Acetylated histones are associated with FMR1 in normal but not fragile X-syndrome cells

Abstract

Mutation of FMR1 results in fragile X mental retardation. The most common FMR1 mutation is expansion of a CGG repeat tract at the 5' end of FMR1, which leads to cytosine methylation and transcriptional silencing. Both DNA methylation and histone deacetylation have been associated with transcriptional inactivity. The finding that the methyl cytosine-binding protein MeCP2 binds to histone deacetylases and represses transcription in vivo supports a model in which MeCP2 recruits histone deacetylases to methylated DNA, resulting in histone deacetylation, chromatin condensation and transcriptional silencing. Here we demonstrate that the 5' end of FMR1 is associated with acetylated histones H3 and H4 in cells from normal individuals, but acetylation is reduced in cells from fragile X patients. Treatment of fragile X cells with 5-aza-2'-deoxycytidine (5-aza-dC) resulted in reassociation of acetylated histones H3 and H4 with FMR1 and transcriptional reactivation, whereas treatment with trichostatin A (TSA) led to almost complete acetylated histone H4 and little acetylated histone H3 reassociation with FMR1, as well as no detectable transcription. Our results represent the first description of loss of histone acetylation at a specific locus in human disease, and advance understanding of the mechanism of FMR1 transcriptional silencing.

Transcriptional repression of BRCA1 by aberrant cytosine methylation, histone hypoacetylation and chromatin condensation of the BRCA1 promoter

“…Our findings are consistent with recent reports of the imprinted human fragile X mental retardation gene, FMR1. The FMR1 gene is inactivated in fragile X cells by cytosine methylation, hypoacetylation of histones H3 and H4 and chromatin condensation in its 5′ regulatory region compared to normal cells (23,24). Similarly, in an in vitro system, a methylated herpes simplex virus thymidine kinase reporter is transcriptionally repressed in stably transfected L-cells and is associated with hypoacetylated histone H4 and DNase I insensitivity (25).…”

DNA methylation in early development

“…In patients with the full phenotype, the CGG repeat has expanded to over 200 and more typically >500-800 copies. The result of this repeat expansion is DNA methylation and silencing of the FMR1 gene (Sutcliffe et al, 1992;Coffee et al, 1999). The transcriptional repression then leads to loss of this important developmental protein.…”

The active FMR1 promoter is associated with a large domain of altered chromatin conformation with embedded local histone modifications

“…Previous studies have analyzed histone modifications in only a small (Ϸ2 kb) region around the FMR1 promoter (14)(15)(16). In light of the findings presented above, we were interested in determining the relationship between histone modifications and the region of altered chromatin conformation detected by 3C.…”

“…The active promoter displays high levels of histone acetylation and H3K4 methylation, whereas the silenced promoter is enriched in H3K9 methylation (14)(15)(16). Further, expression of FMR1 was correlated with formation of DNaseI hypersensitive sites at the promoter, pointing to an open chromatin conformation at this site (18).…”

“…Silencing of FMR1 is caused by expansion of a CGG trinucleotide repeat located in the first exon of the gene (13). This genetic alteration triggers DNA methylation of the repeat plus local changes in histone modifications (14)(15)(16). When combined, these epigenetic alterations induce the silencing of FMR1, leading to the clinical features of fragile X syndrome, the most common form of inherited mental retardation (17).…”