Maternal Rnf12/RLIM is required for imprinted X-chromosome inactivation in mice

Abstract

To ensure that female mammals do not have an excess of X-chromosome gene products, one of the two X chromosomes present in each cell is silenced. During embryogenesis, the imprinted form of this X-chromosome inactivation (XCI) process selectively silences the paternal X following the detection of Xist RNA expression on the paternal X chromosome (Xp) at about the four-cell stage of embryonic development. Later, an embryonic form of XCI occurs in the developing blastocyst of the embryo proper, inactivating either the paternal or maternal X chromosome at random. Using mouse genetics, Shin et al. show that maternal deposits of Rnf12/RLIM ubiquitin ligase are crucial in the initiation of the initial process, but not in the later random X inactivation. Two forms of X-chromosome inactivation ensure the selective silencing of female sex chromosomes in mouse embryos. Imprinted silencing begins with the detection of Xist RNA expression on the paternal X chromosome at about the four-cell stage of development. Later, a random form of inactivation silences either the paternal or the maternal X chromosome. Here it is shown that maternal deposits of the ubiquitin ligase Rnf12/RLIM are required for the imprinted form of X-chromosome inactivation. Two forms of X-chromosome inactivation (XCI) ensure the selective silencing of female sex chromosomes during mouse embryogenesis. Imprinted XCI begins with the detection of Xist RNA expression on the paternal X chromosome (Xp) at about the four-cell stage of embryonic development. In the embryonic tissues of the inner cell mass, a random form of XCI occurs in blastocysts that inactivates either Xp or the maternal X chromosome (Xm)1,2. Both forms of XCI require the non-coding Xist RNA that coats the inactive X chromosome from which it is expressed. Xist has crucial functions in the silencing of X-linked genes, including Rnf12 (refs 3, 4) encoding the ubiquitin ligase RLIM (RING finger LIM-domain-interacting protein). Here we show, by targeting a conditional knockout of Rnf12 to oocytes where RLIM accumulates to high levels, that the maternal transmission of the mutant X chromosome (Δm) leads to lethality in female embryos as a result of defective imprinted XCI. We provide evidence that in Δm female embryos the initial formation of Xist clouds and Xp silencing are inhibited. In contrast, embryonic stem cells lacking RLIM are able to form Xist clouds and silence at least some X-linked genes during random XCI. These results assign crucial functions to the maternal deposit of Rnf12/RLIM for the initiation of imprinted XCI.