This is a phase I, multicenter, non-randomized, adaptive, open-label, multiple ascending, intra-participant, dose-escalation study with a long-term extension (LTE) part and an optional open-label extension (OOE) part. The objective of the study is to investigate the safety, tolerability, PK and PD of RO7248824 administered intrathecally (IT) in participants with AS. Two linked sets of dose escalation cohorts are planned based on two different age groups, namely participants with AS aged ≥ 5 to ≤ 12 years in cohorts A1 to A5 (with at least 2 participants ≤ 8 years old in each cohort) and AS participants aged ≥ 1 to ≤ 4 years in cohorts B1 to B5. The two sets of cohorts will be run in parallel, with each cohort A1 to A5 preceding and gating the linked cohort B1 to B5 (e.g., A1 precedes B1).