The cell cycle inhibitor p27Kip1 also has
cyclin-
cyclin-dependent kinase (CDK)-independent functions. To investigate the significance of these functions in vivo, we generated a knock-in mouse in which four amino acid substitutions in the cdkn1b gene product prevent its interaction with
cyclins and CDKs (p27CK-). In striking contrast to complete deletion of the cdkn1b gene, which causes spontaneous
tumorigenesis only in the pituitary, the p27CK-
protein dominantly caused hyperplastic lesions and
tumors in multiple organs, including the lung, retina, pituitary, ovary, adrenals, spleen, and
lymphomas. Moreover, the high incidence of spontaneous
tumors in the lung and retina was associated with amplification of stem/progenitor cell populations. Therefore, independently of its role as a CDK inhibitor, p27Kip1 promoted stem cell expansion and functioned as a dominant oncogene in vivo. Thus, the p27CK- mouse unveils a dual role for p27 during
tumorigenesis: It is a
tumor suppressor by virtue of its
cyclin-CDK regulatory function, and also an oncogene through a
cyclin-CDK-independent function. This may explain why the cdkn1b gene is rarely inactivated in human
tumors, and the p27CK- mouse in which the
tumor suppressor function is lost but the
cyclin-CDK-independent-oncogenic-function is maintained may represent a more faithful model for the widespread role of p27 misregulation in human
cancers than the p27 null.