The deacylase SIRT5 supports melanoma viability by influencing chromatin dynamics.

Cutaneous melanoma remains the most lethal skin cancer, and ranks third among all malignancies in terms of years of life lost. Despite the advent of immune checkpoint and targeted therapies, only roughly half of patients with advanced melanoma achieve a durable remission. Sirtuin 5 (SIRT5) is a member of the sirtuin family of protein deacylases that regulates metabolism and other biological processes. Germline Sirt5 deficiency is associated with mild phenotypes in mice. Here we showed that SIRT5 was required for proliferation and survival across all cutaneous melanoma genotypes tested, as well as uveal melanoma, a genetically distinct melanoma subtype that arises in the eye and is incurable once metastatic. Likewise, SIRT5 was required for efficient tumor formation by melanoma xenografts and in an autochthonous mouse Braf Pten-driven melanoma model. Via metabolite and transcriptomic analyses, we found that SIRT5 was required to maintain histone acetylation and methylation levels in melanoma cells, thereby promoting proper gene expression. SIRT5-dependent genes notably included MITF, a key lineage-specific survival oncogene in melanoma, and the c-MYC proto-oncogene. SIRT5 may represent a druggable genotype-independent addiction in melanoma.

Reversible chromatin condensation by the DNA repair and demethylation factor thymine DNA glycosylase.

Chromatin structures (and modulators thereof) play a central role in genome organization and function. Herein, we report that thymine DNA glycosylase (TDG), an essential enzyme involved in DNA repair and demethylation, has the capacity to alter chromatin structure directly through its physical interactions with DNA. Using chemically defined nucleosome arrays, we demonstrate that TDG induces decompaction of individual chromatin fibers upon binding and promotes self-association of nucleosome arrays into higher-order oligomeric structures (i.e. condensation). Chromatin condensation is mediated by TDG's disordered polycationic N-terminal domain, whereas its C-terminal domain antagonizes this process. Furthermore, we demonstrate that TDG-mediated chromatin condensation is reversible by growth arrest and DNA damage 45 alpha (GADD45a), implying that TDG cooperates with its binding partners to dynamically control chromatin architecture. Finally, we show that chromatin condensation by TDG is sensitive to the methylation status of the underlying DNA. This new paradigm for TDG has specific implications for associated processes, such as DNA repair, DNA demethylation, and transcription, and general implications for the role of DNA modification 'readers' in controlling chromatin organization.

SMYD3: a regulator of epigenetic and signaling pathways in cancer.

Chromatin modifiers and their implications in oncogenesis have been an exciting area of cancer research. These are enzymes that modify chromatin via post-translational modifications such as methylation, acetylation, sumoylation, phosphorylation, in addition to others. Depending on the modification, chromatin modifiers can either promote or repress transcription. SET and MYN-domain containing 3 (SMYD3) is a chromatin modifier that has been implicated in the development and progression of various cancer types. It was first reported to tri-methylate Histone 3 Lysine 4 (H3K4), a methylation mark known to promote transcription. However, since this discovery, other histone (H4K5 and H4K20, for example) and non-histone (VEGFR, HER2, MAP3K2, ER, and others) substrates of SMYD3 have been described, primarily in the context of cancer. This review aims to provide a background on basic characteristics of SMYD3, such as its protein structure and tissue expression profiles, discuss reported histone and non-histone substrates of SMYD3, and underscore prognostic and functional implications of SMYD3 in cancer. Finally, we briefly discuss ongoing efforts to develop inhibitors of SMYD3 for future therapeutic use. It is our hope that this review will help synthesize existing research on SMYD3 in an effort to propel future discovery.

Nono deficiency compromises TET1 chromatin association and impedes neuronal differentiation of mouse embryonic stem cells.

NONO is a DNA/RNA-binding protein, which plays a critical regulatory role during cell stage transitions of mouse embryonic stem cells (mESCs). However, its function in neuronal lineage commitment and the molecular mechanisms of its action in such processes are largely unknown. Here we report that NONO plays a key role during neuronal differentiation of mESCs. Nono deletion impedes neuronal lineage commitment largely due to a failure of up-regulation of specific genes critical for neuronal differentiation. Many of the NONO regulated genes are also DNA demethylase TET1 targeted genes. Importantly, re-introducing wild type NONO to the Nono KO cells, not only restores the normal expression of the majority of NONO/TET1 coregulated genes but also rescues the defective neuronal differentiation of Nono-deficient mESCs. Mechanistically, our data shows that NONO directly interacts with TET1 via its DNA binding domain and recruits TET1 to genomic loci to regulate 5-hydroxymethylcytosine levels. Nono deletion leads to a significant dissociation of TET1 from chromatin and dysregulation of DNA hydroxymethylation of neuronal genes. Taken together, our findings reveal a key role and an epigenetic mechanism of action of NONO in regulation of TET1-targeted neuronal genes, offering new functional and mechanistic understanding of NONO in stem cell functions, lineage commitment and specification.

ATAD5 deficiency alters DNA damage metabolism and sensitizes cells to PARP inhibition.

Replication factor C (RFC), a heteropentamer of RFC1-5, loads PCNA onto DNA during replication and repair. Once DNA synthesis has ceased, PCNA must be unloaded. Recent findings assign the uloader role primarily to an RFC-like (RLC) complex, in which the largest RFC subunit, RFC1, has been replaced with ATAD5 (ELG1 in Saccharomyces cerevisiae). ATAD5-RLC appears to be indispensable, given that Atad5 knock-out leads to embryonic lethality. In order to learn how the retention of PCNA on DNA might interfere with normal DNA metabolism, we studied the response of ATAD5-depleted cells to several genotoxic agents. We show that ATAD5 deficiency leads to hypersensitivity to methyl methanesulphonate (MMS), camptothecin (CPT) and mitomycin C (MMC), agents that hinder the progression of replication forks. We further show that ATAD5-depleted cells are sensitive to poly(ADP)ribose polymerase (PARP) inhibitors and that the processing of spontaneous oxidative DNA damage contributes towards this sensitivity. We posit that PCNA molecules trapped on DNA interfere with the correct metabolism of arrested replication forks, phenotype reminiscent of defective homologous recombination (HR). As Atad5 heterozygous mice are cancer-prone and as ATAD5 mutations have been identified in breast and endometrial cancers, our finding may open a path towards the therapy of these tumours.

Assaying epigenome functions of PRMTs and their substrates.

Among the widespread and increasing number of identified post-translational modifications (PTMs), arginine methylation is catalyzed by the protein arginine methyltransferases (PRMTs) and regulates fundamental processes in cells, such as gene regulation, RNA processing, translation, and signal transduction. As epigenetic regulators, PRMTs play key roles in pluripotency, differentiation, proliferation, survival, and apoptosis, which are essential biological programs leading to development, adult homeostasis but also pathological conditions including cancer. A full understanding of the molecular mechanisms that underlie PRMT-mediated gene regulation requires the genome wide mapping of each player, i.e., PRMTs, their substrates and epigenetic marks, methyl-marks readers as well as interaction partners, in a thorough and unambiguous manner. However, despite the tremendous advances in high throughput sequencing technologies and the numerous efforts from the scientific community, the epigenomic profiling of PRMTs as well as their histone and non-histone substrates still remains a big challenge owing to obvious limitations in tools and methodologies. This review will summarize the present knowledge about the genome wide mapping of PRMTs and their substrates as well as the technical approaches currently in use. The limitations and pitfalls of the technical tools along with conventional approaches will be then discussed in detail. Finally, potential new strategies for chromatin profiling of PRMTs and histone substrates will be proposed and described.

The Role of Ubiquitination and SUMOylation in Telomere Biology.

Telomeres are a unique structure of DNA repeats covered by proteins at the ends of the chromosomes that protect the coding regions of the genome and function as a biological clock. They require a tight regulation of the factors covering and protecting their structure, as they are shortened with each cell division to limit the ability of cells to replicate uncontrollably. Additionally, they protect the chromosome ends from DNA damage responses and thereby, prevent genomic instability. Telomere dysfunction can lead to chromosomal abnormalities and cancer. Therefore, dysregulation of any of the factors that regulate the integrity of the telomeres will have implications to chromosomal stability, replicative lifespan and may lead to cell transformation. This review will cover the main factors participating in the normal function of the telomeres and how these are regulated by the ubiquitin and SUMO systems. Accumulating evidence indicate that the ubiquitin and SUMO pathways are significant regulators of the shelterin complex and other chromatin modifiers, which are important for telomere structure integrity. Furthermore, the crosstalk between these two pathways has been reported in telomeric DNA repair. A better understanding of the factors contributing to telomere biology, and how they are regulated, is important for the design of new strategies for cancer therapies and regenerative medicine.

Using Yeast to Define the Regulatory Role of Protein Lysine Methylation.

The post-translational modifications (PTM) of proteins are crucial for cells to survive under diverse environmental conditions and to respond to stimuli. PTMs are known to govern a broad array of cellular processes including signal transduction and chromatin regulation. The PTM lysine methylation has been extensively studied within the context of chromatin and the epigenetic regulation of the genome. However, it has also emerged as a critical regulator of non-histone proteins important for signal transduction pathways. While the number of known non-histone protein methylation events is increasing, the molecular functions of many of these modifications are not yet known. Proteomic studies of the model system Saccharomyces cerevisiae suggest lysine methylation may regulate a diversity of pathways including transcription, RNA processing, translation, and signal transduction cascades. However, there has still been relatively little investigation of lysine methylation as a broad cellular regulator beyond chromatin and transcription. Here, we outline our current state of understanding of non-histone protein methylation in yeast and propose ways in which the yeast system can be leveraged to develop a much more complete picture of molecular mechanisms through which lysine methylation regulates cellular functions.

Theory of Active Chromatin Remodeling.

Nucleosome positioning controls the accessible regions of chromatin and plays essential roles in DNA-templated processes. ATP driven remodeling enzymes are known to be crucial for its establishment in vivo, but their nonequilibrium nature has hindered the development of a unified theoretical framework for nucleosome positioning. Using a perturbation theory, we show that the effect of these enzymes can be well approximated by effective equilibrium models with rescaled temperatures and interactions. Numerical simulations support the accuracy of the theory in predicting both kinetic and steady-state quantities, including the effective temperature and the radial distribution function, in biologically relevant regimes. The energy landscape view emerging from our study provides an intuitive understanding for the impact of remodeling enzymes in either reinforcing or overwriting intrinsic signals for nucleosome positioning, and may help improve the accuracy of computational models for its prediction in silico.

Mendelian disorders of the epigenetic machinery: postnatal malleability and therapeutic prospects.

The epigenetic machinery in conjunction with the transcriptional machinery is responsible for maintaining genome-wide chromatin states and dynamically regulating gene expression. Mendelian disorders of the epigenetic machinery (MDEMs) are genetic disorders resulting from mutations in components of the epigenetic apparatus. Though individually rare, MDEMs have emerged as a collectively common etiology for intellectual disability (ID) and growth disruption. Studies in model organisms and humans have demonstrated dosage sensitivity of this gene group with haploinsufficiency as a predominant disease mechanism. The epigenetic machinery consists of three enzymatic components (writers, erasers and chromatin remodelers) as well as one non-enzymatic group (readers). A tally of the entire census of such factors revealed that although multiple enzymatic activities never coexist within a single component, individual enzymatic activities often coexist with a reader domain. This group of disorders disrupts both the chromatin and transcription states of target genes downstream of the given component but also DNA methylation on a global scale. Elucidation of these global epigenetic changes may inform our understanding of disease pathogenesis and have diagnostic utility. Moreover, many therapies targeting epigenetic marks already exist, and some have proven successful in treating cancer. This, along with the recent observation that neurological dysfunction in these disorders may in fact be treatable in postnatal life, suggests that the scientific community should prioritize this group as a potentially treatable cause of ID. Here we summarize the recent expansion and major characteristics of MDEMs, as well as the unique therapeutic prospects for this group of disorders.

KDM2 proteins constrain transcription from CpG island gene promoters independently of their histone demethylase activity.

CpG islands (CGIs) are associated with the majority of mammalian gene promoters and function to recruit chromatin modifying enzymes. It has therefore been proposed that CGIs regulate gene expression through chromatin-based mechanisms, however in most cases this has not been directly tested. Here, we reveal that the histone H3 lysine 36 (H3K36) demethylase activity of the CGI-binding KDM2 proteins contributes only modestly to the H3K36me2-depleted state at CGI-associated gene promoters and is dispensable for normal gene expression. Instead, we discover that KDM2 proteins play a widespread and demethylase-independent role in constraining gene expression from CGI-associated gene promoters. We further show that KDM2 proteins shape RNA Polymerase II occupancy but not chromatin accessibility at CGI-associated promoters. Together this reveals a demethylase-independent role for KDM2 proteins in transcriptional repression and uncovers a new function for CGIs in constraining gene expression.

Stability and sub-cellular localization of DNA polymerase ß is regulated by interactions with NQO1 and XRCC1 in response to oxidative stress.

Protein-protein interactions regulate many essential enzymatic processes in the cell. Somatic mutations outside of an enzyme active site can therefore impact cellular function by disruption of critical protein-protein interactions. In our investigation of the cellular impact of the T304I cancer mutation of DNA Polymerase ß (Polß), we find that mutation of this surface threonine residue impacts critical Polß protein-protein interactions. We show that proteasome-mediated degradation of Polß is regulated by both ubiquitin-dependent and ubiquitin-independent processes via unique protein-protein interactions. The ubiquitin-independent proteasome pathway regulates the stability of Polß in the cytosol via interaction between Polß and NAD(P)H quinone dehydrogenase 1 (NQO1) in an NADH-dependent manner. Conversely, the interaction of Polß with the scaffold protein X-ray repair cross complementing 1 (XRCC1) plays a role in the localization of Polß to the nuclear compartment and regulates the stability of Polß via a ubiquitin-dependent pathway. Further, we find that oxidative stress promotes the dissociation of the Polß/NQO1 complex, enhancing the interaction of Polß with XRCC1. Our results reveal that somatic mutations such as T304I in Polß impact critical protein-protein interactions, altering the stability and sub-cellular localization of Polß and providing mechanistic insight into how key protein-protein interactions regulate cellular responses to stress.

Diverse and dynamic DNA modifications in brain and diseases.

DNA methylation is a class of epigenetic modification essential for coordinating gene expression timing and magnitude throughout normal brain development and for proper brain function following development. Aberrant methylation changes are associated with changes in chromatin architecture, transcriptional alterations and a host of neurological disorders and diseases. This review highlights recent advances in our understanding of the methylome's functionality and covers potential new roles for DNA methylation, their readers, writers, and erasers. Additionally, we examine novel insights into the relationship between the methylome, DNA-protein interactions, and their contribution to neurodegenerative diseases. Lastly, we outline the gaps in our knowledge that will likely be filled through the widespread use of newer technologies that provide greater resolution into how individual cell types are affected by disease and the contribution of each individual modification site to disease pathogenicity.

Defining the NSD2 interactome: PARP1 PARylation reduces NSD2 histone methyltransferase activity and impedes chromatin binding.

NSD2 is a histone methyltransferase that specifically dimethylates histone H3 lysine 36 (H3K36me2), a modification associated with gene activation. Dramatic overexpression of NSD2 in t(4;14) multiple myeloma (MM) and an activating mutation of NSD2 discovered in acute lymphoblastic leukemia are significantly associated with altered gene activation, transcription, and DNA damage repair. The partner proteins through which NSD2 may influence critical cellular processes remain poorly defined. In this study, we utilized proximity-based labeling (BioID) combined with label-free quantitative MS to identify high confidence NSD2 interacting partners in MM cells. The top 24 proteins identified were involved in maintaining chromatin structure, transcriptional regulation, RNA pre-spliceosome assembly, and DNA damage. Among these, an important DNA damage regulator, poly(ADP-ribose) polymerase 1 (PARP1), was discovered. PARP1 and NSD2 have been found to be recruited to DNA double strand breaks upon damage and H3K36me2 marks are enriched at damage sites. We demonstrate that PARP1 regulates NSD2 via PARylation upon oxidative stress. In vitro assays suggest the PARylation significantly reduces NSD2 histone methyltransferase activity. Furthermore, PARylation of NSD2 inhibits its ability to bind to nucleosomes and further get recruited at NSD2-regulated genes, suggesting PARP1 regulates NSD2 localization and H3K36me2 balance. This work provides clear evidence of cross-talk between PARylation and histone methylation and offers new directions to characterize NSD2 function in DNA damage response, transcriptional regulation, and other pathways.

Transcriptional variation in malaria parasites: why and how.

Transcriptional differences enable the generation of alternative phenotypes from the same genome. In malaria parasites, transcriptional plasticity plays a major role in the process of adaptation to fluctuations in the environment. Multiple studies with culture-adapted parasites and field isolates are starting to unravel the different transcriptional alternatives available to Plasmodium falciparum and the underlying molecular mechanisms. Here we discuss how epigenetic variation, directed transcriptional responses and also genetic changes that affect transcript levels can all contribute to transcriptional variation and, ultimately, parasite survival. Some transcriptional changes are driven by stochastic events. These changes can occur spontaneously, resulting in heterogeneity within parasite populations that provides the grounds for adaptation by dynamic natural selection. However, transcriptional changes can also occur in response to external cues. A better understanding of the mechanisms that the parasite has evolved to alter its transcriptome may ultimately contribute to the design of strategies to combat malaria to which the parasite cannot adapt.

Histone Acetyltransferase p300 Induces De Novo Super-Enhancers to Drive Cellular Senescence.

Accumulation of senescent cells during aging contributes to chronic inflammation and age-related diseases. While senescence is associated with profound alterations of the epigenome, a systematic view of epigenetic factors in regulating senescence is lacking. Here, we curated a library of short hairpin RNAs for targeted silencing of all known epigenetic proteins and performed a high-throughput screen to identify key candidates whose downregulation can delay replicative senescence of primary human cells. This screen identified multiple new players including the histone acetyltransferase p300 that was found to be a primary driver of the senescent phenotype. p300, but not the paralogous CBP, induces a dynamic hyper-acetylated chromatin state and promotes the formation of active enhancer elements in the non-coding genome, leading to a senescence-specific gene expression program. Our work illustrates a causal role of histone acetyltransferases and acetylation in senescence and suggests p300 as a potential therapeutic target for senescence and age-related diseases.

BRCA1 Haploinsufficiency Is Masked by RNF168-Mediated Chromatin Ubiquitylation.

BRCA1 functions at two distinct steps during homologous recombination (HR). Initially, it promotes DNA end resection, and subsequently it recruits the PALB2 and BRCA2 mediator complex, which stabilizes RAD51-DNA nucleoprotein filaments. Loss of 53BP1 rescues the HR defect in BRCA1-deficient cells by increasing resection, suggesting that BRCA1's downstream role in RAD51 loading is dispensable when 53BP1 is absent. Here we show that the E3 ubiquitin ligase RNF168, in addition to its canonical role in inhibiting end resection, acts in a redundant manner with BRCA1 to load PALB2 onto damaged DNA. Loss of RNF168 negates the synthetic rescue of BRCA1 deficiency by 53BP1 deletion, and it predisposes BRCA1 heterozygous mice to cancer. BRCA1+/-RNF168-/- cells lack RAD51 foci and are hypersensitive to PARP inhibitor, whereas forced targeting of PALB2 to DNA breaks in mutant cells circumvents BRCA1 haploinsufficiency. Inhibiting the chromatin ubiquitin pathway may, therefore, be a synthetic lethality strategy for BRCA1-deficient cancers.

The lyase activity of bifunctional DNA glycosylases and the 3'-diesterase activity of APE1 contribute to the repair of oxidized bases in nucleosomes.

The vast majority of oxidized bases that form in DNA are subject to base excision repair (BER). The DNA intermediates generated during successive steps in BER may prove mutagenic or lethal, making it critical that they be 'handed' from one BER enzyme to the next in a coordinated fashion. Here, we report that the handoff of BER intermediates that occurs during the repair of naked DNA substrates differs significantly from that in nucleosomes. During BER of oxidized bases in naked DNA, products generated by the DNA glycosylase NTHL1 were efficiently processed by the downstream enzyme, AP-endonuclease (APE1). In nucleosomes, however, NTHL1-generated products accumulated to significant levels and persisted for some time. During BER of naked DNA substrates, APE1 completely bypasses the inefficient lyase activity of NTHL1. In nucleosomes, the NTHL1-associated lyase contributes to BER, even in the presence of APE1. Moreover, in nucleosomes but not in naked DNA, APE1 was able to process NTHL1 lyase-generated substrates just as efficiently as it processed abasic sites. Thus, the lyase activity of hNTHL1, and the 3' diesterase activity of APE1, which had been seen as relatively dispensable, may have been preserved during evolution to enhance BER in chromatin.

RecA-like domain 2 of DNA-dependent ATPase A domain, a SWI2/SNF2 protein, mediates conformational integrity and ATP hydrolysis.

ATP-dependent chromatin remodeling proteins use the energy released from ATP hydrolysis to reposition nucleosomes in DNA-dependent processes. These proteins are classified as SF2 helicases. SMARCAL1, a member of this protein family, is known to modulate both DNA repair and transcription by specifically recognizing DNA molecules possessing double-strand to single-strand transition regions. Mutations in this gene cause a rare autosomal recessive disorder known as Schimke Immuno-Osseous Dysplasia (SIOD).Structural studies have shown that the ATP-dependent chromatin remodeling proteins possess two RecA-like domains termed as RecA-like domain 1 and RecA-like domain 2. Using Active DNA-dependent ATPase A domain (ADAAD), the bovine homolog of SMARCAL1, as a model system we had previously shown that the RecA-like domain 1 containing helicase motifs Q, I, Ia, II, and III are sufficient for ligand binding; however, the Rec A-like domain 2 containing motifs IV, V, and VI are needed for ATP hydrolysis. In the present study, we have focused on the motifs present in the RecA-like domain 2. Our studies demonstrate that the presence of an aromatic residue in motif IV is needed for interaction with DNA in the presence of ATP. We also show that the motif V is required for the catalytic efficiency of the protein and motif VI is needed for interaction with DNA in the presence of ATP. Finally, we show that the SIOD-associated mutation, R820H, present in motif VI results in loss of ATPase activity, and therefore, reduced response to DNA damage.

CREBBP and p300 lysine acetyl transferases in the DNA damage response.

The CREB-binding protein (CREBBP, or in short CBP) and p300 are lysine (K) acetyl transferases (KAT) belonging to the KAT3 family of proteins known to modify histones, as well as non-histone proteins, thereby regulating chromatin accessibility and transcription. Previous studies have indicated a tumor suppressor function for these enzymes. Recently, they have been found to acetylate key factors involved in DNA replication, and in different DNA repair processes, such as base excision repair, nucleotide excision repair, and non-homologous end joining. The growing list of CBP/p300 substrates now includes factors involved in DNA damage signaling, and in other pathways of the DNA damage response (DDR). This review will focus on the role of CBP and p300 in the acetylation of DDR proteins, and will discuss how this post-translational modification influences their functions at different levels, including catalytic activity, DNA binding, nuclear localization, and protein turnover. In addition, we will exemplify how these functions may be necessary to efficiently coordinate the spatio-temporal response to DNA damage. CBP and p300 may contribute to genome stability by fine-tuning the functions of DNA damage signaling and DNA repair factors, thereby expanding their role as tumor suppressors.