SNCA and Parkinson disease: Based on such findings, the pathophysiology of PD may arise from any one or combination of five main possible pathways, namely: (1) α-synuclein protein aggregation mediated pathway, (2) neuroinflammation mediated pathway, (3) neuronal abnormalities, (4) mitochondrial abnormality and mitophagy, and (5) gut-brain microbiome interaction (Figs. 2–4).