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

The ability of extracts of human tumor cells to demethylate O⁶-methylguanine (O⁶-MeG) in DNA was assayed using the synthetic DNA polymer poly(dC,dG,m⁶dG). Cell strains proficient in repair of O⁶-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 endonuclease 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 alkylation. The activity was partially purified from a Mer⁺ colon carcinoma cell strain. Its kinetics parallel the repair of O⁶-MeG in DNA in vivo and suggest that the activity is inactivated during repair of DNA.