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

Genetic background of idiopathic neurodevelopmental delay patients with significant brain deviation volume

BACKGROUND

Significant brain volume deviation is an essential phenotype in children with neurodevelopmental delay (NDD), but its genetic basis has not been fully characterized. This study attempted to analyze the genetic factors associated with significant whole-brain deviation volume (WBDV).

METHODS

We established a reference curve based on 4222 subjects ranging in age from the first postnatal day to 18 years. We recruited only NDD patients without acquired etiologies or positive genetic results. Cranial magnetic resonance imaging (MRI) and clinical exome sequencing (2742 genes) data were acquired. A genetic burden test was performed, and the results were compared between patients with and without significant WBDV. Literature review analyses and BrainSpan analysis based on the human brain developmental transcriptome were performed to detect the potential role of genetic risk factors in human brain development.

RESULTS

We recruited a total of 253 NDD patients. Among them, 26 had significantly decreased WBDV (<-2 standard deviations [SDs]), and 14 had significantly increased WBDV (>+2 SDs). NDD patients with significant WBDV had higher rates of motor development delay (49.8% [106/213] vs . 75.0% [30/40], P  = 0.003) than patients without significant WBDV. Genetic burden analyses found 30 genes with an increased allele frequency of rare variants in patients with significant WBDV. Analyses of the literature further demonstrated that these genes were not randomly identified: burden genes were more related to the brain development than background genes ( P  = 1.656e -9 ). In seven human brain regions related to motor development, we observed burden genes had higher expression before 37-week gestational age than postnatal stages. Functional analyses found that burden genes were enriched in embryonic brain development, with positive regulation of synaptic growth at the neuromuscular junction, positive regulation of deoxyribonucleic acid templated transcription, and response to hormone, and these genes were shown to be expressed in neural progenitors. Based on single cell sequencing analyses, we found TUBB2B gene had elevated expression levels in neural progenitor cells, interneuron, and excitatory neuron and SOX15 had high expression in interneuron and excitatory neuron.

CONCLUSION

Idiopathic NDD patients with significant brain volume changes detected by MRI had an increased prevalence of motor development delay, which could be explained by the genetic differences characterized herein.

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.

Case Report: Exome and RNA Sequencing Identify a Novel de novo Missense Variant in HNRNPK in a Chinese Patient With Au-Kline Syndrome

Au-Kline syndrome is a severe multisystemic syndrome characterized by several congenital defects, including intellectual disability. Loss-of-function and missense variants in the HNRNPK gene are associated with a range of dysmorphic features. This report describes an eleven-year-old Chinese boy with intellectual disability and developmental delays. Family-based whole-exome and Sanger sequencing identified a de novo missense variant in HNRNPK (NM_002140.3: c.143T > A, p. Leu48Val). In silico analysis predicted that this variant would be damaged in a highly conserved residue in the K homology 1 (KH1) domain. Bioinformatic analysis showed that the affinity change (ΔΔG) caused by this variant was -0.033 kcal/mol, indicating that it would have reduced affinity for RNA binding. Transcript analysis of the peripheral blood from this case found 42 aberrantly expressed and 86 aberrantly spliced genes (p-value <0.01). Functional enrichment analysis confirmed that the biological functions of these genes, including protein binding and transcriptional regulation, are associated with HNRNPK. In summary, this study identifies the first Chinese patient with a novel de novo heterozygous HNRNPK gene variant that contributes to Au-Kline syndrome and expands current knowledge of the clinical spectrum of HNRNPK variants.

A novel family illustrating the mild phenotypic spectrum of TUBB2B variants

TUBB2B codes for one of the isotypes of β-tubulin and dominant negative variants in this gene result in distinctive malformations of cortical development (MCD), including dysgyria, dysmorphic basal ganglia and cerebellar anomalies. We present a novel family with a heterozygous missense variant in TUBB2B and an unusually mild phenotype. First, at 21 37 weeks of gestation ultrasonography revealed a fetus with a relatively small head, enlarged lateral ventricles, borderline hypoplastic cerebellum and a thin corpus callosum. The couple opted for pregnancy termination. Exome sequencing on fetal material afterwards identified a heterozygous maternally inherited variant in TUBB2B (NM_178012.4 (TUBB2B):c.530A > T, p.(Asp177Val)), not present in GnomAD and predicted as damaging. The healthy mother had only a language delay in childhood. This inherited TUBB2B variant prompted re-evaluation of the older son of the couple, who presented with a mild delay in motor skills and speech. His MRI revealed mildly enlarged lateral ventricles, a thin corpus callosum, mild cortical dysgyria, and dysmorphic vermis and basal ganglia, a pattern typical of tubulinopathies. This son finally showed the same TUBB2B variant, supporting pathogenicity of the TUBB2B variant. These observations illustrate the wide phenotypic heterogeneity of tubulinopathies, including reduced penetrance and mild expressivity, that require careful evaluation in pre- and postnatal counseling.

Morphological and Advanced Imaging of Epilepsy: Beyond the Basics

The etiology of epilepsy is variable and sometimes multifactorial. Clinical course and response to treatment largely depend on the precise etiology of the seizures. Along with the electroencephalogram (EEG), neuroimaging techniques, in particular, magnetic resonance imaging (MRI), are the most important tools for determining the possible etiology of epilepsy. Over the last few years, there have been many developments in data acquisition and analysis for both morphological and functional neuroimaging of people suffering from this condition. These innovations have increased the detection of underlying structural pathologies, which have till recently been classified as "cryptogenic" epilepsy. Cryptogenic epilepsy is often refractory to anti-epileptic drug treatment. In drug-resistant patients with structural or consistent functional lesions related to the epilepsy syndrome, surgery is the only treatment that can offer a seizure-free outcome. The pre-operative detection of the underlying structural condition increases the odds of successful surgical treatment of pharmacoresistant epilepsy. This article provides a comprehensive overview of neuroimaging techniques in epilepsy, highlighting recent advances and innovations and summarizes frequent etiologies of epilepsy in order to improve the diagnosis and management of patients suffering from seizures, especially young patients and children.