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Functional analysis of XIST and DNMT1 genes in the maintenance of X chromosome inactivation process in human through gene silencing by RNAi

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Author(s):
Stabellini, Raquel
Total Authors: 1
Document type: Doctoral Thesis
Press: São Paulo. 2008. 141 f., ilustrações.
Institution: Universidade de São Paulo (USP). Instituto de Biociências
Defense date:
Examining board members:
Carramaschi, Lygia da Veiga Pereira; Jasiulionis, Miriam Galvonas; Mingroni Netto, Regina Celia; Morgante, Angela Maria Vianna; Vasques, Luciana dos Reis
Advisor: Carramaschi, Lygia da Veiga Pereira
Field of knowledge: Biological Sciences - Biochemistry
Indexed in: Banco de Dados Bibliográficos da USP-DEDALUS; Biblioteca Digital de Teses e Dissertações - USP
Location: Universidade de São Paulo. Biblioteca do Instituto de Biociências; D-1313
Abstract

X chromosome inactivation (XCI) is the phenomenon through which one of the X chromosomes in female mammals is silenced to achieve dosage compensation related to males. It involves the expression of XIST gene exclusively from the inactive X, and the association of its RNA in cis in this chromosome. This leads to a series of epigenetic modifications in the chromatin of the inactive X (Xi) that guarantee a stable maintenance of the transcriptional silence through all the mitoses in the organism. One of these epigenetic modifications is DNA methylation, achieved mainly by the maintenance DNA methylase DNMT1. The roles of XIST and DNMT1 in the maintenance phase of XCI are controversial in humans. Therefore, the main goal of this present work was to analyze some of the possible functions of these genes in this process in untransformed human cells. An experimental system was optimized to study possible disturbances in maintenance of XCI, where the re-expression of genes submitted to this process could be monitored. In this system we identified two genes, MAOA and GYG2, whose pattern of expression on the Xi, differed from what had been previously described. It was demonstrated that low levels of XIST expression were sufficient to keep its RNA associated to the Xi, assuring the silenced state of this chromosome. Besides, evidences have been found that XIST inhibition in human fibroblasts reduces cellular viability. It was possible to demonstrate that DNMT1 is necessary to the maintenance of global genome methylation in untransformed human cells, and the eXISTence of a compensation mechanism involving DNMT3B upregulation. It was also observed that repression of DNMT1 was not sufficient to reactivate genes of the Xi chromosome. Additionally, demethylation of MAOA and XIST promoters was not enough to cause expression of these genes on the inactive and active Xs, respectively. All these results emphasize the requirement of studying the molecular mechanisms of XCI in humans using experimental systems appropriate for the analysis of epigenetic inheritance. (AU)