Applied molecular evolution of O6-benzylguanine-resistant DNA alkyltransferases in human hematopoietic cells.

Applied molecular evolution is a rapidly developing technology that can be used to create and identify novel enzymes that nature has not selected. An important application of this technology is the creation of highly drug-resistant enzymes for cancer gene therapy. Seventeen O(6)-alkylguanine-DNA alkyltransferase (AGT) mutants highly resistant to O(6)-benzylguanine (BG) were identified previously by screening 8 million variants, using genetic complementation in Escherichia coli. To examine the potential of these mutants for use in humans, the sublibrary of AGT clones was introduced to human hematopoietic cells and stringently selected for resistance to killing by the combination of BG and 1,3-bis(2-chloroethyl)-1-nitrosourea. This competitive analysis between the mutants in human cells revealed three AGT mutants that conferred remarkable resistance to the combination of BG and 1,3-bis(2-chloroethyl)-1-nitrosourea. Of these, one was recovered significantly more frequently than the others. Upon further analysis, this mutant displayed a level of BG resistance in human hematopoietic cells greater than that of any previously reported mutant.

Enhanced in vivo repair of O(4)-methylthymine by a mutant human DNA alkyltransferase.

The repair of O(6)-methylguanine (m(6)G) by human O(6)-alkylguanine-DNA alkyltransferase (hAGT) is approximately 5000-fold greater than that for O(4)-methylthymine (m(4)T). To evaluate each adduct's contribution to mutagenesis, we previously created a mutant hAGT with increased specificity for m(4)T in vitro. The mutant and wild-type (WT) hAGT have now been expressed in bacterial strains that allow for the specific detection of A:T-->G:C and G:C-->A:T mutations induced by m(4)T and m(6)G, respectively. After exposure to the mutagenic methylating agent, N-methyl-N'-nitro-N-nitrosoguanidine, A:T-->G:C substitutions were reduced >4-fold in cells expressing the mutant hAGT compared with 1. 1-fold for WT hAGT. G:C-->A:T substitutions were decreased >2.5-fold in cells expressing the mutant hAGT, whereas WT hAGT totally prevented G:C-->A:T mutations. These results demonstrate that the altered substrate specificity of hAGT observed in vitro also occurs in vivo, and that it is responsible for the observed differences in mutations.

Human O(6)-alkylguanine-DNA alkyltransferase: protection against alkylating agents and sensitization to dibromoalkanes.

O(6)-alkylguanine-DNA alkyltransferase (AGT) is a suicide protein that corrects DNA damage by alkylating agents and may also serve to activate environmental carcinogens. We expressed human wild-type and two active mutant AGTs in bacteria that lack endogenous AGT and are also defective in nucleotide excision repair, to examine the ability of the AGTs to protect Escherichia coli from DNA damage by different types of alkylating agents and, oppositely, to sensitize cells to the genotoxic effects of dibromoalkanes (DBAs). Control bacteria carrying the cloning vector alone were extremely sensitive to mutagenesis by low, noncytotoxic doses of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Expression of human wild-type AGT prevented most of this enlarged susceptibility to MNNG mutagenesis. Oppositely, cell killing required much higher MNNG concentrations and prevention by wild-type AGT was much less effective. Mutants V139F and V139F/P140R/L142M protected bacteria against MNNG-induced cytotoxicity more effectively than the wild-type AGT, but protection against the less stringent mutagenesis assay was variable. Subtle differences between wild-type AGT and the two mutant variants were further revealed by assaying protection against mutagenesis by more complex alkylating agents, such as N-ethyl-N-nitrosourea and 1-(2-chloro- ethyl)-3-cyclohexyl-1-nitrosourea. Unlike wild-type and V139F, the triple mutant variant, V139F/P140R/L142M was unaffected by the AGT inhibitor, O(6)-benzylguanine. Wild-type AGT and V139F potentiated the genotoxic effects of DBAs; however, the triple mutant virtually failed to sensitize the bacteria to these agents. These experiments provide evidence that in addition to the active site cysteine at position 145, the proline at position 140 might be important in defining the capacity by which AGTs modulate genotoxicity by environmentally relevant DBAs. The ability of AGTs to activate dibromoalkanes suggests that this DNA repair enzyme could be altered, and if expressed in tumors might be lethal by enhancing the activation of specific chemotherapeutic prodrugs.

Redesigning the substrate specificity of human O(6)-alkylguanine-DNA alkyltransferase. Mutants with enhanced repair of O(4)-methylthymine.

Human O(6)-alkylguanine-DNA alkyltransferase (MGMT) repairs potentially cytotoxic and mutagenic alkylation damage at the O(6)-position of guanine and the O(4)-position of thymine in DNA. We have used random sequence mutagenesis and functional complementation to obtain human MGMT mutants that are resistant to the MGMT inhibitor, O(6)-benzylguanine [Encell, L. P., Coates, M. M., and Loeb, L. A. (1998) Cancer Res. 58, 1013-1020]. Here we describe screening of O(6)-benzylguanine-resistant mutants for altered substrate specificity, i.e., for an increased level of utilization of O(4)-methylthymine (m(4)T) relative to that of O(6)-methylguanine (m(6)G). One mutant identified by the screen, 56-8, containing eight substitutions near the active site (C150Y, S152R, A154S, V155G, N157T, V164M, E166Q, and A170T), was purified and characterized kinetically. The second-order rate constant for repair of m(4)T by the mutant was up to 11.5-fold greater than that of WT MGMT, and the relative m(4)T specificity, k(m(4)T)/k(m(6)G), was as much as 75-fold greater. In competition experiments with both substrates present, the mutant was 277-fold more sensitive to inhibition by m(4)T than WT MGMT. This mutant, and others like it, could help elucidate the complex relationship between adduction at specific sites in DNA and the cytotoxicity and mutagenicity of alkylating agents.

Improving enzymes for cancer gene therapy.

New techniques now make it feasible to tailor enzymes for cancer gene therapy. Novel enzymes with desired properties can be created and selected from vast libraries of mutants containing random substitutions within catalytic domains. In this review, we first consider genes for the ablation of tumors, namely, genes that have been mutated (or potentially can be mutated) to afford enhanced activation of prodrugs and increased sensitization of tumors to specific chemotherapeutic agents. We then consider genes that have been mutated to provide better protection of normal host tissues, such as bone marrow, against the toxicity of specific chemotherapeutic agents. Expression of the mutant enzyme could render sensitive tissues, such as bone marrow, more resistant to specific cytotoxic agents.

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.

Creating novel enzymes by applied molecular evolution.

Recent molecular techniques have made it feasible to simulate evolutionary processes and apply in vitro selection to evolve enzymes with novel properties that may have potential benefits for medical and industrial applications. The characterization of such mutants has also provided new insights into how molecular structure determines enzyme function.