Study of DNL126 in Pediatric Participants With Mucopolysaccharidosis Type IIIA (Sanfilippo Syndrome Type A)

Key Inclusion Criteria: * Confirmed diagnosis of MPS IIIA * For Cohort A2: No more than 1 participant may have predictors of a slow-progressing phenotype * For Cohort A3: Approximately 2 participants will have predictors of the slow-progressing phenotype * For Cohort B1: Have a severe phenotype based on having at least one of the following: * An older sibling with the same genotype and severe MPS IIIA, in the opinion of the investigator * A definitive genotype indicative of severe MPS IIIA, in the opinion of the investigator * Clinical symptoms of MPS IIIA prior to 28 months of age that, in the opinion of the investigator, are indicative of severe MPS IIIA * For Cohort B2: Are an older sibling of a participant in Cohort B1 (who has already been confirmed to be eligible for dosing) with MPS IIIA, the same causative genotype, and who has severe MPS IIIA in the opinion of the investigator Key Exclusion Criteria: * Have unstable or poorly controlled medical condition(s) or significant medical or psychological comorbidity or comorbidities that, in the opinion of the investigator, would interfere with safe participation in the trial or interpretation of study assessments * Have lost the ability to walk independently, in the opinion of the investigator * Are unable to take the majority of nutrition via mouth, in the opinion of the investigator * For Cohort B only: Are homozygous or compound heterozygous for the N-sulfoglucosamine sulfohydrolase (SGSH) S298P mutation or any…