[Genetic instability as a driver for oncogenesis]

DNA damage repair: historical perspectives, mechanistic pathways and clinical translation for targeted cancer therapy

Genomic instability is the hallmark of various cancers with the increasing accumulation of DNA damage. The application of radiotherapy and chemotherapy in cancer treatment is typically based on this property of cancers. However, the adverse effects including normal tissues injury are also accompanied by the radiotherapy and chemotherapy. Targeted cancer therapy has the potential to suppress cancer cells' DNA damage response through tailoring therapy to cancer patients lacking specific DNA damage response functions. Obviously, understanding the broader role of DNA damage repair in cancers has became a basic and attractive strategy for targeted cancer therapy, in particular, raising novel hypothesis or theory in this field on the basis of previous scientists' findings would be important for future promising druggable emerging targets. In this review, we first illustrate the timeline steps for the understanding the roles of DNA damage repair in the promotion of cancer and cancer therapy developed, then we summarize the mechanisms regarding DNA damage repair associated with targeted cancer therapy, highlighting the specific proteins behind targeting DNA damage repair that initiate functioning abnormally duo to extrinsic harm by environmental DNA damage factors, also, the DNA damage baseline drift leads to the harmful intrinsic targeted cancer therapy. In addition, clinical therapeutic drugs for DNA damage and repair including therapeutic effects, as well as the strategy and scheme of relative clinical trials were intensive discussed. Based on this background, we suggest two hypotheses, namely "environmental gear selection" to describe DNA damage repair pathway evolution, and "DNA damage baseline drift", which may play a magnified role in mediating repair during cancer treatment. This two new hypothesis would shed new light on targeted cancer therapy, provide a much better or more comprehensive holistic view and also promote the development of new research direction and new overcoming strategies for patients.

Clinical importance of DNA repair in sporadic colorectal cancer

Colorectal cancer (CRC) is the third major cause of cancer-related deaths worldwide. However, despite the scientific efforts to provide a molecular classification to improve CRC clinical practice management, prognosis and therapeutic decision are still strongly dependent on the TNM staging system. Mismatch repair system deficiencies can occur in many organs, but it is mainly a hallmark of CRC influencing clinical outcomes and response to therapy. This review will discuss the effect of the modulation of other DNA repair pathways (direct, excision and double strand break repairs) in the clinical and pathological aspects of colorectal cancer and its potential as prognostic and predictive biomarkers.

Inactivation of miR-100 combined with arsenic treatment enhances the malignant transformation of BEAS-2B cells via stimulating epithelial -mesenchymal transition

Chronic arsenic treatment induces epithelial-mesenchymal transition (EMT) and promotes tumorigenicity, but the mechanism is unclear. MiR-100 has been shown to be involved in this biologic process. In this study, we hypothesize that inactivation of miR-100 combined with low concentration of arsenic exposure could promote the malignant transformation of human bronchial epithelial cells (BEAS-2B cell) by promoting EMT. To test this hypothesis, BEAS–2B cells were treated with low-dose of As₂O₃ chronically, and lentiviral vectors were used to mediate the inhibition of miR-100 expression. Flow cytometry, cloning formation, and transwell assays were used to examine cell cycle progression, cell proliferation, and cell migration, respectively. The mouse xenograft model was used to investigate the cell malignant growth in vivo, and western blot was used to detect EMT related marker expressions. Our results showed that, the inactivation of miR-100 combined with arsenic treatment significantly promoted the proliferation, viability, and migration of BEAS-2B cells in vitro, and tumorigenesis in vivo. Consistently, the EMT related marker expressions were also significantly increased in corresponding groups. Our data indicate that inactivation of miR-100 combined with chronic arsenic treatment promotes tumorigenicity of BEAS-2B cells via activation of EMT. This novel insight may help us to better understand the pathogenesis of arsenic carcinogenesis.

HDAC turnover, CtIP acetylation and dysregulated DNA damage signaling in colon cancer cells treated with sulforaphane and related dietary isothiocyanates

Histone deacetylases (HDACs) and acetyltransferases have important roles in the regulation of protein acetylation, chromatin dynamics and the DNA damage response. Here, we show in human colon cancer cells that dietary isothiocyanates (ITCs) inhibit HDAC activity and increase HDAC protein turnover with the potency proportional to alkyl chain length, i.e., AITC < sulforaphane (SFN) < 6-SFN < 9-SFN. Molecular docking studies provided insights into the interactions of ITC metabolites with HDAC3, implicating the allosteric site between HDAC3 and its co-repressor. ITCs induced DNA double-strand breaks and enhanced the phosphorylation of histone H2AX, ataxia telangiectasia and Rad3-related protein (ATR) and checkpoint kinase-2 (CHK2). Depending on the ITC and treatment conditions, phenotypic outcomes included cell growth arrest, autophagy and apoptosis. Coincident with the loss of HDAC3 and HDAC6, as well as SIRT6, ITCs enhanced the acetylation and subsequent degradation of critical repair proteins, such as CtIP, and this was recapitulated in HDAC knockdown experiments. Importantly, colon cancer cells were far more susceptible than non-cancer cells to ITC-induced DNA damage, which persisted in the former case but was scarcely detectable in non-cancer colonic epithelial cells under the same conditions. Future studies will address the mechanistic basis for dietary ITCs preferentially exploiting HDAC turnover mechanisms and faulty DNA repair pathways in colon cancer cells vs. normal cells.

Dietary phytochemicals, HDAC inhibition, and DNA damage/repair defects in cancer cells

Genomic instability is a common feature of cancer etiology. This provides an avenue for therapeutic intervention, since cancer cells are more susceptible than normal cells to DNA damaging agents. However, there is growing evidence that the epigenetic mechanisms that impact DNA methylation and histone status also contribute to genomic instability. The DNA damage response, for example, is modulated by the acetylation status of histone and non-histone proteins, and by the opposing activities of histone acetyltransferase and histone deacetylase (HDAC) enzymes. Many HDACs overexpressed in cancer cells have been implicated in protecting such cells from genotoxic insults. Thus, HDAC inhibitors, in addition to unsilencing tumor suppressor genes, also can silence DNA repair pathways, inactivate non-histone proteins that are required for DNA stability, and induce reactive oxygen species and DNA double-strand breaks. This review summarizes how dietary phytochemicals that affect the epigenome also can trigger DNA damage and repair mechanisms. Where such data is available, examples are cited from studies in vitro and in vivo of polyphenols, organosulfur/organoselenium compounds, indoles, sesquiterpene lactones, and miscellaneous agents such as anacardic acid. Finally, by virtue of their genetic and epigenetic mechanisms, cancer chemopreventive agents are being redefined as chemo- or radio-sensitizers. A sustained DNA damage response coupled with insufficient repair may be a pivotal mechanism for apoptosis induction in cancer cells exposed to dietary phytochemicals. Future research, including appropriate clinical investigation, should clarify these emerging concepts in the context of both genetic and epigenetic mechanisms dysregulated in cancer, and the pros and cons of specific dietary intervention strategies.

Evaluating the effects of genetic variants of DNA repair genes using cytogenetic mutagen sensitivity approaches

Mutagen sensitivity, measured in short-term cultures of peripheral blood lymphocytes by cytogenetic endpoints, is an indirect measure for DNA repair capacity and has been used for many years as a biomarker for intrinsic susceptibility for cancer. In this article, we briefly give an overview of the different cytogenetic mutagen sensitivity approaches that have been used successfully to evaluate the biological effects of polymorphisms in DNA repair genes based on a current review of the literature and based on the need for biomarkers that would allow the characterization of the biological and functional significance of such polymorphisms. We also address some of the future challenges facing this emerging area of research.