A dual-fluorescent reporter facilitates identification of thiol compounds that suppress microsatellite instability induced by oxidative stress

The DNA mismatch-repair (MMR) system corrects replicative errors and minimizes mutations that occur at a high rate in microsatellites. Patients with chronic inflammation or inflammation-associated cancer display microsatellite instability (MSI), indicating a possible MMR inactivation. In fact, H2O2-generated oxidative stress inactivates the MMR function and increases mutation accumulation in a reporter microsatellite. However, it remains unclear whether MSI induced by oxidative stress is preventable because of the lack of a sufficiently sensitive detection assay. Here, we developed and characterized a dual-fluorescent system, utilizing DsRed harboring the (CA)13 microsatellite as a reporter and GFP for normalization, in near-isogenic human colorectal cancer cell lines. Via flow cytometry, this reporter sensitively detected H2O2-generated oxidative microsatellite mutations in a dose-dependent manner. The reporter further revealed that glutathione or N-acetylcysteine was better than aspirin and ascorbic acid for suppressing oxidative microsatellite mutations. These two thiol compounds also partially suppressed oxidative frameshift mutations in the coding microsatellites of the hMSH6 and CHK1 genes based on a fluoresceinated PCR-based assay. MSI suppression by N-acetylcysteine appears to be mediated through reduction of oxidative frameshift mutations in the coding microsatellite of hMSH6 and protection of hMSH6 and other MMR protein levels from being decreased by H2O2. Our findings suggest a linkage between oxidative damage, MMR deficiency, and MSI. The two thiol compounds are potentially valuable for preventing inflammation-associated MSI. The dual-fluorescent reporter with improved features will facilitate identification of additional compounds that modulate MSI, which is relevant to cancer initiation and progression.

Abstract 2543: A dual-fluorescent reporter system facilitates identification of thiol compounds that suppress oxidative frameshift mutations

DNA mismatch repair (MMR) system corrects replicative errors, especially in microsatellites, and minimizes mutations caused by DNA-damaging agents. Patients with chronic and cancer-associated inflammation display microsatellite instability (MSI), indicative of an inactivated MMR system. MMR-deficient patients also display frameshift mutations in coding microsatellites of many genes, including oncogenes and tumor suppressors. We previously reported that oxidative stress, which occurs in the inflammatory settings, not only inactivates the MMR function but also increases accumulation of frameshift mutations. However, it remains unclear whether oxidative frameshift mutations are preventable due to the lack of a sufficiently sensitive detection assay. Here, we developed and characterized a dual-fluorescent system, utilizing RFP harboring the (CA)13 microsatellite as a reporter and GFP for normalization, in near-isogeneic human colorectal cancer cell lines. By flow cytometry, the new reporter sensitively detected a low level of H2O2-induced frameshift mutations in MMR-proficient cells. Without a functional MMR system, 0.5 mM H2O2 increased frameshift mutations by approximately 10 folds in the cells. In MMR-deficient cells, our reporter revealed that 5 mM glutathione or N-acetylcysteine suppressed oxidative frameshift mutations by approximately 75% or 90% respectively, without affecting cell viability. In contrast, 0.5 mM ascorbic acid augmented H2O2-induced frameshift mutations by two folds, whereas 1 mM aspirin had no effects. Glutathione and N-acetylcysteine also partially suppressed oxidative frameshift mutations in coding microsatellites of endogenous hMSH6 and CHK1 genes, determined by a fluorescinated PCR-based assay. The new utility of these thiol compounds is potentially valuable for preventing oxidative frameshift mutations in the inflammatory settings. The dual-fluorescent reporter system with improved sensitivity will facilitate identification of additional compounds that modulate frameshift mutations relevant to cancer initiation and progression.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2543. doi:1538-7445.AM2012-2543

Abstract 3899: Anti-cancer drugs increases microsatellite instability in human colorectal cancer cells

Manifested as microsatellite instability (MSI), DNA mismatch repair (MMR) deficiency is known to contribute to colorectal cancer pathogenesis and drug resistance. Growing evidence indicates that MMR deficiency is also widespread among secondary cancers in patients who have received chemotherapy. It is unclear what chemotherapeutic agents cause MMR deficiency. In this study, we aim at identifying the MSI-inducing anti-cancer drugs. To facilitate the identification, we developed an in-vivo dual fluorescent reporter system in a human colorectal cancer cell model using RFP that contains a microsatellite as the MSI reporter and GFP as the cell label. Based on flow cytometric analysis, we found that test anti-cancer drugs increased the MSI frequency in a dose-dependent manner. This was confirmed by fluorescent microscopy. With the NCI recommended markers, we detected a low frequency of MSI in cells after treated with a couple, but not all, of test anti-cancer drugs. Our cell-based MSI reporter assay will facilitate large scale screens of MSI-inducing and MSI-reducing drugs/compounds, which might provide treatment strategies for controlling therapy-related secondary cancers.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3899. doi:10.1158/1538-7445.AM2011-3899