α-synuclein genetic variability: A biomarker for dementia in Parkinson disease by Guella, I, Evans, D, Szu-Tu, C, Nosova, E, Bortnick, S, Goldman, J, Dalrymple-Alford, J, Geurtsen, G, Litvan, I, Ross, O, Middleton, L, Parkkinen, L, Farrer, M