Mapping of two human homologs of a Drosophila heterochromatin protein gene to the X chromosome

Evidence that heteronuclear proteins interact with XIST RNA in vitro

The process of X chromosome inactivation results in the transcriptional silencing of one of the two X chromosomes in mammalian females. A large heterogeneous nuclear RNA that is expressed exclusively from the inactive X chromosome (XIST--X Inactive Specific Transcripts) has been implicated in the inactivation process. The XIST RNA colocalizes with the inactive X chromosome and therefore proteins that interact with the XIST RNA may be involved in the inactivation of the X chromosome. In order to identify such proteins we have used an in vitro UV light cross-linking technique to detect nuclear proteins associating with sections of the XIST RNA. The strongest interaction detected by this technique was between a pair of approximately 40 kDa proteins and a 5' region of the XIST RNA which contains a series of well-conserved tandem repeats. Immunoprecipitation suggested that these proteins may be the heteronuclear proteins hnRNPC1/C2.

Adult phenotype in the mouse can be affected by epigenetic events in the early embryo

Major epigenetic modifications apparently occur during early development in the mouse. The factors that induce such modifications are complex and may involve the various components of a zygote. We have started to explore whether changes in the nucleocytoplasmic composition brought about by micromanipulation can induce phenotypic effects through epigenetic modifications. Nucleocytoplasmic hybrids were therefore prepared by transplanting a female pronucleus into a recipient egg from a different genotype. As a result, the maternal genome was of a different genetic background as compared with the egg cytoplasm. Specifically, experimental zygotes had cytoplasm from the inbred strain C57BL/6, a maternal genome from DBA/2, and a paternal genome from C57BL/6 (termed BDB hybrids). The mirror-image combination, termed DBD, was also made. The reconstituted zygotes were transferred to recipients and allowed to develop to term. Mice born from manipulated zygotes showed transcriptional repression and DNA methylation of major urinary protein genes in their liver, as well as growth deficiency resulting in reduced adult body weight. No altered phenotype was observed in controls in which the maternal pronucleus was simply transplanted back into another zygote of the same genetic background. These results clearly demonstrate phenotypic as well as molecular effects on DNA methylation and expression of at least one gene. Phenotype was therefore no longer predicted by genotype as a result of epigenetic modifications in experimental embryos. What precisely triggers the phenotypic and epigenetic changes is unknown, but presumably, nucleocytoplasmic interactions in hybrid zygotes may be partly responsible.