Cell type-specific conditional regulation of the c-myc proto-oncogene by combining Cre/loxP recombination and tamoxifen-mediated activation.

Development of inducible genetic switches for in vivo use with transgenic mice has revolutionized many areas in modern molecular biology. Combining two techniques, Cre/loxP-based genetic recombination and ligand-dependent activation of a chimeric protein, we generated transgenic mice which allow for the spatiotemporal control of expression and of activity of the proto-oncogene c-myc. To these ends, the gene encoding the tamoxifen-inducible c-mycER(T) fusion protein (mycER(T)) was inserted in the ubiquitously active ROSA 26 gene locus by gene targeting. In the resulting ROSAMER allele, generalized transcription of the mycER(T) gene is prevented by a preceding transcriptional stop sequence which is flanked by loxP sites. Crosses of ROSAMER transgenic mice with Mox2 cre transgenic mice revealed tight control of mycER(T) transcription in various tissues unless the transcriptional stop sequence was removed by cre-mediated excision. Furthermore, we were able to demonstrate tamoxifen-dependent activation of the MycER(T) protein in embryonic fibroblasts derived from such mice. As a proof of principle, we demonstrate that primary neural crest cultures established from ROSAMER mice maintain their proliferative capacity in a 4-OHT-dependent manner. Furthermore, we demonstrate that such neural crest cells retain their differentiation potential as shown by expression of NF 160, a marker of neuronal differentiation upon 4-OHT withdrawal. The transgenic mice produced may thus be valuable tools for studying the cell type-specific effects of c-myc activity in development and disease.