Research
The molecular basis for stability of heterochromatin-mediated silencing in mammals
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* Corresponding author: Richard Festenstein r.festenstein@imperial.ac.uk
1 Gene Control Mechanisms and Disease Group, MRC Clinical Sciences Centre, Imperial College, Hammersmith Hospital, London W12 0NN, UK
2 Genome Function Group, MRC Clinical Sciences Centre, Imperial College, Hammersmith Hospital, London W12 0NN, UK
3 Laboratory for Chromatin Dynamics, Riken Kobe Institute, Centre for Developmental Biology, Kobe, Hyogo 650-0047, Japan
4 Division of Immune Cell Biology, National Institute for Medical Research, Mill Hill, London NW7 1AA, UK
5 Epigenetics and Imprinting Laboratory, Department of Oncology, University of Cambridge, CRUK-CRI, Li Ka Shing Centre, Cambridge CB2 0RE, UK
Epigenetics & Chromatin 2009, 2:14 doi:10.1186/1756-8935-2-14
Published: 4 November 2009Abstract
The archetypal epigenetic phenomenon of position effect variegation (PEV) in Drosophila occurs when a gene is brought abnormally close to heterochromatin, resulting in stochastic silencing of the affected gene in a proportion of cells that would normally express it. PEV has been instrumental in unraveling epigenetic mechanisms. Using an in vivo mammalian model for PEV we have extensively investigated the molecular basis for heterochromatin-mediated gene silencing. Here we distinguish 'epigenetic effects' from other cellular differences by studying ex vivo cells that are identical, apart from the expression of the variegating gene which is silenced in a proportion of the cells. By separating cells according to transgene expression we show here that silencing appears to be associated with histone H3 lysine 9 trimethylation (H3K9me3), DNA methylation and the localization of the silenced gene to a specific nuclear compartment enriched in these modifications. In contrast, histone H3 acetylation (H3Ac) and lysine 4 di or tri methylation (H3K4me2/3) are the predominant modifications associated with expression where we see the gene in a euchromatic compartment. Interestingly, DNA methylation and inaccessibility, rather than H3K9me3, correlated most strongly with resistance to de-repression by cellular activation. These results have important implications for understanding the contribution of specific factors involved in the establishment and maintenance of gene silencing and activation in vivo.