In this study, researchers will learn more about a study drug called BIIB122 in participants with early-stage Parkinson's disease (PD). The study will focus on participants with a specific genetic variant in their LRRK2 gene. The main question researchers are trying to answer is if taking BIIB122 slows the worsening of PD more than placebo in the early stages of PD. To help answer this question, researchers will use a questionnaire called the Movement Disorder Society-Unified Parkinson's Disease Rating Scale, also known as the MDS-UPDRS. * The MDS-UPDRS measures impairment and disability in people living with PD. It was created in the 1980s and is one of the most used rating scales for PD symptoms. * The MDS-UPDRS has 4 parts, and a higher score means more severe PD symptoms. * Part I assesses non-motor experiences of daily living, including but not limited to memory loss, problems sleeping, pain, depression, and anxiety. * Part II measures motor experiences of daily living. * Part III is the results of a motor symptoms exam by a medical professional. * Part IV records PD complications caused by motor symptoms. Researchers will also learn more about the safety of BIIB122. A description of how the study will be done is given below. * Participants will take BIIB122 or a placebo as tablets by mouth. A placebo looks like the study drug but contains no real medicine. * Participants will be in the study for 103 weeks to 187 weeks. This includes the screening and follow-up periods. * Participants will take BIIB122 or placebo 1 time a day for 96 to 180 weeks. * Participants can continue to take certain medications for PD. Participants must be on the same dose of medication for at least 90 days before the study begins. * Participants will visit the clinic less often as the study continues, ranging every 4 weeks to every 24 weeks.