Mutational Analysis of Xist and the X-inactivation Center
- Goodrich, Leeanne
- Advisor(s): Panning, Barbara
In placental mammals, dosage compensation of the sex chromosomes is
achieved through inactivation of one X chromosome in female cells. This X
chromosome inactivation (XCI) requires tight developmental regulation to ensure all but
one X chromosome is silenced.
At the center of this process is Xist, a long non-coding RNA. Upon differentiation
of a female cell, Xist spreads in cis to coat and silence the inactive X chromosome.
While upregulation of Xist, has been shown to be sufficient for X inactivation to occur,
no one has thoroughly investigated whether Xist is necessary for the establishment of X
chromosome inactivation. In this thesis I provide evidence that Xist is not required for
dosage compensation of the X chromosome during epiblast-like cell differentiation. This
result suggests Xist-independent silencing mechanisms for this essential process may
be in place.
A 1-2 Mb region of the X chromosome, termed the X-inactivation center (Xic) is
necessary in two copies for XCI to occur, indicating it is necessary for cells to count the
number of X chromosomes present. I delete one copy of the putative 2 Mb Xic in male
and female mouse embryonic stem cells and present evidence that this deletion is not
well tolerated, suggesting that this region requires finer resolution mapping to identify
the minimal element required for counting.
Finally, I finish with a review which elaborates on studies that enlighten our
understanding of activators and repressors that control XCI. Our findings challenge
existing dogmas in the field and provide the foundation for future work focused on
uncovering the molecular mechanisms behind Xist-independent silencing of the X
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