An X-linked GFP transgene reveals unexpected paternal X-chromosome activity in trophoblastic giant cells of the mouse placenta

A GFP transgene has been integrated on the proximal part of the mouse X chr omosome just distal of Timp and Syn1, During development, this X-linked GFP transgene exhibits widespread green fluorescence throughout the embryonic and adult life of male mice but displays mosaic expression in tissues as a result of X-inactivation in females. In living female embryos, inactivation of the transgene is imprinted in extraembryonic regions and random in the embryo proper, demonstrating that this reporter is behaving in a similar fa shion to the majority of X-linked loci, and so provides a vital readout of X chromosome activity. This is observation is further supported in T16H/X f emale mice harboring the GFP transgene on the normal X chromosome where rep orter inactivation is observed in somatic cells. The differential expressio n of GFP activity facilitates fluorescence activated cell sorting for the p urification of GFP+ vs. GFP- cells from female embryonic tissues, thereby a llowing access to populations of cells that have kept active a particular X chromosome, By tracking the activity of this X-linked GFP transgene, we di scovered that the primary and secondary giant cells of the X/X placenta mai ntain an active paternal copy of this transgene on the presumed silenced pa ternal X-chromosome, This finding implies that the imprint on the paternal X chromosome may be relaxed in these trophectodermal derivatives, (C) 2001 Wiley-Liss, Inc.