Chromosome-mediated gene transfer with the Chinese hamster ovary cell line

Using an improved method of chromosome-mediated gene transfer, we have investigated transfer of the codominantly expressed methotrexate-resistant dihydrofolate reductase (MtxRIIIdhfr) gene into Chinese hamster ovary (CHO) cell recipients. The frequency of dhfr gene transfer with CHO cells varied considerably from clone to clone, ranging from 4 X 10(-7) to 5 X 10(-5). Using appropriate cell recipients we were able to test for linkage of several genetic markers available in the CHO cell line. For example, the mutation resulting in the auxotrophic glyB-CHO cell line has been reported by others to be linked to the dhfr gene. However, we could not demonstrate cotransfer of these two markers when glyB- recipient cells were treated with MtxRIII chromosomes and transformant clones were selected for either methotrexate-resistance (MtxR) or glycine prototrophy. We conclude that these two genes are not closely linked in the hamster genome. However, the genes for thymidine kinase (tk) and galactokinase (gk), which are known to be linked in mammalian genomes, were found to cotransfer into CHO recipients with a frequency of about 50%.