This study will help determine the safety and effectiveness of BIIB122, compared to placebo (an inactive substance), in people with early-stage Parkinson's disease. The use of BIIB122 in this study is investigational. "Investigational" means that the study drug is currently being tested and is not approved by the U.S. Food and Drug Administration (FDA) or any other health authorities around the world for treating people with PD. The study is expected to last a minimum of 1 year to a maximum of 3 years. You may or may not receive direct medical benefit from participating in this study. Your condition may get better, worse, or stay the same. The information obtained from this study, however, could help other patients with your disease in the future.

This study aims to study the natural progression of P-SYN quantification by conducting skin biopsies in patients with Parkinson's Disease (PD) and REM sleep behavior disorder (RBD). Participation will be over an 18-month period and will include skin biopsies and clinical examinations every six months. Subjects will be male or female between the ages of 50-100 and have a diagnosis of PD or RBD.