A focal epilepsy and intellectual disability syndrome is due to a mutation in TBC1D24

A loss-of-function mutation in human Oxidation Resistance 1 disrupts the spatial-temporal regulation of histone arginine methylation in neurodevelopment

BACKGROUND

Oxidation Resistance 1 (OXR1) gene is a highly conserved gene of the TLDc domain-containing family. OXR1 is involved in fundamental biological and cellular processes, including DNA damage response, antioxidant pathways, cell cycle, neuronal protection, and arginine methylation. In 2019, five patients from three families carrying four biallelic loss-of-function variants in OXR1 were reported to be associated with cerebellar atrophy. However, the impact of OXR1 on cellular functions and molecular mechanisms in the human brain is largely unknown. Notably, no human disease models are available to explore the pathological impact of OXR1 deficiency.

RESULTS

We report a novel loss-of-function mutation in the TLDc domain of the human OXR1 gene, resulting in early-onset epilepsy, developmental delay, cognitive disabilities, and cerebellar atrophy. Patient lymphoblasts show impaired cell survival, proliferation, and hypersensitivity to oxidative stress. These phenotypes are rescued by TLDc domain replacement. We generate patient-derived induced pluripotent stem cells (iPSCs) revealing impaired neural differentiation along with dysregulation of genes essential for neurodevelopment. We identify that OXR1 influences histone arginine methylation by activating protein arginine methyltransferases (PRMTs), suggesting OXR1-dependent mechanisms regulating gene expression during neurodevelopment. We model the function of OXR1 in early human brain development using patient-derived brain organoids revealing that OXR1 contributes to the spatial-temporal regulation of histone arginine methylation in specific brain regions.

CONCLUSIONS

This study provides new insights into pathological features and molecular underpinnings associated with OXR1 deficiency in patients.

Reversal of cell, circuit and seizure phenotypes in a mouse model of DNM1 epileptic encephalopathy

Dynamin-1 is a large GTPase with an obligatory role in synaptic vesicle endocytosis at mammalian nerve terminals. Heterozygous missense mutations in the dynamin-1 gene (DNM1) cause a novel form of epileptic encephalopathy, with pathogenic mutations clustering within regions required for its essential GTPase activity. We reveal the most prevalent pathogenic DNM1 mutation, R237W, disrupts dynamin-1 enzyme activity and endocytosis when overexpressed in central neurons. To determine how this mutation impacted cell, circuit and behavioural function, we generated a mouse carrying the R237W mutation. Neurons from heterozygous mice display dysfunctional endocytosis, in addition to altered excitatory neurotransmission and seizure-like phenotypes. Importantly, these phenotypes are corrected at the cell, circuit and in vivo level by the drug, BMS-204352, which accelerates endocytosis. Here, we demonstrate a credible link between dysfunctional endocytosis and epileptic encephalopathy, and importantly reveal that synaptic vesicle recycling may be a viable therapeutic target for monogenic intractable epilepsies.

Motor Speech Phenotypes in Children With Epilepsy: Preliminary Findings

PURPOSE

This exploratory study aimed to characterize motor speech impairments in a small sample of children with epilepsy, both with and without a known seizure etiology. A secondary aim was to evaluate the validity of the Profile for Childhood Apraxia of speech and Dysarthria (ProCAD), a newly developed tool for differential diagnosis of childhood apraxia of speech and dysarthria.

METHOD

Thirteen children with seizure disorders completed a comprehensive speech and language assessment. Three expert speech-language pathologists rated the presence of auditory-perceptual features of motor speech impairment using the ProCAD. Motor speech features, diagnoses, and standardized test scores were compared between children with a known seizure etiology and children with idiopathic epilepsy.

RESULTS

Nine of the 13 children exhibited motor speech impairment; dysarthria was the most common diagnosis. Most children (11/13) exhibited language impairment. Group comparisons showed that children with a known seizure etiology had more atypical motor speech features and lower language scores than children with idiopathic seizures.

CONCLUSION

These preliminary findings suggest a high rate of motor speech impairment among children with epilepsy.

Network Preservation Analysis Reveals Dysregulated Synaptic Modules and Regulatory Hubs Shared Between Alzheimer’s Disease and Temporal Lobe Epilepsy

Background: There is increased prevalence of epilepsy in patients with Alzheimer’s disease (AD). Although shared pathological and clinical features have been identified, the underlying pathophysiology and cause-effect relationships are poorly understood. We aimed to identify commonly dysregulated groups of genes between these two disorders.

Methods: Using publicly available transcriptomic data from hippocampal tissue of patients with temporal lobe epilepsy (TLE), late onset AD and non-AD controls, we constructed gene coexpression networks representing all three states. We then employed network preservation statistics to compare the density and connectivity-based preservation of functional gene modules between TLE, AD and controls and used the difference in significance scores as a surrogate quantifier of module preservation.

Results: The majority (>90%) of functional gene modules were highly preserved between all coexpression networks, however several modules identified in the TLE network showed various degrees of preservation in the AD network compared to that of control. Of note, two synaptic signalling-associated modules and two metabolic modules showed substantial gain of preservation, while myelination and immune system-associated modules showed significant loss of preservation. The genes SCN3B and EPHA4 were identified as central regulatory hubs of the highly preserved synaptic signalling-associated module. GABRB3 and SCN2A were identified as central regulatory hubs of a smaller neurogenesis-associated module, which was enriched for multiple epileptic activity and seizure-related human phenotype ontologies.

Conclusion: We conclude that these hubs and their downstream signalling pathways are common modulators of synaptic activity in the setting of AD and TLE, and may play a critical role in epileptogenesis in AD.

Genetic architecture and phenotypic landscape of deafness and onychodystrophy syndromes

Deafness and onychodystrophy syndromes are a group of phenotypically overlapping syndromes, which include DDOD syndrome (dominant deafness-onychodystrophy), DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation and seizures) and Zimmermann-Laband syndrome (gingival hypertrophy, coarse facial features, hypoplasia or aplasia of nails and terminal phalanges, intellectual disability, and hypertrichosis). Pathogenic variants in four genes, ATP6V1B2, TBC1D24, KCNH1 and KCNN3, have been shown to be associated with deafness and onychodystrophy syndromes. ATP6V1B2 encodes a component of the vacuolar H⁺-ATPase (V-ATPase) and TBC1D24 belongs to GTPase-activating protein, which are all involved in the regulation of membrane trafficking. The overlapping clinical phenotype of TBC1D24- and ATP6V1B2- related diseases and their function with GTPases or ATPases activity indicate that they may have some physiological link. Variants in genes encoding potassium channels KCNH1 or KCNN3, underlying human Zimmermann-Laband syndrome, have only recently been recognized. Although further analysis will be needed, these findings will help to elucidate an understanding of the pathogenesis of these disorders better and will aid in the development of potential therapeutic approaches. In this review, we summarize the latest developments of clinical features and molecular basis that have been reported to be associated with deafness and onychodystrophy disorders and highlight the challenges that may arise in the differential diagnosis.

TBC1D24 and Its Related Epileptic Encephalopathy

TBC1D24, mapped to 16p13.3, encodes a protein containing a Tre2/Bub2/Cdc16 (TBC) domain, belonging to the super-family of Rab GTPase activating proteins (Rab-GAP). These proteins regulate various functions, including the regulation of the traffic of the vesicular membrane. Several TBC1D24 mutations have been related to autosomal recessive neurological disorders, including severe developmental encephalopathies with malignant early childhood epilepsy, benign epilepsy, epileptic encephalopathy, and a complex neurological syndrome characterized by deafness, onychodystrophy, bone and neurological degeneration. Mutations of TBC1D24 have also been reported in patients with nonsyndromic deafness with dominant or recessive inheritance. Mechanisms underlying TBC1D24-associated disorders and the functions of TBC1D24 products in the generation of such complex spectrum of diseases remain partly unclear and future studies are needed to clarify this aspect, in order to improve the management of seizures and for the prevention of complication (including death) of newly diagnosed patients affected by TBC1D24-related disorders.

The axonal endolysosomal and autophagic systems

Neurons, because of their elaborate morphology and the long distances between distal axons and the soma as well as their longevity, pose special challenges to autophagy and to the endolysosomal system, two of the main degradative routes for turnover of defective proteins and organelles. Autophagosomes sequester cytoplasmic or organellar cargos by engulfing them into their lumen before fusion with degradative lysosomes enriched in neuronal somata and participate in retrograde signaling to the soma. Endosomes are mainly involved in the sorting, recycling, or lysosomal turnover of internalized or membrane-bound macromolecules to maintain axonal membrane homeostasis. Lysosomes and the multiple shades of lysosome-related organelles also serve non-degradative roles, for example, in nutrient signaling and in synapse formation. Recent years have begun to shed light on the distinctive organization of the autophagy and endolysosomal systems in neurons, in particular their roles in axons. We review here our current understanding of the localization, distribution, and growing list of functions of these organelles in the axon in health and disease and outline perspectives for future research.

Hereditary Hearing Impairment with Cutaneous Abnormalities

Syndromic hereditary hearing impairment (HHI) is a clinically and etiologically diverse condition that has a profound influence on affected individuals and their families. As cutaneous findings are more apparent than hearing-related symptoms to clinicians and, more importantly, to caregivers of affected infants and young individuals, establishing a correlation map of skin manifestations and their underlying genetic causes is key to early identification and diagnosis of syndromic HHI. In this article, we performed a comprehensive PubMed database search on syndromic HHI with cutaneous abnormalities, and reviewed a total of 260 relevant publications. Our in-depth analyses revealed that the cutaneous manifestations associated with HHI could be classified into three categories: pigment, hyperkeratosis/nail, and connective tissue disorders, with each category involving distinct molecular pathogenesis mechanisms. This outline could help clinicians and researchers build a clear atlas regarding the phenotypic features and pathogenetic mechanisms of syndromic HHI with cutaneous abnormalities, and facilitate clinical and molecular diagnoses of these conditions.

A Novel Variant in the TBC1D24 Lipid-Binding Pocket Causes Autosomal Dominant Hearing Loss: Evidence for a Genotype-Phenotype Correlation

Background: Hereditary hearing loss is a disorder with high genetic and allelic heterogeneity. Diagnostic screening of candidate genes commonly yields novel variants of unknown clinical significance. TBC1D24 is a pleiotropic gene associated with recessive DOORS syndrome, epileptic encephalopathy, myoclonic epilepsy, and both recessive and dominant hearing impairment. Genotype-phenotype correlations have not been established to date but could facilitate diagnostic variant assessment and elucidation of pathomechanisms.

Methods and Results: Whole-exome and gene panel screening identified a novel (c.919A>C; p.Asn307His) causative variant in TBC1D24 in two unrelated Caucasian families with Autosomal dominant (AD) nonsyndromic late-onset hearing loss. Protein modeling on the Drosophila TBC1D24 ortholog Skywalker crystal structure showed close interhelix proximity (6.8Å) between the highly conserved residue p.Asn307 in α18 and the position of the single known pathogenic dominant variation (p.Ser178Leu) in α11 that causes a form of deafness with similar clinical characteristics.

Conclusion: Genetic variants affecting two polar hydrophilic residues in neighboring helices of TBC1D24 cause AD nonsyndromic late-onset hearing loss. The spatial proximity of the affected residues suggests the first genotype-phenotype association in TBC1D24-related disorders. Three conserved residues in α18 contribute to the formation of a functionally relevant cationic phosphoinositide binding pocket that regulates synaptic vesicle trafficking which may be involved in the molecular mechanism of disease.

Novel variants in TBC1D24 associated with epilepsy and deafness: Report of two cases

PURPOSE

To identify the causative variants in two unrelated Chinese patients presenting with epilepsy and deafness.

METHODS

The two patients underwent a thorough examination, including brain MRI, EEG and metabolic studies. Next-generation sequencing (NGS) was performed on genomic DNA samples from the siblings and parents. Sanger sequencing was used to confirm the variants.

RESULTS

Gene sequencing revealed that they carried two novel compound heterozygous missense variants of the TBC1D24: c.116 C > T (p.Ala39Val) and c.827 T > C (p.Ile276Thr) in patient 1; c.404 C > T (p.Pro135Leu) and c.679 T > C (p.Arg227Trp) in patient 2. Audiologic examination showed bilateral sensorineural hearing loss in both patients.

CONCLUSION

We have found novel variants in the TBC1D24 in two Chinese unrelated patients. They result in a rare phenotype, characterized by drug-resistant epilepsy and deafness.

Biallelic loss-of-function variants in TBC1D2B cause a neurodevelopmental disorder with seizures and gingival overgrowth

The family of Tre2-Bub2-Cdc16 (TBC)-domain containing GTPase activating proteins (RABGAPs) is not only known as key regulatorof RAB GTPase activity but also has GAP-independent functions. Rab GTPases are implicated in membrane trafficking pathways, such as vesicular trafficking. We report biallelic loss-of-function variants in TBC1D2B, encoding a member of the TBC/RABGAP family with yet unknown function, as the underlying cause of cognitive impairment, seizures, and/or gingival overgrowth in three individuals from unrelated families. TBC1D2B messenger RNA amount was drastically reduced, and the protein was absent in fibroblasts of two patients. In immunofluorescence analysis, ectopically expressed TBC1D2B colocalized with vesicles positive for RAB5, a small GTPase orchestrating early endocytic vesicle trafficking. In two independent TBC1D2B CRISPR/Cas9 knockout HeLa cell lines that serve as cellular model of TBC1D2B deficiency, epidermal growth factor internalization was significantly reduced compared with the parental HeLa cell line suggesting a role of TBC1D2B in early endocytosis. Serum deprivation of TBC1D2B-deficient HeLa cell lines caused a decrease in cell viability and an increase in apoptosis. Our data reveal that loss of TBC1D2B causes a neurodevelopmental disorder with gingival overgrowth, possibly by deficits in vesicle trafficking and/or cell survival.

TBC1D24 regulates recycling of clathrin-independent cargo proteins mediated by tubular recycling endosomes

Many plasma membrane proteins enter cells by clathrin-independent endocytosis (CIE). Rab family small GTPases play pivotal roles in CIE and following intracellular trafficking of cargo proteins. Here, we provide evidence that TBC1D24, which contains an atypical Rab GAP domain, facilitates formation of tubular recycling endosomes (TREs) that are a hallmark of the CIE cargo trafficking pathway in HeLa cells. Overexpression of TBC1D24 in HeLa cells dramatically increased TREs loaded with CIE cargo proteins, while deletion of TBC1D24 impaired TRE formation and delayed the recycling of CIE cargo proteins back to the plasma membrane. We also found that TBC1D24 binds to Rab22A, through which TBC1D24 regulates TRE-mediated CIE cargo recycling. These findings provide insight into regulatory mechanisms for CIE cargo trafficking.

Presynaptic dysfunction in neurodevelopmental disorders: Insights from the synaptic vesicle life cycle

The activity-dependent fusion, retrieval and recycling of synaptic vesicles is essential for the maintenance of neurotransmission. Until relatively recently it was believed that most mutations in genes that were essential for this process would be incompatible with life, because of this fundamental role. However, an ever-expanding number of mutations in this very cohort of genes are being identified in individuals with neurodevelopmental disorders, including autism, intellectual disability and epilepsy. This article will summarize the current state of knowledge linking mutations in presynaptic genes to neurodevelopmental disorders by sequentially covering the various stages of the synaptic vesicle life cycle. It will also discuss how perturbations of specific stages within this recycling process could translate into human disease. Finally, it will also provide perspectives on the potential for future therapy that are targeted to presynaptic function.

The phenotypic landscape of a Tbc1d24 mutant mouse includes convulsive seizures resembling human early infantile epileptic encephalopathy

Epilepsy, deafness, onychodystrophy, osteodystrophy and intellectual disability are associated with a spectrum of mutations of human TBC1D24. The mechanisms underlying TBC1D24-associated disorders and the functions of TBC1D24 are not well understood. Using CRISPR-Cas9 genome editing, we engineered a mouse with a premature translation stop codon equivalent to human S324Tfs*3, a recessive mutation of TBC1D24 associated with early infantile epileptic encephalopathy (EIEE). Homozygous S324Tfs*3 mice have normal auditory and vestibular functions but show an abrupt onset of spontaneous seizures at postnatal day 15 recapitulating human EIEE. The S324Tfs*3 variant is located in an alternatively spliced micro-exon encoding six perfectly conserved amino acids incorporated postnatally into TBC1D24 protein due to a micro-exon utilization switch. During embryonic and early postnatal development, S324Tfs*3 homozygotes produce predominantly the shorter wild-type TBC1D24 protein isoform that omits the micro-exon. S324Tfs*3 homozygotes show an abrupt onset of seizures at P15 that correlates with a developmental switch to utilization of the micro-exon. A mouse deficient for alternative splice factor SRRM3 impairs incorporation of the Tbc1d24 micro-exon. Wild-type Tbc1d24 mRNA is abundantly expressed in the hippocampus using RNAscope in situ hybridization. Immunogold electron microscopy using a TBC1D24-specific antibody revealed that TBC1D24 is associated with clathrin-coated vesicles and synapses of hippocampal neurons, suggesting a crucial role of TBC1D24 in vesicle trafficking important for neuronal signal transmission. This is the first characterization of a mouse model of human TBC1D24-associated EIEE that can now be used to screen for antiepileptogenic drugs ameliorating TBCID24 seizure disorders.

The epilepsy and intellectual disability-associated protein TBC1D24 regulates the maintenance of excitatory synapses and animal behaviors

Perturbation of synapse development underlies many inherited neurodevelopmental disorders including intellectual disability (ID). Diverse mutations on the human TBC1D24 gene are strongly associated with epilepsy and ID. However, the physiological function of TBC1D24 in the brain is not well understood, and there is a lack of genetic mouse model that mimics TBC1D24 loss-of-function for the study of animal behaviors. Here we report that TBC1D24 is present at the postsynaptic sites of excitatory synapses, where it is required for the maintenance of dendritic spines through inhibition of the small GTPase ARF6. Mice subjected to viral-mediated knockdown of TBC1D24 in the adult hippocampus display dendritic spine loss, deficits in contextual fear memory, as well as abnormal behaviors including hyperactivity and increased anxiety. Interestingly, we show that the protein stability of TBC1D24 is diminished by the disease-associated missense mutation that leads to F251L amino acid substitution. We further generate the F251L knock-in mice, and the homozygous mutants show increased neuronal excitability, spontaneous seizure and pre-mature death. Moreover, the heterozygous F251L knock-in mice survive into adulthood but display dendritic spine defects and impaired memory. Our findings therefore uncover a previously uncharacterized postsynaptic function of TBC1D24, and suggest that impaired dendritic spine maintenance contributes to the pathophysiology of individuals harboring TBC1D24 gene mutations. The F251L knock-in mice represent a useful animal model for investigation of the mechanistic link between TBC1D24 loss-of-function and neurodevelopmental disorders.

EXOME REPORT: Novel mutation in ATP6V1B2 segregating with autosomal dominant epilepsy, intellectual disability and mild gingival and nail abnormalities

Mutations in ATP6V1B2, which encodes the B2 subunit of the vacuolar H + ATPase have previously been associated with Zimmermann-Laband syndrome 2 (ZLS2) and deafness-onychodystrophy (DDOD) syndrome. Recently epilepsy has also been described as a potentially associated phenotype. Here we further uncover the role of ATP61VB2 in epilepsy and report autosomal dominant inheritance of a novel missense variant in ATP6V1B2 in a large Polish family with relatively mild gingival and nail problems, no phalangeal hypoplasia and with generalized epilepsy. In light of our findings and review of the literature, we propose that the ATP6V1B2 gene should be considered in families with autosomal dominant epilepsy both with or without intellectual disability, and that presence of subtle gingival and nail problems may be another characteristic calling card of affected individuals with ATP6V1B2 mutations.

The epilepsy-associated protein TBC1D24 is required for normal development, survival and vesicle trafficking in mammalian neurons

Mutations in the Tre2/Bub2/Cdc16 (TBC)1 domain family member 24 (TBC1D24) gene are associated with a range of inherited neurological disorders, from drug-refractory lethal epileptic encephalopathy and DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, seizures) to non-syndromic hearing loss. TBC1D24 has been implicated in neuronal transmission and maturation, although the molecular function of the gene and the cause of the apparently complex disease spectrum remain unclear. Importantly, heterozygous TBC1D24 mutation carriers have also been reported with seizures, suggesting that haploinsufficiency for TBC1D24 is significant clinically. Here we have systematically investigated an allelic series of disease-associated mutations in neurons alongside a new mouse model to investigate the consequences of TBC1D24 haploinsufficiency to mammalian neurodevelopment and synaptic physiology. The cellular studies reveal that disease-causing mutations that disrupt either of the conserved protein domains in TBC1D24 are implicated in neuronal development and survival and are likely acting as loss-of-function alleles. We then further investigated TBC1D24 haploinsufficiency in vivo and demonstrate that TBC1D24 is also crucial for normal presynaptic function: genetic disruption of Tbc1d24 expression in the mouse leads to an impairment of endocytosis and an enlarged endosomal compartment in neurons with a decrease in spontaneous neurotransmission. These data reveal the essential role for TBC1D24 at the mammalian synapse and help to define common synaptic mechanisms that could underlie the varied effects of TBC1D24 mutations in neurological disease.

TBC1D24 regulates axonal outgrowth and membrane trafficking at the growth cone in rodent and human neurons

Mutations in TBC1D24 are described in patients with a spectrum of neurological diseases, including mild and severe epilepsies and complex syndromic phenotypes such as Deafness, Onycodystrophy, Osteodystrophy, Mental Retardation and Seizure (DOORS) syndrome. The product of TBC1D24 is a multifunctional protein involved in neuronal development, regulation of synaptic vesicle trafficking, and protection from oxidative stress. Although pathogenic mutations in TBC1D24 span the entire coding sequence, no clear genotype/phenotype correlations have emerged. However most patients bearing predicted loss of function mutations exhibit a severe neurodevelopmental disorder. Aim of the study is to investigate the impact of TBC1D24 knockdown during the first stages of neuronal differentiation when axonal specification and outgrowth take place. In rat cortical primary neurons silenced for TBC1D24, we found defects in axonal specification, the maturation of axonal initial segment and action potential firing. The axonal phenotype was accompanied by an impairment of endocytosis at the growth cone and an altered activation of the TBC1D24 molecular partner ADP ribosylation factor 6. Accordingly, acute knockdown of TBC1D24 in cerebrocortical neurons in vivo analogously impairs callosal projections. The axonal defect was also investigated in human induced pluripotent stem cell-derived neurons from patients carrying TBC1D24 mutations. Reprogrammed neurons from a patient with severe developmental encephalopathy show significant axon formation defect that were absent from reprogrammed neurons of a patient with mild early onset epilepsy. Our data reveal that alterations of membrane trafficking at the growth cone induced by TBC1D24 loss of function cause axonal and excitability defects. The axonal phenotype correlates with the disease severity and highlight an important role for TBC1D24 in connectivity during brain development.

A fragment activity assay reveals the key residues of TBC1D15 GTPase-activating protein (GAP) in Chiloscyllium plagiosum

BACKGROUND

GTPase-activating proteins (GAPs) with a TBC (Tre-2/Bub2/Cdc16) domain architecture serve as negative regulators of Rab GTPases. The related crystal structure has been studied and reported by other members of our research group in 2017 (Chen et al. in Protein Sci 26(4):834-846, 2017). The protein crystal structure and sequencing data accession numbers in Protein structure database (PDB) are 5TUB (Shark TBC1D15 GAP) and 5TUC (Sus TBC1D15 GAP), respectively. In this paper, we analyzed the Rab-GAP specificity of TBC1D15 in the evolution and influence of key amino acid residue mutations on Rab-GAP activity.

RESULTS

Sequence alignment showed that five arginine residues of the TBC1D15-GAP domain are conserved among the species Sus/Mus/Homo but have been replaced by glycine or lysine in Shark. A fragment activity assay was conducted by altering the five residues of Shark TBC1D15-GAP to arginine, and the corresponding arginine in TBC1D15 GAP domains from Sus and Homo species were mutated to resemble Shark TBC1D15-GAP. Our data revealed that the residues of G28, K45, K119, K122 and K221 in the Shark TBC1D15-GAP domain had a key role in determining the specificity for Rab7 and Rab11. Mutation of the five residues significantly altered the Shark TBC1D15-GAP activity.

CONCLUSIONS

These results revealed that the substrate specificity of TBC1D15 has had different mechanisms across the evolution of species from lower-cartilaginous fish to higher mammals. Collectively, the data support a different mechanism of Shark TBC1D15-GAP in substrate selection, which provides a new idea for the development of Marine drugs.

Oxidation Resistance 1 Modulates Glycolytic Pathways in the Cerebellum via an Interaction with Glucose-6-Phosphate Isomerase

Glucose metabolism is essential for the brain: it not only provides the required energy for cellular function and communication but also participates in balancing the levels of oxidative stress in neurons. Defects in glucose metabolism have been described in neurodegenerative disease; however, it remains unclear how this fundamental process contributes to neuronal cell death in these disorders. Here, we investigated the molecular mechanisms driving the selective neurodegeneration in an ataxic mouse model lacking oxidation resistance 1 (Oxr1) and discovered an unexpected function for this protein as a regulator of the glycolytic enzyme, glucose-6-phosphate isomerase (GPI/Gpi1). Initially, we present a dysregulation of metabolites of glucose metabolism at the pre-symptomatic stage in the Oxr1 knockout cerebellum. We then demonstrate that Oxr1 and Gpi1 physically and functionally interact and that the level of Gpi1 oligomerisation is disrupted when Oxr1 is deleted in vivo. Furthermore, we show that Oxr1 modulates the additional and less well-understood roles of Gpi1 as a cytokine and neuroprotective factor. Overall, our data identify a new molecular function for Oxr1, establishing this protein as important player in neuronal survival, regulating both oxidative stress and glucose metabolism in the brain.

Downregulation of TBC1 Domain Family Member 24 (BC1D24) Inhibits Breast Carcinoma Growth via IGF1R/PI3K/AKT Pathway

BACKGROUND TBC1 domain family member 24 (TBC1D24) pathogenic mutations affect its binding to ARF6 and then result in severe impairment of neuronal development. However, there are no reports about the expression and function of TBC1D24 in cancer. The aim of the present study was to evaluate the effect of proliferation, migration, and invasion after silencing TBC1D24 expression in breast cancer MCF-7 cells, and to elucidate the potential mechanism of TBC1D24 in breast cancer. MATERIAL AND METHODS The expression of TBC1D24 in breast cancer tissues and the adjacent non-tumor tissues was determined by S-P immunohistochemistry. The malignant behavior, including proliferation, migration, and invasion ability, was determined after silencing TBC1D24 in breast cancer MCF-7 cells. The expression of IGF1R was determined after silencing TBC1D24. The expression of TBC1D24 and IGF1R was detected after transfecting miR-30a mimics or inhibitors. The effect of TBC1D24 on MCF-7 cells growth in vivo was evaluated by a tumor xenograft study. RESULTS TBC1D24 expression was elevated and was associated with poor outcome in breast carcinoma. TBC1D24 high expression was significantly correlated with unfavorable OS and RFS for breast cancer patients (p<0.05). Silencing TBC1D24 inhibited the proliferation, migration, and invasion ability of MCF-7 cells. TBC1D24 and IGF1R expression were decreased when transfected with miR-30a mimics. However, TBC1D24 and IGF1R expression were increased when transfected with miR-30a inhibitors (p<0.05). Knockdown of TBC1D24 inhibited the expression of IGF1R, PI3K, and p-AKT (p<0.05). Knockdown of TBC1D24 abolished tumorigenicity of MCF-7 cells. The average volume and weight of tumors was lower after silencing TBC1D24 expression (P<0.05). CONCLUSIONS Silencing TBC1D24 inhibited MCF-7 cells growth in vitro and in vivo. TBC1D24 promoted breast carcinoma growth through the IGF1R/PI3K/AKT pathway. TBC1D24 is a potential therapeutic target for breast cancer.

Neuronal lysosomes

Lysosomes support diverse cellular functions by acting as sites of macromolecule degradation and nutrient recycling. The degradative abilities of lysosomes are conferred by a lumen that is characterized by an acidic pH and which contains numerous hydrolases that support the breakdown of major cellular macromolecules to yield cellular building blocks (amino acids, nucleic acids, sugars, lipids and metals) that are transported into the cytoplasm for their re-use. In addition to these important hydrolytic and recycling functions, lysosomes also serve as a signaling platform that integrates nutrient and metabolic cues to control signaling via the mTORC1 pathway. Due to their extreme longevity, polarity, demands of neurotransmission and metabolic activity, neurons are particularly sensitive to perturbations in lysosome function. The dependence of neurons on optimal lysosome function is highlighted by insights from human genetics that link lysosome dysfunction to a wide range of both rare and common neurological diseases. How then is lysosome function adapted to the unique demands of neurons? This review will focus on the roles played by lysosomes in distinct neuronal sub-compartments, the regulation of neuronal lysosome sub-cellular localization and the implications of such neuronal lysosome regulation for both physiology and disease.

Homozygous Mutations in TBC1D23 Lead to a Non-degenerative Form of Pontocerebellar Hypoplasia

Pontocerebellar hypoplasia (PCH) represents a group of recessive developmental disorders characterized by impaired growth of the pons and cerebellum, which frequently follows a degenerative course. Currently, there are 10 partially overlapping clinical subtypes and 13 genes known mutated in PCH. Here, we report biallelic TBC1D23 mutations in six individuals from four unrelated families manifesting a non-degenerative form of PCH. In addition to reduced volume of pons and cerebellum, affected individuals had microcephaly, psychomotor delay, and ataxia. In zebrafish, tbc1d23 morphants replicated the human phenotype showing hindbrain volume loss. TBC1D23 localized at the trans-Golgi and was regulated by the small GTPases Arl1 and Arl8, suggesting a role in trans-Golgi membrane trafficking. Altogether, this study provides a causative link between TBC1D23 mutations and PCH and suggests a less severe clinical course than other PCH subtypes.

The role of ESCRT during development and functioning of the nervous system

The endosomal sorting complex required for transport (ESCRT) is made of subcomplexes (ESCRT 0-III), crucial to membrane remodelling at endosomes, nuclear envelope and cell surface. ESCRT-III shapes membranes and in most cases cooperates with the ATPase VPS4 to mediate fission of membrane necks from the inside. The first ESCRT complexes mainly serve to catalyse the formation of ESCRT-III but can be bypassed by accessory proteins like the Alg-2 interacting protein-X (ALIX). In the nervous system, ALIX/ESCRT controls the survival of embryonic neural progenitors and later on the outgrowth and pruning of axons and dendrites, all necessary steps to establish a functional brain. In the adult brain, ESCRTs allow the endosomal turn over of synaptic vesicle proteins while stable ESCRT complexes might serve as scaffolds for the postsynaptic parts. The necessity of ESCRT for the harmonious function of the brain has its pathological counterpart, the mutations in CHMP2B of ESCRT-III giving rise to several neurodegenerative diseases.

TLDc proteins: new players in the oxidative stress response and neurological disease

Oxidative stress (OS) arises from an imbalance in the cellular redox state, which can lead to intracellular damage and ultimately cell death. OS occurs as a result of normal ageing, but it is also implicated as a common etiological factor in neurological disease; thus identifying novel proteins that modulate the OS response may facilitate the design of new therapeutic approaches applicable to many disorders. In this review, we describe the recent progress that has been made using a range of genetic approaches to understand a family of proteins that share the highly conserved TLDc domain. We highlight their shared ability to prevent OS-related cell death and their unique functional characteristics, as well as discussing their potential application as new neuroprotective factors. Furthermore, with an increasing number of pathogenic mutations leading to epilepsy and hearing loss being discovered in the TLDc protein TBC1D24, understanding the function of this family has important implications for a range of inherited neurological diseases.

Unresolved questions regarding human hereditary deafness

Human hearing loss is a common neurosensory disorder about which many basic research and clinically relevant questions are unresolved. This review on hereditary deafness focuses on three examples considered at first glance to be uncomplicated, however, upon inspection, are enigmatic and ripe for future research efforts. The three examples of clinical and genetic complexities are drawn from studies of (i) Pendred syndrome/DFNB4 (PDS, OMIM 274600), (ii) Perrault syndrome (deafness and infertility) due to mutations of CLPP (PRTLS3, OMIM 614129), and (iii) the unexplained extensive clinical variability associated with TBC1D24 mutations. At present, it is unknown how different mutations of TBC1D24 cause non-syndromic deafness (DFNB86, OMIM 614617), epilepsy (OMIM 605021), epilepsy with deafness, or DOORS syndrome (OMIM 220500) that is characterized by deafness, onychodystrophy (alteration of toenail or fingernail morphology), osteodystrophy (defective development of bone), mental retardation, and seizures. A comprehensive understanding of the multifaceted roles of each gene associated with human deafness is expected to provide future opportunities for restoration as well as preservation of normal hearing.

Neuronal Polarity in the Embryonic Mammalian Cerebral Cortex

The cerebral cortex is composed of billions of neurons that can grossly be subdivided into two broad classes: inhibitory GABAergic interneurons and excitatory glutamatergic neurons. The majority of cortical neurons in mammals are the excitatory type and they are the main focus of this review article. Like many of the cells in multicellular organisms, fully differentiated neurons are both morphologically and functionally polarized. However, they go through several changes in polarity before reaching this final mature differentiated state. Neurons are derived from polarized neuronal progenitor/stem cells and their commitment to neuronal fate is decided by cellular and molecular asymmetry during their last division in the neurogenic zone. They migrate from their birthplace using so-called multipolar migration, during which they switch direction of movement several times, and repolarize for bipolar migration when the axon is specified. Therefore, neurons have to break their previous symmetry, change their morphology and adequately respond to polarizing signals during migration in order to reach the correct position in the cortex and start making connections. Finally, the dendritic tree is elaborated and the axon/dendrite morphological polarity is set. Here we will describe the function, establishment and maintenance of polarity during the different developmental steps starting from neural stem cell (NSC) division, neuronal migration and axon specification at embryonic developmental stages.

PLAA Mutations Cause a Lethal Infantile Epileptic Encephalopathy by Disrupting Ubiquitin-Mediated Endolysosomal Degradation of Synaptic Proteins

During neurotransmission, synaptic vesicles undergo multiple rounds of exo-endocytosis, involving recycling and/or degradation of synaptic proteins. While ubiquitin signaling at synapses is essential for neural function, it has been assumed that synaptic proteostasis requires the ubiquitin-proteasome system (UPS). We demonstrate here that turnover of synaptic membrane proteins via the endolysosomal pathway is essential for synaptic function. In both human and mouse, hypomorphic mutations in the ubiquitin adaptor protein PLAA cause an infantile-lethal neurodysfunction syndrome with seizures. Resulting from perturbed endolysosomal degradation, Plaa mutant neurons accumulate K63-polyubiquitylated proteins and synaptic membrane proteins, disrupting synaptic vesicle recycling and neurotransmission. Through characterization of this neurological intracellular trafficking disorder, we establish the importance of ubiquitin-mediated endolysosomal trafficking at the synapse.

TBC1D24 Mutations in a Sibship with Multifocal Polymyoclonus

Background

Advances in molecular genetic technologies have improved our understanding of genetic causes of rare neurological disorders with features of myoclonus.

Case Report

A family with two affected siblings, presenting with multifocal polymyoclonus and neurodevelopmental delay, was recruited for whole-exome sequencing following unyielding diagnostic neurometabolic investigations. Compound heterozygous mutations in TBC1D24, a gene previously associated with various epilepsy phenotypes and hearing loss, were identified in both siblings. The mutations included a missense change c.457G>A (p.Glu157Lys), and a novel frameshift mutation c.545del (p.Thr182Serfs*6).

Discussion

We propose that TBC1D24-related diseases should be in the differential diagnosis for children with polymyoclonus.

Coordination of synaptic vesicle trafficking and turnover by the Rab35 signaling network

Rab35 and the Rab35 network of GAPs, GEFs, and effectors are important regulators of membrane trafficking for a variety of cellular processes, from cytokinesis and phagocytosis to neurite outgrowth. In the past five years, components of this signaling network have also been implicated as critical mediators of synaptic vesicle (SV) recycling and protein homeostasis. Recent studies by several groups, including our own, have demonstrated that Rab35-mediated endosomal sorting is required for the degradation of SV proteins via the ESCRT pathway, thereby eliminating old or damaged proteins from the SV pool. This sorting process is regulated by Rab35 activation in response to neuronal activity, and potentially by an antagonistic signaling relationship between Rab35 and the small GTPase Arf6 that directs SVs into distinct recycling pathways depending on neuronal activity levels. Furthermore, mutations in genes encoding Rab35 regulatory proteins are emerging as causative factors in human neurologic and neurodegenerative diseases, consistent with their important roles in synaptic and neuronal health. Here, we review these recent findings and offer our perspective on how the Rab35 signaling network functions to maintain neurotransmission and synaptic fitness.

Genetic and epigenetic mechanisms of epilepsy: a review

Epilepsy is a common episodic neurological disorder or condition characterized by recurrent epileptic seizures, and genetics seems to play a key role in its etiology. Early linkage studies have localized multiple loci that may harbor susceptibility genes to epilepsy, and mutational analyses have detected a number of mutations involved in both ion channel and nonion channel genes in patients with idiopathic epilepsy. Genome-wide studies of epilepsy have found copy number variants at 2q24.2-q24.3, 7q11.22, 15q11.2-q13.3, and 16p13.11-p13.2, some of which disrupt multiple genes, such as NRXN1, AUTS2, NLGN1, CNTNAP2, GRIN2A, PRRT2, NIPA2, and BMP5, implicated for neurodevelopmental disorders, including intellectual disability and autism. Unfortunately, only a few common genetic variants have been associated with epilepsy. Recent exome-sequencing studies have found some genetic mutations, most of which are located in nonion channel genes such as the LGI1, PRRT2, EFHC1, PRICKLE, RBFOX1, and DEPDC5 and in probands with rare forms of familial epilepsy, and some of these genes are involved with the neurodevelopment. Since epigenetics plays a role in neuronal function from embryogenesis and early brain development to tissue-specific gene expression, epigenetic regulation may contribute to the genetic mechanism of neurodevelopment through which a gene and the environment interacting with each other affect the development of epilepsy. This review focused on the analytic tools used to identify epilepsy and then provided a summary of recent linkage and association findings, indicating the existence of novel genes on several chromosomes for further understanding of the biology of epilepsy.

Invited review: Small GTPases and their GAPs

Widespread utilization of small GTPases as major regulatory hubs in many different biological systems derives from a conserved conformational switch mechanism that facilitates cycling between GTP-bound active and GDP-bound inactive states under control of guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), which accelerate slow intrinsic rates of activation by nucleotide exchange and deactivation by GTP hydrolysis, respectively. Here we review developments leading to current understanding of intrinsic and GAP catalyzed GTP hydrolytic reactions in small GTPases from structural, molecular and chemical mechanistic perspectives. Despite the apparent simplicity of the GTPase cycle, the structural bases underlying the hallmark hydrolytic reaction and catalytic acceleration by GAPs are considerably more diverse than originally anticipated. Even the most fundamental aspects of the reaction mechanism have been challenging to decipher. Through a combination of experimental and in silico approaches, the outlines of a consensus view have begun to emerge for the best studied paradigms. Nevertheless, recent observations indicate that there is still much to be learned. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 431-448, 2016.

TBC1D24 genotype-phenotype correlation: Epilepsies and other neurologic features

Objective:

To evaluate the phenotypic spectrum associated with mutations in TBC1D24.

Methods:

We acquired new clinical, EEG, and neuroimaging data of 11 previously unreported and 37 published patients. TBC1D24 mutations, identified through various sequencing methods, can be found online (http://lovd.nl/TBC1D24).

Results:

Forty-eight patients were included (28 men, 20 women, average age 21 years) from 30 independent families. Eighteen patients (38%) had myoclonic epilepsies. The other patients carried diagnoses of focal (25%), multifocal (2%), generalized (4%), and unclassified epilepsy (6%), and early-onset epileptic encephalopathy (25%). Most patients had drug-resistant epilepsy. We detail EEG, neuroimaging, developmental, and cognitive features, treatment responsiveness, and physical examination. In silico evaluation revealed 7 different highly conserved motifs, with the most common pathogenic mutation located in the first. Neuronal outgrowth assays showed that some TBC1D24 mutations, associated with the most severe TBC1D24-associated disorders, are not necessarily the most disruptive to this gene function.

Conclusions:

TBC1D24-related epilepsy syndromes show marked phenotypic pleiotropy, with multisystem involvement and severity spectrum ranging from isolated deafness (not studied here), benign myoclonic epilepsy restricted to childhood with complete seizure control and normal intellect, to early-onset epileptic encephalopathy with severe developmental delay and early death. There is no distinct correlation with mutation type or location yet, but patterns are emerging. Given the phenotypic breadth observed, TBC1D24 mutation screening is indicated in a wide variety of epilepsies. A TBC1D24 consortium was formed to develop further research on this gene and its associated phenotypes.

MicroRNA-338 Attenuates Cortical Neuronal Outgrowth by Modulating the Expression of Axon Guidance Genes

MicroRNAs (miRs) are small non-coding RNAs that confer robustness to gene networks through post-transcriptional gene regulation. Previously, we identified miR-338 as a modulator of axonal outgrowth in sympathetic neurons. In the current study, we examined the role of miR-338 in the development of cortical neurons and uncovered its downstream mRNA targets. Long-term inhibition of miR-338 during neuronal differentiation resulted in reduced dendritic complexity and altered dendritic spine morphology. Furthermore, monitoring axon outgrowth in cortical cells revealed that miR-338 overexpression decreased, whereas inhibition of miR-338 increased axonal length. To identify gene targets mediating the observed phenotype, we inhibited miR-338 in cortical neurons and performed whole-transcriptome analysis. Pathway analysis revealed that miR-338 modulates a subset of transcripts involved in the axonal guidance machinery by means of direct and indirect gene targeting. Collectively, our results implicate miR-338 as a novel regulator of cortical neuronal maturation by fine-tuning the expression of gene networks governing cortical outgrowth.

Molecular Machines Determining the Fate of Endocytosed Synaptic Vesicles in Nerve Terminals

The cycle of a synaptic vesicle (SV) within the nerve terminal is a step-by-step journey with the final goal of ensuring the proper synaptic strength under changing environmental conditions. The SV cycle is a precisely regulated membrane traffic event in cells and, because of this, a plethora of membrane-bound and cytosolic proteins are devoted to assist SVs in each step of the journey. The cycling fate of endocytosed SVs determines both the availability for subsequent rounds of release and the lifetime of SVs in the terminal and is therefore crucial for synaptic function and plasticity. Molecular players that determine the destiny of SVs in nerve terminals after a round of exo-endocytosis are largely unknown. Here we review the functional role in SV fate of phosphorylation/dephosphorylation of SV proteins and of small GTPases acting on membrane trafficking at the synapse, as they are emerging as key molecules in determining the recycling route of SVs within the nerve terminal. In particular, we focus on: (i) the cyclin-dependent kinase-5 (cdk5) and calcineurin (CN) control of the recycling pool of SVs; (ii) the role of small GTPases of the Rab and ADP-ribosylation factor (Arf) families in defining the route followed by SV in their nerve terminal cycle. These regulatory proteins together with their synaptic regulators and effectors, are molecular nanomachines mediating homeostatic responses in synaptic plasticity and potential targets of drugs modulating the efficiency of synaptic transmission.

The Evolutionarily Conserved Tre2/Bub2/Cdc16 (TBC), Lysin Motif (LysM), Domain Catalytic (TLDc) Domain Is Neuroprotective against Oxidative Stress

Oxidative stress is a pathological feature of many neurological disorders; therefore, utilizing proteins that are protective against such cellular insults is a potentially valuable therapeutic approach. Oxidation resistance 1 (OXR1) has been shown previously to be critical for oxidative stress resistance in neuronal cells; deletion of this gene causes neurodegeneration in mice, yet conversely, overexpression of OXR1 is protective in cellular and mouse models of amyotrophic lateral sclerosis. However, the molecular mechanisms involved are unclear. OXR1 contains the Tre2/Bub2/Cdc16 (TBC), lysin motif (LysM), domain catalytic (TLDc) domain, a motif present in a family of proteins including TBC1 domain family member 24 (TBC1D24), a protein mutated in a range of disorders characterized by seizures, hearing loss, and neurodegeneration. The TLDc domain is highly conserved across species, although the structure-function relationship is unknown. To understand the role of this domain in the stress response, we carried out systematic analysis of all mammalian TLDc domain-containing proteins, investigating their expression and neuroprotective properties in parallel. In addition, we performed a detailed structural and functional study of this domain in which we identified key residues required for its activity. Finally, we present a new mouse insertional mutant of Oxr1, confirming that specific disruption of the TLDc domain in vivo is sufficient to cause neurodegeneration. Our data demonstrate that the integrity of the TLDc domain is essential for conferring neuroprotection, an important step in understanding the functional significance of all TLDc domain-containing proteins in the cellular stress response and disease.

Multiplex families with epilepsy: Success of clinical and molecular genetic characterization

OBJECTIVE

To analyze the clinical syndromes and inheritance patterns of multiplex families with epilepsy toward the ultimate aim of uncovering the underlying molecular genetic basis.

METHODS

Following the referral of families with 2 or more relatives with epilepsy, individuals were classified into epilepsy syndromes. Families were classified into syndromes where at least 2 family members had a specific diagnosis. Pedigrees were analyzed and molecular genetic studies were performed as appropriate.

RESULTS

A total of 211 families were ascertained over an 11-year period in Israel. A total of 169 were classified into broad familial epilepsy syndrome groups: 61 generalized, 22 focal, 24 febrile seizure syndromes, 33 special syndromes, and 29 mixed. A total of 42 families remained unclassified. Pathogenic variants were identified in 49/211 families (23%). The majority were found in established epilepsy genes (e.g., SCN1A, KCNQ2, CSTB), but in 11 families, this cohort contributed to the initial discovery (e.g., KCNT1, PCDH19, TBC1D24). We expand the phenotypic spectrum of established epilepsy genes by reporting a familial LAMC3 homozygous variant, where the predominant phenotype was epilepsy with myoclonic-atonic seizures, and a pathogenic SCN1A variant in a family where in 5 siblings the phenotype was broadly consistent with Dravet syndrome, a disorder that usually occurs sporadically.

CONCLUSION

A total of 80% of families were successfully classified, with pathogenic variants identified in 23%. The successful characterization of familial electroclinical and inheritance patterns has highlighted the value of studying multiplex families and their contribution towards uncovering the genetic basis of the epilepsies.

Molecular Pathways Underlying Projection Neuron Production and Migration during Cerebral Cortical Development

Glutamatergic neurons of the mammalian cerebral cortex originate from radial glia (RG) progenitors in the ventricular zone (VZ). During corticogenesis, neuroblasts migrate toward the pial surface using two different migration modes. One is multipolar (MP) migration with random directional movement, and the other is locomotion, which is a unidirectional movement guided by the RG fiber. After reaching their final destination, the neurons finalize their migration by terminal translocation, which is followed by maturation via dendrite extension to initiate synaptogenesis and thereby complete neural circuit formation. This switching of migration modes during cortical development is unique in mammals, which suggests that the RG-guided locomotion mode may contribute to the evolution of the mammalian neocortical 6-layer structure. Many factors have been reported to be involved in the regulation of this radial neuronal migration process. In general, the radial migration can be largely divided into four steps; (1) maintenance and departure from the VZ of neural progenitor cells, (2) MP migration and transition to bipolar cells, (3) RG-guided locomotion, and (4) terminal translocation and dendrite maturation. Among these, many different gene mutations or knockdown effects have resulted in failure of the MP to bipolar transition (step 2), suggesting that it is a critical step, particularly in radial migration. Moreover, this transition occurs at the subplate layer. In this review, we summarize recent advances in our understanding of the molecular mechanisms underlying each of these steps. Finally, we discuss the evolutionary aspects of neuronal migration in corticogenesis.

Lessons learned from gene identification studies in Mendelian epilepsy disorders

Next-generation sequencing (NGS) technologies are now routinely used for gene identification in Mendelian disorders. Setting up cost-efficient NGS projects and managing the large amount of variants remains, however, a challenging job. Here we provide insights in the decision-making processes before and after the use of NGS in gene identification studies. Genetic factors are thought to have a role in ~70% of all epilepsies, and a variety of inheritance patterns have been described for seizure-associated gene defects. We therefore chose epilepsy as disease model and selected 35 NGS studies that focused on patients with a Mendelian epilepsy disorder. The strategies used for gene identification and their respective outcomes were reviewed. High-throughput NGS strategies have led to the identification of several new epilepsy-causing genes, enlarging our knowledge on both known and novel pathomechanisms. NGS findings have furthermore extended the awareness of phenotypical and genetic heterogeneity. By discussing recent studies we illustrate: (I) the power of NGS for gene identification in Mendelian disorders, (II) the accelerating pace in which this field evolves, and (III) the considerations that have to be made when performing NGS studies. Nonetheless, the enormous rise in gene discovery over the last decade, many patients and families included in gene identification studies still remain without a molecular diagnosis; hence, further genetic research is warranted. On the basis of successful NGS studies in epilepsy, we discuss general approaches to guide human geneticists and clinicians in setting up cost-efficient gene identification NGS studies.

Recessive TBC1D24 Mutations Are Frequent in Moroccan Non-Syndromic Hearing Loss Pedigrees

Mutations in the TBC1D24 gene are responsible for four neurological presentations: infantile epileptic encephalopathy, infantile myoclonic epilepsy, DOORS (deafness, onychodystrophy, osteodystrophy, mental retardation and seizures) and NSHL (non-syndromic hearing loss). For the latter, two recessive (DFNB86) and one dominant (DFNA65) mutations have so far been identified in consanguineous Pakistani and European/Chinese families, respectively. Here we report the results of a genetic study performed on a large Moroccan cohort of deaf patients that identified three families with compound heterozygote mutations in TBC1D24. Four novel mutations were identified, among which, one c.641G>A (p.Arg214His) was present in the three families, and has a frequency of 2% in control Moroccan population with normal hearing, suggesting that it acts as an hypomorphic variant leading to restricted deafness when combined with another recessive severe mutation. Altogether, our results show that mutations in TBC1D24 gene are a frequent cause (>2%) of NSHL in Morocco, and that due to its possible compound heterozygote recessive transmission, this gene should be further considered and screened in other deaf cohorts.

Advancing epilepsy genetics in the genomic era

Epilepsy is a group of disorders characterized by recurrent seizures, and is one of the most common neurological conditions. The genetic basis of epilepsy is clear from epidemiological studies and from rare gene discoveries in large families. The three major classes of epilepsy disorders are genetic generalized, focal and encephalopathic epilepsies, with several specific disorders within each class. Advances in genomic technologies that facilitate genome-wide discovery of both common and rare variants have led to a rapid increase in our understanding of epilepsy genetics. Copy number variant and genome-wide association studies have contributed to our understanding of the complex genetic architecture of generalized epilepsy, while genetic insights into the focal epilepsies and epileptic encephalopathies have come primarily from exome sequencing. It is increasingly clear that epilepsy is genetically heterogeneous, and novel gene discoveries have moved the field beyond the known contribution of ion channels to implicate chromatin remodeling, transcriptional regulation and regulation of the mammalian target of rapamycin (mTOR) protein in the etiology of epilepsy. Such discoveries pave the way for new therapeutics, some of which are already being studied. In this review, we discuss the rapid pace of gene discovery in epilepsy, as facilitated by genomic technologies, and highlight several novel genes and potential therapies.