Teratocarcinoma cells as vehicles for mutant and foreign genes

Mouse malignant teratocarcinoma cells, upon injection into early embryos at the blastocyst stage, become integrated into the developing embryo and stably normalized. In the mosaic mice that are formed, tumor-derived cells can give rise to the full range of normal, functional somatic tissues, and also to germ cells from which progeny are obtained. Totipotent mouse teratocarcinoma cells thus provide a new and essentially parasexual means of generating entire animals. The advantages of in vitro and in vivo technologies can now therefore be combined to synthesize mice with experimentally useful mutations. After mutagenization of the teratocarcinoma cells in culture, and selection or screening for the desired mutant phenotype, cells of mutant clones are microinjected into genetically marked blastocysts for further differentiation and full gene expression within the framework of the organism. An example is the isolation, by 6-thioguanine resistance, of cells deficient in hypoxanthine-guanine phosphoribosyl transferase -- the same deficiency that characterizes human patients with the X-linked Lesch-Nyhan disease. In work in progress, these cells have been cycled into genetically marked blastocysts, where the tumor lineage has successfully given rise to fully differentiated tissue contributions in which the enzyme defect persists. Such experiments present numerous possibilities for introducing specific mutations into mice, toward the ends of studying gene mechanisms responsible for differentiation and of producing animal models of human genetic diseases. Teratocarcinoma cells may also serve as vehicles for introducing foreign genetic material into mice in order to facilitate analyses of gene control mechanisms in development and disease.