Transcript versus transcription? The regulation of X-inactivation by antisense Tsix expression

Numerous sense-antisense gene pairs have been discovered in various organisms. Antisense genes play important roles in establishing parentally imprinted gene expression patterns in mammals. Typically, protein-coding sense genes are reciprocally regulated by their non-coding antisense partners. One example for antisense regulation is the Xist (X-inactive specific transcript) and Tsix gene pair, which is pivotal in X-inactivation. Xist works as a functional RNA molecule that recruits repressive chromatin factors towards one of the female Xs for inactivation. Antisense Tsix transcription negatively regulates Xist and protects one X-chromosome in cis from inactivation by Xist. Albeit, the precise molecular mechanism is still obscure it has been shown that Tsix transcription regulates the chromatin structure by altering histone tail modifications and DNA methylation at the Xist promoter. In addition, Xist and Tsix RNA form an RNA duplexes in vivo and are processed to small RNAs, which have a potential regulatory function. Here we review the latest findings and based on ample experimental data consider models for antisense-mediated gene regulation in X-inactivation.