Where do you express your transgene?

Transgenic mice are an effective model to study (not only) gene function in vivo. Whenever you will be so lucky/unlucky to get a gene to study in vivo, a big double-question remain: where and where are you going to express such transgene? The first "where" addresses the position on genome in which you wish to place the transgene expression cassette, the second "where" entails cells/tissues/organs that you want (or not) to be the expression places. Due to biological complexity, often it is impossible to distinguish between the two side of the same question, and it is a fact that conventional transgenesis leaves to the chance the "2 where" question. Because of that, conventional transgenesis results in random integration of transgenes into the mouse genome, and basically this means NO CONTROL. Several attempts have been made to gain a more efficient transgenesis: concerning the genome position some authors choose to surround the expression cassette by insulator sequences (i.e. MAR ), that buffer transgene from (often tissue-specific) enhancer/silencer effects, and from (often tissue-specific) chromatin silencing; other authors tried to find a good place (locus) in the genome in which perform gene-targeting (i.e. Rosa26).
Recently, the lab of Bernd Kinzel (Novartis), published a technology report in Genesis (vol.45), in which the locus of beta-actin was identified as a good dock for gene expression. Beta-actin is a cytoskeletal building-block expressed in almost every mammalian cell, and it is necessary for life, so only heterozygous transgenic can be developed. To better approach the tissue question, they further engineered the locus by placing a floxed-STOP cassette between the beta-actin promoter and the reporter gene (EGFP). What they observed is that transgene expression was efficiently repressed by STOP, but become activated after Cre-mediated excision of the floxed STOP cassette. Obviously several Cre-mice are available in order to drive the spatio-temporal expression of Cre recombinase. In conclusion, reporter genes can be adopted to make new models to facilitate predictable transgene expression in a spatially and temporally controlled manner.

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Jägle, U., Gasser, J., Müller, M., & Kinzel, B. (2007). Conditional transgene expression mediated by the mouse β-actin locus genesis, 45 (11), 659-666 DOI: 10.1002/dvg.20342