Invasive melanoma in Cdk4-targeted mice.

Many human tumors harbor mutations that result in deregulation of Cdk4 activity. Most of these mutations involve overexpression of D-type cyclins and inactivation of INK4 inhibitors. In addition, a mutation in the Cdk4 protein has been described in patients with familial melanoma (Wolfel, T., Hauer, M., Schneider, J., Serrano, M., Wolfel, C., et al. (1995) Science 269, 1281-1284; Zuo, L., Weger, J., Yang, Q., Goldstein, A. M., Tucker, M. A., et al. (1996) Nat. Genet. 12, 97-99). This mutation, R24C, renders the Cdk4 protein insensitive to inhibition by INK4 proteins including p16(INK4a), a major candidate for the melanoma susceptibility locus. Here we show that knock-in mice expressing a Cdk4 R24C allele are highly susceptible to melanoma development after specific carcinogenic treatments. These tumors do not have mutations in the p19(ARF)/p53 pathway, suggesting a specific involvement of the p16(INK4a)/Cdk4/Rb pathway in melanoma development. Moreover, by using targeted mice deficient for other INK4 inhibitors, we show that deletion of p18(INK4c) but not of p15(INK4b) confers proliferative advantage to melanocytic tumor growth. These results provide an experimental scenario to study the role of Cdk4 regulation in melanoma and to develop novel therapeutic approaches to control melanoma progression.