Adenovirus-Cre-mediated recombination in mammary epithelial early progenitor cells

The transplantation of primary mammary epithelial cells after adenovirus-Cr e-mediated recombination provides a new approach for the study of specific gene function during mammary gland development and in breast cancer. Most m ammary-gland-specific promoters identified to date are regulated by lactoge nic hormones. They are expressed predominantly in lobuloalveolar cells duri ng pregnancy and lactation, but not during early stages of ductal morphogen esis in the mammary epithelial cell progenitors, which are primarily implic ated in tumorigenesis. In transgenic mice these promoters will continually or repeatedly express Cre depending on the hormonal environment precluding the definition of cell lineages. To circumvent these limitations, we have t aken advantage of the unique regenerative capacity of mammary epithelium to reconstitute a mammary gland in an epithelium-cleared fat pad in conjuncti on with transient Cre expression using recombinant adenovirus in primary cu ltures. This approach was validated using mice carrying reporter constructs that exclusively express the LacZ gene after Cre-mediated deletion of a fl oxed DNA fragment. These studies demonstrated that, following recombination , cells that are marked as genetically manipulated contribute to the recons titution of the mammary gland. The presence of beta -galactosidase-expressi ng cells in serial transplants of the primary outgrowths indicated that ear ly progenitor or stem cells were successfully targeted. With the increased availability of floxed alleles, this approach should greatly facilitate the study of gene function during early stages of mammary gland development an d in breast cancer.