Review Article Dna Damage Response And Hematological Malignancy Pdf

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DNA damage response and hematological malignancy.

Hematopoiesis is a highly regulated process that supplies mature blood cells of various lineages. In an adult human, around 10 11 new blood cells are produced daily to ensure homeostasis.

To meet this high regenerative demand, hematopoiesis is structured as a cellular hierarchy composed of cells endowed with different proliferative, differentiation and longevity potentials 1 , 2. Hematopoietic stem cells HSCs are located at the apex of the hierarchy and maintain blood production due to their unique ability to produce more blood stem cells a property defined as self-renewal , as well as to give rise to multipotent progenitors with limited self-renewal capacity 2 , 3.

Short-lived but extremely proliferative lineage-committed progenitors CPs , which are the progeny of multipotent progenitors, generate large numbers of differentiated cells to ensure daily homeostasis 4 , 5. During injury or infection, stem and progenitor compartments undergo expansion via replication to meet the increased demand for particular cell subsets, followed by a return to homeostasis.

All cells in the body, including highly proliferative and long-lived hematopoietic subsets, must constantly contend with different types of DNA damage. Most of this damage is generated by endogenous sources, such as reactive oxygen species ROS. It has been estimated that the average cell experiences approximately , spontaneous DNA lesions daily 6.

The critical importance of the DDR in hematopoiesis is well demonstrated by the severe clinical consequences-including bone marrow BM failure, immunodeficiency and a high incidence of hematological cancers-observed in patients with inherited mutations in DNA damage signaling and repair components [e. Analysis of human HSC DNA isolated from newborn, young and elderly individuals by whole-genome sequencing has shown that long-lived self-renewing HSC serve as a reservoir for DNA-damage accumulation and thus represent a likely cell of origin for hematopoietic malignancies 8 , 9.

Obviously, the short life span of hematopoietic progenitors reduces the risk of leukemogenic-process initiation from this compartment. Definitive analysis of the DDR in the early stages of human hematopoiesis was unattainable, until recently, due to our inability to isolate the distinct, functionally homogeneous cellular subsets that make up the hematopoietic tissue hierarchy.

Fortunately, in the last 5 years, significant advances in cell-surface marker characterization, multicolor flow cytometry and functional clonal assays, which efficiently distinguish different stem and progenitor subsets, have led to the establishment of a detailed hierarchical map of human HSCs and progenitors 3 - 5.

The self-renewal potential of LT-HSCs is most often measured by their ability to sustain multilineage hematopoiesis for at least 16 weeks upon transplantation into a properly conditioned recipient, and to reinitiate hematopoiesis upon secondary transplantation.

Although the basic roadmap of hematopoiesis is largely conserved between mice and humans, their vastly different body masses, life spans and environmental exposures have led to several important differences in hematopoiesis, including species-specific responses to DNA damage Today, scientists have the tools to address the role of single molecules and pathways in HSCs.

As a result, it has become clear that DDR regulators themselves are the key players in HSC-specific processes such as self-renewal and multilineage differentiation. In this review, we summarize the progress made in this rapidly growing field in the last several years, placing special emphasis on the human HSC. The most convincing evidence that unrepaired genomic damage results in age-dependent decreases in HSC regeneration is based on the careful analysis of hematopoiesis and HSC functionality in mice that are deficient in key DNA-repair genes.

Loss of DNA-damage sensors e. Interestingly, young mutant mice had normal numbers of LT-HSCs as defined by surface markers, suggesting that the reason for their functional decline was the inability to self-renew optimally under conditions of BM regeneration.

Thus, mouse models overwhelmingly demonstrate the accumulation of endogenous DNA damage in the genome of LT-HSCs, which eventually impairs their self-renewal. Serial transplantation experiments with cord blood cells also revealed that HSCs and not progenitors accumulate signs of DNA damage that most probably originate from ROS Interestingly, both studies found accumulation of immunophenotypic HSCs with age, with no marked decline in their reconstitution potential, which stands in contrast to the mouse models of HSC aging.

Unfortunately, no analysis of DNA damage-related markers was performed, warranting further exploration. Of note, most of this physiological DNA damage did not colocalize with telomeres, which undergo an age-dependent decline in the same immunophenotype, arguing against telomeres as the origin of DNA damage in human primitive hematopoietic cells Furthermore, a gradual age-dependent increase in DNA alterations was reported in long-lived self-renewing HSCs based on next-generation sequencing analysis, supporting the notion that HSCs are reservoirs of both neutral and potentially dangerous mutations 8 , 9.

In contrast, the study of DNA repair in the human primitive hematopoietic compartment is still in its infancy. Cells of both fractions were exposed to IR, and kinetics of DSB rejoining was measured using the neutral comet assay. To make this observation, Shao et al. The major pathway for DSB repair in quiescent human cells is NHEJ, a rapid but error-prone method that is responsible for most of the genomic structural variants and chromosomal translocations in human cancers Future experiments should be aimed at revealing the molecular mechanism underlying the distinct dynamics of the DDR in human HSCs.

In contrast to the results obtained with human hematopoietic cells, highly purified mouse HSCs showed higher NHEJ activity than committed myeloid progenitors 32 , Several important technical aspects should be considered before interpreting these potentially important interspecies differences.

First, human HSC-containing populations are more heterogeneous than those of mice and additional human HSC-specific surface markers have recently been identified 3. Second, human cord blood and adult murine BM cells might represent developmentally incomparable HSC subsets. In this respect, the role of different niche factors should also be considered and will be discussed further on.

In contrast, BM progenitors are highly cycling, further complicating a direct comparison. Bearing in mind these important experimental caveats, all three studies point to the existence of distinct DNA-repair characteristics in HSCs from the two species. Differences in the basal expression of DNA-repair proteins provide important correlative evidence for the observed differences in DNA-repair activity Table 1. For example, murine HSCs exhibited increased expression of numerous transcripts encoding NHEJ proteins relative to their downstream progeny.

The opposite pattern of expression was evident for HR-repair genes. It is important to bear in mind that the activity of some DNA-repair proteins is regulated post-transcriptionally and in a cell-cycle dependent manner. Furthermore, human cells typically express more Ku and DNA-dependent protein kinase DNA-PK proteins discussed further on than their murine counterparts, adding to the difference in cellular concentration and activity of repair proteins 39 - Accurate analysis of DNA-repair protein levels and their biochemical activity in DNA-repair assays in the specific cellular contexts HSCs and progenitors will undoubtedly reveal the underlying mechanism and significance of the observed interspecies differences.

Thus, a complex network of DDR factors that include sensors, regulators and effectors has evolved to respond effectively to genotoxic stress. Activation of this network results in two primary outcomes: repair of DNA damage and genome restoration or, if damaged DNA cannot be sufficiently repaired, execution of cell death, differentiation or senescence programs 41 - Of the many different lesions that occur in DNA molecules, DSBs induced by IR are the most lethal for the individual cells as well as for the whole hematopoietic tissue This notion is exemplified by the dramatic loss of HSC self-renewal potential as measured by competitive repopulation or serial transplantation assays, upon exposure of the mouse to even relatively low doses of IR Gy 31 , 36 , Thus, IR is the most frequently used trigger to provoke a DDR and delineate the molecular pathways engaged.

In response to DSBs, cells mount a powerful signaling network. It is becoming increasingly clear that many of the central regulators of the DDR play a key role in the regulation of HSC homeostasis 13 , 14 , 25 , 46 - The protein kinase ATM is the primary transducer of the DSB signal via phosphorylation of the numerous nuclear and cytoplasmic substrates that participate in DNA repair, apoptosis and senescence.

As in the case of ATM-knockout mice, antioxidant treatment returned HSCs to the quiescent state and restored their repopulation potential. Indeed, many DNA repair factors are differentially expressed between HSC and different committed progenitor populations 32 , 36 - 38 Table 1.

However, the molecular circuitry that governs DNA repair gene expression in primitive hematopoietic cells is not well understood. How mTOR-dependent regulation of FA repair pathway contributes to its pleiotropic roles in carcinogenesis should be determined in the future. How DNA repair in primitive hematopoietic cells is regulated by the microenvironment is not entirely known. Recently, de Laval and colleagues 54 found that thrombopoietin TPO and its receptor MPL, previously implicated in maintenance of HSC quiescence and self-renewal, are also important for DSB repair in mouse and human primitive hematopoietic cells.

The tumor suppressor p53 is a widely expressed transcription factor that plays a critical role in regulating several DDR pathways, including activation of cell-cycle arrest, induction of apoptosis, senescence, suppression of HR, and others As with additional DDR genes, germline mutations in p53 increase the incidence of hematological malignancy, and somatic aberrations of p53 are frequent in leukemia, lymphoma and myelodysplastic syndrome 58 , The importance of p53 in IR-induced apoptosis has been analyzed in detail both in vitro and in vivo and attributed mainly to its transcriptional target Puma 60 Figure 1B.

However, it has been difficult to establish the function of p53 in HSCs compared to other hematopoietic cells using mouse models. BM progenitors from pknockout mice exhibit increased resistance to DNA damage-induced apoptosis in short-term survival assays 36 , 60 - These observations would suggest little involvement of pdependent apoptosis in the irradiated murine LT-HSCs.

However, meticulous analysis of BM cellularity under the even lower IR dose of 1. Analysis of cell death by Annexin V staining clearly implicated apoptosis as the reason for HSC loss 65 - Although the reason for this persistence of apoptosis was not examined in those studies, Wang et al.

It is likely that the different apoptosis assays cleaved caspase-3 vs. Annexin and variability in the time points after IR hours vs. When human primitive hematopoietic cells derived from cord blood were exposed to 3 Gy of IR, the HSC-enriched fraction underwent significantly higher cell death relative to the downstream CPs in both short-term and long-term viability assays Increased resistance of human HSCs and progenitors to apoptosis accounts for the enhanced hematopoietic regeneration when irradiated HSCs in which p53 had been disabled were transplanted into conditioned recipients Thus, both phenotypic and functional assays indicate that acute genotoxic injury initiates a pdependent apoptosis pathway that ablates primitive hematopoietic cells in both humans and mice.

When attempts were made to examine the function of putatively non-malignant HSCs from pdeficient mice, once again, conflicting results were obtained. Although there were differences in the designs of the competitive repopulation assays, it was suggested that HSC repopulation increased, remained the same or even decreased 70 , 72 - However, when the dosage of p53 was increased, HSCs exhibited reduced competitive repopulation potential Studies of human HSCs in which p53 activity had been disabled by dominant negative mutation or by means of RNA interference extended observations made in mice and provided some clarification of the role of p53 in the regulation of HSC self-renewal.

Head-to-head analysis of human HSCs in which pdependent apoptosis was bypassed by Bcl-2 overexpression established that p53 has both positive and negative roles in the regulation of HSC function. Apoptosis-independent p53 effectors, such as p21 and others, might participate in HSC genome quality control coupled with self-renewal Thus, p53 plays at least two distinct physiological roles to ensure optimal HSC function: apoptosis regulation and prevention of genomic damage accumulation upon HSC self-renewal.

The complex network of p53 transcriptional targets and cofactors involved in the regulation of HSC quiescence, cell-cycle arrest, apoptosis and senescence determines HSC fate based on the severity and persistence of the genotoxic stress Figure 1. In both species, hematopoietic stem and progenitor cells undergo alterations in lineage distribution, gene expression, epigenetics, differentiation potential and repopulation capacity 27 - 29 , Decreased lymphoid cell output by aged HSCs represents one of the best-documented functional changes 76 , In an elegant in-vivo shRNA screen, Wang et al.

Importantly, the ability of DNA damage to induce premature stem cells differentiation was also documented in another stem cell compartment in the mouse, namely, Melanocyte Stem Cells In this case, IR triggered p53 and INK4A independent loss of Melanocyte Stem Cells self-renewal that led to the appearance of ectopically pigmented melanocytes and hair graying of the irradiated animals.

Interestingly, murine Bulge Stem Cells that share with Melanocyte Stem Cells the same niche turned to be resistant to both irradiation induced apoptosis and differentiation 80 pointing to the distinct pathways regulating various tissue stem cells response to DNA damage. An efficient DDR is one of the major evolutionary adaptations of all living organisms, ensuring the transfer of accurate genomic information from one generation to the next. Research into the DDR in the last decades has identified hundreds of proteins that form myriad functional intracellular interactions to accomplish various tasks in the injured cell, including DNA repair, cell-cycle arrest, apoptosis and others.

In the context of long-lived animals, such as humans and rodents, causative relationships between the DDR, tissue regeneration, organismal aging and cancer susceptibility become established. Much of this important knowledge is derived from the study of DDR in the context of hematopoiesis-a vital process sustained by a small fraction of quiescent self-renewing stem cells Figure 1A.

Some generalizations can be made based on the available experimental evidence as outlined in this review. First, HSCs are selectively sensitive to acute and chronic genome damage, which results in a dramatic reduction in their regenerative potential.

The critical differences in DDR between humans and rodents at the level of HSCs, highlighted in this review, preclude direct extrapolation of findings from one species to the other Table 1. Although the driving force for the described interspecies differences is currently unknown, it is tempting to speculate that vastly different lifespan between the two species had major impact on their respective environmental adaptations, including different strategies to deal with DNA damage.

Obviously, exciting discoveries made in the last few years have presented the scientific community with even more burning questions. For instance, why is leukemia a rare disease given that 10 11 new blood cells are produced daily?

If so, what is the molecular basis and physiological significance of this interspecies difference?

DNA repair

Recent intensive genomic sequencing of hematopoietic malignancies has identified recurrent mutations in genes that encode regulators of chromatin structure and function, highlighting the central role that aberrant epigenetic regulation plays in the pathogenesis of these neoplasms. Deciphering the molecular mechanisms for how alterations in epigenetic modifiers, specifically histone and DNA methylases and demethylases, drive hematopoietic cancer could provide new avenues for developing novel targeted epigenetic therapies for treating hematological malignancies. Just as past studies of blood cancers led to pioneering discoveries relevant to other cancers, determining the contribution of epigenetic modifiers in hematologic cancers could also have a broader impact on our understanding of the pathogenesis of solid tumors in which these factors are mutated. Hematopoiesis is a highly dynamic developmental process requiring both self-renewal and a well-regulated differentiation process of hematopoietic stem cells HSCs to maintain the lifelong regeneration of the mammalian blood cells. The ontogeny of the mouse hematopoietic system involves two waves of hematopoiesis during development, beginning with a transient primitive hematopoiesis, which originates from the embryonic mesoderm and progresses to the extraembryonic yolk sac to produce primitive erythrocytes and some myeloid cells around embryonic day 7. As embryonic development progresses, HSCs colonize the fetal liver around E In mammalian adults, HSCs exist in a relatively quiescent state but retain the capabilities of both self-renewal and multipotency, ensuring their lifelong maintenance in the bone marrow while, through a hierarchical cascade of differentiation, giving rise to all types of phenotypically distinct mature blood cells Fig.

DNA damage has been long recognized as causal factor for cancer development. When erroneous DNA repair leads to mutations or chromosomal aberrations affecting oncogenes and tumor suppressor genes, cells undergo malignant transformation resulting in cancerous growth. Genetic defects can predispose to cancer: mutations in distinct DNA repair systems elevate the susceptibility to various cancer types. However, DNA damage not only comprises a root cause for cancer development but also continues to provide an important avenue for chemo- and radiotherapy. Since the beginning of cancer therapy, genotoxic agents that trigger DNA damage checkpoints have been applied to halt the growth and trigger the apoptotic demise of cancer cells. We provide an overview about the involvement of DNA repair systems in cancer prevention and the classes of genotoxins that are commonly used for the treatment of cancer. A better understanding of the roles and interactions of the highly complex DNA repair machineries will lead to important improvements in cancer therapy.

PDF | DNA repair is an important defense mechanism that faces the difficult task of Hematological malignancies are characterized by genomic instability that is possibly The purpose of this review is to summarize the existing knowledge concerning of this article at

Epigenetics of hematopoiesis and hematological malignancies

Hematopoiesis is a highly regulated process that supplies mature blood cells of various lineages. In an adult human, around 10 11 new blood cells are produced daily to ensure homeostasis. To meet this high regenerative demand, hematopoiesis is structured as a cellular hierarchy composed of cells endowed with different proliferative, differentiation and longevity potentials 1 , 2. Hematopoietic stem cells HSCs are located at the apex of the hierarchy and maintain blood production due to their unique ability to produce more blood stem cells a property defined as self-renewal , as well as to give rise to multipotent progenitors with limited self-renewal capacity 2 , 3. Short-lived but extremely proliferative lineage-committed progenitors CPs , which are the progeny of multipotent progenitors, generate large numbers of differentiated cells to ensure daily homeostasis 4 , 5.

Endogenous DNA-damage accrual limits HSC function

Self-renewing and multipotent hematopoietic stem cells HSCs maintain lifelong hematopoiesis. Their enormous regenerative potential coupled with lifetime persistence in the body, in contrast with the Progenitors, demand tight control of HSCs genome stability. Indeed, failure to accurately repair DNA damage in HSCs is associated with bone marrow failure and accelerated leukemogenesis. Human HSCs in comparison with Progenitors exhibit delayed DNA double-strand break rejoining, persistent DDR signaling activation, higher sensitivity to the cytotoxic effects of ionizing radiation and attenuated expression of DNA-repair genes. Nevertheless, physiological significance and the molecular basis of the HSCs-specific DDR features are only partially understood. Taking radiation-induced DDR as a paradigm, this review will focus on the current advances in understanding the role of cell-intrinsic DDR regulators and the cellular microenvironment in balancing stemness with genome stability. Pre-leukemia HSCs and clonal hematopoiesis evolvement will be discussed as an evolutionary compromise between the need for lifelong blood regeneration and DDR.

DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis.

Nijmegen Breakage Syndrome NBS , an autosomal recessive genetic instability syndrome, is caused by hypomorphic mutation of the NBN gene, which codes for the protein nibrin. Cardinal features of NBS are immunodeficiency and an extremely high incidence of hematological malignancies. Recent studies in conditional null mutant mice have indicated disturbances in redox homeostasis due to impaired DSB processing. Clearly this could contribute to DNA damage, chromosomal instability, and cancer occurrence.

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    This review discusses recent findings and highlights the importance of DNA Keywords: DNA repair; DNA damage; hematological malignancies; The publication of this article was funded by the Qatar National Library.

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