A Study to Evaluate the Safety, Tolerability, and Pharmacokinetics of BIIB115

The primary objective of the study is to evaluate the safety and tolerability of single ascending dose of BIIB115 administered via intrathecal (IT) bolus injection to healthy male participants in Part A and multiple ascending doses of BIIB115 administered via IT bolus injection to pediatric Spinal Muscular Atrophy (SMA) participants previously treated with onasemnogene abeparvovec in Part B. The secondary objective of the study is to evaluate the pharmacokinetics (PK) of single-dose of BIIB115 administered via IT bolus injection to healthy male participants in Part A and multiple ascending doses of BIIB115 administered via IT bolus injection to pediatric SMA participants who previously received onasemnogene abeparvovec in Part B.