abnormalities of the nervous system's white matter, which consists of nerve fibers covered by a fatty substance called myelin. Myelin insulates nerve fibers and promotes the rapid transmission of nerve impulses. In particular, TUBB4A-related leukodystrophy involves hypomyelination, which means that the nervous system has a reduced ability to form myelin. In some affected individuals, myelin may also break down, which is known as demyelination.

People with TUBB4A-related leukodystrophy have different combinations of signs and symptoms. Some of these combinations are described as separate disorders. However, the features in some affected individuals do not fit into these defined disorders. Researchers now group all of these cases of leukodystrophy, which have the same genetic cause, as TUBB4A-related leukodystrophy.

At the most severe end of the TUBB4A-related leukodystrophy spectrum is a condition called hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC). This disorder begins in infancy or early childhood. Most affected individuals have delayed development of motor skills, such as sitting and walking, and some are never able to walk on their own. In other cases, motor skills develop normally and then are lost in early childhood (developmental regression). In addition, individuals with H-ABC have other movement abnormalities, such as involuntary muscle contractions (dystonia), uncontrolled movements of the limbs (choreoathetosis), muscle stiffness (rigidity), and difficulty coordinating movements (ataxia). These individuals also often have impaired speech (dysarthria), a weak voice (dysphonia), and swallowing problems (dysphagia). Some develop seizures. Learning difficulty is common in individuals with H-ABC.

H-ABC is characterized by particular brain abnormalities, including hypomyelination. In addition, tissue in certain regions of the brain breaks down (atrophies), most prominently in a region called the putamen, which is part of a group of structures that help control movement (the basal ganglia). Atrophy of brain tissue in another region involved in movement called the cerebellum is common, and atrophy of the cerebrum, which controls most voluntary activity, language, sensory perception, learning, and memory, can also occur.

At the mildest end of the TUBB4A-related leukodystrophy spectrum is a condition called isolated hypomyelination, which begins at any time from late childhood to adulthood. Individuals at this end of the spectrum have mild hypomyelination and sometimes mild atrophy of the cerebellum, but no problems with the basal ganglia. These individuals can have movement problems, dysarthria, and learning difficulty, although these features are typically milder than in H-ABC.

The features in other individuals with TUBB4A-related leukodystrophy fall in between these two extremes. Affected individuals can have varying degrees of hypomyelination and atrophy or impairment of the basal ganglia or other brain regions. Movement problems can also occur. A small group of affected individuals develop muscle stiffness and paralysis of the lower limbs (spastic paraplegia) that slowly worsen. In addition, these individuals may have mild hypomyelination and ataxia without the other movement or learning problems common in H-ABC.  https://medlineplus.gov/genetics/condition/tubb4a-related-leukodystrophy