Mechanisms of Xist-Mediated Gene Silencing During the Initiation and Maintenance of X Chromosome Inactivation

Citation

Strehle, Mackenzie (2024) Mechanisms of Xist-Mediated Gene Silencing During the Initiation and Maintenance of X Chromosome Inactivation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/nxq5-hj97. https://resolver.caltech.edu/CaltechTHESIS:04182024-125718373

Abstract

X chromosome inactivation (XCI) is a critical development process during which one of the two X chromosomes in female mammals is silenced to balance gene expression with males. XCI is initiated by upregulation of the long noncoding RNA (lncRNA) Xist from the future inactive X chromosome (Xi), which recruits a variety of proteins in cis to mediate transcriptional repression that is maintained throughout the lifetime of the organism. Recent studies have demonstrated that silencing following Xist expression is dependent on direct recruitment of the transcriptional silencing protein SHARP (also known as SPEN); however, the mechanism underlying formation of the Xi silencing compartment has remained poorly defined. Similarly, it has long been thought that maintenance of XCI occurs independently of Xist and depends on differential DNA methylation enrichment on the Xi, but the evidence in support of these views is lacking. Here, we show how low copy numbers of Xist can recruit SHARP in super-stoichiometric excess to initiate gene silencing on the X and mediate formation of the silent Xi compartment. We also provide preliminary evidence suggesting that maintenance of XCI is Xist independent, but dependent on DNA methylation and histone deacetylation. Together, these results offer a more holistic view of the molecular mechanisms underlying both initiation and maintenance XCI, as well as provide a framework for further investigation into lncRNA biology and epigenetic regulation more broadly.

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