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 b e associated with many types of tumors. To determine the capacity of c yclin A to function as an oncogene in vivo, wild-type and mutant cycli n A proteins were specifically overexpressed in the mammary glands of transgenic mice using regulatory sequences from the ovine beta-lactogl obulin gene. Several lines of transgenic mice were generated that expr essed 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 lo calized in the nuclei of mammary epithelial cells, and the transgenic mammary glands had an increase in cyclin A- and cdk2-associated H1 kin ase activity. Many mammary epithelial cells in the transgenic glands e xhibited nuclear abnormalities, including multinucleation and karyomeg aly, which were suggestive of preneoplastic alterations. The abnormali ties were more severe in mammary glands of the mutant cyclin A transge nics, 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 we re generated that expressed both the mutant human cyclin A and human c dk2 transgenes, and a more pronounced phenotype resulted. The bigenic mammary glands exhibited focal areas of hyperplasia, as well as a grea ter incidence of apoptosis than observed in the single transgenic glan ds, demonstrating in vivo cooperation between these genes in transform ation and apoptotic signaling pathways.