Mouse embryonic stem cells carrying one or two defective Msh2 alleles respond abnormally to oxidative stress inflicted by low-level radiation

Inhibition of the 5' to 3' exonuclease activity of hEXO1 by 8-oxoguanine

The mismatch repair pathway is responsible for maintaining genomic stability by correcting base-base mismatches and insertion/deletion loops that arise mainly via replication errors. Additionally, the mismatch repair pathway performs a central role in the cellular response to both alkylation and reactive oxygen species induced DNA damage. An important step in mismatch processing is the recruitment of hEXO1, a 5' to 3' exonuclease, by hMSH2-hMSH6 to remove the nascent DNA strand. However, very little is currently known about the capacity of hEXO1 to exonucleolytically process damaged DNA bases. Therefore, we examined whether hEXO1 can degrade double-stranded DNA substrates containing alkylated or oxidized nucleotides. Our results demonstrated that hEXO1 is capable of degrading duplex DNA containing an O6-methylguanine (O6-meG) adduct paired with either a C or a T. Additionally, the hMSH2-hMSH6 complex stimulated hEXO1 exonuclease activity on the O6-meG/T and O6-meG/C DNA substrates. In contrast, hEXO1 exonuclease activity was significantly blocked by the presence of an 8-oxoguanine adduct in both single and double stranded DNA substrates. Further, hMSH2-hMSH6 was not able to alleviate the nucleolytic block caused by the 8-oxoguanine adduct in heteroduplex DNA.

Mutagenic roles of DNA "repair" proteins in antibody diversity and disease-associated trinucleotide repeat instability

While DNA repair proteins are generally thought to maintain the integrity of the whole genome by correctly repairing mutagenic DNA intermediates, there are cases where DNA "repair" proteins are involved in causing mutations instead. For instance, somatic hypermutation (SHM) and class switch recombination (CSR) require the contribution of various DNA repair proteins, including UNG, MSH2 and MSH6 to mutate certain regions of immunoglobulin genes in order to generate antibodies of increased antigen affinity and altered effector functions. Another instance where "repair" proteins drive mutations is the instability of gene-specific trinucleotide repeats (TNR), the causative mutations of numerous diseases including Fragile X mental retardation syndrome (FRAXA), Huntington's disease (HD), myotonic dystrophy (DM1) and several spinocerebellar ataxias (SCAs) all of which arise via various modes of pathogenesis. These healthy and deleterious mutations that are induced by repair proteins are distinct from the genome-wide mutations that arise in the absence of repair proteins: they occur at specific loci, are sensitive to cis-elements (sequence context and/or epigenetic marks) and transcription, occur in specific tissues during distinct developmental windows, and are age-dependent. Here we review and compare the mutagenic role of DNA "repair" proteins in the processes of SHM, CSR and TNR instability.

DNA repair dysfunction in gastrointestinal tract cancers

The DNA repair system surveys the genome, which is always suffering from exposure to both exogenous as well as endogenous mutagens, to maintain the genetic information. The fact that the basis of this DNA repair system is highly conserved, from prokaryote to mammalian cells, suggests the importance of precise genome maintenance mechanisms for organisms. In the past 15 years, considerable progress has been made in understanding how repair processes interact and how disruptions of these mechanisms lead to the accumulation of mutations and carcinogenesis. In 1993, two groups reported that DNA mismatch repair could be associated with hereditary non-polyposis colorectal cancer, indicating a connection between faulty DNA repair function and cancer. More recently, an inherited disorder of DNA glycosylase, which removes mutagenic oxidized base from DNA, has been reported in individuals with a predisposition to multiple colorectal adenomas and carcinomas. This is the first report that directly indicates the role of the repair of oxidative DNA in human inherited cancer. Studies from gene knockout mice have elucidated the principal role of these repair systems in the process of carcinogenesis. Moreover, clinical samples derived from cancer patients have shown the direct involvement. This review focuses on the function of DNA mismatch repair and oxidative DNA/nucleotide repair among various DNA repair systems in cells, both of which are essentially involved in the carcinogenesis of gastrointestinal tract cancer.

Mismatch repair in Trypanosoma brucei: heterologous expression of MSH2 from Trypanosoma cruzi provides new insights into the response to oxidative damage

Trypanosomes are unicellular eukaryotes that cause disease in humans and other mammals. Trypanosoma cruzi and Trypanosoma brucei are the causative agents, respectively, of Chagas disease in the Americas and sleeping sickness in sub-Saharan Africa. To better comprehend the interaction of these parasites with their hosts, understanding the mechanisms involved in the generation of genetic variability is critical. One such mechanism is mismatch repair (MMR), which has a crucial, evolutionarily conserved role in maintaining the fidelity of DNA replication, as well as acting in other cellular processes, such as DNA recombination. Here we have attempted to complement T. brucei MMR through the expression of MSH2 from T. cruzi. Our results show that T. brucei MSH2-null mutants are more sensitive to hydrogen peroxide (H2O2) than wild type cells, suggesting the involvement of MSH2 in the response to oxidative stress in this parasite. This phenotype is reverted by the expression of either the T. cruzi or the T. brucei MSH2 protein in the MSH2-null mutants. In contrast, MMR complementation, as assessed by resistance to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and microsatellite instability, was not achieved by the heterologous expression of T. cruzi MSH2. This finding, associated to the demonstration that mutation of MLH1, another component of the MMR system, did not affect sensitivity of T. brucei cells to H2O2, suggests an additional role of MSH2 in dealing with oxidative damage in these parasites, which may occur independently of MMR.

Detection of complex DNA damage in gamma-irradiated acute lymphoblastic leukemia Pre-b NALM-6 cells

Bistranded complex DNA damage, i.e., double-strand breaks (DSBs) and non-DSB oxidative clustered DNA lesions, is hypothesized to challenge the repair mechanisms of the cell and consequently the genomic integrity. The oxidative clustered DNA lesions may be persistent and may accumulate in human cancer cells for long times after irradiation. To evaluate the detection and possible accumulation of oxidative clustered DNA lesions in leukemia cells exposed to doses equivalent to those used in radiotherapy, we measured the induction of DSBs and three different types of oxidative clustered DNA lesions in NALM-6 cells, a human acute lymphoblastic leukemia (ALL) pre-B cell line, after exposure to (137)Cs gamma rays. For the detection and measurement of DSBs and oxidative clustered DNA lesions, we used an adaptation of the neutral comet assay (single-cell gel electrophoresis) using E. coli repair enzymes (Endo IV, Fpg and Endo III) as enzymatic probes. We found a linear dose response for the induction of DSBs and oxidative clustered DNA lesions. Clustered DNA lesions were more prevalent than prompt DSBs. For each DSB induced by radiation, approximately 2.5 oxidative clustered DNA lesions were detected. To our knowledge, this is the first study to demonstrate the detection and linear induction of oxidative clustered DNA lesions with radiation dose in an ALL cell line. These results point to the biological significance of clustered DNA lesions.

Screening seeds of some Scottish plants for free radical scavenging activity

From a consideration of ethnobotanical and taxonomic information, seeds of 45 Scottish plant species encompassing 23 different families were obtained from authentic seed suppliers. The n-hexane, dichloromethane (DCM) and methanol (MeOH) extracts were assessed, both qualitatively and quantitatively, for free radical scavenging activity in the DPPH assay. The MeOH extracts of 37 species exhibited low to high levels of free radical scavenging activity (RC50 values ranging from 2.00 to 4.7 x 10(-4) mg/mL), and Alliaria petiolata, Prunus padus and Prunus spinosa were the most potent antioxidant extracts. The DCM extracts of 17 species showed similar levels of activity, and among those, Prunus padus and Prunus spinosa extracts were the most active with RC50 values of 2.5 x 10(-4) and 5.0 x 10(-4) mg/mL, respectively. The n-hexane extracts were much less active than the MeOH and DCM extracts, and 17 species, with the exception of Glechoma hederacea (RC50 = 1.94 x 10(-4)) displayed low to moderate levels of free radical scavenging property (RC50 values ranging from 2.00 to 8.7 x 10(-3) mg/mL).

Biallelic mutation of MSH2 in primary human cells is associated with sensitivity to irradiation and altered RAD51 foci kinetics

BACKGROUND

Reports of differential mutagen sensitivity conferred by a defect in the mismatch repair (MMR) pathway are inconsistent in their conclusions. Previous studies have investigated cells established from immortalised human colorectal tumour lines or cells from animal models.

METHODS

We examined primary human MSH2-deficient neonatal cells, bearing a biallelic truncating mutation in MSH2, for viability and chromosomal damage after exposure to DNA-damaging agents.

RESULTS

MSH2-deficient cells exhibit no response to interstrand DNA cross-linking agents but do show reduced viability in response to irradiation. They also show increased chromosome damage and exhibit altered RAD51 foci kinetics after irradiation exposure, indicating defective homologous recombinational repair.

DISCUSSION

The cellular features and sensitivity of MSH2-deficient primary human cells are broadly in agreement with observations of primary murine cells lacking the same gene. The data therefore support the view that the murine model recapitulates early features of MMR deficiency in humans, and implies that the variable data reported for MMR-deficient immortalised human cells may be due to further genetic or epigenetic lesions. We suggest caution in the use of radiotherapy for treatment of malignancies in individuals with functional loss of MSH2.

Mismatch repair deficiencies transforming stem cells into cancer stem cells and therapeutic implications

For the exceptional self-renewal capacity, regulated cell proliferation and differential potential to a wide variety of cell types, the stem cells must maintain the intact genome. The cells under continuous exogenous and endogenous genotoxic stress accumulate DNA errors, drive proliferative expansion and transform into cancer stem cells with a heterogeneous population of tumor cells. These cells are a common phenomenon for the hematological malignancies and solid tumors. In response to DNA damage, the complex cellular mechanisms including cell cycle arrest, transcription induction and DNA repair are activated. The cells when exposed to cytotoxic agents, the apoptosis lead to cell death. However, the absence of repair machinery makes the cells resistant to tumor sensitizing agents and result in malignant transformation. Mismatch repair gene defects are recently identified in hematopoietic malignancies, leukemia and lymphoma cell lines. This review emphasizes the importance of MMR systems in maintaining the stem cell functioning and its therapeutic implications in the eradication of cancer stem cells and differentiated tumor cells as well. The understanding of the biological functions of mismatch repair in the stem cells and its malignant counterparts could help in developing an effective novel therapies leaving residual non-tumorigenic population of cells resulting in potential cancer cures.

The effects of MSH2 deficiency on spontaneous and radiation-induced mutation rates in the mouse germline

Mutation rates at two expanded simple tandem repeat (ESTR) loci were studied in the germline of mismatch repair deficient Msh2 knock-out mice. Spontaneous mutation rates in homozygous Msh2(-/-) males were significantly higher than those in isogenic wild-type (Msh2(+/+)) and heterozygous (Msh2(+/-)) mice. In contrast, the irradiated Msh2(-/-) mice did not show any detectable increases in their mutation rate, whereas significant ESTR mutation induction was observed in the irradiated Msh2(+/+) and Msh2(+/-) animals. Considering these data and the results of other publications, we propose that the Msh2-deficient mice possess a mutator phenotype in their germline and somatic tissues while the loss of a single Msh2 allele does not affect the stability of heterozygotes.

Significance of error-avoiding mechanisms for oxidative DNA damage in carcinogenesis

Reactive oxygen species (ROS) are produced through normal cellular metabolism, and their formation is further enhanced by exposure to ionizing radiation and various chemicals. ROS attack DNA, and the resulting oxidative DNA damage is considered to contribute to aging, carcinogenesis and neurodegeneration. Among various types of oxidative DNA damage, 8-oxo-7,8-dihydroguanine (8-oxoguanine or 8-oxoG) is the most abundant, and plays significant roles in mutagenesis because of its ability to pair with adenine as well as cytosine. Enzymatic activities that may be responsible for preventing 8-oxoG-evoked mutations were identified in mammalian cells. We have focused on the following three enzymes: MTH1, OGG1 and MUTYH. MTH1 is a mammalian ortholog of Escherichia coli MutT, which hydrolyzes 8-oxo-dGTP to its monophosphate form in nucleotide pools, thereby preventing incorporation of the mutagenic substrate into DNA. OGG1, a functional counterpart of E. coli MutM, has an 8-oxoG DNA glycosylase activity. MUTYH, a mammalian ortholog of E. coli MutY, excises an adenine paired with 8-oxoG. These three enzymes are thought to prevent mutagenesis caused by 8-oxoG in mammals. To analyze the functions of mammalian MTH1 (Mth1), OGG1 (Ogg1) and MUTYH (Mutyh) in vivo, we established mutant mice for these three enzymes by targeted mutagenesis, and investigated spontaneous tumorigenesis as well as mutagenesis. Here we discuss our recent investigation of mutagenesis and carcinogenesis in these mutant mice.

Reduced host cell reactivation of oxidative DNA damage in human cells deficient in the mismatch repair gene hMSH2

Germ line mutations in the mismatch repair (MMR) genes hMSH2 and hMLH1 account for approximately 98% of hereditary non-polyposis colorectal cancers. In addition, there is increasing evidence for an involvement of MMR gene expression in the response of cells to UV-induced skin cancer. The link between MMR and skin cancer suggests an involvement of MMR gene expression in the response of skin cells to UV-induced DNA damage. In this report, we have used two reporter gene assays to examine the role of hMSH2 and hMLH1 in the repair of oxidative DNA damage induced by UVA light and DNA damage caused by methylene blue plus visible light (MB+VL). UVA and MB+VL produce 8-hydroxyguanines in DNA that are repaired by base excision repair (BER). AdHCMVlacZ is a replication-deficient recombinant adenovirus that expresses the beta-galactosidase (beta-gal) reporter gene under the control of the human cytomegalovirus (CMV) immediate-early promoter. We show a reduced host cell reactivation for beta-gal expression of UVA-treated and MB+VL-treated AdHCMVlacZ in hMSH2-deficient LoVo human colon adenocarcinoma cells compared to their hMSH2-proficient counterpart SW480 cells, but not in hMLH1-deficient HCT116 human colon adenocarcinoma cells compared to hMLH1-proficient HCT116-chr3 cells. We have also reported previously that enhanced expression of the undamaged AdHCMVlacZ reporter gene is induced by the pre-treatment of cells with lower levels of the DNA-damaging agent and to higher expression levels in transcription-coupled repair (TCR)-deficient compared to TCR-proficient cells. Here we show that pre-treatment of cells with UVA or MB+VL enhanced expression of the undamaged reporter gene to a higher level in LoVo compared to SW480 cells but there was little or no difference in HCT116 compared to HCT116-chr3 cells. These results suggest a substantial involvement of hMSH2 but little or no involvement of hMLH1 in the repair of UVA- and MB+VL-induced oxidative DNA damage by BER.

Different DNA repair strategies to combat the threat from 8-oxoguanine

Oxidative DNA damage is one of the most common threats to genome stability and DNA repair enzymes provide protection from the effects of oxidized DNA bases. In mammalian cells, base excision repair (BER) mediated by the OGG1 and MYH DNA glycosylases prevents the accumulation of 8-oxoguanine (8-oxoG) in DNA. When steady-state levels of DNA 8-oxoG were measured in myh(-/-) and myh(-/-)/ogg1(-/-) mice, an age-dependent accumulation of the oxidized purine was found in lung and small intestine of doubly defective myh(-/-)/ogg1(-/-) mice. Since there is an increased incidence of lung and small intestinal cancer in myh(-/-)/ogg1(-/-) mice, these findings are consistent with a causal role for unrepaired oxidized DNA bases in cancer development. We previously presented in vitro evidence that mismatch repair (MMR) participates in the repair of oxidative DNA damage and msh2(-/-) mouse embryo fibroblasts also have increased steady state levels of DNA 8-oxoG. To investigate whether DNA 8-oxoG also accumulates in vivo, basal levels were measured in several organs of 4-month-old msh2(-/-) mice and their wild-type counterparts. Msh2(-/-) mice had significantly increased levels of DNA 8-oxoG in spleen, heart, liver, lung, kidney and possibly small intestine but not in bone marrow, thymus or brain. The tissue-specificity of DNA 8-oxoG accumulation in msh2(-/-) and other DNA repair defective mice suggests that DNA protection of different organs is mediated by different combinations of repair pathways.

Replication of 2-hydroxyadenine-containing DNA and recognition by human MutSalpha

2-Hydroxyadenine (2-OH-A), a product of DNA oxidation, is a potential source of mutations. We investigated how representative DNA polymerases from the A, B and Y families dealt with 2-OH-A in primer extension experiments. A template 2-OH-A reduced the rate of incorporation by DNA polymerase alpha (Pol alpha) and Klenow fragment (Kf(exo-)). Two Y family DNA polymerases, human polymerase eta (Pol eta) and the archeal Dpo4 polymerase were affected differently. Bypass by Pol eta was very inefficient whereas Dpo4 efficiently replicated 2-OH-A. Replication of a template 2-OH-A by both enzymes was mutagenic and caused base substitutions. Dpo4 additionally introduced single base deletions. Thermodynamic analysis showed that 2-OH-A forms stable base pairs with T, C and G, and to a lesser extent with A. Oligonucleotides containing 2-OH-A base pairs, including the preferred 2-OH-A:T, were recognized by the human MutSalpha mismatch repair (MMR). MutSalpha also recognized 2-OH-A located in a repeat sequence that mimics a frameshift intermediate.

Physical and functional interactions between Escherichia coli MutY glycosylase and mismatch repair protein MutS

Escherichia coli MutY and MutS increase replication fidelity by removing adenines that were misincorporated opposite 7,8-dihydro-8-oxo-deoxyguanines (8-oxoG), G, or C. MutY DNA glycosylase removes adenines from these mismatches through a short-patch base excision repair pathway and thus prevents G:C-to-T:A and A:T-to-G:C mutations. MutS binds to the mismatches and initiates the long-patch mismatch repair on daughter DNA strands. We have previously reported that the human MutY homolog (hMYH) physically and functionally interacts with the human MutS homolog, hMutSalpha (Y. Gu et al., J. Biol. Chem. 277:11135-11142, 2002). Here, we show that a similar relationship between MutY and MutS exists in E. coli. The interaction of MutY and MutS involves the Fe-S domain of MutY and the ATPase domain of MutS. MutS, in eightfold molar excess over MutY, can enhance the binding activity of MutY with an A/8-oxoG mismatch by eightfold. The MutY expression level and activity in mutS mutant strains are sixfold and twofold greater, respectively, than those for the wild-type cells. The frequency of A:T-to-G:C mutations is reduced by two- to threefold in a mutS mutY mutant compared to a mutS mutant. Our results suggest that MutY base excision repair and mismatch repair defend against the mutagenic effect of 8-oxoG lesions in a cooperative manner.

Partial deficiency of DNA-PKcs increases ionizing radiation-induced mutagenesis and telomere instability in human cells

The correct repair of DNA double-strand breaks (DSBs) is essential to maintaining the integrity of the genome. Misrepair of DSBs is detrimental to cells and organisms, leading to gene mutation, chromosomal aberration, and cancer development. Nonhomologous end-joining (NHEJ) is one of the principal rejoining processes in most higher eukaryotic cells. NHEJ is facilitated by DNA-dependent protein kinase (DNA-PK), which is composed of a catalytic subunit, DNA-PKcs, and the heterodimeric DNA binding regulatory complex Ku70/86. Null mutation of DNA-PKcs leads to immunodeficiency, chromosomal aberration, gene mutation, telomeric end-capping failure, and cancer predisposition in animals and cells. However, it is unknown whether partial deficiency of DNA-PKcs as might occur in a fraction of the population (e.g., heterozygotes), influences cellular function. Using small interfering RNA (siRNA) transfection, we established partial deficiency of DNA-PKcs in human cells, ranging from 4 to 85% of control levels. Our results reveal for the first time, that partial deficiency of DNA-PKcs leads to increased ionizing radiation (IR)-induced mutagenesis, cell killing, and telomere dysfunction. Radiation mutagenesis was increased inversely with DNA-PKcs protein level, with the most pronounced effect being observed in cells with protein levels below 50% of controls. A small but statistically significant increase in IR-induced cell killing was observed as DNA-PKcs levels decreased, over the entire range of protein levels. Frequencies of IR-induced telomere-DSB fusion was increased at levels of DNA-PKcs as low as approximately 50%, similar to what would be expected in heterozygous individuals. Taken together, our results suggest that even partial deficiency of DNA repair proteins may represent a considerable risk to genomic stability.

Transcriptome analysis of Aspergillus nidulans exposed to camptothecin-induced DNA damage

We have used an Aspergillus nidulans macroarray carrying sequences of 2,787 genes from this fungus to monitor gene expression of both wild-type and uvsB(ATR) (the homologue of the ATR gene) deletion mutant strains in a time course exposure to camptothecin (CPT). The results revealed a total of 1,512 and 1,700 genes in the wild-type and uvsB(ATR) deletion mutant strains that displayed a statistically significant difference at at least one experimental time point. We characterized six genes that have increased mRNA expression in the presence of CPT in the wild-type strain relative to the uvsB(ATR) mutant strain: fhdA (encoding a forkhead-associated domain protein), tprA (encoding a hypothetical protein that contains a tetratrico peptide repeat), mshA (encoding a MutS homologue involved in mismatch repair), phbA (encoding a prohibitin homologue), uvsC(RAD51) (the homologue of the RAD51 gene), and cshA (encoding a homologue of the excision repair protein ERCC-6 [Cockayne's syndrome protein]). The induced transcript levels of these genes in the presence of CPT require uvsB(ATR). These genes were deleted, and surprisingly, only the DeltauvsC mutant strain was sensitive to CPT; however, the others displayed sensitivity to a range of DNA-damaging and oxidative stress agents. These results indicate that the selected genes when inactivated display very complex and heterogeneous sensitivity behavior during growth in the presence of agents that directly or indirectly cause DNA damage. Moreover, with the exception of UvsC, deletion of each of these genes partially suppressed the sensitivity of the DeltauvsB strain to menadione and paraquat. Our results provide the first insight into the overall complexity of the response to DNA damage in filamentous fungi and suggest that multiple pathways may act in parallel to mediate DNA repair.

DNA mismatch repair mediates protection from mutagenesis induced by short-wave ultraviolet light

To investigate involvement of DNA mismatch repair in the response to short-wave ultraviolet (UVC) light, we compared UVC-induced mutant frequencies and mutational spectra at the Hprt gene between wild type and mismatch-repair-deficient mouse embryonic stem (ES) cells. Whereas mismatch repair gene status did not significantly affect survival of these cells after UVC irradiation, UVC induced substantially more mutations in ES cells that lack the MutSalpha mismatch-recognizing heterodimer than in wild type ES cells. The global UVC-induced mutational spectra at Hprt and the distribution of most spectral mutational hotspots were found to be similar in mismatch-repair-deficient and wild type cells. However, at one predominant spectral hot spot for mutagenesis in wild type cells, the UVC-induced mutation frequency was not affected by the mismatch repair status. Together these data reveal a major role of mismatch repair in controlling mutagenesis induced by UVC light and may suggest the sequence context-dependent direct mismatch repair of misincorporations opposite UVC-induced pyrimidine dimers.

The Helicobacter pylori MutS protein confers protection from oxidative DNA damage

The human gastric pathogenic bacterium Helicobacter pylori lacks a MutSLH-like DNA mismatch repair system. Here, we have investigated the functional roles of a mutS homologue found in H. pylori, and show that it plays an important physiological role in repairing oxidative DNA damage. H. pylori mutS mutants are more sensitive than wild-type cells to oxidative stress induced by agents such as H2O2, paraquat or oxygen. Exposure of mutS cells to oxidative stress results in a significant ( approximately 10-fold) elevation of mutagenesis. Strikingly, most mutations in mutS cells under oxidative stress condition are G:C to T:A transversions, a signature of 8-oxoguanine (8-oxoG). Purified H. pylori MutS protein binds with a high specific affinity to double-stranded DNA (dsDNA) containing 8-oxoG as well as to DNA Holliday junction structures, but only weakly to dsDNA containing a G:A mismatch. Under oxidative stress conditions, mutS cells accumulate higher levels (approximately threefold) of 8-oxoG DNA lesions than wild-type cells. Finally, we observe that mutS mutant cells have reduced colonization capacity in comparison to wild-type cells in a mouse infection model.

Role of the mismatch repair system and p53 in the clastogenicity and cytotoxicity induced by bleomycin

The mismatch repair (MMR) system and p53 protein play a pivotal role in maintaining genomic stability and modulate cell chemosensitivity. Aim of this study was to examine the effects of either MMR-deficiency or p53 inactivation, or both, on cellular responses to bleomycin. The MMR-deficient colon carcinoma cell line HCT116 and its MMR-proficient subline HCT116/3-6, both expressing wild-type p53, were transfected with an expression vector encoding a dominant-negative p53 mutant, or with the empty vector. Four transfected clones, having the following phenotypes, MMR-proficient/p53 wild-type, MMR-proficient/p53 mutant, MMR-deficient/p53 wild-type, MMR-deficient/p53 mutant, were subjected to treatment with bleomycin. Loss of MMR function alone was associated with increased resistance to apoptosis, chromosomal damage and inhibition of colony formation caused by bleomycin. Loss of p53 alone resulted in abrogation of G1 arrest and increased sensitivity to apoptosis and chromosomal damage induced by the drug, but did not affect clonogenic survival after bleomycin treatment. Disabling both p53 and MMR function led to abrogation of G1 arrest and to a moderate impairment of drug-induced apoptosis. Chromosomal damage was reduced in the MMR-deficient/p53 mutant clone with respect to the MMR-proficient/p53 wild-type one, when evaluated 48 h after bleomycin treatment, but was comparable in both clones 96 h after drug exposure. Clonogenic survival of the MMR-deficient/p53 mutant clone was similar to that of the MMR-deficient/p53 wild-type one. The effects of MMR-deficiency on cellular responses to bleomycin were confirmed using the MMR-proficient lymphoblastoid cell line TK6 and its MMR-deficient subline MT1, both expressing wild-type p53. In conclusion, our data show that loss of MMR and p53 function exerts opposite and independent effects on apoptosis and chromosomal damage induced by bleomycin. Moreover, inactivation of MMR confers resistance to the cytotoxic activity of the anticancer agent in cells expressing either wild-type or mutant p53.

Lipid peroxidation and renal cell carcinoma: further supportive evidence and new mechanistic insights

We have recently proposed lipid peroxidation as a unifying mechanistic pathway by which several seemingly unrelated risk/protective factors (obesity, hypertension, diabetes, smoking, oophorectomy/hysterectomy, parity, antioxidants) affect renal cell carcinoma development. In experimental studies, increased lipid peroxidation is a principal mechanistic pathway in renal carcinogenesis induced by different chemicals. In this communication, we provide additional lines of evidence that further support a role for lipid peroxidation on renal cell cancer development. (1) Lipid peroxidation may explain the role of other risk (analgesic use, pre-eclampsia) or protective (alcohol intake, oral contraceptives) factors for renal cell carcinoma. (2) Additional experimental evidence supports lipid peroxidation as an important mechanism in renal carcinogenesis, and (3) Existing evidence support a cross-talk between the lipid peroxidation pathway and other pathways that are relevant to renal carcinogenesis, such as apoptosis, VHL, and possibly other pathways.

MSH2 is essential for the preservation of genome integrity and prevents homeologous recombination in the moss Physcomitrella patens

MSH2 is a central component of the mismatch repair pathway that targets mismatches arising during DNA replication, homologous recombination (HR) and in response to genotoxic stresses. Here, we describe the function of MSH2 in the moss Physcomitrella patens, as deciphered by the analysis of loss of function mutants. Ppmsh2 mutants display pleiotropic growth and developmental defects, which reflect genomic instability. Based on loss of function of the APT gene, we estimated this mutator phenotype to be at least 130 times higher in the mutants than in wild type. We also found that MSH2 is involved in some but not all the moss responses to genotoxic stresses we tested. Indeed, the Ppmsh2 mutants were more tolerant to cisplatin and show higher sensitivity to UV-B radiations. PpMSH2 gene involvement in HR was studied by assessing gene targeting (GT) efficiency with homologous and homeologous sequences. GT efficiency with homologous sequences was slightly decreased in the Ppmsh2 mutant compared with wild type. Strikingly GT efficiency with homeologous sequences decreased proportionally to sequence divergence in the wild type whereas it remained unaffected in the mutants. Those results demonstrate the role of PpMSH2 in the maintenance of genome integrity and in homologous and homeologous recombination.

Mechanisms of therapy-related carcinogenesis

Therapy-related cancers, defined as second primary cancers that arise as a consequence of chemotherapy and/or radiotherapy, are unusual in that they have a well-defined aetiology. Knowledge of the specific nature of the initiating exposure and exactly when it occurred has made it easier to identify crucial genetic events and to model these in vitro and in vivo. As such, the study of therapy-related cancers has led to the elucidation of discrete mechanisms of carcinogenesis, including DNA double-strand-break-induced gene translocation and genomic instability conferred by loss of DNA repair. Unsurprisingly, some of these mechanisms seem to operate in the development of sporadic cancers.

No Major Role for 7,8-Dihydro-8-oxoguanine in Ultraviolet Light-Induced Mutagenesis

Oxidative DNA damage, in particular 7,8-dihydro-8-oxoguanine (8-oxoG), has been suggested to mediate mutation formation and malignant transformation after exposure of the skin to long-wave ultraviolet (UVA) light. It is processed primarily by the base excision repair (BER) pathway. The initial step of BER is the removal of the damaged base by a damage-specific DNA-glycosylase, which is 8-oxoG DNA glycosylase (OGG1) for 8-oxoG. To study the contribution of 8-oxoG to UVA-light mutagenesis, we compared UVA- and UVB-light-induced mutation frequencies in mouse embryonal fibroblasts from OGG1 knockout mice and their OGG1-intact littermates using the ouabain mutagenesis assay. After irradiation with various doses of UVA or UVB radiation, mutations in the Na,K-ATPase gene of single cells were detected by testing for colony-forming ability in a selective medium. OGG1-/- cells did not exhibit an increased frequency of UV-light-induced mutations compared to OGG1+/+ cells after exposure to either UVA or UVB radiation. This indicates that 8-oxoG, which is processed by OGG1, does not contribute significantly to either UVA- or UVB-light-induced mutagenesis.

8-oxoguanine incorporation into DNA repeats in vitro and mismatch recognition by MutSalpha

DNA 8-oxoguanine (8-oxoG) causes transversions and is also implicated in frameshifts. We previously identified the dNTP pool as a likely source of mutagenic DNA 8-oxoG and demonstrated that DNA mismatch repair prevented oxidation-related frameshifts in mononucleotide repeats. Here, we show that both Klenow fragment and DNA polymerase α can utilize 8-oxodGTP and incorporate the oxidized purine into model frameshift targets. Both polymerases incorporated 8-oxodGMP opposite C and A in repetitive DNA sequences and efficiently extended a terminal 8-oxoG. The human MutSα mismatch repair factor recognized DNA 8-oxoG efficiently in some contexts that resembled frameshift intermediates in the same C or A repeats. DNA 8-oxoG in other slipped/mispaired structures in the same repeats adopted configurations that prevented recognition by MutSα and by the OGG1 DNA glycosylase thereby rendering it invisible to DNA repair. These findings are consistent with a contribution of oxidative DNA damage to frameshifts. They also suggest how mismatch repair might reduce the burden of DNA 8-oxoG and prevent frameshift formation.

Tumor suppressor genetics

The observation that mutations in tumor suppressor genes can have haploinsufficient, as well as gain of function and dominant negative, phenotypes has caused a reevaluation of the 'two-hit' model of tumor suppressor inactivation. Here we examine the history of haploinsufficiency and tumor suppressors in order to understand the origin of the 'two-hit' dogma. The two-hit model of tumor suppressor gene inactivation was derived from mathematical modeling of cancer incidence. Subsequent interpretations implied that tumor suppressors were recessive, requiring mutations in both alleles. This model has provided a useful conceptual framework for three decades of research on the genetics and biology of tumor suppressor genes. Recently it has become clear that mutations in tumor suppressor genes are not always completely recessive. Haploinsufficiency occurs when one allele is insufficient to confer the full functionality produced from two wild-type alleles. Haploinsufficiency, however, is not an absolute property. It can be partial or complete and can vary depending on tissue type, other epistatic interactions, and environmental factors. In addition to simple quantitative differences (one allele versus two alleles), gene mutations can have qualitative differences, creating gain of function or dominant negative effects that can be difficult to distinguish from dosage-dependence. Like mutations in many other genes, tumor suppressor gene mutations can be haploinsufficient, dominant negative or gain of function in addition to recessive. Thus, under certain circumstances, one hit may be sufficient for inactivation. In addition, the phenotypic penetrance of these mutations can vary depending on the nature of the mutation itself, the genetic background, the tissue type, environmental factors and other variables. Incorporating these new findings into existing models of the clonal evolution will be a challenge for the future.

Spontaneous and mutagen-induced loss of DNA mismatch repair in Msh2-heterozygous mammalian cells

We have developed a simple procedure that enables the efficient selection of cells that are deficient for DNA mismatch repair (MMR). This selection procedure was used to investigate the frequency of fortuitous MMR-deficient cells in a mouse embryonic stem cell line, heterozygous for the MMR gene Msh2. We found a surprisingly high frequency (3 x 10(-4)) of Msh2-deficient cells. The wild type Msh2 allele was almost invariably lost by loss of heterozygosity. Single treatments with the genotoxic agents ethylnitrosourea, UVC light and mitomycin C resulted in a further increase of the number of Msh2-/- cells in the heterozygous cell line. This increase was not only due to induced loss of the wild type allele but also to a selective growth advantage of preexisting Msh2-/- cells to ethylnitrosourea and UVC. Mitomycin C, in contrast to ethylnitrosourea and UVC, uniquely induced loss of heterozygosity at Msh2. These mechanistically different ways of loss of the wild type Msh2 allele reflect the different repair pathways processing these damages. Heterozygous germ line defects in one of the MMR genes underlie the hereditary nonpolyposis colorectal cancer (HNPCC) syndrome. Based on the results described here we hypothesize that mutagen-induced loss of MMR in the intestine of these patients contributes to the tissue specificity of carcinogenesis in HNPCC patients.

Preservation of DNA integrity and neuronal degeneration

The mismatch repair system (MMR) is an important member of the DNA checkpoint, that includes a number of protein deputed to control genomic stability through cell cycle arrest, DNA repair, and apoptosis. Here we summarize some recent data from our and other groups underlining the contribution to neurodegeneration of MSH2, perhaps the most relevant component of the MMR system. These data suggest that this protein participates not only in the cancer prevention machinery for the body but also in neurodegenerative processes.

Mismatch repair and DNA damage signalling

Postreplicative mismatch repair (MMR) increases the fidelity of DNA replication by up to three orders of magnitude, through correcting DNA polymerase errors that escaped proofreading. MMR also controls homologous recombination (HR) by aborting strand exchange between divergent DNA sequences. In recent years, MMR has also been implicated in the response of mammalian cells to DNA damaging agents. Thus, MMR-deficient cells were shown to be around 100-fold more resistant to killing by methylating agents of the S(N)1type than cells with functional MMR. In the case of cisplatin, the sensitivity difference was lower, typically two- to three-fold, but was observed in all matched MMR-proficient and -deficient cell pairs. More controversial is the role of MMR in cellular response to other DNA damaging agents, such as ionizing radiation (IR), topoisomerase poisons, antimetabolites, UV radiation and DNA intercalators. The MMR-dependent DNA damage signalling pathways activated by the above agents are also ill-defined. To date, signalling cascades involving the Ataxia telangiectasia mutated (ATM), ATM- and Rad3-related (ATR), as well as the stress-activated kinases JNK/SAPK and p38alpha have been linked with methylating agent and 6-thioguanine (TG) treatments, while cisplatin damage was reported to activate the c-Abl and JNK/SAPK kinases in MMR-dependent manner. MMR defects are found in several different cancer types, both familiar and sporadic, and it is possible that the involvement of the MMR system in DNA damage signalling play an important role in transformation. The scope of this article is to provide a brief overview of the recent literature on this subject and to raise questions that could be addressed in future studies.

Diet, lifestyle, and genomic instability in the North Carolina Colon Cancer Study

OBJECTIVE

Microsatellite instability (MSI) is one form of genomic instability that occurs in 10% to 20% of sporadic colon tumors and almost all hereditary nonpolyposis colon cancers. However, little is known about how environmental factors (e.g., diet) may influence MSI in sporadic colon cancer.

METHODS

We used data from a population-based case-control study in North Carolina (486 colon cancer cases and 1,048 controls) to examine associations of diet (total energy, macronutrients, micronutrients, and food groups) with MSI. In-person interviews elicited information on potential colon cancer risk factors, and a previously validated food frequency questionnaire adapted to include regional foods was used to assess diet over the year before diagnosis or interview date. MSI was classified as MSI-high (MSI-H) and MSI-low or microsatellite stable (MSI-L/MSS). Multivariate logistic regression models estimated energy-adjusted and non-energy-adjusted odds ratios (OR).

RESULTS

Ten percent of the cases (n = 49) had MSI-H tumors (29% African American). The strongest associations between diet and MSI were observed in case-control comparisons: there was a robust inverse association between MSI-H status and beta-carotene [OR, 0.4; 95% confidence interval (95% CI), 0.2-0.9] and positive associations with energy-adjusted refined carbohydrates (OR, 2.2; 95% CI, 0.9-5.4) and non-energy-adjusted read meat intake (OR, 2.0; 95% CI, 0.9-4.2). Compared with controls, MSI-L/MSS tumors were statistically significantly associated with energy-adjusted vitamin C, vitamin E, calcium, dietary fiber, and dark green vegetables and positively associated with total energy intake (all Ps for trend < 0.05). In case-case comparisons, no dietary factors were significantly differently related to MSI-H compared with MSI-L/MSS tumors.

CONCLUSION

Refined carbohydrate and red meat consumption may promote development of MSI-H tumors, whereas beta-carotene may be associated with lower risk.

Functional characterization of two human MutY homolog (hMYH) missense mutations (R227W and V232F) that lie within the putative hMSH6 binding domain and are associated with hMYH polyposis

The base excision repair DNA glycosylase MutY homolog (MYH) is responsible for removing adenines misincorporated into DNA opposite guanine or 7,8-dihydro-8-oxo-guanine (8-oxoG), thereby preventing G:C to T:A mutations. Biallelic germline mutations in the human MYH gene predispose individuals to multiple colorectal adenomas and carcinoma. We have recently demonstrated that hMYH interacts with the mismatch repair protein hMSH6, and that the hMSH2/hMSH6 (hMutSalpha) heterodimer stimulates hMYH activity. Here, we characterize the functional effect of two missense mutations (R227W and V232F) associated with hMYH polyposis that lie within, or adjacent to, the putative hMSH6 binding domain. Neither missense mutation affects the physical interaction between hMYH and hMSH6. However, hMYH(R227W) has a severe defect in A/8-oxoG binding and glycosylase activities, while hMYH(V232F) has reduced A/8-oxoG binding and glycosylase activities. The glycosylase activity of the V232F mutant can be partially stimulated by hMutSalpha but cannot be restored to the wild-type level. Both mutants also fail to complement mutY-deficiency in Escherichia coli. These data define the pathogenic mechanisms underlying two further hMYH polyposis-associated mutations.

Differential Radiosensitization in DNA Mismatch Repair-Proficient and -Deficient Human Colon Cancer Xenografts with 5-Iodo-2-pyrimidinone-2′-deoxyribose

PURPOSE

5-iodo-2-pyrimidinone-2'-deoxyribose (IPdR) is a pyrimidinone nucleoside prodrug of 5-iododeoxyuridine (IUdR) under investigation as an orally administered radiosensitizer. We previously reported that the mismatch repair (MMR) proteins (both hMSH2 and hMLH1) impact on the extent (percentage) of IUdR-DNA incorporation and subsequent in vitro IUdR-mediated radiosensitization in human tumor cell lines. In this study, we used oral IPdR to assess in vivo radiosensitization in MMR-proficient (MMR+) and -deficient (MMR-) human colon cancer xenografts.

EXPERIMENTAL DESIGN

We tested whether oral IPdR treatment (1 g/kg/d for 14 days) can result in differential IUdR incorporation in tumor cell DNA and subsequent radiosensitization after a short course (every day for 4 days) of fractionated radiation therapy, by using athymic nude mice with an isogenic pair of human colon cancer xenografts, HCT116 (MMR-, hMLH1-) and HCT116/3-6 (MMR+, hMLH1+). A tumor regrowth assay was used to assess radiosensitization. Systemic toxicity was assessed by daily body weights and by percentage of IUdR-DNA incorporation in normal bone marrow and intestine.

RESULTS

After a 14-day once-daily IPdR treatment by gastric gavage, significantly higher IUdR-DNA incorporation was found in HCT116 (MMR-) tumor xenografts compared with HCT116/3-6 (MMR+) tumor xenografts. Using a tumor regrowth assay after the 14-day drug treatment and a 4-day radiation therapy course (days 11-14 of IPdR), we found substantial radiosensitization in both HCT116 and HCT116/3-6 tumor xenografts. However, the sensitizer enhancement ratio (SER) was substantially higher in HCT116 (MMR-) tumor xenografts (1.48 at 2 Gy per fraction, 1.41 at 4 Gy per fraction), compared with HCT116/3-6 (MMR+) tumor xenografts (1.21 at 2 Gy per fraction, 1.20 at 4 Gy per fraction). No substantial systemic toxicity was found in the treatment groups.

CONCLUSIONS

These results suggest that IPdR-mediated radiosensitization can be an effective in vivo approach to treat "drug-resistant" MMR-deficient tumors as well as MMR-proficient tumors.

An examination of radiation hormesis mechanisms using a multistage carcinogenesis model

A multistage cancer model that describes the putative rate-limiting steps in carcinogenesis is developed and used to investigate the potential impact on cumulative lung cancer incidence of the hormesis mechanisms suggested by Feinendegen and Pollycove. In the model, radiation and endogenous processes damage the DNA of target cells in the lung. Some fraction of the misrepaired or unrepaired DNA damage induces genomic instability and, ultimately, leads to the accumulation of malignant cells. The model explicitly accounts for cell birth and death processes, the clonal expansion of initiated cells, malignant conversion, and a lag period for tumor formation. Radioprotective mechanisms are incorporated into the model by postulating dose and dose-rate-dependent radical scavenging. The accuracy of DNA damage repair also depends on dose and dose rate. As currently formulated, the model is most applicable to low-linear-energy-transfer (LET) radiation delivered at low dose rates. Sensitivity studies are conducted to identify critical model inputs and to help define the shapes of the cumulative lung cancer incidence curves that may arise when dose and dose-rate-dependent cellular defense mechanisms are incorporated into a multistage cancer model. For lung cancer, both linear no-threshold (LNT-), and non-LNT-shaped responses can be obtained. If experiments demonstrate that the effects of DNA damage repair and radical scavenging are enhanced at least three-fold under low-dose conditions, our studies would support the existence of U-shaped responses. The overall fidelity of the DNA damage repair process may have a large impact on the cumulative incidence of lung cancer. The reported studies also highlight the need to know whether or not (or to what extent) multiply damaged DNA sites are formed by endogenous processes. Model inputs that give rise to U-shaped responses are consistent with an effective cumulative lung cancer incidence threshold that may be as high as 300 mGy (4 mGy per year for 75 years) for low-LET radiation.

Oxidative DNA damage and disease: induction, repair and significance

The generation of reactive oxygen species may be both beneficial to cells, performing a function in inter- and intracellular signalling, and detrimental, modifying cellular biomolecules, accumulation of which has been associated with numerous diseases. Of the molecules subject to oxidative modification, DNA has received the greatest attention, with biomarkers of exposure and effect closest to validation. Despite nearly a quarter of a century of study, and a large number of base- and sugar-derived DNA lesions having been identified, the majority of studies have focussed upon the guanine modification, 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-OH-dG). For the most part, the biological significance of other lesions has not, as yet, been investigated. In contrast, the description and characterisation of enzyme systems responsible for repairing oxidative DNA base damage is growing rapidly, being the subject of intense study. However, there remain notable gaps in our knowledge of which repair proteins remove which lesions, plus, as more lesions identified, new processes/substrates need to be determined. There are many reports describing elevated levels of oxidatively modified DNA lesions, in various biological matrices, in a plethora of diseases; however, for the majority of these the association could merely be coincidental, and more detailed studies are required. Nevertheless, even based simply upon reports of studies investigating the potential role of 8-OH-dG in disease, the weight of evidence strongly suggests a link between such damage and the pathogenesis of disease. However, exact roles remain to be elucidated.

Strand-specific processing of 8-oxoguanine by the human mismatch repair pathway: inefficient removal of 8-oxoguanine paired with adenine or cytosine

Genomic DNA and its precursors are susceptible to oxidation during aerobic cellular metabolism, and at least five distinct repair activities target a single common lesion, 7,8-dihydro-8-oxoguanine (8-oxoG). The human mismatch repair (MMR) pathway, which has been implicated in an apoptotic response to covalent DNA damage, is likely to encounter 8-oxoG in both the parental and daughter strand during replication. Here, we show that lesions containing 8-oxoG paired with adenine or cytosine, which are most likely to arise during replication, are not efficiently processed by the mismatch repair system. Lesions containing 8-oxoG paired with thymine or guanine, which are unlikely to arise, are excised in an MSH2/MSH6-dependent manner as effectively as the corresponding mismatches when placed in a context that reflects the daughter strand during replication. Using a newly developed assay based on methylation sensitivity, we characterized strand-excision events opposite 8-oxoG situated to reflect placement in the parental strand. Lesions that efficiently trigger strand excision and resynthesis (8-oxoG paired with thymine or guanine) result in adenine or cytosine insertion opposite 8-oxoG. These latter pairings are poor substrates for further action by mismatch repair, but precursors for alternative pathways with non-mutagenic outcomes. We suggest that the lesions most likely to be encountered by the human mismatch repair pathway during replication, 8-oxoG.A or 8-oxoG.C, are likely to escape processing in either strand by this system. Taken together, these data suggest that the human mismatch repair pathway is not a major contributor to removal of misincorporated 8-oxoG, nor is it likely to trigger repeated attempts at lesion processing.

Fission yeast Uve1 and Apn2 function in distinct oxidative damage repair pathways in vivo

In Schizosaccharomyces pombe, the endonuclease Uve1 functions as the first step in an alternate UV photo-product repair pathway that is distinct from nucleotide excision repair (NER). Based upon the broad substrate specificity of Uve1 in vitro, and the observation that Uve1 mutants accumulate spontaneous mutations at an elevated rate in vivo, we and others have hypothesized that this protein might have a function in a mutation avoidance pathway other than UV photo-product repair. We show here that fission yeast Uve1 also functions in oxidative damage repair in vivo. We have determined the spectrum of spontaneous mutations that arise in uve1 null (uve1 degrees ) cells and have observed that both G-->T(C-->A) and T-->G(A-->C) transversions occur at an increased rate relative to wildtype cells. These mutations are indicative of unrepaired oxidative DNA damage and are very similar to the mutation spectrum observed in 8-oxoguanine glycosylase (OGG1) mutants in Saccharomyces cerevisiae. We have generated an apn2 null (apn2 degrees ) strain and shown that it is mildly sensitive to H(2)O(2). Furthermore we have also shown that apn2 degrees cells have an elevated rate of spontaneous mutation that is similar to uve1 degrees. The phenotype of apn2 degrees uve1 degrees double mutants indicates that these genes define distinct spontaneous mutation avoidance pathways. While uve1 degrees cells show only a modest sensitivity to the oxidizing agent hydrogen peroxide (H(2)O(2)), both uve1 degrees and apn2 degrees cells also display a marked increased in mutation rate following exposure to H(2)O(2) doses. Collectively these data demonstrate that Uve1 is a component of multiple alternate repair pathways in fission yeast and suggest a possible role for Uve1 in a general alternate incision repair pathway in eukaryotes.

Haploinsufficiency for tumour suppressor genes: when you don't need to go all the way

Classical tumour suppressor genes are thought to require mutation or loss of both alleles to facilitate tumour progression. However, it has become clear over the last few years that for some genes, haploinsufficiency, which is loss of only one allele, may contribute to carcinogenesis. These effects can either be directly attributable to the reduction in gene dosage or may act in concert with other oncogenic or haploinsufficient events. Here we describe the genes that undergo this phenomenon and discuss possible mechanisms that allow haploinsufficiency to display a phenotype and facilitate the pathogenesis of cancer.

Folate deficiency, mismatch repair-dependent apoptosis, and human disease

The vitamin that is most commonly deficient in the American diet is folate. Severe folate deficiency in humans is known to cause megaloblastic anemia and developmental defects, and is associated with an increased incidence of several forms of human cancer. Although the exact mechanisms by which this vitamin deficiency may cause these diseases are not known at the present time, recent work has shown that folate deficiency also causes genomic instability and programmed cell death (or apoptosis). Additionally, it is known that the DNA mismatch repair pathway mediates folate deficiency-induced apoptosis. This review will first describe work suggesting that folate deficiency causes genomic instability and apoptosis, then discuss possible mechanisms by which the mismatch repair pathway could trigger folate deficiency-induced apoptosis, which has either protective or destructive effects on tissue.

A subgroup of microsatellite stable colorectal cancers has elevated mutation rates and different responses to alkylating and oxidising agents

An early step in the carcinogenesis of hereditary non-polyposis colorectal cancer (HNPCC) and some sporadic colorectal cancers (CRCs) is the acquisition of a ‘mutator phenotype’ resulting from defects in DNA mismatch repair (MMR) genes, which normally maintain genomic stability. This mutator phenotype causes an approximately 100–1000-fold increase in base substitutions and small insertion/deletion mutations thereby driving carcinogenesis. It also causes genome-wide microsatellite instability (MSI) due to the inability to repair mutations within these small, hard to replicate, repetitive DNA elements. In contrast, less is known about the role of mutator phenotypes in microsatellite stable (MSS) CRC. In this report, we have measured the mutation rates in 11 MSS CRC cell lines to obtain an estimate of the prevalence of mutator phenotypes in MSS carcinogenesis. Of the 11 cell lines, three of them (27%) possess spontaneous hypoxanthine phosphoribosyltransferase mutation rates approximately 10–100-fold above background. When challenged with alkylating and oxidising agents, the degree of survival and apoptotic responses are different, indicating that these cell lines may represent more than one mutator phenotype. These data demonstrate that a significant portion of MSS CRC cell lines has increased mutation rates and that this may play a role in MSS CRC carcinogenesis.

Deficiencies in mouse Myh and Ogg1 result in tumor predisposition and G to T mutations in codon 12 of the K-ras oncogene in lung tumors

Oxidative DNA damage is unavoidably and continuously generated by oxidant byproducts of normal cellular metabolism. The DNA damage repair genes, mutY and mutM, prevent G to T mutations caused by reactive oxygen species in Escherichia coli, but it has remained debatable whether deficiencies in their mammalian counterparts, Myh and Ogg1, are directly involved in tumorigenesis. Here, we demonstrate that deficiencies in Myh and Ogg1 predispose 65.7% of mice to tumors, predominantly lung and ovarian tumors, and lymphomas. Remarkably, subsequent analyses identified G to T mutations in 75% of the lung tumors at an activating hot spot, codon 12, of the K-ras oncogene, but none in their adjacent normal tissues. Moreover, malignant lung tumors were increased with combined heterozygosity of Msh2, a mismatch repair gene involved in oxidative DNA damage repair as well. Thus, oxidative DNA damage appears to play a causal role in tumorigenesis, and codon 12 of K-ras is likely to be an important downstream target in lung tumorigenesis. The multiple oxidative repair genes are required to prevent mutagenesis and tumor formation. The mice described here provide a valuable model for studying the mechanisms of oxidative DNA damage in tumorigenesis and investigating preventive or therapeutic approaches.

Urinary 8-OHdG: a marker of oxidative stress to DNA and a risk factor for cancer, atherosclerosis and diabetics

Reactive oxygen species (ROS) produced either endogenously or exogenously can attack lipid, protein and nucleic acid simultaneously in the living cells. In nuclear and mitochondrial DNA, 8-hydroxydeoxyguanosine (8-OHdG), an oxidized nucleoside of DNA, is the most frequently detected and studied DNA lesion. Upon DNA repair, 8-OHdG is excreted in the urine. Numerous evidences have indicated that urinary 8-OHdG not only is a biomarker of generalized, cellular oxidative stress but might also be a risk factor for cancer, atherosclerosis and diabetes. For example, elevated level of urinary 8-OHdG has been detected in patients with various cancers. In human atherosclerotic plaques, there were increased amounts of oxidatively modified DNA and 8-OHdG. Elevated urinary 8-OHdG and leukocyte DNA were also detected in diabetic patients with hyperglycemia, and the level of urinary 8-OHdG in diabetes correlated with the severity of diabetic nephropathy and retinopathy. We have discussed various methods for determining 8-OHdG in the tissue and urine, including HPLC with and without extraction, and ELISA. Using the ELISA we developed, we found that the normal range of urinary 8-OHdG for females was 43.9 +/- 42.1 ng/mg creatinine and 29.6 +/- 24.5 ng/mg creatinine for males, respectively. We found that the normal value between females and males is significantly different (p < 0.001).

The oxidized deoxynucleoside triphosphate pool is a significant contributor to genetic instability in mismatch repair-deficient cells

Oxidation is a common form of DNA damage to which purines are particularly susceptible. We previously reported that oxidized dGTP is potentially an important source of DNA 8-oxodGMP in mammalian cells and that the incorporated lesions are removed by DNA mismatch repair (MMR). MMR deficiency is associated with a mutator phenotype and widespread microsatellite instability (MSI). Here, we identify oxidized deoxynucleoside triphosphates (dNTPs) as an important cofactor in this genetic instability. The high spontaneous hprt mutation rate of MMR-defective msh2(-/-) mouse embryonic fibroblasts was attenuated by expression of the hMTH1 protein, which degrades oxidized purine dNTPs. A high level of hMTH1 abolished their mutator phenotype and restored the hprt mutation rate to normal. Molecular analysis of hprt mutants showed that the presence of hMTH1 reduced the incidence of mutations in all classes, including frameshifts, and also implicated incorporated 2-oxodAMP in the mutator phenotype. In hMSH6-deficient DLD-1 human colorectal carcinoma cells, overexpression of hMTH1 markedly attenuated the spontaneous mutation rate and reduced MSI. It also reduced the incidence of -G and -A frameshifts in the hMLH1-defective DU145 human prostatic cancer cell line. Our findings indicate that incorporation of oxidized purines from the dNTP pool may contribute significantly to the extreme genetic instability of MMR-defective human tumors.

Role of DNA mismatch repair in apoptotic responses to therapeutic agents

Deficiencies in DNA mismatch repair (MMR) have been found in both hereditary cancer (i.e., hereditary nonpolyposis colorectal cancer) and sporadic cancers of various tissues. In addition to its primary roles in the correction of DNA replication errors and suppression of recombination, research in the last 10 years has shown that MMR is involved in many other processes, such as interaction with other DNA repair pathways, cell cycle checkpoint regulation, and apoptosis. Indeed, a cell's MMR status can influence its response to a wide variety of chemotherapeutic agents, such as temozolomide (and many other methylating agents), 6-thioguanine, cisplatin, ionizing radiation, etoposide, and 5-fluorouracil. For this reason, identification of a tumor's MMR deficiency (as indicated by the presence of microsatellite instability) is being utilized more and more as a prognostic indicator in the clinic. Here, we describe the basic mechanisms of MMR and apoptosis and investigate the literature examining the influence of MMR status on the apoptotic response following treatment with various therapeutic agents. Furthermore, using isogenic MMR-deficient (HCT116) and MMR-proficient (HCT116 3-6) cells, we demonstrate that there is no enhanced apoptosis in MMR-proficient cells following treatment with 5-fluoro-2'-deoxyuridine. In fact, apoptosis accounts for only a small portion of the induced cell death response.

DNA mismatch repair protein Msh6 is required for optimal levels of ultraviolet-B-induced apoptosis in primary mouse fibroblasts

Recent data support a role for DNA mismatch repair in the cellular response to some forms of exogenous DNA damage beyond that of DNA repair; cells with defective DNA mismatch repair have partial or complete failure to undergo apoptosis and/or G2M arrest following specific types of damage. We propose that the DNA mismatch repair Msh2/Msh6 heterodimer, responsible for the detection of DNA damage, promotes apoptosis in normal cells, thus protecting mammals from ultraviolet-induced malignant transformation. Using primary mouse embryonic fibroblasts derived from Msh6+/+ and Msh6-/- mice, we compare the response of DNA-mismatch repair-proficient and -deficient cells to ultraviolet B radiation. In the wild-type mouse embryonic fibroblasts, ultraviolet-B-induced increases in Msh6 protein levels were not dependent on p53. Msh6-/- mouse embryonic fibroblasts were significantly less sensitive to the cytotoxic effects of ultraviolet B radiation. Further comparison of the Msh6+/+ and Msh6-/- mouse embryonic fibroblasts revealed that Msh6-/- mouse embryonic fibroblasts undergo significantly less apoptosis following ultraviolet B irradiation, thus indicating that ultraviolet-B-induced apoptosis is partially Msh6 dependent. These data support a role for Msh6 in protective cellular responses of primary cells to ultraviolet-B-induced mutagenesis and, hence, the prevention of skin cancer.

hMSH2 expression is driven by AP1-dependent regulation through phorbol-ester exposure

Mammalian mismatch repair (MMR) plays a prominent role in genomic stability and toxicity induced by some DNA damaging agents. Advance in the appreciation of regulation mechanisms of the key MMR protein hMSH2 would certainly lead to valuable information on its role and to a better understanding of MMR system dysfunctions with respect to their consequences in cells. We have previously reported that, in myeloid leukemic U937 cell line, the expression of hMSH2 MMR protein is regulated by protein kinase C (PKC) activity. Here we show that the increase of protein level following PKC activation by phorbol ester (TPA) treatment parallels that of hMSH2 mRNA. Our results support the view that the hMSH2 gene is prone to transcriptional regulation upon TPA induction, and that AP-1 is a factor implicated in the transactivation. When losing the AP-1-dependent hMSH2 promoter activity, either by mutating the AP-1 binding sites of the hMSH2 promoter or by using a dominant negative c-Jun factor, the hMSH2 overexpression induced by TPA is abolished both in vitro and in vivo. Thus the control of hMSH2 expression by PKC appears to be dependent, at least partially, on an up-regulation mediated by AP-1 transactivation.

Molecular markers in clinical radiation oncology

Radiation therapy plays a critical role in the management of a majority of patients diagnosed with cancer. Identification of factors that help predict which patients are at risk for relapse within the irradiated field remains an active area of investigation. Although conventional clinical and pathologic factors have been helpful in identifying risk and guiding clinical decision-making for both local and systemic management, there is clearly a need to identify additional prognostic markers, which can aid in refining our treatment strategies and improving outcomes. A substantial amount of research efforts have been devoted to identifying molecular markers for prognostic and therapeutic strategies. The recent emergence of a powerful armamentarium of molecular tools has resulted in rapid expansion of our fund of knowledge and understanding of the molecular biology underlying tumor behavior and response. While a majority of these efforts have been focused on risk factors for metastatic disease and survival, there is a rapidly growing body of literature focused on molecular factors associated with radiation resistance and locoregional failure. In this review, we summarize recent advances and the available literature evaluating molecular markers as they relate to radiation sensitivity of solid tumors. Literature regarding the potential application of expression of genes related to apoptosis, angiogenesis, cell cycle, DNA repair and growth factors will be reviewed. Some of the basic biology and laboratory evidence demonstrating how the marker relates to radiation response and available correlative clinical studies employing these markers as prognostic tools are presented. The majority of molecular markers that have potential clinical significance with respect to radiation sensitivity and local control will be highlighted.

Germline mutations at microsatellite loci in homozygous and heterozygous mutants for mismatch repair and PCNA genes in Drosophila

Microsatellite instability (MSI) is a phenotype associated with the deficient repair of replication errors. Replication errors persist in defective mismatch repair (MMR) conditions, although alterations in components of the replication machinery, such as the proliferating cell nuclear antigen (PCNA) factor, could also increase the replication errors; therefore, MSI is expected in both situations. It also seems that heterozygous individuals for MMR genes have a high risk of cancer, as in the case of human non-polyposis colon carcinoma (HNPCC), characterised by MSI. Thus, here we investigate the effect of heterozygosity for a Msh2-null allele or for altered PCNA alleles, on the stability of microsatellite sequences. The study was carried out in Drosophila germ cells analysing the progeny of individual crosses. We found that one Msh2 disrupted allele is sufficient to produce MSI in germ cells. Although the MSI in Msh2(-/+) individuals was in the same order of magnitude as in Msh2(-/-) individuals, the former manifested a MSI that was four-fold lower. To a lesser extent, PCNA homozygous and heterozygous mutants also show MSI in the germline, which reveals the importance of DNA replication factors to maintain genomic stability in vivo. Furthermore, the high MSI found both in heterozygous Msh2 and PCNA mutants suggests a high degree of genomic instability in individuals bearing a mutant allele of these genes, which could have important implications in cancer susceptibility.

Reduced expression of hMSH2 protein is correlated to poor survival for soft tissue sarcoma patients

BACKGROUND

Deregulation of DNA mismatch repair is a common mechanism for the development of hereditary nonpolyposis colon carcinoma and related familiar cancers, but it also plays a role in the tumorigenesis of sporadic cancers. Although the protein expression of the two main components of DNA mismatch repair, hMSH2 and hMLH1, has been described in soft tissue sarcoma (STS) patients, its prognostic impact is yet to be determined.

METHODS

The authors investigated the expression of the DNA repair proteins hMSH2 and hMLH1 by Western blot analysis in tumor samples of 57 STS patients. The correlation between the expression of hMSH2/hMLH1 and survival was studied in a Cox proportional hazards regression model, which was adjusted for the prognostic effects of staging, tumor entity, and radicality of tumor resection.

RESULTS

Nine of 57 STS (16%) showed reduced expression of hMSH2 and reduced expression of hMLH1 was detected in seven STS patients (12%). In a Kaplan-Meier analysis, the median survival for patients with reduced expression of the hMSH2 protein was 18 months, whereas the patients with a normal expression of hMSH2 survived for an average of 68 months. A multivariate Cox proportional hazards regression model revealed a significant correlation between the reduced expression of the hMSH2 protein and poor survival (relative risk = 4.7; 95% confidence interval [CI]: 1.3-17.2; P = 0.019).

CONCLUSIONS

Reduced expression of the hMSH2 protein is an independent negative prognostic factor for STS patients.