Mechanisms of DNA excision repair

DNA repair mechanisms involved in gemcitabine cytotoxicity and in the interaction between gemcitabine and cisplatin

The influence of DNA repair mechanisms on the interaction between gemcitabine and cisplatin was studied using a panel of Chinese hamster ovary (CHO) cell lines deficient in one of the following repair pathways: base excision repair (BER), nucleotide excision repair (NER), homologous recombination (HR) and non-homologous end joining (NHEJ). NER and HR are known to be involved in platinum-DNA adduct repair. Single agent experiments demonstrated that each of the repair deficient cell lines had a similar sensitivity towards gemcitabine as the parental cell lines, whereas the NER- and HR-deficient lines showed increased sensitivity towards cisplatin. Furthermore, in the parental cell lines, the administration sequence cisplatin followed by gemcitabine was synergistic, whereas the reversed schedule showed additivity and simultaneous administration revealed antagonistic cytotoxicity. In the repair deficient cell lines, using this synergistic schedule of cisplatin followed by gemcitabine, loss of synergy was observed in the NER- and HR-deficient cell lines. However, the magnitude of the effect in the NER-deficient cells was small. The sensitivity to the combination of cisplatin and gemcitabine shown by the BER- and NHEJ-deficient cell lines did not differ significantly from that of the parental cell line. Cellular accumulation of platinum as well as the formation of GG- and AG-intrastrand adducts in the parental line and in the HR-deficient line were not affected by gemcitabine. In conclusion, our results indicate that BER, NER, HR, and NHEJ are most likely incapable of modulating the cytotoxicity of gemcitabine, and that HR is involved in the synergistic interaction between cisplatin and gemcitabine in our cell system.

Ocular manifestations in the inherited DNA repair disorders

Deoxyribonucleic acid (DNA) repair is a fundamental process designed to keep the integrity of genomic DNA that is continuously challenged by intrinsic or environmental induced alterations. Numerous genes involved in DNA repair have been cloned and are involved in different DNA repair pathways: base excision repair, nucleotide excision repair, mismatch repair, DNA recombination. Inherited conditions due to mutations in DNA repair genes include mainly: xeroderma pigmentosum, Cockayne syndrome, Trichothiodystrophy, Bloom syndrome, Rothmund-Thomson syndrome, and Werner syndrome. Minor to major ocular manifestations occur in these syndromes. For example, eyelid skin cancers in xeroderma pigmentosum and retinal dystrophy in Cockayne syndrome are major features of these syndromes. This review focuses on the DNA repair pathways, the general and ocular features of the related syndromes, the laboratory tests useful for diagnosis, and the general processes implied with DNA repair (ultraviolet sensitivity, carcinogenesis, apoptosis, oxydative stress, and premature aging).

UV stimulation of nucleophosmin/B23 expression is an immediate-early gene response induced by damaged DNA

Nucleophosmin/B23 (NPM/B23), a nucleolar protein, was rapidly up-regulated after UV irradiation (at 254 nm; 30 J/m(2)) in NIH 3T3 cells and HeLa/S3 cells. Levels of NPM/B23 mRNA peaked 45-60 min after UV treatment and returned to baseline by 12 h. Transcription inhibitor actinomycin D (5 microg/ml) prevented the UV-induced increase of NPM/B23 mRNA, suggesting that UV induction of NPM/B23 was mediated at the transcriptional level. Moreover, UV-induced NPM/B23 expression was super-induced by cycloheximide (20 microg/ml), which was characteristic of immediate-early gene response. The transcriptional activation of NPM/B23 by UV was also confirmed by NPM/B23 promoter activity assay. Thymine dinucleotide, mimicking the effects of UV-induced DNA damage, was able to trigger NPM/B23 expression in the absence of genomic DNA damage. UV-induced activation of NPM/B23 promoter could not be blocked by UV-inducible pathway inhibitors, such as those of growth factor tyrosine kinase, mitogen-activated protein kinase, AP-1, NF-kappaB, and DNA-dependent kinase. Our results indicate that UV stimulation of NPM/B23 expression may be mediated through a novel UV-inducible pathway and is an immediate-early gene response induced by damaged DNA. Induction of immediate-early gene is an initial step in the regulation of cellular and genomic responses to external stimuli. Our results thus provide important evidence for an involvement of NPM/B23 in the acute response of mammalian cells to environmental stress.

The 2B domain of the Escherichia coli Rep protein is not required for DNA helicase activity

The Escherichia coli Rep protein is a 3' to 5' SF1 DNA helicase required for replication of bacteriophage phiX174 in E. coli, and is structurally homologous to the E. coli UvrD helicase and the Bacillus stearothermophilus PcrA helicase. Previous crystallographic studies of Rep protein bound to single-stranded DNA revealed that it can undergo a large conformational change consisting of an approximately 130 degrees rotation of its 2B subdomain about a hinge region connected to the 2A subdomain. Based on crystallographic studies of PcrA, its 2B subdomain has been proposed to form part of its duplex DNA binding site and to play a role in duplex destabilization. To test the role of the 2B subdomain in Rep-catalyzed duplex DNA unwinding, we have deleted its 2B subdomain, replacing it with three glycines, to form the RepDelta2B protein. This RepDelta2B protein can support phiX174 replication in a rep(-) E. coli strain, although the growth rate of E. coli containing the repDelta2B gene is approximately 1.5-fold slower than with the wild-type rep gene. Pre-steady-state, single-turnover DNA unwinding kinetics experiments show that purified RepDelta2B protein has DNA helicase activity in vitro and unwinds an 18-bp DNA duplex with rates at least as fast as wild-type Rep, and with higher extents of unwinding and higher affinity for the DNA substrate. These studies show that the 2B domain of Rep is not required for DNA helicase activity in vivo or in vitro, and that it does not facilitate DNA unwinding in vitro.

DNA damage, histological changes and DNA repair in larval Japanese medaka (Oryzias latipes) exposed to ultraviolet-B radiation

Cyclobutane dimer formation, photorepair capability and histological damage were compared among four differently pigmented strains of larval Japanese medaka (Oryzias latipes) to determine whether pigmentation modifies the level of UV-B radiation (290-320 nm) inducible damage in these fish. One-day post-hatch medaka were exposed to one of several UV-B fluence rates with or without photoreactivating light for 5 days for 7 h per day. Their DNA was extracted for analysis by ELISA for cyclobutane pyrimidine dimers or the larvae were processed for histological examination. At the higher UV-B fluence rates tested, wild-type melanophore-containing medaka formed significantly more dimers than at least one of the other strains tested. Wild-type medaka also showed significantly less photorepair capability than the white melanophore-lacking medaka. The wild-type larvae had significantly more necrosis than the orange-red melanophore-lacking larvae at the lower UV-B fluence rate tested and at the higher fluence rate used, the wild-type medaka also exhibited significantly more necrosis than the white melanophore-lacking larvae. Of the 19 medaka observed with cellular hyperplasia, six were wild-type. These six individual larvae showed the greatest degree of cellular hyperplasia. Cellular hyperplasia appeared to be greatest at the lowest UV-B fluence rate used. The presence of melanophores in the wild-type medaka may have contributed to an increased level of tissue damage in this strain when compared to the other strains.

Temperature-modulated carcinogenicity of 7,12-dimethylbenz[a]anthracene in rainbow trout

Temperature-modulated hepatic disposition, covalent binding of radiolabeled genotoxin to hepatic DNA, and cancer incidence in rainbow trout (Oncorhyncus mykiss) were assessed after a single exposure to 7,12-dimethylbenz[a]anthracene (DMBA). Fish (2 g) were acclimated at 10, 14, or 18 degrees C for 1 mo and then exposed to 1 ppm DMBA in their water for 20 h. Exposures were at respective acclimation temperatures, or 10 and 18 degrees C acclimated fish were shifted to 14 degrees C for DMBA exposures. After 4 but not 20 h of exposure, hepatic [(3)H]DMBA equivalents increased with temperature for fish exposed at their respective acclimation temperatures (10 or 18 degrees C). Covalent binding of [(3)H]DMBA to hepatic DNA was similar after 3 d in fish exposed at their respective acclimation temperatures. However, in fish exposed at 14 degrees C, after 3 d the concentration of [(3)H]DMBA covalently bound to hepatic DNA was higher in 10 degrees C than 18 degrees C acclimated fish. After 21 d, covalent binding of [(3)H]DMBA to hepatic DNA was less persistent in 18 degrees C than 10 degrees C acclimated, exposed, and reared fish. There were no differences between temperature-shifted groups at that time. Temperature effects on tumor incidence were assessed 9 mo after DMBA waterborne exposures in fish that were reared at (1) their respective acclimation and exposure temperatures, (2) 14 degrees C after exposure at their respective acclimation temperature, and (3) 14 degrees C after 14 degrees C exposures. Incidence of stomach, liver, and swimbladder cancer increased dramatically with rearing temperature. Differences in tumor incidence were less marked in fish reared at the same temperature (14 degrees C). A strong negative correlation between liver tumor incidence and persistence of [(3)H]DMBA equivalents covalently bound to hepatic DNA suggested increased error-prone DNA repair at warmer temperature played an important role in increased tumor incidence.

Myeloid differentiation (MyD) primary response genes in hematopoiesis

Myeloid Differentiation (MyD) primary response and Growth Arrest DNA-Damage (Gadd) genes comprise a set of overlapping genes, including known (IRF-1, EGR-1, Jun) and novel (MyD88, Gadd45alpha MyD118/Gadd45beta, GADD45gamma, MyD116/Gadd34) genes, that have been cloned by virtue of there being co-ordinately induced upon the onset of terminal myeloid differentiation. This review delineates the role MyD genes play in blood cell development, where they function as positive regulators of terminal differentiation, lineage specific blood cell development and control of blood cell homeostasis, including growth inhibition and apoptosis.

Scientists and clinicians test their metal-back to the future with platinum compounds

After more than two decades of extensive use, drugs based on platinum continue to have a major role in cancer treatment. Although systematic approaches to the development of new analogues have produced agents with less toxicity and novel mechanisms of action, to date such approaches have not achieved more cures than can be achieved with the parent compound, cisplatin. Greater gains might be expected from accumulating knowledge about what makes cancer cells sensitive or resistant to platinum-based chemotherapy. Recent information on drug-efflux pathways, including expression of multidrug-resistance protein 2, and on how tumour cells behave when their DNA is distorted by a platinum adduct, suggests new avenues for translational research. The prospects include modulation of cellular handling of platinum compounds and individualised therapy based on expression of molecules that determine platinum sensitivity.

Myeloid differentiation (MyD)/growth arrest DNA damage (GADD) genes in tumor suppression, immunity and inflammation

Myeloid differentiation (MyD) primary response and growth arrest DNA damage (Gadd) genes comprise a set of overlapping genes, including known (IRF-1, EGR-1, Jun) and novel (MyD88, Gadd45alpha, MyD118/Gadd45beta, GADD45gamma, MyD116/ Gadd34) genes, that have been cloned by virtue of being co-ordinately induced upon the onset of terminal myeloid differentiation and following exposure of cells to stress stimuli. In recent years it has become evident that MyD/Gadd play a role in blood cell development, where they function as positive regulators of terminal differentiation, lineage-specific blood cell development and control of blood cell homeostasis, including growth inhibition and apoptosis. MyD/Gadd are also involved in inflammatory responses to invading micro-organisms, and response to environmental stress and physiological stress, such as hypoxia, which results in ischemic tissue damage. An intricate network of interactions among MyD/GADD genes and gene products appears to control their diverse functions. Deregulated growth, increased cell survival, compromised differentiation and deficiencies in DNA repair are hallmarks of malignancy and its progression. Thus, the role MyD/Gadd play in negative growth control, including cell cycle arrest and apoptosis, and in DNA repair, make them attractive molecular targets for tumor suppression. The role MyD/Gadd play in innate immunity and host response to hypoxia also make these genes and gene products attractive molecular targets to treat immunity and inflammation disorders, such as septic shock and ischemic tissue damage.

DNA repair-deficient Xpa and Xpa/p53+/- knock-out mice: nature of the models

Xeroderma pigmentosum (XP) is a rare autosomal recessive disease in which repair of ultraviolet (UV)-induced DNA damage is impaired or is totally absent due to mutations in genes controlling the DNA repair pathway known as nucleotide excision repair (NER). XP is characterized, in part, by extreme sensitivity of the skin to sunlight, and XP patients have a more than 1000-fold increased risk of developing cancer at sun-exposed areas of the skin. To study the role of NER in chemical-induced tumorigenesis in more detail, the authors developed Xpa-/- homozygous knockout mice with a complete defect in NER (designated as Xpa mice or XPA model). Xpa mice develop skin tumors at high frequency when exposed to UV light, and as such, they mimic the phenotype of human XP. Moreover, the Xpa mice also appear to be susceptible to genotoxic carcinogens given orally. Based on these phenotypic characteristics, the Xpa mice were considered to be an attractive candidate mouse model for use in identifying human carcinogens. In an attempt to further increase both the sensitivity and specificity of the XPA model in carcinogenicity testing, the authors crossed Xpa mice with mice having a heterozygous defect in the tumor suppressor gene p53. Xpa/p53+/- double knockout mice develop tumors earlier and with higher incidences upon exposure to carcinogens as compared to their single knockout counterparts. Here the authors describe the development and features of the Xpa mouse and present some examples of the Xpa and Xpa/p53+/- mouse models' sensitivity towards genotoxic carcinogens. It appeared that the Xpa/p53+/- double knockout mouse model is favorable over both the Xpa and p53+/- single knockout models in short-term carcinogenicity testing. In addition to the fact that the double knockout mice respond more robustly to carcinogens, they also appear to respond in a very discriminative way. All compounds identified thus far are true (human) carcinogens, and, therefore, the authors believe that the Xpa/p53+/- mouse model is an excellent candidate for a future replacement of the chronic mouse bioassay, at least for certain classes of chemicals.

Human cells deficient in p53 regulated p21(waf1/cip1) expression exhibit normal nucleotide excision repair of UV-induced DNA damage

Cancer development requires the accumulation of numerous genetic changes, which are believed to initiate through the presence of unrepaired lesions in the genome. In the absence of proficient repair, genotoxic agents can lead to crucial mutations of vital cellular genes via replication of damaged DNA. Many cell cycle regulatory proteins are known to modulate the repair capacity and consequently the fate of cells. We and others have recently shown that p53 tumor suppressor gene product is required for efficient global genomic repair (GGR) but not the transcription coupled repair (TCR) of the nucleotide excision repair (NER) sub-pathways. In order to discern the nature of the p53 modulation to be direct or indirect through a downstream mediator, we have investigated the processing of UV radiation induced lesions in human colon carcinoma, HCT116 cells expressing wild-type p53 but having different p21(waf1cip1) (hereafter p21) genotypes (p21+/+, p21+/-, p21-/-). Following 20 J/m(2) UV, all the three cell lines showed rapid increase in p53 protein but the accompanying increase in the expression of its downstream target protein p21 could only be seen in p21+/+ and p21+/- cells and not in p21-/- cells. Nevertheless, an absence of detectable p21 protein in deficient cells had no demonstrable effect on DNA repair response to UV irradiation, as measured by an immunoassay to detect removal of UV photoproducts from genomic DNA (GGR) and by individual strand specific removal of endonuclease-sensitive CPD from a target gene fragment (TCR). Introduction of cytomegalovirus (CMV)-driven luciferase reporter plasmid, UV damaged in vitro, into the un-irradiated cells of varying p21 background, revealed a relatively small but statistically significant decrease in the reporter expression in the host p21-/- as compared with p21+/+ and p21+/- HCT116 cells. Super-expression of p21 protein upon reintroduction of p21 expression construct, showed an enhanced recovery of UV damaged reporter activity that was not greatly different from a similar enhancement observed with undamaged plasmid reporter DNA. Taken together, the results indicate that (i) the p21 protein does not have a significant role in the repair of genomic DNA at chromosomal level; (ii) the well-established p53 dependent modulation of NER is distinct and independent of its cell cycle checkpoint function; and (iii) the reproducible enhancing effect of p21 expression observed through host cell reactivation (HCR) of extrachromosomal DNA is mainly attributable to an effect exerted on transcription rather than repair.

Mapping polycyclic aromatic hydrocarbon and aromatic amine-induced DNA damage in cancer-related genes at the sequence level

Genomic injury induced by environmental carcinogens, such as polycyclic aromatic hydrocarbons and aromatic amines, is the initial step that can trigger mutagenesis and carcinogenesis. In addition to the physico-chemical property of DNA damaging agents, several important factors such as primary sequence, chromatin structure, methylation, protein association, and transcriptional activity can affect not only the initial level and distribution of DNA damage but also the efficiency of repair. Therefore, mapping the DNA damage induced by environmental agents in cancer-related genes such as p53 and ras at the sequence level provides essential information for assessing their carcinogenic potential. Recently, using the E. coli nucleotide excision enzyme complex, UvrABC nucleases in combination with ligation-mediated polymerase chain reaction, we developed a method to map DNA damage in the p53 and ras genes. These studies led us to conclude that targeted DNA damage, in combination with growth selection, contributes greatly in shaping the mutation spectrum in these genes in human cancer. Here we present the rationale and details of this approach, typical experimental results and necessary precautions.

Camptothecin and Zeocin can increase p53 levels during all cell cycle stages

The ability of DNA damage to stabilize p53 in all cell cycle stages has not been examined in actively growing cells. The chemotherapeutic drug camptothecin is a topoisomerase I poison. Zeocin is a member of the bleomycin/phleomycin family of antibiotics, known to bind DNA. Both increase the level of p53 albeit by different mechanisms. We have utilized centrifugal elutriation to separate exponentially growing ML-1 cells (containing wild-type p53) into cell cycle fractions and have subsequently treated these cells with the two drugs. We provide evidence that both drugs can mediate an increase in p53 protein levels independent of the cell cycle stage. The p53 induced by both drugs was able to bind to DNA; however, only the p53 induced by camptothecin was phosphorylated at serine-392. This is the first demonstration that camptothecin and Zeocin can differentially signal for increased levels of modified p53 during all stages of the cell cycle.

Flavones inhibit the hexameric replicative helicase RepA

Helicases couple the hydrolysis of nucleoside triphosphates (NTPs) to the unwinding of double-stranded nucleic acids and are essential in DNA metabolism. Thus far, no inhibitors are known for helicases except heliquinomycin isolated from Streptomyces sp. As the three-dimensional structure of the hexameric replicative DNA helicase RepA encoded by the broad host-range plasmid RSF1010 is known, this protein served as a model helicase to search for inhibitory compounds. The commercially available flavone derivatives luteolin, morin, myricetin and dimyricetin (an oxidation product of myricetin) inhibited the ATPase and double-stranded DNA unwinding activities of RepA. Dimyricetin was the most effective inhibitor for both activities. Single-stranded DNA-dependent RepA ATPase activity is inhibited non-competitively by all four compounds. This finding contrasts the inhibition of phosphoinositide 3-kinase by flavones that fit into the ATP binding pocket of this enzyme. Myricetin also inhibited the growth of a Gram-positive and a Gram-negative bacterial species. As we found other hexameric and non-hexameric prokaryotic helicases to be differentially sensitive to myricetin, flavones may provide substructures for the design of molecules helpful for unraveling the mechanism of helicase action and of novel pharmacologically useful molecules.

Nucleotide excision repair genes are upregulated by low-dose artificial ultraviolet B: evidence of a photoprotective SOS response?

Nucleotide excision repair is a major mechanism of defense against the carcinogenic effects of ultraviolet light. Ultraviolet B causes sunburn and DNA damage in human skin. Nucleotide excision repair has been studied extensively and described in detail at the molecular level, including identification of many nucleotide excision repair-specific proteins and the genes encoding nucleotide excision repair proteins. In this study, normal human keratinocytes were exposed to increasing doses of ultraviolet B from fluorescent sunlamps, and the effect of this exposure on expression of nucleotide excision repair genes was examined. An RNase protection assay was performed to quantify transcripts from nucleotide excision repair genes, and a slot blot DNA repair activity assay was used to assess induction of the nucleotide excision repair pathway. The activity assay demonstrated that cyclobutane pyrimidine dimers were removed efficiently after exposure to low doses of ultraviolet B, but this activity was delayed significantly at higher doses. All nucleotide excision repair genes examined demonstrated a similar trend: ultraviolet B induces expression of nucleotide excision repair genes at low doses, but downregulates expression at higher doses. In addition, we show that pre-exposure of cells to low-dose ultraviolet protected keratinocytes from apoptosis following high-dose exposure. These data support the notion that nucleotide excision repair is induced in cells exposed to low doses of ultraviolet B, which may protect damaged keratinocytes from cell death; however, exposure to high doses of ultraviolet B downregulates nucleotide excision repair genes and is associated with cell death.

An ERCC1 splicing variant involving the 5'-UTR of the mRNA may have a transcriptional modulatory function

Human ovarian cancer cells and tissues were examined for the presence or absence of a 42-bp splicing variant of ERCC1 gene, and for a possible functional role of this 42-bp sequence. This specific sequence exists in exon I, the 5'-UTR of the gene. Loss of this 42-bp sequence was associated with increased ERCC1 mRNA expression, in an assessment of 121 ovarian cancer specimens (p2<10(-6)). In cells in tissue culture, the absence of the 42-bp segment was associated with a twofold increased ability to drive transcription in a Luciferase reporter system. Protein can be demonstrated in ovarian cancer cells based on EMSA analysis. Computer analysis shows that this 42-bp sequence contains several binding sites, including a core-binding domain for protein RFX1, transcriptional repressor. These preliminary results lay the groundwork in determination of potential roles for a negative regulatory element in NER repair pathway.

Functional interactions and signaling properties of mammalian DNA mismatch repair proteins

The mismatch repair (MMR) system promotes genomic fidelity by repairing base-base mismatches, insertion-deletion loops and heterologies generated during DNA replication and recombination. This function is critically dependent on the assembling of multimeric complexes involved in mismatch recognition and signal transduction to downstream repair events. In addition, MMR proteins coordinate a complex network of physical and functional interactions that mediate other DNA transactions, such as transcription-coupled repair, base excision repair and recombination. MMR proteins are also involved in activation of cell cycle checkpoint and induction of apoptosis when DNA damage overwhelms a critical threshold. For this reason, they play a role in cell death by alkylating agents and other chemotherapeutic drugs, including cisplatin. Inactivation of MMR genes in hereditary and sporadic cancer is associated with a mutator phenotype and inhibition of apoptosis. In the future, a deeper understanding of the molecular mechanisms and functional interactions of MMR proteins will lead to the development of more effective cancer prevention and treatment strategies.

Skin aging: postulated mechanisms and consequent changes in structure and function

Aging is a complex process influenced by telomere shortening and damage to cellular DNA. New insights into age-associated decrements in DNA damage repair are reviewed. Age-associated gross, histologic, and functional cutaneous deficits are delineated. Different treatment options for aged skin are examined.

Disruption of Thermus thermophilus genes by homologous recombination using a thermostable kanamycin-resistant marker

Genes of an extremely thermophilic bacterium, Thermus thermophilus, were disrupted by homologous recombination using a recently developed, thermostable kanamycin-resistant marker. First, the trpE gene was disrupted with various constructions of DNA. The transformation efficiency was exponentially increased as the length of the homologous regions flanking the marker gene increased above the minimum length (200-300 bp). We then disrupted five genes of the nucleotide excision repair system and examined their phenotypes. The convenience and high reliability of this method should prompt its application to the high-throughput systematic disruption of the genes of this thermophilic bacterium.

Effects of solar radiation on cutaneous detoxification pathways

Because of its accessibility the human skin is constantly exposed to solar ultraviolet (UV) radiation. It is increasingly appreciated that exposure of the mammalian skin to UV plays a causal and decisive role in acute and chronic skin damage including the development of skin cancer. UV exposure causes sunburn, pigmentation, hyperplasia, immunosuppression, DNA damage, photoaging and photocarcinogenesis. To cope with constant environmental damage the skin possesses elaborate enzymatic detoxification systems. This paper briefly focuses on the effect of solar radiation, particularly UV spectrum, on detoxification pathways in the skin. Specifically the effect of solar radiation on cytochrome P450, glutathione, superoxide dismutase, glutathione peroxidase, catalase, glutathione-S-transferase and ceruloplasmin has been discussed.

Interactions in the error-prone postreplication repair proteins hREV1, hREV3, and hREV7

Most mutations after DNA damage in yeast Saccharomyces cerevisiae are induced by error-prone translesion DNA synthesis employing scRev1 and DNA polymerase zeta that consists of scRev3 and scRev7 proteins. Recently, the human REV1 (hREV1) and REV3 (hREV3) genes were identified, and their products were revealed to be involved in UV-induced mutagenesis, as observed for their yeast counterparts. Human REV7 (hREV7) was also cloned, and its product was found to interact with hREV3, but the biological function of hREV7 remained unknown. We report here the analyses of precise interactions in the human REV proteins. The interaction between hREV1 and hREV7 was identified by the yeast two-hybrid library screening using a bait of hREV7, which was confirmed by in vitro and in vivo binding assays. The homodimerization of hREV7 was also detected in the two-hybrid analysis. In addition, the precise domains for interaction between hREV7 and hREV1 or hREV3 and for hREV7 homodimerization were determined. Although hREV7 interacts with both hREV1 and hREV3, a stable complex formation of the three proteins was undetectable in vitro. These findings suggest the possibility that hREV7 might play an important role in regulating the enzymatic activities of hREV1 and hREV3 for mutagenesis in response to DNA damage.

Effects of 2-deoxy-D-glucose on DNA repair and mutagenesis in UV-irradiated yeast

We have studied the effects of 2-deoxy-D-Glucose (2-DG) on the recovery of potentially lethal damage (PLDR), repair of chromosomal DNA, cyclobutane pyrimidine dimers (CPDs), reverse mutation and gene-conversion in UVC (254 nm) irradiated yeast. As studied by pulsed-field gel electrophoresis, post-irradiation chromosomal DNA repair kinetics in a phosphate buffer (PB) with 10 mM glucose (G) was biphasic, where the first phase exhibited a decrease and the second phase showed an increase in the band intensities. A post-irradiation treatment in PB + G (10 mM) with 2-DG (10, 20, 50 mM) reduced the decrease in the DNA band intensities in the first phase of DNA repair. As compared to a post-irradiation (125 J/m2) treatment in PB + G (10 mM), a treatment in PB + G (10 mM) + 2-DG (10 mM) showed a decreased PLDR, but increased revertants and gene-convertants.

Adenosine nucleotide modulates the physical interaction between hMSH2 and BRCA1

We have identified the physical interaction between the Breast Cancer susceptibility gene product BRCA1 and the Hereditary Non-Polyposis Colorectal Cancer (HNPCC) and DNA mismatch repair (MMR) gene product hMSH2, both in vitro and in vivo. The BRCA1-hMSH2 association involved several well-defined regions of both proteins which include the adenosine nucleotide binding domain of hMSH2. Moreover, the interaction of BRCA1 with purified hMSH2-hMSH6 appears to be modulated by adenosine nucleotide much like G protein downstream interaction/signaling is modulated by guanosine nucleotide. BARD1, another BRCA1-interacting protein, was also found to interact with hMSH2. In addition, BRCA1 was found to associate with both hMSH3 and hMSH6, the heterodimeric partners of hMSH2. These observations implicate BRCA1/BARD1 as downstream effectors of the adenosine nucleotide-activated hMSH2-hMSH6 signaling complex, and suggest a global role for BRCA1 in DNA damage processing. The functional interaction between BRCA1 and hMSH2 may provide a partial explanation for the background of gynecological and colorectal cancer in both HNPCC and BRCA1 kindreds, respectively.

Development of UV defense mechanisms during growth of spinach seedlings

Changes in UV defense mechanisms were studied during the growth periods of spinach seedlings grown under the white light, which did not contain UV-B light. DNA photolyase activity in the photorepair of cyclobutane pyrimidine dimers in spinach seedlings was high in the early growth phase (cell division phase) and declined thereafter, whereas UV-absorbing substances accumulated throughout the growth period acted as a major UV-defense mechanism in the cell expansion phase.

Higher sensitivity in induction of apoptosis in fibroblast cell lines derived from LEC strain rats to ultraviolet B radiation

When lung fibroblast cell lines from LEC and WKAH rats were irradiated with ultraviolet B (UVB) and assayed for colony formation, LEC rat cells showed a higher sensitivity than did WKAH rat cells. The LEC rat cells were approximately 1.5-fold more sensitive to UVB radiation than were the WKAH rat cells in terms of D37 values, which are the doses of UVB required to reduce cell survival to 37%. When the rat cells were irradiated with UVB in the presence of 0.5 M dimethyl sulfoxide (DMSO), which efficiently scavenges free radicals such as hydroxyl radicals, no significant difference was observed between the survival curves of either LEC or WKAH rat cells irradiated with UVB in the presence of 0.5 M DMSO and those irradiated with UVB in the absence of DMSO. Therefore, formation of free radicals may not be involved in cell death induced by UVB radiation. Flow cytometry showed that the percentage of apoptotic cells in the LEC rat cell population increased with post-incubation time after UVB radiation. The proportion of apoptotic cells in the UVB-irradiated LEC rat cell population increased as the dose of UVB was increased. In contrast, no significant proportion of apoptotic cells was observed in the UVB-irradiated WKAH rat cell population. These results showed a higher sensitivity in induction of apoptosis by UVB radiation in LEC rat cells than in WKAH rat cells.

A novel single-molecule study to determine protein--protein association constants

Atomic force microscopy (AFM) is traditionally used as an imaging technique to gain qualitative information for a biological system. We have successfully used the imaging capabilities of the AFM to determine protein-protein association constants. We have developed a method to measure the molecular weight of a protein based on its volume determined from AFM images. Our volume determination method allows for rapid, accurate analysis of large protein populations. On the basis of the measured volume, the fraction of monomers as dimers was determined for the DNA helicase UvrD, and the dissociation constant (K(d)) for the helicase was calculated. We determined a K(d) for UvrD of 1.4 microM, which is in good agreement with published K(d) data obtained from analytical ultracentrifugation (AUC) studies. Our method provides a rapid method for determining protein-protein association constants.

The role of nucleotide excision repair and loss of p53 in mutagenesis and carcinogenesis

Xpa mice, which have a completely defective nucleotide excision repair (NER) pathway, have a cancer predisposition when exposed to several carcinogens. NER is one of the major DNA repair pathways in the mammalian cell, and is involved in the removal of a wide variety of DNA lesions, such as those induced by UV light, bulky adducts and DNA crosslinks. To study the role of NER in both mutagenesis and carcinogenesis, NER-defective Xpa mice were crossed with transgenic lacZ/pUR288 mutation-indicator mice. Furthermore, the relationship between the tumor suppressor gene p53, NER, induction of mutations and tumor development was studied in Xpa/p53+/-/lacZ triple transgenic mice. Using the genotoxic carcinogens benzo[a]pyrene (B[a]P) and 2-acetylaminofluorene (2-AAF), it is shown that mutations in the inactive (non-transcribed) lacZ reporter gene reliably predict cancer risk. In tissues at risk for the development of tumors, increased mutant frequencies could be found at much earlier stages. A heterozygous loss of p53 appears to act synergistically to a NER defect, both in mutation- as well as tumor-induction. Surprisingly, however, the effect of a heterozygous loss of p53 appeared to be tissue-restricted, being apparent in the bladder but absent in liver and spleen.

The intrinsically unstable life of DNA triplet repeats associated with human hereditary disorders

Expansions of specific DNA triplet repeats are the cause of an increasing number of hereditary neurological disorders in humans. In some diseases, such as Huntington's and several spinocerebellar ataxias, the repetitive DNA sequences are translated into long tracts of the same amino acid (usually glutamine), which alters interactions with cellular constituents and leads to the development of disease. For other disorders, including common genetic disorders such as myotonic dystrophy and fragile X syndrome, the DNA repeat is located in noncoding regions of transcribed sequences and disease is probably caused by altered gene expression. In studies in lower organisms, mammalian cells, and transgenic mice, high frequencies of length changes (increases and decreases) occur in long DNA triplet repeats. These observations are similar to other types of repetitive DNA sequences, which also undergo frequent length changes at genomic loci. A variety of processes acting on DNA influence the genetic stability of DNA triplet repeats, including replication, recombination, repair, and transcription. It is not yet known how these different multienzyme systems interact to produce the genetic mutation of expanded repeats. In vitro studies have identified that DNA triplet repeats can adopt several unusual DNA structures, including hairpins, triplexes, quadruplexes, slipped structures, and highly flexible and writhed helices. The formation of stable unusual structures within the cell is likely to disturb DNA metabolism and be a critical intermediate in the molecular mechanism(s) leading to genetic instabilities of DNA repeats and, hence, to disease pathogenesis.

Interaction of CR6 (GADD45gamma ) with proliferating cell nuclear antigen impedes negative growth control

GADD45, MyD118, and CR6 (also termed GADD45alpha, beta, and gamma) comprise a family of genes that encode for related proteins playing important roles in negative growth control, including growth suppression. Data accumulated suggest that MyD118/GADD45/CR6 serve similar but not identical functions along different apoptotic and growth suppressive pathways. It is also apparent that individual members of the MyD118/GADD45/CR6 family are differentially induced by a variety of genetic and environmental stress agents. The MyD118, CR6, and GADD45 proteins were shown to predominantly localize within the cell nucleus. Recently, we have shown that both MyD118 and GADD45 interact with proliferating cell nuclear antigen (PCNA), a protein that plays a central role in DNA replication, DNA repair, and cell cycle progression, as well as with the universal cyclin-dependent kinase inhibitor p21. In this work we show that also CR6 interacts with PCNA and p21. Moreover, it is shown that CR6 interacts with PCNA via a domain that also mediates interaction of both GADD45 and MyD118 with PCNA. Importantly, evidence has been obtained that interaction of CR6 with PCNA impedes the function of this protein in negative growth control, similar to observations reported for MyD118 and GADD45.

Oestrogen deficiency causes DNA damage in uterine leiomyoma cells: a possible mechanism for shrinkage of fibroids by GnRH agonists

OBJECTIVE

To examine whether gonadotrophin-releasing hormone agonist or oestradiol can directly affect DNA in leiomyoma cells.

DESIGN

In vitro explant culture of leiomyoma cells.

SETTING

University research group.

SAMPLE

Leiomyoma cells were cultured from the specimens of four premenopausal women at myomectomy.

METHODS

The presence of gonadotrophin-releasing hormone receptor in leiomyoma cells was determined by reverse transcriptase-olymerase chain reaction. Leiomyoma cells were treated with gonadotrophin-releasing hormone agonist or cultured in different concentrations of oestrogen, progesterone or fetal calf serum for one, four or seven days.

MAIN OUTCOME MEASURES

Cell number, expression of proliferating cell nuclear antigen, and DNA damage after one, four or seven days of treatment.

RESULTS

Gonadotrophin-releasing hormone receptor messenger ribonucleic acid was detected on cultured leiomyoma cells. Leiomyoma cell growth was not affected by the addition of gonadotrophin-releasing hormone agonist or progesterone, but increased with oestrogen or fetal calf serum supplementation. Overexpression of proliferating cell nuclear antigen was prevented in cultures added with oestrogen or fetal calf serum, but not related to gonadotrophin-releasing hormone agonist treatment. Significant decreases in DNA damage as indicated by decreased comet number were found in the leiomyoma cultures treated with oestrogen or fetal calf serum for four and seven days but not with gonadotrophin-releasing hormone agonist or progesterone. Furthermore, 5% fetal calf serum supplementation was more growth supporting and more significantly reduced the comet number than 250 pM 17 beta-oestradiol.

CONCLUSION

Cell growth, proliferating cell nuclear antigen expression and DNA damage are dependent on oestrogen or fetal calf serum, but independent of gonadotrophin-releasing hormone agonist or progesterone. Our findings suggest that gonadotrophin-releasing hormone agonist-induced leiomyoma shrinkage may be due in part to a mechanism involving DNA damage, and support the hypothesis that gonadotrophin-releasing hormone agonist exerts its action indirectly through oestrogen action on the tumour level.

Prevention of cisplatin-DNA adduct repair and potentiation of cisplatin-induced apoptosis in ovarian carcinoma cells by proteasome inhibitors

Histones H2A and H2B are known to be reversibly post-translationally modified by ubiquitination. We previously observed in cultured tumor cells that proteasome inhibition stabilizes polyubiquitinated proteins, depletes unconjugated ubiquitin, and thereby promotes the deubiquitination of nucleosomal histones in chromatin. Provocative indirect evidence suggests that histone ubiquitination/deubiquitination cycles alter chromatin structure, which may limit accessibility of DNA repair proteins to damaged sites. In the present study, we focused on the relationship between the ubiquitination status of histone H2A, the structure of chromatin, and the efficiency of nucleotide excision repair (NER) of cisplatin-DNA adducts in human ovarian carcinoma cells exposed to the antitumor drug cisplatin. Pretreating cells with the proteasome inhibitor lactacystin (LC) or N-acetyl-leucyl-leucyl-norleucinal (ALLnL) induced deubiquitination of ubiquitinated histone H2A (uH2A) and concomitantly promoted chromatin condensation, increased the extent of cisplatin-DNA adducts, and diminished NER-dependent repair of cisplatin-DNA lesions, compared with control cells treated with cisplatin alone. Both proteasome inhibitors also prevented the increase in ERCC-1 mRNA expression that occurs in cells exposed to cisplatin. Cells treated with the combination of ALLnL and cisplatin underwent apoptosis, as indicated by caspase-dependent poly(ADP-ribose) polymerase (PARP) cleavage, more quickly than cells treated with either agent alone. Additionally, the combination of ALLnL and cisplatin potently increased p53 levels in cell lysates and stimulated the binding of p53 to chromatin. Together, these observations suggest that proteasome inhibition may be exploited therapeutically for its potential to sensitize ovarian tumor cells to cisplatin.

p53 C-terminal interaction with DNA ends and gaps has opposing effect on specific DNA binding by the core

In addition to binding DNA in a sequence-specific manner, the p53 tumour suppressor protein can interact with damaged DNA. In order to understand which structural features in DNA the C-teminal domain recognises we have studied the interaction of p53 protein with different types of DNA oligonucleotides imitating damaged DNA. Here we show that one unpaired nucleotide within double-stranded (ds)DNA is sufficient for recognition by the p53 C-terminus, either as a protruding end or as an internal gap in dsDNA. C-terminal interaction with DNA ends facilitated core domain binding to DNA, whereas interaction with gaps prevented core domain-DNA complexing, implying that p53 might adopt distinct conformations upon binding to different DNA lesions. These observations suggest that both single-strand and double-strand breaks can serve as a target for p53 C-terminal recognition in vivo and indicate that p53 might recruit different repair factors to the sites of damaged DNA depending on the type of lesion.

Measuring DNA damage using capillary electrophoresis with laser-induced fluorescence detection

Damage to cellular DNA is implicated in the early stages of carcinogenesis and in the cytotoxicity of many anticancer agents, including ionizing radiation. Sensitive techniques are required for measuring cellular levels of DNA damage. We describe in detail a novel immunoassay that makes use of the resolving power of capillary electrophoresis and the sensitivity of laser-induced fluorescence detection. An example is given of the detection of thymine glycol in DNA produced by irradiation of human cells with a clinical dose of 2 Gy. A detection limit of approximately 10(-21) mol allowed us to monitor the repair of the lesion and to suggest that the cellular repair response may be inducible.

Lack of correlation between DNA strand breakage and p53 protein levels in human fibroblast strains exposed to ultraviolet lights

The contribution of DNA strand breaks accumulating in the course of nucleotide excision repair to upregulation of the p53 tumor suppressor protein was investigated in human dermal fibroblast strains after treatment with 254 nm ultraviolet (UV) light. For this purpose, fibroblast cultures were exposed to UV and incubated for 3 h in the presence or absence of l-beta-D-arabinofuranosylcytosine (araC) and/or hydroxyurea (HU), and then assayed for DNA strand breakage and p53 protein levels. As expected from previous studies, incubation of normal and ataxia telangiectasia (AT) fibroblasts with araC and HU after UV irradiation resulted in an accumulation of DNA strand breaks. Such araC/HU-accumulated strand breaks (reflecting nonligated repair-incision events) following UV irradiation were not detected in xeroderma pigmentosum (XP) fibroblast strains belonging to complementation groups A and G. Western blot analysis revealed that normal fibroblasts exhibited little upregulation of p53 (approximately 1.2-fold) when incubated without araC after 5 J/m2 irradiation, but showed significant (three-fold) upregulation of p53 when incubated with araC after irradiation. AraC is known to inhibit nucleotide excision repair at both the damage removal and repair resynthesis steps. Therefore, the potentiation of UV-induced upregulation of p53 evoked by araC in normal cells may be a consequence of either persistent bulky DNA lesions or persistent incision-associated DNA strand breaks. To distinguish between these two possibilities, we determined p53 induction in AT fibroblasts (which do not upregulate p53 in response to DNA strand breakage) and in XP fibroblasts (which do not exhibit incision-associated breaks after UV irradiation). The p53 response after treatment with 5 J/m2 UV and incubation with araC was similar in AT, XPA, XPG and normal fibroblasts. In addition, exposure of XPA and XPG fibroblasts to UV (5, 10 or 20 J/m2) followed by incubation without araC resulted in a strong upregulation of p53. We further demonstrated that HU, an inhibitor of replicative DNA synthesis (but not of nucleotide excision repair), had no significant impact on p53 protein levels in UV irradiated and unirradiated human fibroblasts. We conclude that upregulation of p53 at early times after exposure of diploid human fibroblasts to UV light is triggered by persistent bulky DNA lesions, and that incision-associated DNA strand breaks accumulating in the course of nucleotide excision repair and breaks arising as a result of inhibition of DNA replication contribute little (if anything) to upregulation of p53.

Distribution of mutations in the human xeroderma pigmentosum group A gene and their relationships to the functional regions of the DNA damage recognition protein

A series of xeroderma pigmentosum group A cell lines from 19 patients and cell lines from 13 other family members were examined for XPA mutations to find previously unidentified mutations from American and European patients, to establish pedigrees in represented families, and to develop a database for XPA diagnosis. Most mutations were deletions and splice site mutations observed previously in other XPA patients, in exon III, intron III, or exon IV, that resulted in frameshifts within the DNA binding region-including an Afl III RFLP (G to C) in four unrelated families. One new mutation was a point mutation within intron III (A to G) creating a new splice acceptor site that may compete with the original splice acceptor site. Missplicing at this new site inserts 11 nucleotides in the mRNA creating a frameshift. A small amount of normal splicing to give wild-type XPA protein is the likely molecular mechanism for the relatively mild clinical features of this patient. In another patient, a new 2 bp deletion in the RPA70 binding region was identified in the same region as a 20 bp deletion previously characterized in an unrelated patient. Mutations in the DNA binding region of XPA were from patients with the more severe disease often associated with neurological complications, whereas mutations in the C-terminal end of the protein, which interacts with the TFIIH transcription factor, were from patients with milder skin disease only. The rarity of naturally occurring missense mutations in the DNA binding region of XPA suggests that amino acid changes might be sufficiently tolerated that patients would have mild symptoms and escape detection.

Characterization of MyD118, Gadd45, and proliferating cell nuclear antigen (PCNA) interacting domains. PCNA impedes MyD118 AND Gadd45-mediated negative growth control

MyD118 and Gadd45 are related genes encoding for proteins that play important roles in negative growth control, including growth suppression and apoptosis. MyD118 and Gadd45 are related proteins that previously were shown to interact with proliferating cell nuclear antigen (PCNA), implicated in DNA replication, DNA repair, and cell cycle progression. To establish the role of MyD118 and Gadd45 interactions with PCNA, in this work we sought to identify the interacting domains and analyze the significance of this interaction in negative growth control. Using complementary in vivo and in vitro interaction assays the N-terminal (1-46) and middle (100-127) regions of PCNA were identified as harboring MyD118- and Gadd45 interacting domains, whereas PCNA interacting domains within MyD118 and Gadd45 were localized to the C termini of these proteins (amino acids 114-156 and 137-165, respectively). These findings provide first evidence that similar domains within MyD118 and Gadd45 mediate interactions with PCNA. Importantly, ectopic expression of MyD118 or Gadd45 N-terminal peptides, lacking the PCNA interacting domain, was found to suppress colony formation or induce apoptosis more efficiently than the full-length proteins. These findings suggest that interaction of MyD118 or Gadd45 with PCNA, in essence, serves to impede negative growth control.

lon incompatibility associated with mutations causing SOS induction: null uvrD alleles induce an SOS response in Escherichia coli

The uvrD gene in Escherichia coli encodes a 720-amino-acid 3'-5' DNA helicase which, although nonessential for viability, is required for methyl-directed mismatch repair and nucleotide excision repair and furthermore is believed to participate in recombination and DNA replication. We have shown in this study that null mutations in uvrD are incompatible with lon, the incompatibility being a consequence of the chronic induction of SOS in uvrD strains and the resultant accumulation of the cell septation inhibitor SulA (which is a normal target for degradation by Lon protease). uvrD-lon incompatibility was suppressed by sulA, lexA3(Ind(-)), or recA (Def) mutations. Other mutations, such as priA, dam, polA, and dnaQ (mutD) mutations, which lead to persistent SOS induction, were also lon incompatible. SOS induction was not observed in uvrC and mutH (or mutS) mutants defective, respectively, in excision repair and mismatch repair. Nor was uvrD-mediated SOS induction abolished by mutations in genes that affect mismatch repair (mutH), excision repair (uvrC), or recombination (recB and recF). These data suggest that SOS induction in uvrD mutants is not a consequence of defects in these three pathways. We propose that the UvrD helicase participates in DNA replication to unwind secondary structures on the lagging strand immediately behind the progressing replication fork, and that it is the absence of this function which contributes to SOS induction in uvrD strains.

Mutagenesis and carcinogenesis in nucleotide excision repair-deficient XPA knock out mice

Mice with a defect in the xeroderma pigmentosum group A (XPA) gene have a complete deficiency in nucleotide excision repair (NER). As such, these mice mimic the human XP phenotype in that they have a >1000-fold higher risk of developing UV-induced skin cancer. Besides being UV-sensitive, XPA(-/-) mice also develop internal tumors when they are exposed to chemical carcinogens. To investigate the effect of a total NER deficiency on the induction of gene mutations and tumor development, we crossed XPA(-/-) mice with transgenic lacZ/pUR288 mutation-indicator mice. The mice were treated with various agents and chemicals like UV-B, benzo[a]pyrene and 2-aceto-amino-fluorene. Gene mutation induction in several tumor target- and non-target tissues was determined in both the bacterial lacZ reporter gene and in the endogenous Hprt gene. Furthermore, alterations in the p53- and ras genes were determined in UV-induced skin tumors of XPA(-/-) mice. In this work, we review these results and discuss the applicability and reliability of enhanced gene mutant frequencies as early indicators of tumorigenesis.

p53-mediated DNA repair responses to UV radiation: studies of mouse cells lacking p53, p21, and/or gadd45 genes

Human cells lacking functional p53 exhibit a partial deficiency in nucleotide excision repair (NER), the pathway for repair of UV-induced DNA damage. The global genomic repair (GGR) subpathway of NER, but not transcription-coupled repair (TCR), is mainly affected by p53 loss or inactivation. We have utilized mouse embryo fibroblasts (MEFs) lacking p53 genes or downstream effector genes of the p53 pathway, gadd45 (Gadd45a) or p21 (Cdkn1a), as well as MEFs lacking both gadd45 and p21 genes to address the potential contribution of these downstream effectors to p53-associated DNA repair. Loss of p53 or gadd45 had a pronounced effect on GGR, while p21 loss had only a marginal effect, determined by measurements of repair synthesis (unscheduled DNA synthesis), by immunoassays to detect removal of UV photoproducts from genomic DNA, and by assays determining strand-specific removal of CPDs from the mouse dhfr gene. Taken together, the evidence suggests a role for Gadd45, but relatively little role for p21, in DNA repair responses to UV radiation. Recent evidence suggests that Gadd45 binds to UV-damaged chromatin and may affect lesion accessibility. MEFs lacking p53 or gadd45 genes exhibited decreased colony-forming ability after UV radiation and cisplatin compared to wild-type MEFs, indicating their sensitivity to DNA damage. We provide evidence that Gadd45 affects chromatin remodelling of templates concurrent with DNA repair, thus indicating that Gadd45 may participate in the coupling between chromatin assembly and DNA repair.

Absence of evidence for allelic loss or allelic gain for ERCC1 or for XPD in human ovarian cancer cells and tissues

We have previously reported on mRNA expression of ERCC1, XPA and XPD in human ovarian cancer cells and tissues. Several factors can influence mRNA expression for any given gene. Alterations in gene copy number for ERCC1 and/or XPD have been reported to occur in malignant glioma specimens. Human ovarian cancer cell lines and tissues were therefore examined for evidence of altered gene copy number in selected genes within the nucleotide excision repair (NER) pathway. Six ovarian cancer cell lines were studied: A2780, A2780/CP70, SKOV3, MCAS, QvCar3 and Caov4. Cellular sensitivity to cisplatin varies by more than 1 log between some of these cells. In each of these cell lines, the genes examined included ERCC1, XPA, XPB, XPD, XPG, CSB and p53. Genomic DNA was also extracted from ovarian cancer specimens taken from 22 patients and assessed for evidence of allelic loss and/or allelic gain for ERCC1 and XPD. Twelve of the clinical specimens were from patients with platinum-sensitive tumors and ten were from patients with platinum-resistant tumors. In no case could we demonstrate a reproducible variation in gene copy number in any cell line. Among the human tissues studied, there was one case of allelic gain out of 22 specimens. We therefore conclude that alterations in gene copy number is not a common event in human ovarian cancer. Other mechanisms must be invoked to explain differences in mRNA expression for these genes.

A human REV7 homolog that interacts with the polymerase zeta catalytic subunit hREV3 and the spindle assembly checkpoint protein hMAD2

Widespread alteration of the genomic DNA is a hallmark of tumors, and alteration of genes involved in DNA maintenance have been shown to contribute to the tumorigenic process. The DNA polymerase zeta of Saccharomyces cerevisiae is required for error-prone repair following DNA damage and consists of a complex between three proteins, scRev1, scRev3, and scRev7. Here we describe a candidate human homolog of S. cerevisiae Rev7 (hREV7), which was identified in a yeast two-hybrid screen using the human homolog of S. cerevisiae Rev3 (hREV3). The hREV7 gene product displays 23% identity and 53% similarity with scREV7, as well as 23% identity and 54% similarity with the human mitotic checkpoint protein hMAD2. hREV7 is located on human chromosome 1p36 in a region of high loss of heterozygosity in human tumors, although no alterations of hREV3 or hREV7 were found in primary human tumors or human tumor cell lines. The interaction domain between hREV3 and hREV7 was determined and suggests that hREV7 probably functions with hREV3 in the human DNA polymerase zeta complex. In addition, we have identified an interaction between hREV7 and hMAD2 but not hMAD1. While overexpression of hREV7 does not lead to cell cycle arrest, we entertain the possibility that it may act as an adapter between DNA repair and the spindle assembly checkpoint.

Mismatch repair processing of carcinogen-DNA adducts triggers apoptosis

The DNA mismatch repair pathway is well known for its role in correcting biosynthetic errors of DNA replication. We report here a novel role for mismatch repair in signaling programmed cell death in response to DNA damage induced by chemical carcinogens. Cells proficient in mismatch repair were highly sensitive to the cytotoxic effects of chemical carcinogens, while cells defective in either human MutS or MutL homologs were relatively insensitive. Since wild-type cells but not mutant cells underwent apoptosis upon treatment with chemical carcinogens, the apoptotic response is dependent on a functional mismatch repair system. By analyzing p53 expression in several pairs of cell lines, we found that the mismatch repair-dependent apoptotic response was mediated through both p53-dependent and p53-independent pathways. In vitro biochemical studies demonstrated that the human mismatch recognition proteins hMutSalpha and hMutSbeta efficiently recognized DNA damage induced by chemical carcinogens, suggesting a direct participation of mismatch repair proteins in mediating the apoptotic response. Taken together, these studies further elucidate the mechanism by which mismatch repair deficiency predisposes to cancer, i.e., the deficiency not only causes a failure to repair mismatches generated during DNA metabolism but also fails to direct damaged and mutation-prone cells to commit suicide.