A Suv39h-dependent mechanism for silencing S-phase genes in differentiating but not in cycling cells

The Rb/E2F complex represses S-phase genes both in cycling cells and in cells that have permanently exited from the cell cycle and entered a terminal differentiation pathway. Here we show that S-phase gene repression, which involves histone-modifying enzymes, occurs through distinct mechanisms in these two situations. We used chromatin immunoprecipitation to show that methylation of histone H3 lysine 9 (H3K9) occurs at several Rb/E2F target promoters in differentiating cells but not in cycling cells. Furthermore, phenotypic knock-down experiments using siRNAs showed that the histone methyltransferase Suv39h is required for histone H3K9 methylation and subsequent repression of S-phase gene promoters in differentiating cells, but not in cycling cells. These results indicate that the E2F target gene permanent silencing mechanism that is triggered upon terminal differentiation is distinct from the transient repression mechanism in cycling cells. Finally, Suv39h-depleted myoblasts were unable to express early or late muscle differentiation markers. Thus, appropriately timed H3K9 methylation by Suv39h seems to be part of the control switch for exiting the cell cycle and entering differentiation

Differential Cooperation between Heterochromatin Protein HP1 Isoforms and MyoD in Myoblasts*S⃞

Mechanisms of transcriptional repression are important during cell differentiation. Mammalian heterochromatin protein 1 isoforms HP1α, HP1β, and HP1γ play important roles in the regulation of chromatin structure and function. We explored the possibility of different roles for the three HP1 isoforms in an integrated system, skeletal muscle terminal differentiation. In this system, terminal differentiation is initiated by the transcription factor MyoD, whose target genes remain mainly silent until myoblasts are induced to differentiate. Here we show that HP1α and HP1β isoforms, but not HP1γ, interact with MyoD in myoblasts. This interaction is direct, as shown using recombinant proteins in vitro. A gene reporter assay revealed that HP1α and HP1β, but not HP1γ, inhibit MyoD transcriptional activity, suggesting a model in which MyoD could serve as a bridge between nucleosomes and chromatin-binding proteins such as HDACs and HP1. Chromatin immunoprecipitation assays show a preferential recruitment of HP1 proteins on MyoD target genes in proliferating myoblasts. Finally, modulation of HP1 protein level impairs MyoD target gene expression and muscle terminal differentiation. Together, our data show a nonconventional interaction between HP1 and a tissue-specific transcription factor, MyoD. In addition, they strongly suggest that HP1 isoforms play important roles during muscle terminal differentiation in an isoform-dependent manner

Histone Deacetylase Inhibitors in the Treatment of Muscular Dystrophies: Epigenetic Drugs for Genetic Diseases

Histone deacetylases inhibitors (HDACi) include a growing number of drugs that share the ability to inhibit the enzymatic activity of some or all the HDACs. Experimental and preclinical evidence indicates that these epigenetic drugs not only can be effective in the treatment of malignancies, inflammatory diseases and degenerative disorders, but also in the treatment of genetic diseases, such as muscular dystrophies. The ability of HDACi to counter the progression of muscular dystrophies points to HDACs as a crucial link between specific genetic mutations and downstream determinants of disease progression. It also suggests the contribution of epigenetic events to the pathogenesis of muscular dystrophies. Here we describe the experimental evidence supporting the key role of HDACs in the control of the transcriptional networks underlying the potential of dystrophic muscles either to activate compensatory regeneration or to undergo fibroadipogenic degeneration. Studies performed in mouse models of Duchenne muscular dystrophy (DMD) indicate that dystrophin deficiency leads to deregulated HDAC activity, which perturbs downstream networks and can be restored directly, by HDAC blockade, or indirectly, by reexpression of dystrophin. This evidence supports the current view that HDACi are emerging candidate drugs for pharmacological interventions in muscular dystrophies, and reveals unexpected common beneficial outcomes of pharmacological treatment or gene therapy