The taiep rat is a tubulin mutant with an early hypomyelination followed by progressive demyelination of the central nervous system due to a point mutation in the Tubb4a gene. It shows clinical, radiological, and pathological signs like those of the human leukodystrophy hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC). Taiep rats had tremor, ataxia, immobility episodes, epilepsy, and paralysis; the acronym of these signs given the name to this autosomal recessive trait. The aim of this study was to analyze the characteristics of somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) in adult taiep rats and in a patient suffering from H-ABC. Additionally, we evaluated the effects of 4-aminopyridine (4-AP) on sensory responses and locomotion and finally, we compared myelin loss in the spinal cord of adult taiep and wild type (WT) rats using immunostaining. Our results showed delayed SSEPs in the upper and the absence of them in the lower extremities in a human patient. In taiep rats SSEPs had a delayed second negative evoked responses and were more susceptible to delayed responses with iterative stimulation with respect to WT. MEPs were produced by bipolar stimulation of the primary motor cortex generating a direct wave in WT rats followed by several indirect waves, but taiep rats had fused MEPs. Importantly, taiep SSEPs improved after systemic administration of 4-AP, a potassium channel blocker, and this drug induced an increase in the horizontal displacement measured in a novelty-induced locomotor test. In taiep subjects have a significant decrease in the immunostaining of myelin in the anterior and ventral funiculi of the lumbar spinal cord with respect to WT rats. In conclusion, evoked potentials are useful to evaluate myelin alterations in a leukodystrophy, which improved after systemic administration of 4-AP. Our results have a translational value because our findings have implications in future medical trials for H-ABC patients or with other leukodystrophies.

Mutations in tubulin alpha 4a (TUBB4A) result in a spectrum of leukodystrophies, including Hypomyelination with atrophy of basal ganglia and cerebellum (H-ABC), resulting from a recurring mutation p.Asp249Asn (TUBB4AD249N). H-ABC presents with dystonia, motor and cognitive impairment and pathological features of hypomyelination and loss of cerebellar and striatal neurons. We have generated three induced pluripotent stem cell (iPSC) lines from fibroblast and peripheral blood mononuclear cells (PBMCs) of individuals with TUBB4AD249N mutation. The iPSCs were assessed to confirm a normal karyotype, pluripotency, and trilineage differentiation potential. The iPSCs will allow for disease modeling, understanding mechanisms and testing of therapeutic targets.

Hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC) is a neurodegenerative disease with neurodevelopmental delay, motor, and speech regression, pronounced extrapyramidal syndrome, and sensory deficits due to TUBB4A mutation. In 2017, a severe variant was described in 16 Roma infants due to mutation in UFM1. The objective of this study is to expand the clinical manifestations of H-ABC due to UFM1 mutation and suggest clues for clinical diagnosis. Retrospective analysis of all 9 cases with H-ABC due to c.-273_-271delTCA mutation in UFM1 treated during 2013-2020 in a Neuropediatric Ward in Plovdiv, Bulgaria. Presentation is no later than 2 months with inspiratory stridor, impaired sucking, swallowing, vision and hearing, and reduced active movements. By the age of 10 months, a monomorphic disease was observed: microcephaly (6/9), malnutrition (5/9), muscle hypertonia (9/9) and axial hypotonia (4/9), progressing to opisthotonus (6/9), dystonic posturing (5/9), nystagmoid ocular movements (6/9), epileptic seizures (4/9), non-epileptic spells (3/9). Dysphagia (7/9), inspiratory stridor (9/9), dyspnea (5/9), bradypnea (5/9), apnea (2/9) were major signs. Vision and hearing were never achieved or lost by 4-8 mo. Neurodevelopment was absent or minimal with subsequent regression after 2-5 mo. Brain imaging revealed cortical atrophy (7/9), atrophic ventricular dilatation (4/9), macrocisterna magna (5/9), reduced myelination (6/6), corpus callosum atrophy (3/6) and abnormal putamen and caput nuclei caudati. The age at death was between 8 and 18 mo. Roma patients with severe encephalopathy in early infancy with stridor, opisthotonus, bradypnea, severe hearing and visual impairment should be tested for the Roma founder mutation of H-ABC in UFM1.

Mutations in TUBB4A result in a spectrum of leukodystrophy including Hypomyelination with Atrophy of Basal Ganglia and Cerebellum (H-ABC), a rare hypomyelinating leukodystrophy, often associated with a recurring variant p.Asp249Asn (D249N). We have developed a novel knock-in mouse model harboring heterozygous (Tubb4aD249N/+) and the homozygous (Tubb4aD249N/D249N) mutation that recapitulate the progressive motor dysfunction with tremor, dystonia and ataxia seen in H-ABC. Tubb4aD249N/D249N mice have myelination deficits along with dramatic decrease in mature oligodendrocytes and their progenitor cells. Additionally, a significant loss occurs in the cerebellar granular neurons and striatal neurons in Tubb4aD249N/D249N mice. In vitro studies show decreased survival and dysfunction in microtubule dynamics in neurons from Tubb4aD249N/D249N mice. Thus Tubb4aD249N/D249N mice demonstrate the complex cellular physiology of H-ABC, likely due to independent effects on oligodendrocytes, striatal neurons, and cerebellar granule cells in the context of altered microtubule dynamics, with profound neurodevelopmental deficits. Inside human and other animal cells, filaments known as microtubules help support the shape of the cell and move proteins to where they need to be. Defects in microtubules may lead to disease. For example, genetic mutations affecting a microtubule component called TUBB4A cause a rare brain disease in humans known as H-ABC. Individuals with H-ABC display many symptoms including abnormal walking, speech defects, impaired swallowing, and several cognitive defects. Abnormalities in several areas of the brain, including the cerebellum and striatum contribute to these defects. . In these structures, the neurons that carry messages around the brain and their supporting cells, known as oligodendrocytes, die, which causes these parts of the brain to gradually waste away. At this time, there are no therapies available to treat H-ABC. Furthermore, research into the disease has been hampered by the lack of a suitable “model” in mice or other laboratory animals. To address this issue, Sase, Almad et al. generated mice carrying a mutation in a gene which codes for the mouse equivalent of the human protein TUBB4A. Experiments showed that the mutant mice had similar physical symptoms to humans with H-ABC, including an abnormal walking gait, poor coordination and involuntary movements such as twitching and reduced reflexes. H-ABC mice had smaller cerebellums than normal mice, which was consistent with the wasting away of the cerebellum observed in individuals with H-ABC. The mice also lost neurons in the striatum and cerebellum, and oligodendrocytes in the brain and spinal cord. Furthermore, the mutant TUBB4A protein affected the behavior and formation of microtubules in H-ABC mice. The findings of Sase, Almad et al. provide the first mouse model that shares many features of H-ABC disease in humans. This model provides a useful tool to study the disease and develop potential new therapies.