PROJECT SUMMARY Understanding the molecular biology behind weight regulation is imperative to address the growing epidemic of obesity and the urgent need for therapies. The primary cilium has been identified as an important organelle for signaling and function of eukaryotic cells including the cells involved in energy homeostasis. Ciliary dysfunction results in several human syndromes, collectively called ciliopathies, that are associated with diverse phenotypes affecting nearly every tissue and organ system. Some ciliopathies, such as Bardet-Biedl syndrome and Alström syndrome, present with obesity in childhood. Primary cilia are known to be expressed in the weight regulation centers of the hypothalamus. Loss of function in these primary cilia causes disruption in the neuroendocrine signaling pathways involved in energy homeostasis, resulting in obesity. Animal models show that disruption of ciliary gene(s) in neurons secreting pro-opiomelanocortin, the satiety producing neuropeptide, causes hyperphagia and obesity. The number of genes identified to be involved in the function of the primary cilium have increased over time. Recent evidence shows that the inositol phosphatase OCRL, a gene known to cause Oculocerebral syndrome of Lowe (LS), is expressed in the cilia, and may have a role in regulating the levels of phosphoinositol-4-phosphate and trafficking in the cilia. OCRL is also highly expressed in the hypothalamus, especially the cells expressing the satiety neuropeptide, pro-opiomelanocortin (POMC), and growth hormone releasing hormone. We have identified a family of two male siblings with a previously unknown mutation in the C-terminus of OCRL that is only expressed in the brain specific isoform. The proband has severe obesity and other diverse clinical features, different from those seen in LS, but overlapping with ciliopathy. We hypothesize that this loss of function variant limited to the brain-specific isoform causes a diverse phenotype including severe obesity, but does not affect the other somatic tissues. Thus, it provides a unique opportunity to study the brain limited impact of the loss of function of OCRL. This proposal seeks to use this phenotype to establish the role of OCRL as a ciliary gene involved in weight regulation in human and mouse models. We will use our patient-specific induced pluripotent stem cell line, their isogenic control and allelic series to differentiate into arcuate-like hypothalamic neurons expressing POMC to assess the impact of the mutation on the neuropeptide. We will also use conditional knockout of the gene by AAV-mediated RNA interference in the arcuate and paraventricular nuclei of the brain in LS-specific humanized mouse model to assess the phenotype. These studies will contribute a new gene to the growing list of genes involved in weight regulation and ciliary function. It will also expand the phenotype for LS and potentially provide avenues to explore therapies in future.