Identifying pathogenic mechanisms underlying PACS1 Syndrome: implications for neural development

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Project Summary Neurodevelopmental disorders (NDDs) are heterogenous and usually present with complex etiology. Individuals with these conditions present with cognitive impairment accompanied by lifelong deficits; yet remarkably little is known about their neurological basis. Patients and their families are in desperate need for disease-modifying therapies. However, to develop effective treatments, it is imperative to understand the dysregulation of molecular and cellular processes leading to these conditions. PACS1 Syndrome is a NDD caused by a single R203W substitution in the Phosphofurin Acidic Cluster Sorting 1 (PACS1) protein in over 200 patients. Moreover, genome-wide association studies (GWAS) identified the human PACS1 locus as a susceptibility gene in severe early-onset obesity, developmental delay, and bipolar disorder, suggesting a broader role in brain development. PACS1 encodes a multifunctional protein which canonical cytosolic function in the secretory pathway is to direct its cargo to the trans-Golgi Network.However, PACS1 also contains a nuclear localization signal (NLS), shuttles to the nucleus during S phase, and, at least in highly proliferative cancer cells, regulates chromatin stability through interaction with HDAC proteins. Despite previous studies have revealed critical PACS1 functions, the role of PACS1 within the nucleus as well as the effect of the PACS1 R203W variant in the nervous system yet to be determined. Thus, we have generated patient induced pluripotent stem cells (iPSCs)-derived cerebral organoids to investigate the transcriptome of a developing patterned, three-dimensional neural structure with the patient genetic background. By performing single cell RNA Sequencing in organoids during early development, we generated a preliminary dataset that strongly indicate that regulates a gene expression program important for Glutamatergic/GABAergic fate specification. Altogether, evidence regarding nuclear localization of PACS1, in addition to our preliminary data, suggests that PACS1 has an unexplored nuclear function, possibly by regulating gene expression in neural progenitors. Therefore, we hypothesize that PACS1 regulates Glutamatergic/GABAergic balance through a non-canonical nuclear function that decreases as neurogenesis proceeds. Thus, PACS1 deficits result in a shift towards GABAergic fate, generating ectopic GABAergic neurons, subsequent Glutamatergic/GABAergic imbalance, and finally NDD phenotypes. We will test this hypothesis by addressing whether PACS1 possesses context specific functions that differ across neural differentiation (aim1), and deficits in PACS1 nuclear function alter Glutamatergic/GABAergic specification balance (aim2). Results from this proposal will greatly enhance our understanding of how the PACS1 deficits affect the developing nervous system, by uncovering a currently disregarded nuclear function. Moreover, our work will expand our knowledge of the molecular underpinnings and consequences of the Glutamatergic/GABAergic imbalance, that has repeatedly been described as a precursor of many NDDs, pointing towards a possible common mechanism from genetically heterogeneous origins and may ultimately reveal convergent therapy targets.