APOE and early-onset autosomal dominant Alzheimer disease: Cognitive impairment varies across sporadic Parkinson's disease (PD) and the common genetic subtypes glucocerebrosidase (GBA1) and leucine-rich repeat kinase 2 (LRRK2) PD and is influenced by Apolipoprotein E (APOE) polymorphisms and Alzheimer's disease (AD) co-pathology.