Mutations in the CYFIP2 gene, particularly the R87C variant, are associated with severe epileptic encephalopathy, and present challenges for therapeutic development. This study utilized CRISPR/Cas9-edited human pluripotent stem cell (hPSC) lines to investigate the impact of R87C variant on neuronal morphology and function. hPSCs were differentiated into neural progenitor cells (NPCs), cortical neurons (CNs), and cortical organoids. Phenotypic characterization included immunofluorescence, scanning electron microscopy (SEM), high-throughput scanning (HTS), multi-electrode array (MEA) recordings, and Western blotting. Edited hPSC lines maintained pluripotency, and neurogenic differentiation yielded NPCs and CNs without significant differences in neural progenitor marker expression. However, mutated NPCs exhibited reduced motility in cell tracking assays, and SEM revealed altered cell morphology, suggesting an impact on lamellipodia formation. While both mutant and wild-type CNs expressed appropriate neuronal and glial markers and showed similar electrophysiological properties, R87C/R87C cortical organoids displayed decreased CYFIP2 protein levels and, by day 30, showed increased size alongside an absence of SOX2 + cells, suggesting premature depletion of the progenitor pool. These findings highlight a marked divergence between 2D and 3D models, with organoids revealing neurodevelopmental abnormalities not evident in monolayer cultures. Together, our results suggest that the CYFIP2 R87C variant impacts NPC cytoskeletal dynamics and early cortical development, warranting further investigation into its role in epileptic encephalopathy.

Defects in RNA metabolism are a defining feature of neurodevelopmental disease, yet how RNA decay pathways contribute to human brain development remains poorly understood. Mutations in ubiquitously expressed RNA surveillance factors often cause highly tissue-selective disease, highlighting a central paradox in human biology. The RNA exosome is a conserved ribonuclease complex traditionally viewed as a housekeeping machine for RNA turnover, yet recessive mutations in genes encoding structural subunits of the complex disproportionately cause neurological disease, suggesting an instructive role in nervous system development. Here, we show that the RNA exosome regulates the temporal progression of gene expression programs during human cerebellar differentiation. Using CRISPR-engineered human cerebellar organoids modeling EXOSC3 variants, we find that RNA exosome dysfunction does not broadly alter transcript abundance, but instead disrupts transitions between developmental states. Mutant organoids exhibit incomplete and mis-timed resolution of early transcriptional programs, altered lineage specificity, and impaired coordination of maturation-associated gene expression programs, with pronounced effects in neuronal lineages, particularly Purkinje cells and rhombic lip-derivatives. These defects are accompanied by disorganized laminar architecture and reduced coordination of neuronal activity, despite preserved intrinsic excitability. Together, our findings establish RNA surveillance as a key regulator of developmental timing, lineage fidelity, and neurodevelopmental disease.

Vesicle-associated membrane protein-associated protein-B (VAPB) is an endoplasmic reticulum (ER) membrane-bound protein. The P56S mutation in VAPB causes a dominant, familial form of amyotrophic lateral sclerosis (ALS). However, the mechanism by which this mutation leads to motor neuron (MN) degeneration remains unclear. Utilizing inducible pluripotent stem cell (iPSC)-derived MNs expressing either wild-type (WT) or P56S VAPB, we demonstrate that the mutant protein reduces neuronal firing and disrupts ER-mitochondria-associated membranes (ER MAMs), with a time-dependent decline in mitochondrial membrane potential (MMP), hallmarks of MN pathology. These findings were validated in patient-derived iPSC-MNs. Additionally, VAPB P56S MNs show increased susceptibility to ER stress, elevated expression of the Integrated Stress Response (ISR) regulator ATF4 under stress, and reduced global protein synthesis. Notably, pharmacological ISR inhibition using ISRIB rescued ALS-associated phenotypes in both VAPB P56S and patient-derived iPSC-MNs. We present the first evidence that the VAPB P56S mutation activates ISR signaling via mitochondrial dysfunction in human MNs. These findings support ISR modulation as a strategy for ALS intervention and highlight the need for patient stratification in clinical trials.

Gamma-aminobutyric acid type A receptors (GABAARs) are essential for maintaining the excitation-inhibition balance in the central nervous system. Genetic variations of GABAARs result in a variety of neurological disorders, such as epilepsy. A key pathogenic mechanism involves protein misfolding and defective assembly of GABAARs in the endoplasmic reticulum (ER), resulting in impaired surface expression and loss of function. Here, we investigated three trafficking-deficient variants of the GABAAR α1 subunit (GABRA1), including D219N (ClinVar Variation ID: 127232), G251D (Variation ID: 419523), and P260L. We demonstrated that selective pharmacological activation of the IRE1/XBP1s signaling arm of the unfolded protein response using IXA62, IXA554, and IXA105 increases total and surface protein levels of all three α1 variants without affecting wild-type receptor protein levels in HEK293T cells. Patch-clamping recordings further showed that treatment with IXA62, IXA554, and IXA105 increases the peak GABA-evoked current amplitudes in HEK293T cells expressing α1(D219N) and α1(G251D). Mechanistic analyses revealed that IXA62 and IXA554 remodel the GABAAR-associated proteostasis network by promoting folding and anterograde trafficking while inhibiting degradation in HEK293T cells expressing α1(D219N) variant and human iPSC-derived neurons carrying α1(G251D) variant. These results suggest that selective IRE1/XBP1s activation pharmacologically can be further developed to provide a potential therapeutic avenue for genetic epilepsies caused by GABAAR trafficking defects.

Intellectual disability (ID) is a broad diagnostic category that encompasses individuals with impaired cognitive ability. While these disorders have heterogeneous causes, recent developments in next-generation sequencing (NGS) are revealing the prevalence of genetic etiologies. In particular, germline mutations in genes that affect RNA splicing are increasingly common causes of ID disorders. Research to elucidate the functional relationship between splicing and neurodevelopment is critical since molecular therapeutics require a nuanced understanding of the pathological mechanism. In this review, we first summarize the trends that have led to the discovery of the RNA splicing-ID relationship, then discuss recent progress and future directions for research surrounding RNA splicing in neurodevelopment. Finally, we speak on how these results may serve as the foundation for burgeoning therapies.

Integrator (INT) is a metazoan-specific complex that targets promoter-proximally paused RNA polymerase II (RNAPII) for termination, preventing immature RNAPII from entering gene bodies and functionally attenuating transcription of stress-responsive genes. Mutations in INT subunits are associated with many human diseases, including cancer, ciliopathies, and neurodevelopmental disorders, but how reduced INT activity contributes to disease is unknown. Here, we demonstrate that the loss of INT-mediated termination in human cells triggers the integrated stress response (ISR). INT depletion causes upregulation of short genes such as the ISR transcription factor activating transcription factor 3 (ATF3). Further, immature RNAPII that escapes into genes upon INT depletion is prone to premature termination, generating incomplete pre-mRNAs with retained introns. Retroelements within retained introns form double-stranded RNA (dsRNA) that is recognized by protein kinase R (PKR), which drives ATF4 activation and prolonged ISR. Critically, patient cells with INT mutations exhibit dsRNA accumulation and ISR activation, thereby implicating chronic ISR in diseases caused by INT deficiency.

The GRIN genes encoding N-methyl-D-aspartate receptor (NMDAR) subunits are remarkably intolerant to variation. Many pathogenic NMDAR variants result in their protein misfolding, inefficient assembly, reduced surface expression, and impaired function on neuronal membrane, causing neurological disorders including epilepsy and intellectual disability. Here, we investigated the proteostasis maintenance of NMDARs containing epilepsy-associated variations in the GluN2A subunit, including M705V and A727T. In the transfected HEK293T cells, we showed that the two variants were targeted to the proteasome for degradation and had reduced functional surface expression. We demonstrated that the application of BIX, a known small molecule activator of an HSP70 family chaperone BiP (binding immunoglobulin protein) in the endoplasmic reticulum (ER), dose-dependently enhanced the functional surface expression of the M705V and A727T variants in HEK293T cells. Moreover, BIX (10 μM) increased the surface protein levels of the M705V variant in human iPSC-derived neurons. We revealed that BIX promoted folding, inhibited degradation, and enhanced anterograde trafficking of the M705V variant by modest activation of the IRE1 pathway of the unfolded protein response. Our results suggest that adapting the ER proteostasis network restores the folding, trafficking, and function of pathogenic NMDAR variants, representing a potential treatment for neurological disorders resulting from NMDAR dysfunction.

THOC6 variants are the genetic basis of autosomal recessive THOC6 Intellectual Disability Syndrome (TIDS). THOC6 is critical for mammalian Transcription Export complex (TREX) tetramer formation, which is composed of four six-subunit THO monomers. The TREX tetramer facilitates mammalian RNA processing, in addition to the nuclear mRNA export functions of the TREX dimer conserved through yeast. Human and mouse TIDS model systems revealed novel THOC6-dependent, species-specific TREX tetramer functions. Germline biallelic Thoc6 loss-of-function (LOF) variants result in mouse embryonic lethality. Biallelic THOC6 LOF variants reduce the binding affinity of ALYREF to THOC5 without affecting the protein expression of TREX members, implicating impaired TREX tetramer formation. Defects in RNA nuclear export functions were not detected in biallelic THOC6 LOF human neural cells. Instead, mis-splicing was detected in human and mouse neural tissue, revealing novel THOC6-mediated TREX coordination of mRNA processing. We demonstrate that THOC6 is required for key signaling pathways known to regulate the transition from proliferative to neurogenic divisions during human corticogenesis. Together, these findings implicate altered RNA processing in the developmental biology of TIDS neuropathology.

ABSTRACT Vesicle-associated membrane protein-associated protein-B (VAPB) is an ER membrane bound protein. VAPB P56S causes a dominant, familial form of amyotrophic lateral sclerosis (ALS), however, the mechanism through which this mutation causes motor neuron (MN) disease remains unknown. Using inducible wild type (WT) and VAPB P56S expressing iPSC-derived MNs we show that VAPB P56S, but not WT, protein decreased neuronal firing and mitochondrial-ER contact (MERC) with an associated age-dependent decrease in mitochondrial membrane potential (MMP); all typical characteristics of MN-disease. We further show that VAPB P56S expressing iPSC-derived MNs have enhanced age-dependent sensitivity to ER stress. We identified elevated expression of the master regulator of the Integrated Stress Response (ISR) marker ATF4 and decreased protein synthesis in the VAPB P56S iPSC-derived MNs. Chemical inhibition of ISR with the compound, ISRIB, rescued all MN disease phenotype in VAPB P56S MNs. Thus, our results not only support ISR inhibition as a potential therapeutic target for ALS patients, but also provides evidence to pathogenesis.

Recent advances in genetic diagnosis identified variants in genes encoding GABAA receptors as causative for genetic epilepsy. Here, we selected eight disease-associated variants in the α1 subunit of GABAA receptors causing mild to severe clinical phenotypes and showed that they are loss of function, mainly by reducing the folding and surface trafficking of the α1 protein. Furthermore, we sought client protein-specific pharmacological chaperones to restore the function of pathogenic receptors. Applications of positive allosteric modulators, including Hispidulin and TP003, increase the functional surface expression of the α1 variants. Mechanism of action study demonstrated that they enhance the folding, assembly, and trafficking and reduce the degradation of GABAA variants without activating the unfolded protein response in HEK293T cells and human iPSC-derived neurons. Since these compounds cross the blood-brain barrier, such a pharmacological chaperoning strategy holds great promise to treat genetic epilepsy in a GABAA receptor-specific manner.

In biomedical research, particularly for rare diseases (RDs), there is a critical need for model organisms to unravel the mechanistic basis of diseases, perform biomarker studies and develop potential therapeutic interventions. Within Solve-RD, an EU-funded research project with the aim of solving large numbers of previously unsolved RDs, the European Rare Disease Models & Mechanisms Network (RDMM-Europe) has been established.

Childhood apraxia of speech (CAS) is a severe and rare form of speech sound disorder (SSD). CAS is typically sporadic, but may segregate in families with broader speech and language deficits. We hypothesize that genetic changes may be involved in the etiology of CAS. We conduct whole-genome sequencing in 27 families with CAS, 101 individuals in all. We identify 17 genomic regions including 19 unique copy number variants (CNVs). Three variants are shared across families, but the rest are unique; three events are de novo. In four families, siblings with milder phenotypes co-inherited the same CNVs, demonstrating variable expressivity. We independently validate eight CNVs using microarray technology and find many of these CNVs are present in children with milder forms of SSD. Bioinformatic investigation reveal four CNVs with substantial functional consequences (cytobands 2q24.3, 6p12.3-6p12.2, 11q23.2-11q23.3, and 16p11.2). These discoveries show that CNVs are a heterogeneous, but prevalent, cause of CAS. Copy number variations identified in families with childhood apraxia of speech and other speech sound disorders provide a deeper understanding of the genetic basis for naturally acquired speech.

Neuromuscular junctions (NMJs) are specialized synapses that mediate communication between motor neurons and skeletal muscles and are essential for movement. The degeneration of this system can lead to symptoms observed in neuromuscular and motor neuron diseases. Studying these synapses and their degeneration has proven challenging. Prior NMJ studies heavily relied upon the use of mouse, chick, or isolated primary human cells, which have demonstrated limited fidelity for disease modeling. To enable the study of NMJ dysfunction and model genetic diseases, we, and others, have developed methods to generate human NMJs from pluripotent stem cells (PSCs), embryonic stem cells, and induced pluripotent stem cells. However, published studies have highlighted technical limitations associated with these complex in vitro NMJ models. In this study, we developed a robust PSC-derived motor neuron and skeletal muscle co-culture method, and demonstrated its sensitivity in modeling motor neuron disease. Our method spontaneously and reproducibly forms human NMJs. We developed multiwell-multielectrode array (MEA) parameters to quantify the activity of PSC-derived skeletal muscles, as well as measured the electrophysiological activity of functional human PSC-derived NMJs. We further leveraged our method to morphologically and functionally assess NMJs from the familial amyotrophic lateral sclerosis (fALS) PSCs, C9orf72 hexanucleotide (G4C2)n repeat expansion (HRE), SOD1A5V , and TDP43G298S to define the reproducibility and sensitivity of our system. We observed a significant decrease in the numbers and activity of PSC-derived NMJs developed from the different ALS lines compared to their respective controls. Furthermore, we evaluated a therapeutic candidate undergoing clinical trials and observed a variant-dependent rescue of functionality of NMJs. Our newly developed method provides a platform for the systematic investigation of genetic causes of NMJ neurodegeneration and highlights the need for therapeutic avenues to consider patient genotype.

THOC6 is the genetic basis of autosomal recessive THOC6 Intellectual Disability Syndrome (TIDS). THOC6 facilitates the formation of the Transcription Export complex (TREX) tetramer, composed of four THO monomers. The TREX tetramer supports mammalian mRNA processing that is distinct from yeast TREX dimer functions. Human and mouse TIDS model systems allow novel THOC6-dependent TREX tetramer functions to be investigated. Biallelic loss-of-functon(LOF) THOC6 variants do not influence the expression and localization of TREX members in human cells, but our data suggests reduced binding affinity of ALYREF. Impairment of TREX nuclear export functions were not detected in cells with biallelic THOC6 LOF. Instead, mRNA mis-splicing was observed in human and mouse neural tissue, revealing novel insights into THOC6-mediated TREX coordination of mRNA processing. We demonstrate that THOC6 is required for regulation of key signaling pathways in human corticogenesis that dictate the transition from proliferative to neurogenic divisions that may inform TIDS neuropathology.

Abstract Childhood apraxia of speech (CAS) is a severe and rare form of speech sound disorder (SSD) with an estimated prevalence of 0.01-2%. CAS typically occurs in isolation (sporadic), but may segregate in families with broader speech and language deficits. We hypothesized that disruptive genetic changes may be involved in the etiology of CAS, and were resolvable by examination of whole genome sequence. We sequenced 27 families with a CAS child within the Cleveland Family Speech and Reading Study, examining 101 individuals in all. CAS subjects displayed errors on single word articulation (75%), multisyllabic real word repetition (93%), multisyllabic non-word repetition (82%), and oral motor function (82%); many also exhibited problems with fine and gross motor skills. We identified 17 genomic regions including 19 unique structural variants (SVs) present in children with CAS. Three variants were shared across families, but the rest were unique; some events were de novo. In four families, siblings with milder phenotypes co-inherited the same SVs, suggesting that some SVs display variable expressivity. In an independent sample, we replicated eight SVs using microarray technology and found that many of these SVs were present in children with milder forms of SSD. Bioinformatic examination of the deletions/duplications identified four SVs with substantial functional consequences (cytobands 2q24.3, 6p12.3-6p12.2, 11q23.2-11q23.3, and 16p11.2). Of these, the 16p11.2 deletion is the most well-established variant that causes a broad array of neurological features. These discoveries show that SVs are a heterogeneous, but prevalent cause of CAS, identifiable by standard genetic testing.

Tight regulation of mRNA isoform expression is essential for neuronal development, maintenance, and function; however, the repertoire of proteins that govern isoform composition and abundance remains incomplete. Here, we show that the RNA kinase CLP1 regulates mRNA isoform expression through suppression of proximal cleavage and polyadenylation. We found that human stem-cell-derived motor neurons without CLP1 or with the disease-associated CLP1 p.R140H variant had distinct patterns of RNA-polymerase-II-associated cleavage and polyadenylation complex proteins that correlated with polyadenylation site usage. These changes resulted in imbalanced mRNA isoform expression of long genes important for neuronal function that were recapitulated in vivo. Strikingly, we observed the same pattern of reduced mRNA isoform diversity in 3' end sequencing data from brain tissues of patients with neurodegenerative disease. Together, our results identify a previously uncharacterized role for CLP1 in mRNA 3' end formation and reveal an mRNA misprocessing signature in neurodegeneration that may suggest a common mechanism of disease.

Pancreatic β-cells are prone to endoplasmic reticulum (ER) stress due to their role in insulin secretion. They require sustainable and efficient adaptive stress responses to cope with this stress. Whether episodes of chronic stress directly compromise β-cell identity is unknown. We show here under reversible, chronic stress conditions β-cells undergo transcriptional and translational reprogramming associated with impaired expression of regulators of β-cell function and identity. Upon recovery from stress, β-cells regain their identity and function, indicating a high degree of adaptive plasticity. Remarkably, while β-cells show resilience to episodic ER stress, when episodes exceed a threshold, β-cell identity is gradually lost. Single cell RNA-sequencing analysis of islets from type 1 diabetes patients indicates severe deregulation of the chronic stress-adaptation program and reveals novel biomarkers of diabetes progression. Our results suggest β-cell adaptive exhaustion contributes to diabetes pathogenesis.

Balanced mRNA isoform diversity and abundance are spatially and temporally regulated throughout cellular differentiation. The proportion of expressed isoforms contributes to cell type specification and determines key properties of the differentiated cells. Neurons are unique cell types with intricate developmental programs, characteristic cellular morphologies, and electrophysiological potential. Neuron-specific gene expression programs establish these distinctive cellular characteristics and drive diversity among neuronal subtypes. Genes with neuron-specific alternative processing are enriched in key neuronal functions, including synaptic proteins, adhesion molecules, and scaffold proteins. Despite the similarity of neuronal gene expression programs, each neuronal subclass can be distinguished by unique alternative mRNA processing events. Alternative processing of developmentally important transcripts alters coding and regulatory information, including interaction domains, transcript stability, subcellular localization, and targeting by RNA binding proteins. Fine-tuning of mRNA processing is essential for neuronal activity and maintenance. Thus, the focus of neuronal RNA biology research is to dissect the transcriptomic mechanisms that underlie neuronal homeostasis, and consequently, predispose neuronal subtypes to disease. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA in Disease and Development > RNA in Development.

BackgroundIntellectual disability syndromes (IDSs) with or without developmental delays affect up to 3% of the world population. We sought to clinically and genetically characterise a novel IDS segregating in five unrelated consanguineous families.MethodsClinical analyses were performed for eight patients with intellectual disability (ID). Whole-exome sequencing for selected participants followed by Sanger sequencing for all available family members was completed. Identity-by-descent (IBD) mapping was carried out for patients in two Egyptian families harbouring an identical variant. RNA was extracted from blood cells of Turkish participants, followed by cDNA synthesis and real-time PCR forTTC5.ResultsPhenotype comparisons of patients revealed shared clinical features of moderate-to-severe ID, corpus callosum agenesis, mild ventriculomegaly, simplified gyral pattern, cerebral atrophy, delayed motor and verbal milestones and hypotonia, presenting with an IDS. Four novel homozygous variants inTTC5: c.629A>G;p.(Tyr210Cys), c.692C>T;p.(Ala231Val), c.787C>T;p.(Arg263Ter) and c.1883C>T;p.(Arg395Ter) were identified in the eight patients from participating families. IBD mapping revealed that c.787C>T;p.(Arg263Ter) is a founder variant in Egypt. Missense variants c.629A>G;p.(Tyr210Cys) and c.692C>T;p.(Ala231Val) disrupt highly conserved residues of TTC5 within the fifth and sixth tetratricopeptide repeat motifs which are required for p300 interaction, while the nonsense variants are predicted to decreaseTTC5expression. Functional analysis of variant c.1883C>T;p.(Arg395Ter) showed reducedTTC5transcript levels in accordance with nonsense-mediated decay.ConclusionCombining our clinical and molecular data with a recent case report, we identify the core and variable clinical features associated withTTC5loss-of-function variants and reveal the requirement for TTC5 in human brain development and health.