Overexpression of cyclin A in the mammary glands of transgenic mice results in the induction of nuclear abnormalities and increased apoptosis.

Aberrant expression of several cyclin genes has been demonstrated to be associated with many types of tumors. To determine the capacity of cyclin A to function as an oncogene in vivo, wild-type and mutant cyclin A proteins were specifically overexpressed in the mammary glands of transgenic mice using regulatory sequences from the ovine beta-lactoglobulin gene. Several lines of transgenic mice were generated that expressed human cyclin A or a nondegradable mutant version of human cyclin A, in which the amino-terminal 89 amino acids encompassing the cyclin destruction box were removed. The cyclin A transgene products were localized in the nuclei of mammary epithelial cells, and the transgenic mammary glands had an increase in cyclin A- and cdk2-associated H1 kinase activity. Many mammary epithelial cells in the transgenic glands exhibited nuclear abnormalities, including multinucleation and karyomegaly, which were suggestive of preneoplastic alterations. The abnormalities were more severe in mammary glands of the mutant cyclin A transgenics, which expressed a stabilized cyclin A protein. In situ analysis of mid-lactation mammary gland sections revealed increased numbers of apoptotic cells in the transgenic glands. Double transgenic animals were generated that expressed both the mutant human cyclin A and human cdk2 transgenes, and a more pronounced phenotype resulted. The bigenic mammary glands exhibited focal areas of hyperplasia, as well as a greater incidence of apoptosis than observed in the single transgenic glands, demonstrating in vivo cooperation between these genes in transformation and apoptotic signaling pathways.