| In mammals,X-chromosome inactivation(XCI),compensates for the imbalance of the X chromosomes between the sexes.XCI is initiated by Xist RNA,which is located in the region of Xic.Xist is a long non-coding RNA that encodes approximately 17 kb from one of the X chromosomes.Xist RNA coats on X chromosome in cis,recruits PRC1/PRC2 by binding to proteins,and these PRC1/PRC2 recruit H3K27me3 and H2AK119ub1,then the CpG island of X-linked genes will be modified,and silenced.The fail of XCI and the over-expression of Xist RNA will initiate diseases,such as the lethal of embryo,Sj?gren's syndrome and cancers etc.,demonstrating that Xist plays an important role in the development of embryo and the growth of individuals.Xic locates on Xq13.3 in human,and Xist contains several tandem repeats,named repeat A-F.Besides,the minimal promoter of Xist(Xist P1)is in the area of 5'-end of Xist,and Xist P1 contained a highly conserved sequence in humans,mice,rabbits and horses.In previous report,the results showed that Xist P1 contributes to the Xist RNA activation by binding to the common transcription factors SP1,YY1 and TBP in cells.Xist are transcribed of two chromosomes prior to X inactivation,and then one of chromosomes is repression of Xist and reactivation.However,the role of the Xist P1 has not yet been investigated in individual animals.There is limited genetic evidence to support this conclusion in animal level.The diversity in the regulation of XCI initiates between mammals.For mice,the paternal X-chromosome(Xp)is inactive during 4-cells and 8-cells stage,which is named imprinted XCI(iXCI).Then,from morula to blastocyst stage the inner cell mass(ICM)become reactivation,and one of the paternal X-chromosome and maternal X-chromosome is inactive randomly.The pattern of XCI in human as follow.No iXCI occurs during early embryo,then one of the paternal X-chromosome and maternal X-chromosome is inactive in the blastocyst stage,which is defined as random inactivation.In rabbit,the pattern of XCI is familiar to human,the different of XCI in human and mice is limited to generate mice as a animal model to demonstrate the function of Xist in human.As a classic model organism,rabbits are similar with human in the aspects of physiological and anatomical.Shorter gestational cycles and lower feeding costs are also defined as the advantages of rabbits as a model.In additions,rabbits are an appropriate animal model for biomedical research,as they show more similarities to human beings in terms of XCI pattern and the conserved gene sequence than mice.Therefore,this study generate Xist P1 gene knockout rabbit though CRISPR/Cas9 system.In this study,We have successfully obtained Xist P1 knockout rabbits,and we provided the first evidence that the P1 is non-essential for the regulation of Xist expression and for inducing the process of XCI in animal individuals.The result is different with the previous report,that the Xist P1 is necessary to the transcription of Xist.The Xist P1 knockout rabbits are closer to the process of XCI in human,and this novel animal model would provide a convenient way to explore the further functions of Xist. |
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