Critical Role of the UBL Domain in Stimulating the E3 Ubiquitin Ligase Activity of UHRF1 toward Chromatin

Mechanisms of Inheritance of Chromatin States: From Yeast to Human

In this article I review mechanisms that underpin epigenetic inheritance of CpG methylation and histone H3 lysine 9 methylation (H3K9me) in chromatin in fungi and mammals. CpG methylation can be faithfully inherited epigenetically at some sites for a lifetime in vertebrates and, remarkably, can be propagated for millions of years in some fungal lineages. Transmission of methylation patterns requires maintenance-type DNA methyltransferases (DNMTs) that recognize hemimethylated CpG DNA produced by replication. DNMT1 is the maintenance enzyme in vertebrates; we recently identified DNMT5 as an ATP-dependent CpG maintenance enzyme found in fungi and protists. In vivo, CpG methylation is coupled to H3K9me. H3K9me is itself reestablished after replication via local histone H3-H4 tetramer recycling involving mobile and nonmobile chaperones, de novo nucleosome assembly, and read-write mechanisms that modify naive nucleosomes. Additional proteins recognize hemimethylated CpG or fully methylated CpG-containing motifs and enhance restoration of methylation by recruiting and/or activating the maintenance methylase.

Investigating the efficacy of immune checkpoint inhibitors in clear cell renal cell carcinoma based on methylation cross talk scoring

Methylation processes in different molecular contexts (DNA, RNA, and histones) are controlled by different regulatory factors and serve as critical determinants in cancer development. However, the mechanistic links between these epigenetic modifications during malignant transformation, metastasis, disease relapse, and therapeutic resistance remain incompletely understood. In this research, we investigated the transcriptional and genetic alterations of regulators associated with 3 major types of methylation modifications in clear cell renal cell carcinoma. Utilizing ChIP/MeRIP-seq and 450K methylation array data, we identified genes regulated by multiple methylation modifications and constructed a scoring model to quantify the methylation patterns for each patient. Our findings indicate that patients with a low score may be more likely to respond to immunotherapy, whereas patients with a high score may be more sensitive to targeted therapy, such as RITA, Pazopanib, Irlotinib, SU-11274, BRD-K16762525, and FCCP. In conclusion, the score model can serve as a valuable biomarker to guide clinical selection of immunotherapy and targeted drugs and help to improve personalized clear cell renal cell carcinoma treatment.

Tudor-Containing Methyl-Lysine and Methyl-Arginine Reader Proteins: Disease Implications and Chemical Tool Development

Tudor domains are histone readers that can recognize various methylation marks on lysine and arginine. This recognition event plays a key role in the recruitment of other epigenetic effectors and the control of gene accessibility. The Tudor-containing protein family contains 42 members, many of which are involved in the development and progression of various diseases, especially cancer. The development of chemical tools for this family will not only lead to a deeper understanding of the biological functions of Tudor domains but also lay the foundation for therapeutic discoveries. In this review, we discuss the role of several Tudor domain-containing proteins in a range of relevant diseases and progress toward the development of chemical tools such as peptides, peptidomimetics, or small-molecules that bind Tudor domains. Overall, we highlight how Tudor domains are promising targets for therapeutic development and would benefit from the development of novel chemical tools.

DNA hypomethylation promotes UHRF1-and SUV39H1/H2-dependent crosstalk between H3K18ub and H3K9me3 to reinforce heterochromatin states

Mono-ubiquitination of lysine 18 on histone H3 (H3K18ub), catalyzed by UHRF1, is a DNMT1 docking site that facilitates replication-coupled DNA methylation maintenance. Its functions beyond this are unknown. Here, we genomically map simultaneous increases in UHRF1-dependent H3K18ub and SUV39H1/H2-dependent H3K9me3 following DNMT1 inhibition. Mechanistically, transient accumulation of hemi-methylated DNA at CpG islands facilitates UHRF1 recruitment and E3 ligase activity toward H3K18. Notably, H3K18ub enhances SUV39H1/H2 methyltransferase activity and, in colon cancer cells, nucleates new H3K9me3 domains at CpG island promoters of DNA methylation-silenced tumor suppressor genes (TSGs). Disrupting UHRF1 enzyme activity prevents H3K9me3 accumulation while promoting PRC2-dependent H3K27me3 as a tertiary layer of gene repression in these regions. By contrast, disrupting H3K18ub-dependent SUV39H1/H2 activity enhances the transcriptional activating and antiproliferative effects of DNMT1 inhibition. Collectively, these findings reveal roles for UHRF1 and H3K18ub in regulating a hierarchy of repressive histone methylation signaling and rationalize a combination strategy for epigenetic cancer therapy.

UHRF1 ubiquitin ligase activity supports the maintenance of low-density CpG methylation

The RING E3 ubiquitin ligase UHRF1 is an established cofactor for DNA methylation inheritance. The model posits that nucleosomal engagement through histone and DNA interactions directs UHRF1 ubiquitin ligase activity toward lysines on histone H3 tails, creating binding sites for DNMT1 through ubiquitin interacting motifs (UIM1 and UIM2). However, the extent to which DNMT1 relies on ubiquitin signaling through UHRF1 in support of DNA methylation maintenance remains unclear. Here, with integrative epigenomic and biochemical analyses, we reveal that DNA methylation maintenance at low-density cytosine-guanine dinucleotides (CpGs) is particularly vulnerable to disruption of UHRF1 ubiquitin ligase activity and DNMT1 ubiquitin reading activity through UIM1. Hypomethylation of low-density CpGs in this manner induces formation of partially methylated domains (PMDs), a methylation signature observed across human cancers. In contrast, UIM2 disruption completely abolishes the DNA methylation maintenance function of DNMT1 in a CpG density-independent manner. In the context of DNA methylation recovery following acute DNMT1 depletion, we further reveal a 'bookmarking' function for UHRF1 ubiquitin ligase activity in support of DNA re-methylation. Collectively, these studies show that DNMT1-dependent DNA methylation inheritance is a ubiquitin-regulated process that is partially reliant on UHRF1 and suggest a disrupted UHRF1-DNMT1 ubiquitin signaling axis contributes to PMD formation in cancers.

Aberrant cytoplasmic expression of UHRF1 restrains the MHC-I-mediated anti-tumor immune response

Immunotherapy successfully complements traditional cancer treatment. However, primary and acquired resistance might limit efficacy. Reduced antigen presentation by MHC-I has been identified as potential resistance factor. Here we show that the epigenetic regulator ubiquitin-like with PHD and ring finger domains 1 (UHRF1), exhibits altered expression and aberrant cytosolic localization in cancerous tissues, where it promotes MHC-I ubiquitination and degradation. Cytoplasmic translocation of UHRF1 is induced by its phosphorylation on a specific serine in response to signals provided by factors present in the tumor microenvironment (TME), such as TGF-β, enabling UHRF1 to bind MHC-I. Downregulation of MHC-I results in suppression of the antigen presentation pathway to establish an immune hostile TME. UHRF1 inactivation by genetic deletion synergizes with immune checkpoint blockade (ICB) treatment and induces an anti-tumour memory response by evoking low-affinity T cells. Our study adds to the understanding of UHRF1 in cancer immune evasion and provides a potential target to synergize with immunotherapy and overcome immunotherapeutic resistance.

Design of linked-domain protein inhibitors of UBE2D as tools to study cellular ubiquitination

Ubiquitin (Ub) is a post-translational modification that largely controls proteostasis through mechanisms spanning transcription, translation, and notably, protein degradation. Ub conjugation occurs through a hierarchical cascade of three enzyme classes (E1, E2, and E3s) involving >1000 proteins that regulate the ubiquitination of proteins. The E2 Ub-conjugating enzymes are the midpoint, yet their cellular roles remain under-characterized, partly due to a lack of inhibitors. For example, the cellular roles of the promiscuous E2 UBE2D/UBCH5 are not well described. Here, we develop a highly selective, multivalent, engineered protein inhibitor for the UBE2D family that simultaneously targets the RING- and backside-binding sites. In HeLa cells, these inhibitors phenocopy knockdown of UBE2D by reducing the IC50 to cisplatin and whole-cell proteomics reveal an increased abundance of ~20% of the identified proteins, consistent with reduced Ub degradation and proteotoxic stress. These precision tools will enable new studies probing UBE2D's central role in proteome management.

To Ub or not to Ub: The epic dilemma of histones that regulate gene expression and epigenetic cross-talk

A dynamic array of histone post-translational modifications (PTMs) regulate diverse cellular processes in the eukaryotic chromatin. Among them, histone ubiquitination is particularly complex as it alters nucleosome surface area fostering intricate cross-talk with other chromatin modifications. Ubiquitin signaling profoundly impacts DNA replication, repair, and transcription. Histones can undergo varied extent of ubiquitination such as mono, multi-mono, and polyubiquitination, which brings about distinct cellular fates. Mechanistic studies of the ubiquitin landscape in chromatin have unveiled a fascinating tapestry of events that orchestrate gene regulation. In this review, we summarize the key contributors involved in mediating different histone ubiquitination and deubiquitination events, and discuss their mechanism which impacts cell transcriptional identity and DNA damage response. We also focus on the proteins bearing epigenetic reader modules critical in discerning site-specific histone ubiquitination, pivotal for establishing complex epigenetic crosstalk. Moreover, we highlight the role of histone ubiquitination in different human diseases including neurodevelopmental disorders and cancer. Overall the review elucidates the intricate orchestration of histone ubiquitination impacting diverse cellular functions and disease pathogenesis, and provides insights into the current challenges of targeting them for therapeutic interventions.

Oncogenic Roles of UHRF1 in Cancer

Ubiquitin-like with PHD and RING finger domains 1 (UHRF1) is an essential protein involved in the maintenance of repressive epigenetic marks, ensuring epigenetic stability and fidelity. As an epigenetic regulator, UHRF1 comprises several functional domains (UBL, TTD, PHD, SRA, RING) that are collectively responsible for processes like DNA methylation, histone modification, and DNA repair. UHRF1 is a downstream effector of the RB/E2F pathway, which is nearly universally deregulated in cancer. Under physiological conditions, UHRF1 protein levels are cell cycle-dependent and are post-translationally regulated by proteasomal degradation. Conversely, UHRF1 is overexpressed and serves as an oncogenic driver in multiple cancers. This review focuses on the functional domains of UHRF1, highlighting its key interacting proteins and oncogenic roles in solid tumors including retinoblastoma, osteosarcoma, lung cancer, and breast cancer. Additionally, current therapeutic strategies targeting UHRF1 domains or its interactors are explored, providing an insight on potential clinical applications.

Structure of human DPPA3 bound to the UHRF1 PHD finger reveals its functional and structural differences from mouse DPPA3

DNA methylation maintenance is essential for cell fate inheritance. In differentiated cells, this involves orchestrated actions of DNMT1 and UHRF1. In mice, the high-affinity binding of DPPA3 to the UHRF1 PHD finger regulates UHRF1 chromatin dissociation and cytosolic localization, which is required for oocyte maturation and early embryo development. However, the human DPPA3 ortholog functions during these stages remain unclear. Here, we report the structural basis for human DPPA3 binding to the UHRF1 PHD finger. The conserved human DPPA3 85VRT87 motif binds to the acidic surface of UHRF1 PHD finger, whereas mouse DPPA3 binding additionally utilizes two unique α-helices. The binding affinity of human DPPA3 for the UHRF1 PHD finger was weaker than that of mouse DPPA3. Consequently, human DPPA3, unlike mouse DPPA3, failed to inhibit UHRF1 chromatin binding and DNA remethylation in Xenopus egg extracts effectively. Our data provide novel insights into the distinct function and structure of human DPPA3.

UHRF1 poly-auto-ubiquitination induced by the anti-cancer drug, thymoquinone, is involved in the DNA repair machinery recruitment

DNA methylation is one of the most important epigenetic mark involved in many physiologic cellular processes and pathologies. During mitosis, the transmission of DNA methylation patterns from a mother to the daughter cells is ensured through the action of the Ubiquitin-like, containing PHD and RING domains, 1/DNA methyltransferase 1 (UHRF1/DNMT1) tandem. UHRF1 is involved in the silencing of many tumor suppressor genes (TSGs) via mechanisms that remain largely to be deciphered. The present study investigated the role and the regulation of UHRF1 poly-ubiquitination induced by thymoquinone, a natural anti-cancer drug, known to enhance or re-activate the expression of TSGs. We found that the auto-ubiquitination of UHRF1, induced by TQ, is mediated by reactive oxygen species, and occurs following DNA damage. We demonstrated that the poly-ubiquitinated form of UHRF1 is K63-linked and can still silence the tumor suppressor gene p16INK4A/CDKN2A. We further showed that TQ-induced auto-ubiquitination is mediated via the activity of Tip60. Since this latter is known as a nuclear receptor co-factor, we investigated if the glucocorticoid receptor (GR) might be involved in the regulation of UHRF1 ubiquitination. Activation of the GR, with dexamethasone, did not influence auto-ubiquitination of UHRF1. However, we could observe that TQ induced a K48-linked poly-ubiquitination of GR, probably involved in the proteosomal degradation pathway. Mass-spectrometry analysis of FLAG-HA-tagged UHRF1 identified UHRF1 partners involved in DNA repair and showed that TQ increased their association with UHRF1, suggesting that poly-ubiquitination of UHRF1 is involved in the DNA repair process. We propose that poly-ubiquitination of UHRF1 serves as a scaffold to recruit the DNA repair machinery at DNA damage sites.

Non-canonical functions of UHRF1 maintain DNA methylation homeostasis in cancer cells

DNA methylation is an essential epigenetic chromatin modification, and its maintenance in mammals requires the protein UHRF1. It is yet unclear if UHRF1 functions solely by stimulating DNA methylation maintenance by DNMT1, or if it has important additional functions. Using degron alleles, we show that UHRF1 depletion causes a much greater loss of DNA methylation than DNMT1 depletion. This is not caused by passive demethylation as UHRF1-depleted cells proliferate more slowly than DNMT1-depleted cells. Instead, bioinformatics, proteomics and genetics experiments establish that UHRF1, besides activating DNMT1, interacts with DNMT3A and DNMT3B and promotes their activity. In addition, we show that UHRF1 antagonizes active DNA demethylation by TET2. Therefore, UHRF1 has non-canonical roles that contribute importantly to DNA methylation homeostasis; these findings have practical implications for epigenetics in health and disease.

UHRF1 ubiquitin ligase activity supports the maintenance of low-density CpG methylation

The RING E3 ubiquitin ligase UHRF1 is an established cofactor for DNA methylation inheritance. Nucleosomal engagement through histone and DNA interactions directs UHRF1 ubiquitin ligase activity toward lysines on histone H3 tails, creating binding sites for DNMT1 through ubiquitin interacting motifs (UIM1 and UIM2). Here, we profile contributions of UHRF1 and DNMT1 to genome-wide DNA methylation inheritance and dissect specific roles for ubiquitin signaling in this process. We reveal DNA methylation maintenance at low-density CpGs is vulnerable to disruption of UHRF1 ubiquitin ligase activity and DNMT1 ubiquitin reading activity through UIM1. Hypomethylation of low-density CpGs in this manner induces formation of partially methylated domains (PMD), a methylation signature observed across human cancers. Furthermore, disrupting DNMT1 UIM2 function abolishes DNA methylation maintenance. Collectively, we show DNMT1-dependent DNA methylation inheritance is a ubiquitin-regulated process and suggest a disrupted UHRF1-DNMT1 ubiquitin signaling axis contributes to the development of PMDs in human cancers.

CDCA7 is a hemimethylated DNA adaptor for the nucleosome remodeler HELLS

Mutations of the SNF2 family ATPase HELLS and its activator CDCA7 cause immunodeficiency-centromeric instability-facial anomalies (ICF) syndrome, characterized by hypomethylation at heterochromatin. The unique zinc-finger domain, zf-4CXXC_R1, of CDCA7 is widely conserved across eukaryotes but is absent from species that lack HELLS and DNA methyltransferases, implying its specialized relation with methylated DNA. Here we demonstrate that zf-4CXXC_R1 acts as a hemimethylated DNA sensor. The zf-4CXXC_R1 domain of CDCA7 selectively binds to DNA with a hemimethylated CpG, but not unmethylated or fully methylated CpG, and ICF disease mutations eliminated this binding. CDCA7 and HELLS interact via their N-terminal alpha helices, through which HELLS is recruited to hemimethylated DNA. While placement of a hemimethylated CpG within the nucleosome core particle can hinder its recognition by CDCA7, cryo-EM structure analysis of the CDCA7-nucleosome complex suggests that zf-4CXXC_R1 recognizes a hemimethylated CpG in the major groove at linker DNA. Our study provides insights into how the CDCA7-HELLS nucleosome remodeling complex uniquely assists maintenance DNA methylation.

The UHRF1 protein is a key regulator of retrotransposable elements and innate immune response to viral RNA in human cells

While epigenetic mechanisms such as DNA methylation and histone modification are known to be important for gene suppression, relatively little is still understood about the interplay between these systems. The UHRF1 protein can interact with both DNA methylation and repressive chromatin marks, but its primary function in humans has been unclear. To determine what that was, we first established stable UHRF1 knockdowns (KD) in normal, immortalized human fibroblasts using targeting shRNA, since CRISPR knockouts (KO) were lethal. Although these showed a loss of DNA methylation across the whole genome, transcriptional changes were dominated by the activation of genes involved in innate immune signalling, consistent with the presence of viral RNA from retrotransposable elements (REs). We confirmed using mechanistic approaches that 1) REs were demethylated and transcriptionally activated; 2) this was accompanied by activation of interferons and interferon-stimulated genes and 3) the pathway was conserved across other adult cell types. Restoring UHRF1 in either transient or stable KD systems could abrogate RE reactivation and the interferon response. Notably, UHRF1 itself could also re-impose RE suppression independent of DNA methylation, but not if the protein contained point mutations affecting histone 3 with trimethylated lysine 9 (H3K9me3) binding. Our results therefore show for the first time that UHRF1 can act as a key regulator of retrotransposon silencing independent of DNA methylation.

Mechanistic insights into UHRF1‑mediated DNA methylation by structure‑based functional clarification of UHRF1 domains (Review)

Epigenetic modification is crucial for transmitting genetic information, while abnormalities in DNA methylation modification are primarily associated with cancer and neurological diseases. As a multifunctional epigenetic modifier, ubiquitin like with PHD and ring finger domains 1 (UHRF1) mainly affects cell energy metabolism and cell cycle control. It also inhibits the transcription of tumor suppressor genes through DNA and/or histone methylation modifications, promoting the occurrence and development of cancer. Therefore, comprehensively understanding the molecular mechanism of the epigenetic modification of UHRF1 in tumors will help identify targets for inhibiting the expression and function of UHRF1. Notably, each domain of UHRF1 functions as a whole and differently. Thus, the abnormality of any domain can lead to a change in phenotype or disease. However, the specific regulatory mechanism and proteins of each domain have not been fully elucidated. The present review aimed to contribute to the study of the regulatory mechanism of UHRF1 to a greater extent in different cancers and provide ideas for drug research by clarifying the function of UHRF1 domains.

Natural and Synthetic Anticancer Epidrugs Targeting the Epigenetic Integrator UHRF1

Cancer is one of the leading causes of death worldwide, and its incidence and mortality are increasing each year. Improved therapeutic strategies against cancer have progressed, but remain insufficient to invert this trend. Along with several other risk factors, abnormal genetic and epigenetic regulations play a critical role in the initiation of cellular transformation, as well as tumorigenesis. The epigenetic regulator UHRF1 (ubiquitin-like, containing PHD and RING finger domains 1) is a multidomain protein with oncogenic abilities overexpressed in most cancers. Through the coordination of its multiple domains and other epigenetic key players, UHRF1 regulates DNA methylation and histone modifications. This well-coordinated dialogue leads to the silencing of tumor-suppressor genes (TSGs) and facilitates tumor cells' resistance toward anticancer drugs, ultimately promoting apoptosis escape and uncontrolled proliferation. Several studies have shown that the downregulation of UHRF1 with natural compounds in tumor cells induces the reactivation of various TSGs, inhibits cell growth, and promotes apoptosis. In this review, we discuss the underlying mechanisms and the potential of various natural and synthetic compounds that can inhibit/minimize UHRF1's oncogenic activities and/or its expression.

UHRF1 is essential for proper cytoplasmic architecture and function of mouse oocytes and derived embryos

Ubiquitin-like with PHD and RING finger domains 1 (UHRF1) is a protein essential for the maintenance of DNA methylation in somatic cells. However, UHRF1 is predominantly localized in the cytoplasm of mouse oocytes and preimplantation embryos, where it may play a role unrelated to the nuclear function. We herein report that oocyte-specific Uhrf1 KO results in impaired chromosome segregation, abnormal cleavage division, and preimplantation lethality of derived embryos. Our nuclear transfer experiment showed that the phenotype is attributable to cytoplasmic rather than nuclear defects of the zygotes. A proteomic analysis of KO oocytes revealed the down-regulation of proteins associated with microtubules including tubulins, which occurred independently of transcriptomic changes. Intriguingly, cytoplasmic lattices were disorganized, and mitochondria, endoplasmic reticulum, and components of the subcortical maternal complex were mislocalized. Thus, maternal UHRF1 regulates the proper cytoplasmic architecture and function of oocytes and preimplantation embryos, likely through a mechanism unrelated to DNA methylation.

Epigenetic signaling and crosstalk in regulation of gene expression and disease progression

Chromatin modifications - including DNA methylation, modification of histones and recruitment of noncoding RNAs - are essential epigenetic events. Multiple sequential modifications converge into a complex epigenetic landscape. For example, promoter DNA methylation is recognized by MeCP2/methyl CpG binding domain proteins which further recruit SETDB1/SUV39 to attain a higher order chromatin structure by propagation of inactive epigenetic marks like H3K9me3. Many studies with new information on different epigenetic modifications and associated factors are available, but clear maps of interconnected pathways are also emerging. This review deals with the salient epigenetic crosstalk mechanisms that cells utilize for different cellular processes and how deregulation or aberrant gene expression leads to disease progression.

The ANIP1-OsWRKY62 module regulates both basal defense and Pi9-mediated immunity against Magnaporthe oryzae in rice

During effector-triggered immunity (ETI) against the devastating rice blast pathogen Magnaporthe oryzae, Pi9 functions as an intracellular resistance protein sensing the pathogen-secreted effector AvrPi9 in rice. Importantly, the underlying recognition mechanism(s) between Pi9 and AvrPi9 remains elusive. In this study, we identified a rice ubiquitin-like domain-containing protein (UDP), AVRPI9-INTERACTING PROTEIN 1 (ANIP1), which is directly targeted by AvrPi9 and also binds to Pi9 in plants. Phenotypic analysis of anip1 mutants and plants overexpressing ANIP1 revealed that ANIP1 negatively modulates rice basal defense against M. oryzae. ANIP1 undergoes 26S proteasome-mediated degradation, which can be blocked by both AvrPi9 and Pi9. Moreover, ANIP1 physically associates with the rice WRKY transcription factor OsWRKY62, which also interacts with AvrPi9 and Pi9 in plants. In the absence of Pi9, ANIP1 negatively regulates OsWRKY62 abundance, which can be promoted by AvrPi9. Accordingly, knocking out of OsWRKY62 in a non-Pi9 background decreased immunity against M. oryzae. However, we also observed that OsWRKY62 plays negative roles in defense against a compatible M. oryzae strain in Pi9-harboring rice. Pi9 binds to ANIP1 and OsWRKY62 to form a complex, which may help to keep Pi9 in an inactive state and weaken rice immunity. Furthermore, using competitive binding assays, we showed that AvrPi9 promotes Pi9 dissociation from ANIP1, which could be an important step toward ETI activation. Taken together, our results reveal an immune strategy whereby a UDP-WRKY module, targeted by a fungal effector, modulates rice immunity in distinct ways in the presence or absence of the corresponding resistance protein.

Activity-based CRISPR scanning uncovers allostery in DNA methylation maintenance machinery

Allostery enables dynamic control of protein function. A paradigmatic example is the tightly orchestrated process of DNA methylation maintenance. Despite the fundamental importance of allosteric sites, their identification remains highly challenging. Here, we perform CRISPR scanning on the essential maintenance methylation machinery-DNMT1 and its partner UHRF1-with the activity-based inhibitor decitabine to uncover allosteric mechanisms regulating DNMT1. In contrast to non-covalent DNMT1 inhibition, activity-based selection implicates numerous regions outside the catalytic domain in DNMT1 function. Through computational analyses, we identify putative mutational hotspots in DNMT1 distal from the active site that encompass mutations spanning a multi-domain autoinhibitory interface and the uncharacterized BAH2 domain. We biochemically characterize these mutations as gain-of-function, exhibiting increased DNMT1 activity. Extrapolating our analysis to UHRF1, we discern putative gain-of-function mutations in multiple domains, including key residues across the autoinhibitory TTD-PBR interface. Collectively, our study highlights the utility of activity-based CRISPR scanning for nominating candidate allosteric sites, and more broadly, introduces new analytical tools that further refine the CRISPR scanning framework.

The termination of UHRF1-dependent PAF15 ubiquitin signaling is regulated by USP7 and ATAD5

UHRF1-dependent ubiquitin signaling plays an integral role in the regulation of maintenance DNA methylation. UHRF1 catalyzes transient dual mono-ubiquitylation of PAF15 (PAF15Ub2), which regulates the localization and activation of DNMT1 at DNA methylation sites during DNA replication. Although the initiation of UHRF1-mediated PAF15 ubiquitin signaling has been relatively well characterized, the mechanisms underlying its termination and how they are coordinated with the completion of maintenance DNA methylation have not yet been clarified. This study shows that deubiquitylation by USP7 and unloading by ATAD5 (ELG1 in yeast) are pivotal processes for the removal of PAF15 from chromatin. On replicating chromatin, USP7 specifically interacts with PAF15Ub2 in a complex with DNMT1. USP7 depletion or inhibition of the interaction between USP7 and PAF15 results in abnormal accumulation of PAF15Ub2 on chromatin. Furthermore, we also find that the non-ubiquitylated form of PAF15 (PAF15Ub0) is removed from chromatin in an ATAD5-dependent manner. PAF15Ub2 was retained at high levels on chromatin when the catalytic activity of DNMT1 was inhibited, suggesting that the completion of maintenance DNA methylation is essential for the termination of UHRF1-mediated ubiquitin signaling. This finding provides a molecular understanding of how the maintenance DNA methylation machinery is disassembled at the end of the S phase.

Mechanisms of chromatin-based epigenetic inheritance

Multi-cellular organisms such as humans contain hundreds of cell types that share the same genetic information (DNA sequences), and yet have different cellular traits and functions. While how genetic information is passed through generations has been extensively characterized, it remains largely obscure how epigenetic information encoded by chromatin regulates the passage of certain traits, gene expression states and cell identity during mitotic cell divisions, and even through meiosis. In this review, we will summarize the recent advances on molecular mechanisms of epigenetic inheritance, discuss the potential impacts of epigenetic inheritance during normal development and in some disease conditions, and outline future research directions for this challenging, but exciting field.

Probing protein ubiquitination in live cells

The reversible attachment of ubiquitin governs the interaction, activity and degradation of proteins whereby the type and target of this conjugation determine the biological response. The investigation of this complex and multi-faceted protein ubiquitination mostly relies on painstaking biochemical analyses. Here, we employ recombinant binding domains to probe the ubiquitination of proteins in living cells. We immobilize GFP-fused proteins of interest at a distinct cellular structure and detect their ubiquitination state with red fluorescent ubiquitin binders. With this ubiquitin fluorescent three-hybrid (ubiF3H) assay we identified HP1β as a novel ubiquitination target of UHRF1. The use of linkage specific ubiquitin binding domains enabled the discrimination of K48 and K63 linked protein ubiquitination. To enhance signal-to-noise ratio, we implemented fluorescence complementation (ubiF3Hc) with split YFP. Using in addition a cell cycle marker we could show that HP1β is mostly ubiquitinated by UHRF1 during S phase and deubiquitinated by the protease USP7. With this complementation assay we could also directly detect the ubiquitination of the tumor suppressor p53 and monitor its inhibition by the anti-cancer drug Nutlin-3. Altogether, we demonstrate the utility of the ubiF3H assay to probe the ubiquitination of specific proteins and to screen for ligases, proteases and small molecules controlling this posttranslational modification.

H3K27me3 shapes DNA methylome by inhibiting UHRF1-mediated H3 ubiquitination

DNA methylation and histone lysine tri-methylation at H3K27 (H3K27me3) are two chromatin modifications for transcriptional gene silencing, which play important roles in diverse biological processes, including cell fate determination and cell lineage commitment. These two marks are largely mutually exclusive and target distinct sets of genes in the mammalian genome. However, how H3K27me3 shapes the DNA methylome remains elusive. Here, we report that the loss of H3K27me3 modification leads to increased DNA methylation at previously marked H3K27me3 sites, indicating that H3K27me3 negatively regulates DNA methylation. Genome-wide analysis of H3 ubiquitination, essential for recruitment and activation of DNA methyltransferase DNMT1, reveals the absence of H3 ubiquitination at H3K27me3 marked nucleosomes. Moreover, loss of H3K27me3 modification induces an increase in H3K18 ubiquitination at the corresponding hyper-methylated loci. Importantly, we show that H3K27me3 directly inhibits UHRF1-mediated H3 ubiquitination toward nucleosomes in a defined biochemical assay. Taken together, our findings reveal a general mechanism for H3K27me3-mediated shaping of the mammalian DNA methylome via modulation of H3 ubiquitination.

Molecular basis of hUHRF1 allosteric activation for synergistic histone modification binding by PI5P

Chromatin marks are recognized by distinct binding modules, many of which are embedded in multidomain proteins. How the different functionalities of such complex chromatin modulators are regulated is often unclear. Here, we delineated the interplay of the H3 amino terminus- and K9me-binding activities of the multidomain hUHRF1 protein. We show that the phosphoinositide PI5P interacts simultaneously with two distant flexible linker regions connecting distinct domains of hUHRF1. The binding is dependent on both, the polar head group, and the acyl part of the phospholipid and induces a conformational rearrangement juxtaposing the H3 amino terminus and K9me3 recognition modules of the protein. In consequence, the two features of the H3 tail are bound in a multivalent, synergistic manner. Our work highlights a previously unidentified molecular function for PI5P outside of the context of lipid mono- or bilayers and establishes a molecular paradigm for the allosteric regulation of complex, multidomain chromatin modulators by small cellular molecules.

Identifying Specific Protein Interactors of Nucleosomes Carrying Methylated Histones Using Quantitative Mass Spectrometry

Chemical modification of histone proteins by methylation plays a central role in chromatin regulation by recruiting epigenetic "readers" via specialized binding domains. Depending on the degree of methylation, the exact modified amino acid, and the associated reader proteins histone methylations are involved in the regulation of all DNA-based processes, such as transcription, DNA replication, and DNA repair. Here we present methods to identify histone methylation readers using a mass spectrometry-linked nucleosome affinity purification approach. We provide detailed protocols for the generation of semisynthetic methylated histones, their assembly into biotinylated nucleosomes, and the identification of methylation-specific nucleosome-interacting proteins from nuclear extracts via nucleosome pull-downs and label-free quantitative proteomics. Due to their versatility, these protocols allow the identification of readers of various histone methylations, and can also be adapted to different cell types and tissues, and other types of modifications.

Genetic Studies on Mammalian DNA Methyltransferases

Cytosine methylation at the C5-position-generating 5-methylcytosine (5mC)-is a DNA modification found in many eukaryotic organisms, including fungi, plants, invertebrates, and vertebrates, albeit its levels vary greatly in different organisms. In mammals, cytosine methylation occurs predominantly in the context of CpG dinucleotides, with the majority (60-80%) of CpG sites in their genomes being methylated. DNA methylation plays crucial roles in the regulation of chromatin structure and gene expression and is essential for mammalian development. Aberrant changes in DNA methylation and genetic alterations in enzymes and regulators involved in DNA methylation are associated with various human diseases, including cancer and developmental disorders. In mammals, DNA methylation is mediated by two families of DNA methyltransferases (Dnmts), namely Dnmt1 and Dnmt3 proteins. Over the last three decades, genetic manipulations of these enzymes, as well as their regulators, in mice have greatly contributed to our understanding of the biological functions of DNA methylation in mammals. In this chapter, we discuss genetic studies on mammalian Dnmts, focusing on their roles in embryogenesis, cellular differentiation, genomic imprinting, and human diseases.

The UHRF protein family in epigenetics, development, and carcinogenesis

The UHRF protein family consists of multidomain regulatory proteins that sense modification status of DNA and/or proteins and catalyze the ubiquitylation of target proteins. Through their functional domains, they interact with other molecules and serve as a hub for regulatory networks of several important biological processes, including maintenance of DNA methylation and DNA damage repair. The UHRF family is conserved in vertebrates and plants but is missing from fungi and many nonvertebrate animals. Mammals commonly have UHRF1 and UHRF2, but, despite their high structural similarity, the two paralogues appear to have distinct functions. Furthermore, UHRF1 and UHRF2 show different expression patterns and different outcomes in gene knockout experiments. In this review, we summarize the current knowledge on the molecular function of the UHRF family in various biological pathways and discuss their roles in epigenetics, development, gametogenesis, and carcinogenesis, with a focus on the mammalian UHRF proteins.

Mechanically transduced immunosorbent assay to measure protein-protein interactions

Measuring protein-protein interaction (PPI) affinities is fundamental to biochemistry. Yet, conventional methods rely upon the law of mass action and cannot measure many PPIs due to a scarcity of reagents and limitations in the measurable affinity ranges. Here, we present a novel technique that leverages the fundamental concept of friction to produce a mechanical signal that correlates to binding potential. The mechanically transduced immunosorbent (METRIS) assay utilizes rolling magnetic probes to measure PPI interaction affinities. METRIS measures the translational displacement of protein-coated particles on a protein-functionalized substrate. The translational displacement scales with the effective friction induced by a PPI, thus producing a mechanical signal when a binding event occurs. The METRIS assay uses as little as 20 pmols of reagents to measure a wide range of affinities while exhibiting a high resolution and sensitivity. We use METRIS to measure several PPIs that were previously inaccessible using traditional methods, providing new insights into epigenetic recognition.

TIP60 governs the auto‑ubiquitination of UHRF1 through USP7 dissociation from the UHRF1/USP7 complex

Tat interactive protein, 60 kDa (TIP60) is an important partner of ubiquitin-like, containing PHD and RING finger domains 1 (UHRF1), ensuring various cellular processes through its acetyltransferase activity. TIP60 is believed to play a tumor suppressive role, partly explained by its downregulated expression in a number of cancers. The aim of the present study was to investigate the role and mechanisms of action of TIP60 in the regulation of UHRF1 expression. The results revealed that TIP60 overexpression downregulated the UHRF1 and DNA methyltransferase 1 (DNMT1) expression levels. TIP60 interfered with USP7-UHRF1 association and induced the degradation of UHRF1 in an auto-ubiquitination-dependent manner. Moreover, TIP60 activated the p73-mediated apoptotic pathway. Taken together, the data of the present study suggest that the tumor suppressor role of TIP60 is mediated by its regulation to UHRF1.

The multi-functionality of UHRF1: epigenome maintenance and preservation of genome integrity

During S phase, the cooperation between the macromolecular complexes regulating DNA synthesis, epigenetic information maintenance and DNA repair is advantageous for cells, as they can rapidly detect DNA damage and initiate the DNA damage response (DDR). UHRF1 is a fundamental epigenetic regulator; its ability to coordinate DNA methylation and histone code is unique across proteomes of different species. Recently, UHRF1’s role in DNA damage repair has been explored and recognized to be as important as its role in maintaining the epigenome. UHRF1 is a sensor for interstrand crosslinks and a determinant for the switch towards homologous recombination in the repair of double-strand breaks; its loss results in enhanced sensitivity to DNA damage. These functions are finely regulated by specific post-translational modifications and are mediated by the SRA domain, which binds to damaged DNA, and the RING domain. Here, we review recent studies on the role of UHRF1 in DDR focusing on how it recognizes DNA damage and cooperates with other proteins in its repair. We then discuss how UHRF1’s epigenetic abilities in reading and writing histone modifications, or its interactions with ncRNAs, could interlace with its role in DDR.

Navigating the DNA methylation landscape of cancer

DNA methylation is a chemical modification that defines cell type and lineage through the control of gene expression and genome stability. Disruption of DNA methylation control mechanisms causes a variety of diseases, including cancer. Cancer cells are characterized by aberrant DNA methylation (i.e., genome-wide hypomethylation and site-specific hypermethylation), mainly targeting CpG islands in gene expression regulatory elements. In particular, the early findings that a variety of tumor suppressor genes (TSGs) are targets of DNA hypermethylation in cancer led to the proposal of a model in which aberrant DNA methylation promotes cellular oncogenesis through TSGs silencing. However, recent genome-wide analyses have revealed that this classical model needs to be reconsidered. In this review, we will discuss the molecular mechanisms of DNA methylation abnormalities in cancer as well as their therapeutic potential.

Control of Genome through Variative Nature of Histone-Modifying Ubiquitin Ligases

Covalent attachment of ubiquitin residue is not only the proteasomal degradation signal, but also a widespread posttranslational modification of cellular proteins in eukaryotes. One of the most important targets of the regulatory ubiquitination are histones. Localization of ubiquitin residue in different regions of the nucleosome attracts a strictly determined set of cellular factors with varied functionality. Depending on the type of histone and the particular lysine residue undergoing modification, histone ubiquitination can lead both to transcription activation and to gene repression, as well as contribute to DNA repair via different mechanisms. An extremely interesting feature of the family of RING E3 ubiquitin ligases catalyzing histone ubiquitination is the striking structural diversity of the domains providing high specificity of modification very similar initial targets. It is obvious that further elucidation of peculiarities of the ubiquitination system involved in histone modification, as well as understanding of physiological role of this process in the maintenance of homeostasis of both single cells and the entire organism, will substantially expand the possibilities of treating a number of socially significant diseases.

Chromatin remodeling in replication-uncoupled maintenance DNA methylation and chromosome stability: Insights from ICF syndrome studies

Immunodeficiency, centromeric instability, and facial anomalies (ICF) syndrome is characterized by frequent appearance of multiradial chromosomes, which are distinctive chromosome fusions that occur at hypomethylated pericentromeric regions comprising repetitive sequences, in activated lymphocytes. The syndrome is caused by mutations in DNMT3B, ZBTB24, CDCA7, or HELLS. De novo DNA methylation is likely defective in patients with ICF syndrome harboring mutations in DNMT3B, whereas accumulating evidence suggests that replication‐uncoupled maintenance DNA methylation of late‐replicating regions is impaired in patients with ICF syndrome harboring mutations in ZBTB24, CDCA7, or HELLS. ZBTB24 is a transcriptional activator of CDCA7, and CDCA7 and HELLS compose a chromatin remodeling complex and are involved in the maintenance DNA methylation through an interaction with UHRF1 in a feed‐forward manner. Furthermore, our recent studies possibly provided the missing link between DNA hypomethylation and the formation of the abnormal chromosomes; it could occur via aberrant transcription from the hypomethylated regions, followed by pathological R‐loop formation. The homologous‐recombination dominant condition caused by a defect in nonhomologous end joining observed in several types of ICF syndrome could facilitate the formation of multiradial chromosomes. Here, the latest knowledge regarding maintenance DNA methylation and chromosome stability provided by those studies is reviewed.

The interplay between DNA and histone methylation: molecular mechanisms and disease implications

Methylation of cytosine in CpG dinucleotides and histone lysine and arginine residues is a chromatin modification that critically contributes to the regulation of genome integrity, replication, and accessibility. A strong correlation exists between the genome-wide distribution of DNA and histone methylation, suggesting an intimate relationship between these epigenetic marks. Indeed, accumulating literature reveals complex mechanisms underlying the molecular crosstalk between DNA and histone methylation. These in vitro and in vivo discoveries are further supported by the finding that genes encoding DNA- and histone-modifying enzymes are often mutated in overlapping human diseases. Here, we summarize recent advances in understanding how DNA and histone methylation cooperate to maintain the cellular epigenomic landscape. We will also discuss the potential implication of these insights for understanding the etiology of, and developing biomarkers and therapies for, human congenital disorders and cancers that are driven by chromatin abnormalities.

A tri-functional amino acid enables mapping of binding sites for posttranslational-modification-mediated protein-protein interactions

Posttranslational modification (PTM), through the recruitment of effector proteins (i.e., "readers") that signal downstream events, plays key roles in regulating a variety of cellular processes. To understand how a PTM is recognized, it is necessary to find its readers and, importantly, the location of the binding pockets responsible for PTM recognition. Although various methods have been developed to identify PTM readers, it remains a challenge to directly map the PTM-binding regions, especially for intrinsically disordered domains. Here, we demonstrate a photo-crosslinkable, clickable, and cleavable tri-functional amino acid, ADdis-Cys, that when coupled with mass spectrometry (ADdis-Cys-MS) can not only identify PTM readers from complex proteomes but also simultaneously map their PTM-recognition modules. Using ADdis-Cys-MS, we successfully identify the binding sites of several reader-PTM interactions, among which we discover human C1QBP as a histone chaperone. This robust method should find wide applications in examining other histone or non-histone PTM-mediated protein-protein interactions.

Thymoquinone Is a Multitarget Single Epidrug That Inhibits the UHRF1 Protein Complex

Silencing of tumor suppressor genes (TSGs) through epigenetic mechanisms, mainly via abnormal promoter DNA methylation, is considered a main mechanism of tumorigenesis. The abnormal DNA methylation profiles are transmitted from the cancer mother cell to the daughter cells through the involvement of a macromolecular complex in which the ubiquitin-like containing plant homeodomain (PHD), and an interesting new gene (RING) finger domains 1 (UHRF1), play the role of conductor. Indeed, UHRF1 interacts with epigenetic writers, such as DNA methyltransferase 1 (DNMT1), histone methyltransferase G9a, erasers like histone deacetylase 1 (HDAC1), and functions as a hub protein. Thus, targeting UHRF1 and/or its partners is a promising strategy for epigenetic cancer therapy. The natural compound thymoquinone (TQ) exhibits anticancer activities by targeting several cellular signaling pathways, including those involving UHRF1. In this review, we highlight TQ as a potential multitarget single epidrug that functions by targeting the UHRF1/DNMT1/HDAC1/G9a complex. We also speculate on the possibility that TQ might specifically target UHRF1, with subsequent regulatory effects on other partners.

Staying true to yourself: mechanisms of DNA methylation maintenance in mammals

DNA methylation is essential to development and cellular physiology in mammals. Faulty DNA methylation is frequently observed in human diseases like cancer and neurological disorders. Molecularly, this epigenetic mark is linked to other chromatin modifications and it regulates key genomic processes, including transcription and splicing. Each round of DNA replication generates two hemi-methylated copies of the genome. These must be converted back to symmetrically methylated DNA before the next S-phase, or the mark will fade away; therefore the maintenance of DNA methylation is essential. Mechanistically, the maintenance of this epigenetic modification takes place during and after DNA replication, and occurs within the very dynamic context of chromatin re-assembly. Here, we review recent discoveries and unresolved questions regarding the mechanisms, dynamics and fidelity of DNA methylation maintenance in mammals. We also discuss how it could be regulated in normal development and misregulated in disease.

Mitotic inheritance of DNA methylation: more than just copy and paste

Decades of investigation on DNA methylation have led to deeper insights into its metabolic mechanisms and biological functions. This understanding was fueled by the recent development of genome editing tools and our improved capacity for analyzing the global DNA methylome in mammalian cells. This review focuses on the maintenance of DNA methylation patterns during mitotic cell division. We discuss the latest discoveries of the mechanisms for the inheritance of DNA methylation as a stable epigenetic memory. We also highlight recent evidence showing the rapid turnover of DNA methylation as a dynamic gene regulatory mechanism. A body of work has shown that altered DNA methylomes are common features in aging and disease. We discuss the potential links between methylation maintenance mechanisms and disease-associated methylation changes.

Discovery of small molecules targeting the tandem tudor domain of the epigenetic factor UHRF1 using fragment-based ligand discovery

Despite the established roles of the epigenetic factor UHRF1 in oncogenesis, no UHRF1-targeting therapeutics have been reported to date. In this study, we use fragment-based ligand discovery to identify novel scaffolds for targeting the isolated UHRF1 tandem Tudor domain (TTD), which recognizes the heterochromatin-associated histone mark H3K9me3 and supports intramolecular contacts with other regions of UHRF1. Using both binding-based and function-based screens of a ~ 2300-fragment library in parallel, we identified 2,4-lutidine as a hit for follow-up NMR and X-ray crystallography studies. Unlike previous reported ligands, 2,4-lutidine binds to two binding pockets that are in close proximity on TTD and so has the potential to be evolved into more potent inhibitors using a fragment-linking strategy. Our study provides a useful starting point for developing potent chemical probes against UHRF1.

Chromatin Regulation through Ubiquitin and Ubiquitin-like Histone Modifications

Chromatin functions are influenced by the addition, removal, and recognition of histone post-translational modifications (PTMs). Ubiquitin and ubiquitin-like (UBL) PTMs on histone proteins can function as signaling molecules by mediating protein-protein interactions. Fueled by the identification of novel ubiquitin and UBL sites and the characterization of the writers, erasers, and readers, the breadth of chromatin functions associated with ubiquitin signaling is emerging. Here, we highlight recently appreciated roles for histone ubiquitination in DNA methylation control, PTM crosstalk, nucleosome structure, and phase separation. We also discuss the expanding diversity and functions associated with histone UBL modifications. We conclude with a look toward the future and pose key questions that will drive continued discovery at the interface of epigenetics and ubiquitin signaling.

A role for LSH in facilitating DNA methylation by DNMT1 through enhancing UHRF1 chromatin association

LSH, a SNF2 family DNA helicase, is a key regulator of DNA methylation in mammals. How LSH facilitates DNA methylation is not well defined. While previous studies with mouse embryonic stem cells (mESc) and fibroblasts (MEFs) derived from Lsh knockout mice have revealed a role of Lsh in de novo DNA methylation by Dnmt3a/3b, here we report that LSH contributes to DNA methylation in various cell lines primarily by promoting DNA methylation by DNMT1. We show that loss of LSH has a much bigger effect in DNA methylation than loss of DNMT3A and DNMT3B. Mechanistically, we demonstrate that LSH interacts with UHRF1 but not DNMT1 and facilitates UHRF1 chromatin association and UHRF1-catalyzed histone H3 ubiquitination in an ATPase activity-dependent manner, which in turn promotes DNMT1 recruitment to replication fork and DNA methylation. Notably, UHRF1 also enhances LSH association with the replication fork. Thus, our study identifies LSH as an essential factor for DNA methylation by DNMT1 and provides novel insight into how a feed-forward loop between LSH and UHRF1 facilitates DNMT1-mediated maintenance of DNA methylation in chromatin.

The histone and non-histone methyllysine reader activities of the UHRF1 tandem Tudor domain are dispensable for the propagation of aberrant DNA methylation patterning in cancer cells

The chromatin-binding E3 ubiquitin ligase ubiquitin-like with PHD and RING finger domains 1 (UHRF1) contributes to the maintenance of aberrant DNA methylation patterning in cancer cells through multivalent histone and DNA recognition. The tandem Tudor domain (TTD) of UHRF1 is well-characterized as a reader of lysine 9 di- and tri-methylation on histone H3 (H3K9me2/me3) and, more recently, lysine 126 di- and tri-methylation on DNA ligase 1 (LIG1K126me2/me3). However, the functional significance and selectivity of these interactions remain unclear. In this study, we used protein domain microarrays to search for additional readers of LIG1K126me2, the preferred methyl state bound by the UHRF1 TTD. We show that the UHRF1 TTD binds LIG1K126me2 with high affinity and selectivity compared to other known methyllysine readers. Notably, and unlike H3K9me2/me3, the UHRF1 plant homeodomain (PHD) and its N-terminal linker (L2) do not contribute to multivalent LIG1K126me2 recognition along with the TTD. To test the functional significance of this interaction, we designed a LIG1K126me2 cell-penetrating peptide (CPP). Consistent with LIG1 knockdown, uptake of the CPP had no significant effect on the propagation of DNA methylation patterning across the genomes of bulk populations from high-resolution analysis of several cancer cell lines. Further, we did not detect significant changes in DNA methylation patterning from bulk cell populations after chemical or genetic disruption of lysine methyltransferase activity associated with LIG1K126me2 and H3K9me2. Collectively, these studies identify UHRF1 as a selective reader of LIG1K126me2 in vitro and further implicate the histone and non-histone methyllysine reader activity of the UHRF1 TTD as a dispensable domain function for cancer cell DNA methylation maintenance.

Molecular mechanisms of eukaryotic origin initiation, replication fork progression, and chromatin maintenance

Eukaryotic DNA replication is a highly dynamic and tightly regulated process. Replication involves several dozens of replication proteins, including the initiators ORC and Cdc6, replicative CMG helicase, DNA polymerase α-primase, leading-strand DNA polymerase ε, and lagging-strand DNA polymerase δ. These proteins work together in a spatially and temporally controlled manner to synthesize new DNA from the parental DNA templates. During DNA replication, epigenetic information imprinted on DNA and histone proteins is also copied to the daughter DNA to maintain the chromatin status. DNA methyltransferase 1 is primarily responsible for copying the parental DNA methylation pattern into the nascent DNA. Epigenetic information encoded in histones is transferred via a more complex and less well-understood process termed replication-couple nucleosome assembly. Here, we summarize the most recent structural and biochemical insights into DNA replication initiation, replication fork elongation, chromatin assembly and maintenance, and related regulatory mechanisms.

Alternative splicing and allosteric regulation modulate the chromatin binding of UHRF1

UHRF1 is an important epigenetic regulator associated with apoptosis and tumour development. It is a multidomain protein that integrates readout of different histone modification states and DNA methylation with enzymatic histone ubiquitylation activity. Emerging evidence indicates that the chromatin-binding and enzymatic modules of UHRF1 do not act in isolation but interplay in a coordinated and regulated manner. Here, we compared two splicing variants (V1, V2) of murine UHRF1 (mUHRF1) with human UHRF1 (hUHRF1). We show that insertion of nine amino acids in a linker region connecting the different TTD and PHD histone modification-binding domains causes distinct H3K9me3-binding behaviour of mUHRF1 V1. Structural analysis suggests that in mUHRF1 V1, in contrast to V2 and hUHRF1, the linker is anchored in a surface groove of the TTD domain, resulting in creation of a coupled TTD-PHD module. This establishes multivalent, synergistic H3-tail binding causing distinct cellular localization and enhanced H3K9me3-nucleosome ubiquitylation activity. In contrast to hUHRF1, H3K9me3-binding of the murine proteins is not allosterically regulated by phosphatidylinositol 5-phosphate that interacts with a separate less-conserved polybasic linker region of the protein. Our results highlight the importance of flexible linkers in regulating multidomain chromatin binding proteins and point to divergent evolution of their regulation.

Thymoquinone and Difluoromethylornithine (DFMO) Synergistically Induce Apoptosis of Human Acute T Lymphoblastic Leukemia Jurkat Cells Through the Modulation of Epigenetic Pathways

Thymoquinone (TQ), a natural anticancer agent exerts cytotoxic effects on several tumors by targeting multiple pathways, including apoptosis. Difluoromethylornithine (DFMO), an irreversible inhibitor of the ornithine decarboxylase (ODC) enzyme, has shown promising inhibitory activities in many cancers including leukemia by decreasing the biosynthesis of the intracellular polyamines. The present study aimed to investigate the combinatorial cytotoxic effects of TQ and DFMO on human acute T lymphoblastic leukemia Jurkat cells and to determine the underlying mechanisms. Here, we show that the combination of DFMO and TQ significantly reduced cell viability and resulted in significant synergistic effects on apoptosis when compared to either DFMO or TQ alone. RNA-sequencing showed that many key epigenetic players including Ubiquitin-like containing PHD and Ring finger 1 (UHRF1) and its 2 partners DNA methyltransferase 1 (DNMT1) and histone deacetylase 1 (HDAC1) were down-regulated in DFMO-treated Jurkat cells. The combination of DFMO and TQ dramatically decreased the expression of UHRF1, DNMT1 and HDAC1 genes compared to either DFMO or TQ alone. UHRF1 knockdown led to a decrease in Jurkat cell viability. In conclusion, these results suggest that the combination of DFMO and TQ could be a promising new strategy for the treatment of human acute T lymphoblastic leukemia by targeting the epigenetic code.

Modes of allosteric regulation of the ubiquitination machinery

Ubiquitination is a post-translational modification crucial for cellular signaling. A diverse range of enzymes constitute the machinery that mediates attachment of ubiquitin onto target proteins. This diversity allows the targeting of various proteins in a highly regulated fashion. Many of the enzymes have multiple domains or subunits that bind allosteric effectors and exhibit large conformational rearrangements to facilitate regulation. Here we consider recent examples of ubiquitin itself as an allosteric effector of RING and RBR E3 ligases, as well as advances in the understanding of allosteric regulatory elements within HECT E3 ligases.