Synaptic Dysregulation and Neurodevelopmental Consequences in Mouse and Human Genetic Models of Intellectual Disability

This dissertation investigates the cellular and molecular mechanisms underlying neurodevelopmental disorders associated with chromatin-modifying genes and large-scale gene deletions, focusing on Kabuki Syndrome Types 1 and 2 (KS1 and KS2) and Miller-Dieker Syndrome (MDS). Intellectual disability (ID) is often linked to alterations in synaptic structure and function, as proper cognitive development relies on the precise formation and regulation of synaptic connections within the brain. An essential aspect of synaptic regulation involves maintaining a balance between excitatory and inhibitory (E/I) signaling, which is crucial for neuronal communication, network stability, and ultimately, cognitive function. Disruptions in E/I balance can lead to neuronal hyperexcitability or reduced synaptic connectivity, both of which are associated with cognitive deficits in neurodevelopmental disorders. By examining both shared and distinct features of these disorders in mouse models and human neurons derived from patient iPSCs, this research provides a comprehensive view of how disruptions in chromatin regulation and large gene networks impact synaptic architecture, neuronal excitability, and overall neuronal morphology. In KS1, mutations in KMT2D lead to disruptions in the balance of excitatory and inhibitory (E/I) synapses, characterized by increased inhibitory synaptic markers, reduced stability of excitatory postsynaptic components, and altered calcium signaling in excitatory neurons. Similarly, human neurons with KMT2D mutations showed increased presynaptic vesicle clustering and reduced levels of the postsynaptic protein PSD95, suggesting compromised synaptic stability and plasticity that can alter network connectivity. As KMT2D is a chromatin-modifying gene, it plays a pivotal role in regulating the expression of a broad set of genes, a subset of which are likely involved in synaptic function and neuronal maturation. Alterations in KMT2D activity may therefore disrupt biological pathways critical for synaptic development, leading to imbalances in synaptic input that may contribute to the cognitive impairments seen in KS1. It is these synaptic abnormalities which likely underlie the cognitive deficits observed in KS1, as E/I imbalance is a hallmark of neurodevelopmental disorders with cognitive impairments. In KS2, mutations in UTX affected only inhibitory vesicle marker vGAT and reduced excitatory calcium event frequency, revealing unique but overlapping mechanisms of dysfunction compared to KS1. UTX, like KMT2D, is involved in chromatin modification, and its role in gene regulation also likely extends to similar pathways influencing synaptic formation and neuronal development. Understanding the distinctions between how KMT2D and UTX affect these common pathways is essential, as they may guide the development of syndrome-specific therapeutic strategies. In contrast to KS1 and KS2, MDS results from large-scale deletions of over 26 genes on chromosome 17p13.3, affecting broad gene networks rather than a single chromatin modifier. This deletion impacts multiple essential neurodevelopmental genes, including LIS1, YWHAE, and CRK, which together regulate processes such as neuronal migration, synaptic function, and cytoskeletal organization. The overall diversity of genes affected in MDS suggests that the disorder arises from compounded effects potentially across multiple pathways, each contributing to neuronal connectivity and excitability in distinct ways. MDS neurons exhibit a distinct hyperexcitable phenotype, marked by increased synaptic density, elevated calcium event frequency and amplitude, and altered dendritic complexity. This synaptic hyperexcitability and increased synaptic density can disrupt the E/I balance, potentially driving the cognitive and developmental impairments characteristic of MDS. MDS neurons exhibit a distinct hyperexcitable phenotype, marked by increased synaptic density, elevated calcium event frequency and amplitude, and altered dendritic complexity. These findings suggest that MDS pathophysiology may stem from cumulative effects across various pathways and gene networks, leading to extensive disruptions in neuronal function that drive its severe cognitive and developmental impairments. This work underscores the broader implications of studying KS1, KS2, and MDS together, as it reveals how chromatin modifiers and large-scale gene networks intersect to maintain synaptic balance and neuronal health. By exploring both the commonalities and the disorder-specific mechanisms, this thesis highlights the diverse outcomes of chromatin dysregulation and large-scale gene deletion, with potential shared therapeutic targets for intervention. These findings enhance our understanding of the molecular and cellular underpinnings of these neurodevelopmental disorders, offering a foundation for developing tailored therapies to address the unique synaptic and functional deficits associated with each condition and providing broader insights into the role of chromatin and gene network regulation in neurodevelopment.