APOE and early-onset autosomal dominant Alzheimer disease: The ε4 allele of apolipoprotein E (APOE), APOEε4, has been identified as one of the greatest genetic risk factors for Alzheimer’s disease (AD) and related dementias (ADRDs) (Rohn, 2014; Halliday et al., 2016; Riedel et al., 2016; Koizumi et al., 2018; Zhao et al., 2020).