SNCA and major depressive disorder: Nonmotor symptoms such as depression and cognitive impairments also occur in PD patients.10,203 PD is characterized by a loss of dopaminergic neurons in the substantia nigra and the accumulation of intracellular protein inclusions called Lewy body, composed of alpha-synuclein.202,204 Although the normal neuronal function of α-synuclein protein is not fully understood, it plays a role in synaptic vesicle dynamics, mitochondrial function, and intracellular trafficking.