X-chromosome inactivation (XCI), the process of one female X chromosome being silenced to achieve gene dosage parity between the sexes, is an epigenetic phenomenon regulated by non-coding RNAs. Xist RNA is expressed in differentiated cells from the inactive X-chromosome and causes transcriptional silencing in cis. Tsix, an RNA antisense to Xist, antagonizes Xist function in cis uniquely in undifferentiated cells.
Two forms of XCl occur during female mouse development. Imprinted XCI begins during pre-implantation embryogenesis where the paternally inherited X chromosome (XP) expresses Xist RNA, thereby silencing the XP. At implantation, imprinted XCl remains in the extraembryonic tissues while the epiblast cells reactivate the XP. At E6.5, either the XP or XM inactivate in a stochastic manner, termed random XCl.
Little is known regarding how this switch is made from imprinted to random XCl in the epiblast and what regulates imprinted XCl. Herein, I demonstrate that the DNA element DXPas34 is required to maintain an active XM in the imprinted extraembryonic tissues. Maternal germ line transmission of a DXPas34 deletion allele results in embryonic lethality at the implantation stage, while paternal germ line transmission has no phenotype.
Next, I address the question why epiblast derived cells do not undergo imprinted XCl. I sought to determine a fundamental question of imprinted XCl, is the X-chromosome imprint erased in mouse ES cells? Using inducible CDX2 transgenic ES cells, I demonstrate that mouse embryonic stem cells largely erase the X-chromosome imprint; however, remnants of a slight memory can be observed.
While the pluripotency factors OCT4, SOX2, and NANOG repress Xist and activate Tsix during X-reactivation, these factors are also expressed in the pre-implantation embryo where Xist is expressed. I investigate why OCT4 does not repress Xist in the preimplantation embryo and what regulates the transition from imprinted to random XCl. I provide evidence that CDX2 acts directly as a molecular switch, potentially activating Xist expression in the pre-implantation embryo by preventing OCT4 binding to Xist intronl. Taken together, these findings demonstrate that cell-type specific transcription factors achieve Xist expression on the XP and Tsix expression on the XM.
|Advisor:||Lee, Jeannie T.|
|School Location:||United States -- Massachusetts|
|Source:||DAI-B 72/01, Dissertation Abstracts International|
|Keywords:||DXPas34, Embryo, Female, Stem cells, X-chromosome inactivation|
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