Engineering human DNA alkyltransferases for gene therapy using random sequence mutagenesis

O6-Alkylguanine-DNA alkyltransferase (AGT) is a suicide enzyme that repairs alkylation damage at the O6 position of guanine in DNA. The essentiality of a limited number of amino acid residues at the active site has been determined by site-directed mutagenesis. We used random mutagenesis techniques to create a plasmid library of > 10(6) human AGT mutants with substitutions at residues 150-172 and selected for clones with mutations rendering Escherichia coli resistant to both the alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and the AGT inhibitor, O6-benzylguanine (BG). On sequencing surviving clones resistant to MNNG in the presence and absence of BG, we found that a majority of the clones contained multiple substitutions at mostly nonconserved positions. We selected nine mutants resistant to a combination of MNNG and BG, and the survival of these mutants was as much as 341-fold higher than that of cells harboring wild-type AGT under the same conditions. Each of the mutants contained at least three amino acid substitutions and as many as eight, suggesting that maximum resistance to MNNG in the presence of BG requires even more substitutions than resistance to MNNG alone. BG is being tested clinically as a way to sensitize tumors to chemotherapeutic alkylating agents. Therefore, our BG-resistant mutants hold strong potential as gene therapy candidates for protecting normal tissue in patients receiving BG in combination with alkylating agents for the treatment of cancer.