Methotrexate Resistance in an in vivo Mouse Tumor Due to a Non-Active-Site Dihydrofolate Reductase Mutation

A series of methotrexate (MTX)-resistant L1210 leukemia murine ascites tumors were developed in vivo and analyzed for drug resistance. Three of 20 tumors studied expressed an altered dihydrofolate reductase (DHFR) and each was identical, having a C to T base transition at nucleotide 46 in the DHFR gene as demonstrated by PCR and direct sequencing. This transition results in a Gly to Trp substitution at amino acid 15 of the enzyme. Purified altered enzyme displays significantly lower binding affinity for the antifolates MTX, trimetrexate, edatrexate, and trimethoprim with respective Kivalues 165-, 76-, 30-, and 28-fold higher than values obtained for enzyme isolated from parental tumor (wild-type enzyme). Substrate (dihydrofolate) and cofactor (NADPH) binding is also diminished for the mutant enzyme, although to a lesser extent (17.3- and 3.6-fold higher Km, respectively). Gly-15 is highly conserved for all vertebrate species of DHFR but has no known interaction(s), either directly or indirectly, with bound cofactor, substrate, or inhibitor. Protein molecular modeling reveals that the affected residue is 9-12 Å away from the enzyme active site and located in a region analogous to the mobile Met-20 loop domain characterized for Escherichia coli DHFR.