2-Aminopurine as a probe for mismatch repair in mammalian cells and its relationship to DNA methylation.

We have used a series of clonally related mammalian cell lines with different levels of DNA methylation and the known inducer of DNA mismatch repair in Escherichia coli, 2-aminopurine (2AP), to test for the presence of methylation-directed DNA repair in mammalian cells. While a number of these hypomethylated clones showed increased sensitivity to 2AP, another similarly hypomethylated clone was resistant. DNA replication and both the immediate and delayed classes of DNA methylation were strongly inhibited and to similar extents in both sensitive and resistant clones. The inclusion of deoxycytidine reversed most of the 2AP-induced inhibition of replication and methylation without reducing 2AP toxicity. While 2AP-induced DNA mismatches are repaired by a methylation-directed process in E. coli, this analogue has major secondary effects in mammalian cells unrelated to its toxicity. Genomic methylation levels are also not a determinant of resistance or sensitivity to this base analogue in mammalian cells.