Repair of O6-methylguanine in DNA by demethylation is lacking in Mer− human tumor cell strains

Abstract

The ability of extracts of human tumor cells to demethylate O6-methylguanine (O ⁶ -MeG) in DNA was assayed using the synthetic DNA polymer poly(dC,dG,m ⁶ dG). Cell strains proficient in repair of O ⁶ -MeG in vivo (Mer+ phenotype) contained a methyltransferase activity while repair deficient cells (Mer ⁻ phenotype) had little or no activity. Mixing extracts of different Mer ⁻ strains did not result in the appearance of the activity. Extracts of Mer ⁻ cells did not inhibit the activity in extracts of Mer ⁺ cells. Both Mer ⁺ and Mer ⁻ strains contained methylnitrosourea-damage-specific endonudease activity. The data suggest that the Mer ⁺ strains are deficient in methyltransferase and that this is the fundamental reason for their hypersensitivity to the cytotoxic effects of DNA alkyla-tion. The activity was partially purified from a Mer ⁺ colon carcinoma cell strain. Its kinetics parallel the repair of O ⁶ -MeG in DNA in vivo and suggest that the activity is inactivated during repair of DNA.