Heat-induced and spontaneous expression of Hsp70.1Luciferase transgene copies localized on Xp22 in female bovine cells

BACKGROUND: Expression of several copies of the heat-inducible Hsp70.1Luciferase (LUC) transgene inserted at a single X chromosome locus of a bull (Bos taurus) was assessed in females after X-chromosome inactivation (XCI). Furthermore, impact of the chromosomal environment on the spontaneous expression of these transgene copies before XCI was studied during early development in embryos obtained after in vitro fertilization (IVF), when the locus was carried by the X chromosome inherited from the bull, and after somatic cell nuclear transfer (SCNT) cloning, when the locus could be carried by the inactive Xi or the active Xa chromosome in a female donor cell, or by the (active) X in a male donor cell. FINDINGS: Transgene copies were mapped to bovine Xp22. In XX(LUC )female fibroblasts, i.e. after random XCI, the proportions of late-replicating inactive and early-replicating active X(LUC )chromosomes were not biased and the proportion of cells displaying an increase in the level of immunostained luciferase protein after heat-shock induction was similar to that in male fibroblasts. Spontaneous transgene expression occurred at the 8-16-cell stage both in transgenic (female) embryos obtained after IVF and in male and female embryos obtained after SCNT. CONCLUSIONS: The X(LUC )chromosome is normally inactivated but at least part of the inactivated X-linked Hsp70.1Luciferase transgene copies remains heat-inducible after random XCI in somatic cells. Before XCI, the profile of the transgenes' spontaneous expression is independent of the epigenetic origin of the X(LUC )chromosome since it is similar in IVF female, SCNT male and SCNT female embryos.