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Evidence for de novo imprinted X-chromosome inactivation independent of meiotic inactivation in mice


In mammals, one of the two X chromosomes is inactivated in females to enable dosage compensation for X-linked gene products1. In rodents and marsupials, only the X chromosome of paternal origin (Xp) is silenced during early embryogenesis. This could be due to a carry-over effect of the X chromosome's passage through the male germ line, where it becomes transiently silenced together with the Y chromosome, during meiotic sex chromosome inactivation (MSCI)2. Here we show that XIST (X inactive specific transcript) transgenes, located on autosomes, do not undergo MSCI in the male germ line of mice and yet can induce imprinted cis-inactivation when paternally inherited, with identical kinetics to the Xp chromosome. This suggests that MSCI is not necessary for imprinted X-chromosome inactivation in mice. We also show that the Xp is transcribed, like autosomes, at zygotic gene activation rather than being ‘pre-inactivated’3. We propose that expression of the paternal Xist gene at zygotic gene activation is sufficient to trigger cis-inactivation of the X chromosome, or of an autosome carrying a Xist transgene.

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Figure 1: Imprinted X-inactivation and autosomal Xist transgenes.
Figure 2: Histone H3 modifications and Polycomb proteins in female embryos carrying paternally transmitted Tg53 Xist transgenes.
Figure 3: Early transcriptional activity followed by silencing during imprinted X inactivation.
Figure 4: Absence of XY body marks and meiotic inactivation of the Tg53 transgene on chromosome 13 in pachytene-stage spermatocytes from male Tg53 testes.


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We would like to thank G. Almouzni, P. A. Defossez and N. Takagi for critical reading of the manuscript; members of our group and V. Colot for discussions; D. Reinberg and D. Papadopoulou for gifts of antibodies; and M. Blanche for help with mice. I.O. was supported by the CNRS; E.H. and I.O. were financed by the CNRS (ATIP programme), the French Ministry of Research (Action Concertée Incitative), the Curie Institute (Program Incitatif et Collaboratif) and the EU Network of Excellence (Epigenome). P.A. was supported by contracts from ARC by the Ministry of Research under the ACI programme and the EU Network of Excellence (Epigenome).

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Correspondence to Edith Heard.

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Supplementary information

Supplementary Data

Analysis of Xist expression patterns in blastocysts derived from transgenic males or females. (DOC 21 kb)

Supplementary Discussion

A possible explanation for the evolution of imprinted X inactivation in rodents is provided. (DOC 20 kb)

Supplementary Methods

Additional details of methods used in this study, which could not be mentioned in the main text. (DOC 25 kb)

Supplementary Figure Legends

Text to accompany the below Supplementary Figures (DOC 26 kb)

Supplementary Figure 1

Paternally and maternally transmitted autosomal Xist YAC transgenes. (PDF 6056 kb)

Supplementary Figure 2

Additional marks of imprinted X inactivation on females embryos carrying paternally transmitted Tg53 Xist transgenes. (PDF 17739 kb)

Supplementary Figure 3

Transcriptional activity of the paternal X chromosome assayed by gene specific RNA FISH. (PDF 15342 kb)

Supplementary Figure 4

Transcriptional activity of the paternal X chromosome assayed by Cot-1 RNA FISH. (PDF 13679 kb)

Supplementary Figure 5

Chromatin marks associated with the XY body in pachytene stage spermatocytes do not affect the chromosome 13 pair in Tg53 testis. (PDF 9902 kb)

Supplementary Figure 6

Analysis of transgene induced cis-inactivation in embryos derived from homozygous males. (PDF 6222 kb)

Supplementary Tables

Supplementary Table 1 details Xist expression patterns in pre-implantation embryos derived from WT females and YAC Tg53 males. Supplementary Table 2 details antibodies and dilutions used. (DOC 88 kb)

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Okamoto, I., Arnaud, D., Le Baccon, P. et al. Evidence for de novo imprinted X-chromosome inactivation independent of meiotic inactivation in mice. Nature 438, 369–373 (2005).

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