O6-methylguanine methyltransferase activity and sensitivity of Japanese tumor cell strains to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)- 3-nitrosourea hydrochloride.

Using 40 tumor cell strains derived from various organs of Japanese tumor patients and also 12 normal cell strains, we have measured the activity of O6-methylguanine-DNA methyltransferase (MT), which can repair O6-methylguanine produced in DNA by alkylating agents. Then, the lethal sensitivities of the strains to the anti-tumor drug 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-)2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) were measured. The MT activity was assayed by measuring the 3H radioactivity transferred from the substrate DNA containing [methyl-3H]-O6-methylguanine to acceptor molecules in the cell extracts. Extracts from the majority of tumor strains and all normal cell strains contained substantial MT activity of varying degree, while the extracts of 6 tumor strains showed virtually undetectable MT activity. Hence these 6 strains were assigned as Mer-, the phenotype which is characterized by the inability to repair O6-methylguanine in DNA due to the lack of MT. The Mer- tumor strains were much more sensitive to ACNU than the rest of Mer+ strains, as measured by colony-forming ability. Furthermore, with all the tumor and normal strains tested, a good correlation was observed between MT activity and cellular resistance to ACNU. These results indicate that the frequency of Mer- strain is about 15% among Japanese tumor cell strains so far analyzed, and further suggest that MT may be the only system able to repair lethal DNA damage induced by ACNU.