Mammalian Trithorax and polycomb-group homologues are antagonistic regulators of homeotic development

Proteolysis of MLL family proteins is essential for taspase1-orchestrated cell cycle progression

Taspase1 was identified as the threonine endopeptidase that cleaves mixed-lineage leukemia (MLL) for proper Hox gene expression in vitro. To investigate its functions in vivo, we generated Taspase1(-/-) mice. Taspase1 deficiency results in noncleavage (nc) of MLL and MLL2 and homeotic transformations. Remarkably, our in vivo studies uncover an unexpected role of Taspase1 in the cell cycle. Taspase1(-/-) animals are smaller in size. Taspase1(-/-) mouse embryonic fibroblasts (MEFs) exhibit impaired proliferation, and acute deletion of Taspase1 leads to a marked reduction of thymocytes. Taspase1 deficiency incurs down-regulation of Cyclin Es, As, and Bs and up-regulation of p16(Ink4a) . We show that MLL and MLL2 directly target E2Fs for Cyclin expression. The uncleaved precursor MLL displays a reduced histone H3 methyl transferase activity in vitro. Accordingly, chromatin immunoprecipitation assays demonstrate a markedly decreased histone H3 K4 trimethylation at Cyclin E1 and E2 genes in Taspase1(-/-) cells. Furthermore, MLL(nc/nc;2nc/nc) MEFs are also impaired in proliferation. Our data are consistent with a model in which precursor MLLs, activated by Taspase1, target to Cyclins through E2Fs to methylate histone H3 at K4, leading to activation. Lastly, Taspase1(-/-) cells are resistant to oncogenic transformation, and Taspase1 is overexpressed in many cancer cell lines. Thus, Taspase1 may serve as a target for cancer therapeutics.

Complexity beneath the silence

Polycomb group (PcG)-mediated silencing by proteins that are conserved across plants and animals is a key feature of eukaryotic gene regulation. Investigation of PcG-mediated gene silencing has revealed a surprising degree of complexity in the molecular mechanisms that recruit the protein complexes, repress expression, and maintain the epigenetic silent state of target genes. This review summarizes our current understanding of the mechanism of PcG-mediated gene silencing in animals and higher plants.

The control of histone lysine methylation in epigenetic regulation

Histone lysine methylation plays a fundamental role in chromatin organization and function. This epigenetic mark is involved in many biological processes such as heterochromatin formation, chromosome X inactivation, genomic imprinting and transcriptional regulation. Here, we review recent advances in how histone lysine methylation participates in these biological events, and the enzymes that control histone lysine methylation and demethylation.

Crucial role of MLL for the maintenance of memory T helper type 2 cell responses

The Mixed-Lineage Leukemia (MLL) gene, a mammalian homolog of the Drosophila trithorax, is implicated in regulating the maintenance of Hox gene expression and hematopoiesis. The physiological functions of MLL in the immune system remain largely unknown. Although MLL(+/-) CD4 T cells differentiate normally into antigen-specific effector Th1/Th2 cells in vitro, the ability of memory Th2 cells to produce Th2 cytokines was selectively reduced. Furthermore, histone modifications at the Th2 cytokine gene loci were not properly maintained in MLL(+/-) memory Th2 cells. The reduced expression of MLL in memory Th2 cells resulted in decreased GATA3 expression accompanied with impaired GATA3 locus histone modifications. The direct association of MLL with the GATA3 locus and the Th2 cytokine gene loci was demonstrated. Memory Th2 cell-dependent allergic airway inflammation was decreased in MLL(+/-) Th2 cell-transferred mice. Thus, a crucial role for MLL in the maintenance of memory Th2 cell function is indicated.

Distinct roles of Polycomb group gene products in transcriptionally repressed and active domains of Hoxb8

To address the molecular mechanisms underlying Polycomb group (PcG)-mediated repression of Hox gene expression, we have focused on the binding patterns of PcG gene products to the flanking regions of the Hoxb8 gene in expressing and non-expressing tissues. In parallel, we followed the distribution of histone marks of transcriptionally active H3 acetylated on lysine 9 (H3-K9) and methylated on lysine 4 (H3-K4), and of transcriptionally inactive chromatin trimethylated on lysine 27 (H3-K27). Chromatin immunoprecipitation revealed that the association of PcG proteins, and H3-K9 acetylation and H3-K27 trimethylation around Hoxb8 were distinct in tissues expressing and not expressing the gene. We show that developmental changes of these epigenetic marks temporally coincide with the misexpression of Hox genes in PcG mutants. Functional analyses, using mutant alleles impairing the PcG class 2 component Rnf2 or the Suz12 mutation decreasing H3-K27 trimethylation, revealed that interactions between class 1 and class 2 PcG complexes, mediated by trimethylated H3-K27, play decisive roles in the maintenance of Hox gene repression outside their expression domain. Within the expression domains, class 2 PcG complexes appeared to maintain the transcriptionally active status via profound regulation of H3-K9 acetylation. The present study indicates distinct roles for class 2 PcG complexes in transcriptionally repressed and active domains of Hoxb8 gene.

Gene expression profiling data in lymphoma and leukemia: review of the literature and extrapolation of pertinent clinical applications

CONTEXT

Gene expression (GE) analyses using microarrays have become an important part of biomedical and clinical research in hematolymphoid malignancies. However, the methods are time-consuming and costly for routine clinical practice.

OBJECTIVES

To review the literature regarding GE data that may provide important information regarding pathogenesis and that may be extrapolated for use in diagnosing and prognosticating lymphomas and leukemias; to present GE findings in Hodgkin and non-Hodgkin lymphomas, acute leukemias, and chronic myeloid leukemia in detail; and to summarize the practical clinical applications in tables that are referenced throughout the text.

DATA SOURCE

PubMed was searched for pertinent literature from 1993 to 2005.

CONCLUSIONS

Gene expression profiling of lymphomas and leukemias aids in the diagnosis and prognostication of these diseases. The extrapolation of these findings to more timely, efficient, and cost-effective methods, such as flow cytometry and immunohistochemistry, results in better diagnostic tools to manage the diseases. Flow cytometric and immunohistochemical applications of the information gained from GE profiling assist in the management of chronic lymphocytic leukemia, other low-grade B-cell non-Hodgkin lymphomas and leukemias, diffuse large B-cell lymphoma, nodular lymphocyte-predominant Hodgkin lymphoma, and classic Hodgkin lymphoma. For practical clinical use, GE profiling of precursor B acute lymphoblastic leukemia, precursor T acute lymphoblastic leukemia, and acute myeloid leukemia has supported most of the information that has been obtained by cytogenetic and molecular studies (except for the identification of FLT3 mutations for molecular analysis), but extrapolation of the analyses leaves much to be gained based on the GE profiling data.

Multiple epigenetic maintenance factors implicated by the loss of Mll2 in mouse development

Epigenesis is the process whereby the daughters of a dividing cell retain a chromatin state determined before cell division. The best-studied cases involve the inheritance of heterochromatic chromosomal domains, and little is known about specific gene regulation by epigenetic mechanisms. Recent evidence shows that epigenesis pivots on methylation of nucleosomes at histone 3 lysines 4, 9 or 27. Bioinformatics indicates that mammals have several enzymes for each of these methylations, including at least six histone 3 lysine 4 methyltransferases. To look for evidence of gene-specific epigenetic regulation in mammalian development, we examined one of these six, Mll2, using a multipurpose allele in the mouse to ascertain the loss-of-function phenotype. Loss of Mll2 slowed growth, increased apoptosis and retarded development, leading to embryonic failure before E11.5. Using chimera experiments, we demonstrated that Mll2 is cell-autonomously required. Evidence for gene-specific regulation was also observed. Although Mox1 and Hoxb1 expression patterns were correctly established, they were not maintained in the absence of Mll2, whereas Wnt1 and Otx2were. The Mll2 loss-of-function phenotype is different from that of its sister gene Mll, and they regulate different Hox complex genes during ES cell differentiation. Therefore, these two closely related epigenetic factors play different roles in development and maintain distinct gene expression patterns. This suggests that other epigenetic factors also regulate particular patterns and that development entails networks of epigenetic specificities.

Expression of Polycomb-group genes in human ovarian follicles, oocytes and preimplantation embryos

Mammalian oocytes possess unique properties with respect to their ability to regulate and reprogram chromatin structure and epigenetic information. Proteins containing the conserved chromodomain motif that is common to the Polycomb-group (Pc-G) proteins and the heterochromatin-associated protein HP1, play essential roles in these processes and more specifically, in X-chromosome inactivation in female zygotes and extra-embryonic tissues and in the regulation of genomic imprinting. To characterize the potential role of these proteins in the regulation of epigenetic events during early human development, we utilized a degenerate PCR priming assay to assess the expression of mRNAs of chromodomain proteins in cDNA samples derived from the human female germline and preimplantation embryos. Expression of mRNAs of HP1 genes was observed in ovarian follicles, (HP1 (HSalpha), HP1 (HSbeta), HP1 (HSgamma)), mature oocytes (HP1 (HSalpha), HP1 (HSbeta)), cleavage stage preimplantation embryos (HP1 (HSalpha), HP1 (HSbeta), HP1 (HSgamma)) and blastocysts (HP1 (HSalpha), HP1 (HSgamma)). Transcripts for three Pc-G genes, which are essential for early mammalian development (Yin Yang 1 (YY1), Enhancer of Zeste-2 (EZH2) and Embryonic Ectoderm Development (EED)) and that are essential for the regulation of X-inactivation and certain imprinted genes (EED) were revealed by gene-specific-PCR expression analysis of human ovarian follicles, oocytes and preimplantation embryos. YY1 and EZH2 transcripts were additionally detected in metaphase II oocytes.

Histone Ubiquitylation and the Regulation of Transcription

The small (76 amino acids) and highly conserved ubiquitin protein plays key roles in the physiology of eukaryotic cells. Protein ubiquitylation has emerged as one of the most important intracellular signaling mechanisms, and in 2004 the Nobel Prize was awarded to Aaron Ciechanower, Avram Hersko, and Irwin Rose for their pioneering studies of the enzymology of ubiquitin attachment. One of the most common features of protein ubiquitylation is the attachment of polyubiquitin chains (four or more ubiquitin moieties attached to each other), which is a widely used mechanism to target proteins for degradation via the 26S proteosome. However, it is noteworthy that the first ubiquitylated protein to be identified was histone H2A, to which a single ubiquitin moiety is most commonly attached. Following this discovery, other histones (H2B, H3, H1, H2A.Z, macroH2A), as well as many nonhistone proteins, have been found to be monoubiquitylated. The role of monoubiquitylation is still elusive because a single ubiquitin moiety is not sufficient to target proteins for turnover, and has been hypothesized to control the assembly or disassembly of multiprotein complexes by providing a protein-binding site. Indeed, a number of ubiquitin-binding domains have now been identified in both polyubiquitylated and monoubiquitylated proteins. Despite the early discovery of ubiquitylated histones, it has only been in the last five or so years that we have begun to understand how histone ubiquitylation is regulated and what roles it plays in the cell. This review will discuss current research on the factors that regulate the attachment and removal of ubiquitin from histones, describe the relationship of histone ubiquitylation to histone methylation, and focus on the roles of ubiquitylated histones in gene expression.

HOXA1 is required for E-cadherin-dependent anchorage-independent survival of human mammary carcinoma cells

Forced expression of HOXA1 is sufficient to stimulate oncogenic transformation of immortalized human mammary epithelial cells and subsequent tumor formation. We report here that the expression and transcriptional activity of HOXA1 are increased in mammary carcinoma cells at full confluence. This confluence-dependent expression of HOXA1 was abrogated by incubation of cells with EGTA to produce loss of intercellular contact and rescued by extracellular addition of Ca2+. Increased HOXA1 expression at full confluence was prevented by an E-cadherin function-blocking antibody and attachment of non-confluent cells to a substrate by homophilic ligation of E-cadherin increased HOXA1 expression. E-cadherin-directed signaling increased HOXA1 expression through Rac1. Increased HOXA1 expression consequent to E-cadherin-activated signaling decreased apoptotic cell death and was required for E-cadherin-dependent anchorage-independent proliferation of human mammary carcinoma cells. HOXA1 is therefore a downstream effector of E-cadherin-directed signaling required for anchorage-independent proliferation of mammary carcinoma cells.

In vivo transcriptional profile analysis reveals RNA splicing and chromatin remodeling as prominent processes for adult neurogenesis

Neural stem cells and neurogenesis persist in the adult mammalian brain subventricular zone (SVZ). Cells born in the rodent SVZ migrate to the olfactory bulb (Ob) where they differentiate into interneurons. To determine the gene expression and functional profile of SVZ neurogenesis, we performed three complementary sets of transcriptional analysis experiments using Affymetrix GeneChips: (1) comparison of adult mouse SVZ and Ob gene expression profiles with those of the striatum, cerebral cortex, and hippocampus; (2) profiling of SVZ stem cells and ependyma isolated by fluorescent-activated cell sorting (FACS); and (3) analysis of gene expression changes during in vivo SVZ regeneration after anti-mitotic treatment. Gene Ontology (GO) analysis of data from these three separate approaches showed that in adult SVZ neurogenesis, RNA splicing and chromatin remodeling are biological processes as statistically significant as cell proliferation, transcription, and neurogenesis. In non-neurogenic brain regions, RNA splicing and chromatin remodeling were not prominent processes. Fourteen mRNA splicing factors including Sf3b1, Sfrs2, Lsm4, and Khdrbs1/Sam68 were detected along with 9 chromatin remodeling genes including Mll, Bmi1, Smarcad1, Baf53a, and Hat1. We validated the transcriptional profile data with Northern blot analysis and in situ hybridization. The data greatly expand the catalogue of cell cycle components, transcription factors, and migration genes for adult SVZ neurogenesis and reveal RNA splicing and chromatin remodeling as prominent biological processes for these germinal cells.

MLL: how complex does it get?

The mixed lineage leukemia (MLL) gene encodes a very large nuclear protein homologous to Drosophila trithorax (trx). MLL is required for the proper maintenance of HOX gene expression during development and hematopoiesis. The exact regulatory mechanism of HOX gene expression by MLL is poorly understood, but it is believed that MLL functions at the level of chromatin organization. MLL was identified as a common target of chromosomal translocations associated with human acute leukemias. About 50 different MLL fusion partners have been isolated to date, and while similarities exist between groups of partners, there exists no unifying property shared by all the partners. MLL gene rearrangements are found in leukemias with both lymphoid and myeloid phenotypes and are often associated with infant and secondary leukemias. The immature phenotype of the leukemic blasts suggests an important role for MLL in the early stages of hematopoietic development. Mll homozygous mutant mice are embryonic lethal and exhibit deficiencies in yolk sac hematopoiesis. Recently, two different MLL-containing protein complexes have been isolated. These and other gain- and loss-of-function experiments have provided insight into normal MLL function and altered functions of MLL fusion proteins. This article reviews the progress made toward understanding the function of the wild-type MLL protein. While many advances in understanding this multifaceted protein have been made since its discovery, many challenging questions remain to be answered.

Roles of a trithorax group gene, MLL, in hematopoiesis

The mixed-lineage leukemia (MLL) gene is a trithorax group (trxG) gene that was originally identified at chromosomal translocations in patients developing acute leukemia. Although Polycomb group (PcG) genes, which counteract trxG genes, were found to play essential roles in hematopoiesis, little has been understood about the roles of trxG genes in hematopoiesis except for MLL. MLL has been found fused with 1 of more than 30 different partner genes to yield a diverse collection of MLL fusion oncoproteins that lead to the aberrant expression of HOX genes. Recent studies have revealed that MLL assembles, as do some trxG proteins, into a chromatin-modifying transcriptional regulatory supercomplex to regulate epigenetic pathways, including the methylation of histone H3 lysine 4, which is conferred by the Su (var)3-9, enhancer of zeste, and tritho-rax (SET) domain. Other studies also indicated that MLL plays a nonredundant and essential role in definitive hematopoiesis and induces the proliferation and differentiation of hematopoietic progenitors by maintaining appropriate up-regulation of HOX genes. Further progress in the field will provide novel insights into trxG- and PcG-mediated hematopoiesis and help us understand the epigenetic process by which developing stem cells coordinate proliferation and differentiation.

Polycomb homologs are involved in teratogenicity of valproic acid in mice

BACKGROUND

Valproic acid (VPA) is widely used to treat epilepsy and bipolar disorder and is also a potent teratogen, but its teratogenic mechanisms are unknown. We have attempted to describe a fundamental role of the Polycomb group (Pc-G) in VPA-induced transformations of the axial skeleton.

METHODS

Pregnant NMRI mice were given a single subcutaneous injection of vehicle or VPA (800 mg/kg) on gestation day (GD) 8. The expression of genes encoding Polycomb and trithorax groups was measured by quantitative real-time RT-PCR using total RNA isolated from the embryos exposed to vehicle or VPA for 1, 3, and 6 hr. In addition, the use of two less teratogenic antiepileptic chemicals valpromide (VPD) and valnoctamide (VCD) provide reliable evidence to support the relationship between VPA teratogenicity and the Polycomb group.

RESULTS

At a teratogenic level, VPA inhibits the expression of the Polycomb group genes, including Eed, Ezh2, Zfp144, Bmi1, Cbx2, Rnf2, and YY1 in the mouse embryos. In contrast, neither VPD nor VCD have significant effects on the expression of those genes affected by VPA. The trithorax group (trx-G) gene MLL, which is known to be required to maintain homeobox gene expression such as the Polycomb gene, is not affected by a teratogenic dose of VPA.

CONCLUSIONS

We propose that, during embryonic development, VPA may affect the gene silencing pathway mediated by the Polycomb group complex. The epigenetic mechanism of VPA teratogenicity on anteroposterior patterning is suspected.

Mammalian polycomb-mediated repression of Hox genes requires the essential spliceosomal protein Sf3b1

Polycomb group (PcG) proteins are responsible for the stable repression of homeotic (Hox) genes by forming multimeric protein complexes. We show (1) physical interaction between components of the U2 small nuclear ribonucleoprotein particle (U2 snRNP), including Sf3b1 and PcG proteins Zfp144 and Rnf2; and (2) that Sf3b1 heterozygous mice exhibit skeletal transformations concomitant with ectopic Hox expressions. These alterations are enhanced by Zfp144 mutation but repressed by Mll mutation (a trithorax-group gene). Importantly, the levels of Sf3b1 in PcG complexes were decreased in Sf3b1-heterozygous embryos. These findings suggest that Sf3b1-PcG protein interaction is essential for true PcG-mediated repression of Hox genes.

Epigenetic regulation of cellular memory by the Polycomb and Trithorax group proteins

During the development of multicellular organisms, cells become different from one another by changing their genetic program in response to transient stimuli. Long after the stimulus is gone, "cellular memory" mechanisms enable cells to remember their chosen fate over many cell divisions. The Polycomb and Trithorax groups of proteins, respectively, work to maintain repressed or active transcription states of developmentally important genes through many rounds of cell division. Here we review current ideas on the protein and DNA components of this transcriptional memory system and how they interact dynamically with each other to orchestrate cellular memory for several hundred genes.

HOXA genes are included in genetic and biologic networks defining human acute T-cell leukemia (T-ALL)

Using a combination of molecular cytogenetic and large-scale expression analysis in human T-cell acute lymphoblastic leukemias (T-ALLs), we identified and characterized a new recurrent chromosomal translocation, targeting the major homeobox gene cluster HOXA and the TCRB locus. Real-time quantitative polymerase chain reaction (RQ-PCR) analysis showed that the expression of the whole HOXA gene cluster was dramatically dysregulated in the HOXA-rearranged cases, and also in MLL and CALM-AF10-related T-ALL cases, strongly suggesting that HOXA genes are oncogenic in these leukemias. Inclusion of HOXA-translocated cases in a general molecular portrait of 92 T-ALLs based on large-scale expression analysis shows that this rearrangement defines a new homogeneous subgroup, which shares common biologic networks with the TLX1- and TLX3-related cases. Because T-ALLs derive from T-cell progenitors, expression profiles of the distinct T-ALL subgroups were analyzed with respect to those of normal human thymic subpopulations. Inappropriate use or perturbation of specific molecular networks involved in thymic differentiation was detected. Moreover, we found a significant association between T-ALL oncogenic subgroups and ectopic expression of a limited set of genes, including several developmental genes, namely HOXA, TLX1, TLX3, NKX3-1, SIX6, and TFAP2C. These data strongly support the view that the abnormal expression of developmental genes, including the prototypical homeobox genes HOXA, is critical in T-ALL oncogenesis.

The pathophysiology of HOX genes and their role in cancer

The HOM-C clustered prototype homeobox genes of Drosophila, and their counterparts, the HOX genes in humans, are highly conserved at the genomic level. These master regulators of development continue to be expressed throughout adulthood in various tissues and organs. The physiological and patho-physiological functions of this network of genes are being avidly pursued within the scientific community, but defined roles for them remain elusive. The order of expression of HOX genes within a cluster is co-ordinated during development, so that the 3' genes are expressed more anteriorly and earlier than the 5' genes. Mutations in HOXA13 and HOXD13 are associated with disorders of limb formation such as hand-foot-genital syndrome (HFGS), synpolydactyly (SPD), and brachydactyly. Haematopoietic progenitors express HOX genes in a pattern characteristic of the lineage and stage of differentiation of the cells. In leukaemia, dysregulated HOX gene expression can occur due to chromosomal translocations involving upstream regulators such as the MLL gene, or the fusion of a HOX gene to another gene such as the nucleoporin, NUP98. Recent investigations of HOX gene expression in leukaemia are providing important insights into disease classification and prediction of clinical outcome. Whereas the oncogenic potential of certain HOX genes in leukaemia has already been defined, their role in other neoplasms is currently being studied. Progress has been hampered by the experimental approach used in many studies in which the expression of small subsets of HOX genes was analysed, and complicated by the functional redundancy implicit in the HOX gene system. Attempts to elucidate the function of HOX genes in malignant transformation will be enhanced by a better understanding of their upstream regulators and downstream target genes.

When epigenetics kills: MLL fusion proteins in leukemia

Chromosomal aberrations that affect the MLL (Mixed Lineage Leukemia) gene at the locus 11q23 are associated with an aggressive subtype of leukemia. These alterations create MLL fusion derivatives with an active transforming potential. This review summarizes recent advances in our knowledge about normal and malignant MLL proteins with special emphasis on epigenetic processes affected by these molecules.

An Mll-dependent Hox program drives hematopoietic progenitor expansion

Chromosomal translocations disrupting the Mixed lineage leukemia (Mll) gene result in leukemia, with aberrant expression of some native Mll target genes (reviewed in). The Mll gene encodes a Trithorax-group chromatin regulator that is essential for the development of hematopoietic stem cells (HSCs) during embryogenesis. Like Trithorax, MLL positively regulates clustered homeodomain or Hox genes, yet the role of Hox genes collectively in the development of the mammalian hematopoietic system has been difficult to ascertain because of redundancy among Hox paralogs. Here, we show that in the absence of MLL, early hematopoietic progenitors develop despite reduced expression of HoxA, HoxB, and HoxC genes. However, these progenitors exhibit a marked reduction in their ability to generate hematopoietic colonies, a subsequent process requiring cell division and differentiation. Reactivation of a subset of Hox genes or, remarkably, reexpression of a single Hox gene in Mll-deficient progenitors rescued hematopoietic-colony frequency and growth. In contrast, expression of other MLL target genes such as Pitx2 or expression of anti-apoptotic BCL-2 failed to rescue hematopoietic-colony frequency. Furthermore, our results highlight a shared function of Hox proteins at this point in the development of the hematopoietic system.

Hoxc8 early enhancer of the Indonesian coelacanth, Latimeria menadoensis

Hoxc8 early enhancer controls the initiation and establishment phase of Hoxc8 expression in the mouse. Comparative studies indicate the presence of Hoxc8 early enhancer sequences in different vertebrate clades including mammals, birds and fish. Previous studies have shown differences between teleost and mammalian Hoxc8 early enhancers with respect to sequence and organization of protein binding elements. This raises the question of when the Hoxc8 early enhancer arose and how it has become modified in different vertebrate lineages. Here, we describe Hoxc8 early enhancer from the Indonesian coelacanth, Latimeria menadoensis. Coelacanths are the only extant lobefinned fish whose genome is tractable to genome analysis. The Latimeria Hoxc8 early enhancer sequence more closely resembles that of the mouse than that of Fugu or zebrafish. When assayed for enhancer activity by reporter gene analysis in transgenic mouse embryos, Latimeria Hoxc8 early enhancer directs expression to the posterior neural tube and mesoderm similar to that of the mouse enhancer. These observations support a close relationship between coelacanths and tetrapods and place the origin of a common Hoxc8 early enhancer sequence within the sarcopterygian lineage. The divergence of teleost (actinopterygii) Hoxc8 early enhancer may reflect a case of relaxed selection or other forms of instability induced by genome duplication events.

Comparative cis-regulatory analyses identify new elements of the mouse Hoxc8 early enhancer

The Hoxc8 early enhancer is a 200 bp region that controls the early phase of Hoxc8 expression during mouse embryonic development. This enhancer defines the domain of Hoxc8 expression in the neural tube and mesoderm of the posterior regions of the developing embryo. Five distinct cis-acting elements, A-E, were previously shown to govern early phase Hoxc8 expression. Significant divergence between mammalian and fish Hoxc8 early enhancer sequences and activities suggested additional cis-acting elements. Here we describe four additional cis-acting elements (F-I) within the 200 bp Hoxc8 early enhancer region identified by comparative regulatory analysis and transgene-mutation studies. These elements affect posterior neural tube and mesoderm expression of the reporter gene, either singly or in combination. Surprisingly, these new elements are missing from the zebrafish and Fugu Hoxc8 early enhancer sequences. Considering that fish enhancers direct robust reporter expression in transgenic mouse embryos, it is tempting to postulate that fish and mammalian Hoxc8 early enhancers utilize different sets of elements to direct Hoxc8 early expression. These observations reveal a remarkable plasticity in the Hoxc8 early enhancer, suggesting different modes of initiation and establishment of Hoxc8 expression in different species. We postulate that extensive restructuring and remodeling of Hox cis-regulatory regions occurring in different taxa lead to relatively different Hox expression patterns, which in turn may act as a driving force in generating diverse axial morphologies.

Malformations of the axial skeleton inMuseum Vrolik I: Homeotic transformations and numerical anomalies

The Museum Vrolik collection of anatomical specimens in Amsterdam, The Netherlands, comprises over 5,000 specimens of human and animal anatomy, embryology, pathology, and congenital anomalies. Recently, we rediagnosed a subset of the collection comprising dried human trunk skeletons and cranial base preparations presenting with homeotic transformations (vertebral phenotypic shifts) and numerical vertebral anomalies. We identified 11 trunk skeletons with either anterior or posterior homeotic transformations (AHT or PHT), 5 trunk skeletons with either less or more than the normal number of vertebrae, and well over a hundred cranial base preparations with either AHT (atlas-assimilation) or PHT (occipital vertebra). We found that, although homeotic transformations and numerical anomalies are distinct conditions, both can be described in terms of mismatch between homeotic patterning and morphological segmentation of the paraxial mesoderm. Therefore these two processes are perhaps not as tightly linked as they may seem on the basis of recent molecular studies. In homeotic transformations there is a constant mismatch between homeotic patterning and morphological segmentation throughout the affected region of the vertebral column. In numerical anomalies there is a variable mismatch between homeotic patterning and morphological segmentation, either because of stretching or squeezing of the homeotic pattern or because of oligo- or polysegmentation of the presomitic mesoderm (PSM). Homeotic transformations of the axial skeleton have an incidence of about 1%-5%, apart from their occurrence in malformation syndromes. Of the various etiological possibilities, explaining their frequent but mostly sporadic occurrence, maternal hyperthermia seems an attractive candidate.

The robotic mouse: unravelling the function of AF4 in the cerebellum

The devastating nature and lack of effective treatments associated with neurodegenerative diseases have stimulated a world-wide search for the elucidation of their molecular basis to which mouse models have made a major contribution. In combination with transgenic and knockout technologies, large-scale mouse mutagenesis is a powerful approach for the identification of new genes and associated signalling pathways controlling neuronal cell death and survival. Here we review the characterization of the robotic mouse, a novel model of autosomal dominant cerebellar ataxia isolated from an ENU-mutagenesis programme, which develops adult-onset region-specific Purkinje cell loss and cataracts, and displays defects in early T-cell maturation and general growth retardation. The mutated protein, Af4, is a member of the AF4/LAF4/FMR2 (ALF) family of putative transcription factors previously implicated in childhood leukaemia and FRAXE mental retardation. The mutation, which lies in a highly conserved region among the ALF family members, significantly reduces the binding affinity of Af4 to the E3 ubiquitin-ligase Siah-1a, isolated with Siah-2 as interacting proteins in the brain. This leads to a markedly slower turnover of mutant Af4 by the ubiquitin-proteasome pathway and consequently to its abnormal accumulation in the robotic mouse. Importantly, the conservation of the Siah-binding domain of Af4 in all other family members reveals that Siah-mediated proteasomal degradation is a common regulatory mechanism that controls the levels, and thereby the function, of the ALF family. The robotic mouse represents a unique model in which to study the newly revealed role of Af4 in the maintenance of vital functions of Purkinje cells in the cerebellum and further the understanding of its implication in lymphopoeisis.

Maintenance of gene expression patterns

In development, cells pass on established gene expression patterns to daughter cells over multiple rounds of cell division. The cellular memory of the gene expression state is termed maintenance, and the proteins required for this process are termed maintenance proteins. The best characterized are proteins of the Polycomb and trithorax Groups that are required for silencing and maintenance of activation of target loci, respectively. These proteins act through DNA elements termed maintenance elements. Here, we re-examine the genetics and molecular biology of maintenance proteins. We discuss molecular models for the maintenance of activation and silencing, and the establishment of epigenetic marks, and suggest that maintenance proteins may play a role in propagating the mark through DNA synthesis.

Self-renewal and solid tumor stem cells

Solid tumors arise in organs that contain stem cell populations. The tumors in these tissues consist of heterogeneous populations of cancer cells that differ markedly in their ability to proliferate and form new tumors. In both breast cancers and central nervous system tumors, cancer cells differ in their ability to form tumors. While the majority of the cancer cells have a limited ability to divide, a population of cancer stem cells that has the exclusive ability to extensively proliferate and form new tumors can be identified based on marker expression. Growing evidence suggests that pathways that regulate the self-renewal of normal stem cells are deregulated in cancer stem cells resulting in the continuous expansion of self-renewing cancer cells and tumor formation. This suggests that agents that target the defective self-renewal pathways in cancer cells might lead to improved outcomes in the treatment of these diseases.

Expression of leukemic MLL fusion proteins in Drosophila affects cell cycle control and chromosome morphology

The Mixed Lineage Leukemia (MLL) gene is involved in lymphoblastic and myeloid leukemia through chromosome translocations leading to fusion of MLL to partner genes, or through internal MLL rearrangements. MLL is the mammalian counterpart of the Drosophila trithorax (trx) gene, involved in maintaining active gene expression states. We have used transgenic Drosophila to assess the molecular targets and cellular processes affected by MLL and two of its leukemic fusion proteins. We find that whereas expression of normal human MLL in flies does not result in phenotypic alterations, overexpressing the human MLL-AF9 and MLL-AF4 proteins causes larval to pupal lethality, which interestingly resembles the phenotypes displayed by certain Drosophila trx mutant alleles. MLL-AF9 and MLL-AF4 transgenic flies exhibit antagonistic alterations in cell cycle progression. Additionally, flies expressing MLL-AF9 display impairment in higher order chromatin integrity, evidenced in decondensation of mitotic figures. The effects of MLL fusion proteins in Drosophila suggest that alteration of chromatin structure by MLL fusion proteins may contribute to the lethal phenotype. Our results indicate that the mode(s) of action of MLL-AF9 in Drosophila varies from that of MLL-AF4. Taken together, the expression of MLL fusion proteins in Drosophila provides a new and powerful system to reveal and characterize biological activities associated with MLL fusion proteins.

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.

Leukemia proto-oncoprotein MLL forms a SET1-like histone methyltransferase complex with menin to regulate Hox gene expression

MLL (for mixed-lineage leukemia) is a proto-oncogene that is mutated in a variety of human leukemias. Its product, a homolog of Drosophila melanogaster trithorax, displays intrinsic histone methyltransferase activity and functions genetically to maintain embryonic Hox gene expression. Here we report the biochemical purification of MLL and demonstrate that it associates with a cohort of proteins shared with the yeast and human SET1 histone methyltransferase complexes, including a homolog of Ash2, another Trx-G group protein. Two other members of the novel MLL complex identified here are host cell factor 1 (HCF-1), a transcriptional coregulator, and the related HCF-2, both of which specifically interact with a conserved binding motif in the MLL(N) (p300) subunit of MLL and provide a potential mechanism for regulating its antagonistic transcriptional properties. Menin, a product of the MEN1 tumor suppressor gene, is also a component of the 1-MDa MLL complex. Abrogation of menin expression phenocopies loss of MLL and reveals a critical role for menin in the maintenance of Hox gene expression. Oncogenic mutant forms of MLL retain an ability to interact with menin but not other identified complex components. These studies link the menin tumor suppressor protein with the MLL histone methyltransferase machinery, with implications for Hox gene expression in development and leukemia pathogenesis.

The biological and clinical significance of MLL abnormalities in haematological malignancies

The MLL (Mixed Lineage Leukaemia or Myeloid/Lymphoid Leukaemia) gene on chromosome 11q23 is frequently involved in chromosomal translocations associated with human acute leukaemias. These translocations lead to fusion genes generally resulting in novel chimeric proteins containing the amino terminus of MLL fused in-frame to one of about 30 distinct partner proteins. Abnormalities involving the MLL gene are observed in leukaemias of either lymphoid or myeloid lineage derivation, as well as in poorly differentiated or biphenotypic leukaemias. They are frequently seen in infant patients, and patients with therapy-related secondary AML following treatment with inhibitors of topoisomerase II (epipodophyllotoxins). In the majority of cases, abnormalities involving the MLL gene are associated with a very poor prognostic outcome. In this review, we will discuss some of the recent advances in MLL research resulting from biological as well as clinical studies.

Hox expression in AML identifies a distinct subset of patients with intermediate cytogenetics

We previously reported that favorable and poor prognostic chromosomal rearrangements in acute myeloid leukemia (AML) were associated with distinct levels of HOX expression. We have now analyzed HOX expression in 50 independent adult AML patients (median age=62 years), together with FLT3 and FLT3-ligand mRNA levels, and FLT3 mutation determination. By cluster analysis, we could divide AMLs into cases with low, intermediate and high HOX expression. Cases with high expression were uniquely restricted to a subset of AMLs with intermediate cytogenetics (P=0.0174). This subset has significantly higher levels of FLT3 expression and appears to have an increase of FLT3 mutations (44%), while CEBPalpha mutations were infrequent (6%). FLT3 mRNA levels were correlated with the expression of multiple HOX genes, whereas FLT3 mutations were correlated with HOXB3. In some cases, FLT3 was expressed at levels equivalent to GAPDH in the absence of genomic amplification. We propose that high HOX expression may be characteristically associated with a distinct biologic subset of AML. The apparent global upregulation of HOX expression could be due to growth-factor signaling or, alternatively, these patterns may reflect a particular stage of differentiation of the leukemic cells.

The histone methyltransferases Trithorax and Ash1 prevent transcriptional silencing by Polycomb group proteins

Transcriptional on and off states of HOX genes and other developmental control genes are maintained by antagonistic regulators encoded by trithorax group (trxG) and Polycomb group (PcG) genes. The trxG proteins Ash1 and hTRX and the PcG repressor E(z) are histone methyltransferases (HMTases) that methylate distinct lysine residues in the N-terminal tail of histone H3. trxG proteins are generally thought to function as activators of HOX genes, but how histone methylation by Ash1 and Trx promotes HOX gene transcription is not clear. Here, we show that in ash1 and trx mutants expression of HOX genes is lost within their normal expression domains, but we find that, contrary to expectation, this expression is restored in ash1 and trx mutants that also lack PcG gene function. Moreover, such trxG PcG double mutants show severe misexpression of HOX genes and, hence, ectopic activation of HOX genes caused by the removal of PcG gene function also occurs in the absence of ash1 and trx function. Together, these results suggest that the Ash1 and Trx HMTases are not "coactivators" required for transcriptional activation of HOX genes, but function specifically as anti-repressors. We propose that histone methylation by Ash1 and Trx is required continuously throughout development to prevent inappropriate PcG silencing of HOX genes in cells in which they must stay transcriptionally active.

Definitive Hematopoiesis Requires the Mixed-Lineage Leukemia Gene

The Mixed-Lineage Leukemia (MLL) gene encodes a Trithorax-related chromatin-modifying protooncogene that positively regulates Hox genes. In addition to their well-characterized roles in axial patterning, Trithorax and Polycomb family proteins perform less-understood functions in vertebrate hematopoiesis. To define the role of MLL in the development of the hematopoietic system, we examined the potential of cells lacking MLL. Mll-deficient cells could not develop into lymphocytes in adult RAG-2 chimeric animals. Similarly, in vitro differentiation of B cells required MLL. In chimeric embryos, Mll-deficient cells failed to contribute to fetal liver hematopoietic stem cell/progenitor populations. Moreover, we show that aorta-gonad-mesonephros (AGM) cells from Mll-deficient embryos lacked hematopoietic stem cell (HSC) activity despite their ability to generate hematopoietic progeny in vitro. These results demonstrate an intrinsic requirement for MLL in definitive hematopoiesis, where it is essential for the generation of HSCs in the embryo.

Hoxa9 and Meis1 are key targets for MLL-ENL-mediated cellular immortalization

MLL fusion proteins are oncogenic transcription factors that are associated with aggressive lymphoid and myeloid leukemias. We constructed an inducible MLL fusion, MLL-ENL-ERtm, that rendered the transcriptional and transforming properties of MLL-ENL strictly dependent on the presence of 4-hydroxy-tamoxifen. MLL-ENL-ERtm-immortalized hematopoietic cells required 4-hydroxy-tamoxifen for continuous growth and differentiated terminally upon tamoxifen withdrawal. Microarray analysis performed on these conditionally transformed cells revealed Hoxa9 and Hoxa7 as well as the Hox coregulators Meis1 and Pbx3 among the targets upregulated by MLL-ENL-ERtm. Overexpression of the Hox repressor Bmi-1 inhibited the growth-transforming activity of MLL-ENL. Moreover, the enforced expression of Hoxa9 in combination with Meis1 was sufficient to substitute for MLL-ENL-ERtm function and to maintain a state of continuous proliferation and differentiation arrest. These results suggest that MLL fusion proteins impose a reversible block on myeloid differentiation through aberrant activation of a limited set of homeobox genes and Hox coregulators that are consistently expressed in MLL-associated leukemias.

Taspase1: a threonine aspartase required for cleavage of MLL and proper HOX gene expression

The Mixed-Lineage Leukemia gene (MLL/HRX/ALL1) encodes a large nuclear protein homologous to Drosophila trithorax that is required for the maintenance of HOX gene expression. MLL is cleaved at two conserved sites generating N320 and C180 fragments, which heterodimerize to stabilize the complex and confer its subnuclear destination. Here, we purify and clone the protease responsible for cleaving MLL. We entitle it Taspase1 as it initiates a class of endopeptidases that utilize an N-terminal threonine as the active site nucleophile to proteolyze polypeptide substrates following aspartate. Taspase1 proenzyme is intramolecularly proteolyzed generating an active 28 kDa alpha/22 kDa beta heterodimer. RNAi-mediated knockdown of Taspase1 results in the appearance of unprocessed MLL and the loss of proper HOX gene expression. Taspase1 coevolved with MLL/trithorax as Arthropoda and Chordata emerged from Metazoa suggesting that Taspase1 originated to regulate complex segmental body plans in higher organisms.

The anthropoid postcranial axial skeleton: Comments on development, variation, and evolution

Within-species phenotypic variation is the raw material on which natural selection acts to shape evolutionary change, and understanding more about the developmental genetics of intraspecific as well as interspecific phenotypic variation is an important component of the Evo-Devo agenda. The axial skeleton is a useful system to analyze from such a perspective. Its development is increasingly well understood, and between-species differences in functionally important developmental parameters are well documented. I present data on intraspecific variation in the axial postcranial skeleton of some Primates, including hominoids (apes and humans). Hominoid species are particularly valuable, because counts of total numbers of vertebrae, and hence original somite numbers, are available for large samples. Evolutionary changes in the axial skeleton of various primate lineages, including bipedal humans, are reviewed, and hypotheses presented to explain the changes in terms of developmental genetics. Further relevant experiments on model organisms are suggested in order to explore more fully the differences in developmental processes between primate species, and hence to test these hypotheses.

Divergence of Hoxc8 early enhancer parallels diverged axial morphologies between mammals and fishes

There is considerable interest in understanding how cis-regulatory modifications drive morphological changes across species. Because developmental regulatory genes, including Hox genes, are remarkably conserved, their noncoding regulatory regions are likely sources for variations. Modifications of Hox cis-regulatory elements have potential to alter Hox gene expression and, hence, axial morphologies. In vertebrates, differences in the axial levels of Hox gene expression correlate with differences in the number and relative position of thoracic vertebrae. Variation in cis-regulatory elements of Hox genes can be identified by comparative sequence and reporter gene analyses in transgenic mouse embryos. Using these approaches, we show a remarkable divergence of the Hoxc8 early enhancers between mammals and fishes representing diverse axial morphologies. Extensive restructuring of the Hoxc8 early enhancer including nucleotide substitutions, inversion, and divergence result in distinct patterns of reporter gene expression along the embryonic axis. Our results provide an evolutionary perspective on how the enhancer elements are engineered and support the hypothesis that remodeling of Hox regulatory elements in different species has played a significant role in generating morphological diversity.

MLL fusion partners AF4 and AF9 interact at subnuclear foci

The MLL gene is involved in translocations associated with both acute lymphoblastic and acute myelogenous leukemia. These translocations fuse MLL with one of over 30 partner genes. Collectively, the MLL partner genes do not share a common structural motif or biochemical function. We have identified a protein interaction between the two most common MLL fusion partners AF4 and AF9. This interaction is restricted to discrete nuclear foci we have named 'AF4 bodies'. The AF4 body is non-nucleolar and is not coincident with any known nuclear structures we have examined. The AF4-AF9 interaction is maintained by the MLL-AF4 fusion protein, and expression of the MLL-AF4 fusion can alter the subnuclear localization of AF9. In view of other research indicating that other MLL fusion partners also interact with one another, these results suggest that MLL fusion partners may participate in a web of protein interactions with a common functional goal. The disruption of this web of interactions by fusion with MLL may be important to leukemogenesis.

B-cell development in the presence of the MLL/AF4 oncoprotein proceeds in the absence of HOX A7 and HOX A9 expression

Infant acute lymphoblastic leukemia (ALL) is frequently characterized by the t(4;11)(q21;q23) cytogenetic abnormality encoding the MLL/AF4 oncogene, increased HOX gene expression and a pro-B/monocytoid phenotype. We have previously established a novel MLL/AF4-positive cell line, B-lineage 3 (BLIN-3), which retains several features of normal B-lineage development (functional Ig gene rearrangement and apoptotic sensitivity to stromal cell withdrawal) not generally observed in infant ALL. We now use microarray analysis to identify patterns of gene expression in BLIN-3 that may modulate MLL/AF4 oncogenesis and contribute to the retention of normal B-lineage developmental characteristics. Comparison of 6815 expressed genes in BLIN-3 with published microarray data on leukemic blasts from t(4;11) patients indicated that BLIN-3 was unique in lacking the expression of certain HOX-A cluster genes. These results were validated by RT-PCR showing no expression of HOX A7 or HOX A9 in BLIN-3. A HOX C8 promoter reporter was active in BLIN-3, indicating that lack of HOX gene expression in BLIN-3 was not due to a nonfunctional MLL/AF4. Our results suggest that B-lineage development can proceed in t(4;11) leukemic blasts in the absence of HOX-A gene expression.

Transformation of myeloid progenitors by MLL oncoproteins is dependent on Hoxa7 and Hoxa9

Transcriptional deregulation through the production of dominant-acting chimeric transcription factors derived from chromosomal translocations is a common theme in the pathogenesis of acute leukemias; however, the essential target genes for acute leukemogenesis are unknown. We demonstrate here that primary myeloid progenitors immortalized by various MLL oncoproteins exhibit a characteristic Hoxa gene cluster expression profile, which reflects that preferentially expressed in the myeloid clonogenic progenitor fraction of normal bone marrow. Continued maintenance of this MLL-dependent Hoxa gene expression profile is associated with conditional MLL-associated myeloid immortalization. Moreover, Hoxa7 and Hoxa9 were specifically required for efficient in vitro myeloid immortalization by an MLL fusion protein but not other leukemogenic fusion proteins. Finally, in a bone marrow transduction/transplantation model, Hoxa9 is essential for MLL-dependent leukemogenesis in vivo, a primary requirement detected at the earliest stages of disease initiation. Thus, a genetic reliance on Hoxa7 and Hoxa9 in MLL-mediated transformation demonstrates a gain-of-function mechanism for MLL oncoproteins as upstream constitutive activators that promote myeloid transformation via a Hox-dependent mechanism.

DLX genes as targets of ALL-1: DLX 2,3,4 down-regulation in t(4;11) acute lymphoblastic leukemias

Dlx genes constitute a gene family thought to be essential in morphogenesis and development. We show here that in vertebrate cells, Dlx genes appear to be part of a regulatory cascade initiated by acute lymphoblastic leukemia (ALL)-1, a master regulator gene whose disruption is implicated in several human acute leukemias. The expression of Dlx2, Dlx3, Dlx5, Dlx6, and Dlx7 was absent in All-1 -/- mouse embryonic stem cells and reduced in All-1 +/- cells. In leukemic patients affected by the t(4;11)(q21;q23) chromosomal abnormality, the expression of DLX2, DLX3, and DLX4 was virtually abrogated. Our data indicate that Dlx genes are downstream targets of ALL-1 and could be considered as important tools for the study of the early leukemic cell phenotype.

MLL repression domain interacts with histone deacetylases, the polycomb group proteins HPC2 and BMI-1, and the corepressor C-terminal-binding protein

The MLL (mixed-lineage leukemia) gene is involved in many chromosomal translocations associated with acute myeloid and lymphoid leukemia. We previously identified a transcriptional repression domain in MLL, which contains a region with homology to DNA methyltransferase. In chromosomal translocations, the MLL repression domain is retained in the leukemogenic fusion protein and is required for transforming activity of MLL fusion proteins. We explored the mechanism of action of the MLL repression domain. Histone deacetylase 1 interacts with the MLL repression domain, partially mediating its activity; binding of Cyp33 to the adjacent MLL-PHD domain potentiates this binding. Because the MLL repression domain activity was only partially relieved with the histone deacetylase inhibitor trichostatin A, we explored other protein interactions with this domain. Polycomb group proteins HPC2 and BMI-1 and the corepressor C-terminal-binding protein also bind the MLL repression domain. Expression of exogenous BMI-1 potentiates MLL repression domain activity. Functional antagonism between Mll and Bmi-1 has been shown genetically in murine knockout models for Mll and Bmi-1. Our new data suggest a model whereby recruitment of BMI-1 to the MLL protein may be able to modulate its function. Furthermore, repression mediated by histone deacetylases and that mediated by polycomb group proteins may act either independently or together for MLL function in vivo.

Polycomb group genes as epigenetic regulators of normal and leukemic hemopoiesis

Epigenetic modification of chromatin structure underlies the differentiation of pluripotent hemopoietic stem cells (HSCs) into their committed/differentiated progeny. Compelling evidence indicates that Polycomb group (PcG) genes play a key role in normal and leukemic hemopoiesis through epigenetic regulation of HSC self-renewal/proliferation and commitment. The PcG proteins are constituents of evolutionary highly conserved molecular pathways regulating cell fate in several other tissues through diverse mechanisms, including 1) regulation of self-renewal/proliferation, 2) regulation of senescence/immortalization, 3) interaction with the initiation transcription machinery, 4) interaction with chromatin-condensation proteins, 5) modification of histones, 6) inactivation of paternal X chromosome, and 7) regulation of cell death. It is therefore not surprising that PcG genes lead to pleiotropic phenotypes when mutated and have been associated with malignancies in several systems in both mice and humans. Although much remains to be learned regarding the PcG mechanism(s) of action, advances in identifying the functional domains and enzymatic activities of these multimeric protein complexes have provided insights into how PcG proteins accomplish such processes. Some of the new insights into a role for the PcG cellular memory system in regulating normal and leukemic hemopoiesis are reviewed here, with special emphasis on their potential involvement in epigenetic regulation of gene expression through modification of chromatin structure.

Polycomb group regulation of Hox gene expression in C. elegans

Polycomb group (PcG) chromatin proteins regulate homeotic genes in both animals and plants. In Drosophila and vertebrates, PcG proteins form complexes and maintain early patterns of Hox gene repression, ensuring fidelity of developmental patterning. PcG proteins in C. elegans form a complex and mediate transcriptional silencing in the germline, but no role for the C. elegans PcG homologs in somatic Hox gene regulation has been demonstrated. Surprisingly, we find that the PcG homologs MES-2 [E(Z)] and MES-6 (ESC), along with MES-3, a protein without known homologs, do repress Hox expression in C. elegans. mes mutations cause anteroposterior transformations and disrupt Hox-dependent neuroblast migration. Thus, as in Drosophila, vertebrates, and plants, C. elegans PcG proteins regulate key developmental patterning genes to establish positional identity.

Hox in hair growth and development

The evolutionarily conserved Hox gene family of transcriptional regulators has originally been known for specifying positional identities along the longitudinal body axis of bilateral metazoans, including mouse and man. It is believed that subsequent to this archaic role, subsets of Hox genes have been co-opted for patterning functions in phylogenetically more recent structures, such as limbs and epithelial appendages. Among these, the hair follicle is of particular interest, as it is the only organ undergoing cyclical phases of regression and regeneration during the entire life span of an organism. Furthermore, the hair follicle is increasingly capturing the attention of developmental geneticists, as this abundantly available miniature organ mimics key aspects of embryonic patterning and, in addition, presents a model for studying organ renewal. The first Hox gene shown to play a universal role in hair follicle development is Hoxc13, as both Hoxc13-deficient and overexpressing mice exhibit severe hair growth and patterning defects. Differential gene expression analyses in the skin of these mutants, as well as in vitro DNA binding studies performed with potential targets for HOXC13 transcriptional regulation in human hair, identified genes encoding hair-specific keratins and keratin-associated proteins (KAPs) as major groups of presumptive Hoxc13 downstream effectors in the control of hair growth. The Hoxc13 mutant might thus serve as a paradigm for studying hair-specific roles of Hoxc13 and other members of this gene family, whose distinct spatio-temporally restricted expression patterns during hair development and cycling suggest discrete functions in follicular patterning and hair cycle control. The main conclusion from a discussion of these potential roles vis-à-vis current expression data in mouse and man, and from the perspective of the results obtained with the Hoxc13 transgenic models, is that members of the Hox family are likely to fulfill essential roles of great functional diversity in hair that require complex transcriptional control mechanisms to ensure proper spatio-temporal patterns of Hox gene expression at homeostatic levels.

The oncoprotein MLL-ENL disturbs hematopoietic lineage determination and transforms a biphenotypic lymphoid/myeloid cell

Mixed-lineage leukemia (MLL) fusion proteins are associated with a unique class of leukemia that is characterized by the simultaneous expression of lymphoid-specific as well as myeloid-specific genes. Here we report the first experimental model of MLL. Murine bone marrow cells were retrovirally transduced to express the MLL-eleven nineteen leukemia (MLL-ENL) fusion protein. When cultivated in flt-3 ligand, stem cell factor and interleukin-7 (IL-7) in a stroma-free culture system MLL-ENL-transduced as well as control cells showed a wave of B-lymphopoiesis. Whereas the controls exhausted their proliferative capacity in a CD19+/B220+ state, a continuously proliferating CD19-/B220+ cell population emerged in the MLL-ENL-transduced cultures. Despite the lymphoid surface marker, these cells were of monocytoid morphology. The immortalized cells contained unrearranged retrovirus, expressed MLL-ENL mRNA and were able to grow in syngenic recipients. From the diseased animals an MLL-ENL positive, B220+/CD19- cell type could be reisolated and cultivated in vitro. In analogy to human MLL, MLL-ENL-transformed cells not only coexpressed lymphocyte-specific (rag1, rag2, pax5, Tdt) and monocyte-specific genes (lysozyme, c-fms), but also showed rearrangements of the genomic immunoglobulin locus. This model shows that MLL-ENL influences events of early lineage determination and it will enable the investigation of the underlying molecular processes.

A t(11;15) fuses MLL to two different genes, AF15q14 and a novel gene MPFYVE on chromosome 15

The mixed lineage leukemia gene (MLL, also known as HRX, ALL-1 and Htrx) located at 11q23 is involved in translocations with over 40 different chromosomal bands in a variety of leukemia subtypes. Here we report our analysis of a rare but recurring translocation, t(11;15)(q23;q14). This translocation has been described in a small subset of cases with both acute myeloblastic leukemia and ALL. Recent studies have shown that MLL is fused to AF15q14 in the t(11;15). Here we analyse a sample from another patient with this translocation and confirm the presence of an MLL-AF15q14 fusion. However, we have also identified and cloned another fusion transcript from the same patient sample. In this fusion transcript, MLL is fused to a novel gene, MLL partner containing FYVE domain (MPFYVE). Both MLL-AF15q14 and MLL-MPFYVE are in-frame fusion transcripts with the potential to code for novel fusion proteins. MPFYVE is also located on chromosome 15, approximately 170 kb telomeric to AF15q14. MPFYVE contains a highly conserved motif, the FYVE domain which, in other proteins, has been shown to bind to phosphotidyl-inositol-3 phosphate (PtdIns(3)P). The MLL-MPFYVE fusion may be functionally important in the leukemia process in at least some patients containing this translocation.

Proteolytic cleavage of MLL generates a complex of N- and C-terminal fragments that confers protein stability and subnuclear localization

The mixed-lineage leukemia gene (MLL, ALL1, HRX) encodes a 3,969-amino-acid nuclear protein homologous to Drosophila trithorax and is required to maintain proper Hox gene expression. Chromosome translocations in human leukemia disrupt MLL (11q23), generating chimeric proteins between the N terminus of MLL and multiple translocation partners. Here we report that MLL is normally cleaved at two conserved sites (D/GADD and D/GVDD) and that mutation of these sites abolishes the proteolysis. MLL cleavage generates N-terminal p320 (N320) and C-terminal p180 (C180) fragments, which form a stable complex that localizes to a subnuclear compartment. The FYRN domain of N320 directly interacts with the FYRC and SET domains of C180. Disrupting the interaction between N320 and C180 leads to a marked decrease in the level of N320 and a redistribution of C180 to a diffuse nuclear pattern. These data suggest a model in which a dynamic post-cleavage association confers stability to N320 and correct nuclear sublocalization of the complex, to control the availability of N320 for target genes. This predicts that MLL fusion proteins of leukemia which would lose the ability to complex with C180 have their stability conferred instead by the fusion partners, thus providing one mechanism for altered target gene expression.

Leukemia proto-oncoprotein MLL is proteolytically processed into 2 fragments with opposite transcriptional properties

MLL (mixed lineage leukemia; also ALL-1 or HRX) is a proto-oncogene that is mutated in a variety of acute leukemias. Its product is normally required for the maintenance of Hox gene expression during embryogenesis and hematopoiesis through molecular mechanisms that remain poorly defined. Here we demonstrate that MLL (mixed lineage leukemia) is proteolytically processed into 2 fragments (MLL(N) and MLL(C)) that display opposite transcriptional properties and form an intramolecular MLL complex in vivo. Proteolytic cleavage occurs at 2 amino acids (D2666 and D2718) within a consensus processing sequence (QXD/GZDD, where X is a hydrophobic amino acid and Z is an alanine or a valine) that is conserved in TRX, the Drosophila homolog of MLL, and in the MLL-related protein MLL2, suggesting that processing is important for MLL function. Processed MLL(N) and MLL(C) associate with each other via N-terminal (1253-2254 amino acids) and C-terminal (3602-3742 amino acids) intramolecular interaction domains. MLL processing occurs rapidly within a few hours after translation and is followed by the phosphorylation of MLL(C). MLL(N) displays transcriptional repression activity, whereas MLL(C) has strong transcriptional activation properties. Leukemia-associated MLL fusion proteins lack the MLL processing sites, do not undergo cleavage, and are unable to interact with MLL(C). These observations suggest that posttranslational modifications of MLL may participate in regulating its activity as a transcription factor and that this aspect of its function is perturbed by leukemogenic fusions.

Cooperativity in transcription factor binding to the coactivator CREB-binding protein (CBP). The mixed lineage leukemia protein (MLL) activation domain binds to an allosteric site on the KIX domain

The interactions between cAMP-response element-binding protein (CREB)-binding protein (CBP) and gene-specific transcription factors play an important role in activation of transcription from numerous genes. Cooperative interactions between CBP and multiple transcriptional activators may provide a mechanism for synergistic increases in transcriptional activation. Here we report the characterization of ternary complexes formed by the KIX domain of CBP and the transactivation domain of the trithorax group protein mixed lineage leukemia protein (MLL), together with either the phosphorylated kinase-inducible domain (pKID) of CREB or the activation domain from c-Myb. Isothermal titration calorimetry experiments show that KIX in complex with the MLL activation domain binds the c-Myb activation domain and pKID domain with 2-fold higher affinity than does the KIX domain alone. Thus, the activation domains of Myb and MLL or of pKID and MLL bind cooperatively to KIX. The thermodynamics of these interactions imply different mechanisms of binding cooperativity for the two ternary complexes; the KIX.MLL.pKID complex is stabilized by entropy increases, whereas the enhancement of Myb binding in the presence of the MLL activation domain is due to more favorable enthalpy. NMR experiments show that the MLL-binding site on KIX is distinct from the surface that binds the pKID and c-Myb activation domains. Our data indicate that KIX can directly mediate cooperative interactions between pairs of transcriptional regulatory proteins. In the case of MLL and c-Myb, both proteins are involved in proliferation of hematopoietic cells and leukemogenesis, and synergistic interactions mediated by CBP may play a functional role.