MLL/COMPASS in transcription, cell cycle and disease control

Liang, Kaiwei (2016). MLL/COMPASS in transcription, cell cycle and disease control. Doctor of Philosophy (PhD) thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f6c1

Abstract

The Super Elongation Complex (SEC) contains the RNA polymerase II C-terminal domain (CTD) kinase CDK9 and has a crucial role in MLL leuke-mogenesis. To develop therapies for these aggressive leukemias, it will be necessary to better understand the roles of MLL and SEC in transcription elongation and to decipher how they are misregulated in the disease state. To this end, I have undertaken studies to investigate the possibility of additional SEC-like complexes, to investigate roles for CTD kinases in mitosis, and to investigate how MLL protein levels are controlled. First, I characterized the roles of the novel CTD kinases CDK12 and CDK13 with regard to protein-protein interactions, CTD phosphorylation and gene expression. I found that CDK12/13 are involved in RNA processing through physically associating with RNA processing factors, and regulating their genes’ expression and al-ternative splicing. Second, I identified a novel role for CDK9 in the cell cycle, finding that CDK9 is crucial for Pol II release from mitotic chromatin by tran-scription elongation in early mitosis. Blocking of CDK9 kinase activity leads to retention of engaged Pol II in mitosis and delays in cell cycle progression, suggesting a CDK9-mediated transcriptional activation step preceding tran-scriptional silencing in early mitosis. Third, I established a genome-wide shRNA screen to study the regulation of MLL protein and found that the in-terleukin 1 pathway specifically regulates wild-type MLL degradation through the downstream IRAK kinases. IRAK inhibitor stabilizes wild-type MLL and specifically abrogates MLL leukemia cell proliferation, suggesting that inhibition of the IL-1 pathway may be a promising strategy for MLL leu-kemia treatment. Together, these studies contribute to our understanding of CTD kinases at different steps in the cell cycle, to the diverse levels of mis-regulation of MLL in leukemia, and provide potential therapeutic targets for MLL and CTD kinase-associated cancers.