Mechanisms of carcinogenicity/chemotherapy by O6-methylguanine

Alkylating agents are a structurally diverse group of compounds that cause a wide range of biological effects, including cell death, mutation and cancer. DNA damaged by these agents contains widely different amounts of 12 alkylated purines/pyrimidines and two phosphotriester isomers. The biological effects appear to be mediated predominantly by attack at the O(6) position of guanine. DNA extracted from various normal human tissues contains detectable levels of O(6)-alkylguanine, the source of which has not been defined. Given that, following DNA replication, this lesion cannot only generate point mutations but can also initiate mismatch repair-mediated DNA recombination and cell death, it seems worthwhile to consider the possible contribution of these events and cell killing to the aetiology of human cancer. There is increasing evidence that point mutations are not the only mechanism involved in malignant transformation by alkylating agents. Some cancer chemotherapeutic agents exploit the cytotoxic effects of O6-alkylguanine and an understanding of the processing of this lesion has allowed strategies to be developed that should increase the effectiveness of such agents

Variability and regulation of O6-alkylguanine-DNA alkyltransferase

O6-Alkylguanine-DNA alkyltransferase (ATase) confers resistance to many of the biological effects of certain classes of alkylating agents by repairing the DNA lesions responsible. The role of ATase in the mutagenic and toxic effects of the carcinogenic and antitumour alkylating agents are of interest in relation to the prevention and treatment of cancer in man. In this commentary we specifically focus on the variation in ATase levels and our current understanding of the factors involved in the regulation of ATase expression

A novel DNA damage recognition protein in Schizosaccharomyces pombe

Toxic and mutagenic O(6)-alkylguanine adducts in DNA are repaired by O(6)-alkylguanine-DNA alkyltransferases (MGMT) by transfer of the alkyl group to a cysteine residue in the active site. Comparisons in silico of prokaryotes and lower eukaryotes reveal the presence of a group of proteins [alkyltransferase-like (ATL) proteins] showing amino acid sequence similarity to MGMT, but where the cysteine at the putative active site is replaced by tryptophan. To examine whether ATL proteins play a role in the biological effects of alkylating agents, we inactivated the gene, referred to as atl1(+), in Schizosaccharomyces pombe, an organism that does not possess a functional MGMT homologue. The mutants are substantially more susceptible to the toxic effects of the methylating agents, N-methyl-N-nitrosourea, N-methyl-N′nitro-N-nitrosoguanidine and methyl methanesulfonate and longer chain alkylating agents including N-ethyl-N-nitrosourea, ethyl methanesulfonate, N-propyl-N-nitrosourea and N-butyl-N-nitrosourea. Purified Atl1 protein does not transfer methyl groups from O(6)-methylguanine in [(3)H]-methylated DNA but reversibly inhibits methyl transfer by human MGMT. Atl1 binds to short single-stranded oligonucleotides containing O(6)-methyl, -benzyl, -4-bromothenyl or -hydroxyethyl-guanine but does not remove the alkyl group or base and does not cleave the oligonucleotide in the region of the lesion. This suggests that Atl1 acts by binding to O(6)-alkylguanine lesions and signalling them for processing by other DNA repair pathways. This is the first report describing an activity that protects S.pombe against the toxic effects of O(6)-alkylguanine adducts and the biological function of a family of proteins that is widely found in prokaryotes and lower eukaryotes