Induction of an error-prone mode of DNA repair in UV-irradiated monkey kidney cells

Use of an infectious Simian virus 40-based shuttle vector to analyse UV-induced mutagenesis in monkey cells

SV40 based shuttle vectors able to be packaged as pseudovirions have been used either as naked DNA or as pseudovirus to analyse the mutation frequency and the UV-induced mutation spectra obtained after transfection or infection of COS7 monkey cells. The frequency of supF spontaneous mutants was similar whatever the state of the vector, indicating that the transfection step is not responsible for the high spontaneous mutation frequency when using shuttle vectors. Nevertheless the UV-induced mutation frequency of the supF gene was higher when transfected DNA was replicated into COS7 cells than when pseudovirus infection was performed. The UV induced mutation spectra was basically similar in both situations but a new hot-spot at nucleotide 110 was obtained after pseudovirus infection. UV-pretreated and control COS7 cells were infected with untreated or UV-damaged pi SVPC7 shuttle virus and the survival and the supF mutation frequency were analysed in the progeny. The survival of UV-damaged pseudovirus replicated in 10 J/m2 UV-pretreated cells was 2-fold higher than in untreated cells. This increase in the survival was accompanied by a slight enhancement in the number of supF mutants.

Mutagenicity of a unique thymine-thymine dimer or thymine-thymine pyrimidine pyrimidone (6-4) photoproduct in mammalian cells

The mutagenic properties of UV-induced photoproducts, both the cis-syn thymine-thymine dimer (TT) and the thymine-thymine pyrimidine pyrimidone (6-4) photoproduct [T(6-4)T] were studied in mammalian cells using shuttle vectors. A shuttle vector able to replicate in both mammalian cells and bacteria was produced in its single-stranded DNA form. A unique photoproduct was inserted at a single restriction site and after recircularization of the single-stranded DNA vector, this latter was transfected into simian COS7 cells. After DNA replication the vector was extracted from cells and used to transform bacteria. Amplified DNA was finally analyzed without any selective screening, DNA from randomly picked bacterial colonies being directly sequenced. Our results show clearly that both lesions are mutagenic, but at different levels. Mutation frequencies of 2 and 60% respectively were observed with the TT dimer and the T(6-4)T. With the TT dimer the mutations were targeted on the 3'-T. With the T(6-4)T a large variety of mutations were observed. A majority of G-->T transversions were semi-targeted to the base before the 5'-T of the photoproduct. These kinds of mutations were not observed when the same plasmid was transfected directly into SOS-induced JM105 bacteria or when the T(6-4)T oligonucleotide inserted in a different plasmid was replicated in SOS-induced SMH10 Escherichia coil bacteria. These semi-targeted mutations are therefore the specific result of bypass of the T(6-4)T lesion in COS7 cells by one of the eukaryotic DNA polymerases.

A putative SOS repair gene (dinF-like) in a hyperthermophilic archaeon

The hyperthermophilic archaeon, Pyrococcus (strain IFREMER AL585), contains an ORF that encodes a polypeptide with a high similarity to the Escherichia coli dinF (DNA damage-inducible) gene product. The conservation of this protein between Archaea and Bacteria suggests that a SOS repair system might operate in Archaea.

Enhanced reactivation of nitrous acid treated adenovirus is not associated with enhanced mutagenesis in pretreated with heavy metals HeLa cells

The reversion frequency of an adenovirus 2 temperature-sensitive growth mutant treated with different doses of nitrous acid was determined after infection of control. UV-irradiated, cadmium chloride and zinc chloride treated HeLa cells. No enhanced mutagenesis was observed.

Mutation spectrum of 4-nitroquinoline 1-oxide-damaged single-stranded shuttle vector DNA transfected into monkey cells

4-Nitroquinoline 1-oxide (4NQO) is a potent mutagen and carcinogen which induces two main guanine adducts at positions C8 and N2. We recently determined the mutation spectrum induced by the ultimate metabolite of 4NQO, acetoxy-4-aminoquinolone 1-oxide in the M13lacZ'/E. coli lacZ delta M15 alpha-complementation assay. Our data suggested that dGuo-C8-AQO induces (per se or via AP sites) G to Pyr transversions. Here we report our study on 4NQO mutagenesis in monkey cells. 4NQO lesions were induced in vitro on a single-stranded (ss) DNA shuttle vector carrying the supF tRNA gene. This vector was able to replicate both in mammalian cells and in bacteria. The mutations induced in monkey cells were screened by the white/blue beta-galactosidase activity assay in E. coli. We took advantage of the peculiar feature of ss supF DNA in which the extent of secondary structure may be a function of the temperature, with the dependence of the 4NQO-specific adduct spectrum on DNA secondary structure. We reasoned that mutational spectra derived from damage induced in the presence (20 degrees C) or absence (70 degrees C) of DNA secondary structure should be different. The result of sequencing a total of 89 induced and spontaneous mutants confirmed that the spectra are statistically different. These data suggest that the two 4NQO guanine adducts may induce different mutations.

Alteration of irradiated shuttle vector processing by exposure of human lymphoblast host cells to single or split gamma-ray doses

The repair of damaged DNA by mammalian cells exposed to single or split doses of radiation was probed with shuttle vector pZ189. Human lymphoblast hosts who received a single 120 cGy dose 2 h before transfection with 2500 cGy-damaged pZ189 yielded a two-fold higher frequency of progeny plasmids with mutations in their supF-tRNA target genes than did unirradiated host cells. Delaying transfection for 12 h, however, reduced the mutation frequency by half versus unirradiated controls. Plasmid survival was also affected by the time between host cell irradiation and transfection. Splitting doses of 50-500 cGy into two equal fractions separated by 4 h lowered mutation frequency and increased plasmid survival compared with equivalent acute doses; increasing the interval between dose fractions to 8 h, however, lowered plasmid survival compared with acute doses. Sequence analyses of the target gene in mutant plasmids revealed increased multiple-base substitution mutations among progenies recovered from irradiated hosts, indicating enhanced excision repair. These findings support modulation of mammalian cell DNA repair by ionizing radiation, disclose the transient nature of the effect of radiation on DNA repair, and demonstrate a quantitative difference in the effectiveness of single and split doses.

Replication of damaged DNA and the molecular mechanism of ultraviolet light mutagenesis

On UV irradiation of Escherichia coli cells, DNA replication is transiently arrested to allow removal of DNA damage by DNA repair mechanisms. This is followed by a resumption of DNA replication, a major recovery function whose mechanism is poorly understood. During the post-UV irradiation period the SOS stress response is induced, giving rise to a multiplicity of phenomena, including UV mutagenesis. The prevailing model is that UV mutagenesis occurs by the filling in of single-stranded DNA gaps present opposite UV lesions in the irradiated chromosome. These gaps can be formed by the activity of DNA replication or repair on the damaged DNA. The gap filling involves polymerization through UV lesions (also termed bypass synthesis or error-prone repair) by DNA polymerase III. The primary source of mutations is the incorporation of incorrect nucleotides opposite lesions. UV mutagenesis is a genetically regulated process, and it requires the SOS-inducible proteins RecA, UmuD, and UmuC. It may represent a minor repair pathway or a genetic program to accelerate evolution of cells under environmental stress conditions.

A damage-responsive DNA binding protein regulates transcription of the yeast DNA repair gene PHR1

The PHR1 gene of Saccharomyces cerevisiae encodes the DNA repair enzyme photolyase. Transcription of PHR1 increases in response to treatment of cells with 254-nm radiation and chemical agents that damage DNA. We report here the identification of a damage-responsive DNA binding protein, termed photolyase regulatory protein (PRP), and its cognate binding site, termed the PHR1 upstream repression sequence, that together regulate induction of PHR1 transcription after DNA damage. PRP activity, monitored by electrophoretic-mobility-shift assay, was detected in cells during normal growth but disappeared within 30 min after irradiation. Copper-phenanthroline footprinting of PRP-DNA complexes revealed that PRP protects a 39-base-pair region of PHR1 5' flanking sequence beginning 40 base pairs upstream from the coding sequence. A prominent feature of the foot-printed region is a 22-base-pair palindrome. Deletion of the PHR1 upstream repression sequence increased the basal level expression of PHR1 in vivo and decreased induction after exposure of cells to UV radiation or methyl methanesulfonate, whereas insertion of the PRP binding site between the CYC1 upstream activation sequence and "TATA" sequence reduced basal level expression and conferred damage responsiveness upon a reporter gene. Thus these observations establish that PRP is a damage-responsive repressor of PHR1 transcription.

Expression of the yeast PHR1 gene is induced by DNA-damaging agents

The PHR1 gene of Saccharomyces cerevisiae encodes a photolyase which repairs specifically and exclusively pyrimidine dimers, the most frequent lesions induced in DNA by far-UV radiation. We have asked whether expression of PHR1 is modulated in response to UV-induced DNA damage and to DNA-damaging agents that induce lesions structurally dissimilar to pyrimidine dimers. Using a PHR1-lacZ fusion gene in which expression of beta-galactosidase is regulated by PHR1 5' regulatory elements, we found that exposure of cells to 254-nm light, 4-nitroquinoline-N-oxide, methyl methanesulfonate, and N-methyl-N'-nitro-N-nitrosoguanidine induced synthesis of increased amounts of fusion protein. In contrast to these DNA-damaging agents, neither heat shock nor exposure to photoreactivating light elicited a response. Induction by far-UV radiation was evident both when the fusion gene was carried on a multicopy plasmid and when it replaced the endogenous chromosomal copy of PHR1, and it was accompanied by an increase in the steady-state concentration of PHR1-lacZ mRNA. Northern (RNA) blot analysis of PHR1 mRNA encoded by the chromosomal locus was consistent with either enhanced transcription of PHR1 after DNA damage or stabilization of the transcripts. Neither the intact PHR1 or RAD2 gene was required for induction. Comparison of the region of PHR1 implicated in regulation of its expression with other damage-inducible genes from yeast cells revealed a common conserved sequence that is present in the PHR1, RAD2, and RNR2 genes and is required for damage inducibility of the latter two genes. These sequences may constitute elements of a damage-responsive regulon in S. cerevisiae.

Expression of the yeast PHR1 gene is induced by DNA-damaging agents

The PHR1 gene of Saccharomyces cerevisiae encodes a photolyase which repairs specifically and exclusively pyrimidine dimers, the most frequent lesions induced in DNA by far-UV radiation. We have asked whether expression of PHR1 is modulated in response to UV-induced DNA damage and to DNA-damaging agents that induce lesions structurally dissimilar to pyrimidine dimers. Using a PHR1-lacZ fusion gene in which expression of beta-galactosidase is regulated by PHR1 5' regulatory elements, we found that exposure of cells to 254-nm light, 4-nitroquinoline-N-oxide, methyl methanesulfonate, and N-methyl-N'-nitro-N-nitrosoguanidine induced synthesis of increased amounts of fusion protein. In contrast to these DNA-damaging agents, neither heat shock nor exposure to photoreactivating light elicited a response. Induction by far-UV radiation was evident both when the fusion gene was carried on a multicopy plasmid and when it replaced the endogenous chromosomal copy of PHR1, and it was accompanied by an increase in the steady-state concentration of PHR1-lacZ mRNA. Northern (RNA) blot analysis of PHR1 mRNA encoded by the chromosomal locus was consistent with either enhanced transcription of PHR1 after DNA damage or stabilization of the transcripts. Neither the intact PHR1 or RAD2 gene was required for induction. Comparison of the region of PHR1 implicated in regulation of its expression with other damage-inducible genes from yeast cells revealed a common conserved sequence that is present in the PHR1, RAD2, and RNR2 genes and is required for damage inducibility of the latter two genes. These sequences may constitute elements of a damage-responsive regulon in S. cerevisiae.

Mechanisms and consequences of mutation induction in mammalian cells

Mutations have been studied for several decades in order to understand biological processes of great significance and the selection of better-adapted species. Our knowledge both of mutation spectra induced by genotoxic agents and the mechanisms involved in DNA damage processing is more advanced in bacteria than in animal cells. However, the use of new technologies such as shuttle vectors or the polymerase chain reaction will undoubtedly allow rapid progress in the next few years. Shuttle vectors consist of target sequences for monitoring mutagenic activity and additional sequences permitting DNA replication and selection, both in bacteria and in mammalian cells. These plasmids are very efficient in allowing the production of mutation spectra of a particular genotoxin in animal cells. In most cases, base substitutions occur predominantly at the sites of base damage and the type of substitution depends on the kind of damage. This has been well characterized using ultraviolet (UV) light as a mutagen. UV-induced mutations are targeted opposite pyrimidine-pyrimidine sites, where the two major UV lesions are produced. The direct relationships existing between mutation and cancer are exemplified by some hereditary diseases where deficiency in an enzymatic repair system is linked to a high incidence of tumours. Similarly, activation of some cellular proto-oncogenes occurs via specific point mutations. A correlation does exist between the mutation spectra found in model systems and the specific mutation found in the activated oncogene in tumours induced by a given genotoxin. This is particularly well illustrated in the DNA repair deficiency syndrome, xeroderma pigmentosum. The specific mutations found in activated ras oncogenes isolated from UV-stimulated skin tumours correlate well with the mutagenic properties of unrepaired UV-induced DNA lesions.

Respective roles of pyrimidine dimer and pyrimidine (6-4) pyrimidone photoproducts in UV mutagenesis of simian virus 40 DNA in mammalian cells

UV light induces DNA lesions which are mutagenic in mammalian cells. We used simian virus 40 tsB201 (unable to produce viral capsid at the restrictive temperature of 41 degrees C because of a point mutation in the VP1 gene) to analyze the mutagenic potency of the two major UV-induced lesions, pyrimidine dimers (Py-Py) and pyrimidine (6-4) pyrimidones [Py(6-4)Py], which are formed on the same nucleotide sites. The mutagenesis criterion was the reversion toward a wild-type growth phenotype. After UV irradiation (mainly at 254 nm), part of the DNA was treated with the photoreactivating enzyme of Escherichia coli, which monomerizes Py-Py but does not modify the Py(6-4)Py photoproduct. Higher survival and lower mutation frequency rates for the photoreactivated DNA indicated that the two lesions were lethal and mutagenic. The VP1 gene of some mutants was entirely sequenced. The mutation spectra showed that the two lesions did not induce the same mutation hot spots, although some sites were common to both. The induced mutation hot spots were not only correlated with lesion hot spots but seemed partially directed by local DNA structures.

UV-enhanced reactivation of UV-damaged SV40 is due to the restoration of viral early gene function

Mammalian cells respond to UV-radiation by inducing an increased ability to support the survival of UV-damaged virus. We have tested whether the induction of enhanced viral reactivation (ER) reflects heightened UV-resistance of specific viral functions. For this, we examined the extent of ER for SV40 containing UV-damage in three functionally distinct regions of the SV40 genome: (i) the viral regulatory region, (ii) the early genes region and (iii) the late genes region. ER corresponding to a dose reduction factor of 43% was observed for damage in the early genes region. No ER was observed for damage in the regulatory or late genes regions. We conclude that ER in SV40 reverses the lethal disruption of an essential function peculiar to the viral early genes region. This function is almost certainly transcription.

From simian virus 40 to transient shuttle vectors in mutagenesis studies

Exogenous DNA probes are frequently used to study mutagenesis in mammalian cells. Experimental protocols utilizing simian virus 40 (SV40) and transient shuttle vectors able to replicate in mammalian cells as well as in bacteria are described. The main interests and the limits of the 2 genetic assays are discussed from results obtained with both systems. Despite some minor discrepancies, results obtained are very similar using either method. The complementarity of the 2 assays will allow a better comprehension of the mechanisms by which mutations may arise in mammalian cells.

Homologous recombination is not enhanced in UV-irradiated normal and repair-deficient human fibroblasts

Many mammalian cells exhibit damage-inducible phenomena that resemble the bacterial SOS functions. However, whereas RecA plays a prominent role in the prokaryotic SOS response, in mammalian cells so far no enhanced recombination as a result of treatment with DNA-damaging agents of the cells, rather than of infecting viruses, has been found. In order to study recombination as a UV-inducible cellular phenomenon we infected UV-irradiated normal and repair-deficient human fibroblasts with a mixed population of adenovirus 5 (Ad5) mutants that carried a deletion in the E1A or the E2A gene. Wild-type recombinant progeny viruses were readily obtained, but no enhanced recombination was observed at any UV dose given to the cells, nor at any time point between -6 h and +4 days between irradiation and infection. Control experiments, in which we infected unirradiated cells with UV-irradiated Ad5 deletion mutants (a test for recombination targeted at UV-damaged DNA) showed a strong increase in wild-type recombinant viruses when both deletion mutants had been irradiated compared to the additive effect of irradiation of either one of the mutants alone. Therefore, this study shows that UV irradiation results in an enhanced recombination activity in cells that is specifically targeted to damaged DNA, but it does not cause a general (untargeted) recombinational response (enhanced recombination) in the cell.

Radiation-induced recovery processes in cultured marsupial cells

The ultraviolet sensitivity of Potorous tridactylus male kidney (PtK-2) cells is markedly increased by post irradiation treatment for 24 h with 5 microM emetine (which inhibits protein synthesis by acting on the 40S ribosomal subunit), or with 5 microM cycloheximide (which inhibits by interaction with the 60S subunit), or with the RNA polymerase II inhibitor 5,6-dichloro-1-beta-ribofuranosylbenzimidazole at 50 microM. All 3 treatments give the same sensitivity, while unirradiated cells are little affected. Shortening the time of treatment, or delaying application of the drugs decreases their effect on the same time schedule. Preiirradiation of cells, with no drug treatment in the following 8 h, diminishes the sensitivity to a subsequent irradiation with protein synthesis blocked afterwards. Photoreactivation immediately following such preirradiation eliminates its desensitizing effect. Inhibiting protein synthesis after irradiation also markedly reduces the frequency of UV-induced mutants in the surviving population. These facts suggest that gene expression in the period following irradiation facilitates recovery from radiation damage, with an increased probability of mutation, reminiscent of the "SOS response" in Escherichia coli.

Preirradiation of host cells does not alter blockage of simian virus 40 replication forks by pyrimidine dimers

Do damage-inducible responses in mammalian cells alter the interaction of lesions with replication forks? We have previously demonstrated that preirradiation of the host cell mitigates UV inhibition of SV40 DNA replication; this mitigation can be detected within the first 30 min after the test irradiation. Here we test the hypotheses that this mitigation involves either (1) rapid dimer removal, (2) rapid synthesis of daughter strands past lesions (trans-dimer synthesis), or (3) continued progression of the replication fork beyond a dimer. Cells preirradiated with UV were infected with undamaged SV40, and the effects of UV upon viral DNA synthesis were measured within the first hour after a subsequent test irradiation. In preirradiated cells, as well as in non-preirradiated cells, pyrimidine dimers block elongation of daughter strands; daughter strands grow only to a size equal to the interdimer distance along the parental strands. There is, within this first hour after UV, no evidence for trans-dimer synthesis, nor for more rapid dimer removal either in the bulk of the parental DNA or in molecules in the replication pool. Progression of the replication forks was analyzed by electron microscopy of replicating SV40 molecules. Dimers block replication-fork progression in preirradiated cells to the same extent as in non-preirradiated cells. These experiments argue strongly against the hypotheses that preirradiation of host cells results in either the rapid removal of dimers, trans-dimer synthesis, or continued replication-fork progression beyond dimers.

Enhanced reactivation of UV-irradiated adenovirus 2 is not associated with enhanced mutagenesis in carcinogen-pretreated HeLa cells

Treatment of HeLa cells with low doses of the carcinogens aflatoxin B1, methyl methanesulfonate (MMS) or ethyl methanesulfonate (EMS) increases the survival rate of UV-irradiated adenovirus 2 (ade2). This increase is maximal if the time interval between cell treatment and virus infection is delayed by 36 h. No enhanced mutagenesis was found measuring the reversion frequency of a temperature-sensitive mutant of ade2 grown in HeLa cells treated with the same carcinogens. The enhanced viral reactivation observed does not, therefore, display a significant error-prone component.

Preirradiation of host (monkey) cells mitigates the effects of UV upon simian virus 40 DNA replication

We are examining the effects of preirradiation of host (monkey) cells upon the replication of UV-damaged SV40. Control cells and cells preirradiated with low fluences (5 or 10 J/m2) of UV were infected with undamaged SV40, and the immediate effects of a subsequent irradiation were determined. UV inhibited total SV 40 DNA synthesis (incorporation of thymidine into viral DNA) in both preirradiated and control cells, but the extent of inhibition was less in the preirradiated cells. A test fluence of 60 J/m2 to SV40 replicating in preirradiated cells reduced synthesis only as much as a test fluence of 25 J/m2 in control cells. The fraction of recently replicated SV40 molecules that re-entered the replication pool and subsequently completed one round of replication in the first 2 h after UV was also decreased less in the preirradiated cells. Thus preirradiation of the host cell mitigates the immediate inhibitory effects of a subsequent UV exposure upon SV40 replication.

An immortalized xeroderma pigmentosum, group C, cell line which replicates SV40 shuttle vectors

We have established and characterized an immortalized xeroderma pigmentosum (XP), group C, cell line. Transformation of the human fibroblasts was carried out with a recombinant plasmid, pLAS-wt, containing SV40 DNA encompassing the entire early region with a defective origin of DNA replication. The transformed XP cell line, XP4PA-SVwt, and the normal transformed fibroblasts AS3-SVwt, both express SV40 T antigen together with enhanced levels of the transformation-associated cellular protein, p53. XP4PA-SVwt retains the XP UV-repair defective phenotype as demonstrated by low levels of unscheduled DNA synthesis and by the reduced survival of irradiated SV40 virus. Analysis of cellular DNA shows a single major, stable, integration site of pLAS-wt in the XP4PA-SVwt cells. The T antigen in these cells supports efficiently the replication of SV40 based shuttle vectors and should prove suitable for the introduction, expression and selection of genes related to DNA repair and to the study of mutagenesis using defined molecular probes.

Cellular response to DNA damage is enhanced by the pR plasmid in mouse cells and in Escherichia coli

The pR plasmid, which enhances the survival of Escherichia coli C600 exposed to UV light by induction of the SOS regulatory mechanism, showed the same effect when it transformed mouse LTA cells (tk-, aprt-). With Tn5 insertion mutagenesis which inactivates UV functions in the pR plasmid, we recognized two different regions of the plasmid, uvp1 and uvp2. These pR UVR- mutants exhibited the same effect in LTA transformed cells, demonstrating that resistance to UV light, carried by the pR plasmid, was really due to the expression of these two regions, which were also in the mouse cells. Statistical analysis showed that the expression of the uvp1 and uvp2 regions significantly increased (P less than 0.01) the survival upon exposure to UV light in mouse cells and bacteria. These results might suggest the presence of an inducible repair response to DNA damage in mouse LTA cells.

Enhanced mutagenesis of UV-irradiated simian virus 40 occurs in mitomycin C-treated host cells only at a low multiplicity of infection

Treatment of monkey kidney cells with mitomycin C (MMC) 24 h prior to infection with UV-irradiated simian virus 40 (SV40) enhanced both virus survival and virus mutagenesis. The use of SV40 as a biological probe has been taken as an easy method to analyse SOS response of mammalian cells to the stress caused by DNA damage or inhibition of DNA replication. The mutation assay we used was based on the reversion from a temperature-sensitive phenotype (tsA58 mutant) to a wild-type phenotype. The optimal conditions for producing enhanced survival and mutagenesis in the virus progeny were determined with regard to the multiplicity of infection (MOI). Results showed that the level of enhanced mutagenesis observed for UV-irradiated virus grown in MMC-treated cells was an inverse function of the MOI, while enhanced survival was observed at nearly the same level regardless of the MOI. For the unirradiated virus, almost no increase in the mutation of virus progeny issued from MMC-treated cells was observed, while a small amount of enhanced virus survival was obtained. These results show that enhanced virus mutagenesis and enhanced virus survival can be dissociated under some experimental conditions. Enhanced virus mutagenesis, analogous to the error-prone replication of phages in SOS-induced bacteria, was observed, at least for SV40, only when DNA of both virus and host cells was damaged and when infection occurred with a small number of viral particles. We therefore hypothesize that an error-prone replication mode of UV-damaged templates is observed in induced monkey kidney cells.

2-(N-acetoxy-N-acetylamino)fluorene mutagenesis in mammalian cells: sequence-specific hot spot

Mutations induced by 2-(N-acetoxy-N-acetylamino)fluorene were studied using temperature-sensitive simian virus 40 (SV40) mutants as probe in monkey kidney cells. In vitro treatment of the SV40 virions with 2-(N-acetoxy-N-acetylamino)fluorene increased mutagenesis and decreased survival in the viral progeny. A lethal hit of approximately 85 acetylaminofluorene adducts per SV40 genome was calculated. UV irradiation of cells prior to infection did not modify the results. Molecular analysis of independent SV40 revertants showed that 2-(N-acetoxy-N-acetylamino)fluorene induces base substitutions that are located not opposite putative acetylaminofluorene adducts but next to them. Moreover, a hot spot of mutation restoring a true wild-type genotype was observed in 10 of the 16 revertants analyzed. This hot spot, not targeted opposite a major DNA lesion, was not observed using UV light as damaging agent in the same genetic assay. Two models involving the stabilization, by acetylaminofluorene adducts, of the secondary structure of a specific quasipalindromic SV40 sequence are proposed to explain this sequence-specific hot spot.

The use of recombinant DNA techniques to study radiation-induced damage, repair and genetic change in mammalian cells

A brief Introduction is given to appropriate elements of recombinant DNA techniques and applications to problems in radiobiology are reviewed with illustrative detail. Examples are included of studies with both 254 nm ultraviolet light (u.v.) and ionizing radiation (i.r.) and the review progresses from the molecular analysis of DNA damage in vitro through to the nature of consequent cellular responses. The section on the Molecular distribution of DNA damage (section 2) focuses on the use of defined DNA molecules to assess the nature, sites and frequency of radiation damage. Recombinant DNA techniques have also been used in the study of enzyme-DNA interactions, to comment upon the rôle of specific types and sites of damage in producing cellular responses. The use of DNA-mediated gene transfer to assess damage and repair (section 3) indicates that recombinant DNA molecules can be used to implicate (or reject) specific types of DNA damage in gene inactivation. Some gene-transfer assays may also be able to confirm the presence of specific repair functions in mammalian cells. Restriction endonucleases are essential for the construction of recombinant DNA molecules, but their ability to cut DNA at specific sequences is also being exploited to implicate the double-strand break as an important type of damage leading to the well-characterized responses of irradiated cells. The DNA double strand break: use of restriction endonucleases to model radiation damage (section 4) documents experiments showing that blunt-ended cuts introduced into cellular DNA are able to produce chromosome aberrations and cell death. Assays based upon the introduction of restriction endonuclease-cut plasmids into radiosensitive and normal cells suggest that sensitivity is in some instances, e.g. the radiosensitive disorder ataxia-telangiectasia, a result of excessive degradation of DNA around broken ends. Identification and cloning of DNA repair genes (section 5) reviews the successful cloning of one human repair gene and the putative identification of others, as well as the lack of success in identifying genes complementing radiosensitive human disorders. Analysis of radiation-induced genetic change (section 6) links the types of DNA damage observed in defined DNA molecules with the types of mutations occurring in irradiated prokaryotes. In mammalian cells recombinant DNA techniques have allowed the nature of mutational changes to be determined for the first time: to date it seems that u.v. produces mainly small (point) mutations while i.r. produces mainly large changes (deletions/rearrangements).(ABSTRACT TRUNCATED AT 400 WORDS)

Benzo[a]pyrene diol epoxide induces viral reactivation at concentrations that block DNA elongation in mammalian cells

The survival of UV-irradiated Simian virus 40 (SV40) in CV-1P African green monkey kidney cells treated with (+/-)7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BP-diol epoxide I) was studied. Enhanced survival of UV damaged SV40 was detected when CV-1P cells were treated with dose levels of BP-diol epoxide I corresponding to the exponential portion (0.33-1.11 microM) of a CV-1P cell survival curve. Dose levels of BP-diol epoxide I corresponding to the shoulder region (less than or equal to 0.16 microM) of a CV-1P survival curve did not induce viral reactivation. The shoulder region concentrations of BP-diol epoxide I selectively inhibited DNA initiation while the concentrations on the exponential portion of the curve preferentially inhibited DNA elongation. It was shown in a time course of enhanced viral survival at 0.66 microM BP-diol epoxide I that the reactivation response was fully induced by 24 h. In conclusion, the viral reactivation response was associated with concentrations of BP-diol epoxide I which induced lethal damage and preferentially inhibited DNA elongation.

Inhibition of SV40 DNA replication by benzo[a]pyrene diol epoxide adducts: two recovery modes

Anti-benzo[a]pyrene diol epoxide (BPDE) adducts produced in vitro in SV40 initially inhibit SV40 DNA replication in vivo, in cells unexposed to BPDE. A single adduct in a replicon is probably sufficient to block DNA replication. The recovery process appears to begin immediately after infection. The rate of recovery of replicative capacity is inversely related to the initial adduct number. Holding the infected cells temporarily under conditions that prevent viral DNA replication results subsequently in increased recovery, proportional to the holding time. The mechanism of recovery appears to be constitutive and prereplicative. In addition, there is a second mode of recovery which is induced by pretreatment of the host cells with BPDE before infection. The effect of pretreatment is similar to that of extending the holding time before replication: the first molecules begin to replicate earlier but the subsequent rate of recovery is unchanged. The induced mechanism may be either a limited stoichiometric repair process or a slow replicative bypass.

Enhanced mutagenesis of UV-irradiated simian virus 40 occurs in mitomycin C-treated host cells only at a low multiplicity of infection

Treatment of monkey kidney cells with mitomycin C (MMC) 24 h prior to infection with UV-irradiated simian virus 40 (SV40) enhanced both virus survival and virus mutagenesis. The use of SV40 as a biological probe has been taken as an easy method to analyse SOS response of mammalian cells to the stress caused by DNA damage or inhibition of DNA replication. The mutation assay we used was based on the reversion from a temperature-sensitive phenotype (tsA58 mutant) to a wild-type phenotype. The optimal conditions for producing enhanced survival and mutagenesis in the virus progeny were determined with regard to the multiplicity of infection (MOI). Results showed that the level of enhanced mutagenesis observed for UV-irradiated virus grown in MMC-treated cells was an inverse function of the MOI, while enhanced survival was observed at nearly the same level regardless of the MOI. For the unirradiated virus, almost no increase in the mutation of virus progeny issued from MMC-treated cells was observed, while a small amount of enhanced virus survival was obtained. These results show that enhanced virus mutagenesis and enhanced virus survival can be dissociated under some experimental conditions. Enhanced virus mutagenesis, analogous to the error-prone replication of phages in SOS-induced bacteria, was observed, at least for SV40, only when DNA of both virus and host cells was damaged and when infection occurred with a small number of viral particles. We therefore hypothesize that an error-prone replication mode of UV-damaged templates is observed in induced monkey kidney cells.

Cellular response to DNA damage is enhanced by the pR plasmid in mouse cells and in Escherichia coli

The pR plasmid, which enhances the survival of Escherichia coli C600 exposed to UV light by induction of the SOS regulatory mechanism, showed the same effect when it transformed mouse LTA cells (tk-, aprt-). With Tn5 insertion mutagenesis which inactivates UV functions in the pR plasmid, we recognized two different regions of the plasmid, uvp1 and uvp2. These pR UVR- mutants exhibited the same effect in LTA transformed cells, demonstrating that resistance to UV light, carried by the pR plasmid, was really due to the expression of these two regions, which were also in the mouse cells. Statistical analysis showed that the expression of the uvp1 and uvp2 regions significantly increased (P less than 0.01) the survival upon exposure to UV light in mouse cells and bacteria. These results might suggest the presence of an inducible repair response to DNA damage in mouse LTA cells.

Timing of ultraviolet light induced mutagenesis in simian virus 40 relative to viral DNA replication in monkey cells

Simian virus 40 (SV40) was used to probe ultraviolet light (UV)-induced mutation in mammalian cells. Viral mutations were scored as reversions of early and late temperature-sensitive (ts) mutants to the wild-type (WT) phenotype. When virus was exposed to moderate or high UV doses, WT revertants were obtained at a frequency related to the square of the dose from two early (tsA) and one late (tsBC) mutant grown at the restrictive temperature. The reversions generated in the progeny of UV-irradiated early mutants presumably arose before the onset of viral DNA replication because, at the non-permissive temperature, tsA mutants are unable to express the functions responsible for the initiation of viral DNA synthesis. Moreover, the early mutant tsA209 underwent similar levels of induced reversion at the permissive and restrictive temperatures, suggesting that the pre-replicative mutational pathway might predominate for moderately and heavily irradiated virus, even under conditions where DNA synthesis can be initiated. The analysis of bursts from revertant plaques produced at the restrictive temperature was consistent with this interpretation. Although the mechanism of pre-replicative mutagenesis is not known, it is likely to be mediated by cellular activities owing to the low genetic complexity of the virus.

Enhancement of genetic transformation frequencies of mammalian cell cultures by damage to the cell DNA

Ultraviolet (UV)-light and 5-fluorodeoxyuridine (FUdR), two known DNA damaging agents, were found to enhance the frequency of stable plasmid transformations in several different animal cell lines. Combined treatment with the two agents was more effective than treatment with either agent alone. A correlation between the transformability of a cell line in the absence of treatment and its response to damaging treatment was also observed. Southern blot analysis of transformed clones indicated that the stimulation in transformation frequency was not due to an increased number of copies of the integrated plasmid in the transformed cells.

Reactivation and mutagenesis of herpes virus in 5-azacytidine-treated monkey kidney cells

Enhanced survival of UV-damaged herpes simplex virus and Simian virus 40 was investigated in CV-1 monkey cells treated with inhibitors of DNA methylation such as 5-azacytidine and ethionine. Survival of UV-irradiated virus was higher in treated cells than in untreated cells. Survival of herpes virus irradiated with 60Co gamma-ray was not enhanced in the treated cells. The frequency of forward mutation of herpes virus increased in 5-azacytidine-treated cells. Relative content of methylcytosine was reduced in the cells treated with 5-azacytidine. Therefore a mechanism similar to UV-enhanced reactivation of virus was operating in the cells with hypomethylated DNA.

Molecular analysis of mutagenesis in mammalian cells

Mammalian cells are constantly facing various types of mutagens. However, due to the high complexity of the cell genome, the molecular analysis of mutagenesis has not yet been possible. Therefore, we have used simian virus 40 (SV40) as a biological and molecular probe to characterize mutagenesis at the nucleotide level. By using a reversion assay from a temperature-sensitive phenotype towards a wild-type phenotype, we have analysed mutagenesis induced by u.v.-light and by apurinic sites (Ap sites). We report here experiments allowing us to quantify and to compare the mutagenic efficiency of various DNA lesions measured on the SV40 genome. The Ap sites are very mutagenic in this type of assay. The molecular analysis of u.v.-induced mutagenesis reveals that mutations correspond to single base-pair substitutions always located opposite Py-Py lesions. The mutations are almost equally distributed between transition and transversion types, and between the 5' and the 3' side of the Py-Py targets. These results demonstrate for the first time in animal cells the existence of targeted mutations induced by u.v.-light. We propose therefore, the use of SV40 as an efficient biological and molecular probe for assaying mutagenic pathways in mammalian cells.

Survival and mutagenesis of ultraviolet irradiated simian virus 40 in foetal human fibroblasts

Survival and mutagenesis of UV-irradiated, temperature-sensitive simian virus 40 mutants (SV40) have been studied after infection of human fibroblasts. Survival of the viral progeny obtained after 6,8 or 10 days at permissive temperature decrease as a function of the UV-dose delivered to the virus. In cels which have been pretreated with 10 Jm-2 of UV 24 hours before infection, progeny survival was increased as compared to survival in control cells. The reactivation factor varies from one to ten, depending on the number of lytic cycles carried out at permissive temperature. The level of mutation frequency, as measured by the reversion from a temperature sensitive growth phenotype towards a wild type phenotype, increases with the dose of UV-irradiation given to the virus. Moreover, the mutation frequency is increased in the viral progeny produced in UV-irradiated human cells. Similar experiments carried out with SV40-transformed human fibroblasts, which constitutively express SV40 T antigen, gave comparable results. These experiments show that, as in monkey cells, a new error-prone recovery pathway can be induced by pretreating human cells with UV-light before infection.

Specific transcripts are elevated in Saccharomyces cerevisiae in response to DNA damage

Differential hybridization has been used to identify genes in Saccharomyces cerevisiae displaying increased transcript levels after treatment of cells with UV irradiation or with the mutagen/carcinogen 4-nitroquinoline-1-oxide (NQO). We describe the isolation and characterization of four DNA damage responsive genes obtained from screening ca. 9,000 yeast genomic clones. Two of these clones, lambda 78A and pBR178C, contain repetitive elements in the yeast genome as shown by Southern hybridization analysis. Although the genomic hybridization pattern is distinct for each of these two clones, both of these sequences hybridize to large polyadenylated transcripts ca. 5 kilobases in length. Two other DNA damage responsive sequences, pBRA2 and pBR3016B, are single-copy genes and hybridize to 0.5- and 3.2-kilobase transcripts, respectively. Kinetic analysis of the 0.5-kilobase transcript homologous to pBRA2 indicates that the level of this RNA increases more than 15-fold within 20 min after exposure to 4-nitroquinoline-1-oxide. Moreover, the level of this transcript is significantly elevated in cells containing the rad52-1 mutation which are deficient in DNA strand break repair and gene conversion. These results provide some of the first evidence that DNA damage stimulates transcription of specific genes in eucaryotic cells.

Specific transcripts are elevated in Saccharomyces cerevisiae in response to DNA damage

Differential hybridization has been used to identify genes in Saccharomyces cerevisiae displaying increased transcript levels after treatment of cells with UV irradiation or with the mutagen/carcinogen 4-nitroquinoline-1-oxide (NQO). We describe the isolation and characterization of four DNA damage responsive genes obtained from screening ca. 9,000 yeast genomic clones. Two of these clones, lambda 78A and pBR178C, contain repetitive elements in the yeast genome as shown by Southern hybridization analysis. Although the genomic hybridization pattern is distinct for each of these two clones, both of these sequences hybridize to large polyadenylated transcripts ca. 5 kilobases in length. Two other DNA damage responsive sequences, pBRA2 and pBR3016B, are single-copy genes and hybridize to 0.5- and 3.2-kilobase transcripts, respectively. Kinetic analysis of the 0.5-kilobase transcript homologous to pBRA2 indicates that the level of this RNA increases more than 15-fold within 20 min after exposure to 4-nitroquinoline-1-oxide. Moreover, the level of this transcript is significantly elevated in cells containing the rad52-1 mutation which are deficient in DNA strand break repair and gene conversion. These results provide some of the first evidence that DNA damage stimulates transcription of specific genes in eucaryotic cells.

Apurinic sites cause mutations in simian virus 40

SV40 has been used as a molecular probe to study the mutagenicity of apurinic sites (Ap) in mammalian cells. Untreated or UV-irradiated monkey kidney cells were transfected with depurinated DNA from the temperature-sensitive tsB201 SV40 late mutant which grows normally at the permissive temperature of 33 degrees C but which is unable to grow at 41 degrees C. Phenotypic revertants were screened at 41 degrees C for their ability to grow at the restrictive temperature and the mutation frequency was calculated in the viral progeny. Ap sites were introduced into DNA by heating at 70 degrees C under acid conditions (pH 4.8). This treatment induces one Ap site per SV40 genome per 15 min of heating as measured by alkaline denaturation or by treatment with the T4-encoded UV-specific endonuclease which possesses Ap-endonuclease activity. The experiments reported here show that Ap sites strongly decrease virus survival with a lethal hit corresponding roughly to 3 Ap lesions per SV40 genome, and indicate for the first time that apurinic sites produced by heating are highly mutagenic in animal cells. UV irradiation of the host cells 24 h prior to transfection with depurinated DNA did not modify the mutation frequency in the virus progeny.

Genetic changes in mammalian cells reminiscent of an SOS response

Prior to the isolation of mammalian DNA repair genes and identification of their gene products, the comparison between the bacterial SOS response and various similar reactions in mammalian cells remains rather speculative. The increasing number of observed phenomena including enhanced DNA repair, virus induction, induced cellular differentiation, and neoplastic transformation, all following DNA damage or arrest of replication, are, however, suggestive of an SOS-like system of growth control and may form an entry into this fascinating area.

Heat-enhanced reactivation of UV-irradiated adenovirus 2 is not associated with enhanced mutagenesis in HeLa cells

The reversion frequency of an adenovirus 2 temperature-sensitive growth mutant irradiated with different doses of UV light was determined after infection of control, UV-irradiated and heat-shocked HeLa cells. No enhancement of mutagenesis by treatment of the cells was observed. Heat-enhanced viral reactivation does not therefore display a significant error-prone component.

Hyperthermia enhances the reactivation of irradiated adenovirus in HeLa cells

The reactivation of U.V.-irradiated adenovirus 2 in HeLa cells is enhanced 8-9 fold if the cells are given a brief hyperthermic shock before infection. Maximum reactivation is achieved by heating for 10 min at 45.5 degrees C and with a delay of 36 h between heating and infection. The induction process requires protein synthesis only during the 3 h period immediately following heating; cycloheximide does not prevent the expression of enhanced reactivation if added to the cells after this time. Heat-enhanced reactivation exhibits properties similar in some respects to radiation-enhanced reactivation and indicates an increased capacity of the heated cells to tolerate DNA damage.

Exposure of nondividing populations of primary human fibroblasts to UV (254 nm) radiation induces a transient enhancement in capacity to repair potentially lethal cellular damage

Nondividing (arrested) populations of primary human fibroblasts from normal individuals exposed to an initial dose (1.5 or 3 J X m-2) of far-UV (254 nm) radiation and then incubated in medium containing low (0.5%) serum develop enhanced resistance to inactivation of cloning efficiency by a second (challenge) dose of UV. The resistance develops within 2-4 days, after which there is a decline. Resistance develops to a higher degree and more rapidly (1-2 days) in cells derived from patients with the variant form of xeroderma pigmentosum. Excision-deficient cells from xeroderma pigmentosum complementation group A individuals also develop UV resistance after a lower (0.2 J X m-2) exposure to UV. Enhanced UV resistance does not develop in UV-irradiated cell populations incubated with the protein synthesis inhibitor cycloheximide (5 microM). These observations are consistent with the interpretation that exposure of human fibroblasts to low doses of UV induces synthesis of a protein involved in a metabolic pathway that transiently enhances the capacity of cells to repair potentially lethal damage resulting from a subsequent dose of UV.

Targeted mutagenesis of SV40 DNA induced by UV light

UV light is a potent mutagen in most living cells, but molecular analysis of its mode of action in animal cells has not been possible because of the high complexity of the cell genome. Therefore, we have used simian virus 40 (SV40) as a biological probe to study the mutagenic effect of UV-irradiation at the molecular level. A thermosensitive SV40 mutant of the large T antigen (tsA58), which is defective at high temperature for initiation of viral DNA replication and for repression of early transcription, was UV-irradiated in vitro, then replicated in monkey kidney cells. Revertants of tsA58 were selected for growth at the nonpermissive temperature, then the reversion sites were mapped by using the marker rescue technique, and precisely localized by DNA sequencing. We report here that all revertants analysed showed one or two base pair substitutions localized in the C-terminal half of the T antigen gene. The original tsA58 mutation was still present in all revertant genomes. In 16 DNAs sequenced, seven reversion sites were found on the large T antigen gene, all of which localized opposite a possible UV-induced pyrimidine-pyrimidine lesion, suggesting targeted mutagenesis. UV-irradiation of the host cell before infection did not change the pattern of reversion sites at the molecular level, although it strongly increased the mutation frequency. These results demonstrate for the first time in animal cells the specificity of UV-induced mutagenesis.

Lack of enhanced mutation of UV- and gamma-irradiated herpes virus in UV-irradiated CV-1 monkey cells

The frequency of forward mutation of unirradiated, UV-irradiated or gamma-irradiated herpes virus was determined after infecting UV-irradiated or unirradiated CV-1 monkey kidney cells, to investigate the correlation between UV-enhanced reactivation (UVER) and mutagenesis. UV-irradiation to cells had no effect on mutation frequency of irradiated virus even in the conditions in which UVER was maximally expressed for the survival of UV-irradiated virus.

Influence of confluent holding time on UV light mutagenesis in human diploid fibroblasts

We have investigated the induction of mutants resistant to 6-thioguanine (6TG) following 254 nm ultraviolet light exposure of density-inhibited cultures of human diploid fibroblasts. Phenotypic expression of 6TG resistance was maximal within 9 days and remained stable through 19 days after irradiation. In reconstruction studies, complete recovery of 6TG-resistant mutants occurred at cell densities of up to 35 000 cells per 100-mm petri dish. The induced mutation frequency increased linearly with dose over the range of 3-9 J/m2; the D0 of the survival curve was 4.2 J/m2. Delaying subculture to low density for 1.5-24 h after irradiation produced unexpected alterations in induced mutation frequencies. An increase in UV-induced mutations of approximately 3-fold was observed in cultures maintained in confluence for 3 h. This trend was reversed with longer holding times: the mutation frequency declined sharply in cultures held for 6 h as compared to the 3-h value, and thereafter showed a steady and gradual diminution to background levels. These data suggest that the repair of potentially mutagenic damage is a complex phenomenon which can lead to an increase or decrease in mutation frequency as a function of holding time. Although the decline in mutation frequency observed following longer holding intervals is consistent with the notion of an error-free process, we hypothesize that the increased mutation frequency produced by a short holding period reflects the existence of a cell-mediated process which enhances the mutagenic potential of at least some UV-induced DNA photoproducts.

Evidence for an indirect effect of radiation on mammalian chromosomes. III. UV- and x-ray-induced sister chromatid exchanges in heterokaryons

The hypothesis that chromosomes may be damaged indirectly by radiation was examined by assaying sister chromatid exchange, (SCE) frequency in heterokaryons between irradiated and unirradiated mouse and Chinese hamster cells. One cell line was UV or x irradiated, then fused to unirradiated BrdU-labeled cells of the other line; SCEs in the unirradiated set were scored in heterokaryons. A dose-dependent increase was consistently observed; the magnitude of which suggested that 25% of UV-induced and up to 60% of x-ray-induced SCEs are indirectly induced. Medium transfer experiments, cell mixing, and fusion with irradiated chick erythrocyte ghosts suggested that unirradiated chromosomes in heterokaryons are damaged by a stable, nondiffusible cytoplasmic component contributed by the irradiated cell.