Inhibition of excision-repair of ultraviolet damage in human cells by exposure to methyl methanesulfonate

Differential repair of 1-beta-D-arabinofuranosylcytosine-detectable sites in DNA of human fibroblasts exposed to ultraviolet light and 4-nitroquinoline 1-oxide

The extent of DNA excision repair was determined in dermal fibroblast strains from clinically normal and xeroderma pigmentosum (XP; complementation group A) human donors after single or combined exposures to 254-nm ultraviolet light and 4-nitroquinoline 1-oxide (4NQO). The repair was monitored by incubation of the treated cultures in the presence of 1-beta-D-arabinofuranosylcytosine (araC), a potent inhibitor of long-patch excision repair, followed by quantitation of araC-accumulated DNA single-strand breaks (representing repair events) by velocity sedimentation analysis in alkaline sucrose gradients. The amount of repair in normal fibroblast strains increased as a function of UV fluence and reached a plateau at 15 J/m2; strand breaks were not detected when these same cultures were irradiated with as much as 60 J/m2 UV and incubated in the absence of araC, implying that an initial (incision) step is rate-limiting in the repair of UV damage. In normal fibroblasts (i) the incidence of araC-detectable lesions removed during fixed intervals following exposure to 4NQO (4 microM; 30 min) was approximately 2.5 times greater than that seen following irradiation with repair-saturating fluences (greater than or equal to 15 J/m2) of UV-rays; and (ii) the amount of repair in cultures treated simultaneously with 4NQO (0.5-6 microM; 30 min) and a repair-saturating fluence of UV (20 J/m2) was found to approach the sum of that arising from exposure to each separately. The XP cells (XP12BE) exhibited a deficiency in the removal of araC-detectable DNA lesions following exposure to either of the carcinogens. Since araC is known to inhibit the repair of alkali-stable 4NQO-DNA adducts (i.e., lesions assumed to be removed by the UV-like excision pathway) but not that of alkali-labile sites (i.e., DNA lesions operated on by the X-ray-like repair pathway), our results strongly imply that the multistep excision-repair pathway operative on UV photoproducts in human fibroblasts differs from that responsible for removing alkali-stable (araC-detectable) 4NQO adducts by at least one step, presumably the rate-limiting incision reaction mediated by a lesion-recognizing endonuclease.

Analysis of interactions between mutagens, II. Ethyl methanesulfonate and ultraviolet light in Saccharomyces cerevisiae

The results of this study indicate the existence of a strong interaction between ethyl methanesulfonate (EMS) and ultraviolet light (UV) for cell killing in the yeast Saccharomyces cerevisiae. Conversely, mutation and gene conversion frequencies observed for the combined treatment of EMS and UV do not deviate significantly from that expected on the basis of simple additivity. Studies involving repair-deficient mutants (rad mutants) reveal that the synergistic interaction for cell killing depends on RAD52 function (recombinational repair), but not on RAD3 function (excision repair). On the basis of this analysis, the interaction between EMS and UV in S. cerevisiae might arise from the inhibition of double-strand break repair by one, or both agents.

Inhibition of DNA polymerase activity by methyl methanesulfonate

Methyl methanesulfonate (MMS) inhibits both thymidine incorporation into DNA in mitogen-activated human lymphocytes and deoxythymidine triphosphate incorporation into template DNA by DNA polymerase-alpha in a cell-free system. When MMS-modified DNA was used as the template for DNA synthesis utilizing unmodified DNA polymerase-alpha, nucleotide incorporation into template DNA was not inhibited. When unmodified DNA was used as the template for DNA synthesis utilizing MMS-modified DNA polymerase-alpha, nucleotide incorporation was differentially inhibited dependent on the MMS concentration. An analysis of the kinetics of DNA polymerase-alpha inhibition showed that incorporation of all 4 deoxynucleoside triphosphates into DNA template was noncompetitively inhibited by MMS, which is consistent with nonspecific MMS modification of the enzyme. These data indicate that MMS modification of DNA polymerase-alpha alone is sufficient to inhibit the incorporation of deoxynucleoside triphosphates into template DNA in vitro. The data further indicate that alkylation of both DNA polymerase-alpha and DNA template synergistically increases inhibition of DNA synthesis.

Interaction of UV and N-methyl-N'-nitro-N-nitrosoguanidine: cytotoxicity and mutagenicity in V79 cells

Killing and mutation by UV in the MNNG-exposed population of V79 cells, as well as by MNNG in the UV-irradiated population of these cells have been studied. It was observed that pretreatment with MNNG increased the killing and mutation by UV, whereas, pretreatment with UV had no effect upon killing and mutation by MNNG. The increase in sensitivity to UV due to pretreatment with MNNG was lost if UV exposure was delayed for 24 h after MNNG treatment.

Single-strand breaks in DNA of hamster and human cells exposed to methyl methanesulfonate and ultraviolet light

The number of single-strand breaks produced in DNA after exposure to UV light or to methyl methanesulfonate (MMS) was additive when cells were exposed to both agents in close succession. Repair of the damage from either agent was partially inhibited by cytosine arabinoside, resulting in higher break frequencies under all conditions of exposure. Exposure to both agents followed by growth in cytosine arabinoside resulted in break frequencies that were approximately the same as the sum of those from each agent individually. These findings contrast with previous results in which pyrimidine dimer excision and repair replication after exposure to UV light were inhibited by MMS. These observations are not due to cell permeability changes after alkylation, but can be explained if the complex of excision-repair proteins is only partially inactivated by alkylation. Initial incisions to start repair would still occur but only limited amounts of repair replication would ensue without actual removal of the pyrimidine dimers.

Sensitivity of carcinogen-treated DNA to inactivation by ultraviolet radiation

The DNA of bacteriophage SPO2c12 was treated with methylmethane sulfonate (MMS), beta-propiolactone (BPL), 2-anthramine (AA) or benzo[a]pyrene (BP) and then exposed to 254-nm radiation. Competent Bacillus subtilis host cells were transfected with DNA subjected to the carcinogen-UV treatment or with DNA treated with carcinogen only. Survival curves were obtained for loss of plaque-forming ability as a function of UV dose. The UV sensitivity of DNA treated with MMS, BPL or AA was not significantly different from that of untreated DNA. The results indicate that in competent B. subtilis the pathways for repair of alkylating agent damage and for repair of UV damage are probably different.

Inhibition of DNA synthesis by an electrophilic metabolite of benzo[a]pyrene

Mitogen-stimulated scheduled DNA synthesis and DNA excision repair in human lymphocytes, as well as DNA polymerase a activity in a cell-free system, were inhibited by an electrophilic metabolite of benzo[a]pyrene. This metabolite, (+/-)-anti-(7r,8t)-dihydroxy-(9,10t)-epoxy-7,8,9,10-tetrahyd robenzo[a]pyrene (BPDE), covalently binds to cellular macromolecules and is mutagenic, carcinogenic, and cytotoxic. Human lymphocytes treated with BPDE at concentrations greater than 500-800 ng/ml showed decreases in both mitogen-stimulated DNA synthesis and excision repair of damaged DNA but did not exhibit overt cytotoxicity (excluded trypan blue and maintained an adenylate charge of greater than 0.7). Formation of, and total concentration of, BPDE-DNA adducts was not correlated with inhibition of DNA synthesis. DNA polymerase alpha studies using a cell-free system showed that enzymatic activity was not diminished when purified polymerase was treated with BPDE prior to the addition of template DNA. When the template DNA concentration was varied, BPDE inhibition of enzyme activity was uncompetitive. BPDE inhibition of enzyme activity was found to be noncompetitive when concentrations of dATP, dCTP, or dTTP were varied and competitive when the concentration of dGTP was varied. The data indicate that BPDE competitively inhibits interaction of dGTP with the template-DNA polymerase alpha complex.

Inhibition of unscheduled DNA synthesis in human lymphocytes by chemical carcinogens

Freshly isolated human peripheral lymphocytes were treated with an alkylating agent immediately after collection and subsequently treated with UV radiation. This system was used because it represents a method for assaying damage in cells immediately after their removal from the host. The amount of UV-induced repair was measured as unscheduled DNA synthesis (UDS) by incorporation of [3H]deoxythymidine into the cellular DNA. The alkylating agents beta-propiolactone (BPL) and methyl methane-sulfonate (MMS) inhibited UDS at concentrations of 0.08 mM and 0.6 mM, respectively. Lower concentrations had no effect. Lymphocytes allowed to remain in culture medium after treatment with the alkylating agents did not recover the ability to perform UV-induced UDS even when cells were irradiated 48 h after carcinogen treatment. The decrease in UV-induced UDS resulting from alkylating agent treatment could not be attributed to cell death.

Replicon size and excision repair as factors in the inhibition and recovery of DNA synthesis from ultraviolet damage

Initiation of DNA replication and chain growth, analyzed by alkaline sucrose gradient sedimentation, was interrupted to different extents in different cell types by irradiation with ultraviolet light. Within the first hour of irradiation DNA replication was reduced in a manner that depended on the average number of lesions per replicating unit (replicon). At low numbers of lesions per replicon, inhibition of replicon initiation was the predominant response; at higher numbers of lesions per replicon, blockage of chain growth was also observed. After irradiation with a dose that initially blocked chain growth, the rate at which cells recovered their ability to synthesize increasingly more and larger size DNA was a function both of replicon size and of excision repair capacity. Cells with small replicons recovered more rapidly than cells with large replicons, and excision repair-deficient cells recovered less rapidly than excision-competent cells. These observations indicate that excision repair capacity and replicon size play major roles in the response of DNA replication to ultraviolet damage.

Inhibitors of poly(ADP-ribose) polymerase enhance DNA strand breaks, excision repair, and sister chromatid exchanges induced by alkylating agents

Benzamide and 3-aminobenzamide, inhibitors of poly(ADP-ribose) polymerase, synergistically enhanced the frequencies of unscheduled DNA synthesis and sister chromatid exchanges in Chinese hamster ovary (CHO) cells treated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). These inhibitors also increased methyl methanesulfonate (MMS)- or MNNG-induced DNA strand breaks and significantly retarded the rejoining of strand breaks in CHO and HeLa S3 cells. These results suggest that poly(ADP-ribose) is important in the repair of DNA damage after exposure to alkylating agents and exercises a regulatory role in stabilizing chromatin structure whenever strand breaks occur in DNA.

Perturbations of enzymic uracil excision due to purine damage in DNA

Phage PBS-2 DNA, which contains uracil in place of thymine, was selectively damaged and then used as substrate for purified Bacillus subtilis uracil-DNA glycosylase. This enzyme releases uracil from DNA in a limited processive manner. Irradiation by ultraviolet light (greater than 305 nm) in the presence of isopropanol and a free radical photoinitiator introduced covalently bound 8-(2-hydroxy-2-propyl)purines into DNA. Methylation by dimethylsulfate yielded 7-methylguanine. Apurinic sites were produced by gentle heating of methylated DNA. Rates of enzymic release of uracil from DNA varied among these three substrates. The Vmax was markedly decreased for DNA containing 8-(2-hydroxy-2-propyl)purines and apurinic sites but was unaffected by the presence of larger quantities of 7-methylguanine. This suggests that certain types of damaged DNA moieties may decrease the capacity for uracil excision. Therefore, interference with enzymic excision of this potentially mutagenic base may constitute a common mechanism of action of the reaction products of several unrelated DNA damaging agents.

Xeroderma pigmentosum patients from Egypt: II. Preliminary correlations of epidemiology, clinical symptoms and molecular biology

Xeroderma pigmentosum (XP) occurs with high frequency in Egypt and a continuation of our field studies has identified representatives of the 3 major complementation groups A, C, and variant. Group A patients, with one exception, showed very early onset of sun sensitivity and development of skin cancers, and microcephaly and mental retardation. The exceptional group A patient was 35 yr old, with normal stature and intelligence who had 2 normal children. DNA repair was as low in his cells as in other group A cases. Group C patients showed a slightly slower onset of sun sensitivity and had no central nervous system disorders. The variants showed later onset of sun sensitivity and no skin cancers evident at the time of observation (about 20 yr of age). No sun sensitivity was present in the 25 heterozygotes we observed, nor reportedly in the additional 60 not yet observed. This indicates that only homozygosity for XP genes increases risk of skin cancer. Cell cultures from both normal persons and these XP patients reached in vitro "senescence" at similar passage levels. Groups A and C appear to have lost different major gene products that are involved in the excision of UV damage from DNA, but the residual repair in XP-C cells facilitates more recovery of DNA synthesis than in other groups. This may contribute to the higher in vitro survival in culture and milder clinical symptoms in group C as compared to group A. XP variants appear to have lost a gene product that permits normal cells to replicate, uninterrupted by DNA damage, and consequently synthesize DNA in smaller pieces than normal.