The gold standard for a definitive diagnosis of Parkinson’s disease (PD) is the pathologic finding of aggregated alpha-synuclein into Lewy bodies and for Alzheimer’s disease (AD) aggregated amyloid into plaques and hyperphosphorylated tau into tangles. Implicit in this clinico-pathologic-based nosology is the assumption that pathological protein aggregation at autopsy reflect pathogenesis at disease onset. While these aggregates may in exceptional cases be on a causal pathway in humans (e.g., aggregated alpha-synuclein in SNCA gene multiplication...