SNCA and Parkinson disease: An extension to Braak’s hypothesis is the dichotomization of PD into two subtypes: a brain-first (top-down) type, in which α-synuclein pathology starts in the brain with consecutive spreading to the peripheral autonomic nervous system; and a body-first (bottom-up) type, in which the pathology starts in the peripheral autonomic nervous system and then spreads to the brain [18].