High-throughput clonal selection of recombinant CHO cells using a dominant selectable and amplifiable metallothionein-GFP fusion protein.

Transfected mammalian cells can be used for the production of fully processed recombinant proteins for medical and industrial purposes. However, the isolation of high-producing clones is traditionally time-consuming. Therefore, we developed a high-throughput screening method to reduce the time and effort required to isolate high-producing cells. This involved the construction of an expression vector containing the amplifiable gene metallothionein (MT), fused in-frame to green fluorescent protein (GFP). The fusion gene (MTGFP) confers metal resistance similar to that of the wild-type metallothionein and expression can be monitored using either flow cytometry or a fluorometer to measure green fluorescence. Expression of MTGFP acted as a dominant selectable marker allowing rapid and more efficient selection of clones at defined metal concentrations than with the antibiotic G418. Cells harboring MTGFP responded to increasing metal concentrations with a corresponding increase in fluorescence. There was also a corresponding increase in recombinant protein production, indicating that MTGFP could be used as a selectable and amplifiable gene for the coexpression of foreign genes. Using our expression vector encoding MTGFP, we demonstrate a high-throughput clonal selection protocol for the rapid isolation of high-producing clones from transfected CHO cells. We were able to isolate cell lines reaching specific productivities of >10 microg hGH/10(6) cells/day within 4 weeks of transfection. The advantage of this method is that it can be easily adapted for automated procedures using robotic handling systems.