INACTIVATION OF AN X-LINKED TRANSGENE IN MURINE EXTRAEMBRYONIC AND ADULT TISSUES

Transgenes located on the X chromosome have been used to study the mec hanisms involved in X-chromosome inactivation. Analysis of the transge nic mouse strain M-TKneo1 carrying a neomycin resistance gene inserted in the X chromosome showed that, in adult somatic tissues, this trans gene is subject to X-inactivation and to de novo methylation as other endogenous X-linked genes. During mouse embryogenesis, X-linked genes show a preferential paternal inactivation in extraembryonic tissues, w hereas these genes are subject to random inactivation in embryonic tis sues. It has been suggested that, in the mouse, the extraembryonic tis sues carry a parental imprint at the time of inactivation. The study o f the neo transgene expression in extra-embryonic endoderm has shown n ot only that neo is inactivated but also that, at the RNA level, pater nal inactivation of the transgene seems essentially complete. The diff erences between our results and previously obtained results with a mou se alpha-fetoprotein transgene, which was only inactivated in neonatal tissues but not in extraembryonic tissues, are discussed.