In vivo YY1 knockdown effects on genomic imprinting.

The YY1 transcription factor is predicted to control several imprinted domains, including the Peg3, Gnas and Xist/Tsix regions. To test this possibility, we have used RNA interference strategies to generate transgenic mouse lines that express reduced levels of the cellular YY1 protein. As predicted, lowering YY1 levels resulted in global expression changes in these three imprinted domains. In neonatal brains, most imprinted genes of the Peg3 domain were up-regulated. In the Gnas domain, Nespas was down-regulated, whereas three other imprinted transcripts were up-regulated, including Nesp, Gnasxl and Exon1A. In the Xist/Tsix domain, no obvious change was detected in the expression levels of the two genes in female mice. However, male mice showed low-level coordinated, up- and down-regulation of Xist and Tsix, respectively, suggesting potential de-repression of Xist in a subset of male cell populations. YY1 knockdown also changed the methylation levels at the imprinting control regions (ICRs) of these domains in a target-specific manner. In addition, breeding experiments indicated that the birth weights of 20% of the transgenic females were much lower than those of normal female littermates. We surmise that this gender-specific outcome is caused by the YY1 knockdown effect on the Xist locus of females. In sum, these results demonstrate that YY1 indeed functions as a trans factor for transcriptional regulation and DNA methylation of these imprinted domains in vivo.