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

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.

Structure-related properties of the mutagenic lesion 6-O-methylguanine in DNA

Chemical probing of the structures of a few very similar 30 base-pair duplexes containing a 6-O-methylguanine (meG) residue at the 16th position reveals that the modified base simultaneously perturbs the helical structure in two ways; it preferentially unstacks the 3' neighbouring base residue (thymine in this study) on the same strand and it unstacks the pyrimidine to which it is base-paired. Depending on its neighbouring 5' base residue and the base-pairing pyrimidine, this perturbation can extend to a few base-pairs in both 3' and 5' directions from the abnormal base-pair. These perturbations can be detected by cleavage at the site for the restriction enzyme MaeII. The unstaking of the C in the meG.C and A.C base-pairs may explain the de novo methylation of these helices by the human DNA-(cytosine-5-)methyltransferase. Interestingly, the kinetics of repair of the 6-O-methylguanine-containing dinucleotides by the cloned human methylguanine methyltransferase appears to be largely determined by the strength of the stacking interaction between the 6-O-methylguanine and the 5' neighbouring base.

Mutagenesis by O6 meG residues within codon 12 of the human Ha-ras proto-oncogene in monkey cells

The first or/and the second guanines of the human Ha-ras codon 12 (normally GGC) were substituted by O6 meG residues and the modified sequence was subsequently introduced into an SV40-based shuttle vector able to replicate in both simian cells and bacteria. After replication in simian COS7 cells (proficient in O6-alkyl-guanine transferase), plasmid DNA was extracted and mutations were screened in E. coli DH5 alpha cells. The vast majority of the mutations induced by O6 meG were G----A transitions. The mutation frequency observed at the second guanine of codon 12 (12G2 position: 3.75% +/- 0.4) was higher than the one observed at the first guanine (12G1 position: 1.09% +/- 0.6). This difference was confirmed by the results obtained when two adjacent O6 meG residues were positioned within codon 12. The higher mutation frequency observed for the 12G2 position could be attributed to differential repair or/and variation in polymerase fidelity. These results are in agreement with animal experiments where alkylating agents gave rise to mutation on G2 position of codon 12.

O6-alkylguanine-DNA alkyltransferase activity in human malignant melanoma

O6-alkylguanine has been known to be the major lesion in DNA for the cytotoxicity of alkylating agents and it is repaired by O6-Alkylguanine-DNA alkyltransferase (O6-AGT). To examine the relation of O6-AGT to the clinical characteristics in malignant melanoma (MM), O6-AGT activity in 13 human MM tissues was measured. The activity in tumor tissues varied widely from 0 to 0.11 pmol/mg protein. The activity in normal skin tissues was lower and less variable than in the tumors. The activity was not related to the tumor size or clinical stage of melanomas, but it was higher in tumors after chemotherapy with alkylating agents than in those without chemotherapy. In metastatic tissues, in primary tumors of the patients with metastases and in tumors of the patients with bad prognosis, the activity was also high. Two of the tumors, having the highest O6-AGT activity, were both transplantable to nude mice. These results suggest two possibilities; melanomas exposed to the alkylating agents may change to have high O6-AGT activity, followed by the resistance to such agents, or O6-AGT in melanomas may be originally diverse. O6-AGT activity in the tumour tissue may represent the effect of alkylating agents and can be used in selecting the methods of therapy.

Isolation and partial characterization of murine O6-alkylguanine-DNA-alkyltransferase: comparative sequence and structural properties

A cDNA encoding murine O6-alkylguanine-DNA-alkyltransferase (ATase) has been sequenced after isolation from total liver RNA by the polymerase chain reaction using oligonucleotide primers derived from the rat ATase cDNA sequence. Functionally active murine ATase protein has been expressed in Escherichia coli at high levels (about 2% of total protein) and purified to apparent homogeneity (molecular mass 26 kDa). In liquid hybridization experiments, anti-human ATase polyclonal antibodies inhibited human but not rat or mouse ATase, whereas anti-rat polyclonal antibodies inhibited rat and mouse but not human ATase. Both antibodies detected all mammalian ATases tested by western analysis so far. These results indicate some common epitopes and at least one unique human epitope. We compared the amino-acid sequence of the murine ATase with those of other mammalian and bacterial ATases. The proteins of this family all have a large domain (approximately 70 amino acids) of highly conserved residues flanking the sequence PCHRV, which contains the alkyl-accepting cysteine residue of the active site. No evidence was found in the sequences for helix-turn-helix, leucine-zipper, or zinc-finger motifs for DNA recognition and binding. Nuclear localization signals (basic-residue-rich regions) could not be uniquely identified in the mammalian members of the family. Outside of the conserved PCHRV region, there were major differences between prokaryotic and eukaryotic proteins at the primary structure level: there was a series of proline-rich motifs, but these also varied between sequences.

Widespread adaptive response against environmental methylating agents in microorganisms

Many bacterial species have adaptive responses which protect against the toxicity and mutagenicity of methylating agents. Induced 3-methyladenine-DNA glycosylase and O6-methylguanine-DNA methyltransferase activities increase the cellular capacity of E. coli, B. subtilis, and M. luteus to repair toxic and mutagenic methylated base derivatives in DNA. The DNA methyltransferase or Ada protein of E. coli regulates the response and is converted into a strong transcriptional activator by self-methylation on repair of a methylphosphotriester in DNA. The multiple functions of the E. coli Ada protein (39 kDa) are split between two proteins, AdaA (24 kDa) and AdaB (20 kDa), in B. subtilis. Proteins (39 kDa) recognised by anti-Ada antibodies are efficiently induced in several enterobacterial species and correlate with increased DNA methyltransferase activities. In contrast, an "Ada-related" protein is only weakly induced in Salmonella typhimurium and no increase in DNA repair activity is detectable. The existence of adaptive responses in diverged bacterial species suggests the frequent occurrence of methylating agents in the environment. Several direct-acting methylating agents which are known to arise in the environment have been shown to induce the response. These include abundantly occurring methyl chloride, the antibiotic streptozotocin, the precursors of the known labile inducers N-methyl-N'-nitrosourea and N-methyl-N'-nitro-N-nitrosoguanidine and as shown in this paper, methyl radicals which may arise by the irradiation or oxidation of methyl compounds.

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.