Molecular, Cellular and Behavioral Impact of the R203W PACS1 Syndrome Mutation

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Project Summary PACS1 Syndrome is a recently identified neurodevelopmental disorder caused by a recurrent de novo missense mutation in PACS1 (p.Arg203Trp). Patients carrying this missense mutation share several developmental deficits, including intellectual disability, seizures and autism. The mechanism by which PACS1R203W causes PACS1 Syndrome is unknown and no curative treatment is available. PACS1 is a multifunctional sorting protein that facilitates retrograde trafficking from endosomes to the trans-Golgi network, for delivery of proteins to the primary cilium and for genome integrity. This multifunctionality depends on a small segment of PACS1 called the furin- binding region (FBR), which binds a broad range of client proteins and signaling molecules. The R203W mutation is located in the FBR, and our biophysical studies reveal a change in the FBR dynamics when the R203W substitution is present, suggesting the possibility of an altered interaction between PACS1 and one or more of its client proteins in PACS1 Syndrome. Our preliminary studies strongly suggest PACS1R203W increases binding to the deacetylase HDAC6 to profoundly disturb membrane traffic and impair neuron development. Consequently, PACS1R203W reduces acetylation of known HDAC6 substrates, including α-tubulin, disrupting centrosome positioning and leading to Golgi fragmentation and increased dendritic arborization in pyramidal neurons. This dendritic overbranching is coupled to reduced inhibitory currents in L2/3 cortical neurons resulting in an increased excitatory:inhibitory (E:I) ratio, similar to that found in other neurodevelopmental disorders, suggesting that PACS1R203W severely affects neuronal function and behavior. Our long-term goal is to understand how PACS1R203W causes disease and to use this information to develop effective therapies. The objective of this particular application is to determine how PACS1R203W and HDAC6 combine to dysregulate neuronal arborization and synaptic transmission. We hypothesize that the aberrant interaction between PACS1R203W and HDAC6 alters organellar positioning, which contributes to excessive dendrite arborization and dysregulated synaptic activity. Guided by strong preliminary data, we will test our hypothesis by pursuing three specific aims: 1) Determine how the R203W mutation alters PACS1 structure and dynamics for influencing client protein interactions, 2) Determine how PACS1R203W and HDAC6 combine to dysregulate Golgi positioning and dendrite arborization, and 3) Determine how PACS1R203W alters synaptic activity and behavior. The approach is innovative because we will characterize, from the atomic structure to the whole-organism, the mechanism by which the recurrent R203W substitution causes neuronal dysfunction. This research is significant because it may identify new targets and therapeutic approaches to treat this debilitating disorder.