Quantitative assessment of the role of O6-methylguanine in the initiation of carcinogenesis by methylating agents.

Abstract

Induction of transformation, cell lethality and DNA lesions were quantitatively compared in Syrian hamster embryo cells (HEC) treated with 3 different methylating agents: N-methyl-N''-nitro-N-nitrosoguanidine (MNNG), N-methyl-N-nitrosourea (MNU), or methyl methanesulfonate (MMS). Each induced transformation in a dose-dependent manner. On a molar basis, MNNG was .simeq. 100- and 500-fold more effective than MNU and MMS, respectively. For each carcinogen the induction and repair of O6- and N7-methylguanine (O6- and N7-MeGua) relative to total guanine content was compared. At concentrations that induced equivalent transformation frequencies, the induction of O6-MeGua was the same for all 3 carcinogens, but N7-MeGua induction was 30-fold higher with MMS than with MNNG or MNU. The capacity to repair methylation lesions in HEC is limited because only between 50-70% of both O6- and N7-MeGua lesions were removed from the DNA within 24 h after treatment, independent of methylating carcinogen. No consistent effect on either the rate of DNA replication or the size distribution of nascent strands correlated with O6-MeGua induction. O6-MeGua is the critical lesion for initiation of carcinogenesis by methylating agents. The frequency of transformation relative to O6-MeGua induction is 40- to 750-fold more than that of mutation. Based on the quantitative data for induction of O6-MeGua and transformation, the target size for initiation of carcinogenesis was calculated as a minimum of 104 nucleotides. One of many genes can initiate carcinogenesis or initiation is not the result of a single base mutation.