Autism spectrum disorder Cell Models for Research

Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by deficits in social communication and repetitive behaviors. According to the World Health Organization (WHO), approximately 1 in 100 children worldwide is diagnosed with ASD. The prevalence has increased over time, partly due to improved diagnostic criteria and awareness. ASD affects individuals across all ethnic and socioeconomic groups, with a male-to-female ratio of about 4:1. The condition imposes a significant lifelong burden on individuals, families, and healthcare systems. There is no cure, and current treatments focus on behavioral and educational interventions. The National Cancer Institute (NCI) does not track ASD, but the Centers for Disease Control and Prevention (CDC) provides surveillance data. The economic cost of ASD in the US is estimated to exceed $268 billion annually, including medical care, special education, and lost productivity. The heterogeneity of ASD, with varying genetic and environmental contributions, underscores the need for robust research models to understand its pathophysiology and develop targeted therapies.

ASD is ideal for mechanistic studies due to its strong genetic component. Hundreds of genes have been implicated, including SHANK3, NRXN1, and MECP2. The availability of patient-derived induced pluripotent stem cells (iPSCs) and gene-editing technologies allows the creation of isogenic cell models that recapitulate specific genetic mutations. These models enable researchers to study neuronal development, synaptic function, and network activity in a controlled environment. Open questions include the convergence of genetic pathways, the role of environmental factors, and the identification of biomarkers for early diagnosis and treatment response. Gene-edited cell models provide a platform to address these questions, facilitating drug discovery and personalized medicine approaches.

While ASD is not a cancer, its pathogenesis involves several key signaling pathways that are also relevant to neurodevelopment. These pathways include:

• • Synaptic signaling pathways: Genes encoding postsynaptic scaffolding proteins (e.g., SHANK3) and presynaptic neurexins (e.g., NRXN1) are critical for synapse formation and function.
• • mTOR signaling: Dysregulation of the mTOR pathway, often due to mutations in TSC1/TSC2, leads to abnormal protein synthesis and synaptic plasticity.
• • Wnt signaling: Altered Wnt signaling affects neuronal migration and differentiation.
• • Fragile X mental retardation protein (FMRP) pathway: FMRP regulates translation of many synaptic proteins; its loss leads to Fragile X syndrome, a common monogenic cause of ASD.

These pathways are interconnected and contribute to the excitatory/inhibitory imbalance observed in ASD.

Data from ClinVar and large-scale sequencing studies. Frequencies represent approximate contributions to ASD cases.

Key signaling networks implicated in ASD include:

• • Synaptic signaling: Genes such as SHANK3, NRXN1, and NLGN3/4 are involved in synapse formation and maintenance. Disruptions lead to altered synaptic transmission.
• • mTOR pathway: Overactivation of mTOR due to TSC mutations leads to excessive protein synthesis and synaptic dysfunction.
• • Wnt/β-catenin pathway: Mutations in CTNNB1 and other Wnt components affect neuronal migration and cortical patterning.
• • MAPK/ERK pathway: Altered signaling affects neuronal differentiation and plasticity.
• • Calcium signaling: Mutations in CACNA1C and other calcium channels disrupt neuronal excitability.

These networks converge on synaptic function and plasticity, highlighting potential therapeutic targets.

Organoids offer a more physiologically relevant 3D architecture and can recapitulate early neurodevelopmental processes. However, they are more complex and less reproducible than 2D cultures.

Animal models for ASD include:

• • Genetic mouse models: Knockout or knock-in mice for genes like SHANK3, NRXN1, and MECP2. These models exhibit ASD-like behaviors.
• • Rat models: Some rat models with SHANK3 mutations show social deficits.
• • Non-human primate models: CRISPR-edited monkeys with SHANK3 mutations are being developed for closer recapitulation of human neurobiology.
• • Environmental models: Maternal immune activation (MIA) models induce ASD-like phenotypes in offspring.

These models are valuable for studying behavioral outcomes and testing therapeutic interventions.

CRISPR-based gene editing enables the creation of isogenic cell lines with precise mutations. For example:

• • SHANK3 knockout SH-SY5Y cells: Generated by CRISPR-Cas9, these cells lack functional SHANK3 protein, allowing study of synaptic deficits.
• • NRXN1 heterozygous knockout iPSC-derived neurons: Mimic haploinsufficiency seen in patients.
• • Isogenic pairs: A patient-derived iPSC line with a mutation and its corrected counterpart (via CRISPR) provide a controlled comparison.

Commercially available, sequence-verified gene-edited cell models accelerate research by providing reproducible and validated tools. These models are essential for drug screening and functional genomics.

Gene-edited cells allow functional validation of ASD candidate genes. For example:

• • Knockout of SHANK3 in neurons reduces synaptic density and alters dendritic spine morphology.
• • Knock-in of a patient-specific mutation in SCN2A in iPSC-derived neurons leads to altered sodium currents.
• • CRISPR screens can identify genes that modify the phenotype of ASD mutations, revealing potential therapeutic targets.

Isogenic pairs (mutant vs. corrected) are used for high-throughput drug screening. For instance:

• • Screening compounds that rescue synaptic deficits in SHANK3 knockout neurons.
• • Testing drugs that modulate mTOR activity in TSC-mutant cells.
• • Assessing drug efficacy on patient-specific mutations, enabling precision medicine approaches.

These models also help identify mechanisms of drug resistance, as cells may adapt to chronic treatment.

CRISPR-based synthetic lethality screens can identify genes that are essential only in the context of an ASD mutation. For example:

• • In SHANK3 knockout cells, screening for genes whose knockdown leads to cell death may reveal novel therapeutic targets.
• • Proteomic and transcriptomic profiling of gene-edited cells can identify biomarkers for diagnosis or treatment response.
• • These biomarkers can be validated in patient samples, aiding in early detection and monitoring.

These resources provide valuable data for hypothesis generation and validation.