This is a multicenter, open-label, Phase 1/2 study to assess the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD) and clinical efficacy of DNL126 in participants with Sanfilippo syndrome Type A (MPS IIIA). The core study period is 25 weeks (approximately 6 months); followed by an open-label extension (OLE), which extends through Week 97 (approximately 18 months); and a long-term extension (LTE), which extends through Week 193 (Year 4). Participants with MPS IIIA will be enrolled in two planned cohorts, and additional participants with MPS IIIA may be enrolled in three optional cohorts.