Preferential DNA repair of alkali-labile sites within the active insulin gene

Role of DNA repair in carcinogen-induced ras mutation

In this contribution we discuss the gene- and cell type-specific repair of miscoding DNA alkylation products as a risk parameter in both mutation induction and malignant transformation by N-nitroso carcinogens. Upon exposure to N-nitroso compounds such as N-methyl-N-nitrosourea (MeNU) or N-ethyl-N-nitrosourea (EtNU), about a dozen different alkylation products are formed in cellular DNA. Among these are O(6)-methylguanine (O(6)-MeGua) and O(6)-ethylguanine (O(6)-EtGua), respectively, which differ only by one CH(2) group in their alkyl residue and, when unrepaired, cause G:C-->A:T transition mutations by anomalous base pairing during DNA replication. We have analyzed the global and gene-specific repair of O(6)-MeGua and O(6)-EtGua in target cell DNA, ras gene mutation frequencies, and tumor incidence, in the model of mammary carcinogenesis induced in 50-day-old female Sprague-Dawley rats by a single application of MeNU or EtNU. Both carcinogens induce histologically indistinguishable mammary adenocarcinomas at high yield. In the target mammary epithelia, O(6)-MeGua is repaired at similar slow rates in both transcriptionally active genes (Ha-ras, beta-actin), silent genes (lgE heavy chain), and in bulk DNA, by the one-step repair protein O(6)-alkylguanine-DNA alkyltransferase (MGMT; low level of expression in the target cells). The slow repair of O(6)-MeGua translates into a high frequency of mutations at the central position of Ha-ras codon 12 (GGA) in MeNU-induced tumors. O(6)-EtGua, however, is removed approximately 20 times faster than O(6)-MeGua selectively from transcribed genes via an MGMT independent, as yet uncharacterized excision mechanism. Accordingly, no Ha-ras codon 12 mutations are found in the EtNU-induced mammary tumors. Neither MeNU- nor EtNU-induced tumors exhibit mutations at codons 13 and 61 of Ha-ras or at codons 12, 13 and 61 of Ki-ras. While a moderate surplus MGMT activity of the target cells - contributed by a bacterial MGMT transgene (ada) - significantly counteracts mammary tumorigenesis in MeNU-exposed rats, this is not the case in the EtNU-treated animals. Differential repair of structurally distinct DNA lesions in transcribed or (temporarily) silent genes thus determines the probability of mutation and, together with cell type-specific and interindividual differences in DNA repair capacity, influences carcinogenic risk.

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.

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.

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.

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.

Age-related studies on the removal of 7-methylguanine from DNA of mouse kidney tissue following N-methyl-N-nitrosourea treatment

To investigate the effects of age on DNA repair of alkylation damage, C57BL/6NNia mice ranging from 9 months to 29 months of age were injected by the intraperitoneal route with single doses of N-methyl-N-nitrosourea (MNU). The rates of removal of 7-methylguanine (m7Gua) in nuclear DNA from kidney were determined at various intervals from 1 to 288 h after injection of either 25 mg or 50 mg MNU per kg body weight. Reversed phase HPLC with electrochemical detection was used to monitor adduct disappearance from DNA hydrolysates. The kinetics of m7Gua removal from DNA were at least biphasic. Evidence was obtained that there was a rapid removal of m7Gua occurring in the first 24 h after MNU administration, followed by a slow phase of removal with a t1/2 greater than 150 h. We assume that these two phases of m7Gua removal correspond to active repair of DNA by N-alkylglycosylases and to passive elimination via spontaneous hydrolysis, respectively. Young and old kidney tissues all exhibited significant repair of m7Gua (55-73% of the induced adducts were removed in the first 24 h), but a substantial fraction of m7Gua was removed slowly, indicating that there are methylated bases which were refractory to repair processes. At both doses of MNU studied, old tissues showed active repair of m7Gua that, within the limits of detection, had similar initial rates of removal as young tissues. However, old kidney did not remove this adduct with the same overall efficiency as young kidney. Therefore, the amount of m7Gua in the repair-resistant fraction was greater in the senescent tissues. The biochemical mechanisms responsible for the less efficient DNA repair in senescent kidney are not known, but we suggest that such differences are due in part to structural alterations in the chromatin.

N-methyl-N-nitrosourea-induced mutations in human cells. Effects of the transcriptional activity of the target gene

In this study we addressed the question as to whether the mutagenesis by methylating agents is affected by the transcriptional activity of the damaged gene. An Epstein-Barr virus (EBV)-derived shuttle vector system was developed where the genetic target for mutation analysis, the bacterial gpt gene, is under the control of an eukaryotic inducible promoter in plasmid pF1-EBV and lacks the eukaryotic promoter in plasmid pF2-EBV. Two human cell lines that episomically maintain these shuttle vectors were established. In clone 6NT cells, which contain pF1-EBV plasmid, the gpt gene is actively transcribed and the transcription rate is regulated by zinc ions. In clone 3 cells, which harbor pF2-EBV plasmid, the gpt gene is not transcribed. Following treatment of both cell lines with the potent alkylating carcinogen N-methyl-N-nitrosourea (MNU), G.C to A.T transitions were the major mutagenic event, consistent with the miscoding potential of O6-methylguanine. The mutations were predominantly generated in the non-transcribed DNA strand of the active gpt gene. The same strand-bias was observed when the gpt gene was transcriptionally inactive, indicating that MNU-induced strand-specific formation of mutations is not due to transcription. Our data identify as major determinants of this phenomenon the sequence-specificity of MNU mutagenesis and the conformational properties of the target protein. Differences in mutation distribution were observed between the transcriptionally active and inactive gpt gene. This finding suggests that the organization of active genes in chromatin might modulate DNA alkylation and/or DNA repair.

Effects of N-methyl-N-nitrosourea on transcriptionally active and inactive nucleosomes: macromolecular damage and DNA repair

We previously reported a separation, on an organomercurial column, of transcriptionally inactive nucleosomes (class 1) from those containing active gene sequences (classes 2 and 3). In this paper, we analyzed nucleosomal damage caused by exposure of HeLa S3 cells in suspension culture to the directly alkylating carcinogen N-methyl-N-nitrosourea (MNU). The extent and site of methylation induced by the compound in nucleosomal DNA and RNA were determined by cell incubation in the presence of [3H]MNU. The highest amount of damage was detected in DNA of class 3 nucleosomes, while RNA alkylation was comparable in all nucleosomal classes. Cellular capacity for repair of MNU-induced DNA strand breaks (estimated after a short pulse with [3H]thymidine) was found to be higher in active nucleosomal fractions (classes 2 and 3) than in the inactive fraction (class 1). Our data support the postulate that chromatin primary structure plays a role in modulating carcinogen damage to chromosomal macromolecules and in DNA strand breakage and repair mechanisms. Some of these initial steps are believed to be critical in the process of carcinogenesis.

Exposure of pancreatic islets to different alkylating agents decreases mitochondrial DNA content but only streptozotocin induces long-lasting functional impairment of B-cells

Pancreatic B-cells exposed in vivo or in vitro to streptozotocin (SZ), the N-nitrosourea derivative of glucosamide, present a long-lasting impairment in the production and release of insulin while other cell functions are better preserved. This functional impairment is associated with a defective mitochondrial function. To further study the mechanisms behind SZ actions, mouse pancreatic islets were exposed in vitro to SZ (1.5 mM) or to different concentrations of methyl methanesulfonate (MMS; 2, 4 and 6 mM). The effect of the aglucone moiety of SZ, nitroso-N-methylurea (NMU; 2, 4 and 6 mM) was also tested. Islets were either studied immediately after exposure to the drugs (day 0) or after six days in culture following toxin treatment (day 6). On day 0 the islets showed a decrease in the NAD + NADH content, decreased glucose oxidation rates and an impaired insulin release in response to glucose. Six days after exposure to SZ there was still impaired glucose oxidation and insulin release, and decreased islet insulin mRNA and insulin content, but the NAD + NADH content was again similar to the control group. On the other hand, islets which survived for 6 days in culture following exposure to either MMS or NMU were able to regain normal B-cell function. The mouse islets exposed to SZ, NMU and MMS showed on day 6 a 30-40% decrease in the content of the mitochondrial DNA encoded cytochrome b mRNA and a 60-70% decrease in total mitochondrial DNA, as evaluated by dot and Southern blot analysis. Only SZ decreased the insulin mRNA content whereas both MMS and NMU decreased the glucagon mRNA content. As a whole, the data obtained indicate that SZ, NMU and MMS induce damage to the mitochondrial genome, and this may contribute to the B-cell dysfunction observed after SZ treatment. It is conceivable that the glucose moiety of SZ may direct the methylation to other intracellular sites besides the mitochondrial DNA, thus explaining the different functional responses of islets following exposure to SZ and NMU.