| Summary: | X-chromosome inactivation ensures equal expression of mammalian male and female X-linked genes by transcriptionally silencing one X chromosome in each female cell. The pivotal molecule responsible for the silencing is a long non-coding RNA XIST; however, an all-encompassing model explaining how XIST induces silencing of the whole X chromosome is yet to emerge. This thesis aims to broaden our understanding of XIST action in humans by leveraging an inducible XIST transgene capable of silencing downstream reporters to identify sequences within XIST and XIST-interacting proteins critical for gene silencing.
First, we demonstrate that the repeat A region of XIST is necessary and sufficient to induce gene silencing, at least locally, irrespective of the makeup of the surrounding chromatin, and that XIST induces silencing of a distal gene in one of the HT1080 cell lines. Second, we show that individual repeats of a consensus repeat A sequence contribute additively to silencing. Mutations within a construct consisting of two repeat A units both demonstrate that the two palindromic sequences within the repeat A units spanning ‘ATCG’ and ‘ATAC’ tetranucleotides are critical for repeat A function and add to the evidence that the first palindrome forms a hairpin, rather than engaging in pairing between repeat A units.
Third, we explore which proteins are critical for XIST-induced silencing. We show that histone deacetylation, an early mark of an X-chromosome inactivation, is likely a consequence, and not the cause of XIST-induced silencing. We next demonstrate that in the transgenic HT1080 system, gene silencing is not accompanied by recruitment of the H3K27me3 repressive histone mark and XIST induces silencing independently of its previously reported associations with the polycomb repressive complex 2 (PRC2). Finally, we performed siRNA-mediated knock-down of 31 proteins previously implicated to play a role in X-chromosome inactivation. Our results show that proteins involved in XIST RNA localization (YY1), chromatin organization (SATB2, HNRNPU), and cell cycle (ATM), as well as an E3 ubiquitin ligase (SPOP) contribute to XIST-induced gene silencing in the HT1080 system. Thus, we demonstrate that the repeat A alone induces gene silencing and identify candidate pathways critical for its function. === Medicine, Faculty of === Medical Genetics, Department of === Graduate
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