Dynamics and Memory of Heterochromatin in Living Cells

Howard Hughes Medical Institute, Department of Developmental Biology, Stanford University School of Medicine, Stanford, CA 94305, USA.
Cell (Impact Factor: 32.24). 06/2012; 149(7):1447-60. DOI: 10.1016/j.cell.2012.03.052
Source: PubMed


Posttranslational histone modifications are important for gene regulation, yet the mode of propagation and the contribution to heritable gene expression states remains controversial. To address these questions, we developed a chromatin in vivo assay (CiA) system employing chemically induced proximity to initiate and terminate chromatin modifications in living cells. We selectively recruited HP1α to induce H3K9me3-dependent gene silencing and describe the kinetics and extent of chromatin modifications at the Oct4 locus in fibroblasts and pluripotent cells. H3K9me3 propagated symmetrically and continuously at average rates of ~0.18 nucleosomes/hr to produce domains of up to 10 kb. After removal of the HP1α stimulus, heterochromatic domains were heritably transmitted, undiminished through multiple cell generations. Our data enabled quantitative modeling of reaction kinetics, which revealed that dynamic competition between histone marking and turnover, determines the boundaries and stability of H3K9me3 domains. This framework predicts the steady-state dynamics and spatial features of the majority of euchromatic H3K9me3 domains over the genome.

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Available from: Nathaniel A Hathaway
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    • "For example, JAK2- mediated phosphorylation of H3Y41, a site located at the DNA entry/ exit point [55], was found to be refractory to HP1α chromatin association by perturbing an interaction between the HP1α chromoshadow domain and H3 in this region. While H3Y41 was shown to relieve HP1-mediated gene repression, it remains to be determined whether H3Y41 phosphorylation negates HP1α interaction in the context of H3K9me3 [56] [57]. Interestingly, neighboring H3R42 was recently identified as a substrate for CARM1 and PRMT6-mediated asymmetric dimethylation (H3R42me2a) [58]. "
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    • "Theoretical models based on a combination of such feedback loops have suggested the existence of two discrete chromatin states that can stably co-exist ( " bistability " ) for a certain range of conditions (Schreiber & Bernstein, 2002; Dodd et al, 2007; Angel et al, 2011). Hathaway et al have proposed an alternative, " monostable " model of heterochromatin propagation through interactions between neighboring nucleosomes (Hathaway et al, 2012). However, direct evidence on how such epigenetic networks might operate in living cells is lacking. "
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    • "However, a recent study demonstrated that the maintenance of induced heterochromatin is not dependent on DNA methylation. Using a chromatin in vivo assay (CiA), which enables induction and termination of chromatin modifications in living cells, the authors selectively targeted HP1α to induce a H3K9me3 heterochromatic domain at the Oct4 locus (Hathaway et al., 2012). Interestingly, they found that after removal of HP1α these heterochromatic domains were heritably transmitted over multiple cell divisions independently of DNA methylation, suggesting that H3K9me3 is the epigenetic mark required for inheritance of heterochromatic state. "
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