Revealing the Mechanism of Xist-mediated Silencing

Citation

Chen, Chun-Kan (2018) Revealing the Mechanism of Xist-mediated Silencing. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z94J0C9J. https://resolver.caltech.edu/CaltechTHESIS:11032017-102218314

Abstract

Xist initiates XCI by spreading across the future inactive X-chromosome, excluding RNA polymerase II, recruiting the polycomb repressive complex and its associated repressive chromatin modifications, and repositioning active genes into a transcriptionally silenced nuclear compartment. While much is known about the events that occur during XCI, the mechanism by which Xist carries out these various roles remains unclear. Here we identify ten proteins that directly associate with Xist, and we further show that three of these proteins are required for Xist-mediated transcriptional silencing. One of these proteins, SHARP, which is known to interact with the SMRT co-repressor that activates HDAC3, is not only essential for silencing, but is also required for the exclusion of PolII from the inactive X. We show that both SMRT and HDAC3 are required for Xist-mediated silencing and RNA polymerase II exclusion. Another of these proteins, LBR, is required for repositioning actively transcribed genes into the Xist-silenced compartment. We further show that Xist, through its interaction with LBR, a protein that is anchored in the inner nuclear membrane, would effectively reposition Xist-coated DNA to the nuclear lamina, thereby changing the accessibility of other genes on the X-chromosome to enable Xist to spread to active genes across the entire chromosome to silence chromosome-wide transcription. Together, these results present an integrative picture of how Xist can scaffold multiple proteins to orchestrate the complex functions required for the establishment of the inactive X-chromosome.

Thesis Files