International Mammalian Genome Society

The 14th International Mouse Genome Conference (2000)

I23. Role of Antisense RNA at the Xist Locus in X-Inactivation

Takashi Sado1, 2, Hiroyuki Sasaki1, and En Li2
1Hum. Genet., Natl. Inst. Genet., Japan;

In early development of female mammals, one of the two X-chromosomes becomes transcriptionally silenced. While X-inactivation in the embryonic lineage is basically random, the paternally derived X chromosome is preferentially inactivated in the extraembryonic lineages in rodents. Several lines of evidence suggest that in the extraembryonic tissues, the maternal X chromosome is imprinted to be extremely resistant to inactivation rather than the paternal X chromosome being imprinted to be inactivated. Since antisense RNA is often associated with imprinted genes, we surmised that there might be transcription occurring at the Xist locus in an opposite direction to Xist, which could regulate not only expression of Xist in cis but also the imprinting related to X-inactivation in the extraembryonic tissues. We assumed that such antisense RNA involved in imprinted X-inactivation, if present, could be expressed in the placenta in both male and female fetuses. We attempted to examine this possibility, and found that such antisense transcription indeed takes place at the Xist locus in the placenta. Although Lee et al. recently described the presence of unprocessed antisense RNA, Tsix, at the Xist locus, our results indicated that the major products of the antisense RNA are subject to processing. In addition, the antisense RNA appeared to be imprinted so as to occur from the maternal X chromosome in blastocysts, implying that it is involved in imprinted X-inactivation. Targeted disruption of the antisense RNA in male ES cells caused accumulation of Xist RNA on the X chromosome in a subset of the differentiated population. The role of the antisense RNA in X-inactivation will be discussed.

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