Activating and inhibitory functions for the histone lysine methyltransferase G9a in T helper cell differentiation and function

The H3K9 dimethyltransferases EHMT1/2 protect against pathological cardiac hypertrophy

Cardiac hypertrophic growth in response to pathological cues is associated with reexpression of fetal genes and decreased cardiac function and is often a precursor to heart failure. In contrast, physiologically induced hypertrophy is adaptive, resulting in improved cardiac function. The processes that selectively induce these hypertrophic states are poorly understood. Here, we have profiled 2 repressive epigenetic marks, H3K9me2 and H3K27me3, which are involved in stable cellular differentiation, specifically in cardiomyocytes from physiologically and pathologically hypertrophied rat hearts, and correlated these marks with their associated transcriptomes. This analysis revealed the pervasive loss of euchromatic H3K9me2 as a conserved feature of pathological hypertrophy that was associated with reexpression of fetal genes. In hypertrophy, H3K9me2 was reduced following a miR-217-mediated decrease in expression of the H3K9 dimethyltransferases EHMT1 and EHMT2 (EHMT1/2). miR-217-mediated, genetic, or pharmacological inactivation of EHMT1/2 was sufficient to promote pathological hypertrophy and fetal gene reexpression, while suppression of this pathway protected against pathological hypertrophy both in vitro and in mice. Thus, we have established a conserved mechanism involving a departure of the cardiomyocyte epigenome from its adult cellular identity to a reprogrammed state that is accompanied by reexpression of fetal genes and pathological hypertrophy. These results suggest that targeting miR-217 and EHMT1/2 to prevent H3K9 methylation loss is a viable therapeutic approach for the treatment of heart disease.

Interplay Between Inflammation and Epigenetic Changes in Cancer

Immune responses can suppress tumorigenesis, but also contribute to cancer initiation and progression suggesting a complex interaction between the immune system and cancer. Epigenetic alterations, which are heritable changes in gene expression without changes to the DNA sequence, also play a role in carcinogenesis through silencing expression of tumor suppressor genes and activating oncogenic signaling. Interestingly, epithelial cells at sites of chronic inflammation undergo DNA methylation alterations that are similar to those present in cancer cells, suggesting that inflammation may initiate cancer-specific epigenetic changes in epithelial cells. Furthermore, epigenetic changes occur during immune cell differentiation and participate in regulating the immune response, including the regulation of inflammatory cytokines. Cancer cells utilize epigenetic silencing of immune-related genes to evade the immune response. This chapter will detail the interactions between inflammation and epigenetics in tumor initiation, promotion, and immune evasion and how these connections are being leveraged in cancer prevention and treatment.

Epigenetic control of adult stem cell function

Mammalian embryonic development is a tightly regulated process that, from a single zygote, produces a large number of cell types with hugely divergent functions. Distinct cellular differentiation programmes are facilitated by tight transcriptional and epigenetic regulation. However, the contribution of epigenetic regulation to tissue homeostasis after the completion of development is less well understood. In this Review, we explore the effects of epigenetic dysregulation on adult stem cell function. We conclude that, depending on the tissue type and the epigenetic regulator affected, the consequences range from negligible to stem cell malfunction and disruption of tissue homeostasis, which may predispose to diseases such as cancer.

Identification of an NKX3.1-G9a-UTY transcriptional regulatory network that controls prostate differentiation

The NKX3.1 homeobox gene plays essential roles in prostate differentiation and prostate cancer. We show that loss of function of Nkx3.1 in mouse prostate results in down-regulation of genes that are essential for prostate differentiation, as well as up-regulation of genes that are not normally expressed in prostate. Conversely, gain of function of Nkx3.1 in an otherwise fully differentiated nonprostatic mouse epithelium (seminal vesicle) is sufficient for respecification to prostate in renal grafts in vivo. In human prostate cells, these activities require the interaction of NKX3.1 with the G9a histone methyltransferase via the homeodomain and are mediated by activation of target genes such as UTY (KDM6c), the male-specific paralog of UTX (KDM6a) We propose that an NKX3.1-G9a-UTY transcriptional regulatory network is essential for prostate differentiation, and we speculate that disruption of such a network predisposes to prostate cancer.

Histone methylation, alternative splicing and neuronal differentiation

Alternative splicing, as well as chromatin structure, greatly contributes to specific transcriptional programs that promote neuronal differentiation. The activity of G9a, the enzyme responsible for mono- and di-methylation of lysine 9 on histone H3 (H3K9me1 and H3K9me2) in mammalian euchromatin, has been widely implicated in the differentiation of a variety of cell types and tissues. In a recent work from our group (Fiszbein et al., 2016) we have shown that alternative splicing of G9a regulates its nuclear localization and, therefore, the efficiency of H3K9 methylation, which promotes neuronal differentiation. We discuss here our results in the light of a report from other group (Laurent et al. 2015) demonstrating a key role for the alternative splicing of the histone demethylase LSD1 in controlling specific gene expression in neurons. All together, these results illustrate the importance of alternative splicing in the generation of a proper equilibrium between methylation and demethylation of histones for the regulation of neuron-specific transcriptional programs.

The Costimulatory Receptor OX40 Inhibits Interleukin-17 Expression through Activation of Repressive Chromatin Remodeling Pathways

T helper 17 (Th17) cells are prominently featured in multiple autoimmune diseases, but the regulatory mechanisms that control Th17 cell responses are poorly defined. Here we found that stimulation of OX40 triggered a robust chromatin remodeling response and produced a "closed" chromatin structure at interleukin-17 (IL-17) locus to inhibit Th17 cell function. OX40 activated the NF-κB family member RelB, and RelB recruited the histone methyltransferases G9a and SETDB1 to the Il17 locus to deposit "repressive" chromatin marks at H3K9 sites, and consequently repressing IL-17 expression. Unlike its transcriptional activities, RelB acted independently of both p52 and p50 in the suppression of IL-17. In an experimental autoimmune encephalomyelitis (EAE) disease model, we found that OX40 stimulation inhibited IL-17 and reduced EAE. Conversely, RelB-deficient CD4(+) T cells showed enhanced IL-17 induction and exacerbated the disease. Our data uncover a mechanism in the control of Th17 cells that might have important clinic implications.

G9a regulates group 2 innate lymphoid cell development by repressing the group 3 innate lymphoid cell program

Antignano, Zaph, and collaborators show that the lysine methyltransferase G9a plays a critical role in determining the developmental programs of group 2 and 3 innate lymphoid cells.

Innate lymphoid cells (ILCs) are emerging as important regulators of homeostatic and disease-associated immune processes. Despite recent advances in defining the molecular pathways that control development and function of ILCs, the epigenetic mechanisms that regulate ILC biology are unknown. Here, we identify a role for the lysine methyltransferase G9a in regulating ILC2 development and function. Mice with a hematopoietic cell–specific deletion of G9a (Vav.G9a^−/− mice) have a severe reduction in ILC2s in peripheral sites, associated with impaired development of immature ILC2s in the bone marrow. Accordingly, Vav.G9a^−/− mice are resistant to the development of allergic lung inflammation. G9a-dependent dimethylation of histone 3 lysine 9 (H3K9me2) is a repressive histone mark that is associated with gene silencing. Genome-wide expression analysis demonstrated that the absence of G9a led to increased expression of ILC3-associated genes in developing ILC2 populations. Further, we found high levels of G9a-dependent H3K9me2 at ILC3-specific genetic loci, demonstrating that G9a-mediated repression of ILC3-associated genes is critical for the optimal development of ILC2s. Together, these results provide the first identification of an epigenetic regulatory mechanism in ILC development and function.

The lysine methyltransferase Ehmt2/G9a is dispensable for skeletal muscle development and regeneration

Background

Euchromatic histone-lysine N-methyltransferase 2 (G9a/Ehmt2) is the main enzyme responsible for the apposition of H3K9 di-methylation on histones. Due to its dual role as an epigenetic regulator and in the regulation of non-histone proteins through direct methylation, G9a has been implicated in a number of biological processes relevant to cell fate control. Recent reports employing in vitro cell lines indicate that Ehmt2 methylates MyoD to repress its transcriptional activity and therefore its ability to induce differentiation of activated myogenic cells.

Methods

To further investigate the importance of G9a in modulating myogenic regeneration in vivo, we crossed Ehmt2^floxed mice to animals expressing Cre recombinase from the Myod locus, resulting in efficient knockout in the entire skeletal muscle lineage (Ehmt2^ΔmyoD).

Results

Surprisingly, despite a dramatic drop in the global levels of H3K9me2, knockout animals did not show any developmental phenotype in muscle size and appearance. Consistent with this finding, purified Ehmt2^ΔmyoD satellite cells had rates of activation and proliferation similar to wild-type controls. When induced to differentiate in vitro, Ehmt2 knockout cells differentiated with kinetics similar to those of control cells and demonstrated normal capacity to form myotubes. After acute muscle injury, knockout mice regenerated as efficiently as wildtype. To exclude possible compensatory mechanisms elicited by the loss of G9a during development, we restricted the knockout within adult satellite cells by crossing Ehmt2^floxed mice to Pax7^CreERT2 and also found normal muscle regeneration capacity.

Conclusions

Thus, Ehmt2 and H3K9me2 do not play significant roles in skeletal muscle development and regeneration in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0093-7) contains supplementary material, which is available to authorized users.

CXXC finger protein 1 is critical for T-cell intrathymic development through regulating H3K4 trimethylation

T-cell development in the thymus is largely controlled by an epigenetic program, involving in both DNA methylation and histone modifications. Previous studies have identified Cxxc1 as a regulator of both cytosine methylation and histone 3 lysine 4 trimethylation (H3K4me3). However, it is unknown whether Cxxc1 plays a role in thymocyte development. Here we show that T-cell development in the thymus is severely impaired in Cxxc1-deficient mice. Furthermore, we identify genome-wide Cxxc1-binding sites and H3K4me3 modification sites in wild-type and Cxxc1-deficient thymocytes. Our results demonstrate that Cxxc1 directly controls the expression of key genes important for thymocyte survival such as RORγt and for T-cell receptor signalling including Zap70 and CD8, through maintaining the appropriate H3K4me3 on their promoters. Importantly, we show that RORγt, a direct target of Cxxc1, can rescue the survival defects in Cxxc1-deficient thymocytes. Our data strongly support a critical role of Cxxc1 in thymocyte development.

Longitudinal genome-wide methylation study of Roux-en-Y gastric bypass patients reveals novel CpG sites associated with essential hypertension

BACKGROUND

Essential hypertension is a significant risk factor for cardiovascular diseases. Emerging research suggests a role of DNA methylation in blood pressure physiology. We aimed to investigate epigenetic associations of promoter related CpG sites to essential hypertension in a genome-wide methylation approach.

METHODS

The genome-wide methylation pattern in whole blood was measured in 11 obese patients before and six months after Roux-en-Y gastric bypass surgery using the Illumina 450 k beadchip. CpG sites located within 1500 bp of the transcriptional start site of adjacent genes were included in our study, resulting in 124 199 probes investigated in the subsequent analysis. Percent changes in methylation states and SBP measured before and six months after surgery were calculated. These parameters were correlated to each other using the Spearman's rank correlation method (Edgeworth series approximation). To further investigate the detected relationship between candidate CpG sites and systolic blood pressure levels, binary logistic regression analyses were performed in a larger and independent cohort of 539 individuals aged 19-101 years to elucidate a relationship between EH and the methylation state in candidate CpG sites.

RESULTS

We identified 24 promoter associated CpG sites that correlated with change in SBP after RYGB surgery (p < 10(-16)). Two of these CpG loci (cg00875989, cg09134341) were significantly hypomethylated in dependency of EH (p < 10(-03)). These results were independent of age, BMI, ethnicity and sex.

CONCLUSIONS

The identification of these novel CpG sites may contribute to a further understanding of the epigenetic regulatory mechanisms underlying the development of essential hypertension.

Alternative Splicing of G9a Regulates Neuronal Differentiation

Chromatin modifications are critical for the establishment and maintenance of differentiation programs. G9a, the enzyme responsible for histone H3 lysine 9 dimethylation in mammalian euchromatin, exists as two isoforms with differential inclusion of exon 10 (E10) through alternative splicing. We find that the G9a methyltransferase is required for differentiation of the mouse neuronal cell line N2a and that E10 inclusion increases during neuronal differentiation of cultured cells, as well as in the developing mouse brain. Although E10 inclusion greatly stimulates overall H3K9me2 levels, it does not affect G9a catalytic activity. Instead, E10 increases G9a nuclear localization. We show that the G9a E10(+) isoform is necessary for neuron differentiation and regulates the alternative splicing pattern of its own pre-mRNA, enhancing E10 inclusion. Overall, our findings indicate that by regulating its own alternative splicing, G9a promotes neuron differentiation and creates a positive feedback loop that reinforces cellular commitment to differentiation.

Regulation of IL-4 Expression in Immunity and Diseases

IL-4 was first identified as a T cell-derived growth factor for B cells. Studies over the past several decades have markedly expanded our understanding of its cellular sources and function. In addition to T cells, IL-4 is produced by innate lymphocytes, such as NTK cells, and myeloid cells, such as basophils and mast cells. It is a signature cytokine of type 2 immune response but also has a nonimmune function. Its expression is tightly regulated at several levels, including signaling pathways, transcription factors, epigenetic modifications, microRNA, and long noncoding RNA. This chapter will review in detail the molecular mechanism regulating the cell type-specific expression of IL-4 in physiological and pathological type 2 immune responses.

Integrin α4β1 controls G9a activity that regulates epigenetic changes and nuclear properties required for lymphocyte migration

The mechanical properties of the cell nucleus change to allow cells to migrate, but how chromatin modifications contribute to nuclear deformability has not been defined. Here, we demonstrate that a major factor in this process involves epigenetic changes that underpin nuclear structure. We investigated the link between cell adhesion and epigenetic changes in T-cells, and demonstrate that T-cell adhesion to VCAM1 via α4β1 integrin drives histone H3 methylation (H3K9me2/3) through the methyltransferase G9a. In this process, active G9a is recruited to the nuclear envelope and interacts with lamin B1 during T-cell adhesion through α4β1 integrin. G9a activity not only reorganises the chromatin structure in T-cells, but also affects the stiffness and viscoelastic properties of the nucleus. Moreover, we further demonstrated that these epigenetic changes were linked to lymphocyte movement, as depletion or inhibition of G9a blocks T-cell migration in both 2D and 3D environments. Thus, our results identify a novel mechanism in T-cells by which α4β1 integrin signaling drives specific chromatin modifications, which alter the physical properties of the nucleus and thereby enable T-cell migration.

Progressive Chromatin Condensation and H3K9 Methylation Regulate the Differentiation of Embryonic and Hematopoietic Stem Cells

Epigenetic regulation serves as the basis for stem cell differentiation into distinct cell types, but it is unclear how global epigenetic changes are regulated during this process. Here, we tested the hypothesis that global chromatin organization affects the lineage potential of stem cells and that manipulation of chromatin dynamics influences stem cell function. Using nuclease sensitivity assays, we found a progressive decrease in chromatin digestion among pluripotent embryonic stem cells (ESCs), multipotent hematopoietic stem cells (HSCs), and mature hematopoietic cells. Quantitative high-resolution microscopy revealed that ESCs contain significantly more euchromatin than HSCs, with a further reduction in mature cells. Increased cellular maturation also led to heterochromatin localization to the nuclear periphery. Functionally, prevention of heterochromatin formation by inhibition of the histone methyltransferase G9A resulted in delayed HSC differentiation. Our results demonstrate global chromatin rearrangements during stem cell differentiation and that heterochromatin formation by H3K9 methylation regulates HSC differentiation.

Functional Role of G9a Histone Methyltransferase in Cancer

Post-translational modifications of DNA and histones are epigenetic mechanisms, which affect the chromatin structure, ultimately leading to gene expression changes. A number of different epigenetic enzymes are actively involved in the addition or the removal of various covalent modifications, which include acetylation, methylation, phosphorylation, ubiquitination, and sumoylation. Deregulation of these processes is a hallmark of cancer. For instance, G9a, a histone methyltransferase responsible for histone H3 lysine 9 (H3K9) mono- and dimethylation, has been observed to be upregulated in different types of cancer and its overexpression has been associated with poor prognosis. Key roles played by these enzymes in various diseases have led to the hypothesis that these molecules represent valuable targets for future therapies. Several small molecule inhibitors have been developed to specifically block the epigenetic activity of these enzymes, representing promising therapeutic tools in the treatment of human malignancies, such as cancer. In this review, the role of one of these epigenetic enzymes, G9a, is discussed, focusing on its functional role in regulating gene expression as well as its implications in cancer initiation and progression. We also discuss important findings from recent studies using epigenetic inhibitors in cell systems in vitro as well as experimental tumor growth and metastasis assays in vivo.

The Histone Methyltransferase Inhibitor A-366 Uncovers a Role for G9a/GLP in the Epigenetics of Leukemia

Histone methyltransferases are epigenetic regulators that modify key lysine and arginine residues on histones and are believed to play an important role in cancer development and maintenance. These epigenetic modifications are potentially reversible and as a result this class of enzymes has drawn great interest as potential therapeutic targets of small molecule inhibitors. Previous studies have suggested that the histone lysine methyltransferase G9a (EHMT2) is required to perpetuate malignant phenotypes through multiple mechanisms in a variety of cancer types. To further elucidate the enzymatic role of G9a in cancer, we describe herein the biological activities of a novel peptide-competitive histone methyltransferase inhibitor, A-366, that selectively inhibits G9a and the closely related GLP (EHMT1), but not other histone methyltransferases. A-366 has significantly less cytotoxic effects on the growth of tumor cell lines compared to other known G9a/GLP small molecule inhibitors despite equivalent cellular activity on methylation of H3K9me2. Additionally, the selectivity profile of A-366 has aided in the discovery of a potentially important role for G9a/GLP in maintenance of leukemia. Treatment of various leukemia cell lines in vitro resulted in marked differentiation and morphological changes of these tumor cell lines. Furthermore, treatment of a flank xenograft leukemia model with A-366 resulted in growth inhibition in vivo consistent with the profile of H3K9me2 reduction observed. In summary, A-366 is a novel and highly selective inhibitor of G9a/GLP that has enabled the discovery of a role for G9a/GLP enzymatic activity in the growth and differentiation status of leukemia cells.

Regulation of cell differentiation and function by the euchromatin histone methyltranserfases G9a and GLP

The euchromatin histone methyltransferases (EHMTs) are an evolutionarily conserved protein family that are known for their ability to dimethylate histone 3 at lysine 9 in euchromatic regions of the genome. In mammals there are two EHMT proteins, G9a, encoded by EHMT2, and GLP, encoded by EHMT1. EHMTs have diverse roles in the differentiation of different tissues and cell types and are involved in adult-specific processes like memory, drug addiction, and immune response. This review discusses recent findings from rodent and Drosophila models that are beginning to reveal the broad biological role and complex mechanistic functioning of EHMT proteins.

Deciphering the epigenetic code of T lymphocytes

The multiple lineages and differentiation states that constitute the T-cell compartment all derive from a common thymic precursor. These distinct transcriptional states are maintained both in time and after multiple rounds of cell division by the concerted actions of a small set of lineage-defining transcription factors that act in conjunction with a suite of chromatin-modifying enzymes to activate, repress, and fine-tune gene expression. These chromatin modifications collectively provide an epigenetic code that allows the stable and heritable maintenance of the T-cell phenotype. Recently, it has become apparent that the epigenetic code represents a therapeutic target for a variety of immune cell disorders, including lymphoma and acute and chronic inflammatory diseases. Here, we review the recent advances in epigenetic regulation of gene expression, particularly as it relates to the T-cell differentiation and function.

H3K9MTase G9a is essential for the differentiation and growth of tenocytes in vitro

Cell differentiation is controlled by specific transcription factors. The functions and expression levels of these transcription factors are regulated by epigenetic modifications, such as histone modifications and cytosine methylation of the genome. In tendon tissue, tendon-specific transcription factors have been shown to play functional roles in the regulation of tenocyte differentiation. However, the effects of epigenetic modifications on gene expression and differentiation in tenocytes are unclear. In this study, we investigated the epigenetic regulation of tenocyte differentiation, focusing on the enzymes mediating histone 3 lysine 9 (H3K9) methylation. In primary mouse tenocytes, six H3K9 methyltransferase (H3K9MTase) genes, i.e., G9a, G9a-like protein (GLP), PR domain zinc finger protein 2 (PRDM2), SUV39H1, SUV39H2, and SETDB1/ESET were all expressed, with increased mRNA levels observed during tenocyte differentiation. In mouse embryos, G9a and Prdm2 mRNAs were expressed in tenocyte precursor cells, which were overlapped with or were adjacent to cells expressing a tenocyte-specific marker, tenomodulin. Using tenocytes isolated from G9a-flox/flox mice, we deleted G9a by infecting the cells with Cre-expressing adenoviruses. Proliferation of G9a-null tenocytes was significantly decreased compared with that of control cells infected with GFP-expressing adenoviruses. Moreover, the expression levels of tendon transcription factors gene, i.e., Scleraxis (Scx), Mohawk (Mkx), Egr1, Six1, and Six2 were all suppressed in G9a-null tenocytes. The tendon-related genes Col1a1, tenomodulin, and periostin were also downregulated. Consistent with this, Western blot analysis showed that tenomodulin protein expression was significantly suppressed by G9a deletion. These results suggested that expression of the H3K9MTase G9a was essential for the differentiation and growth of tenocytes and that H3K9MTases may play important roles in tendinogenesis.

Regulation of CD4 T-cell differentiation and inflammation by repressive histone methylation

Repressive epigenetic modifications such as dimethylation and trimethylation histone H3 at lysine 9 (H3K9me2 and H3K9me3) and H3K27me3 have been shown to be critical for embryonic stem (ES) cell differentiation by silencing cell lineage-promiscuous genes. CD4(+) T helper (T(H)) cell differentiation is a powerful model to study the molecular mechanisms associated with cellular lineage choice in adult cells. Naïve T(H) cells have the capacity to differentiate into one of the several phenotypically and functionally distinct and stable lineages. Although some repressive epigenetic mechanisms have a critical role in T(H) cell differentiation in a similar manner to that in ES cells, it is clear that there are disparate functions for certain modifications between ES cells and T(H) cells. Here we review the role of repressive histone modifications in the differentiation and function of T(H) cells in health and disease.

KMT1E-mediated chromatin modifications at the FcγRIIb promoter regulate thymocyte development

This work examines the role the lysine methyltransferase KMT1E (Setdb1) in thymocyte development. We have developed and described a T cell-specific conditional knockout of Setdb1. A partial block was seen at the double-positive to single-positive transition, causing reduced numbers of single-positive T cells in the thymus and periphery. Knockout thymocytes had reduced numbers of CD69(+) and T-cell receptor TCRβ(+) cells and increased numbers of apoptotic cells in the double-positive compartment, suggesting an alteration in the selection process. Transcriptional profiling of thymocytes revealed that Setdb1 deletion derepresses expression of FcγRIIb, the inhibitory Fc receptor. We demonstrate that a KMT1E-containing complex directly interacts with the FcγRIIb promoter and that histone H3 at lysine 9 tri-methylation at this promoter is dependent on Setdb1 expression. Derepression of FcγRIIb causes exacerbated signaling through the TCR complex, with specifically increased phosphorylation of ZAP70, affecting selection. This work identifies KMT1E as a novel repressor of FcγRIIb and identifies an underappreciated role of FcγRIIb in fine tuning thymocyte development.

Transcriptional regulation of T helper type 2 differentiation

Considerable progress has been made in recent years towards our understanding of the molecular mechanisms of transcriptional regulation of T helper type 2 (Th2) cell differentiation. Additional transcription factors and chromatin-modifying factors were identified and shown to promote Th2 cell differentiation and inhibit differentiation into other subsets. Analyses of mice lacking several cis-regulatory elements have yielded more insight into the regulatory mechanism of Th2 cytokine genes. Gene deletion studies of several chromatin modifiers confirmed their impact on CD4 T-cell differentiation. In addition, recent genome-wide analyses of transcription factor binding and chromatin status revealed unexpected roles of these factors in Th2-cell differentiation. In this review, these recent findings and their implication are summarized.

Analogues of the Natural Product Sinefungin as Inhibitors of EHMT1 and EHMT2

A series of analogues of the natural product sinefungin lacking the amino acid moiety was synthesized and probed for their ability to inhibit EHMT1 and EHMT2. This study led to inhibitors 3b and 4d of methyltransferase activity of EHMT1 and EHMT2 and it demonstrates that such analogues constitute an interesting scaffold to develop selective methyltransferase inhibitors. Surprisingly, the inhibition was not competitive toward AdoMet.

Kruppel-associated box (KRAB) proteins in the adaptive immune system

The ability of adaptive immune system to protect higher vertebrates from pathogens resides in the ability of B and T cells to express different antigen specific receptors and to respond to different threats by activating distinct differentiation and/or activation pathways. In the past 10 years, the major role of epigenetics in controlling molecular mechanisms responsible for these peculiar features and, more in general, for lymphocyte development has become evident. KRAB-ZFPs is the widest family of mammalian transcriptional repressors, which function through the recruitment of the co-factor KRAB-Associated Protein 1 (KAP1) that in turn engages histone modifiers inducing heterochromatin formation. Although most of the studies on KRAB proteins have been performed in embryonic cells, more recent reports highlighted a relevant role for these proteins also in adult tissues. This article will review the role of KRAB-ZFP and KAP1 in the epigenetic control of mouse and human adaptive immune cells.

The methyltransferase G9a regulates HoxA9-dependent transcription in AML

Lehnertz et al. identify the histone methyltransferase G9a as a selective regulator of fast proliferating myeloid progenitors with no discernible function in hematopoietic stem cells. Loss of G9a significantly delays disease progression and reduces leukemia stem cell frequency in mouse models of acute myeloid leukemia. G9a interacts with the leukemogenic transcription factor HoxA9 and regulates HoxA9-dependent transcription. These results highlight G9a inhibition as a means to counteract the proliferation and self-renewal of AML cells.

Chromatin modulators are emerging as attractive drug targets, given their widespread implication in human cancers and susceptibility to pharmacological inhibition. Here we establish the histone methyltransferase G9a/EHMT2 as a selective regulator of fast proliferating myeloid progenitors with no discernible function in hematopoietic stem cells (HSCs). In mouse models of acute myeloid leukemia (AML), loss of G9a significantly delays disease progression and reduces leukemia stem cell (LSC) frequency. We connect this function of G9a to its methyltransferase activity and its interaction with the leukemogenic transcription factor HoxA9 and provide evidence that primary human AML cells are sensitive to G9A inhibition. Our results highlight a clinical potential of G9A inhibition as a means to counteract the proliferation and self-renewal of AML cells by attenuating HoxA9-dependent transcription.

Histone methyltransferase and histone methylation in inflammatory T-cell responses

During immune responses, T cells require tightly controlled expression of transcriptional programs to regulate the balance between beneficial and harmful immunity. These transcriptional programs are critical for the lineage specification of effector T cells, the production of effector cytokines and molecules, and the development and maintenance of memory T cells. An emerging theme is that post-translational modification of histones by methylation plays an important role in orchestrating the expression of transcriptional programs in T cells. In this article, we provide a broad overview of histone methylation signatures for effector molecules and transcription factors in T cells, and the functional importance of histone methyltransferases in regulating T-cell immune responses.

Methyltransferase G9A regulates T cell differentiation during murine intestinal inflammation

Inflammatory bowel disease (IBD) pathogenesis is associated with dysregulated CD4⁺ Th cell responses, with intestinal homeostasis depending on the balance between IL-17-producing Th17 and Foxp3⁺ Tregs. Differentiation of naive T cells into Th17 and Treg subsets is associated with specific gene expression profiles; however, the contribution of epigenetic mechanisms to controlling Th17 and Treg differentiation remains unclear. Using a murine T cell transfer model of colitis, we found that T cell-intrinsic expression of the histone lysine methyltransferase G9A was required for development of pathogenic T cells and intestinal inflammation. G9A-mediated dimethylation of histone H3 lysine 9 (H3K9me2) restricted Th17 and Treg differentiation in vitro and in vivo. H3K9me2 was found at high levels in naive Th cells and was lost following Th cell activation. Loss of G9A in naive T cells was associated with increased chromatin accessibility and heightened sensitivity to TGF-β1. Pharmacological inhibition of G9A methyltransferase activity in WT T cells promoted Th17 and Treg differentiation. Our data indicate that G9A-dependent H3K9me2 is a homeostatic epigenetic checkpoint that regulates Th17 and Treg responses by limiting chromatin accessibility and TGF-β1 responsiveness, suggesting G9A as a therapeutic target for treating intestinal inflammation.

Epigenetic modifications induced by Blimp-1 Regulate CD8⁺ T cell memory progression during acute virus infection

The transcription factor Blimp-1 regulates the overall accumulation of virus-specific CD8⁺ T cells during acute viral infections. We found that increased proliferation and survival of Blimp-1-deficient CD8⁺ T cells resulted from sustained expression of CD25 and CD27 and persistent cytokine responsiveness. Silencing of Il2ra and Cd27 reduced the Blimp-1-deficient CD8⁺ T cell response. Genome-wide chromatin immunoprecipitation (ChIP) sequencing analysis identified Il2ra and Cd27 as direct targets of Blimp-1. At the peak of the antiviral response, but not earlier, Blimp-1 recruited the histone-modifying enzymes G9a and HDAC2 to the Il2ra and Cd27 loci, thereby repressing expression of these genes. In the absence of Blimp-1, Il2ra and Cd27 exhibited enhanced histone H3 acetylation and reduced histone H3K9 trimethylation. These data elucidate a central mechanism by which Blimp-1 acts as an epigenetic regulator and enhances the numbers of short-lived effector cells while suppressing the development of memory-precursor CD8⁺ T cells.

Epigenetic control of cytokine gene expression: regulation of the TNF/LT locus and T helper cell differentiation

Epigenetics encompasses transient and heritable modifications to DNA and nucleosomes in the native chromatin context. For example, enzymatic addition of chemical moieties to the N-terminal "tails" of histones, particularly acetylation and methylation of lysine residues in the histone tails of H3 and H4, plays a key role in regulation of gene transcription. The modified histones, which are physically associated with gene regulatory regions that typically occur within conserved noncoding sequences, play a functional role in active, poised, or repressed gene transcription. The "histone code" defined by these modifications, along with the chromatin-binding acetylases, deacetylases, methylases, demethylases, and other enzymes that direct modifications resulting in specific patterns of histone modification, shows considerable evolutionary conservation from yeast to humans. Direct modifications at the DNA level, such as cytosine methylation at CpG motifs that represses promoter activity, are another highly conserved epigenetic mechanism of gene regulation. Furthermore, epigenetic modifications at the nucleosome or DNA level can also be coupled with higher-order intra- or interchromosomal interactions that influence the location of regulatory elements and that can place them in an environment of specific nucleoprotein complexes associated with transcription. In the mammalian immune system, epigenetic gene regulation is a crucial mechanism for a range of physiological processes, including the innate host immune response to pathogens and T cell differentiation driven by specific patterns of cytokine gene expression. Here, we will review current findings regarding epigenetic regulation of cytokine genes important in innate and/or adaptive immune responses, with a special focus upon the tumor necrosis factor/lymphotoxin locus and cytokine-driven CD4+ T cell differentiation into the Th1, Th2, and Th17 lineages.

Control of the hippo pathway by Set7-dependent methylation of Yap

Methylation of nonhistone proteins is emerging as a regulatory mechanism to control protein function. Set7 (Setd7) is a SET-domain-containing lysine methyltransferase that methylates and alters function of a variety of proteins in vitro, but the in vivo relevance has not been established. We found that Set7 is a modifier of the Hippo pathway. Mice that lack Set7 have a larger progenitor compartment in the intestine, coinciding with increased expression of Yes-associated protein (Yap) target genes. Mechanistically, monomethylation of lysine 494 of Yap is critical for cytoplasmic retention. These results identify a methylation-dependent checkpoint in the Hippo pathway.

Roles of repressive epigenetic machinery in lineage decision of T cells

DNA methylation and histone modifications are central to epigenetic gene regulation, which has been shown to play a crucial role in development. Epigenetics has often been discussed in the context of the maintenance of cell identity because of the heritable nature of gene expression status. Indeed, crucial roles of the epigenetic machinery in establishment and maintenance of particular lineages during early development have been well documented. However, unexpected observation of a developmental plasticity retained in mature T lymphocytes, in particular in CD4(+) T-cell subsets, by recent studies is accelerating studies that focus on roles of each epigenetic pathway in cell fate decisions of T lymphocytes. Here, we focus on the repressive epigenetic machinery, i.e. DNA methylation, histone deacetylation, H3K9 methylation and Polycomb repressive complexes, and briefly review the studies examining the role of these mechanisms during T-lymphocyte differentiation. We also discuss the current challenges faced when analysing the function of the epigenetic machinery and potential directions to overcome the problems.

G9a is transactivated by C/EBPβ to facilitate mitotic clonal expansion during 3T3-L1 preadipocyte differentiation

In 3T3-L1 preadipocyte differentiation, the CCAAT/enhancer-binding protein-β (C/EBPβ) is an important early transcription factor that activates cell cycle genes during mitotic clonal expansion (MCE), sequentially activating peroxisome proliferator-activated receptor-γ (PPARγ) and C/EBPα during terminal differentiation. Although C/EBPβ acquires its DNA binding activity via dual phosphorylation at about 12-16 h postinduction, the expression of PPARγ and C/EBPα is not induced until 36-72 h. The delayed expression of PPARγ and C/EBPα ensures the progression of MCE, but the mechanism responsible for the delay remains elusive. We provide evidence that G9a, a major euchromatic methyltransferase, is transactivated by C/EBPβ and represses PPARγ and C/EBPα through H3K9 dimethylation of their promoters during MCE. Inhibitor- or siRNA-mediated G9a downregulation modestly enhances PPARγ and C/EBPα expression and adipogenesis in 3T3-L1 preadipocytes. Conversely, forced expression of G9a impairs the accumulation of triglycerides. Thus, this study elucidates an epigenetic mechanism for the delayed expression of PPARγ and C/EBPα.

G9a histone methyltransferase activity in retinal progenitors is essential for proper differentiation and survival of mouse retinal cells

In vertebrate retinal development, various transcription factors are known to execute essential activities in gene regulation. Although epigenetic modification is considered to play a pivotal role in retinal development, the exact in vivo role of epigenetic regulation is still poorly understood. We observed that G9a histone methyltransferase, which methylates histone H3 at lysine 9 (H3K9), is substantially expressed in the mouse retina throughout development. To address in vivo G9a function in the mouse retina, we ablated G9a in retinal progenitor cells by conditional gene knock-out (G9a Dkk3 CKO). The G9a Dkk3 CKO retina exhibited severe morphological defects, including photoreceptor rosette formation, a partial loss of the outer nuclear layer, elevated cell death, and persistent cell proliferation. Progenitor cell-related genes, including several cyclins, Hes1, Chx10, and Lhx2, are methylated on histone H3K9 in the wild-type retina, but they were defective in H3K9 methylation and improperly upregulated at late developmental stages in the G9a Dkk3 CKO retina. Notably, conditional depletion of G9a in postmitotic photoreceptor precursors (G9a Crx CKO) led to the development of an almost normal retina, indicating that G9a activity mainly in retinal progenitor cells, but not in photoreceptor precursors, is essential for normal terminal differentiation of and survival of the retina. Our results suggest that proper epigenetic marks in progenitor cells are important for subsequent appropriate terminal differentiation and survival of retinal cells by repressing progenitor cell-related genes in differentiating retinal cells.

Recruitment of histone methyltransferase G9a mediates transcriptional repression of Fgf21 gene by E4BP4 protein

The liver responds to fasting-refeeding cycles by reprogramming expression of metabolic genes. Fasting potently induces one of the key hepatic hormones, fibroblast growth factor 21 (FGF21), to promote lipolysis, fatty acid oxidation, and ketogenesis, whereas refeeding suppresses its expression. We previously reported that the basic leucine zipper transcription factor E4BP4 (E4 binding protein 4) represses Fgf21 expression and disrupts its circadian oscillations in cultured hepatocytes. However, the epigenetic mechanism for E4BP4-dependent suppression of Fgf21 has not yet been addressed. Here we present evidence that histone methyltransferase G9a mediates E4BP4-dependent repression of Fgf21 during refeeding by promoting repressive histone modification. We find that Fgf21 expression is up-regulated in E4bp4 knock-out mouse liver. We demonstrate that the G9a-specific inhibitor BIX01294 abolishes suppression of the Fgf21 promoter activity by E4BP4, whereas overexpression of E4bp4 leads to increased levels of dimethylation of histone 3 lysine 9 (H3K9me2) around the Fgf21 promoter region. Furthermore, we also show that E4BP4 interacts with G9a, and knockdown of G9a blocks repression of Fgf21 promoter activity and expression in cells overexpressing E4bp4. A G9a mutant lacking catalytic activity, due to deletion of the SET domain, fails to inhibit the Fgf21 promoter activity. Importantly, acute hepatic knockdown by adenoviral shRNA targeting G9a abolishes Fgf21 repression by refeeding, concomitant with decreased levels of H3K9me2 around the Fgf21 promoter region. In summary, we show that G9a mediates E4BP4-dependent suppression of hepatic Fgf21 by enhancing histone methylation (H3K9me2) of the Fgf21 promoter.

Mechanisms and dynamics of heterochromatin formation during mammalian development: closed paths and open questions

Early embryonic development in mammals is characterized by major changes in the components of the chromatin and its remodeling. The embryonic chromatin and the nuclear organization in the mouse preimplantation embryo display particular features that are dramatically different from somatic cells. These include the highly specific organization of the pericentromeric heterochromatin within the nucleus and the suggested lack of conventional heterochromatin. We postulate that the plasticity of the cells in the early embryo relies on the distinctive heterochromatin features that prevail during early embryogenesis. Here, we review some of these features and discuss recent findings on the mechanisms driving heterochromatin formation after fertilization, in particular, the emerging role of RNA as a regulator of heterochromatic loci also in mammals. Finally, we believe that there are at least three major avenues that should be addressed in the coming years: (i) Is heterochromatin a driving force in development? (ii) Does it have a role in lineage allocation? (iii) How can heterochromatin "regulate" epigenetic reprogramming?

G9a, a multipotent regulator of gene expression

Lysine methylation of histone and non-histone substrates by the methyltransferase G9a is mostly associated with transcriptional repression. Recent studies, however, have highlighted its role as an activator of gene expression through mechanisms that are independent of its methyltransferase activity. Here we review the growing repertoire of molecular mechanisms and substrates through which G9a regulates gene expression. We also discuss emerging evidence for its wide-ranging functions in development, pluripotency, cellular differentiation and cell cycle regulation that underscore the complexity of its functions. The deregulated expression of G9a in cancers and other human pathologies suggests that it may be a viable therapeutic target in various diseases.

B lymphocyte-induced maturation protein-1 contributes to intestinal mucosa homeostasis by limiting the number of IL-17-producing CD4+ T cells

The transcription factor B lymphocyte-induced maturation protein-1 (Blimp-1) plays important roles in embryonic development and immunity. Blimp-1 is required for the differentiation of plasma cells, and mice with T cell-specific deletion of Blimp-1 (Blimp-1CKO mice) develop a fatal inflammatory response in the colon. Previous work demonstrated that lack of Blimp-1 in CD4(+) and CD8(+) T cells leads to intrinsic functional defects, but little is known about the functional role of Blimp-1 in regulating differentiation of Th cells in vivo and their contribution to the chronic intestinal inflammation observed in the Blimp1CKO mice. In this study, we show that Blimp-1 is required to restrain the production of the inflammatory cytokine IL-17 by Th cells in vivo. Blimp-1CKO mice have greater numbers of IL-17-producing TCRβ(+)CD4(+)cells in lymphoid organs and in the intestinal mucosa. The increase in IL-17-producing cells was not restored to normal levels in wild-type and Blimp-1CKO-mixed bone marrow chimeric mice, suggesting an intrinsic role for Blimp-1 in constraining the production of IL-17 in vivo. The observation that Blimp-1-deficient CD4(+) T cells are more prone to differentiate into IL-17(+)/IFN-γ(+) cells and cause severe colitis when transferred to Rag1-deficient mice provides further evidence that Blimp-1 represses IL-17 production. Analysis of Blimp-1 expression at the single cell level during Th differentiation reveals that Blimp-1 expression is induced in Th1 and Th2 but repressed by TGF-β in Th17 cells. Collectively, the results described here establish a new role for Blimp-1 in regulating IL-17 production in vivo.

G9a functions as a molecular scaffold for assembly of transcriptional coactivators on a subset of glucocorticoid receptor target genes

Histone H3 lysine-9 methyltransferase G9a/EHMT2/KMT1C is a key corepressor of gene expression. However, activation of a limited number of genes by G9a (independent of its catalytic activity) has also been observed, although the precise molecular mechanisms are unknown. By using RNAi in combination with gene expression microarray analysis, we found that G9a functions as a positive and a negative transcriptional coregulator for discrete subsets of genes that are regulated by the hormone-activated Glucocorticoid Receptor (GR). G9a was recruited to GR-binding sites (but not to the gene body) of its target genes and interacted with GR, suggesting recruitment of G9a by GR. In contrast to its corepressor function, positive regulation of gene expression by G9a involved G9a-mediated enhanced recruitment of coactivators CARM1 and p300 to GR target genes. Further supporting a role for G9a as a molecular scaffold for its coactivator function, the G9a-specific methyltransferase inhibitor UNC0646 did not affect G9a coactivator function but selectively decreased G9a corepressor function for endogenous target genes. Overall, G9a functioned as a coactivator for hormone-activated genes and as a corepressor in support of hormone-induced gene repression, suggesting that the positive or negative actions of G9a are determined by the gene-specific regulatory environment and chromatin architecture. These findings indicate distinct mechanisms of G9a coactivator vs. corepressor functions in transcriptional regulation and provide insight into the molecular mechanisms of G9a coactivator function. Our results also suggest a physiological role of G9a in fine tuning the set of genes that respond to glucocorticoids.

Maintenance of gene silencing by the coordinate action of the H3K9 methyltransferase G9a/KMT1C and the H3K4 demethylase Jarid1a/KDM5A

Chromatin remodeling is essential for controlling the expression of genes during development. The histone-modifying enzyme G9a/KMT1C can act both as a coactivator and a corepressor of transcription. Here, we show that the dual function of G9a as a coactivator vs. a corepressor entails its association within two distinct protein complexes, one containing the coactivator Mediator and one containing the corepressor Jarid1a/KDM5A. Functionally, G9a is important in stabilizing the Mediator complex for gene activation, whereas its repressive function entails a coordinate action with the histone H3 lysine 4 (H3K4) demethylase Jarid1a for the maintenance of gene repression. The essential nature of cross-talk between the histone methyltransferase G9a and the demethylase Jarid1a is demonstrated on the embryonic E(y)-globin gene, where the concurrent introduction of repressive histone marks (dimethylated H3K9 and dimethylated H3K27) and removal of activating histone mark (trimethylated H3K4) is required for maintenance of gene silencing. Taken together with our previous demonstration of cross-talk between UTX and MLL2 to mediate activation of the adult β(maj)-globin gene, these data suggest a model where "active" and "repressive" cross-talk between histone-modifying enzymes coexist on the same multigene locus and play a crucial role in the precise control of developmentally regulated gene expression.

Application of epigenome-modifying small molecules in induced pluripotent stem cells

Recent breakthroughs in generating induced pluripotent stem cells (iPSCs) using four defined factors have revealed the potential utility of stem cells in biological research and clinical applications. However, the low efficiency and slow kinetics of reprogramming related to producing these cells and underlying safety issues, such as viral integration and genetic and epigenetic abnormalities of iPSCs, hamper the further application of iPSCs in laboratory and clinical settings. Previous studies have suggested that reprogramming efficiency can be enhanced and that reprogramming kinetics can be accelerated by manipulating epigenetic status. Herein, we review recent studies on the application of epigenome-modifying small molecules in enhancing reprogramming and functionally replacing some reprogramming factors. We mainly focus on studies that have used small molecules to interfere with epigenome-modifying enzymes, such as DNA methyltransferase, histone acetyltransferase, and histone methyltransferase. The potential use of these small molecules in inducing iPSCs and new ways to identify small molecules of higher potency and fewer side effects are also discussed.

Gene-specific patterns of coregulator requirements by estrogen receptor-α in breast cancer cells

Progesterone receptor (PgR) controls the menstrual cycle, pregnancy, embryonic development, and homeostasis, and it plays important roles in breast cancer development and progression. However, the requirement of coregulators for estrogen-induced expression of the PgR gene has not been fully explored. Here we used RNA interference to demonstrate dramatic differences in requirements of 10 different coregulators for estrogen-regulated expression of six different genes, including PgR and the well-studied TFF1 (or pS2) gene in MCF-7 breast cancer cells. Full estrogen-induced expression of TFF1 required all ten coregulators, but PgR induction required only four of the 10 coregulators. Chromatin immunoprecipitation studies demonstrated several mechanisms responsible for the differential coregulator requirements. Actin-binding coregulator Flightless-I, required for TFF1 expression and recruited to that gene by estrogen receptor-α (ERα), is not required for PgR expression and not recruited to that gene. Protein acetyltransferase tat-interactive protein 60 and ATP-dependent chromatin remodeler Brahma Related Gene 1 are recruited to both genes but are required only for TFF1 expression. Histone methyltransferase G9a is recruited to both genes and required for estrogen-induced expression of TFF1 but negatively regulates estrogen-induced expression of PgR. In contrast, histone methyltransferase myeloid/lymphoid or mixed-lineage leukemia 1 (MLL1), pioneer factor Forkhead box A1, and p160 coregulator steroid receptor coactivator-3 are required for expression of and are recruited to both genes. Depletion of MLL1 decreased ERα binding to the PgR and TFF1 genes. In contrast, depletion of G9a enhanced ERα binding to the PgR gene but had no effect on ERα binding to the TFF1 gene. These studies suggest that differential promoter architecture is responsible for promoter-specific mechanisms of gene regulation.

A distinct mechanism for coactivator versus corepressor function by histone methyltransferase G9a in transcriptional regulation

Histone methyltransferase G9a has been understood primarily as a corepressor of gene expression, but we showed previously that G9a positively regulates nuclear receptor-mediated transcription in reporter gene assays. Here, we show that endogenous G9a contributes to the estradiol (E(2))-dependent induction of some endogenous target genes of estrogen receptor (ER)α in MCF-7 breast cancer cells while simultaneously limiting the E(2)-induced expression of other ERα target genes. Thus, G9a has a dual and selective role as a coregulator for ERα target genes. The ERα binding regions associated with the pS2 gene, which requires G9a for E(2)-induced expression, are transiently occupied by G9a at 15 min after beginning E(2) treatment, suggesting that G9a coactivator function is by direct interaction with ERα target genes. Transient reporter gene assays with deletion mutants of G9a demonstrated that domains previously associated with the corepressor functions of G9a (C-terminal methyltransferase domain, ankyrin repeat domain, and cysteine-rich domain) were unnecessary for G9a coactivator function in ERα-mediated transcription. In contrast, the N-terminal domain of G9a was necessary and sufficient for enhancement of ERα-mediated transcription and for E(2)-induced occupancy of G9a on ERα binding sites associated with endogenous target genes of ERα. In addition to a previously identified activation domain, this region contains a previously uncharacterized ligand-dependent ERα binding function, indicating how G9a is recruited to the target genes. Therefore, the coactivator and corepressor functions of G9a involve different G9a domains and different molecular mechanisms.

Maintaining CD4-CD8 lineage integrity in T cells: where plasticity serves versatility

The divergence of the two αβ T cell subsets defined by the mutually exclusive expression of CD4 and CD8 glycoproteins is an important event during the intrathymic differentiation of T lymphocytes. This reviews briefly summarizes the mechanisms that promote commitment to the CD4 or CD8 lineage in the thymus, and discusses the transcription factor circuits and epigenetic mechanisms that concur to maintain lineage integrity in post-thymic cells and yet allow effector cell differentiation.

Testosterone response of hepatic gene expression in female mice having acquired testosterone-unresponsive immunity to Plasmodium chabaudi malaria

Blood-stage malaria of Plasmodium chabaudi is characterized by its responsiveness to testosterone (T): T suppresses development of protective immunity, whereas once acquired immunity is T-unresponsive. Here, we have analyzed the liver, a T target and lymphoid organ with anti-malaria activity, for its T-responsiveness of gene expression in immune mice. Using Affymetrix microarray technology, in combination with quantitative RT-PCR, we have identified (i) T-unresponsive expression of newly acquired mRNAs encoding diverse sequences of IgG- and IgM-antibodies, (ii) 24 genes whose expression has become T-unresponsive including those encoding the T(H)2 response promoting EHMT2 and the erythrocyte membrane protein band 7.2 STOM, (iii) T-unresponsive expression of mRNAs for the cytokines IL-1β, IL-6, TNFα, and IFNγ, as well as iNOS, which are even not inducible by infection, and (iv) 35 genes retaining their T-responsiveness, which include those encoding the infection-inducible acute phase proteins SAA1, SAA2, and ORM2 as well as those of liver metabolism which encode the T-downregulated female-prevalent enzymes CYP2B9, CYP2B13, CYP3A41, CYP7A1, and SULT2A2 and the T-upregulated male-prevalent enzymes CYP2D9, CYP7B1, UGT2B1, HSD3B2, HSD3B5, respectively. The mRNA of the latter T-metabolizing enzyme is even 5-fold increased by T, suggesting a decrease in the effective T concentrations in the liver of immune mice. Collectively, our data suggest that the liver, which has acquired a selective T-unresponsiveness of gene expression, contributes to the acquired T-unresponsive, antibody-mediated protective immunity to blood-stage malaria of P. chabaudi.

H3K9 methyltransferase G9a and the related molecule GLP

The discovery of Suv39h1, the first SET domain-containing histone lysine methyltransferase (HKMT), was reported in 2000. Since then, research on histone methylation has progressed rapidly. Among the identified HKMTs in mammals, G9a and GLP are the primary enzymes for mono- and dimethylation at Lys 9 of histone H3 (H3K9me1 and H3K9me2), and exist predominantly as a G9a-GLP heteromeric complex that appears to be a functional H3K9 methyltransferase in vivo. Recently, many important studies have reported that G9a and GLP play critical roles in various biological processes. The physiological relevance of G9a/GLP-mediated epigenetic gene regulation is discussed.

The contribution of epigenetic memory to immunologic memory

Memory T lymphocytes are distinct from antigen-inexperienced naïve T cells in that memory T cells can respond more rapidly when they re-encounter a pathogen. Work over the past decade has begun to define the epigenetic underpinnings of the transcriptional component of the memory T cell response. An emerging theme is the persistence of an active chromatin signature at relevant gene loci in resting memory T cells, even when those genes are transcriptionally inactive. This gives strength to the concept of gene poising, and has shown that memory T lymphocytes are an ideal model in which to further define various mechanisms of epigenetic poising.