The cell cycle of cultured cells appears to be regulated by opposing actions of the
cyclins together with their partners, the
cyclin-dependent kinases (Cdk), and their inhibitors (Cki). Consistent with this situation null mutations in the genes for
cyclin D1 and Cki p27(Kip1) in mice give opposite phenotypes of
dwarfism and
gigantism. To test their genetic interactions, we generated mice nullizygous for both genes. Correction of
cyclin D1 or p27 null to wild-type phenotypes was observed for many but not all traits. These included, for
cyclin D1(-/-) mice,
body weight, early lethality,
retinal hypoplasia, and male aggressiveness and, for p27(-/-) mice,
body weight,
retinal hyperplasia, and embryo implantation. p27(-/-) traits that were not corrected were the aberrant estrus cycles, luteal cell proliferation, and susceptibility to
pituitary tumors. This mutual correction of these phenotypes is the first genetic demonstration of the interaction of these inhibitory and stimulatory cell cycle-regulatory molecules in vivo. The molecular basis for the correction was analyzed in the neonatal retina.
Retinal cellularity was rescued in the
cyclin D1 null mouse by loss of p27 with only a partial restoration of phosphorylation of
retinoblastoma protein (Rb) and Cdk4 activity but with a dramatic elevation of Cdk2 activity. Our data provide in vivo genetic validation of cell culture experiments that indicated that p27 acts as a negative regulator of
cyclin E-Cdk2 activity and that it can be titrated away by
cyclin D-Cdk4 complexes. It also supports the suggestion that the
cyclin E/Cdk2 pathway can largely bypass Rb in regulating the cell cycle in vivo.