Liver Glycogen Storage Disease type IX has an overall estimated prevalence of 1 in 100,000 individuals, accounting for approximately 25% of all GSD cases. Liver GSD IX is caused by deficiency of the liver enzyme phosphorylase kinase (PhK) and presents with hepatomegaly, elevated liver enzymes, and hypoglycemia. PhK is a complex, hetero-tetrameric enzyme comprised of four subunits - α, β, γ, and δ - each with tissue specific isoforms encoded by different genes. The genes PHKA2, PHKB, and PHKG2 encode the liver specific isoform PhK subunits α2, β, and γ2 respectively. Until the recent availability of gene panels and exome sequencing, the diagnosis of Liver GSD IX did not allow for differentiation of these subtypes. There is growing evidence that patients with the second most common subtype, PHKG2 GSD IX (GSD IX γ2) develop severe liver disease. Of published case reports, 95.8% of patients with GSD IX γ2 reported features of liver fibrosis and/or cirrhosis, placing individuals with GSD IX γ2 at higher risk for liver failure, hepatocellular carcinoma and death. Despite the life-threatening severity of GSD IX γ2, there has been minimal research to understand disease progression or options for treatment. The first goal of this project is to characterize the phenotype of the first mouse model of PHKG2 GSD IX (GSD IX γ2). Preliminary evidence from male and female 3-month-old Phkg2-/- mice is encouraging. I have discovered that knockout mice have significantly elevated liver glycogen content, serum ALP, AST, ALT, urine Hex4, and features of hepatocyte enlargement and fibrosis compared to wild type controls. I will continue to characterize the Phkg2-/- mouse phenotype at 6, 9- and 12-months of age. By identifying the time of onset and progression of liver disease in our model, I will better understand the ideal time to deliver therapy. The second goal of this project is to evaluate the efficacy of a novel AAV gene therapy approach for reducing liver disease progression in the Phkg2-/- mouse model. Current liver-directed gene therapies utilize a recombinant adeno-associated virus serotype 8 (AAV8), based on preclinical efficacy in mice. However, recent studies have demonstrated that AAV8 has greater tropism for murine versus human hepatocytes. Our group has identified a novel capsid with high transduction rates for both human and murine hepatocytes (AAVhum.8) – making it an excellent vector for preclinical evaluation with high potential for clinical translation. The results of this project will characterize the first GSD IX γ2 mouse model, will provide preclinical evidence for a non-surgical, long-term, therapeutic option for patients with GSD IX γ2, and will inform the treatment of other pediatric genetic liver diseases.