• The participant has a parent, caregiver or legal representative (hereinafter caregiver) who is reliable, competent and at least 18 years of age. The caregiver is willing and able to accompany the participant to clinic visits and to be available to the Investigational Site by phone or email if needed and who (in the opinion of the Investigator) is and will remain sufficiently knowledgeable of participant's ongoing condition to respond to any inquiries about the participant from personnel from the Study Site.

• A caregiver must be able to consent for the participant according to International Council on Harmonisation (ICH) and local regulations.

• Ability to comply with all study requirements.

• Have adequate supportive psychosocial circumstances.

• Able to tolerate blood draws.

• Able to undergo lumbar puncture (LP) and IT injection, under sedation or anesthesia if needed and as determined appropriate by the Investigator.

• Stable medical status for at least 4 weeks prior to Screening and at the time of enrollment.

• Body weight of ≥ 7 kg

• Participant must be ≥ 1 to ≤ 12 years of age at the time of signing of the informed consent by the caregiver.

• Clinical diagnosis of AS confirmed by a molecular diagnosis with genotypic classification of either Ubiquitin-protein ligase E3A (UBE3A) mutation of the maternal allele or deletion on the maternally inherited chromosome 15q11q13 that includes the UBE3A gene and is less than 7 megabyte (Mb) in size.

⁃ Reproductive Status:

⁃ Some of the provisions that follow may have limited applicability based on the age range of study participants (i.e., up to the age of 12) and the nature of the disease understudy. These provisions are nonetheless included for purposes of completeness in order:

⁃ A) Female Participants

⁃ A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies:

• Women of non-childbearing potential.

• Women of childbearing potential who agree to remain abstinent (refrain from heterosexual intercourse) or use acceptable contraceptive methods during the treatment period and for at least 6 months after the final dose of RO7248824 (RG6091). The following are acceptable contraceptive methods: bilateral tubal occlusion/ ligation, male sexual partner who is sterilized, established proper use of hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices and copper intrauterine devices, male or female condom with or without spermicide; and cap, diaphragm, or sponge with spermicide.

⁃ B) Male Participants

⁃ During the treatment period and for at least 6 months after the final dose of RO7248824 (RG6091), consent has to be provided to:

• Remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures such as a condom, with a female partner of childbearing potential, or pregnant female partner, to avoid exposing the embryo.

⁃ The reliability of sexual abstinence for male and/or female enrollment eligibility needs to be evaluated in relation to the duration of the clinical study and the preferred and usual lifestyle of the participant. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or post-ovulation methods) and withdrawal are not acceptable methods of preventing drug exposure.

⁃ \- Current or prior participations in the LTE part of Study BP41674.