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DNA triplet repeats mediate heterochromatin-protein-1-sensitive variegated gene silencing


Gene repression is crucial to the maintenance of differentiated cell types in multicellular organisms, whereas aberrant silencing can lead to disease. The organization of DNA into chromatin and heterochromatin1 is implicated in gene silencing. In chromatin, DNA wraps around histones, creating nucleosomes. Further condensation of chromatin, associated with large blocks of repetitive DNA sequences, is known as heterochromatin. Position effect variegation (PEV) occurs when a gene is located abnormally close to heterochromatin, silencing the affected gene in a proportion of cells1. Here we show that the relatively short triplet-repeat expansions found in myotonic dystrophy and Friedreich's ataxia confer variegation of expression on a linked transgene in mice. Silencing was correlated with a decrease in promoter accessibility and was enhanced by the classical PEV modifier heterochromatin protein 1 (HP1). Notably, triplet-repeat-associated variegation was not restricted to classical heterochromatic regions but occurred irrespective of chromosomal location. Because the phenomenon described here shares important features with PEV, the mechanisms underlying heterochromatin-mediated silencing might have a role in gene regulation at many sites throughout the mammalian genome and modulate the extent of gene silencing and hence severity in several triplet-repeat diseases.

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Figure 1: CTG or GAA triplet-repeat expansions confer position-independent variegation on a heterochromatin-sensitive reporter.
Figure 2: Triplet-repeat-associated gene silencing correlated with inaccessible chromatin at the promoter but not the enhancer.
Figure 3: Increased nucleosomal density in triplet-repeat proximal regions.
Figure 4: Triplet-repeat-associated variegation is enhanced by overexpression of HP1β.


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We thank P. Fraser and especially C. Osborne (Babraham Insitute) for help with the RNA FISH; S. Sharghi Namini, S. Uribe Lewis and D. Kazazi for technical assistance; M. Walport, N. Dillon, L. Aragorn and D. Kioussis for critical reading of an earlier version of the manuscript; and M. McCarthy for advice on statistical analysis. A.S. was a Wellcome Travelling Fellow and C.E. a Wellcome Clinical Fellow. This work was supported by the Muscular Dystrophy Campaign (UK), the MRC UK and a EU Framework 5 grant—GeneXtra.

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Correspondence to Richard Festenstein.

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Saveliev, A., Everett, C., Sharpe, T. et al. DNA triplet repeats mediate heterochromatin-protein-1-sensitive variegated gene silencing. Nature 422, 909–913 (2003).

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