br Acknowledgments The work in the Vancurova lab is

Acknowledgments The work in the Vancurova lab is supported by St John’s University, and by NIHCA202775 grant.
HDACs are Epigenetic Achilles’ Heels in Tumors with ICLs On the one hand, proficient and accurate DNA repair mechanisms ensure the genomic integrity of cells. On the other hand, the DDR and ensuing repair processes limit the efficacy of anticancer therapy with DNA-damaging agents 1, 2. Hence, small molecules that modulate the DDR and repair mechanisms hold the promise to sensitize tumor cells to therapy and to improve the survival rates of patients [3]. To identify such agents it is necessary to understand how cells respond to DNA damage. DNA associates with histones and other nuclear proteins to form a regulatory complex termed chromatin 4, 5. Post-translational modifications, including lysine acetylation of histones, control chromatin. Histone acetylation relaxes and histone deacetylation compacts chromatin. Relaxed chromatin permits transcription and the access of repair proteins to DNA lesions, for example, to interstrand crosslinks (ICLs, see Glossary) and to single-strand (ss)/double-strand (ds) DNA breaks 4, 5. Histone acetyltransferases (HATs) and histone deacetylases (HDACs), modulate histone acetylation antagonistically 4, 5, 6, 7 (Box 1). In addition to modifying histones, HATs and HDACs regulate physiologically relevant non-histone proteins in the nucleus and cytosol [8]. Thus, pharmacological inhibitors of HATs and HDACs alter gene expression, signaling, protein stability, cell cycle control, and ultimately cell fate decisions 4, 5, 6, 7, 8. Current evidence shows that HDACi themselves propel cellular DNA damage in malignant cutaneous T cell lymphoma (CTCL), glioma, breast cancer, colon cancer, and acute promyelocytic leukemia cells 9, 10, 11, 12. Suberoylanilide hydroxamic lamotrigine (SAHA), an HDACi against Zn2+-dependent HDACs that is effective in the μM range, leads to a rapid induction of replicative stress, dormant DNA replication origin firing, and dsDNA damage 10, 11. Data collected with RNAi against HDAC3 and its specific inhibitor RGFP966 demonstrate that HDAC3 affects replication fork speed and chromatin integrity 10, 11. In acute promyelocytic leukemia cells, pan- and class I HDACi also cause dsDNA damage and apoptosis [12]. The extent of these cytotoxic processes correlates with the affinity of HDACi for class I HDACs and with cell cycle progression [12]. Dysregulation of DNA repair proteins by HDACi (Table 1, Table 2) could be the cause of HDACi-induced DNA damage in rapidly dividing cells, which constantly face replicative stress and DNA damage 2, 9, 10, 11. Furthermore, genetic evidence shows that class I HDAC activity is necessary to regulate the timing of DNA replication origin firing [13]. HDACs regulate ICL repair proteins and the survival of cells with ICLs (Table 1, Table 2). This turns HDACs, which are often overexpressed in cancer cells 5, 8, 14, into epigenetic Achilles’ heels in tumor cells carrying ICLs. Recent findings demonstrate that protein acetylation, HDACs, and HDACi crucially determine DNA repair, which represents a pivotal survival mechanism of cancer cells carrying chemotherapy-induced ICLs. We summarize these key observations and their implications in the following sections.
ICL Inducers and ICL Formation ICLs are typically generated by environmental factors. Examples are acrolein and unsaturated aldehydes, psoralens in some plants, air pollutants and their metabolites such as 1,2,3,4-diepoxybutane (DEB), and the naturally occurring antitumor antibiotic mitomycin C (MMC; a proapoptotic ICL inducer) 1, 2, 15, 16, 17. Other prototypical classes of ICL inducers are nitrogen mustards and chloroethylnitrosoureas (CNUs) 1, 16. Whereas MMC and nitrogen mustards react mainly with the N7 of guanine to form ICLs [18], CNUs form various adducts with DNA through the transfer of chloroethylene ions [2] (Figure 1). The most important nucleophilic site for CNUs is the O6-position of guanine, generating O6-chloroethyl-guanine (O6-ClEtG). O6-ClEtG is an unstable DNA adduct that rapidly converts to N1-O6-ethanoguanine DNA [2]. By binding to a cytosine in the complementary DNA strand, the latter can generate a stable secondary DNA lesion, the N3-guanine–N3-cytosine ICL [2] (Figure 1). Metal complexes, including the chemotherapeutics cisplatin, oxaliplatin, and carboplatin, also form ICLs between guanines in 5'-GC-sequences within the major groove of DNA 16, 18.