DNA CpG hypomethylation induces heterochromatin reorganization involving the histone variant macroH2A

Abstract

In mammalian heterochromatin, cytosine bases of CpG dinucleotides are symmetrically modified by methylation. Patterns of CpG methylation are maintained by the action of Dnmt1, the mammalian maintenance cytosine methyltransferase enzyme. We genetically manipulated the levels of CpG methylation and found that extensive chromatin alterations occur in pericentric heterochromatin. Homozygous mutations in Dnmt1 cause severe hypomethylation of pericentric heterochromatin and concomitant chromatin reorganization involving the histone variant macroH2A. Demethylation-induced alterations in macroH2A localization occur in both interphase and mitotic embryonic stem (ES) cells. Heterochromatin protein 1 (HP1) marks interphase pericentric heterochromatin (chromocenters). MacroH2A immunostaining in Dnmt1–/– cells becomes coincident with chromocenters detected by HP1 content. MacroH2A, but not HP1, is enriched in nuclease-resistant chromatin fractions extracted from Dnmt1–/– cells. Normal localization of macroH2A was restored upon reintroduction of a Dnmt1 transgene into Dnmt1–/– cells. MacroH2A localization was also affected in T-antigen-transformed fibroblasts subjected to the conditional mutation of Dnmt1. Together, these results suggest that pericentric heterochromatin can be maintained in the absence of CpG methylation, but in a significantly altered configuration.

The Relationship between Global Methylation Level, Loss of Heterozygosity, and Microsatellite Instability in Sporadic Colorectal Cancer

“…Eden et al have shown that DNA hypomethylation increased the rate of LOH in mouse embryonic fibroblasts (14). Recently, it has also been reported that DNA CpG hypomethylation induces a significant reorganization of constitutive pericentric heterochromatin (36). Our observation that LOH significantly correlated with global hypomethylation might be explained by the possibility that global hypomethylation may affect the molecular composition of heterochromatin and chromatin conformation, and may promote LOH by destabilizing the genome.…”

The X and Y Chromosomes Assemble into H2A.Z, Containing Facultative Heterochromatin, following Meiosis

“…Rather than being structurally static as first believed, it is becoming clear that these large highly compacted domains are dynamic since their protein composition can change during cellular differentiation (10,26,50), between transitions from proliferation to quiescence (21), and when the levels of CpG methylation are altered (37). This argues that the function(s) of constitutive heterochromatin may vary with the cellular context.…”

Maintenance of self‐renewal ability of mouse embryonic stem cells in the absence of DNA methyltransferases Dnmt1, Dnmt3a and Dnmt3b

“…However, it is possible that some unrecognized changes in heterochromatin occur in TKO cells. Indeed, the redistribution of the histone variant macroH2A into pericentromeric heterochromatin in hypomethylated Dnmt1 -/-ES cells has recently been reported (Ma et al 2005), indicating that CpG methylation modulates some properties of heterochromatic domains. Although the apparent contribution of CpG methylation to the steady-state heterochromatin structure in ES cells may be small, it is possible that CpG methylation might serve as a mark for anchoring heterochromatin during dynamic chromatin remodeling or during multiple replication processes.…”