Understanding Heterochromatin Biology through Histone Mutagenesis

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  • March 21, 2019
Creator
  • Penke, Taylor
    • Affiliation: School of Medicine, Curriculum in Genetics and Molecular Biology
Abstract
  • The relatively large genomes of eukaryotic cells must be organized and compacted within the nucleus while maintaining DNA accessibility for essential processes such as transcription, replication, and DNA repair. This organization is accomplished in large part through the interaction of DNA with histone proteins to form a structure known as chromatin. Chromatin organization is regulated through epigenetic mechanisms, such as histone post-translational modifications (PTMs) or the differential incorporation of variant or canonical histones into chromatin. These processes are regulated differently throughout the genome, leading to functionally distinct chromatin environments. Regions where DNA is “open” or more accessible are collectively referred to as euchromatin, whereas “closed” or inaccessible regions are classified as heterochromatin. Proper heterochromatin formation is essential for regulating numerous cellular processes including, cell division, nuclear organization, gene expression, and DNA replication. A defining feature of heterochromatin is methylation of lysine nine on histone H3 (H3K9me), a histone PTM that recruits Heterochromatin Protein 1 (HP1). Although H3K9 methyltransferases and HP1 are necessary for proper heterochromatin structure, the specific contribution of H3K9 to heterochromatin function and animal development is unknown. Using our recently developed platform to engineer histone genes in Drosophila, I generated H3K9R mutant flies, separating the functions of H3K9 and non-histone substrates of H3K9 methyltransferases. I observed that H3K9 plays an essential role in regulating the structure of pericentromeric heterochromatin and the repression of transposons, but not protein-coding gene expression. Furthermore, I generated a K9R mutation in the variant histone H3.3, revealing functional redundancies between variant H3.3K9 and canonical H3K9, though to differing extents in heterochromatin and euchromatin. Finally, I have used the H3K9R mutant as a tool to uncover general principles of genome regulation. Several previous studies have identified correlations between histone PTMs, transcription, and DNA replication; however, no causative relationship between these processes has been identified. The H3K9R mutant specifically disrupts pericentromeric heterochromatin providing a unique opportunity to determine the direct consequences of altered chromatin structure on replication. We demonstrated that changes in chromatin accessibility and most likely transcription are required but not sufficient for altered replication, influencing the framework through which we view genome regulation.
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  • Duronio, Robert
  • Sekelsky, Jeff
  • McKay, Daniel
  • Matera, Gregory
  • Strahl, Brian
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill
Graduation year
  • 2018
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