PROJECT SUMMARY/ABSTRACT Alagille syndrome (ALGS) is a genetic, multisystem developmental disorder affecting the liver biliary system, cardiovascular system, kidney and other organs. The most common phenotype observed in ALGS patients is a severe decrease in the number of bile ducts in the liver (called bile duct paucity). Similar to other diseases involving intrahepatic biliary system, bile duct paucity and its consequences often lead to cholestatic phenotypes (like severe itching) and end-stage liver disease in ALGS patients. Dominant mutations in a gene called JAG1, which encodes a ligand in the Notch signaling pathway, were identified as the cause of ALGS in 1997, and were later shown to be responsible for about 95% of ALGS cases. However, despite the wealth of knowledge on the function and regulation of Notch pathway and despite the passage of more than 2 decades from the discovery of JAG1 in ALGS patients, there is still no mechanism-based therapy for this disease. The only cure for ALGS liver disease and diseases of bile duct paucity is liver transplantation. However, the shortage in liver donor poses a significant challenge to this patient population. Moreover, involvement of other organ systems—including the cardiovascular and renal systems—in many ALGS patients makes them poor candidates for liver transplantation. Moreover, for those who do receive a liver transplant, a higher frequency of cardiovascular and kidney complications are observed, and the long-term effects of immunosuppressive therapy can be problematic. Therefore, development of therapies that can correct or decrease the degree of bile duct paucity and help avoid or delay transplant is a major unmet clinical/medical need of patients with ALGS and other diseases with bile duct paucity. Building on our recently published and preliminary data, the current application proposes preclinical studies to directly address this need. We have established a mouse model for ALGS by removing one copy of Jag1 on a C57BL/6 genetic background. This model allowed us to identify two dosage-sensitive genetic modifiers of the Jag1 heterozygosity in the liver, which can rescue the bile duct paucity and liver phenotypes without causing side effects. Moreover, our preliminary data generated in collaboration with an industry partner indicate that postnatal reduction in the expression of one of these modifiers by a novel approach can significantly improve the liver phenotypes in our ALGS mouse model. In the two Aims of this proposal, we will determine the optimal dosage, long-term benefits, and the potential side effects of our treatment strategy in Jag1 heterozygous and other genetic models of ALGS with more severe liver involvement and/or multiple organ involvement. If successful, our research will establish a strategy for augmentation of the biliary tree in bile duct paucity models and will pave the way for clinical studies which might one day help ALGS patients avoid liver transplantation.