Generation of helper-free amphotropic retroviruses that transduce a dominant-acting, methotrexate-resistant dihydrofolate reductase gene.

We constructed several retroviruses which transduced a mutant dihydrofolate reductase gene that was resistant to methotrexate inhibition and functioned as a dominant selectable marker. The titer of dihydrofolate reductase-transducing virus produced by virus-producing cells could be increased to very high levels by selection of the cells in increasing concentrations of methotrexate. Helper virus-free dihydrofolate reductase-transducing virus was also generated by using a broad-host-range amphotropic retroviral packaging system. Cell lines producing helper-free dihydrofolate reductase-transducing virus with a titer of 4 X 10(6) per ml were generated. These retroviral vectors should have general utility for high-efficiency transduction of genes in cultured cells and in animals.