This Phase 2a, multicenter, randomized, 12-week double-blind, placebo-controlled, parallel-group study, followed by an OLE, is designed to evaluate the safety, tolerability, and pharmacodynamic effects of BIIB122 in participants with LRRK2-PD. LRRK2-PD is defined as Parkinson's Disease (PD) in individuals who are heterozygous or homozygous carriers of a pathogenic LRRK2 variant that increases LRRK2 kinase activity.