Safety, Tolerability, and Pharmacokinetics of UX053 in Patients With Glycogen Storage Disease Type III (GSD III)

Key Inclusion Criteria: * Confirmed diagnosis of GSD III by gene sequencing or enzymatic testing * Alanine aminotransferase at or below 5 times normal during the three months prior to dosing * Willing and able to comply with standard dietary management of GSD III Inclusion Criteria for Participants Rescreening Into OL-RD Cohorts After Treatment with UX053 in SAD Cohort: * If a significant rise in ALT occurs after the prior dose, ALT should show a decreasing trend toward the subject's baseline value * Total bilirubin, platelets and international normalized ratio (INR) is within normal limits Key Exclusion Criteria: * History of liver transplant or currently awaiting liver transplant * History of cirrhosis * Active Hepatitis B or C * Severe kidney impairment * History of liver cancer or large liver tumors * History of any cancer within the past 3 years * Known history of HIV infection * Known severe allergy to polyethylene glycol (PEG), polysorbate, or mRNA vaccine * Heart failure that causes marked limitation in physical activity * Poorly controlled diabetes * Poorly controlled hypothyroidism * Treatment with immunosuppressive medications such as those used to treat chronic autoimmune conditions and solid organ transplants * Pregnant or nursing, or planning to become pregnant during the study Exclusion Criteria for Participants Rescreening Into OL-RD Cohorts After Treatment with UX053 in SAD Cohort: * New or worsening symptoms of liver disease (including new or worsening hepatomegaly) along with any increase in transaminase levels * Receipt of any blood product administration (eg, packed red blood cells, platelet, FFP) for management of consumptive coagulopathy * An ALT level that is ≥ 8x ULN and \> 2x the participants baseline value in the absence of an alternative explanation Note: Additional inclusion/exclusion criteria may apply, per protocol