DNA templated cellular processes require alteration of chromatin structure to dynamically facilitate access to packaged DNA. The post-translational chemical modification of histone proteins is one critical mechanism that alters chromatin structure either directly or via the recruitment of effector proteins. We have identified a novel histone kinase complex in yeast containing the conserved S-phase replication initiation kinase Cdc7, its activating protein Dbf4, and a number of additional factors. We have found that this complex phosphorylates histone H3 on threonine 45 (H3T45). The site lies at a critical location, where DNA contacts the histone octamer at the entry and exit points of the nucleosome. Therefore, modification of this site has the potential to dramatically alter DNA-histone contacts, providing access for the unwrapping of DNA. Surprisingly, genome- wide studies reveal that H3T45 phosphorylation occurs not only at the expected origins of DNA replication, but it also marks the promoters of specific genes important for cell growth. Furthermore, we find that Cdc7 physically associates with target promoters and is required for RNA polymerase recruitment and full transcription at these locations. The goals of this proposal are to test our hypothesis that the Cdc7-Dbf4 complex establishes an epigenomic state, where under favorable nutritional conditions, the modified chromatin architecture facilitates robust transcription that drives cell growth and proliferation. Furthermore, we will test a model that proposes that the modification, or subsequent nucleosome remodeling, regulates promoter activity via altering the chromatin-binding dynamics of a potential repressor complex. If our hypothesis is correct we will have identified the H3T45 residue as a gatekeeper of the nucleosome, regulating DNA accessibility at target locations for transcription. Importantly, we will have uncovered a novel regulatory network by which gene expression is coordinated to ensure the necessary cell growth that accompanies cell proliferation. Of particular importance, Cdc7 and Dbf4 are misregulated in a variety of cancers and their upregulation is indicative of lower relapse-free survival. As uncontrolled cell growth, division and DNA replication are associated with the proliferation of cancer cells we anticipate that conservation of function of this epigenomic mark will provide a biomarker of diseased states and will offer a highly unique target for cancer therapeutics.
The transformation of normal cells to cancerous cells requires a number of regulatory changes, including an increased growth and proliferative potential. Our lab has identified a new marker of proliferating cells, a chemical mark on a histone protein. Histones are critical to the packaging of genetic material in our cells, and combined with DNA make up the fundamental unit of our chromosomes. This proposal focuses on manipulating the generation and function of this new mark, which may be critical to both cell growth and division. Its deregulation could play a vital role in the uncontrolled cell growth exhibited by cancer cells and thus provides a novel target for therapeutic intervention.
