Hematopoietic Precursor Cells Transiently Reestablish Permissiveness for X Inactivation

Abstract

Xist is the trigger for X inactivation in female mammals. The long noncoding Xist RNA localizes along one of the two female X chromosomes and initiates chromosome-wide silencing in the early embryo. In differentiated cells, Xist becomes dispensable for the maintenance of the inactive X, and its function for initiation of silencing is lost. How Xist mediates gene repression remains an open question. Here, we use an inducible Xist allele in adult mice to identify cells in which Xist can cause chromosome-wide silencing. We show that Xist has the ability to initiate silencing in immature hematopoietic precursor cells. In contrast, hematopoietic stem cells and mature blood cells are unable to initiate ectopic X inactivation. This indicates that pathways critical for silencing are transiently activated in hematopoietic differentiation. Xist-responsive cell types in normal female mice show a change of chromatin marks on the inactive X. However, dosage compensation is maintained throughout hematopoiesis. Therefore, Xist can initiate silencing in precursors with concomitant maintenance of dosage compensation. This suggests that Xist function is restricted in development by the limited activity of epigenetic pathways rather than by a change in the responsiveness of chromatin between embryonic and differentiated cell types.

Supplemental material for this article may be found at http://mcb.asm.org/.

We thank M. Dezic, R. Flannery, and C. Beard for mouse husbandry; G. Stengl for cell sorting; C. Cobaleda for advice with pro-B cell cultures; N. Brockdorff for the Mecp2 probe; T. Jenuwein and H. Beug for helpful discussions; and R. Grosschedl for critical reading of the manuscript.

This research was supported by Boehringer Ingelheim, the Austrian GEN-AU initiative (financed by the Ministry of Science) and EU FP6 funding for the Epigenome Network of Excellence.