Examining APOE ε4 alongside PD-specific genes such as GBA1 therefore, helps to determine whether observed associations with neuropsychiatric dimensions reflect mechanisms specific to PD-linked genetics (GBA1) or are driven by genetic susceptibility to dementia more generally (APOE ε4); several longitudinal and cohort studies report independent and sometimes additive effects of GBA1 and APOE ε4 on cognitive outcomes, supporting this comparative approach28. Here, APOE is linked to Parkinson disease.