[IMRT combined to IGRT: increase of the irradiated volume. Consequences?]

Image-guided radiotherapy (IGRT) combined or not with intensity-modulated radiation therapy (IMRT) are new and very useful techniques. However, these new techniques are responsible of irradiation at low dose in large volumes. The control of alignment, realignment of the patient and target positioning in external beam radiotherapy are increasingly performed by radiological imaging devices. The management of this medical imaging depends on the practice of each radiotherapy centre. The physical doses due to the IGRT are however quantifiable and traceable. In one hand, these doses appear justified for a better targeting and could be considered negligible in the context of radiotherapy. On the other hand, the potential impact of these low doses should deserve the consideration of professionals. It appears important therefore to report and consider not only doses in target volumes and in "standard" organs at risk, but also the volume of all tissue receiving low doses of radiation. The recent development of IMRT launches the same issue concerning the effects of low doses of radiation. Indeed, IMRT increases the volume of healthy tissue exposed to radiation. At low dose (<100mGy), many parameters have to be considered for health risk estimations: the induction of genes and activation of proteins, bystander effect, radio-adaptation, the specific low-dose radio-hypersensitivity and individual radiation sensitivity. With the exception of the latter, the contribution of these parameters is generally protective in terms of carcinogenesis. An analysis of secondary cancers arising out of field appears to confirm such notion. The risk of secondary tumours is not well known in these conditions of treatment associating IMRT and IGRT. It is therefore recommended that the dose due to imaging during therapeutic irradiation be reported.

Biological effects of essential oils--a review

Since the middle ages, essential oils have been widely used for bactericidal, virucidal, fungicidal, antiparasitical, insecticidal, medicinal and cosmetic applications, especially nowadays in pharmaceutical, sanitary, cosmetic, agricultural and food industries. Because of the mode of extraction, mostly by distillation from aromatic plants, they contain a variety of volatile molecules such as terpenes and terpenoids, phenol-derived aromatic components and aliphatic components. In vitro physicochemical assays characterise most of them as antioxidants. However, recent work shows that in eukaryotic cells, essential oils can act as prooxidants affecting inner cell membranes and organelles such as mitochondria. Depending on type and concentration, they exhibit cytotoxic effects on living cells but are usually non-genotoxic. In some cases, changes in intracellular redox potential and mitochondrial dysfunction induced by essential oils can be associated with their capacity to exert antigenotoxic effects. These findings suggest that, at least in part, the encountered beneficial effects of essential oils are due to prooxidant effects on the cellular level.

XRCC4 in G1 suppresses homologous recombination in S/G2, in G1 checkpoint-defective cells

Non-homologous end joining (NHEJ) and homologous recombination (HR) are two pathways that can compete or cooperate for DNA double-strand break (DSB) repair. NHEJ was previously shown to act throughout the cell cycle whereas HR is restricted to late S/G2. Paradoxically, we show here that defect in XRCC4 (NHEJ) leads to over-stimulation of HR when cells were irradiated in G1, not in G2. However, XRCC4 defect did not modify the strict cell cycle regulation for HR (i.e. in S/G2) as attested by (i) the formation of Rad51 foci in late S/G2 whatever the XRCC4 status, and (ii) the fact that neither Rad51 foci nor HR (gene conversion plus single-strand annealing) events induced by ionizing radiation were detected when cells were maintained blocked in G1. Finally, both gamma-H2AX analysis and pulse field gel electrophoresis showed that following irradiation in G1, some DSBs reached S/G2 in NHEJ-defective cells. Taken together, our results show that when cells are defective in G1/S arrest, DSB produced in G1 and left unrepaired by XRCC4 can be processed by HR but in late S/G2.

Cadmium: cellular effects, modifications of biomolecules, modulation of DNA repair and genotoxic consequences (a review)

Cadmium is an important toxic environmental heavy metal. Occupational and environmental pollution with cadmium results mainly from mining, metallurgy industry and manufactures of nickel-cadmium batteries, pigments and plastic stabilizers. Important sources of human intoxication are cigarette smoke as well as food, water and air contaminations. In humans, cadmium exposures have been associated with cancers of the prostate, lungs and testes. Acute exposures are responsible for damage to these organs. Chronic intoxication is associated with obstructive airway disease, emphysema, irreversible renal failure, bone disorders and immuno-suppression. At the cellular level, cadmium affects proliferation, differentiation and causes apoptosis. It has been classified as a carcinogen by the International Agency for Research on Cancer (IARC). However, it is weakly genotoxic. Indirect effects of cadmium provoke generation of reactive oxygen species (ROS) and DNA damage. Cadmium modulates also gene expression and signal transduction, reduces activities of proteins involved in antioxidant defenses. Several studies have shown that it interferes with DNA repair. The present review focuses on the effects of cadmium in mammalian cells with special emphasis on the induction of damage to DNA, membranes and proteins, the inhibition of different types of DNA repair and the induction of apoptosis. Current data and hypotheses on the mechanisms involved in cadmium genotoxicity and carcinogenesis are outlined.

The debate on the use of linear no threshold for assessing the effects of low doses

From December 2004 to July 2005, three reports on the effects of low doses of ionising radiation were released: ICRP (2004), the joint report of the French Academies of Science and Medicine (Tubiana et al 2005), and a report from the American Academy of Sciences (BEIR VII 2005). These reports quote the same recent articles on the biological effects of low doses, yet their conclusions diverge. The French report concludes that recent biological data show that the efficacy of defense mechanisms is modulated by dose and dose rate and that linear no threshold (LNT) is no longer plausible. The ICRP and the BEIR VII reports recognise that there are biologic arguments against LNT but feel that there are not sufficient biological proofs against it to change risk assessment methodology and subsequent regulatory policy based on LNT. They point out the remaining uncertainties and the lack of mechanistic explanations of phenomena such as low dose hyperlethality or the adaptive response. In this context, a critical analysis of the available data is necessary. The epidemiological data and the experimental data challenge the validity of the LNT hypothesis for assessing the carcinogenic effect of low doses, but do not allow its exclusion. Therefore, the main criteria for selecting the most reliable dose-effect relationship from a scientific point of view should be based on biological data. Their analysis should help one to understand the current controversy.

Antigenotoxic effects of three essential oils in diploid yeast (Saccharomyces cerevisiae) after treatments with UVC radiation, 8-MOP plus UVA and MMS

Essential oils (EOs) extracted from medicinal plants such as Origanum compactum, Artemisia herba alba and Cinnamomum camphora are known for their beneficial effects in humans. The present study was undertaken to investigate their possible antigenotoxic effects in an eukaryotic cell system, the yeast Saccharomyces cerevisiae. The EOs alone showed some cytotoxicity and cytoplasmic petite mutations, i.e. mitochondrial damage, but they were unable to induce nuclear genetic events. In combination with exposures to nuclear mutagens such as 254-nm UVC radiation, 8-methoxypsoralen (8-MOP) plus UVA radiation and methylmethane sulfonate (MMS), treatments with these EOs produced a striking increase in the amount of cytoplasmic petite mutations but caused a significant reduction in revertants and mitotic gene convertants induced among survivors of the diploid tester strain D7. In a corresponding rho0 strain, the level of nuclear genetic events induced by the nuclear mutagens UVC and 8-MOP plus UVA resulted in the same reduced level as the combined treatments with the EOs. This clearly suggests a close relationship between the enhancement of cytoplasmic petites (mitochondrial damage) in the presence of the EOs and the reduction of nuclear genetic events induced by UVC or 8-MOP plus UVA. After MMS plus EO treatment, induction of these latter events was comparable at least per surviving fraction in wildtype and rho0 cells, and apparently less dependent on cytoplasmic petite induction. Combined treatments with MMS and EOs clearly triggered switching towards late apoptosis/necrosis indicating an involvement of this phenomenon in EO-induced cell killing and concomitant decreases in nuclear genetic events. After UVC and 8-MOP plus UVA plus EO treatments, little apoptosis and necrosis were observed. The antigenotoxic effects of the EOs appeared to be predominantly linked to the induction of mitochondrial dysfunction.

Recent reports on the effect of low doses of ionizing radiation and its dose-effect relationship

Recently, the risk associated with low doses of ionizing radiation has gained new interest. Here, we analyze and discuss the major differences between two reports recently published on this issue; the report of the French Academy of Sciences and of the French Academy of Medicine published in March 2005, and the BEIR VII-Phase 2 Report of the American National Academy of Sciences published as a preliminary version in July 2005. The conclusion of the French Report is that the linear no-threshold relationship (LNT) may greatly overestimate the carcinogenic effect of low doses (<100 mSv) and even more that of very low doses (<10 mSv), such as those delivered during X-ray examinations. Conversely, the conclusion of the BEIR VII report is that LNT should be used for assessing the detrimental effects of these low and very low doses. The causes of these diverging conclusions should be carefully examined. They seem to be mostly associated with the interpretation of recent biological data. The point of view of the French Report is that these recent data are incompatible with the postulate on which LNT is implicitly based, namely the constancy of the carcinogenic effect per unit dose, irrespective of dose and dose rate.

Cytotoxicity and gene induction by some essential oils in the yeast Saccharomyces cerevisiae

In order to get an insight into the possible genotoxicity of essential oils (EOs) used in traditional pharmacological applications we tested five different oils extracted from the medicinal plants Origanum compactum, Coriandrum sativum, Artemisia herba alba, Cinnamomum camphora (Ravintsara aromatica) and Helichrysum italicum (Calendula officinalis) for genotoxic effects using the yeast Saccharomyces cerevisiae. Clear cytotoxic effects were observed in the diploid yeast strain D7, with the cells being more sensitive to EOs in exponential than in stationary growth phase. The cytotoxicity decreased in the following order: Origanum compactum>Coriandrum sativum>Artemisia herba alba>Cinnamomum camphora>Helichrysum italicum. In the same order, all EOs, except that derived from Helichrysum italicum, clearly induced cytoplasmic petite mutations indicating damage to mitochondrial DNA. However, no nuclear genetic events such as point mutations or mitotic intragenic or intergenic recombination were induced. The capacity of EOs to induce nuclear DNA damage-responsive genes was tested using suitable Lac-Z fusion strains for RNR3 and RAD51, which are genes involved in DNA metabolism and DNA repair, respectively. At equitoxic doses, all EOs demonstrated significant gene induction, approximately the same as that caused by hydrogen peroxide, but much lower than that caused by methyl methanesulfonate (MMS). EOs affect mitochondrial structure and function and can stimulate the transcriptional expression of DNA damage-responsive genes. The induction of mitochondrial damage by EOs appears to be closely linked to overall cellular cytotoxicity and appears to mask the occurrence of nuclear genetic events. EO-induced cytotoxicity involves oxidative stress, as is evident from the protection observed in the presence of ROS inhibitors such as glutathione, catalase or the iron-chelating agent deferoxamine.

[Mechanisms of repair and radiation-induced mutagenesis in higher eukaryotes]

Cells of higher eukaryotes possess several very efficient systems for the repair of radiation-induced lesions in DNA. Different strategies have been adopted at the cellular level to remove or even tolerate various types of lesions in order to assure survival and limit the mutagenic consequences. In mammalian cells, the main DNA repair systems comprise direct reversion of damage, excision of damage and exchange mechanisms with intact DNA. Among these, the direct ligation of single strand breaks (SSB) by a DNA ligase and the multi-enzymatic repair systems of mismatch repair, base and nucleotide excision repair as well as the repair of double strand breaks (DSB) by homologous recombination or non homologous end-joining are the most important systems. Most of these processes are error-free except the non homologous end-joining pathway used mainly for the repair of DSB. Moreover, certain lesions can be tolerated by more or less accurately acting polymerases capable of performing translesional DNA syntheses. The DNA repair systems are intimately integrated in the network of cellular regulation. Some of their components are DNA damage inducible. Radiation-induced mutagenesis is largely due to unrepaired DNA damage but also involves error-prone repair processes like the repair of DSB by non-homologous end-joining. Generally, mammalian cells are well prepared to repair radiation-induced lesions. However, some questions remain to be asked about mechanistic details and efficiencies of the systems for removing certain types of radiation-damage and about their order and timing of action. The answers to these questions would be important for radioprotection as well as radiotherapy.

Radiation-induced arrest of cells in G2 phase elicits hypersensitivity to DNA double-strand break inducers and an altered pattern of DNA cleavage upon re-irradiation

PURPOSE

To determine how radiation-induced arrest in G2 affects the response of mammalian cells to a challenging dose of radiation or to antitumour drugs producing DNA double-strand breaks.

MATERIALS AND METHODS

V79 fibroblast survival to 5 Gy gamma-rays followed at intervals by 3 Gy irradiation or by contact with an equitoxic dose of neocarzinostatin or etoposide, was measured by clonogenic assays. The pattern of radiation-induced DNA double-strand breaks was determined by filter elution and CFGE (continuous field gel electrophoresis) or PFGE (pulsed-field gel electrophoresis) in G2-arrested cells as well as in nonpre-irradiated asynchronous or synchronized cells. The cell-cycle phase specificity of drug susceptibility was determined in synchronized HeLa cells.

RESULTS

Cell kill by radiation-drug combined treatment varied markedly with the time elapsed after priming irradiation. Pre-irradiated, G2-arrested V79 fibroblasts demonstrated excess double-stranded DNA cleavage upon re-irradiation and hypersensitivity to drugs and radiation, although maximum resistance to both neocarzinostatin and etoposide in synchronized HeLa cells was in G2. This effect occurred in the megabase range only, with a peak around 4 Mbp; no change in the electrophoretic migration profile of DNA was observed below 1 Mbp. Moreover, the DNA migration profile and the yield of DNA cleavage in G2-arrested cells were close to those expected from S-phase cells.

CONCLUSION

The available data suggest that mechanisms operating within the radiation-induced G2 block promote susceptibility to DNA double-strand break inducers at this stage. It is also proposed that the conformation of DNA in cells accumulated in G2 following irradiation bears resemblance to that for cells in S phase, due either to active repair mechanisms or to inhibition of chromosome disentanglement at the S-G2 transition.

DNA damage-inducible and RAD52-independent repair of DNA double-strand breaks in Saccharomyces cerevisiae

Chromosomal repair was studied in stationary-phase Saccharomyces cerevisiae, including rad52/rad52 mutant strains deficient in repairing double-strand breaks (DSBs) by homologous recombination. Mutant strains suffered more chromosomal fragmentation than RAD52/RAD52 strains after treatments with cobalt-60 gamma irradiation or radiomimetic bleomycin, except after high bleomycin doses when chromosomes from rad52/rad52 strains contained fewer DSBs than chromosomes from RAD52/RAD52 strains. DNAs from both genotypes exhibited quick rejoining following gamma irradiation and sedimentation in isokinetic alkaline sucrose gradients, but only chromosomes from RAD52/RAD52 strains exhibited slower rejoining (10 min to 4 hr in growth medium). Chromosomal DSBs introduced by gamma irradiation and bleomycin were analyzed after pulsed-field gel electrophoresis. After equitoxic damage by both DNA-damaging agents, chromosomes in rad52/rad52 cells were reconstructed under nongrowth conditions [liquid holding (LH)]. Up to 100% of DSBs were eliminated and survival increased in RAD52/RAD52 and rad52/rad52 strains. After low doses, chromosomes were sometimes degraded and reconstructed during LH. Chromosomal reconstruction in rad52/rad52 strains was dose dependent after gamma irradiation, but greater after high, rather than low, bleomycin doses with or without LH. These results suggest that a threshold of DSBs is the requisite signal for DNA-damage-inducible repair, and that nonhomologous end-joining repair or another repair function is a dominant mechanism in S. cerevisiae when homologous recombination is impaired.

Disruption and functional analysis of six ORFs on chromosome IV: YDR013w, YDR014w, YDR015c, YDR018c, YDR020c, YDR021w (FAL1)

The disruption of six novel yeast genes has been realized in two genetic backgrounds. Six open reading frames (ORFs) from chromosome IV, YDR013w, YDR014w, YDR015c, YDR018c, YDR020c and YDR021w, were disrupted using the KanMX4 marker and PCR-targeting with long flanking regions homologous (LFH) to the target locus. The deletants were verified at the molecular level, using PCR and Southern analysis. Sporulation and tetrad analysis revealed that ORFs YDR013w and YDR021w (also known as FAL1) are essential genes. Microscopical observations showed that ydr013wDelta haploid cells were blocked after one or two cell cycles and presented heterogeneous bud sizes. The ydr021wDelta haploid cells gave rise to microcolonies of about 20 cells. The other four ORFs are non-essential. Basic phenotypic analysis of the non-lethal deletant strains did not reveal any significant differences in cell morphology, growth on different media and temperatures, sporulation and mating efficiency between parental and mutant strains in the FY1679 background.

[DNA-dependent protein kinase (DNA-PK), a key enzyme in the re-ligation of double-stranded DNA breaks]

Repair pathways of DNA are now better defined, and some important findings have been discovered in the last few years. DNA non-homologous end-joining (NEHJ) is a crucial process in the repair of radiation-induced double-strand breaks (DSBs). NHEJ implies at least three steps: the DNA free-ends must get closer, preparation of the free-ends by exonucleases and then a transient hybridisation in a region of DNA with weak homology. DNA-dependent protein kinase (DNA-PK) is the key enzyme in this process. DNA-PK is a nuclear serine/threonine kinase that comprises three components: a catlytic subunit (DNA-PKCS) and two regulatory subunits, DNA-binding proteins, Ku80 and Ku70. The severe combined immunodeficient (scid) mice are deficient in DNA-PKCS: this protein is involved both in DNA repair and in the V(D)J recombination of immunoglobulin and T-cell receptor genes. It is a protein-kinase of the P13-kinase family and which can phosphorylates Ku proteins, p53 and probably some other proteins still unknown. DNA-PK is an important actor of DSBs repair (induced by ionising radiations or by drugs like etoposide), but obviously it is not the only mechanism existing in the cell for this function. Some others, like homologous recombination, seem also to have a great importance for cell survival.

DNA photodamage, repair, gene induction and genotoxicity following exposures to 254 nm UV and 8-methoxypsoralen plus UVA in a eukaryotic cell system

The induction and repair of different types of photodamage and photogenotoxicity in eukaryotic cells have been the subject of many studies. Little is known about possible links between these phenomena and the induction of DNA damage-inducible genes. We explored this relationship using the yeast Saccharomyces cerevisiae, a pertinent eukaryotic model. Previous results showed that the photogenotoxic potential of 8-methoxypsoralen (8-MOP) plus UVA is higher than that of UV (254 nm). Moreover, the induction of the ribonucleotide reductase gene RNR2 by UV and 8-MOP plus UVA in an RNR2-LACZ fusion strain and the formation of DNA double-strand breaks (dsb) as repair intermediates after such treatments suggest that the latter process could involve a signal for gene induction. To further substantiate this, we measured the induction of the DNA repair gene RAD51 in RAD51-LACZ fusion strains using the dsb repair and recombination deficient mutant rad52 and the corresponding wild type, and we determined the formation of dsb by pulsed-field gel electrophoresis. After treatments, the resealing of dsb formed as repair intermediates was impaired in the rad52 mutant. At equal doses, i.e. the same number of lesions, the induction of the RAD51 gene by UV or 8-MOP plus UVA was significantly reduced in the rad52 mutant as compared with the wild type. The same was true when equitoxic doses were used. Thus, the RAD52 repair pathway appears to play an important role not only in dsb repair but also in gene induction. Furthermore, the signaling pathways initiated by DNA damage and its processing are somewhat linked to the photogenotoxic response.

Mitotic recombination and localized DNA double-strand breaks are induced after 8-methoxypsoralen and UVA irradiation in Saccharomyces cerevisiae

Mitotic recombination within the ARG4 gene of Saccharomyces cerevisiae was analysed after treatment of cells with the recombinogenic agent 8-methoxypsoralen (8-MOP) plus UVA. The appearance of DNA double-strand breaks (DSBs) in the ARG4 region during post-treatment incubation was also tested. The results obtained after 8-MOP plus UVA treatment indicate that in mitotic cells: (1) recombination at the ARG4 locus is increased 30 - 500 fold per survivor depending on the strains and the doses employed, (2) the increase of recombination results essentially from gene conversion events which involve the RV site located in the 5' region of the ARG4 gene twice as often as the Bgl site at the 3' end, (3) depending on 8-MOP/UVA dose, ectopic gene conversion is associated with reciprocal translocation, (4) DSBs occur preferentially in the ARG 5' region during post-treatment incubation, as well as in other intergenic regions containing both promoters or/and terminators of transcription, and (5) changes in sequence content in the 5' region of ARG4, which influences positions and frequencies of DSBs formed during repair, are correlated with a modification of the local chromatin structure.

[Radiation-induced DNA fragmentation evaluated by anti-poly(ADP-ribose)immunolabeling in CHO cells. Standardization with pulsed-field electrophoresis]

The poly (ADP-ribose) polymerase is an ubiquitous nuclear protein capable of binding specifically to DNA strand breaks. It synthesizes ADP-ribose polymers proportionally to DNA breaks. The actual method of reference to determine DNA double strand breaks is pulsed-field gel electrophoresis, but this requires many cells. It thus appeared of interest to use poly (ADP-ribos)ylation to follow and estimate gamma-ray-induced DNA fragmentation at the level of isolated cells after gamma-irradiation in chinese hamster ovary cells (CHO-K1). The results obtained by the immunolabelling technique of ADP-ribose polymers were compared to those obtained by pulsed-field gel electrophoresis. They show that poly (ADP-ribos)ylation reflects the occurrence of radiation-induced DNA strand breaks. A clear relationship exists between the amount of ADP-ribose polymers detected and DNA double strand breaks after gamma-irradiation.

Lack of ceramide generation in TF-1 human myeloid leukemic cells resistant to ionizing radiation

The mechanism(s) by which ionizing radiation (IR) induces cell death is of fundamental importance in understanding cell sensitivity and resistance. Here we evaluated the response to IR of two subclones of the autonomous human erythromyeloblastic cell line TF-1: TF-1-34 (which expresses CD34) and TF-1-33 (which lacks CD34). In clonogenic assays, TF-1-34 cells were found to be relatively less sensitive to IR compared to TF-1-33 cells based on the D0 determination (3.01 vs 1.56 Gy). Furthermore, after IR at 12 Gy, TF-1-33 cell viability decreased by approximately 50% within 24 h, whereas TF-1-34 cell growth was unaffected during this time. Gradual loss of TF-1-34 cell viability was observed only after 48 h. Morphological and molecular analysis revealed that TF-1-33 cells died of apoptosis, and TF-1-34 cells of delayed reproductive cell death. While IR produced comparable amounts of DNA double strand breaks (DSB) in both cell lines, TF-1-34 retained DSB much longer than TF-1-33 suggesting that radioresistance and the defective apoptotic response of TF-1-34 cells was not related to a higher DNA repair capacity. However, the lack of an apoptotic response in TF-1-34 was correlated to the absence of a sphingomyelin (SM)-ceramide (CER) signaling pathway. Indeed, IR triggered in TF-1-33 cells but not in TF-1-34, SM hydrolysis (25% at 12 Gy) and CER generation (>50%) through the activation of neutral but not acid sphingomyelinase. Synthetic cell permeate CER induced apoptosis in both TF-1-33 and TF-1-34 cells. This study indicates that alterations of the SM-CER signaling pathway can significantly influence the cell death process as well as the survival of acute myeloid leukemia cells after IR exposure.

Enzymatic recognition and biological effects of photodynamic damage induced in DNA by 1,6-dioxapyrene plus UVA

The specific recognition of DNA modifications by repair endonucleases was used to characterize DNA damage induced by 1,6-dioxapyrene (1,6-DP) in the presence of ultraviolet light at 365 nm (UVA) in the plasmid YEplac181. Under cell free conditions, 1,6-DP plus UVA generated lesions are recognized by the UvrABC endonuclease, the proteins Nth, Nfo and Fpg. The number of UvrABC sensitive sites was at least ten-fold higher than that of Fpg or Nth sensitive sites. Moreover, 1,6-DP plus UVA generated single-strand breaks which are the second most frequent lesions. To investigate the biological effect of DNA damage, YEplac181 DNA was treated with 1,6-DP plus UVA and transformed into Escherichia coli or Saccharomyces cerevisiae. In Escherichia coli, the transformation efficiency of 1,6-DP plus UVA treated DNA was greatly reduced in the uvrA mutant compared to that in the wild-type strain. However, the transforming efficiency was not affected in Fpg-deficient strains. In Saccharomyces cerevisiae, the transformation efficiency of 1,6-DP plus UVA treated YEplac181 was greatly reduced in the rad14::URA3 strain. The photobiological effect of 1,6-DP plus UVA was also analysed in haploid yeast strains of various repair capacities. The results show that the yeast strain defective in the nucleotide excision repair pathway (rad14::URA3) is hypersensitive to 1,6-DP plus UVA treatment as compared to the parental wild-type strain. It is confirmed that the lethal effect of 1,6-DP plus UVA on wild-type yeast is strongly oxygen dependent, whereas the survival of the rad14::URA3 mutant only exhibits a minor oxygen dependence. To conclude, our data show that the photodynamic DNA lesions induced by 1,6-DP plus UVA can be recognized and repaired in pro- and eukaryotic cells by the nucleotide excision repair pathway.

Single strand breaks and mutagenesis in yeast induced by photodynamic treatment with chloroaluminum phthalocyanine

Photodynamic treatment of the yeast Kluyveromyces marxianus with the sensitizer aluminum phthalocyanine results in loss of clonogenicity. In this paper the effect of this treatment on DNA of this yeast was investigated by searching for single strand breaks and forward mutations. Using the alkaline step elution technique it was found that illumination of the yeast in the presence of aluminum phthalocyanine resulted in an increase in single strand breaks. These could, partially, be repaired by post-incubating illuminated cells in growth medium. At comparable survival levels, photodynamic treatment with aluminum phthalocyanine induced fewer single strand breaks than X-ray treatment. By using a medium containing 5-fluoroorotic acid, mutants in the uracil biosynthetic pathway were selected. Photodynamic treatment resulted in a light dose dependent increase of the mutation frequency. The observed mutagenicity of photodynamic treatment of the yeast with phthalocyanine was lower than the mutagenicity of UVC and X-ray treatment at equal colony forming capacity, indicating that photodynamic treatment is the least mutagenic of those treatments. It is concluded that photodynamic treatment of K. marxianus results in DNA damage. Saccharomyces cerevisiae rad14 and rad52 mutants were used to determine the effect of the nucleotide excision repair and recombinational repair pathways, respectively, on survival after photodynamic treatment. Our data indicate that DNA damage is not the main determinant for cell killing by photodynamic treatment and that the type of damage induced is apparently not subject to RAD14- or RAD52 controlled repair.

Preferential incision of interstrand crosslinks induced by 8-methoxypsoralen plus UVA in yeast during the cell cycle

Interstrand crosslink (ICL) induction by 8-methoxypsoralen plus UVA and the incision step of the repair have been investigated during the mitotic cell cycle of haploid Saccharomyces cerevisiae. Cells were synchronised by elutriation and events were examined at the level of the MAT alpha and the HML alpha loci in a SIR strain. The DNA sequence of these two loci is identical, but the MAT alpha locus may be replicated earlier in S phase and is transcriptionally active while the HML alpha locus may be replicated later in S phase and is transcriptionally inactive because of Sir repression that creates a heterochromatin-like structure at this locus. ICL were induced to similar extents in both loci during the stages of the cell cycle examined, and these levels were identical to those reported for asynchronous cultures. Preferential incisions occurred for ICL in the MAT alpha locus compared to those in the HML alpha locus, independently of the cell cycle phase studied. The levels of incision were comparable for events in the early G1 phase (eG1), late G1 phase (lG1), early S phase (eS), middle S phase (mS), late S phase (lS) or G2 phase (G2). Thus the preferential incision of ICL observed previously in asynchronous cell culture is maintained throughout the cell cycle and, surprisingly, occurs equally well in G1. Here the opportunities for recombination to further process the incised damaged are substantially limited compared to those in the S and G2 phases.

Photoreaction of 5-methoxypsoralen with thymidine and the thymine moiety of isolated and Saccharomyces cerevisiae DNA. Characterization and measurement of the two cis-syn furan-side monocycloadducts

The photoreaction of the furan-side moiety of 5-methoxypsoralen (5-MOP) with thymidine used as a DNA model compound was investigated in the dry state. Under these conditions, two main fluorescent photoadducts were formed and isolated by HPLC. The two modified nucleosides were characterized as the two cis-syn diastereoisomers of furan-side monoadducts of 5-MOP to thymidine on the basis of spectroscopic measurements including UV, fluorescence, 1H-NMR and circular dichroism analysis. The identification and quantification of the latter photoproducts within naked DNA exposed to photoexcited 5-MOP were achieved by enzymatic digestion completed by HPLC separation and fluorescence detection. Similarly, the two cis-syn furan-side monoadducts were found to be formed in the DNA of Saccharomyces cerevisiae cells after incubation with 5-MOP and subsequent exposure to 365 nm at an incident dose of 38.4 kJ m-2. Under these conditions, the rate of induction of two diastereoisomeric photoadducts was as low as one modification per 10(6) and 2 x 10(5) bases, respectively.

Preferential repair in Saccharomyces cerevisiae rad mutants after induction of interstrand cross-links by 8-methoxypsoralen plus UVA

The gene specific induction and the incision step of the removal of 8-methoxypsoralen (8-MOP) plus UVA-induced interstrand cross-links (ICL) was measured in repair mutants of Saccharomyces cerevisiae. Events were examined at the MAT alpha and HML alpha loci in mutants deficient in the repair of ICL, namely rad1, rad2 delta, rad52, pso2 and the rad16 mutant which is impaired in the removal of UV-induced pyrimidine dimers from the silent HML alpha locus. Previously, we observed in a wild-type strain (K107) preferential repair concerning the incision of 8-MOP photo-induced ICL. The present study indicates that the two mutants rad1 and rad2 delta show no repair in either locus, due presumably to their deficiency in the incision step of ICL repair. The rad52 mutant which is defective in recombination, is proficient in the preferential incision of ICL at the MAT alpha locus versus the HML alpha locus. The same is true for the pso2 mutant which also lacks the ability to perform complete repair of ICL. The rad16 mutant is unable to repair ICL in the silent locus HML alpha but is proficient in repair (i.e. the incision of ICL) in the transcriptionally active MAT alpha locus.

Preferential repair in yeast after induction of interstrand cross-links by 8-methoxypsoralen plus UVA

The gene-specific induction and removal of 8-methoxypsoralen (8-MOP) plus UVA-induced interstrand cross-links (ICL) was studied using the genetic system MAT alpha and HML alpha in Saccharomyces cerevisiae. We first examined events in a SIR alpha haploid strain (K107) in which these identical sequences are respectively transcriptionally active (MAT alpha) and inactive (HML alpha). Induction and repair of ICL was then studied in a sir3 mutant in which HML alpha is derepressed so that MAT alpha and HML alpha are both transcriptionally active. In the SIR strain at low levels of damage, no preferential repair of ICL occurred for MAT alpha versus HML alpha, whereas at high levels of ICL, those at MAT alpha were clearly repaired more rapidly than those at HML alpha. Similar experiments with the sir3 mutant revealed that the repair of ICL from both MAT alpha and HML alpha loci proceeded at the same rate at both low and high levels of damage. These data suggest that 8-MOP plus UVA-induced ICL are subject to preferential repair in yeast and that for the MAT alpha and HML alpha loci, this is dependent on their transcriptional status (i.e., the transcribed sequences are repaired more rapidly than the identical non-transcribed ones).

Induction of double-strand breaks in Chinese hamster ovary cells at two different dose rates of gamma-irradiation

Using pulsed-field gel electrophoresis (PFGE) analysis we investigated the existence of a dose rate effect of gamma-irradiation on the measured presence of DNA double-strand breaks (DSB) in a repair competent (K1) and a repair deficient (mutant xrs6) Chinese hamster ovary (CHO) cell line. The fraction of DNA fragments released from cells embedded in agarose during PFGE after gamma-irradiation was taken as a measure of DSB induction. In CHO-K1 cells DSB were present at a significantly higher rate when gamma-irradiation was delivered at a high dose rate of 22 Gy/min (HDR) than at a medium dose rate of 0.45 Gy/min (MDR) at 37 degrees C. However, the same amount of DSB was found when irradiation was performed at the two dose rates at 4 degrees C. The DSB yield was also identical at both dose rates in the DSB repair deficient mutant xrs6. The results indicate that there is an apparent dose rate effect for gamma-ray induced DSB in repair competent CHO cells due to partial repair of DSB taking place during gamma-ray exposures at MDR but not at HDR. This repair of DSB was inhibited upon irradiation at 4 degrees C and in repair deficient xrs6 cells.

Pulsed-field gel electrophoresis analysis of the repair of psoralen plus UVA induced DNA photoadducts in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, double-strand breaks (DSB) have been observed during the DNA repair of psoralen plus UVA induced lesions. In the present paper, we analyzed this repair step in some detail using pulsed-field gel electrophoresis (CHEF) to get a better understanding of this phenomenon with regard to the type of lesions induced and the repair pathways involved. The results confirm that, during post-treatment incubation of Saccharomyces cerevisiae cells, DSB are formed. Their appearance is dose-dependent and the rate of induction is comparable in large (chromosome IV) and small (chromosome III) chromosomes. The formation of DSB is evidenced by the breakage of linear chromosomes III and IV, but also, after high doses, by the linearization of a circular form of chromosome III. The induction of DSB appears to be highly dependent on the induction of interstrand cross-links since they are clearly present after treatments with 8-MOP plus 365 nm radiation (inducing monoadducts and cross-linking in DNA), but practically absent after treatment with 8-MOP plus 405 nm radiation (inducing predominantly monoadducts) at comparable levels of photoadducts. The occurrence of DSB is dependent on the RAD2 and RAD52, but not on the RAD6 gene. It is likely that the specific processing of DNA lesions involving DSB is related to the genotoxic consequences observed.

Involvement of the PS03 gene of Saccharomyces cerevisiae in intrachromosomal mitotic recombination and gene amplification

Using a genetic system of haploid strains of Saccharomyces cerevisiae carrying a duplication of the his4 region on chromosome III, the pso3-1 mutation was shown to decrease the rate of spontaneous mitotic intrachromosomal recombination 2- to 13-fold. As previously found for the rad52-1 mutant, the pso3-1 mutant is specifically affected in mitotic gene conversion. Moreover, both mutations reduce the frequency of spontaneous recombination. However, the two mutations differ in the extent to which they affect recombinations between either proximally or distally located markers on the two his4 heteroalleles. In addition, amplifications of the his4 region were detected in the pso3-1 mutant. We suggest that the appearance of these amplifications is a consequence of the inability of the pso3-1 mutant to perform mitotic gene conversion.

Comparison of the effects of DNA topoisomerase inhibitors on lymphoblasts from normal and Fanconi anemia donors

DNA topoisomerases modify supercoiled DNA through concerted breaking and rejoining of the DNA strands and consequently play a key role in DNA biosynthesis and processing. It has been suggested that topoisomerases may facilitate access to damaged sites of excision repair enzymes due to their property to relax supercoiled DNA. We show here that treatment with nalidixic acid and novobiocin, which affects topoisomerase II activity among other targets, impairs the incision of 8-methoxypsoralen photoinduced DNA interstrand cross-links in normal human fibroblasts. Since cells derived from Fanconi anemia (FA) demonstrate hypersensitivity to DNA cross-linking agents associated with a reduced repair efficiency of cross-links, we compared the effects of different topoisomerase I and II inhibitors on FA and normal lymphoblasts. No differences were found in growth inhibition or induction of chromosome aberrations between FA and normal cells. The specificity of inhibitors is questionable and even if topoisomerases are indeed inhibited alternative pathways may be involved. However, our observations provisionally suggested that topoisomerases activities are normal in FA cells.

Induction of the genes RAD54 and RNR2 by various DNA damaging agents in Saccharomyces cerevisiae

The relationship between the induction of the genes RAD54 and RNR2 and the induction and repair of specific DNA lesions was studied in the yeast Saccharomyces cerevisiae using Rad54-lacZ and RNR2-lacZ fusion strains. Gene induction was followed by measuring beta-galactosidase activity. At comparable levels of furocoumarin-DNA photoadducts, RAD54 was more effectively induced by bifunctional than by monofunctional furocoumarins indicating that mixtures of monoadducts (MA) and interstrand cross-links (CL) provide a stronger inducing signal than MA. RNR2 induction kinetics were measured in relation to cell growth and survival responses after treatment with the furocoumarins 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP), 3-carbethoxypsoralen (3-CPs), 7-methyl-pyrido[3,4-c]psoralen (MePyPs) and 4,4',6-trimethylangelicin (TMA), benzo[a]pyrene (B(a)P and 1,6-dioxapyrene (1,6-DP) plus UVA, 254 nm UV radiation and cobalt-60 gamma-radiation. Induction of RNR2 took place during the DNA repair period before resumption of cell growth and clearly increased with increasing equitoxic dose levels. Treatments with furocoumarin plus 365 nm radiation (UVA) and 254 nm (UV) radiation were effective inducers whereas gene induction was relatively weak after gamma-radiation and absent after the induction of oxidative damage by B(a)P and 1,6-DP and UVA. The results suggest that it is the specific processing of different DNA lesions that determines the potency of the induction signal. Apparently, DNA lesions such as CL, and probably also closely located MA or pyrimidine dimers in opposite DNA strands involving the formation of double-strand breaks as repair intermediates, are most effective inducers.

Biophysical and biological properties of newly synthesized dioxinocoumarin derivatives. II. Dark and photoinduced effects on T7 phage, yeast and HeLa cells

The dioxinocoumarin derivatives 5H-[2]benzopyrano-[3,4-g][1,4]benzodioxin-5-one (I), 5H-[2]benzopyrano-[3,4-g][2,3]-dihydro-[1,4]benzodioxin-5-on e II, 6H-[2]benzopyrano[3,4-f]-1,4-benzodioxin-6-one (III) and 6H-[2]benzopyrano[3,4-f]-2,3-dihydro-1,4-benzodioxin-6-one (IV) were synthesized. Their biological effect was studied in the presence and absence of UVA radiation, and compared with that of 8-methoxypsoralen (8-MOP) and angelicin derivatives on T7 phage, diploid yeast (Saccharomyces cerevisiae) and HeLa cells. The photobiological activities of compounds I and III were stronger than that of 8-MOP in phage inactivation and DNA synthesis inhibition in HeLa cells, whereas compounds II and IV, with a saturated dioxin ring, showed very poor activity. The photosensitizing activity of dioxinocoumarins on phage inactivation decreased by a factor of two to three in the absence of oxygen. Treatments with compound I and UVA in the presence of oxygen modified the helical structure and stability of phage DNA and proteins. Compounds I and II were more active than IV for photoinduced cell killing in yeast, although always less active than 8-MOP. At comparable photocytotoxic levels, compounds I and III were as strong inducers of cytoplasmic "petite" mutants in yeast as angelicin, suggesting a possible monofunctional mode of action with cellular DNA.

Activated ras oncogene and specifically acquired resistance to cisplatin in human mammary epithelial cells: induction of DNA cross-links and their repair

A human non-malignant mammary epithelial cell line, HBL100, and the ras-transformed HBL100/ras1 cell line were examined for their sensitivity to cis-diamminedichloroplatinum(II) (cisplatin). The clonogenic cell survival assay showed that HBL100/ras1 exhibited a 2.7-fold increased resistance compared to the parental HBL100 cell line. The responses to other agents interacting with DNA, such as mitomycin C, 8-methoxypsoralen plus UVA or doxorubicin, were very similar in both cell lines. The same is true for ionizing radiation (Alapetite et al., Int J. Radiat. Biol., 59, 385-396, 1991). In other words, the mechanism of acquired resistance in HBL100 appears to be limited to cisplatin. No difference was observed between the two cell lines in cisplatin uptake as determined by atomic absorption spectrometry. Alkaline elution showed that less interstrand cross-links were formed by this drug in the resistant HBL100/ras1 cells compared to HBL100 and, moreover, the removal of these adducts was clearly more efficient in the former cell line. This was confirmed by an in vitro excision repair assay which revealed a 2.2-fold increase in DNA repair activity in the extracts from HBL100/ras1 versus HBL100 cells. It is concluded that the transformation of human epithelial HBL100 cells by the ras gene resulted in an acquired resistance apparently limited to cisplatin, a feature associated with a reduced proportion of induced interstrand cross-links and a higher efficiency in their removal. The mechanism of involvement of the ras gene product in this process is still a matter of speculation.

Detection of pyrimidine dimers and monoadducts induced by 7-methylpyrido(3,4-c) psoralen and UVA in Chinese hamster V79 cells by enzymatic cleavage and high-pressure liquid chromatography

The photochemotherapeutically active psoralen derivative 7-methylpyrido(3,4-c) psoralen (MePyPs) has been recently shown to be able to photoinduce monoadducts of the C4-cycloaddition type as well as pyrimidine dimers in DNA in vitro. In the present study, we report on the induction of these two types of photolesions in mammalian cells in culture. The MePyPs photocycloadducts were quantified in V79 Chinese hamster cells after treatment with MePyPs plus UVA following enzymatic hydrolysis of the DNA by DNase I, S1 nuclease and acidic phosphatase treatments. Concomitantly induced pyrimidine dimers were determined by two methods, high-pressure liquid chromatography and alkaline gel electrophoresis after dimer-specific endonucleolytic cleavage. The results show that, in Chinese hamster cells treated with MePyPs plus UVA, the yield of pyrimidine dimers is approximately 5-10% that of MePyPs-DNA photocycloadducts. Because psoralen monoadditions to DNA alone are generally not considered as being very phototoxic, a synergistic interaction of monoadditions with pyrimidine dimers may be expected to occur in order to explain the high photobiological effectiveness of this psoralen derivative.

Repair of DNA double-strand breaks induced in Saccharomyces cerevisiae using different gamma-ray dose-rates: a pulsed-field gel electrophoresis analysis

We investigated the effects of gamma-ray exposures at high dose-rate (HDR, 23.2 Gy/min) and low dose-rate (LDR, 0.47 Gy/min) on survival and the induction of DNA double-strand breaks (dsb) in a diploid wild-type (D7) and the repair-deficient mutant strain rad52/rad52 of Saccharomyces cerevisiae. Analysis by pulsed-field gel electrophoresis (PFGE) using a contour homogeneous electric field apparatus revealed that, at HDR, in the range 0-400 Gy, dsb are induced as a linear function of gamma-ray dose. Liquid holding recovery in non-nutrient medium (LHR) for 48 h of wild-type cells treated at HDR, significantly increased survival and reduced the yield of dsb. Such changes did not occur in rad52/rad52 cells defective in the repair of dsb. Thus, in gamma-irradiated wild-type cells, an efficient repair of dsb is taking place during LHR. Treatments of wild-type cells at LDR resulted in higher survival and an approximately two-fold lower yield of dsb than at HDR. Such a dose-rate effect was absent in rad52/rad52 cells suggesting that, in wild-type cells during LDR exposures, significant amounts of dsb can be repaired. This repair could be very much accentuated by 48-h LHR of wild-type cells treated at LDR. The relationship observed between gamma-ray survival and dsb repair clearly indicates that increases in survival of wild-type cells, during LDR as compared with HDR exposures and after LHR, are strongly related to the repair of dsb.

Photosensitized cross-linking and cleavage of pBR322 and M13 DNA: comparison of 4,4',6-trimethylangelicin and 3-carbethoxypsoralen

The furocourmarins 3-carbethoxypsoralen (3-CP) and 4,4',6-trimethylangelicin (TMA) were generally believed to be incapable of cross-linking DNA upon irradiation with ultraviolet light. Denaturation of photosensitized pBR322 DNA, either supercoiled or previously linearized with a restriction enzyme, proved that 3-CP was indeed monofunctional, but that TMA produced cross-links. Identical conclusions were reached with double stranded M13 DNA which had been linearized with EcoR 1. Both sensitizers also induced partial DNA cleavage. In contrast to 3-CP, photosensitization with TMA made the DNA resistant to enzymatic cleavage.

Evidence for preferential repair of 3-carbethoxypsoralen plus UVA induced DNA lesions in the active MAT alpha locus in Saccharomyces cerevisiae using the UvrABC assay

The occurrence of preferential repair in Saccharomyces cerevisiae of the active MAT alpha locus compared with the inactive HML alpha locus was confirmed after 254 nm UV irradiation. Experiments carried out using the UvrABC excinuclease assay with the monofunctional furocoumarin 3-carbethoxypsoralen (3-CPs) plus UVA radiation which induce mainly monoadducts in DNA demonstrated preferential repair of the active MAT alpha locus compared with the inactive HML alpha locus in a SIR+ strain. However, as after 254 nm UV irradiation, no difference in the rate of removal of 3-CPs plus UVA induced lesions was observed between the two loci in the sir-3 mutant in which both loci are active. Thus, it appears that 3-CPs plus UVA induced monoadducts as well as pyrimidine dimers are subject to preferential repair.

Photobiological activities of 1,6-dioxapyrene in pro- and eukaryotic cells

The photobiological effect of a new pyrene derivative, 1,6-dioxapyrene (1,6-DP), was studied in Salmonella typhimurium (strain TA100) and in the diploid strain D7 of the yeast Saccharomyces cerevisiae. In Salmonella, 1,6-DP shows little mutagenicity in the dark in comparison to benzo[a]pyrene (B[a]P). This mutagenic activity decreases in the presence of liver S9 homogenates from Aroclor induced XVIInc/Z mice. However, in combination with 365 nm (UVA) radiation and in the absence of S9 mix, 1,6-DP behaves as an effective photodynamic compound inducing lethal and mutagenic effects in both organisms. In yeast, its activity, like that of B[a]P, is highly dependent on the presence of oxygen. For the same incident dose of UVA, 1,6-DP is, however, at least 6 times more effective than B[a]P in inducing cytotoxic and mutagenic effects. At equitoxic doses, 1,6-DP is as photomutagenic as B[a]P, suggesting that in both cases mutagenicity is due to similar mechanisms. Spectrophotometric measurements indicate physical interaction of 1,6-DP with DNA in the dark. Laser flash photolysis experiments show that 1,6-DP generates singlet oxygen with a quantum yield of 0.17. In vitro 1,6-DP produces oxidative damage to guanine bases specific for singlet oxygen mediated reactions. Alkaline step elution analysis of 1,6-DP plus UVA treated yeast cells indicates a decrease in average molecular weights in DNA and an induction of single strand breaks (ssb) originating from alkali labile sites. This effect is enhanced by D2O and is thus likely to be due to the production of singlet oxygen. The strand breaks appear to differ from those induced by gamma-rays because little, if any, repair of these ssb occurs during 30 min of post-treatment incubation in complete growth medium. These results suggest that the photobiological effects of 1,6-DP are due to oxidative damage in DNA mostly induced by singlet oxygen.

Repair of the two diastereoisomer photoadducts formed between 7-methylpyrido(3,4-c)psoralen (MePyPs) and thymidine in yeast cells: a chemical approach

When analysing the repair of psoralen plus UVA-induced photoadducts in DNA, it must be realized that, in most cases, different isomers are formed. The monofunctional psoralen derivative 7-methylpyrido(3,4-c)psoralen (MePyPs) is known for its high antiproliferative activity at the cellular level and interesting photochemotherapeutic properties. To understand its photobiological efficiency in more detail, the induction of specific photoadducts in DNA and their repair were analysed in a eukaryotic cell system, the yeast Saccharomyces cerevisiae. After photoaddition of MePyPs, two main diastereoisomers were characterized after enzymatic hydrolysis of the DNA and analysis by high performance liquid chromatography. One diastereoisomer was more effectively repaired in yeast than the other during post-treatment incubation, suggesting that the two diastereoisomers may be recognized differently by cellular enzymatic repair systems.

Evaluation of phototoxic and photogenotoxic risk associated with the use of photosensitizers in suntan preparations: application to tanning preparations containing bergamot oil

Bases for the elaboration of a standardized protocol are proposed for studying phototoxic effects of skin tanning preparations containing photosensitizing agents. The experimental procedure includes in vivo phototoxicity tests, evaluation of the photogenotoxic risk and determination of the photosensitizer concentration in plasma after topical application. This procedure was carried out with tanning preparations containing a well-known photosensitizer, 5-methoxypsoralen, as a component of bergamot oil. The whole study has been performed using topical application of the commercial suntan product, i.e. containing the sunscreens and all other components. Whereas the exposure to solar simulated radiation never triggered any phototoxic response, a photosensitizing effect was observed for skin type I volunteers exposed to high doses of ultraviolet A. The transepidermal penetration resulted in a 5-methoxypsoralen concentration of 1-4 ng/ml in the suction blister fluid. The photogenotoxicity of this suction blister fluid containing 5-methoxypsoralen and also other ingredients of the tanning preparation was assayed on yeast cells and was found to be rather low. 5-Methoxypsoralen was also detected in plasma after repeated applications but at low concentrations (about 1 ng/ml) which do not present a potential risk for systemic ocular effects.

The pso4-1 mutation reduces spontaneous mitotic gene conversion and reciprocal recombination in Saccharomyces cerevisiae

Spontaneous mitotic recombination was examined in the haploid pso4-1 mutant of Saccharomyces cerevisiae and in the corresponding wild-type strain. Using a genetic system involving a duplication of the his4 gene it was shown that the pso4-1 mutation decreases at least fourfold the spontaneous rate of mitotic recombination. The frequency of spontaneous recombination was reduced tenfold in pso4-1 strains, as previously observed in the rad52-1 mutant. However, whereas the rad52-1 mutation specifically reduces gene conversion, the pso4-1 mutation reduces both gene conversion and reciprocal recombination. Induced mitotic recombination was also studied in pso4-1 mutant and wild-type strains after treatment with 8-methoxypsoralen plus UVA and 254 nm UV irradiation. Consistent with previous results, the pso4-1 mutation was found strongly to affect recombination induction.

The binding of bridged bis-pyridinium oximes to DNA and its relevance to the induction of mitochondrial dysfunction in yeast

Bis-pyridium oximes and methoximes from a newly synthesized series are weak DNA binders (K = 3.10(4) M-1 under physiological conditions). From the number of binding sites per phosphate, 0.25, the ionic strength dependence of the binding constant and the negative electric dichroism, it is concluded that monointercalation is the mode of association. In contrast to methoxy compounds, the oxime derivatives are able both to induce the mutated "petite" phenotype in yeast S. cerevisiae and to cause "in vitro" extensive condensation of single stranded DNA. This reaction is postulated to be relevant to the mutational process that leads to "peptide" cells. The absence of nuclear mutation is interpreted in terms of sequestration of the drug in mitochondria under the effect of the organelle inner membrane electrochemical potential.

New aspects of the repair and genotoxicity of psoralen photoinduced lesions in DNA

Several approaches are described aiming at a better understanding of the genotoxicity of psoralen photoinduced lesions in DNA. Psoralens can photoinduce different types of photolesions including 3,4- and 4',5'-monoadducts and interstrand cross-links, oxidative damage (in the case of 3-carbethoxypsoralen (3-CPs)) and even pyrimidine dimers (in the case of 7-methylpyrido(3,4-c)psoralen (MePyPs)). The characterization and detection of different types of lesions has been essential for the analysis of their possible contributions to genotoxicity. For example, oxidative damage photoinduced by 3-CPs can be detected by the formamidopyrimidine glycosylase (FPG) protein. Furthermore, it is shown how the presence of MePyPs induced monoadducts may interfere with the photoreactivation of concomitantly induced pyrimidine dimers, how the ratio of monoadducts and interstrand cross-links (CL) affects the occurrence of double-strand breaks during the repair of photolesions and genotoxicity. In vitro treatment of yeast plasmids, followed by transformation, also indicates that the repair of photoadducts on exogenous DNA differs for 8-methoxy-psoralen (8-MOP) induced mono- and diadducts and for monoadducts alone. The recombinational rad52 dependent pathway is not needed for the repair of 8-MOP induced monoadducts. The results obtained suggest that the genotoxic effects of psoralens are conditioned by the nature, number, ratio and sequence distribution of the photolesions induced in DNA.

Non-specific incision of DNA due to the presence of 8-methoxypsoralen photoinduced interstrand cross-links in Saccharomyces cerevisiae

The repair of DNA interstrand cross-links (CL) induced by 8-methoxypsoralen (8-MOP) plus UVA irradiation was analyzed by the alkaline step elution technique. A double-exposure protocol was used with 8-MOP, starting with exposure to monochromatic 405-nm radiation inducing only DNA monoadducts (MA), followed, after washing out of unbound 8-MOP molecules, by a second exposure to 365-nm radiation inducing varying relative amounts of CL at a constant level of total photoadducts. In the range of doses used for the second exposure, repair of CL took place; however, in the presence of increased relative amounts of CL induced non-specific incision of DNA occurred. This endonucleolytic cleavage appears to be related to the increased mutagenic and recombinogenic effects observed at increased levels of CL.

Repair of exogenous (plasmid) DNA damaged by photoaddition of 8-methoxypsoralen in the yeast Saccharomyces cerevisiae

The contribution of different repair pathways to the repair of 8-methoxypsoralen (8-MOP) plus UVA induced lesions on a centromeric plasmid (YCp50) was investigated in the yeast Saccharomyces cerevisiae using the lithium acetate transformation method. The pathways of excision-resynthesis (RAD1) and recombination (RAD52) were found to be involved in the repair of exogenous as well as of genomic DNA. Mutants in RAD6 and PSO2 genes showed the same transformation efficiency with 8-MOP plus UVA treated plasmid as wild-type cells suggesting that these latter pathways involved in mutagenesis are not operating on plasmid DNA although required for the repair of 8-MOP photoadducts induced in genomic DNA. These results indicate that DNA-repair gene products may be differently involved in the repair of exogenous and endogenous DNA depending on the repair system and the nature of the DNA damage considered.

Metabolism of 5-methoxypsoralen by Saccharomyces cerevisiae

Incubation of methoxypsoralen (5-MOP) in the presence of diploid yeast cells (Saccharomyces cerevisiae) before UV-A exposure leads to an incubation-time dependent decrease of photoinduced genotoxic effects. The reduction in photoinduced genotoxicity is stronger in cells grown in the presence of 20% glucose and containing high levels of cytochrome P-450 than in cells grown in the presence of 0.5% glucose and containing undetectable levels of cytochrome P-450. Inhibition of P-450 activity by specific inhibitors, such as tetrahydrofuran and metyrapone, strongly affects the observed decrease in 5-MOP genotoxicity, indicating the involvement of P-450 in 5-MOP metabolism. As demonstrated by spectrophotometric and chromatographic (HPLC) analysis during incubation of 5-MOP with P-450 containing yeast cells, 5-MOP gradually disappears from the cell supernatant of the incubation mixture. The reduction in the chromatographic peak corresponding to 5-MOP is accompanied by the appearance of a new peak that probably corresponds to a metabolite. As shown by the use of P-450 specific inhibitors, the metabolite appears to be due to P-450 mediated 5-MOP metabolisation. Its UV absorption spectrum suggests an alteration of the pyrone moiety of the 5-MOP molecule.

Mitochondrial uptake of bridged bis-methylpyridinium aldoximes and induction of the "petite" phenotype in yeast

The 3,3'-[omega,omega'-alkanediylbis(oxy)]bis[2- (hydroxyimino)methyl]-1-methylpyridinium derivatives bearing a linking chain of 4, 5 and 6 methylene groups are accumulated in mitochondria with increasing efficiency under the effect of the electrical potential. Accumulation does not take place with derivatives carrying a 2 and 3 methylene-long linking chain. The uptake process is saturable. The efficiency of the various derivatives to induce the "petite" phenotype in yeast reflects the uptake rate observed with purified mitochondria.