PURINE SALVAGE RESCUE BY XANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE (XGPRT) POTENTIATES METHOTREXATE RESISTANCE CONFERRED BY TRANSFER OF A MUTATED DIHYDROFOLATE-REDUCTASE GENE

We have previously shown that successful gene transfer of a mutated di hydrofolate reductase (DHFR) cDNA confers resistance to methotrexate ( MTX) upon infected cells. We constructed a retrovirus vector, DC/SV6S3 1GPT, which carries both the Escherichia coli xanthine-guanine phospho ribosyltransferase gene and the mutated Serine 31 DHFR gene. Mouse fib roblast NIH3T3 cells infected with DC/SV6S31 GPT are more resistant to MTX than cells infected with DC/SV6S31, which carries the Serine 31 D H FR and the neomycin resistance gene cDNA. The mechanism of this augm ented resistance is the increased salvaging of purines due to expressi on of xanthine-guanine phosphoribosyltransferase, as the augmentation does not occur when dialyzed serum, containing little xanthine or guan ine, is used for cytotoxicity assays. These results indicate that coex pression of a metabolically related gene can potentiate the resistance carried by a drug resistance gene. This vector may be useful in clini cal gene therapy to protect bone marrow from the toxic effects of MTX.