Precise control of transcription, the copying of the genetic DNA code to an expressed RNA molecule, is fundamental for all processes of an organism. Transcriptional regulation is dependent on the activity of proteins and protein complexes including general and cell specific transcription factors and co-activators. Studying how these transcriptional regulators function is crucial for understanding processes such as development and disease. This PhD thesis focuses on the function of one evolutionarily conserved transcriptional co-activator, called the Mediator complex, and in particular on the role of its kinase module. This kinase module consists of four proteins: Cdk8, cyclin C, Med12 and Med13 in lower metazoans and CDK8 or CDK19, cyclin C, MED12 or MED12L and MED13 or MED13L in vertebrates. The Mediator kinase module regulates transcription through various mechanisms, including association with transcription factors and regulation of enhancer-dependent transcription. Kinase module deregulation is implicated in developmental disorders and cancer, but the molecular mechanisms underlying these diseases remain poorly understood. CDK8 was found to be dispensable for cell-autonomous survival but required for mouse embryonic development at the pre-implantation stage. Cdk8-CycC-mediated regulation of transcription was dependent on Med12-Med13 while Cdk8 and CycC depletion caused distinct and even opposite effects on gene expression as compared to Med12 and Med13 depletion in fruit fly cells. This work identified highly similar effects on transcription after depletion of fruit fly Med12 or Med13, suggesting limited Med12-independent functions for Med13. Kinase module subunit hierarchy was conserved in human colon cancer cells where depletion of MED12 or double depletion of the redundant MED13 and MED13L also resulted in highly similar transcriptional responses. Med12 and Med13 were found to be activators of innate immunity genes that are dependent on the serpent/GATA transcription factor in fruit fly cells and larvae. In human colon cancer cells, MED12, MED13 and MED13L were disproportionally required for the expression of genes associated with cancer-acquired super-enhancers. MED12 or MED13 and MED13L depletion caused a dramatic decrease in the expression of the super-enhancer associated MYC oncogene and impaired proliferation of colon cancer cells, suggesting that targeting of these kinase module subunits is a possible future therapeutic opportunity. This thesis extends the present understanding of the relationships between Mediator kinase module subunits in metazoan species and identifies kinase module-dependent functions in development, immunity and cancer-acquired transcription. Further research should be directed at studying the molecular mechanisms of kinase module functions in vivo and at the potential to target this complex with the goal of treating human diseases such as cancer.
|Award date||1 Sep 2017|
|Place of Publication||Helsinki|
|Publication status||Published - 1 Sep 2017|
|MoE publication type||G5 Doctoral dissertation (article)|
Fields of Science
- 1182 Biochemistry, cell and molecular biology
- 3111 Biomedicine