BACKGROUND:Stem cell homeostasis is a central aspect of tissue maintenance and regeneration in response to injury or stress, which is perhaps best exemplified in hematopoietic stem and progenitor cells (HSPCs). Adult HSPCs show extensive self-renewal capacity and allow for long-term multi-lineage differentiation in the hematopoietic system. If deregulated, this process can result in a variety of disorders including myeloproliferative diseases and loss of immune function. Recent evidence suggests that DNA damage accumulation or a defective response to genotoxic stress are key contributors to aberrant expansion and concomitant functional exhaustion of HSPCs, which is further associated with altered expression of hematopoietic differentiation factors and age-associated stem cell decline. Determining the factors that affect HSPC function with regard to both DNA damage accumulation and epigenetic deregulation is, thus, essential for our understanding of HSPC homeostasis in aging and disease. The (histone) deacetylase SIRT1 was shown to be critically involved in several aspects of the DNA damage response as well as in the transcriptional regulation of a variety of key developmental regulators, placing it at the crossroads of DNA damage and epigenetic gene regulation. Supporting a role for SIRT1 in the epigenetic (de)regulation of stem cell fate, SIRT1 was shown to be part of a novel polycomb repressive complex, PRC4, which has distinctive chromatin modifying activity and is expressed specifically in undifferentiated cells including embryonic stem cells. Notably, polycomb Group (PcG)-mediated gene regulation is known to play a critical role in HSPC maintenance. Together, these observations call for an in depth investigation of SIRT1 function in hematopoietic stem cells.RESULTS AND FUTURE DIRECTIONS:To unequivocally address the role of Sirt1 in HSPCs, we induced ablation of Sirt1 in adult mice, thereby avoiding possible developmental defects associated with conventional Sirt1 knock-out mice. In so doing, we found that Sirt1 ablation promotes aberrant expansion of HSPCs in vivo specifically in response to hematopoietic stress, as shown for cytotoxic and genotoxic agents. Sirt1 ablation in HSPCs further led to a reduced ability to repair DNA damage, resulting in genomic instability and a progressive loss of long-term progenitors following stress-induced proliferative expansion or DNA damage. Through gene expression profiling, we implicate inappropriate induction of the PcG target gene Hoxa9 as the mechanism that promotes HSPC expansion in the absence of Sirt1. Hoxa9 is a critical regulator of HSPC maintenance and Hoxa9 overexpression confers a growth advantage to HSPCs. Using chromatin immunoprecipitation (ChIP), we found Sirt1 is enriched at the Hoxa9 gene body. Notably, Sirt1 deletion caused a significant increase in Hoxa9-associated K16-acetylated histone H4 (H4K16Ac), which is a modification associated with transcriptional activation and a known preferred enzymatic target for Sirt1 deacetylase activity. Increased H4K16-Ac was previously shown to be inversely correlated with PcG-associated repressive marks, and consistent with this notion, H3K27 trimethylation was reduced in the absence of Sirt1. Moreover, we observed a strikingly similar pattern of enrichment for both H3K27me3 and Sirt1 across the Hoxa9 gene. While the physiological consequences of PcG-associated Sirt1 remained unexplored, our data now provide first in vivo evidence that Sirt1 can modulate the expression of PcG-target genes to promote the maintenance of hematopoietic stem cells. Together, our findings demonstrate a dual role for Sirt1 in HSPC homeostasis, both via epigenetic regulation of a key developmental gene and by promoting genome stability in adult stem cells.IMPLICATIONS: The tight regulation of HSPC proliferation is a central aspect of normal mammalian development and immune function. Moreover, perturbed quiescence and aberrant proliferation of HSPCs, as observed here, are key steps in malignant transformation of both acute and chronic myeloid leukemia, which have furthermore been linked to aberrant Hoxa9 expression. Our work suggest that (pharmacological) activation of Sirt1 may provide a means to interfere with HSPC expansion, concomitant exhaustion and possibly malignant transformation, particularly in situations of chronic hematopoietic stress. Taken together our data, thus, add to the understanding of stress-induced changes in HSPC homeostasis with implications for HSPC maintenance with age as well as during malignant transformation.