Spontaneous and UVR-induced multiple metastatic melanomas in Cdk4R24C/R24C/TPras mice

Human melanoma susceptibility is often characterized by germline inactivating CDKN2A (INK4A/ARF) mutations, or mutations that activate CDK4 by preventing its binding to and inhibition by INK4A. We have previously shown that a single neonatal UVR dose delivered to mice that carry melanocyte-specific activation of Hras (TPras) increases melanoma penetrance from 0 to 57%. Here we report that activated Cdk4 cooperates with activated Hras to greatly increase susceptibility to melanoma in mice. While UVR treatment failed to induce melanomas in Cdk4R24C/R24C mice, it greatly increased the penetrance and decreased the age of onset of melanoma development in Cdk4R24C/R24C/TPras animals, compared to TPras alone. This increased penetrance was dependant on the threshold of Cdk4 activation, as Cdk4R24C/+/TPras animals did not show an increase in UVR-induced melanoma penetrance compared to TPras alone. In addition, Cdk4R24C/R24C/TPras mice invariably developed multiple primary lesions, which occurred rarely in TPras mice. These results indicate that germline defects abrogating the pRb pathway may enhance UVR-induced melanoma. TPras and Cdk4R24C/R24C/TPras tumors were comparable histopathologically, but the latter were larger and more aggressive, and cultured cells derived from such melanomas were also larger and had higher levels of nuclear atypia. Moreover, the melanomas in Cdk4R24C/R24C/TPras mice, but not TPras mice, readily metastasized to regional lymph nodes. Thus it appears that in the mouse, Hras activation initiates UVR-induced melanoma development, while the cell cycle defect introduced by mutant Cdk4 contributes to tumor progression, producing larger, more aggressive, metastatic tumors.