Allelic contribution of Nrxn1α to autism-relevant behavioral phenotypes in mice

Clinical phenotypic spectrum of NRXN1 microdeletions and their association with epilepsy: A systematic review and meta-analysis

OBJECTIVE

NRXN1 microdeletions are associated with an increased genetic risk for various neuropsychiatric disorders, with diverse breakpoints complicating research, diagnosis, and treatment. This study aims to investigate the deletion rate and penetrance of NRXN1 microdeletions across different clinical phenotypes through meta-analysis while exploring their relationship with epilepsy and summarizing the characteristics of NRXN1 biallelic variations.

METHODS

For meta-analysis, a systematic review of published studies was conducted to calculate NRXN1 microdeletion rates and penetrance across different disorders, with comparisons to control groups. For systematic review, data from 401 cases across 57 studies were analyzed to compare microdeletion characteristics in patients with and without epilepsy, alongside a review of NRXN1 biallelic variation clinical features.

RESULTS

NRXN1 microdeletion carriers had a 3.20-fold higher disease risk compared to noncarriers. The deletion rate was elevated in patients with autism, schizophrenia, and Tourette syndrome relative to controls. Additionally, NRXN1 microdeletions were more prevalent in epilepsy patients with comorbidities than in those with epilepsy alone. Among epilepsy patients, 81.3% had comorbidities. Deletions involving exons 1-6 were more frequent in patients with epilepsy, of whom 71.42% were diagnosed with genetic generalized epilepsy (GGE). Among those with NRXN1 biallelic variations, 53.84% had epilepsy, and all experienced generalized seizures.

SIGNIFICANCE

Understanding genotype-phenotype associations in NRXN1 microdeletion-related diseases is critical for early diagnosis and management. Our study shows that NRXN1 microdeletions have been associated with various neuropsychiatric disorders and exhibit incomplete penetrance. In epilepsy, patients with NRXN1 microdeletions are associated with mental comorbidities and generalized seizure types, particularly involving exon 1-6 deletions, and are common in patients with GGE.

Targeted splicing approach for alleviation of a neurexin 1 haploinsufficiency model

NRXN1 encoding the synaptic organizing protein neurexin 1 (Nrxn1) is among the strongest risk genes for autism spectrum disorders as well as other neuropsychiatric disorders. The most common contributing mutation is a deletion in one allele. While mice lacking one form of the protein, Nrxn1α, have been characterized, information is lacking on animal models with heterozygous deletion of all isoforms, as well as on therapeutic approaches directly targeting Nrxn1. We report that Nrxn1+/- mice with a deletion affecting all isoforms, α, β and γ, show deficits in excitatory synaptic transmission affecting presynaptic and postsynaptic properties at hippocampal CA3-CA1 synapses, and show increased repetitive behaviors. Based on previous studies indicating that exclusion of the insert at Nrxn1 splice site 5 (S5) boosts synaptic transmission, we tested S5 exclusion as a therapeutic approach. Genetic exclusion of S5 in the remaining Nrxn1 allele alleviated the deficits, restoring miniature excitatory postsynaptic current frequency, paired pulse ratio, AMPA/NMDA ratio, and repetitive behaviors to wild type levels and partially restoring Nrxn1 protein level in Nrxn1ΔS5/- compared to Nrxn1+/- mice. These data suggest that S5 exclusion may be a beneficial therapeutic direction in cases of neuropsychiatric disorders involving NRXN1.

Single-cell technology grows up: Leveraging high-resolution omics approaches to understand neurodevelopmental disorders

The identification of hundreds of neurodevelopmental disorder (NDD) genes in the last decade led to numerous genetic models for understanding NDD gene mutation consequences and delineating putative neurobiological mediators of disease. In parallel, single-cell and single-nucleus genomic technologies have been developed and implemented to create high-resolution atlases of cell composition, gene expression, and circuit connectivity in the brain. Here, we discuss the opportunities to leverage mutant models (or human tissue, where available) and genomics approaches to systematically define NDD etiology at cellular resolution. We review progress in applying single-cell and spatial transcriptomics to interrogate developmental trajectories, cellular composition, circuit activity, and connectivity across human tissue and NDD models. We discuss considerations for implementing these approaches at scale to maximize insights and facilitate reproducibility. Finally, we highlight how standardized application of these technologies promises to not only define etiologies of individual disorders but also identify molecular, cellular, and circuit level convergence across NDDs.

Independent genetic strategies define the scope and limits of CDKL5 deficiency disorder reversal

Cyclin-dependent kinase-like 5 (CDKL5) deficiency disorder (CDD) is a neurodevelopmental syndrome caused by mutations in the X-linked CDKL5 gene. The early onset of CDD suggests that CDKL5 is essential during development, but post-developmental re-expression rescues multiple CDD-related phenotypes in hemizygous male mice. Since most patients are heterozygous females, studies in clinically relevant female models are essential. Here, we systematically compare phenotype reversal across age and sex using two independent mouse models of CDD. We find that early re-activation of endogenous Cdkl5 in heterozygous females reverses most phenotypes, except working memory. Later re-expression improves several traits but has limited effects on cognitive function. Seizure prevention is more effective with early intervention in heterozygous females but becomes limited after seizure onset. These findings demonstrate the robust potential of CDKL5 re-expression to reverse CDD-related phenotypes in both sexes while underscoring the critical impact of age and disease stage in designing clinical trials.

Decoding the genetic landscape of autism: A comprehensive review

BACKGROUND

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition characterized by heterogeneous symptoms and genetic underpinnings. Recent advancements in genetic and epigenetic research have provided insights into the intricate mechanisms contributing to ASD, influencing both diagnosis and therapeutic strategies.

AIM

To explore the genetic architecture of ASD, elucidate mechanistic insights into genetic mutations, and examine gene-environment interactions.

METHODS

A comprehensive systematic review was conducted, integrating findings from studies on genetic variations, epigenetic mechanisms (such as DNA methylation and histone modifications), and emerging technologies [including Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-Cas9 and single-cell RNA sequencing]. Relevant articles were identified through systematic searches of databases such as PubMed and Google Scholar.

RESULTS

Genetic studies have identified numerous risk genes and mutations associated with ASD, yet many cases remain unexplained by known factors, suggesting undiscovered genetic components. Mechanistic insights into how these genetic mutations impact neural development and brain connectivity are still evolving. Epigenetic modifications, particularly DNA methylation and non-coding RNAs, also play significant roles in ASD pathogenesis. Emerging technologies like CRISPR-Cas9 and advanced bioinformatics are advancing our understanding by enabling precise genetic editing and analysis of complex genomic data.

CONCLUSION

Continued research into the genetic and epigenetic underpinnings of ASD is crucial for developing personalized and effective treatments. Collaborative efforts integrating multidisciplinary expertise and international collaborations are essential to address the complexity of ASD and translate genetic discoveries into clinical practice. Addressing unresolved questions and ethical considerations surrounding genetic research will pave the way for improved diagnostic tools and targeted therapies, ultimately enhancing outcomes for individuals affected by ASD.

Rare heterozygous genetic variants of NRXN and NLGN gene families involved in synaptic function and their association with neurodevelopmental disorders

The interaction of neurexins (NRXNs) in the presynaptic membrane with postsynaptic cell adhesion molecules called neuroligins (NLGNs) is critical for this synaptic function. Impaired synaptic functions are emphasized in neurodevelopmental disorders to uncover etiological factors. We evaluated variants in NRXN and NLGN genes encoding molecules located directly at the synapse in patients with neuropsychiatric disorders using clinical exome sequencing and chromosomal microarray. We presented detailed clinical findings of cases carrying heterozygous NRXN1 (c.190C > T, c.1679C > T and two copy number variations [CNVs]), NRXN2 (c.808dup, c.1901G > T), NRXN3 (c.3889C > T), and NLGN1 (c.269C > G, c.473T > A) gene variants. In addition, three novel variants were identified in the NRXN1 (c.1679C > T), NRXN3 [c.3889C > T (p.Pro1297Ser)], and NLGN1 [c.473T > A (p.Ile158Lys)] genes. We emphasize the clinical findings of CNVs of the NRXN1 gene causing a more severe clinical presentation than single nucleotide variants of the NRXN1 gene in this study. We detected an NRXN2 gene variant (c.808dup) with low allelic frequency in two unrelated cases with the same diagnosis. We emphasize the importance of this variant for future studies. We suggest that NRXN2, NRXN3, and NLGN1 genes, which are less frequently reported than NRXN1 gene variants, may also be associated with neurodevelopmental disorders.

Structural neuroplasticity after sleep loss modifies behavior and requires neurexin and neuroligin

Structural neuroplasticity (changes in the size, strength, number, and targets of synaptic connections) can be modified by sleep and sleep disruption. However, the causal relationships between genetic perturbations, sleep loss, neuroplasticity, and behavior remain unclear. The C. elegans GABAergic DVB neuron undergoes structural plasticity in adult males in response to adolescent stress, which rewires synaptic connections, alters behavior, and is dependent on conserved autism-associated genes NRXN1/nrx-1 and NLGN3/nlg-1. We find that four methods of sleep deprivation transiently induce DVB neurite extension in day 1 adults and increase the time to spicule protraction, which is the functional and behavioral output of the DVB neuron. Loss of nrx-1 and nlg-1 prevent DVB structural plasticity and behavioral changes at day 1 caused by adolescent sleep loss. Therefore, nrx-1 and nlg-1 mediate the morphologic and behavioral consequences of sleep loss, providing insight into the relationship between sleep, neuroplasticity, behavior, and neurologic disease.

Neurexin dysfunction in neurodevelopmental and neuropsychiatric disorders: a PRIMSA-based systematic review through iPSC and animal models

Background

Neurexins, essential synaptic proteins, are linked to neurodevelopmental and neuropsychiatric disorders like autism spectrum disorder (ASD) and schizophrenia.

Objective

Through this systematic review, we aimed to shed light on the relationship between neurexin dysfunction and its implications in neurodevelopmental and neuropsychiatric manifestations. Both animal and human-induced pluripotent stem cell (hiPSC) models served as our primary investigative platforms.

Methods

Utilizing the PRISMA 2020 guidelines, our search strategy involved scouring articles from the PubMed and Google Scholar databases covering a span of two decades (2003-2023). Of the initial collection, 27 rigorously evaluated studies formed the essence of our review.

Results

Our review suggested the significant ties between neurexin anomalies and neurodevelopmental and neuropsychiatric outcomes, most notably ASD. Rodent-based investigations delineated pronounced ASD-associated behaviors, and hiPSC models derived from ASD-diagnosed patients revealed the disruptions in calcium dynamics and synaptic activities. Additionally, our review underlined the integral role of specific neurexin variants, primarily NRXN1, in the pathology of schizophrenia. It was also evident from our observation that neurexin malfunctions were implicated in a broader array of these disorders, including ADHD, intellectual challenges, and seizure disorders.

Conclusion

This review accentuates the cardinal role neurexins play in the pathological process of neurodevelopmental and neuropsychiatric disorders. The findings underscore a critical need for standardized methodologies in developing animal and hiPSC models for future studies, aiming to minimize heterogeneity. Moreover, we highlight the need to expand research into less studied neurexin variants (i.e., NRXN2 and NRXN3), broadening the scope of our understanding in this field. Our observation also projects hiPSC models as potent tools for bridging research gaps, promoting translational research, and fostering the development of patient-specific therapeutic interventions.

Altered motor learning and coordination in mouse models of autism spectrum disorder

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder with increasing prevalence. Over 1,000 risk genes have now been implicated in ASD, suggesting diverse etiology. However, the diagnostic criteria for the disorder still comprise two major behavioral domains - deficits in social communication and interaction, and the presence of restricted and repetitive patterns of behavior (RRBs). The RRBs associated with ASD include both stereotyped repetitive movements and other motor manifestations including changes in gait, balance, coordination, and motor skill learning. In recent years, the striatum, the primary input center of the basal ganglia, has been implicated in these ASD-associated motor behaviors, due to the striatum's role in action selection, motor learning, and habit formation. Numerous mouse models with mutations in ASD risk genes have been developed and shown to have alterations in ASD-relevant behaviors. One commonly used assay, the accelerating rotarod, allows for assessment of both basic motor coordination and motor skill learning. In this corticostriatal-dependent task, mice walk on a rotating rod that gradually increases in speed. In the extended version of this task, mice engage striatal-dependent learning mechanisms to optimize their motor routine and stay on the rod for longer periods. This review summarizes the findings of studies examining rotarod performance across a range of ASD mouse models, and the resulting implications for the involvement of striatal circuits in ASD-related motor behaviors. While performance in this task is not uniform across mouse models, there is a cohort of models that show increased rotarod performance. A growing number of studies suggest that this increased propensity to learn a fixed motor routine may reflect a common enhancement of corticostriatal drive across a subset of mice with mutations in ASD-risk genes.

Bridging the translational gap: what can synaptopathies tell us about autism?

Multiple molecular pathways and cellular processes have been implicated in the neurobiology of autism and other neurodevelopmental conditions. There is a current focus on synaptic gene conditions, or synaptopathies, which refer to clinical conditions associated with rare genetic variants disrupting genes involved in synaptic biology. Synaptopathies are commonly associated with autism and developmental delay and may be associated with a range of other neuropsychiatric outcomes. Altered synaptic biology is suggested by both preclinical and clinical studies in autism based on evidence of differences in early brain structural development and altered glutamatergic and GABAergic neurotransmission potentially perturbing excitatory and inhibitory balance. This review focusses on the NRXN-NLGN-SHANK pathway, which is implicated in the synaptic assembly, trans-synaptic signalling, and synaptic functioning. We provide an overview of the insights from preclinical molecular studies of the pathway. Concentrating on NRXN1 deletion and SHANK3 mutations, we discuss emerging understanding of cellular processes and electrophysiology from induced pluripotent stem cells (iPSC) models derived from individuals with synaptopathies, neuroimaging and behavioural findings in animal models of Nrxn1 and Shank3 synaptic gene conditions, and key findings regarding autism features, brain and behavioural phenotypes from human clinical studies of synaptopathies. The identification of molecular-based biomarkers from preclinical models aims to advance the development of targeted therapeutic treatments. However, it remains challenging to translate preclinical animal models and iPSC studies to interpret human brain development and autism features. We discuss the existing challenges in preclinical and clinical synaptopathy research, and potential solutions to align methodologies across preclinical and clinical research. Bridging the translational gap between preclinical and clinical studies will be necessary to understand biological mechanisms, to identify targeted therapies, and ultimately to progress towards personalised approaches for complex neurodevelopmental conditions such as autism.

Alternative splicing and heparan sulfation converge on neurexin-1 to control glutamatergic transmission and autism-related behaviors

Neurexin synaptic organizing proteins are central to a genetic risk pathway in neuropsychiatric disorders. Neurexins also exemplify molecular diversity in the brain, with over a thousand alternatively spliced forms and further structural heterogeneity contributed by heparan sulfate glycan modification. Yet, interactions between these modes of post-transcriptional and post-translational modification have not been studied. We reveal that these regulatory modes converge on neurexin-1 splice site 5 (S5): the S5 insert increases the number of heparan sulfate chains. This is associated with reduced neurexin-1 protein level and reduced glutamatergic neurotransmitter release. Exclusion of neurexin-1 S5 in mice boosts neurotransmission without altering the AMPA/NMDA ratio and shifts communication and repetitive behavior away from phenotypes associated with autism spectrum disorders. Thus, neurexin-1 S5 acts as a synaptic rheostat to impact behavior through the intersection of RNA processing and glycobiology. These findings position NRXN1 S5 as a potential therapeutic target to restore function in neuropsychiatric disorders.