Xist expression from an Xist YAC transgene carried on the mouse Y chromosome

Abstract

We have constructed mouse transgenic lines carrying a YAC clone encompassing the Xist gene in order to investigate the factors influencing Xist expression and the initiation of X-inactivation. Two transgenic lines were derived, one carrying four copies integrated at an autosomal site and a second line carrying four copies integrated at a single site on the Y chromosome. Xist expression was not observed in mice carrying the autosomal insertion. However, Xist expression from the Y-inserted transgenes was observed and at levels commensurate with that found in normal female mice. Methylation sites in the autosomal transgene both 5' and 3' of the Xist gene are hypermethylated and appear to reflect methylation patterns observed on the active X chromosome. For the Y-linked transgene, methylation sites 5' and 3' of the Xist gene are hypomethylated reflecting patterns found on the inactive X chromosome. However, the 5' and 3' methylation levels have been decoupled at the active transgenic locus. The data suggest that sequences in the vicinity of Xist can initiate some of the features that are associated with the initiation process of X-inactivation.

Xist imprinting is promoted by the hemizygous (unpaired) state in the male germ line

“…We obtained six independent Tg lines. In contrast to the lack of phenotype observed previously with YAC Tgs (39)(40)(41), intriguing phenotypes were observed in five of the six hemizygous transgenic lines, here named Tg2080, Tg2082, Tg2086, Tg2087, Tg2091, and Tg2092 (Fig. 1B).…”

Imprinted X chromosome inactivation offers up a double dose of epigenetics

“…Strangely, the transgenic lines reported previously did not exhibit any abnormalities (12)(13)(14), suggesting that they are incapable of maintaining Xicdependent repression autosomally. In the report from Sun et al, the smaller BAC transgenes confer several phenotypes, including sterility, indicating that they contain all of the necessary sequences to establish and faithfully maintain repression of the neighboring genes.…”

Position effect of translocations involving the inactive X chromosome: physical linkage to XIC/XIST does not lead to long-range de novo inactivation in human differentiated cells

“…Indeed, it has been recently reported that Xist introduced in an autosome is sufficient by itself for X inactivation in cis, suggesting that Xic is composed of the Xist gene alone (Herzing et al, 1997) and that the Xist gene alone plays a direct role in X-chromosome choosing (Marahrens et al, 1998) and initiation of X inactivation through Xist RNA stabilization (Panning et al, 1997;Sheardown et al, 1997). However, further studies are needed to confirm the above findings, because similar experiments where Xic segments were introduced onto autosomes did not result in Xist induction (Heard et al, 1996;Matsuura et al, 1996).…”