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Early-Onset Familial Alzheimer’s Disease (EOFAD) is a dominantly inherited neurodegenerative disorder elicited by over 300 mutations in the PSEN1, PSEN2, and APP genes. Hallmark pathological changes and symptoms observed, namely the accumulation of misfolded Amyloid-β (Aβ) in plaques and Tau aggregates in neurofibrillary tangles associated with memory loss and cognitive decline, are understood to be temporally accelerated manifestations of the more common sporadic Late-Onset Alzheimer’s Disease. The complete penetrance of EOFAD-causing mutations has allowed for experimental models which have proven integral to the overall understanding of AD. However, the failure of pathology-targeting therapeutic development suggests that the formation of plaques and tangles may be symptomatic and not describe the etiology of the disease^,. In this work, we used an integrative, multi-omics approach and systems-level analysis in hiPSC-derived neurons to generate a mechanistic disease model for EOFAD. Using patient-specific cells from donors harboring mutations in PSEN1 differentiated into neurons, we characterized the disease-related gene expression and chromatin accessibility changes by RNA-Seq, ATAC-Seq, and histone methylation ChIP-Seq. Here, we show that the defining disease-causing mechanism of EOFAD is dedifferentiation, causing neurons to traverse the lineage-defining chromatin landscape along an alternative axis to a mixed-lineage cell state with gene signature profiles indicative of less-defined ectoderm as well as non-ectoderm lineages via REST-mediated repression of neuronal lineage specification …