Quiescent hematopoietic stem cells accumulate DNA damage during aging that is repaired upon entry into cell cycle

Accumulation of DNA damage in the aged hematopoietic stem cell compartment

Aging is associated with loss of functional potential of multiple tissue systems, and there has been significant interest in understanding how tissue-specific cells contribute to this decline. DNA damage accumulation has been widely associated with aging in differentiated cell types. However, tissue-specific stem cells were once thought to be a geno-protected population, as damage accrued in a stem cell population has the potential to be inherited by differentiated progeny, as well as propagated within the stem cell compartment through self-renewal divisions. This review will discuss the evidence for DNA damage accumulation in the aged HSC compartment, potential drivers, and finally the consequences of the acquired damage.

Inflamm-Aging of Hematopoiesis, Hematopoietic Stem Cells, and the Bone Marrow Microenvironment

All hematopoietic and immune cells are continuously generated by hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) through highly organized process of stepwise lineage commitment. In the steady state, HSCs are mostly quiescent, while HPCs are actively proliferating and contributing to daily hematopoiesis. In response to hematopoietic challenges, e.g., life-threatening blood loss, infection, and inflammation, HSCs can be activated to proliferate and engage in blood formation. The HSC activation induced by hematopoietic demand is mediated by direct or indirect sensing mechanisms involving pattern recognition receptors or cytokine/chemokine receptors. In contrast to the hematopoietic challenges with obvious clinical symptoms, how the aging process, which involves low-grade chronic inflammation, impacts hematopoiesis remains undefined. Herein, we summarize recent findings pertaining to functional alternations of hematopoiesis, HSCs, and the bone marrow (BM) microenvironment during the processes of aging and inflammation and highlight some common cellular and molecular changes during the processes that influence hematopoiesis and its cells of origin, HSCs and HPCs, as well as the BM microenvironment. We also discuss how age-dependent alterations of the immune system lead to subclinical inflammatory states and how inflammatory signaling might be involved in hematopoietic aging. Our aim is to present evidence supporting the concept of “Inflamm-Aging,” or inflammation-associated aging of hematopoiesis.

Aging of Stem and Progenitor Cells: Mechanisms, Impact on Therapeutic Potential, and Rejuvenation

It was once suggested that adult or tissue-specific stem cells may be immortal; however, several recently published data suggest that their efficacy is limited by natural aging in common with most other somatic cell types. Decreased activity of stem cells in old age raises questions as to whether the age of the donor should be considered during stem cell transplantation and at what age the donor stem cells should be harvested to ensure the largest possible number of viable, functional, and non-altered stem cells. Although stem cells remain active into old age, changes in stem cells and their microenvironments inhibit their regenerative potential. The impact of aging on stem cell populations differs between tissues and depends on a number intrinsic and extrinsic factors, including systemic changes associated with immune system alterations. In this review, we describe key mechanisms of stem and progenitor cell aging and techniques that are currently used to identify signs of stem cells aging. Furthermore, we focus on the impact of aging on the capacity for proliferation, differentiation, and clinical use of stem cells. Finally, we detail the aging of embryonic, mesenchymal, and induced pluripotent stem cells, with particular emphasis on aging mechanisms and rejuvenation.

Hematopoietic Stem Cells Count and Remember Self-Renewal Divisions

The ability of cells to count and remember their divisions could underlie many alterations that occur during development, aging, and disease. We tracked the cumulative divisional history of slow-cycling hematopoietic stem cells (HSCs) throughout adult life. This revealed a fraction of rarely dividing HSCs that contained all the long-term HSC (LT-HSC) activity within the aging HSC compartment. During adult life, this population asynchronously completes four traceable symmetric self-renewal divisions to expand its size before entering a state of dormancy. We show that the mechanism of expansion involves progressively lengthening periods between cell divisions, with long-term regenerative potential lost upon a fifth division. Our data also show that age-related phenotypic changes within the HSC compartment are divisional history dependent. These results suggest that HSCs accumulate discrete memory stages over their divisional history and provide evidence for the role of cellular memory in HSC aging.

Quantification of in vivo progenitor mutation accrual with ultra-low error rate and minimal input DNA using SIP-HAVA-seq

Assaying in vivo accrual of DNA damage and DNA mutations by stem cells and pinpointing sources of damage and mutations would further our understanding of aging and carcinogenesis. Two main hurdles must be overcome. First, in vivo mutation rates are orders of magnitude lower than raw sequencing error rates. Second, stem cells are vastly outnumbered by differentiated cells, which have a higher mutation rate—quantification of stem cell DNA damage and DNA mutations is thus best performed from small, well-defined cell populations. Here we report a mutation detection technique, based on the “duplex sequencing” principle, with an error rate below ∼10⁻¹⁰ and that can start from as little as 50 pg DNA. We validate this technique, which we call SIP-HAVA-seq, by characterizing Caenorhabditis elegans germline stem cell mutation accrual and asking how mating affects that accrual. We find that a moderate mating-induced increase in cell cycling correlates with a dramatic increase in accrual of mutations. Intriguingly, these mutations consist chiefly of deletions in nonexpressed genes. This contrasts with results derived from mutation accumulation lines and suggests that mutation spectrum and genome distribution change with replicative age, chronological age, cell differentiation state, and/or overall worm physiological state. We also identify single-stranded gaps as plausible deletion precursors, providing a starting point to identify the molecular mechanisms of mutagenesis that are most active. SIP-HAVA-seq provides the first direct, genome-wide measurements of in vivo mutation accrual in stem cells and will enable further characterization of underlying mechanisms and their dependence on age and cell state.

The aging hematopoietic stem cell niche: Phenotypic and functional changes and mechanisms that contribute to hematopoietic aging

The hematopoietic system has the remarkable ability to provide a lifelong supply of mature cells that make up the entire blood and immune system. However, similar to other adult stem cell niches, the hematopoietic system is vulnerable to the detrimental effects of aging. This is a substantial health concern as the trend for population aging continues to increase. Identifying mechanisms that underlie hematopoietic aging is vital for understanding hematopoietic-related diseases. In this review, we first discuss the cellular hierarchy of the hematopoietic system and the components that make up the surrounding hematopoietic niche. We then provide an overview of the major phenotypes associated with hematopoietic aging and discuss recent research investigating cell-intrinsic and cell-extrinsic mechanisms of hematopoietic stem cell (HSCs) aging. We end by discussing the exciting new concept of possibly reversing the HSC aging process along with outstanding questions that remain to be answered.

Endogenous DNA Damage Leads to p53-Independent Deficits in Replicative Fitness in Fetal Murine Fancd2-/- Hematopoietic Stem and Progenitor Cells

Our mechanistic understanding of Fanconi anemia (FA) pathway function in hematopoietic stem and progenitor cells (HSPCs) owes much to their role in experimentally induced DNA crosslink lesion repair. In bone marrow HSPCs, unresolved stress confers p53-dependent apoptosis and progressive cell attrition. The role of FA proteins during hematopoietic development, in the face of physiological replicative demand, remains elusive. Here, we reveal a fetal HSPC pool in Fancd2^−/− mice with compromised clonogenicity and repopulation. Without experimental manipulation, fetal Fancd2^−/− HSPCs spontaneously accumulate DNA strand breaks and RAD51 foci, associated with a broad transcriptional DNA-damage response, and constitutive activation of ATM as well as p38 stress kinase. Remarkably, the unresolved stress during rapid HSPC pool expansion does not trigger p53 activation and apoptosis; rather, it constrains proliferation. Collectively our studies point to a role for the FA pathway during hematopoietic development and provide a new model for studying the physiological function of FA proteins.

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Fancd2^−/− fetal HSPCs show spontaneous deficits on replicative stress in development

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Fancd2^−/− FL HSPCs show activated DNA-damage responses and strand-break accumulation

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Fancd2^−/− FL deficits occur without apoptosis and independent of p53 activation

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MAPK (p38) inhibition rescues Fancd2^−/− progenitor defects in vitro and in vivo

Do DNA Double-Strand Breaks Drive Aging?

DNA double-strand breaks (DSBs) are rare, but highly toxic, lesions requiring orchestrated and conserved machinery to prevent adverse consequences, such as cell death and cancer-causing genome structural mutations. DSBs trigger the DNA damage response (DDR) that directs a cell to repair the break, undergo apoptosis, or become senescent. There is increasing evidence that the various endpoints of DSB processing by different cells and tissues are part of the aging phenotype, with each stage of the DDR associated with specific aging pathologies. In this Perspective, we discuss the possibility that DSBs are major drivers of intrinsic aging, highlighting the dynamics of spontaneous DSBs in relation to aging, the distinct age-related pathologies induced by DSBs, and the segmental progeroid phenotypes in humans and mice with genetic defects in DSB repair. A model is presented as to how DSBs could drive some of the basic mechanisms underlying age-related functional decline and death.

In vivo transduction of primitive mobilized hematopoietic stem cells after intravenous injection of integrating adenovirus vectors

Current protocols for hematopoietic stem/progenitor cell (HSPC) gene therapy, involving the transplantation of ex vivo genetically modified HSPCs are complex and not without risk for the patient. We developed a new approach for in vivo HSPC transduction that does not require myeloablation and transplantation. It involves subcutaneous injections of granulocyte-colony-stimulating factor/AMD3100 to mobilize HSPCs from the bone marrow (BM) into the peripheral blood stream and the IV injection of an integrating, helper-dependent adenovirus (HD-Ad5/35⁺⁺) vector system. These vectors target CD46, a receptor that is uniformly expressed on HSPCs. We demonstrated in human CD46 transgenic mice and immunodeficient mice with engrafted human CD34⁺ cells that HSPCs transduced in the periphery home back to the BM where they stably express the transgene. In hCD46 transgenic mice, we showed that our in vivo HSPC transduction approach allows for the stable transduction of primitive HSPCs. Twenty weeks after in vivo transduction, green fluorescent protein (GFP) marking in BM HSPCs (Lin^-Sca1⁺Kit^- cells) in most of the mice was in the range of 5% to 10%. The percentage of GFP-expressing primitive HSPCs capable of forming multilineage progenitor colonies (colony-forming units [CFUs]) increased from 4% of all CFUs at week 4 to 16% at week 12, indicating transduction and expansion of long-term surviving HSPCs. Our approach was well tolerated, did not result in significant transduction of nonhematopoietic tissues, and was not associated with genotoxicty. The ability to stably genetically modify HSPCs without the need of myeloablative conditioning is relevant for a broader clinical application of gene therapy.

Mutant IDH1 Downregulates ATM and Alters DNA Repair and Sensitivity to DNA Damage Independent of TET2

Mutations in the isocitrate dehydrogenase-1 gene (IDH1) are common drivers of acute myeloid leukemia (AML) but their mechanism is not fully understood. It is thought that IDH1 mutants act by inhibiting TET2 to alter DNA methylation, but there are significant unexplained clinical differences between IDH1- and TET2-mutant diseases. We have discovered that mice expressing endogenous mutant IDH1 have reduced numbers of hematopoietic stem cells (HSCs), in contrast to Tet2 knockout (TET2-KO) mice. Mutant IDH1 downregulates the DNA damage (DD) sensor ATM by altering histone methylation, leading to impaired DNA repair, increased sensitivity to DD, and reduced HSC self-renewal, independent of TET2. ATM expression is also decreased in human IDH1-mutated AML. These findings may have implications for treatment of IDH-mutant leukemia.

When stem cells grow old: phenotypes and mechanisms of stem cell aging

All multicellular organisms undergo a decline in tissue and organ function as they age. An attractive theory is that a loss in stem cell number and/or activity over time causes this decline. In accordance with this theory, aging phenotypes have been described for stem cells of multiple tissues, including those of the hematopoietic system, intestine, muscle, brain, skin and germline. Here, we discuss recent advances in our understanding of why adult stem cells age and how this aging impacts diseases and lifespan. With this increased understanding, it is feasible to design and test interventions that delay stem cell aging and improve both health and lifespan.

Aging of hematopoietic stem cells: DNA damage and mutations?

Aging in the hematopoietic system and the stem cell niche contributes to aging-associated phenotypes of hematopoietic stem cells (HSCs), including leukemia and aging-associated immune remodeling. Among others, the DNA damage theory of aging of HSCs is well established, based on the detection of a significantly larger amount of γH2AX foci and a higher tail moment in the comet assay, both initially thought to be associated with DNA damage in aged HSCs compared with young cells, and bone marrow failure in animals devoid of DNA repair factors. Novel data on the increase in and nature of DNA mutations in the hematopoietic system with age, the quality of the DNA damage response in aged HSCs, and the nature of γH2AX foci question a direct link between DNA damage and the DNA damage response and aging of HSCs, and rather favor changes in epigenetics, splicing-factors or three-dimensional architecture of the cell as major cell intrinsic factors of HSCs aging. Aging of HSCs is also driven by a strong contribution of aging of the niche. This review discusses the DNA damage theory of HSC aging in the light of these novel mechanisms of aging of HSCs.

Aging, Clonality, and Rejuvenation of Hematopoietic Stem Cells

Aging is associated with reduced organ function and increased disease incidence. Hematopoietic stem cell (HSC) aging driven by both cell intrinsic and extrinsic factors is linked to impaired HSC self-renewal and regeneration, aging-associated immune remodeling, and increased leukemia incidence. Compromised DNA damage responses and the increased production of reactive oxygen species (ROS) have been previously causatively attributed to HSC aging. However, recent paradigm-shifting concepts, such as global epigenetic and cytoskeletal polarity shifts, cellular senescence, as well as the clonal selection of HSCs upon aging, provide new insights into HSC aging mechanisms. Rejuvenating agents that can reprogram the epigenetic status of aged HSCs or senolytic drugs that selectively deplete senescent cells provide promising translational avenues for attenuating hematopoietic aging and, potentially, alleviating aging-associated immune remodeling and myeloid malignancies.

The bulk of the hematopoietic stem cell population is dispensable for murine steady-state and stress hematopoiesis

Long-term repopulating (LT) hematopoietic stem cells (HSCs) are the most undifferentiated cells at the top of the hematopoietic hierarchy. The regulation of HSC pool size and its contribution to hematopoiesis are incompletely understood. We depleted hematopoietic stem and progenitor cells (HSPCs) in adult mice in situ and found that LT-HSCs recovered from initially very low levels (<1%) to below 10% of normal numbers but not more, whereas progenitor cells substantially recovered shortly after depletion. In spite of the persistent and massive reduction of LT-HSCs, steady-state hematopoiesis was unaffected and residual HSCs remained quiescent. Hematopoietic stress, although reported to recruit quiescent HSCs into cycle, was well tolerated by HSPC-depleted mice and did not induce expansion of the small LT-HSC compartment. Only upon 5-fluorouracil treatment was HSPC-depleted bone marrow compromised in reconstituting hematopoiesis, demonstrating that HSCs and early progenitors are crucial to compensate myeloablation. Hence, a contracted HSC compartment cannot recover in situ to its original size, and normal steady-state blood cell generation is sustained with <10% of normal LT-HSC numbers without increased contribution of the few residual cells.

DNA Damage and Aging Around the Clock

The hematopoietic system undergoes many changes during aging, but the causes and molecular mechanisms behind these changes are not well understood. Wang et al. have recently implicated a circadian rhythm gene, Per2, as playing a role in the DNA damage response and in the expression of lymphoid genes in aged hematopoietic stem cells.

To wake up cancer stem cells, or to let them sleep, that is the question

Cancer stem cells (CSCs) generate transient-amplifying cells and thereby contribute to cancer propagation. A fuller understanding of the biological features of CSCs is expected to lead to the development of new anticancer therapies capable of eradicating this life-threatening disease. Cancer stem cells are known to maintain a non-proliferative state and to enter the cell cycle only infrequently. Given that conventional anticancer therapies preferentially target dividing cells, CSCs are resistant to such treatments, with those remaining after elimination of bulk cancer cells potentially giving rise to disease relapse and metastasis as they re-enter the cell cycle after a period of latency. Targeting of the switch between quiescence and proliferation in CSCs is therefore a potential strategy for preventing the reinitiation of malignancy, underscoring the importance of elucidation of the mechanisms by which these cells are maintained in the quiescent state. The fundamental properties of CSCs are thought to be governed cooperatively by internal molecules and cues from the external microenvironment (stem cell niche). Several such intrinsic and extrinsic regulators are responsible for the control of cell cycle progression in CSCs. In this review, we address two opposite approaches to the therapeutic targeting of CSCs - wake-up and hibernation therapies - that either promote or prevent the entry of CSCs into the cell cycle, respectively, and we discuss the potential advantages and risks of each strategy.

Terminal differentiation induction as DNA damage response in hematopoietic stem cells by GADD45A

Hematopoietic stem cells (HSCs) sustain lifelong blood cell regeneration by balancing their ability for self-renewal with their ability to differentiate into all blood cell types. To prevent organ exhaustion and malignant transformation, long-lived HSCs, in particular, must be protected from exogenous and endogenous stress, which cause severe DNA damage. When DNA is damaged, distinct DNA repair mechanisms and cell fate controls occur in adult HSCs compared with committed cells. Growth arrest and DNA damage-inducible 45 alpha (GADD45A) is known to coordinate a variety of cellular stress responses, indicating the molecule is an important stress mediator. So far, the function of GADD45A in hematopoietic stem and progenitor cells is controversial and appears highly dependent on the cell type and stress stimulus. Recent studies have analyzed its role in cell fate decision control of prospectively isolated HSCs and have revealed unexpected functions of GADD45A, as discussed here. The upregulation of GADD45A by DNA damage-causing conditions results in enhanced HSC differentiation, probably to efficiently eliminate aberrant HSCs from the system. These findings, in concert with a few studies on other stem cell systems, have led us to propose DNA damage-induced differentiation as a novel DNA damage response mechanism in stem cells that circumvents the fatal consequences of cumulative DNA damage in the stem cell compartment.

DNA Damage Response in Hematopoietic Stem Cell Ageing

Maintenance of tissue-specific stem cells is vital for organ homeostasis and organismal longevity. Hematopoietic stem cells (HSCs) are the most primitive cell type in the hematopoietic system. They divide asymmetrically and give rise to daughter cells with HSC identity (self-renewal) and progenitor progenies (differentiation), which further proliferate and differentiate into full hematopoietic lineages. Mammalian ageing process is accompanied with abnormalities in the HSC self-renewal and differentiation. Transcriptional changes and epigenetic modulations have been implicated as the key regulators in HSC ageing process. The DNA damage response (DDR) in the cells involves an orchestrated signaling pathway, consisting of cell cycle regulation, cell death and senescence, transcriptional regulation, as well as chromatin remodeling. Recent studies employing DNA repair-deficient mouse models indicate that DDR could intrinsically and extrinsically regulate HSC maintenance and play important roles in tissue homeostasis of the hematopoietic system. In this review, we summarize the current understanding of how the DDR determines the HSC fates and finally contributes to organismal ageing.

In vivo blunt-end cloning through CRISPR/Cas9-facilitated non-homologous end-joining

The CRISPR/Cas9 system facilitates precise DNA modifications by generating RNA-guided blunt-ended double-strand breaks. We demonstrate that guide RNA pairs generate deletions that are repaired with a high level of precision by non-homologous end-joining in mammalian cells. We present a method called knock-in blunt ligation for exploiting these breaks to insert exogenous PCR-generated sequences in a homology-independent manner without loss of additional nucleotides. This method is useful for making precise additions to the genome such as insertions of marker gene cassettes or functional elements, without the need for homology arms. We successfully utilized this method in human and mouse cells to insert fluorescent protein cassettes into various loci, with efficiencies up to 36% in HEK293 cells without selection. We also created versions of Cas9 fused to the FKBP12-L106P destabilization domain in an effort to improve Cas9 performance. Our in vivo blunt-end cloning method and destabilization-domain-fused Cas9 variant increase the repertoire of precision genome engineering approaches.

The effects of proliferation and DNA damage on hematopoietic stem cell function determine aging

In most of the mammalian tissues, homeostasis as well as injury repair depend upon a small number of resident adult stem cells. The decline in tissue/organ function in aged organisms has been directly linked with poorly functioning stem cells. Altered function of hematopoietic stem cells (HSCs) is at the center of an aging hematopoietic system, a tissue with high cellular turnover. Poorly engrafting, myeloid-biased HSCs with higher levels of DNA damage accumulation are the hallmark features of an aged hematopoietic system. These cells show a higher proliferation rate than their younger counterparts. It was proposed that quiescence of these cells over long period of time leads to accumulation of DNA damage, eventually resulting in poor function/pathological conditions in hematopoietic system. However, various mouse models with premature aging phenotype also show highly proliferative HSCs. This review examines the evidence that links proliferation of HSCs with aging, which leads to functional changes in the hematopoietic system. Developmental Dynamics 245:739-750, 2016. © 2016 Wiley Periodicals, Inc.

Per2 induction limits lymphoid-biased haematopoietic stem cells and lymphopoiesis in the context of DNA damage and ageing

Ageing-associated impairments in haemato-lymphopoiesis are associated with DNA damage accumulation and reduced maintenance of lymphoid-biased (Ly-biased) compared with myeloid-biased (My-biased) haematopoietic stem cells (HSCs). Here, in vivo RNAi screening identifies period circadian clock 2 (Per2) as a critical factor limiting the maintenance of HSCs in response to DNA damage and ageing. Under these conditions, Per2 is activated predominantly in Ly-biased HSCs and stimulates DNA damage signalling and p53-dependent apoptosis in haematopoietic cells. Per2 deletion ameliorates replication stress and DNA damage responses in haematopoietic cells, thereby improving the maintenance of Ly-biased HSCs, lymphopoiesis, and immune function in ageing mice without increasing the accumulation of DNA damage. Per2-deficient mice retain Batf/p53-dependent induction of differentiation of HSCs in response to DNA damage and exhibit an elongated lifespan. Together, these results identify Per2 as a negative regulator of Ly-biased HSCs and immune functions in response to DNA damage and ageing.

TGF-β Inhibition Rescues Hematopoietic Stem Cell Defects and Bone Marrow Failure in Fanconi Anemia

Fanconi anemia (FA) is an inherited DNA repair disorder characterized by progressive bone marrow failure (BMF) from hematopoietic stem and progenitor cell (HSPC) attrition. A greater understanding of the pathogenesis of BMF could improve the therapeutic options for FA patients. Using a genome-wide shRNA screen in human FA fibroblasts, we identify transforming growth factor-β (TGF-β) pathway-mediated growth suppression as a cause of BMF in FA. Blocking the TGF-β pathway improves the survival of FA cells and rescues the proliferative and functional defects of HSPCs derived from FA mice and FA patients. Inhibition of TGF-β signaling in FA HSPCs results in elevated homologous recombination (HR) repair with a concomitant decrease in non-homologous end-joining (NHEJ), accounting for the improvement in cellular growth. Together, our results suggest that elevated TGF-β signaling contributes to BMF in FA by impairing HSPC function and may be a potential therapeutic target for the treatment of FA.

Identification of early myeloid progenitors as immunosuppressive cells

Growing evidence suggests that hematopoietic stem/progenitor cells (HSPCs), precursors of mature immune cells, may play a direct role in immunosurveillance. Early myeloid progenitors are the major components of HSPCs and they often undergo extensive expansion in stress as a result of myeloid-biased hematopoiesis. Yet, the precise function of early myeloid progenitors remains unclear. Here we show that during tumor progression, mouse granulocyte/macrophage progenitors (GMPs) but not common myeloid progenitors (CMPs) are markedly expanded within the bone marrow and blood of mice. Interestingly, both GMPs and CMPs freshly isolated from either tumor-bearing or naïve animals are capable of inhibiting polyclonal stimuli- and alloantigen-induced T cell proliferation, with tumor host-derived cells having elevated activities. Strikingly, these early myeloid progenitor cells even display much stronger suppressive capacity than the classical myeloid-derived suppressive cells. Analysis of GMPs indicates that they express iNOS and can secrete high levels of NO. Further studies unusing iNOS specific inhibitors reveal that the immunosuppression of GMPs is, to a large extent, NO-dependent. GMPs can also efficiently induce regulatory T cell development. These studies demonstrate that early myeloid progenitors can act as immunosuppressive cells. This finding provides novel insights into the functional diversity and plasticity of early myeloid progenitor cells.

Roles of IDH1/2 and TET2 mutations in myeloid disorders

Mutations of the epigenetic enzymes isocitrate dehydrogenase (IDH) 1 and 2, and the methylcytosine dioxygenase 'ten-eleven translocation 2' (TET2), are common in human myeloid malignancies and drivers of these disorders but the underlying mechanisms remain obscure. This review examines mutant IDH1/2 and TET2 enzymes in the context of responses to DNA damage and their potential involvement in age-related genomic instability. The clinical relevance of these findings and their potential application in novel therapeutic strategies is also discussed.

Repression of p53-target gene Bbc3/PUMA by MYSM1 is essential for the survival of hematopoietic multipotent progenitors and contributes to stem cell maintenance

p53 is a central mediator of cellular stress responses, and its precise regulation is essential for the normal progression of hematopoiesis. MYSM1 is an epigenetic regulator essential for the maintenance of hematopoietic stem cell (HSC) function, hematopoietic progenitor survival, and lymphocyte development. We recently demonstrated that all developmental and hematopoietic phenotypes of Mysm1 deficiency are p53-mediated and rescued in the Mysm1(-/-)p53(-/-) mouse model. However, the mechanisms triggering p53 activation in Mysm1(-/-) HSPCs, and the pathways downstream of p53 driving different aspects of the Mysm1(-/-) phenotype remain unknown. Here we show the transcriptional activation of p53 stress responses in Mysm1(-/-) HSPCs. Mechanistically, we find that the MYSM1 protein associates with p53 and colocalizes to promoters of classical p53-target genes Bbc3/PUMA (p53 upregulated modulator of apoptosis) and Cdkn1a/p21. Furthermore, it antagonizes their p53-driven expression by modulating local histone modifications (H3K27ac and H3K4me3) and p53 recruitment. Using double-knockout mouse models, we establish that PUMA, but not p21, is an important mediator of p53-driven Mysm1(-/-) hematopoietic dysfunction. Specifically, Mysm1(-/-)Puma(-/-) mice show full rescue of multipotent progenitor (MPP) viability, partial rescue of HSC quiescence and function, but persistent lymphopenia. Through transcriptome analysis of Mysm1(-/-)Puma(-/-) MPPs, we demonstrate strong upregulation of other p53-induced mediators of apoptosis and cell-cycle arrest. The full viability of Mysm1(-/-)Puma(-/-) MPPs, despite strong upregulation of many other pro-apoptotic mediators, establishes PUMA as the essential non-redundant effector of p53-induced MPP apoptosis. Furthermore, we identify potential mediators of p53-dependent but PUMA-independent Mysm1(-/-)hematopoietic deficiency phenotypes. Overall, our study provides novel insight into the cell-type-specific roles of p53 and its downstream effectors in hematopoiesis using unique models of p53 hyperactivity induced by endogenous stress. We conclude that MYSM1 is a critical negative regulator of p53 transcriptional programs in hematopoiesis, and that its repression of Bbc3/PUMA expression is essential for MPP survival, and partly contributes to maintaining HSC function.

Normal and Neoplastic Stem Cells

A stem cell is broadly defined as a cell that retains the capacity to self-renew, a feature that confers the ability to continuously make identical daughter cells or additional cells that will differentiate into downstream progeny. This highly regulated genetic program to retain "stemness" is under active investigation. Research in our laboratory has explored similarities and differences in embryonic, tissue-specific, and neoplastic stem cells and their terminally differentiated counterparts. In this review, we will focus on the contributions of our laboratory, in particular on the studies that identified the mouse hematopoietic stem cell (HSC) and the human leukemic stem cell. These studies have led to significant improvements in both preclinical and clinical research, including improved clinical bone marrow transplantation protocols, isolation of nonleukemic HSCs, a cancer immunotherapy currently in clinical trials, and development of a HSC reporter mouse. These studies and the current follow-up research by us and others will continue to identify the properties, function, and regulation of both normal and neoplastic stem cells.

Metabolic Features of Cancer Treatment Resistance

A major barrier to achieving durable remission and a definitive cure in oncology patients is the emergence of tumor resistance, a common outcome of different disease types, and independent from the therapeutic approach undertaken. In recent years, subpopulations of slow-cycling cells endowed with enhanced tumorigenic potential and multidrug resistance have been isolated in different tumors, and mounting experimental evidence suggests these resistant cells are responsible for tumor relapse. An in-depth metabolic characterization of resistant tumor stem cells revealed that they rely more on mitochondrial respiration and less on glycolysis than other tumor cells, a finding that challenges the assumption that tumors have a primarily glycolytic metabolism and defective mitochondria. The demonstration of a metabolic program in resistant tumorigenic cells that may be present in the majority of tumors has important therapeutic implications and is a critical consideration as we address the challenge of identifying new vulnerabilities that might be exploited therapeutically.

Genetic and Epigenetic Mechanisms That Maintain Hematopoietic Stem Cell Function

All hematopoiesis cells develop from multipotent progenitor cells. Hematopoietic stem cells (HSC) have the ability to develop into all blood lineages but also maintain their stemness. Different molecular mechanisms have been identified that are crucial for regulating quiescence and self-renewal to maintain the stem cell pool and for inducing proliferation and lineage differentiation. The stem cell niche provides the microenvironment to keep HSC in a quiescent state. Furthermore, several transcription factors and epigenetic modifiers are involved in this process. These create modifications that regulate the cell fate in a more or less reversible and dynamic way and contribute to HSC homeostasis. In addition, HSC respond in a unique way to DNA damage. These mechanisms also contribute to the regulation of HSC function and are essential to ensure viability after DNA damage. How HSC maintain their quiescent stage during the entire life is still matter of ongoing research. Here we will focus on the molecular mechanisms that regulate HSC function.

Stem Cell-Specific Mechanisms Ensure Genomic Fidelity within HSCs and upon Aging of HSCs

Whether aged hematopoietic stem and progenitor cells (HSPCs) have impaired DNA damage repair is controversial. Using a combination of DNA mutation indicator assays, we observe a 2- to 3-fold increase in the number of DNA mutations in the hematopoietic system upon aging. Young and aged hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) do not show an increase in mutation upon irradiation-induced DNA damage repair, and young and aged HSPCs respond very similarly to DNA damage with respect to cell-cycle checkpoint activation and apoptosis. Both young and aged HSPCs show impaired activation of the DNA-damage-induced G1-S checkpoint. Induction of chronic DNA double-strand breaks by zinc-finger nucleases suggests that HSPCs undergo apoptosis rather than faulty repair. These data reveal a protective mechanism in both the young and aged hematopoietic system against accumulation of mutations in response to DNA damage.

ΔNp63α induces quiescence and downregulates the BRCA1 pathway in estrogen receptor-positive luminal breast cancer cell line MCF7 but not in other breast cancer cell lines

Despite apparent resection of tumors, breast cancer patients often suffer relapse due to remnant dormant tumor cells. Although quiescence of cancer stem cells is thought as one of the mechanisms regulating dormancy, the mechanism underlying quiescence is unclear. Since ΔNp63α, an isoform of p51/p63, is crucial in the maintenance of stem cells within mammary epithelium, we investigated its roles in the regulation of dormancy in normal and malignant breast cells. Inducible expression of ΔNp63α in MCF7 estrogen receptor positive (ER+) luminal breast cancer cells led to quiescence and acquisition of progenitor-like properties. Judging from mRNA-microRNA microarray analysis, activation of bone morphogenetic protein (BMP) signaling and inhibition of Wnt signaling emerged as prominent mechanisms underlying ΔNp63α-dependent induction of quiescence and acquisition of stemness in MCF7. More interestingly, through Ingenuity Pathway analysis, we found for the first time that BRCA1 pathway was the most significantly downregulated pathway by ΔNp63α expression in quiescent MCF7 cells, where miR-205 was a downstream mediator. Furthermore, ΔNp63α-expressing MCF7 cells exhibited resistance to paclitaxel and doxorubicin. Expression of ΔNp63α in normal MCF10A basal cells increased proliferation and stemness, but did not affect more aggressive luminal (T47D) and basal (MDA-MB-231) cells with p53 mutation. Gene expression datasets analyses suggested that ΔNp63 expression is associated with relapse-free survival of luminal A/B-type patients, but not of the other subtypes. Our results established a cell type-specific function of ΔNp63α in induction of quiescence and downregulation of the BRCA1 pathway which suggested a role of ΔNp63α in the dormancy of luminal breast cancers.

Highly proliferative primitive fetal liver hematopoietic stem cells are fueled by oxidative metabolic pathways

Hematopoietic stem cells (HSCs) in the fetal liver (FL) unlike adult bone marrow (BM) proliferate extensively, posing different metabolic demands. However, metabolic pathways responsible for the production of energy and cellular building blocks in FL HSCs have not been described. Here, we report that FL HSCs use oxygen dependent energy generating pathways significantly more than their BM counterparts. RNA-Seq analysis of E14.5 FL versus BM derived HSCs identified increased expression levels of genes involved in oxidative phosphorylation (OxPhos) and the citric acid cycle (TCA). We demonstrated that FL HSCs contain more mitochondria than BM HSCs, which resulted in increased levels of oxygen consumption and reactive oxygen species (ROS) production. Higher levels of DNA repair and antioxidant pathway gene expression may prevent ROS-mediated (geno)toxicity in FL HSCs. Thus, we here for the first time highlight the underestimated importance of oxygen dependent pathways for generating energy and building blocks in FL HSCs.

HIV-1 Vpr- and Reverse Transcription-Induced Apoptosis in Resting Peripheral Blood CD4 T Cells and Protection by Common Gamma-Chain Cytokines

HIV-1 infection leads to the progressive depletion of the CD4 T cell compartment by various known and unknown mechanisms. In vivo, HIV-1 infects both activated and resting CD4 T cells, but in vitro, in the absence of any stimuli, resting CD4 T cells from peripheral blood are resistant to infection. This resistance is generally attributed to an intracellular environment that does not efficiently support processes such as reverse transcription (RT), resulting in abortive infection. Here, we show that in vitro HIV-1 infection of resting CD4 T cells induces substantial cell death, leading to abortive infection. In vivo, however, various microenvironmental stimuli in lymphoid and mucosal tissues provide support for HIV-1 replication. For example, common gamma-chain cytokines (CGCC), such as interleukin-7 (IL-7), render resting CD4 T cells permissible to HIV-1 infection without inducing T cell activation. Here, we find that CGCC primarily allow productive infection by preventing HIV-1 triggering of apoptosis, as evidenced by early release of cytochrome c and caspase 3/7 activation. Cell death is triggered both by products of reverse transcription and by virion-borne Vpr protein, and CGCC block both mechanisms. When HIV-1 RT efficiency was enhanced by SIVmac239 Vpx protein, cell death was still observed, indicating that the speed of reverse transcription and the efficiency of its completion contributed little to HIV-1-induced cell death in this system. These results show that a major restriction on HIV-1 infection in resting CD4 T cells resides in the capacity of these cells to survive the early steps of HIV-1 infection.

IMPORTANCE

A major consequence of HIV-1 infection is the destruction of CD4 T cells. Here, we show that delivery of virion-associated Vpr protein and the process of reverse transcription are each sufficient to trigger apoptosis of resting CD4 T cells isolated from peripheral blood. While these 2 mechanisms have been previously described in various cell types, we show for the first time their concerted effect in inducing resting CD4 T cell depletion. Importantly, we found that cytokines such as IL-7 and IL-4, which are particularly active in sites of HIV-1 replication, protect resting CD4 T cells from these cytopathic effects and, primarily through this protection, rather than through enhancement of specific replicative steps, they promote productive infection. This study provides important new insights for the understanding of the early steps of HIV-1 infection and T cell depletion.

Reactive oxygen species contribute to dysfunction of bone marrow hematopoietic stem cells in aged C57BL/6 J mice

Background

Stem cells of intensely regenerative tissues are susceptible to cellular damage. Although the response to this process in hematopoietic stem cells (HSCs) is crucial, the mechanisms by which hematopoietic homeostasis is sustained are not completely understood. Aging increases reactive oxygen species (ROS) levels and inflammation, which contribute to increased proliferation, senescence and/or apoptosis, leading to self-renewal premature exhaustion. In this study, we assessed ROS production, DNA damage, apoptosis, senescence and plasticity in young, middle and aged (2-, 12- and 24-month-old, respectively) C57BL/6 J mice.

Results

Aged HSCs showed an increase in intracellular superoxide anion (1.4-fold), hydrogen peroxide (2-fold), nitric oxide (1.6-fold), peroxynitrite/hidroxil (2.6-fold) compared with young cells. We found that mitochondria and NADPHox were the major sources of ROS production in the three groups studied, whereas CYP450 contributed in middle and aged, and xanthine oxidase only in aged HSCs. In addition, we observed DNA damage and apoptosis in the middle (4.2- and 2-fold, respectively) and aged (6- and 4-fold, respectively) mice; aged mice also exhibited a significantly shorter telomere length (−1.8-fold) and a lower expression of plasticity markers.

Conclusion

These data suggest that aging impairs the functionality of HSCs and that these age-associated alterations may affect the efficacy of aged HSC recovery and transplantation.

Frequent Somatic Mutation in Adult Intestinal Stem Cells Drives Neoplasia and Genetic Mosaicism during Aging

Adult stem cells may acquire mutations that modify cellular behavior, leading to functional declines in homeostasis or providing a competitive advantage resulting in premalignancy. However, the frequency, phenotypic impact, and mechanisms underlying spontaneous mutagenesis during aging are unclear. Here, we report two mechanisms of genome instability in adult Drosophila intestinal stem cells (ISCs) that cause phenotypic alterations in the aging intestine. First, we found frequent loss of heterozygosity arising from mitotic homologous recombination in ISCs that results in genetic mosaicism. Second, somatic deletion of DNA sequences and large structural rearrangements, resembling those described in cancers and congenital diseases, frequently result in gene inactivation. Such modifications induced somatic inactivation of the X-linked tumor suppressor Notch in ISCs, leading to spontaneous neoplasias in wild-type males. Together, our findings reveal frequent genomic modification in adult stem cells and show that somatic genetic mosaicism has important functional consequences on aging tissues.

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The aging Drosophila intestine is genetically mosaic

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Somatic recombination, genomic deletions, and rearrangements occur in aging ISCs

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Somatic inactivation of the tumor-suppressor Notch causes male-specific neoplasia

Replication stress caused by low MCM expression limits fetal erythropoiesis and hematopoietic stem cell functionality

Replicative stress during embryonic development influences ageing and predisposition to disease in adults. A protective mechanism against replicative stress is provided by the licensing of thousands of origins in G1 that are not necessarily activated in the subsequent S-phase. These ‘dormant' origins provide a backup in the presence of stalled forks and may confer flexibility to the replication program in specific cell types during differentiation, a role that has remained unexplored. Here we show, using a mouse strain with hypomorphic expression of the origin licensing factor mini-chromosome maintenance (MCM)3 that limiting origin licensing in vivo affects the functionality of hematopoietic stem cells and the differentiation of rapidly-dividing erythrocyte precursors. Mcm3-deficient erythroblasts display aberrant DNA replication patterns and fail to complete maturation, causing lethal anemia. Our results indicate that hematopoietic progenitors are particularly sensitive to replication stress, and full origin licensing ensures their correct differentiation and functionality.

What causes hematopoietic stem cell loss of functionality? Here, Alvarez et al. show that loss of origin licensing factor MCM3 induces replicative stress (RS), causing aberrant erythrocyte maturation, but mice strains with higher tolerance to RS can overcome this defect.

Comparative long-term effects of interferon α and hydroxyurea on human hematopoietic progenitor cells

Interferon α (IFNα) is used clinically to restore polyclonal hematopoiesis in patients with the myeloproliferative neoplasms polycythemia vera and essential thrombocythemia and to improve chemosensitivity in chronic myeloid leukemia patients. However, the mechanisms by which IFNα affects disease-initiating hematopoietic stem and progenitor cells (HSPCs) remain poorly understood. Although IFNα has been found to transiently impair quiescence of murine hematopoietic stem cells, its effects on human HSPCs have not been studied in vivo. Here, we compared bone marrow serially obtained from patients with myeloproliferative neoplasms before and during pegylated IFNα treatment against marrow serially obtained from patients on hydroxyurea. The percentage of HSPCs actively undergoing cell cycle was increased after pegylated IFNα treatment in a majority of patients compared with hydroxyurea-treated controls, suggesting that IFNα promotes cell division. Furthermore, transcriptional profiling revealed that cell cycle-associated genes were induced, whereas genes involved in HSPC quiescence were repressed during IFNα treatment. Compared with hydroxyurea-treated controls, pegylated IFNα-treated patients had similar numbers of HSPCs, but increased numbers of hematopoietic progenitors as determined by colony formation assay, indicating an increase in myeloid proliferation/differentiation. These effects occurred regardless of JAK2 mutational status. Together, these data provide the first in vivo evidence that pegylated IFNα promotes cell division and differentiation of human HSPCs.

Single-cell transcriptomic reconstruction reveals cell cycle and multi-lineage differentiation defects in Bcl11a-deficient hematopoietic stem cells

BACKGROUND

Hematopoietic stem cells (HSCs) are a rare cell type with the ability of long-term self-renewal and multipotency to reconstitute all blood lineages. HSCs are typically purified from the bone marrow using cell surface markers. Recent studies have identified significant cellular heterogeneities in the HSC compartment with subsets of HSCs displaying lineage bias. We previously discovered that the transcription factor Bcl11a has critical functions in the lymphoid development of the HSC compartment.

RESULTS

In this report, we employ single-cell transcriptomic analysis to dissect the molecular heterogeneities in HSCs. We profile the transcriptomes of 180 highly purified HSCs (Bcl11a (+/+) and Bcl11a (-/-)). Detailed analysis of the RNA-seq data identifies cell cycle activity as the major source of transcriptomic variation in the HSC compartment, which allows reconstruction of HSC cell cycle progression in silico. Single-cell RNA-seq profiling of Bcl11a (-/-) HSCs reveals abnormal proliferative phenotypes. Analysis of lineage gene expression suggests that the Bcl11a (-/-) HSCs are constituted of two distinct myeloerythroid-restricted subpopulations. Remarkably, similar myeloid-restricted cells could also be detected in the wild-type HSC compartment, suggesting selective elimination of lymphoid-competent HSCs after Bcl11a deletion. These defects are experimentally validated in serial transplantation experiments where Bcl11a (-/-) HSCs are myeloerythroid-restricted and defective in self-renewal.

CONCLUSIONS

Our study demonstrates the power of single-cell transcriptomics in dissecting cellular process and lineage heterogeneities in stem cell compartments, and further reveals the molecular and cellular defects in the Bcl11a-deficient HSC compartment.

Fine-tuning the ubiquitin code at DNA double-strand breaks: deubiquitinating enzymes at work

Ubiquitination is a reversible protein modification broadly implicated in cellular functions. Signaling processes mediated by ubiquitin (ub) are crucial for the cellular response to DNA double-strand breaks (DSBs), one of the most dangerous types of DNA lesions. In particular, the DSB response critically relies on active ubiquitination by the RNF8 and RNF168 ub ligases at the chromatin, which is essential for proper DSB signaling and repair. How this pathway is fine-tuned and what the functional consequences are of its deregulation for genome integrity and tissue homeostasis are subject of intense investigation. One important regulatory mechanism is by reversal of substrate ubiquitination through the activity of specific deubiquitinating enzymes (DUBs), as supported by the implication of a growing number of DUBs in DNA damage response processes. Here, we discuss the current knowledge of how ub-mediated signaling at DSBs is controlled by DUBs, with main focus on DUBs targeting histone H2A and on their recent implication in stem cell biology and cancer.

[Hematopoietic stem cell exhaustion and advanced glycation end-products in the unexplained anemia of the elderly]

INTRODUCTION

More than 10% of the aged 65 years and over in the western world suffers anemia and in one third of them the cause of the anemia remains obscure. The unexplained anemia of the elderly (UAE) is considered an exclusion diagnosis, without the existence of a clear consensus to its clinical or experimental approach. There is an association between aging and anemia in studies performed in animals and in humans.

OBJECTIVES

To determine if there is evidence in the literature that supports hematopoietic stem cells (HSC) exhaustion and the advanced glycation end-products (AGE's) as a cause of UAE.

METHOD

A total of 32 combined texts (28 for HSC exhaustion and 4 for AGEs) were selected after an intensive review. Conclusions were associated with causes and effects of the HSC exhaustion and circulating AGE's over aging and anemia.

RESULTS

Only three works try to establish an association between UAE and HSC exhaustion, two of them disagreed in their conclusions, with the third one differing in the type of study. There is a relationship between anemia and AGEs increase and accumulation.

CONCLUSIONS

There is evidence in the literature that links the aging molecular and cellular mechanisms with the HSC exhaustion and the increase of AGE's. Furthermore; there is some evidence that both conditions determine the emergence of anemia associated with age in animals and in humans. There is little evidence in the literature to clarify the relationship between aging and UAE.

Genome instability of ageing stem cells--Induction and defence mechanisms

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Stem cell function is declining with increasing age.

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DNA lesions and mutations accumulate in ageing stem cells.

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Inability to repair DNA can lead to premature depletion of stem cell pools.

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Checkpoint function preserves genomic integrity at young age.

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Enforced checkpoint induction contributes to stem cell ageing.

The mammalian organism is comprised of tissue types with varying degrees of self-renewal and regenerative capacity. In most organs self-renewing tissue-specific stem and progenitor cells contribute to organ maintenance, and it is vital to maintain a functional stem cell pool to preserve organ homeostasis. Various conditions like tissue injury, stress responses, and regeneration challenge the stem cell pool to re-establish homeostasis (Fig. 1). However, with increasing age the functionality of adult stem cells declines and genomic mutations accumulate. These defects affect different cellular response pathways and lead to impairments in regeneration, stress tolerance, and organ function as well as to an increased risk for the development of ageing associated diseases and cancer. Maintenance of the genome appears to be of utmost importance to preserve stem cell function and to reduce the risk of ageing associated dysfunctions and pathologies. In this review, we discuss the causal link between stem cell dysfunction and DNA damage accrual, different strategies how stem cells maintain genome integrity, and how these processes are affected during ageing.

Increased oxidative stress and apoptosis in peripheral blood mononuclear cells of fructose-fed rats

BACKGROUND

Measuring of oxidative stress in peripheral blood mononuclear cells is a suitable model of dietary induced systemic oxidative stress. Thus, we aimed to evaluate whether a chronic high fructose intake could induce oxidative damage in peripheral blood and bone marrow mononuclear cells of rats.

METHODS

Animals were randomly assigned to the following groups: Control group (standard rat chow and tap water n=8), and Fructose group (standard rat chow and a 10% fructose solution in the drinking water n=8). Reactive oxygen species and cytokines were measure using flow cytometry in peripheral blood and bone-marrow mononuclear cells. Apoptotic cell death and the advanced oxidation protein products (AOPP) were also determined.

RESULTS

We observed a significant increase in ROS production in peripheral blood mononuclear cells of fructose group as compared to control rats. Apoptosis and the AOPP were higher in those animals underwent high fructose intake. Serum levels of IL-6 and IL-12 were also increased after 12 weeks of high fructose intake.

CONCLUSION

We concluded that fructose intake leads to systemic oxidative stress and pro-inflammatory condition which affect peripheral blood mononuclear cells and bone-marrow mononuclear cells viability.

Excessive proliferation and impaired function of primitive hematopoietic cells in bone marrow due to senescence post chemotherapy in a T cell acute lymphoblastic leukemia model

BACKGROUND

In clinic settings, rel apsed leukemic patients are found to be more fragile to chemotherapy due to delayed or incomplete hematopoietic recovery, and hematopoiesis of these patients seem to be impaired.

METHODS

We established a leukemia therapy model with a non-irradiated T cell acute lymphoblastic leukemia mouse model combined with cytarabine and cyclophosphamide. Dynamic kinetics and functional status of both primitive hematopoietic cells and leukemic cells in a leukemia host under the chemotherapy stress were comprehensively investigated.

RESULTS

We successfully established the leukemia therapy model with T lymphoblastic phenotype. After treatment with cytarabine and cyclophosphamide, the frequency of L(-)K(+)S(+) hematopoietic cells tides with the therapy, and stabled when the disease remission, then reduced when relapsed, while leukemic cells showed a delayed but consistent regeneration. Combination of chemotherapy significantly promote an early and transient entrance of L(-)K(+)S(+) hematopoietic cells into active proliferation and induction of apoptosis on L(-)K(+)S(+) cells in vivo. Moreover, in the competitive bone marrow transplantation assays, hematopoietic cells showed gradually diminished regenerative capacity. Testing of senescence-associated beta-galactosidase (SA-β gal) status showed higher levels in L(-)K(+)S(+) hematopoietic cells post therapy when compared with the control. Gene expression analysis of hematopoietic primitive cells revealed up-regulated p16, p21, and down-regulated egr1 and fos.

CONCLUSION

We conclude that primitive hematopoietic cells in bone marrow enter proliferation earlier than leukemic cells after chemotherapy, and gradually lost their regenerative capacity partly by senescence due to accelerated cycling.

The Fanconi anemia pathway is required for efficient repair of stress-induced DNA damage in haematopoietic stem cells

Within regenerating tissues, aging is characterized by a progressive general deterioration of organ function, thought to be driven by the gradual depletion of functional adult stem cells. Although there are probably multifactorial mechanisms that result in compromized stem cell functionality with advancing age, the accumulation of DNA damage within the stem cell compartment is likely to make a major contribution to this process. However, the physiologic source of DNA damage within the different tissue specific stem cell compartments remains to be determined, as does the fate of stem cells exposed to such damage. Using the haematopoietic system as a model organ, we have recently shown that certain forms of physiologic stress, such as infection-associated inflammation and extensive blood loss, leads to the induction of biologically relevant levels of DNA damage in haematopoietic stem cells (HSCs) by dramatically increasing the proliferative index of this normally quiescent cell population. (1) We were also able to demonstrate that such stress-associated DNA damage was sufficient to completely deplete HSCs and promote severe aplastic anemia (SAA) in the Fanconi anemia (FA) knockout mouse model, which has compromized replication-associated DNA repair. In this "Extra Views" article, we extend this previous work to show that FA mice do not spontaneously develop a haematopoietic phenotype consistent with SAA, even at extreme old age. This suggests that HSC quiescence restricts the acquisition of DNA damage during aging and preserves the functional integrity of the stem cell pool. In line with this hypothesis, we provide an extended time course analysis of the response of FA knockout mice to chronic inflammatory stress and show that enforced HSC proliferation leads to a highly penetrant SAA phenotype, which closely resembles the progression of the disease in FA patients.