Chapter 3 - The ATM–Chk2 and ATR–Chk1 Pathways in DNA Damage Signaling and Cancer

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DNA damage is a key factor both in the evolution and treatment of cancer. Genomic instability is a common feature of cancer cells, fuelling accumulation of oncogenic mutations, while radiation and diverse genotoxic agents remain important, if imperfect, therapeutic modalities. Cellular responses to DNA damage are coordinated primarily by two distinct kinase signaling cascades, the ATM–Chk2 and ATR–Chk1 pathways, which are activated by DNA double-strand breaks (DSBs) and single-stranded DNA respectively. Historically, these pathways were thought to act in parallel with overlapping functions; however, more recently it has become apparent that their relationship is more complex. In response to DSBs, ATM is required both for ATR–Chk1 activation and to initiate DNA repair via homologous recombination (HRR) by promoting formation of single-stranded DNA at sites of damage through nucleolytic resection. Interestingly, cells and organisms survive with mutations in ATM or other components required for HRR, such as BRCA1 and BRCA2, but at the cost of genomic instability and cancer predisposition. By contrast, the ATR–Chk1 pathway is the principal direct effector of the DNA damage and replication checkpoints and, as such, is essential for the survival of many, although not all, cell types. Remarkably, deficiency for HRR in BRCA1- and BRCA2-deficient tumors confers sensitivity to cisplatin and inhibitors of poly(ADP-ribose) polymerase (PARP), an enzyme required for repair of endogenous DNA damage. In addition, suppressing DNA damage and replication checkpoint responses by inhibiting Chk1 can enhance tumor cell killing by diverse genotoxic agents. Here, we review current understanding of the organization and functions of the ATM–Chk2 and ATR–Chk1 pathways and the prospects for targeting DNA damage signaling processes for therapeutic purposes.

Introduction

Cells in multicellular organisms are continuously exposed to DNA damage arising from a variety of endogenous and exogenous sources. These include reactive oxygen species, ultraviolet light, background radiation, and environmental mutagens. To protect their genomes from this assault, cells have evolved complex mechanisms, collectively referred to as DNA damage responses, that act to rectify damage and minimize the probability of lethal or permanent genetic damage. The cellular response to DNA damage encompasses multiple repair mechanisms and checkpoint responses that can delay cell cycle progression or modulate DNA replication. Collectively, these processes are essential to maintain genome stability.

DNA damage responses are orchestrated by multiple signal transduction processes, key among which are the ATM–Chk2 and ATR–Chk1 pathways. Activation of these pathways is crucial for the proper coordination of checkpoint and DNA repair processes; however, they can also modulate other biological outcomes such as apoptosis or cell senescence. In recent years, it has become evident that DNA damage responses are central both for the evolution and therapy of cancer. Inherited defects in DNA damage responses predispose to cancer by enhancing the accumulation of oncogenic mutations, while genome instability is also common in sporadic cancers. More recently, it has become apparent that oncogenic mutations elicit spontaneous DNA damage that can suppress the evolution of incipient cancer cells. Escape from this tumor suppressive barrier may be a major factor in selecting for additional genetic changes during tumor progression such as mutation of the p53 tumor suppressor, the most frequent alteration in human cancer.

Conversely, radiation and genotoxic chemotherapies remain a mainstay of conventional cancer treatment and are likely to remain so for the foreseeable future. Such therapies are, however, imperfect and can incur severe side effects. As a result, much current interest is focused on understanding how normal and tumor cells respond to DNA damage and determining whether DNA damage responses could be exploited or manipulated for therapeutic purposes. Two concepts in particular have attracted attention in recent years. First, inherent defects in genome stability mechanisms, such as homologous recombination, can confer tumor sensitivity to specific genotoxic agents or inhibition of complementary repair pathways. Second, evidence suggests that pharmacological suppression of DNA damage or checkpoint responses can enhance the efficacy of conventional genotoxic agents. Although promising, a full understanding of the biology and functions of the DNA damage signaling pathways will be crucial for the future success of such approaches.

Section snippets

Activation of the ATM–Chk2 and ATR–Chk1 DNA Pathways

DNA damage responses are controlled by biochemical pathways whose principal components and general organization have been conserved from yeasts to humans (Rhind and Russell, 2000). In vertebrates, the two main signaling pathways activated by DNA damage consist of the ATM–Chk2 and ATR–Chk1 protein kinases (Sancar et al., 2004). ATM and ATR are large kinases with sequence similarity to lipid kinases of the phosphatidylinositol-3-kinase (PI3K) family, but which phosphorylate only protein

Checkpoint Functions of the ATM–Chk2 and ATR–Chk1 Pathways

DNA damage or DNA synthesis inhibition in vertebrate cells evokes the activation of multiple, mechanistically distinct checkpoint responses that facilitate repair and promote cell survival (Kastan and Bartek, 2004). As shown in Fig. 2, DNA damage induces cell cycle delays at the G1/S and G2/M transitions (the G1 and G2 checkpoints), and a transient decrease in the rate of DNA synthesis (the intra-S checkpoint). Of these, the G1 checkpoint is unique in depending primarily on the function of the

The Three Rs of Damage Signaling: Resection, Recombination, and Repair

In eukaryotes DNA DSBs are repaired via two main mechanisms; nonhomologous end-joining (NHEJ) and homologous recombination repair (HRR). NHEJ occurs throughout the cell cycle; however, because HRR requires a sister chromatid to serve as a template, this mechanism is restricted to the S and G2 phases. Unlike NHEJ, HRR requires extensive DNA damage processing to generate tracts of ssDNA that, once coated with Rad51 recombinase, invade the homologous DNA duplex to initiate repair. Such

ATM–Chk2 and ATR–Chk1 Pathway Alterations in Cancer

The importance of genome stability for preventing carcinogenesis is evident both from human cancer predisposition syndromes that result from inherited loss-of-function mutations in DNA damage response genes and from the occurrence of sporadic mutations affecting such genes in cancers in otherwise genetically normal individuals (Kastan and Bartek, 2004). Examples of both have been found to affect ATM and Chk2 in human cancer, whereas ATR and Chk1 appear to be mutated only rarely. Important

Exploiting Homologous Recombinational Repair (HRR) Defects for Cancer Therapy

In recent years it has emerged that in addition to predisposing to cancer as a result of increased genomic instability, defective HRR may also render tumor cells inherently vulnerable to specific conventional anticancer agents and also to new strategies based on inhibition of complementary repair pathways. Thus, BRCA1- and BRCA2-deficient tumor cells have been found to be hypersensitive to cross-linking agents such as cisplatin in vitro (Bhattacharyya et al., 2000, Yuan et al., 1999), most

DNA Damage Signaling as a Barrier to Tumorigenesis

Predisposition syndromes demonstrate that genomic instability can promote cancer; however, in recent years an alternative paradigm has emerged in which DNA damage and the resulting downstream signaling processes act as a tumor suppression mechanism (Halazonetis et al., 2008). This concept originated with the observation that premalignant or early stage lesions in several cancer types, including bladder, breast, lung, and colon, frequently showed evidence of DNA damage as judged by the presence

Checkpoint Suppression as a Therapeutic Principle

Radiation and genotoxic chemotherapies remain the mainstays of cancer treatment. Although new, molecularly targeted, drugs like imatinib have revolutionized treatment of rare cancers such as chronic myeloid leukemia (Agrawal et al., 2010), there seems little prospect that conventional therapies will be replaced in the treatment of other, more common malignancies in the foreseeable future. Such treatments are, however, of limited efficacy and toxic not only to tumor cells but also to normal

Future Perspectives

Genomic instability has long been recognized as a cardinal feature, and arguably an important cause of, cancer, however, in recent years it has also emerged as a potential Achilles heel that offers new therapeutic opportunities. Thus far this prospect has been most evident in cancers that arise in predisposed individuals, for example in BRCA1 and BRCA2 mutation carriers, where impairment of one particular form of DNA repair specifically in tumor cells creates sensitivity to both existing and

Acknowledgments

The authors wish to thank Cancer Research-UK, the Beatson Institute for Cancer Research, and the Royal College of Radiologists (LMT) for financial support.

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