Aβ: Amyloid-β; Aβ1-42: Amyloid-β 1-42; α-syn: Alpha-synuclein; AD: Alzheimer's disease; AQT: A quick test of cognitive speed; CSF: Cerebrospinal fluid; HC: Healthy control; LB: Lewy body; NfL: Neurofilament light; PD: Parkinson's disease; PDD: Parkinson's disease dementia; PET: Positron emission tomography; Tau: Tubulin-associated unit; p-Tau: Phosphorylated Tau; t-tau: Total Tau; TUG: Timed up and go; UPDRS III: Unified Parkinson's disease rating scale part III; YKL-40: Chitinase-3-like protein 1 (CHI3L1). Here, SNCA is linked to early-onset autosomal dominant Alzheimer disease.