Inducible XIST-dependent X-chromosome inactivation in human somatic cells is reversible

doi:10.1073/pnas.0610946104

Inducible XIST-dependent X-chromosome inactivation in human somatic cells is reversible

*Department of Medical Genetics, University of British Columbia, Vancouver, BC, Canada V6T1Z3; and
^‡University of Massachusetts Medical Center, Worcester, MA 01655

Edited by Stanley M. Gartler, University of Washington, Seattle, WA, and approved April 25, 2007 (received for review December 10, 2006)

Abstract

During embryogenesis, the XIST RNA is expressed from and localizes to one X chromosome in females and induces chromosome-wide silencing. Although many changes to inactive X heterochromatin are known, the functional relationships between different modifications are not well understood, and studies of the initiation of X-inactivation have been largely confined to mouse. We now present a model system for human XIST RNA function in which induction of an XIST cDNA in somatic cells results in localized XIST RNA and transcriptional silencing. Chromatin immunoprecipitation and immunohistochemistry shows that this silencing need only be accompanied by a subset of heterochromatic marks and that these can differ between integration sites. Surprisingly, silencing is XIST-dependent, remaining reversible over extended periods. Deletion analysis demonstrates that the first exon of human XIST is sufficient for both transcript localization and the induction of silencing and that, unlike the situation in mice, the conserved repeat region is essential for both functions. In addition to providing mechanistic insights into chromosome regulation and formation of facultative heterochromatin, this work provides a tractable model system for the study of chromosome silencing and suggests key differences from mouse embryonic X-inactivation.

Footnotes

^§To whom correspondence should be addressed at:
Department of Medical Genetics, Molecular Epigenetics Group, University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC, Canada V6T 1Z3.
E-mail: cbrown{at}interchange.ubc.ca
Author contributions: J.C.C., J.B.L., and C.J.B. designed research; J.C.C., L.L.H., S.E.L.B., N.P.T., and C.J.B. performed research; J.C.C., L.L.H., N.P.T., J.B.L., and C.J.B. analyzed data; and J.C.C. and C.J.B. wrote the paper.
↵ ^†Present address: Section of Research Pavillion Pasteur, Curie Institute, 26 Rue d'Ulm, 75248 Paris Cedex 05, France.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Abbreviations:

H3K27m3,

trimethylation of lysine 27 of histone H3;

H3K4m2,

dimethylation of lysine 4 of histone H3;

H4K20m1,

monomethylation of lysine 20 of histone H4;

H3K9m2,

dimethylation of lysine 9 of histone H3;

FRT,

Flp recombination target;

DOX,

doxycycline.