Expression of the truncated E. coli O6-methylguanine methyltransferase gene in repair-deficient human cells and restoration of cellular resistance to alkylating agents

Programming of Cell Resistance to Genotoxic and Oxidative Stress

Different organisms, cell types, and even similar cell lines can dramatically differ in resistance to genotoxic stress. This testifies to the wide opportunities for genetic and epigenetic regulation of stress resistance. These opportunities could be used to increase the effectiveness of cancer therapy, develop new varieties of plants and animals, and search for new pharmacological targets to enhance human radioresistance, which can be used for manned deep space expeditions. Based on the comparison of transcriptomic studies in cancer cells, in this review, we propose that there is a high diversity of genetic mechanisms of development of genotoxic stress resistance. This review focused on possibilities and limitations of the regulation of the resistance of normal cells and whole organisms to genotoxic and oxidative stress by the overexpressing of stress-response genes. Moreover, the existing experimental data on the effect of such overexpression on the resistance of cells and organisms to various genotoxic agents has been analyzed and systematized. We suggest that the recent advances in the development of multiplex and highly customizable gene overexpression technology that utilizes the mutant Cas9 protein and the abundance of available data on gene functions and their signal networks open new opportunities for research in this field.

Chemical biology of mutagenesis and DNA repair: cellular responses to DNA alkylation

The reaction of DNA-damaging agents with the genome results in a plethora of lesions, commonly referred to as adducts. Adducts may cause DNA to mutate, they may represent the chemical precursors of lethal events and they can disrupt expression of genes. Determination of which adduct is responsible for each of these biological endpoints is difficult, but this task has been accomplished for some carcinogenic DNA-damaging agents. Here, we describe the respective contributions of specific DNA lesions to the biological effects of low molecular weight alkylating agents.

Intracellular localization and function of DNA repair methyltransferase in human cells

An antibody preparation specific for human O6-methylguanine-DNA methyltransferase (EC 2.1.1.63) was obtained by immunoaffinity purification on two types of affinity columns with the purified human and mouse methyltransferase proteins as ligands. The antibodies were used in Western blotting analysis of fractionated cell extracts. More than 90% of the methyltransferase protein was recovered in the cytoplasmic fractions with both human HeLa S3 cells and MR-M cells, the latter overproducing the enzyme 36 times as much as the former. Cytoplasmic localization of the methyltransferase in HeLa S3 cells was further confirmed by in situ immunostaining. By Western blotting analysis of fractionated cell extracts from HeLa S3 cells treated with alkylating agents, we found that amounts of the enzyme decreased more rapidly in the nuclear fraction than in the cytoplasmic fraction, and recovery of the enzyme level in the cytoplasmic fraction was slower than that in the other. These results suggest that the methyltransferase protein is degraded in the nucleus after it commits the repair reaction and that the cytoplasmic enzyme is transported into the nucleus as the nuclear methyltransferase is used up in this manner.

On the quantitative relationship between O6-methylguanine residues in genomic DNA and production of sister-chromatid exchanges, mutations and lethal events in a Mer- human tumor cell line

O6-Methylguanine (m6G) is an altered base produced in DNA by SN1 methylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). This lesion is repaired by the protein O6-methylguanine-DNA methyltransferase (MGMT) in normal human cell lines, but is not repaired in certain human tumor lines that are termed Mex- or Mer-. Compared with repair-proficient cell lines, such repair-deficient tumor lines are hypersensitive to the production by MNNG of sister-chromatid exchanges (SCE), mutations and lethality. We report here that MNNG treatment produces 1 SCE for every 42 +/- 10 m6G formed in the genome of Mer- tumor cells, 1 6TG-resistant mutant for every 8 (range of 5-14) m6G produced statistically in the coding region of the hypoxanthine phosphoribosyltransferase gene, and 1 lethal event per 6650 +/- 1200 m6G. In addition, in vitro base mismatch incision at m6G: BrU pairs was similar to that at m6G: T pairs, the lesions that likely initiate SCE production. We conclude that m6G residues in genomic DNA are very recombinogenic as well as highly mutagenic in Mer- human tumor cells. The results are interpreted in terms of the relationship between methylation-induced SCE and G: T mismatch recognition.

Expression in mammalian cells of the Escherichia coli O6 alkylguanine-DNA-alkyltransferase gene ogt reduces the toxicity of alkylnitrosoureas

V79 Chinese hamster cells expressing either the O6-alkylguanine-DNA-alkyltransferase (ATase) encoded by the E. coli ogt gene or a truncated version of the E. coli ada gene have been exposed to various alkylnitrosoureas to investigate the contribution of ATase repairable lesions to the toxicity of these compounds. Both ATases are able to repair O6-alkylguanine (O6-AlkG) and O4-alkylthymine (O4-AlkT) but the ogt ATase is more efficient in the repair of O4-methylthymine (O4-MeT) and higher alkyl derivatives of O6-AlkG than is the ada ATase. Expression of the ogt ATase provided greater protection against the toxic effects of the alkylating agents then the ada ATase particularly with N-ethyl-N-nitrosourea (ENU) and N-butyl-N-nitrosourea (BNU) to which the ada ATase expressing cells were as sensitive as parent vector transfected cells. Although ogt was expressed at slightly higher levels than the truncated ada in the transfected cells, this could not account for the differential protection observed. For-N-methyl-N-nitrosourea (MNU) the increased protection in ogt-transfected cells is consistent with O4-MeT acting as a toxic lesion. For the longer chain alkylating agents and chloroethylating agents, the protection afforded by the ogt protein may be a consequence of the more efficient repair of O6-AlkG, O4-AlkT or both of these lesions in comparison with the ada-encoded ATase.

Correlation between DNA methylation and expression of O6-methylguanine-DNA methyltransferase gene in cultured human tumor cells

Approximately 20% of human tumor cell strains are deficient in a DNA repair protein, O6-methylguanine-DNA methyltransferase (MGMT), and are called Mer- strains. In an attempt to determine the molecular basis for the extinction of MGMT expression in Mer- human cells, the distribution of DNA methylation sites in and around the exon sequences of the repair gene was compared in 6 Mer+ (repair-proficient) and 12 Mer- cell lines. Southern blot analysis of the genomic DNA digested with isoschizomeric restriction endonucleases MspI and HpaII to detect 5-methylcytosine in CCGG sequences indicated that the DNA of all the Mer+ cells but of none of the Mer- cells is heavily methylated in the exon-containing regions. The methylation pattern contradicts the general belief that inactive genes are hypermethylated compared to hypomethylation of transcriptionally active genes. It appears that the regulation of the MGMT gene in human cells is much more complex than simply dictated by its methylation level.

Effect of O6-alkylguanine-DNA alkyltransferase on the frequency and spectrum of mutations induced by N-methyl-N'-nitro-N-nitrosoguanidine in the HPRT gene of diploid human fibroblasts

N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) reacts with 12 nucleophilic sites in DNA to induce a variety of lesions, but O6-methylguanine (O6-MeG) and O4-methylthymine are the most effective premutagenic lesions produced, mispairing with thymine and guanine, respectively. O6-MeG is repaired by O6-alkylguanine-DNA alkyltransferase (AGT), which removes the methyl group from the O6 position and transfers it to itself, rendering the transferase inactive. When diploid human fibroblasts were exposed to 25 microM, O6-benzylguanine (O6-BzG) in the medium for 3 h, their level of AGT activity was dramatically reduced, to a level of at most 1.6% of the control. Populations of cells pretreated with this level of O6-BzG for 2 h or not pretreated, were exposed to MNNG at a concentration of 2, 4 or 6 microM in the presence or absence of O6-BzG and assayed for survival of colony-forming ability and the frequency of 6-thioguanine-resistant cells (mutations induced in the HPRT gene). O6-BzG (25 microM) was also present in the appropriate half of the cells during the 24 h immediately following exposure to MNNG. This 27-h exposure to O6-BzG alone had no cytotoxic or mutagenic effect on the cells but significantly increased the cytotoxicity and mutagenicity of MNNG, increasing the mutant frequency to that found previously in human cells constitutively devoid of AGT activity. At doses of 2 microM and 4 microM MNNG, the mutant frequency observed with the AGT-depleted cells was 120 x 10(-6) and 240 x 10(-6), respectively; in the cells with abundant AGT activity, these values were 10 x 10(-6) and 20 x 10(-6), respectively. DNA-sequence analysis of the coding region of the HPRT gene in 36 independent mutants obtained from MNNG-treated AGT-depleted populations and 36 from the control populations showed that even though AGT repair lowered the frequency of mutants by more than 90%, it did not affect the kinds of mutations induced by MNNG nor the strand distribution of the premutagenic guanine lesions. In mutants from the AGT-depleted cells, there were 26 base substitutions and 13 putative splice site mutations; in the control, there were 25 base substitutions and 11 splice site mutations. All but two substitutions involved G.C with 92% being G.C----A.T. In both sets, approximately 73% of the premutagenic lesions were located in the nontranscribed strand.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of human O6-methylguanine-DNA methyltransferase in Chinese hamster ovary cells and restoration of cellular resistance to certain N-nitroso compounds

We have constructed a plasmid in which the expression of human O6-methylguanine-DNA methyltransferase (MGMT) cDNA is driven by the Rous sarcoma virus promoter sequence. Transfection of this plasmid into Chinese hamster ovary (CHO) cells results in expression of MGMT and in cellular resistance to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 1-(2-chloroethyl)-1-nitrosourea (CNU), but not to N-nitroso-N-ethylurea. The specific activity of MGMT in transfected CHO cells correlated well with their resistance to MNNG and CNU. Southern analysis showed that the plasmid had been integrated into the CHO cell genome. Western analysis of extracts from transfected CHO cells using an antibody against a peptide corresponding to the carboxyl-terminal end of the human MGMT protein demonstrated a single band with a molecular size of 24-25 kDa; no such band was observed in extracts from wild-type CHO cells. These transfected cells may therefore be used to study the role of MGMT in the repair of alkylating DNA lesions and to determine its importance in carcinogenesis as well as in chemotherapy.

Regulation of O6-methylguanine-DNA methyltransferase expression in the Burkitt's lymphoma cell line Raji

We have investigated the expression of the DNA-repair enzyme O6-methylguanine-DNA methyltransferase in the Burkitt's lymphoma cell line Raji. An existing mutant Raji cell line which lacks thymidine kinase activity had previously been shown to be Mex- and to no longer express O6-methylguanine-DNA methyltransferase. We report here that in addition to the methyltransferase and thymidine kinase, a third enzyme with an unrelated function, galactokinase, is also not expressed in Raji cells. The control of thymidine kinase expression is post-transcriptional and it is possible that galactokinase and methyltransferase can share a common post-transcriptional regulation with thymidine kinase.

Alkylation damage, DNA repair and mutagenesis in human cells

17 human cell lines that differ significantly in level of O6-alkylguanine-DNA alkyltransferase (AGT) activity were identified by comparing their sensitivity to the cytotoxic effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and determining the level of AGT activity in cell extracts from the various lines by measuring the decrease in radiolabeled O6-methylguanine from DNA, using high-performance liquid chromatography. 9 lines exhibited high levels of AGT activity, 2 showed an intermediate level (25-50% of the mean of those with the higher levels), and 6 exhibited very low or virtually undetectable levels of AGT. Included were several lines that are very deficient in capacity for nucleotide excision repair. When representatives from the 3 categories of cell lines defined by the level of AGT activity were compared for sensitivity to the cytotoxic and mutagenic effect of MNNG, they showed an inverse correlation between the degree of cell killing and frequency of mutants induced and the level of AGT activity. The cells' capacity for nucleotide excision repair did not affect these results. Exposure of cells with a high level of AGT activity to O6-methylguanine in the medium reduced the AGT activity 60-80%. These pre-treated cells exhibited a significantly higher frequency of MNNG-induced mutants than did cells that were not pre-treated, suggesting that the O6-methylguanine lesion in DNA is responsible for a significant proportion of the mutations induced. Cell strains containing substrates for assaying intrachromosomal homologous recombination were constructed using parental cell lines from each of the 3 categories of AGT activity. These strains showed an inverse correlation between the level of AGT activity and the frequency of MNNG-induced recombination. When various cell lines representing the 3 categories of AGT activity were compared for sensitivity to ethylnitrosourea, the results were consistent with AGT and nucleotide excision repair playing a role in preventing cell killing and mutation induction by this agent.

Mismatch binding proteins and tolerance to alkylating agents in human cells

The Mex- (Mer-) phenotype of human cells is characterised by a sensitivity to agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-methyl-N-nitrosourea (MNU). The hypersensitivity of Mex- cells is a consequence of their failure to express the DNA-repair enzyme m6-Gua-DNA methyltransferase. Resistance to MNNG and MNU may be acquired by Mex- cells either by reexpression of a methyltransferase function or by an ill-defined process of tolerance in which the cytotoxic potential of m6-Gua is circumvented without the altered base being removed from DNA. It has been suggested that tolerance might involve an altered mismatch correcting function. We have investigated proteins which recognise and bind specifically to DNA fragments containing single-base mismatches. Cell-free extracts of a Burkitt's lymphoma cell line (Raji) contain two such mismatch binding activities. Neither protein appears to have a high affinity for m6-Gua-containing base pairs. The data indicate that m6-Gua-containing base pairs might be poor substrates for mismatch repair processes in human cells.

Active site amino acid sequence of the bovine O6-methylguanine-DNA methyltransferase

An O6-methylguanine-DNA methyltransferase has been partially purified from calf thymus by conventional biochemical techniques. The enzyme was specifically radioactively labelled at the cysteine residue of the active site and further purified by attachment to a solid support. Following digestion with trypsin, a radioactive peptide containing the active site region of the protein was purified by size fractionation, ion exchange chromatography and reverse phase HPLC. The technique yielded an essentially homogeneous oligopeptide which was subjected to amino acid sequencing. The sequence adjacent to the acceptor cysteine residue of the bovine protein exhibits striking homology to the C-terminal methyl acceptor site of the E. coli Ada protein and the proposed acceptor sites of the E. coli Ogt and the B. subtilis Dat1 proteins.

Isolation of a Bacillus subtilis mutant defective in constitutive O6-alkylguanine-DNA alkyltransferase

A mutant of Bacillus subtilis defective in the constitutive activity of O6-alkylguanine-DNA alkyltransferase was isolated from a strain (ada-1) deficient in the adaptive response to DNA alkylation. Cells carrying the mutation dat-1 which was responsible for the defect in constitutive activity exhibited hypersensitivity for lethality and mutagenesis when challenged with methyl-nitroso compounds. The constitutive activity is independent of the adaptive response, and seems to function as a basal defense against environmental alkylating agents.

Yeast gene RAD52 can substitute for phage T4 gene 46 or 47 in carrying out recombination and DNA repair

The RAD52 gene of Saccharomyces cerevisiae and genes 46 and 47 of bacteriophage T4 are essential for most recombination and recombinational repair in their respective organisms. The RAD52 gene was introduced into expression vectors that were used to transform Escherichia coli. The expression of RAD52 was then induced, and the ability of RAD52 to complement phage mutants defective in gene 46 or 47 was determined with respect to the three criteria of phage growth, recombination, and recombinational repair. RAD52 gene expression was found to allow growth of gene 46 and 47 mutants under otherwise restrictive conditions, as measured by plaque formation and burst size. Expression of the RAD52 gene also restored the ability of gene 46 and 47 mutants to undergo recombination of rII markers. Furthermore, the RAD52 gene restored the ability of gene 46 and 47 mutants to undergo recombinational repair after UV irradiation. The published DNA sequence of gene RAD52 was compared with the published sequences of genes 46 and 47. Although overall sequence similarities were only marginally significant, RAD52 and gene 46 had substantial sequence similarity over a limited region.

Comparative analysis of O6-methylguanine methyltransferase activity and cellular sensitivity to alkylating agents in cell strains derived from a variety of animal species

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.