Examining The Roles of Histone Methyltransferases in Heterochromatin Formation

The eukaryotic genome is meticulously stored inside the cell to control gene expression, protect the genome integrity, and facilitate cell division. The storage of genome is often achieved through epigenetic modifications. These modifications divide the genome into euchromatin and heterochromatin, each associated with gene expression and gene repression, respectively. Epigenetic modifications are often covalent modifications onto histone proteins or methylated DNA. On their own, some modifications can alter chromatin states, but most require specific epigenetic machinery that often termed epigenetic reader proteins. The epigenetic reader protein recognizes a specific epigenetic modification to alter chromatin states. For heterochromatin, these modifications often require heterochromatin protein 1 (HP1) to function. Heterochromatin represents a highly condensed form of chromatin that is often devoid of any transcriptional activities. Trimethylation of histone 3 lysine 9 (H3K9me3) and the presence of HP1 are the hallmarks and core players in heterochromatin formation. H3K9me3 recruits HP1, which when bound, can serve as a scaffold protein for more heterochromatin machinery. One key group of proteins HP1 recruits are the histone methyltransferases, the enzymes that catalyze the methylation of H3K9. The recruitment of HP1 often results in more H3K9me3 modifications, which can induce a positive feedback loop that results in a heterochromatin state. This group of histone methyltransferases all belong to the SET-domain family of proteins and demonstrate overlapping roles in heterochromatin formation. The redundancy within this group of enzymes calls for a better understanding in differentiating individual H3K9 methyltransferases. Does the redundancy suggest overlap in functionality or individual methyltransferases contribute to heterochromatin formation differently. The central hypothesis for this project is that different H3K9 methyltransferases can have different impacts on heterochromatin formation. By combining CiA-Oct4 cell line and Molecular biosystem, we have studied the intricacy of heterochromatin formation. Our SETDB1 knockdown cell lines show that SETDB1 contributes heavily to heterochromatin formation. SETDB1 knockdown cell lines have also shown reductions in H3K9me3 accumulation and impaired heterochromatin formation kinetics.Doctor of Philosoph