Elucidating the interplay between AD genetic variants using human iPSC-derived brain cells - PROJECT SUMMARY Alzheimer’s disease (AD), the leading cause of dementia, has an estimated heritability of approximately 70%. Two genetic varieties of AD exist: monogenic and polygenic AD. Monogenic AD is very rare, accounting for <1% of all people with AD, and is caused by pathogenic mutations in amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2), leading to autosomal dominant familial early-onset AD (EOAD), typically at ≤65 years. On the other hand, late-onset AD (LOAD) accounts for most cases of AD and is influenced by numerous genetic variants (polygenic form). A single fully penetrant mutation does not cause LOAD but is the result of the collective effect of multiple, more common genetic variants that increase a person's susceptibility to AD. Each individual genetic risk variant is neither necessary nor sufficient to cause AD, which explains the absence of a typical autosomal dominant pattern of inheritance in the families of most individuals with AD. The ε4 allele of apolipoprotein E (APOE) is the most potent genetic risk factor for LOAD. APOE is the primary apolipoprotein expressed in the human brain and is involved in cholesterol/lipid homeostasis, and regulates Ab plaque load. ABCA7 is another major risk genes of LOAD. The ABCA7 gene encodes the ATP-binding cassette (ABC) transporter A7, which is thought to play a role in lipid metabolism and mediates phospholipid export. ABCA7 common variants, which include rs3764650, rs3752246, and rs12151021, are established late-onset AD susceptibility loci. Notably rs3752246, results in a missense variant p.Ala1527Gly, potentially impacting secondary structure stability and function. In vitro and in vivo studies have shown that ABCA7 loss of function exacerbates amyloid-β (Aβ) pathology by increasing APP processing and reducing the phagocytic clearance of Aβ in macrophages and microglia. In this proposal, we will investigate the effects of the ABCA7 p.Ala1527Gly (ABCA7AG) variant and its interaction with the APOE genotype to understand their contributions to AD pathogenesis using two-dimensional (2D) and three-dimensional (3D) human cellular models. Notably, we have found reduced ABCA7 protein levels and increased Aβ42/40 ratio in ABCA7AG/APOE33 iPSC-derived human excitatory neurons, suggesting that p.Ala1527Gly results in loss of function. Interestingly, we found that ABCA7 protein levels did not decrease in ABCA7AG/APOE44 neurons, suggesting that APOE44, not APOE33, selectively interact with pathways regulating ABCA7 levels. The central hypothesis of this proposal is that ABCA7 p.Ala1527Gly results in decreased expression levels and a loss of function in association with APOE33 but not APOE44. Thus, we propose to elucidate the functional impact of ABCA7 p.Ala1527Gly and its interaction with the APOE genotype in excitatory neurons and microglia, the cell types that express ABCA7 at the highest levels in the brain.
