Characterization of cDNA encoding mouse DNA repair protein O6-methylguanine-DNA methyltransferase and high-level expression of the wild-type and mutant proteins in Escherichia coli

The alkyltransferase-like ybaZ gene product enhances nucleotide excision repair of O(6)-alkylguanine adducts in E. coli

O(6)-methylguanine adducts are potent pre-mutagenic lesions owing to their high capacity to direct mis-insertion of thymine when bypassed by replicative DNA polymerases. The strong mutagenic potential of these adducts is prevented by alkyltransferases such as Ada and Ogt in Escherichia coli that transfer the methyl group to one of their cysteine residues. Alkyl residues larger than methyl are generally weak substrates for reversion by alkyltransferases. In this paper we have investigated the genotoxic potential of the O(6)-alkylguanine adducts formed by ethylene and propylene oxide using single-adducted plasmid probes. Our work shows that the ybaZ gene product, a member of the alkyltransferase-like protein family, strongly enhances the repair by nucleotide excision repair of the larger O(6)-alkylguanine adducts that are otherwise poor substrates for alkyltransferases. The YbaZ protein is shown to interact with UvrA. This factor may thus enhance the efficiency of nucleotide excision repair in a way similar to the Transcription-Repair Coupling factor Mfd, by recruiting the UvrA(2).UvrB complex to the adduct site via its interaction with UvrA.

MGMT: a personal perspective

This review describes the history of studies on alkylation damage of mammalian genomes and its carcinogenic consequences that led to the discovery of a unique DNA repair protein, named MGMT. MGMT repairs O(6)-alkylguanine, a critical mutagenic lesion induced by alkylating agents. The follow-up studies in mammalian cells following the discovery of the ubiquitous repair protein in E. coli are summarized.

Interaction of mammalian O(6)-alkylguanine-DNA alkyltransferases with O(6)-benzylguanine

Human O(6)-alkylguanine-DNA alkyltransferase (hAGT) activity is a major factor in providing resistance to cancer chemotherapeutic alkylating agents. Inactivation of hAGT by O(6)-benzylguanine (BG) is a promising strategy for overcoming this resistance. Previous studies, which have focused on the region encompassed by residues Pro138 to Gly173, have identified more than 100 individual mutations located at 23 discrete sites at which alterations can render AGT less sensitive to BG. We have now extended the examination of possible sites in hAGT at which alterations might lead to BG resistance to include the residues from Val130 to Asn137, which also make up part of the binding pocket into which BG is postulated to fit. A further 21 mutations located at positions Gly132, Met134, Arg135, and Gly136 were found to lower sensitivity to BG. Mutants R135L, R135Y, and G136P were the most strikingly resistant, with a 50-fold increase in the amount of BG needed to obtain 50% inactivation. These results therefore increase the number of sites at which BG resistance can occur in response to a single amino acid change to 27. Although mammalian AGTs are very similar in amino acid sequence, mouse AGT (mAGT) is significantly less sensitive to BG than rat AGT (rAGT) or hAGT. Construction of chimeric proteins in which portions came from the rAGT and the mAGT indicated that the difference in inactivation resided solely in the amino acids located in the sequence from residues 150 to 188. Individual mutations of the three residues where rAGT and mAGT differ in this region showed that the principal reason for the reduced ability of the mAGT to react with BG was the presence of a histidine residue at position 161, which is occupied by asparagine in rAGT and hAGT. These experiments indicate that many minor changes in amino acids forming all parts of the nucleoside binding pocket of AGT can alter its ability to react with BG and that the possibility that polymorphisms or variants may occur reducing the effectiveness of combination therapy with BG and alkylating agents must be considered.

Reduction of BCNU toxicity to lung cells by high-level expression of O(6)-methylguanine-DNA methyltransferase

1,3-Bis(2-chloroethyl)-1-nitrosourea (BCNU) is an important cause of pulmonary toxicity. BCNU alkylates DNA at the O(6) position of guanine. O(6)-methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that removes alkyl groups from the O(6) position of guanine. To determine whether overexpression of MGMT in a lung cell reduces BCNU toxicity, the MGMT gene was transfected into A549 cells, a lung epithelial cell line. Transfected A549 cell populations demonstrated high levels of MGMT RNA, MGMT protein, and DNA repair activity. The overexpression of MGMT in lung epithelial cells provided protection from the cytotoxic effects of BCNU. Control A549 cells incubated with 100 microM BCNU had a cell survival rate of 12.5 +/- 1.2%; however, A549 cells overexpressing MGMT had a survival rate of 71.8 +/- 2.7% (P < 0.001). We also demonstrated successful transfection of MGMT into human pulmonary artery endothelial cells and a primary culture of rat type II alveolar epithelial cells with overexpression of MGMT, resulting in significant protection from BCNU toxicity. These data suggest that overexpression of DNA repair proteins such as MGMT in lung cells may protect the lung cells from cytotoxic effects of cancer chemotherapy drugs such as BCNU.

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.

Amplification of the DNA repair gene O6-methylguanine-DNA methyltransferase associated with resistance to alkylating drugs in a mammalian cell line

The cytotoxic action of such alkylating chemotherapeutic drugs as 2-chloroethyl-N-nitrosourea (CNU) derivatives is countered by the repair protein O6-methylguanine-DNA methyltransferase (MGMT), which removes O6-alkylguanine induced in the DNA by these agents. Resistance to these drugs is often correlated with the MGMT levels in normal and tumor cells of human and rodent origin. Exposure of mouse 3T3 cells to increasing concentrations of CNU, and subsequent selection of resistant cells, led to the isolation of clones with 5-10 times higher levels of MGMT activity than in the control. The increased MGMT expression at both mRNA and protein levels resulted from 5- to 10-fold amplification of the Mgmt gene. Amplification of this gene was not associated with concomitant amplification of another alkylation damage repair gene, N-methylpurine-DNA glycosylase. No amplification of at least three other genes on chromosome 7 (which contains the Mgmt gene) was observed in the drug-resistant cells. Furthermore, the amplified Mgmt sequence was not associated with a homogeneously staining region, or double minute chromosomes, nor present as episomal DNA. In situ hybridization of metaphase chromosomes of the drug-resistant cells indicated both translocation and localized amplification of the Mgmt gene.

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.

Purification and biochemical characterization of recombinant N-methylpurine-DNA glycosylase of the mouse

The mouse N-methylpurine-DNA glycosylase (MPG), responsible for the removal of most N-alkyladducts in DNA, was purified to homogeneity as a recombinant nonfusion protein from Escherichia coli. Only 10-15% of the protein was present in the soluble form in E. coli cells. The N-terminal amino acid sequence of the purified protein which lacks 48 residues from the amino terminus of the wild type protein was identical to that predicted from the nucleotide sequence. The glycosylase hydrolyzes 3-methyladenine (m3A), 7-methylguanine(m7G), and 3-methylguanine (m3G) from DNA, and the Km and kcat values were 130 nM and 0.8 min-1 for m3A, and 860 nM and 0.2 min-1 for m7G, respectively, when methylated calf thymus DNA was used as the substrate. A comparison of kcat/Km values for different bases indicates that the enzyme was more efficient in excising both m3A and m3G than m7G from methylated DNA. The enzyme showed moderate binding affinities (KA) for both methylated (5.8 x 10(7) M-1) and nonmethylated DNAs (4.2 x 10(7) M-1). The mouse protein has an extinction coefficient E280nm1% of 10.5 and a pI of 9.3. The enzyme activity was optimal in the presence of 100 mM NaCl, with a broad pH optimum of 8.5-9.5. The enzymatic release of both m3A and m7G was stimulated 50-75% by 0.5 mM MgCl2 and 0.02 mM spermine but inhibited by higher concentrations of these agents. Product inhibition by 40-50% of the reaction occurred in the presence of 10 mM m3A or m7G. However, 1.0 mM m3A stimulated release of m7G. The enzyme was inhibited by 60% in the presence of 0.9 mg/mL DNA which, at the same time, protected it from thermal inactivation.

Requirement of the Pro-Cys-His-Arg sequence for O6-methylguanine-DNA methyltransferase activity revealed by saturation mutagenesis with negative and positive screening

O6-Methylguanine-DNA methyltransferase catalyzes transfer of a methyl group from O6-methylguanine and O4-methylthymine of DNA to a cysteine residue of the enzyme protein, thereby repairing the mutagenic and carcinogenic lesions in a single-step reaction. There are highly conserved amino acid sequences around the methyl-accepting cysteine site in eleven molecular species of methyltransferases. To elucidate the significance of the conserved sequence, amino acid substitutions were introduced by site-directed mutagenesis of the cloned DNA for Escherichia coli Ogt methyltransferase, and the activity and stability of mutant forms of the enzyme were examined. When cysteine-139, to which methyl transfer occurs, was replaced by other amino acids, all of the mutants showed the methyltransferase-negative phenotype. Methyltransferase-positive revertants, isolated from one of the negative mutants, had restored codons for cysteine. Thus the cysteine residue is essential for acceptance of the methyl group and is not replaceable by other amino acids. Using this negative and positive selection procedure, the analysis was extended to other residues near the acceptor site. At the histidine-140 and arginine-141 sites, all the positive revertants isolated carried codons for amino acids identical to those of the wild-type protein. At proline-138, five substitutions (serine, glutamine, threonine, histidine, and alanine) exhibited the positive phenotype but levels of methyltransferase activity in extracts of cells harboring these mutant forms were very low. This suggests that the proline residue at this site is important for maintaining the proper conformation of the protein. With valine-142 substitutions there were seven types of positive revertants, among which mutants carrying isoleucine, cysteine, leucine, and alanine showed relatively high levels of methyltransferase activity. These results indicate that the sequence Pro-Cys-His-Arg is a sine qua non for methyltransferase to exert its function.

Specificities of human, rat and E. coli O6-methylguanine-DNA methyltransferases towards the repair of O6-methyl and O6-ethylguanine in DNA

The behaviour of highly purified bacterial expressed rat O6-methylguanine-DNA methyltransferase (MGMT) towards the repair of CGCm6GAGCTCGCG and CGCe6GAGCTCGCG (km6G/ke6G = 1.45, where k is the second order repair rate constant determined, m6G and e6G are O6-methyl and O6-ethylguanine) is similar to that of E. coli 39kD Ada protein (km6G/ke6G = 1.6). However, the human MGMT is very different (km6G/ke6G = 163). The preferential repair of O6-ethylguanine lesion by the rat MGMT appears not to be related to the lack of the initiator methionine in the expressed protein since similar results were obtained from N-terminal Glutathione-S-transferase (GST) fused protein (GSTMGMT) which retains the methionine. The possible relationship between these findings and the differences observed in the primary amino acid sequence of these proteins is discussed. In addition the preferential repair of O6-ethylguanine substrate by the 39kD Ada protein as compared to the catalytic C-terminus alone (different by 134 times) suggests that the N-terminus plays a crucial role in the repair of O6-ethylguanine. This is in contrast to the minor effects of the GST domain when fused to the N-terminus of mammalian MGMT.

Site directed mutagenesis of two cysteine residues in the E. coli ogt O6-alkylguanine DNA alkyltransferase protein

The E. coli ogt O6-alkylguanine-DNA alkyltransferase has two cysteine residues positioned identically with respect to cysteines in the E. coli ada O6-alkylguanine-DNA alkyltransferase. In order to assess their function, these residues were each substituted by a glycine to generate altered forms of the ogt protein. Mutagenesis of cysteine-139, located within a 'PCHRV' region of homology, eliminated functional activity confirming that this residue is the methyl-accepting cysteine in the active site of the protein. Substitution of cysteine 102 within the sequence 'LRTIPCG' had little effect on the ogt protein activity demonstrating that this cysteine is not directly involved with the transfer of O6-methylguanine adducts.

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.