Chk2 Molecular Interaction Map and Rationale for Chk2 Inhibitors Yves Pommier, John N.Weinstein, Mirit I. Aladjem, and Kurt W. Kohn Abstract To organize the rapidly accumulatinginformationonbioregulatory networks related to the histone g-H2AX-ATM-Chk2-p53-Mdm2 pathways in concise and unambiguous diagrams, we used the molecular interaction map notation (http://discover.nci.nih.gov/min).Molecular interaction maps are particularly useful for networks that include protein-protein binding and posttransla- tional modifications (e.g., phosphorylation). Both are important for nearly all of the proteins involved in DNA double-strand break signaling.Visualizing the regulatory circuits underlying cellular signaling may help identify key regulatory reactions and defects that can serve as targets for anticancer drugs. DNA double-strand breaks (DSB) are among the most severe genomic lesions. Their repair requires cells to arrest cell cycle progression to avoid further damage during replication or transcription (1–5). Cell cycle arrest may also allow the chromatin to switch from a metabolic (replicative) state to a repair state. DSB can be produced directly by ionizing radiation and certain anticancer drugs that bind to DNA (e.g., bleomycin, neocarcinostatin, topoisomerase II inhibitors, such as etoposide or doxorubicin). DSB can also result from conversion of single- strand breaks by DNA polymerase collisions or ‘‘replication fork collapse’’ at sites of damaged DNA. Replication DSB occur normally in cancer cells, but their frequency is markedly enhanced by topoisomerase I inhibitors such as the campto- thecin derivatives (e.g., topotecan and irinotecan; ref. 6) and DNA alkylating agents. The importance of Chk2, a protein involved in cell cycle arrest due to DSB, is indicated by its conservation in eukaryotes (7 – 10). However, yeast and vertebrate cells can survive without Chk2, although they are defective in cell cycle checkpoints and apoptosis after irradiation. Chk2 activation by camptothecin treatment has been reported to be defective in several colon carcinoma cell lines from the NCI 60 cell line screen (11). Chk2 inactivation in humans with normal p53 leads to Li-Fraumeni syndrome (12), supporting a genetic connection between Chk2 and p53 and the existence of a Chk2-p53 axis [see the mole- cular interaction map (MIM) Fig. 1]. Families with Chk2 deficiencies have been identified among patients with increased risk of breast cancer (10, 13, 14). Chk2 functions as a kinase relay for ATM (refs. 9, 15, 16; see MIM), whose gene (ataxia telangiectasia mutant ) is mutated in ataxia telangiectasia patients (2). It is generally accepted that DNA damage triggers two main pathways (16). DSB activate the ATM-Chk2 axis, whereas replication lesions activate the ataxia telangiectasia and rad3-related kinase (ATR)–Chk1 axis. However, there is crosstalk between the two pathways and certain lesions activate both. In such cases, the ATM-Chk2 axis is activated first and transiently, whereas the ATR-Chk1 axis is activated secondarily and in a more sustained way (17). The association of Chk1 with replication damage is consistent with the cell cycle–dependent expression of Chk1, which increases during S-phase and peaks in G 2 . In contrast, Chk2 is expressed throughout the cell cycle, consistent with its broader role in response to DSB produced both during S-phase and indepen- dently of replication (18). Although Chk2 shares no sequence homology with Chk1, crosstalk between the Chk2 and Chk1 pathways is also indicated by the observation that several Chk2 substrates (e.g., Cdc25C and p53, highlighted in blue in Fig. 1) are also Chk1 substrates and are phosphorylated by both kinases on the same amino acid residues (9). Chk2 MIM Foreword. To organize the rapidly accumulating informa- tion on bioregulatory networks in concise and unambiguous diagrams, we crafted the MIM notation (ref. 19). 1 MIMs are particularly useful for networks that include protein-protein binding and posttranslational modifications (e.g., phosphory- lation). Both are important for nearly all of the proteins involved in DNA damage signaling by the ATM-Chk2 axis. The MIM allows one to trace pathways and loops in the networks, thereby helping to interpret data and formulate testable hypotheses (Fig. 1). Molecular species and events in the MIM fall into three categories indicated to the right of the MIM: (a ) chromatin, (b ) sensors and kinases, and (c ) effectors. Symbol definitions are shown below the MIM. Arabic numbers in the map refer to text and literature references listed in the Supplementary Data and 1 http://discover.nci.nih.gov/mim. Molecular Pathways Authors’Affiliation: Laboratory of Molecular Pharmacology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland Grantsupport: IntramuralResearchProgramoftheNIH,NationalCancerInstitute, Center forCancerResearch. Note: Supplementary data for this article are available at Clinical Cancer Research Online (http://clincancerres.aacrjournals.org/) and at the Discover Website. 1 Requests for reprints: Yves Pommier, NIH, Room 5068, Building 37, Bethesda, MD 20892-4255. E-mail: pommier@nih.gov. F 2006 American Associationfor Cancer Research. doi:10.1158/1078-0432.CCR-06-0743 www.aacrjournals.org ClinCancerRes2006;12(9)May1,2006 2657 Research. on May 28, 2020. © 2006 American Association for Cancer clincancerres.aacrjournals.org Downloaded from