Heterogeneity of nitrogen mustard-induced DNA damage and repair at the level of the gene in Chinese hamster ovary cells

Two azobenzene derivatives CAB/ACB as reusable sunscreen: UV absorptive capacity and biosafety evaluation

Exposure to UV radiation may cause harmful effects to the skin such as damage, aging and cancer, which can be prevented by using sunscreens. Here, two azobenzene derivatives, 4-cholesterocarbonyl-4'-(N,N,N-triethylaminebutyloxyl bromide) azobenzene (CAB) and 4-cholesterocarbonyl-4'-(N,N-diethylaminebutyloxyl) azobenzene (ACB) were studied as reusable sunscreen candidates. Biocompatibility studies including apoptosis, cytotoxicity and in vivo phototoxicity revealed that the two compounds were rather safe, except ACB, which showed a weak phototoxicity in vitro. Both CAB and ACB have good UV absorption capacity not only in their solution state (dimethylsulfoxide, DMSO) but also in the cosmetic cream state. A commercial sunscreen, avobenzone was decomposed upon UV irradiation and lost its UV protection ability, while that of CAB and ACB could be quickly recovered upon visible light irradiation, implying that they can serve as a new type of reusable sunscreen.

Mitochondria, oxidative DNA damage, and aging

Protection from reactive oxygen species (ROS) and from mitochondrial oxidative damage is well known to be necessary to longevity. The relevance of mitochondrial DNA (mtDNA) to aging is suggested by the fact that the two most commonly measured forms of mtDNA damage, deletions and the oxidatively induced lesion 8-oxo-dG, increase with age. The rate of increase is species-specific and correlates with maximum lifespan. It is less clear that failure or inadequacies in the protection from reactive oxygen species (ROS) and from mitochondrial oxidative damage are sufficient to explain senescence. DNA containing 8-oxo-dG is repaired by mitochondria, and the high ratio of mitochondrial to nuclear levels of 8-oxo-dG previously reported are now suspected to be due to methodological difficulties. Furthermore, MnSOD -/+ mice incur higher than wild type levels of oxidative damage, but do not display an aging phenotype. Together, these findings suggest that oxidative damage to mitochondria is lower than previously thought, and that higher levels can be tolerated without physiological consequence. A great deal of work remains before it will be known whether mitochondrial oxidative damage is a "clock" which controls the rate of aging. The increased level of 8-oxo-dG seen with age in isolated mitochondria needs explanation. It could be that a subset of cells lose the ability to protect or repair mitochondria, resulting in their incurring disproportionate levels of damage. Such an uneven distribution could exceed the reserve capacity of these cells and have serious physiological consequences. Measurements of damage need to focus more on distribution, both within tissues and within cells. In addition, study must be given to the incidence and repair of other DNA lesions, and to the possibility that repair varies from species to species, tissue to tissue, and young to old.

Altering DNA base excision repair: use of nuclear and mitochondrial-targeted N-methylpurine DNA glycosylase to sensitize astroglia to chemotherapeutic agents

Primary astrocyte cultures were used to investigate the modulation of DNA repair as a tool for sensitizing astrocytes to genotoxic agents. Base excision repair (BER) is the principal mechanism by which mammalian cells repair alkylation damage to DNA and involves the processing of relatively nontoxic DNA adducts through a series of cytotoxic intermediates during the course of restoring normal DNA integrity. An adenoviral expression system was employed to target high levels of the BER pathway initiator, N-methylpurine glycosylase (MPG), to either the mitochondria or nucleus of primary astrocytes to test the hypothesis that an alteration in BER results in increased alkylation sensitivity. Increasing MPG activity significantly increased BER kinetics in both the mitochondria and nuclei. Although modulating MPG activity in mitochondria appeared to have little effect on alkylation sensitivity, increased nuclear MPG activity resulted in cell death in astrocyte cultures treated with methylnitrosourea (MNU). Caspase-3 cleavage was not detected, thus indicating that these alkylation sensitive astrocytes do not undergo a typical programmed cell death in response to MNU. Astrocytes were found to express relatively high levels of antiapoptotic Bcl-2 and Bcl-XL and very low levels of proapoptotic Bad and Bid suggesting that the mitochondrial pathway of apoptosis may be blocked making astrocytes less vulnerable to proapoptotic stimuli compared with other cell types. Consequently, this unique characteristic of astrocytes may be responsible, in part, for resistance of astrocytomas to chemotherapeutic agents.

Palmitate induced mitochondrial deoxyribonucleic acid damage and apoptosis in l6 rat skeletal muscle cells

A major characteristic of type 2 diabetes mellitus (T2DM) is insulin resistance in skeletal muscle. A growing body of evidence indicates that oxidative stress that results from increased production of reactive oxygen species and/or reactive nitrogen species leads to insulin resistance, tissue damage, and other complications observed in T2DM. It has been suggested that muscular free fatty acid accumulation might be responsible for the mitochondrial dysfunction and insulin resistance seen in T2DM, although the mechanisms by which increased levels of free fatty acid lead to insulin resistance are not well understood. To help resolve this situation, we report that saturated fatty acid palmitate stimulated the expression of inducible nitric oxide (NO) synthase and the production of reactive oxygen species and NO in L6 myotubes. Additionally, palmitate caused a significant dose-dependent increase in mitochondrial DNA (mtDNA) damage and a subsequent decrease in L6 myotube viability and ATP levels at concentrations as low as 0.5 mM. Furthermore, palmitate induced apoptosis, which was detected by DNA fragmentation, caspase-3 cleavage, and cytochrome c release. N-acetyl cysteine, a precursor compound for glutathione formation, aminoguanidine, an inducible NO synthase inhibitor, and 5,10,15,20-tetrakis(4-sulphonatophenyl) porphyrinato iron (III), a peroxynitrite inhibitor, all prevented palmitate-induced mtDNA damage and diminished palmitate-induced cytotoxicity. We conclude that exposure of L6 myotubes to palmitate induced mtDNA damage and triggered mitochondrial dysfunction, which caused apoptosis. Additionally, our findings indicate that palmitate-induced mtDNA damage and cytotoxicity in skeletal muscle cells were caused by overproduction of peroxynitrite.

Preferential in vivo DNA repair of melphalan-induced damage in human genes is greatly affected by the local chromatin structure

To investigate the molecular mechanisms of action of the nitrogen mustard melphalan in patients treated for multiple myeloma, the in vivo induction and repair of melphalan-induced DNA damage was measured in genes with different transcriptional activity (b-actin>p53>N-ras>d-globin) from leukocytes of 20 multiple myeloma patients following chemotherapeutic administration of high-dose melphalan (200mg/m(2)) and autologous blood stem cell transplantation. Heterogeneous repair was found among the studied genes. The extent of repair was always in the order: b-actin>p53>N-ras>d-globin, correlating with the gene transcriptional state. Similar findings were obtained using peripheral blood mononuclear cells (PBMC) from healthy volunteers following in vitro treatment with melphalan, indicating that these results are not malignant disease-specific. Following in vitro treatment of PBMC from healthy volunteers with alpha-amanitin, an inhibitor of RNA polymerase II that can also induce condensation of chromatin structure, a significant inhibition of the removal of melphalan-induced damage in the three active genes but not in the silent d-globin gene was found, suggesting that transcription and/or chromatin structure may play important roles in the preferential DNA repair. When the in vivo DNA damage formation and repair in multiple myeloma patients following chemotherapeutic administration of melphalan was measured in the two strands of the active genes, no strand bias was found, indicating that the global genome repair subpathway of nucleotide excision repair may play a crucial role in the repair of these adducts. These results were also confirmed in PBMC from healthy volunteers following in vitro treatment with melphalan. Using micrococcal nuclease digestion of nuclei isolated from PBMC of multiple myeloma patients before the chemotherapeutic treatment, as well as from PBMC of healthy volunteers, we probed the chromatin structure in each gene and found that the "looseness" of the chromatin structure correlated with the levels of the gene-specific repair, being again in the order: b-actin>p53>N-ras>d-globin. To conclude, the in vivo gene-specific repair of melphalan-induced damage in humans is greatly affected by the local chromatin structure.

DNA repair and chromatin structure in genetic diseases

Interaction of DNA repair proteins with damaged DNA in eukaryotic cells is influenced by the packaging of DNA into chromatin. The basic repeating unit of chromatin, the nucleosome, plays an important role in regulating accessibility of repair proteins to sites of damage in DNA. There are a number of different pathways fundamental to the DNA repair process. Elucidation of the proteins involved in these pathways and the mechanisms they utilize for interacting with damaged nucleosomal and nonnucleosomal DNA has been aided by studies of genetic diseases where there are defects in the DNA repair process. Two of these diseases are xeroderma pigmentosum (XP) and Fanconi anemia (FA). Cells from patients with these disorders are similar in that they have defects in the initial steps of the repair process. However, there are a number of important differences in the nature of these defects. One of these is in the ability of repair proteins from XP and FA cells to interact with damaged nucleosomal DNA. In XP complementation group A (XPA) cells, for example, endonucleases present in a chromatin-associated protein complex involved in the initial steps in the repair process are defective in their ability to incise damaged nucleosomal DNA, but, like the normal complexes, can incise damaged naked DNA. In contrast, in FA complementation group A (FA-A) cells, these complexes are equally deficient in their ability to incise damaged naked and similarly damaged nucleosomal DNA. This ability to interact with damaged nucleosomal DNA correlates with the mechanism of action these endonucleases use for locating sites of damage. Whereas the FA-A and normal endonucleases act by a processive mechanism of action, the XPA endonucleases locate sites of damage distributively. Thus the mechanism of action utilized by a DNA repair enzyme may be of critical importance in its ability to interact with damaged nucleosomal DNA.

Excision repair of nitrogen mustard-DNA adducts in Saccharomyces cerevisiae

The bifunctional alkylating anticancer drug nitrogen mustard forms a variety of DNA lesions, including monoadducts and intrastrand and interstrand crosslinks. Although it is known that nucleotide excision repair (NER) is important in processing these adducts, the role of the other principal excision repair pathway, base excision repair (BER) is less well defined. Using isogenic Saccharomyces cerevisiae strains disrupted for a variety of NER and BER genes we have examined the relative importance of the two pathways in the repair of nitrogen mustard adducts. As expected, NER defective cells (rad4 and rad14 strains) are extremely sensitive to the drug. One of the BER mutants, a 3-methyladenine glycosylase defective (mag1) strain also shows significant hypersensitivity. Using a rad4/mag1 double mutant it is shown that the two excision repair pathways are epistatic to each other for nitrogen mustard sensitivity. Furthermore, both rad14 and mag1 disruptants show elevated levels of nitrogen mustard-induced forward mutation. Measurements of repair rates of nitrogen mustard N-alkylpurine adducts in the highly transcribed RPB2 gene demonstrate defects in the processing of mono-adducts in rad4, rad14 and mag1 strains. However, there are differences in the kinetics of adduct removal in the NER mutants compared to the mag1 strain. In the mag1 strain significant repair occurs within 1 h with evidence of enhanced repair on the transcribed strand. Adducts however accumulate at later times in this strain. In contrast, in the NER mutants repair is only evident at times greater than 1 h. In a mag1/rad4 double mutant damage accumulates with no evidence of repair. Comparison of the rates of repair in this gene with those in a different genomic region indicate that the contributions of NER and BER to the repair of nitrogen mustard adducts may not be the same genome wide.

Base excision repair of N-methylpurines in a yeast minichromosome. Effects of transcription, dna sequence, and nucleosome positioning

Base excision repair of dimethyl sulfate induced N-methylpurines (NMPs) was measured in a yeast minichromosome that has a galactose-inducible GAL1:URA3 fusion gene, a constitutively expressed HIS3 gene, and varied regions of chromatin structure. Removal rates of NMPs varied dramatically (>20-fold) at different sites along three selected fragments encompassing a total length of 1775 base pairs. Repair of NMPs was not coupled to transcription, because the transcribed strands of HIS3 and induced GAL1:URA3 were not repaired faster than the nontranscribed strands. However, the repair rate of NMPs was significantly affected by the nearest neighbor nucleotides. Slow repair occurred at NMPs between purines, especially guanines, whereas fast repair occurred at NMPs between pyrimidines. NMPs between a purine and pyrimidine were repaired at moderate rates. Moreover, a rough correlation between nucleosome positions and repair rates exists in some but not all regions that were analyzed.

Adduct formation, mutagenesis and nucleotide excision repair of DNA damage produced by reactive oxygen species and lipid peroxidation product

Reactive oxygen species are formed constantly in living organisms, as products of the normal metabolism, or as a result of many different environmental influences. Here we review the knowledge of formation of DNA damage, the mutations caused by reactive oxygen species and the role of the excision repair processes, that protect the organism from oxidative DNA damage. In particular, we have focused on recent studies that demonstrate the important role of nucleotide excision repair. We propose two major roles of nucleotide excision repair as 1) a backup when base excision repair of small oxidative lesions becomes saturated, and as 2) a primary repair pathway for DNA damage produced by lipid peroxidation products.

Proficient deoxyribonucleic acid repair of methylation damage in hamster ERCC-gene mutants

Three major pathways, nucleotide excision repair (NER), base excision repair (BER) and O6-methylguanine-DNA methyltransferase (MGMT), are responsible for the removal of most adducts to DNA and thus for the survival of cells influenced by deoxyribonucleic acid (DNA) adduct-forming chemicals. We have evaluated host cell reactivation and cell survival of wild type Chinese hamster ovary cells and of mutants in the NER-genes ERCC1, ERCC2, and ERCC4 after treatment with the methylating compounds dimethylsulfate and methylnitrosourea. No effect of the three genes could be demonstrated, i.e., survival and host cell reactivation after methylation damage in the mutants and the wild type cells were similar. Gene-specific repair experiments confirmed the proficient removal of methyl lesions. We conclude that the three nucleotide excision repair genes are immaterial to the repair of methylation damage. This suggests that NER does not play a role in the removal of methylation in mammalian cells and that BER and MGMT are responsible for the survival of such cells, when they are challenged with methylation of DNA.

Preferential accumulation of single-stranded regions in telomeres of human fibroblasts

We have demonstrated recently that chronic hyperoxic treatment accelerates the rate of aging of fibroblasts and the rate of telomere shortening in parallel. It was hypothesized that accelerated telomere shortening is due to preferential accumulation of oxidative damage in telomeres. To test this hypothesis, we measured the accumulation of sites sensitive to S1 nuclease treatment in telomeres, in minisatellites, and in the bulk of the genome of fibroblasts under different models of oxidative stress as well as after treatment with the alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine. A comparison with qualitative data obtained by alkaline electrophoresis reveals that the sites transferred to double-strand breaks by treatment with low concentrations of S1 nuclease are, in fact, single-stranded regions in the DNA. These regions may resemble single-stranded overhangs, gaps, or conventional single-strand breaks. The frequency of single-stranded regions is significantly higher in telomeres than in minisatellites or in the bulk of the genome under all conditions tested. Those regions induced in minisatellites or in the overall genome by a bolus dose of hydrogen peroxide are completely repaired within 24 h. On the other hand, 50 +/- 12% of H2O2-induced single-stranded regions remain unrepaired for at least 19 days in telomeres of human fibroblasts, leading to a significant increase of the telomeric steady-state level of these lesions. This preferential accumulation might significantly contribute to telomere shortening.

Repair of oxidative damage in nuclear DNA sequences with different transcriptional activities

This study was designed to investigate the repair of oxidative damage in nuclear DNA sequences with different transcriptional activities. Chinese hamster ovary (CHO) cells were treated with the oxygen radical generator hypoxanthine/xanthine oxidase (Hyp/XO). Damage and repair were evaluated in 14-kb restriction fragments containing either the DHFR gene, a 3'-non-transcribed flanking region, or the c-fos gene using a quantitative Southern blot technique. Damage to the sugar-phosphate backbone and abasic sites were detected by measuring their lability in alkali conditions. Lesions in DNA bases were identified using the bacterial repair enzyme endonuclease III, which predominantly recognizes damage to thymines and cytosines, and formamidopyrimidine-DNA glycosylase, which recognizes 8-oxoguanine and purines with fractured imidazole rings. The results showed that similar amounts of all types of oxidative damage were produced in both the transcribed and non-transcribed sequences following a 1-h exposure to the radical generator. Repair in all sequences was rapid, with approximately 60% removal of lesions observed by 1 h. Therefore, within these sequences, the repair of oxidative lesions is much faster than that of other types of damage, such as those induced by alkylating toxins and UV irradiation, and the repair is not affected appreciably by transcriptional status.

Gene-specific nuclear and mitochondrial repair of formamidopyrimidine DNA glycosylase-sensitive sites in Chinese hamster ovary cells

This study examines the capacity of a mammalian cell to repair, at the gene level, DNA base lesions generated by photoactivation of acridine orange. Chinese hamster ovary fibroblasts were exposed to acridine orange and visible light, and gene-specific DNA repair was measured in the dihydrofolate reductase (DHFR) gene and in the mitochondrial genome. DNA lesions were recognized by Escherichia coli formamidepyrimidine-DNA glycosylase (FPG) which removes predominantly 8-oxodG and the corresponding formamidopyrimidine ring opened bases, and subsequently cleaves the DNA at the resulting apurinic site. FPG-recognized DNA lesions increased linearly with increasing photo-activation of AO, while cell survival was not affected by light alone and was negligibly affected by preincubation with AO in the dark. The frequency of induction of FPG-sensitive DNA damage by photoactivation of AO was similar in the transcribed and non-transcribed nuclear DNA as well as in the mitochondrial DNA. FPG-sensitive sites in the DHFR gene were repaired quickly, with 84% of adducts repaired within 4 h. The lesion frequency, kinetics and percent of repair of non-transcribed genomic DNA did not differ significantly from repair in the active DHFR gene up to 1 h postexposure. At late time points, transcribed DNA was repaired faster than the non-transcribed DNA. Mitochondrial DNA was efficiently repaired, at a rate similar to that in the active nuclear DNA.

Ionizing radiation-induced DNA strand breakage and rejoining in specific genomic regions as determined by an alkaline unwinding/Southern blotting method

A recently developed, combined alkaline unwinding/Southern blotting assay was utilized to examine DNA damage and repair induced by ionizing radiation within specific large-scale genomic regions. Following treatment of MCF-7 breast tumour cells with 2-10-Gy gamma-rays, strand breakage and rejoining were measured in bulk DNA, in the centromeric alpha-satellite region of chromosome 17, and in the chromatin regions containing the unexpressed beta-globin gene and the expressed c-myc oncogene, which is known to be important for growth in the MCF-7 cell line. Damage in both the c-myc and beta-globin regions was markedly greater than in either alpha-satellite or bulk DNA. However, the kinetics of strand break repair were approximately the same in c-myc as in alpha-satellite or bulk DNA. Surprisingly, the radiomimetic antibiotic bleomycin, which also induces free-radical-mediated strand breakage, showed considerably less heterogeneity of DNA damage among the genomic regions examined than did radiation. The results suggest that actively transcribed genes, as well as at least some inactive genes, are surrounded by large-scale domains of radiosensitive chromatin. With no apparent enhancement of rejoining, the increased incidence of strand breaks in these regions persists until rejoining is essentially complete. Changes in the integrity of specific chromatin regions may be an important aspect of DNA damage-induced cell death.

Gene specific DNA repair of damage induced in familial Alzheimer disease cells by ultraviolet irradiation or by nitrogen mustard

We have measured gene specific DNA repair in a normal human fibroblast cell line, and in fibroblast lines from two patients with familial Alzheimer disease (AD). Cells were treated with either ultraviolet radiation (UV) or the chemotherapeutic alkylating agent, nitrogen mustard (HN2). DNA damage formation and repair were studied in the active dihydrofolate reductase (DHFR) gene for the main lesions introduced by each of these two types of DNA damaging agents. The gene specific repair of UV induced cyclobutane pyrimidine dimers in the human DHFR gene was 86% complete in the AD cells after 24 h of repair incubation. This repair efficiency was similar to what we and others have found in normal human fibroblasts. After treatment of the AD cells with HN2, we found the frequency of HN2 induced lesions in the DHFR gene to be similar to the frequency in the transcriptionally inactive delta-globin gene. The gene specific repair of HN2 induced lesions in the DHFR gene was completed within 8-24 h in the normal fibroblast line and in the familial AD line, and the repair kinetics were similar for both cell lines. These results indicate that familial AD fibroblasts have normal gene specific repair of both UV induced and HN2 induced DNA damage in active genes.

DNA damage and mutagenesis induced by nitrogen mustards

The nitrogen mustards are bifunctional alkylating agents which, although used extensively in cancer chemotherapy, are themselves highly carcinogenic. All nitrogen mustards induce monofunctional guanine-N7 adducts, as well as interstrand N7-N7 crosslinks involving the two guanines in GNC.GNC (5'-->3'/5'-->3') sequences. In addition, the aromatic mustards melphalan and chlorambucil also induce substantial alkylation at adenine N3, while cyclophosphamide forms phosphotriesters with relatively high frequency. Nitrogen mustards are genotoxic in virtually every assay, and produce a wide array of mutations, including base substitutions at both G.C and A.T base pairs, intragenic as well as multilocus deletions, and chromosomal rearrangements. Mutational spectra generated by these agents in various model systems vary widely, and no single lesion has been implicated as being primarily responsible for mustard-induced mutagenesis. On the contrary, adducts of both adenine and guanine, and monofunctional as well as bifunctional adducts, appear to be involved. Further, it is still not known which types of mutation are responsible for mustard-induced cancers, since no genes have yet been identified which are consistently altered in these malignancies.

Down-regulation of c-myc gene expression with induction of high molecular weight DNA fragments by fluorodeoxyuridine

5-Fluoro-2'-deoxyuridine (FdUrd), a potent inhibitor of thymidylate synthase, induces extensive bulk DNA damage at drug concentrations that produce significant in vitro growth inhibition of human ileocecal carcinoma (HCT-8) cells. Constant- and pulsed-field gel electrophoresis (CFGE and PFGE), to detect size distribution of DNA double-strand breaks and repair kinetics, in parallel with northern and western blot analyses, to quantitate c-myc gene and protein expression, were utilized to analyze drug effects. At 24-hr post in vitro drug treatment, when maximum bulk DNA damage was detected, FdUrd produced a broad range of high molecular weight DNA fragments, clustering between 0.1 and 5.7 megabases in size, and resulted in a decrease in the level of c-myc transcripts and protein with no significant effect on the level of v-myc and H-ras. These effects preceded the observed cellular growth inhibition. Addition of the reduced folate leucovorin potentiated the effects induced by FdUrd, indicating that thymidylate synthase inhibition is an important initial step in drug effect followed by DNA fragmentation and suppression of c-myc expression. Changes in the integrity of the genetic materials and regulatory genes occurred prior to the observed cell growth inhibition by FdUrd, suggesting that these molecular alterations by FdUrd may be associated with subsequent FdUrd-induced cell growth inhibition.

Intragenomic heterogeneity of DNA damage formation and repair: a review of cellular responses to covalent drug DNA interaction

Chemical DNA interaction and its processing can now be studied at the level of specific genomic regions. Such investigations have revealed important new information about the molecular biology of the cellular responses to genomic insult and especially of the repair processes. They also have demonstrated that both the formation and repair of DNA damage display patterns of intragenomic heterogeneity. Therefore, mechanistic studies should involve examination of DNA damage formation and repair in specific genomic sequences besides in the overall genome to provide clues to the way in which specific modifications of DNA or chromatin could have specific biological effects. This review primarily focuses on studies done to elucidate the nature of DNA damage induction and intragenomic processing provoked by covalent drug-DNA modification in mammalian cells. The involvement of DNA damage formation and cellular processing as critical factors for genomic injury is exemplified by studies of the novel alkylating morpholinyl anthracyclines and the bifunctional alkylating agent nitrogen mustard as a prototype agent for covalent drug DNA interaction.

Repair of N-methylpurines in DNA from lymphocytes of patients with amyotrophic lateral sclerosis

We have previously reported reduced ability of ALS fibroblasts to repair genomic DNA damage produced by alkylating agents. This report presents our experience of studying DNA repair in lymphocytes from ALS patients. The repair of N-methylpurines produced by treatment with the alkylating agent, methyl methanesulfonate, was studied in T-lymphocytes from patients with sporadic and familial ALS, and appropriate controls. Repair of damage was quantitated by using alkaline elution for genomic DNA repair, and methoxyamine protection of abasic sites in DNA fragments for gene-specific repair in the dihydrofolate reductase (dhfr) gene, at time points 0, 6 h and 24 h. No significant repair rate differences were observed between ALS and control lymphocytes in either genomic or gene-specific DNA repair. The possible reasons for the discrepancy with our earlier results in lymphocytes and fibroblasts are discussed.

Quantitation of 2-amino-3-methylimidazo[4,5-f]quinoline and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline DNA adducts in specific sequences using alkali or uvrABC excinuclease

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MelQx) are carcinogens found in cooked meats that form DNA adducts upon metabolic activation. Purified DNA from Chinese hamster ovary (CHO) cells was reacted in vitro with the active metabolites N-acetoxy-IQ or N-acetoxy-MelQx, and the adduct levels in the 5' dihydrofolate reductase (DHFR) gene and downstream region were quantitated by Southern hybridization. Adducted and restricted DNA was treated with Escherichia coli uvrABC excinuclease or alkali (0.1 N NaOH, 37 degrees C, 60 min) to incise DNA at IQ and MelQx adduct sites. The DNA was then denatured with formamide, electrophoresed on a neutral agarose gel, transferred to a support membrane, and hybridized with sequence-specific DNA probes. Both uvrABC and alkali reduced the intensity of Southern hybridization in proportion to the number of IQ or MelQx adducts in DNA, indicating that these adducts are substrates for uvrABC and that they form alkali-labile lesions in DNA. IQ and MelQx adduct levels were the same in the 5' DHFR gene and in the downstream region. Southern hybridization analysis of pBR322 containing known levels of IQ or MelQx adducts showed that the efficiency of cutting IQ or MelQx adducts by uvrABC excinuclease and alkali was approximately 30% and 15%, respectively. 32P-postlabeling studies examining adduct level in bulk DNA further showed that the adduct profiles were identical in pBR322, CHO DNA, and cultured CHO cells exposed to the reactive metabolites of IQ or MelQx. The results indicate that IQ and MelQx adducts can be quantitated in specific genomic sequences and that this method should be directly applicable to studies of gene-specific repair of these adducts in cultured cells.

Influence of DNA repair defects (rad1, rad52) on nitrogen mustard mutagenesis in yeast

Nitrogen mustard (HN2) mutagenesis of a plasmid-borne copy of the Saccharomyces cerevisiae SUP4-o gene was examined in a repair-proficient yeast strain and isogenic derivatives defective for excision (rad1) or DNA double-strand break (rad52) repair. The excision repair deficiency sensitized the cells to killing by HN2 and abolished mutation induction. Inactivation of RAD52 had no influence on the lethality of HN2 treatment but diminished the induced mutation frequency by 50% at all doses tested. DNA sequence analysis of HN2-induced SUP4-o mutations suggested that RAD52 contributed to the production of basepair substitutions at G.C sites. The rad52 defect appeared to alter the distribution of G.C-->A.T transitions in SUP4-o relative to the distribution for the wild-type strain. This difference did not seem to be due to an effect of RAD52 on the relative fractions of HN2-induced transitions at localized (flanked by A.T pairs) or contiguous (flanked by at least one G.C pair) G.C sites but instead to an influence on the strand specificity of HN2 mutagenesis. In the repair-proficient strain, the transitions showed a small bias for sites having the guanine on the transcribed strand and this preference was eliminated by inactivation of RAD52.

DNA damage by drugs and radiation: what is important and how is it measured?

DNA is the most important target for drug and radiation induced cell killing. The mode of cell killing by cytotoxic drugs and radiation has been derived by correlating the type and quantity of DNA damage induced with lethality. Cytotoxic drugs can be classified by their main mode of action, while ionising radiation causes a range of lesions with the DNA double-strand break (dsb) being the most significant. Strand-breaks are measured from the reduction in the size of DNA molecules following treatment. Molecule size can be derived from the rate that DNA fragments sediment when centrifuged, elute through filters or migrate under electrophoresis. The effect of strand-breaks on DNA loop supercoiling allow a sensitive assay of DNA damage. Specific assays for base damage and drug adducts include changes in chromatographic mobility or binding by specific antibodies. By comparing the levels of damage in the genome overall with damage in specific gene targets, regions susceptible to damage induction, and varying in repair efficiency, have been revealed.

Two expressed human genes sustain slightly more DNA damage after alkylating agent treatment than an inactive gene

Alkylating agent damage was quantified in human T-lymphocytes by calculating gene-specific lesion frequencies and repair rates. At 3 time points after exposure to methyl methanesulfonate (0, 6, and 24 h), T-lymphocyte DNA was extracted, digested with HindIII, and divided into 2 aliquots. Apurinic sites were formed in the DNA fragments of both aliquots by heat-induced liberation of the N-methylpurines. The methoxyamine-treated aliquot provided gene fragments which were refractory to alkaline hydrolysis (full-length fragments), while the fragments in the untreated aliquot were cleaved at apurinic sites by hydroxide. After Southern blotting, lesion frequencies were calculated by comparing the band intensity of the full-length fragment to its unprotected counterpart. The restriction fragments analyzed were from the constitutively active dihydrofolate reductase (dhfr) plus hypoxanthine phosphoribosyltransferase (hprt) genes and from the transcriptionally inactive Duchenne muscular dystrophy gene (dmd). In decreasing order, the fragments containing the most lesions per kb of DNA were: hprt greater than dhfr greater than dmd. T-Lymphocytes from 2 females had 30% more heat-labile N-methylpurines in the active X-linked hprt gene than in the inactive X-linked dmd gene. The lesion frequency found in the male's lone hprt allele was the highest observed. These lesion frequency differences are discussed in terms of chromatin structure. After 6 and 24 h, no significant repair rate differences were observed among the 3 genes.

Mechanisms of resistance to chlorambucil in chronic lymphocytic leukemia

The postulated biochemical mechanisms responsible for clinical resistance to chlorambucil (CLB) in chronic lymphocytic leukemia (CLL) have been examined. The total sulfhydryl, non-protein-bound sulfhydryl, protein-bound sulfhydryl (PSH) and glutathione (GSH) levels, in addition to glutathione S-transferase (GST) activities, were measured in the leukemic cells of 18 CLL patients. In addition, the formation and repair of DNA cross-links were measured following incubation of the cells with 100 microM chlorambucil in vitro. These parameters were then correlated with the subsequent clinical responses of the patients, as measured by the percent fall in lymphocyte count 3 weeks following 0.9 mg/kg chlorambucil. No correlations were observed between any of the individual parameters and clinical response, although a slight positive correlation was observed between the PSH:GSH ratio and clinical response. These findings suggest that multiple mechanisms may contribute to CLB-resistance in CLL.