Using hiPSCs to investigate the protective mechanisms of the ApoEch mutation

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Although the majority of AD patient are sporadic, genome-wide association studies and related approaches have identified numerous genetic factors that modulate the risk of AD onset and progression. While the majority of these risk factors are associated with increased AD risk, several variants have been identified that reduce the risk of AD. As such, variation in Apolipoprotein E (ApoE), a cholesterol transport protein secreted primarily by astrocytes in the CNS, has been identified as one the more powerful of these risk factors. ApoE has three common alleles which differ with respect to two amino acid residues. Compared to the risk neutral ApoE3/3 genotype, each additional copy of the ApoE4 increases risk of AD, while the presence the APO2 allele is protective. More recently, an additional rare variant of APOE—the ApoE Christchurch (ApoCh)—was identified as potentially mitigating neurodegeneration in a PSEN1 mutation carrier. As such, understanding the protective mechanisms of the ApoCh variant will have a significant effect on therapeutic interventions. We and others have used hiPSC-based models, which are highly complementary to existing animal and primary cell culture models, to study AD is a simplified and accessible system. As such, we will use our collective experience in stem cell bioengineering, neurodegenerative disease modeling, and genome editing to elucidate the potential mechanisms by which ApoEch mitigates AD risk. In the first aim, we will use our recently developed highly efficient gene editing approach to introduce the ApoEch mutation into isogenic hiPSCs. In the second aim, we will perform biochemical, molecular, and transcriptome analysis of neural cultures derived from these isogenic lines to test the hypotheses that ApoEch exerts its protection effects through (i) modulation of Aβ processing, secretion, and uptake and (ii) alteration in tau hyperphosphorylation and internalization. In addition, we will use RNA-seq analysis to identify signaling pathways, gene regulatory networks, and transcriptional target that are independently influenced by the presence of the ApoEch mutation and disease status. Overall, a better understanding of the mechanism by which ApoEch enhances neuroprotection against AD will have a significant translational impact on the design of therapeutic interventions.