The mismatch repair-mediated cell cycle checkpoint response to fluorodeoxyuridine

REV3 promotes cellular tolerance to 5-fluorodeoxyuridine by activating translesion DNA synthesis and intra-S checkpoint

The drug floxuridine (5-fluorodeoxyuridine, FUdR) is an active metabolite of 5-Fluorouracil (5-FU). It converts to 5-fluorodeoxyuridine monophosphate (FdUMP) and 5-fluorodeoxyuridine triphosphate (FdUTP), which on incorporation into the genome inhibits DNA replication. Additionally, it inhibits thymidylate synthase, causing dTMP shortage while increasing dUMP availability, which induces uracil incorporation into the genome. However, the mechanisms underlying cellular tolerance to FUdR are yet to be fully elucidated. In this study, we explored the mechanisms underlying cellular resistance to FUdR by screening for FUdR hypersensitive mutants from a collection of DT40 mutants deficient in each genomic maintenance system. We identified REV3, which is involved in translesion DNA synthesis (TLS), to be a critical factor in FUdR tolerance. Replication using a FUdR-damaged template was attenuated in REV3-/- cells, indicating that the TLS function of REV3 is required to maintain replication on the FUdR-damaged template. Notably, FUdR-exposed REV3-/- cells exhibited defective cell cycle arrest in the early S phase, suggesting that REV3 is involved in intra-S checkpoint activation. Furthermore, REV3-/- cells showed defects in Chk1 phosphorylation, which is required for checkpoint activation, but the survival of FUdR-exposed REV3-/- cells was further reduced by the inhibition of Chk1 or ATR. These data indicate that REV3 mediates DNA checkpoint activation at least through Chk1 phosphorylation, but this signal acts in parallel with ATR-Chk1 DNA damage checkpoint pathway. Collectively, we reveal a previously unappreciated role of REV3 in FUdR tolerance.

Lack of mismatch repair enhances resistance to methylating agents for cells deficient in oxidative demethylation

The human AlkB homologs, ALKBH2 and ALKBH3, respond to methylation damage to maintain genomic integrity and cellular viability. Both ALKBH2 and ALKBH3 are direct reversal repair (DRR) enzymes that remove 1meA and 3meC lesions commonly generated by alkylating chemotherapeutic agents. Thus, the existence of deficiencies in ALKBH proteins can be exploited in synergy with chemotherapy. In this study, we investigated possible interactions between ALKBH2 and ALKBH3 with other proteins that could alter damage response and discovered an interaction with the mismatch repair (MMR) system. To test whether the lack of active MMR impacts ALKBH2 and/or ALKBH3 response to methylating agents, we generated cells deficient in ALKBH2, ALKBH3, or both in addition to Mlh homolog 1 (MLH1), another MMR protein. We found that MLH1koALKBH3ko cells showed enhanced resistance towards SN1- and SN2-type methylating agents, whereas MLH1koALKBH2ko cells were only resistant to SN1-type methylating agents. Concomitant loss of ALKBH2 and ALKBH3 (ALKBH2ko3ko) rendered cells sensitive to SN1- and SN2-agents, but the additional loss of MLH1 enhanced resistance to both types of damage. We also showed that ALKBH2ko3ko cells have an ATR-dependent arrest at the G2/M checkpoint, increased apoptotic signalling, and replication fork stress in response to methylation. However, these responses were not observed with the loss of functional MLH1 in MLH1koALKBH2ko3ko cells. Finally, in MLH1koALKBH2ko3ko cells, we observed elevated mutant frequency in untreated and temozolomide treated cells. These results suggest that obtaining a more accurate prognosis of chemotherapeutic outcome requires information on the functionality of ALKBH2, ALKBH3, and MLH1.

Enterohemorrhagic Escherichia coli effector EspF triggers oxidative DNA lesions in intestinal epithelial cells

Attaching/effacing (A/E) pathogens induce DNA damage and colorectal cancer by injecting effector proteins into host cells via the type III secretion system (T3SS). EspF is one of the T3SS-dependent effector proteins exclusive to A/E pathogens, which include enterohemorrhagic Escherichia coli. The role of EspF in the induction of double-strand breaks (DSBs) and the phosphorylation of the repair protein SMC1 has been demonstrated previously. However, the process of damage accumulation and DSB formation has remained enigmatic, and the damage response is not well understood. Here, we first showed a compensatory increase in the mismatch repair proteins MutS homolog 2 (MSH2) and MSH6, as well as poly(ADP-ribose) polymerase 1, followed by a dramatic decrease, threatening cell survival in the presence of EspF. Flow cytometry revealed that EspF arrested the cell cycle at the G2/M phase to facilitate DNA repair. Subsequently, 8-oxoguanine (8-oxoG) lesions, a marker of oxidative damage, were assayed by ELISA and immunofluorescence, which revealed the accumulation of 8-oxoG from the cytosol to the nucleus. Furthermore, the status of single-stranded DNA (ssDNA) and DSBs was confirmed. We observed that EspF accelerated the course of DNA lesions, including 8-oxoG and unrepaired ssDNA, which were converted into DSBs; this was accompanied by the phosphorylation of replication protein A 32 in repair-defective cells. Collectively, these findings reveal that EspF triggers various types of oxidative DNA lesions with impairment of the DNA damage response and may result in genomic instability and cell death, offering novel insight into the tumorigenic potential of EspF.IMPORTANCEOxidative DNA lesions play causative roles in colitis-associated colon cancer. Accumulating evidence shows strong links between attaching/effacing (A/E) pathogens and colorectal cancer (CRC). EspF is one of many effector proteins exclusive to A/E pathogens with defined roles in the induction of oxidative stress, double-strand breaks (DSBs), and repair dysregulation. Here, we found that EspF promotes reactive oxygen species generation and 8-oxoguanine (8-oxoG) lesions when the repair system is activated, contributing to sustained cell survival. However, infected cells exposed to EspF presented 8-oxoG, which results in DSBs and ssDNA accumulation when the cell cycle is arrested at the G2/M phase and the repair system is defective or saturated by DNA lesions. In addition, we found that EspF could intensify the accumulation of nuclear DNA lesions through oxidative and replication stress. Overall, our work highlights the involvement of EspF in DNA lesions and DNA damage response, providing a novel avenue by which A/E pathogens may contribute to CRC.

Veliparib (ABT-888), a PARP inhibitor potentiates the cytotoxic activity of 5-fluorouracil by inhibiting MMR pathway through deregulation of MSH6 in colorectal cancer stem cells

OBJECTIVE

Sensitization of mismatch repair (MMR)-deficient colorectal cancer (CRC) cells by 5-Fluorouracil (5-FU) is well-documented. But not much is known about the treatment of MMR-proficient CRC cancer stem cells (CRC-CSCs). Here, we investigated whether a PARP inhibitor (ABT-888) can enhance the 5-FU-mediated apoptosis in CRC-CSCs through MMR pathway inhibition.

METHODS

The anti-cancer action of 5-FU+ABT-888 combination in CRC-CSCs has been studied by using in vitro, ex vivo, and in vivo preclinical model systems.

RESULTS

5-FU caused DNA damage in CRC-CSCs, and ABT-888 enhanced the accumulation of DNA mismatches by downregulating the MMR pathway, triggering S-phase arrest, and finally apoptosis and cell death in 5-FU-pre-treated MMR-proficient-CRC-CSCs at much lower concentrations than their individual treatments. After 5-FU treatment, PARylated-PARP1 activated MMR pathway by interacting with MSH6. But, upon ABT-888 treatment in 5-FU-pre-exposed CSCs, PARylation was inhibited, as a result of which PARP1 could not interact with MSH6, and other MMR proteins were downregulated. The role of MSH6 in PARP1-mediated MMR activation, was confirmed by silencing MSH6 gene, which resulted in MMR pathway shutdown. Similar results were obtained in ex vivo and in vivo model systems.

CONCLUSIONS

5-FU+ABT-888 combination enhanced CRC-CSCs death by increasing DNA damage accumulation and simultaneously inhibiting the MMR pathway in MMR-proficient cells. But this study does not discuss whether the combination treatment will increase the sensitivity of MMR-deficient CSCs, for which further research will be performed in the future.

Effects of deficient mismatch repair on the prognosis of patients with stage II and stage III colon cancer during different postoperative periods

Background

We evaluated the prognostic role of deficient mismatch repair (dMMR) systems in stage II and stage III colon cancer patients during different postoperative periods. We also assessed whether patients aged ≥75 could benefit from chemotherapy.

Methods

This retrospective study was conducted across three medical centers in China. Kaplan–Meier survival methods and Cox proportional hazards models were used to evaluate the differences in overall survival (OS) and disease-free survival (DFS) rates. Propensity score matching was performed to reduce imbalances in the baseline characteristics of the patients. Landmark analysis was performed to evaluate the role of dMMR during different postoperative periods.

Results

The median follow-up time for all patients was 45.0 months (25–75 IQR: 38.0–82.5). There was no significant OS (p = 0.350) or DFS (p = 0.752) benefit associated with dMMR for stage II and III patients during the first postoperative year. However, significant OS (p < 0.001) and DFS (p < 0.001) benefits were observed from the second postoperative year until the end of follow-up. These differences remained after propensity score matching. Moreover, chemotherapy produced no OS (HR = 0.761, 95% CI: 0.43–1.34, p = 0.341) or DFS (HR = 0.98, 95% CI: 0.51–1.88, p = 0.961) benefit for patients aged ≥75 years.

Conclusion

The benefits of dMMR in stage III patients were observed from the second postoperative year until the end of follow-up. However, the prognosis of patients with dMMR is not different from that of patients with proficient mismatch repair (pMMR) during the first postoperative year. In addition, elderly patients aged ≥75 years obtained no significant survival benefits from postoperative chemotherapy.

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.

Neoadjuvant Chemotherapy Induces IL34 Signaling and Promotes Chemoresistance via Tumor-Associated Macrophage Polarization in Esophageal Squamous Cell Carcinoma

The tumor microenvironment (TME) plays a key role in the efficacy of neoadjuvant chemotherapy (NAC) in solid tumors including esophageal squamous cell carcinoma (ESCC). However, the TME profile of ESCC treated with NAC is not fully understood. In this study, we investigated the effect of NAC on the TME especially tumor-associated macrophages (TAM), the important immunosuppressive components of the TME, in ESCC. We quantified the expression of CD163, a crucial marker of TAM, in pretherapeutic biopsy and surgically resected ESCC specimens from patients who received NAC (n = 33) or did not receive NAC (n = 12). We found that NAC dramatically increased the expression of CD163 on TAMs in ESCC. Colony-stimulating factor 1 (CSF-1) and IL34 are crucial cytokines that recruit monocytes into tumor sites and differentiate them into TAMs. Interestingly, NAC significantly upregulated the expression of IL34 but not CSF-1 on tumor cells, and the frequencies of CD163⁺ TAMs were significantly correlated with IL34 expression in ESCC after NAC. The expression of IL34 in NAC-nonresponsive patients was significantly higher than that in NAC-responsive patients, and patients with IL34-high ESCC exhibited worse prognosis as compared with patients with IL34-low ESCC. We also demonstrated that 5-fluorouracil (5-FU)/cisplatin preferentially increased mRNA expression of IL34 on human ESCC cell lines. Human peripheral blood monocytes co-cultured with ESCC cells treated with 5-FU/cisplatin increased the expression of CD163, which was attenuated by the treatment with CSF-1R inhibitors. These data suggest that IL34 expression by NAC shifts the TME toward CD163⁺ TAM-rich immunosuppressive and chemo-insensitive microenvironment in ESCC. IMPLICATIONS: The blockade of IL34 signaling may offer a novel therapeutic strategy against chemoresistance in ESCC by inhibiting M2-TAM polarization.

Duration of FOLFOX Adjuvant Chemotherapy in High-Risk Stage II and Stage III Colon Cancer With Deficient Mismatch Repair

Background

We evaluated the impact of 3 months of mFOLFOX6 adjuvant chemotherapy or surgery alone in comparison with 6 months of mFOLFOX6 on disease-free survival (DFS) in deficient mismatch repair (dMMR) colon cancer (CC) patients.

Methods

This retrospective study identified a cohort of patients with high-risk stage II and III dMMR CC who underwent curative surgery between May 2011 and July 2019. DFS was compared using the Kaplan-Meier survival methods and Cox proportional hazards models. Propensity-score matching was performed to reduce imbalance in baseline characteristics.

Results

A total of 242 dMMR CC patients were identified; 66 patients received 6 months of mFOLFOX6, 87 patients received 3 months of mFOLFOX6, and 89 patients were treated with surgery alone. The 3-year DFS rate was 72.8% in 3-month therapy group and 86.1% in 6-month therapy group, with a hazard ratio (HR) of 2.78 (95CI%, 1.18 to 6.47; P= 0.019). The difference in DFS between surgery alone group and 6-month therapy group was also observed but was nonsignificant (HR= 2.30, 95%CI, 0.99 to 5.38; P=0.054). The benefit of 6-month therapy in DFS compared with 3-month therapy group was pronounced for patients with stage III (HR=2.81, 95%CI, 1.03 to 7.67; P=0.044) but not for high-risk stage II patients. Propensity score matched analysis confirmed a DFS benefit in the 6-month therapy group.

Conclusion

This study suggested that a 6-month duration of mFOLFOX6 adjuvant chemotherapy in dMMR CC patients may be associated with improved DFS compared with 3-month therapy, particularly in patients with stage III. The observational nature of the study implies caution should be taken in the interpretation of these results.

KSHV LANA acetylation-selective acidic domain reader sequence mediates virus persistence

Viruses modulate biochemical cellular pathways to permit infection. A recently described mechanism mediates selective protein interactions between acidic domain readers and unacetylated, lysine-rich regions, opposite of bromodomain function. Kaposi´s sarcoma (KS)-associated herpesvirus (KSHV) is tightly linked with KS, primary effusion lymphoma, and multicentric Castleman's disease. KSHV latently infects cells, and its genome persists as a multicopy, extrachromosomal episome. During latency, KSHV expresses a small subset of genes, including the latency-associated nuclear antigen (LANA), which mediates viral episome persistence. Here we show that LANA contains two tandem, partially overlapping, acidic domain sequences homologous to the SET oncoprotein acidic domain reader. This domain selectively interacts with unacetylated p53, as evidenced by reduced LANA interaction after overexpression of CBP, which acetylates p53, or with an acetylation mimicking carboxyl-terminal domain p53 mutant. Conversely, the interaction of LANA with an acetylation-deficient p53 mutant is enhanced. Significantly, KSHV LANA mutants lacking the acidic domain reader sequence are deficient for establishment of latency and persistent infection. This deficiency was confirmed under physiological conditions, on infection of mice with a murine gammaherpesvirus 68 chimera expressing LANA, where the virus was highly deficient in establishing latent infection in germinal center B cells. Therefore, LANA's acidic domain reader is critical for viral latency. These results implicate an acetylation-dependent mechanism mediating KSHV persistence and expand the role of acidic domain readers.

The mismatch repair-dependent DNA damage response: Mechanisms and implications

An important role for the DNA mismatch repair (MMR) pathway in maintaining genomic stability is embodied in its conservation through evolution and the link between loss of MMR function and tumorigenesis. The latter is evident as inheritance of mutations within the major MMR genes give rise to the cancer predisposition condition, Lynch syndrome. Nonetheless, how MMR loss contributes to tumorigenesis is not completely understood. In addition to preventing the accumulation of mutations, MMR also directs cellular responses, such as cell cycle checkpoint or apoptosis activation, to different forms of DNA damage. Understanding this MMR-dependent DNA damage response may provide insight into the full tumor suppressing capabilities of the MMR pathway. Here, we delve into the proposed mechanisms for the MMR-dependent response to DNA damaging agents. We discuss how these pre-clinical findings extend to the clinical treatment of cancers, emphasizing MMR status as a crucial variable in selection of chemotherapeutic regimens. Also, we discuss how loss of the MMR-dependent damage response could promote tumorigenesis via the establishment of a survival advantage to endogenous levels of stress in MMR-deficient cells.

Aberrations in DNA repair pathways in cancer and therapeutic significances

Cancer cells show various types of mutations and aberrant expression in genes involved in DNA repair responses. These alterations induce genome instability and promote carcinogenesis steps and cancer progression processes. These defects in DNA repair have also been considered as suitable targets for cancer therapies. A most effective target so far clinically demonstrated is "homologous recombination repair defect", such as BRCA1/2 mutations, shown to cause synthetic lethality with inhibitors of poly(ADP-ribose) polymerase (PARP), which in turn is involved in DNA repair as well as multiple physiological processes. Different approaches targeting genomic instability, including immune therapy targeting mismatch-repair deficiency, have also recently been demonstrated to be promising strategies. In these DNA repair targeting-strategies, common issues could be how to optimize treatment and suppress/conquer the development of drug resistance. In this article, we review the extending framework of DNA repair response pathways and the potential impact of exploiting those defects on cancer treatments, including chemotherapy, radiation therapy and immune therapy.

Hypomethylation of mismatch repair genes MLH1 and MSH2 is associated with chemotolerance of breast carcinoma: Clinical significance

BACKGROUND AND OBJECTIVES

The aim of the study was to understand the importance of mismatch repair genes MLH1 and MSH2 in chemotolerance and prognosis of breast carcinoma (BC).

METHODS

First, the alterations (deletion/methylation/expression) of MLH1 and MSH2 were analyzed in 45 neoadjuvant chemotherapy (NACT)-treated and 133 pretherapeutic BC samples. The chemotolerant BC cells were characterized by treating two BC cell lines MCF-7 and MDA MB 231 with two anthracycline antitumor antibiotics, doxorubicin and nogalamycin.

RESULTS

The deletion frequencies were 32% to 38% in MLH1/MSH2 genes and promoter methylation frequencies were 49% to 62% in MLH1 and 41% to 51% in MSH2 in both NACT-treated and pretherapeutic samples. The overall alteration of MLH1 and MSH2 was 58% to 71% in the samples. Reduced messenger RNA (mRNA) and protein expression were found in both the genes and it showed concordance with the molecular alterations. NACT-treated patients showed better prognosis. The chemotherapeutic drug induced increased mRNA/protein expression of the genes in BC cell lines was due to their promoter hypomethylation, as analyzed by quantitative methylation assay. This phenomenon was also evident in NACT-treated BC samples.

CONCLUSION

MLH1/MSH2 genes play a critical role in the development of BC. Hypomethylation of MLH1/MSH2 genes might be important in chemotolerance of the disease.

Methoxyamine Enhances 5-Fluorouracil-Induced Radiosensitization in Colon Cancer Cell Line HT29

Objective

This study intended to observe the effects of methoxyamine (Mx) on cytotoxic effects and DNA damage caused by 5-Fluorouracil (5-FU) in combination with gamma radiation in a human colon cancer cell line, HT29.

Materials and Methods

In this experimental study, HT29 cells were cultured as a monolayer and treated with different concentrations of 5-FU along with 1 mM Mx for 24 hours. Next, the cells were irradiated with 2 Gy gamma radiation. After the treatments, we assessed for DNA damage, cytotoxicity, and viability by alkaline comet, clonogenic survival, and trypan blue dye exclusion assays.

Results

Cytotoxicity and DNA damage increased with increasing 5-FU concentration. The 1 mM Mx concentration had no significant effect on cytotoxicity and DNA damage from 5-FU; however, it increased the cytotoxic and genotoxic effects of different concentrations of 5-FU when used in combination with 2 Gy gamma radiation.

Conclusion

Mx combined with 5-FU enhanced the radiosensitivity of colon cancer cells.

DNA mismatch repair and the DNA damage response

This review discusses the role of DNA mismatch repair (MMR) in the DNA damage response (DDR) that triggers cell cycle arrest and, in some cases, apoptosis. Although the focus is on findings from mammalian cells, much has been learned from studies in other organisms including bacteria and yeast [1,2]. MMR promotes a DDR mediated by a key signaling kinase, ATM and Rad3-related (ATR), in response to various types of DNA damage including some encountered in widely used chemotherapy regimes. An introduction to the DDR mediated by ATR reveals its immense complexity and highlights the many biological and mechanistic questions that remain. Recent findings and future directions are highlighted.

Identification of reproducible drug-resistance-related dysregulated genes in small-scale cancer cell line experiments

Researchers usually measure only a few technical replicates of two types of cell line, resistant or sensitive to a drug, and use a fold-change (FC) cut-off value to detect differentially expressed (DE) genes. However, the FC cut-off lacks statistical control and is biased towards the identification of genes with low expression levels in both cell lines. Here, viewing every pair of resistant-sensitive technical replicates as an experiment, we proposed an algorithm to identify DE genes by evaluating the reproducibility of the expression difference or FC between every two independent experiments without overlapping samples. Using four small datasets of cancer cell line resistant or sensitive to a drug, we demonstrated that this algorithm could efficiently capture reproducible DE genes significantly enriched in biological pathways relevant to the corresponding drugs, whereas many of them could not be found by the FC and other commonly used methods. Therefore, the proposed algorithm is an effective complement to current approaches for analysing small cancer cell line data.

Milestones of Lynch syndrome: 1895-2015

Lynch syndrome, which is now recognized as the most common hereditary colorectal cancer condition, is characterized by the predisposition to a spectrum of cancers, primarily colorectal cancer and endometrial cancer. We chronicle over a century of discoveries that revolutionized the diagnosis and clinical management of Lynch syndrome, beginning in 1895 with Warthin's observations of familial cancer clusters, through the clinical era led by Lynch and the genetic era heralded by the discovery of causative mutations in mismatch repair (MMR) genes, to ongoing challenges.

Trifluridine Induces p53-Dependent Sustained G2 Phase Arrest with Its Massive Misincorporation into DNA and Few DNA Strand Breaks

Trifluridine (FTD) is a key component of the novel oral antitumor drug TAS-102, which consists of FTD and a thymidine phosphorylase inhibitor. Like 5-fluoro-2'-deoxyuridine (FdUrd), a deoxynucleoside form of 5-fluorouracil metabolite, FTD is sequentially phosphorylated and not only inhibits thymidylate synthase activity, but is also incorporated into DNA. Although TAS-102 was effective for the treatment of refractory metastatic colorectal cancer in clinical trials, the mechanism of FTD-induced cytotoxicity is not completely understood. Here, we show that FTD as well as FdUrd induce transient phosphorylation of Chk1 at Ser345, and that this is followed by accumulation of p53 and p21 proteins in p53-proficient human cancer cell lines. In particular, FTD induced p53-dependent sustained arrest at G2 phase, which was associated with a proteasome-dependent decrease in the Cyclin B1 protein level and the suppression of CCNB1 and CDK1 gene expression. In addition, a p53-dependent increase in p21 protein was associated with an FTD-induced decrease in Cyclin B1 protein. Although numerous ssDNA and dsDNA breaks were induced by FdUrd, few DNA strand breaks were detected in FTD-treated HCT-116 cells despite massive FTD misincorporation into genomic DNA, suggesting that the antiproliferative effect of FTD is not due to the induction of DNA strand breaks. These distinctive effects of FTD provide insights into the cellular mechanism underlying its antitumor effect and may explain the clinical efficacy of TAS-102.

Functional interplay between ATM/ATR-mediated DNA damage response and DNA repair pathways in oxidative stress

To maintain genome stability, cells have evolved various DNA repair pathways to deal with oxidative DNA damage. DNA damage response (DDR) pathways, including ATM-Chk2 and ATR-Chk1 checkpoints, are also activated in oxidative stress to coordinate DNA repair, cell cycle progression, transcription, apoptosis, and senescence. Several studies demonstrate that DDR pathways can regulate DNA repair pathways. On the other hand, accumulating evidence suggests that DNA repair pathways may modulate DDR pathway activation as well. In this review, we summarize our current understanding of how various DNA repair and DDR pathways are activated in response to oxidative DNA damage primarily from studies in eukaryotes. In particular, we analyze the functional interplay between DNA repair and DDR pathways in oxidative stress. A better understanding of cellular response to oxidative stress may provide novel avenues of treating human diseases, such as cancer and neurodegenerative disorders.

Aberrant methylation of the MSH3 promoter and distal enhancer in esophageal cancer patients exposed to first-hand tobacco smoke

PURPOSE

Polymorphisms in MSH3 gene confer risk of esophageal cancer when in combination with tobacco smoke exposure. The purpose of this study was to investigate the methylation status of MSH3 gene in esophageal cancer patients in order to further elucidate possible role of MSH3 in esophageal tumorigenesis.

METHODS

We applied nested methylation-specific polymerase chain reaction to investigate the methylation status of the MSH3 promoter in tumors and matching adjacent normal-looking tissues of 84 esophageal cancer patients from a high-risk South African population. The Cancer Genome Atlas data were used to examine DNA methylation profiles at 17 CpG sites located in the MSH3 locus.

RESULTS

Overall, promoter methylation was detected in 91.9 % of tumors, which was significantly higher compared to 76.0 % in adjacent normal-looking esophageal tissues (P = 0.008). When samples were grouped according to different demographics (including age, gender and ethnicity) and smoking status of patients, methylation frequencies were found to be significantly higher in tumor tissues of Black subjects (P = 0.024), patients of 55-65 years of age (P = 0.032), males (P = 0.037) and tobacco smokers (P = 0.015). Furthermore, methylation of the MSH3 promoter was significantly more frequent in tumor samples from smokers compared to tumor samples from non-smokers [odds ratio (OR) = 31.9, P = 0.031]. The TCGA data confirmed significantly higher DNA methylation level at the MSH3 promoter region in tumors (P = 0.0024). In addition, we found evidence of an aberrantly methylated putative MSH3-associated distal enhancer element.

CONCLUSION

Our results suggest that methylation of MSH3 together with exposure to tobacco smoke is involved in esophageal carcinogenesis. Due to the active role of the MSH3 protein in modulating chemosensitivity of cells, methylation of MSH3 should further be examined in association with the outcome of esophageal cancer treatment using anticancer drugs.

Base excision repair AP endonucleases and mismatch repair act together to induce checkpoint-mediated autophagy

Cellular responses to DNA damage involve distinct DNA repair pathways, such as mismatch repair (MMR) and base excision repair (BER). Using Caenorhabditis elegans as a model system, we present genetic and molecular evidence of a mechanistic link between processing of DNA damage and activation of autophagy. Here we show that the BER AP endonucleases APN-1 and EXO-3 function in the same pathway as MMR, to elicit DNA-directed toxicity in response to 5-fluorouracil, a mainstay of systemic adjuvant treatment of solid cancers. Immunohistochemical analyses suggest that EXO-3 generates the DNA nicks required for MMR activation. Processing of DNA damage via this pathway, in which both BER and MMR enzymes are required, leads to induction of autophagy in C. elegans and human cells. Hence, our data show that MMR- and AP endonuclease-dependent processing of 5-fluorouracil-induced DNA damage leads to checkpoint activation and induction of autophagy, whose hyperactivation contributes to cell death.

MSH3 mismatch repair protein regulates sensitivity to cytotoxic drugs and a histone deacetylase inhibitor in human colon carcinoma cells

BACKGROUND

MSH3 is a DNA mismatch repair (MMR) gene that undergoes frequent somatic mutation in colorectal cancers (CRCs) with MMR deficiency. MSH3, together with MSH2, forms the MutSβ heteroduplex that interacts with interstrand cross-links induced by drugs such as cisplatin. To date, the impact of MSH3 on chemosensitivity is unknown.

METHODS

We utilized isogenic HCT116 (MLH1-/MSH3-) cells where MLH1 is restored by transfer of chromosome 3 (HCT116+ch3) and also MSH3 by chromosome 5 (HCT116+3+5). We generated HCT116+3+5, SW480 (MLH1+/MSH3+) and SW48 (MLH1-/MSH3+) cells with shRNA knockdown of MSH3. Cells were treated with 5-fluorouracil (5-FU), SN-38, oxaliplatin, or the histone deacetylase (HDAC) inhibitor PCI-24781 and cell viability, clonogenic survival, DNA damage and apoptosis were analyzed.

RESULTS

MSH3-deficient vs proficient CRC cells showed increased sensitivity to the irinotecan metabolite SN-38 and to oxaliplatin, but not 5-FU, as shown in assays for apoptosis and clonogenic survival. In contrast, suppression of MLH1 attenuated the cytotoxic effect of 5-FU, but did not alter sensitivity to SN-38 or oxaliplatin. The impact of MSH3 knockdown on chemosensitivity to SN-38 and oxaliplatin was maintained independent of MLH1 status. In MSH3-deficient vs proficient cells, SN-38 and oxaliplatin induced higher levels of phosphorylated histone H2AX and Chk2, and similar results were found in MLH1-proficient SW480 cells. MSH3-deficient vs proficient cells showed increased 53BP1 nuclear foci after irradiation, suggesting that MSH3 can regulate DNA double strand break (DSB) repair. We then utilized PCI-24781 that interferes with homologous recombination (HR) indicated by a reduction in Rad51 expression. The addition of PCI-24781 to oxaliplatin enhanced cytotoxicity to a greater extent compared to either drug alone.

CONCLUSION

MSH3 status can regulate the DNA damage response and extent of apoptosis induced by chemotherapy. The ability of MSH3 to regulate chemosensitivity was independent of MLH1 status. PCI-24781-mediated impairment of HR enhanced oxaliplatin sensitivity, suggesting that reduced DSB repair capacity may be contributory.

CCT244747 is a novel potent and selective CHK1 inhibitor with oral efficacy alone and in combination with genotoxic anticancer drugs

PURPOSE

Many tumors exhibit defective cell-cycle checkpoint control and increased replicative stress. CHK1 is critically involved in the DNA damage response and maintenance of replication fork stability. We have therefore discovered a novel potent, highly selective, orally active ATP-competitive CHK1 inhibitor, CCT244747, and present its preclinical pharmacology and therapeutic activity.

EXPERIMENTAL DESIGN

Cellular CHK1 activity was assessed using an ELISA assay, and cytotoxicity a SRB assay. Biomarker modulation was measured using immunoblotting, and cell-cycle effects by flow cytometry analysis. Single-agent oral CCT244747 antitumor activity was evaluated in a MYCN-driven transgenic mouse model of neuroblastoma by MRI and in genotoxic combinations in human tumor xenografts by growth delay.

RESULTS

CCT244747 inhibited cellular CHK1 activity (IC(50) 29-170 nmol/L), significantly enhanced the cytotoxicity of several anticancer drugs, and abrogated drug-induced S and G(2) arrest in multiple tumor cell lines. Biomarkers of CHK1 (pS296 CHK1) activity and cell-cycle inactivity (pY15 CDK1) were induced by genotoxics and inhibited by CCT244747 both in vitro and in vivo, producing enhanced DNA damage and apoptosis. Active tumor concentrations of CCT244747 were obtained following oral administration. The antitumor activity of both gemcitabine and irinotecan were significantly enhanced by CCT244747 in several human tumor xenografts, giving concomitant biomarker modulation indicative of CHK1 inhibition. CCT244747 also showed marked antitumor activity as a single agent in a MYCN-driven neuroblastoma.

CONCLUSION

CCT244747 represents the first structural disclosure of a highly selective, orally active CHK1 inhibitor and warrants further evaluation alone or combined with genotoxic anticancer therapies.

Identification of DNA repair pathways that affect the survival of ovarian cancer cells treated with a poly(ADP-ribose) polymerase inhibitor in a novel drug combination

Floxuridine (5-fluorodeoxyuridine, FdUrd), a U.S. Food and Drug Administration-approved drug and metabolite of 5-fluorouracil, causes DNA damage that is repaired by base excision repair (BER). Thus, poly(ADP-ribose) polymerase (PARP) inhibitors, which disrupt BER, markedly sensitize ovarian cancer cells to FdUrd, suggesting that this combination may have activity in this disease. It remains unclear, however, which DNA repair and checkpoint signaling pathways affect killing by these agents individually and in combination. Here we show that depleting ATR, BRCA1, BRCA2, or RAD51 sensitized to ABT-888 (veliparib) alone, FdUrd alone, and FdUrd + ABT-888 (F+A), suggesting that homologous recombination (HR) repair protects cells exposed to these agents. In contrast, disabling the mismatch, nucleotide excision, Fanconi anemia, nonhomologous end joining, or translesion synthesis repair pathways did not sensitize to these agents alone (including ABT-888) or in combination. Further studies demonstrated that in BRCA1-depleted cells, F+A was more effective than other chemotherapy+ABT-888 combinations. Taken together, these studies 1) identify DNA repair and checkpoint pathways that are important in ovarian cancer cells treated with FdUrd, ABT-888, and F+A, 2) show that disabling HR at the level of ATR, BRCA1, BRCA2, or RAD51, but not Chk1, ATM, PTEN, or FANCD2, sensitizes cells to ABT-888, and 3) demonstrate that even though ABT-888 sensitizes ovarian tumor cells with functional HR to FdUrd, the effects of this drug combination are more profound in tumors with HR defects, even compared with other chemotherapy + ABT-888 combinations, including cisplatin + ABT-888.

FANCJ expression predicts the response to 5-fluorouracil-based chemotherapy in MLH1-proficient colorectal cancer

PURPOSE

Fanconi anemia protein, FANCJ, directly interacts with MLH1, a key protein involved in DNA mismatch repair. Deficient mismatch repair, or microsatellite instability, is a potent marker for the ineffectiveness of 5-fluorouracil (5-FU) in colorectal cancer (CRC). We investigated the significance of FANCJ expression in CRC, focusing on the effects of 5-FU-based adjuvant chemotherapy.

METHODS

Clinicopathologic features and immunohistochemical expression of FANCJ and MLH1 were studied in 219 patients with CRC. We also analyzed 5-FU sensitivity in CRC cell lines with varying levels of FANCJ expression.

RESULTS

FANCJ expression was elevated in tumor tissues compared with normal epithelial tissue. High expression of FANCJ was significantly associated with 5-FU resistance measured by the SDI test (P < 0.05) and poor recurrence-free survival (RFS) (P < 0.05). Among patients with stage II/III tumors who received 5-FU, patients with tumors exhibiting high FANCJ expression had significantly worse RFS than did patients with tumors exhibiting low FANCJ expression (P < 0.01). Among patients who did not receive adjuvant chemotherapy, FANCJ expression was not correlated with RFS (P = 0.76). High FANCJ expression was correlated with 5-FU resistance in tumors with normal MLH1 expression (P < 0.05) but not in tumors not expressing MLH1 (P = 0.9). In vitro, FANCJ overexpression was correlated with 5-FU resistance in MLH1-proficient HCT116 3-6 cells but not in MLH1-deficient HCT116 cells.

CONCLUSIONS

FANCJ could be a useful biomarker to predict the response to 5-FU and prognosis of CRC, particularly in tumors with normal MLH1 expression.

P38MAPK is a major determinant of the balance between apoptosis and autophagy triggered by 5-fluorouracil: implication in resistance

5-Fluorouracil (5-FU), together with other drugs such as oxaliplatin, is one of the most important pharmacological agents in the treatment of colorectal cancer. Although mitogen-activated protein kinases (MAPKs) have been extensively connected with resistance to platinum compounds, no role has been established in 5-FU resistance. Here we demonstrate that p38MAPK activation is a key determinant in the cellular response to 5-FU. Thus, inhibition of p38MAPKα by SB203580 compound or by short-hairpin RNA interference-specific knockdown correlates with a decrease in the 5-FU-associated apoptosis and chemical resistance in both HaCaT and HCT116 cells. Activation of p38MAPK by 5-FU was dependent on canonical MAP2K, MAPK kinase (MKK)-3 and MKK6. In addition, ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3 related (ATR) showed a redundancy of function for the final activation of p38MAPK. Resistance associated with p38MAPK inhibition correlates with an autophagic response that was mediated by a decrease in p53-driven apoptosis, without effect onto p53-dependent autophagy. Moreover, the results with colorectal cancer-derived cell lines with different p53 status and patterns of resistance to 5-FU suggest that de novo and acquired resistance was controlled by similar mechanisms. In summary, our data demonstrate a critical role for the p38MAPK signaling pathway in the cellular response to 5-FU by controlling the balance between apoptosis and autophagy.

ATR-Chk1 signaling pathway and homologous recombinational repair protect cells from 5-fluorouracil cytotoxicity

5-Fluorouracil (5-FU) has long been a mainstay antimetabolite chemotherapeutic drug for the treatment of major solid tumors, particularly colorectal cancer. 5-FU is processed intracellularly to yield active metabolites that compromise RNA and DNA metabolism. However, the mechanisms responsible for its cytotoxicity are not fully understood. From the phenotypic analysis of mutant chicken B lymphoma DT40 cells, we found that homologous recombinational repair (HRR), involving Rad54 and BRCA2, and the ATR-Chk1 signaling pathway, involving Rad9 and Rad17, significantly contribute to 5-FU tolerance. 5-FU induced γH2AX nuclear foci, which were colocalized with the key HRR factor Rad51, but not with DNA double-strand breaks (DSBs), in a dose-dependent manner as cells accumulated in the S phase. Inhibition of Chk1 kinase by UCN-01 increased 5-FU-induced γH2AX and enhanced 5-FU cytotoxicity not only in wild-type cells but also in Rad54- or BRCA2-deficient cells, suggesting that HRR and Chk1 kinase have non-overlapping roles in 5-FU tolerance. 5-FU-induced Chk1 phosphorylation was significantly impaired in Rad9- or Rad17-deficient cells, and severe γH2AX nuclear foci and DSBs were formed, which was followed by apoptosis. Finally, inhibition of Chk1 kinase by UCN-01 increased 5-FU-induced γH2AX nuclear foci and enhanced 5-FU cytotoxicity in Rad9- or Rad17-deficient cells. These results suggest that Rad9- and Rad17-independent activation of the ATR-Chk1 signaling pathway also significantly contributes to 5-FU tolerance.

Checkpoint signaling, base excision repair, and PARP promote survival of colon cancer cells treated with 5-fluorodeoxyuridine but not 5-fluorouracil

The fluoropyrimidines 5-fluorouracil (5-FU) and FdUrd (5-fluorodeoxyuridine; floxuridine) are the backbone of chemotherapy regimens for colon cancer and other tumors. Despite their widespread use, it remains unclear how these agents kill tumor cells. Here, we have analyzed the checkpoint and DNA repair pathways that affect colon tumor responses to 5-FU and FdUrd. These studies demonstrate that both FdUrd and 5-FU activate the ATR and ATM checkpoint signaling pathways, indicating that they cause genotoxic damage. Notably, however, depletion of ATM or ATR does not sensitize colon cancer cells to 5-FU, whereas these checkpoint pathways promote the survival of cells treated with FdUrd, suggesting that FdUrd exerts cytotoxicity by disrupting DNA replication and/or inducing DNA damage, whereas 5-FU does not. We also found that disabling the base excision (BER) repair pathway by depleting XRCC1 or APE1 sensitized colon cancer cells to FdUrd but not 5-FU. Consistent with a role for the BER pathway, we show that small molecule poly(ADP-ribose) polymerase 1/2 (PARP) inhibitors, AZD2281 and ABT-888, remarkably sensitized both mismatch repair (MMR)-proficient and -deficient colon cancer cell lines to FdUrd but not to 5-FU. Taken together, these studies demonstrate that the roles of genotoxin-induced checkpoint signaling and DNA repair differ significantly for these agents and also suggest a novel approach to colon cancer therapy in which FdUrd is combined with a small molecule PARP inhibitor.

Both hMutSα and hMutSß DNA mismatch repair complexes participate in 5-fluorouracil cytotoxicity

BACKGROUND

Patients with advanced microsatellite unstable colorectal cancers do not show a survival benefit from 5-fluorouracil (5-FU)-based chemotherapy. We and others have shown that the DNA mismatch repair (MMR) complex hMutSα binds 5-FU incorporated into DNA. Although hMutSß is known to interact with interstrand crosslinks (ICLs) induced by drugs such as cisplatin and psoralen, it has not been demonstrated to interact with 5-FU incorporated into DNA. Our aim was to examine if hMutSß plays a role in 5-FU recognition.

METHODS

We compared the normalized growth of 5-FU treated cells containing either or both mismatch repair complexes using MTT and clonogenic assays. We utilized oligonucleotides containing 5-FU and purified baculovirus-synthesized hMutSα and hMutSß in electromobility shift assays (EMSA) and further analyzed binding using surface plasmon resonance.

RESULTS

MTT and clonogenic assays after 5-FU treatment demonstrated the most cytotoxicity in cells with both hMutSα and hMutSß, intermediate cytotoxicity in cells with hMutSα alone, and the least cytotoxicity in cells with hMutSß alone, hMutSß binds 5-FU-modified DNA, but its relative binding is less than the binding of 5-FU-modified DNA by hMutSα.

CONCLUSION

Cytotoxicity induced by 5-FU is dependent on intact DNA MMR, with relative cell death correlating directly with hMutSα and/or hMutSß 5-FU binding ability (hMutSα>hMutSß). The MMR complexes provide a hierarchical chemosensitivity for 5-FU cell death, and may have implications for treatment of patients with certain MMR-deficient tumors.

DNA mismatch repair proficiency executing 5-fluorouracil cytotoxicity in colorectal cancer cells

BACKGROUND

5-fluorouracil (5FU)-based chemotherapy is the standard treatment for advanced stage colorectal cancer (CRC) patients. Several groups including ours have reported that stage II-III colorectal cancer patients whose tumors retain DNA Mismatch repair (MMR) function derive a benefit from 5FU, but patients with tumors that lost MMR function do not. Although MMR recognition of 5FU incorporated in DNA has been demonstrated biochemically, it has not been demonstrated within cells to execute 5FU cytotoxicity.

AIM

To establish an efficient construction model for 5FU within DNA and demonstrate that 5FU incorporated into DNA can trigger cellular cytotoxicity executed by the DNA MMR system.

METHODS

We constructed a 5FdU-containing heteroduplex plasmid (5FdU plasmid) and 5FdU-containing linear dsDNA (5FdU linear DNA), and transfected these into MMR-proficient, hMLH1-/- and hMSH6-/- cells. We observed cell growth characteristics of both transfectants for 5FU-induced cytotoxicity.

RESULTS

MMR- proficient cells transfected with the 5FdU plasmid but not the 5FdU linear DNA showed reduced cell proliferation by MTS and clonogenic assays, and demonstrated cell morphological change consistent with apoptosis. In MMR-deficient cells, neither the 5FdU plasmid nor 5FdU linear DNA induced cell growth or morphological changes different from controls.

CONCLUSION

5FdU as heteroduplex DNA in plasmid but not linear form triggered cytotoxicity in a MMR-dependent manner. Thus 5FU incorporated into DNA, separated from its effects on RNA, can be recognized by DNA MMR to trigger cell death.

Poly(ADP-Ribose) polymerase inhibition synergizes with 5-fluorodeoxyuridine but not 5-fluorouracil in ovarian cancer cells

5-Fluorouracil (5-FU) and 5-fluorodeoxyuridine (FdUrd, floxuridine) have activity in multiple tumors, and both agents undergo intracellular processing to active metabolites that disrupt RNA and DNA metabolism. These agents cause imbalances in deoxynucleotide triphosphate levels and the accumulation of uracil and 5-FU in the genome, events that activate the ATR- and ATM-dependent checkpoint signaling pathways and the base excision repair (BER) pathway. Here, we assessed which DNA damage response and repair processes influence 5-FU and FdUrd toxicity in ovarian cancer cells. These studies revealed that disabling the ATM, ATR, or BER pathways using small inhibitory RNAs did not affect 5-FU cytotoxicity. In stark contrast, ATR and a functional BER pathway protected FdUrd-treated cells. Consistent with a role for the BER pathway, the poly(ADP-ribose) polymerase (PARP) inhibitors ABT-888 (veliparib) and AZD2281 (olaparib) markedly synergized with FdUrd but not with 5-FU in ovarian cancer cell lines. Furthermore, ABT-888 synergized with FdUrd far more effectively than other agents commonly used to treat ovarian cancer. These findings underscore differences in the cytotoxic mechanisms of 5-FU and FdUrd and suggest that combining FdUrd and PARP inhibitors may be an innovative therapeutic strategy for ovarian tumors.

UNG-initiated base excision repair is the major repair route for 5-fluorouracil in DNA, but 5-fluorouracil cytotoxicity depends mainly on RNA incorporation

Cytotoxicity of 5-fluorouracil (FU) and 5-fluoro-2′-deoxyuridine (FdUrd) due to DNA fragmentation during DNA repair has been proposed as an alternative to effects from thymidylate synthase (TS) inhibition or RNA incorporation. The goal of the present study was to investigate the relative contribution of the proposed mechanisms for cytotoxicity of 5-fluoropyrimidines. We demonstrate that in human cancer cells, base excision repair (BER) initiated by the uracil–DNA glycosylase UNG is the major route for FU–DNA repair in vitro and in vivo. SMUG1, TDG and MBD4 contributed modestly in vitro and not detectably in vivo. Contribution from mismatch repair was limited to FU:G contexts at best. Surprisingly, knockdown of individual uracil–DNA glycosylases or MSH2 did not affect sensitivity to FU or FdUrd. Inhibitors of common steps of BER or DNA damage signalling affected sensitivity to FdUrd and HmdUrd, but not to FU. In support of predominantly RNA-mediated cytotoxicity, FU-treated cells accumulated ~3000- to 15 000-fold more FU in RNA than in DNA. Moreover, FU-cytotoxicity was partially reversed by ribonucleosides, but not deoxyribonucleosides and FU displayed modest TS-inhibition compared to FdUrd. In conclusion, UNG-initiated BER is the major route for FU–DNA repair, but cytotoxicity of FU is predominantly RNA-mediated, while DNA-mediated effects are limited to FdUrd.

Drug resistance in lung cancer

Resistance to chemotherapy drugs is a major problem in cancer treatment. Scientific advances made in the last two decades have resulted in the identification of genes and molecular signaling mechanisms that contribute to drug resistance. This has resulted in a better understanding of the biology of cancer cells and the way these cells adapt or undergo subtle molecular changes thereby protecting themselves from the cytotoxic effects of the anticancer drugs. Based on the knowledge gained to-date new molecularly targeted drugs are being developed and tested in clinical studies, in an attempt to overcome drug resistance and improve drug efficacy. Despite these attempts the overall 5-year survival of patients diagnosed with cancer, such as lung cancer, remains dismal and is less than 15%. It is evident that additional mechanisms contributing to drug resistance exist which are yet to be discovered. It is hoped that identification of new targets and understanding their contribution to drug resistance will provide opportunities for innovative therapies in overcoming drug resistance. In an attempt to broaden our knowledge and understanding on drug resistance we have, in this review article, summarized the most common mechanisms associated with drug resistance in lung cancer.

Differential expression of a novel gene BRE (TNFRSF1A modulator/BRCC45) in response to stress and biological signals

Stress-responsive genes play critical roles in many biological functions that includes apoptosis, survival, differentiation and regeneration. We have identified a novel stress-responsive gene called BRE which interacts with TNF-receptor-1 and blocks the apoptotic effect of TNF-alpha. BRE enhances tumor growth in vivo and is up-regulated in hepatocellular and esophageal carcinomas. BRE also regulates the ubiquitination of the DNA repair complex BRCC, and the synthesis of steroid hormones. Here, we examined BRE-mRNA in cells after treatments with UV and ionizing radiation (IR). UV and IR treatment alone suppressed BRE-mRNA levels by more than 90% at 24 h, while hydroxyurea, fluorodeoxyuridine, aphidicolin, known inhibitors of S-phase DNA synthesis, had no significant effect. BRE protein expression was unaltered in cells treated with TNF-alpha, Interleukin-1 and Dexamethasone, while a threefold increase was observed following chorionic gonadotropin exposure. Although BRE plays a regulatory role in many different pathways, yet its expression is apparently under very stringent control.

Abrogation of microsatellite-instable tumors using a highly selective suicide gene/prodrug combination

A substantial fraction of sporadic and inherited colorectal and endometrial cancers in humans is deficient in DNA mismatch repair (MMR). These cancers are characterized by length alterations in ubiquitous simple sequence repeats, a phenotype called microsatellite instability. Here we have exploited this phenotype by developing a novel approach for the highly selective gene therapy of MMR-deficient tumors. To achieve this selectivity, we mutated the VP22FCU1 suicide gene by inserting an out-of-frame microsatellite within its coding region. We show that in a significant fraction of microsatellite-instable (MSI) cells carrying the mutated suicide gene, full-length protein becomes expressed within a few cell doublings, presumably resulting from a reverting frameshift within the inserted microsatellite. Treatment of these cells with the innocuous prodrug 5-fluorocytosine (5-FC) induces strong cytotoxicity and we demonstrate that this owes to multiple bystander effects conferred by the suicide gene/prodrug combination. In a mouse model, MMR-deficient tumors that contained the out-of-frame VP22FCU1 gene displayed strong remission after treatment with 5-FC, without any obvious adverse systemic effects to the mouse. By virtue of its high selectivity and potency, this conditional enzyme/prodrug combination may hold promise for the treatment or prevention of MMR-deficient cancer in humans.

Participation of DNA repair in the response to 5-fluorouracil

The anti-metabolite 5-fluorouracil (5-FU) is employed clinically to manage solid tumors including colorectal and breast cancer. Intracellular metabolites of 5-FU can exert cytotoxic effects via inhibition of thymidylate synthetase, or through incorporation into RNA and DNA, events that ultimately activate apoptosis. In this review, we cover the current data implicating DNA repair processes in cellular responsiveness to 5-FU treatment. Evidence points to roles for base excision repair (BER) and mismatch repair (MMR). However, mechanistic details remain unexplained, and other pathways have not been exhaustively interrogated. Homologous recombination is of particular interest, because it resolves unrepaired DNA intermediates not properly dealt with by BER or MMR. Furthermore, crosstalk among DNA repair pathways and S-phase checkpoint signaling has not been examined. Ongoing efforts aim to design approaches and reagents that (i) approximate repair capacity and (ii) mediate strategic regulation of DNA repair in order to improve the efficacy of current anticancer treatments.