Cell-Type-Specific Transgenesis in the Mouse

Since the early 1980s, when the first transgenic mice were generated, thousands of genetically modified mouse lines have been created. Early on, Jaenisch established proof of principle, showing that viral integration into the mouse genome and germline transmission of those exogenous sequences were possible (Proc Natl Acad Sci USA 71:1250–1254, 1974). Gordon et al. (Proc Natl Acad Sci USA 77:7380–7384, 1980) and Brinster et al. (Cell 27:223–231, 1981) subsequently used cloned genes to create “transgenic constructs” in which the exogenous DNA was randomly inserted into different sites in the mouse genome, stably maintained, and transmitted through the germline to the progeny. The utility of the process quickly became apparent when a transgene carrying the metallothionein-1 (Mt-1) promoter linked to thymidine kinase was able to drive expression in the mouse liver when promoter activity was induced by administration of metals. In an attempt to find stronger and more reliable promoters, viral promoter elements from SV40 or cytomegalovirus were incorporated. However, while these promoters were able to drive high levels of expression, for many applications they proved to be too blunt an instrument as they drove ubiquitous expression in many, if not all cell types, making it very hard to discern organ-specific or cell-type-specific effects due to transgene expression. Thus the need to find cell-type-specific promoters that could reproducibly drive high levels of transgene expression in a particular cell type, e.g., cardiomyocyte, became apparent. One such example is the α myosin heavy-chain (MHC) promoter, which has been used extensively to drive transgene expression in a cardiomyocyte-specific manner in the mouse. This chapter, while not written as a typical methods section, will describe the necessary components of the α myosin promoter. In addition, common problems associated with transgenic mouse lines will be addressed.