The adaptive imbalance in base excision-repair enzymes generates microsatellite instability in chronic inflammation

Predictive power of biomarkers of oxidative stress and inflammation in patients with hepatitis C virus-associated hepatocellular carcinoma

BACKGROUND

This study evaluated the relationship between inflammation, intra-hepatic oxidative stress, oxidative DNA damage and the progression of liver carcinogenesis in hepatitis C virus (HCV)-infected humans.

METHODS

Non-cancerous liver tissues were collected from 30 patients with an HCV-associated solitary hepatocellular carcinoma (HCC) who received curative tumor removal. After surgery, the patients were followed at monthly intervals at the outpatient clinic. Distribution of the inflammatory cells (CD68+), the number of 8-hydroxydeoxyguanosine (8-OHdG) DNA adducts and 4-hydroxynonenal (HNE) protein adducts and the expression of apurinic/apyrimidinic endonuclease (APE) were determined by immunohistochemical analysis in serial liver sections from tumor-free parenchyma at the surgical margin around the tumor.

RESULTS

Significant positive correlations were observed between the number of CD68+ cells, the amount of HNE protein adducts, and the number of 8-OHdG adducts in liver tissue of patients with HCC and HCV. The cumulative disease-free survival was significantly shorter in patients with the highest percentage of 8-OHdG-positive hepatocytes. Using a Cox proportional hazard model, 8-OHdG, HNE and CD68 were determined to be good biomarkers for predicting disease-free survival in patients with HCC and HCV.

CONCLUSIONS

These results support the hypothesis that HCV-induced inflammation causes oxidative DNA damage and promotes hepatocarcinogenesis which directly affects the clinical outcome. Since patients with greater intra-hepatic oxidative stress had a higher incidence of HCC recurrence, we suggest that oxidative stress biomarkers could potentially be used as a useful clinical diagnostic tool to predict the duration of disease-free survival in patients with HCV-associated HCC.

Base excision repair sensitizes cells to sulfur mustard and chloroethyl ethyl sulfide

DNA repair generally functions to improve survival and reduce mutagenesis of cells that have suffered DNA damage. In this study we examine the role of nucleotide excision repair (NER) and base excision repair (BER) in recovery, mutagenesis and DNA repair in response to DNA damage inflicted by the mustard compounds, sulfur mustard (SM) and chloroethyl ethyl sulfide (CEES) in bacteria and mammalian cells. SM and CEES are compared because SM produces cross-links and monoadducts, whereas CEES produces only monoadducts that are similar to those produced by SM, thus allowing the examination of which types of lesions may be responsible for the effects seen. We find that the presence of a functional NER pathway increases survival and reduces mutagenesis, whereas the presence of a functional BER pathway reduces survival, increases mutagenesis, and decreases repair. The deleterious effects of BER appear to be due to an interaction between the DNA glycosylases and the lesions produced by SM and CEES. Possible mechanisms for BER-mediated sensitization by glycosylase action on mustard lesions are discussed.

MGMT: key node in the battle against genotoxicity, carcinogenicity and apoptosis induced by alkylating agents

O(6)-methylguanine-DNA methyltransferase (MGMT) plays a crucial role in the defense against alkylating agents that generate, among other lesions, O(6)-alkylguanine in DNA (collectively termed O(6)-alkylating agents [O(6)AA]). The defense is highly important, since O(6)AA are common environmental carcinogens, are formed endogenously during normal cellular metabolism and possibly inflammation, and are being used in cancer therapy. O(6)AA induced DNA damage is subject to repair, which is executed by MGMT, AlkB homologous proteins (ABH) and base excision repair (BER). Although this review focuses on MGMT, the mechanism of repair by ABH and BER will also be discussed. Experimental systems, in which MGMT has been modulated, revealed that O(6)-methylguanine (O(6)MeG) and O(6)-chloroethylguanine are major mutagenic, carcinogenic, recombinogenic, clastogenic and killing lesions. O(6)MeG-induced clastogenicity and cell death require MutS alpha-dependent mismatch repair (MMR), whereas O(6)-chloroethylguanine-induced killing occurs independently of MMR. Extensive DNA replication is required for O(6)MeG to provoke cytotoxicity. In MGMT depleted cells, O(6)MeG induces apoptosis almost exclusively, barely any necrosis, which is presumably due to the remarkable ability of secondarily formed DNA double-strand breaks (DSBs) to trigger apoptosis via ATM/ATR, Chk1, Chk2, p53 and p73. Depending on the cellular background, O(6)MeG activates both the death receptor and the mitochondrial apoptotic pathway. The inter-individual expression of MGMT in human lymphocytes is highly variable. Given the key role of MGMT in cellular defense, determination of MGMT activity could be useful for assessing a patient's drug sensitivity. MGMT is expressed at highly variable amounts in human tumors. In gliomas, a correlation was found between MGMT activity, MGMT promoter methylation and response to O(6)AA. Although the human MGMT gene is inducible by glucocorticoids and genotoxins such as radiation and alkylating agents, the role of this induction in the protection against carcinogens and the development of chemotherapeutic alkylating drug resistance are still unclear. Modulation of MGMT expression in tumors and normal tissue is currently being investigated as a possible strategy for improving cancer therapy.

Microsatellite instability in long-standing ulcerative colitis

OBJECTIVE

Ulcerative colitis (UC) is a chronic inflammatory disease of the colon associated with a high risk of developing colorectal cancer. Several genetic alterations have been documented in dysplasia and cancer developing in UC. Concerning microsatellite instability (MSI), many contradictory results have been published. We therefore analysed a large, well-characterized UC material for MSI to elucidate its significance in long-standing UC.

MATERIAL AND METHODS

From 33 patients, a total of 159 microdissected lesions and 165 mucosa samples obtained adjacent to the tissue blocks were analysed for MSI using the five standard markers recommended by the National Cancer Institute; D2S123, D5S346, D17S250, BAT-25 and BAT-26. In addition, 12 of the patients were investigated by a mini-satellite marker at the D1S7 locus.

RESULTS

High-level MSI (MSI-H) was detected in one villous adenoma with high-grade dysplasia and right-sided location. This represents 3.6% (1/28) of dysplastic mucosa investigated. No other lesions showed MSI in the five standard markers or at the D1S7 locus.

CONCLUSIONS

This study suggests that MSI is rare in UC-related neoplasia as well as non-neoplastic lesions, and does not contribute to the development of dysplasia.

Methylating agents and DNA repair responses: Methylated bases and sources of strand breaks

The chemical methylating agents methylmethane sulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) have been used for decades as classical DNA damaging agents. These agents have been utilized to uncover and explore pathways of DNA repair, DNA damage response, and mutagenesis. MMS and MNNG modify DNA by adding methyl groups to a number of nucleophilic sites on the DNA bases, although MNNG produces a greater percentage of O-methyl adducts. There has been substantial progress elucidating direct reversal proteins that remove methyl groups and base excision repair (BER), which removes and replaces methylated bases. Direct reversal proteins and BER, thus, counteract the toxic, mutagenic, and clastogenic effects of methylating agents. Despite recent progress, the complexity of DNA damage responses to methylating agents is still being discovered. In particular, there is growing understanding of pathways such as homologous recombination, lesion bypass, and mismatch repair that react when the response of direct reversal proteins and BER is insufficient. Furthermore, the importance of proper balance within the steps in BER has been uncovered with the knowledge that DNA structural intermediates during BER are deleterious. A number of issues complicate the elucidation of the downstream responses when direct reversal is insufficient or BER is imbalanced. These include inter-species differences, cell-type-specific differences within mammals and between cancer cell lines, and the type of methyl damage or BER intermediate encountered. MMS also carries a misleading reputation of being a radiomimetic, that is, capable of directly producing strand breaks. This review focuses on the DNA methyl damage caused by MMS and MNNG for each site of potential methylation to summarize what is known about the repair of such damage and the downstream responses and consequences if the damage is not repaired.

Decreased levels of lipid peroxidation-induced DNA damage in the onset of atherogenesis in apolipoprotein E deficient mice

Increased oxidative stress and subsequent lipid peroxidation (LPO) are thought to be critical events in the formation of atherosclerotic lesions in apolipoprotein E deficient mice (ApoE-KO). LPO derived reactive aldehydes react with DNA to form exocyclic etheno-DNA adducts. These pro-mutagenic DNA lesions are known to be involved in the initiation of carcinogenesis, but their role in the development of atherosclerosis is unknown. In the present study we show that levels of the LPO derived 1,N(6)-ethenodeoxyadenosine (varepsilondA) and 3,N(4)-ethenodeoxycytidine (varepsilondC) were both significantly lower in aorta of 12 weeks old ApoE-KO mice as compared to their wild type controls (1.6+/-0.3 versus 3.2+/-0.8 varepsilondA per 10(8) parent nucleotides, P=0.04 and 4.8+/-0.8 versus 9.2+/-2.1 for varepsilondC, P=0.02). Moreover, levels of both DNA adduct types were inversely related with total plasma cholesterol levels. Consequently, lowest etheno-DNA adduct levels were observed in ApoE-KO mice on a high fat diet. Hypercholesterolemia has previously been associated with increased expression of base excision repair (BER) enzymes, which could explain the lower levels of etheno-DNA adducts in ApoE-KO mice as compared to wild type controls. Indeed, increased staining for the BER-specific DNA repair enzyme apurinic/apyrimidinic endonuclease (Ape1/Ref1) was observed by immunohistochemistry in the endothelium and the first layers of arterial smooth muscle cells of ApoE-KO mice as compared to their wild type counterparts. A high fat diet further increased overall Ape1/Ref1 protein expression in ApoE-KO mice. Although these data suggest no role for increased LPO derived DNA damage in the onset of atherogenesis in ApoE-KO mice, the potentially modulating role of Ape1/Ref1 in the arterial wall deserves further attention.

Accumulation of oxidatively generated DNA damage in the brain: a mechanism of neurotoxicity

Unrepaired or erroneously repaired DNA lesions drive genomic instability and contribute to cellular and organ decline. Since delayed neuropathologies are common in survivors of smoke inhalation injuries, we asked whether the integrity of brain DNA might be compromised by acute exposure to combustion smoke. Although many studies demonstrate that the brain is equipped to repair oxidatively damaged DNA, to date, the capacity for accurate DNA repair under conditions of disrupted oxygenation and oxidative stress has not been defined. We show that DNA adducts detectable by their ability to block PCR amplification form in the rat hippocampus after acute exposure to smoke. To identify the different types of adducts and to dissect their temporal formation and repair profiles in vivo in the brain, we used DNA-modifying enzymes to convert specific adducts into strand breaks prior to PCR amplification. Using this strategy, we detected formation of oxidative DNA adducts early on after smoke inhalation, while mismatched bases emerged at the later recovery times, potentially due to an erroneous DNA repair process. Erroneous repair can be mutagenic and because the initial smoke-induced oxidative damage to DNA is extensive, compromised fidelity of DNA repair may underlie neurotoxicity and contribute to delayed death of hippocampal neurons.

Heterogeneous microsatellite instability observed within epithelium of ulcerative colitis

Microsatellite instability (MSI) has been associated with colitic cancer. However, reported frequency of MSI was varied and the association of MSI with mismatch repair (MMR) deficiency was unclear. In addition, the occurrence of genetic alterations in stromal cells within ulcerative colitis (UC) is still controversial. We therefore sampled 164 microareas in various pathological lesions of UC with or without colitic cancer and studied the MSI status in relation to the DNA repair protein expressions. A total of 129 microfoci from colorectal tissue of 5 colitic cancer patients and 35 microfoci of 7 UC patients (without neoplasm) were carefully sampled by laser-capture microdissection. MSI was analyzed in each microsamples. The protein expression of MMR genes (MLH1, MSH2, MSH6), O(6)-methylguanine-DNA methyltransferase and p53 were assessed by immunohistochemical analysis. Variety of di-nulcleotide microsatellite markers was altered in individual microfoci from different morphological epithelial lesions, in full range of nonneoplastic epithelium to colitic cancer. Interestingly, MSI was not observed in stromal cells at any sites, including those within colitic cancer/dysplasia lesions. Expression of the MMR proteins was not lost in any of the lesions examined. Microsatellite alterations rather seem to be related to the initiation than to the progression of colitic cancer.

Estimating the effect of human base excision repair protein variants on the repair of oxidative DNA base damage

Epidemiologic studies have revealed a complex association between human genetic variance and cancer risk. Quantitative biological modeling based on experimental data can play a critical role in interpreting the effect of genetic variation on biochemical pathways relevant to cancer development and progression. Defects in human DNA base excision repair (BER) proteins can reduce cellular tolerance to oxidative DNA base damage caused by endogenous and exogenous sources, such as exposure to toxins and ionizing radiation. If not repaired, DNA base damage leads to cell dysfunction and mutagenesis, consequently leading to cancer, disease, and aging. Population screens have identified numerous single-nucleotide polymorphism variants in many BER proteins and some have been purified and found to exhibit mild kinetic defects. Epidemiologic studies have led to conflicting conclusions on the association between single-nucleotide polymorphism variants in BER proteins and cancer risk. Using experimental data for cellular concentration and the kinetics of normal and variant BER proteins, we apply a previously developed and tested human BER pathway model to (i) estimate the effect of mild variants on BER of abasic sites and 8-oxoguanine, a prominent oxidative DNA base modification, (ii) identify ranges of variation associated with substantial BER capacity loss, and (iii) reveal nonintuitive consequences of multiple simultaneous variants. Our findings support previous work suggesting that mild BER variants have a minimal effect on pathway capacity whereas more severe defects and simultaneous variation in several BER proteins can lead to inefficient repair and potentially deleterious consequences of cellular damage.

Stress dysfunctions as a unifying paradigm for illness: repairing relationships instead of individuals as a new gateway for medicine

Stress has been implicated as a risk factor for most diseases, but a mechanistic explanation behind such associations remains elusive. As emergent responses to stress, adaptations range from acute responses where extant system capabilities mitigate current stress, to longer-term responses where system plasticity buffers against future stress. The long compendium of human ailments manifests through a much shorter set of symptoms that may operate through the stress axis. We propose a unifying ontology for human illnesses that classifies stress dysfunctions according to types of Darwinian dysfunction - inadequate response with adequate adaptation, inadequate adaptation, inappropriate adaptation, and epiphenomena of adaptation. Examples include cancer as a bystander effect of increased biologic plasticity in response to stress, and infectious illness as a manifestation of mutually escalating stress in an otherwise commensal relationship between hosts and microbes. We explore the contributing role of man-made stresses that have emerged as humans increasingly remodel their environment. Examples include biologic decompensation associated with reliance on technology to buffer stress, and behavioral stress caused by the dislocation of kin networks that promotes illegitimate signaling. Dysfunctional relationships engender stress not only among humans, but also among individual organs; heart failure, renal failure, and carotid stenosis may represent examples of such conditions. If stress dysfunction is the Occam's razor of human illnesses, and derangements in biologic relationships induce stress dysfunctions, then the study of relationships - an incarnation of systems biology - may represent a new gateway for medicine.

Chronic inflammation and oxidative stress in the genesis and perpetuation of cancer: role of lipid peroxidation, DNA damage, and repair

BACKGROUND AND AIMS

Chronic inflammation, induced by biological, chemical, and physical factors, was associated with increased risk of human cancer at various sites. Chronic inflammatory processes induce oxidative/nitrosative stress and lipid peroxidation (LPO), thereby generating excess reactive oxygen species (ROS), reactive nitrogen species (RNS), and DNA-reactive aldehydes. Miscoding etheno- and propano-modified DNA bases are generated inter alia by reaction of DNA with these major LPO products. Steady-state levels of LPO-derived (etheno-) DNA adducts in organs affected by persistent inflammatory processes were investigated as potential lead markers for assessing progression of inflammatory cancer-prone diseases.

RESULTS

Using ultrasensitive and specific detection methods for the analysis of human tissues, cells, and urine, etheno-DNA adduct levels were found to be significantly elevated in the affected organs of subjects with chronic pancreatitis, ulcerative colitis, and Crohn's disease. Patients with alcohol-related liver diseases showed excess hepatic DNA damage progressively increasing from hepatitis, fatty liver, to liver cirrhosis. Ethenodeoxyadenosine excreted after DNA repair in urine of hepatitis B virus-related chronic hepatitis and liver cirrhosis patients was increased up to 90-fold. Putative mechanisms that may control DNA damage in inflamed tissues including impaired or imbalanced DNA repair pathways are reviewed.

CONCLUSION

Persistent oxidative/nitrosative stress and excess LPO are induced by inflammatory processes in a self-perpetuating process and cause progressive accumulation of DNA damage in target organs. Together with deregulation of cell homeostasis, the resulting genetic changes act as driving force in chronic inflammation-associated human disease pathogenesis. Thus steady-state levels of DNA damage caused by ROS, RNS, and LPO end products provide promising molecular signatures for risk prediction and potential targets and biomarkers for preventive measures.

Genetic damage in cultured human keratinocytes stressed by long-term exposure to areca nut extracts

Chewing betel quid (BQ) is a popular habit worldwide. A causal association between BQ chewing and oral cancer has been well documented. Emerging evidence indicates that sustained exposure to stress induces epigenetic reprogramming of some mammalian cells and increases the mutation rate to accelerate adaptation to stressful environments. In this study, we first confirmed that 24-h treatment with areca nut extracts (ANE; a major component of BQ) at doses over 40 microg/ml induced mutations at the hypoxanthine phosphoribisyltransferase (HPRT) locus in human keratinocytes (HaCaT cells). We then investigated whether the stress of long-term exposure to sublethal doses of ANE (0, 5 and 20 microg/ml for 35 passages) could enhance genetic damage to HaCaT cells. Compared to cells exposed to 0 or 5 microg/ml ANE, cells exposed to 20 microg/ml ANE were slightly but significantly more resistant to a 72-h treatment with ANE and its major ingredients, arecoline and arecaidine, but did not develop cross-resistance to other BQ ingredients or alcohol. The cells that received 20 microg/ml ANE for 35 passages also had a significantly increased mutation frequency at the HPRT locus and an increased frequency in the appearance of micronuclei compared to lower doses. Moreover, increased intracellular levels of reactive oxygen species and 8-hydroxyguanosine in cells exposed to 20 microg/ml ANE suggested that long-term ANE exposure results in the accumulation of oxidative damage. However, cells subjected to long-term treatment of 20 microg/ml ANE contained higher levels of glutathione than unexposed cells. Therefore, after long-term exposure to sublethal doses of ANE, intracellular antioxidative activity may also be enhanced in response to increased oxidative stress. These results suggest that stress caused by long-term ANE exposure enhances oxidative stress and genetic damage in human keratinocytes.

Cancer surveillance in inflammatory bowel disease: new molecular approaches

PURPOSE OF REVIEW

Patients with chronic inflammatory bowel disease, such as ulcerative colitis and Crohn's disease, have an increased risk of colorectal cancer. Life-long colonoscopy surveillance is performed to detect the presence of dysplasia, but this approach is expensive and time-consuming. Thus, there is intensive research to identify molecular factors with prognostic value. This review summarizes recent research, with a special emphasis on the mechanisms underlying these molecular alterations.

RECENT FINDINGS

The role of chromosomal instability in the progression to inflammatory bowel disease-associated colorectal cancer is clear and likely relates to chronic cycles of injury, inflammation, repair and telomere shortening. The role of microsatellite instability has been a subject of discussion, and data suggest that microsatellite instability in inflammatory bowel disease might be different from microsatellite instability in sporadic colorectal cancer. Methylation, as a mechanism of gene silencing, also plays a role in ulcerative colitis tumorigenesis. Chronic inflammation has been linked to p53 activation and oxidative stress, contributing to the extensive genomic DNA damage observed in ulcerative colitis.

SUMMARY

Improved understanding of the molecular biology of cancer progression in inflammatory bowel disease will hopefully lead to the identification of useful prognostic biomarkers. Efforts are needed to prove the clinical utility of the most promising markers now identified.

Oxidative and nitrative DNA damage in animals and patients with inflammatory diseases in relation to inflammation-related carcinogenesis

Infection and chronic inflammation are proposed to contribute to carcinogenesis through inflammation-related mechanisms. Infection with hepatitis C virus, Helicobacter pylori and the liver fluke, Opisthorchis viverrini (OV), are important risk factors for hepatocellular carcinoma (HCC), gastric cancer and cholangiocarcinoma, respectively. Inflammatory bowel diseases (IBDs) and oral diseases, such as oral lichen planus (OLP) and leukoplakia, are associated with colon carcinogenesis and oral squamous cell carcinoma (OSCC), respectively. We performed a double immunofluorescence labeling study and found that nitrative and oxidative DNA lesion products, 8-nitroguanine and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), were formed and inducible nitric oxide synthase (iNOS) was expressed in epithelial cells and inflammatory cells at the site of carcinogenesis in humans and animal models. Antibacterial, antiviral and antiparasitic drugs dramatically diminished the formation of these DNA lesion markers and iNOS expression. These results suggest that oxidative and nitrative DNA damage occurs at the sites of carcinogenesis, regardless of etiology. Therefore, it is considered that excessive amounts of reactive nitrogen species produced via iNOS during chronic inflammation may play a key role in carcinogenesis by causing DNA damage. On the basis of our results, we propose that 8-nitroguanine is a promising biomarker to evaluate the potential risk of inflammation-mediated carcinogenesis.

Ntg1p, the base excision repair protein, generates mutagenic intermediates in yeast mitochondrial DNA

Mitochondrial DNA is predicted to be highly prone to oxidative damage due to its proximity to free radicals generated by oxidative phosphorylation. Base excision repair (BER) is the primary repair pathway responsible for repairing oxidative damage in nuclear and mitochondrial genomes. In yeast mitochondria, three N-glycosylases have been identified so far, Ntg1p, Ogg1p and Ung1p. Ntg1p, a broad specificity N-glycosylase, takes part in catalyzing the first step of BER that involves the removal of the damaged base. In this study, we examined the role of Ntg1p in maintaining yeast mitochondrial genome integrity. Using genetic reporters and assays to assess mitochondrial mutations, we found that loss of Ntg1p suppresses mitochondrial point mutation rates, frameshifts and recombination rates. We also observed a suppression of respiration loss in the ntg1-Delta cells in response to ultraviolet light exposure implying an overlap between BER and UV-induced damage in the yeast mitochondrial compartment. Over-expression of the BER AP endonuclease, Apn1p, did not significantly affect the mitochondrial mutation rate in the presence of Ntg1p, whereas Apn1p over-expression in an ntg1-Delta background increased the frequency of mitochondrial mutations. In addition, loss of Apn1p also suppressed mitochondrial point mutations. Our work suggests that both Ntg1p and Apn1p generate mutagenic intermediates in the yeast mitochondrial genome.

Deregulation of homologous recombination DNA repair in alkylating agent-treated stem cell clones: a possible role in the aetiology of chemotherapy-induced leukaemia

Chemotherapeutic regimes involving alkylating agents, such as methylators and crosslinking nitrogen mustards, represent a major risk factor for acute myeloid leukaemia. A high frequency of microsatellite instability and evidence of MSH2 loss in alkylating chemotherapy-related acute myeloid leukaemia (t-AML) suggests that DNA mismatch repair (MMR) dysfunction may be an initiating event in disease evolution. Subsequent accumulation of secondary genetic changes as a result of DNA MMR loss may ultimately lead to the gross chromosomal abnormalities seen in t-AML. Homologous recombination repair (HRR) maintains chromosomal stability by the repair of DNA double-strand breaks, and is therefore a possible target for deregulation in MMR dysfunctional t-AML. In order to test this hypothesis Msh2- proficient and -deficient murine embryonic stem (ES) cells were used to examine the effects of MMR status and methylating agent treatment on cellular expression of DNA double-strand break repair genes. HRR gene expression was significantly deregulated in Msh2 null ES cell clones compared to wild-type clones. Furthermore, some Msh2 null clones expressed high levels of Rad51 specifically, a critical component of HRR. Such Rad51 superexpressing clones were also observed when expression was determined in monocytic myeloid cells differentiated from ES cells. A deregulated HRR phenotype could be partially recapitulated in MMR-competent wild-type cells by treatment with the methylating agent, N-methyl-N-nitrosourea. Furthermore, treatment with melphalan, a leukaemogenic DNA crosslinking chemotherapy nitrogen mustard predicted to elicit HRR, selected against cells with deregulated HRR. These data suggest a t-AML mechanism whereby DNA MMR loss promotes the emergence of HRR gene superexpressing clones, with concomitant chromosomal instability. However, melphalan selection against clones with deregulated HRR suggests that persistence and expansion of unstable clones may require additional genetic alterations that promote cell survival.

Frequent genomic alterations in epithelium measured by microsatellite instability following allogeneic hematopoietic cell transplantation in humans

Although typically found in cancers, frameshift mutations in microsatellites have also been detected in chronically inflamed tissues. Allogeneic hematopoietic cell transplantation (HCT) may potentially produce chronic tissue stress through graft-versus-host reactions. We examined non-neoplastic epithelial tissues (colon, buccal) obtained 1 to 5061 days after human allogeneic HCT for the presence of genomic alterations at 3 tetranucleotide and 3 mononucleotide microsatellite loci. Novel bands indicative of microsatellite instability (MSI) at tetranucleotide repeats were detected in laser-microdissected colonic crypts and in buccal smears of 75% and 42% of patients who received an allograft, respectively. In contrast, no MSI was found in similar tissues from control subjects and from patients after intensive chemotherapy or in buccal cells from patients after autologous HCT. The MSI found in colon, which was often affected by graft-versus-host disease, was not due to loss of expression or nitrosylation of DNA repair proteins. MSI in clinically intact oral mucosa was more frequently found at later time points after HCT. MSI was also found in 3 posttransplant squamous cell cancers examined. Our data show that genomic alterations in epithelium regularly occur after allogeneic HCT and may be implicated in the evolution of posttransplantation diseases, including secondary cancer.

The p53 tumor suppressor network is a key responder to microenvironmental components of chronic inflammatory stress

Activation of the p53 network plays a central role in the inflammatory stress response associated with ulcerative colitis and may modulate cancer risk in patients afflicted with this chronic disease. Here, we describe the gene expression profiles associated with four microenvironmental components of the inflammatory response (NO*, H2O2, DNA replication arrest, and hypoxia) that result in p53 stabilization and activation. Isogenic HCT116 and HCT116 TP53-/- colon cancer cells were exposed to the NO* donor Sper/NO, H2O2, hypoxia, or hydroxyurea, and their mRNA was analyzed using oligonucleotide microarrays. Overall, 1,396 genes changed in a p53-dependent manner (P < 0.001), with the majority representing a "unique" profile for each condition. Only 14 genes were common to all four conditions. Included were eight known p53 target genes. Hierarchical sample clustering distinguished early (1 and 4 hours) from late responses (8, 12, and 24 hours), and each treatment was differentiated from the others. Overall, NO* and hypoxia stimulated similar transcriptional responses. Gene ontology analysis revealed cell cycle as a key feature of stress responses and confirmed the similarity between NO* and hypoxia. Cell cycle profiles analyzed by flow cytometry showed that NO* and hypoxia induced quiescent S-phase and G2-M arrest. Using a novel bioinformatic algorithm, we identified several putative p53-responsive elements among the genes induced in a p53-dependent manner, including four [KIAA0247, FLJ12484, p53CSV (HSPC132), and CNK (PLK3)] common to all exposures. In summary, the inflammatory stress response is a complex, integrated biological network in which p53 is a key molecular node regulating gene expression.

IL-10 deficiency leads to somatic mutations in a model of IBD

Individuals with inflammatory bowel disease (IBD) are at increased risk of developing gastrointestinal cancer. Here, we have tested the possibility that chronic inflammation could trigger mutations. For this, we have used IL-10-deficient (IL-10-/-) mice, which spontaneously develop intestinal inflammation, in combination with a transgenic gpt gene and red/gam gene (gpt+IL-10-/-), which is a well-characterized mutation reporter locus. The total mutation frequency in the colon of gpt+IL-10-/- mice was about five times higher than that in normal gpt+IL-10+/+ mice. In the particular case of G:C to A:T transitions, the frequency of mutations in gpt+IL-10-/- mice was 4.1 times higher than that in control mice. Interestingly, the frequency of small deletions and insertions was also strikingly increased (approximately 10 times). The majority of the deletion or insertion mutations were observed in the monotonous base runs or adjacent repeats of short tandem sequences. In contrast, the frequency of large deletions, detected by loss of the Spi marker present in the red/gam transgene, was similar among the mouse strains. Finally, as a control, the mutation frequency in non-inflamed tissues, such as the liver, were similar between gpt+IL-10-/- mice and gpt+IL-10+/+ mice. Our data demonstrate that the chronic inflammatory environment in the colon promotes the generation of mutations.

Chronic inflammation promotes retinoblastoma protein hyperphosphorylation and E2F1 activation

Chronic inflammation contributes to tumorigenesis. The retinoblastoma protein (pRb), in its hyperphosphorylated form, releases E2 promoter binding factor-1 (E2F1), which drives cell proliferation. Here, we show that pRb is hyperphosphorylated in both mouse and human colitis. In turn, pRb hyperphosphorylation is associated with release of E2F1 from pRb, resulting in the activation of E2F1 target molecules involved in proliferation and apoptosis. These observations provide insight into the in vivo mechanisms associated with chronic colon inflammation and increased colon cancer risk.

Molecular profiling of ulcerative colitis-associated neoplastic progression

Fundamental differences exist between ulcerative colitis (UC)-associated and sporadic forms of colorectal cancer, including preexisting inflammation, type of dysplasia, and timing of molecular events in carcinogenesis. Transcriptional alterations that occur in UC-associated neoplasia in the progression from normal mucosa through dysplastic epithelium to invasive cancer have not been described. We used Affymetrix U95Av2 microarrays to assess differential gene expression in the neoplastic progression of UC tissue from the colonic mucosa of individuals with benign UC, UC-dysplasia-associated lesions or masses, and UC adenocarcinoma. By correlating transcript alterations across tissue types using a mixed statistical model, we identified 699 genes exhibiting altered expression with dysplasia development. A different expression profile was observed in progression to adenocarcinoma with 392 transcripts exhibiting differential expression. There were 224 transcripts common to both dysplasia and adenocarcinoma. Most of the differentially expressed genes described herein were not previously known to play a role in neoplastic progression in UC, including transcripts affecting cell proliferation and apoptosis, signal transduction and signaling, and DNA repair. The altered expression of five transcripts was confirmed by quantitative real-time reverse-transcription polymerase chain reaction. Based on comparisons with previous studies on sporadic colorectal carcinoma, several similarities were found. There were, however, important differences that suggest that different molecular events may occur in the development of UC-associated neoplasia. Several of these genes demonstrated similar changes in dysplastic and cancerous tissue and may be involved in early cancer formation. Identification of these genes as potential clinical biomarkers may lead to improved early disease diagnosis.

Expression of base excision DNA repair genes as a biomarker of oxidative DNA damage

Oxidative stress induced DNA damage is considered to be the most common insult affecting the genome. Moreover, it is recognized as a common pathway to mutations and is suggested to play a major role in the development of chronic diseases such as cancer. However, current analytical methods used to detect oxidative DNA damage have been hampered by both technical and biological obstacles. These include spurious oxidation during DNA isolation and processing, and the inherent removal of damaged bases by numerous operating DNA repair systems. The removal of oxidized bases is performed predominantly by the base excision repair (BER) pathway and it has been shown that induction of DNA repair genes occurs in response to oxidative stress. Here, we demonstrate the utility of measuring changes in expression of BER genes as a sensitive in vivo biomarker for oxidative DNA damage.

Can mycoplasma-mediated oncogenesis be responsible for formation of conventional renal cell carcinoma?

OBJECTIVES

To investigate the association between Mycoplasma sp. infection and conventional renal cell carcinoma (RCC).

METHODS

Normal kidney, renal intratubular neoplasia, and tumor tissue samples from 33 patients with RCC and 35 healthy controls were studied. Molecular DNA analysis was done after nested polymerase chain reaction performed in two steps with seven primers (four outer and three inner) that can recognize at least 15 different Mycoplasma sp.

RESULTS

Mycoplasma sp. DNA was detected in normal kidney, renal intratubular neoplasia, and tumor tissue samples at a ratio of 36%, 67%, and 82%, respectively. In 6 of the 33 patients with RCC, no Mycoplasma sp. was detected from any of the three tissue samples. Mycoplasma sp. DNA was detected in only 5 (14%) of the 35 samples from the control group.

CONCLUSIONS

The relationship between mycoplasma infection and conventional RCC has been investigated for the first time, and a significantly high existence of Mycoplasma sp. DNA was found in the tissues of patients with conventional RCC compared with that found in a healthy control group. This suggests that mycoplasma-mediated multistage carcinogenesis may play a role in the development of RCC.

Identification and quantification of the depurinating DNA adducts formed in mouse skin treated with dibenzo[a,l]pyrene (DB[a,l]P) or its metabolites and in rat mammary gland treated with DB[a,l]P

Dibenzo[a,l]pyrene (DB[a,l]P) is the most potent carcinogenic polycyclic aromatic hydrocarbon and has been identified in the environment. Comparative tumorigenicity studies in mouse skin and rat mammary gland indicate that DB[a,l]P is slightly more potent than DB[a,l]P-11,12-dihydrodiol and much more potent than (+/-)-syn-DB[a,l]P-11,12-dihydrodiol-13,14-epoxide {(+/-)-syn-DB[a,l]PDE} and (+/-)-anti-DB[a,l]PDE. We report here the identification and quantification of the depurinating adducts formed in mouse skin treated with DB[a,l]P, DB[a,l]P-11,12-dihydrodiol, (+/-)-syn-DB[a,l]PDE, or (+/-)-anti-DB[a,l]PDE and rat mammary gland treated with DB[a,l]P. The biologically formed adducts were compared with standard adducts by their retention times on HPLC and their spectra obtained by fluorescence line-narrowing spectroscopy at low temperature. In mouse skin treated with DB[a,l]P, depurinating adducts comprised 99% of the total adducts. Most of the depurinating adducts were formed by one-electron oxidation, with 63% at Ade and 12% at Gua. The remainder were formed by the diol epoxide, with 18% at Ade and 6% at Gua. When mouse skin was treated with DB[a,l]P-11,12-dihydrodiol, depurinating adducts comprised 80% of the total, and the predominant one was with Ade (69%). Treatment of skin with (+/-)-syn-DB[a,l]PDE resulted in 32% depurinating adducts, primarily at Ade (25%), whereas treatment with (+/-)-anti-DB[a,l]PDE produced 97% stable adducts. The formation of depurinating adducts following treatment of rat mammary gland with DB[a,l]P resulted in approximately 98% depurinating adducts, with the major adducts formed by one-electron oxidation. Only one depurinating diol epoxide adduct was formed. Tumorigenicity, mutations, and DNA adduct data suggest that depurinating Ade adducts play a major role in the initiation of tumors by DB[a,l]P.

Identifying and defusing weapons of mass inflammation in carcinogenesis

The continued cancer risks associated with chronic inflammation necessitate the identification of inflammatory molecules and the cancer pathways they affect. Evidence indicates that there are multiple mechanisms linking inflammation to cancer and that there are multiple targets for chemoprevention. Here, we review some of the key factors and the cancer pathways they disturb as a necessary prerequisite to the identification of targets for chemoprevention.

Human AP endonuclease suppresses DNA mismatch repair activity leading to microsatellite instability

The multifunctional mammalian apurinic/apyrimidinic (AP) endonuclease (APE) participates in the repair of AP sites in the cellular DNA as well as participating in the redox regulation of the transcription factor function. The function of APE is considered as the rate-limiting step in DNA base excision repair. Paradoxically, an unbalanced increase in APE protein leads to genetic instability. Therefore, we investigated the mechanisms of genetic instability that are induced by APE. Here, we report that the overexpression of APE protein disrupts the repair of DNA mismatches, which results in microsatellite instability (MSI). We found that expression of APE protein led to the suppression of the repair of DNA mismatches in the normal human fibroblast cells. Western blot analysis revealed that hMSH6 protein was markedly reduced in the APE-expressing cells. Moreover, the addition of purified Mutalpha (MSH2 and MSH6 complex) to the extracts from the APE-expressing cells led to the restoration of mismatch repair (MMR) activity. By performing MMR activity assay and MSI analysis, we found that the co-expression of hMSH6 and APE exhibited the microsatellite stability, whereas the expression of APE alone generated the MSI-high phenotype. The APE-mediated decrease in MMR activity described here demonstrates the presence of a new and highly effective APE-mediated mechanism for MSI.

The multifunctional DNA repair/redox enzyme Ape1/Ref-1 promotes survival of neurons after oxidative stress

Although correlative studies demonstrate a reduction in the expression of apurinic/apyrimidinic endonuclease/redox effector factor (Ape1/Ref-1 or Ape1) in neural tissues after neuronal insult, the role of Ape1 in regulating neurotoxicity remains to be elucidated. To address this issue, we examined the effects of reducing Ape1 expression in primary cultures of hippocampal and sensory neurons on several endpoints of neurotoxicity induced by H2O2. Ape1 is highly expressed in hippocampal and sensory neurons grown in culture as indicated by immunohistochemistry, immunoblotting and activity. Exposing hippocampal or sensory neuronal cultures to 25 or 50 nM small interfering RNA to Ape1 (Ape1siRNA), respectively, for 48 h, causes a reduction in immunoreactive Ape1 by approximately 65 and 54%, and an equivalent loss in endonuclease activity. The reduced expression of Ape1 is maintained for up to 5 days after the siRNA in the medium is removed, whereas exposing cultures to scrambled sequence siRNA (SCsiRNA) has no effect of Ape1 protein levels. The reduction in Ape1 significantly reduces cell viability in cultures 24 h after a 1-h exposure to 25-300 microM H2O2, compared to SCsiRNA treated controls. In cells treated with SCsiRNA, exposure to 300 microM H2O2 reduced cell viability by 40 and 30% in hippocampal and sensory neuronal cultures, respectively, whereas cultures treated with Ape1siRNA lost 93 and 80% of cells after the peroxide. Reduced Ape1 levels also increase caspase-3 activity in the cells, 2-3-fold, 60min after a 1-h exposure to 100 microM H2O2 in the cultures. Exposing neuronal cultures with reduced expression of Ape1 to 65 microM H2O2 (hippocampal) or 300 microM H2O2 (sensory) for 1h results in a 3-fold and 1.5-fold increase in the phosphorylation of histone H2A.X compared to cells exposed to SCsiRNA. Overexpressing wild-type Ape1 in hippocampal and sensory cells using adenoviral expression constructs results in significant increase in cell viability after exposure to various concentrations of H2O2. The C65A repair competent/redox incompetent Ape1 when expressed in the hippocampal and sensory cells conferred only partial protection on the cells. These data support the notion that both of functions of Ape1, redox and repair are necessary for optimal levels of neuronal cell survival.

Molecular phenotype of inflammatory bowel disease-associated neoplasms with microsatellite instability

BACKGROUND & AIMS

Patients with inflammatory bowel disease (IBD) are at increased risk of developing colorectal cancer (CRC). We sought to determine the frequency of high-level microsatellite instability (MSI-H) and the mutational and methylation profile of MSI-H IBD-related neoplasms (IBDNs).

METHODS

A total of 124 IBDNs (81 cancers, 43 dysplasias) from 78 patients were studied for the frequency of MSI-H and hypermethylation of 3 target genes: MLH1 , HPP1 , and RAB-32 . Fifteen MSI-H IBDNs were characterized according to their profile of frameshift mutations in 28 mononucleotide repeats and compared with 46 sporadic MSI-H CRCs.

RESULTS

Nineteen of 124 IBDNs were MSI-H. The frequency of frameshift mutations in coding mononucleotide repeats was significantly lower in MSI-H IBDNs than in sporadic MSI-H CRCs for TGFBR2 (7 of 14 vs 34 of 43 samples; P = .047) and ACVR2 (3 of 14 vs 25 of 43 samples; P = .029). In contrast, ICA1 was mutated in 3 of 9 MSI-H IBDNs vs 2 of 54 sporadic MSI-H CRCs ( P = .028). HPP1 and RAB32 methylation was independent of MSI status and was observed in 4 of 59 and 0 of 64 nondysplastic mucosae, 20 of 38 and 1 of 25 dysplasias, and 28 of 61 and 20 of 60 carcinomas, respectively.

CONCLUSIONS

The profiles of coding microsatellite mutations (instabilotypes) differ significantly between MSI-H IBDNs and MSI-H sporadic CRCs. Specifically, TGFBR2 and ACVR2 mutations are significantly rarer in MSI-H IBDNs than in MSI-H sporadic CRCs. Furthermore, HPP1 methylation occurs early, in 7% of nondysplastic and approximately half of dysplastic mucosae, whereas RAB32 methylation occurs at the transition to invasive growth, being rarer in dysplasias.

Cytochrome P450 CYP2E1, but not nicotinamide adenine dinucleotide phosphate oxidase, is required for ethanol-induced oxidative DNA damage in rodent liver

The occurrence of malignant tumors of the upper gastrointestinal tract and liver is, based largely on epidemiological evidence, causally related to the consumption of ethanol. It is widely recognized that oxidants play a key role in alcohol-induced liver injury; however, it is unclear how oxidants may be involved in DNA damage. We asked whether nicotinamide adenine dinucleotide phosphate oxidase, cytochrome P450 CYP2E1, or both are responsible for the production of DNA damage. The rodent Tsukamoto-French model of intragastric ethanol infusion was used. Wistar rats, Cyp2e1-, p47(phox)-null, and hCyp2e1 transgenic mice were used. The abundance of oxidative DNA adducts, mutagenic apurinic/apyrimidinic sites, and expression of base excision DNA repair genes was determined. In rats and wild-type mice, ethanol treatment for 4 weeks led to an increase in oxidative DNA damage and induction of expression of the base excision DNA repair genes that are known to remove oxidative DNA lesions. No increase in either of the endpoints was observed in ethanol-treated Cyp2e1-null mice, whereas the magnitude of response in p47(phox)-null mice and transgenic hCyp2e1 was identical to that in wild types. The increase in expression of DNA repair genes was completely abolished by treatment with the P450 inhibitor 1-aminobenzotriazole. In conclusion, the data support the hypothesis that oxidative stress to DNA is induced in liver by ethanol. Furthermore, although it was shown that nicotinamide adenine dinucleotide phosphate oxidase-derived oxidants are critical for the development of ethanol-induced liver injury, CYP2E1 is required for the induction of oxidative stress to DNA, and thus may play a key role in ethanol-associated hepatocarcinogenesis.

Microenvironmental influences on mutagenesis in mammary epithelial cells

Tumor progression may be viewed as an evolutionary process at the cellular level. Because blood supply to solid tumors is inadequate, the cancer cells face a hostile microenvironment characterized by hypoxia or anoxia, acidic extracellular pH and nutrient deficiencies. It has been proposed that these factors result in increased levels of spontaneous mutagenesis and thereby contribute to tumor progression. We have examined spontaneous mutagenesis in vitro and in vivo, using previously characterized cell lines (mammary epithelial cells [ME] and mammary fibroblast cells [MFib]) from the mammary gland of the BigBluetrade mark rat, carrying a transgene construct suitable for the detection of mutations. Cells were exposed in vitro to control conditions, low pH, or to glucose deprivation, under normoxic or hypoxic culture conditions, and were also grown as xenografted tumors in immune-deficient mice. We examined cell survival and mutant frequency/spectrum at the cII locus. Significant increases in mutant frequency were observed in ME cells exposed to hypoxia alone or in combination with no glucose; the latter condition also resulted in reduced clonogenic survival. Cells grown as xenografts and then recovered and expanded in culture also had elevated frequencies of spontaneous mutations. We observed a shift in the spontaneous mutation spectrum between the ME cells and the MET cells (cultured in vitro or isolated from mouse xenograft tumors). These results support the concept that the tumor microenvironment contributes to tumor progression by enhancing spontaneous mutagenesis, that different cell types from the same organ can respond differently to these stresses and that differences in microenvironment may influence the types of mutations that arise.

DNA methyltransferase Dnmt1 and mismatch repair

We have examined the relationship between DNA mismatch repair and deficiency of DNA methylation in a mouse embryonic cell line, Dnmt1-/- ES, with homologous deletion of the gene coding for the maintenance DNA methyltransferase Dnmt1. With the use of a sensitive PCR for the assay of two mononucleotide- and three dinucleotide microsatellite markers that exhibited instabilities in mismatch repair-deficient cells, significantly higher frequencies of instability were detected at three of the five markers in Dnmt1-/- ES than the wild-type ES. The data suggest that Dnmt1 enzyme plays an integrating role in DNA replication and/or repair. The implication that Dnmt1 enzyme and/or cytosine methylation may participate in the strand discrimination of mismatch repair during eukaryotic DNA replication is discussed.

Inflammation and cancer IV. Colorectal cancer in inflammatory bowel disease: the role of inflammation

Patients with ulcerative colitis and Crohn's disease are at increased risk for developing colorectal cancer. To date, no known genetic basis has been identified to explain colorectal cancer predisposition in these inflammatory bowel diseases. Instead, it is assumed that chronic inflammation is what causes cancer. This is supported by the fact that colon cancer risk increases with longer duration of colitis, greater anatomic extent of colitis, the concomitant presence of other inflammatory manifestations such as primary sclerosing cholangitis, and the fact that certain drugs used to treat inflammation, such as 5-aminosalicylates and steroids, may prevent the development of colorectal cancer. The major carcinogenic pathways that lead to sporadic colorectal cancer, namely chromosomal instability, microsatellite instability, and hypermethylation, also occur in colitis-associated colorectal cancers. Unlike normal colonic mucosa, however, inflamed colonic mucosa demonstrates abnormalities in these molecular pathways even before any histological evidence of dysplasia or cancer. Whereas the reasons for this are unknown, oxidative stress likely plays a role. Reactive oxygen and nitrogen species produced by inflammatory cells can interact with key genes involved in carcinogenic pathways such as p53, DNA mismatch repair genes, and even DNA base excision-repair genes. Other factors such as NF-kappaB and cyclooxygenases may also contribute. Administering agents that cause colitis in healthy rodents or genetically engineered cancer-prone mice accelerates the development of colorectal cancer. Mice genetically prone to inflammatory bowel disease also develop colorectal cancer especially in the presence of bacterial colonization. These observations offer compelling support for the role of inflammation in colon carcinogenesis.

p53 functions in the incorporation step in DNA base excision repair in mouse liver mitochondria

The tumor suppressor p53 protein stimulates nuclear base excision repair (BER) in vitro. In response to certain cellular stresses, p53 translocates to mitochondria, where it can trigger an apoptotic response. However, a potential role for p53 in modulating mitochondrial DNA repair has not yet been examined. In this study, we show that p53 also modulates mitochondrial BER. Uracil-initiated BER incorporation, which measures flux through the entire BER pathway, was lower in mitochondrial extracts from nonstressed p53 knockout mice than in wild type. The addition of recombinant p53 complemented the BER incorporation in p53 knockout extracts and stimulated BER in wt extracts. The activities of three major mitochondrial DNA glycosylases were similar in extracts from wild-type and knockout animals. Likewise, AP endonuclease activity was unaffected by the absence of p53. Gel shift experiments with recombinant p53 demonstrated that p53 did not bind to the uracil-containing substrate used in the repair assay. Polymerase gamma gap-filing activity was less efficient in p53 knockout extracts, but it was complemented with the addition of recombinant p53. Thus, we conclude that p53 may participate in mtBER by stimulating the repair synthesis incorporation step.

The evolution to stool DNA testing for colorectal cancer

Despite a variety of screening strategies and recent trends showing death rate stabilization, colorectal cancer still remains the second leading cause of overall cancer death. Current screening tools suffer from performance limitations, low patient acceptability, and marginal reliable access within the health care system. Noninvasive strategies present the lowest risk with the highest potential for patient satisfaction. However, serious implementation barriers exist requiring consistent programmatic screening, strict patient adherence, and poor sensitivity for adenomas. Colonoscopy remains an invasive screening test with the best sensitivity and specificity, but faces large financial costs, manpower requirements, patient access and adherence. Development of advanced molecular techniques identifying altered DNA markers in exfoliated colonocytes signify early or precancerous growth. Stool-based DNA testing provides an entirely noninvasive population-based screening strategy which patients can perform easier than faecal occult blood testing (FOBT). Large-scale prospective randomized control trials currently pending should help characterize accurate test performance, screening intervals, cost-effectiveness, direct comparison to FOBT and analysis of patient adherence. As tumour development pathways and potential target genes are further elucidated, refinements in multi-assay stool-based DNA testing portend enhanced test characteristics to detect and treat this genetically heterogeneous disease.

Hijacking of the human alkyl-N-purine-DNA glycosylase by 3,N4-ethenocytosine, a lipid peroxidation-induced DNA adduct

Lipid peroxidation generates aldehydes, which react with DNA bases, forming genotoxic exocyclic etheno(epsilon)-adducts. E-bases have been implicated in vinyl chloride-induced carcinogenesis, and increased levels of these DNA lesions formed by endogenous processes are found in human degenerative disorders. E-adducts are repaired by the base excision repair pathway. Here, we report the efficient biological hijacking of the human alkyl-N-purine-DNA glycosylase (ANPG) by 3,N(4)-ethenocytosine (epsilonC) when present in DNA. Unlike the ethenopurines, ANPG does not excise, but binds to epsilonC when present in either double-stranded or single-stranded DNA. We developed a direct assay, based on the fluorescence quenching mechanism of molecular beacons, to measure a DNA glycosylase activity. Molecular beacons containing modified residues have been used to demonstrate that the epsilonC.ANPG interaction inhibits excision repair both in reconstituted systems and in cultured human cells. Furthermore, we show that the epsilonC.ANPG complex blocks primer extension by the Klenow fragment of DNA polymerase I. These results suggest that epsilonC could be more genotoxic than 1,N(6)-ethenoadenine (epsilonA) residues in vivo. The proposed model of ANPG-mediated genotoxicity of epsilonC provides a new insight in the molecular basis of lipid peroxidation-induced cell death and genome instability in cancer.

Tumbling down a different pathway to genetic instability

Ulcerative colitis (UC), a chronic inflammatory condition associated with a predisposition to colon cancer, is frequently characterized by DNA damage in the form of microsatellite instability (MSI). A new report links inflammation in UC with increases in the DNA repair enzymes 3-methyladenine DNA glycosylase and apurinic/apyrimidinic endonuclease, and, paradoxically, with increased MSI. These findings may represent a novel mechanism contributing to MSI in chronic inflammation.