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 heterochromatin 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 cells. 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.

Human CD2 locus control region (LCR) sequences are shown here to be essential for establishing an open chromatin configuration. Transgenic mice carrying an hCD2 mini-gene attached only to the 3' CD2 transcriptional enhancer exhibited variegated expression when the transgene integrated in the centromere. In contrast, mice carrying a transgene with additional 3' sequences showed no variegation even when the latter integrated in centromeric positions. This result suggests that LCRs operate by ensuring an open chromatin configuration and that a short region, with no enhancer activity, functions in the establishment, maintenance, or both of an open chromatin domain.

Heterochromatin protein 1 (HP1beta), a key component of condensed DNA, is strongly implicated in gene silencing and centromeric cohesion. Heterochromatin has been considered a static structure, stabilizing crucial aspects of nuclear organization and prohibiting access to transcription factors. We demonstrate here, by fluorescence recovery after photobleaching, that a green fluorescent protein-HP1beta fusion protein is highly mobile within both the euchromatin and heterochromatin of ex vivo resting murine T cells. Moreover, T cell activation greatly increased this mobility, indicating that such a process may facilitate (hetero)chromatin remodeling and permit access of epigenetic modifiers and transcription factors to the many genes that are consequently derepressed.

Methylation of histone H3 lysine 4 (H3K4me) is an evolutionarily conserved modification whose role in the regulation of gene expression has been extensively studied. In contrast, the function of H3K4 acetylation (H3K4ac) has received little attention because of a lack of tools to separate its function from that of H3K4me. Here we show that, in addition to being methylated, H3K4 is also acetylated in budding yeast. Genetic studies reveal that the histone acetyltransferases (HATs) Gcn5 and Rtt109 contribute to H3K4 acetylation in vivo. Whilst removal of H3K4ac from euchromatin mainly requires the histone deacetylase (HDAC) Hst1, Sir2 is needed for H3K4 deacetylation in heterochomatin. Using genome-wide chromatin immunoprecipitation (ChIP), we show that H3K4ac is enriched at promoters of actively transcribed genes and located just upstream of H3K4 tri-methylation (H3K4me3), a pattern that has been conserved in human cells. We find that the Set1-containing complex (COMPASS), which promotes H3K4me2 and -me3, also serves to limit the abundance of H3K4ac at gene promoters. In addition, we identify a group of genes that have high levels of H3K4ac in their promoters and are inadequately expressed in H3-K4R, but not in set1Δ mutant strains, suggesting that H3K4ac plays a positive role in transcription. Our results reveal a novel regulatory feature of promoter-proximal chromatin, involving mutually exclusive histone modifications of the same histone residue (H3K4ac and H3K4me).

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Heterochromatin protein 1 (HP1), a component of condensed chromatin, was discovered more than 10 years ago and subsequently found to play important roles in chromosomal biology and gene silencing. Consistent with the hypothesis that post-translational modifications of histones may functionally 'mark' DNA sequences, HP1 was found to bind to 'silent' chromatin via the methylated lysine 9 (K9) residue on the histone H3 tail that protrudes from the nucleosome. The discovery of several HP1-associating proteins has given us insight into how HP1 may function. Although initially found to localise predominantly at heterochromatin, recent data suggest that HP1 also localises and dynamically participates in gene regulation in euchromatin. Moreover, the initial definition of HP1 as a gene repressor may need to be revisited, as HP1 has been shown, in some cases, to localise at transcriptionally active chromosomal sites. Here we review current knowledge on HP1 and explore possible mechanisms whereby HP1 might exert divergent effects on gene regulation.

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An intronic silencer within the CD4 gene is the critical cis regulatory element for T cell subset-specific expression of CD4. We have combined transfection studies with gene targeting in mice to identify several key sequences within the silencer core that are required for gene silencing during thymocyte development. In mice, mutations in individual sites resulted in variegated, but heritable, derepression of CD4 in mature CD8(+) T lymphocytes, whereas compound mutations resulted in full derepression. These results indicate that there is partial redundancy in recruiting a chromatin remodeling machinery that results in epigenetic silencing. Mutations in single sites also resulted in partial derepression of CD4 in immature double-negative thymocytes, but there was no apparent variegation. These findings suggest two distinct modes of CD4 silencer function at different developmental stages: active repression in CD4(-)CD8(-) thymocytes, in which silencing must be reversible, and epigenetic gene silencing upon differentiation to the CD8(+) cytotoxic T cell lineage.