Role of Transvection in X-Inactivation

Early in mammalian female embryonic development, an unidentiffed developmental signal causes heterochromatin to form along nearly the entire length of one X chromosome. We generated Xist knockout mice and showed that the X-linked Xist locus is required in cis for X-inactivation in the extraembryonic tissue of females (Marahrens et al., 1997). Analysis of the somatic cells of mutant females indicated that the Xist mutation caused primary nonrandom X-inactivation [Marahrens et al., 1998). This was confirmed by following the expression of an X-linked GFP gene through early mouse development. We proposed that (1) the two Xist loci in female cells physically associate with one another resulting in one Xist allele being remodeled into heterochromatin and (2) that the heterochromatic Xist locus physically interacts with heterochromatic LINE elements dispersed throughout the X chromosome stimulating heterochromatin to spread from LINE sequence outward into unique sequence until the entire X chromosome is transcriptionally inactivated (Marahrens, 1999). Xist-LINE interactions may allow the Xist RNA, which is required for heterochromatin formation, to be transferred to distant chromosomal regions. More recently, we have been examining the phenotype caused by a Xist deletion on the active X chromosome. At the chromosomal level we have been examining the replication timing, stability' of gene silencing, and chromatin structure. We have also been characterizing the phenotype of embryos of Xist+/- mothers. Our results indicate that a Xist deletion in the nontranscribed allele of the active X chromosome causes the chromatin structure throughout the entire inactive X chromosome to be altered. This appears to result in a destabilization of gene silencing in adult females resulting in growth retardation of their embryos when they are pregnant. Our results can only be explained by a transvection-based mechanism.