The pattern of methylated purines formed in DNA of intact and regenerating liver of rats treated with the carcinogen dimethylnitrosamine

Investigating the biochemical impact of DNA damage with structure-based probes: abasic sites, photodimers, alkylation adducts, and oxidative lesions

DNA sustains a wide variety of damage, such as the formation of abasic sites, pyrimidine dimers, alkylation adducts, or oxidative lesions, upon exposure to UV radiation, alkylating agents, or oxidative conditions. Since these forms of damage may be acutely toxic or mutagenic and potentially carcinogenic, it is of interest to gain insight into how their structures impact biochemical processing of DNA, such as synthesis, transcription, and repair. Lesion-specific molecular probes have been used to study polymerase-mediated translesion DNA synthesis of abasic sites and TT dimers, while other probes have been developed for specifically investigating the alkylation adduct O(6)-Bn-G and the oxidative lesion 8-oxo-G. In this review, recent examples of lesion-specific molecular probes are surveyed; their specificities of incorporation opposite target lesions compared to unmodified nucleotides are discussed, and limitations of their applications under physiologically relevant conditions are assessed.

Methylation of DNA guanine via the 1-carbon pool in dimethylnitrosamine-treated rats

Treatment of rats with radioactive methionine and nonradioactive dimethylnitrosamine resulted in the formation of radioactive 7-methylguanine in rat-liver DNA. By comparing the specific activity of administered [14C-Me]-dimethylnitrosamine to the specific activity of isolated 7-methylguanine it was determined that following 20 mg/kg dimethylnitrosamine DNA methylation via the 1-carbon pool may account for up to 30% of the total 7-methylguanine formed.

Alkylation of protein by methyl methanesulfonate and 1-methyl-1-nitrosourea in vitro

Methylation of horse heart cytochrome c has been examined in vitro with [methyl-14C]methanesulfonate (MMS) and [1-methyl-14C]-1-nitrosourea (MNU) as alkylating agents. Analysis of protein hydrolyzates by an automatic amino acid analyzer indicates that, at pH 9.0 with MMS, epsilon-N-monomethyl-lysine is found to be the only major methylated basic amino acid. On the other hand, the identity of the predominant basic amino acid residue which is [methyl-14C]-labeled by MNU cannot be determined at present. Peptide mapping of chymotryptic digests of cytochrome c after reaction with MMS reveals a lack of specificity in methylation of a specific lysine residue in this hemoprotein.

Effect of storage and dose on MMS-induced deletions. Complementation analysis of X-chromosomal recessive lethals in the zeste-white and maroon-like regions of Drosophila melanogaster

The effect of storage on MMS-induced recessive lethals in the zeste-white (3A1-3C2) and the maroon-like (18F4-20F) regions was studied by complementation analysis. (1) Without any exception, all 52 mutants (from unstored spermatozoa) mapped in the zeste-white region were restricted to single complementation units. Furthermore, none of an additional 15 lethals, sampled from sperm that had been stored in females for 9-12 days, was associated with a deletion. (2) Of 34 mutations induced by 8.5 X 10(-2) mM MMS in the maroon-like (mal) region, 4 spanned 2 or more complementation units, and thus are considered to be deletions. A high dose of 2.5 mM MMS provided 55 lethals for analysis of which 4 were deletions. There was no evidence for any difference in the frequency of deletions as the MMS concentration was enhanced from 8.5 X 10(-2) mM to 2.5 mM. However, with storage, 47.1% lethals (16 of 34 mutants induced by 2.5 mM MMS) mapped in the mal region were found to involve large structural changes. (3) A high proportion of double mutants in both the zeste-white (z w) and the maroon-like regions was found among the chromosomes analyzed. These double mutants have one lethal positioned within the region studied and the other outside it. Clearly, the proportion of double mutants increased with dose, from 6.3 to 41.7% in z w and from 14.7 to 61.8% in the mal section. Apurinic sites in DNA reacted with MMS are considered as the likely primary lesions responsible for the storage effect on MMS-induced recessive lethals in the mal region. Thus, the ability of MMS to produce delayed deletion lethals seems to correlate with preference for alkylation of base nitrogens. An interesting aspect for further analysis is the apparent infrequency in the zeste-white region of alkylation-induced chromosomal breakage, as observed by various investigators for MMS, EMS and MNNG.

A differential DNA-repair test using mixtures of E. coli K12 strains in liquid suspension and animal-mediated assays

The feasibility of performing tests for repairable DNA damage in animal assay procedures was investigated by using repair-proficient and repair-deficient derivatives of E. coli K12 strain 343/113, including mutations in the uvrB, recA, polA and dam genes. To avoid variations in the relative recovery of viable cells from different samples, the strains were further marked with auxotrophic growth requirements, so that mixtures could be treated and the survival of each strain determined individually on media containing the corresponding growth factors. Spot tests were performed with the various strains to re-assess the necessity of using a combination of repair deficiencies, when genotoxic agents of differing mode of action are to be detected. Liquid suspension tests on mixtures of the different strains, furthermore, confirmed that the survival of the individual strains can be determined separately on selective media after treatment with methyl methanesulfonate (MMS) and methyl nitrosourea (MNU). These tests were also used to demonstrate that dimethyl nitrosamine (DMNA) is activated by Aroclor-1254-induced rat-liver S9 fractions to genotoxic products, as measured by the low survival of a recA derivative compared with the repair-proficient wild-type strain. Intrasanguineous host-mediated assays using the present derivatives of E. coli K12/343/113 showed that the various strains, injected simultaneously into mice, could be recovered in amounts sufficient for the individual determination of the relative survival in liver, spleen, lungs, kidneys, pancreas and the blood stream of the host animals. Using a mixture of the repair-proficient parent and the recA derivative inoculated into mice that were subsequently treated with MMS, NMU or DMNA, we found that these chemicals induce a larger decrease in survival in the recA strain as compared with the wild-type in cells recovered from the liver and the spleen. The order of genotoxic potency so determined was DMNA greater than MNU greater than MMS; this is similar to the ranking of the carcinogenicity of these compounds in rodents and probably also reflects the various degrees of DNA alkylation in cells of the livers of the treated animals. The general usefulness of the host-mediated differential DNA repair assay for detecting genotoxic factors in various organs of animals remains to be assessed by using chemical mutagens of different modes of action.

Promutagen activation in partially hepatectomized mice

Mutagenic activity of aflatoxin B1 (AFB1) was followed in the Ames Salmonella/microsome test using liver S-9 from control and partially hepatectomized (PH) mice. The S-9 preparations from the PH mice were capable of inducing significantly greater AFB1 mutagenic activity to strain TA100 than S-9 preparations from either unoperated control, sham-operated control, or Aroclor 1254-induced mice. Increased activity was observed with S-9 preparations made at both 42 h and 148 h following PH.

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 estimation of the extent of alkylation of DNA following treatment of mammalian cells with non-radioactive alkylating agents

Alkaline sucrose sedimentation has been used to quantitate phosphotriester formation following treatment of human cells with the monofunctional alkylating agents methyl and ethyl methanesulfonate. These persistent alkaline-labile lesions are not repaired during short-term culture conditions and thus serve as a useful and precise index of the total alkylation of the DNA. Estimates of alkylation by this procedure compare favorably with direct estimates by use of labeled alkylating agents.

In vivo replication of hepatic deoxyribonucleic acid of rats treated with dimethylnitrosamine: presence of dimethylnitrosamine-induced O6-methylguanine, N7-methylguanine, and N3-methyladenine in the replicated hybrid deoxyribonucleic acid

Experiments were designed to determine whether some chemical lesions such as O6-methylguanine, N7-methylguanine, and N3-methyladenine induced in rat liver DNA by the hepatocarcinogen dimethylnitrosamine permit replication in vivo. For this purpose, [14C]dimethylnitrosamine was administered to methylate the parental strand of liver DNA. Four hours later, a time period when the carcinogen cannot be detected in either the liver or the blood, rats were subjected to partial hepatectomy in order to induce DNA replication. During the S phase, 5-bromo-2-deoxyuridine was administered to render the newly made strands heavy. The rebanded, hybrid, hepatic DNA of density 1.714 g/cm3 and greater was pooled from the neutral cesium chloride gradient, dialyzed, and lyophilized. The hybrid DNA was then treated with S1 nuclease to digest any single-stranded regions. The results obtained indicated the presence of O6-methylguanine, N7-methylguanine, and N3-methyladenine in S1 nuclease resistant, hybrid DNA. The results are interpreted to indicate that these chemical lesions permitted in vivo DNA replication.

Genetic effects of dimethyl sulfate, diethyl sulfate, and related compounds

DMS and DES are monofunctional alkylating agents that have been shown to induce mutations, chromosomal aberrations, and other genetic alterations in a diversity of organisms. They have also been shown to be carcinogenic in animals. As an alkylating agent, DMS is a typical SN2 agent, attacking predominantly nitrogen sites in nucleic acids. DES is capable of SN1 alkylations as well as SN2 and thereby causes some alkylation on oxygen sites including the O6-position of guanine which is thought to be significant in mutagenesis by direct mispairing. The mutagenicity of DMS is better explained in terms of indirect, repair-dependent processes. With respect to both alkylating activity and genetic effects, striking similarities are found between DMS and MMS and between DES and EMS. In most systems where they have been tested, both DMS and DES are mutagenic. Results of many of the mutagenesis studies involving these compounds and other alkylating sulfuric acid esters are summarized in Tables 6, 7, 8, 9 and 10 of this review. Most data are consistent with these agents acting primarily as base-pair substitution mutagens. In the case of DES, strong specificity for G.C to A.T transitions has been reported in some systems but has not been clearly supported in some others. Low levels of frameshift mutations of the deletion type are also likely. In addition to the induction of mutations, recombinogenic and clastogenic effects have been described.

In vivo covalent binding of organic chemicals to DNA as a quantitative indicator in the process of chemical carcinogenesis

The covalent binding of chemical carcinogens to DNA of mammalian organs is expressed per unit dose, and a 'Covalent-Binding Index', CBI, is defined. CBI for various carcinogens span over 6 orders of magnitude. A similar range is observed for the carcinogenic potency in long-term bioassays on carcinogenicity. For the assessment of a risk from exposure to a carcinogen, the total DNA dmaage can be estimated if the actual dose is also accounted for. A detailed description is given for planning and performing a DNA-binding assay. A complete literature survey on DNA binding in vivo (83 compounds) is given with a calculation of CBI, where possible, 153 compounds are listed where a covalent binding to any biological macromolecule has been shown in vivo or in vitro. Recent, so far unpublished findings with aflatoxin M1, macromolecule-bound aflatoxin B1, diethylstilbestrol, and 1,2-epithiobutyronitrile are included. A comparison of CBI for rat-liver DNA with hepatocarcinogenic potency reveals a surprisingly good quantitative correlation. Refinements for a DNA-binding assay are proposed. Possibilites and limitations in the use of DNA binding in chemical carcinogenesis are discussed extensively.

Effect of a single treatment with the alkylating carcinogens dimethylnitrosamine and methyl methanesulphonate on liver regenerating after partial hepatectomy. IV. Effect on methylase-mediated methylation of DNA

The possibility that carcinogens may affect methylase-mediated methylation of replicating DNA was investigated. A system eminently suitable for this purpose is liver regenerating after partial hepatectomy, as one injection of dimethylnitrosamine (DMN) given during the ensuing period of increased DNA synthesis induces hepatocellular carcinoma. Methylation of DNA by DNA methylase normally occurs only in proportion to DNA synthesis. Therefore simultaneous measurements were made of synthesis (incorporation of [14C]adenine into DNA adenine, or of d[5-3H]cytidine into DNA cytosine), and of methylation (incorporation of [methyl-3H]methionine into 5-methylcytosine of DNA) in liver regenerating after partial hepatectomy. After treatment with DMN, the ratio of methylation: synthesis remained within the normal range. Methyl methanesulphonate (MMS), a compound which damages DNA in regenerating liver in a similar but not identical way to DMN and which does not induce tumors in liver even when given after partial hepatectomy, caused an increase in methylation in relation to synthesis. These experiments therefore do not support the view that altered DNA methylase activity is involved in carcinogenesis.

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

An enzyme extract from Micrococcus luteus excises 3-methyladenine and 3-methylguanine but not O6-methylguanine, 7-methylguanine, 1-methyladenine or 7-methyladenine from DNA reacted with N-methyl-N-nitrosourea. The extract was used to detect lesions in the DNA of Chinese hamster cells treated in culture with N-methyl-N-nitrosourea. It was concluded that 3-methyladenine is excised from these cells with a half-life of about 2.3 h.

The action of rat cytosol enzymes on some methylated nucleic acid components produced by the carcinogenic N-nitroso compounds

The stabilities of several alkylated nucleic acid components have been examined in the presence of cytosol extracts from a variety of rat tissues. An activity capable of demethylating O6-methyldeoxyguanosine was readily detectable in all tissues examined; arranged in approximate order of decreasing specific activity these are as follows: small intestine, spleen, kidney, lung, liver, skin, heart and brain. The in vitro requirements for the activity derived from liver and the observations that O6-methylguanine and its deoxynucleoside 5'-monophosphate are insensitive to the action of these extracts suggests that this activity may be due to an enzyme which resembles adenosine deaminase. In contrast to the ready degradation of O6-methyldeoxyguanosine the corresponding ethyl derivative was degraded very much more slowly but there was no evidence for other activities against the O4- and O2-methyldeoxythymidines. Similarly, no demethylation of the N-substituted deoxynucleosides, 3-methyldeoxycytidine 3-methldeoxythymidine, 1-methyldeoxyadenosine and 7-methyldeoxyguanosine, was detected.

Synthesis and properties of O6-methyldeoxyguanylic acid and its copolymers with deoxycytidylic acid

This paper describes the synthesis of O6-methyldeoxyguanosine triphosphate (m6dGTP) and its copolymerization to high molecular weight polymer with deoxycytidylic acid. The monomer, m6dGTP, was synthesized from deoxyguanosine first protected by acetylation of the sugar hydroxyls, and then chlorinated in the 6-position with POCl3. The product, 6-chloro-3',5'-di-O-acetyl deoxyguanosine, was converted to O6-methyldeoxyguanosine with sodium methoxide and phosphorylated in the 5' position with carrot phosphotransferase. Monophosphate was converted chemically to the triphosphate and copolymerized with dCTP by terminal deoxynucleotidyl transferase. The resulting template, which contained O6-methylguanine, was tested for its ability to direct RNA synthesis by bacterial RNA polymerase. The presence of O6-methylguanine was shown to lead to the misincorporation of UMP in the product polymer, thus strengthening the hypothesis that O6-methylguanine is a promutagenic base.

Effect of hypophysectomy on persistence of methylated purines in rat liver deoxyribonucleic acid after administration of dimethylnitrosamine

The formation of methylated purines in DNA following dimethylnitrosamine administration was studied in control and hypophysectomized rats. When given the same dose of this carcinogen (in mg/kg body weight) the formation of the major product 7-methylguanine and of the minor products 1-, 3- and 7-methyladenine and 3-methylguanine was slightly greater in the livers of hypophysectomized rats than in controls. The rate of loss of these products from the DNA was not affected by hypophysectomy. O6-Methylguanine levels were significantly greater in the hepatic DNA of hypophysectomized rats compared to controls after doses of dimethylnitrosamine ranging from 1 to 20 mg/kg. This difference was due to a slower rate of loss of this purine from the DNA in the hypothysectomized rats. Growth hormone treatment increased the rate of removal of O6-methylguanine in the hypophysectomized rats but did not restore the activity to that found in controls. The possible significance of these results in the induction of tumors by dimethylnitrosamine is discussed.

Formation and subsequent removal of O6-methylguanine from deoxyribonucleic acid in rat liver and kidney after small doses of dimethylnitrosamine

1. The amounts of 7-methylguanine and O(6)-methylguanine present in the DNA of liver and kidney of rats 4h and 24h after administration of low doses of dimethylnitrosamine were measured. 2. O(6)-Methylguanine was rapidly removed from liver DNA so that less than 15% of the expected amount (on the basis of 7-methylguanine found) was present within 4h after doses of 0.25mg/kg body wt. or less. Within 24h of administration of dimethylnitrosamine at doses of 1mg/kg or below, more than 85% of the expected amount of O(6)-methylguanine was removed. Removal was most efficient (defined in terms of the percentage of the O(6)-methylguanine formed that was subsequently lost within 24h) after doses of 0.25-0.5mg/kg body wt. At doses greater or less than this the removal was less efficient, even though the absolute amount of O(6)-methylguanine lost during 24h increased with the dose of dimethylnitrosamine over the entire range of doses from 0.001 to 20mg/kg body wt. 3. Alkylation of kidney DNA after intraperitoneal injections of 1-50mug of dimethylnitrosamine/kg body wt. occurred at about one-tenth the extent of alkylation of liver DNA. Removal of O(6)-methylguanine from the DNA also took place in the kidney, but was slower than in the liver. 4. After oral administration of these doses of dimethylnitrosamine, the alkylation of kidney DNA was much less than after intraperitoneal administration and represented only 1-2% of that found in the liver. 5. Alkylation of liver and kidney DNA was readily detectable when measured 24h after the final injection in rats that received daily injections of 1mug of [(3)H]dimethylnitrosamine/kg for 2 or 3 weeks. After 3 weeks, O(6)-methylguanine contents in the liver DNA were about 1% of the 7-methylguanine contents. The amount of 7-methylguanine in the liver DNA was 10 times that in the kidney DNA, but liver O(6)-methylguanine contents were only twice those in the kidney. 6. Extracts able to catalyse the removal of O(6)-methylguanine from alkylated DNA in vitro were isolated from liver and kidney. These extracts did not lead to the loss of 7-methylguanine from DNA. 7. The possible relevance of the formation and removal of O(6)-methylguanine in DNA to the risk of tumour induction by exposure to low concentrations of dimethylnitrosamine is discussed.

Excision of 7-bromomethylbenz[a]anthracene--DNA adducts in replicating mammalian cells

The excision of 7-bromomethylbenz[a]anthracene--DNA adducts was studied in two cell lines (HeLa S-3 and Chinese hamster V-79379A). In both cell lines, carcinogen-modified adenine residues were excised more readily than the modified guanine residues and the percentage of the total products excised decreased after treatment with higher concentrations of carcinogen. At the highest concentrations used in the Chinese hamster cells, neither DNA synthesis nor excision was detected. The lowest concentration used for these cells permitted almost 100% survival and all the DNA was replicated in a 30-h interval even though 50% of the initial damage was still present. The two- to threefold lower sensitivity of the Chinese hamster cells (compared with the Hela cells) to the carcinogen is attributed to this capacity for replication of DNA on a damaged template since the two cell lines' capacities for excision of the chemical damage were found to be comparable.

Excision of O6-methylguanine from DNA of various mouse tissues following a single injection of N-methyl-Nitrosourea

The persistence of O6-methylguanine produced by a single dose of N-methyl-N-nitrosourea (MNU) was determined in DNA of various murine tissues and compared with the location of tumours induced by MNU and related alkylating carcinogens in this species. A/J and C3HeB/FeJ mice received a single intravenous injection of MNU (10 mg/kg) and were killed at different time intervals ranging from 4 h to 7 days. The rate rate of loss of O6-methylguanine from brain DNA was considerably slower than from liver DNA; tumours have been found in both organs after administration of MNU and other alkylnitrosoureas. There was no difference in the rate of excision from cerebral DNA of A/J and C3HeB/FeJ mice, although these strains differ significantly in their susceptibility to the neurooncogenic effect of MNU and related carcinogens. Excision of O6-methylguanine from hepatic DNA was significantly slower in A/J than in C3HeB/FeJ mice; both strains habe been found to develop hepatic carcinomas following MNU administration. Seven days after the injection of 3H-MNU, O6-methylguanine concentrations were highest in brain and lung DNA, lowest in the liver, and intermediate in kidney, spleen, small intestine and stomach. The lung is a principal target organ for tumour induction by MNU and other carcinogens in mice; however, neural tumours are usually induced at a low incidence. The results obtained do not contradict the hypothesis that O6-alkylation of guanine in DNA is a critical event in the initiation of tumour induction by alkylating agents. However, the location of tumours produced in mice does not seem to depend solely on the formation and persistence of O6-alkylguanine in DNA.

All oxygens in nucleic acids react with carcinogenic ethylating agents

Over 80% of ethylnitrosourea and ethylnitrosoguanidine modification of nucleic acids is on oxygens. The reactivity of oxygens (other than ribose and phosphate) in single-stranded RNA is: O2 of C greater than O2 of U greater than O6 of G greater than O4 of U. In double-stranded DNA the order is: O2 of T equals O6 of G greater than O4 of T greater than O2 of C. Oxygen reactivity of single-stranded DNA resembles RNA. The glycosidic bond of O2-alkylpyrimidines is labilised.

Effect of a single treatment with the alkylating carcinogens dimethylnitrosamine and methyl methanesulphonate on liver regenerating after partial hepatectomy. III. Effect on DNA synthesis in vivo and on DNA polymerase activity assayed in vitro

A single treatment with dimethylnitrosamine (DMN) but not with methyl methanesulphonate (MMS) induces liver cell carcinoma if given during the period of restorative hyperplasia following partial hepatectomy, a higher incidence of tumours being induced if the carcinogen is given during the period of DNA synthesis (24 h after the operation) than if given early in the prereplicative stage (at 6 h). To study the effect of treatment with DMN and with MMS on the regenerating liver, DNA replication was measured in vivo in partially hepatectomised animals treated with the methylating agents, and DNA polymerase activity was assayed in vitro.

Synthesis and properties of poly(O6-methylguanylic acid) and poly(O6-ethylguanylic acid)

The nucleotide analogues, O6-methyl- and O6-ethylguanosine diphosphate, have been synthesized and polymerized to high-molecular-weight homopolymers with polynucleotide phosphorylase. The ultraviolet spectra of these polymers show marked hypochromicity, which suggests that they possess considerable secondary structures. Graphs of optical denisty vs. temperature in 0.15 M NaC1 indicate that cooperative melting occurs for both polymers, and that the secondary structure of poly(O6-methylguanosine monophosphate) is somewhat more stable than that of poly (O6-ethyl-guanosine monophosphate). Mixing experiments show that these analogue polymers no longer form helical structures with poly(C), nor do thye form helices with poly(U). We would conclude from these data that environmental mutagens and carcinogens which react at the O6 position of guanine not only disrupt normal base-pairing relationships, but may also affect the secondary structure of nucleic acids.

Repair replication of DNA in the intact animal following treatment with dimethylnitrosamine and with methyl methanesulphonate, studied by fractionation of nuclei in a zonal centrifuge

To relate repair of lesions in DNA with carcinogenesis, it is necessary to study repair in the intact animal under conditions which will and will not induce cancer. The methods used to study the replications stage of repair in isolated cells are not suitable for use in the intact animal. A method is presented which depends on the fact that during cell replication the nuclei enlarge, and may be separated from nuclei of non-replicating cells by rate zonal centrifugation on a sucrose gradient in a zonal rotor. Thus incorporation of [3H]thymidine as a result of de novo synthesis of DNA in replicating nuclei can be separated from incorporation due to repair synthesis in non-replicating nuclei. Treatment of animals with dimethylnitrosamine or with methyl methanesulphonate produced a "repair-type" profile, which contrasted with that given by liver nuclei from untreated animals, or from animals in which DNA synthesis had been reduced by treatment with cycloheximide, a compound which is not known to cause direct damage to DNA. Evidence is presented which suggests that the method is a rapid sensitive test for the occurrence of repair replication of some kind in the liver of the intact animal.

Studies on DNA repair in early spermatid stages of male mice after in vivo treatment with methyl-, ethyl-, propyl-, and isopropyl methanesulfonate

In vivo DNA repair occurring in early spermatid stages of the mouse has been studied with four mutagens that are chemical homologs: MMS, EMS, PMS and IMS. Using the well-studied sequence of events that occurs during spermatogenesis and spermiogenesis in the mouse, aatids was measured by the unscheduled incorporation of [3H]dT into these germ cells which were recovered from the caudal epididymides 16 days after chemical treatment. Purification of the caudal sperm DNA at this time verified that the [3H]dT was incorporated into the DNA. For each chemical mutagen a study was made on the level of DNA repair occurring in early spermatids as a function of the administered, in vivo dose. Within experimental errors, all four chemicals produced a linear increase in DNA repair in early spermatids with increasing dose. Only the highest dose of MMS (100 mg/kg) produced a greater repair response than expected for a linear curve. At equimolar doses the most effective chemical in inducing DNA repair was MMS, followed by EMS, IMS and PMS. When testicular injections of [3H]dT were given at the same time as the intraperitoneal injections of the mutagens, the amount of unscheduled incorporation of [3H]dT into the DNA of early spermatids was maximized. Since [3H]dT has been shown to be available for incorporation into germ-cell DNA for only approximately 1 h after injection, all four mutagens must reach the DNA of early spermatids and begin producing "repairable" lesions within 1 h after treatment. The amount of DNA repair occurring at later times after chemical treatment of early spermatids was studied by testicular injections of [3H]dT 1/2, 1, 2 and 3 days after chemical treatment. Repair was still occurring in the early spermatids at 3 days post-treatment; this repair is most likely a manifestation of the finite rate of the repair process rather than resulting from newly alkylated DNA. For MMS and EMS there was a rapid decrease in the level of DNA repair in the first 1/2 day following treatment. This was followed by a much slower, exponential decrease in the level of repair out to 3 days post-treatment. The curves suggest that the amount of repair is proportional to the number of repairable lesions still present in the DNA. For PMS and IMS the level of repair decreases rapidly in the first 1/2 day after treatment and thereafter remains relatively constant through 3 days post-treatment. With all four mutagens, DNA repair in early spermatids was detectable at doses 5 to 10 times lower than those required to observe other genetic end points such as dominant lethals, translocations and specific-locus mutations in any germ-cell stage. The sensitivity of detection of in vivo DNA repair in the germ cells of male mice makes such a system a useful adjunct to other genetic tests for studying chemical mutagenesis in mammals.

Some mechanisms operative in carcinogenesis a review

Multiple factors contribute to the development of neoplasia. Sometimes a single agency can bring about a tumour if it has many different effects, but at other times a tumour arises more insidiously due to a succession of events [240,241] which by themselves may be innocent. Alterations in the genome of the cell are at the fore-front of our interest because they can be brought about by most of the carcinogenic agents we know. The cell can repair some such alterations but both forward and destructive mutations do appear. The roles of cell proliferation, cell differentiation, the immune mechanism and carcinogen-activating enzymes are beginning to be understood. The effects of dose, route of administration, and of other agents given at the same time [242-245] must not be lost sight of. Other factors no doubt will be added as we begin to look at the structure and function of cell-surface membranes [246-248], at host susceptibility genetics [26, 249], and at the generation of carcinogens inside the body [250,251]. We are only beginning to understand carcinogenesis. In no single instance do we as yet know how a tumour comes about in full details of molecular biology. It is possible that fully rational treatment of cancer will not be possible until we have such an understanding. Once a tumour becomes independent of carcinogenic factors, it continues to develop in a bizarre fashion which makes its study and treatment by all means other than surgery difficult.

Heterogeneous distribution of DNA alkylation products in rat liver chromatin after in vivo administration of N,N-di[14C]methylnitrosamine

Poly-l-lysine (PL) binds to about 50% of chromatin DNA, rendering it resistant to degradation by DNAase I. Separation of the unbound DNA as acid-soluble nucleotides allows the fractionation of chromatin DNA into two zones. After in vivo administration of N,N-di[14C]methylnitrosamine, the amount of alkylation in DNA was found to be lower in the polylysine-binding regions. Some possible reasons for this heterogeneous distribution are discussed.

Studies on liver chromatin from rats treated with dimethylnitrosamine

Shortly after injecting a single low dose of N,N-dimethylnitrosamine into rats, the DNA, RNA and histones are methylated, the level in the DNA greatly exceeding that in the histones. The composition of the chromatin and the electrophoretic profiles of the histone and non-histone proteins are not detectably different from those obtained from control animals. Electric birefringence studies suggest that methylation may result in both interparticle cross-linking and some localised loosening of the DNA-protein complex complex.

Binding of [-14C]aflatoxin B1 to cellular macromolecules in the rat and hamster

The uptake and binding of ring-labelled [-14C]aflatoxin B1 (AFB1) by rat and hamster liver and kidney has been studied, the former species being extremely sensitive to the carcinogenic action of AFB, whereas the latter is resistant. In contrast to an earlier report (Lijinsky et al, Cancer Res., 30 (1970) 2280-2283, binding of the carcinogen to nucleic acids was far greater than that to protein. Rat liver DNA bound ten times and rRNA twenty times more carcinogen than protein. There were also differences in the amount of carcinogen bound to rat liver nucleic acids compared to those of the hamster, the latter species binding lower amounts of the carcinogen. Rat liver DNA bound four times and rRNA ten times as much AFB1 6 h after carcinogen administration whereas liver protein bound AFB1 was similar for the two species. Not only was there a difference in the amount of AFB1 bound but whereas in the rat, liver nucleic acid bound carcinogen decayed with time, no such fall was seen in the hamster, this remaining at a low level throughout the 48-h time period studied. In contrast, reaction of the carcinogen with kidney macromolecules was similar for the two species. The much higher binding of AFB1 to nucleic acids than to protein might account for the potent carcinogenicity of this compound in the rat, particularly since liver protein binding does not differ between a susceptible and a resistant species. A further important factor in determining carcinogenic sensitivity may be the removal of nucleic acid bound radioactivity with time, a possible repair process.

Increased urinary excretion of labelled deoxyribonucleosides in rats with stable pre-labelled DNA treated with methyl methanesulphonate or nitrogen mustard

After the DNA of newborn female rats had been labelled by repeated injections of [14C]orotate (totalling 36 mu Ci) during the first 3 weeks of life, approximately 1,000,000 dpm were found in the DNA of the liver, lungs, kidneys, gut, brain, heart and spleen of 8-week-old rats. Methyl methanesulphonate (MMS) (80 mg/kg) and di-(2-chloroethyl)methylamine (HN2) (5 mg/kg) injection increased the amount of 14C-labelled DNA pyrimidine nucleosides excreted in the urine to 5000 dpm from 350 dpm before injection. The effect on RNA products was much less marked.

N-nitrosomorpholine and N-nitrosobutylamine-stimulated DNA synthesis in BHK-21/C13 cells

Non-semiconservative DNA synthesis was examined in BHK-21/C13 cells after treatment with the aliphatic N-nitrosobutylamine (NB) and the heterocyclic N-nitrosomorpholine (NM). The extent of repair synthesis after alkylation was compared quentitatively and calculations were made of the amount of DNA damage per cell and number of bases inserted per damaged site. Of the chromosome aberrations caused by NM the most predominant were dicentrics. Other aberrations included fragments, gaps, breaks and exchanges of both the chromatid and chromosome type. Unlike NM, NB elicted a higher frequency of chromatid breaks.

Methylation of DNA in various organs of C57B1 mice by a carcinogenic dose of N-methyl-N-nitrosourea and stabiltty of some methylation products up to 18 hours

Female 6-8-week-old (57B1 mice were injected i.p. with N-methyl-N-nitrosourea (MNUS) (14C or 3H-methyl-labelled) in saline (80 mg/kg) and DNA was isolated from bone marrow, small bowel, kidneys, liver, lungs, spleen and thymus at various times thereafter up to 18 h. Methylation of DNA was found in all organs examined, and by analyses using column or paper chromatography of DNA hydrolysates, the extent of methylation of DNA purines was determined. Methylated guanine residues (at N-3, N-7 and 0-6 positions) were stable in DNA up to 18 h, but methylated adenines (at N-3 or N-7) were removed from DNA of all organs examined; the overall half-life of methyladenines was about 3 h, but removal appeared to occur in a biphasic manner, with a proportion of methyladenine remaining relatively stable. This relative stability was somewhat more marked in bone marrow than in other organs.

Chemical carcinogenesis in the nervous system. Preferential accumulation of O6-methylguanine in rat brain deoxyribonucleic acid during repetitive administration of N-methyl-N-nitrosourea

The alkylation of purine bases in DNA of several rat tissues was determined during weekly injections (10 mg/kg) of N-[3H]methyl-N-nitrosourea, a dose schedule known to selectively induce tumours of the nervous system. Each group of animals was killed 1 week after the final injection, and the DNA hydrolysates were analysed by chromatography on Sephadex G-10. After five weekly applications, O6-methylguanine had accumulated in brain DNA to an extent which greatly exceeded that in kidney, spleen and intestine. In the liver, the final O6-methylguanine concentration was less than 1% of that in brain. Between the first and the fifth injection, the O6-methylguanine/7-methylguanine ratio in cerebral DNA increased from 0.28 to 0.68. In addition, 3-methylguanine was found to accumulate in brain DNA whereas in the other organs no significant quantities of this base were detectable. The results are compatible with the hypothesis that O6-alkylation of guanine in DNA plays a major role in the induction of tumours by N-methyl-N-nitrosourea and related carcinogens. The kinetics of the increase of O6-methylguanine in cerebral DNA suggest that there is no major cell fraction in the brain which is capable of excising chemically methylated bases from DNA. This repair deficiency could be a determining factor in the selective induction of nervous-system tumours by N-methyl-N-nitrosourea and other neuro-oncogenic compounds.

Effect of a single treatment with the alkylating carcinogens dimethynitrosamine, diethylnitrosamine and methyl methanesulphonate, on liver regenerating after partial hepatectomy. I. Test for induction of liver carcinomas

A single injection of dimethylnitrosamine (DMN), 12.0-15.6 mg-kg, given to 100 g female rats 24 h after partial hepatectomy, induced hepatocellular carcinoma. No animals receiving DMN without partial hepatectomy developed liver carcinomas. Previous evidence had suggested that the incidence of tumours was highest when DMN was administered during the wave of DNA replication which follows partial hepatectomy. The present experiments made this suggestive evidence statistically significant. A single treatment with diethylnitrosamine (DEN) induced liver cell cancer when given to intact or to partially hepatectomised rats. No tumors developed when another alkylating carcinogen, methyl methanesulphonate (MMS), was administered after partial hepatectomy. The significance of these results in relation to the mechanism of initiation of carcinogenesis is discussed.

Effect of a single treatment with the alkylating carcinogens dimethylnitrosamine, diethylnitrosamine and methyl methanesulphonate, on liver regenerating after partial hepatectomy. II. Alkylation of DNA and inhibition of DNA replication

Experiments were carried out to determine whether replication of alkylated DNA could be involved in the initiation of hepatocellular carcinoma which results from a single administration of dimethylnitrosamine (DMN) given after partial hepatectomy. The incidence of tumours is higher when DMN is given during the wave of DNA synthesis induced by the operation than when given in the early prereplicative stage. Therefore the alkylation of DNA in the regenerating liver by DMN given at these times and the effect of DMN on DNA synthesis were investigated. The extent, duration and pattern of alkylation of DNA, including the formation of 0-6-methylguanine, were similar whether DMN was given in the early pre-replicative stage (6 h after the operation) or during the period of DNA synthesis (at 24 h). DMN given a 6 h very greatly reduced the wave of DNA replication which would otherwise have ensued. When given at 24 h, by which time DNA synthesis was already taking place, DMN reduced the rate of incorporation of (-3H)thymidine after 1-2 h delay. However, in neither case was DNA synthesis reduced to the level occurring in normal intact liver. Treatment with diethylnitrosamine (DEN) at 6 h or at 24 h had a similar effect to DMN on the wave of DNA replication induced by partial hepatectomy. Methyl methanesulphonate (MMS given in the early pre-replicative stage delayed the wave of DNA synthesis by about 8 h, but when it did take place the extent of synthesis was as great as in untreated animals. When given during the period of DNA replication, MMS rapidly reduced the rate of synthesis. As in the case of the nitrosamines, synthesis was not reduced to the level occuring in normal intact animals. The difference from the nitrosamines lies in the nature of the alkylated bases formed in DNA. The fact that a single treatment with DMN induces cancer in partially hepatectomised animals but not in intact adult animals is not considered to be due to a gross difference in the nature of the alkylation of DNA. The experiments described support the concept that replication of DNA containing bases which are likely to mispair during replication may be necessary to 'fix' the lesion and thus cause a permanent inheritable change in the genetic material.

The specificity of different classes of ethylating agents toward various sites of HeLa cell DNA in vitro and in vivo

The sites and extent of ethyl products of neutral ethylation of HeLa cell DNA by [14-C]diethyl sulfate, [14-C]ethyl methanesulfonate, and [14-C]ethylnitrosourea have been determined in vitro and in vivo, and found to differ significantly depending on the ethylating agents. Diethyl sulfate and ethyl methanesulfonate ethylate the bases of HeLa cell DNA in the following order: 7-ethylguanine greater than 3-ethyladenine greater than 1-ethyladenine, 7-ethyladenine greater than 3-ethylguanine, 3-ethylcytosine, O-6-ethylguanine. Ethyl bases accounted for 84-87% of the total ethyl groups associated with HeLa cell DNA. Ethylnitrosourea, in contrast, has particular affinity for the O-6 position of guanine. It ethylates the bases of HeLa cell DNA in the following order: O-6-ethylguanine, 7-ethylguanine greater than 3-ethyladenine greater than 3-ethylguanine, 3-ethylthymine greater than 1-ethyladenine, 7-ethyladenine, 3-ethylcytosine. Ethylation of the bases only accounts for 30% of the total ethylation in the case of ethylnitrosourea. The remaining 70% of the [14-C]ethyl groups, introduced in vivo and in vitro, are in the form of phosphotriesters which after perchloric acid hydrolysis are found as [14-CA1ethanol and [14-C]ethyl phosphate. In contrast, phosphotriesters amounted to only 8-20% of total ethylation in in vivo or in vitro diethyl sulfate and ethyl methanesulfonate treated HeLa cell DNA, and 25% of the total methylation in in vitro methylnitrosourea treated HeLa cell DNA. Alkylation at the N-7 and N-3 positions of purines in DNA destabilizes the glycosidic linkages. Part of 7-ethylguanine and 3-ethyladenine are found to be spontaneously released during the ethylation reaction. Incorporation of the 14-C of the alkylating agents into normal DNA bases of HeLa cells can be eliminated by performing the alkylations, in the presence of cytosine arabinoside, for 1 hr.