Transgenic Mice and Animals

The function of genes has traditionally been studied by observing the effects of spontaneous mutations in whole organisms and more recently by analyzing the effects of targeted mutations in cultured cells. The advent of gene cloning and in vivo mutagenesis now make it possible to produce specific mutations in whole animals. Mice with extra copies or altered copies of a gene in their genome can be generated by trangenesis which is now a well established procedure. To produce transgenic mice, a cloned gene is introduced into the mouse genome by microinjection into the male pronucleus of a fertilized egg, which is then implanted into the uterus of a pseudopregnant female mouse. In some of the eggs the injected DNA becomes integrated randomly into the genome, giving rise to a mouse that has an extra genetic element of known structure, the transgene.

The transgene to be studied in detail needs to be introduced onto a stable, well characterized genetic background. However, it is difficult to prepare transgenic embryos successfully in inbred strains of mice, and transgenic mice are routinely prepared in F2 embryos (that is, the embryo formed after the mating of two F1 animals). The transgene must then be bred onto a well characterized genetic background; this requires 10 generations of backcrossing with an inbred strain to assure that the integrated transgene is largely (>99%) free of heterogeneous genes from the founder mouse of the transgenic mouse line.

This technique allows one to study the impact of a newly discovered gene on development, to identify the regulatory regions of a gene required for its normal tissue-specific expression, to determine the effects of its overexpression or expression in inappropriate tissues, and to find out the impact of mutations on gene function. Transgenic mice have been particularly useful in studying the role of T-cell and B-cell receptors in lymphocyte development.