A case of tubulinopathy presenting with porencephaly caused by a novel missense mutation in the TUBA1A gene

Insights on the Role of α- and β-Tubulin Isotypes in Early Brain Development

Tubulins are the highly conserved subunit of microtubules which involve in various fundamental functions including brain development. Microtubules help in neuronal proliferation, migration, differentiation, cargo transport along the axons, synapse formation, and many more. Tubulin gene family consisting of multiple isotypes, their differential expression and varied post translational modifications create a whole new level of complexity and diversity in accomplishing manifold neuronal functions. The studies on the relation between tubulin genes and brain development opened a new avenue to understand the role of each tubulin isotype in neurodevelopment. Mutations in tubulin genes are reported to cause brain development defects especially cortical malformations, referred as tubulinopathies. There is an increased need to understand the molecular correlation between various tubulin mutations and the associated brain pathology. Recently, mutations in tubulin isotypes (TUBA1A, TUBB, TUBB1, TUBB2A, TUBB2B, TUBB3, and TUBG1) have been linked to cause various neurodevelopmental defects like lissencephaly, microcephaly, cortical dysplasia, polymicrogyria, schizencephaly, subcortical band heterotopia, periventricular heterotopia, corpus callosum agenesis, and cerebellar hypoplasia. This review summarizes on the microtubule dynamics, their role in neurodevelopment, tubulin isotypes, post translational modifications, and the role of tubulin mutations in causing specific neurodevelopmental defects. A comprehensive list containing all the reported tubulin pathogenic variants associated with brain developmental defects has been prepared to give a bird's eye view on the broad range of tubulin functions.

Epilepsy in Tubulinopathy: Personal Series and Literature Review

Mutations in tubulin genes are responsible for a large spectrum of brain malformations secondary to abnormal neuronal migration, organization, differentiation and axon guidance and maintenance. Motor impairment, intellectual disability and epilepsy are the main clinical symptoms. In the present study 15 patients from a personal cohort and 75 from 21 published studies carrying mutations in TUBA1A, TUBB2B and TUBB3 tubulin genes were evaluated with the aim to define a clinical and electrophysiological associated pattern. Epilepsy shows a wide range of severity without a specific pattern. Mutations in TUBA1A (60%) and TUBB2B (74%) and TUBB3 (25%) genes are associated with epilepsy. The accurate analysis of the Electroencephalogram (EEG) pattern in wakefulness and sleep in our series allows us to detect significant abnormalities of the background activity in 100% of patients. The involvement of white matter and of the inter-hemispheric connection structures typically observed in tubulinopathies is evidenced by the high percentage of asynchronisms in the organization of sleep activity recorded. In addition to asymmetries of the background activity, excess of slowing, low amplitude and Magnetic Resonance (MR) imaging confirm the presence of extensive brain malformations involving subcortical and midline structures. In conclusion, epilepsy in tubulinopathies when present has a favorable evolution over time suggesting a not particularly aggressive therapeutic approach.

β-Tubulinopathy caused by a mutation of the TUBB2B gene: magnetic resonance imaging findings of the brain

Patients with mutations in tubulin-related genes usually present with brain malformations, intellectual disability, epilepsy, microcephaly and ocular abnormalities. In these patients the diagnosis can be suggested by neuroimaging findings. We report a 5-year-old patient with characteristic magnetic resonance imaging findings including malformation of cortical development, fused basal ganglia, large head of the caudate nuclei, absent anterior limbs of the internal capsules, corpus callosum dysgenesis and dysplastic cerebellar vermis. Sequencing of the TUBB2B gene confirmed a heterozygous mutation: c. 260C>A (p. Pro87Gln).

The mutational and phenotypic spectrum of TUBA1A-associated tubulinopathy

BACKGROUND

The TUBA1A-associated tubulinopathy is clinically heterogeneous with brain malformations, microcephaly, developmental delay and epilepsy being the main clinical features. It is an autosomal dominant disorder mostly caused by de novo variants in TUBA1A.

RESULTS

In three individuals with developmental delay we identified heterozygous de novo missense variants in TUBA1A using exome sequencing. While the c.1307G > A, p.(Gly436Asp) variant was novel, the two variants c.518C > T, p.(Pro173Leu) and c.641G > A, p.(Arg214His) were previously described. We compared the variable phenotype observed in these individuals with a carefully conducted review of the current literature and identified 166 individuals, 146 born and 20 fetuses with a TUBA1A variant. In 107 cases with available clinical information we standardized the reported phenotypes according to the Human Phenotype Ontology. The most commonly reported features were developmental delay (98%), anomalies of the corpus callosum (96%), microcephaly (76%) and lissencephaly (agyria-pachygyria) (70%), although reporting was incomplete in the different studies. We identified a total of 121 specific variants, including 15 recurrent ones. Missense variants cluster in the C-terminal region around the most commonly affected amino acid position Arg402 (13.3%). In a three-dimensional protein model, 38.6% of all disease-causing variants including those in the C-terminal region are predicted to affect the binding of microtubule-associated proteins or motor proteins. Genotype-phenotype analysis for recurrent variants showed an overrepresentation of certain clinical features. However, individuals with these variants are often reported in the same publication.

CONCLUSIONS

With 166 individuals, we present the most comprehensive phenotypic and genotypic standardized synopsis for clinical interpretation of TUBA1A variants. Despite this considerable number, a detailed genotype-phenotype characterization is limited by large inter-study variability in reporting.