Multiple functions of the PR-Set7 histone methyltransferase: from transcription to the cell cycle

2012-11-15Within the eukaryotic nucleus, DNA is packaged via its interaction with histones and non-histone proteins into a structure known as chromatin. Chromatin is dynamic in nature, and can become more condensed or more accessible depending on, among other things, the post-translational modifications present on the histone proteins. Histone-modifying enzymes can alter the configuration of chromatin and are known to play essential roles in DNA-templated processes, including gene transcription, cell cycle progression and DNA damage repair pathways. This dissertation presents novel findings on the functions and binding partners of the PR-Set7 histone H4 lysine 20 monomethyltransferase. ❧ Several lines of evidence have been reported implicating PR-Set7 in both transcriptional repression and activation. Chapter 1 presents our efforts to more clearly elucidate the role of PR-Set7 and H4K20me1 in transcription. In short, we found that depletion of PR-Set7 or its catalytic activity resulted in the de-repression of newly identified PR-Set7 target genes, strongly suggesting that PR-Set7 and H4K20me1 function in the transcriptional repression of specific genes. ❧ A ‘histone code’ hypothesis has been proposed, in which specific combinations of post-translational modifications on histone tails may function coordinately to regulate distinct chromatin-templated processes. We previously reported a novel trans-tail histone code involving monomethylated H4K20 and H3K9. We found that global H3K9 monomethylation requires PR-Set7 but does not require its catalytic activity. We therefore predicted that PR-Set7 recruits an unidentified H3K9 methyltransferase to establish this novel histone code. In Chapter 2 we identify this H3K9 methyltransferase as Riz1 (PRDM2/KMT8). Riz1 is a tumor suppressor, and is frequently mutated in human cancers by a frameshift mutation resulting in the expression of a truncated protein lacking the C-terminal domain found to be required for PR-Set7 binding and proper localization. Finally, forced ectopic expression of wild type Riz1 in cancer cell lines carrying this Riz1 truncation mutation resulted in cell cycle arrest and apoptosis. These data indicate that Riz1 is an important tumor suppressor and imply that direct Riz1 binding to PR-Set7 is required for proper Riz1 localization and function. ❧ In Chapter 3 we demonstrate that PR-Set7 interacts with class I HDAC proteins in vivo, and that treatment with class I HDAC inhibitors results in the selective de-repression of PR-Set7 target genes. Furthermore, we found that PR-Set7 directly binds HDAC3 in vitro and that this interaction may be functionally significant in epithelial-mesenchymal transition. Together, these results suggest that PR-Set7 and class I HDACs cooperate to regulate transcription of specific human genes. ❧ Lastly, in Chapter 4 we present evidence that after osmotic stress, PR-Set7 is phosphorylated at serine 29 by JNK kinase, and that this modification protects PR-Set7 from degradation after UV irriadiation. Collectively, this dissertation presents novel findings regarding PR-Set7’s binding to other histone-modifying enzymes, Riz1 and HDAC3. These findings help to improve our understanding of PR-Set7 function during transcriptional repression and cell cycle progression, as well as provide insights into how perturbation of PR-Set7 may contribute to cancer development and progression