Role of CDK4 in Development and Cancer

Abstract

The mammalian cell cycle is divided into four distinct phases: G1, S, G2, and M. The transition from G1 to S and G2 to M is tightly controlled by a set of protein complexes containing cyclin-dependent kinases (Cdks) and cyclins. In G1-phase, Cdk4/6 and Cdk2 bind to and form complexes with D-type and E-type cyclins, respectively. The active Cdk-cyclin complexes phosphorylate retinoblastoma proteins (pRb), enabling transcription of E2F-reponsive genes necessary for S-phase transition. Dysregulation of the cyclin-Cdk-pRb-E2F pathway is frequently found in a variety of human tumors. Previously, our laboratory generated a mouse strain harboring, an activated allele of Cdk4, Cdk4-R24C. The Cdk4-R24C mutation was initially identified in familial melanomas. Mice homozygous for the Cdk4-R24C allele develop spontaneous tumors and die earlier than their wild-type counterparts. The incidence of spontaneous melanoma in Cdk4R24C/R24C mice is very low, at approximately 2%. We sought to determine if cooperation with other oncogenic mutations may be required for development of melanomas in mice. We demonstrate here that mice carrying a melanocytic expression of oncogenic HRAS (G12V) on Cdk4R24C background develop spontaneous cutaneous melanomas with an increased incidence (approximately 30%). Furthermore, Tyr-HRas:Cdk4R24C/R24C mice, upon treatment with carcinogens DMBA/TPA, display an increase in the formation of nevi and develop melanomas with a shorter latency than the Tyr-HRas:Cdk4+/+ mice. Therefore, we demonstrate that the activated CDK4 protein cooperates with oncogenic HRAS (G12V) in the development of melanoma. Previously, our laboratory also generated a mouse strain, lacking expression of Cdk4 gene. This mouse strain, Cdk4Neo/Neo, contained disrupted Cdk4 alleles due to an insertion neomycin gene in the Cdk4 locus. The loss of Cdk4 affects the development of various organs such as ovary, testis and pancreas. Additionally, these mice develop a disease phenotype closely resembling insulin-deficient diabetes in humans. Furthermore, it has previously been shown that in human T-cells, the loss of Cdk4 results in cytokine unresponsiveness. Based on these observations, we sought to determine the role of Cdk4 in thymocytes development by utilizing the Cdk4Neo/Neo mice. We observed a decreased cellularity in the thymus of the Cdk4Neo/Neo mice when compared to that of wildtype mice. Furthermore, we observed a significant decrease in the proliferation of Cdk4Neo/Neo thymocytes to anti-CD3 antibody stimulation. This defect in proliferation may be a result of a decreased level of pRb phosphorylation at Cdk4 consensus site. Interestingly, we observed a significantly higher expression of CD25 receptors on double positive thymocytes of Cdk4Neo/Neo mice. Taken together, these findings suggest that Cdk4 plays an important role in thymocyte proliferation and development in mice.