Interrogating the role of <i>APOE4</i> in Alzheimer’s disease and Down syndrome using human induced pluripotent stem cells (hiPSC)‐derived cerebral organoids

Abstract Background Nearly all adults with Down syndrome (DS) will develop Alzheimer’s disease (AD) brain pathology by their 40s, and most will become demented by age 50‐60. The inheritance of the apolipoprotein E (apoE) ε4 allele ( APOE4 ) is the strongest risk factor for AD other than age, whereas the ε3 allele ( APOE3 ) does not change AD risk. ApoE is a cholesterol carrier protein, and APOE4 ‐positive astrocytes have been demonstrated to exhibit impaired cholesterol transport. ApoE4 has also been shown to catalyze amyloid‐beta (Aβ) polymerization. As such, the APOE4 genotype is associated with earlier and more rapid cognitive decline in individuals with DS. Understanding the molecular changes in AD neuropathology resulting from APOE4 in DS would aid in the development of therapies to alleviate dementia risk, especially in individuals with DS. Method We used CRISPR‐Cas9 editing to modify DS hiPSCs, or isogenic control hiPSCs disomic for chromosome 21, from an APOE3/3 genotype to APOE4/4 , or to knock out the APOE gene. We developed cerebral organoids (COs) to test the effects of APOE4 on a three‐dimensional in vitro model system of AD pathology in DS. As a preliminary investigation, we also exogenously treated COs with each apoE isoform for 90 days in the culture medium to examine their effects on AD‐related pathologies. Result Our preliminary investigation of exogenous apoE exposure of DS and control COs revealed a significant increase in apoE expression/accumulation in DS organoids, but not in control organoids, at 90 days in vitro . Amyloid‐beta production/deposition were also differentially regulated in DS and control organoids, as evaluated by immunoblotting and immunohistochemical analyses. There were also significant size differences between DS and HC organoids. These initial findings support continued investigation of APOE genotype and AD pathology in DS and control COs. Conclusion Interrogation of APOE4 ‐driven AD pathologies will enable the development of more targeted and proactive therapies to improve the quality of life for individuals who harbor this risk allele. Understanding differences in AD pathology in individuals with DS could enable the prevention of dementia in individuals with DS, and further elucidate the pathways by which APOE acts in conferring AD risk.