Key Inclusion Criteria:  Confirmed diagnosis of Wilson disease Ongoing copper chelator (ie, penicillamine, trientine) and/or zinc therapy for at least 12 months at screening, with no medication or dose changes for at least 6 months at screening. Stable Wilson disease as evidenced by stable laboratory values during the Screening Period Ongoing restriction of high copper containing foods for at least 12 months at Screening, continued through study participation. Willing and able to comply with all study procedures and requirements, including frequent blood collection, total urine collection over a 24-hour period, patient-reported outcome assessments, and long-term follow-up  Key Exclusion Criteria:  Detectable pre-existing antibodies to the AAV9 capsid. Stage 1 only: History of copper chelator or zinc therapy noncompliance, in the Investigator's judgment, within 12 months prior to Screening. History of liver transplant. Decompensated hepatic cirrhosis or presence of advanced liver disease as evidenced by portal hypertension, ascites, splenomegaly, esophageal varices, hepatic encephalopathy, or a liver biopsy with evidence of stage III fibrosis. Significant hepatic inflammation as evidenced by imaging or any of the laboratory abnormalities. Model for End-Stage Liver Disease (MELD) score > 13. Wilson Index score > 11 Hemoglobin < 9 g/dL History of prolonged QT syndrome (QT interval > 450 msec on electrocardiogram [ECG]) Presence of Stage 3 or higher chronic kidney disease based on estimated glomerular filtration rate < 60 mL/min/1.73 m2. Marked neurological deficit or compromise that, in the Investigator's opinion, would interfere with the subject's safety or ability to participate in the study. Moderate to severe depression, recent or active suicidal ideation with intent or suicidal behavior, psychosis, or unstable psychiatric illness. Participation in another gene transfer study or use of another gene transfer product before or during study participation. Subjects with known hypersensitivity to amide-containing local anesthetics are excluded from participating in the optional liver biopsy substudy.  Note: Other protocol defined Inclusion/ Exclusion criteria may apply