Cloning and expresion of cDNA for rat O6-methylguanine-DNA methyltransferase

Multifaceted roles of alkyltransferase and related proteins in DNA repair, DNA damage, resistance to chemotherapy, and research tools

O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a widely distributed, unique DNA repair protein that acts as a single agent to directly remove alkyl groups located on the O(6)-position of guanine from DNA restoring the DNA in one step. The protein acts only once, and its alkylated form is degraded rapidly. It is a major factor in counteracting the mutagenic, carcinogenic, and cytotoxic effects of agents that form such adducts including N-nitroso-compounds and a number of cancer chemotherapeutics. This review describes the structure, function, and mechanism of action of AGTs and of a family of related alkyltransferase-like proteins, which do not act alone to repair O(6)-alkylguanines in DNA but link repair to other pathways. The paradoxical ability of AGTs to stimulate the DNA-damaging ability of dihaloalkanes and other bis-electrophiles via the formation of AGT-DNA cross-links is also described. Other important properties of AGTs include the ability to provide resistance to cancer therapeutic alkylating agents, and the availability of AGT inhibitors such as O(6)-benzylguanine that might overcome this resistance is discussed. Finally, the properties of fusion proteins in which AGT sequences are linked to other proteins are outlined. Such proteins occur naturally, and synthetic variants engineered to react specifically with derivatives of O(6)-benzylguanine are the basis of a valuable research technique for tagging proteins with specific reagents.

Phosphorylation of methylated-DNA-protein-cysteine S-methyltransferase at serine-204 significantly increases its resistance to proteolytic digestion

In a previous paper [Lim, Park, Jee, Lee and Paik (1999) J. Cancer Res. Clin. Oncol. 125, 493-499], we showed two major forms of active DNA-6-O-methylguanine:protein-L-cysteine S-methyltransferase (MGMT; EC 2.1.1.63) in the liver with N-nitrosodiethylamine (DEN)-induced carcinogenesis: these were 26 and 24 kDa species. Here we show that a 2 kDa C-terminal fragment was cleaved from the 26 kDa species in vitro by thrombin or microsomal fractions isolated from DEN-treated rat livers. When Ser(204) of the 26 kDa protein was replaced with Ala by site-directed mutagenesis, phosphorylation of the protein was completely abolished, indicating Ser(204) to be the site of phosphorylation. We also show that the phosphorylation was performed by Ca(2+)-independent protein kinase isoenzymes, and that the phosphorylated rat MGMT protein was resistant to digestion by protease(s) whose activity was increased during DEN-induced hepatocarcinogenesis and also by digestion with endopeptidase Glu-C (V8 protease).

The potential role of glycine-160 of human O6-alkylguanine-DNA alkyltransferase in reaction with O6-benzylguanine as determined by site-directed mutagenesis and molecular modelling comparisons

O6-Alkylguanine DNA-alkyltransferase (ATase) repairs toxic, mutagenic and carcinogenic O6-alkylguanine (O6-alkG) lesions in DNA by a highly conserved reaction involving the stoichiometric transfer of the alkyl group to the active centre cysteine residue of the ATase protein. In the Escherichia coli Ada ATase, which is effectively refactory to inhibition by O6-benzylguanine (O6-BzG), the residue corresponding to glycine-160 (G160) for the mammalian proteins of this class is replaced by a tryptophan (W). Therefore, to investigate the potential role of the G160 of the human ATase (hAT) protein in determining sensitivity to O6-BzG, site-directed mutagenesis was used to produce a mutant protein (hATG160W) substituted at position 160 with a W residue. The hATG160W mutant was found to be stably expressed and was 3- and 5-fold more sensitive than hAT to inactivation by O6-BzG, in the absence and presence of additional calf-thymus DNA respectively. A similar, DNA dependent increased sensitivity of the hATG160W mutant relative to wild-type was also found for O6-methylguanine mediated inactivation. The potential role of the W160 residue in stabilising the binding of the O6-alkG to the protein is discussed in terms of a homology model of the structure of hAT. The region occupied by G/W-160 forms the site of a putative hinge that could be important in the conformational change that is likely to occur on DNA binding. Three sequence motifs have been identified in this region which may influence O6-BzG access to the active site; YSGG or YSGGG in mammals (YAGG in E. coli Ogt, YAGS in Dat from Bacillus subtilis), YRWG in E. coli Ada and Salmonella typhimurium (but YKWS in Saccharomyces cerevisiae) or YRGGF in AdaB from B. Subtilis. Finally,conformational and stereoelectronic analysis of the putative transition states for the alkyl transfer from a series of inactivators of hAT, including O6-BzG was undertaken to rationalise the unexpected weak inhibition shown by the alpha-pi-unsaturated electrophiles.

Roles of DNA repair methyltransferase in mutagenesis and carcinogenesis

Alkylation of DNA at the O6-position of guanine is one of the most critical events leading to induction of mutation as well as cancer. An enzyme, O6-methylguanine-DNA methyltransferase, is present in various organisms, from bacteria to human cells, and appears to be responsible for preventing the occurrence of such mutations. The enzyme transfers methyl groups from O6-methylguanine and other methylated moieties of the DNA to its own molecule, thereby repairing DNA lesions in a single-step reaction. To elucidate the role of methyltransferase in preventing cancer, animal models with altered levels of enzyme activity were generated. Transgenic mice carrying extra copies of the foreign methyltransferase gene showed a decreased susceptibility to alkylating carcinogens, with regard to tumor formation. By means of gene targeting, mouse lines defective in both alleles of the methyltransferase gene were established. Administration of methylnitrosourea to these gene-targeted mice led to early death while normal mice treated in the same manner showed no untoward effects. Numerous tumors were formed in the gene-defective mice exposed to a low dose of methylnitrosourea, while none or only few tumors were induced in the methyltransferase-proficient mice. It seems apparent that the DNA repair methyltransferase plays an important role in lowering a risk of occurrence of cancer in organisms.

Application of antisense ribonucleic acid complementary to O6-methylguanine-deoxyribonucleic acid methyltransferase messenger ribonucleic acid for therapy of malignant gliomas

OBJECTIVE

A derivative of chloroethylnitrosoureas, 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU), is a drug of choice for the chemotherapy of human malignant brain tumors. However, the cytocidal effect of ACNU is effectively repressed through repair of ACNU-mediated deoxyribonucleic acid lesions by O6-methylguanine-deoxyribonucleic acid methyltransferase (MGMT). Because a variety of human tumors, including brain tumors, contain high levels of MGMT activity, we investigated the effect of antisense ribonucleic acid (RNA) complementary to MGMT messenger RNA on ACNU resistance in tumor cells.

METHODS

We established a stable ACNU-resistant clone, C6AR, from the rat glioma cell line C6 exposed to a stepwise increasing concentration of ACNU. We transfected a plasmid deoxyribonucleic acid-encoding antisense MGMT RNA under the control of the human metallothionein promoter into C6AR cells and determined the effect of the antisense RNA on ACNU resistance of tumor cells by a colony-forming efficiency assay.

RESULTS

C6AR cells expressed abundant MGMT messenger RNA, although the transcription level of the MGMT gene in parental C6 cells was below the lower limits of detection under the same assay conditions. ACNU resistance of C6AR cells was significantly repressed by transfected gene-dependent antisense MGMT RNA expression that resulted in decreased survival of the tumor cells.

CONCLUSION

ACNU resistance resulting from the expression of MGMT in rat glioma cells is significantly overcome by the expression of antisense MGMT RNA. This result suggests that the antisense MGMT RNA system might be a useful strategy for overcoming ACNU resistance in the treatment of intractable malignant gliomas.

DNA-repair methyltransferase as a molecular device for preventing mutation and cancer

Alkylation of DNA at the 0(6) position of guanine is regarded as one o f the most critical events leading to induction of mutations and cancers in organisms. Once 0(6)-methylguanine is formed, it can pair with thymine during DNA replication, the result being a conversion of the guanine.cytosine to an adenine.thymine pair in DNA, and such mutations are often found in tumors induced by alkylating agents. To counteract such effects, organisms possess a mechanism to repair 0(6)-methylguanine in DNA. An enzyme, 0(6)-methylguanine-DNA methyltransferase, is present in various organism, from bacteria to human cells, and appears to be responsible for preventing the occurrence of such mutations. The enzyme transfers methyl groups from 0(6)-methylguanine and other methylated moieties of the DNA to its own molecule, thereby repairing DNA lesions in a single-step reaction. To elucidate the role of methyltransferase in preventing cancers, animal models with altered levels of enzyme activity were generated. Transgenic mice carrying the foreign methyltransferase gene with functional promoters had higher levels of methyltransferase activity and showed a decreased susceptibility to N-nitroso compounds in regard to liver carcinogenesis. Mouse lines deficient in the methyltransferase gene, which were established by gene targeting, exhibited an extraordinarily high sensitivity to an alkylating carcinogen.

Temporal cell-type-specific mRNA expression of O6-methylguanine-DNA methyltransferases in liver of rats treated with dimethylnitrosamine

The intercellular distribution of O6-methylguanine-DNA methyltransferase (MGMT) mRNA expression has been investigated at the individual cell level in the liver of rats treated with dimethylnitrosamine using in situ hybridization. Male BDIV rats were orally administered with a single dose of dimethylnitrosamine (5 or 10 mg/kg) and were killed at 24, 48, and 96 hours after exposure. Constitutive MGMT mRNA expression was found in bile duct cells, vascular endothelial cells, and fibrous cells; however, weak or negative expression was detected in hepatocytes and Kupffer cells. On the other hand, after exposure to dimethylnitrosamine, inducible MGMT mRNA was rapidly expressed in hepatocytes, especially in the centrilobular area at earlier time points, confirming our previous studies in which enzymatic activity has been measured in different purified cell populations obtained by differential centrifugation. Immunohistochemical detection of the O6-methylguanine-DNA adduct formation was demonstrated in a dose-dependent fashion and was mainly found in the centrilobular hepatocytes where high levels of MGMT mRNA were present. These results suggest a correlation between the induction of this repair activity and DNA damage; however, the relationship between MGMT mRNA and enzyme activity remains to be fully established for all cell types.

Expression of the E.coli ada gene in S.cerevisiae provides cellular resistance to N-methyl-N'-nitro-N-nitrosoguanidine in rad6 but not in rad52 mutants

The Escherichia coli ada gene protein coding region under the control of the yeast alcohol dehydrogenase promoter in the extrachromosomally replicating yeast expression vectors pADHO6C and pVT103LO6C was introduced into the wild-type yeast strains, YNN-27 and FF-18733, and the repair deficient mutants LN-1 (rad1-1), VV-5 (rad6-1), C5-6 (rad52-1) and FF-18742 (rad52::URA3). This resulted in the expression of 3950, 1900, 1870, 1620, 1320 and 1420 fmol ada-encoded ATase/mg protein respectively: transformation with the parent vectors resulted in ATase activities of 3-17 fmol/mg protein. The wild-types, rad1-1 and rad6-1 yeast expressing the bacterial ATase showed increased resistance to the toxic and mutagenic effects of N-methyl-N'-nitro-N- nitrosoguanidine (MNNG). Expression of ATase in the rad52-1 and rad52::URA3 mutants neither complemented their sensitivity, nor reduced the mutagenic effects of this agent. These results suggest that whilst a portion of the toxic and mutagenic lesions induced by MNNG can be repaired in yeast by the E.coli Ada protein in a RAD1- and RAD6-independent manner, the RAD52 gene product may be essential for the complete functioning of the Ada ATase. This is the first suggestion of a possible cofactor requirement for ATase.

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.

Organization and expression of the human gene for O6-methylguanine-DNA methyltransferase

O6-Methylguanine-DNA methyltransferase plays an important role in cellular defence against mutagens and carcinogens with alkylating activity. Certain tumor-derived cell lines, termed Mer-, are defective in the enzyme activity and have an increased sensitivity to alkylating agents. We cloned the genomic sequence coding for the human O6-methylguanine-DNA methyltransferase and elucidated the structure. The gene consisted of 5 exons and spanned more than 170 kb, while mRNA for the enzyme was 950 nucleotides long. No or only little mRNA for the enzyme was formed in Mer- cells, though there was no gross difference in the coding and promoter regions of the gene between Mer+ and Mer- cells. The putative promoter region, derived from Mer+ cells, was placed upstream of the chloramphenicol acetyltransferase reporter gene and the constructs were introduced into Mer+ and Mer- cells. In Mer- cells, a lowered level of transient expression of the gene was observed as compared with Mer+ cells, but this difference alone does not account for the in vivo difference of expression of the gene in the two types of cells; there might be difference in cis-acting elements. The DNA sequence in the 5' upstream region of the gene was extremely GC-rich and there were no consensus sequences, such as the TATA and CAAT boxes. There were lower levels of methylation in the putative promoter of various Mer- cells, as compared with findings in Mer+ cells. Methylation in this region may be involved in regulating expression of the gene.

C-terminally truncated human O6-alkylguanine-DNA alkyltransferase retains activity

A cDNA encoding the human O6-alkylguanine-DNA alkyltransferase (ATase; EC 2.1.1.63; methylated-DNA: protein-cysteine methyltransferase) has been manipulated to generate a C-terminally deleted protein which retains full methyl-transfer activity. The elimination of 22 amino-acid residues from the C-terminus was achieved by endonuclease-SacI digestion of the 623 bp cDNA coding sequence and ligation of a SacI/HindIII linker containing an in-frame stop codon. The truncated protein was characterized by its reduced molecular mass in immunoblots probed with an antiserum against the full-length protein and by fluorography after incubation with [3H]methylated calf thymus DNA. The rate of methyl transfer was virtually identical for the full-length and truncated ATases. The construction of such a truncated, yet still functional, ATase, with a molecular mass of 19.7 kDa should facilitate a detailed n.m.r. structural study and help to determine the functional significance of the C-terminal domain of mammalian ATases.

The Saccharomyces cerevisiae MGT1 DNA repair methyltransferase gene: its promoter and entire coding sequence, regulation and in vivo biological functions

We previously cloned a yeast DNA fragment that, when fused with the bacterial lacZ promoter, produced O6-methylguanine DNA repair methyltransferase (MGT1) activity and alkylation resistance in Escherichia coli (Xiao et al., EMBO J. 10,2179). Here we describe the isolation of the entire MGT1 gene and its promoter by sequence directed chromosome integration and walking. The MGT1 promoter was fused to a lacZ reporter gene to study how MGT1 expression is controlled. MGT1 is not induced by alkylating agents, nor is it induced by other DNA damaging agents such as UV light. However, deletion analysis defined an upstream repression sequence, whose removal dramatically increased basal level gene expression. The polypeptide deduced from the complete MGT1 sequence contained 18 more N-terminal amino acids than that previously determined; the role of these 18 amino acids, which harbored a potential nuclear localization signal, was explored. The MGT1 gene was also cloned under the GAL1 promoter, so that MTase levels could be manipulated, and we examined MGT1 function in a MTase deficient yeast strain (mgt1). The extent of resistance to both alkylation-induced mutation and cell killing directly correlated with MTase levels. Finally we show that mgt1 S.cerevisiae has a higher rate of spontaneous mutation than wild type cells, indicating that there is an endogenous source of DNA alkylation damage in these eukaryotic cells and that one of the in vivo roles of MGT1 is to limit spontaneous mutations.

Isolation and partial characterisation of a Chinese hamster O6-alkylguanine-DNA alkyltransferase cDNA

The cDNA encoding Chinese hamster O6-alkylguanine-DNA-alkyltransferase (ATase) has been isolated from a library prepared from RNA isolated from V79 lung fibroblasts which had an upregulated level of this repair activity following stepwise selection with a chloroethylating agent (1, 2). Expression of the cDNA in E. coli produced functionally active ATase at levels of 2.5% of total cellular protein as determined by in vitro assay. The recombinant hamster protein has a molecular weight of 28 kDa as estimated by SDS-PAGE and fluorography and this was identical to that in the upregulated cells. The characteristic PCHRV pentapeptide of the alkyl acceptor site has been identified and there is a 68 amino acid residue region which is 90% conserved across all the mammalian proteins so far analysed: in contrast, the N- and C-terminal domains diverge by as much as 50% between species. Polyclonal antibodies to the human and rat ATases hybridised to the hamster protein on western analysis suggesting at least one common epitope shared across species. However, in antibody inhibition experiments neither of the antisera cross reacted with the hamster ATase in a way which interfered with functional activity whereas the anti-human antibodies inhibited the human ATase and the anti-rat antibodies inhibited the rat and mouse ATases. There may therefore be significant tertiary structural differences between the hamster protein and the other mammalian ATases.

Critical comparison of consensus methods for molecular sequences

Consensus methods are recognized as valuable tools for data analysis, especially when some sort of data aggregation is desired. Although consensus methods for sequences play a vital role in molecular biology, researchers pay little heed to the features and limitations of such methods, and so there are risks that criteria for constructing consensus sequences will be misused or misunderstood. To understand better the issues involved, we conducted a critical comparison of nine consensus methods for sequences, of which eight were used in papers appearing in this journal. We report the results of that comparison, and we make recommendations which we hope will assist researchers when they must select particular consensus methods for particular applications.