Wide spectrum of tumors in knock-in mice carrying a Cdk4 protein insensitive to INK4 inhibitors

We have introduced a point mutation in the first coding exon of the locus e ncoding the cyclin-dependent kinase 4 (Cdk4) by homologous recombination in embryonic stem cells. This mutation (replacement of Arg24 by Cys) was firs t found in patients with hereditary melanoma and renders Cdk4 insensitive t o INK4 inhibitors. Here, we report that primary embryonic fibroblasts expre ssing the mutant Cdk4R24C kinase are immortal and susceptible to transforma tion by Ras oncogenes. Moreover, homozygous Cdk4(R24C/R24C) mutant mice dev elop multiple tumors with almost complete penetrance. The most common neopl asia (endocrine tumors and hemangiosarcomas) are similar to those found in pRb(+/-) and p53(-/-) mice. This Cdk4 mutation cooperates with p53 and p27( Kip1) deficiencies in decreasing tumor latency and favoring development of specific tumor types. These results provide experimental evidence for a cen tral role of Cdk4 regulation in cancer and provide a valuable model for tes ting the potential anti-tumor effect of Cdk4 inhibitors in vivo.