Gene expression variability between randomly and targeted transgene integration events in tobacco suspension cell lines

Genome editing tools such as zinc-finger nucleases provide novel strategies for genetic manipulation in plants. Unlike agrobacterium-mediated or direct gene transfer, which introduce genes randomly into the genome and thereby potentially resulting in high variation of gene expression, the targeted gene addition provides predictable integration of DNA sequences into a specified location of the plant genome. We investigated whether various independent cell lines that all contain a transgene placed in the same genomic locus by zinc-finger nuclease-mediated homologous recombination (HR) would yield a more reproducible and homogeneous level of expression compared to integration events generated via agrobacterium-mediated transformation at random sites. The variance of gene expression of targeted HR events and random integration events was analyzed in Nicotiana tabacum L cv. Bright Yellow 2 (BY-2) suspension cells by measuring protein amount produced from the transgene by flow cytometry, thus providing the first report on positional effects of marker gene expression in a quickly proliferating plant suspension cell line. Marker protein levels of targeted HR and single-copy random events covered a similar range; however, the uniformity of protein expression in a given cell line was significantly higher in targeted events than in lines with randomly inserted transgene; the same is true for the overall viability of protoplasts from HR lines. In conclusion, using targeted insertion into a qualified locus of a well-characterized line leads to more reliable results than random insertion into the genome.