Genetic dissection identifies Necdin as a driver gene in a mouse model of paternal 15q duplications

Investigation of a mouse model of Prader-Willi Syndrome with combined disruption of Necdin and Magel2

Prader-Willi syndrome (PWS) is a multigenic disorder caused by the loss of 7 contiguous paternally expressed genes. Mouse models with inactivation of all PWS genes are lethal. KO mouse models for each candidate gene have been generated, but they lack the functional interactions between PWS genes. Here, we revealed an interplay between Necdin and Magel2 PWS genes and generated a mouse model (named Del Ndn-Magel2 mice) with a deletion including both genes. A subset of Del Ndn-Magel2 mice showed neonatal lethality. Behaviorally, surviving mutant mice exhibited sensory delays during infancy and alterations in social exploration at adulthood. Del Ndn-Magel2 mice had a lower body weight before weaning, persisting after weaning in males only, with reduced fat mass and improved glucose tolerance as well as altered puberty. Adult mutant mice displayed increased ventilation and a persistent increase in apneas following a hypercapnic challenge. Transcriptomics analyses revealed a dysregulation of key circadian genes and alterations of genes associated with axonal function similar to patients with PWS. At neuroanatomical levels, Del Ndn-Magel2 mice had an impaired maturation of oxytocin neurons and a disrupted development of melanocortin circuits. Together, these data indicate that the Del Ndn-Magel2 mouse is a pertinent and genetically relevant model of PWS.

Neurodevelopmental impact of CNV models in ASD: Recent advances and future directions

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by social communication impairments and restricted, repetitive behaviors. ASD exhibits a strong genetic basis, with rare and common genetic variants contributing to its etiology. Copy number variations (CNVs), deletions or duplications of chromosomal segments, have emerged as key contributors to ASD risk. Rare CNVs often demonstrate large effect sizes and can directly cause ASD, while common variants collectively exert subtle influences. Recent advances have identified numerous ASD-associated CNVs, including recurrent loci such as 1q21.1, 2p16.3, 7q11.23, 15q11.2, 15q11-q13, 16p11.2 and 22q11.2. Mouse models carrying these CNVs have provided profound insights into the underlying neurobiological mechanisms. Recent studies integrating transcriptomic, proteomic, and functional imaging approaches have revealed alterations in synaptic function, neuronal differentiation, myelination, metabolic pathways, and circuit connectivity. Notably, investigations leveraging conditional knockout models, high magnetic field MRI, and single-cell analyses highlight disruptions in excitatory-inhibitory balance, white matter integrity, and dynamic gene regulatory networks. Parallel human-based approaches, including iPSC-derived neurons, cerebral organoids, and large-scale single-nucleus sequencing, are combined with animal model data. These integrative strategies promise to refine our understanding of ASD's genetic architecture, bridging the gap between fundamental discoveries in model organisms and clinically relevant biomarkers, subtypes, and therapeutic targets in humans. This review summarizes key findings from recent CNV mouse model studies and highlights emerging technologies applied to human ASD samples. Finally, we outline prospects for translating findings from mouse studies to humans. By illuminating both unique and convergent genetic mechanisms, these advances offer a critical framework for unraveling etiological complexity in ASD.

Ash1l loss-of-function results in structural birth defects and altered cortical development

The histone methyltransferase ASH1L plays a crucial role in regulating gene expression across various organ systems during development, yet its role in brain development remains largely unexplored. Over 130 individuals with autism harbour heterozygous loss-of-function ASH1L variants, and population studies confirm it as a high-risk autism gene. Previous studies on Ash1l deficient mice have reported autistic-like behaviours and provided insights into the underlying neuropathophysiology. In this study, we used mice with a cre-inducible deletion of Ash1l exon 4, which results in a frame shift and premature stop codon (p.V1693Afs*2). Our investigation evaluated the impact of Ash1l loss-of-function on survival and craniofacial skeletal development. Using a tamoxifen-inducible cre strain, we targeted Ash1l knockout early in cortical development [Emx1-Cre-ERT2; embryonic Day (e) 10.5]. Immunohistochemistry was utilized to assess cortical lamination, while EdU incorporation aided in birthdating cortical neurons. Additionally, single-cell RNA sequencing was employed to compare cortical cell populations and identify genes with differential expression. At e18.5, the proportion of homozygous Ash1l germline knockout embryos appeared normal; however, no live Ash1l null pups were present at birth (e18.5: n = 77, P = 0.90; p0: n = 41, P = 0.00095). Notably, Ash1l-/- exhibited shortened nasal bones (n = 31, P = 0.017). In the cortical-specific knockout model, SATB2 neurons showed increased numbers (n = 6/genotype, P = 0.0001) and were distributed through the cortical plate. Birthdating revealed generation of ectopically placed deep layer neurons that express SATB2 (e13.5 injection: n = 4/genotype, P = 0.0126). Single cell RNA sequencing revealed significant differences in gene expression between control and mutant upper layer neurons, leading to distinct clustering. Pseudotime analysis indicated that the mutant cluster followed an altered cell differentiation trajectory. This study underscores the essential role of Ash1l in postnatal survival and normal craniofacial development. In the cortex, ASH1L exerts broad effects on gene expression and is indispensable for determining the fate of upper layer cortical neurons. These findings provide valuable insights into the potential mechanisms of ASH1L neuropathology, shedding light on its significance in neurodevelopmental disorders like autism.

Correlations of brain structure with the social behavior of 15q11-13 duplication mice, an animal model of autism

Duplication of chromosome 15q11-13 has been reported to be one of the most frequent cytogenetic copy number variations in autism spectrum disorder (ASD), and a mouse model of paternal 15q11-13 duplication was generated, termed 15q dup mice. While previous studies have replicated some of the behavioral and brain structural phenotypes of ASD separately, the relationship between brain structure and behavior has rarely been examined. In this study, we performed behavioral experiments related to anxiety and social behaviors and magnetic resonance imaging (MRI) using the same set of 15q dup and wild-type mice. 15q dup mice showed increased anxiety and a tendency toward alterations in social behaviors, as reported previously, as well as variability in terms of sociability. MRI analysis revealed that a lower sociability index was correlated with a smaller gray matter volume in the right medial entorhinal cortex. These results may help to understand how variability in behavioral phenotypes of ASD arises even in individuals with the same genetic background and to determine the individual differences in neurodevelopmental trajectory correlated with specific brain structures that underlie these phenotypes.

Mechanisms of copy number variants in neuropsychiatric disorders: From genes to therapeutics

Copy number variants (CNVs) are genomic imbalances strongly linked to the aetiology of neuropsychiatric disorders such as schizophrenia and autism. By virtue of their large size, CNVs often contain many genes, providing a multi-genic view of disease processes that can be dissected in model systems. Thus, CNV research provides an important stepping stone towards understanding polygenic disease mechanisms, positioned between monogenic and polygenic risk models. In this review, we will outline hypothetical models for gene interactions occurring within CNVs and discuss different approaches used to study rodent and stem cell disease models. We highlight recent work showing that genetic and pharmacological strategies can be used to rescue important aspects of CNV-mediated pathophysiology, which often converges onto synaptic pathways. We propose that using a rescue approach in complete CNV models provides a new path forward for precise mechanistic understanding of complex disorders and a tangible route towards therapeutic development.

CNTNAP2 intracellular domain (CICD) generated by γ-secretase cleavage improves autism-related behaviors

As the most prevalent neurodevelopmental disorders in children, autism spectrum disorders (ASD) are characterized by deficits in language development, social interaction, and repetitive behaviors or inflexible interests. Contactin associated protein like 2 (CNTNAP2), encoding a single transmembrane protein (CNTNAP2) with 1331 amino acid residues, is a widely validated ASD-susceptible gene. Cntnap2-deficient mice also show core autism-relevant behaviors, including the social deficits and repetitive behavior. However, the cellular mechanisms underlying dysfunction CNTNAP2 and ASD remain elusive. In this study, we found a motif within the transmembrane domain of CNTNAP2 was highly homologous to the γ-secretase cleavage site of amyloid-β precursor protein (APP), suggesting that CNTNAP2 may undergo proteolytic cleavage. Further biochemical analysis indicated that CNTNAP2 is cleaved by γ-secretase to produce the CNTNAP2 intracellular domain (CICD). Virally delivery of CICD to the medial prefrontal cortex (mPFC) in Cntnap2-deficient (Cntnap2^-/-) mice normalized the deficit in the ASD-related behaviors, including social deficit and repetitive behaviors. Furthermore, CICD promoted the nuclear translocation of calcium/calmodulin-dependent serine protein kinase (CASK) to regulate the transcription of genes, such as Prader Willi syndrome gene Necdin. Whereas Necdin deficiency led to reduced social interaction in mice, virally expression of Necdin in the mPFC normalized the deficit in social preference of Cntnap2^-/- mice. Our results thus reveal a critical function of CICD and highlight a role of the CNTNAP2-CASK-Necdin signaling pathway in ASD.

Virtual reality-based real-time imaging reveals abnormal cortical dynamics during behavioral transitions in a mouse model of autism

Functional connectivity (FC) can provide insight into cortical circuit dysfunction in neuropsychiatric disorders. However, dynamic changes in FC related to locomotion with sensory feedback remain to be elucidated. To investigate FC dynamics in locomoting mice, we develop mesoscopic Ca2+ imaging with a virtual reality (VR) environment. We find rapid reorganization of cortical FC in response to changing behavioral states. By using machine learning classification, behavioral states are accurately decoded. We then use our VR-based imaging system to study cortical FC in a mouse model of autism and find that locomotion states are associated with altered FC dynamics. Furthermore, we identify FC patterns involving the motor area as the most distinguishing features of the autism mice from wild-type mice during behavioral transitions, which might correlate with motor clumsiness in individuals with autism. Our VR-based real-time imaging system provides crucial information to understand FC dynamics linked to behavioral abnormality of neuropsychiatric disorders.

The role of UBE3A in the autism and epilepsy-related Dup15q syndrome using patient-derived, CRISPR-corrected neurons

Chromosome 15q11-q13 duplication syndrome (Dup15q) is a neurodevelopmental disorder caused by maternal duplications of this region. Autism and epilepsy are key features of Dup15q. UBE3A, which encodes an E3 ubiquitin ligase, is likely a major driver of Dup15q because UBE3A is the only imprinted gene expressed solely from the maternal allele. Nevertheless, the exact role of UBE3A has not been determined. To establish whether UBE3A overexpression is required for Dup15q neuronal deficits, we generated an isogenic control line for a Dup15q patient-derived induced pluripotent stem cell line. Dup15q neurons exhibited hyperexcitability compared with control neurons, and this phenotype was generally prevented by normalizing UBE3A levels using antisense oligonucleotides. Overexpression of UBE3A resulted in a profile similar to that of Dup15q neurons except for synaptic phenotypes. These results indicate that UBE3A overexpression is necessary for most Dup15q cellular phenotypes but also suggest a role for other genes in the duplicated region.

Molecular and behavioral consequences of Ube3a gene overdosage in mice

Chromosome 15q11.2-q13.1 duplication syndrome (Dup15q syndrome) is a severe neurodevelopmental disorder characterized by intellectual disability, impaired motor coordination, and autism spectrum disorder. Chromosomal multiplication of the UBE3A gene is presumed to be the primary driver of Dup15q pathophysiology, given that UBE3A exhibits maternal monoallelic expression in neurons and that maternal duplications typically yield far more severe neurodevelopmental outcomes than paternal duplications. However, studies into the pathogenic effects of UBE3A overexpression in mice have yielded conflicting results. Here, we investigated the neurodevelopmental impact of Ube3a gene overdosage using bacterial artificial chromosome-based transgenic mouse models (Ube3aOE) that recapitulate the increases in Ube3a copy number most often observed in Dup15q. In contrast to previously published Ube3a overexpression models, Ube3aOE mice were indistinguishable from wild-type controls on a number of molecular and behavioral measures, despite suffering increased mortality when challenged with seizures, a phenotype reminiscent of sudden unexpected death in epilepsy. Collectively, our data support a model wherein pathogenic synergy between UBE3A and other overexpressed 15q11.2-q13.1 genes is required for full penetrance of Dup15q syndrome phenotypes.

Small Extracellular Vesicles in Milk Cross the Blood-Brain Barrier in Murine Cerebral Cortex Endothelial Cells and Promote Dendritic Complexity in the Hippocampus and Brain Function in C57BL/6J Mice

Human milk contains large amounts of small extracellular vesicles (sEVs) and their microRNA cargos, whereas infant formulas contain only trace amounts of sEVs and microRNAs. We assessed the transport of sEVs across the blood-brain barrier (BBB) and sEV accumulation in distinct regions of the brain in brain endothelial cells and suckling mice. We further assessed sEV-dependent gene expression profiles and effects on the dendritic complexity of hippocampal granule cells and phenotypes of EV depletion in neonate, juvenile and adult mice. The transfer of sEVs across the BBB was assessed by using fluorophore-labeled bovine sEVs in brain endothelial bEnd.3 monolayers and dual chamber systems, and in wild-type newborn pups fostered to sEV and cargo tracking (ECT) dams that express sEVs labeled with a CD63-eGFP fusion protein for subsequent analysis by serial two-photon tomography and staining with anti-eGFP antibodies. Effects of EVs on gene expression and dendritic architecture of granule cells was analyzed in hippocampi from juvenile mice fed sEV and RNA-depleted (ERD) and sEV and RNA-sufficient (ERS) diets by using RNA-sequencing analysis and Golgi-Cox staining followed by integrated neuronal tracing and morphological analysis of neuronal dendrites, respectively. Spatial learning and severity of kainic acid-induced seizures were assessed in mice fed ERD and ERS diets. bEnd.3 cells internalized sEVs by using a saturable transport mechanism and secreted miR-34a across the basal membrane. sEVs penetrated the entire brain in fostering experiments; major regions of accumulation included the hippocampus, cortex and cerebellum. Two hundred ninety-five genes were differentially expressed in hippocampi from mice fed ERD and ERS diets; high-confidence gene networks included pathways implicated in axon guidance and calcium signaling. Juvenile pups fed the ERD diet had reduced dendritic complexity of dentate granule cells in the hippocampus, scored nine-fold lower in the Barnes maze test of spatial learning and memory, and the severity of seizures was 5-fold higher following kainic acid administration in adult mice fed the ERD diet compared to mice fed the ERS diet. We conclude that sEVs cross the BBB and contribute toward optimal neuronal development, spatial learning and memory, and resistance to kainic acid-induced seizures in mice.