Neurodevelopmental disorders Cell Models for Research

Neurodevelopmental disorders (NDDs) encompass a group of conditions with onset in the developmental period, including intellectual disability, autism spectrum disorder (ASD), and attention-deficit/hyperactivity disorder (ADHD). According to the World Health Organization (WHO), approximately 1 in 6 children globally has a developmental disability, with ASD affecting about 1 in 100 children worldwide. The exact prevalence varies by region and diagnostic criteria. NDDs often lead to lifelong impairments in cognition, communication, and behavior, imposing significant burdens on individuals, families, and healthcare systems. Early diagnosis and intervention are critical, but many NDDs lack effective treatments, highlighting the need for better models to understand pathophysiology and develop therapies.

NDDs are ideal for mechanistic studies due to their strong genetic component and the availability of well-characterized patient cohorts and public datasets. Many NDDs are monogenic, such as Fragile X syndrome (FMR1), Rett syndrome (MECP2), and Dravet syndrome (SCN1A), providing clear targets for gene editing. Additionally, induced pluripotent stem cell (iPSC) technology allows patient-derived neurons to be generated, but these models often have high variability. Gene-edited cell lines, such as CRISPR knockouts and knock-ins in neural cell lines or iPSCs, offer isogenic controls that reduce variability and enable precise dissection of gene function. Open questions include the role of specific genetic variants in neuronal signaling, synaptic plasticity, and network activity, which can be addressed using engineered models.

Several key pathways are implicated in NDDs:

• • Synaptic signaling: Many NDD genes encode proteins involved in synaptic transmission, such as neuroligins, neurexins, and glutamate receptors. Disruptions affect excitatory/inhibitory balance.
• • Transcriptional regulation: Genes like MECP2 and FMR1 regulate gene expression. Loss of function leads to widespread transcriptional changes.
• • mTOR signaling: Mutations in TSC1/TSC2 cause tuberous sclerosis, leading to hyperactivation of mTOR and altered neuronal growth.
• • Ion channel function: Mutations in SCN1A, SCN2A, and KCNQ2 affect neuronal excitability, leading to epilepsy and cognitive deficits.
• • Wnt and Notch pathways: These are critical for neurodevelopment; dysregulation can cause structural brain abnormalities.

Data from ClinVar and NCBI Gene.

Key signaling networks in NDDs include:

• • mTOR pathway: Hyperactivation leads to abnormal protein synthesis and synaptic dysfunction. Key nodes: PI3K, AKT, mTORC1, S6K.
• • MAPK/ERK pathway: Involved in synaptic plasticity and learning. Mutations in RAS-MAPK components cause RASopathies with NDD features.
• • Wnt/β-catenin: Regulates neural stem cell proliferation and differentiation. Dysregulation contributes to autism and schizophrenia.
• • GABAergic/glutamatergic balance: Imbalance in excitatory/inhibitory transmission is a common theme. Genes like GABRB3 and GRIN2B are implicated.
• • Synaptic scaffolding: Proteins like SHANK3 and DLG4 organize postsynaptic density; mutations disrupt synaptic signaling.

Common cell lines used in NDD research:

Organoids, particularly brain organoids, recapitulate early neurodevelopment and are valuable for studying NDDs. They can be derived from iPSCs and genetically edited to introduce or correct mutations.

Animal models for NDDs include:

• • Genetically engineered mouse models (GEMMs): Knockout or knock-in mice for genes like FMR1, MECP2, and SCN1A. These recapitulate key phenotypes.
• • Induced models: Chemical or viral-induced models, e.g., valproic acid exposure to induce autism-like features.
• • Patient-derived xenograft (PDX) models: Less common for NDDs but used in cancer; for NDDs, chimeric mouse models with human neurons are emerging.
• • Non-human primate models: CRISPR-edited monkeys with MECP2 mutations have been generated, offering closer recapitulation of human disease.

CRISPR-based gene editing has revolutionized NDD research by enabling the creation of isogenic cell lines. These models have precise genetic modifications in a controlled background, reducing variability and allowing functional studies.

Examples of gene-edited models:

• • FMR1 knockout SH-SY5Y cells: To study Fragile X syndrome mechanisms.
• • MECP2 R306C knock-in iPSC-derived neurons: To model Rett syndrome.
• • SCN1A haploinsufficient neurons: To study Dravet syndrome.
• • TSC2 knockout neural stem cells: To investigate tuberous sclerosis.

Commercially available, sequence-verified gene-edited cell lines are available from various vendors, providing ready-to-use models for drug screening and mechanistic studies. These models accelerate research by eliminating the need for time-consuming gene editing and validation.

Gene-edited cell lines are essential for functional genomics studies. For example, knocking out a candidate gene in a neural cell line and assessing neuronal morphology, synaptic activity, or gene expression can validate its role in NDD. Conversely, introducing a patient-specific mutation into a wild-type background can determine causality. CRISPR screens using pooled libraries can identify genes that modify disease phenotypes, providing new therapeutic targets.

Isogenic pairs (wild-type vs. mutant) are ideal for high-throughput drug screening. For example, screening compounds that rescue synaptic deficits in FMR1 knockout neurons can identify potential treatments for Fragile X. Similarly, drug resistance can be modeled by exposing mutant cells to increasing concentrations of a drug and selecting resistant clones, which can then be analyzed for secondary mutations.

CRISPR-based synthetic lethality screens can identify genes that, when knocked out, are lethal only in the context of a specific NDD mutation. This approach can reveal vulnerabilities that can be targeted therapeutically. Additionally, gene-edited cells can be used to identify biomarkers by comparing protein or RNA expression between mutant and wild-type cells, leading to potential diagnostic or prognostic markers.