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Molecular and Cellular Biology, July 2003, p. 4461-4470, Vol. 23, No. 13
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.13.4461-4470.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Silencing of Mouse Aprt Is a Gradual Process in Differentiated Cells

Center for Research on Occupational and Environmental Toxicology,¹ Department of Molecular and Medical Genetics,² Division of Molecular Medicine, Oregon Health and Sciences University, Portland, Oregon 97239,⁴ Department of Pathology and Laboratory Medicine, University of Kentucky, Lexington, Kentucky 40536³

Received 20 December 2002/ Returned for modification 18 March 2003/ Accepted 3 April 2003

Mouse Aprt constructs that are highly susceptible to DNA methylation-associated inactivation in embryonal carcinoma cells were transfected into differentiated cells, where they were expressed. Construct silencing was induced by either whole-cell fusion of the expressing differentiated cells with embryonal carcinoma cells or by treatment of the differentiated cells with the DNA demethylating agent 5-aza-2'-deoxycytidine. Induction of silencing was enhanced significantly by the presence of a methylation center fragment positioned upstream of a truncated promoter comprised of two functional Sp1 binding sites. Initial silencing of the Aprt constructs was unstable, as evidenced by high spontaneous reversion frequencies (10^-2). Stably silenced subclones with spontaneous reversion frequencies of <10^-5 were isolated readily from the unstably silenced clones. These reversion frequencies were enhanced significantly by treatment of the cells with 5-aza-2'-deoxycytidine. A bisulfite sequence analysis demonstrated that CpG methylation initiated within the methylation center region on expressing alleles and that the induction of silencing allowed methylation to spread towards and eventually into the promoter region. Combined with the induction of revertants by 5-aza-2'-deoxycytidine, this result suggested that stabilization of silencing was due to an increased density of CpG methylation. All allelic methylation patterns were variegated, which is consistent with a gradual and evolving process. In total, our results demonstrate that silencing of mouse Aprt is a gradual process in the differentiated cells.

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