Tubb4b is required for multi-ciliogenesis in the mouse

Cilia are microtubule (MT)-based organelles present on the surface of nearly all vertebrate cells. MTs are polymers of α- and β-tubulins that are each encoded by multiple, individual isotype genes. Tubulin isotype composition is thought to influence MT behaviors. Ciliary MTs differ from other MTs in the cell in terms of organization, stability and post-translational modifications. However, little is known about the tubulin isotypes that build ciliary MTs and the functional requirements for tubulin isotypes in cilia have not been examined in vertebrates. Here, we have tested the role of the β-tubulin isotype genes in the mouse that harbor a conserved amino acid motif associated with ciliated organisms. We found that Tubb4b localizes to cilia in multi-ciliated cells (MCCs) specifically. In respiratory and oviduct MCCs, Tubb4b is asymmetrically localized within multi-cilia, indicating that the tubulin isotype composition changes along the length of the ciliary axonemal MTs. Deletion of Tubb4b resulted in striking structural defects within the axonemes of multi-cilia, without affecting primary cilia. These studies show that Tubb4b is essential for the formation of a specific MT-based subcellular organelle and sheds light on the requirements of tubulin isotypes in cilia.

A new genetic cause of spastic ataxia: the p.Glu415Lys variant in TUBA4A

Tubulinopathies encompass neurodevelopmental disorders caused by mutations in genes encoding for different isotypes of α- and β-tubulins, the structural components of microtubules. Less frequently, mutations in tubulins may underlie neurodegenerative disorders. In the present study, we report two families, one with 11 affected individuals and the other with a single patient, carrying a novel, likely pathogenic, variant (p. Glu415Lys) in the TUBA4A gene (NM_006000). The phenotype, not previously described, is that of spastic ataxia. Our findings widen the phenotypic and genetic manifestations of TUBA4A variants and add a new type of spastic ataxia to be taken into consideration in the differential diagnosis.

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.

Testicular androgens determining the incidence of spike-wave discharges in taiep rats: A model of H-ABC leukodystrophy

Absence seizures are characterized as a generalized type of epilepsy that occurs during childhood. Importantly, absence seizures in children often discontinue after puberty. There is limited availability of animal models in which electroencephalography (EEG) can be performed in the long term; however, two absence seizure models, GAERS and WAG/Rij albino rats, are available. The taiep rat is a myelin mutant rat with tubulinopathy due to a tubulin β 4A gene mutation and characteristic spike-wave discharges (SWDs) that mimic absence seizure epilepsy in humans and the above rat models. This study aimed to analyze spike-wave discharges after an orchiectomy was performed on postnatal day (PND) 2 or PND 90 in adult rats; and SWDs was recorded in both groups on PND 104. The results suggest that androgens play a critical role in susceptibility to SWDs. In fact, orchiectomy during the neonatal period significantly reduced the frequency of spike-wave discharges. However, if an orchiectomy was performed in adulthood, then SWDs were significantly increased. The mean duration of spike-wave discharges did not differ among the groups tested. Acute administration of testosterone (1 mg/kg) did not change the frequency or duration of spike-wave discharges in the control group or both orchiectomized groups. Overall, this study is the first to show a dichotomic influence of testicular androgens on spike-wave discharges. These findings will have implications in children with this type of generalized epilepsy and may explain the disappearance of absence epilepsy in two-thirds of patients after puberty.

Lissencephaly in an epilepsy cohort: Molecular, radiological and clinical aspects

INTRODUCTION

Lissencephaly is a rare malformation of cortical development due to abnormal transmantle migration resulting in absent or reduced gyration. The lissencephaly spectrum consists of agyria, pachygyria and subcortical band heterotopia. In this study we compared genetic aetiology, neuroradiology, clinical phenotype and response to antiepileptic drugs in patients with epilepsy and lissencephaly spectrum malformations.

METHODS

The study group consisted of 20 patients - 13 males and 7 females, aged 18 months to 21 years at the time of data collection. Genetic testing was performed by oligonucleotide array comparative genomic hybridization (microarray), multiplex ligation-dependent probe amplification (MLPA), targeted gene panels and whole exome/genome sequencing. All neuroradiological investigations were re-evaluated and the malformations were classified by the same neuroradiologist. Clinical features and response to anti-epileptic drugs (AEDs) were evaluated by retrospective review of medical records.

RESULTS

In eleven patients (55%) mutations in PAFAH1B1 (LIS1) or variable microdeletions of 17p13.3 including the PAFAH1B1 gene were detected. Four patients (20%) had tubulin encoding gene mutations (TUBA1A, TUBG1 and TUBGCP6). Mutations in DCX, DYNC1H1, ADGRG1 and WDR62 were identified in single patients. In one patient, a possibly pathogenic intragenic deletion in TRIO was detected. A clear radiologic distinction could be made between tubulinopathies and PAFAH1B1 related lissencephaly. The majority of the patients had therapy resistant epilepsy and epileptic spasms was the most prominent seizure type. The best therapeutic response to seizure control in our cohort was obtained by the ketogenic diet, vigabatrin, clobazam, phenobarbital and valproate.

CONCLUSION

The most common genetic aetiologies in our cohort of 20 individuals with epilepsy and lissencephaly spectrum were intragenic deletions or single nucleotide mutations in PAFAH1B1 or larger deletions in 17p13.3, encompassing PAFAH1B1, followed by mutations in tubulin encoding genes. Radiological findings could reliably predict molecular results only in agyria with a posterior to anterior gradient. Radiological and molecular findings did not correlate consistently with severity of clinical outcome or therapeutic response.

Second trimester fetal MRI features in a fetus with TUBB3 gene mutation

Tubulinopathies are a heterogeneous group of complex cortical malformations that are associated with mutations in tubulin genes. TUBB3 gene mutation is associated with a broader spectrum of central nervous system malformations and constitutes about 10% of all tubulinopathies. The diagnosis may not be immediately apparent on imaging, though the differential diagnosis may be narrowed based on imaging findings and allow for more directed genetic testing. We report a 22-year-old gravida-1 nulliparous female whose routine second trimester fetal ultrasound revealed ventriculomegaly and possible agenesis of the corpus callosum. Fetal magnetic resonance imaging showed severe lateral and third ventriculomegaly and a dysplastic, z-shaped brainstem without any evidence of ocular abnormalities. Genetic testing revealed a pathogenic mutation in TUBB3.

Expanding the Phenotype of TUBB2A-Related Tubulinopathy: Three Cases of a Novel, Heterozygous TUBB2A Pathogenic Variant p.Gly98Arg

Tubulinopathies are a group of conditions caused by variants in 6 tubulin genes that present with a spectrum of brain malformations. One of these conditions is TUBB2A-related tubulinopathy. Currently, there are 9 reported individuals with pathogenic variants within the TUBB2A gene, with common manifestations including, but not limited to, global developmental delay, seizures, cortical dysplasia, and dysmorphic corpus callosum. We report 3 patients identified by exome and genome sequencing to have a novel, pathogenic, missense variant in TUBB2A (p.Gly98Arg). They presented similarly with intellectual disability, hypotonia, and global developmental delay and varied with respect to the type of cortical brain malformation, seizure history, diagnosis of autism spectrum disorder, and other features. This case series expands the natural history of TUBB2A-related tubulinopathy while describing the presentation of a novel, pathogenic, missense variant in 3 patients.

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.

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

BACKGROUND

Tubulinopathies include a wide spectrum of disorders ranging from abnormal ocular movement to severe brain malformations, and typically present as diffuse agyria or perisylvian pachygyria with microcephaly, agenesis of the corpus callosum, and cerebellar hypoplasia. They are caused by the dysfunction of tubulins encoded by tubulin-related genes, and the TUBA1A gene encoding alpha-1A tubulin is most frequently responsible for this clinical entity. Porencephaly is relatively rare among patients with the TUBA1A mutations. Mild case of tubulinopathy associated with porencephaly caused by a novel TUBA1A mutation.

CASE REPORT

The patient, a 10-month-old girl, presented with gross motor delay at 4 months of age and convulsions at 7 months of age. Brain magnetic resonance imaging showed porencephaly, occipital polymicrogyria, hypoplasia of the corpus callosum, volume loss of the white matter, dysgenesis of anterior limbs of internal capsules, non-separative basal ganglia, cerebellar hypoplasia, and dysplastic brainstem. We identified a novel de novo heterozygous missense mutation in the TUBA1A gene, c.381C > A (p.Asp127Glu), by whole-exome sequencing.

DISCUSSION

Microtubules composed of tubulins regulate not only neuronal migration but also cell division or axon guidance. Accordingly, tubulinopathy affects the cortical lamination, brain size, callosal formation, and white matter as seen in the present case. In contrast to the previously reported cases, the present case showed milder cortical dysgenesis with a rare manifestation of porencephaly. The genotype-phenotype correlation is still unclear, and this study expands the phenotypic range of tubulinopathy.

TUBB2B Mutation in an Adult Patient with Myoclonus-Dystonia

Background

Tubulin mutations are a cause of neuronal migrational disorders referred to as tubulinopathies. Mutations in tubulin genes can have a severe impact on microtubule function and result in heterogeneous clinical presentations. Current understanding of the clinical spectrum of tubulinopathies is predominantly based on research in fetal tissue and early-childhood cases.

Methods

Testing of candidate genes followed by whole-exome sequencing was performed in an adult woman with a neurodevelopmental, hyperkinetic movement disorder, to identify the underlying genetic cause. Bioinformatic modeling and a systematic review of literature was conducted to investigate genotype-phenotype correlations.

Results

The patient was found to carry a heterozygous, de novo c.722G>A, p.R241H mutation in a conserved domain of TUBB2B, encoding the β-isoform of tubulin. In silico analysis indicated that this mutation was pathogenic. On neuroimaging, the patient had asymmetric pachygyria and dysmorphic basal ganglia. Her neurological examination demonstrated mild cognitive impairment, myoclonus-dystonia, and skeletal anomalies.

Conclusions

Here, we report the unique phenotype of an adult TUBB2B mutation carrier. This case illustrates a relatively mild phenotype compared to previously described fetal and early childhood cases. This highlights the importance of obtaining molecular genetic testing in individuals with a high probability of a genetic disease, including undiagnosed adult patients.

Cerebral palsy and seizures in a child with tubulinopathy pattern dysgenesis and focal cortical dysplasia

A 7-year-old boy with a history of spasticity, global developmental delay, and seizures was given the general diagnosis of cerebral palsy at an early age. Chromosomal array analysis performed at an outside center was normal. The patient's family sought neurodevelopmental pediatric care at a new institution following a move out of state. Electroencephalography confirmed abnormal epileptogenic activity. Brain magnetic resonance imaging showed findings consistent with a tubulin gene defect (tubulinopathy) and of focal cortical dysplasia, as well as evidence of a remote occipital lobe injury. This case report describes the various brain magnetic resonance findings suggestive of a tubulin gene defect and raises the possibility of focal cortical dysplasia manifesting as a result of tubulin dysfunction.

Differential diagnosis of ventriculomegaly and brainstem kinking on fetal MRI

BACKGROUND

Fetal ventriculomegaly is a common and frequently leading neuroimaging finding in complex brain malformations. Here we report on pre- and postnatal neuroimaging findings in three fetuses with prenatal ventriculomegaly and brainstem kinking. We aim to identify key neuroimaging features that may allow the prenatal differentiation between diseases associated with fetal ventriculomegaly and brainstem kinking.

METHODS

All pre- and postnatal magnetic resonance imaging (MRI) data were qualitatively evaluated for infra- and supratentorial abnormalities. Data about clinical features and genetic findings were collected from clinical histories.

RESULTS

In all three patients, fetal MRI showed ventriculomegaly and brainstem kinking. In two patients, postnatal MRI also showed supratentorial migration abnormalities and eye abnormalities were found. In these children, the diagnosis of α-dystroglycanopathy was genetically confirmed. In the third patient, basal ganglia had an abnormal shape on MRI suggesting a tubulinopathy.

CONCLUSION

The differential diagnosis of prenatal ventriculomegaly and brainstem kinking includes α-dystroglycanopathies, X-linked hydrocephalus due to mutations in L1CAM, and tubulinopathies. The prenatal differentiation between these diseases may be difficult. The presence of ocular abnormalities on prenatal neuroimaging may favor α-dystroglycanopathies, while dysplastic basal ganglia may suggest a tubulinopathy. However, in some patients the final differentiation between these diseases is possible only postnatally.