SNCA and Parkinson disease: Preclinical evidence showed that the caudo-rostral spread of α-synuclein pathology from the gastrointestinal tract to the brain could occur after intestinal inoculation of α-synuclein pre-formed fibrils or α-synuclein derived from human PD brain lysate (Kim et al., 2019; Challis et al., 2020; Holmqvist et al., 2014).