CHK2 kinase: cancer susceptibility and cancer therapy - two sides of the same coin?

Potentiation of the novel topoisomerase I inhibitor indenoisoquinoline LMP-400 by the cell checkpoint and Chk1-Chk2 inhibitor AZD7762

Novel topoisomerase I (Top1) inhibitors are in clinical development to circumvent the drawbacks of camptothecins (CPT). Here, we report molecular investigations into LMP-400, an indenoisoquinoline Top1 inhibitor in phase 1 clinical trial, by itself and in combination with the cell-cycle checkpoint inhibitor AZD7762. We examined drug effects on DNA replication and killing of cancer cells and found that LMP-400 showed synergistic antiproliferative activity when combined with AZD7762 in human colon carcinoma cells. Inhibition of S-phase progression and bromodeoxyuridine incorporation were similarly induced by LMP-400 and CPT and were abrogated by AZD7762. Replication studied by single DNA molecule analyses and immunofluorescence microscopy (molecular combing) showed rapid inhibition of fork progression in response to LMP-400 treatment with subsequent recapitulation after AZD7762 addition. AZD7762 inhibited both the activation/autophosphosphorylation of Chk1 and Chk2 at nanomolar concentrations in LMP-400-treated cells. This potent dual inhibition of Chk1 and Chk2 by AZD7762 was below the drug concentrations required to abrogate cell-cycle inhibition and produce synergism with LMP-400. Also, the synergism was independent of Chk2 both in Chk2-complemented cells and Chk2 knockout cells, suggesting additional mechanisms for cell-cycle abrogation by AZD7762. Together, our findings show a rationale for combining cell-cycle checkpoint inhibitors with the novel non-CPT indenoisoquinoline Top1 inhibitors.

Chk2 mediates RITA-induced apoptosis

Reactivation of the p53 tumor-suppressor protein by small molecules like Nutlin-3 and RITA (reactivation of p53 and induction of tumor cell apoptosis) is a promising strategy for cancer therapy. The molecular mechanisms involved in the responses to RITA remain enigmatic. Several groups reported the induction of a p53-dependent DNA damage response. Furthermore, the existence of a p53-dependent S-phase checkpoint has been suggested, involving the checkpoint kinase Chk1. We have recently shown synergistic induction of apoptosis by RITA in combination with Nutlin-3, and we observed concomitant Chk2 phosphorylation. Therefore, we investigated whether Chk2 contributes to the cellular responses to RITA. Strikingly, the induction of apoptosis seemed entirely Chk2 dependent. Transcriptional activity of p53 in response to RITA required the presence of Chk2. A partial rescue of apoptosis observed in Noxa knockdown cells emphasized the relevance of p53 transcriptional activity for RITA-induced apoptosis. In addition, we observed an early p53- and Chk2-dependent block of DNA replication upon RITA treatment. Replicating cells seemed more prone to entering RITA-induced apoptosis. Furthermore, the RITA-induced DNA damage response, which was not a secondary effect of apoptosis induction, was strongly attenuated in cells lacking p53 or Chk2. In conclusion, we identified Chk2 as an essential mediator of the cellular responses to RITA.

Structure-guided evolution of potent and selective CHK1 inhibitors through scaffold morphing

Pyrazolopyridine inhibitors with low micromolar potency for CHK1 and good selectivity against CHK2 were previously identified by fragment-based screening. The optimization of the pyrazolopyridines to a series of potent and CHK1-selective isoquinolines demonstrates how fragment-growing and scaffold morphing strategies arising from a structure-based understanding of CHK1 inhibitor binding can be combined to successfully progress fragment-derived hit matter to compounds with activity in vivo. The challenges of improving CHK1 potency and selectivity, addressing synthetic tractability, and achieving novelty in the crowded kinase inhibitor chemical space were tackled by multiple scaffold morphing steps, which progressed through tricyclic pyrimido[2,3-b]azaindoles to N-(pyrazin-2-yl)pyrimidin-4-amines and ultimately to imidazo[4,5-c]pyridines and isoquinolines. A potent and highly selective isoquinoline CHK1 inhibitor (SAR-020106) was identified, which potentiated the efficacies of irinotecan and gemcitabine in SW620 human colon carcinoma xenografts in nude mice.

X-ray structures of checkpoint kinase 2 in complex with inhibitors that target its gatekeeper-dependent hydrophobic pocket

The serine/threonine checkpoint kinase 2 (Chk2) is an attractive molecular target for the development of small molecule inhibitors to treat cancer. Here, we report the rational design of Chk2 inhibitors that target the gatekeeper-dependent hydrophobic pocket located behind the adenine-binding region of the ATP-binding site. These compounds exhibit IC(50) values in the low nanomolar range and are highly selective for Chk2 over Chk1. X-ray crystallography was used to determine the structures of the inhibitors in complex with the catalytic kinase domain of Chk2 to verify their modes of binding.

Association of reactive oxygen species-mediated signal transduction with in vitro apoptosis sensitivity in chronic lymphocytic leukemia B cells

BACKGROUND

Chronic lymphocytic leukemia (CLL) is a B cell malignancy with a variable clinical course and unpredictable response to therapeutic agents. Single cell network profiling (SCNP) utilizing flow cytometry measures alterations in signaling biology in the context of molecular changes occurring in malignancies. In this study SCNP was used to identify proteomic profiles associated with in vitro apoptotic responsiveness of CLL B cells to fludarabine, as a basis for ultimately linking these with clinical outcome.

METHODOLOGY/PRINCIPAL FINDING

SCNP was used to quantify modulated-signaling of B cell receptor (BCR) network proteins and in vitro F-ara-A mediated apoptosis in 23 CLL samples. Of the modulators studied the reactive oxygen species, hydrogen peroxide (H₂O₂), a known intracellular second messenger and a general tyrosine phosphatase inhibitor stratified CLL samples into two sub-groups based on the percentage of B cells in a CLL sample with increased phosphorylation of BCR network proteins. Separately, in the same patient samples, in vitro exposure to F-ara-A also identified two sub-groups with B cells showing competence or refractoriness to apoptotic induction. Statistical analysis showed that in vitro F-ara-A apoptotic proficiency was highly associated with the proficiency of CLL B cells to undergo H₂O₂-augmented signaling.

CONCLUSIONS/SIGNIFICANCE

This linkage in CLL B cells among the mechanisms governing chemotherapy-induced apoptosis increased signaling of BCR network proteins and a likely role of phosphatase activity suggests a means of stratifying patients for their response to F-ara-A based regimens. Future studies will examine the clinical applicability of these findings and also the utility of this approach in relating mechanism to function of therapeutic agents.

Structural characterization of inhibitor complexes with checkpoint kinase 2 (Chk2), a drug target for cancer therapy

Chk2 (checkpoint kinase 2) is a serine/threonine kinase that participates in a series of signaling networks responsible for maintaining genomic integrity and responding to DNA damage. The development of selective Chk2 inhibitors has recently attracted much interest as a means of sensitizing cancer cells to current DNA-damaging agents used in the treatment of cancer. Additionally, selective Chk2 inhibitors may reduce p53-mediated apoptosis in normal tissues, thereby helping to mitigate adverse side effects from chemotherapy and radiation. Thus far, relatively few selective inhibitors of Chk2 have been described and none have yet progressed into clinical trials. Here, we report crystal structures of the catalytic domain of Chk2 in complex with a novel series of potent and selective small molecule inhibitors. These compounds exhibit nanomolar potencies and are selective for Chk2 over Chk1. The structures reported here elucidate the binding modes of these inhibitors to Chk2 and provide information that can be exploited for the structure-assisted design of novel chemotherapeutics.

Physiological relevance of cell cycle kinases

The basic biology of the cell division cycle and its control by protein kinases was originally studied through genetic and biochemical studies in yeast and other model organisms. The major regulatory mechanisms identified in this pioneer work are conserved in mammals. However, recent studies in different cell types or genetic models are now providing a new perspective on the function of these major cell cycle regulators in different tissues. Here, we review the physiological relevance of mammalian cell cycle kinases such as cyclin-dependent kinases (Cdks), Aurora and Polo-like kinases, and mitotic checkpoint regulators (Bub1, BubR1, and Mps1) as well as other less-studied enzymes such as Cdc7, Nek proteins, or Mastl and their implications in development, tissue homeostasis, and human disease. Among these functions, the control of self-renewal or asymmetric cell division in stem/progenitor cells and the ability to regenerate injured tissues is a central issue in current research. In addition, many of these proteins play previously unexpected roles in metabolism, cardiovascular function, or neuron biology. The modulation of their enzymatic activity may therefore have multiple therapeutic benefits in human disease.

A phospho-proteomic screen identifies substrates of the checkpoint kinase Chk1

Background

The cell-cycle checkpoint kinase Chk1 is essential in mammalian cells due to its roles in controlling processes such as DNA replication, mitosis and DNA-damage responses. Despite its paramount importance, how Chk1 controls these functions remains unclear, mainly because very few Chk1 substrates have hitherto been identified.

Results

Here, we combine a chemical genetics approach with high-resolution mass spectrometry to identify novel Chk1 substrates and their phosphorylation sites. The list of targets produced reveals the potential impact of Chk1 function not only on processes where Chk1 was already known to be involved, but also on other key cellular events such as transcription, RNA splicing and cell fate determination. In addition, we validate and explore the phosphorylation of transcriptional co-repressor KAP1 Ser473 as a novel DNA-damage-induced Chk1 site.

Conclusions

By providing a substantial set of potential Chk1 substrates, we present opportunities for studying unanticipated functions for Chk1 in controlling a wide range of cellular processes. We also refine the Chk1 consensus sequence, facilitating the future prediction of Chk1 target sites. In addition, our identification of KAP1 Ser473 phosphorylation as a robust readout for Chk1 activity could be used to explore the in vivo effects of Chk1 inhibitors that are being developed for clinical evaluation.

Functional and molecular interactions between ERK and CHK2 in diffuse large B-cell lymphoma

Distinct oncogenic signalling cascades have been associated with non-Hodgkin lymphoma. ERK1/2 signalling elicits both transcriptional and post-transcriptional effects through phosphorylation of numerous substrates. Here we report a novel molecular relationship between ERK1/2 and CHK2, a protein kinase that is a key mediator of the DNA damage checkpoint that responds to DNA double-strand breaks. Our studies are the first to demonstrate the co-localization and overexpression of ERK1/2 and CHK2 in diffuse large B-cell lymphoma (DLBCL). The physical interaction between ERK and CHK2 was highly dependent on phosphorylated Thr 68 of CHK2. Concurrent administration of an ERK inhibitor enhances the antitumour activity of CHK2 inhibition in both a human DLBCL xenograft model as well as primary human DLBCL cells. Our data suggest a functional interaction between ERK and CHK2 and support the potential combined therapeutic targeting of ERK and CHK2 in human DLBCL.

Chk2 is a kinase that is a potential chemotherapeutic target. Here, Chk2 and the kinase ERK are shown to functionally interact, and are elevated in expression in human diffuse B-cell lymphomas. Combinatorial inhibition of the kinases was also shown to block tumour growth in an in vivo mouse model.

CHEK2 genomic and proteomic analyses reveal genetic inactivation or endogenous activation across the 60 cell lines of the US National Cancer Institute

CHEK2 encodes a serine/threonine kinase (Chk2) activated by ATM in response to DNA double-strand breaks. On the one hand, CHEK2 has been described as a tumor suppressor with proapoptotic, cell-cycle checkpoint and mitotic functions. On the other hand, Chk2 is also commonly activated (phosphorylated at T68) in cancers and precancerous lesions. Here, we report an extensive characterization of CHEK2 across the panel of 60 established cancer cell lines from the NCI Anticancer Screen (the NCI-60) using genomic and proteomic analyses, including exon-specific mRNA expression, DNA copy-number variation (CNV) by aCGH, exome sequencing, as well as western blot analyses for total and activated (pT68-Chk2) Chk2. We show that the high heterogeneity of Chk2 levels in cancer cells is primarily due to its inactivation (owing to low gene expression, alternative splicing, point mutations, copy-number alterations and premature truncation) or reduction of protein levels. Moreover, we observe that a significant percentage of cancer cells (12% of the NCI-60 and HeLa cells) show high endogenous Chk2 activation, which is always associated with p53 inactivation, and which is accompanied by downregulation of the Fanconi anemia and homologous recombination pathways. We also report the presence of activated Chk2 (pT68-Chk2) along with histone γ-H2AX in centrosomes.

Chk2 and p53 are haploinsufficient with dependent and independent functions to eliminate cells after telomere loss

The mechanisms that cells use to monitor telomere integrity, and the array of responses that may be induced, are not fully defined. To date there have been no studies in animals describing the ability of cells to survive and contribute to adult organs following telomere loss. We developed assays to monitor the ability of somatic cells to proliferate and differentiate after telomere loss. Here we show that p53 and Chk2 limit the growth and differentiation of cells that lose a telomere. Furthermore, our results show that two copies of the genes encoding p53 and Chk2 are required for the cell to mount a rapid wildtype response to a missing telomere. Finally, our results show that, while Chk2 functions by activating the p53-dependent apoptotic cascade, Chk2 also functions independently of p53 to limit survival. In spite of these mechanisms to eliminate cells that have lost a telomere, we find that such cells can make a substantial contribution to differentiated adult tissues.

Checkpoint kinase inhibitors: a patent review (2009 - 2010)

INTRODUCTION

Cells that suffer DNA damage activate the checkpoint kinases CHK1 and CHK2, which signal to initiate repair processes, limit cell-cycle progression and prevent cell replication, until the damaged DNA is repaired. Due to their potential application as novel anticancer therapies, inhibitors of CHK1 and CHK2 have become the focus of numerous drug discovery projects.

AREAS COVERED

This patent review examines the chemical structures and biological activities of recently reported CHK1 and CHK2 inhibitors. The chemical abstract and patent databases SciFinder and esp@cenet were used to locate patent applications that were published between September 2008 and December 2010, claiming chemical structures for use as CHK1 or CHK2 inhibitors.

EXPERT OPINION

This is an exciting time for checkpoint kinase inhibitors, with several currently in Phase I or II clinical trials. Many of the CHK1 inhibitors contained within this patent review have shown preclinical efficacy in combination with DNA-damaging chemotherapies. CHK1 inhibitors have recently been demonstrated to be efficacious as single agents in preclinical models of tumors with constitutive activation of CHK1 or high intrinsic DNA damage due to replication stress. The level of newly published patent applications covering CHK1 and CHK2 inhibitors remains high and a diverse range of scaffolds has been claimed.

Inactivation of chk2 and mus81 leads to impaired lymphocytes development, reduced genomic instability, and suppression of cancer

Chk2 is an effector kinase important for the activation of cell cycle checkpoints, p53, and apoptosis in response to DNA damage. Mus81 is required for the restart of stalled replication forks and for genomic integrity. Mus81^{Δex3-4/Δex3-4} mice have increased cancer susceptibility that is exacerbated by p53 inactivation. In this study, we demonstrate that Chk2 inactivation impairs the development of Mus81^{Δex3-4/Δex3-4} lymphoid cells in a cell-autonomous manner. Importantly, in contrast to its predicted tumor suppressor function, loss of Chk2 promotes mitotic catastrophe and cell death, and it results in suppressed oncogenic transformation and tumor development in Mus81^{Δex3-4/Δex3-4} background. Thus, our data indicate that an important role for Chk2 is maintaining lymphocyte development and that dual inactivation of Chk2 and Mus81 remarkably inhibits cancer.

Failure to repair DNA damage has been associated with a number of human syndromes, neurodegenerative diseases, immunodeficiency, and cancer. In addition, radiotherapy and many cancer chemotherapeutic drugs induce DNA damage, thus allowing the killing of tumors. Recent data indicated Mus81's role in maintaining genomic integrity and suppressing cancer. Furthermore, inactivation of p53, the most frequently inactivated tumor suppressor in cancer, leads to synergistic tumorigenesis in Mus81 mutant mice. As Chk2 is important for p53 activation, we have examined the effect of its inactivation on the phenotypes associated with Mus81 loss of function. We report that Chk2 is essential for the development of lymphoid cells deficient for Mus81. Chk2 inactivation increased spontaneous cell death of Mus81 deficient cells and impaired the development of T and B-cell lineages. Chk2 inactivation also reduced the frequency of Mus81-deficient cells that carry elevated levels of spontaneous genomic instability. Importantly, inactivation of Chk2 protected Mus81 mutant mice from developing spontaneous tumorigenesis. These data indicate potential therapeutic benefits for the inactivation of Chk2 and Mus81.

Two new CHEK2 germ-line variants detected in breast cancer/sarcoma families negative for BRCA1, BRCA2, and TP53 gene mutations

CHEK2 gene mutations occur in a subset of patients with familial breast cancer, acting as moderate/low penetrance cancer susceptibility alleles. Although CHEK2 is no longer recognized as a major determinant of the Li-Fraumeni syndrome, a hereditary condition predisposing to cancer at multiple sites, it cannot be ruled out that mutations of this gene play a role in malignancies arising in peculiar multi-cancer families. To assess the contribution of CHEK2 to the breast cancer/sarcoma phenotype, we screened for germ-line sequence variations of the gene among 12 probands from hereditary breast/ovarian cancer families with one case of sarcoma that tested wild-type for mutations in the BRCA1, BRCA2, and TP53 genes. Two cases harbored previously unreported mutations in CHEK2, the c.507delT and c.38A>G, leading to protein truncation (p.Phe169LeufsX2) and amino acid substitution (p.His13Arg), respectively. These mutations were not considered common polymorphic variants, as they were undetected in 230 healthy controls of the same ethnic origin. While the c.38A>G encodes a mutant protein that behaves in biochemical assays as the wild-type form, the c.507delT is a loss-of-function mutation. The identification of two previously unreported CHEK2 variants, including a truncating mutation leading to constitutional haploinsufficiency, in individuals belonging to families selected for breast cancer/sarcoma phenotype, supports the hypothesis that the CHEK2 gene may act as a factor contributing to individual tumor development in peculiar familial backgrounds.

Detecting differential allelic expression using high-resolution melting curve analysis: application to the breast cancer susceptibility gene CHEK2

BACKGROUND

The gene CHEK2 encodes a checkpoint kinase playing a key role in the DNA damage pathway. Though CHEK2 has been identified as an intermediate breast cancer susceptibility gene, only a small proportion of high-risk families have been explained by genetic variants located in its coding region. Alteration in gene expression regulation provides a potential mechanism for generating disease susceptibility. The detection of differential allelic expression (DAE) represents a sensitive assay to direct the search for a functional sequence variant within the transcriptional regulatory elements of a candidate gene. We aimed to assess whether CHEK2 was subject to DAE in lymphoblastoid cell lines (LCLs) from high-risk breast cancer patients for whom no mutation in BRCA1 or BRCA2 had been identified.

METHODS

We implemented an assay based on high-resolution melting (HRM) curve analysis and developed an analysis tool for DAE assessment.

RESULTS

We observed allelic expression imbalance in 4 of the 41 LCLs examined. All four were carriers of the truncating mutation 1100delC. We confirmed previous findings that this mutation induces non-sense mediated mRNA decay. In our series, we ruled out the possibility of a functional sequence variant located in the promoter region or in a regulatory element of CHEK2 that would lead to DAE in the transcriptional regulatory milieu of freely proliferating LCLs.

CONCLUSIONS

Our results support that HRM is a sensitive and accurate method for DAE assessment. This approach would be of great interest for high-throughput mutation screening projects aiming to identify genes carrying functional regulatory polymorphisms.

Anticancer therapy with checkpoint inhibitors: what, where and when?

Research into inhibitors of the protein kinases controlling the cellular response to DNA damage has reached an exciting stage, particularly for the checkpoint kinases CHK1 and CHK2. Selective inhibitors are now being tested in clinical trials in cancer patients. In this review, we highlight recent data from cellular and in vivo preclinical models that provide insight into the clinical contexts for checkpoint kinase inhibition (e.g. the timing of treatment and what type of inhibitor would be most appropriate). Although it has been shown that CHK1 inhibition potentiates the efficacy of various DNA-damaging therapies, the context for selective CHK2 inhibition is not yet as well defined. Distinct effects of selective CHK1 or CHK2 inhibition are observed when combined with DNA-damaging agents. It has also been shown that both CHK1 and CHK2 inhibitors potentiate the effects of other molecular targeted therapeutics [e.g. poly(ADP-ribose) polymerase inhibitors]. We also consider the single-agent activity of checkpoint kinase inhibitors for tumours with defined genetic backgrounds.

CCT241533 is a potent and selective inhibitor of CHK2 that potentiates the cytotoxicity of PARP inhibitors

CHK2 is a checkpoint kinase involved in the ATM-mediated response to double-strand DNA breaks. Its potential as a drug target is still unclear, but inhibitors of CHK2 may increase the efficacy of genotoxic cancer therapies in a p53 mutant background by eliminating one of the checkpoints or DNA repair pathways contributing to cellular resistance. We report here the identification and characterization of a novel CHK2 kinase inhibitor, CCT241533. X-ray crystallography confirmed that CCT241533 bound to CHK2 in the ATP pocket. This compound inhibits CHK2 with an IC(50) of 3 nmol/L and shows minimal cross-reactivity against a panel of kinases at 1 μmol/L. CCT241533 blocked CHK2 activity in human tumor cell lines in response to DNA damage, as shown by inhibition of CHK2 autophosphorylation at S516, band shift mobility changes, and HDMX degradation. CCT241533 did not potentiate the cytotoxicity of a selection of genotoxic agents in several cell lines. However, this compound significantly potentiates the cytotoxicity of two structurally distinct PARP inhibitors. Clear induction of the pS516 CHK2 signal was seen with a PARP inhibitor alone, and this activation was abolished by CCT241533, implying that the potentiation of PARP inhibitor cell killing by CCT241533 was due to inhibition of CHK2. Consequently, our findings imply that CHK2 inhibitors may exert therapeutic activity in combination with PARP inhibitors.

Loss of the tumour-suppressor genes CHK2 and BRCA1 results in chromosomal instability

CHK2 (checkpoint kinase 2) and BRCA1 (breast cancer early-onset 1) are tumour-suppressor genes that have been implicated previously in the DNA damage response. Recently, we have identified CHK2 and BRCA1 as genes required for the maintenance of chromosomal stability and have shown that a Chk2-mediated phosphorylation of Brca1 is required for the proper and timely assembly of mitotic spindles. Loss of CHK2, BRCA1 or inhibition of its Chk2-mediated phosphorylation inevitably results in the transient formation of abnormal spindles that facilitate the establishment of faulty microtubule-kinetochore attachments associated with the generation of lagging chromosomes. Importantly, both CHK2 and BRCA1 are lost at very high frequency in aneuploid lung adenocarcinomas that are typically induced in knockout mice exhibiting chromosomal instability. Thus these results suggest novel roles for Chk2 and Brca1 in mitosis that might contribute to their tumour-suppressor functions.

Calcineurin ensures a link between the DNA replication checkpoint and microtubule-dependent polarized growth

Microtubules are central to eukaryotic cell morphogenesis. Microtubule plus-end tracking proteins (+TIPs) transport polarity factors to the cell cortex, thereby playing a key role in both microtubule dynamics and cell polarity. However, the signalling pathway linking +TIPs to cell polarity control remains elusive. Here we show that the fission yeast checkpoint kinase Cds1 (Chk2 homologue) delays the transition of growth polarity from monopolar to bipolar (termed NETO; new-end take-off). The +TIPs CLIP170 homologue Tip1 and kinesin Tea2 are responsible for this delay, which is accompanied by a reduction in microtubule dynamics at the cell tip. Remarkably, microtubule stabilization occurs asymmetrically, prominently at the non-growing cell end, which induces abnormal accumulation of the polarity factor Tea1. Importantly, NETO delay requires activation of calcineurin, which is carried out by Cds1, resulting in Tip1 dephosphorylation. Thus, our study establishes a critical link between calcineurin and checkpoint-dependent cell morphogenesis.

The ATM-Chk2 and ATR-Chk1 pathways in DNA damage signaling and cancer

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.

Global dissociation of HuR-mRNA complexes promotes cell survival after ionizing radiation

Ionizing radiation (IR) triggers adaptive changes in gene expression. Here, we show that survival after IR strongly depends on the checkpoint kinase Chk2 acting upon its substrate HuR, an RNA-binding protein that stabilizes and/or modulates the translation of target mRNAs. Microarray analysis showed that in human HCT116 colorectal carcinoma cells (WT), IR-activated Chk2 triggered the dissociation of virtually all of HuR-bound mRNAs, since IR did not dissociate HuR target mRNAs in Chk2-null (CHK2-/-) HCT116 cells. Accordingly, several HuR-interacting mRNAs encoding apoptosis- and proliferation-related proteins (TJP1, Mdm2, TP53BP2, Bax, K-Ras) dissociated from HuR in WT cells, but remained bound and showed altered post-transcriptional regulation in CHK2-/- cells. Use of HuR mutants that were not phosphorylatable by Chk2 (HuR(3A)) and HuR mutants mimicking constitutive phosphorylation by Chk2 (HuR(3D)) revealed that dissociation of HuR target transcripts enhanced cell survival. We propose that the release of HuR-bound mRNAs via an IR-Chk2-HuR regulatory axis improves cell outcome following IR.

Histone tyrosine phosphorylation comes of age

Histones were discovered over a century ago and have since been found to be the most extensively posttranslationally modified proteins, although tyrosine phosphorylation of histones had remained elusive until recently. The year 2009 proved to be a landmark year for histone tyrosine (Y) phosphorylation as five research groups independently discovered this modification. Three groups describe phosphorylation of Y142 in the variant histone H2A.X, where it may be involved in the cellular decision making process to either undergo DNA repair or apoptosis in response to DNA damage. Further, one group suggests that phosphorylation of histone H3 on Y99 is crucial for its regulated proteolysis in yeast, while another found that Y41 phosphorylation modulates chromatin architecture and oncogenesis in mammalian cells. These pioneering studies provide the initial conceptual framework for further analyses of the diverse roles of tyrosine phosphorylation on different histones, with far reaching implications for human health and disease.

Rare, evolutionarily unlikely missense substitutions in CHEK2 contribute to breast cancer susceptibility: results from a breast cancer family registry case-control mutation-screening study

Introduction

Both protein-truncating variants and some missense substitutions in CHEK2 confer increased risk of breast cancer. However, no large-scale study has used full open reading frame mutation screening to assess the contribution of rare missense substitutions in CHEK2 to breast cancer risk. This absence has been due in part to a lack of validated statistical methods for summarizing risk attributable to large numbers of individually rare missense substitutions.

Methods

Previously, we adapted an in silico assessment of missense substitutions used for analysis of unclassified missense substitutions in BRCA1 and BRCA2 to the problem of assessing candidate genes using rare missense substitution data observed in case-control mutation-screening studies. The method involves stratifying rare missense substitutions observed in cases and/or controls into a series of grades ordered a priori from least to most likely to be evolutionarily deleterious, followed by a logistic regression test for trends to compare the frequency distributions of the graded missense substitutions in cases versus controls. Here we used this approach to analyze CHEK2 mutation-screening data from a population-based series of 1,303 female breast cancer patients and 1,109 unaffected female controls.

Results

We found evidence of risk associated with rare, evolutionarily unlikely CHEK2 missense substitutions. Additional findings were that (1) the risk estimate for the most severe grade of CHEK2 missense substitutions (denoted C65) is approximately equivalent to that of CHEK2 protein-truncating variants; (2) the population attributable fraction and the familial relative risk explained by the pool of rare missense substitutions were similar to those explained by the pool of protein-truncating variants; and (3) post hoc power calculations implied that scaling up case-control mutation screening to examine entire biochemical pathways would require roughly 2,000 cases and controls to achieve acceptable statistical power.

Conclusions

This study shows that CHEK2 harbors many rare sequence variants that confer increased risk of breast cancer and that a substantial proportion of these are missense substitutions. The study validates our analytic approach to rare missense substitutions and provides a method to combine data from protein-truncating variants and rare missense substitutions into a one degree of freedom per gene test.

Structure-based design of potent and selective 2-(quinazolin-2-yl)phenol inhibitors of checkpoint kinase 2

Structure-based design was applied to the optimization of a series of 2-(quinazolin-2-yl)phenols to generate potent and selective ATP-competitive inhibitors of the DNA damage response signaling enzyme checkpoint kinase 2 (CHK2). Structure-activity relationships for multiple substituent positions were optimized separately and in combination leading to the 2-(quinazolin-2-yl)phenol 46 (IC(50) 3 nM) with good selectivity for CHK2 against CHK1 and a wider panel of kinases and with promising in vitro ADMET properties. Off-target activity at hERG ion channels shown by the core scaffold was successfully reduced by the addition of peripheral polar substitution. In addition to showing mechanistic inhibition of CHK2 in HT29 human colon cancer cells, a concentration dependent radioprotective effect in mouse thymocytes was demonstrated for the potent inhibitor 46 (CCT241533).

Loss of ATM/Chk2/p53 pathway components accelerates tumor development and contributes to radiation resistance in gliomas

Maintenance of genomic integrity is essential for adult tissue homeostasis and defects in the DNA-damage response (DDR) machinery are linked to numerous pathologies including cancer. Here, we present evidence that the DDR exerts tumor suppressor activity in gliomas. We show that genes encoding components of the DDR pathway are frequently altered in human gliomas and that loss of elements of the ATM/Chk2/p53 cascade accelerates tumor formation in a glioma mouse model. We demonstrate that Chk2 is required for glioma response to ionizing radiation in vivo and is necessary for DNA-damage checkpoints in the neuronal stem cell compartment. Finally, we observed that the DDR is constitutively activated in a subset of human GBMs, and such activation correlates with regions of hypoxia.

The Role of Wild-Type p53 in Cisplatin-Induced Chk2 Phosphorylation and the Inhibition of Platinum Resistance with a Chk2 Inhibitor

The major obstacle in platinum chemotherapy is the repair of platinum-damaged DNA that results in increased resistance, reduced apoptosis, and finally treatment failure. Our research goal is to determine and block the mechanisms of platinum resistance. Our recent studies demonstrate that several kinases in the DNA-repair pathway are activated after cells are exposed to cisplatin. These include ATM, p53, and Chk2. The increased Chk2 phosphorylation is modulated by p53 in a wild-type p53 model. Overexpression of p53 by cDNA transfection in wt-p53 (but not p53 deficient) cells doubled the amount of Chk2 phosphorylation 48 hours after cisplatin treatment. p53 knockdown by specific siRNA greatly reduced Chk2 phosphorylation. We conclude that wild-type p53, in response to cisplatin stimulation, plays a role in the upstream regulation of Chk2 phosphorylation at Thr-68. Cells without normal p53 function survive via an alternative pathway in response to the exogenous influence of cisplatin. We strongly suggest that it is very important to include the p53 mutational status in any p53 involved studies due to the functional differentiation of wt p53 and p53 mutant. Inhibition of Chk2 pathway with a Chk2 inhibitor (C3742) increased cisplatin efficacy, especially those with defective p53. Our findings suggest that inhibition of platinum resistance can be achieved with a small-molecule inhibitor of Chk2, thus improving the therapeutic indices for platinum chemotherapy.

RAD51C: a novel cancer susceptibility gene is linked to Fanconi anemia and breast cancer

Germline mutations in many of the genes that are involved in homologous recombination (HR)-mediated DNA double-strand break repair (DSBR) are associated with various human genetic disorders and cancer. RAD51 and RAD51 paralogs are important for HR and in the maintenance of genome stability. Despite the identification of five RAD51 paralogs over a decade ago, the molecular mechanism(s) by which RAD51 paralogs regulate HR and genome maintenance remains obscure. In addition to the known roles of RAD51C in early and late stages of HR, it also contributes to activation of the checkpoint kinase CHK2. One recent study identifies biallelic mutation in RAD51C leading to Fanconi anemia-like disorder. Whereas a second study reports monoallelic mutation in RAD51C associated with increased risk of breast and ovarian cancer. These reports show RAD51C is a cancer susceptibility gene. In this review, we focus on describing the functions of RAD51C in HR, DNA damage signaling and as a tumor suppressor with an emphasis on the new roles of RAD51C unveiled by these reports.

Cooperative functions of Chk1 and Chk2 reduce tumour susceptibility in vivo

Although the linkage of Chk1 and Chk2 to important cancer signalling suggests that these kinases have functions as tumour suppressors, neither Chk1+/- nor Chk2-/- mice show a predisposition to cancer under unperturbed conditions. We show here that Chk1+/-Chk2-/- and Chk1+/-Chk2+/- mice have a progressive cancer-prone phenotype. Deletion of a single Chk1 allele compromises G2/M checkpoint function that is not further affected by Chk2 depletion, whereas Chk1 and Chk2 cooperatively affect G1/S and intra-S phase checkpoints. Either or both of the kinases are required for DNA repair depending on the type of DNA damage. Mouse embryonic fibroblasts from the double-mutant mice showed a higher level of p53 with spontaneous DNA damage under unperturbed conditions, but failed to phosphorylate p53 at S23 and further induce p53 expression upon additional DNA damage. Neither Chk1 nor Chk2 is apparently essential for p53- or Rb-dependent oncogene-induced senescence. Our results suggest that the double Chk mutation leads to a high level of spontaneous DNA damage, but fails to eliminate cells with damaged DNA, which may ultimately increase cancer susceptibility independently of senescence.

Distinct patterns of DNA damage response and apoptosis correlate with Jak/Stat and PI3kinase response profiles in human acute myelogenous leukemia

Background

Single cell network profiling (SCNP) utilizing flow cytometry measures alterations in intracellular signaling responses. Here SCNP was used to characterize Acute Myeloid Leukemia (AML) disease subtypes based on survival, DNA damage response and apoptosis pathways.

Methodology and Principal Findings

Thirty four diagnostic non-M3 AML samples from patients with known clinical outcome were treated with a panel of myeloid growth factors and cytokines, as well as with apoptosis-inducing agents. Analysis of induced Jak/Stat and PI3K pathway responses in blasts from individual patient samples identified subgroups with distinct signaling profiles that were not seen in the absence of a modulator. In vitro exposure of patient samples to etoposide, a DNA damaging agent, revealed three distinct “DNA damage response (DDR)/apoptosis” profiles: 1) AML blasts with a defective DDR and failure to undergo apoptosis; 2) AML blasts with proficient DDR and failure to undergo apoptosis; 3) AML blasts with proficiency in both DDR and apoptosis pathways. Notably, AML samples from clinical responders fell within the “DDR/apoptosis” proficient profile and, as well, had low PI3K and Jak/Stat signaling responses. In contrast, samples from clinical non responders had variable signaling profiles often with in vitro apoptotic failure and elevated PI3K pathway activity. Individual patient samples often harbored multiple, distinct, leukemia-associated cell populations identifiable by their surface marker expression, functional performance of signaling pathway in the face of cytokine or growth factor stimulation, as well as their response to apoptosis-inducing agents.

Conclusions and Significance

Characterizing and tracking changes in intracellular pathway profiles in cell subpopulations both at baseline and under therapeutic pressure will likely have important clinical applications, potentially informing the selection of beneficial targeted agents, used either alone or in combination with chemotherapy.

Interdependent phosphorylation within the kinase domain T-loop Regulates CHK2 activity

Chk2 is a critical regulator of the cellular DNA damage repair response. Activation of Chk2 in response to IR-induced damage is initiated by phosphorylation of the Chk2 SQ/TQ cluster domain at Ser¹⁹, Ser³³, Ser³⁵, and Thr⁶⁸. This precedes autophosphorylation of Thr³⁸³/Thr³⁸⁷ in the T-loop region of the kinase domain an event that is a prerequisite for efficient kinase activity. We conducted an in-depth analysis of phosphorylation within the T-loop region (residues 366–406). We report four novel phosphorylation sites at Ser³⁷², Thr³⁷⁸, Thr³⁸⁹, and Tyr³⁹⁰. Substitution mutation Y390F was defective for kinase function. The substitution mutation T378A ablated the IR induction of kinase activity. Interestingly, the substitution mutation T389A demonstrated a 6-fold increase in kinase activity when compared with wild-type Chk2. In addition, phosphorylation at Thr³⁸⁹ was a prerequisite to phosphorylation at Thr³⁸⁷ but not at Thr³⁸³. Quantitative mass spectrometry analysis revealed IR-induced phosphorylation and subcellular distribution of Chk2 phosphorylated species. We observed IR-induced increase in phosphorylation at Ser³⁷⁹, Thr³⁸⁹, and Thr³⁸³/Thr³⁸⁹. Phosphorylation at Tyr³⁹⁰ was dramatically reduced following IR. Exposure to IR was also associated with changes in the ratio of chromatin/nuclear localization. IR-induced increase in chromatin localization was associated with phosphorylation at Thr³⁷², Thr³⁷⁹, Thr³⁸³, Thr³⁸⁹, Thr³⁸³/Thr³⁸⁷, and Thr³⁸³/Thr³⁸⁹. Chk2 hyper-phosphorylated species at Thr³⁸³/Thr³⁸⁷/Thr³⁸⁹ and Thr³⁸³/Thr³⁸⁷/Thr³⁸⁹/Tyr³⁹⁰ relocalized from almost exclusively chromatin to predominately nuclear expression, suggesting a role for phosphorylation in regulation of chromatin targeting and egress. The differential impact of T-loop phosphorylation on Chk2 ubiquitylation suggests a co-dependence of these modifications. The results demonstrate that a complex interdependent network of phosphorylation events within the T-loop exchange region regulates dimerization/autophosphorylation, kinase activation, and chromatin targeting/egress of Chk2.

Enhancement of p53-mutant human colorectal cancer cells radiosensitivity by flavonoid fisetin

PURPOSE

The aim of this study was to investigate whether fisetin is a potential radiosensitizer for human colorectal cancer cells, which are relatively resistant to radiotherapy.

METHODS AND MATERIALS

Cell survival was examined by clonogenic survival assay, and DNA fragmentation was assessed by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay. The effects of treatments on cell cycle distribution and apoptosis were examined by flow cytometry. Western blot analysis was performed to ascertain the protein levels of gamma-H2AX, phospho-Chk2, active caspase-3, PARP cleavage, phospho-p38, phospho-AKT, and phospho-ERK1/2.

RESULTS

Fisetin pretreatment enhanced the radiosensitivity of p53-mutant HT-29 human colorectal cancer cells but not human keratocyte HaCaT cells; it also prolonged radiation-induced G(2)/M arrest, enhanced radiation-induced cell growth arrest in HT-29 cells, and suppressed radiation-induced phospho-H2AX (Ser-139) and phospho-Chk2 (Thr-68) in p53-mutant HT-29 cells. Pretreatment with fisetin enhanced radiation-induced caspase-dependent apoptosis in HT-29 cells. Fisetin pretreatment augmented radiation-induced phosphorylation of p38 mitogen-activated protein kinase, which is involved in caspase-mediated apoptosis, and SB202190 significantly reduced apoptosis and radiosensitivity in fisetin-pretreated HT-29 cells. By contrast, both phospho-AKT and phospho-ERK1/2, which are involved in cell proliferation and antiapoptotic pathways, were suppressed after irradiation combined with fisetin pretreatment.

CONCLUSIONS

To our knowledge, this study is the first to provide evidence that fisetin exerts a radiosensitizing effect in p53-mutant HT-29 cells. Fisetin could potentially be developed as a novel radiosensitizer against radioresistant human cancer cells.

Importance of DNA damage checkpoints in the pathogenesis of human cancers

All forms of life on earth must cope with constant exposure to DNA-damaging agents that may promote cancer development. As a biological barrier, known as DNA damage response (DDR), cells are provided with both DNA repair mechanisms and highly conserved cell cycle checkpoints. The latter are responsible for the control of cell cycle phase progression with ATM, ATR, Chk1, and Chk2 as the main signaling molecules, thus dealing with both endogenous and exogenous sources of DNA damage. As cell cycle checkpoint and also DNA repair genes, such as BRCA1 and BRCA2, are frequently mutated, we here discuss their fundamental roles in the pathogenesis of human cancers. Importantly, as current evidence also suggests a role of MAPK's (mitogen activated protein kinases) in cell cycle checkpoint control, we describe in this review both the ATR/ATM-Chk1/Chk2 signaling pathways as well as the regulation of cell cycle checkpoints by MAPK's as molecular mechanisms in DDR, and how their dysfunction is related to cancer development. Moreover, since damage to DNA might be the common underlying mechanism for the positive outcome of chemotherapy, we also discuss targeting anticancer treatments on cell cycle checkpoints as an important issue emerging in drug discovery.

Design and evaluation of 3,6-di(hetero)aryl imidazo[1,2-a]pyrazines as inhibitors of checkpoint and other kinases

A range of 3,6-di(hetero)arylimidazo[1,2-a]pyrazine ATP-competitive inhibitors of CHK1 were developed by scaffold hopping from a weakly active screening hit. Efficient synthetic routes for parallel synthesis were developed to prepare analogues with improved potency and ligand efficiency against CHK1. Kinase profiling showed that the imidazo[1,2-a]pyrazines could inhibit other kinases, including CHK2 and ABL, with equivalent or better potency depending on the pendant substitution. These 3,6-di(hetero)aryl imidazo[1,2-a]pyrazines appear to represent a general kinase inhibitor scaffold.

Depletion of securin induces senescence after irradiation and enhances radiosensitivity in human cancer cells regardless of functional p53 expression

PURPOSE

Radiotherapy is one of the best choices for cancer treatment. However, various tumor cells exhibit resistance to irradiation-induced apoptosis. The development of new strategies to trigger cancer cell death besides apoptosis is necessary. This study investigated the role of securin in radiation-induced apoptosis and senescence in human cancer cells.

METHODS AND MATERIALS

Cell survival was determined using clonogenic assays. Western blot analysis was used to analyze levels of securin, caspase-3, PARP, p53, p21, Rb, gamma-H2AX, and phospho-Chk2. Senescent cells were analyzed using a beta-galactosidase staining assay. A securin-expressed vector (pcDNA-securin) was stably transfected into securin-null HCT116 cells. Securin gene knockdown was performed by small interfering RNA and small hairpin RNA in HCT116 and MDA-MB-231 cells, respectively.

RESULTS

Radiation was found to induce apoptosis in securin wild type HCT116 cells but induced senescence in securin-null cells. Restoration of securin reduced senescence and increased cell survival in securin-null HCT116 cells after irradiation. Radiation-induced gamma-H2AX and Chk2 phosphorylation were induced transiently in securin-wild-type cells but exhibited sustained activation in securin-null cells. Securin gene knockdown switches irradiation-induced apoptosis to senescence in both HCT116 p53-null and MDA-MB-231 cells.

CONCLUSIONS

Our results demonstrated that the level of securin expression plays a determining role in the radiosensitivity and fate of cells. Depletion of securin impairs DNA repair after irradiation, increasing DNA damage and promoting senescence in the residual surviving cells regardless of functional p53 expression. The knockdown of securin may contribute to a novel radiotherapy protocol for the treatment of human cancer cells that are resistant to irradiation.

A DNA damage-activated checkpoint kinase phosphorylates tau and enhances tau-induced neurodegeneration

Hyperphosphorylation of the microtubule associated protein tau is detected in the brains of individuals with a range of neurodegenerative diseases including Alzheimer's disease (AD). An imbalance in phosphorylation and/or dephosphorylation of tau at disease-related sites has been suggested to initiate the abnormal metabolism and toxicity of tau in disease pathogenesis. However, the mechanisms underlying abnormal phosphorylation of tau in AD are not fully understood. Here, we show that the DNA damage-activated Checkpoint kinase 2 (Chk2) is a novel tau kinase and enhances tau toxicity in a transgenic Drosophila model. Overexpression of Drosophila Chk2 increases tau phosphorylation at Ser262 and enhances tau-induced neurodegeneration in transgenic flies expressing human tau. The non-phosphorylatable Ser262Ala mutation abolishes Chk2-induced enhancement of tau toxicity, suggesting that the Ser262 phosphorylation site is involved in the enhancement of tau toxicity by Chk2. In vitro kinase assays revealed that human Chk2 and a closely related checkpoint kinase 1 (Chk1) directly phosphorylate human tau at Ser262. We also demonstrate that Drosophila Chk2 does not modulate the activity of the fly homolog of microtubule affinity regulating kinase, which has been shown to be a physiological tau Ser262 kinase. Since accumulation of DNA damage has been detected in the brains of AD patients, our results suggest that the DNA damage-activated kinases Chk1 and Chk2 may be involved in tau phosphorylation and toxicity in the pathogenesis of AD.

Oxaliplatin responses in colorectal cancer cells are modulated by CHK2 kinase inhibitors

BACKGROUND AND PURPOSE

Checkpoint kinase 2 (CHK2) is activated by DNA damage and can contribute to p53 stabilization, modulating growth arrest and/or apoptosis. We investigated the contribution of CHK2 to oxaliplatin-mediated toxicity in a colorectal cancer model.

EXPERIMENTAL APPROACH

We evaluated the ability of CHK2 small molecule inhibitors to potentiate oxaliplatin-induced toxicity. The role of CHK2 in oxaliplatin-induced apoptosis was investigated in HCT116 cells that were wild-type (WT) or KO for CHK2. Small molecule inhibitors of CHK2 were used in combination studies with oxaliplatin in this cell model.

KEY RESULTS

In oxaliplatin-treated CHK2 KO cells, accelerated apoptosis was accompanied by attenuated p53 stabilization and p21(WAF-1) up-regulation correlating with increased Bax expression, cytochrome c release and elevated caspase activity. The higher levels of apoptosis in CHK2 KO cells were restored to control (WT) levels when CHK2 was re-introduced. This 'uncoupling' of p53 stabilization and Bax up-regulation in CHK2 KO cells suggested oxaliplatin-induced apoptosis was due to a p53-independent response. Combination studies revealed that CHK2 inhibitor II or debromohymenialdisine antagonized the responses to oxaliplatin. This inhibitory effect correlated with decreases in apoptosis, p53 stabilization and DNA inter-strand cross-link formation, and was dependent on the presence (but not activity) of CHK2.

CONCLUSIONS AND IMPLICATIONS

Combinations of CHK2 inhibitors with oxaliplatin should further sensitize cells to oxaliplatin treatment. However, these inhibitors produced an antagonistic effect on the response to oxaliplatin, which was reversed on the re-introduction of CHK2. These observations may have implications for the use of oxaliplatin in colorectal cancer therapy in combination with therapies targeting CHK2.

Naphthalimides induce G(2) arrest through the ATM-activated Chk2-executed pathway in HCT116 cells

Naphthalimides, particularly amonafide and 2-(2-dimethylamino)-6-thia-2-aza-benzo[def]chrysene-1,3-diones (R16), have been identified to possess anticancer activities and to induce G(2)-M arrest through inhibiting topoisomerase II accompanied by Chk1 degradation. The current study was designed to precisely dissect the signaling pathway(s) responsible for the naphthalimide-induced cell cycle arrest in human colon carcinoma HCT116 cells. Using phosphorylated histone H3 and mitotic protein monoclonal 2 as mitosis markers, we first specified the G(2) arrest elicited by the R16 and amonafide. Then, R16 and amonafide were revealed to induce phosphorylation of the DNA damage sensor ataxia telangiectasia-mutated (ATM) responding to DNA double-strand breaks (DSBs). Inhibition of ATM by both the pharmacological inhibitor caffeine and the specific small interference RNA (siRNA) rescued the G(2) arrest elicited by R16, indicating its ATM-dependent characteristic. Furthermore, depletion of Chk2, but not Chk1 with their corresponding siRNA, statistically significantly reversed the R16- and amonafide-triggered G(2) arrest. Moreover, the naphthalimides phosphorylated Chk2 in an ATM-dependent manner but induced Chk1 degradation. These data indicate that R16 and amonafide preferentially used Chk2 as evidenced by the differential ATM-executed phosphorylation of Chk1 and Chk2. Thus, a clear signaling pathway can be established, in which ATM relays the DNA DSBs signaling triggered by the naphthalimides to the checkpoint kinases, predominantly to Chk2,which finally elicits G(2) arrest. The mechanistic elucidation not only favors the development of the naphthalimides as anticancer agents but also provides an alternative strategy of Chk2 inhibition to potentiate the anticancer activities of these agents.

The preclinical pharmacology and therapeutic activity of the novel CHK1 inhibitor SAR-020106

Genotoxic antitumor agents continue to be the mainstay of current cancer chemotherapy. These drugs cause DNA damage and activate numerous cell cycle checkpoints facilitating DNA repair and the maintenance of genomic integrity. Most human tumors lack functional p53 and consequently have compromised G(1)-S checkpoint control. This has led to the hypothesis that S and G(2)-M checkpoint abrogation may selectively enhance genotoxic cell killing in a p53-deficient background, as normal cells would be rescued at the G(1)-S checkpoint. CHK1 is a serine/threonine kinase associated with DNA damage-linked S and G(2)-M checkpoint control. SAR-020106 is an ATP-competitive, potent, and selective CHK1 inhibitor with an IC(50) of 13.3 nmol/L on the isolated human enzyme. This compound abrogates an etoposide-induced G(2) arrest with an IC(50) of 55 nmol/L in HT29 cells, and significantly enhances the cell killing of gemcitabine and SN38 by 3.0- to 29-fold in several colon tumor lines in vitro and in a p53-dependent fashion. Biomarker studies have shown that SAR-020106 inhibits cytotoxic drug-induced autophosphorylation of CHK1 at S296 and blocks the phosphorylation of CDK1 at Y15 in a dose-dependent fashion both in vitro and in vivo. Cytotoxic drug combinations were associated with increased gammaH2AX and poly ADP ribose polymerase cleavage consistent with the SAR-020106-enhanced DNA damage and tumor cell death. Irinotecan and gemcitabine antitumor activity was enhanced by SAR-020106 in vivo with minimal toxicity. SAR-020106 represents a novel class of CHK1 inhibitors that can enhance antitumor activity with selected anticancer drugs in vivo and may therefore have clinical utility.

A novel Chk inhibitor, XL-844, increases human cancer cell radiosensitivity through promotion of mitotic catastrophe

Check point kinases (Chk) play a major role in facilitating DNA repair upon radiation exposure. We tested the potency of a novel inhibitor of Chk1 and Chk2, XL-844 (provided by Exelixis Inc., CA, USA), to radiosensitize human cancer cells grown in culture and investigated the underlying mechanisms. HT-29 cells (a human colon cancer line) were exposed to XL-844, radiation, or both, and assessed for clonogenic cell survival. Treatment-dependent effects on phosphorylated forms of Chk proteins were assessed by Western blots. Further mechanistic investigations in HT-29 cells included cell cycle analysis by flowcytometry and assessment of DNA repair kinetics by immuno-cytochemistry (ICC) for nuclear appearance of the phosphorylated form of histone 2AX protein (γ-H2AX) staining. Cells undergoing mitotic catastrophe were identified by irregular pattern of mitotic spindle markers α and γ-tubulin staining by ICC. XL-844 enhanced radiosensitivity in a dose and schedule-dependent manner and the enhancement factor was 1.42 at 0.5 survival fraction. Mechanistically XL-844 abrogated radiation-induced Chk2 phosphorylation, induced pan-nuclear γ-H2AX, and prolonged the presence of radiation-induced γ-H2AX foci, and promoted mitotic catastrophe. In conclusion, our data showed that inhibition of Chk2 activity by XL-844 enhanced cancer cell radiosensitivity that was associated with inhibition of DNA repair and induction of mitotic catastrophe.

Identification and characterisation of 2-aminopyridine inhibitors of checkpoint kinase 2

5-(Hetero)aryl-3-(4-carboxamidophenyl)-2-aminopyridine inhibitors of CHK2 were identified from high throughput screening of a kinase-focussed compound library. Rapid exploration of the hits through straightforward chemistry established structure-activity relationships and a proposed ATP-competitive binding mode which was verified by X-ray crystallography of several analogues bound to CHK2. Variation of the 5-(hetero)aryl substituent identified bicyclic dioxolane and dioxane groups which improved the affinity and the selectivity of the compounds for CHK2 versus CHK1. The 3-(4-carboxamidophenyl) substituent could be successfully replaced by acyclic omega-aminoalkylamides, which made additional polar interactions within the binding site and led to more potent inhibitors of CHK2. Compounds from this series showed activity in cell-based mechanistic assays for inhibition of CHK2.

Chk2 protects against radiation-induced genomic instability

The murine Chk2 kinase is activated after exposure to ionizing radiation and is necessary for p53-dependent apoptosis, but the role Chk2 plays in determining genomic stability is poorly understood. By analyzing the sensitivity of Chk2-deficient murine and human cells to a range of DNA-damaging agents, we show that Chk2 deficiency results in resistance to agents that generate double-strand breaks but not to other forms of damage. Surprisingly, the absence of Chk2 results in increased sensitivity to UV-radiation-induced DNA damage. Defective apoptosis after radiation-induced DNA damage may result in genomic instability; therefore, the consequences of Chk2 deficiency on genomic instability were assayed using an in vitro screen. Gene amplification was not detected in untreated Chk2(-/-) cells, but the rate of gene amplification after irradiation was elevated and was similar to that found in p53 compromised cells. A synergistic increase in genomic instability was seen after disruption of both Chk2 and p53 function, indicating that the two proteins have non-redundant roles in regulating genome stability after irradiation. The data demonstrate that Chk2 functions to maintain genome integrity after radiation-induced damage and has important implications for the use of Chk2 inhibitors as adjuvant cancer therapy.

Promyelocytic leukemia nuclear bodies link the DNA damage repair pathway with hepatitis B virus replication: implications for hepatitis B virus exacerbation during chemotherapy and radiotherapy

The mechanism responsible for hepatitis B virus (HBV) exacerbation during chemotherapy and radiotherapy remains unknown. We investigated whether the activation of DNA repair pathways influences HBV replication. The upregulation of the promyelocytic leukemia (PML) protein and its associated PML nuclear body (PML-NB) by chemotherapy and irradiation-induced DNA repair signaling correlated with the upregulation of HBV pregenomic transcription, HBV-core expression, and HBV DNA replication. The HBV-core protein and HBV DNA localized to PML-NBs, where they associated with PML and histone deacetylase 1 (HDAC1). Chemotherapy and radiotherapy affected the interactions between PML, HBV-core, and HDAC1. The enhanced protein-protein interaction between PML and HBV-core inhibited PML-mediated apoptosis and decreased PML-associated HDAC activity. The reversal of HDAC-mediated repression on the HBV covalently closed circular DNA basal core promoter resulted in the amplification of HBV-core and pregenomic expression. These results suggest that PML in PML-NBs links the DNA damage response with HBV replication and may cooperate with HBV-core and HDAC1 on the HBV covalently closed circular DNA basal core promoter to form a positive feedback loop for HBV exacerbation during chemotherapy and radiotherapy.

Human papillomaviruses activate the ATM DNA damage pathway for viral genome amplification upon differentiation

Human papillomaviruses (HPV) are the causative agents of cervical cancers. The infectious HPV life cycle is closely linked to the differentiation state of the host epithelia, with viral genome amplification, late gene expression and virion production restricted to suprabasal cells. The E6 and E7 proteins provide an environment conducive to DNA synthesis upon differentiation, but little is known concerning the mechanisms that regulate productive viral genome amplification. Using keratinocytes that stably maintain HPV-31 episomes, and chemical inhibitors, we demonstrate that viral proteins activate the ATM DNA damage response in differentiating cells, as indicated by phosphorylation of CHK2, BRCA1 and NBS1. This activation is necessary for viral genome amplification, as well as for formation of viral replication foci. In contrast, inhibition of ATM kinase activity in undifferentiated keratinocytes had no effect on the stable maintenance of viral genomes. Previous studies have shown that HPVs induce low levels of caspase 3/7 activation upon differentiation and that this is important for cleavage of the E1 replication protein and genome amplification. Our studies demonstrate that caspase cleavage is induced upon differentiation of HPV positive cells through the action of the DNA damage protein kinase CHK2, which may be activated as a result of E7 binding to the ATM kinase. These findings identify a major regulatory mechanism responsible for productive HPV replication in differentiating cells. Our results have potential implications for the development of anti-viral therapies to treat HPV infections.

Over 100 types of human papillomavirus (HPV) have been identified, and approximately one-third of these infect epithelial cells of the genital mucosa. A subset of these HPV types are the causative agents of cervical and other anogenital cancers. The infectious life cycle of HPV is dependent on differentiation of the host epithelial cell, with viral genome amplification and virion production restricted to differentiated suprabasal cells. While normal keratinocytes exit the cell cycle upon differentiation, HPV positive suprabasal cells are able to re-enter S-phase to mediate productive replication. The mechanisms regulating the activation of differentiation-dependent viral replication are largely unknown. In this study, we demonstrate that HPV induces an ATM-dependent DNA damage response that is essential for viral genome amplification in differentiating cells. In addition, we have found that ATM signaling to its downstream target CHK2 is critical for providing an environment that is conducive to HPV productive replication. Our findings identify an important regulatory mechanism by which HPV controls replication during the productive phase of the life cycle and may identify new targets for the development of therapeutics to treat HPV-induced infections.