Manual Advances in DNA Repair in Cancer Therapy: 72 (Cancer Drug Discovery and Development)

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The second end of the broken chromosome is captured and anneals to the complementary strand of the donor DNA molecule, resulting in the formation of two Holliday junctions HJs. After DNA synthesis and ligation of both strands, the double HJ is either dissolved or is dismantled by the catalytic action of resolvases in order to complete repair [18].

Thus, repair by HR is error-free since it copies the missing genetic information from the undamaged sister chromatid, whereas NHEJ is error-prone since DNA ends without sequence homology are religated with the risk of causing mutations [19]. Given that a single unrepaired DSB has the potential to kill a cell, inhibition of repair by compounds that target factors involved in NHEJ or HR will increase the sensitivity of cancer cells to DSB-inducing anticancer agents.

The fact that cells with a compromised DDR are hypersensitive to DNA damage-inducing agents is currently under vigorous investigation for use in targeted cancer therapy. More precisely, during their pathogenesis, many cancer cells acquire defects in a certain DNA repair pathway and become dependent on a compensatory mechanism in order to survive.


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Furthermore, highly proliferative cancer cells are inherently hypersensitive to DNA damage because S-phase, in which DNA replication takes place, is the most vulnerable period of the cell cycle. As previously mentioned, cell-cycle checkpoint activation in response to DSBs gives a cell time for DNA repair before entry into S-phase or mitosis. Consequently, cell-cycle checkpoints reduce the efficacy of DNA-damaging agents used in cancer therapy.

Biology-driven cancer drug development: back to the future

Therefore, selective abrogation of checkpoint signalling sensitises cancer cells to chemo- and radio-therapy, potentiating cancer treatment [20]. In the late s, long before the discovery of cell-cycle checkpoints, the first attempts to sensitise cancer cells to standard cytotoxic therapy were made using ordinary compounds such as caffeine [22].

On the molecular level, KU, like most kinase inhibitors, competes with the ATP-binding site of the enzyme, thereby inhibiting the catalytic activity of ATM [26]. Compounds selectively targeting ATR have long been awaited, particularly when inhibitors of CHK1, a direct downstream target of ATR, had proven to be clinically effective [28]. NU is a pyrimidine analogue originally discovered as an adenosine triphosphate ATP competitive inhibitor of CDKs, but recently reported also to inhibit ATR at low micromolar concentrations and to confer cisplatin cytotoxicity independently of CDK inhibition [29, 30].

Preclinical testing of VE using pancreatic cancer cells demonstrated its chemo- and radio-sensitisation properties [32]. ETP was discovered by screening compounds with a previously reported activity against the related PI3Ks using a cell-based system assaying for ATR activity [33]. Six Phase II clinical trials of UCN, either as a single agent or in combination with other drugs, in patients with different types of advanced cancer have already been completed.

Another promising drug that interferes with checkpoint activation is the WEE1 tyrosine kinase inhibitor MK Merck , which was discovered by screening a chemical library [40]. MK is already under investigation in a Phase II trial combined with carboplatin in order to assess the benefit for patients with pmutated epithelial ovarian cancer. Last but not least, efforts to target CDC25 phosphatases, which also represent key molecules in checkpoint regulation, led to the discovery of several CDC25 inhibitors, amongst which the most potent are quinonoid-based derivatives such as the bis-quinone compound IRC [41, 42].

In summary, several SMIs that interfere with checkpoint activation show great promise of advancing in clinical studies and eventually being used as chemo- or radio-sensitisers as well as monotherapeutic agents in cancer treatment. Nevertheless, since many of the SMIs have only very recently been discovered, their safety, tolerability and efficacy when used alone or in combination has to be further investigated.

Impairing the repair of DSBs using drugs that either inhibit the enzymatic activity or interfere with protein-protein interactions of repair factors provides another approach to sensitising cancer cells for chemo- and radio-therapy. Unfortunately, neither of them has progressed into clinical development.

Introduction

More recent attempts to find novel DSB repair inhibitors led to the identification of mirin, the first inhibitor of the MRN complex that acts by blocking the nuclease activity of MRE11 [46]. Although peptides blocking protein-protein interactions represent an interesting concept for inhibiting DSB repair, their potential application in the clinics has yet to be established. In summary, safety, tolerability, pharmacokinetics and efficacy of most of the aforementioned SMIs have still to be carefully validated before they may enter clinical trials to examine their benefit for cancer therapy.

Importantly, normal cells survive the treatment owing to functional HR, providing the kind of selectivity that is considered the ultimate goal of cancer therapy. Today, 7 years after PARPi were first established for cancer therapy and despite some quite promising clinical studies, none of them has gained official approval for the treatment of cancer patients.

This decision was possibly driven by economic concerns rather than by clinical issues [64]. Notwithstanding all setbacks, clinical development and research on the mechanism of action of PARPi is still ongoing table 2. Despite controversies about its effectiveness as a PARPi, Sanofi's iniparib is under clinical investigation as a single agent and in combination with chemotherapeutic regimens in patients with recurrent solid tumours NCT , nonsmall-cell lung cancer NCT and ovarian cancer NCT [65, 66]. This finding clearly demonstrates positive responses of a subpopulation of sporadic cancers to PARPi therapy and also underlines the importance of classifying patients according to biomarkers in order to predict the efficacy of PARPi.

Such potential biomarkers also include deficiency of the phosphatase and tensin homologue PTEN tumour suppressor. Several ways of identifying HR defects are under investigation, including gene expression profiling and gene copy number analysis of DNA repair factors [70, 71]. As for most cancer therapies, a major challenge of using PARPis is the acquired resistance of initially PARPi-sensitive cancer cells due, for example, to the loss of 53BP1 a p53 binding protein or to overexpression of multidrug-resistance efflux transporters [72, 73]. Thus, despite considerable efforts to develop PARPi for clinical use, conventional DNA-damaging chemo- and radio-therapy largely remains the mainstay of cancer treatment.

However, several onging preclinical and clinical studies employ PARPi both as monotherapy and as chemo- or radio-sensitisers, because an improvement of current anti-cancer regimes is long-awaited. As intensive basic research is leading towards a better understanding of cellular functions and their underlying genetic networks, more and more genetic interactions become apparent as potential targets for synthetic lethality in cancer therapy. Moreover, synthetic lethality with components of the cell-cycle checkpoint machinery could be exploited in cancers harbouring activated oncogenes, since oncogene-induced replication stress activates the ATR-CHK1 signalling pathway.

For example, exacerbated toxicity was reported upon inhibition of CHK1 in lymphoma cells with upregulated c-Myc expression [77]. This finding underscores the concept that cancers with elevated levels of replication stress rely on intact checkpoint signalling for cell survival. Finally, disruption of the FA repair pathway was shown to be synthetically lethal with abrogated checkpoint signalling. More precisely, inactivation of ATM or CHK1 resulted in reduced viability of FA-deficient cells, illustrating the concept that checkpoint signalling and FA are mutually compensatory pathways in the maintenance of genome integrity [79, 80].

These observations highlight the usefulness of SMIs, as currently tested for CHK1, to treat tumours bearing a specific genetic background. Although many of the strategies that are based on the concept of synthetic lethality have so far only been investigated in preclinical settings, some hold great promise of entering clinical trials soon.

There is increasing evidence that haploinsufficiency of DDR components promotes genome instability and drives tumourigenesis. Dosage insufficiencies of DNA repair genes might, however, only be unmasked once a cell is challenged with an increased load of DNA damage such as oncogene-induced replicative stress [81, 82]. Synthetic lethal approaches might therefore be applicable not only in cancer cells with deficiencies, but also in those bearing haploinsufficiencies for DDR factors.

Evidence from gene targeting studies in mice revealed that, for example, the loss of one allele of ATR or CtIP is sufficient to cause increased chromosomal aberrations, genomic instability and tumour susceptibility [83, 84]. This indicates that heterozygous carriers of DDR defects are more prone to develop tumours once the threshold of endogenous DNA damage is increased as, for example, in precancerous lesions [85]. However, scientists are just beginning to unravel how haploinsufficiency of DDR genes contributes to carcinogenesis and how these may be exploited for novel synthetic lethal approaches in cancer therapy.

To date, DSB-inducing agents have been the core components of conventional cancer therapy, confirming the rationale of inflicting excessive DNA damage in order to kill cancer cells. However, most chemotherapeutic regimens cause severe side effects that limit their therapeutic potential.

As summarised in this review, SMIs and synthetic lethal approaches targeting the individual genetic profile of the tumours are under clinical development, with the aim to improve the patients' benefit by increasing the efficacy while lowering the toxicity of cancer treatments. A prerequisite for personalised therapy is the molecular characterisation of tumours with reliable biomarkers to assign patients the appropriate treatment. In order to stratify cancer patients according to their DNA repair status, tumour biopsies can be analysed with immunohistochemistry, fluorescence in-situ hybridisation FISH , gene sequencing, expression profiling and other methods [86].

Relevant biomarker assays should ideally predict the functionality of DNA repair pathways, rather than just providing information about mutations or expression levels of proteins involved in the DDR.

DNA Damage Response/Repair in Cancer Stem Cells — Potential vs. Controversies

Certainly, such a detailed molecular profiling of cancer versus normal tissue from a given patient is critical to maximise the potential of personalised cancer drugs in terms of both therapeutic success and cost-effectiveness. Recent in-vitro and in-vivo research has deepened our knowledge about synthetic genetic interactions and put forward alternative ways to treat cancer. Furthermore, by utilising ribonucleic acid RNA interference technologies, screens for synthetic lethal interactions of cancer-specific defects in DNA repair pathways have augmented the discovery of targets for cancer therapy.

Recently discovered inhibitors of RPA and RAD51 are promising candidates, which are in preclinical testing in order to be approved for the use in clinical trials soon [48, 50]. These observations highlight the therapeutic potential of miRNA mimics or inhibitors in future approaches for cancer therapy [94]. In summary, as the concept of personalised medicine emerges, tumour-specific defects of DSB repair pathways represent a promising therapeutic target to be exploited for the selective elimination of cancer cells. Thus, there is an air of optimism for targeted cancer therapy through exploiting the DDR of tumour cells in the clinics.

Acknowledgement: We wish to thank D. We also would like to apologize to all authors whose significant contributions could not be cited due to space limitations. Correspondence: Professor Alessandro A. Hallmarks of cancer: the next generation. Repair and genetic consequences of endogenous DNA base damage in mammalian cells. Annu Rev Genet. DNA damage, aging, and cancer. N Engl J Med. The DNA-damage response in human biology and disease. The effects of deregulated DNA damage signalling on cancer chemotherapy response and resistance.

Staff Profiles

The DNA damage response and cancer therapy. DNA repair dysregulation from cancer driver to therapeutic target. Nat Rev Cancer. FEBS Lett. The multifaceted mismatch-repair system.

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Nat Rev Mol Cell Biol. DNA interstrand crosslinks: natural and drug-induced DNA adducts that induce unique cellular responses. The DNA damage response: making it safe to play with knives. Mol Cell. Playing the end game: DNA double-strand break repair pathway choice. Double-strand break end resection and repair pathway choice. Mitotic homologous recombination maintains genomic stability and suppresses tumorigenesis. Sensing and repairing DNA double-strand breaks. Glioma stem cells promote radioresistance by preferential activation of the DNA damage response. Targeting Chk1 in pdeficient triple-negative breast cancer is therapeutically beneficial in human-in-mouse tumor models.

J Clin Invest. Proc Soc Exp Biol Med. Cancer Res. Caffeine inhibits the checkpoint kinase ATM. Curr Biol. Identification and characterization of a novel and specific inhibitor of the ataxia-telangiectasia mutated kinase ATM. Small molecule inhibitors of the PI3-kinase family. Curr Top Microbiol Immunol. Identification and evaluation of a potent novel ATR inhibitor, NU, in breast and ovarian cancer cell lines.

Br J Cancer. Identification of novel purine and pyrimidine cyclin-dependent kinase inhibitors with distinct molecular interactions and tumor cell growth inhibition profiles. J Med Chem. Nat Chem Biol. The novel ATR inhibitor VE increases sensitivity of pancreatic cancer cells to radiation and chemotherapy. Cancer Biol Ther. A cell-based screen identifies ATR inhibitors with synthetic lethal properties for cancer-associated mutations.

Nat Struct Mol Biol. Purification, physico-chemical properties, structural determination and biological activities. J Antibiot. J Biol Chem. Structural basis for Chk1 inhibition by UCN Mol Cancer Ther. Clin Cancer Res. CDC25 phosphatase inhibitors: an update. Mini Rev Med Chem. Int J Cancer. Bioorg Med Chem Lett. A forward chemical genetic screen reveals an inhibitor of the MreRadNbs1 complex. Novel irreversible small molecule inhibitors of replication protein A display single-agent activity and synergize with cisplatin.

Identification of specific inhibitors of human RAD51 recombinase using high-throughput screening. ACS Chem Biol. UV light from the sun, as well as various chemical reagents, can react with DNA and induce nucleotide chemical modifications. In addition, ionizing radiation generated by the cosmos, X-rays, and exposure to radioactive substances, as well as treatment with certain chemotherapeutic drugs, can induce base modifications, interstrand cross links, and DNA single - and double-strand breaks DSBs , which can all lead to genomic instability.

It is estimated that each cell is confronted with approximately 10 4 —10 5 DNA lesions per day, indicating that clearance of genomic injuries constitutes the maintenance of proper genome function a demanding task [ 2 ]. Thus, maintenance of genomic stability through damage repair is essential for cell and organism longevity. Without genomic stability, replication errors and external stress as well as direct forms of DNA damage can induce mutations, which decrease cell survival, cause altered gene expression, and therefore can lead to cellular transformation. In response to the wide diversity of potential DNA lesions, eukaryotic cells developed a number of highly conserved DNA repair mechanisms that can recognize and repair different types of DNA damage with varying fidelity and mutagenic consequences Table 1.

Irrespective of the type of lesion and the repair mechanism, DNA damage is rapidly sensed and activates evolutionarily conserved signaling pathways, known collectively as the DNA damage response DDR.

DDR components can be separated into four functional groups, extensively described in other chapters of the current edition: damage sensors, signal transducers, repair effectors, and arrest or death effectors. In brief, cells contain multiple DNA repair mechanisms including: base excision repair BER that removes damaged bases caused by small chemical alterations base modifications , mismatch repair MMR that recognizes and removes mispaired base incorporation errors and base damage arising from replication errors, nucleotide excision repair NER that corrects bulky helix-distorting lesions caused by chemicals and ionizing radiations, and cross-link repair ICL that removes interstrand cross links.

These most challenging DSBs may be restored by a different degree of repair fidelity, related to the pathway chosen according to the phase of the cell cycle.

Mechanisms of DNA Damage and Repair

While the the almost error-free HR repair dominates in dividing cells, the G1 phase acting NHEJ is error-prone, as genome has not yet undergone duplication; hence, a template for recombination used in HR is not yet available. These two pathways seem to repair the majority of chemotherapy- and radiotherapy-induced damage [ 2 - 7 ].

In a rapid overview, regarding the restoration processes of DSBs, HR allows cells to repair DNA damage in an error-free manner and can be performed only during S and G2 phases of the cell cycle due to the requirement for the undamaged sister chromatid as a template. Each stage of this multistep pathway requires the sequential involvement of a number of distinct enzymes [ 8 - 11 ]. X, forms nuclear foci on the sites of DNA damage and is necessary for the assembly of repair complexes [ 12 , 13 ]. Such genome instability likely contributes to aging and age-related disease and it constitutes an essential step in the development of cancer [ 14 , 15 ].

Cancer is broadly defined as a group of diseases characterized by uncontrolled growth and spread of abnormal cells bearing genetic or epigenetic alterations resulting in high morbidity and mortality. As a result, an apparent target of antitumor therapy was proved to be the destruction of tumor genetic material. Tearing up the DNA of carcinoma cells, usually combined with tumor removal by surgery, is suggested as one of the most effective therapeutic schemes resulting in residual cancer cell death. Radiation therapy and common chemotherapeutics are DNA damaging agents targeting the genome of carcinoma cells and nowadays they represent conventional treatment schemes.

A common practice in cancer treatment includes a combination of surgery, chemotherapy, and radiotherapy, depending on the lesion type and the clinical picture of the patient. Nevertheless, cancer still plagues humanity as a largely incurable disease. Poor prognosis and low survival rate are even more prominent in cases where malignancy is detected at a late stage.

Another challenge for oncologists arises from frequent metastasis and tumor recurrence, which further frustrates effective treatment protocols currently available. The underlying mechanisms are not clearly understood in detail but a lot of work is accumulating worldwide, aiming to elucidate CSCs resistance etiology to DNA-damaging agents [ 1 , 23 ]. CSCs seem to differentiate into a diverse panel of progeny cells that make up the tumor, and reproduce the original tumor after xenotransplantation.

There are several theories regarding the origin of CSCs [ 24 ]. CSCs can be distinguished from other cells within the tumor by symmetry of their cell division and alterations in their gene expression. CSCs possess stem-like properties, such as self-renewal, proliferation and differentiation abilities, expression of pluripotency factors e. Markedly, CSCs express surface markers, which seem to be tissue-type-specific and in some cases tumor-subtype-specific. Nevertheless, novel combinations of CSC markers are continuously being added to this group or combinations not previously defined are also described Frangou et al.

Therefore, expression of CSC surface markers can only be a manmade criterion to describe tumor stem cells and some CSCs may not fulfil these criteria. Apart from the theory suggesting that CSCs derive from transformation of normal stem cells, an alternative theory about the origin of CSCs suggests that they may arise from transformation of normal somatic cells. The epithelial to mesenchymal transition EMT of cancer cells during the metastatic process may provide a mechanism by which cancer cells may gain stem-like characteristics [ 29 , 30 ]. In addition, another role attributed to normal stem cells in the various tissues is their implication in the repair process of damaged tissue.

In order to fulfill this task, normal stem cells have to overcome genotoxic insults and in turn proliferate to restore eradicated tissue cells. Since much evidence favors cancer originating from stem cells, it is not surprising that many of survival and proliferation pathways of stem cells have aberrant expression in cancer cells. Another aspect to consider is that tumorigenesis is followed by angiogenesis and by cancer cell invasion in other tissues metastasis as part of the disease progression.

Not surprisingly, CSCs have been associated with the induction of tumor vascularization through the expression of vascular-related factors and by their contribution to metastasis through the induction of the Epithelial to Mesenchymal Transition EMT program [ 30 , 32 ]. As previously mentioned, mechanisms helping cells to escape cytostatic and cytotoxic effects after application of DNA-damaging treatment approaches are far from clearly understood despite intensive and extensive studies. It is also suggested that CSCs possess specific intracellular molecular properties assisting them to avoid treatment-derived cytotoxicity [ 23 ].

Various mechanisms account for CSCs drug resistance.

The good news and the bad news

A comprehensive example comes from studies on breast cancer stem cells. The mechanisms involved possibly include decreased induction of reactive oxygen species and activation of the DNA damage checkpoint response [ 34 ]. Moreover, an in vitro study has documented that during a fractionated course of radiation, the number of CSCs with activation of Jagged-1 and Notch-1 increased, suggesting the possible induction of radiotherapy resistance via the Notch signaling pathway [ 33 ].

Understanding the mechanisms of chemo- and radiation-resistance of CSCs may pave the way towards discovering a set of signaling pathways unique to CSCs. Targeting such set of pathways would ideally provide more effective and presumably personalized therapeutic potential towards complete tumor eradication. Despite difficulties and though such ideal pathway has not been found yet, elucidation of developmental pathways that control survival, proliferation, and differentiation of stem cells is under extensive investigation.

Among these characteristics, the multifaceted protection of genome integrity by a prompt activation of the DNA damage sensor and repair machinery is one of the key features rendering resistance to applied genotoxic insults. These pathways seem to be distinctly regulated in CSCs, resulting in significant enhancement of DNA repair capability and finally radio- and chemoresistance [ 23 , 39 - 43 ].

This is a highly evolving topic as the anticipated information about the basic mechanisms governing DNA repair processes is also expected to contribute in predicting and improving therapy responses and the clinical outcome in cancer patients treated with DNA-damaging agents.

Accumulating reports tend to indicate that there are CSCs in almost all tumor types reviewed in [ 22 ]. For example, using the CD as the brain stem cell marker, Bao et al. All of these findings shed new light on the mechanisms of an accelerated tumor cell proliferation with an increase in the percentage of radioresistant CSCs. The use of radiation therapy and chemotherapeutic drugs aim to provoke DNA damage in cancer cells. If the damage is quite extensive and cannot be repaired, cell death is inevitable [ 44 ].

Research on radio- and chemoresistance of CSCs after exposure to DNA-damaging agents has been extensively conducted and there is great evidence demonstrating that this subpopulation in tumors protects itself from DNA-damaging treatment by multiple mechanisms. First of all, CSCs are considered to have an enhanced DNA repair capability and consequently they are protected more effectively than the rest of tumor cells. Numerous studies suggest that the resistance of cancer stem cells to therapy is mediated by more robust DNA damage response and repair pathways [ 45 - 48 ].

Special regulation and elongation of cell cycle is also regarded to be another protection mechanism incorporated, providing CSCs more available time to repair damaged DNA. It is well documented that depending on the type and tumor stage, CSCs adopt distinct main and auxiliary mechanisms to protect their genome and overcome insults. The indirect pathway of radiation-induced damage includes the generation of chemically reactive free radicals, including the product of oxygen metabolism called reactive oxygen species ROS.

These products play an important physiological role and participate in many signaling events regulating cell proliferation, migration, angiogenesis, wound healing, and metabolism [ 52 ]. Both normal and cancer cells can control ROS level by balancing their production and elimination by ROS-scavenging molecules such as glutathione, peroxidase, catalase, superoxide dismutase, thioredoxin, etc.

An excessive production of ROS in response to irradiation may lead to their interaction with critical cell macromolecules including DNA, lipids, and proteins, leading to cell death [ 54 ]. High resistance of CSC populations in breast and gastrointestinal carcinomas to genotoxic stress is related to a more efficient ROS scavenging system and lower levels of ROS production after irradiation as compared to non-CSC populations [ 49 ]. The role of ROS scavenging in CSC radioresistance is supported by the observation that pharmacological depletion of ROS scavengers in tumor progenitors by treatment with buthionine sulfoximine BSO , which inhibits glutamate-cysteine ligase, markedly decreased clonogenic properties and radioresistance of CSCs [ 49 ].

The functional link between stem cell markers and ROS metabolism was first demonstrated by Ishimoto and colleagues who showed that CD44 interacts with glutamate-cysteine transporter xCT and controls the intracellular level of ROS scavenger glutathione in gastrointestinal cancer cells [ 50 ]. These preclinical results are supported by recent clinical studies, which showed that high expression of CD44 in tumors was correlated with resistance to radiation therapy and associated with early recurrence in HNSCC patients [ 55 - 57 ].

The activity of aldehyde dehydrogenase ALDH enzymes is also highly correlated with the existence of cancer stem cells in tumors [ 58 ]. Furthermore, there is a relationship between poor clinical prognosis in breast and prostate cancer and increased expression of ALDH1 [ 59 ].

These observations suggest that ALDH activity can be crucial for regulation of cell radio-sensitivity. Other studies have found overexpression of ALDH1 in cyclophosphamide-resistant leukemic and colonic cancer cells [ 62 ]. Thus, overexpression of the detoxification enzyme ALDH1 may also contribute to the resistance of CSCs to various cancer treatments, including chemo- and radiation therapy resistance.

During cell exposure to genotoxic agents its expression is increased in response to reactive oxygen radicals ROS production. Activation of Ref-1 redox domain decreases ROS levels resulting in the enhancement of carcinoma cell stemness and self-renewal. On the other hand, inhibition of the Ref-1 domain, which is responsible for ROS scavenging, results in increased intracellular ROS levels, activation of p53, and promotion of cancer cell differentiation and cell death.

Radiation-induced cell death may occur as a result of direct and indirect energy transfer to critical cellular structures including chromatin, plasma membrane and mitochondria. These cell-cycle arrest mechanisms allow the recruitment of either DNA repair effectors or in case of irreversible damage and repair failure, of proapoptotic molecules [ 65 ]. This enhancement of DNA repair capacity can be either direct, through elevated DNA repair mechanisms, or indirect, through delayed cell-cycle progression. Several studies indicate that ATM activity may play a role in normal stem cell maintenance and proliferation.

Two main roles are attributed to ATM: a role in stem cell survival and an implication, as part of DDR, in pathways classically linked to stem cell maintenance [ 68 , 69 ]. ATM is required to maintain normal self-renewal and proliferation of NSCs due to its role in controlling the redox status. Moreover, it has been shown that ATM plays a central role in terminal differentiation of a human neural stem cell line model through its function in DDR [ 73 ]. In addition, ATM protein is a major player in signaling pathways classically implicated in stem cell maintenance.

These cellular functions are prerequisites ensuring the accurate developmental progress and homeostasis maintenance of adult tissues [ 76 ]. A second hint comes from the observation of the wild-type pinduced phosphatase 1 WIP1 function. This phenomenon may be responsible for CSCs expansion as the derived CSC progenitors retain the ability to sustain tumor growth [ 80 ]. In accordance to this hypothesis, it was recently shown that wild-type p53 downregulates GLl-1 function by sequestering the co-activator TATA Binding Protein Associated Factor 9 TAF9 , a process comprising an inhibitory loop controlling stem cell and tumor cell numbers [ 81 ].

The delicate control of GLI-1 activity may thus be part of the mechanisms controlling the precursors and stem cell numbers and preventing tumorigenesis [ 82 , 83 ]. Whether ATM kinase may directly modulate SHH signaling, therefore contributing to the maintenance of stem cell identity, remains to be elucidated. In particular, upon therapy-induced DNA damage, temporal halt of proliferation through cell-cycle elongation may provide cancer stem cells with increased time for repair. Upon genome insults restoration, the replisome is reactivated for genome duplication. Through such pathways genotoxic resistance may be triggered in CSCs.

Therefore, by specific inhibition of the DNA damage checkpoint response, a cell-cycle break may occur in CSCs, driving them again towards proliferation and thereby specifically sensitizing them to genotoxic insults caused by radiotherapy. In this context, ATM may serve as a useful candidate target for eliminating cancer stem cells in the tumor.

Utilization of this idea was pioneered by Bao and collaborators in glioblastoma multiforme GBM. CD is a marker for both neural stem cells and brain cancer stem cells. In consistency to these results, two distinct grade IV glioma cell lines, varying in CSC content low and high, respectively , were preincubated with a nontoxic concentration of the ATM inhibitors KU and KU and then irradiated.