GSE162659
Epigenetic traits inscribed in chromatin accessibility in aged hematopoietic stem cells (EZH2_KO_HSC)
Hematopoietic stem cells (HSCs) exhibit considerable cell-intrinsic changes with age. Epigenetic alterations are one of the hallmarks of HSC aging, and profiling of DNA methylation and histone modifications has provided potential mechanisms that contribute to HSC aging. Chromatin accessibility reflects a comprehensive transcriptional network operating in cells; however, it has not yet been investigated in HSC aging. Here we performed an integrated analysis of aged HSCs on transcriptome, chromatin accessibilities, and histone modifications. Alterations in chromatin accessibility preferentially took place in HSCs with aging, the cells at the top of hematopoietic hierarchy, suggesting that the age-associated alterations in chromatin accessibility are memorized in HSCs and are inherited to downstream progenitor cells. However, most genes with differentially accessible regions (DARs) were not actively transcribed and kept poised for activation in aged HSCs. Motifs of ATF/CREB, STAT, and CNC family transcription factors were significantly enriched at DARs in aged HSCs. These transcription factors are activated in response to external stresses such as cytokine and inflammation signals and oxidative stresses, suggesting that the long-term exposure to such stress signals have changed chromatin accessibility in HSCs to augment responses by such trained HSCs to subsequent stimuli. In contrast, aged HSC-specific gene expression occurred mainly at gene loci with poised accessible regions but not DARs without accompanying drastic chromatin reorganization, suggesting that altered cell-extrinsic stimuli or signals from aged niche largely account for this process. Our findings provide key epigenetic molecular insights into HSC aging and serve as a reference for future analysis.  Overall design: RNA-seq of Ezh2 knock out HSCs