Figure 1. The recombinase mediated cassette exchange (RMCE) principle and strategy for efficient selection of correctly targeted subclones. A single copy of a tagging cassette is initially integrated into the genome of the cell of choice through random integration or viral mediated delivery of the DNA sequence (the tagging step). A reporter gene can be used to identify tagged cell clones with desired expression levels. The cassette exchange-targeting step through RMCE is performed by co-transfecting an Flp-encoding vector and a targeting vector containing the gene of interest (GOI). The latter is flanked by two heterospecific Flp recognition target sites (FRT) that are identical to those flanking the tagging cassette. Additionally, the vector contains an ATG start codon to restore the nonfunctional Δmarker-gene of the tagging cassette. Once the Flp enzyme has exchanged the tagging for the targeting cassette, the ATG is placed in frame with the marker gene and cells can easily be selected by the addition of the respective marker drug.10 Importantly, because of the incompatibility of the two heterospecific FRT sites, excision reaction of the tagging cassette is excluded.

By tagging the genomic locus of interest, the heterotypic and incompatible recognition targets such as the Flp recognition target (FRT) and FRTmut are introduced into a genomic locus (Figure 1).^6,7 This creates a cassette acceptor allele, i.e., this tagged locus can now be used to integrate and exchange different DNA cassettes of choice.

To target the genomic locus, a targeting vector containing the desired transgene flanked by the same set of heterotypic recognition target sites as the tagged cassette is transfected together with an expression plasmid that encodes for the recombinase. A double reciprocal crossover recombination event will lead to the exchange of the reporter cassette with the cassette of interest, the targeting cassette. This strategy can hence be termed the tag and target strategy.⁸

Figure 2. Timelines to generate tagged high expression clones and constitute a producer cell line after targeting with the gene of interest. Standard protocols to generate conventional producer cell lines rely on the establishment of a different cell line for the expression of a different product. Commonly, more than 24 months are needed for the development and full characterization of a biotechnologically relevant cell line. In contrast, the establishment of different cell lines that express different products with biotechnological value by the application of RMCE technology can be achieved one-month after targeting of an already tagged cell clone. This represents a tremendous advantage and opens up the possibility of creating a panel of cell lines that express different genes of interest in a predictive manner.

The examples given below illustrate the performance of this technology for the establishment and exploitation of biopharmaceutical producer cell lines.