Project Summary/Abstract Cyclin C was cloned, along with other G1 cyclins, as a cDNA which can rescue the proliferative block in yeast Saccharomyces cerevisiae lacking CLN1, CLN2 and CLN3 genes. Subsequent work revealed that cyclin C, acting together with its kinase partners, the cyclin-dependent kinase 8 (CDK8) and CDK19, plays a role in regulating gene transcription. Cyclin C serves as a regulatory partner of CDK8 and CDK19, and activates the kinase activity of these proteins. The two paralog kinases show substantial overall amino acid identity, particularly within their catalytic domains. Cyclin C, CDK8 and CDK19 represent parts of the mediator complex, a large multisubunit protein complex that regulates gene transcription by linking RNA polymerase II to sequence-specific transcription factors. In addition, cyclin C-CDK8/19 was shown to phosphorylate various transcription factors and to regulate their stability and activity. Growing evidence indicates that cyclin C-CDK8 and C-CDK19 kinases may represent potential anti-cancer targets. CDK8 has been identified as an oncogene in several human cancers. Cyclin C, CDK8 and CDK19 are overexpressed in a wide range of tumor types. Importantly, higher expression of these three proteins was found to be associated with poor clinical outcome. Consistent with tumor-promoting roles for cyclin C-CDK8/19, treatment of mice bearing xenografts of several human tumor types with small molecule CDK8/19 inhibitors resulted in a potent anti-tumor effect without apparent systemic toxicity. To understand the molecular function of cyclin C in vivo, we generated conditional cyclin C knockout mice (cyclin CF/F). To test the impact of cyclin C-CDK8/19 inhibition on T cell development and adaptive anti-tumor immunity, we crossed cyclin CF/F mice with CD4-Cre animals. The latter strain expresses Cre recombinase at an early stage of T cell development, and drives deletion of the ‘floxed’ sequences in all T cell subsets. In our preliminary analyses we established that cyclin C functions as a gate- keeper of T cell differentiation, likely by directly phosphorylating a lineage-specific transcription factor. By doing so, cyclin C affects signaling networks that regulate the immune response. Hence, this novel function of cyclin C may have a profound effect on the anti-tumor immune response and on tumorigenesis. In the proposed work we will extend these studies. In Aim 1, we will study the exact molecular function of cyclin C- CDK8 and C-CDK19 in the T cell lineage using a combination of biochemical in vitro studies, cell culture experiments, analyses of ex vivo cultured T cells as well as mouse genetic approaches. In Aim 2, we will study how ablation of cyclin C impacts tumorigenesis in vivo, using mouse cancer models and combination treatments with other anti-cancer modalities. In Aim 3, we will extend these cancer studies to cyclin C catalytic partners, using conditional Cdk19 knockout mice (Cdk19F/F), that my laboratory generat...