SIRT7: a novel molecular target for personalized cancer treatment?

The Sirtuin family of NAD+-dependent enzymes assumes a pivotal role in orchestrating adaptive responses to environmental fluctuations and stress stimuli, operating at both genomic and metabolic levels. Within this family, SIRT7 emerges as a versatile player in tumorigenesis, displaying both pro-tumorigenic and tumor-suppressive functions in a context-dependent manner. While other sirtuins, such as SIRT1 and SIRT6, exhibit a similar dual role in cancer, SIRT7 stands out due to distinctive attributes that sharply distinguish it from other family members. Among these are a unique key role in regulation of nucleolar functions, a close functional relationship with RNA metabolism and processing -exceptional among sirtuins- and a complex multienzymatic nature, which provides a diverse range of molecular targets. This review offers a comprehensive overview of the current understanding of the role of SIRT7 in various malignancies, placing particular emphasis on the intricate molecular mechanisms employed by SIRT7 to either stimulate or counteract tumorigenesis. Additionally, it delves into the unique features of SIRT7, discussing their potential and specific implications in tumor initiation and progression, underscoring the promising avenue of targeting SIRT7 for the development of innovative anti-cancer therapies.

Lack of the Histone Deacetylase SIRT1 Leads to Protection against Endoplasmic Reticulum Stress through the Upregulation of Heat Shock Proteins

Histone deacetylase SIRT1 represses gene expression through the deacetylation of histones and transcription factors and is involved in the protective cell response to stress and aging. However, upon endoplasmic reticulum (ER) stress, SIRT1 impairs the IRE1α branch of the unfolded protein response (UPR) through the inhibition of the transcriptional activity of XBP-1 and SIRT1 deficiency is beneficial under these conditions. We hypothesized that SIRT1 deficiency may unlock the blockade of transcription factors unrelated to the UPR promoting the synthesis of chaperones and improving the stability of immature proteins or triggering the clearance of unfolded proteins. SIRT1+/+ and SIRT1-/- fibroblasts were exposed to the ER stress inducer tunicamycin and cell survival and expression of heat shock proteins were analyzed 24 h after the treatment. We observed that SIRT1 loss significantly reduced cell sensitivity to ER stress and showed that SIRT1-/- but not SIRT1+/+ cells constitutively expressed high levels of phospho-STAT3 and heat shock proteins. Hsp70 silencing in SIRT1-/- cells abolished the resistance to ER stress. Furthermore, accumulation of ubiquitinated proteins was lower in SIRT1-/- than in SIRT1+/+ cells. Our data showed that SIRT1 deficiency enabled chaperones upregulation and boosted the proteasome activity, two processes that are beneficial for coping with ER stress.

SIRT7 regulates NUCKS1 chromatin binding to elicit metabolic and inflammatory gene expression in senescence and liver aging

Sirtuins, a class of highly conserved histone/protein deacetylases, are heavily implicated in senescence and aging. The regulation of sirtuin proteins is tightly controlled both transcriptionally and translationally and via localization within the cell. While Sirtiun proteins are implicated with aging, how their levels are regulated during aging across cell types and eliciting tissue specific age-related cellular changes is unclear. Here, we demonstrate that SIRT7 is targeted for degradation during senescence and liver aging. To uncover the significance of SIRT7 loss, we performed proteomics analysis and identified a new SIRT7 interactor, the HMG box protein NUCKS1. We found that the NUCKS1 transcription factor is recruited onto chromatin during senescence and this is mediated by SIRT7 loss. Further, depletion of NUCKS1 delayed senescence upon DNA damage leading to reduction of inflammatory gene expression. Examination of NUCKS1 transcriptional regulation during senescence revealed gene targets of transcription factors NFKB1, RELA, and CEBPβ. Consistently, in both Sirt7 KO mouse liver and in naturally aged livers, Nucks1 was recruited to chromatin. Further, Nucks1 was bound at promoters and enhancers of age-related genes, including transcription factor Rela, and, moreover, these bound sites had increased accessibility during aging. Overall, our results uncover NUCKS1 as a novel interactor of SIRT7, and show that loss of SIRT7 during senescence and liver aging promotes NUCKS1 chromatin binding to regulate metabolic and inflammatory genes.

SIRT7 and p53 interaction in embryonic development and tumorigenesis

p53 is a hallmark tumor suppressor due in part to its role in cell cycle progression, DNA damage repair, and cellular apoptosis; its protein activity interrelates with the Sirtuin family of proteins, major regulators of the cellular response to metabolic, oxidative, and genotoxic stress. In the recent years, mammalian Sirtuin 7 (SIRT7) has emerged as a pivotal regulator of p53, fine-tuning its activity in a context dependent manner. SIRT7 is frequently overexpressed in human cancer, yet its precise role in tumorigenesis and whether it involves p53 regulation is insufficiently understood. Depletion of SIRT7 in mice results in impaired embryo development and premature aging. While p53 activity has been suggested to contribute to tissue specific dysfunction in adult Sirt7 -/- mice, whether this also applies during development is currently unknown. By generating SIRT7 and p53 double-knockout mice, here we show that the demise of SIRT7-deficient embryos is not the result of p53 activity. Notably, although SIRT7 is commonly considered an oncogene, SIRT7 haploinsufficiency increases tumorigenesis in p53 knockout mice. Remarkably, in specific human tumors harboring p53 mutation, we identified that SIRT7 low expression correlates with poor patient prognosis. Transcriptomic analysis unveils a previously unrecognized interplay between SIRT7 and p53 in epithelial-to-mesenchymal transition (EMT) and extracellular matrix regulation with major implications for our understanding of embryonic development and tumor progression.

A complex interplay between H2A.Z and HP1 isoforms regulates pericentric heterochromatin

Pericentric heterochromatin (PCH) plays an essential role in the maintenance of genome integrity and alterations in PCH have been linked to cancer and aging. HP1 α, β, and γ, are hallmarks of constitutive heterochromatin that are thought to promote PCH structure through binding to heterochromatin-specific histone modifications and interaction with a wide range of factors. Among the less understood components of PCH is the histone H2A variant H2A.Z, whose role in the organization and maintenance of PCH is poorly defined. Here we show that there is a complex interplay between H2A.Z and HP1 isoforms in PCH. While the loss of HP1α results in the accumulation of H2A.Z.1 in PCH, which is associated with a significant decrease in its mobile fraction, H2A.Z.1 binds preferentially to HP1β in these regions. Of note, H2A.Z.1 downregulation results in increased heterochromatinization and instability of PCH, reflected by accumulation of the major epigenetic hallmarks of heterochromatin in these regions and increased frequency of chromosome aberrations related to centromeric/pericentromeric defects. Our studies support a role for H2A.Z in genome stability and unveil a key role of H2A.Z in the regulation of heterochromatin-specific epigenetic modifications through a complex interplay with the HP1 isoforms.

SIRT1 regulates DNA damage signaling through the PP4 phosphatase complex

The Sirtuin family of NAD+-dependent enzymes plays an important role in maintaining genome stability upon stress. Several mammalian Sirtuins have been linked directly or indirectly to the regulation of DNA damage during replication through Homologous recombination (HR). The role of one of them, SIRT1, is intriguing as it seems to have a general regulatory role in the DNA damage response (DDR) that has not yet been addressed. SIRT1-deficient cells show impaired DDR reflected in a decrease in repair capacity, increased genome instability and decreased levels of γH2AX. Here we unveil a close functional antagonism between SIRT1 and the PP4 phosphatase multiprotein complex in the regulation of the DDR. Upon DNA damage, SIRT1 interacts specifically with the catalytical subunit PP4c and promotes its inhibition by deacetylating the WH1 domain of the regulatory subunits PP4R3α/β. This in turn regulates γH2AX and RPA2 phosphorylation, two key events in the signaling of DNA damage and repair by HR. We propose a mechanism whereby during stress, SIRT1 signaling ensures a global control of DNA damage signaling through PP4.

Validation of a DNA methylation microarray for 285,000 CpG sites in the mouse genome

The mouse has been extensively used as the model organism in many studies to characterize biological pathways, drug effects and to mimic human diseases. The similar DNA sequences between both species facilitates these type of experiments. However, much less is known about the mouse epigenome, particularly for DNA methylation. Progress in delivering mouse DNA methylomes has been slow due to the currently available time-consuming and expensive methodologies. Following the great acceptance of the human DNA methylation microarrays, we have herein validated a newly developed DNA methylation microarray (Infinium Mouse Methylation BeadChip) that interrogates 280,754 unique CpG sites within the mouse genome. The CpGs included in the platform cover CpG islands, shores, shelfs and open sea sequences; and loci surrounding transcription start sites and gene bodies. From the functional standpoint, mouse ENCODE representative DNase hypersensitivity sites (rDHSs) and candidate cis-Regulatory Elements (cCREs) are also included. Herein, we show that the profiled mouse DNA methylation microarray provides reliable values among technical replicates; matched results from fresh frozen versus formalin-fixed samples; detects hemimethylated X-chromosome and imprinted CpG sites; and is able to determine CpG methylation changes in mouse cell lines treated with a DNA demethylating agent or upon genetic disruption of a DNA methyltransferase. Most important, using unsupervised hierarchical clustering and t-SNE approaches, the platform is able to classify all types of normal mouse tissues and organs. These data underscore the great features of the assessed microarray to obtain comprehensive DNA methylation profiles of the mouse genome.

O-147 Differential seminal metabolomic signature is related to sperm quality

Study question

What is the entire metabolomic profile of human semen and does the metabolic composition differ between men with good-quality and low-quality semen?

Summary answer

Human semen contains ∼700 different metabolites, and the metabolomic signature differs between normozoospermic men and men with altered seminal parameters. 

What is known already

Semen contains a wide diversity of metabolites as has been identified in single and targeted metabolite studies. The full composition of metabolites in human semen, however, is not known. The knowledge of the complete metabolic signature in semen and whether there are differences between metabolic composition and seminal quality could enhance our knowledge of possible factors involved in reduced sperm quality and male infertility.

Study design, size, duration

Case-control study, where a total of 100 men (age= 29.73±8.9 years) from March 2019 to March 2020 participated. The study was approved by the Ethics Committee of Investigación Biomédica de Andalucia.

Participants/materials, setting, methods

Semen samples from 69 normozoospermic and 31 oligozoospermic men were collected at the University Hospital and sperm biobank (Ceifer Biobank - NextClinics). The complete metabolome from unprocessed seminal samples was analysed by Ultrahigh Performance Liquid Chromatography-Tandem Mass Spectroscopy (UPLC-MS/MS). Raw data were extracted, peak-identified and quality control processed using Metabolon’s hardware and software (metabolon.com). Multiple regression models controlling for age and sample collection centres were applied using R software. 

Main results and the role of chance

In total, 695 different metabolites were detected in the seminal samples, where docosahexaenoate (DHA; 22:6n3, PubChem ID 445580), choline phosphate (1014), dihomo-linolenate (20:3n3 or n6, 5280581), docosapentaenoate (n6 DPA; 22:5n6, 6441454), adenosine 3’,5’-cyclic monophosphate (cAMP, 6076) and N-acetylalliin (122164824) metabolites were the most prevalent. The seminal metabolomic profiles differed significantly between men with normal and low sperm parameters. The most abundant metabolites in normozoospermic men belonged to Lipid Super-Pathway, while Nucleotide Super-Pathway was predominant in semen samples with low quality (p < 0.05). More specifically, the leading Sub-Pathway in normozoospermic men was Long Chain Polyunsaturated Fatty Acid (n3 and n6), whereas Purine and Pyrimidine Metabolism Sub-Pathway prevailed in low-quality semen samples, where DHA and cAMP dominated in men with normal and low seminal quality parameters, respectively (p < 0.05 in all comparisons). 

Limitations, reasons for caution

This is the first study presenting the entire metabolome signature of unprocessed human semen, and these preliminary results need to be confirmed in a bigger sample size.

Wider implications of the findings

Semen analyses applied in clinics do not evaluate the functional status of sperm, leaving the infertility causes due to male factor frequently unknown. Our study results could help to understand the molecular background of reduced seminal quality and male infertility and lead to identification of molecular biomarkers of functional sperm.

Trial registration number

not applicable

A synthetic mRNA cell reprogramming method using CYCLIN D1 promotes DNA repair, generating improved genetically stable human induced pluripotent stem cells

A key challenge for clinical application of induced pluripotent stem cells (iPSC) to accurately model and treat human pathologies depends on developing a method to generate genetically stable cells to reduce long-term risks of cell transplant therapy. Here, we hypothesized that CYCLIN D1 repairs DNA by highly efficient homologous recombination (HR) during reprogramming to iPSC that reduces genetic instability and threat of neoplastic growth. We adopted a synthetic mRNA transfection method using clinically compatible conditions with CYCLIN D1 plus base factors (OCT3/4, SOX2, KLF4, LIN28) and compared with methods that use C-MYC. We demonstrate that CYCLIN D1 made iPSC have (a) lower multitelomeric signal, (b) reduced double-strand DNA breaks, (c) correct nuclear localization of RAD51 protein expression, and (d) reduced single-nucleotide polymorphism (SNP) changes per chromosome, compared with the classical reprogramming method using C-MYC. CYCLIN D1 iPSC have reduced teratoma Ki67 cell growth kinetics and derived neural stem cells successfully engraft in a hostile spinal cord injury (SCI) microenvironment with efficient survival, differentiation. We demonstrate that CYCLIN D1 promotes double-stranded DNA damage repair predominantly through HR during cell reprogramming to efficiently produce iPSC. CYCLIN D1 reduces general cell stress associated with significantly lower SIRT1 gene expression and can rescue Sirt1 null mouse cell reprogramming. In conclusion, we show synthetic mRNA transfection of CYCLIN D1 repairs DNA during reprogramming resulting in significantly improved genetically stable footprint in human iPSC, enabling a new cell reprogramming method for more accurate and reliable generation of human iPSC for disease modeling and future clinical applications.

Shikimic acid protects skin cells from UV-induced senescence through activation of the NAD+-dependent deacetylase SIRT1

UV radiation is one of the main contributors to skin photoaging by promoting the accumulation of cellular senescence, which in turn induces a proinflammatory and tissue-degrading state that favors skin aging. The members of the sirtuin family of NAD⁺-dependent enzymes play an anti-senescence role and their activation suggests a promising approach for preventing UV-induced senescence in the treatment of skin aging. A two-step screening designed to identify compounds able to protect cells from UV-induced senescence through sirtuin activation identified shikimic acid (SA), a metabolic intermediate in many organisms, as a bona-fide candidate. The protective effects of SA against senescence were dependent on specific activation of SIRT1 as the effect was abrogated by the SIRT1 inhibitor EX-527. Upon UV irradiation SA induced S-phase accumulation and a decrease in p16^INK4A expression but did not protect against DNA damage or increased polyploidies. In contrast, SA reverted misfolded protein accumulation upon senescence, an effect that was abrogated by EX-527. Consistently, SA induced an increase in the levels of the chaperone BiP, resulting in a downregulation of unfolded protein response (UPR) signaling and UPR-dependent autophagy, avoiding their abnormal hyperactivation during senescence. SA did not directly activate SIRT1 in vitro, suggesting that SIRT1 is a downstream effector of SA signaling specifically in the response to cellular senescence. Our study not only uncovers a shikimic acid/SIRT1 signaling pathway that prevents cellular senescence, but also reinforces the role of sirtuins as key regulators of cell proteostasis.

Sirtuins in female meiosis and in reproductive longevity

Transmission of genetic material through high-quality gametes to progeny requires accurate homologous chromosome recombination and segregation during meiosis. A failure to accomplish these processes can have major consequences in reproductive health, including infertility, and development disorders in offspring. Sirtuins, a family of NAD⁺ -dependent protein deacetylases and ADP-ribosyltransferases, play key roles in genome maintenance, metabolism, and aging. In recent years, Sirtuins have emerged as regulators of several reproductive processes and interventions aiming to target Sirtuin activity are of great interest in the reproductive biology field. Sirtuins are pivotal to protect germ cells against oxidative stress, a major determinant influencing ovarian aging and the quality of gametes. Sirtuins also safeguard the integrity of the genome through epigenetic programs required for regulating gene repression, DNA repair, and chromosome segregation, among others. Although these functions are relatively well characterized in many somatic tissues, how they contribute to reproductive functions is not well understood. This review summarizes our current knowledge on the role of Sirtuins in female reproductive systems and discusses the underlying molecular pathways. In addition, we highlight the importance of Sirtuins as antiaging factors in the ovary and summarize current preclinical efforts to identify treatments to extend female reproductive longevity.

First Simulations of Axion Minicluster Halos

We study the gravitational collapse of axion dark matter fluctuations in the postinflationary scenario, so-called axion miniclusters, with N-body simulations. Largely confirming theoretical expectations, overdensities begin to collapse in the radiation-dominated epoch and form an early distribution of miniclusters with masses up to 10^{-12}  M_{⊙}. After matter-radiation equality, ongoing mergers give rise to a steep power-law distribution of minicluster halo masses. The density profiles of well-resolved halos are Navarro-Frenk-White-like to good approximation. The fraction of axion dark matter in these bound structures is ∼0.75 at redshift z=100.

SirT7 auto-ADP-ribosylation regulates glucose starvation response through mH2A1

Sirtuins are key players of metabolic stress response. Originally described as deacetylases, some sirtuins also exhibit poorly understood mono-adenosine 5'-diphosphate (ADP)-ribosyltransferase (mADPRT) activity. We report that the deacetylase SirT7 is a dual sirtuin, as it also features auto-mADPRT activity. SirT7 mADPRT occurs at a previously undefined active site, and its abrogation alters SirT7 chromatin distribution. We identify an epigenetic pathway by which ADP-ribosyl-SirT7 is recognized by the ADP-ribose reader mH2A1.1 under glucose starvation, inducing SirT7 relocalization to intergenic regions. SirT7 promotes mH2A1 enrichment in a subset of nearby genes, many of them involved in second messenger signaling, resulting in their specific up- or down-regulation. The expression profile of these genes under calorie restriction is consistently abrogated in SirT7-deficient mice, resulting in impaired activation of autophagy. Our work provides a novel perspective on sirtuin duality and suggests a role for SirT7/mH2A1.1 axis in glucose homeostasis and aging.

Sirtuin 1 Inhibiting Thiocyanates (S1th)-A New Class of Isotype Selective Inhibitors of NAD+ Dependent Lysine Deacetylases

Sirtuin 1 (Sirt1) is a NAD⁺ dependent lysine deacetylase associated with the pathogenesis of various diseases including cancer. In many cancer types Sirt1 expression is increased and higher levels have been associated with metastasis and poor prognosis. However, it was also shown, that Sirt1 can have tumor suppressing properties and in some instances even a dual role for the same cancer type has been reported. Increased Sirt1 activity has been linked to extension of the life span of cells, respectively, organisms by promoting DNA repair processes and downregulation of tumor suppressor proteins. This may have the downside of enhancing tumor growth and metastasis. In mice embryonic fibroblasts depletion of Sirt1 was shown to decrease levels of the DNA damage sensor histone H2AX. Impairment of DNA repair mechanisms by Sirt1 can promote tumorigenesis but also lower chemoresistance toward DNA targeting therapies. Despite many biological studies, there is currently just one small molecule Sirt1 inhibitor in clinical trials. Selisistat (EX-527) reached phase III clinical trials for treatment of Huntington's Disease. New small molecule Sirt1 modulators are crucial for further investigation of the contradicting roles of Sirt1 in cancer. We tested a small library of commercially available compounds that were proposed by virtual screening and docking studies against Sirt1, 2 and 3. A thienopyrimidone featuring a phenyl thiocyanate moiety was found to selectively inhibit Sirt1 with an IC₅₀ of 13 μM. Structural analogs lacking the thiocyanate function did not show inhibition of Sirt1 revealing this group as key for the selectivity and affinity toward Sirt1. Further analogs with higher solubility were identified through iterative docking studies and in vitro testing. The most active compounds (down to 5 μM IC₅₀) were further studied in cells. The ratio of phosphorylated γH2AX to unmodified H2AX is lower when Sirt1 is depleted or inhibited. Our new Sirtuin 1 inhibiting thiocyanates (S1th) lead to similarly lowered γH2AX/H2AX ratios in mouse embryonic fibroblasts as Sirt1 knockout and treatment with the reference inhibitor EX-527. In addition to that we were able to show antiproliferative activity, inhibition of migration and colony forming as well as hyperacetylation of Sirt1 targets p53 and H3 by the S1th in cervical cancer cells (HeLa). These results reveal thiocyanates as a promising new class of selective Sirt1 inhibitors.

SIRT1/2 orchestrate acquisition of DNA methylation and loss of histone H3 activating marks to prevent premature activation of inflammatory genes in macrophages

Sirtuins 1 and 2 (SIRT1/2) are two NAD-dependent deacetylases with major roles in inflammation. In addition to deacetylating histones and other proteins, SIRT1/2-mediated regulation is coupled with other epigenetic enzymes. Here, we investigate the links between SIRT1/2 activity and DNA methylation in macrophage differentiation due to their relevance in myeloid cells. SIRT1/2 display drastic upregulation during macrophage differentiation and their inhibition impacts the expression of many inflammation-related genes. In this context, SIRT1/2 inhibition abrogates DNA methylation gains, but does not affect demethylation. Inhibition of hypermethylation occurs at many inflammatory loci, which results in more drastic upregulation of their expression upon macrophage polarization following bacterial lipopolysaccharide (LPS) challenge. SIRT1/2-mediated gains of methylation concur with decreases in activating histone marks, and their inhibition revert these histone marks to resemble an open chromatin. Remarkably, specific inhibition of DNA methyltransferases is sufficient to upregulate inflammatory genes that are maintained in a silent state by SIRT1/2. Both SIRT1 and SIRT2 directly interact with DNMT3B, and their binding to proinflammatory genes is lost upon exposure to LPS or through pharmacological inhibition of their activity. In all, we describe a novel role for SIRT1/2 to restrict premature activation of proinflammatory genes.

SIRT7 mediates L1 elements transcriptional repression and their association with the nuclear lamina

Long interspersed elements-1 (LINE-1, L1) are retrotransposons that hold the capacity of self-propagation in the genome with potential mutagenic outcomes. How somatic cells restrict L1 activity and how this process becomes dysfunctional during aging and in cancer cells is poorly understood. L1s are enriched at lamin-associated domains, heterochromatic regions of the nuclear periphery. Whether this association is necessary for their repression has been elusive. Here we show that the sirtuin family member SIRT7 participates in the epigenetic transcriptional repression of L1 genome-wide in both mouse and human cells. SIRT7 depletion leads to increased L1 expression and retrotransposition. Mechanistically, we identify a novel interplay between SIRT7 and Lamin A/C in L1 repression. Our results demonstrate that SIRT7-mediated H3K18 deacetylation regulates L1 expression and promotes L1 association with elements of the nuclear lamina. The failure of such activity might contribute to the observed genome instability and compromised viability in SIRT7 knockout mice. Overall, our results reveal a novel function of SIRT7 on chromatin organization by mediating the anchoring of L1 to the nuclear envelope, and a new functional link of the nuclear lamina with transcriptional repression.

Chromatin regulation by Histone H4 acetylation at Lysine 16 during cell death and differentiation in the myeloid compartment

Histone H4 acetylation at Lysine 16 (H4K16ac) is a key epigenetic mark involved in gene regulation, DNA repair and chromatin remodeling, and though it is known to be essential for embryonic development, its role during adult life is still poorly understood. Here we show that this lysine is massively hyperacetylated in peripheral neutrophils. Genome-wide mapping of H4K16ac in terminally differentiated blood cells, along with functional experiments, supported a role for this histone post-translational modification in the regulation of cell differentiation and apoptosis in the hematopoietic system. Furthermore, in neutrophils, H4K16ac was enriched at specific DNA repeats. These DNA regions presented an accessible chromatin conformation and were associated with the cleavage sites that generate the 50 kb DNA fragments during the first stages of programmed cell death. Our results thus suggest that H4K16ac plays a dual role in myeloid cells as it not only regulates differentiation and apoptosis, but it also exhibits a non-canonical structural role in poising chromatin for cleavage at an early stage of neutrophil cell death.

Complex role of SIRT6 in NF-κB pathway regulation

The NF-κB pathway regulates cell physiology under stress conditions. We have recently described a novel NF-κB regulatory mechanism, by which SIRT6 induces cysteine monoubiquitination of the methyltransferase SUV39H1. This causes SUV39H1 dissociation from the gene encoding the NF-κB inhibitor IκBα,  increasing its expression and leading to NF-κB pathway inactivation.

Strong-Isospin-Breaking Correction to the Muon Anomalous Magnetic Moment from Lattice QCD at the Physical Point

All lattice-QCD calculations of the hadronic-vacuum-polarization contribution to the muon's anomalous magnetic moment to date have been performed with degenerate up- and down-quark masses. Here we calculate directly the strong-isospin-breaking correction to a_{μ}^{HVP} for the first time with physical values of m_{u} and m_{d} and dynamical u, d, s, and c quarks, thereby removing this important source of systematic uncertainty. We obtain a relative shift to be applied to lattice-QCD results obtained with degenerate light-quark masses of δa_{μ}^{HVP,m_{u}≠m_{d}}=+1.5(7)%, in agreement with estimates from phenomenology.

Numerical experiments using deflation with the HISQ action

We report on numerical experiments using deflation to compute quark propagators for the highly improved staggered quark (HISQ) action. The method is tested on HISQ gauge configurations, generated by the MILC collaboration, with lattice spacings of 0.15 fm, with a range of volumes, and sea quark masses down to the physical quark mass.

Raising the list of SirT7 targets to a new level

Sirtuins are crucial proteins involved in sensing and coordinating the response to different forms of stress, mainly through NAD⁺ -dependent deacetylation of proteins. For that reason, sirtuins are directly involved in many human pathologies including cancer, diabetes, cardiovascular and neurodegenerative diseases. SirT7, one of the less well-understood sirtuins, has been associated with ribosome biogenesis, gene expression, metabolism and cancer. Despite the wide range of these functions, only a handful of targets for SirT7 have so far been described. In this issue, Zhang et al. report the first proteomic screening of SirT7 substrates. Using stable isotope labeling with amino acids in cell culture (SILAC), coupled with quantitative mass spectrometry, they have identified a comprehensive list of candidates involved in a variety of functions, ranging from maintenance of chromatin architecture to gene silencing and metabolism. A selected group of these candidates has been validated by in vitro co-immunoprecipitation and deacetylation experiments. Predictive tools have identified additional candidates. The identification of these novel targets not only suggests new ways of understanding the physiological role of SirT7, but also provides new evidence to add to our existing knowledge of the global impact of sirtuins in cell homeostasis.

Loss of SIRT2 leads to axonal degeneration and locomotor disability associated with redox and energy imbalance

Sirtuin 2 (SIRT2) is a member of a family of NAD⁺‐dependent histone deacetylases (HDAC) that play diverse roles in cellular metabolism and especially for aging process. SIRT2 is located in the nucleus, cytoplasm, and mitochondria, is highly expressed in the central nervous system (CNS), and has been reported to regulate a variety of processes including oxidative stress, genome integrity, and myelination. However, little is known about the role of SIRT2 in the nervous system specifically during aging. Here, we show that middle‐aged, 13‐month‐old mice lacking SIRT2 exhibit locomotor dysfunction due to axonal degeneration, which was not present in young SIRT2 mice. In addition, these Sirt2^−/− mice exhibit mitochondrial depletion resulting in energy failure, and redox dyshomeostasis. Our results provide a novel link between SIRT2 and physiological aging impacting the axonal compartment of the central nervous system, while supporting a major role for SIRT2 in orchestrating its metabolic regulation. This underscores the value of SIRT2 as a therapeutic target in the most prevalent neurodegenerative diseases that undergo with axonal degeneration associated with redox and energetic dyshomeostasis.

The microRNA-449 family inhibits TGF-β-mediated liver cancer cell migration by targeting SOX4

BACKGROUND & AIMS

Modulation of microRNA expression is a potential treatment for hepatocellular carcinoma (HCC). Therefore, the epigenetically regulated microRNA-449 family (miR-449a, miR-449b, miR-449c) was characterized with regards to its functional effects and target genes in HCC.

METHODS

After transfection of miR-449a, miR-449b, and/or miR-449c, tumor-relevant functional effects were analyzed using in vitro assays and a xenograft mouse model. Binding specificities, target genes, and regulated pathways of each miRNA were identified by microarray analyses. Target genes were validated by luciferase reporter assays and expression analyses in vitro. Furthermore, target gene expression was analyzed in 61 primary human HCCs compared to normal liver tissue.

RESULTS

Tumor suppressive effects, binding specificities, target genes, and regulated pathways of miR-449a and miR-449b differed from those of miR-449c. Transfection of miR-449a, miR-449b, and/or miR-449c inhibited cell proliferation and migration, induced apoptosis, and reduced tumor growth to different extents. Importantly, miR-449a, miR-449b, and, to a lesser degree, miR-449c directly targeted SOX4, which codes for a transcription factor involved in epithelial-mesenchymal transition and HCC metastasis, and thereby inhibited TGF-β-mediated cell migration.

CONCLUSIONS

This study provides detailed insights into the regulatory network of the epigenetically regulated miRNA-449 family and, for the first time, describes distinct tumor suppressive effects and target specificities of miR-449a, miR-449b, and miR-449c. Our results indicate that particularly miR-449a and miR-449b may be considered for miRNA replacement therapy to prevent HCC progression and metastasis.

LAY SUMMARY

In this study, we demonstrated that the microRNA-449 family acts as a tumor suppressor in liver cancer by causing cell death and inhibiting cell migration. These effects are caused by downregulation of the oncogene SOX4, which is frequently overexpressed in liver cancer. We conclude that the microRNA-449 family may be a target for liver cancer therapy.

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, β, 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 HP1β. 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 HP1β 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 HP1β 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.

SIRT7 promotes genome integrity and modulates non-homologous end joining DNA repair

Sirtuins, a family of protein deacetylases, promote cellular homeostasis by mediating communication between cells and environment. The enzymatic activity of the mammalian sirtuin SIRT7 targets acetylated lysine in the N‐terminal tail of histone H3 (H3K18Ac), thus modulating chromatin structure and transcriptional competency. SIRT7 deletion is associated with reduced lifespan in mice through unknown mechanisms. Here, we show that SirT7‐knockout mice suffer from partial embryonic lethality and a progeroid‐like phenotype. Consistently, SIRT7‐deficient cells display increased replication stress and impaired DNA repair. SIRT7 is recruited in a PARP1‐dependent manner to sites of DNA damage, where it modulates H3K18Ac levels. H3K18Ac in turn affects recruitment of the damage response factor 53BP1 to DNA double‐strand breaks (DSBs), thereby influencing the efficiency of non‐homologous end joining (NHEJ). These results reveal a direct role for SIRT7 in DSB repair and establish a functional link between SIRT7‐mediated H3K18 deacetylation and the maintenance of genome integrity.

Arachidonic and oleic acid exert distinct effects on the DNA methylome

Abnormal fatty acid metabolism and availability are landmarks of metabolic diseases, which in turn are associated with aberrant DNA methylation profiles. To understand the role of fatty acids in disease epigenetics, we sought DNA methylation profiles specifically induced by arachidonic (AA) or oleic acid (OA) in cultured cells and compared those with published profiles of normal and diseased tissues. THP-1 monocytes were stimulated with AA or OA and analyzed using Infinium HumanMethylation450 BeadChip (Illumina) and Human Exon 1.0 ST array (Affymetrix). Data were corroborated in mouse embryonic fibroblasts. Comparisons with publicly available data were conducted by standard bioinformatics. AA and OA elicited a complex response marked by a general DNA hypermethylation and hypomethylation in the 1-200 μM range, respectively, with a maximal differential response at the 100 μM dose. The divergent response to AA and OA was prominent within the gene body of target genes, where it correlated positively with transcription. AA-induced DNA methylation profiles were similar to the corresponding profiles described for palmitic acid, atherosclerosis, diabetes, obesity, and autism, but relatively dissimilar from OA-induced profiles. Furthermore, human atherosclerosis grade-associated DNA methylation profiles were significantly enriched in AA-induced profiles. Biochemical evidence pointed to β-oxidation, PPAR-α, and sirtuin 1 as important mediators of AA-induced DNA methylation changes. In conclusion, AA and OA exert distinct effects on the DNA methylome. The observation that AA may contribute to shape the epigenome of important metabolic diseases, supports and expands current diet-based therapeutic and preventive efforts.

KAT6B Is a Tumor Suppressor Histone H3 Lysine 23 Acetyltransferase Undergoing Genomic Loss in Small Cell Lung Cancer

Recent efforts to sequence human cancer genomes have highlighted that point mutations in genes involved in the epigenetic setting occur in tumor cells. Small cell lung cancer (SCLC) is an aggressive tumor with poor prognosis, where little is known about the genetic events related to its development. Herein, we have identified the presence of homozygous deletions of the candidate histone acetyltransferase KAT6B, and the loss of the corresponding transcript, in SCLC cell lines and primary tumors. Furthermore, we show, in vitro and in vivo, that the depletion of KAT6B expression enhances cancer growth, while its restoration induces tumor suppressor-like features. Most importantly, we demonstrate that KAT6B exerts its tumor-inhibitory role through a newly defined type of histone H3 Lys23 acetyltransferase activity.

The embryonic linker histone H1 variant of Drosophila, dBigH1, regulates zygotic genome activation

Histone H1 is an essential chromatin component. Metazoans usually contain multiple stage-specific H1s. In particular, specific variants replace somatic H1s during early embryogenesis. In this regard, Drosophila was an exception because a single dH1 was identified that, starting at cellularization, is detected throughout development in somatic cells. Here, we identify the embryonic H1 of Drosophila, dBigH1. dBigH1 is abundant before cellularization occurs, when somatic dH1 is absent and the zygotic genome is inactive. Upon cellularization, when the zygotic genome is progressively activated, dH1 replaces dBigH1 in the soma, but not in the primordial germ cells (PGCs) that have delayed zygotic genome activation (ZGA). In addition, a loss-of-function mutant shows premature ZGA in both the soma and PGCs. Mutant embryos die at cellularization, showing increased levels of active RNApol II and zygotic transcripts, along with DNA damage and mitotic defects. These results show an essential function of dBigH1 in ZGA regulation.

The diversity of histone versus nonhistone sirtuin substrates

The members of the Sir2 family, or sirtuins, are major regulators of the response to different types of stress. The members of the family have adapted to increasing complexities throughout evolution and have become diversified by increasing their number, specificity, and localization and acquiring novel functions. Sirtuins have been consistently implicated in the cross-talk between the genomic information and environment from the prokaryotes onward. Evidence suggests that in the transition to eukaryotes, histones became one of the basic and most conserved targets of the family, to the extent that in yeast and mammals, sirtuins were originally described as NAD(+)-dependent histone deacetylases and classified as class III histone deacetylases. A growing number of studies have determined that sirtuins also target a wide range of nonhistone proteins. Many of these targets are also directly or indirectly related to chromatin regulation. The number of targets has grown considerably in the last decade but has provoked an ill-founded discussion that neglects the importance of histones as sirtuin targets. In this review, we summarize our knowledge regarding the range of sirtuin targets described to date and discuss the different functional implications of histone and nonhistone targets throughout evolution.

The tumor suppressor SirT2 regulates cell cycle progression and genome stability by modulating the mitotic deposition of H4K20 methylation

The establishment of the epigenetic mark H4K20me1 (monomethylation of H4K20) by PR-Set7 during G2/M directly impacts S-phase progression and genome stability. However, the mechanisms involved in the regulation of this event are not well understood. Here we show that SirT2 regulates H4K20me1 deposition through the deacetylation of H4K16Ac (acetylation of H4K16) and determines the levels of H4K20me2/3 throughout the cell cycle. SirT2 binds and deacetylates PR-Set7 at K90, modulating its chromatin localization. Consistently, SirT2 depletion significantly reduces PR-Set7 chromatin levels, alters the size and number of PR-Set7 foci, and decreases the overall mitotic deposition of H4K20me1. Upon stress, the interaction between SirT2 and PR-Set7 increases along with the H4K20me1 levels, suggesting a novel mitotic checkpoint mechanism. SirT2 loss in mice induces significant defects associated with defective H4K20me1-3 levels. Accordingly, SirT2-deficient animals exhibit genomic instability and chromosomal aberrations and are prone to tumorigenesis. Our studies suggest that the dynamic cross-talk between the environment and the genome during mitosis determines the fate of the subsequent cell cycle.

A big step for SIRT7, one giant leap for Sirtuins… in cancer

Recently reporting in Nature, Barber et al. demonstrated that SIRT7 maintains critical features that define cancer cells by removing the acetylation mark on lysine 18 of histone H3. Interestingly, hypoacetylation of H3K18 has been described as a general marker of tumor prognosis and oncoviral transformation.

Combined bottom-up and top-down mass spectrometry analyses of the pattern of post-translational modifications of Drosophila melanogaster linker histone H1

Linker histone H1 is a major chromatin component that binds internucleosomal DNA and mediates the folding of nucleosomes into a higher-order structure, namely the 30-nm chromatin fiber. Multiple post-translational modifications (PTMs) of core histones H2A, H2B, H3 and H4 have been identified and their important contribution to the regulation of chromatin structure and function is firmly established. In contrast, little is known about histone H1 modifications and their function. Here we address this question in Drosophila melanogaster, which, in contrast to most eukaryotic species, contains a single histone H1 variant, dH1. For this purpose, we combined bottom-up and top-down mass-spectrometry strategies. Our results indicated that dH1 is extensively modified by phosphorylation, methylation, acetylation and ubiquitination, with most PTMs falling in the N-terminal domain. Interestingly, several dH1 N-terminal modifications have also been reported in specific human and/or mouse H1 variants, suggesting that they have conserved functions. In this regard, we also provide evidence for the contribution of one of such conserved PTMs, dimethylation of K27, to heterochromatin organization during mitosis. Furthermore, our results also identified multiple dH1 isoforms carrying several phosphorylations and/or methylations, illustrating the high structural heterogeneity of dH1. In particular, we identified several non-CDK sites at the N-terminal domain that appear to be hierarchically phosphorylated. This study provides the most comprehensive PTM characterization of any histone H1 variant to date.

Drosophila melanogaster linker histone dH1 is required for transposon silencing and to preserve genome integrity

Histone H1 is an intrinsic component of chromatin, whose important contribution to chromatin structure is well-established in vitro. Little is known, however, about its functional roles in vivo. Here, we have addressed this question in Drosophila, a model system offering many advantages since it contains a single dH1 variant. For this purpose, RNAi was used to efficiently deplete dH1 in flies. Expression-profiling shows that dH1 depletion affects expression of a relatively small number of genes in a regional manner. Furthermore, depletion up-regulates inactive genes, preferentially those located in heterochromatin, while active euchromatic genes are down-regulated, suggesting that the contribution of dH1 to transcription regulation is mainly structural, organizing chromatin for proper gene-expression regulation. Up-regulated genes are remarkably enriched in transposons. In particular, R1/R2 retrotransposons, which specifically integrate in the rDNA locus, are strongly up-regulated. Actually, depletion increases expression of transposon-inserted rDNA copies, resulting in synthesis of aberrant rRNAs and enlarged nucleolus. Concomitantly, dH1-depleted cells accumulate extra-chromosomal rDNA, show increased γH2Av content, stop proliferation and activate apoptosis, indicating that depletion causes genome instability and affects proliferation. Finally, the contributions to maintenance of genome integrity and cell proliferation appear conserved in human hH1s, as their expression rescues proliferation of dH1-depleted cells.

The dual role of sirtuins in cancer

Among the greatest challenges facing organisms is that of detecting and effectively responding to life-threatening environmental changes that are intimately associated with metabolic fluctuations and certain forms of stress. These conditions have been linked to the onset of many human pathologies, including cancer. Over the past decade, members of the Sir2 family, or sirtuins, have been described as major players in sensing and coordinating stress response. Evidence has imputed mammalian sirtuins in carcinogenesis, although the mechanisms involved seem to be more diverse and complex than previously anticipated. Some sirtuins, such as SirT2 and SirT6, seem to work as tumor suppressors, but others, such as SirT1, are apparently bifunctional: operating as both tumor suppressors and oncogenic factors depending on the context and the study conditions. The mechanisms underlying these apparently contradictory activities are not well understood, although recent findings suggest that they might actually be two sides of the same coin. In this review, the authors summarize current knowledge on the functional implications of sirtuins in cancer and discuss possible explanations for their functional duality.

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.

Acquired chronic hepatocerebral degeneration due to cirrhosis from non-alcoholic steatohepatitis

INTRODUCTION AND OBJECTIVE

Acquired chronic hepatocerebral degeneration, acquired hepatolenticular degeneration or pseudo-Wilson is an infrequent disorder with a hepatic origin. Cases in the literature are scarce and it is frequently confused with hepatic encephalopathy and Wilson s disease. The aim of this essay is to report a patient suffering from this disorder due to cirrhosis from non-alcoholic steatohepatitis.

CASE REPORT

We present a 54-year-old man diagnosed from cirrhosis grade B9 of the Child Pugh classification. He progressively developed a picture with bradylalia, mild postural and action tremor and spatial and temporal disorientation. Further studies demonstrated an increase of the values of hepatic transaminases and a hyperintensity in the basal nuclei in the cerebral magnetic resonance imaging. Clinical and radiological data established the diagnosis of hepatocerebral degeneration.

CONCLUSIONS

Acquired chronic hepatocerebral degeneration is a disorder rarely reported in the literature that it is usually confused with other diseases. We alert about the need of having this diagnosis into account with patients developing neurological symptoms after hepatic disease.

The conserved role of sirtuins in chromatin regulation

The members of the Sir2 family, or Sirtuins, have garnered considerable attention because of their key roles as metabolic sensors and mediators of cell survival under stress. Sirtuins may play roles in myriad human pathologies such as cancer, neurological diseases, malaria, leishmaniasis and hormone-related disorders. They are present from prokaryotes to humans and show a high degree of functional diversification that has led to two different enzymatic activities, a wide range of substrates and a highly diversified pattern of cellular localization. Throughout chromatin evolution, Sirtuins have maintained an intimate functional relationship in regulating its structure and function via their targeting of histones, particularly H4K16Ac, as well as other non-histone chromatin proteins. This link permitted fast communication from metabolic fluctuations to chromatin allowing efficient adaptation to environmental stimuli. Therefore, understanding the common path of Sirtuins and chromatin development over the course of evolution might be important for understanding not only the remarkable diversity of functions of these proteins in mammals, but also the path followed by chromatin evolution. Herein is provided an overview of current knowledge of Sirtuin function, from bacteria to humans, including a discussion on its implications for chromatin dynamics, organization and integrity.

Calorie restriction and the exercise of chromatin

Since the earliest stages of evolution, organisms have faced the challenge of sensing and adapting to environmental changes for their survival under compromising conditions such as food depletion or stress. Implicit in these responses are mechanisms developed during evolution that include the targeting of chromatin to allow or prevent expression of fundamental genes and to protect genome integrity. Among the different approaches to study these mechanisms, the analysis of the response to a moderate reduction of energy intake, also known as calorie restriction (CR), has become one of the best sources of information regarding the factors and pathways involved in metabolic adaptation from lower to higher eukaryotes. Furthermore, responses to CR are involved in life span regulation-conserved from yeast to mammals-and therefore have garnered major research interest. Herein we review current knowledge of responses to CR at the molecular level and their functional link to chromatin.

[Quality of neurological care in the emergency services: a study from the community-hospital]

INTRODUCTION

The medical conditions shared by hospital emergency services and community-hospital neurology clinics (CHNC) have not been described, and the quality of the medical care received in these conditions has not been evaluated in our environment.

METHODS

Over a 2 month period, those patients presenting at any of the seven CHNC in a Health Care Area 1 of Madrid due to previously attended medical conditions in the emergency services were systematically registered. The area neurologists of the CHNC collected administrative and clinical variables and made a judgment on the medical care (primary outcome measure) and diagnoses (secondary outcome measure) received.

RESULTS

A total of 181 patients were included (mean age: 58 years; 60% women). The inclusion rate was one patient per working day, and 31% of patients were visited out of the established quota number of patients for the clinic. The most frequent reasons for visiting the emergency room were: headache (20%), focal neurological syndrome (16%) and loss of consciousness (14%). The most frequent diagnoses at the CHNC were: primary headache (19 %), stroke (11%) and epilepsy (9 %). Emergency care was deemed correct in 56 % of patients. When the patients with intervention were compared to those with no intervention, participation of the neurology service in the emergency room was associated to a greater percentage of correct diagnoses (59% vs. 41%; p=0.019) and care (69% vs. 47%; p=0.003).

CONCLUSIONS

The medical conditions shared with the emergency services represent a small but relevant proportion of the patients assisted in the CHNC. Some of these conditions (primary headaches, syncopes) should be canalized into primary health care. Others (epilepsy) require a circuit between emergency room and CNNC, but the appointment system should be adapted. The intervention of a neurologist in the emergency room raises the quality of the care.

Characterization of Drosophila melanogaster JmjC+N histone demethylases

In this article, we characterize histone demethylase activity of the entire family of JmjC+N proteins of Drosophila melanogaster. Our results show that Lid (little imaginal discs), which is structurally homologous to JARID1, demethylates H3K4me3. However, contrary to what would be inferred from its demethylase activity, lid contributes to the establishment of transcriptionally competent chromatin states as: (i) is required for histone H3 acetylation; (ii) contributes to expression of the homoeotic gene Ultrabithorax (Ubx); and (iii) antagonizes heterochromatin-mediated gene silencing (PEV). These results, which are consistent with the identification of lid as a trithorax group (trxG) gene, are discussed in the context of current models for the contribution of H3K4me3 to the regulation of gene expression. Here, we also show that the two Drosophila JMJD2 homologues, dJMJD2(1)/CG15835 and dJMJD2(2)/CG33182, are capable of demethylating both H3K9me3 and H3K36me3. dJMJD2(1)/CG15835 regulates heterochromatin organization, as its over-expression induces spreading of HP1, out of heterochromatin, into euchromatin, without affecting the actual pattern of histone modifications of heterochromatin. dJMJD2(1)/CG15835 is excluded from heterochromatin and localizes to multiple euchromatic sites, where it regulates H3K36 methylation. These results indicate that dJMJD2(1)/CG15835 contributes to delimit hetero- and euchromatic territories through the regulation of H3K36 methylation in euchromatin. On the other hand, dJARID2/CG3654 shows no demethylase activity on H3K4me3, H3K9me3, H3K27me3, H3K36me3 and H4K20me3.

Activation properties of GAGA transcription factor

GAGA is a Drosophila transcription factor that has been involved in many nuclear activities. We present evidence that GAGA factor enhances transcription by stabilizing pre-initiation complex (PIC) and promoting reinitiation. Formation of PIC prior to GAGA addition prevents activation suggesting that GAGA is required early in the formation of activated complexes. GAGA stimulation of transcription can be attributed in part to a stabilization of PIC. All these properties depend on the GAGA C-terminal glutamine-rich domain and, in addition to other roles and together with previous data, support a role of GAGA as a transcription factor.

SIRT1 regulates the histone methyl-transferase SUV39H1 during heterochromatin formation

In contrast to stably repressive, constitutive heterochromatin and stably active, euchromatin, facultative heterochromatin has the capacity to alternate between repressive and activated states of transcription. As such, it is an instructive source to understand the molecular basis for changes in chromatin structure that correlate with transcriptional status. Sirtuin 1 (SIRT1) and suppressor of variegation 3-9 homologue 1 (SUV39H1) are amongst the enzymes responsible for chromatin modulations associated with facultative heterochromatin formation. SUV39H1 is the principal enzyme responsible for the accumulation of histone H3 containing a tri-methyl group at its lysine 9 position (H3K9me3) in regions of heterochromatin. SIRT1 is an NAD+-dependent deacetylase that targets histone H4 at lysine 16 (refs 3 and 4), and through an unknown mechanism facilitates increased levels of H3K9me3 (ref. 3). Here we show that the mammalian histone methyltransferase SUV39H1 is itself targeted by the histone deacetylase SIRT1 and that SUV39H1 activity is regulated by acetylation at lysine residue 266 in its catalytic SET domain. SIRT1 interacts directly with, recruits and deacetylates SUV39H1, and these activities independently contribute to elevated levels of SUV39H1 activity resulting in increased levels of the H3K9me3 modification. Loss of SIRT1 greatly affects SUV39H1-dependent H3K9me3 and impairs localization of heterochromatin protein 1. These findings demonstrate a functional link between the heterochromatin-related histone methyltransferase SUV39H1 and the histone deacetylase SIRT1.