The excision of N-methyl-N-nitrosourea-induced lesions from the DNA of Chinese hamster cells as measured by the loss of sites sensitive to an enzyme extract that excises 3-methylpurines but not O6-methylguanine

Detection of base damage in DNA in human blood exposed to ionizing radiation at biologically relevant doses

The alkaline elution technique for the detection of DNA damage has been adapted to allow application on unlabelled blood cells. Both the induction and subsequent repair have been studied of two classes of DNA damage, viz, single-strand breaks and base damage recognized by the gamma-endonuclease activity in a cell-free extract of Micrococcus luteus bacteria. The high sensitivity of the assay permitted the measurement of induction and repair of base damage after in vitro exposure of full blood under aerobic conditions to biologically relevant doses of gamma-rays (1.5-4.5 Gy). After a radiation dose of 3 Gy about 50% of the base damage was removed within 1.5 h of repair. Base damage could still be detected at 24 h after exposure to 15 Gy.

Repair of N-methylpurines in specific DNA sequences in Chinese hamster ovary cells: absence of strand specificity in the dihydrofolate reductase gene

We have developed a quantitative method for examining the removal of N-methylpurines from specific genes to investigate their possible differential repair throughout the genome. Chinese hamster ovary cells were exposed to dimethyl sulfate, and the isolated DNA was treated with an appropriate restriction endonuclease. The DNA was heated to convert remaining N-methylpurines to apurinic sites to render them alkaline-labile. Duplicate samples heated in the presence of methoxyamine to protect the apurinic sites from alkaline hydrolysis provided controls to assess total DNA. After alkaline hydrolysis, agarose gel electrophoresis, Southern transfer, and probing for the fragment of interest, the ratios of band intensities of the test DNA sample to its methoxyamine-treated control counterpart were calculated to yield the percentage of fragments containing no alkaline-labile sites. The frequency of N-methylpurines was measured at different times after dimethyl sulfate treatment to study repair. We found no differences between the rates of repair of N-methylpurines in the active dihydrofolate reductase gene and a nontranscribed region located downstream from it in treated cells. Also, similar rates of repair were observed in the transcribed and nontranscribed strands of the gene, in contrast to previous results for the removal of cyclobutane pyrimidine dimers. Thus, there does not appear to be a coupling of N-methylpurine repair to transcription in Chinese hamster ovary cells. However, the repair in the dihydrofolate reductase domain appears to be somewhat more efficient than that in the genome overall. Our method permits the quantifying at the defined gene level of abasic sites or of any DNA adduct that can be converted to them.

O6-methylguanine repair in liver cells in vivo: comparison between G1- and S-phase of the cell cycle

To compare the formation and persistence of alkylated DNA bases in the G1- and S-phase compartments in liver in vivo, regenerating rat liver was exposed to [14C]dimethylnitrosamine (0.57 mg/kg, IP injection) or N-[methyl14C]-N-nitrosourea (3.3 mg/kg, intraportal injection) during the G1 phase of the cell cycle (12 h after partial hepatectomy), or at 24 h after partial hepatectomy with 30% hepatocytes in DNA synthesis, or at 43 h after partial hepatectomy, 4 h after an hydroxyurea block from 14 to 39 h after operation with 80% hepatocytes in DNA synthesis. At 120 min after dimethylnitrosamine and 90 s, 5, 10, or 60 min after the intraportal pulse of N-methyl-N-nitrosourea the molar fractions of 7-methylguanine (7megua), O6-methylguanine (O6megua), and 3-methyladenine (3mead) and of metabolically labeled guanine were determined from DNA hydrolysates by Sephadex-G10 radiochromatography. After dimethylnitrosamine only minor differences were observed for 7megua formation in the three groups; the 3mead/7megua ratio remained constant irrespective of the number of cells in S phase. In contrast, the O6megua/7megua ratio revealed a loss of O6megua, the extent of which appeared proportional to the fraction of DNA-synthesizing cells in the liver. The rapid loss of O6megua in S-phase cells was confirmed after intraportal administration of N-methyl-N-nitrosourea. During the first 10 min after the methylnitrosourea pulse the O6megua/7megua ratio was constant in G1 cells and dropped from 90 s to 10 min by about 15% in liver containing 30% S-phase cells and by about 40% with 80% cells in DNA synthesis. DNA-synthesizing hepatocytes are apparently endowed with a higher O6megua DNA transferase activity than nonproliferating liver cells. The rapid, though exhaustible elimination of O6megua during S-phase might result in partial protection of DNA-synthesizing cells from base-mispairing and/or from hypomethylation at G-C sites.

DNA-purine methylation in hepatic chromatin following exposure to dimethylnitrosamine or methylnitrosourea

The investigations reported in this paper were designed to analyze the patterns of DNA-purine methylation in hepatic chromatin following in vivo exposure to the carcinogenic alkylating agents dimethylnitrosamine (DMN) or methylnitrosourea (MNU). Male Sprague-Dawley rats were exposed to [14C]DMN (8 mumoles, 1.0 microCi per mumole per 100 g) or [3H]MNU (15 mumoles, 10 microCi per mumole per 100 g) via gastric intubation. Hepatic chromatin was fractionated into portions having characteristics of template-active euchromatin (S2) and template-repressed heterochromatin (P2) by digestion with DNase II followed by MgCl2 precipitation. Specific DNA purines were identified at 24 hr post-intubation using an isocratic high pressure liquid chromatographic system. A qualitatively similar pattern of 7-methylguanine, O6-methylguanine, 1-methyladenine and 3-methyladenine alkylation was observed in DNA from total chromatin versus heterochromatin at 24 hr following exposure to either carcinogen. These assessments were made at times following carcinogen exposure which produced maximal quantitative differences in alkylation of euchromatin versus heterochromatin DNA. Similar patterns of DNA purine alkylation were observed in total chromatin and heterochromatin. These observations suggest that, once the reactive species is generated and access to chromatin DNA occurs, a similar pattern of DNA-purine alkylation is produced in different regions of the genome.

Effect of bacterial host repair systems on the viability of hydroxylamine and methyl methanesulfonate treated T4 and lambda bacteriophages

Survival of HA1 or MMS-treated T4 and lambda phages was estimated in bacterial cells differing in their ability to repair DNA. It has been found that the mismatch repair system of the bacterial host, which involves mutS mutR MutL uvrE and dam loci, does not excise, or does so to only a limited extent, the nonpaired bases from DNA of HA or MMS-treated phages. Mutation in polA, both in the polymerase as well as in the 5' leads to 3' exonuclease activity, have a small effect on survival of HA-treated phages, whereas mutation in the polymerase activity has a pronounced effect on survival of MMS-treated phages. There was a difference in the effect of polA mutations on survival of MMS-treated T4 and lambda phages; the survival of the former was less affected than the latter. Induction of SOS response has no effect on repair of HA and MMS-treated phages. Pretreatment of bacterial host (including the ada- mutant) with low doses of alkylating agents increases the survival of MMS (but not HA)-treated phages; pretreatment of bacteria with HA has no effect on survival of HA-treated phages. Three lines of evidence: the different inactivation rates of MMS-treated T4 and lambda phages, variation in the effect of polA mutations on survival of T4 and lambda phages, and a different level of adaptive response in ada- cells towards of MMS-treated T4 and lambda phages, suggest that the patterns of DNA methylation in T4 and lambda phages are different.

DNA repair assays as tests for environmental mutagens. A report of the U.S. EPA Gene-Tox Program

A literature review was undertaken to determine the usefulness of DNA repair assays, other than unscheduled DNA synthesis, as screening techniques for mutagenic carcinogens. 92 reports were found to contain useful data for 49 chemicals using 6 techniques, namely, (1) cesium chloride equilibrium density gradients to study repair replication, (2) benzoylated naphthoylated diethylaminoethyl cellulose columns to study repair replication, (3) 313-nm irradiation of DNA containing bromodeoxyuridine to study repair replication, (4) alkaline elution to study repair of single-strand breaks and crosslinks, (5) alkaline sucrose gradients to study repair of single-strand breaks, and (6) direct assays for removal of adducts from DNA. Almost all of the 49 chemicals studied were known mutagens or carcinogens and/or known inducers of DNA repair, 9 compounds failed to elicit DNA repair by at least 1 assay technique, and at least 3 of these were not tested by the most appropriate and sensitive method. Nevertheless, although valid for studying repair phenomena in eukaryotic cells, these assays are not considered useful for screening. They are time-consuming, expensive, and/or require highly specialized skills. Despite the high frequency of positive reports, it is obvious from the literature that repair assays will fail to detect, or will detect with low efficiency, those agents whose main action is either intercalation or induction of strand breaks. For these and other reasons, DNA repair as a basis for screening for mutagenic carcinogens is not considered to be a useful concept.

Repair of 3-methyladenine and 7-methylguanine in nuclear DNA of Chlamydomonas: requirement for protein synthesis

The removal of 3-methyladenine and 7-methylguanine from nuclear DNA was determined following exposure of Chlamydomonas reinhardi to methyl methanesulfonate (MMS). The amount of 3-methyladenine in DNA was determined using an extract from Micrococcus luteus that has a 3-methyladenine-DNA glycosylase. The amount of 7-methylguanine was estimated by heating the DNA for 30 min at 70 degrees followed by alkaline hydrolysis of the resulting apurinic sites. The molecular weight of the DNA was determined using alkaline sucrose gradients. The 3-methyladenine is removed with a half-life of 2--3 h whereas the 7-methylguanine is removed with a half-life of 10--12 h. The rate of removal of the 7-methylguanine is more than an order of magnitude faster than the estimated non-enzymatic hydrolysis rate indicating the probability of enzymatic repair. Addition of cycloheximide immediately after MMS treatment inhibits the removal of 3-methyladenine and 7-methylguanine from DNA. If cycloheximide is added 1.5 h after treatment with MMS, there is much less inhibition of the removal of 3-methyladenine. These results are interpreted to mean that MMS induces the synthesis of 1 or more proteins that are required for the repair of 3-methyladenine from Chlamydomonas DNA.

Quantitative aspects of the formation and loss of DNA interstrand crosslinks in Chinese hamster cells following treatment with cis-diamminedichloroplatinum(II) (cisplatin). I. Proportion of DNA-platinum reactions involved in DNA crosslinking

The presence of interstrand crosslinks in the DNA of cis-diamminedichloroplatinum(II) (cisplatin)-treated Chinese hamster cells was demonstrated by the formation of a species of hybrid-labelled DNA in cells that had been density and radioactively prelabelled with a combination of [3H]thymidine and 2'-deoxy-5-bromouridine (BrdUrd) prior to treatment. The extent of crosslinking and of total binding of platinum to the DNA increased linearly with dose over a wide range of concentrations of cisplatin. Simultaneous measurement of the size of co-isolated 14C-labelled DNA from untreated cells permitted an estimate of the size of the crosslinked DNA and hence of the proportion of bound platinum involved in a crosslinking reaction. No increase in the proportion of crosslinking reactions occurred during several hours post treatment incubation of cells when crosslinks were measured by this technique, in contrast to other studies using different techniques to measure DNA interested crosslinking in cis-platin treated cells (see following paper, Pera, M.F., Jr., Rawlings, C.J., Shackleton, J. and Roberts, J.J (1981) Biochim. Biophys, Acta 655, 152--166). The enforced high concentrations of cisplatin required by this method to detect crosslinks precludes studies of their possible repair.

Quantitative aspects of the formation and loss of DNA interstrand crosslinks in Chinese hamster cells following treatment with cis-diamminedichloroplatinum(II) (cisplatin). II. Comparison of results from alkaline elution, DNA renaturation and DNA sedimentation studies

The formation of interstrand crosslinks in the DNA of cis-diamminedichloroplatinum(II) (cisplatin)-treated Chinese hamster cells has been demonstrated by three independent, highly sensitive techniques: namely those of alkaline elution, renaturation of crosslinked DNA and alkaline sucrose gradient velocity sedimentation. Simultaneous measurement of DNA break frequency by the first two methods and a computer model combined with the third enabled the calculation of crosslink frequency after application of low doses of cisplatin. Good agreement was found between the values obtained by the three methods used here, and also with those obtained by direct measurement of the amount of crosslinked hybrid DNA in density-labelled cells treated with higher doses of cis-platin (Roberts, J.J. and Friedlos, F. (1981) Biochim. Biophys, Acta 655, 146--151). When measured by all three methods described here, crosslinking was found to increase during several hours after treatment and then to decrease with a half-life of between 12 and 24 h. For low initial levels of crosslinking, this was largely attributed to an excision repair process, since the formation of breaks in DNA was only minimal.

Human lymphoblasts contain DNA glycosylase activity excising N-3 and N-7 methyl and ethyl purines but not O6-alkylguanines or 1-alkyladenines

Cultured human lymphoblasts (CCRF-CEM line) have DNA glycosylase activities, the specificities of which were investigated by using high-performance liquid chromatography. In addition to 3-methyladenine, 3-ethyladenine, 7-methylguanine, 7-ethylguanine, 3-methylguanine, and 3-ethylguanine also were excised from alkylated double-stranded DNA and deoxypolynucleotides, but 1-methyladenine, 1-ethyladenine, O6-methylguanine, and O6-ethylguanine were not. The glycosylase activity was generally greater for the methylated than for the corresponding ethylated purines and was also greater toward 3-alkyladenine than toward 3-alkyladenine than toward 3-alkylguanine. 7-Methylguanine and 7-ethylguanine were excised to similar but low extents. However, in molar terms, the release of 7-methylguanine was similar to that of 3-methyladenine.

DNA repair in lymphoblastoid cell lines established from human genetic disorders

Lymphoblastoid cell lines (LCLs) established from chromosomal breakage syndromes or related genetic disorders have been used to study the effects of mutagens on human lymphoid cells. The disorders studied include xeroderma pigmentosum, ataxia telangiectasia, Fanconi's anemia, Bloom's syndrome and Cockayne's syndrome. Three approaches were used to assess the cells' ability to cope with a particular mutagen: (1) assaying recovery of DNA synthetic capabilities as measured by [3H]thymidine (dT) incorporation; (2) measurements of classical excision DNA repair by isopyknic sedimentation of DNA density labeled with 5-bromo-2-deoxyuridine (BrdU); (3) determining cell survival by colony formation in microtiter plates. LCLs established from xeroderma pigmentosum showed increased sensitivities to ultraviolet (354 nm) light and N-acetoxy-2-acetylaminofluorene (AAAF) as determined by DNA synthesis or colony formation and had diminished levels of excision-repair. Cockayne's syndrome LCLs, on the other hand, had increased sensitivities to ultraviolet (UV) light, AAAF and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) while showing near normal levels of DNA-repair after treatment with each agent. An LCL established from ataxia telangiectasia had decreased DNA repair synthesis and defective colony-forming ability following treatment with MNNG. LCLs, in addition to ease of establishment, appear likely to provide useful material for the study of DNA repair replication and its relationship to carcinogenesis.

A method for the rapid quantitation of methylated hepatic DNA-purines using high pressure liquid chromatography

An isocratic high pressure liquid chromatography system that can be employed for the separation and quantitation of four of the major methylated purines commonly found in DNA after exposure to carcinogenic/mutagenic alkylating agents (1-methyladenine, 3-methyladenine, O6-methylguanine and 7-methylguanine) is presented in this paper. Data from experiments in which rats were treated with methyl-N-nitrosourea have been used to illustrate the applicability of the method. The amounts of the methylated bases observed were consistent with values reported in the literature.