Stabilization of Suv39H1 by SirT1 is part of oxidative stress response and ensures genome protection.

Sirtuins are NAD-dependent deacetylases that sense oxidative stress conditions and promote a protective cellular response. The Sirtuin SirT1 is involved in facultative heterochromatin formation through an intimate functional relationship with the H3K9me3 methyltransferase Suv39h1, a chromatin organization protein. However, SirT1 also regulates Suv39h1-dependent constitutive heterochromatin (CH) through an unknown mechanism; interestingly, SirT1 does not significantly localize in these regions. Herein, we report that SirT1 controls global levels of Suv39h1 by increasing its half-life through inhibition of Suv39h1 lysine 87 polyubiquitination by the E3-ubiquitin ligase MDM2. This in turn increases Suv39h1 turnover in CH and ensures genome integrity. Stress conditions that lead to SirT1 upregulation, such as calorie restriction, also induce higher levels of Suv39h1 in a SirT1-dependent manner in vivo. These observations reflect a direct link between oxidative stress response and Suv39h1 and support a dynamic view of heterochromatin, in which its structure adapts to cell physiology.

Implications of noncoding regulatory functions in the development of insulinomas.

Insulinomas are rare neuroendocrine tumors arising from pancreatic ß cells, characterized by aberrant proliferation and altered insulin secretion, leading to glucose homeostasis failure. With the aim of uncovering the role of noncoding regulatory regions and their aberrations in the development of these tumors, we coupled epigenetic and transcriptome profiling with whole-genome sequencing. As a result, we unraveled somatic mutations associated with changes in regulatory functions. Critically, these regions impact insulin secretion, tumor development, and epigenetic modifying genes, including polycomb complex components. Chromatin remodeling is apparent in insulinoma-selective domains shared across patients, containing a specific set of regulatory sequences dominated by the SOX17 binding motif. Moreover, many of these regions are H3K27me3 repressed in ß cells, suggesting that tumoral transition involves derepression of polycomb-targeted domains. Our work provides a compendium of aberrant cis-regulatory elements affecting the function and fate of ß cells in their progression to insulinomas and a framework to identify coding and noncoding driver mutations.

An integrated multi-omics approach identifies the landscape of interferon-α-mediated responses of human pancreatic beta cells.

Interferon-α (IFNα), a type I interferon, is expressed in the islets of type 1 diabetic individuals, and its expression and signaling are regulated by T1D genetic risk variants and viral infections associated with T1D. We presently characterize human beta cell responses to IFNα by combining ATAC-seq, RNA-seq and proteomics assays. The initial response to IFNα is characterized by chromatin remodeling, followed by changes in transcriptional and translational regulation. IFNα induces changes in alternative splicing (AS) and first exon usage, increasing the diversity of transcripts expressed by the beta cells. This, combined with changes observed on protein modification/degradation, ER stress and MHC class I, may expand antigens presented by beta cells to the immune system. Beta cells also up-regulate the checkpoint proteins PDL1 and HLA-E that may exert a protective role against the autoimmune assault. Data mining of the present multi-omics analysis identifies two compound classes that antagonize IFNα effects on human beta cells.

The impact of proinflammatory cytokines on the ß-cell regulatory landscape provides insights into the genetics of type 1 diabetes.

The early stages of type 1 diabetes (T1D) are characterized by local autoimmune inflammation and progressive loss of insulin-producing pancreatic ß cells. Here we show that exposure to proinflammatory cytokines reveals a marked plasticity of the ß-cell regulatory landscape. We expand the repertoire of human islet regulatory elements by mapping stimulus-responsive enhancers linked to changes in the ß-cell transcriptome, proteome and three-dimensional chromatin structure. Our data indicate that the ß-cell response to cytokines is mediated by the induction of new regulatory regions as well as the activation of primed regulatory elements prebound by islet-specific transcription factors. We find that T1D-associated loci are enriched with newly mapped cis-regulatory regions and identify T1D-associated variants disrupting cytokine-responsive enhancer activity in human ß cells. Our study illustrates how ß cells respond to a proinflammatory environment and implicate a role for stimulus response islet enhancers in T1D.

Assay for Transposase Accessible Chromatin (ATAC-Seq) to Chart the Open Chromatin Landscape of Human Pancreatic Islets.

The regulatory mechanisms that ensure an accurate control of gene transcription are central to cellular function, development and disease. Such mechanisms rely largely on noncoding regulatory sequences that allow the establishment and maintenance of cell identity and tissue-specific cellular functions.The study of chromatin structure and nucleosome positioning allowed revealing transcription factor accessible genomic sites with regulatory potential, facilitating the comprehension of tissue-specific cis-regulatory networks. Recently a new technique coupled with high-throughput sequencing named Assay for Transposase Accessible Chromatin (ATAC-seq) emerged as an efficient method to chart open chromatin genome wide. The application of such technique to different cell types allowed unmasking tissue-specific regulatory elements and characterizing cis-regulatory networks. Herein we describe the implementation of the ATAC-seq method to human pancreatic islets, a tissue playing a central role in the control of glucose metabolism.

An HP1 isoform-specific feedback mechanism regulates Suv39h1 activity under stress conditions.

The presence of H3K9me3 and heterochromatin protein 1 (HP1) are hallmarks of heterochromatin conserved in eukaryotes. The spreading and maintenance of H3K9me3 is effected by the functional interplay between the H3K9me3-specific histone methyltransferase Suv39h1 and HP1. This interplay is complex in mammals because the three HP1 isoforms, HP1α, ß, and γ, are thought to play a redundant role in Suv39h1-dependent deposition of H3K9me3 in pericentric heterochromatin (PCH). Here, we demonstrate that despite this redundancy, HP1α and, to a lesser extent, HP1γ have a closer functional link to Suv39h1, compared to HP1ß. HP1α and γ preferentially interact in vivo with Suv39h1, regulate its dynamics in heterochromatin, and increase Suv39h1 protein stability through an inhibition of MDM2-dependent Suv39h1-K87 polyubiquitination. The reverse is also observed, where Suv39h1 increases HP1α stability compared HP1ß and γ. The interplay between Suv39h1 and HP1 isoforms appears to be relevant under genotoxic stress. Specifically, loss of HP1α and γ isoforms inhibits the upregulation of Suv39h1 and H3K9me3 that is observed under stress conditions. Reciprocally, Suv39h1 deficiency abrogates stress-dependent upregulation of HP1α and γ, and enhances HP1ß levels. Our work defines a specific role for HP1 isoforms in regulating Suv39h1 function under stress via a feedback mechanism that likely regulates heterochromatin formation.

Mammalian HP1 Isoforms Have Specific Roles in Heterochromatin Structure and Organization.

HP1 is a structural component of heterochromatin. Mammalian HP1 isoforms HP1α, HP1ß, and HP1γ play different roles in genome stability, but their precise role in heterochromatin structure is unclear. Analysis of Hp1α-/-, Hp1ß-/-, and Hp1γ-/- MEFs show that HP1 proteins have both redundant and unique functions within pericentric heterochromatin (PCH) and also act globally throughout the genome. HP1α confines H4K20me3 and H3K27me3 to regions within PCH, while its absence results in a global hyper-compaction of chromatin associated with a specific pattern of mitotic defects. In contrast, HP1ß is functionally associated with Suv4-20h2 and H4K20me3, and its loss induces global chromatin decompaction and an abnormal enrichment of CTCF in PCH and other genomic regions. Our work provides insight into the roles of HP1 proteins in heterochromatin structure and genome stability.