Mutation Research/DNA Repair Reports最新文献

Recently, hydrogen peroxide and its free-radical product, the hydroxyl radical (OH ·) have been identified as major sources of DNA damage in living organisms. They occur as ubiquitous metabolic by-products and, in humans, cause several thousand damages in a cell's DNA per day. They are thought to be a major source of DNA damage leading to aging and cancer in multicellular organisms. This raises two questions. First, what pathways are used in repair of DNA damages caused by H₂O₂ and OH·? Second, a new theory has been proposed that sexual reproduction (sex) evolved to promote repair of DNA in the germ line of organisms. If this theory is correct, then the type of repair specifically available during the sexual process should be able to deal with important natural lesions such as those produced by H₂O₂ and OH·. Does this occur?

We examined repair of hydrogen peroxide damage to DNA, using a standard bacteriophage T4 test system in which sexual reproduction is either permitted or not permitted. Post-replication recombinational repair and denV-dependent excision repair are not dependent on sex. Both of these processes had little or no effect on lethal H₂O₂ damage. Also, an enzyme important in repair of H₂O₂-induced DNA damage in the E. coli host cells, exonuclease III, was not utilized in repair of lethal H₂O₂ damage to the phage. However, multiplicity reactivation, a recombinational form of repair depending on the sexual interaction of two or more of the bacteriophage, was found to repair lethal H₂O₂ damages efficiently.

Our results lend support to the repair hypothesis of sex. Also the homology-dependent recombinational repair utilized in the phage sexual process may be analogous to the homology-dependent recombination which is widespread in diploid eucaryotes. The recombinational repair pathway found in phage T4 may thus be a widely applicable model for repair of the ubiquitous DNA damage caused by endogenous oxidative reactions.

Two intraspecific human cell hybrids, HD2 and HD1A, produced from fusion between HeLa cells and xeroderma pigmentosum fibroblasts, express XPD-like rates of excision repair and hypersensitivity to UV-radiation. In the present paper we described unusual patterns of UV-induced mutation in both cell lines. Though HD2 very closely resembles XPD both phenotypically and genetically, in UV-dose response it is hypomutable at the loci for ouabain and diphtheria toxin resistance. At equitoxic dose, however, it shows normal mutability, HD1A, by contrast, is hypermutable as a function either of UV dose or in terms of equitoxicity for these genes. HD1A's mutator phenotype is a dominant characteristic and is not associated with grossly abnormal DNA precursor pool imbalance. The possibility remains that DNA polymerase infidelity underlies its hypermutability.

Using 26 cultured cell lines derived from 17 different animal species, we have measured both the activity of O⁶-methylguanine (O⁶-MeG) methyltransferase (MT) in cell extracts and the sensitivity of the strains to the lethal effects of the alkylating agents, N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) and 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU). The MT activity was assayed by measuring the amount of ³H radioactivity transferred from methyl-[³H]-labeled O⁶-MeG in DNA to acceptor protein molecules in the extracts. In all the 21 mammalian cell strains, lethal sensitivity to ACNU as measured by colony-forming ability correlated well with cellular MT activity, indicating that the major lethal ACNU damage is reparable by the MT. On the other hand, MNNG sensitivity did not necessarily correlate with the MT activity.

Lymphocytes from patients with ataxia telangiectasia and Down's syndrome show a greater frequency of chromosome aberrations after ionising radiation than do lymphocytes from normal people. The connection between Down's syndrome and Alzheimer's disease (AD) is well-known. In view of this and of a study showing a greater sensitivity of AD than of normal lymphoblastoid cells to X-irradiation (measured by viability), we have examined repair of AD lymphocytes (hydroxyurea-treated) by measuring [³H]thymidine incorporation after γ-irradiation. We have found no difference in incorporation between AD and normal lymphocytes from age-matched individuals. However, incorporation decreases with age in γ-irradiated cells and to a lesser extent in unirradiated cells.

The limited DNA excision repair that occurs in the chromatin of UV-irradiated growth arrested cells isolated from a xeroderma pigmentosum (XP) complementation group C patient is clustered in localized regions. The repaired DNA was found to be more sensitive to nicking by endogenous nucleases than the bulk of the DNA. The extra-sensitivity does not change with increasing amounts of DNA damage or repair activity in the locally-repaired regions and is retained through a 24-h chase period. We suggest that these results are due to the occurrence of DNA repair limited to pre-existing, non-transient chromatin fractions that contain actively transcribed DNA. A similar extra-sensitivity of repaired DNA was not detected in cells of normal or XP complementation group A strains that exhibit either normal or limited repair located randomly throughout their genomes. The association between endogenous nuclease sensitivity and clustered repair probably defines a normal excision repair pathway that is specific for selected chromatin domains. The repair defect in XP-C strains may be one in pathways targeted for other endogenous nuclease-resistant domains.

From the Chinese hamster ovary line CHO-9 a resistant variant, Cl 3, was isolated after treatment with N-methyl-N′-nitro-N-nitrosoguanidine (MNNG). Cl 3 cells were much more resistant to the cytotoxic effects of MNNG (D₁₀ of 1.8 μg/ml MNNG as compared to 0.23 μg/ml for parental line) and other methylating N-nitroso compounds, but they had the same sensitivity to various other alkylating agents. MNNG was equally effective in sensitive parent line and resistant variant in inducing sister-chromatid exchanges (SCEs) and mutations to 6-thioguanine resistance. The increased resistance of Cl 3 was not due to reduced cellular uptake of MNNG, to a more efficient repair of methylated purine bases, or to differences in MNNG-induced inhibition of DNA synthesis.

It is concluded that the resistant variant has some unknown tolerance mechanism which alters the cytotoxic, but not the SCE- and mutation-inducing effects of methylating n-nitroso compounds.

We have constructed a truncated E. coliO⁶-methylguanine methyltransferase (MT) gene (ada gene) to express the MT activity for O⁶-methylguanine and O⁴-methylthymine but not for methylphosphotriester in human cells and transferred it into Mer⁻HeLle MR cells. The transfectant cells expressed the truncated O⁶-E. coli MT were resistant to alkylating agents as same as the transfectant cells with the intact ada gene in cell killing, sister-chromatid exchange induction and host-cell reactivation of adenovirus 5. The results strongly suggest that methylphosphotriester may not contribute to the biological effect of alkylating agents in human cells.

As a first step to the cloning of the Fanconi anemia (FA) gene, we have attempted to correct the sensitivity of FA cells to DNA crosslinking agents by the introduction of wild-type DNA. The protocol involved the introduction of both genomic and pRSV neo DNA, selection for G418-resistant colonies and the subsequent selection of mitomycin C-resistant cells from the latter. Preliminary experiments indicated that untransformed FA cells were not suitable recipients for the introduction of foreign DNA, so all experiments were performed with an SV40-transformed FA cell line. Approximately 40 000 G418-resistant colonies were obtained in 5 separate experiments at an overall frequency of about 5 × 10⁻⁴. These were then selected in mitomycin C and 15 colonies were recovered. Colonies were obtained with wild-type DNA (both human and rodent) and with FA DNA at about the same frequency of 2× 10⁻⁷. Colonies were isolated and shown to have a stable, partial (from 25 to 90% of wild-type) resistance to mitomycin C. One colony was also shown to be partially resistant to two other DNA crosslinking agents, diepoxybutane and nitrogen mustard. This clone also had an intermediate level of spontaneous and MMC-induced chromosome aberrations. pRSVneo, but not rodent, DNA could be demonstrated in the high molecular weight fraction of several colonies. Thus, it is likely that these colonies represent partial revertants rather than transfectants. These mitomycin C-resistant FA cells should be useful for the biochemical analysis of the FA mutation.