Safety and Pharmacodynamic Effects of BIIB122 in Participants With LRRK2-Associated Parkinson's Disease (LRRK2-PD)

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