SNCA and Parkinson disease: Constipation is the most common nonmotor symptom – affecting over 70% of PD patients – and often precedes motor symptoms by over a decade, with α-synuclein aggregates potentially originating in gut submucosal neurons years earlier (Abbott et al., 2001; Bridi and Hirth, 2018; Cersosimo and Benarroch, 2012; Hawkes et al., 2010; Li et al., 2023; Shannon et al., 2011).