From Synapse to Symptom: Modelling the pathobiology of STXBP1 variants

In this thesis, several experimental approaches are taken to model STXBP1 syndrome, in order to unravel the underlying pathobiological mechanisms and which, ultimately, can be used to identify and test therapeutic options. In chapter 2, we use primary mouse Stxbp1 null neurons to model the homozygous MUNC18-1(L446F) variant, and show it has a unique gain-of-function effect on synaptic transmission, in contrast to other (heterozygous) pathogenic variants. In chapters 3 and 4, we investigate the potential of iPSC-derived human neurons for modelling of synaptic disorders: first, we show that by creating autaptic neuron cultures, synaptic morphology and functionality can be probed with single-neuron precision. Next, we further explore the optimal strategies to use iPSC-neurons for disease modelling. Moreover, we investigate the attainable statistical power for different study designs, and observe that generally much higher sample sizes are necessary to reach sufficient statistical power than is currently common in the field. In chapter 5, we apply one of the discussed strategies to STXBP1 syndrome patient-derived neurons. We show that whereas MUNC18-1 protein levels are reduced in all patient neurons, inter-patient clustering is observed at the proteome and network activity level, indicating shared as well as divergent effects are observed. In chapter 6, we perform EEG analysis using recently developed analysis tools, and observe quantitative differences in brain activity dynamics in STXBP1 syndrome patients. Finally, in chapter 7, we return to the Stxbp1 null mice, to investigate a connection between MUNC18-1 and the endocytic protein Dynamin-1. We show that in absence of MUNC18-1, levels of Dynamin-1 are substantially reduced, which is associated with a defect in endocytosis, but does not explain the compromised viability of these neurons. Taken together, mouse models are used to address fundamental questions about MUNC18-1 and to model a unique pathogenic variant. Subsequently, the use of iPSC technology to generate a human in vitro model, and the application of novel analytical methods for EEG recordings, are explored as additions to the toolbox of disease models for STXBP1 syndrome and, potentially, neurodevelopmental disorders more generally.