Solution structure of the nonmethyl-CpG-binding CXXC domain of the leukaemia-associated MLL histone methyltransferase

Interaction Between PHF8 and a Segment of KDM2A, Which Is Controlled by the Phosphorylation Status at a Specific Serine in an Intrinsically Disordered Region of KDM2A, Regulates rRNA Transcription and Cell Proliferation in a Breast Cancer Cell Line

Mild starvation due to low concentrations of an inhibitor of glycolysis, 2-deoxy-D-glucose, activates AMP-activated protein kinase (AMPK) and lysine-specific demethylase 2A (KDM2A) to reduce rRNA transcription and cell proliferation in breast cancer cells. However, the mechanisms of how AMPK regulates KDM2A are unknown. Here, we found that PHD finger protein 8 (PHF8) interacted with KDM2A and contributed to the reduction in rRNA transcription and cell proliferation by 2-deoxy-D-glucose in a breast cancer cell line, MCF-7. We analyzed how KDM2A bound PHF8 in detail and found that PHF8 interacted with KDM2A via two regions of KDM2A. One of the regions contained an intrinsically disordered region (IDR). IDRs can show rapidly switchable protein-protein interactions. Deletion of the PHF8-binding region activated KDM2A to reduce rRNA transcription, and 2-deoxy-D-glucose reduced the interaction between PHF8 and the KDM2A fragment containing the PHF8-binding region. A 2-deoxy-D-glucose or AMPK activator dephosphorylated KDM2A at Ser731, which is located on the N-terminal side of the PHF8-binding region. Replacement of Ser731 by Ala decreased binding of PHF8 to the KDM2A fragment that contains the PHF8-binding region and Ser731 and reduced rRNA transcription and cell proliferation. These results suggest that the mode of interaction between KDM2A and PHF8 is regulated via dephosphorylation of KDM2A through AMPK to control rRNA transcription, and control of the phosphorylation state of Ser731 would be a novel target for breast cancer therapy.

Elucidating the role of MLL1 nsSNPs: Structural and functional alterations and their contribution to leukemia development

(1) BACKGROUND

The Mixed lineage leukemia 1 (MLL1) gene, located on chromosome 11q23, plays a pivotal role in histone lysine-specific methylation and is consistently associated with various types of leukemia. Non-synonymous Single Nucleotide Polymorphisms (nsSNPs) have been tied to numerous diseases, including cancers, and have become valuable cancer biomarkers. There's a notable gap in studies probing the influence of SNPs on MLL1 protein structure, function, and subsequent modifications.

(2) METHODS

We utilized an array of bioinformatics tools, including PredictSNP, InterPro, ConSurf, I-Mutant2.0, MUpro, Musitedeep, Project HOPE, RegulomeDB, Mutpred2, and both CScape and CScape Somatic, to meticulously analyze the consequences of nsSNPs in the MLL1 gene.

(3) RESULTS

Out of 2,097 nsSNPs analyzed, 62 were determined to be significantly pathogenic by the PredictSNP tool, with ten crucial MLL1 functional domains identified using InterPro. Additionally, 50 of these nsSNPs had high conservation scores, hinting at potential effects on protein structure and function, while 32 were found to undermine MLL1 protein stability. Notably, four nsSNPs were deemed oncogenic, with two identified as cancer drivers. The nsSNP, D2724G, between the MLL1 protein's FY-rich domains, could disrupt proteolytic cleavage, altering gene expression patterns and potentially promoting cancer.

(4) CONCLUSIONS

Our research provides a comprehensive assessment of nsSNPs' impact in the MLL1 protein structure and function and consequently on leukemia development, suggesting potential avenues for personalized treatment, early detection, improved prognosis, and a deeper understanding of hematological malignancy genesis.

HBO1, a MYSTerious KAT and its links to cancer

The histone acetyltransferase HBO1, also known as KAT7, is a major chromatin modifying enzyme responsible for H3 and H4 acetylation. It is found within two distinct tetrameric complexes, the JADE subunit-containing complex and BRPF subunit-containing complex. The HBO1-JADE complex acetylates lysine 5, 8 and 12 of histone H4, and the HBO1-BRPF complex acetylates lysine 14 of histone H3. HBO1 regulates gene transcription, DNA replication, DNA damage repair, and centromere function. It is involved in diverse signaling pathways and plays crucial roles in development and stem cell biology. Recent work has established a strong relationship of HBO1 with the histone methyltransferase MLL/KMT2A in acute myeloid leukemia. Here, we discuss functional and pathological links of HBO1 to cancer, highlighting the underlying mechanisms that may pave the way to the development of novel anti-cancer therapies.

H3K4me2 distinguishes a distinct class of enhancers during the maternal-to-zygotic transition

After egg fertilization, an initially silent embryonic genome is transcriptionally activated during the maternal-to-zygotic transition. In zebrafish, maternal vertebrate pluripotency factors Nanog, Pou5f3 (OCT4 homolog), and Sox19b (SOX2 homolog) (NPS) play essential roles in orchestrating embryonic genome activation, acting as "pioneers" that open condensed chromatin and mediate acquisition of activating histone modifications. However, some embryonic gene transcription still occurs in the absence of these factors, suggesting the existence of other mechanisms regulating genome activation. To identify chromatin signatures of these unknown pathways, we profiled the histone modification landscape of zebrafish embryos using CUT&RUN. Our regulatory map revealed two subclasses of enhancers distinguished by presence or absence of H3K4me2. Enhancers lacking H3K4me2 tend to require NPS factors for de novo activation, while enhancers bearing H3K4me2 are epigenetically bookmarked by DNA hypomethylation to recapitulate gamete activity in the embryo, independent of NPS pioneering. Thus, parallel enhancer activation pathways combine to induce transcriptional reprogramming to pluripotency in the early embryo.

Novel KMT2B gene mutation in MUC4 positive low-grade fibromyxoid sarcoma

BACKGROUND

Low-grade Fibromyxoid Sarcoma(LGFM)is a rare fibrosarcoma, which mainly occurs in young people and is mostly seen in the trunk and limbs. The tumor is usually FUS-CREB3L2 fusion caused by t(7;16)(q32-34;p11)chromosome translocation, and rarely FUS-CREB3L1 and EWSR1-CREB3L1 fusion. MUC4 diffuse strong positive can be used as a specific index of LGFM. LGFM is similar to Sclerosing Epithelioid Fibrosarcoma(SEF) and may have the same origin.

CASE PRESENTATION

We report a case of LGFM in the chest wall. A female who is 59 years old. In 2016, CT showed dense nodule shadow and focal thickening of the left pleura, the patient underwent surgery, Pathological report that low to moderate malignant fibrosarcoma(fibromyxoid type). The CT re-examination in 2021 showed that the tumors on the left chest wall were significantly larger than before. Pathological examination showed the disease is composed of alternating collagen like and mucinous areas. Under high-power microscope, the tumor cells are consistent in shape, spindle or short spindle, and the tumor cells are arranged in bundles. In local areas, the density of tumor cells is significantly increased, mixed with collagen fibers, and small focal SEF appear. The result of immunohistochemistry showed that SMA, Desmin, CD34, STAT6, S100, SOX10, HMB45 and Melan A were negative, EMA was weakly positive, MUC4 was diffuse and strongly positive, and Ki67 index was low (3%).

CONCLUSION

Sequencing results showed that MET, EGFR, KMT2B and RET gene were mutated in LGFM, and KMT2B gene had cancer promoting effect, but there was no literature report in LGFM, which may be of certain significance for the diagnosis and treatment of LGFM.

Mechanisms of Secondary Leukemia Development Caused by Treatment with DNA Topoisomerase Inhibitors

Leukemia is a blood cancer originating in the blood and bone marrow. Therapy-related leukemia is associated with prior chemotherapy. Although cancer therapy with DNA topoisomerase II inhibitors is one of the most effective cancer treatments, its side effects include development of secondary leukemia characterized by the chromosomal rearrangements affecting AML1 or MLL genes. Recurrent chromosomal translocations in the therapy-related leukemia differ from chromosomal rearrangements associated with other neoplasias. Here, we reviewed the factors that drive chromosomal translocations induced by cancer treatment with DNA topoisomerase II inhibitors, such as mobility of ends of double-strand DNA breaks formed before the translocation and gain of function of fusion proteins generated as a result of translocation.

Clustered PHD domains in KMT2/MLL proteins are attracted by H3K4me3 and H3 acetylation-rich active promoters and enhancers

Histone lysine-specific methyltransferase 2 (KMT2A-D) proteins, alternatively called mixed lineage leukemia (MLL1-4) proteins, mediate positive transcriptional memory. Acting as the catalytic subunits of human COMPASS-like complexes, KMT2A-D methylate H3K4 at promoters and enhancers. KMT2A-D contain understudied highly conserved triplets and a quartet of plant homeodomains (PHDs). Here, we show that all clustered (multiple) PHDs localize to the well-defined loci of H3K4me3 and H3 acetylation-rich active promoters and enhancers. Surprisingly, we observe little difference in binding pattern between PHDs from promoter-specific KMT2A-B and enhancer-specific KMT2C-D. Fusion of the KMT2A CXXC domain to the PHDs drastically enhances their preference for promoters over enhancers. Hence, the presence of CXXC domains in KMT2A-B, but not KMT2C-D, may explain the promoter/enhancer preferences of the full-length proteins. Importantly, targets of PHDs overlap with KMT2A targets and are enriched in genes involved in the cancer pathways. We also observe that PHDs of KMT2A-D are mutated in cancer, especially within conserved folding motifs (Cys4HisCys2Cys/His). The mutations cause a domain loss-of-function. Taken together, our data suggest that PHDs of KMT2A-D guide the full-length proteins to active promoters and enhancers, and thus play a role in positive transcriptional memory.

Myostatin Mutation Enhances Bovine Myogenic Differentiation through PI3K/AKT/mTOR Signalling via Removing DNA Methylation of RACK1

Myostatin (MSTN) is a negative regulator of skeletal muscle development and plays an important role in muscle development. Fluctuations in gene expression influenced by DNA methylation are critical for homeostatic responses in muscle. However, little is known about the mechanisms underlying this fluctuation regulation and myogenic differentiation of skeletal muscle. Here we report a genome-wide analysis of DNA methylation dynamics in bovine skeletal muscle myogenesis after myostatin editing. We show that, after myostatin editing, an increase in TETs (DNA demethylases) and a concomitant increase in the receptor for activated C kinase 1 (RACK1) control the myogenic development of skeletal muscle. Interestingly, enhancement of PI3K/AKT/mTOR signaling by RACK1 appears to be an essential driver of myogenic differentiation, as it was associated with an increase in myogenic differentiation marker factors (MyHC and MyoG) during muscle differentiation. Overall, our results suggest that loss of myostatin promotes the myogenic differentiation response in skeletal muscle by decreasing DNA methylation of RACK1.

Role of the MOZ/MLL-mediated transcriptional activation system for self-renewal in normal hematopoiesis and leukemogenesis

Homeostasis in the blood system is maintained by the balance between self-renewing stem cells and nonstem cells. To promote self-renewal, transcriptional regulators maintain epigenetic information during multiple rounds of cell division. Mutations in such transcriptional regulators cause aberrant self-renewal, leading to leukemia. MOZ, a histone acetyltransferase, and MLL, a histone methyltransferase, are transcriptional regulators that promote the self-renewal of hematopoietic stem cells. Gene rearrangements of MOZ and MLL generate chimeric genes encoding fusion proteins that function as constitutively active forms. These MOZ and MLL fusion proteins constitutively activate transcription of their target genes and cause aberrant self-renewal in committed hematopoietic progenitors, which normally do not self-renew. Recent progress in the field suggests that MOZ and MLL are part of a transcriptional activation system that activates the transcription of genes with nonmethylated CpG-rich promoters. The nonmethylated state of CpGs is normally maintained during cell divisions from the mother cell to the daughter cells. Thus, the MOZ/MLL-mediated transcriptional activation system replicates the expression profile of mother cells in daughter cells by activating the transcription of genes previously transcribed in the mother cell. This review summarizes the functions of the components of the MOZ/MLL-mediated transcriptional activation system and their roles in the promotion of self-renewal.

Small-Molecule Inhibitors of the MLL1 CXXC Domain, an Epigenetic Reader of DNA Methylation

The CXXC domain is a reader of DNA methylation which preferentially binds to unmethylated CpG DNA motifs. Chromosomal translocations involving the MLL1 gene produce in-frame fusion proteins in which the N-terminal portion of the MLL1 protein harboring its CXXC domain is fused to the C-terminal portion of multiple partners. For the MLL-AF9 fusion, mutations which disrupt CXXC domain-DNA binding abrogate the ability to cause leukemia in mice. Based on this, we initiated an effort to develop small-molecule inhibitors of the MLL1 CXXC domain as a novel approach to therapy. We developed a fluorescence polarization-based assay for MLL CXXC domain-DNA binding and screened a library of Cys-reactive molecules. For the most potent hit from this screen, we have synthesized a library of analogs to explore the structure-activity relationship, defined the binding site using chemical shift perturbations in NMR spectra, and explored the selectivity of compounds across the CXXC domain family.

TET3 CXXC domain is critical for post-fertilization demethylation and expression of pluripotency genes in pig embryos

Enzymes of the Ten-eleven translocation (TET) family are considered to play an important role in the regulation of DNA methylation patterns by converting 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). Known as a maternal transcript enriched in mature oocytes, TET3 has been suggested to initiate DNA demethylation of the paternal genome in zygotes. Previous studies in mouse cells indicate that the N-terminal CXXC domain of TET3 is important in catalyzing the oxidation of 5mC through its potential DNA binding ability; however, it is not clear whether the DNA binding capacity of CXXC domain is required for the 5hmC conversion in mammalian embryos. Here, we identified TET3 isoforms in porcine oocytes and investigated the role of the oocyte specific TET3 isoform (pTET3L) in controlling postfertilization demethylation in porcine embryos. The pTET3L possessed sequences representing a known DNA binding domain, the CXXC, and injection of the TET3 CXXC fused with GFP into mature porcine oocytes resulted in exclusive localization of the GFP-CXXC in the pronuclei. The CXXC overexpression reduced the 5mC level in zygotes and enhanced the DNA demethylation of the NANOG promoter in 2-cell stage embryos. Furthermore, there was an increase in the transcript abundance of NANOG and ESRRB in blastocysts developed from GFP-CXXC injected oocytes. Targeted knockdown of pTET3L resulted in the downregulation of pluripotency genes in subsequently developed blastocysts. The findings indicate that the CXXC domain of TET3 serves as a critical component for the postfertilization demethylation of porcine embryos and coordinates proper expression of pluripotency related genes in blastocysts. Summary sentence: TET3 isoform containing CXXC domain is the predominant isoform in porcine oocytes and orchestrates post-fertilization demethylation and proper expression of pluripotency genes in porcine embryos.

Missense variants causing Wiedemann-Steiner syndrome preferentially occur in the KMT2A-CXXC domain and are accurately classified using AlphaFold2

Wiedemann-Steiner syndrome (WDSTS) is a neurodevelopmental disorder caused by de novo variants in KMT2A, which encodes a multi-domain histone methyltransferase. To gain insight into the currently unknown pathogenesis of WDSTS, we examined the spatial distribution of likely WDSTS-causing variants across the 15 different domains of KMT2A. Compared to variants in healthy controls, WDSTS variants exhibit a 61.9-fold overrepresentation within the CXXC domain–which mediates binding to unmethylated CpGs–suggesting a major role for this domain in mediating the phenotype. In contrast, we find no significant overrepresentation within the catalytic SET domain. Corroborating these results, we find that hippocampal neurons from Kmt2a-deficient mice demonstrate disrupted histone methylation (H3K4me1 and H3K4me3) preferentially at CpG-rich regions, but this has no systematic impact on gene expression. Motivated by these results, we combine accurate prediction of the CXXC domain structure by AlphaFold2 with prior biological knowledge to develop a classification scheme for missense variants in the CXXC domain. Our classifier achieved 92.6% positive and 92.9% negative predictive value on a hold-out test set. This classification performance enabled us to subsequently perform an in silico saturation mutagenesis and classify a total of 445 variants according to their functional effects. Our results yield a novel insight into the mechanistic basis of WDSTS and provide an example of how AlphaFold2 can contribute to the in silico characterization of variant effects with very high accuracy, suggesting a paradigm potentially applicable to many other Mendelian disorders.

Wiedemann-Steiner syndrome (WDSTS) is a neurodevelopmental pediatric disorder caused by the genetic disruption of the histone methyltransferase KMT2A. Since KMT2A has many different domains that perform different functions, we reasoned that by identifying the domains most enriched for WDSTS-causing genetic variants we would gain insights into the incompletely understood molecular pathogenesis of WDSTS. We discovered that the CXXC domain—which binds unmethylated CpGs—shows by far the greatest enrichment, suggesting that loss of the CpG-binding ability of KMT2A plays a central role in WDSTS. Next, to understand specific rules underlying the genetic disruption of the CXXC domain, we combined prior knowledge about the function/structure of the domain with 3D structure prediction by AlphaFold2 to develop an effect classifier for CXXC missense variants. We found that this classifier exhibits accurate performance, and we therefore applied it to provide classifications for any such variant that can possibly arise, in order to aid in the interpretation of such variants in the clinic. Our work provides novel insights into WDSTS and suggests a strategy for missense variant classification that can potentially be applied to many other pediatric genetic disorders.

Direct targeted therapy for MLL-fusion-driven high-risk acute leukaemias

BACKGROUND

Improving the poor prognosis of infant leukaemias remains an unmet clinical need. This disease is a prototypical fusion oncoprotein-driven paediatric cancer, with MLL (KMT2A)-fusions present in most cases. Direct targeting of these driving oncoproteins represents a unique therapeutic opportunity. This rationale led us to initiate a drug screening with the aim of discovering drugs that can block MLL-fusion oncoproteins.

METHODS

A screen for inhibition of MLL-fusion proteins was developed that overcomes the traditional limitations of targeting transcription factors. This luciferase reporter-based screen, together with a secondary western blot screen, was used to prioritize compounds. We characterized the lead compound, disulfiram (DSF), based on its efficient ablation of MLL-fusion proteins. The consequences of drug-induced MLL-fusion inhibition were confirmed by cell proliferation, colony formation, apoptosis assays, RT-qPCR, in vivo assays, RNA-seq and ChIP-qPCR and ChIP-seq analysis. All statistical tests were two-sided.

RESULTS

Drug-induced inhibition of MLL-fusion proteins by DSF resulted in a specific block of colony formation in MLL-rearranged cells in vitro, induced differentiation and impeded leukaemia progression in vivo. Mechanistically, DSF abrogates MLL-fusion protein binding to DNA, resulting in epigenetic changes and down-regulation of leukaemic programmes setup by the MLL-fusion protein.

CONCLUSION

DSF can directly inhibit MLL-fusion proteins and demonstrate antitumour activity both in vitro and in vivo, providing, to our knowledge, the first evidence for a therapy that directly targets the initiating oncogenic MLL-fusion protein.

Epigenetics and Testicular Cancer: Bridging the Gap Between Fundamental Biology and Patient Care

Testicular cancer is the most common solid tumor affecting young males. Most testicular cancers are testicular germ cell tumors (TGCTs), which are divided into seminomas (SGCTs) and non-seminomatous testicular germ cell tumors (NSGCTs). During their development, primordial germ cells (PGCs) undergo epigenetic modifications and any disturbances in their pattern might lead to cancer development. The present study provides a comprehensive review of the epigenetic mechanisms–DNA methylation, histone post-translational modifications, bivalent marks, non-coding RNA–associated with TGCT susceptibility, initiation, progression and response to chemotherapy. Another important purpose of this review is to highlight the recent investigations regarding the identification and development of epigenetic biomarkers as powerful tools for the diagnostic, prognostic and especially for epigenetic-based therapy.

MLL::AF9 degradation induces rapid changes in transcriptional elongation and subsequent loss of an active chromatin landscape

MLL rearrangements produce fusion oncoproteins that drive leukemia development, but the direct effects of MLL-fusion inactivation remain poorly defined. We designed models with degradable MLL::AF9 where treatment with small molecules induces rapid degradation. We leveraged the kinetics of this system to identify a core subset of MLL::AF9 target genes where MLL::AF9 degradation induces changes in transcriptional elongation within 15 minutes. MLL::AF9 degradation subsequently causes loss of a transcriptionally active chromatin landscape. We used this insight to assess the effectiveness of small molecules that target members of the MLL::AF9 multiprotein complex, specifically DOT1L and MENIN. Combined DOT1L/MENIN inhibition resembles MLL::AF9 degradation, whereas single-agent treatment has more modest effects on MLL::AF9 occupancy and gene expression. Our data show that MLL::AF9 degradation leads to decreases in transcriptional elongation prior to changes in chromatin landscape at select loci and that combined inhibition of chromatin complexes releases the MLL::AF9 oncoprotein from chromatin globally.

Spectroscopic and in vitro investigations of Fe2+/α-Ketoglutarate-dependent enzymes involved in nucleic acid repair and modification

The activation of molecular oxygen for the highly selective functionalization and repair of DNA and RNA nucleobases is achieved by α-ketoglutarate (α-KG)/iron-dependent dioxygenases. Enzymes of special interest are the human homologs AlkBH of Escherichia coli EcAlkB and ten-eleven translocation (TET) enzymes. These enzymes are involved in demethylation or dealkylation of DNA and RNA, although additional physiological functions are continuously being revealed. Given their importance, studying enzyme-substrate interactions, turnover and kinetic parameters is pivotal for the understanding of the mode of action of these enzymes. Diverse analytical methods, including X-ray crystallography, UV/Vis absorption, electron paramagnetic resonance (EPR), circular dichroism (CD) and NMR spectroscopy have been employed to study the changes in the active site and the overall enzyme structure upon substrate, cofactor and inhibitor addition. Several methods are now available to assess activity of these enzymes. By discussing limitations and possibilities of these techniques for EcAlkB, AlkBH and TET we aim to give a comprehensive synopsis from a bioinorganic point of view, addressing researchers from different disciplines working in the highly interdisciplinary and rapidly evolving field of epigenetic processes and DNA/RNA repair and modification.

Structure, Activity and Function of the MLL2 (KMT2B) Protein Lysine Methyltransferase

The Mixed Lineage Leukemia 2 (MLL2) protein, also known as KMT2B, belongs to the family of mammalian histone H3 lysine 4 (H3K4) methyltransferases. It is a large protein of 2715 amino acids, widely expressed in adult human tissues and a paralog of the MLL1 protein. MLL2 contains a characteristic C-terminal SET domain responsible for methyltransferase activity and forms a protein complex with WRAD (WDR5, RbBP5, ASH2L and DPY30), host cell factors 1/2 (HCF 1/2) and Menin. The MLL2 complex is responsible for H3K4 trimethylation (H3K4me3) on specific gene promoters and nearby cis-regulatory sites, regulating bivalent developmental genes as well as stem cell and germinal cell differentiation gene sets. Moreover, MLL2 plays a critical role in development and germ line deletions of Mll2 have been associated with early growth retardation, neural tube defects and apoptosis that leads to embryonic death. It has also been involved in the control of voluntary movement and the pathogenesis of early stage childhood dystonia. Additionally, tumor-promoting functions of MLL2 have been detected in several cancer types, including colorectal, hepatocellular, follicular cancer and gliomas. In this review, we discuss the main structural and functional aspects of the MLL2 methyltransferase with particular emphasis on transcriptional mechanisms, gene regulation and association with diseases.

Genome-wide investigation of the dynamic changes of epigenome modifications after global DNA methylation editing

Chromatin properties are regulated by complex networks of epigenome modifications. Currently, it is unclear how these modifications interact and if they control downstream effects such as gene expression. We employed promiscuous chromatin binding of a zinc finger fused catalytic domain of DNMT3A to introduce DNA methylation in HEK293 cells at many CpG islands (CGIs) and systematically investigated the dynamics of the introduced DNA methylation and the consequent changes of the epigenome network. We observed efficient methylation at thousands of CGIs, but it was unstable at about 90% of them, highlighting the power of genome-wide molecular processes that protect CGIs against DNA methylation. Partially stable methylation was observed at about 1000 CGIs, which showed enrichment in H3K27me3. Globally, the introduced DNA methylation strongly correlated with a decrease in gene expression indicating a direct effect. Similarly, global but transient reductions in H3K4me3 and H3K27ac were observed after DNA methylation but no changes were found for H3K9me3 and H3K36me3. Our data provide a global and time-resolved view on the network of epigenome modifications, their connections with DNA methylation and the responses triggered by artificial DNA methylation revealing a direct repressive effect of DNA methylation in CGIs on H3K4me3, histone acetylation, and gene expression.

The role of reciprocal fusions in MLL-r acute leukemia: studying the chromosomal translocation t(6;11)

Leukemia patients bearing t(6;11)(q27;q23) translocations can be divided in two subgroups: those with breakpoints in the major breakpoint cluster region of MLL (introns 9-10; associated mainly with AML M1/4/5), and others with breakpoints in the minor breakpoint cluster region (introns 21-23), associated with T-ALL. We cloned all four of the resulting fusion genes (MLL-AF6, AF6-MLL, exMLL-AF6, AF6-shMLL) and subsequently transfected them to generate stable cell culture models. Their molecular function was tested by inducing gene expression for 48 h in a Doxycycline-dependent fashion. Here, we present our results upon differential gene expression (DGE) that were obtained by the "Massive Analyses of cDNA Ends" (MACE-Seq) technology, an established 3'-end based RNA-Seq method. Our results indicate that the PHD/BD domain, present in the AF6-MLL and the exMLL-AF6 fusion protein, is responsible for chromatin activation in a genome-wide fashion. This led to strong deregulation of transcriptional processes involving protein-coding genes, pseudogenes, non-annotated genes, and RNA genes, e.g., LincRNAs and microRNAs, respectively. While cooperation between the MLL-AF6 and AF6-MLL fusion proteins appears to be required for the above-mentioned effects, exMLL-AF6 is able to cause similar effects on its own. The exMLL-AF6/AF6-shMLL co-expressing cell line displayed the induction of a myeloid-specific and a T-cell specific gene signature, which may explain the T-ALL disease phenotype observed in patients with such breakpoints. This again demonstrated that MLL fusion proteins are instructive and allow to study their pathomolecular mechanisms.

SET1/MLL family of proteins: functions beyond histone methylation

The SET1 family of enzymes are well known for their involvement in the histone 3 lysine 4 (H3K4) methylation, a conserved trait of euchromatin associated with transcriptional activation. These methyltransferases are distinct, and involved in various biological functions in the cell. Impairment in the function of SET1 family members leads to a number of abnormalities such as skeletal and neurological defects, leukaemogenesis and even lethality. Tremendous progress has been made in understanding the unique biological roles and the mechanism of SET1 enzymes in context with H3K4 methylation/canonical functions. However, in recent years, several studies have indicated the novel role of SET1 family proteins, other than H3K4 methylation, which are equally important for cellular functions. In this review, we focus on these non-canonical function of SET1 family members.

Restoring MLL reactivates latent tumor suppression-mediated vulnerability to proteasome inhibitors

MLL undergoes multiple distinct chromosomal translocations to yield aggressive leukemia with dismal outcomes. Besides their well-established role in leukemogenesis, MLL fusions also possess latent tumor-suppressive activity, which can be exploited as effective cancer treatment strategies using pharmacological means such as proteasome inhibitors (PIs). Here, using MLL-rearranged xenografts and MLL leukemic cells as models, we show that wild-type MLL is indispensable for the latent tumor-suppressive activity of MLL fusions. MLL dysfunction, shown as loss of the chromatin accumulation and subsequent degradation of MLL, compromises the latent tumor suppression of MLL-AF4 and is instrumental for the acquired PI resistance. Mechanistically, MLL dysfunction is caused by chronic PI treatment-induced epigenetic reprogramming through the H2Bub-ASH2L-MLL axis and can be specifically restored by histone deacetylase (HDAC) inhibitors, which induce histone acetylation and recruits MLL on chromatin to promote cell cycle gene expression. Our findings not only demonstrate the mechanism underlying the inevitable acquisition of PI resistance in MLL leukemic cells, but also illustrate that preventing the emergence of PI-resistant cells constitutes a novel rationale for combination therapy with PIs and HDAC inhibitors in MLL leukemias.

C1188D mutation abolishes specific recognition between MLL1-CXXC domain and CpG site by inducing conformational switch of flexible N-terminal

Mixed lineage leukemia protein (MLL1 protein) recognizes the CpG site via its CXXC domain and is frequently associated with leukemia. The specific recognition is abolished by C1188D mutation, which also prevents MLL-related leukemia. In this paper, multiple molecular dynamic (MD) simulations were performed to investigate the mechanism of recognition and influences of C1188D mutation. Started from fully dissociated DNA and MLL1-CXXC domain, remarkably, the center of mass (COM) of MLL1-CXXC domain quickly concentrates on the vicinity of the CpG site in all 53 short MD simulations. Extended simulations of the wild type showed that the native complex formed in 500 ns among 4 of 53 simulations. In contrast, the C1188D mutant COM distributed broadly around the DNA and the native complex was not observed in any of the extended simulations. Simulations on the apo MLL1-CXXC domain further suggest that the wild type protein remained predominantly in an open form that closely resembles its structure in the native complex whereas C1188D mutant formed predominantly compact structures in which the N- terminal bends to D1188. This conformational switch hinders the formation of encounter complex, thus abolishes the recognition. Our study also provides clues to the study mechanism of recognition, by the CXXC domain from proteins like DNA methyltransferase and ten-eleven translocation enzymes.

Understanding the interplay between CpG island-associated gene promoters and H3K4 methylation

The precise regulation of gene transcription is required to establish and maintain cell type-specific gene expression programs during multicellular development. In addition to transcription factors, chromatin, and its chemical modification, play a central role in regulating gene expression. In vertebrates, DNA is pervasively methylated at CG dinucleotides, a modification that is repressive to transcription. However, approximately 70% of vertebrate gene promoters are associated with DNA elements called CpG islands (CGIs) that are refractory to DNA methylation. CGIs integrate the activity of a range of chromatin-regulating factors that can post-translationally modify histones and modulate gene expression. This is exemplified by the trimethylation of histone H3 at lysine 4 (H3K4me3), which is enriched at CGI-associated gene promoters and correlates with transcriptional activity. Through studying H3K4me3 at CGIs it has become clear that CGIs shape the distribution of H3K4me3 and, in turn, H3K4me3 influences the chromatin landscape at CGIs. Here we will discuss our understanding of the emerging relationship between CGIs, H3K4me3, and gene expression.

The molecular functions of common and atypical MLL fusion protein complexes

Mixed-lineage leukemia (MLL) fuses with a variety of partners to produce a functionally altered MLL complex that is not expressed in normal cells, which transforms normal hematopoietic progenitors into leukemia cells. Because more than 80 fusion partners have been identified to date, the molecular functions of MLL fusion protein complexes appear diverse. However, over the past decade, the common functions utilized for leukemic transformation have begun to be elucidated. It appears that most (if not all) MLL fusion protein complexes utilize the AF4/ENL/P-TEFb and DOT1L complexes to some extent. Based on an understanding of the underlying molecular mechanisms, several molecular targeting drugs are being developed, opening paths to novel therapies. Here, we review the recent progress made in identifying the molecular functions of various MLL fusions and categorize the numerous fusion partners into several functionally-distinct groups to help discern commonalities and differences among various MLL fusion protein complexes.

Study of KMT2B (MLL2) gene expression changes in patients with breast cancer

Aim: This study aimed to demonstrate misregulation of KMT2B gene expression in breast cancer tissue. Materials & methods: Cancerous and marginal tissue samples were collected from 43 female patients. After RNA extraction and cDNA synthesis, quantitative-PCR was used to evaluate the expression level of the KMT2B gene. REST, Sigma plot and SPSS software were used to analyze data. Results: KMT2B gene expression was significantly decreased in tumor tissue compared with marginal tissue (p = 0.02). No significant correlation was found between expression levels of KMT2B and clinical parameters of patients (p > 0.05) Conclusion: Our study demonstrated that downregulation of KMT2B is associated with breast cancer and its misregulation may play an important role in tumorigenesis.

Why are so many MLL lysine methyltransferases required for normal mammalian development?

The mixed lineage leukemia (MLL) family of proteins became known initially for the leukemia link of its founding member. Over the decades, the MLL family has been recognized as an important class of histone H3 lysine 4 (H3K4) methyltransferases that control key aspects of normal cell physiology and development. Here, we provide a brief history of the discovery and study of this family of proteins. We address two main questions: why are there so many H3K4 methyltransferases in mammals; and is H3K4 methylation their key function?

Rewiring the Epigenetic Networks in MLL-Rearranged Leukemias: Epigenetic Dysregulation and Pharmacological Interventions

Leukemias driven by chromosomal translocation of the mixed-lineage leukemia gene (MLL or KMT2A) are highly prevalent in pediatric oncology. The poor survival rate and lack of an effective targeted therapy for patients with MLL-rearranged (MLL-r) leukemias emphasize an urgent need for improved knowledge and novel therapeutic approaches for these malignancies. The resulting chimeric products of MLL gene rearrangements, i.e., MLL-fusion proteins (MLL-FPs), are capable of transforming hematopoietic stem/progenitor cells (HSPCs) into leukemic blasts. The ability of MLL-FPs to reprogram HSPCs toward leukemia requires the involvement of multiple chromatin effectors, including the histone 3 lysine 79 methyltransferase DOT1L, the chromatin epigenetic reader BRD4, and the super elongation complex. These epigenetic regulators constitute a complicated network that dictates maintenance of the leukemia program, and therefore represent an important cluster of therapeutic opportunities. In this review, we will discuss the role of MLL and its fusion partners in normal HSPCs and hematopoiesis, including the links between chromatin effectors, epigenetic landscapes, and leukemia development, and summarize current approaches to therapeutic targeting of MLL-r leukemias.

RNA Polymerase II-Dependent Transcription Initiated by Selectivity Factor 1: A Central Mechanism Used by MLL Fusion Proteins in Leukemic Transformation

Cancer cells transcribe RNAs in a characteristic manner in order to maintain their oncogenic potentials. In eukaryotes, RNA is polymerized by three distinct RNA polymerases, RNA polymerase I, II, and III (RNAP1, RNAP2, and RNAP3, respectively). The transcriptional machinery that initiates each transcription reaction has been purified and characterized. Selectivity factor 1 (SL1) is the complex responsible for RNAP1 pre-initiation complex formation. However, whether it plays any role in RNAP2-dependent transcription remains unclear. Our group previously found that SL1 specifically associates with AF4 family proteins. AF4 family proteins form the AEP complex with ENL family proteins and the P-TEFb elongation factor. Similar complexes have been independently characterized by several different laboratories and are often referred to as super elongation complex. The involvement of AEP in RNAP2-dependent transcription indicates that SL1 must play an important role in RNAP2-dependent transcription. To date, this role of SL1 has not been appreciated. In leukemia, AF4 and ENL family genes are frequently rearranged to form chimeric fusion genes with MLL. The resultant MLL fusion genes produce chimeric MLL fusion proteins comprising MLL and AEP components. The MLL portion functions as a targeting module, which specifically binds chromatin containing di-/tri-methylated histone H3 lysine 36 and non-methylated CpGs. This type of chromatin is enriched at the promoters of transcriptionally active genes which allows MLL fusion proteins to selectively bind to transcriptionally-active/CpG-rich gene promoters. The fusion partner portion, which recruits other AEP components and SL1, is responsible for activation of RNAP2-dependent transcription. Consequently, MLL fusion proteins constitutively activate the transcription of previously-transcribed MLL target genes. Structure/function analysis has shown that the ability of MLL fusion proteins to transform hematopoietic progenitors depends on the recruitment of AEP and SL1. Thus, the AEP/SL1-mediated gene activation pathway appears to be the central mechanism of MLL fusion-mediated transcriptional activation. However, the molecular mechanism by which SL1 activates RNAP2-dependent transcription remains largely unclear. This review aims to cover recent discoveries of the mechanism of transcriptional activation by MLL fusion proteins and to introduce novel roles of SL1 in RNAP2-dependent transcription by discussing how the RNAP1 machinery may be involved in RNAP2-dependent gene regulation.

Novel sub-cellular localizations and intra-molecular interactions may define new functions of Mixed Lineage Leukemia protein

Mixed-lineage leukemia (MLL) protein is the best-characterized member of SET family of histone 3 lysine 4 methyltransferase, known for its transcriptional-activation role during development. mll gene rearrangements cause multiple kinds of aggressive leukemia in both children and adults. An important 'first' step in understanding the role of MLL in leukemogenesis would be to identify its localization throughout the cell cycle. In order to fully understand the breath of MLL functions in proliferating cells, we have analyzed its sub-cellular localization during the cell cycle. Our results show that MLL localizes to nucleolus and centrosome in interphase. During mitosis, it localizes to centrosomes and midbody in addition to previously reported spindle apparatus. Our results show that MLL_N is required to translocate MLL_C to the nucleolus. These finding suggest functional roles for MLL in nucleolus and mitosis. We also show how MLL-fusion proteins (MLL-FPs) localize to the same sub-cellular organelles like endogenous MLL. Our results indicate that MLL-fusion proteins may not only disturb the cell homeostasis by gain-of-function of the chimeric protein, but also by interfering with the functions of endogenous MLL.

Genome-wide expert annotation of the epigenetic machinery of the plant-parasitic nematodes Meloidogyne spp., with a focus on the asexually reproducing species

Background

The renewed interest in epigenetics has led to the understanding that both the environment and individual lifestyle can directly interact with the epigenome to influence its dynamics. Epigenetic phenomena are mediated by DNA methylation, stable chromatin modifications and non-coding RNA-associated gene silencing involving specific proteins called epigenetic factors.

Multiple organisms, ranging from plants to yeast and mammals, have been used as model systems to study epigenetics. The interactions between parasites and their hosts are models of choice to study these mechanisms because the selective pressures are strong and the evolution is fast.

The asexually reproducing root-knot nematodes (RKN) offer different advantages to study the processes and mechanisms involved in epigenetic regulation. RKN genomes sequencing and annotation have identified numerous genes, however, which of those are involved in the adaption to an environment and potentially relevant to the evolution of plant-parasitism is yet to be discovered.

Results

Here, we used a functional comparative annotation strategy combining orthology data, mining of curated genomics as well as protein domain databases and phylogenetic reconstructions.

Overall, we show that (i) neither RKN, nor the model nematode Caenorhabditis elegans possess any DNA methyltransferases (DNMT) (ii) RKN do not possess the complete machinery for DNA methylation on the 6th position of adenine (6mA) (iii) histone (de)acetylation and (de)methylation pathways are conserved between C. elegans and RKN, and the corresponding genes are amplified in asexually reproducing RKN (iv) some specific non-coding RNA families found in plant-parasitic nematodes are dissimilar from those in C. elegans. In the asexually reproducing RKN Meloidogyne incognita, expression data from various developmental stages supported the putative role of these proteins in epigenetic regulations.

Conclusions

Our results refine previous predictions on the epigenetic machinery of model species and constitute the most comprehensive description of epigenetic factors relevant to the plant-parasitic lifestyle and/or asexual mode of reproduction of RKN. Providing an atlas of epigenetic factors in RKN is an informative resource that will enable researchers to explore their potential role in adaptation of these parasites to their environment.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4686-x) contains supplementary material, which is available to authorized users.

Recognition of Oxidized 5-Methylcytosine Derivatives in DNA by Natural and Engineered Protein Scaffolds

Methylation of genomic cytosine to 5-methylcytosine is a central regulatory element of mammalian gene expression with important roles in development and disease. 5-methylcytosine can be actively reversed to cytosine via oxidation to 5-hydroxymethyl-, 5-formyl-, and 5-carboxylcytosine by ten-eleven-translocation dioxygenases and subsequent base excision repair or replication-dependent dilution. Moreover, the oxidized 5-methylcytosine derivatives are potential epigenetic marks with unique biological roles. Key to a better understanding of these roles are insights into the interactions of the nucleobases with DNA-binding protein scaffolds: Natural scaffolds involved in transcription, 5-methylcytosine-reading and -editing as well as general chromatin organization can be selectively recruited or repulsed by oxidized 5-methylcytosines, forming the basis of their biological functions. Moreover, designer protein scaffolds engineered for the selective recognition of oxidized 5-methylcytosines are valuable tools to analyze their genomic levels and distribution. Here, we review recent structural and functional insights into the molecular recognition of oxidized 5-methylcytosine derivatives in DNA by selected protein scaffolds.

DNA methylation regulates discrimination of enhancers from promoters through a H3K4me1-H3K4me3 seesaw mechanism

BACKGROUND

DNA methylation at promoters is largely correlated with inhibition of gene expression. However, the role of DNA methylation at enhancers is not fully understood, although a crosstalk with chromatin marks is expected. Actually, there exist contradictory reports about positive and negative correlations between DNA methylation and H3K4me1, a chromatin hallmark of enhancers.

RESULTS

We investigated the relationship between DNA methylation and active chromatin marks through genome-wide correlations, and found anti-correlation between H3K4me1 and H3K4me3 enrichment at low and intermediate DNA methylation loci. We hypothesized "seesaw" dynamics between H3K4me1 and H3K4me3 in the low and intermediate DNA methylation range, in which DNA methylation discriminates between enhancers and promoters, marked by H3K4me1 and H3K4me3, respectively. Low methylated regions are H3K4me3 enriched, while those with intermediate DNA methylation levels are progressively H3K4me1 enriched. Additionally, the enrichment of H3K27ac, distinguishing active from primed enhancers, follows a plateau in the lower range of the intermediate DNA methylation level, corresponding to active enhancers, and decreases linearly in the higher range of the intermediate DNA methylation. Thus, the decrease of the DNA methylation switches smoothly the state of the enhancers from a primed to an active state. We summarize these observations into a rule of thumb of one-out-of-three methylation marks: "In each genomic region only one out of these three methylation marks {DNA methylation, H3K4me1, H3K4me3} is high. If it is the DNA methylation, the region is inactive. If it is H3K4me1, the region is an enhancer, and if it is H3K4me3, the region is a promoter". To test our model, we used available genome-wide datasets of H3K4 methyltransferases knockouts. Our analysis suggests that CXXC proteins, as readers of non-methylated CpGs would regulate the "seesaw" mechanism that focuses H3K4me3 to unmethylated sites, while being repulsed from H3K4me1 decorated enhancers and CpG island shores.

CONCLUSIONS

Our results show that DNA methylation discriminates promoters from enhancers through H3K4me1-H3K4me3 seesaw mechanism, and suggest its possible function in the inheritance of chromatin marks after cell division. Our analyses suggest aberrant formation of promoter-like regions and ectopic transcription of hypomethylated regions of DNA. Such mechanism process can have important implications in biological process in where it has been reported abnormal DNA methylation status such as cancer and aging.

Epigenetic activation of the drug transporter OCT2 sensitizes renal cell carcinoma to oxaliplatin

Renal cell carcinoma (RCC) is known for its multidrug resistance. Using data obtained from the cancer transcriptome database Oncomine and the proteome database The Human Protein Atlas, we identified the repression of organic cation transporter OCT2 as a potential factor contributing to oxaliplatin resistance in RCC. By analyzing OCT2 expression in collected patient tissues and commercial tissue microarray specimens, we demonstrated OCT2 repression in RCC at both transcription and protein levels. Epigenetic analysis revealed that the repressed OCT2 promoter in RCC is characterized by hypermethylated CpG islands and the absence of H3K4 methylation. Further mechanistic studies showed that DNA hypermethylation blocked MYC activation of OCT2 by disrupting its interaction with the E-Box motif, which prevented MYC from recruiting MLL1 to catalyze H3K4me3 at the OCT2 promoter and resulted in repressed OCT2 transcription. Targeting this mechanism, we designed a sequential combination therapy and demonstrated that epigenetic activation of OCT2 by decitabine sensitizes RCC cells to oxaliplatin both in vitro and in xenografts. Our study highlights the potential of translating "omics" data into the development of targeted therapies.

Molecular and cellular issues of KMT2A variants involved in Wiedemann-Steiner syndrome

Variants in KMT2A, encoding the histone methyltransferase KMT2A, are a growing cause of intellectual disability (ID). Up to now, the majority of KMT2A variants are non-sense and frameshift variants causing a typical form of Wiedemann-Steiner syndrome. We studied KMT2A gene in a cohort of 200 patients with unexplained syndromic and non-syndromic ID and identified four novel variants, one splice and three missense variants, possibly deleterious. We used primary cells from the patients and molecular approaches to determine the deleterious effects of those variants on KMT2A expression and function. For the putative splice variant c.11322-1G>A, we showed that it led to only one nucleotide deletion and loss of the C-terminal part of the protein. For two studied KMT2A missense variants, c.3460C>T (p.(Arg1154Trp)) and c.8558T>G (p.(Met2853Arg)), located at the cysteine-rich CXXC domain and the transactivation domain of the protein, respectively, we found altered KMT2A target genes expression in patient's fibroblasts compared to controls. Furthermore, we found a disturbed subcellular distribution of KMT2A for the c.3460C>T mutant. Taken together, our results demonstrated the deleterious impact of the splice variant and of the missense variants located at two different functional domains and suggested reduction of KMT2A function as the disease-causing mechanism.

Bivalent Epigenetic Control of Oncofetal Gene Expression in Cancer

Multiple mechanisms of epigenetic control that include DNA methylation, histone modification, noncoding RNAs, and mitotic gene bookmarking play pivotal roles in stringent gene regulation during lineage commitment and maintenance. Experimental evidence indicates that bivalent chromatin domains, i.e., genome regions that are marked by both H3K4me3 (activating) and H3K27me3 (repressive) histone modifications, are a key property of pluripotent stem cells. Bivalency of developmental genes during the G₁ phase of the pluripotent stem cell cycle contributes to cell fate decisions. Recently, some cancer types have been shown to exhibit partial recapitulation of bivalent chromatin modifications that are lost along with pluripotency, suggesting a mechanism by which cancer cells reacquire properties that are characteristic of undifferentiated, multipotent cells. This bivalent epigenetic control of oncofetal gene expression in cancer cells may offer novel insights into the onset and progression of cancer and may provide specific and selective options for diagnosis as well as for therapeutic intervention.

A quantitative homogeneous assay for global DNA methylation levels using CpG-binding domain- and methyl-CpG-binding domain-fused luciferase

Global DNA methylation levels have been considered as biomarkers for cancer diagnostics because transposable elements that constitute approximately 45% of the human genome are hypomethylated in cancer cells. We have previously reported a homogeneous assay for measuring methylated CpG content of genomic DNA based on bioluminescence resonance energy transfer (BRET) using methyl-CpG-binding domain (MBD)-fused luciferase (MBD-luciferase). In this study, a homogeneous assay for measuring unmethylated CpG content of genomic DNA in the same platform was developed using CXXC domain-fused luciferase (CXXC-luciferase) that specifically recognizes unmethylated CpG. In this assay, CXXC-luciferase recognizes unmethylated CpG on genomic DNA, whereby BRET between luciferase and the fluorescent DNA intercalating dye is detected. We demonstrated that the BRET signal depended on the genomic DNA concentration (R² = 0.99) and unmethylated CpG content determined by the bisulfite method (R² = 0.97). There was a significant negative correlation between the BRET signal of the CXXC-luciferase-based assay and that of the MBD-luciferase-based assay (R² = 0.92). Moreover, we demonstrated that the global DNA methylation level determined using the bisulfite method was dependent on the ratio of the BRET signal in the MBD-luciferase-based assay to the total BRET signal in the MBD-luciferase- and CXXC-luciferase-based assays (R² = 0.99, relative standard deviation < 2.2%, and analysis speed < 35 min). These results demonstrated that global DNA methylation levels can be quantified by calculating the BRET signal ratio without any calibration curve.

Ten-eleven translocation 1 functions as a mediator of SOD3 expression in human lung cancer A549 cells

Superoxide dismutase (SOD) 3, one of the SOD isozymes, plays a pivotal role in extracellular redox homeostasis. The expression of SOD3 is regulated by epigenetics in human lung cancer A549 cells and human monocytic THP-1 cells; however, the molecular mechanisms governing SOD3 expression have not been elucidated in detail. Ten-eleven translocation (TET), a dioxygenase of 5-methylcytosine (5mC), plays a central role in DNA demethylation processes and induces target gene expression. In the present study, TET1 expression was abundant in U937 cells, but its expression was weakly expressed in A549 and THP-1 cells. These results are consistent with the expression pattern of SOD3 and its DNA methylation status in these cells. Moreover, above relationship was also observed in human breast cancer cells, human prostate cancer cells, and human skin fibroblasts. The overexpression of TET1-catalytic domain (TET1-CD) induced the expression of SOD3 in A549 cells, and this was accompanied by the direct binding of TET1-CD to the SOD3 promoter region. Furthermore, in TET1-CD-transfected A549 cells, the level of 5-hydroxymethylcytosine within that region was significantly increased, whereas the level of 5mC was decreased. The results of the present study demonstrate that TET1 might function as one of the key molecules in SOD3 expression through its 5mC hydroxylation in A549 cells.

[A CxxC domain that binds to unmethylated CpG is required for KDM2A to control rDNA transcription]

  Dysfunction of ribosome biogenesis is commonly found in cancers. Because the transcription of ribosomal RNA genes (rDNA) is a rate-limiting step in ribosome biogenesis and is elevated in many cancer cells, ribosomal RNA transcription can be a target for cancer therapy. In eukaryotes, ribosomal RNA is transcribed specifically in nucleoli by RNA polymerase I but not by RNA polymerase II. Therefore, ribosomal RNA transcription by RNA polymerase I would have a distinct nature compared to transcription by RNA polymerase II. Genomic DNA with proteins including histones constitutes chromatin. The structure of chromatin has plasticity and is regulated by chemical modifications of chromatin's components. We had reported that histone demethylase KDM2A reduced ribosomal RNA transcription in response to starvation. In this symposium, we reported our recent results showing the mechanism by which KDM2A was recruited to rDNA chromatin. We found that KDM2A bound to a rDNA promoter with unmethylated CpG dinucleotides via KDM2A CxxC-zinc finger motif. This binding was required for KDM2A to demethylate histone in the rDNA promoter and reduce rDNA transcription resulting from starvation. Further, this binding was detected before starvation, independent of the demethylase activity. We also found that the histone demethylation by KDM2A in response to starvation was detected only in the rDNA promoter, but not in a gene promoter transcribed by Pol II, the P2RX4 promoter. These results suggest that it is important to consider genome regions and cell conditions when developing epigenetic drugs.

Transcriptional activation by MLL fusion proteins in leukemogenesis

Chromosomal translocations involving the mixed lineage leukemia (MLL) gene cause aggressive leukemia. Fusion proteins of MLL and a component of the AF4 family/ENL family/P-TEFb complex (AEP) are responsible for two-thirds of MLL-associated leukemia cases. MLL-AEP fusion proteins trigger aberrant self-renewal of hematopoietic progenitors by constitutively activating self-renewal-related genes. MLL-AEP fusion proteins activate transcription initiation by loading the TATA-binding protein (TBP) to the TATA element via selectivity factor 1. Although AEP retains transcription elongation and mediator recruiting activities, the rate-limiting step activated by MLL-AEP fusion proteins appears to be the TBP-loading step. This is contrary to prevailing views, in which the recruitment of transcription elongation activities are emphasized. Here, I review recent advances towards elucidating the mechanisms underlying gene activation by MLL-AEP fusion proteins in leukemogenesis.

TBP loading by AF4 through SL1 is the major rate-limiting step in MLL fusion-dependent transcription

Gene rearrangement of the mixed lineage leukemia (MLL) gene causes leukemia by inducing the constitutive expression of a gene subset normally expressed only in the immature haematopoietic progenitor cells. MLL gene rearrangements often generate fusion products of MLL and a component of the AF4 family/ENL family/P-TEFb (AEP) complex. MLL-AEP fusion proteins have the potential of constitutively recruiting the P-TEFb elongation complex. Thus, it is hypothesized that relieving the promoter proximal pausing of RNA polymerase II is the rate-limiting step of MLL fusion-dependent transcription. AEP also has the potential to recruit the mediator complex via MED26. We recently showed that AEP activates transcription initiation by facilitating TBP loading to the TATA element through the SL1 complex. In the present study, we show that the key activity responsible for the oncogenic property of MLL-AEP fusion proteins is the TBP loading activity, and not the mediator recruitment or transcriptional elongation activities. Thus, we propose that TBP loading by AF4 through SL1 is the major rate-limiting step in MLL fusion-dependent transcription.

Are there specific readers of oxidized 5-methylcytosine bases?

5-methylcytosine (5mC) was long thought to be the only enzymatically created modified DNA base in mammalian cells. The discovery of 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine as reaction products of the TET family 5mC oxidases has prompted extensive searches for proteins that specifically bind to these oxidized bases. However, only a few of such "reader" proteins have been identified and verified so far. In this review, we discuss potential biological functions of oxidized 5mC as well as the role the presumed reader proteins may play in interpreting the genomic signals of 5mC oxidation products.

MLL1 and MLL1 fusion proteins have distinct functions in regulating leukemic transcription program

Mixed lineage leukemia protein-1 (MLL1) has a critical role in human MLL1 rearranged leukemia (MLLr) and is a validated therapeutic target. However, its role in regulating global gene expression in MLLr cells, as well as its interplay with MLL1 fusion proteins remains unclear. Here we show that despite shared DNA-binding and cofactor interacting domains at the N terminus, MLL1 and MLL-AF9 are recruited to distinct chromatin regions and have divergent functions in regulating the leukemic transcription program. We demonstrate that MLL1, probably through C-terminal interaction with WDR5, is recruited to regulatory enhancers that are enriched for binding sites of E-twenty-six (ETS) family transcription factors, whereas MLL-AF9 binds to chromatin regions that have no H3K4me1 enrichment. Transcriptome-wide changes induced by different small molecule inhibitors also highlight the distinct functions of MLL1 and MLL-AF9. Taken together, our studies provide novel insights on how MLL1 and MLL fusion proteins contribute to leukemic gene expression, which have implications for developing effective therapies in the future.

Drugging the methylome: DNA methylation and memory

Over the past decade, since epigenetic mechanisms were first implicated in memory formation and synaptic plasticity, dynamic DNA methylation reactions have been identified as integral to long-term memory formation, maintenance, and recall. This review incorporates various new findings that DNA methylation mechanisms are important regulators of non-Hebbian plasticity mechanisms, suggesting that these epigenetic mechanisms are a fundamental link between synaptic plasticity and metaplasticity. Because the field of neuroepigenetics is so young and the biochemical tools necessary to probe gene-specific questions are just now being developed and used, this review also speculates about the direction and potential of therapeutics that target epigenetic mechanisms in the central nervous system and the unique pharmacokinetic and pharmacodynamic properties that epigenetic therapies may possess. Mapping the dynamics of the epigenome in response to experiential learning, even a single epigenetic mark in isolation, remains a significant technical and bioinformatic hurdle facing the field, but will be necessary to identify changes to the methylome that govern memory-associated gene expression and effectively drug the epigenome.

Epigenetics, TET proteins, and hypoxia in epithelial-mesenchymal transition and tumorigenesis

Hypoxia in tumors is primarily a pathophysiologic consequence of structurally and functionally disturbed microcirculation with inadequate supply of oxygen. Tumor hypoxia is strongly associated with tumor propagation, malignant progression, and resistance to therapy. Aberrant epigenetic regulation plays a crucial role in the process of hypoxia-driven malignant progression. Convert of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) by ten-eleven translocation (TET) family enzymes plays important biological functions in embryonic stem cells, development, aging and disease. Recent reports showed that level of 5hmC and TET proteins was altered in various types of cancers. There is a strong correlation between loss of 5hmC and cancer development but research to date indicates that loss of TET activity is associated with the cancer phenotype but it is not clear whether TET proteins function as tumor suppressors or oncogenes. While loss of TET1 and TET2 expression is associated with solid cancers, implying a tumor suppressor role, TET1 exhibits a clear oncogenic role in the context of genomic rearrangements such as in MLL-fusion rearranged leukemia. Interestingly, hypoxia increases global 5hmC levels and upregulates TET1 expression in a HIF1α-dependent manner. Recently, hypoxia-induced TET1 has been demonstrated to play another important role for regulating hypoxia-responsive gene expression and epithelial-mesenchymal transition (EMT) by serving as a transcription co-activator. Furthermore, hypoxia-induced TET1 also regulates glucose metabolism and hypoxia-induced EMT through enhancing the expression of insulin induced gene 1 (INSIG1). The roles and mechanisms of action of 5hmC and TET proteins in ES cell biology and during embryonic development, as well as in cancer biology, will be the main focus in this review.

Hypoxia, Epithelial-Mesenchymal Transition, and TET-Mediated Epigenetic Changes

Tumor hypoxia is a pathophysiologic outcome of disrupted microcirculation with inadequate supply of oxygen, leading to enhanced proliferation, epithelial-mesenchymal transition (EMT), metastasis, and chemo-resistance. Epigenetic changes induced by hypoxia are well documented, and they lead to tumor progression. Recent advances show that DNA demethylation mediated by the Ten-eleven translocation (TET) proteins induces major epigenetic changes and controls key steps of cancer development. TET enzymes serve as 5mC (5-methylcytosine)-specific dioxygenases and cause DNA demethylation. Hypoxia activates the expression of TET1, which also serves as a co-activator of HIF-1α transcriptional regulation to modulate HIF-1α downstream target genes and promote epithelial-mesenchymal transition. As HIF is a negative prognostic factor for tumor progression, hypoxia-activated prodrugs (HAPs) may provide a favorable therapeutic approach to lessen hypoxia-induced malignancy.