Mechanisms of DNA excision repair

Characterization of the mutational profile of (+)-7R,8S-dihydroxy-9S, 10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene at the hypoxanthine (guanine) phosphoribosyltransferase gene in repair-deficient Chinese hamster V-H1 cells

Earlier studies have shown that the profile of mutations induced by (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (+)-BPDE at the hypoxanthine (guanine) phosphoribosyltransferase (hprt) gene of Chinese hamster V79 cells was dependent on the concentration of (+)-BPDE. In the present study, we examined the effect of the concentration of (+)-BPDE on its mutational profile at the hprt gene in repair-deficient V-H1 cells (a derivative of V79 cells) to explore the role of DNA repair in the dose-dependent mutational profile of (+)-BPDE. Independent hprt mutant clones were isolated after exposing V-H1 cells to dimethylsulfoxide (DMSO) or to low (4-6 nM; 95% cell survival) or high (40-48 nM; 31% cell survival) concentrations of (+)-BPDE in DMSO. The mutation frequencies for the DMSO control and for the low and high concentration groups were 0.1, 2.1 and 32.9 mutant colonies/10(5) survivors, respectively. The profile of mutations at the hprt gene was characterized for 148 (+)-BPDE-induced mutant clones and the results from the present study were compared with those obtained earlier with V79 cells. The data indicated that: (i) V-H1 cells were approximately 9-fold more sensitive to the cytotoxic effects of (+)-BPDE than V79 cells; (ii) the mutation frequency in V-H1 cells was similar to that observed in V79 cells following exposure to similar concentrations of (+)-BPDE; (iii) (+)-BPDE-induced mutations at guanine on the transcribed strand of the hprt gene were common in V-H1 cells but were extraordinarily rare in V79 cells; (iv) (+)-BPDE-induced mutations at adenine on the transcribed strand of the hprt gene were common in both V-H1 and V79 cells; (v) although exposure of V79 cells to different doses of (+)-BPDE resulted in a dose-dependent mutational profile at the hprt gene, this was not observed in V-H1 cells. Our observations indicate a defect in the transcription-coupled repair of (+)-BPDE-DNA adducts in V-H1 cells and that the repair activity deficient in V-H1 cells is essential for the dose-dependent mutational profile observed with (+)-BPDE in V79 cells.

Recombinational repair of DNA damage in Escherichia coli and bacteriophage lambda

Although homologous recombination and DNA repair phenomena in bacteria were initially extensively studied without regard to any relationship between the two, it is now appreciated that DNA repair and homologous recombination are related through DNA replication. In Escherichia coli, two-strand DNA damage, generated mostly during replication on a template DNA containing one-strand damage, is repaired by recombination with a homologous intact duplex, usually the sister chromosome. The two major types of two-strand DNA lesions are channeled into two distinct pathways of recombinational repair: daughter-strand gaps are closed by the RecF pathway, while disintegrated replication forks are reestablished by the RecBCD pathway. The phage lambda recombination system is simpler in that its major reaction is to link two double-stranded DNA ends by using overlapping homologous sequences. The remarkable progress in understanding the mechanisms of recombinational repair in E. coli over the last decade is due to the in vitro characterization of the activities of individual recombination proteins. Putting our knowledge about recombinational repair in the broader context of DNA replication will guide future experimentation.

Impaired nucleotide excision repair in UV-irradiated human oral keratinocytes immortalized with type 16 human papillomavirus genome

We previously reported that 'high risk' human papillomaviruses (HPV) induce genetic instability in human oral keratinocytes. To understand the mechanisms of HPV-induced genetic instability, we determined the nucleotide excision repair (NER) capacity of normal (NHOK) and human papillomavirus type-16 immortalized oral keratinocytes (HOK-16B) by strand-specific removal of UV-induced cyclobutane pyrimidine dimers (CPDs) from a 16 Kb fragment of the p53 gene. In NHOK the NER activity was initiated in both DNA strands immediately, although the process in the non-transcribed strand was notably slower than that of the transcribed strand. In HOK-16B cells the initiation of CPDs removal was delayed for at least 8 h in both DNA strands, and the process was significantly slower than that in NHOK. UV-irradiation enhanced the p53 protein level more than 30-fold in NHOK, but it did not significantly alter the protein level in the HOK-16B cells. UV-irradiation also increased the p21WAF1/CIP1 protein level only in NHOK. These data indicate that 'high risk' HPV induces genetic instability by impairing NER capacity of cells. Impaired NER activity of HOK-16B cells may be implicated with their inability to enhance active p53 when challenged by genotoxic stress.

An oligomeric form of E. coli UvrD is required for optimal helicase activity

Pre-steady-state chemical quenched-flow techniques were used to study DNA unwinding catalyzed by Escherichia coli UvrD helicase (helicase II), a member of the SF1 helicase superfamily. Single turnover experiments, with respect to unwinding of a DNA oligonucleotide, were used to examine the DNA substrate and UvrD concentration requirements for rapid DNA unwinding by pre-bound UvrD helicase. In excess UvrD at low DNA concentrations (1 nM), the bulk of the DNA is unwound rapidly by pre-bound UvrD complexes upon addition of ATP, but with time-courses that display a distinct lag phase for formation of fully unwound DNA, indicating that unwinding occurs in discrete steps, with a "step size" of four to five base-pairs as previously reported. Optimum unwinding by pre-bound UvrD-DNA complexes requires a 3' single-stranded (ss) DNA tail of 36-40 nt, whereas productive complexes do not form readily on DNA with 3'-tail lengths </=16 nt. A 5'-ssDNA tail is neither sufficient nor does it stimulate unwinding, even in the presence of a 3'-ssDNA tail. Nitrocellulose filter binding studies show that UvrD binding affinity also increases with increasing 3'-ssDNA tail length, showing apparent positive cooperativity for binding to DNA with a 40 nt 3'-ssDNA tail. Single turnover DNA unwinding experiments performed at higher DNA concentrations (50 nM) show a sigmoidal dependence of the extent of unwinding on the pre-incubated [UvrD], also indicating cooperativity. These results indicate that the form of the UvrD helicase with optimal helicase activity is oligomeric with at least two sites for binding the DNA substrate, where one site contacts regions of the 3'-ssDNA tail that are distal from the single-stranded/double-stranded DNA junction.

A role for RAD51 and homologous recombination in Trypanosoma brucei antigenic variation

Antigenic variation is an immune evasion strategy used by African trypanosomes, in which the parasites periodically switch the expression of VSG genes that encode their protective variant surface glycoprotein coat. Two main routes exist for VSG switching: changing the transcriptional status between an active and an inactive copy of the site of VSG expression, called the bloodstream VSG expression site, or recombination reactions that move silent VSGs or VSG copies into the actively transcribed expression site. Nothing is known about the proteins that control and catalyze these switching reactions. This study describes the cloning of a trypanosome gene encoding RAD51, an enzyme involved in DNA break repair and genetic exchange, and analysis of the role of the enzyme in antigenic variation. Trypanosomes genetically inactivated in the RAD51 gene were shown to be viable, and had phenotypes consistent with lacking functional expression of an enzyme of homologous recombination. The mutants had an impaired ability to undergo VSG switching, and it appeared that both recombinational and transcriptional switching reactions were down-regulated, indicating that RAD51 either catalyzes or regulates antigenic variation. Switching events were still detectable, however, so it appears that trypanosome factors other than RAD51 can also provide for antigenic variation.

DNA damage and DNA damage-inducible protein Gadd45 following ischemia in the P7 neonatal rat

Cerebral ischemia in adult rodents leads to the production of several types of lesions in the genomic DNA, followed by the activation of the damage-response indicator Gadd45. Our purpose was to investigate the structural changes that occur in chromatin DNA and repair processes after ischemic injury in neonatal brain. Neonatal ischemia was induced by the permanent left MCA occlusion in association with 1 h occlusion of the left common carotid artery in 7-day-old Wistar pups. Oligonucleosome fragments that are recognized as the characteristic DNA ladder was observed in a delayed fashion. Double-strand breaks result in high molecular weight fragments of 50- and 300-kbp as demonstrated by pulsed-field gel electrophoresis, and visualized by the TUNEL assay at 24 h of recovery. In contrast, DNA single-strand breaks, shown by the use of DNA polymerase I-mediated biotin-dATP nick translation were not so abundant. Gadd45 immunoreactivity was sequentially increased in vulnerable neurons in the infarct (4 to 24 h) and in sublethally injured neurons in the penumbra (24-48 h). Taken together, these findings suggest that Gadd45 responds to DNA damage following neonatal ischemia. Furthermore, repairing processes seem to be more active in the penumbra and therefore Gadd45 could have also a protective role in cerebral ischemia.

The influence of UV radiation on protistan evolution

Ultraviolet radiation has provided an evolutionary challenge to life on Earth. Recent increases in surficial ultraviolet B fluxes have focused attention on the role of UV radiation in protistan ecology, cancer, and DNA damage. Exploiting this new wealth of data, I examine the possibility that ultraviolet radiation may have played a significant role in the evolution of the first eukaryotes, that is, protists. Protists probably arose well before the formation of a significant ozone shield, and thus were probably subjected to substantial ultraviolet A, ultraviolet B, and ultraviolet C fluxes early in their evolution. Evolution consists of the generation of heritable variations and the subsequent selection of these variants. Ultraviolet radiation has played a role both as a mutagen and as a selective agent. In its role as a mutagen, it may have been crucial in the origin of sex and as a driver of molecular evolution. As a selective agent, its influence has been broad. Discussed in this paper are the influence of ultraviolet radiation on biogeography, photosynthesis, and desiccation resistance.

Nuclear localization of the Arabidopsis blue light receptor cryptochrome 2

The cryptochrome blue light photoreceptor family of Arabidopsis thaliana consists of two members, CRY1 and CRY2 (PHH1). CRY2 contains a putative nuclear localization signal (NLS) within its C-terminal region. We examined whether CRY2 is localized in the nucleus and whether the C-terminal region of CRY2 is involved in nuclear targeting. Total cellular and nuclear protein extracts from Arabidopsis were subjected to immunoblot analysis with CRY2-specific antibodies. Strong CRY2 signals were obtained in the nuclear fraction. Fusion proteins consisting of the green fluorescent protein (GFP) and different fragments of CRY2 were expressed in parsley protoplasts and the localization of the fusion proteins was determined by fluorescence and confocal laser scanning microscopy. GFP-fusions containing the entire CRY2 protein or its C-terminal region were found exclusively in the nucleus. We conclude from these results that CRY2 is localized in the nucleus and that nuclear localization is mediated by the C-terminal region of CRY2.

Human exonuclease 1 functionally complements its yeast homologues in DNA recombination, RNA primer removal, and mutation avoidance

Yeast exonuclease 1 (Exo1) is induced during meiosis and plays an important role in DNA homologous recombination and mismatch correction pathways. The human homolog, an 803-amino acid protein, shares 55% similarity to the yeast Exo1. In this report, we show that the enzyme functionally complements Saccharomyces cerevisiae Exo1 in recombination of direct repeat DNA fragments, UV resistance, and mutation avoidance by in vivo assays. Furthermore, the human enzyme suppresses the conditional lethality of a rad27Delta mutant, symptomatic of defective RNA primer removal. The purified recombinant enzyme not only displays 5'-3' double strand DNA exonuclease activity, but also shows an RNase H activity. This result indicates a back-up function of exonuclease 1 to flap endonuclease-1 in RNA primer removal during lagging strand DNA synthesis.

Deficient nucleotide excision repair increases base-pair substitutions but decreases TGGC frameshifts induced by methylglyoxal in Escherichia coli

To investigate the mutation spectrum of a well-known mutagen, methylglyoxal, and the influence of nucleotide excision repair (NER) on methylglyoxal-induced mutations, we treated wild-type and NER-deficient (uvrA or uvrC) Escherichia coli strains with methylglyoxal, and analyzed mutations in the chromosomal lacI gene. In the three strains, the cell death and the mutation frequency increased according to the dose of methylglyoxal added to the culture medium. The frequencies of methylglyoxal-induced base-pair substitutions were higher in the NER-deficient strains than in the wild-type strain, in the presence and absence of mucAB gene. Paradoxically, the frequency of methylglyoxal-induced TGGC frameshifts was higher in the wild-type strain than in the NER-deficient strains. When the methylglyoxal-induced mutation spectra in the presence and absence of mucAB gene are compared, the ratios of base-pair substitutions to frameshifts were increased by the effects of mucAB gene. In the three strains, more than 75% of the base-pair substitutions occurred at G:C sites, independent of the mucAB gene. When the mucAB gene was present, G:C-->T:A transversions were predominant, followed by G:C-->A:T transitions. When the mucAB gene was absent, the predominant mutations differed in the three strains: in the wild-type and uvrC strains, G:C-->A:T transitions were predominant, followed by G:C-->T:A transversions, while in the uvrA strains, G:C-->T:A transversions were predominant, followed by G:C-->A:T transitions. These results suggest that NER may be involved in both the repair and the fixation of methylglyoxal-induced mutations.

Defective DNA repair in cells with human T-cell leukemia/bovine leukemia viruses: role of tax gene

BACKGROUND

Human T-cell leukemia virus (HTLV)/bovine leukemia virus (BLV) group retroviruses, which cause hematopoietic cancers, encode a unique protein, Tax, involved in the transformation of infected cells. Our purpose was to determine whether the mechanism by which Tax protein induces transformation in HTLV- or BLV-infected cells involves DNA damage.

METHODS

We used a micronucleus assay to measure chromosomal damage and alkali denaturation analysis to test host-cell DNA integrity in cells infected with HTLV, BLV, or simian T-lymphotropic virus or in cells transfected with the tax gene of HTLV or BLV. Controls included uninfected cells and cells infected with other oncogenic retroviruses or oncogenic DNA viruses. We used a plasmid reactivation assay to examine whether the damage might be due to the inhibition of DNA repair. To ascertain which of several repair pathways might be inhibited, chemical methods were used to selectively introduce lesions repaired by specific pathways into the reporter plasmid.

RESULTS

The presence of Tax was associated with DNA damage. HTLV- or BLV-infected or tax-transfected cells showed normal ability to repair damage induced by deoxyribonuclease I or psoralen but markedly decreased ability to repair damage induced by UV light, quercetin, or hydrogen peroxide.

CONCLUSIONS

These data suggest that the DNA repair pathway most inhibited by Tax is base-excision repair of oxidative damage. To our knowledge, this is the first report demonstrating inhibition of DNA repair by any retrovirus and suggests that this inhibition of DNA repair may contribute to the mechanism of cell transformation by the HTLV/BLV group of viruses.

p21Waf1/Cip1/Sdi1 induces permanent growth arrest with markers of replicative senescence in human tumor cells lacking functional p53

We have shown previously that wild type p53 can rapidly induce replicative senescence in EJ human bladder carcinoma cells lacking functional p53. A major effector of p53 functions is p21Waf1/Cip1/Sdi1, a potent cyclin-dependent kinase inhibitor. p21Waf1/Cip1/Sdi1 has been shown to be involved in both p53 dependent and independent control of cell proliferation, differentiation and death. To directly investigate the effects of p21Waf1/Cip1/Sdi1 in the p53 response observed in EJ tumor cells, we established p21Waf1/Cip1/Sdi1 inducible lines using the tetracycline-regulatable vector system. p21Waf1/Cip1/Sdi1 induction caused irreversible cell cycle arrest in both G1 and G2/M, and diminished Cdk2 kinase activity. In addition, p21Waf1/Cip1/Sdi1 induction led to morphological alterations characteristic of cells undergoing replicative senescence with morphological, biochemical and ultrastructural markers of the senescent phenotype. Furthermore, sustained p21Waf1/Cip1/Sdi1 induction sensitized EJ cells to apoptotic cell death induced by mitomycin C, a cross-linking DNA damaging agent. These findings support the function of p21Waf1/Cip1/Sdi1 as an inducer of replicative senescence and a major mediator of this phenomenon in response to p53. Moreover, our results imply that therapeutic intervention in human cancers might be aimed at sustained elevation of p21Waf1/Cip1/Sdi1 expression.

Transcriptionally active and inactive mouse beta-globin gene loci are repaired at similar rates after ultraviolet irradiation

It has been demonstrated, by Northern blot and in situ hybridization, that the mouse erythroleukaemia cell line F4N-Sofia constitutively expresses the beta-globin genes. The recently developed quantitative assay for DNA repair has been used to study the overall repair rate in the beta-globin gene domain in this cell line after ultraviolet irradiation and to compare it with the repair rate of the same chromatin domain in mouse Ehrlich ascites tumour cells which do not express the beta-globin genes. The results showed that in both cases the 5'-end of the domain was repaired preferentially and that the repair rates in the two cell lines were very similar despite the different transcription state of the genes.

Increased sensitivity of human colon cancer cells to DNA cross-linking agents after GRP78 up-regulation

We have shown earlier that pre-treatment of V79 Chinese hamster cells with 6-aminonicotinamide (6-AN) or 2-deoxyglucose (2-dG) results in over-expression of the Mr 78,000 glucose-regulated stress protein (GRP78) and the subsequent development of resistance to inhibitors of topoisomerase II. These phenomena also occur in V79-derived cell lines that are deficient in poly(ADP-ribose) (p(ADPR)) metabolism. In contrast, over-expression of GRP78 under the conditions outlined above is found to be associated with hypersensitivity to several clinically-relevant DNA cross-linking agents, namely, 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU), cisplatin, and melphalan. We have also previously shown that pre-treatment with 6-AN, an inhibitor of p(ADPR) metabolism, causes an increase in the life span in BCNU-treated mice bearing L1210 tumors. These observations prompted us to examine whether 6-AN pre-treatment can result in the over-expression of GRP78 in human colon cancer cell lines and, if so, whether this increase is associated with sensitization to DNA cross-linking agents outlined above. Following treatment of three colon cancer cell lines, HCT116, SW480, and VACO-8, for 48 h with 0.1 mM 6-AN, cytosolic GRP78 levels were elevated approximately 4.2 times, 8 times, and 2.5 times for each cell line respectively, as measured by Western immunoblotting. To determine sensitivity after GRP78 up-regulation, the cells were washed and grown for 412 h in growth medium devoid of 6-AN, before being treated with DNA cross-linking agents. The 412 h time period allowed p(ADPR) metabolism to return to normal while GRP78 levels remained elevated, thus allowing us to associate GRP78 over-expression with sensitivity to those agents. After treating cells for 1 h with BCNU, cisplatin, or melphalan, cell sensitivity was determined by clonogenic survival assay or a fluorescence-based cytotoxicity assay. Based on changes in IC50 values, 6-AN caused an increase in sensitivity for HCT116, SW480, and VACO-8 cells of 1.5, 2.3, and 1.0 times, respectively, for BCNU, 4.8, 3.8, and 2.6 for cisplatin, and 6.4, 3.7, and 2.2 times for melphalan. Thus, our results show that over-expression of GRP78 in human tumor cell lines is associated with increased sensitivity to clinically useful chemotherapy agents. This sensitization occurred in three different tumor cell lines, each bearing a separate genetic defect associated with altered sensitivity.

The effect of the Saccharomyces cerevisiae endo-exonuclease NUD1 gene expression on the resistance of HeLa cells to DNA-damaging agents

HeLa cells transiently transfected with a mammalian expression DNA vector expressing the Saccharomyces cerevisiae endo-exonuclease (EE) NUD1 gene have exhibited changes in cell survival frequencies after treatment with different DNA-damaging agents as compared to HeLa cells transfected with a control plasmid. The NUD1-transfected cells showed a dose-dependent increase in sensitivity to UV irradiation resulting in up to 58% decrease in cell survival. In response to gamma-irradiation NUD1 transfected cells featured an increased survival at doses equal to and greater than 2.0 Gy, reaching a maximum enhancement in survival frequency of 17%. At the same time, the NUD1-transfectants featured an increase in resistance to 0.25 microM-0.5 microM cis-platin (up to 58% increase in cell survival) and 1.0 mM EMS (11% increase). At higher concentrations of EMS NUD1 expression resulted in a decreased cell survival of the transfected cells (17% decrease for 2.5 mM EMS). No difference in cell survival frequencies between the NUD1-transfectants and the controls was observed after treatment with different concentrations of chlorambucil and mechlorethamine. These results suggest possible roles played by EEs in different DNA repair pathways--being stimulatory for the repair of certain types of DNA lesions, such as double strand breaks (DSBs), and interfering with the endogenous DNA repair systems for the repair of other types of lesions. Furthermore, these results also provide additional indirect evidence for the role of EEs in homologous recombination.

A region near the C-terminal end of Escherichia coli DNA helicase II is required for single-stranded DNA binding

The role of the C terminus of Escherichia coli DNA helicase II (UvrD), a region outside the conserved helicase motifs, was investigated by using three mutants: UvrDDelta107C (deletion of the last 107 C-terminal amino acids), UvrDDelta102C, and UvrDDelta40C. This region, which lacks sequence similarity with other helicases, may function to tailor UvrD for its specific in vivo roles. Genetic complementation assays demonstrated that mutant proteins UvrDDelta107C and UvrDDelta102C failed to substitute for the wild-type protein in methyl-directed mismatch repair and nucleotide excision repair. UvrDDelta40C protein fully complemented the loss of helicase II in both repair pathways. UvrDDelta102C and UvrDDelta40C were purified to apparent homogeneity and characterized biochemically. UvrDDelta102C was unable to bind single-stranded DNA and exhibited a greatly reduced single-stranded DNA-stimulated ATPase activity in comparison to the wild-type protein (kcat = 0.01% of the wild-type level). UvrDDelta40C was slightly defective for DNA binding and was essentially indistinguishable from wild-type UvrD when single-stranded DNA-stimulated ATP hydrolysis and helicase activities were measured. These results suggest a role for a region near the C terminus of helicase II in binding to single-stranded DNA.

Solvent-exposed residues in the Tet repressor (TetR) four-helix bundle contribute to subunit recognition and dimer stability

Dimerization specificity of Tet repressor (TetR) can be altered by changes in the core of the four-helix bundle that mediates protein-protein recognition. We demonstrate here that the affinity of subunit interaction depends also on the solvent-exposed residues at positions 128 and 179'-184', which interact across the dimerization surface. TetR(B) and (D), two naturally occurring sequence variants, differ at position 128 with respect to the monomer-monomer distances in the crystal structures and the charge of the amino acids, being glutamate in TetR(B) and arginine in TetR(D). In vivo analysis of chimeric TetR(B/D) variants revealed that the single E128R exchange does not alter the dimerization specificity of TetR(B) to the one of TetR(D). When combined with specificity mutations in alpha10, it is, however, able to increase dimerization efficiency of the TetR(B/D) chimera with TetR(D). A loss of contact analysis revealed a positive interaction between Arg-128 and residues located at positions 179'-184' of the second monomer. We constructed a hyperstable TetR(B) variant by replacing residues 128 and 179-184 by the respective TetR(D) sequence. These results establish that in addition to a region in the hydrophobic core residues at the solvent-exposed periphery of the dimerization surface participate in protein-protein recognition in the TetR four-helix bundle.

Modulation of excision repair cross complementation group 1 (ERCC-1) mRNA expression by pharmacological agents in human ovarian carcinoma cells

Excision repair cross complementation group 1 (ERCC-1) is a DNA repair gene that is essential for life, and it appears to be a marker gene for nucleotide excision repair activity. Overexpression of ERCC-1 during cisplatin-based chemotherapy is associated with clinical and cellular drug resistance. We therefore began to assess the influence of various pharmacological agents on the induction of ERCC-1 mRNA in A2780/CP70 human ovarian carcinoma cells. Cisplatin exposure in culture resulted in a 4- to 6-fold induction for the steady-state level of ERCC-1 mRNA in A2780/CP70 cells. ERCC-1 mRNA induction was concentration and time dependent. Cyclosporin A and herbimycin A, which suppress c-fos and c-jun gene expressions, respectively, blocked the cisplatin-induced increase in ERCC-1 mRNA. This effect of cyclosporin A or herbimycin A on the down-regulation of ERCC-1 correlates with enhanced cytotoxicity of cisplatin in this system. The products of c-fos and c-jun are components of the transcription factor AP-1 (activator protein 1). 12-O-Tetradecanoylphorbol 13-acetate (TPA), a known AP-1 agonist, induced ERCC-1 mRNA to the same extent as cisplatin, but did not synergize with cisplatin in this regard. The TPA effect was biphasic, with an initial increase during the first 1-6 hr, followed by decreasing mRNA levels at 24-72 hr. These data suggest that the effects of these pharmacological agents on ERCC-1 gene expression may be mediated through the modulation of AP-1 activities.

Xeroderma pigmentosum and the role of UV-induced DNA damage in skin cancer

Xeroderma pigmentosum (XP) is a rare, autosomal recessive disease that is characterized by the extreme sensitivity of the skin to sunlight. Compared to normal individuals, XP patients have a more than 1000-fold increased risk of developing cancer on sun-exposed areas of the skin. Genetic and molecular analyses have revealed that the repair of ultraviolet (UV)-induced DNA damage is impaired in XP patients owing to mutations in genes that form part of a DNA-repair pathway known as nucleotide excision repair (NER). Two other diseases, Cockayne syndrome (CS) and the photosensitive form of trichothiodystrophy (TTD), are linked to a defect in the NER pathway. Strikingly, although CS and TTD patients are UV-sensitive, they do not develop skin cancer. The recently developed animal models that mimic the human phenotypes of XP, CS and TTD will contribute to a better understanding of the etiology of these diseases and the role of UV-induced DNA damage in the development of skin cancer.

Role of Saccharomyces cerevisiae chromatin assembly factor-I in repair of ultraviolet radiation damage in vivo

In vitro, the protein complex Chromatin Assembly Factor-I (CAF-I) from human or yeast cells deposits histones onto DNA templates after replication. In Saccharomyces cerevisiae, the CAC1, CAC2, and CAC3 genes encode the three CAF-I subunits. Deletion of any of the three CAC genes reduces telomeric gene silencing and confers an increase in sensitivity to killing by ultraviolet (UV) radiation. We used double and triple mutants involving cac1Delta and yeast repair gene mutations to show that deletion of the CAC1 gene increases the UV sensitivity of cells mutant in genes from each of the known DNA repair epistasis groups. For example, double mutants involving cac1Delta and excision repair gene deletions rad1Delta or rad14Delta showed increased UV sensitivity, as did double mutants involving cac1Delta and deletions of members of the RAD51 recombinational repair group. cac1Delta also increased the UV sensitivity of strains with defects in either the error-prone (rev3Delta) or error-free (pol30-46) branches of RAD6-mediated postreplicative DNA repair but did not substantially increase the sensitivity of strains carrying null mutations in the RAD6 or RAD18 genes. Deletion of CAC1 also increased the UV sensitivity and rate of UV-induced mutagenesis in rad5Delta mutants, as has been observed for mutants defective in error-free postreplicative repair. Together, these data suggest that CAF-I has a role in error-free postreplicative damage repair and may also have an auxiliary role in other repair mechanisms. Like the CAC genes, RAD6 is also required for gene silencing at telomeres. We find an increased loss of telomeric gene silencing in rad6Delta cac1Delta and rad18Delta cac1Delta double mutants, suggesting that CAF-I and multiple factors in the postreplicative repair pathway influence chromosome structure.

Nucleotide excision repair affects the stability of long transcribed (CTG*CAG) tracts in an orientation-dependent manner in Escherichia coli

The influence of nucleotide excision repair (NER), the principal in vivo repair system for DNA damages, was investigated in Escherichia coli with uvrA, uvrB and uvrAuvrB mutants with the triplet repeat sequences (TRS) involved in myotonic dystrophy, the fragile X syndrome and Friedreich's ataxia. (CTG*CAG)175was more stable when the (CTG) strand was transcribed than when the (CAG) strand was transcribed in the alternate orientation. A lack of the UvrA protein dramatically increases the instability of this TRS in vivo as compared with the stability of the same sequence in uvrB mutant, which produces an intact UvrA protein. We propose that transcription transiently dissociates the triplet repeat complementary strands enabling the non-transcribed strand to fold into a hairpin conformation which is then sufficiently stable that replication bypasses the hairpin to give large deletions. If the TRS was not transcribed, fewer deletions were observed. Alternatively, in the uvrA-mutant, the hairpins existing on the lagging strand will suffer bypass DNA synthesis to generate deleted molecules. Hence, NER, functionally similar in both prokaryotes and eukaryotes, is an important factor in the genetic instabilities of long transcribed TRS implicated in human hereditary neuro-logical diseases.

Genetic susceptibility to tobacco carcinogenesis

Lung cancer risk is thus defined by the balance between metabolic activation and detoxification of xenobiotic compounds and by the efficiency of DNA repair. It is most likely that multiple susceptibility factors must be accounted for to represent the true dimensions of gene-environment interactions. The ability to identify smokers with the highest risks of developing cancer has substantial preventive implications. These subgroups could be targeted for the most intensive screening and smoking cessation interventions and could be enrolled into chemoprevention trials. Studying susceptibility to common cancers and widely prevalent exposures may provide further insights into the basic mechanisms of carcinogenesis. Issues that will need to be addressed in the very near future include risk communication to study subjects and the ethical, legal, and social consequences of such testing.

Structure, expression, and function of human pituitary tumor-transforming gene (PTTG)

Despite advances in characterizing the pathophysiology and genetics of pituitary tumors, molecular mechanisms of their pathogenesis are poorly understood. Recently, we isolated a transforming gene [pituitary tumor-transforming gene (PTTG)] from rat pituitary tumor cells. Here we describe the cloning of human PTTG, which is located on chromosome 5q33 and shares striking sequence homology with its rat counterpart. Northern analysis revealed PTTG expression in normal adult testis, thymus, colon, small intestine, brain, lung, and fetal liver, but most abundant levels of PTTG mRNA were observed in several carcinoma cell lines. Stable transfection of NIH 3T3 cells with human PTTG cDNA caused anchorage-independent transformation in vitro and induced in vivo tumor formation when transfectants were injected into athymic mice. Overexpression of PTTG in transfected NIH 3T3 cells also stimulated expression and secretion of basic fibroblast growth factor, a human pituitary tumor growth-regulating factor. A proline-rich region, which contains two PXXP motifs for the SH3 domain-binding site, was detected in the PTTG protein sequence. When these proline residues were changed by site-directed mutagenesis, PTTG in vitro transforming and in vivo tumor-inducing activity, as well as stimulation of basic fibroblast growth factor, was abrogated. These results indicate that human PTTG, a novel oncogene, may function through SH3-mediated signal transduction pathways and activation of growth factor(s).

The Martian and extraterrestrial UV radiation environment--1. Biological and closed-loop ecosystem considerations

The Martian surface is exposed to both UVC radiation (<280 nm) and higher doses of UVB (280-315 nm) compared to the surface of the Earth. Terrestrial organisms have not evolved to cope with such high levels of UVC and UVB and thus any attempts to introduce organisms to Mars, particularly in closed-loop life support systems that use ambient sunlight, must address this problem. Here we examine the UV radiation environment of Mars with respect to biological systems. Action spectra and UV surface fluxes are used to estimate the UV stress that both DNA and chloroplasts would experience. From this vantage point it is possible to consider appropriate measures to address the problem of the Martian UV environment for future long term human exploration and settlement strategies. Some prospects for improving the UV tolerance of organisms are also discussed. Existing artificial ecosystems such as Biosphere 2 can provide some insights into design strategies pertinent to high UV environments. Some prospects for improving the UV tolerance of organisms are also discussed. The data also have implications for the establishment of closed-loop ecosystems using natural sunlight on the lunar surface and elsewhere in the Solar System.

Nucleotide excision repair: from E. coli to man

Nucleotide excision repair is both a 'wide spectrum' DNA repair pathway and the sole system for repairing bulky damages such as UV lesions or benzo[a]pyrene adducts. The mechanisms of nucleotide excision repair are known in considerable detail in Escherichia coli. Similarly, in the past 5 years important advances have been made towards understanding the biochemical mechanisms of excision repair in humans. The overall strategy of the repair is the same in the two species: damage recognition through a multistep mechanism involving a molecular matchmaker and an ATP-dependent unwinding of the damaged duplex; dual incisions at both sides of the lesion by two different nucleases, the 3' incision being followed by the 5'; removal of the damaged oligomer; resynthesis of the repair patch, whose length matches the gap size. Despite these similarities, the two systems are biochemically different and do not even share structural homology. E. coli excinuclease employs three proteins in contrast to 16/17 polypeptides in man; the excised fragment is longer in man: the procaryotic excinuclease is not able by itself to remove the excised oligomer whereas the human enzyme does. Thus, the excinuclease mode of action is well conserved throughout evolution, but not the biochemical tools: this represents a case of evolutionary convergence.

Human DNA repair systems: an overview

DNA repair systems act to maintain genome integrity in the face of replication errors, environmental insults, and the cumulative effects of age. More than 70 human genes directly involved in the five major pathways of DNA repair have been described, including chromosomal location and cDNA sequence. However, a great deal of information as to the precise functions of these genes and their role in human health is still lacking. Hence, we summarize what is known about these genes and their contra part in bacterial, yeast, and rodent systems and discuss their involvement in human disease. While some associations are already well understood, it is clear that additional diseases will be found which are linked to DNA repair defects or deficiencies.

The C-terminal domain of p53 catalyzes DNA-renaturation and strand exchange toward annealing between intact ssDNAs and toward eliminating damaged ssDNA from duplex formation through preferential recognition of damaged DNA by a duocarmycin

The C-terminal domain of p53 may bind single-stranded (ss) DNA ends and catalyze renaturation of ss complementary DNA molecules, suggesting a possible direct role for p53 in DNA repair (Proc. Natl. Acad. Sci. USA, 92, 9455-9459, 1995). We found that DU-86, a duocarmycin derivative which alkylates DNA, bound ssDNA and enhanced the DNA binding activity of the p53 C-terminus. DU-86 weakened p53-mediated catalysis of complementary ssDNA renaturation. p53 C-terminus catalyzed DNA strand transfer toward annealing between intact ssDNAs and toward eliminating DU-86-damaged ssDNA from duplex formation. These results suggest that p53, via the C-terminal domain, may play a direct role in DNA repair by preferential recognization and elimination of damaged DNA.

Cell cycle control and cancer

This review consists of two parts. In the first part normal mechanisms regulating the progression of cells through the cell cycle are briefly reviewed. Besides mitogenic stimulation, cyclin kinase inhibition, the G1 restriction point and the prb pathway, accuracy of DNA replication and DNA repair, the G2 to M transition, apoptosis and the p 53 pathway, proteolytic, in particular ubiquitin-dependent mechanisms involved in the initiation of DNA synthesis in the separation of sister chromatids and in the telophase to GO/G1 transition, are discussed. In the second part oncogene and tumor suppressor gene products are briefly characterized. Aberrations of cell cycle control mechanisms associated with cancer are grouped as follows: deregulation of protooncogenes by translocations juxtaposing protooncogenes to immunoglobulin--or T cell receptor genes; translocations producing chimeric proteins unique to cancer cells; inversions and amplifications resulting in over expression of regulator genes; and deletions and mutations of tumor suppressor genes. It is emphasized that cancer is the result of a multistep process and that uncontrolled cell production and other alterations are, as a rule, late phenomena.

Novel mutations in the RAD3 and SSL1 genes perturb genome stability by stimulating recombination between short repeats in Saccharomyces cerevisiae

Maintaining genome stability requires that recombination between repetitive sequences be avoided. Because short, repetitive sequences are the most abundant, recombination between sequences that are below a certain length are selectively restricted. Novel alleles of the RAD3 and SSL1 genes, which code for components of a basal transcription and UV-damage-repair complex in Saccharomyces cerevisiae, have been found to stimulate recombination between short, repeated sequences. In double mutants, these effects are suppressed, indicating that the RAD3 and SSL1 gene products work together in influencing genome stability. Genetic analysis indicates that this function is independent of UV-damage repair and mutation avoidance, supporting the notion that RAD3 and SSL1 together play a novel role in the maintenance of genome integrity.

Nucleotide excision repair in the third kingdom

Nucleotide excision repair, a general repair mechanism for removing DNA damage, is initiated by dual incisions bracketing the lesion. In procaryotes, the dual incisions result in excision of the damage in 12- to 13-nucleotide-long oligomers, and in eucaryotes they result in excision of the damage in the form of 24- to 32-nucleotide-long oligomers. We wished to find out if Archaea perform excision repair. Using cell extracts from Methanobacterium thermoautotrophicum, we found that this organism removes UV-induced (6-4) photoproducts in the form of 10- to 11-mers by incising the sixth to seventh phosphodiester bond 5' to the damage and the fourth phosphodiester bond 3' to the damage.

Cancer--a degenerative disorder?

Cancer is primarily a disease of ageing epithelia, and of ageing individuals. We now possess detailed insights into the changes in cell regulatory genes and DNA repair systems which accumulate with time and which manifest in malignancy. These demonstrate how cancer is frequently characterized by degenerative change in the genotype, from the most subtle base pair mutations to gross aneuploidy, and by deterioration in cell and tissue regulatory control, be it of proliferation, programmed cell death or signalling. Cancer may thus be as much a phenomenon of loss or deterioration of normal genomic control as of the acquisition of new, neoplastic functions. This distinction may be more than semantic, not least because it governs our approach to the search for therapeutic strategies. This essay considers the concept of cancer as a degenerative disease and its implications, and proposes the neologism aldoplasia to describe this phenomenon of cancer biology.

Platinum-DNA adduct, nucleotide excision repair and platinum based anti-cancer chemotherapy

Clinical studies performed by several groups suggest that platinum-DNA adduct--measured in malignant or non-malignant cells from cancer patients--may be an important marker for clinical biological effect of platinum-based chemotherapy. DNA repair is clearly an important effector of resistance to platinum-based DNA-damaging agents in tissue culture, although its role in effecting clinical resistance to these agents is not completely clear. In recent years, it has become apparent that DNA repair is an extremely complex process. Processes within DNA repair that may contribute to one or more drug resistance phenotypes include 0-6-alkytransferase activity, base excision repair, mismatch repair, nucleotide excision repair (NER), and gene specific repair. Clearly, several of these processes may concurrently show increased activity within any single cell, or tumor, at any one time. For platinum compounds, in vitro data clearly show that NER is the DNA repair pathway responsible for the repair of cisplatin-DNA damage. One critical gene within NER is ERCC1. Data exist in human ovarian cancer and in human gastric cancer that ERCC1 may be a useful marker for clinical drug resistance when platinum-based systemic chemotherapy is utilized. Although the data suggest that the relative ERCC1 mRNA level may be a good marker for NER activity in human ovarian cancer, it is unclear whether expression of this gene has any relationship to other pathways of DNA repair.

Changes in proliferating cell nuclear antigen, a protein involved in DNA repair, in vulnerable hippocampal neurons following global cerebral ischemia

Proliferating cell nuclear antigen (PCNA) is required for completion of the DNA synthesis step of DNA replication as well as nucleotide excision repair (NER) of damaged DNA. We investigated the expression of PCNA mRNA and the levels of PCNA protein in the adult rat hippocampus following normo- and hypothermic global forebrain ischemia. Hypothermia protected the CA1 neurons from ischemic damage. A constitutive expression of PCNA mRNA and protein was detected in all hippocampal subfields, as well as in other brain regions. During reperfusion, PCNA mRNA levels were up-regulated in the vulnerable CA1 subfield at 36 h following normothermic ischemia. In hypothermia, this induction appeared already after 18 h. Following normothermic ischemia, nuclear PCNA immunoreactivity was largely abolished during reperfusion in the vulnerable CA1 neurons, prior to cell death. In contrast, total PCNA protein content of this region, as measured by Western blotting, remained largely unchanged. In the CA3 region, a transient decrease in nuclear PCNA immunoreactivity was observed. In the dentate gyrus region, no down-regulation of nuclear or total PCNA protein was observed during reperfusion. Following hypothermic ischemia, the PCNA protein levels did not decrease in any of the hippocampal subregions. In contrast, no change in the levels of Ref-1, a protein involved in base excision DNA repair (BER), was observed following normo- or hypothermic ischemia. Our findings indicate an altered functional state of PCNA protein in the ischemia-sensitive CA1 neurons suggesting that DNA repair processes are affected in these post-mitotic cells following ischemia. Impaired DNA repair may play a role in the development of postischemic neuronal damage.

The requirement of yeast Ssl2 (Rad25) for the repair of cisplatin-damaged DNA

Cisplatin is one of the most widely used anticancer agents. Cisplatin-induced cytotoxicity results from its ability to form cisplatin-DNA adducts within the cellular genome which can inhibit the transcription of genes and the replication of DNA. Cisplatin-adducts are primarily removed by the nucleotide excision repair (NER) pathway. The SSL2 (RAD25) gene of Saccharomyces cerevisiae, a homolog of the XPB (ERCC3) gene in humans, is involved in the nucleotide excision repair of UV-damaged DNA and is also required for cell viability. However, the role of Ssl2 (Rad25) in cisplatin sensitivity has not been examined. In this study, we have demonstrated that a yeast strain carrying the mutant allele SSL2-XP, a truncated form of SSL2 (RAD25) at the carboxyl terminus to mimic the human XPB (ERCC3) mutation, has increased cellular sensitivity to cisplatin in comparison to wild type cells. Analysis by host cell reactivation (HCR) assay further shows that Ssl2 (Rad25) is required for the repair of cisplatin-damaged DNA.

Cisplatin induction of ERCC-1 mRNA expression in A2780/CP70 human ovarian cancer cells

ERCC-1 is a critical gene within the nucleotide excision repair pathway, and cells without a functional ERCC-1 do not perform cisplatin-DNA adduct repair. We therefore investigated the cisplatin effect on ERCC-1 mRNA expression in vitro. In response to a 1-h cisplatin exposure, A2780/CP70 human ovarian cancer cells showed a 6-fold increase in steady-state level of ERCC-1 mRNA. This rise was attributable to increased transcription as measured by nuclear run-on assays and a 60% increase in ERCC-1 mRNA half-life. The increase in ERCC-1 mRNA was preceded by a 4-5-fold rise in mRNA expressions of c-fos and c-jun, a 14-fold increase in c-Jun protein phosphorylation, and an increase in in vitro nuclear extract binding activity to the AP-1-like site of ERCC-1. These data suggest that the induction of ERCC-1 expression in A2780/CP70 cells exposed to cisplatin results from two major factors: (a) an increase in the expression of transactivating factors that bind the AP-1-like site in the 5'-flanking region of ERCC-1 and (b) an increase in the level of c-Jun phosphorylation that enhances its transactivation property.

DNA repair in Mycobacterium tuberculosis. What have we learnt from the genome sequence?

The genome sequence of Mycobacterium tuberculosis was analysed by searching for homologues of genes known to be involved in the reversal or repair of DNA damage in Escherichia coli and related organisms. Genes necessary to perform nucleotide excision repair (NER), base excision repair (BER), recombination, and SOS repair and mutagenesis were identified. In particular, all of the genes known to be directly involved in the repair of oxidative and alkylative damage are present in M. tuberculosis. In contrast, we failed to identify homologues of genes involved in mismatch repair. This finding has potentially significant implications with respect to genome stability, strain variability at repeat loci and the emergence of chromosomally encoded drug resistance mutations.

The ability of a variety of polymerases to synthesize past site-specific cis-syn, trans-syn-II, (6-4), and Dewar photoproducts of thymidylyl-(3'-->5')-thymidine

The role of photoproduct structure, 3' --> 5' exonuclease activity, and processivity on polynucleotide synthesis past photoproducts of thymidylyl-(3' --> 5')-thymidine was investigated. Both Moloney murine leukemia virus reverse transcriptase and 3' --> 5' exonuclease-deficient (exo-) Vent polymerase were blocked by all photoproducts, whereas Taq polymerase could slowly bypass the cis-syn dimer. T7 RNA polymerase was able to bypass all the photoproducts in the order cis-syn > Dewar > (6-4) > trans-syn-II. Klenow fragment could not bypass any of the photoproducts, but an exo- mutant could bypass the cis-syn dimer to a greater extent than the others. Likewise T7 DNA polymerase, composed of the T7 gene 5 protein and Escherichia coli thioredoxin, was blocked by all the photoproducts, but the exo- mutant Sequenase 2.0 was able to bypass them all in the order cis-syn > Dewar > trans-syn-II > (6-4). No bypass occurred with an exo- gene 5 protein in the absence of the thioredoxin processivity factor. Bypass of the cis-syn and trans-syn-II products by Sequenase 2.0 was essentially non-mutagenic, whereas about 20% dTMP was inserted opposite the 5'-T of the Dewar photoproduct. A mechanism involving a transient abasic site is proposed to account for the preferential incorporation of dAMP opposite the 3'-T of the photoproducts.

Characterization of gene-specific DNA repair by primary cultures of rat hepatocytes

At present, almost all the information on gene-specific DNA repair in mammals comes from studies with transformed cell lines and proliferating primary cells obtained from rodents and humans. In the present study, we measured the repair of specific DNA regions in primary cultures of nondividing rat hepatocytes (parenchymal cells). DNA damage was induced by irradiating the primary cultures of hepatocytes with ultraviolet (UV) light, and the presence of cyclobutane pyrimidine dimers (CPDs) was measured by using T4 endonuclease V in the following: a 21-kb BamHI fragment containing the albumin gene, a 14-kb BamHI fragment containing the H-ras gene, and the genome overall. The frequency of CPDs in the two BamHI fragments and the genome overall were similar and ranged from 0.5 to 1.3 CPDs per 10 kb for UV doses of 5-30 J/m2. However, the removal of CPDs from the DNA fragment containing the albumin gene was significantly higher than from that of the genome overall and the DNA fragment containing the H-ras gene. Within 24 hr, approximately 67% of the CPDs was removed from the DNA fragment containing the albumin gene versus less than 40% for the genome overall and the DNA fragment containing the H-ras gene. The lower repair observed for the 14-kb fragment containing the H-ras gene is probably indicative of repair of the nontranscribed region of this fragment because the H-ras gene makes up only 2.4 kb of the 14-kb fragment. Primary cultures of hepatocytes removed CPDs from the transcribed strand of albumin fragment more efficiently than from the nontranscribed strand; however, no differences were observed in the repair of the two strands of the fragment containing the H-ras gene. These results demonstrate that primary cultures of nondividing rat hepatocytes show differential repair of UV-induced DNA damage that is comparable to what has been reported for transformed, proliferating mammalian cell lines.

The mutagenesis proteins UmuD' and UmuC prevent lethal frameshifts while increasing base substitution mutations

Error-prone DNA repair consists of replicative filling-in of DNA gaps carrying lesions. We have reconstituted E. coli SOS error-prone repair using purified DNA polymerase III holoenzyme, SSB, RecA, UmuD', a UmuC fusion protein, and a gap lesion plasmid. In the absence of UmuDC, or without SOS induction, replication skips over the lesion, forming mostly one-nucleotide deletions. These cause translational frameshifts that usually inactivate genes. UmuD' and UmuC, in the presence of RecA and SSB, stimulate translesion replication and change its mutagenic specificity such that deletions are prevented and base substitutions are increased. This results in mutagenic but nondetrimental gap repair and provides an effective mechanism for generating genetic variation in bacteria adapting to environmental stress.

Nucleotide excision repair proteins may be involved in the fixation of glyoxal-induced mutagenesis in Escherichia coli

To investigate the influence of nucleotide excision repair (NER) on glyoxal-induced mutations, we treated wild-type and NER-deficient (uvrC) Escherichia coli strains with glyoxal, and analyzed mutations in the chromosomal lacI gene. In both strains, the cell death and the mutation frequency increased according to the dose of glyoxal added to the culture medium, and cell death was induced to a similar level in both strains. Interestingly, the frequency of glyoxal-induced mutations in the wild-type strain was higher than that in the uvrC strain. Particularly, the frequency of base-pair substitutions was 4.7-fold higher in the wild-type strain. In the wild-type strain, G:C-->T:A transversions were predominant, followed by G:C-->A:T and A:T-->T:A mutations. In the uvrC strain, G:C-->A:T transitions were predominant, followed by G:C-->T:A transversions. All the base-pair substitutions except for G:C-->A:T transitions were >4-fold higher in the wild-type strain than in the uvrC strain. These results suggest that NER may be involved in the fixation of glyoxal-induced base-pair substitutions.

Rapid changes of nucleotide excision repair gene expression following UV-irradiation and cisplatin treatment of Dictyostelium discoideum

Organisms use different mechanisms to detect and repair different types of DNA damage, and different species vary in their sensitivity to DNA damaging agents. The cellular slime mold Dictyostelium discoideum has long been recognized for its unusual resistance to UV and ionizing radiation. We have recently cloned three nucleotide excision repair (NER) genes from Dictyostelium , the rep B, D and E genes (the homologs of the human xeroderma pigmentosum group B, D and E genes, respectively). Each of these genes has a unique pattern of expression during the multicellular development of this organism. We have now examined the response of these genes to DNA damage. The rep B and D DNA helicase genes are rapidly and transiently induced in a dose dependent manner following exposure to both UV-light and the widely used chemotherapeutic agent cisplatin. Interestingly, the rep E mRNA level is repressed by UV but not by cisplatin, implying unique signal transduction pathways for recognizing and repairing different types of damage. Cells from all stages of growth and development display the same pattern of NER gene expression following exposure to UV-light. These results suggest that the response to UV is independent of DNA replication, and that all the factors necessary for rapid transcription of these NER genes are either stable throughout development, or are continuously synthesized. It is significant that the up-regulation of the rep B and D genes in response to UV and chemical damage has not been observed to occur in cells from other species. We suggest that this rapid expression of NER genes is at least in part responsible for the unusual resistance of Dictyostelium to DNA damage.

Saccharomyces cerevisiae exonuclease-1 plays a role in UV resistance that is distinct from nucleotide excision repair

Two closely related genes, EXO1 and DIN 7, in the budding yeast Saccharomyces cerevisiae have been found to be sequence homologs of the exo1 gene from the fission yeast Schizosaccharomyces pombe . The proteins encoded by these genes belong to the Rad2/XPG and Rad27/FEN-1 families, which are structure-specific nucleases functioning in DNA repair. An XPG nuclease deficiency in humans is one cause of xeroderma pigmentosum and those afflicted display a hypersensitivity to UV light. Deletion of the RAD2 gene in S. cerevisiae also causes UV hypersensitivity, due to a defect in nucleotide excision repair (NER), but residual UV resistance remains. In this report, we describe evidence for the residual repair of UV damage to DNA that is dependent upon Exo1 nuclease. Expression of the EXO1 gene is UV inducible. Genetic analysis indicates that the EXO1 gene is involved in a NER-independent pathway for UV repair, as exo1 rad2 double mutants are more sensitive to UV than either the rad2 or exo1 single mutants. Since the roles of EXO1 in mismatch repair and recombination have been established, double mutants were constructed to examine the possible relationship between the role of EXO1 in UV resistance and its roles in other pathways for repair of UV damaged DNA. The exo1 msh2 , exo1 rad51 , rad2 rad51 and rad2 msh2 double mutants were all more sensitive to UV than their respective pairs of single mutants. This suggests that the observed UV sensitivity of the exo1 deletion mutant is unlikely to be due to its functional deficiencies in MMR, recombination or NER. Further, it suggests that the EXO1 , RAD51 and MSH2 genes control independent mechanisms for the maintenance of UV resistance.

Species identification of Mycoplasma bovis and Mycoplasma agalactiae based on the uvrC genes by PCR

The DNA repair genes uvrC from Mycoplasma bovis and Mycoplasma agalactiae type strains were cloned and their nucleotide sequences were established. These sequences were used to design polymerase chain reaction (PCR) primer pairs for M. bovis and M. agalactiae. Each primer pair amplified a 1-6 kb fragment of the uvrC gene in the respective species. The specificity of the primer pairs for the two species was demonstrated through the lack of cross-amplifications in heterologous PCR reactions and in reactions using DNA from other mycoplasma species. Subsequent restriction enzyme analysis of the amplified uvrC gene segments from type and field strains of M. bovis and M. agalactiae showed that the uvrC genes are well conserved in both species but differ significantly between the two species. The diagnostic PCR assay enabled unambiguous identification of M. bovis and M. agalactiae strains isolated from geographically diverse places, even in cases where 16S rRNA gene sequence analysis was unable to discriminate between the two species.

Transient global ischemia triggers expression of the DNA damage-inducible gene GADD45 in the rat brain

Using in situ hybridization, Northern blot analysis, Western blot analysis, and immunocytochemistry, mRNA and protein expression of the novel DNA damage-inducible gene GADD45 was examined in the rat brain at 0.5, 2, 4, 8, 16, 24, 48, and 72 hours after 15 minutes of transient global ischemia. Transient ischemia produced by the four-vessel occlusion method resulted in DNA double-strand breaks and delayed neuronal cell death in vulnerable neurons of the hippocampal CA1 sector, the hilus, dorsal caudate-putamen, and thalamus, as shown by in situ DNA nick end-labeling and histologic staining. GADD45 mRNA was transiently increased in less-vulnerable regions such as the parietal cortex (up to 8 hours after ischemia) and dentate granule cells (up to 24 hours after ischemia) but was persistently increased in vulnerable neurons such as CA1 pyramidal neurons (up to 48 hours). GADD45 immunoreactivity was increased in both vulnerable and less-vulnerable regions at earlier reperfusion periods (4 to 16 hours), but thereafter immunoreactivity was decreased below control levels in most vulnerable regions before delayed cell death and DNA double-strand breaks. At 72 hours after transient ischemia, a moderate increase in GADD45 immunoreactivity was still detectable in some CA3 neurons and in a few surviving neurons in the CA1 region. Double staining performed at 16 to 72 hours after ischemia revealed that GADD45 immunoreactivity was persistently increased in neurons that did not develop DNA damage. Because GADD45 protein may participate in the DNA excision repair process and because it has been shown that this protein is also overexpressed in neurons that survive focal ischemia and kainate-induced epileptic seizures, the results reported here support the hypothesis that GADD45 could have a protective role in neuronal injury.