Identification of a human histone acetyltransferase related to monocytic leukemia zinc finger protein

Disruption of the histone acetyltransferase MYST4 leads to a Noonan syndrome-like phenotype and hyperactivated MAPK signaling in humans and mice

Epigenetic regulation of gene expression, through covalent modification of histones, is a key process controlling growth and development. Accordingly, the transcription factors regulating these processes are important targets of genetic diseases. However, surprisingly little is known about the relationship between aberrant epigenetic states, the cellular process affected, and their phenotypic consequences. By chromosomal breakpoint mapping in a patient with a Noonan syndrome-like phenotype that encompassed short stature, blepharoptosis, and attention deficit hyperactivity disorder, we identified haploinsufficiency of the histone acetyltransferase gene MYST histone acetyltransferase (monocytic leukemia) 4 (MYST4), as the underlying cause of the phenotype. Using acetylation, whole genome expression, and ChIP studies in cells from the patient, cell lines in which MYST4 expression was knocked down using siRNA, and the Myst4 querkopf mouse, we found that H3 acetylation is important for neural, craniofacial, and skeletal morphogenesis, mainly through its ability to specifically regulating the MAPK signaling pathway. This finding further elucidates the complex role of histone modifications in mammalian development and adds what we believe to be a new mechanism to the pathogenic phenotypes resulting from misregulation of the RAS signaling pathway.

Epigenetic regulation of neurogenesis in the adult mammalian brain

Epigenetic regulation represents a fundamental mechanism to maintain cell-type-specific gene expression during development and serves as an essential mediator to interface the extrinsic environment and the intrinsic genetic programme. Adult neurogenesis occurs in discrete regions of the adult mammalian brain and is known to be tightly regulated by various physiological, pathological and pharmacological stimuli. Emerging evidence suggests that various epigenetic mechanisms play important roles in fine-tuning and coordinating gene expression during adult neurogenesis. Here we review recent progress in our understanding of various epigenetic mechanisms, including DNA methylation, histone modifications and non-coding RNAs, as well as cross-talk among these mechanisms, in regulating different aspects of adult mammalian neurogenesis.

Mapping of a novel autosomal recessive hypotrichosis locus on chromosome 10q11.23–22.3

Autosomal recessive hypotrichosis is a rare form of human genetic disorder characterized by sparse to absent hair on scalp and rest of the body of affected individuals. Over the past few years at least five autosomal recessive forms of hypotrichosis loci have been mapped on different human chromosomes. In the present study, we report localization of another novel autosomal recessive hypotrichosis locus on human chromosome 10q11.23-22.3 in a four generation consanguineous Pakistani family. All the four patients in the family showed typical features of hereditary hypotrichosis including sparse hair on the scalp and rest of the body. Human genome scan using highly polymorphic microsatellite markers mapped the disease locus to a large region on chromosome 10. This novel locus maps to 29.81 cM (28.5 Mb) region, flanked by markers D10S538 and D10S2327 on chromosome 10q11.23-22.3. A maximum multipoint LOD score of 3.26 was obtained with several markers in this region. DNA sequence analysis of exons and splice-junction sites of four putative candidate genes (P4HA1, ZNF365, ZMYND17, MYST4), located in the linkage interval, were sequenced but were negative for functional sequence variants.

Histone modification therapy of cancer

The state of modification of histone tails plays an important role in defining the accessibility of DNA for the transcription machinery and other regulatory factors. It has been extensively demonstrated that the posttranslational modifications of the histone tails, as well as modifications within the nucleosome domain, regulate the level of chromatin condensation and are therefore important in regulating gene expression and other nuclear events. Together with DNA methylation, they constitute the most relevant level of epigenetic regulation of cell functions. Histone modifications are carried out by a multipart network of macromolecular complexes endowed with enzymatic, regulatory, and recognition domains. Not surprisingly, epigenetic alterations caused by aberrant activity of these enzymes are linked to the establishment and maintenance of the cancer phenotype and, importantly, are potentially reversible, since they do not involve genetic mutations in the underlying DNA sequence. Histone modification therapy of cancer is based on the generation of drugs able to interfere with the activity of enzymes involved in histone modifications: new drugs have recently been approved for use in cancer patients, clinically validating this strategy. Unfortunately, however, clinical responses are not always consistent and do not parallel closely the results observed in preclinical models. Here, we present a brief overview of the deregulation of chromatin-associated enzymatic activities in cancer cells and of the main results achieved by histone modification therapeutic approaches.

MYST family histone acetyltransferases take center stage in stem cells and development

Acetylation of histones is an essential element regulating chromatin structure and transcription. MYST (Moz, Ybf2/Sas3, Sas2, Tip60) proteins form the largest family of histone acetyltransferases and are present in all eukaryotes. Surprisingly, until recently this protein family was poorly studied. However, in the last few years there has been a substantial increase in interest in the MYST proteins and a number of key studies have shown that these chromatin modifiers are required for a diverse range of cellular processes, both in health and disease. Translocations affecting MYST histone acetyltransferases can lead to leukemia and solid tumors. Some members of the MYST family are required for the development and self-renewal of stem cell populations; other members are essential for the prevention of inappropriate heterochromatin spreading and for the maintenance of adequate levels of gene expression. In this review we discuss the function of MYST proteins in vivo.

CD25 blockade protects T cells from activation-induced cell death (AICD) via maintenance of TOSO expression

CD25 monoclonal antibody binding to the alpha-chain of the Interleukin-2 (IL-2) receptor, blocks high-affinity IL-2 binding, thereby preventing complete T-cell activation and being of ample importance in transplantation medicine and potentially the treatment of autoimmune disease. However, CD25 antibodies do not only block T-cell activation but also prevent activation-induced cell death (AICD) attributing a dual function to IL-2. In this study, the modulation of the genomic expression profile of human peripheral blood mononuclear cells (PBMC) with therapeutic concentrations of humanized anti-CD25 mAb was investigated. PBMC were stimulated with CD3 antibody OKT-3 together with recombinant IL-2 in the absence or presence of anti-CD25 mAb. RNA was extracted and subjected to microarray analysis on U133A microarrays (Affymetrix). Anti-CD25 treatment inhibited several genes typically expressed during T-cell activation including granzyme B, signalling lymphocyte activation molecule, family member 1 (SLAMF1), CD40-Ligand (CD40-L), IL-9 and interferon (IFN)-gamma. Interestingly, anti-CD25 mAb also blocked the expression of several genes important for susceptibility to apoptosis, such as death receptor 6 (DR6) or reversed IL-2-mediated repression of anti-apoptotic genes, such as Fas apoptotic inhibitory molecule 3 (FAIM3)/TOSO. Functional significance of DR6 and TOSO expression in IL-2-dependent T-cell activation was subsequently evaluated by RNA interference in AICD: While siRNA specifically directed against DR6 did not modulate FAS-L-mediated apoptosis induction in primary T cells, down-regulation of TOSO significantly increased susceptibility to apoptosis, emphasizing an important role for TOSO in IL-2-mediated AICD.

Epigenetics in cancer: targeting chromatin modifications

Posttranslational modifications to histones affect chromatin structure and function resulting in altered gene expression and changes in cell behavior. Aberrant gene expression and altered epigenomic patterns are major features of cancer. Epigenetic changes including histone acetylation, histone methylation, and DNA methylation are now thought to play important roles in the onset and progression of cancer in numerous tumor types. Indeed dysregulated epigenetic modifications, especially in early neoplastic development, may be just as significant as genetic mutations in driving cancer development and growth. The reversal of aberrant epigenetic changes has therefore emerged as a potential strategy for the treatment of cancer. A number of compounds targeting enzymes that regulate histone acetylation, histone methylation, and DNA methylation have been developed as epigenetic therapies, with some demonstrating efficacy in hematological malignancies and solid tumors. This review highlights the roles of epigenetic modifications to histones and DNA in tumorigenesis and emerging epigenetic therapies being developed for the treatment of cancer.

Alcohol-induced protein hyperacetylation: Mechanisms and consequences

Although the clinical manifestations of alcoholic liver disease are well-described, little is known about the molecular basis of liver injury. Recent studies have indicated that ethanol exposure induces global protein hyperacetylation. This reversible, post-translational modification on the epsilon-amino groups of lysine residues has been shown to modulate multiple, diverse cellular processes ranging from transcriptional activation to microtubule stability. Thus, alcohol-induced protein hyperacetylation likely leads to major physiological consequences that contribute to alcohol-induced hepatotoxicity. Lysine acetylation is controlled by the activities of two opposing enzymes, histone acetyltransferases and histone deacetylases. Currently, efforts are aimed at determining which enzymes are responsible for the increased acetylation of specific substrates. However, the greater challenge will be to determine the physiological ramifications of protein hyperacetylation and how they might contribute to the progression of liver disease. In this review, we will first list and discuss the proteins known to be hyperacetylated in the presence of ethanol. We will then describe what is known about the mechanisms leading to increased protein acetylation and how hyperacetylation may perturb hepatic function.

The histone acetyl transferase activity of monocytic leukemia zinc finger is critical for the proliferation of hematopoietic precursors

The monocytic leukemia zinc finger (MOZ) gene encodes a large multidomain protein that contains, besides other domains, 2 coactivation domains for the transcription factor Runx1/acute myeloid leukemia 1 and a histone acetyl transferase (HAT) catalytic domain. Recent studies have demonstrated the critical requirement for the complete MOZ protein in hematopoietic stem cell development and maintenance. However, the specific function of the HAT activity of MOZ remains unknown, as it has been shown that MOZ HAT activity is not required either for its role as Runx1 coactivator or for the leukemic transformation induced by MOZ transcriptional intermediary factor 2 (TIF2). To assess the specific requirement for this HAT activity during hematopoietic development, we have generated embryonic stem cells and mouse lines carrying a point mutation that renders the protein catalytically inactive. We report in this study that mice exclusively lacking the HAT activity of MOZ exhibit significant defects in the number of hematopoietic stem cells and hematopoietic committed precursors as well as a defect in B-cell development. Furthermore, we demonstrate that the failure to maintain a normal number of hematopoietic precursors is caused by the inability of HAT(-/-) cells to expand. These results indicate a specific role of MOZ-driven acetylation in controlling a desirable balance between proliferation and differentiation during hematopoiesis.

Molecular architecture of quartet MOZ/MORF histone acetyltransferase complexes

The monocytic leukemia zinc finger protein MOZ and the related factor MORF form tetrameric complexes with ING5 (inhibitor of growth 5), EAF6 (Esa1-associated factor 6 ortholog), and the bromodomain-PHD finger protein BRPF1, -2, or -3. To gain new insights into the structure, function, and regulation of these complexes, we reconstituted them and performed various molecular analyses. We found that BRPF proteins bridge the association of MOZ and MORF with ING5 and EAF6. An N-terminal region of BRPF1 interacts with the acetyltransferases; the enhancer of polycomb (EPc) homology domain in the middle part binds to ING5 and EAF6. The association of BRPF1 with EAF6 is weak, but ING5 increases the affinity. These three proteins form a trimeric core that is conserved from Drosophila melanogaster to humans, although authentic orthologs of MOZ and MORF are absent in invertebrates. Deletion mapping studies revealed that the acetyltransferase domain of MOZ/MORF is sufficient for BRPF1 interaction. At the functional level, complex formation with BRPF1 and ING5 drastically stimulates the activity of the acetyltransferase domain in acetylation of nucleosomal histone H3 and free histones H3 and H4. An unstructured 18-residue region at the C-terminal end of the catalytic domain is required for BRPF1 interaction and may function as an "activation lid." Furthermore, BRPF1 enhances the transcriptional potential of MOZ and a leukemic MOZ-TIF2 fusion protein. These findings thus indicate that BRPF proteins play a key role in assembling and activating MOZ/MORF acetyltransferase complexes.

Two essential MYST-family proteins display distinct roles in histone H4K10 acetylation and telomeric silencing in trypanosomes

Chromatin modification is important for virtually all aspects of DNA metabolism but little is known about the consequences of such modification in trypanosomatids, early branching protozoa of significant medical and veterinary importance. MYST-family histone acetyltransferases in other species function in transcription regulation, DNA replication, recombination and repair. Trypanosoma brucei HAT3 was recently shown to acetylate histone H4K4 and we now report characterization of all three T. brucei MYST acetyltransferases (HAT1–3). First, GFP-tagged HAT1–3 all localize to the trypanosome nucleus. While HAT3 is dispensable, both HAT1 and HAT2 are essential for growth. Strains with HAT1 knock-down display mitosis without nuclear DNA replication and also specific de-repression of a telomeric reporter gene, a rare example of transcription control in an organism with widespread and constitutive polycistronic transcription. Finally, we show that HAT2 is responsible for H4K10 acetylation. By analogy to the situation in Saccharomyces cerevisiae, we discuss low-level redundancy of acetyltransferase function in T. brucei and suggest that two MYST-family acetyltransferases are essential due to the absence of a Gcn5 homologue. The results are also consistent with the idea that HAT1 contributes to establishing boundaries between transcriptionally active and repressed telomeric domains in T. brucei.

The interaction of ETV6 (TEL) and TIP60 requires a functional histone acetyltransferase domain in TIP60

The ets-family transcription factor ETV6 (TEL) has been shown to be the target of a large number of balanced chromosomal translocations in various hematological malignancies and in some soft tissue tumors. Furthermore, ETV6 is essential for hematopoietic stem cell function. We identified ETV6 interacting proteins using the yeast two hybrid system. One of these proteins is the HIV Tat interacting protein (TIP60), a histone acetyltransferase (HAT) containing the highly conserved MYST domain. TIP60 functions as a corepressor of ETV6 in reporter gene assays. Fluorescently tagged ETV6 and TIP60 colocalize in the nucleus and an increase in nuclear localization of ETV6 was seen when TIP60 was cotransfected. ETV6 interacts with TIP60 through a 63 amino acids region located in the central domain of ETV6 between the pointed and the ets domain. The ETV6 interacting region of TIP60 mapped to the C2HC zinc finger of the TIP60 MYST domain. The interaction of TIP60 with full length ETV6 required an intact acetyltransferase domain of TIP60. Interestingly, the MYST domains of MOZ and MORF were also able to interact with portions of ETV6. These observations suggest that MYST domain HATs regulate ETV6 transcriptional activity and may therefore play critical roles in leukemogenesis and possibly in normal hematopoietic development.

Investigation of MYST4 histone acetyltransferase and its involvement in mammalian gametogenesis

Background

Various histone acetylases (HATs) play a critical role in the regulation of gene expression, but the precise functions of many of those HATs are still unknown. Here we provide evidence that MYST4, a known HAT, may be involved in early mammalian gametogenesis.

Results

Although MYST4 mRNA transcripts are ubiquitous, protein expression was restricted to select extracts (including ovary and testis). Immunohistochemistry experiments performed on ovary sections revealed that the MYST4 protein is confined to oocytes, granulosa and theca cells, as well as to cells composing the blood vessels. The transcripts for MYST4 and all-MYST4-isoforms were present in oocytes and in in vitro produced embryos. In oocytes and embryos the MYST4 protein was localized in both the cytoplasm and nucleus. Within testis sections, the MYST4 protein was specific to only one cell type, the elongating spermatids, where it was exclusively nuclear.

Conclusion

We established that MYST4 is localized into specialized cells of the ovary and testis. Because the majority of these cells are involved in male and female gametogenesis, MYST4 may contribute to important and specific acetylation events occurring during gametes and embryo development.

The MYST family of histone acetyltransferases and their intimate links to cancer

The histone acetyltransferases (HATs) of the MYST family are highly conserved in eukaryotes and carry out a significant proportion of all nuclear acetylation. These enzymes function exclusively in multisubunit protein complexes whose composition is also evolutionarily conserved. MYST HATs are involved in a number of key nuclear processes and play critical roles in gene-specific transcription regulation, DNA damage response and repair, as well as DNA replication. This suggests that anomalous activity of these HATs or their associated complexes can easily lead to severe cellular malfunction, resulting in cell death or uncontrolled growth and malignancy. Indeed, the MYST family HATs have been implicated in several forms of human cancer. This review summarizes the current understanding of these enzymes and their normal function, as well as their established and putative links to oncogenesis.

MOZ and MORF, two large MYSTic HATs in normal and cancer stem cells

Genes of the human monocytic leukemia zinc-finger protein MOZ (HUGO symbol, MYST3) and its paralog MORF (MYST4) are rearranged in chromosome translocations associated with acute myeloid leukemia and/or benign uterine leiomyomata. Both proteins have intrinsic histone acetyltransferase activity and are components of quartet complexes with noncatalytic subunits containing the bromodomain, plant homeodomain-linked (PHD) finger and proline-tryptophan-tryptophan-proline (PWWP)-containing domain, three types of structural modules characteristic of chromatin regulators. Although leukemia-derived fusion proteins such as MOZ-TIF2 promote self-renewal of leukemic stem cells, recent studies indicate that murine MOZ and MORF are important for proper development of hematopoietic and neurogenic progenitors, respectively, thereby highlighting the importance of epigenetic integrity in safeguarding stem cell identity.

MOZ-TIF2 repression of nuclear receptor-mediated transcription requires multiple domains in MOZ and in the CID domain of TIF2

Background

Fusion of the MOZ and TIF2 genes by an inv (8) (p11q13) translocation has been identified in patients with acute mixed-lineage leukemia. Characterization of the molecular structure of the MOZ-TIF2 fusion protein suggested that the fusion protein would effect on nuclear receptor signaling.

Results

A series of deletions from the N-terminus of the MOZ-TIF2 fusion protein demonstrated that the MOZ portion is essential for nuclear localization of the fusion protein. Transient expression of MOZ-TIF2 dramatically decreased both basal and estradiol inducible reporter gene activity in an estrogen receptor element (ERE) driven luciferase reporter system and decreased androgen-inducible reporter gene activity in an androgen receptor element (ARE) luciferase reporter system. Deletions in the MOZ portion of the MOZ-TIF2 fusion protein reduced the suppression in the ER reporter system. Stable expression of MOZ-TIF2 inhibited retinoic acid (RA) inducible endogenous CD11b and C/EBPβ gene response. The suppression of the reporter systems was released with either a CID domain deletion or with mutations of leucine-rich repeats in the TIF2 portion of MOZ-TIF2. The co-expression of TIF2, but not CBP, with MOZ-TIF2 partially restored the inhibition of the reporter systems. In addition, analysis of protein interactions demonstrated MOZ-TIF2 interaction with the C-terminus of CBP through both the MOZ and TIF2 portions of the fusion protein.

Conclusion

MOZ-TIF2 inhibited nuclear receptor-mediated gene response by aberrant recruitment of CBP and both the MOZ and TIF2 portions are required for this inhibition.

Males absent on the first (MOF): from flies to humans

Histone modifications such as acetylation, methylation and phosphorylation have been implicated in fundamental cellular processes such as epigenetic regulation of gene expression, organization of chromatin structure, chromosome segregation, DNA replication and DNA repair. Males absent on the first (MOF) is responsible for acetylating histone H4 at lysine 16 (H4K16) and is a key component of the MSL complex required for dosage compensation in Drosophila. The human ortholog of MOF (hMOF) has the same substrate specificity and recent purification of the human and Drosophila MOF complexes showed that these complexes were also highly conserved through evolution. Several studies have shown that loss of hMOF in mammalian cells leads to a number of different phenotypes; a G2/M cell cycle arrest, nuclear morphological defects, spontaneous chromosomal aberrations, reduced transcription of certain genes and an impaired DNA repair response upon ionizing irradiation. Moreover, hMOF is involved in ATM activation in response to DNA damage and acetylation of p53 by hMOF influences the cell's decision to undergo apoptosis instead of a cell cycle arrest. These data, highlighting hMOF as an important component of many cellular processes, as well as links between hMOF and cancer will be discussed.

The transcriptional coactivator Querkopf controls adult neurogenesis

The adult mammalian brain maintains populations of neural stem cells within discrete proliferative zones. Understanding of the molecular mechanisms regulating adult neural stem cell function is limited. Here, we show that MYST family histone acetyltransferase Querkopf (Qkf, Myst4, Morf)-deficient mice have cumulative defects in adult neurogenesis in vivo, resulting in declining numbers of olfactory bulb interneurons, a population of neurons produced in large numbers during adulthood. Qkf-deficient mice have fewer neural stem cells and fewer migrating neuroblasts in the rostral migratory stream. Qkf gene expression is strong in the neurogenic subventricular zone. A population enriched in multipotent cells can be isolated from this region on the basis of Qkf gene expression. Neural stem cells/progenitor cells isolated from Qkf mutant mice exhibited a reduced self-renewal capacity and a reduced ability to produce differentiated neurons. Together, our data show that Qkf is essential for normal adult neurogenesis.

MOZ-TIF2 alters cofactor recruitment and histone modification at the RARbeta2 promoter: differential effects of MOZ fusion proteins on CBP- and MOZ-dependent activators

MOZ-TIF2 and MOZ-CBP are leukemogenic fusion proteins associated with therapy-induced acute myeloid leukemia. These proteins are thought to subvert normal gene expression in differentiating hematopoietic progenitor cells. We have previously shown that MOZ-TIF2 inhibits transcription by CREB-binding protein (CBP)/p300-dependent activators such as nuclear receptors and p53. Here we have shown that MOZ-TIF2 associates with the RARbeta2 promoter in vivo, resulting in altered recruitment of CBP/p300, aberrant histone modification, and down-regulation of the RARbeta2 gene. In contrast, MOZ-TIF2 up-regulated transcription mediated by the MOZ/MYST3-dependent activator AML1/RUNX1. Both wild type MOZ and MOZ-TIF2 were found to colocalize with AML1, and MOZ-TIF2 was recruited to an AML1 target promoter. A MOZ-CBP fusion protein showed similar functions to MOZ-TIF2 in that it inhibited retinoic acid receptor-mediated transcription but enhanced AML1 reporter activation. Although it contains almost the entire CBP sequence, MOZ-CBP does not appear to associate with PML bodies. In summary, our results indicate that leukemogenic MOZ fusion proteins have differential effects on the activities of CBP-dependent and MOZ-dependent activators because of their ability to alter cofactor recruitment and chromatin modification at target promoters.

MYST family histone acetyltransferases in the protozoan parasite Toxoplasma gondii

The restructuring of chromatin precedes tightly regulated events such as DNA transcription, replication, and repair. One type of chromatin remodeling involves the covalent modification of nucleosomes by histone acetyltransferase (HAT) complexes. The observation that apicidin exerts antiprotozoal activity by targeting a histone deacetyltransferase has prompted our search for more components of the histone modifying machinery in parasitic protozoa. We have previously identified GNAT family HATs in the opportunistic pathogen Toxoplasma gondii and now describe the first MYST (named for members MOZ, Ybf2/Sas3, Sas2, and Tip60) family HATs in apicomplexa (TgMYST-A and -B). The TgMYST-A genomic locus is singular and generates a approximately 3.5-kb transcript that can encode two proteins of 411 or 471 amino acids. TgMYST-B mRNA is approximately 7.0 kb and encodes a second MYST homologue. In addition to the canonical MYST HAT catalytic domain, both TgMYST-A and -B possess an atypical C2HC zinc finger and a chromodomain. Recombinant TgMYST-A exhibits a predilection to acetylate histone H4 in vitro at lysines 5, 8, 12, and 16. Antibody generated to TgMYST-A reveals that both the long and short (predominant) versions are present in the nucleus and are also plentiful in the cytoplasm. Moreover, both TgMYST-A forms are far more abundant in rapidly replicating parasites (tachyzoites) than encysted parasites (bradyzoites). A bioinformatics survey of the Toxoplasma genome reveals numerous homologues known to operate in native MYST complexes. The characterization of TgMYST HATs represents another important step toward understanding the regulation of gene expression in pathogenic protozoa and provides evolutionary insight into how these processes operate in eukaryotic cells in general.

Translocation (8;18;16)(p11;q21;p13). A new variant of t(8;16)(p11;p13) in acute monoblastic leukemia: case report and review of the literature

A complex three-way t(8;18;16)(p11;q21;p13) was detected in a 15-month-old patient with acute myeloid leukemia (AML). The patient had typical clinical manifestation and bone marrow features of AML subtype M5b associated with t(8;16)(p11;p13). Therefore, we believe that the t(8;18;16) is a new variant of t(8;16) related to AML M4/M5. We also review other t(8;16)(p11;p13) variants reported in the literature.

Control of MEF2 Transcriptional Activity by Coordinated Phosphorylation and Sumoylation

A eukaryotic protein is often subject to regulation by multiple modifications like phosphorylation, acetylation, ubiquitination, and sumoylation. How these modifications are coordinated in vivo is an important issue that is poorly understood but is relevant to many biological processes. We recently showed that human MEF2D (myocyte enhancer factor 2D) is sumoylated on Lys-439. Adjacent to the sumoylation motif is Ser-444, which like Lys-439 is highly conserved among MEF2 proteins from diverse species. Here we present [corrected] several lines of evidence to demonstrate that Ser-444 of MEF2D is required for sumoylation of Lys-439. Histone deacetylase 4 (HDAC4) stimulated this modification by acting through Ser-444. In addition, phosphorylation of Ser-444 by Cdk5, a cyclin-dependent kinase known to inhibit MEF2 transcriptional activity, stimulated sumoylation. Opposing the actions of HDAC4 and Cdk5, calcineurin (also known as protein phosphatase 2B) dephosphorylated Ser-444 and inhibited sumoylation of Lys-439. This phosphatase, however, exerted minimal effects on the phosphorylation catalyzed by ERK5, an extracellular signal-regulated kinase known to activate MEF2D. These results identify [corrected] an essential role for Ser-444 in MEF2D sumoylation and reveal [corrected] a novel mechanism by which calcineurin selectively "edits" phosphorylation at different sites, thereby reiterating that interplay between different modifications represents a general mechanism for coordinated regulation of eukaryotic protein functions in vivo.

A Decade of Histone Acetylation: Marking Eukaryotic Chromosomes with Specific Codes

Post-translational modification of histones, a major protein component of eukaryotic chromosomes, contributes to the epigenetic regulation of gene expression. Distinct patterns of histone modification are observed at specific chromosomal regions and affect various reactions on chromosomes (transcription, replication, repair, and recombination). Histone modification has long been proposed to have a profound effect on eukaryotic gene expression since its discovery in 1964. Verification of this idea, however, was difficult until the identification of enzymes responsible for histone modifications. Ten years ago (1995), histone acetyltransferases (HATs), which acetylate lysine residues in histone amino-terminal tail regions, were isolated. HATs are involved in the regulation of both promoter-specific transcription and long-range/chromosome-wide transcription. Analyses of HATs and other modification enzymes have revealed mechanisms of epigenetic regulation that are mediated by post-translational modifications of histones. Here we review some major advances in the field, with emphasis on the lysine specificity of the acetylation reaction and on the regulation of gene expression over broad regions.

Role of histone acetylation in the control of gene expression

Histone proteins play structural and functional roles in all nuclear processes. They undergo different types of covalent modifications, defined in their ensemble as epigenetic because changes in DNA sequences are not involved. Histone acetylation emerges as a central switch that allows interconversion between permissive and repressive chromatin domains in terms of transcriptional competence. The mechanisms underlying the histone acetylation-dependent control of gene expression include a direct effect on the stability of nucleosomal arrays and the creation of docking sites for the binding of regulatory proteins. Histone acetyltransferases and deacetylases are, respectively, the enzymes devoted to the addition and removal of acetyl groups from lysine residues on the histone N-terminal tails. The enzymes exert fundamental roles in developmental processes and their deregulation has been linked to the progression of diverse human disorders, including cancer.

Identification of the Ankyrin Repeat Proteins ANKRA and RFXANK as Novel Partners of Class IIa Histone Deacetylases

Eighteen human histone deacetylases (HDACs) have been identified, and according to their sequence similarity to yeast homologs, these enzymes are grouped into distinct classes. Within class II, HDAC4, HDAC5, HDAC7, and HDAC9 share similar domain organization both within the N-terminal extension and the C-terminal catalytic domain, thus forming a subclass known as class IIa. These HDACs function as signal-responsive transcriptional corepressors. To gain further insight into their function and regulation, we utilized an N-terminal fragment of HDAC4 as bait in yeast two-hybrid screens, which uncovered myocyte enhancer factor 2C, 14-3-3zeta, and ankyrin repeat family A protein (ANKRA). ANKRA is a poorly characterized protein with an ankyrin repeat domain similar to RFXANK, a subunit of the trimeric transcription factor RFX. Mutations on genes of the RFX subunits and the coactivator CIITA are responsible for the bare lymphocyte syndrome, an immunodeficiency disorder attributed to the lack of major histocompatibility complex class II (MHCII) antigens. Through its ankyrin repeat domain, RFXANK interacted with HDAC4. Two RFXANK-binding sites were found on HDAC4 with one located within residues 118-279 and another within residues 448-666. Interestingly, this deacetylase also interacted with CIITA. Consistent with the physical interaction with RFXANK and CIITA, HDAC4 and homologs repressed MHCII expression. These results identify ANKRA, RFXANK, and CIITA as novel targets of class IIa HDACs and suggest that these deacetylases play a role in regulating MHCII expression.

Cyclin-dependent kinase 11p58interacts with HBO1 and enhances its histone acetyltransferase activity

CDK11(p58), a 58kDa protein of the PITSLRE kinase family, plays an important role in cell cycle progression, and is closely related to cell apoptosis. To gain further insight into the function of CDK11(p58), we screened a human fetal liver cDNA library for its interacting proteins using the yeast two-hybrid system. Here we report that histone acetyltransferase (HAT) HBO1, a MYST family protein, interacts with CDK11(p58) in vitro and in vivo. CDK11(p58) and HBO1 colocalize in the cell nucleus. Recombinant CDK11(p58) enhances the HAT activity of HBO1 significantly in vitro. Meanwhile, overexpression of CDK11(p58) in mammalian cells leads to the enhanced HAT activity of HBO1 towards free histones. Thus, we conclude that CDK11(p58) is a new interacting protein and a novel regulator of HBO1. Both of the proteins may be involved in the regulation of eukaryotic transcription.

MOZ-TIF2 inhibits transcription by nuclear receptors and p53 by impairment of CBP function

Chromosomal rearrangements associated with acute myeloid leukemia (AML) include fusions of the genes encoding the acetyltransferase MOZ or MORF with genes encoding the nuclear receptor coactivator TIF2, p300, or CBP. Here we show that MOZ-TIF2 acts as a dominant inhibitor of the transcriptional activities of CBP-dependent activators such as nuclear receptors and p53. The dominant negative property of MOZ-TIF2 requires the CBP-binding domain (activation domain 1 [AD1]), and coimmunoprecipitation and fluorescent resonance energy transfer experiments show that MOZ-TIF2 interacts with CBP directly in vivo. The CBP-binding domain is also required for the ability of MOZ-TIF2 to extend the proliferative potential of murine bone marrow lineage-negative cells in vitro. We show that MOZ-TIF2 displays an aberrant nuclear distribution and that cells expressing this protein have reduced levels of cellular CBP, leading to depletion of CBP from PML bodies. In summary, our results indicate that disruption of the normal function of CBP and CBP-dependent activators is an important feature of MOZ-TIF2 action in AML.

Uterine leiomyomata with t(10;17) disrupt the histone acetyltransferase MORF

Benign uterine leiomyomata are the most common tumors in women of reproductive age. One recurring chromosomal aberration in uterine leiomyomata is rearrangement of 10q22. Chromosome 10 breakpoints were mapped by fluorescence in situ hybridization to intervals ranging from 8.9 to 72.1 kb within the third intron of MORF (monocytic leukemia zinc finger protein-related factor or MYST4) in four uterine leiomyomata tested. Additional Southern hybridization experiments confirmed that the breakpoint lies within the third intron and narrowed the interval to 2.1 kb in one uterine leiomyomata. MORF is a member of the MYST family of histone acetyltransferase and previously has been found rearranged in some types of acute myeloid leukemia (AML). This is the first instance in which disruption of a histone acetyltransferase has been reported in another tumor type. The breakpoints in uterine leiomyomata would fall in the NH2-terminal portion of the protein between a conserved domain found in histones H1 and H5 and the PHD zinc fingers, the CH2CH zinc finger, or the CoA binding site, which is distinct from the breakpoints reported in AML. Mapping of the 17q21 breakpoint by fluorescence in situ hybridization within a specific region in three tumors revealed several positional candidates including GCN5L2, a gene with histone acetyltransferase activity similar to those fused to MORF in AML. Of note, two of three uterine leiomyomata were of the cellular subtype. Involvement of MORF in four uterine leiomyomata with chromosomal rearrangements involving 10q22 and 17q21 suggests a role for this histone acetyltransferase and altered chromatin regulation in uterine mesenchymal neoplasia.

CBP and p300: HATs for different occasions

The transcriptional coactivators CREB binding protein (CBP) and p300 are key regulators of RNA polymerase II-mediated transcription. Genetic alterations in the genes encoding these regulatory proteins and their functional inactivation have been linked to human disease. Findings in patients, knockout mice and cell-based studies indicate that the ability of these multidomain proteins to acetylate histones and other proteins is critical for many biological processes. Furthermore, despite their high degree of homology, accumulating evidence indicates that CBP and p300 are not completely redundant but also have unique roles in vivo. Recent studies suggest that these functional differences could be due to differential association with other proteins or differences in substrate specificity between these acetyltransferases. Inactivation of the acetyltransferase function of either CBP or p300 in various experimental systems will no doubt teach us more about the specific biological roles of these proteins. Given the wide range of human diseases in which CBP and/or p300 have been implicated, understanding the mechanisms that regulate their activity in vivo could help to develop novel approaches for the development of therapeutic strategies.

Role of the tetradecapeptide repeat domain of human histone deacetylase 6 in cytoplasmic retention

Histone deacetylase 6 (HDAC6) contains tandem catalytic domains and a ubiquitin-binding zinc finger and displays deacetylase activity toward acetylated microtubules. Here we show that unlike its orthologs from Caenorhabditis elegans, Drosophila, and mouse, human HDAC6 possesses a tetradecapeptide repeat domain located between the second deacetylase domain and the C-terminal ubiquitin-binding motif. Related to this structural difference, the cytoplasmic localization of human, but not murine, HDAC6 is resistant to treatment with leptomycin B (LMB). Although it is dispensable for the deacetylase and ubiquitin binding activities of human HDAC6, the tetradecapeptide repeat domain displays acetyl-microtubule targeting ability. Moreover, it forms a unique structure and is required for the LMB-resistant cytoplasmic localization of human HDAC6. Besides the tetradecapeptide repeat domain, human HDAC6 possesses two LMB-sensitive nuclear export signals and a nuclear localization signal. These results thus indicate that the cytoplasmic localization for murine and human HDAC6 proteins is differentially regulated and suggest that the tetradecapeptide repeat domain serves as an important sequence element to stably retain human HDAC6 in the cytoplasm.

moz regulates Hox expression and pharyngeal segmental identity in zebrafish

In vertebrate embryos, streams of cranial neural crest (CNC) cells migrate to form segmental pharyngeal arches and differentiate into segment-specific parts of the facial skeleton. To identify genes involved in specifying segmental identity in the vertebrate head, we screened for mutations affecting cartilage patterning in the zebrafish larval pharynx. We present the positional cloning and initial phenotypic characterization of a homeotic locus discovered in this screen. We show that a zebrafish ortholog of the human oncogenic histone acetyltransferase MOZ (monocytic leukemia zinc finger) is required for specifying segmental identity in the second through fourth pharyngeal arches. In moz mutant zebrafish, the second pharyngeal arch is dramatically transformed into a mirror-image duplicated jaw. This phenotype resembles a similar but stronger transformation than that seen in hox2 morpholino oligo (hox2-MO) injected animals. In addition, mild anterior homeotic transformations are seen in the third and fourth pharyngeal arches of moz mutants. moz is required for maintenance of most hox1-4 expression domains and this requirement probably at least partially accounts for the moz mutant homeotic phenotypes. Homeosis and defective Hox gene expression in moz mutants is rescued by inhibiting histone deacetylase activity with Trichostatin A. Although we find early patterning of the moz mutant hindbrain to be normal, we find a late defect in facial motoneuron migration in moz mutants. Pharyngeal musculature is transformed late, but not early, in moz mutants. We detect relatively minor defects in arch epithelia of moz mutants. Vital labeling of arch development reveals no detectable changes in CNC generation in moz mutants, but later prechondrogenic condensations are mispositioned and misshapen. Mirror-image hox2-dependent gene expression changes in postmigratory CNC prefigure the homeotic phenotype in moz mutants. Early second arch ventral expression of goosecoid (gsc) in moz mutants and in animals injected with hox2-MOs shifts from lateral to medial, mirroring the first arch pattern. bapx1, which is normally expressed in first arch postmigratory CNC prefiguring the jaw joint, is ectopically expressed in second arch CNC of moz mutants and hox2-MO injected animals. Reduction of bapx1 function in wild types causes loss of the jaw joint. Reduction of bapx1 function in moz mutants causes loss of both first and second arch joints, providing functional genetic evidence that bapx1 contributes to the moz-deficient homeotic pattern. Together, our results reveal an essential embryonic role and a crucial histone acetyltransferase activity for Moz in regulating Hox expression and segmental identity, and provide two early targets, bapx1 and gsc, of moz and hox2 signaling in the second pharyngeal arch.

The parent-of-origin effect of 10q22 in pre-eclamptic females coincides with two regions clustered for genes with down-regulated expression in androgenetic placentas

By affected sib-pair linkage analysis of 24 families with pre-eclampsia, we confirm a susceptibility locus on chromosome 10q22.1 in Dutch females: a multipoint non-parametric linkage score of 3.6 near marker D10S1432 was obtained. Haplotype analysis showed a parent-of-origin effect: maximal allele sharing in the affected sibs was found for maternally derived alleles in all families, but not for the paternally derived alleles. As matrilineal inheritance suggests the presence of maternally expressed imprinted genes, while imprinting operates predominantly in (extra)embryonic tissues, all genes (n=132) known on 10q22 between GATA121A08 and D10S580 were screened for seven sequence-related features associated with imprinting and subsequently tested for expression in first trimester placenta. Placental expression of genes selected in this way (n=55) was compared with expression in androgenetic placentas of identical gestational age. Two regions on 10q22 were identified with developmentally co-repressed genes with non-random chromosomal distribution. Interestingly, these two clusters, near CTNNA3 and KCNMA1 and each containing five genes with down-regulated expression in androgenetic placentas, coincided with the regions with maximal maternal allele sharing seen in the pre-eclamptic sisters. Our linkage and expression data are compatible with the concept that pre-eclampsia involves maternally expressed imprinted genes that operate in the first trimester placenta.

The diverse superfamily of lysine acetyltransferases and their roles in leukemia and other diseases

Acetylation of the epsilon-amino group of lysine residues, or N(epsilon)-lysine acetylation, is an important post-translational modification known to occur in histones, transcription factors and other proteins. Since 1995, dozens of proteins have been discovered to possess intrinsic lysine acetyltransferase activity. Although most of these enzymes were first identified as histone acetyltransferases and then tested for activities towards other proteins, acetyltransferases only modifying non-histone proteins have also been identified. Lysine acetyltransferases form different groups, three of which are Gcn5/PCAF, p300/CBP and MYST proteins. While members of the former two groups mainly function as transcriptional co-activators, emerging evidence suggests that MYST proteins, such as Esa1, Sas2, MOF, TIP60, MOZ and MORF, have diverse roles in various nuclear processes. Aberrant lysine acetylation has been implicated in oncogenesis. The genes for p300, CBP, MOZ and MORF are rearranged in recurrent leukemia-associated chromosomal abnormalities. Consistent with their roles in leukemogenesis, these acetyltransferases interact with Runx1 (or AML1), one of the most frequent targets of chromosomal translocations in leukemia. Therefore, the diverse superfamily of lysine acetyltransferases executes an acetylation program that is important for different cellular processes and perturbation of such a program may cause the development of cancer and other diseases.

Cloning and characterization of a novel gene, SHPRH, encoding a conserved putative protein with SNF2/helicase and PHD-finger domains from the 6q24 region

Here we report the identification of a novel transcript containing SNF2, PHD-finger, RING-finger, helicase, and linker histone domains mapping to the q24 band region of human chromosome 6. These domains are characteristic of several DNA repair proteins, transcription factors, and helicases. We have cloned both human and mouse homologs of this novel gene using interexon PCR and RACE technologies. The human cDNA, termed SHPRH, is 6018 bp and codes for a putative protein of 1683 amino acids. The mouse cDNA, termed Shprh, is 7225 bp and codes for a putative protein of 1616 amino acids. The deduced amino acid sequences of the two proteins share 86% identity. Both genes are expressed ubiquitously, with a transcript size of approximately 7.5 kb. Mapping of this gene to 6q24, a region reported to contain a tumor suppressor locus, prompted us to evaluate SHPRH by mutation analysis in tumor cell lines. We have identified one truncating and three missense mutations, thus suggesting SHPRH as a possible candidate for the tumor suppressor gene.

The MYST family of histone acetyltransferases

Multiple chromatin modifying proteins and multisubunit complexes have been characterized in recent years. Histone acetyltransferase (HAT) activities have been the most thoroughly studied, both biochemically and functionally. This review sums up the current knowledge on a specific group of proteins that is extremely well conserved throughout evolution, the MYST family of histone acetyltransferases. These proteins play critical roles in various nuclear functions and the control of cell proliferation.

t(10;16)(q22;p13) and MORF-CREBBP fusion is a recurrent event in acute myeloid leukemia

Recently, it was shown that t(10;16)(q22;p13) fuses the MORF and CREBBP genes in a case of childhood acute myeloid leukemia (AML) M5a, with a complex karyotype containing other rearrangements. Here, we report a new case with the MORF-CREBBP fusion in an 84-year-old patient diagnosed with AML M5b, in which the t(10;16)(q22;p13) was the only cytogenetic aberration. This supports that this is a recurrent pathogenic translocation in AML.

Genomic characterization of MOZ/CBP and CBP/MOZ chimeras in acute myeloid leukemia suggests the involvement of a damage-repair mechanism in the origin of the t(8;16)(p11;p13)

The t(8;16)(p11;p13), which is strongly associated with acute myeloid leukemia (AML) displaying monocytic differentiation, erythrophagocytosis by the leukemic cells, and a poor response to chemotherapy, fuses the MOZ gene (8p11) with the CBP gene (16p13). Although genomic rearrangements of MOZ and CBP have been detected using fluorescence in situ hybridization and Southern blot analyses, characterization of the breakpoints at the sequence level has never been performed. We have sequenced the breakpoints in four t(8;16)-positive AML cases with the aim to identify molecular genetic mechanisms underlying the origin of this translocation. In addition, an exon/intron map of the MOZ gene was constructed, which was found to be composed of 17 exons. Long-range-PCR with CBP forward primers in exon 2 and MOZ reverse primers in exon 17 as well as with a MOZ forward primer in exon 16 and a CBP reverse primer in intron 2 successfully amplified CBP/MOZ and MOZ/CBP hybrid genomic DNA fragments in all four AMLs. The breaks clustered in both CBP intron 2 and MOZ intron 16, and were close to repetitive elements, and in one case an Alu-Alu junction for the CBP/MOZ hybrid was identified. Additional genomic events (i.e., deletions, duplications, and insertions) in the breakpoint regions in both the MOZ and CBP genes were found in all four cases. Thus, the t(8;16) does not originate through a simple end-to-end fusion. The findings of multiple breaks and rearrangements rather suggest the involvement of a damage-repair mechanism in the origin of this translocation.

A novel fusion variant of the MORF and CBP genes detected in therapy-related myelodysplastic syndrome with t(10;16)(q22;p13)

We report a case of therapy-related myelodysplastic syndrome (t-MDS) with t(10;16)(q22;p13), in which novel fusion transcripts of the MORF and CBP genes were detected. In one MORF-CBP fusion transcript, exon 15 of the MORF gene was fused in frame with exon 5 of the CBP gene. In a reciprocal CBP-MORF transcript, exon 4 of the CBP gene was fused in frame with exon 16 of the MORF gene. This is the first reported case of t-MDS associated with t(10;16), and provides molecular evidence that the novel MORF-CBP and/or CBP-MORF fusion protein(s) might play an important role in the development of t-MDS.

Che-1 affects cell growth by interfering with the recruitment of HDAC1 by Rb

DNA tumor virus oncoproteins bind and inactivate Rb by interfering with the Rb/HDAC1 interaction. Che-1 is a recently identified human Rb binding protein that inhibits the Rb growth suppressing function. Here we show that Che-1 contacts the Rb pocket region and competes with HDAC1 for Rb binding site, removing HDAC1 from the Rb/E2F complex in vitro and from the E2F target promoters in vivo. Che-1 overexpression activates DNA synthesis in quiescent NIH-3T3 cells through HDAC1 displacement. Consistently, Che-1-specific RNA interference affects E2F activity and cell proliferation in human fibroblasts but not in the pocket protein-defective 293 cells. These findings indicate the existence of a pathway of Rb regulation supporting Che-1 as the cellular counterpart of DNA tumor virus oncoproteins.

Identification of a polymorphic, neuron-specific chromatin remodeling complex

A variety of chromatin remodeling complexes are thought to assist sequence-specific transcription factors. The complexes described to date are expressed ubiquitously, suggesting that they have general transcriptional functions. We show that vertebrate neurons have a specialized chromatin remodeling complex, bBAF, specifically containing the actin-related protein, BAF53b, which is first expressed in postmitotic neurons at about murine embryonic day 12.5 (E12.5). BAF53b is combinatorially assembled into polymorphic complexes with ubiquitous subunits including the two ATPases BRG1 and BRM. We speculate that bBAF complexes create neuronal-specific patterns of chromatin accessibility, thereby imparting new regulatory characteristics to ubiquitous sequence-specific transcription factors in neurons.

Identification of a transcriptionally active peroxisome proliferator-activated receptor alpha -interacting cofactor complex in rat liver and characterization of PRIC285 as a coactivator

Peroxisome proliferator-activated receptor alpha (PPAR alpha) plays a central role in the cell-specific pleiotropic responses induced by structurally diverse synthetic chemicals designated as peroxisome proliferators. Transcriptional regulation by liganded nuclear receptors involves the participation of cofactors that form multiprotein complexes to achieve cell- and gene-specific transcription. Here we report the identification of such a transcriptionally active PPAR alpha-interacting cofactor (PRIC) complex from rat liver nuclear extracts that interacts with full-length PPAR alpha in the presence of ciprofibrate, a synthetic ligand, and leukotriene B(4), a natural ligand. The liganded PPAR alpha-PRIC complex enhanced transcription from a peroxisomal enoyl-CoA hydratase/l-3-hydroxyacyl-CoA dehydrogenase bifunctional enzyme gene promoter template that contains peroxisome proliferator response elements. Rat liver PRIC complex comprises some 25 polypeptides, and their identities were established by mass spectrometry and limited sequence analysis. Eighteen of these peptides contain one or more LXXLL motifs necessary for interacting with nuclear receptors. PRIC complex includes known coactivators or coactivator-binding proteins (CBP, SRC-1, PBP, PRIP, PIMT, TRAP100, SUR-2, and PGC-1), other proteins that have not previously been described in association with transcription complexes (CHD5, TOG, and MORF), and a few novel polypeptides designated PRIC300, -285, -215, -177, and -145. We describe the cDNA for PRIC285, which contains five LXXLL motifs. It interacts with PPAR alpha and acts as a coactivator by moderately stimulating PPAR alpha-mediated transcription in transfected cells. We conclude that liganded PPAR alpha recruits a distinctive multiprotein complex from rat liver nuclear extracts. The composition of this complex may provide insight into the basis of tissue and species sensitivity to peroxisome proliferators.

Tumor suppressor genes in normal and malignant hematopoiesis

Over the last decade, a growing number of tumor suppressor genes have been discovered to play a role in tumorigenesis. Mutations of p53 have been found in hematological malignant diseases, but the frequency of these alterations is much lower than in solid tumors. These mutations occur especially as hematopoietic abnormalities become more malignant such as going from the chronic phase to the blast crisis of chronic myeloid leukemia. A broad spectrum of tumor suppressor gene alterations do occur in hematological malignancies, especially structural alterations of p15(INK4A), p15(INK4B) and p14(ARF) in acute lymphoblastic leukemia as well as methylation of these genes in several myeloproliferative disorders. Tumor suppressor genes are altered via different mechanisms, including deletions and point mutations, which may result in an inactive or dominant negative protein. Methylation of the promoter of the tumor suppressor gene can blunt its expression. Chimeric proteins formed by chromosomal translocations (i.e. AML1-ETO, PML-RARalpha, PLZF-RARalpha) can produce a dominant negative transcription factor that can decrease expression of tumor suppressor genes. This review provides an overview of the current knowledge about the involvement of tumor suppressor genes in hematopoietic malignancies including those involved in cell cycle control, apoptosis and transcriptional control.

MOZ and MORF histone acetyltransferases interact with the Runt-domain transcription factor Runx2

The monocytic leukemia zinc finger protein MOZ and its homologue MORF have been implicated in leukemogenesis. Both MOZ and MORF are histone acetyltransferases with weak transcriptional repression domains and strong transcriptional activation domains, suggesting that they may function as transcriptional coregulators. Here we describe that MOZ and MORF both interact with Runx2 (or Cbfa1), a Runt-domain transcription factor that is known to play important roles in T cell lymphomagenesis and bone development. Through its C-terminal SM (serine- and methionine-rich) domain, MORF binds to Runx2 in vitro and in vivo. Consistent with this, the SM domain of MORF also binds to Runx1 (or AML1), a Runx2 homologue that is frequently altered by leukemia-associated chromosomal translocations. While MORF does not acetylate Runx2, its SM domain potentiates Runx2-dependent transcriptional activation. Moreover, endogenous MORF is required for transcriptional activation by Runx2. Intriguingly, Runx2 negatively regulates the transcriptional activation potential of the SM domain. Like that of MORF, the SM domain of MOZ physically and functionally interacts with Runx2. These results thus identify Runx2 as an interaction partner of MOZ and MORF and suggest that both acetyltransferases are involved in regulating transcriptional activation mediated by Runx2 and its homologues.

Identification of HDAC10, a novel class II human histone deacetylase containing a leucine-rich domain

Histone acetylation is important for regulating chromatin structure and gene expression. Three classes of mammalian histone deacetylases have been identified. Among class II, there are five known members, namely HDAC4, HDAC5, HDAC6, HDAC7 and HDAC9. Here we describe the identification and characterization of a novel class II member termed HDAC10. It is a 669 residue polypeptide with a bipartite modular structure consisting of an N-terminal Hda1p-related putative deacetylase domain and a C-terminal leucine-rich domain. HDAC10 is widely expressed in adult human tissues and cultured mammalian cells. It is enriched in the cytoplasm and this enrichment is not sensitive to leptomycin B, a specific inhibitor known to block the nuclear export of other class II members. The leucine-rich domain of HDAC10 is responsible for its cytoplasmic enrichment. Recombinant HDAC10 protein possesses histone deacetylase activity, which is sensitive to trichostatin A, a specific inhibitor for known class I and class II histone deacetylases. When tethered to a promoter, HDAC10 is able to repress transcription. Furthermore, HDAC10 interacts with HDAC3 but not with HDAC4 or HDAC6. These results indicate that HDAC10 is a novel class II histone deacetylase possessing a unique leucine-rich domain.

The yeast SAS (something about silencing) protein complex contains a MYST-type putative acetyltransferase and functions with chromatin assembly factor ASF1

It is well established that acetylation of histone and nonhistone proteins is intimately linked to transcriptional activation. However, loss of acetyltransferase activity has also been shown to cause silencing defects, implicating acetylation in gene silencing. The something about silencing (Sas) 2 protein of Saccharomyces cerevisiae, a member of the MYST (MOZ, Ybf2/Sas3, Sas2, and TIP60) acetyltransferase family, promotes silencing at HML and telomeres. Here we identify a ~450-kD SAS complex containing Sas2p, Sas4p, and the tf2f-related Sas5 protein. Mutations in the conserved acetyl-CoA binding motif of Sas2p are shown to disrupt the ability of Sas2p to mediate the silencing at HML and telomeres, providing evidence for an important role for the acetyltransferase activity of the SAS complex in silencing. Furthermore, the SAS complex is found to interact with chromatin assembly factor Asf1p, and asf1 mutants show silencing defects similar to mutants in the SAS complex. Thus, ASF1-dependent chromatin assembly may mediate the role of the SAS complex in silencing.

Histone deacetylase 4 possesses intrinsic nuclear import and export signals

Nucleocytoplasmic trafficking of histone deacetylase 4 (HDAC4) plays an important role in regulating its function, and binding of 14-3-3 proteins is necessary for its cytoplasmic retention. Here, we report the identification of nuclear import and export sequences of HDAC4. While its N-terminal 118 residues modulate the nuclear localization, residues 244 to 279 constitute an authentic, strong nuclear localization signal. Mutational analysis of this signal revealed that three arginine-lysine clusters are necessary for its nuclear import activity. As for nuclear export, leucine-rich sequences located in the middle part of HDAC4 do not function as nuclear export signals. By contrast, a hydrophobic motif (MXXLXVXV) located at the C-terminal end serves as a nuclear export signal that is necessary for cytoplasmic retention of HDAC4. This motif is required for CRM1-mediated nuclear export of HDAC4. Furthermore, binding of 14-3-3 proteins promotes cytoplasmic localization of HDAC4 by both inhibiting its nuclear import and stimulating its nuclear export. Unlike wild-type HDAC4, a point mutant with abrogated MEF2-binding ability remains cytoplasmic upon exogenous expression of MEF2C, supporting the notion that direct MEF2 binding targets HDAC4 to the nucleus. Therefore, HDAC4 possesses intrinsic nuclear import and export signals for its dynamic nucleocytoplasmic shuttling, and association with 14-3-3 and MEF2 proteins affects such shuttling and thus directs HDAC4 to the cytoplasm and the nucleus, respectively.

Replication factors MCM2 and ORC1 interact with the histone acetyltransferase HBO1

The minichromosome maintenance (MCM) proteins, together with the origin recognition complex (ORC) proteins and Cdc6, play an essential role in eukaryotic DNA replication through the formation of a pre-replication complex at origins of replication. We used a yeast two-hybrid screen to identify MCM2-interacting proteins. One of the proteins we identified is identical to the ORC1-interacting protein termed HBO1. HBO1 belongs to the MYST family, characterized by a highly conserved C2HC zinc finger and a putative histone acetyltransferase domain. Biochemical studies confirmed the interaction between MCM2 and HBO1 in vitro and in vivo. An N-terminal domain of MCM2 is necessary for binding to HBO1, and a C2HC zinc finger of HBO1 is essential for binding to MCM2. A reverse yeast two-hybrid selection was performed to isolate an allele of MCM2 that is defective for interaction with HBO1; this allele was then used to isolate a suppressor mutant of HBO1 that restores the interaction with the mutant MCM2. This suppressor mutation was located in the HBO1 zinc finger. Taken together, these findings strongly suggest that the interaction between MCM2 and HBO1 is direct and mediated by the C2HC zinc finger of HBO1. The biochemical and genetic interactions of MYST family protein HBO1 with two components of the replication apparatus, MCM2 and ORC1, suggest that HBO1-associated HAT activity may play a direct role in the process of DNA replication.

Fusion genes in leukemia: an emerging network

The molecular analysis of recurring chromosome rearrangements, especially of translocations and inversions, has provided us with valuable insight into the pathogenesis of hematological malignancies. Many translocations result in the fusion of genes located at the translocation breakpoints. In recent years we have witnessed a rapid rise in the number of chromosome translocations in leukemias being characterized at the molecular level. However, the number of genes being newly identified as translocation fusion genes has not risen at the same pace. This is due to the fact that several genes are involved in more than one translocation forming fusion genes with a number of other partner genes. Not only does one find star-shaped topologies, with one gene forming fusions with several others (e.g. ETV6/PDGFRB, ETV6/JAK2, ETV6/ABL etc.), but also networks connecting several genes with more than one fusion partner (e.g. ETV6/RUNX1 (AML1), RUNX1/CBFA2T1 (ETO), ETV6/EVI1, RUNX1/EVI1, ETV6/ABL, BCR/ABL). The emergence of such networks with the "recycling" of genes in new fusion combinations suggests that there is a rather limited number of genes which can be altered to cause leukemia.

HATs on and beyond chromatin

The role of histone acetylation as a key mechanism of transcriptional regulation has been well established. Recent advances suggest that histone acetyltransferases also play important roles in histone-modulated processes such as DNA replication, recombination and repair. In addition, acetylation of transcriptional cofactors and other proteins is an efficient means of regulating a diverse range of molecular interactions. As new histone acetyltransferases and substrates are rapidly emerging, it is becoming apparent that protein acetylation may rival phosphorylation as a mechanism to transduce cellular regulatory signals.