This study is researching an experimental drug called ALN-SNCA (called "study drug"). The study is focused on people with early Parkinson's disease, a disorder of the nervous system that affects movement. Parkinson's disease is caused by a gradual loss of nerve cells in the brain, especially those due to the harmful build-up of a protein called α-synuclein. The aim of the study is to see if the study drug is safe and tolerated well enough to continue testing it in future studies and what side effects may happen from taking the study drug. The study is looking at several other research questions, including: * Whether the study drug can lower the level of α-synuclein protein in the Cerebrospinal Fluid (also referred to as "CSF", the fluid that surrounds the brain and spinal cord) * How much study drug is in the blood, urine, and CSF at different times * Compatible research to better understand the study drug (ALN-SNCA) and Parkinson's disease, including (but not limited to), whether the study drug can slow down the progression of Parkinson's disease symptoms