Mechanisms of biomolecular assembly in the establishment and maintenance of X-inactivation

The formation of heterochromatin, the silenced part of the genome, is essential for embryonic development, and defects in heterochromatin mechanisms can lead to chromosome abnormalities that underlie many diseases, including cancer. However, the requirements for the transmission of heterochromatin and its maintenance as heritable epigenetic memory remain poorly understood. My work using super-resolution microscopy, genome-editing and kinetic modelling offered critical insights into the process of heterochromatin formation during the silencing of one X chromosome in female mammals mediated by the RNA, Xist. We identified that propagation of heterochromatin is regulated by Xist-supercomplexes, biomolecular assemblies that concentrate effector proteins superstoichiometrically to the RNA. We also found that protein-protein interactions are central to propagation and maintenance of silencing. This work challenged the standing view that silencing spreads linearly across the DNA sequence. Employing these methodologies as well as biophysical and structural studies, this proposal aims at identifying the molecular composition, concentration and dynamic changes in Xist-supercomplexes required to turn off transcription and establish epigenetic memory and understand how protein-protein interactions in biomolecular assemblies regulate the silencing machinery. This research will provide unprecedented insights into factors that establish epigenetic memory which will be crucial to address epigenetic instability in disease.