Title: Dynamic regulation of histone lysine methylation via the ubiquitin-proteasome system
Description: Lysine methylation is an important post-translational modification found on histones that is added and removed by histone lysine methyltransferases and demethylases, respectively. Lysine methylation occurs in a specific and well-regulated manner, and plays key roles in regulating important biological processes such as transcription, DNA damage and cell cycle. Regulation of the protein abundance of these methylation enzymes particularly by the ubiquitin-proteasome system has emerged as a key mechanism by which the histone methylation status of the cell can be regulated, allowing cells to respond rapidly to specific developmental and environmental cues. In my thesis, I focus on two histone lysine demethylases, KDM4A and PHF8, both of which appear to be regulated by E3 ligases; this regulation impacts their function in the cell.

Chapter 2 shows that KDM4A is targeted for proteasomal degradation by the SCFFBXO22, and mis-regulation of KDM4A results in changes in global histone 3 lysine 9 and 36 (H3K9 and H3K36) methylation levels and impacts the transcription of a KDM4A target gene, ASCL2. Chapter 3 shows how PHF8 is targeted for proteasomal degradation by the APCCDC20 via a novel, previously unreported LxPKxLF motif on PHF8. I also found that similar to other APCCDC20 substrates like Cyclin B, PHF8 is an important G2-M regulator, loss of which results in cell cycle defects such as prolonged G2 and defective M phases. To further interrogate PHF8 biology, Chapter 4 describes the generation of a PHF8 conditional knockout mouse. PHF8 biology is interesting and relevant to human disease, as mutations are found in X-linked intellectual disability and autism. Complete loss of PHF8 by full body knockout in the mouse appears to be embryonically lethal, underscoring its key role in early development.

This mouse model would allow us to extensively study the biochemistry and biology of PHF8 in the context of development and especially in brain function, where it is anticipated to play key roles. Overall, my dissertation work provides mechanistic and biological insights into how histone demethylases are dynamically regulated by the ubiquitin-proteasome system, providing an extra dimension to our understanding of how chromatin marks can be regulated.