Beatrice Bodega

A Long ncRNA Links Copy Number Variation to a Polycomb/Trithorax Epigenetic Switch in FSHD Muscular Dystrophy

Summary Repetitive sequences account for more than 50% of the human genome. Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal-dominant disease associated with reduction in the copy number of the D4Z4 repeat mapping to 4q35. By an unknown mechanism, D4Z4 deletion causes an epigenetic switch leading to de-repression of 4q35 genes. Here we show that the Polycomb group of epigenetic repressors targets D4Z4 in healthy subjects and that D4Z4 deletion is associated with reduced Polycomb silencing in FSHD patients. We identify DBE-T, a chromatin-associated noncoding RNA produced selectively in FSHD patients that coordinates de-repression of 4q35 genes. DBE-T recruits the Trithorax group protein Ash1L to the FSHD locus, driving histone H3 lysine 36 dimethylation, chromatin remodeling, and 4q35 gene transcription. This study provides insights into the biological function of repetitive sequences in regulating gene expression and shows how mutations of such elements can influence the progression of a human genetic disease.

Chromatin regulated interchange between polycomb repressive complex 2 (PRC2)-Ezh2 and PRC2-Ezh1 complexes controls myogenin activation in skeletal muscle cells

BackgroundPolycomb group (PcG) genes code for chromatin multiprotein complexes that are responsible for maintaining gene silencing of transcriptional programs during differentiation and in adult tissues. Despite the large amount of information on PcG function during development and cell identity homeostasis, little is known regarding the dynamics of PcG complexes and their role during terminal differentiation.ResultsWe show that two distinct polycomb repressive complex (PRC)2 complexes contribute to skeletal muscle cell differentiation: the PRC2-Ezh2 complex, which is bound to the myogenin (MyoG) promoter and muscle creatine kinase (mCK) enhancer in proliferating myoblasts, and the PRC2-Ezh1 complex, which replaces PRC2-Ezh2 on MyoG promoter in post-mitotic myotubes. Interestingly, the opposing dynamics of PRC2-Ezh2 and PRC2-Ezh1 at these muscle regulatory regions is differentially regulated at the chromatin level by Msk1 dependent methyl/phospho switch mechanism involving phosphorylation of serine 28 of the H3 histone (H3S28ph). While Msk1/H3S28ph is critical for the displacement of the PRC2-Ezh2 complex, this pathway does not influence the binding of PRC2-Ezh1 on the chromatin. Importantly, depletion of Ezh1 impairs muscle differentiation and the chromatin recruitment of MyoD to the MyoG promoter in differentiating myotubes. We propose that PRC2-Ezh1 is necessary for controlling the proper timing of MyoG transcriptional activation and thus, in contrast to PRC2-Ezh2, is required for myogenic differentiation.ConclusionsOur data reveal another important layer of epigenetic control orchestrating skeletal muscle cell terminal differentiation, and introduce a novel function of the PRC2-Ezh1 complex in promoter setting.

Remodeling of the chromatin structure of the facioscapulohumeral muscular dystrophy (FSHD) locus and upregulation of FSHD-related gene 1 (FRG1) expression during human myogenic differentiation

BackgroundFacioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant neuromuscular disorder associated with the partial deletion of integral numbers of 3.3 kb D4Z4 DNA repeats within the subtelomere of chromosome 4q. A number of candidate FSHD genes, adenine nucleotide translocator 1 gene (ANT1), FSHD-related gene 1 (FRG1), FRG2 and DUX4c, upstream of the D4Z4 array (FSHD locus), and double homeobox chromosome 4 (DUX4) within the repeat itself, are upregulated in some patients, thus suggesting an underlying perturbation of the chromatin structure. Furthermore, a mouse model overexpressing FRG1 has been generated, displaying skeletal muscle defects.ResultsIn the context of myogenic differentiation, we compared the chromatin structure and tridimensional interaction of the D4Z4 array and FRG1 gene promoter, and FRG1 expression, in control and FSHD cells. The FRG1 gene was prematurely expressed during FSHD myoblast differentiation, thus suggesting that the number of D4Z4 repeats in the array may affect the correct timing of FRG1 expression. Using chromosome conformation capture (3C) technology, we revealed that the FRG1 promoter and D4Z4 array physically interacted. Furthermore, this chromatin structure underwent dynamic changes during myogenic differentiation that led to the loosening of the FRG1/4q-D4Z4 array loop in myotubes. The FRG1 promoter in both normal and FSHD myoblasts was characterized by H3K27 trimethylation and Polycomb repressor complex binding, but these repression signs were replaced by H3K4 trimethylation during differentiation. The D4Z4 sequences behaved similarly, with H3K27 trimethylation and Polycomb binding being lost upon myogenic differentiation.ConclusionWe propose a model in which the D4Z4 array may play a critical chromatin function as an orchestrator of in cis chromatin loops, thus suggesting that this repeat may play a role in coordinating gene expression.

Lamin A/C sustains PcG protein architecture, maintaining transcriptional repression at target genes

Reduction of lamin A/C, which is evolutionarily required for the modulation of Polycomb group (PcG) protein–dependent transcriptional repression by sustaining PcG protein nuclear architecture, leads to PcG protein diffusion and to muscle differentiation.

Expression Profiling of FSHD-1 and FSHD-2 Cells during Myogenic Differentiation Evidences Common and Distinctive Gene Dysregulation Patterns

BACKGROUND Determine global gene dysregulation affecting 4q-linked (FSHD-1) and non 4q-linked (FSHD-2) cells during early stages of myogenic differentiation. This approach has been never applied to FSHD pathogenesis. METHODOLOGY/PRINCIPAL FINDINGS By in vitro differentiation of FSHD-1 and FSHD-2 myoblasts and gene chip analysis we derived that gene expression profile is altered only in FSHD-1 myoblasts and FSHD-2 myotubes. The changes seen in FSHD-1 regarded a general defect in cell cycle progression, probably due to the upregulation of myogenic markers PAX3 and MYOD1, and a deficit of factors (SUV39H1 and HMGB2) involved in D4Z4 chromatin conformation. On the other hand, FSHD-2 mytubes were characterized by a general defect in RNA metabolism, protein synthesis and degradation and, to a lesser extent, in cell cycle. Common dysregulations regarded genes involved in response to oxidative stress and in sterol biosynthetic process. Interestingly, our results also suggest that miRNAs might be implied in both FSHD-1 and FSHD-2 gene dysregulation. Finally, in both cell differentiation systems, we did not observe a gradient of altered gene expression throughout the 4q35 chromosome. CONCLUSIONS/SIGNIFICANCE FSHD-1 and FSHD-2 cells showed, in different steps of myogenic differentiation, a global deregulation of gene expression rather than an alteration of expression of 4q35 specific genes. In general, FSHD-1 and FSHD-2 global gene deregulation interested common and distinctive biological processes. In this regard, defects of cell cycle progression (FSHD-1 and to a lesser extent FSHD-2), protein synthesis and degradation (FSHD-2), response to oxidative stress (FSHD-1 and FSHD-2), and cholesterol homeostasis (FSHD-1 and FSHD-2) may in general impair a correct myogenesis. Taken together our results recapitulate previously reported defects of FSHD-1, and add new insights into the gene deregulation characterizing both FSHD-1 and FSHD-2, in which miRNAs may play a role.

Evolutionary genomic remodelling of the human 4q subtelomere (4q35.2)

BackgroundIn order to obtain insights into the functionality of the human 4q35.2 domain harbouring the facioscapulohumeral muscular dystrophy (FSHD) locus, we investigated in African apes genomic and chromatin organisations, and the nuclear topology of orthologous regions.ResultsA basic block consisting of short D4Z4 arrays (10–15 repeats), 4q35.2 specific sequences, and approximately 35 kb of interspersed repeats from different LINE subfamilies was repeated at least twice in the gorilla 4qter. This genomic organisation has undergone evolutionary remodelling, leading to the single representation of both the D4Z4 array and LINE block in chimpanzee, and the loss of the LINE block in humans. The genomic remodelling has had an impact on 4qter chromatin organisation, but not its interphase nuclear topology. In comparison with humans, African apes show very low or undetectable levels of FRG1 and FRG2 histone 4 acetylation and gene transcription, although histone deacetylase inhibition restores gene transcription to levels comparable with those of human cells, thus indicating that the 4qter region is capable of acquiring a more open chromatin structure. Conversely, as in humans, the 4qter region in African apes has a very peripheral nuclear localisation.ConclusionThe 4q subtelomere has undergone substantial genomic changes during evolution that have had an impact on chromatin condensation and the regions transcriptional regulation. Consequently, the 4qter genes in African apes and humans seem to be subjected to a different strategy of regulation in which LINE and D4Z4 sequences may play a pivotal role. However, the effect of peripheral nuclear anchoring of 4qter on these regulation mechanisms is still unclear. The observed differences in the regulation of 4qter gene expression between African apes and humans suggest that the human 4q35.2 locus has acquired a novel functional relevance.

A cytosolic Ezh1 isoform modulates a PRC2-Ezh1 epigenetic adaptive response in postmitotic cells

The evolution of chromatin-based epigenetic cell memory may be driven not only by the necessity for cells to stably maintain transcription programs, but also by the need to recognize signals and allow plastic responses to environmental stimuli. The mechanistic role of the epigenome in adult postmitotic tissues, however, remains largely unknown. In vertebrates, two variants of the Polycomb repressive complex (PRC2–Ezh2 and PRC2–Ezh1) control gene silencing via methylation of histone H3 on Lys27 (H3K27me). Here we describe a reversible mechanism that involves a novel isoform of Ezh1 (Ezh1β). Ezh1β lacks the catalytic SET domain and acts in the cytoplasm of skeletal muscle cells to control nuclear PRC2–Ezh1 activity in response to atrophic oxidative stress, by regulating Eed assembly with Suz12 and Ezh1α (the canonical isoform) at their target genes. We report a novel PRC2–Ezh1 function that utilizes Ezh1β as an adaptive stress sensor in the cytoplasm, thus allowing postmitotic cells to maintain tissue integrity in response to environmental changes.

Forced expression of RDH10 gene retards growth of HepG2 cells

The constitutive over-expression of the retinol dehydrogenase 10 (RDH10) gene, involved in retinoic acid (RA) biosynthesis, produced in HepG2 cells a significant antiproliferative response, but not signs of apoptosis. An indirect assay based on the Chloramphenicol AcetylTransferase (CAT) reporter gene driven by a retinoic acid responsive elements (RARE) suggests in genetically modified HepG2 cells an increase of the endogenous RA concentration. Furthermore, the growth arrest of HepG2 cells over-expressing the RDH10 gene was associated with the up-regulation and down-regulation of, respectively, RARβ/p21Cip1 and CycE/CdK2 mRNAs. These results indicated that forced expression of RDH10 produces antiproliferative effects highly comparable to those achieved by retinoids treatment and thus the development of a gene therapy, finalized at the restoration of the enzymatic and receptorial machinery of the RA pathway, could be a possible curative strategy for hepatocellular carcinoma (HCC).

Evolutionary history of linked D4Z4 and Beta satellite clusters at the FSHD locus (4q35).

We performed a detailed genomic investigation of the chimpanzee locus syntenic to human chromosome 4q35.2, associated to the facioscapulohumeral dystrophy. Two contigs of approximately 150 kb and 200 kb were derived from PTR chromosomes 4q35 and 3p12, respectively: both regions showed a very similar sequence organization, including D4Z4 and Beta satellite linked clusters. Starting from these findings, we derived a hypothetical evolutionary history of human 4q35, 10q26 and 3p12 chromosome regions focusing on the D4Z4–Beta satellite linked organization. The D4Z4 unit showed an open reading frame (DUX4) at both PTR 4q35 and 3p12 regions; furthermore some subregions of the Beta satellite unit showed a high degree of conservation between chimpanzee and humans. In conclusion, this paper provides evidence that at the 4q subtelomere the linkage between D4Z4 and Beta satellite arrays is a feature that appeared late during evolution and is conserved between chimpanzee and humans.

How Polycomb-Mediated Cell Memory Deals With a Changing Environment

Cells and tissues are continuously exposed to a changing microenvironment, hence the necessity of a flexible modulation of gene expression that in complex organism have been achieved through specialized chromatin mechanisms. Chromatin‐based cell memory enables cells to maintain their identity by fixing lineage specific transcriptional programs, ensuring their faithful transmission through cell division; in particular PcG‐based memory system evolved to maintain the silenced state of developmental and cell cycle genes. In evolution the complexity of this system have increased, particularly in vertebrates, indicating combinatorial and dynamic properties of Polycomb proteins, in some cases even overflowing outside the cell nucleus. Therefore, their function may not be limited to the imposition of rigid states of genetic programs, but on the ability to recognize signals and allow plastic transcriptional changes in response to different stimuli. Here, we discuss the most novel PcG mediated memory functions in facing and responding to the challenges posed by a fluctuating environment.