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.

Inclusion Criteria:

• For heterozygous pathogenic LRRK2 mutation carriers: ≥ 30 to ≤ 80 years
• For homozygous pathogenic LRRK2 mutation carriers: ≥ 30 years
• Have screening genetic test results verifying the presence of a pathogenic LRRK2 variant.
• Have a clinical diagnosis of PD meeting the Movement Disorder Society Clinical Diagnostic Criteria.

Exclusion Criteria:

• Have a history of any clinically significant neurological disorder other than PD, including, but not limited to, stroke and dementia, in the opinion of the investigator, within 5 years of the screening visit.
• Have clinical evidence of atypical parkinsonism (eg, multiple-system atrophy or progressive supranuclear palsy) or evidence of drug-induced parkinsonism.
• Have previously participated or are currently participating in the BIIB122 LUMA study (Study 283PD201).
• Have previously participated or are currently participating in a gene therapy study for PD.
• Have a history of brain surgical intervention for PD (eg, deep-brain stimulation, pallidotomy).
• Have any physical condition that may confound the motor assessment (MDS-UPDRS) over time (eg, severe arthritis, severe dyskinesias, traumatic injuries with permanent physical disability).
• Abnormal vitals including Blood Pressure, Heart Rate, or Body Temperature
• Have abnormal PFT results at screening

Note: Other protocol defined Inclusion/Exclusion criteria may apply