Relationship of DNA repair phenotypes of human fibroblast and tumor strains to killing by N-methyl-N'-nitro-N-nitrosoguanidine.

Abstract

Two DNA repair assays were used to group human cells. (a) The first assay, survival of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-treated adenovirus infecting cellular monolayers, was previously used to define the Mer phenotype of the strain. Strains that supported the growth of MNNG-treated viruses as well as did human fibroblasts were "Mer+"; those that gave rise to clearly less virus survival were "Mer-." (b) The second assay, data from which are presented in this paper, was that of post-MNNG colony-forming ability, and defined the Rem phenotype of the strain. Strains having post-MNNG colony-forming ability like that of human fibroblasts were "Rem+"; more sensitive strains were "Rem-". In all, 22 human cell strains were analyzed for their post-MNNG colony-forming ability. The most resistant strains (eight Mer+ Rem+ strains) had an average inactivation slope of 0.32 "lethal hit"/microM and were those fully able to repair O6-methylguanine (O6mGua) produced in their DNA by a 5 microM dose of MNNG. The most sensitive strains (9 Mer- Rem- strains) had an average inactivation slope of 7.0 "lethal hits"/microM, and were strains that failed to repair O6mGua. Five strains of intermediate sensitivity (Mer+ Rem-) had an average inactivation slope of 0.93 "lethal hit"/microM and were able to repair some labeled O6mGua produced by a 5 microM dose of labeled MNNG, but they repaired significantly less labeled O6mGua if pretreated with unlabeled MNNG. Representative strains from each group were treated with MNNG and assayed for ability: (a) to perform DNA repair synthesis (and DNA repair replication); (b) to support the growth of MNNG-treated adenoviruses; and (c) to restore control levels of tertiary structure to their DNA as assayed by nucleoid sedimentation. The results support the hypothesis that a lesion (both produced by agents that produce O6mGua and repaired by cell strains that repair O6mGua, but not by those that do not) is a lesion lethal to Mer- Rem- strains. This lesion may also initiate induction of excess DNA repair synthesis, the relaxed conformation of nucleoids, the reduced ability to repair MNNG-treated adenovirus, and sister chromatid exchanges as well.