Establishment and maintenance of gene expression patterns of X-linked genes.

Abstract

X chromosome inactivation equalizes the dose of X-linked genes between male and female mammals. This process is a chromosome-wide, long-range genetic effect which transcriptionally silences one X chromosome per diploid female cell. Within somatic tissues, the choice of which X homolog is inactivated is believed to be random. The basic mechanisms that regulate X inactivation during early development and throughout the life of an organism are largely unknown. Most studies of X inactivation have relied on qualitative or semi-quantitative measures of gene expression. This study documents the development of techniques for discriminating the expression of homologous alleles of X-linked genes to a high sensitivity and the application of such assays to better understand the regulation of gene expression from the X chromosome. The assays were used to examine the levels and variation of X chromosome gene expression in genetically homogenous, interspecific F1 hybrid female mice of different ages. The four primary findings were: (1) non-genetic or stochastic variation had a significant impact on the choice of homolog inactivated in the context of different X chromosome controlling elements; (2) within a tissue sample, X-linked genes demonstrated significant positively correlated expression; (3) the variations in gene expression were age-independent suggesting X inactivation is rigorously maintained; (4) evidence was obtained for a role of the X inactive transcript (Xist) in the maintenance of X chromosome inactivation.