Key Inclusion Criteria:
1. Clinical diagnosis of AS with molecular result indicating either paternal UPD of 15q11.2-q13 or ID of the maternal 15q11.2-q13 region, provided by the Investigator and confirmed by either a qualified central vendor or a qualified local geneticist at the site.
2. The participant's caregiver(s)/legally-authorized representative (LAR) must have given written informed consent and any authorizations required by local law and be able and willing to comply with all study requirements.
3. Medically stable and can undergo sedation and/or general anesthesia without intubation.
4. Male or female between 2 and ≤ 50 years of age, depending on the specific cohort, at the time of the in-clinic Screening visit.
5. If applicable, is currently receiving stable doses of concomitant medications typically prescribed for AS, such as anti-epileptic medication, behavioral management medications, sleep medications, gabapentin, cannabidiol, and special diets, supplements, or nutritional support for at least 8 weeks prior to the Baseline visit. If recent changes (\< 8 weeks stable) in medications, the participant may be allowed per Investigator judgment if the change is not expected to have an impact on the signs and symptoms of AS.
6. LAR/caregiver(s) agree(s) not to post any of the participant's personal medical data or information related to the study on any website or social media site (e.g., Facebook, Instagram, X, YouTube, TikTok, WhatsApp) from the time of enrollment until they are notified that the study is completed.
Key Exclusion Criteria:
1. Participant has a clinical diagnosis of AS with molecular confirmation of a UBE3A deletion or UBE3A mutation.
2. Any clinically significant abnormalities in medical history (e.g., major surgery within 3 months of Screening), or on physical examination for which treatment with an antisense oligonucleotide (ASO) would be contraindicated or which, in the opinion of the Investigator, could confound the results of this study.
3. Known brain or spinal disease that would interfere with the lumbar puncture (LP) procedure, cerebrospinal fluid (CSF) circulation, or presence of other factors that would affect the safety of the LP procedure, including tumors or abnormalities by MRI or computed tomography (CT), subarachnoid hemorrhage, suggestion of raised intracranial pressure (ICP) on magnetic resonance imaging (MRI) or ophthalmic examination, Chiari malformation, obstructive hydrocephalus, syringomyelia, tethered spinal cord syndrome, or connective tissue disorders such as Ehlers-Danlos syndrome and Marfan syndrome.
4. Any laboratory abnormalities or any other clinically significant abnormalities that would, as assessed by the Investigator, at Screening or Baseline, render a participant unsuitable for inclusion.
5. Previous treatment with an oligonucleotide (including small interfering ribonucleic acid \[siRNA\] and ASOs) or gene therapy or gene editing. This exclusion criterion does not apply to approved nucleic acid-based vaccines, including messenger Ribonucleic Acid (mRNA) vaccines, which are allowed.
Other inclusion/exclusion criteria may apply
