Chromatin-related properties of CBP fused to MLL generate a myelodysplastic-like syndrome that evolves into myeloid leukemia

Conditional MLL-CBP targets GMP and models therapy-related myeloproliferative disease

Chromosomal translocations that fuse the mixed lineage leukemia (MLL) gene with multiple partners typify acute leukemias of infancy as well as therapy-related leukemias. We utilized a conditional knockin strategy to bypass the embryonic lethality caused by MLL-CBP expression and to assess the immediate effects of induced MLL-CBP expression on hematopoiesis. Within days of activating MLL-CBP, the fusion protein selectively expanded granulocyte/macrophage progenitors (GMP) and enhanced their self-renewal/proliferation. MLL-CBP altered the gene expression program of GMP, upregulating a subset of genes including Hox a9. Inhibition of Hox a9 expression by RNA interference demonstrated that MLL-CBP required Hox a9 for its enhanced cell expansion. Following exposure to sublethal gamma-irradiation or N-ethyl-N-nitrosourea (ENU), MLL-CBP mice developed myelomonocytic hyperplasia and progressed to fatal myeloproliferative disorders. These represented the spectrum of therapy-induced acute myelomonocytic leukemia/chronic myelomonocytic leukemia/myelodysplastic/myeloproliferative disorder similar to that seen in humans possessing the t(11;16). This model of MLL-CBP therapy-related myeloproliferative disease demonstrates the selectivity of this MLL fusion for GMP cells and its ability to initiate leukemogenesis in conjunction with cooperating mutations.

Binding to nonmethylated CpG DNA is essential for target recognition, transactivation, and myeloid transformation by an MLL oncoprotein

The MLL gene is a frequent target for leukemia-associated chromosomal translocations that generate dominant-acting chimeric oncoproteins. These invariably contain the amino-terminal 1,400 residues of MLL fused with one of a variety of over 30 distinct nuclear or cytoplasmic partner proteins. Despite the consistent inclusion of the MLL amino-terminal region in leukemia oncoproteins, little is known regarding its molecular contributions to MLL-dependent oncogenesis. Using high-resolution mutagenesis, we identified three MLL domains that are essential for in vitro myeloid transformation via mechanisms that do not compromise subnuclear localization. These include the CXXC/Basic domain and two novel domains of unknown function. Point mutations in the CXXC domain that eliminate myeloid transformation by an MLL fusion protein also abolished recognition and binding of nonmethylated CpG DNA sites in vitro and transactivation in vivo. Our results define a critical role for the CXXC DNA binding domain in MLL-associated oncogenesis, most likely via epigenetic recognition of CpG DNA sites within the regulatory elements of target genes.

New insight into the molecular mechanisms of MLL-associated leukemia

Rearrangements of the MLL gene (ALL1, HRX, and Hrtx) located at chromosome band 11q23 are commonly involved in adult and pediatric cases of primary acute leukemias and also found in cases of therapy-related secondary leukemias. Studies on mouse models of MLL translocation and cell lines containing MLL rearrangements showed that the MLL gene linked chromosomal rearrangements to cellular differentiation and tumor tropism. Moreover, recent structural/functional studies on MLL and aberrant MLL proteins provided new clues and suggested that different mechanisms might be included in leukemogenesis by MLL rearrangements. The connection between these different mechanisms will help us understand globally how aberrant MLL oncogenes affect the normal cellular processes at molecular level.

Characterization of genomic breakpoints in MLL and CBP in leukemia patients with t(11;16)

The recurring chromosome translocation t(11;16)(q23;p13) is detected in leukemia patients, virtually all of whom have received previous chemotherapy with topoisomerase (topo) II inhibitors. In the t(11;16), 3' CBP, on 16p13, is fused to 5' MLL, on 11q23, resulting in an MLL-CBP fusion gene that plays an important role in leukemogenesis. In this study, we cloned genomic breakpoints of the MLL and CBP genes in the t(11;16) in the SN-1 cell line and in five patients with therapy-related leukemia, all of whom had received topo II inhibitors for previous tumors. In all patients except one, both the genomic MLL-CBP and the reciprocal fusions were cloned. Genomic breakpoints in MLL occurred in the 8.3-kb breakpoint cluster region in all patients, whereas the breakpoints in CBP clustered in an 8.2-kb region of intron 3 in four patients. Genomic breakpoints in MLL occurred in intron 11 near the topo II cleavage site in the SN-1 cell line and in one patient, and they were close to LINE repetitive sequences in two other patients. In the remaining two patients, genomic breakpoints were in intron 9 in Alu repeats. Genomic breakpoints in CBP occurred in and around Alu repeats in one and two patients, respectively. In two patients, the breaks were near LINE repetitive sequences, suggesting that repetitive DNA sequences may play a role. No specific recombination motifs were identified at or near the breakpoint junctions. No topo II cleavage sites were detected in introns 2 and 3 of CBP. However, there were deletions and duplications at the breakpoints in both MLL and CBP and microhomologies or nontemplated nucleotides at most of the genomic fusion junctions, suggesting that a nonhomologous end-joining repair mechanism was involved in the t(11;16).

Mechanisms controlling pathogenesis and survival of leukemic stem cells

Stem cells are an integral component of normal mammalian physiology and have been intensively studied in many systems. Intriguingly, substantial evidence indicates that stem cells also play an important role in the initiation and pathogenesis of at least some cancers. In particular, myeloid leukemias have been extensively characterized with regard to stem and progenitor cell involvement. Thus, as a focal point for both scientific and therapeutic endeavors, leukemic stem cells (LSC) represent a critical area of investigation. LSC appear to retain many characteristics of normal hematopoietic stem cells (HSC) as evidenced by a hierarchical developmental pattern, a mostly quiescent cell cycle profile, and an immunophenotype very similar to HSC. Consequently, defining unique properties of LSC remains a high priority in order to elucidate the molecular mechanisms driving stem cell transformation, and for developing therapeutic strategies that specifically target the LSC population. In this review, we discuss emerging concepts in the field and describe how various molecular and cellular characteristics of leukemia cells might be exploited as a means to preferentially ablate malignant stem cells.

MLL: a histone methyltransferase disrupted in leukemia

Rearrangements of the MLL gene, which is located at chromosome 11q23, are associated with aggressive acute leukemias in both children and adults. MLL regulates Hox gene expression through direct promoter binding and histone modification. MLL rearrangements occurring in leukemia include MLL fusion genes, partial tandem duplications of MLL and MLL amplification. MLL fusions and amplification upregulate Hox expression, apparently resulting in a block of hematopoietic differentiation. Future therapies for MLL-associated leukemia might involve blocking Hox gene upregulation by using fusion proteins or inhibiting the activity of Hox proteins themselves.

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.

Molecular requirements for gene expression mediated by targeted histone acetyltransferases

Histone acetyltransferases (HATs) play fundamental roles in regulating gene expression. HAT complexes with distinct subunit composition and substrate specificity act on chromatin-embedded genes with different promoter architecture and chromosomal locations. Because requirements for HAT complexes vary, a central question in transcriptional regulation is how different HAT complexes function in different chromosomal contexts. Here, we have tested the ability of targeted yeast HATs to regulate gene expression of an epigenetically silenced locus. Of a panel of HAT fusion proteins targeted to a telomeric reporter gene, Sas3p and Gcn5p selectively increased expression of the silenced gene. Reporter gene expression was not solely dependent on acetyltransferase activity of the targeted HAT. Further analysis of Gcn5p-mediated gene expression revealed collateral requirements for HAT complex subunits Spt8p and Spt3p, which interact with TATA-binding protein, and for a gene-specific transcription factor. These data demonstrate plasticity of gene expression mediated by HATs upon encountering novel promoter architecture and chromatin context. The telomeric location of the reporter gene used in these studies also provides insight into the molecular requirements for heterochromatin boundary formation and for overcoming transcriptional silencing.

BRD7, a novel bromodomain gene, inhibits G1-S progression by transcriptionally regulating some important molecules involved in ras/MEK/ERK and Rb/E2F pathways

Bromodomain is a 110 amino acid domain. It is evolutionally conserved and is found in proteins strongly implicated in signal-dependent transcriptional regulation. BRD7 is a novel bromodomain gene and it is downexpressed in nasopharyngeal carcinoma (NPC) biopsies and cell lines; its function is poorly understood. In the present study, tet-on inducible expression system was used to investigate the role of BRD7 in cell growth and cell cycle progression. We found that ectopic expression of BRD7 in NPC cells inhibited cell growth and cell cycle progression from G1 to S. We further performed cell cycle cDNA array to screen potential transcriptional targets of BRD7 in cell cycle. Thirteen important signaling molecules, mainly implicated in ras/MEK/ERK and Rb/E2F pathways, were differentially expressed by induction of BRD7. Moreover, we observed that BRD7 could regulate the promoter activity of E2F3, one of its targets. Taken together, the present study indicated that BRD7 inhibited G1-S progression by transcriptionally regulating some important molecules involved in ras/MEK/ERK and Rb/E2F pathways and suggested that BRD7 may present a promising candidate of NPC trade mark associated tumor suppressor gene.

The small oligomerization domain of gephyrin converts MLL to an oncogene

The MLL (mixed lineage leukemia) gene forms chimeric fusions with a diverse set of partner genes as a consequence of chromosome translocations in leukemia. In several fusion partners, a transcriptional activation domain appears to be essential for conferring leukemogenic capacity on MLL protein. Other fusion partners, however, lack such domains. Here we show that gephyrin (GPHN), a neuronal receptor assembly protein and rare fusion partner of MLL in leukemia, has the capacity as an MLL-GPHN chimera to transform hematopoietic progenitors, despite lack of transcriptional activity. A small 15–amino acid tubulin-binding domain of GPHN is necessary and sufficient for this activity in vitro and in vivo. This domain also confers oligomerization capacity on MLL protein, suggesting that such activity may contribute critically to leukemogenesis. The transduction of MLL-GPHN into hematopoietic progenitor cells caused myeloid and lymphoid lineage leukemias in mice, suggesting that MLL-GPHN can target multipotent progenitor cells. Our results, and other recent data, provide a mechanism for oncogenic conversion of MLL by fusion partners encoding cytoplasmic proteins.

The role of the MLL gene in infant leukemia

The MLL gene is a major player in leukemia, particularly in infant leukemia and in secondary, therapy-related acute leukemia. The normal MLL gene plays a key role in developmental regulation of gene expression (including HOX genes), and in leukemia this function is subverted by breakage, recombination, and chimeric fusion with one of 40 or more alternative partner genes. In infant leukemias, the chromosome translocations involving MLL arise during fetal hematopoiesis, possibly in a primitive lymphomyeloid stem cell. In general, these leukemias have a very poor prognosis. The malignancy of these leukemias is all the more dramatic considering their very short preclinical natural history or latency. These data raise fundamental issues of how such divergent MLL chimeric genes transform cells, why they so rapidly evolve to a malignant status, and what alternative or novel therapeutic strategies might be considered. We review here progress in tackling these questions.

Functional contribution of EEN to leukemogenic transformation by MLL-EEN fusion protein

The EEN (extra eleven nineteen) gene was originally cloned from a case of acute myeloid leukemia M5 subtype with translocation t (11; 19)(q23; p13), in which EEN was fused with MLL. To explore the involvement of EEN in leukemogenesis caused by MLL-EEN, we studied the transformation potential of the MLL-EEN fusion protein. MLL-EEN had oncogenic features, while, as a control, MLLDelta, the truncated form of MLL lacking the EEN moiety, did not show any oncogenic potential. MLL-EEN exerted a dominant-negative effect over wild-type EEN in terms of subcellular localization. Normally, EEN was found in the cytoplasm, but the MLL-EEN fusion protein was located in the nucleus, and EEN could be delocalized by MLL-EEN. This interaction is via a coiled-coil dimerization domain of EEN, which is reserved in the fusion protein. In addition, MLL-EEN might act as a potential transcriptional factor with the MLL part providing the DNA-binding domain and the EEN part providing the transcription activation domain, though EEN seems to have no direct role in transcriptional regulation. As an aberrant transcriptional factor, MLL-EEN could transactivate the promoter of HoxA7, a potential target gene of MLL.

Structural mechanism of the bromodomain of the coactivator CBP in p53 transcriptional activation

Lysine acetylation of the tumor suppressor protein p53 in response to a wide variety of cellular stress signals is required for its activation as a transcription factor that regulates cell cycle arrest, senescence, or apoptosis. Here, we report that the conserved bromo-domain of the transcriptional coactivator CBP (CREB binding protein) binds specifically to p53 at the C-terminal acetylated lysine 382. This bromodomain/acetyl-lysine binding is responsible for p53 acetylation-dependent coactivator recruitment after DNA damage, a step essential for p53-induced transcriptional activation of the cyclin-dependent kinase inhibitor p21 in G1 cell cycle arrest. We further present the three-dimensional nuclear magnetic resonance structure of the CBP bromodomain in complex with a lysine 382-acetylated p53 peptide. Using structural and biochemical analyses, we define the molecular determinants for the specificity of this molecular recognition.

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.

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.

E mu-BRD2 transgenic mice develop B-cell lymphoma and leukemia

Transgenic mice with lymphoid-restricted overexpression of the double bromodomain protein bromodomain-containing 2 (Brd2) develop splenic B-cell lymphoma and, upon transplantation, B-cell leukemia with leukemic infiltrates in liver and lung. Brd2 is a nuclear-localized transcription factor kinase that is most closely related to TATA box binding protein-associated factor, 250 kDa (TAF(II)250) and the Drosophila developmental protein female sterile homeotic. Constitutive expression of BRD2 in the lymphoid compartment increases cyclin A transcription, "priming" transgenic B cells for proliferation. Mice stochastically develop an aggressive B-cell lymphoma with the features of B-1 cells, including CD5 and surface IgM expression. The B-cell lymphoma is monoclonal for immunoglobulin gene rearrangement and is phenotypically stable. The lymphoblasts are very large and express a transcriptome that is similar to human non-Hodgkin lymphomas. Both a wild-type BRD2 transgene and a kinase-null point mutant drive lymphomagenesis; therefore we propose that, rather than kinase activity, Brd2-mediated recruitment of E2 promoter binding factors (E2Fs) and a specific histone acetyltransferase to the cyclin A promoter by both types of transgene is a mechanistic basis for neoplasia. This report is the first to describe a transgenic mouse model for constitutive expression of a protein with more than one bromodomain.

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.

FLT3 and MLL intragenic abnormalities in AML reflect a common category of genotoxic stress

MLL rearrangements in acute myeloid leukemia (AML) include translocations and intragenic abnormalities such as internal duplication and breakage induced by topoisomerase II inhibitors. In adult AML, FLT3 internal tandem duplications (ITDs) are more common in cases with MLL intragenic abnormalities (33%) than those with MLL translocation (8%). Mutation/deletion involving FLT3 D835 are found in more than 20% of cases with MLL intragenic abnormalities compared with 10% of AML with MLL translocation and 5% of adult AML with normal MLL status. Real-time quantification of FLT3 in 141 cases of AML showed that all cases with FLT3 D835 express high level transcripts, whereas FLT3-ITD AML can be divided into cases with high-level FLT3 expression, which belong essentially to the monocytic lineage, and those with relatively low-level expression, which predominantly demonstrate PML-RARA and DEK-CAN. FLT3 abnormalities in CBF leukemias with AML1-ETO or CBFbeta-MYH11 were virtually restricted to cases with variant CBFbeta-MYH11 fusion transcripts and/or atypical morphology. These data suggest that the FLT3 and MLL loci demonstrate similar susceptibility to agents that modify chromatin configuration, including topoisomerase II inhibitors and abnormalities involving PML and DEK, with consequent errors in DNA repair. Variant CBFbeta-MYH11 fusions and bcr3 PML-RARA may also be initiated by similar mechanisms.

Dimerization of MLL fusion proteins immortalizes hematopoietic cells

MLL fusion proteins are leukemogenic, but their mechanism is unclear. Induced dimerization of a truncated MLL immortalizes bone marrow and imposes a reversible block on myeloid differentiation associated with upregulation of Hox a7, a9, and Meis1. Both dimerized MLL and exon-duplicated MLL are potent transcriptional activators, suggesting a link between dimerization and partial tandem duplication of DNA binding domains of MLL. Dimerized MLL binds with higher affinity than undimerized MLL to a CpG island within the Hox a9 locus. However, MLL-AF9 is not dimerized in vivo. The data support a model in which either MLL dimerization/exon duplication or fusion to a transcriptional activator results in Hox gene upregulation and ultimately transformation.

Dimerization contributes to oncogenic activation of MLL chimeras in acute leukemias

MLL is a histone methyltransferase that can be converted into an oncoprotein by acquisition of transcriptional effector domains following heterologous protein fusions with a variety of nuclear transcription factors, cofactors, or chromatin remodeling proteins in acute leukemias. Here we demonstrate an alternative mechanism for activation of MLL following fusions with proteins (AF1p/Eps15 and GAS7) that normally reside in the cytoplasm. The coiled-coil oligomerization domains of these proteins are necessary and sufficient for leukemogenic transformation induced by the respective MLL fusion proteins. Furthermore, homodimerization of MLL by synthetic dimerization modules mimics bona fide MLL fusion proteins resulting in Hox gene activation and enhanced self-renewal of hematopoietic progenitors. Our studies support an oligomerization-dependent mechanism for oncogenic conversion of MLL, presumably in part by recruitment of accessory factors through the dimerized MLL moiety of the chimeric protein.

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.

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.

MOZ-TIF2-induced acute myeloid leukemia requires the MOZ nucleosome binding motif and TIF2-mediated recruitment of CBP

The MOZ-TIF2 fusion is associated with acute myeloid leukemia (AML) with inv(8)(p11q13). MOZ is a MYST family histone acetyltransferase (HAT), whereas TIF2 is a nuclear receptor coactivator that associates with CREB binding protein (CBP). Here we demonstrate that MOZ-TIF2 has transforming properties in vitro and causes AML in a murine bone marrow transplant assay. The C2HC nucleosome recognition motif of MOZ is essential for transformation, whereas MOZ HAT activity is dispensable. However, MOZ-TIF2 interaction with CBP through the TIF2 CBP interaction domain (CID) is essential for transformation. These results indicate that nucleosomal targeting by MOZ and recruitment of CBP by TIF2 are critical requirements for MOZ-TIF2 transformation and indicate that MOZ gain of function contributes to leukemogenesis.

Transcriptional activation is a key function encoded by MLL fusion partners

Chromosomal translocations that fuse the mixed lineage leukemia gene (MLL) to a variety of unrelated partner genes are frequent in pediatric leukemias. The novel combination of genetic material leads to the production of active oncoproteins that depend on the contributions of both constituents. In a search for a common function amongst the diverse group of MLL fusion partners we constructed artificial fusions joining MLL with generic transactivator and repressor domains (acidic blob, GAL4 transactivator domain, Herpes simplex VP16 activation domain, KRAB repressor domain). Of all constructs tested, only MLL-VP16 was able to transform primary bone marrow cells and to induce a block of early myeloid differentiation like an authentic MLL fusion. Interestingly, the transformation capability of the artificial MLL fusions was correlated with the transcriptional potential of the resulting chimeric protein but it was not related to the strength of the isolated transactivation domain that was joined to MLL. These results prove for the first time that a general biological function - transactivation - might be the common denominator of many MLL fusion partners.

MLL-GAS7 transforms multipotent hematopoietic progenitors and induces mixed lineage leukemias in mice

A specific association with mixed lineage leukemias suggests that MLL oncoproteins may selectively target early multipotent hematopoietic progenitors or stem cells. We demonstrate here that a representative MLL fusion protein, MLL-GAS7, impairs the differentiation and enhances the in vitro growth of murine hematopoietic cells with multipotent features. The multilineage differentiation potential of these cells was suggested by their immuno-phenotypes and transcriptional programs and confirmed by their ability to induce three pathologically distinct leukemias in mice, including an acute biphenotypic leukemia (ABL) that recapitulates the distinctive hallmark features of many MLL-associated leukemias in humans. This experimental modeling of ABL in mice highlights its origin from multipotential progenitors that arrest at a bipotential stage specifically targeted or induced by MLL oncogenes.

Common mechanism for oncogenic activation of MLL by forkhead family proteins

The mixed lineage leukemia (MLL) gene undergoes fusions with a diverse set of genes as a consequence of chromosomal translocations in acute leukemias. Two of these partner genes code for members of the forkhead subfamily of transcription factors designated FKHRL1 and AFX. We demonstrate here that MLL-FKHRL1 enhances the self-renewal of murine myeloid progenitors in vitro and induces acute myeloid leukemias in syngeneic mice. The long latency (mean = 157 days), reduced penetrance, and hematologic features of the leukemias were very similar to those observed for the forkhead fusion protein MLL-AFX and contrasted with the more aggressive features of leukemias induced by MLL-AF10. Transformation mediated by MLL-forkhead fusion proteins required 2 conserved transcriptional effector domains (CR2 and CR3), each of which alone was not sufficient to activate MLL. A synthetic fusion of MLL with FKHR, a third mammalian forkhead family member that contains both effector domains, was also capable of transforming hematopoietic progenitors in vitro. A comparable requirement for 2 distinct transcriptional effector domains was also displayed by VP16, which required its proximal minimal transactivation domain (MTD/H1) and distal H2 domain to activate the oncogenic potential of MLL. The functional importance of CR2 was further demonstrated by its ability to substitute for H2 of VP16 in domain-swapping experiments to confer oncogenic activity on MLL. Our results, based on bona fide transcription factors as partners for MLL, unequivocally establish a transcriptional effector mechanism to activate its oncogenic potential and further support a role for fusion partners in determining pathologic features of the leukemia phenotype.

The human LASP1 gene is fused to MLL in an acute myeloid leukemia with t(11;17)(q23;q21)

The MLL gene at chromosome 11q23 is frequently rearranged in acute leukemia. Here we report the identification of a new MLL fusion partner in the case of an infant with AML-M4 and a t(11;17)(q23;q21) translocation. Fluorescence in situ hybridization (FISH) and RT-PCR analyses indicated a rearrangement of the MLL gene, but no fusion with previously identified MLL fusion partners at 17q, such as AF17 or MSF. Rapid amplification of cDNA ends (RACE) revealed an in-frame fusion of MLL to LASP1, a gene that is amplified and overexpressed in breast cancer. Retroviral transduction of myeloid progenitors demonstrated that MLL/LASP1 is the fourth known fusion of MLL with a cytoplasmic protein that has no in vitro transformation capability, thus establishing a potential subgroup among the MLL fusion proteins.

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.

The promiscuous MLL gene links chromosomal translocations to cellular differentiation and tumour tropism

MLL is a promiscuous gene involved in a diversity of chromosomal fusions in haematological malignancies, usually resulting from chromosomal translocations. MLL-associated chromosomal rearrangements usually occur in tumours of specific haematological lineages, suggesting a crucial role for the MLL fusion partner in determining disease phenotype (or tumour tropism). The MLL gene is homologous to Drosophila trithorax, and is likewise involved in embryo pattern formation. Common themes linking several of the MLL partners include a possible involvement in embryo patterning via Hox gene regulation and chromatin remodelling. These findings reinforce the link between developmental regulation and chromosomal translocations, and indicate the role of chromosomal translocation in activating genes capable of determining tumour phenotype in leukaemias and sarcomas.

MLL-AFX requires the transcriptional effector domains of AFX to transform myeloid progenitors and transdominantly interfere with forkhead protein function

MLL-AFX is a fusion gene created by t(X;11) chromosomal translocations in a subset of acute leukemias of either myeloid or lymphoid derivation. It codes for a chimeric protein consisting of MLL fused to AFX, a forkhead transcription factor that normally regulates genes involved in apoptosis and cell cycle progression. We demonstrate here that forced expression of MLL-AFX enhances the self-renewal of hematopoietic progenitors in vitro and induces acute myeloid leukemias after long latencies in syngeneic recipient mice. MLL-AFX interacts with the transcriptional coactivator CBP, which is also a fusion partner for MLL in human leukemias. A potent minimal transactivation domain (CR3) at the C terminus of AFX mediates interactions with the KIX domain of CBP and is necessary for transformation of myeloid progenitors by MLL-AFX. However, CR3 alone is not sufficient, suggesting that simple acquisition of a transactivation domain per se does not activate the oncogenic potential of MLL. Rather, two conserved transcriptional effector domains (CR2 and CR3) of AFX are required for full oncogenicity of MLL-AFX and also endow it with the potential to competitively interfere with transcription and apoptosis mediated by wild-type forkhead proteins. Furthermore, a dominant-negative mutant of AFX containing CR2 and CR3 enhances the growth of myeloid progenitors in vitro, although considerably less effectively than does MLL-AFX. Taken together, these data suggest that recruitment of transcriptional cofactors utilized by forkhead proteins is a critical requirement for oncogenic action of MLL-AFX, which may impact both MLL- and forkhead-dependent transcriptional pathways.

MLL5, a homolog of Drosophila trithorax located within a segment of chromosome band 7q22 implicated in myeloid leukemia

Proteins encoded by Polycomb and Trithorax-group (Pc-G and Trx-G) genes regulate developmental fates by maintaining or repressing HOX gene expression, respectively. In a search for candidate myeloid leukemia tumor suppressor genes from a approximately 2.5 Mb commonly-deleted segment within chromosome band 7q22, we identified a novel human Trithorax (Trx) family member named MLL5. Trx-G genes encode proteins that modulate transcriptional programs through protein-protein interactions that are mediated by PHD and SET domains, and by binding to DNA via A-T hooks and methyltransferase homology motifs. MLL5 is a homolog of the Drosophila gene CG9007; it encodes a 6.5 kb mRNA that is expressed widely. MLL5 includes a SET domain and a single PHD finger, but lacks A-T hooks and methyltransferase homology domains that are found in MLL. The leukemia cell line RCV-ACV-A carries a heterozygous missense mutation within the PHD domain; however, no mutations within the MLL5 coding region were detected in primary leukemias. MLL5 is a novel mammalian Trx-G gene that might modulate transcription by protein association.

MLL-SEPTIN6 fusion recurs in novel translocation of chromosomes 3, X, and 11 in infant acute myelomonocytic leukaemia and in t(X;11) in infant acute myeloid leukaemia, and MLL genomic breakpoint in complex MLL-SEPTIN6 rearrangement is a DNA topoisomerase II cleavage site

We examined the MLL translocation in two cases of infant AML with X chromosome disruption. The G-banded karyotype in the first case suggested t(X;3)(q22;p21)ins(X;11)(q22;q13q25). Southern blot analysis showed one MLL rearrangement. Panhandle PCR approaches were used to identify the MLL fusion transcript and MLL genomic breakpoint junction. SEPTIN6 from chromosome band Xq24 was the partner gene of MLL. MLL exon 7 was joined in-frame to SEPTIN6 exon 2 in the fusion transcript. The MLL genomic breakpoint was in intron 7; the SEPTIN6 genomic breakpoint was in intron 1. Spectral karyotyping revealed a complex rearrangement disrupting band 11q23. FISH with a probe for MLL confirmed MLL involvement and showed that the MLL-SEPTIN6 junction was on the der(X). The MLL genomic breakpoint was a functional DNA topoisomerase II cleavage site in an in vitro assay. In the second case, the karyotype revealed t(X;11)(q22;q23). Southern blot analysis showed two MLL rearrangements. cDNA panhandle PCR detected a transcript fusing MLL exon 8 in-frame to SEPTIN6 exon 2. MLL and SEPTIN6 are vulnerable to damage to form recurrent translocations in infant AML. Identification of SEPTIN6 and the SEPTIN family members hCDCrel and MSF as partner genes of MLL suggests a common pathway to leukaemogenesis.

Fusion of MLL and MSF in adult de novo acute myelomonocytic leukemia (M4) with t(11;17)(q23;q25)

The MLL gene at chromosome band 11q23 is frequently rearranged and fused to partner genes in acute leukemias. Previously, the MSF gene, also called AF17q25, has been cloned as a fusion partner of the MLL gene in therapy-related or infant acute myelogenous leukemias with t(11;17)(q23;q25). MSF belongs to the septin family of proteins, which includes other MLL fusion partners, hCDCrel1 and Septin 6, and has also been implicated in the pathogenesis of human ovarian tumor and murine T-cell lymphoma. We describe here a 64-year-old man with de novo acute myelomonocytic leukemia (French-American-British subtype M4) with t(11;17)(q23;q25). His leukemia was successfully induced into a first remission, which, however, lasted only briefly. A second remission was never attained, and the patient died of sepsis 16 months after the diagnosis of leukemia. Examination of his leukemic cells at diagnosis revealed an MLL gene rearrangement, by Southern blotting, and an MLL-MSF fusion transcript, by the reverse transcriptase polymerase chain reaction (RT-PCR) method. Sequence analysis of the RT-PCR product further revealed that MLL exon 5 was fused in-frame to MSF exon 3. Further clinical and molecular analyses of acute leukemias with the MLL-MSF transcript may shed more light on the clinical characteristics and molecular mechanisms of the MLL-septin type leukemias.

A murine model of CML blast crisis induced by cooperation between BCR/ABL and NUP98/HOXA9

Constitutive activation of tyrosine kinases, such as the BCR/ABL fusion associated with t(9;22)(q34;q22), is a hallmark of chronic myeloid leukemia (CML) syndromes in humans. Expression of BCR/ABL is both necessary and sufficient to cause a chronic myeloproliferative syndrome in murine bone marrow transplantation models, and absolutely depends on kinase activity. Progression of CML to acute leukemia (blast crisis) in humans has been associated with acquisition of secondary chromosomal translocations, including the t(7;11)(p15;p15) resulting in the NUP98/HOXA9 fusion protein. We demonstrate that BCR/ABL cooperates with NUP98/HOXA9 to cause blast crisis in a murine model. The phenotype depends both on expression of BCR/ABL and NUP98/HOXA9, but tumors retain sensitivity to the ABL inhibitor STI571 in vitro and in vivo. This paradigm is applicable to other constitutively activated tyrosine kinases such as TEL/PDGFbetaR. These experiments document cooperative effects between constitutively activated tyrosine kinases, which confer proliferative and survival properties to hematopoietic cells, with mutations that impair differentiation, such as the NUP98/HOXA9, giving rise to the acute myeloid leukemia (AML) phenotype. Furthermore, these data indicate that despite acquisition of additional mutations, CML blast crisis cells retain their dependence on BCR/ABL for proliferation and survival.

Molecular, cytogenetic and genetic abnormalities in MDS and secondary AML

Myelodysplasia (MDS) is a clonal disease, which increases with age, suggesting that multiple steps are required for the evolution of the condition. Approximately 30% of MDS evolve into acute myelogenous leukemia (AML). In this review, we intend to delineate the genetic events, which may drive this sequence and therefore we will focus primarily on cytogenetic abnormalities where the genes have been identified and oncogenes and suppressor genes that have been implicated. In terms of the biological mechanisms, which characterise this process, it is generally thought that the MDS cell has impaired differentiation, and has increased apoptosis. As the disease progresses in addition, the cells have increased proliferation. As the disease evolves, the population of cells, which predominate remain immature, have decreased apoptosis and in many cases, upregulate anti-apoptotic genes and have deregulated proliferation as the number of blast cells increase. Etiological factors, which contribute to the development of leukemia, include therapeutic agents administered for a primary malignancy. The cytogenetic abnormalities, predisposition factors and genes involved in secondary leukemia will also be reviewed.

The AF10 leucine zipper is required for leukemic transformation of myeloid progenitors by MLL-AF10

The t(10;11)(p12;q23) chromosomal translocation in human acute myeloid leukemia results in the fusion of the MLL and AF10 genes. The latter codes for a novel leucine zipper protein, one of many MLL fusion partners of unknown function. In this report, we demonstrate that retroviral-mediated transduction of an MLL-AF10 complementary DNA into primary murine myeloid progenitors enhanced their clonogenic potential in serial replating assays and led to their efficient immortalization at a primitive stage of myeloid differentiation. Furthermore, MLL-AF10-transduced cells rapidly induced acute myeloid leukemia in syngeneic or severe combined immunodeficiency recipient mice. Structure/function analysis showed that a highly conserved 82-amino acid portion of AF10, comprising 2 adjacent alpha-helical domains, was sufficient for immortalizing activity when fused to MLL. Neither helical domain alone mediated immortalization, and deletion of the 29-amino acid leucine zipper within this region completely abrogated transforming activity. Similarly, the minimal oncogenic domain of AF10 exhibited transcriptional activation properties when fused to the MLL or GAL4 DNA-binding domains, while neither helical domain alone did. However, transcriptional activation per se was not sufficient because a second activation domain of AF10 was neither required nor competent for transformation. The requirement for alpha-helical transcriptional effector domains is similar to the oncogenic contributions of unrelated MLL partners ENL and ELL, suggesting a general mechanism of myeloid leukemogenesis by a subset of MLL fusion proteins, possibly through specific recruitment of the transcriptional machinery.

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.

Unique molecular and cellular features of acute myelogenous leukemia stem cells

It is well known in the field of acute myelogenous leukemia (AML) that many different translocations and genetic aberrancies are found with the various forms of the disease. Indeed, specific translocations are often associated with disease subtypes that manifest themselves through the accumulation of immature myeloid cells at varying stages of differentiation. Moreover, the differentiation state of myeloid blast populations has been utilized as a means of categorizing different AML subtypes (French, American, British, or FAB classification system). Thus, the notion that AML is a family of related but distinct diseases is a common view. Interestingly, however, studies in recent years that have formalized the concept of a leukemic stem cell (LSC) have also begun to define shared developmental, cellular and molecular features amongst the malignant stem cells that give rise to different AML subtypes. Moreover, some of these conserved features appear to be unique to the leukemia stem/progenitor cell population, and are not found in normal hematopoietic stem cells (HSCs). This article will summarize data emerging from the study of LSCs and suggest how distinct molecular and cellular characteristics of the LSC population may provide new opportunities for AML therapy.

Genetic lesions and perturbation of chromatin architecture: a road to cell transformation

Differential gene expression is a rigorously precise procedure that defines the developmental program of cells, tissues, organs, and of the entire organism. The correct execution of this program requires the participation of multiple and complex groups of regulators. In addition to transcription factors, which are key tools in ontogenesis by providing sequential switch of different genes, the structure of the chromatin is a dominant determinant leading to gene expression. Through the novel and insightful work of several investigators, it appears that the architecture of the chromatin spanning the genes can and does influence the efficiency of RNA transcription, and therefore of gene expression. Several new enzymatic complexes have been identified that reversibly modify the chromatin architecture by methylation, phosphorylation, and acetylation of the nucleosomal core proteins. These enzymes are crucial for the proper balance and maintenance of gene expression, and are often the target of mutations and alterations in human cancer. Here, we review briefly the current models proposing how some of these enzymes normally modify the chromatin structure and how their functional disruption leads to inappropriate gene expression and cell transformation.

The MT domain of the proto-oncoprotein MLL binds to CpG-containing DNA and discriminates against methylation

Alterations of the proto-oncogene MLL (mixed lineage leukemia) are characteristic for a high proportion of acute leukemias, especially those occurring in infants. The activation of MLL is achieved either by an internal tandem duplication of 5' MLL exons or by chromosomal translocations that create chimeric proteins with the N-terminus of MLL fused to a variety of different partner proteins. A domain of MLL with significant homology to the eukaryotic DNA methyltransferases (MT domain) has been found to be essential for the transforming potential of the oncogenic MLL derivatives. Here we demonstrate that this domain specifically recognizes DNA with unmethylated CpG sequences. In gel mobility shifts, the presence of CpG was sufficient for binding of recombinant GST-MT protein to DNA. The introduction of 5-methylCpG on one or both DNA strands precluded an efficient interaction. In surface plasmon resonance a KD of approximately 3.3 x 10(-8) M was determined for the GST-MT/DNA complex formation. Site selection experiments and DNase I footprinting confirmed CpG as the target of the MT domain. Finally, this interaction was corroborated in vivo in reporter assays utilizing the DNA-binding properties of the MT domain in a hybrid MT-VP16 transactivator construct.

Molecular mechanisms of leukemogenesis mediated by MLL fusion proteins

The MLL (Mixed Lineage Leukemia) gene is a common target for chromosomal translocations associated with human acute leukemias. These translocations result in a gain of MLL function by generating novel chimeric proteins containing the amino-terminus of MLL fused in-frame with one of 30 distinct partner proteins. Structure/function studies using an in vitro myeloid progenitor immortalization assay have revealed that at least four nuclear partner proteins contribute transcriptional effector properties to MLL to produce a range of chimeric transcription factors with leukemogenic potential. Mouse models suggest that expression of an MLL fusion protein is necessary but not sufficient for leukemogenesis. Interestingly, whilst all MLL fusion proteins tested so far phenocopy each other with respect to in vitro immortalization, the latency period required for the onset of acute leukemia in vivo is variable and partner protein dependent. We discuss potential mechanisms that may account for the ability of distinct MLL fusion proteins to promote short or long latency leukemogenesis.

Common themes in the pathogenesis of acute myeloid leukemia

The pathogenesis of acute myeloid leukemia is associated with the appearance of oncogenic fusion proteins generated as a consequence of specific chromosome translocations. Of the two components of each fusion protein, one is generally a transcription factor, whereas the other partner is more variable in function, but often involved in the control of cell survival and apoptosis. As a consequence, AML-associated fusion proteins function as aberrant transcriptional regulators that interfere with the process of myeloid differentiation, determine a stage-specific arrest of maturation and enhance cell survival in a cell-type specific manner. The abnormal regulation of transcriptional networks occurs through common mechanisms that include recruitment of aberrant co-repressor complexes, alterations in chromatin remodeling, and disruption of specific subnuclear compartments. The identification and analysis of common and specific target genes regulated by AML fusion proteins will be of fundamental importance for the full understanding of acute myeloid leukemogenesis and for the implementation of disease-specific drug design.

The elongation domain of ELL is dispensable but its ELL-associated factor 1 interaction domain is essential for MLL-ELL-induced leukemogenesis

The MLL-ELL chimeric gene is the product of the (11;19)(q23p13.1) translocation associated with de novo and therapy-related acute myeloid leukemias (AML). ELL is an RNA polymerase II elongation factor that interacts with the recently identified EAF1 (ELL associated factor 1) protein. EAF1 contains a limited region of homology with the transcriptional activation domains of three other genes fused to MLL in leukemias, AF4, LAF4, and AF5q31. Using an in vitro transformation assay of retrovirally transduced myeloid progenitors, we conducted a structure-function analysis of MLL-ELL. Whereas the elongation domain of ELL was dispensable, the EAF1 interaction domain of ELL was critical to the immortalizing properties of MLL-ELL in vitro. To confirm these results in vivo, we transplanted mice with bone marrow transduced with MLL fused to the minimal EAF1 interaction domain of ELL. These mice all developed AML, with a longer latency than mice transplanted with the wild-type MLL-ELL fusion. Based on these results, we generated a heterologous MLL-EAF1 fusion gene and analyzed its transforming potential. Strikingly, we found that MLL-EAF1 immortalized myeloid progenitors in the same manner as that of MLL-ELL. Furthermore, transplantation of bone marrow transduced with MLL-EAF1 induced AML with a shorter latency than mice transplanted with the MLL-ELL fusion. Taken together, these results indicate that the leukemic activity of MLL-ELL requires the EAF1 interaction domain of ELL, suggesting that the recruitment by MLL of a transactivation domain similar to that in EAF1 or the AF4/LAF4/AF5q31 family may be a critical common feature of multiple 11q23 translocations. In addition, these studies support a critical role for MLL partner genes and their protein-protein interactions in 11q23 leukemogenesis.

Structure of AF3p21, a new member of mixed lineage leukemia (MLL) fusion partner proteins-implication for MLL-induced leukemogenesis

The Mixed Lineage Leukemia (MLL) gene is frequently rearranged in leukemia, especially in infantile leukemia and therapy-related leukemia. The MLL gene is localized at chromosome 11q23, and is involved in almost all of the chromosomal translocations involving 11q23. Twenty-four fusion partner genes have been identified to date, and the N-terminus of MLL fuses in-frame to the partner genes in all cases. Some of the MLL fusion partner genes encode transcription factors; others encode small GTP binding protein interacting molecules or cytoplasmic proteins, the functions of which are presently unknown. As a result of the diverse features of the MLL fusion partners, the underlying mechanism for leukemogenesis remains obscure. We cloned the MLL fusion partner gene from leukemic cells from a therapy-related leukemia patient with t(3;11)(p21;q23) and designated the gene AF3p21. This patient had a long latency period (9 years) before developing secondary leukemia. The AF3p21 gene encodes a nuclear protein with a molecular mass of 80 kDa, and this protein has SH3 and proline-rich domains. Among MLL fusion partners identified to date, only AF10 and AF17 have a homo-oligomerization domain. AF3p21 also has a homo-oligomerization domain, which was revealed by using a mammalian two-hybrid system. These results suggest that one possible role of the MLL fusion partners is to form an oligomer of truncated MLL. In this review, current knowledge about MLL-involved leukemogenesis is outlined.