Transcription factor fusions in acute leukemia: variations on a theme

A direct comparison between AML1-ETO and ETO2-GLIS2 leukemia fusion proteins reveals context-dependent binding and regulation of target genes and opposite functions in cell differentiation

The ETO-family transcriptional corepressors, including ETO, ETO2, and MTGR1, are all involved in leukemia-causing chromosomal translocations. In every case, an ETO-family corepressor acquires a DNA-binding domain (DBD) to form a typical transcription factor—the DBD binds to DNA, while the ETO moiety manifests transcriptional activity. A directly comparative study of these “homologous” fusion transcription factors may clarify their similarities and differences in regulating transcription and leukemogenesis. Here, we performed a side-by-side comparison between AML1-ETO and ETO2-GLIS2, the most common fusion proteins in M2-and M7-subtypes of acute myeloid leukemia, respectively, by inducible expression of them in U937 leukemia cells. We found that, although AML1-ETO and ETO2-GLIS2 can use their own DBDs to bind DNA, they share a large proportion of genome-wide binding regions dependent on other cooperative transcription factors, including the ETS-, bZIP- and bHLH-family proteins. AML1-ETO acts as either transcriptional repressor or activator, whereas ETO2-GLIS2 mainly acts as activator. The repressor-versus-activator functions of AML1-ETO might be determined by the abundance of cooperative transcription factors/cofactors on the target genes. Importantly, AML1-ETO and ETO2-GLIS2 differentially regulate key transcription factors in myeloid differentiation including PU.1 and C/EBPβ. Consequently, AML1-ETO inhibits, but ETO2-GLIS2 facilitates, myeloid differentiation of U937 cells. This function of ETO2-GLIS2 is reminiscent of a similar effect of MLL-AF9 as previously reported. Taken together, this directly comparative study between AML1-ETO and ETO2-GLIS2 in the same cellular context provides insights into context-dependent transcription regulatory mechanisms that may underlie how these seemingly “homologous” fusion transcription factors exert distinct functions to drive different subtypes of leukemia.

MOZ is critical for the development of MOZ/MLL-fusion-induced leukemia through regulation of Hoxa9/Meis1 expression

Monocytic leukemia zinc finger protein (MOZ, MYST3, or KAT6A) is a MYST-type acetyltransferase involved in chromosomal translocation in acute myelogenous leukemia (AML) and myelodysplastic syndrome. MOZ is established as essential for hematopoiesis; however, the role of MOZ in AML has not been addressed. We propose that MOZ is critical for AML development induced by MLL-AF9, MLL-AF10, or MOZ-TIF2 fusions. Moz-deficient hematopoietic stem/progenitor cells (HSPCs) transduced with an MLL-AF10 fusion gene neither formed colonies in methylcellulose nor induced AML in mice. Moz-deficient HSPCs bearing MLL-AF9 also generated significantly reduced colony and cell numbers. Moz-deficient HSPCs expressing MOZ-TIF2 could form colonies in vitro but could not induce AML in mice. By contrast, Moz was dispensable for colony formation by HOXA9-transduced cells and AML development caused by HOXA9 and MEIS1, suggesting a specific requirement for MOZ in AML induced by MOZ/MLL fusions. Expression of the Hoxa9 and Meis1 genes was decreased in Moz-deficient MLL fusion-expressing cells, while expression of Meis1, but not Hoxa9, was reduced in Moz-deficient MOZ-TIF2 AML cells. AML development induced by MOZ-TIF2 was rescued by introducing Meis1 into Moz-deficient cells carrying MOZ-TIF2. Meis1 deletion impaired MOZ-TIF2–mediated AML development. Active histone modifications were also severely reduced at the Meis1 locus in Moz-deficient MOZ-TIF2 and MLL-AF9 AML cells. These results suggest that endogenous MOZ is critical for MOZ/MLL fusion-induced AML development and maintains active chromatin signatures at target gene loci.

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MOZ is critical for MOZ/MLL fusion-mediated AML development, Meis1 induction by MOZ fusions, and Hoxa9/Meis1 induction by MLL fusions.

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Endogenous MOZ is required to maintain MOZ-target and active histone modifications at the Meis1 gene locus.

Diagnosis of rare subtypes of acute myeloid leukaemia and related neoplasms

The diagnosis of acute myeloid leukaemia and related neoplasms in adults is challenging as this requires the integration of clinical findings, morphology, immunophenotype, cytogenetics, and molecular genetic findings. Lack of familiarity with rare subtypes of acute leukaemia hinders the diagnosis. In this review, we will describe diagnostic findings of several rare acute myeloid leukaemias and related neoplasms that primarily occur in adults including information on presentation, morphology, immunophenotype, genetics, differential diagnosis, and prognosis. Leukaemias discussed include blastic plasmacytoid dendritic cell neoplasm, acute myeloid leukaemia with t(6;9) (p23;q34.1); DEK-NUP214, acute myeloid leukaemia with inv(3)(q21.3q26.2) or t(3;3)(q21.3;q26.2); GATA2, MECOM, acute myeloid leukaemia with BCR-ABL1, acute leukaemias of ambiguous lineage, acute myeloid leukaemia with mutated RUNX1, pure erythroid leukaemia, acute panmyelosis with myelofibrosis, and acute basophilic leukaemia. Case studies with morphological features of the nine subtypes of acute myeloid leukaemia and related neoplasms have been included, and additional evidence available since publication of the 2016 World Health Organization Classification has been added to each subtype.

Allogeneic hematopoietic cell transplantation improves outcome of adults with t(6;9) acute myeloid leukemia: results from an international collaborative study

Acute myeloid leukemia (AML) with t(6;9)(p22;q34) is a distinct entity accounting for 1-2% of AML cases. A substantial proportion of these patients have a concomitant FLT3-ITD. While outcomes are dismal with intensive chemotherapy, limited evidence suggests allogeneic hematopoietic cell transplantation (allo-HCT) may improve survival if performed early during first complete remission. We report on a cohort of 178 patients with t(6;9)(p22;q34) within an international, multicenter collaboration. Median age was 46 years (range: 16-76), AML was de novo in 88%, FLT3-ITD was present in 62%, and additional cytogenetic abnormalities in 21%. Complete remission was achieved in 81% (n=144), including 14 patients who received high-dose cytarabine after initial induction failure. With a median follow up of 5.43 years, estimated overall survival at five years was 38% (95%CI: 31-47%). Allo-HCT was performed in 117 (66%) patients, including 89 in first complete remission. Allo-HCT in first complete remission was associated with higher 5-year relapse-free and overall survival as compared to consolidation chemotherapy: 45% (95%CI: 35-59%) and 53% (95%CI: 42-66%) versus 7% (95%CI: 3-19%) and 23% (95%CI: 13-38%), respectively. For patients undergoing allo-HCT, there was no difference in overall survival rates at five years according to whether it was performed in first [53% (95%CI: 42-66%)], or second [58% (95%CI: 31-100%); n=10] complete remission or with active disease/relapse [54% (95%CI: 34-84%); n=18] (P=0.67). Neither FLT3-ITD nor additional chromosomal abnormalities impacted survival. In conclusion, outcomes of t(6;9)(p22;q34) AML are poor with chemotherapy, and can be substantially improved with allo-HCT.

The incidence, genetic characteristics, and prognosis of leukemia with concurrent pathogenic fusion genes: a series of 25 cases from a large cohort of leukemia patients

Recurrent fusion genes (FGs) with clinical significances in leukemias are mainly mutually exclusive, and the coexistence of different FGs has been rarely reported. In this study, we retrospectively analyzed the incidence, genetic characteristics, and prognosis of leukemias with concurrent pathogenic FGs, which commonly reported in hematological malignancies in 8226 leukemia patients. A total of 25 patients with coexistence of double FGs were identified, accounting for 0.30% of all cases enrolled. More than half of the cases (14/25, 56%) were diagnosed as chronic myeloid leukemia in accelerated or blast phase, another six and five cases were acute myeloid leukemia and acute lymphocytic leukemia, respectively. Most cases (20/25, 80%) carried constitutively activated tyrosine kinases FGs (BCR-ABL1 or ETV6-PDGFRB) and transcription factors associated FGs simultaneously. Of the 11 patients with contemporaneous karyotype, 5 (45%) showed visible chromosomal abnormalities corresponding to both FGs. The concurrency of FGs was often associated with disease progressions. The prognosis was pessimistic for patients with concurrent FGs, even with the combination of targeted therapy and chemotherapy. Performing allogeneic hematopoietic stem cell transplantation as soon as possible after complete remission can ameliorate the dismal prognosis.

Bcl-2 Family Overexpression and Chemoresistance in Acute Myeloid Leukemia

The family of Bcl-2 proteins is one of the most responsible for apoptosis pathway, that is a critical process to the maintenance of tissue homeostasis. Bcl-2 is an essential apoptotic regulator belonging to a family of functionally and structurally related proteins known as the Bcl-2 family. Some members of this family act as anti-apoptotic regulators, whereas others act in pro-apoptotic function. The relationship between the pro and anti-apoptotic proteins can regulate whether cells begin the apoptosis or remain its life cycle. Increasing of Bcl-2 expression has been found in some hematologic diseases, such as Acute Myeloid Leukemia (AML) and their effects on responsiveness to anticancer therapy have been recently described. Thus, this review aims to discuss apoptosis and the role of the Bcl-2 family of proteins in chemoresistance when overexpressed in patients committed with Acute Myeloid Leukemia submitted to chemotherapy treatment.

Clinical implications of molecular markers in acute myeloid leukemia

The recently updated World Health Organization (WHO) Classification of myeloid neoplasms and leukemia reflects the fact that research in the underlying pathogenic mechanisms of acute myeloid leukemia (AML) has led to remarkable advances in our understanding of the disease. Gene mutations now allow us to explore the enormous diversity among cytogenetically defined subsets of AML, particularly the large subset of cytogenetically normal AML. Despite the progress in unraveling the tumor genome, only a small number of recurrent mutations have been incorporated into risk-stratification schemes and have been proven to be clinically relevant, targetable lesions. We here discuss the utility of molecular markers in AML in prognostication and treatment decision making, specifically highlighting the aberrations included in the current WHO classification.

Domain retention in transcription factor fusion genes and its biological and clinical implications: a pan-cancer study

Genomic rearrangements involving transcription factors (TFs) can form fusion proteins resulting in either enhanced, weakened, or even loss of TF activity. Functional domain (FD) retention is a critical factor in the activity of transcription factor fusion genes (TFFGs). A systematic investigation of FD retention in TFFGs and their outcome (e.g. expression changes) in a pan-cancer study has not yet been completed. Here, we examined the FD retention status in 386 TFFGs across 13 major cancer types and identified 83 TFFGs involving 67 TFs that retained FDs. To measure the potential biological relevance of TFs in TFFGs, we introduced a Major Active Isofusion Index (MAII) and built a prioritized TFFG network using MAII scores and the observed frequency of fusion positive samples. Interestingly, the four TFFGs (PML-RARA, RUNX1-RUNX1T1, TMPRSS2-ERG, and SFPQ-TFE3) with the highest MAII scores showed 50 differentially expressed target genes (DETGs) in fusion-positive versus fusion-negative cancer samples. DETG analysis revealed that they were involved in tumorigenesis-related processes in each cancer type. PLAU, which encodes plasminogen activator urokinase and serves as a biomarker for tumor invasion, was found to be consistently activated in the samples with the highest MAII scores. Among the 50 DETGs, 21 were drug targetable genes. Fourteen of these 21 DETGs were expressed in acute myeloid leukemia (AML) samples. Accordingly, we constructed an AML-specific TFFG network, which included 38 DETGs in RUNX1-RUNX1T1 or PML-RARA positive samples. In summary, this study revealed several TFFGs and their potential target genes, and provided insights into the clinical implications of TFFGs.

Mutation Analysis of Isocitrate Dehydrogenase (IDH1/2) and DNA Methyltransferase 3A (DNMT3A) in Thai Patients with Newly Diagnosed Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a clonal hematopoietic stem/progenitor cell disorder which features several genetic mutations. Recurrent genetic alterations identified in AML are recognized as causes of the disease, finding application as diagnostic, prognostic and monitoring markers, with potential use as targets for cancer therapy. Here, we performed a pyrosequencing technique to investigate common mutations of IDH1, IDH2 and DNMT3A in 81 newly diagnosed AML patients. The prevalences of IDH1, IDH2 and DNMT3A mutations were 6.2%, 18.5%, and 7.4%, respectively. In addition, exclusive mutations in IDH1 codon 132 (R132H, R132C, R132G and R132S) were identified in all IDH1-mutated cases indicating that these are strongly associated with AML. Interestingly, higher median blast cell counts were significantly associated with IDH1/2 and DNMT3A mutations. In summary, we could establish a routine robust pyrosequencing method to detect common mutations in IDH1/2 and DNMT3A and demonstrate the frequency of those mutations in adult Thai AML patients.

Targeted Cancer Therapy: Vital Oncogenes and a New Molecular Genetic Paradigm for Cancer Initiation Progression and Treatment

It has been declared repeatedly that cancer is a result of molecular genetic abnormalities. However, there has been no working model describing the specific functional consequences of the deranged genomic processes that result in the initiation and propagation of the cancer process during carcinogenesis. We no longer need to question whether or not cancer arises as a result of a molecular genetic defect within the cancer cell. The legitimate questions are: how and why? This article reviews the preeminent data on cancer molecular genetics and subsequently proposes that the sentinel event in cancer initiation is the aberrant production of fused transcription activators with new molecular properties within normal tissue stem cells. This results in the production of vital oncogenes with dysfunctional gene activation transcription properties, which leads to dysfunctional gene regulation, the aberrant activation of transduction pathways, chromosomal breakage, activation of driver oncogenes, reactivation of stem cell transduction pathways and the activation of genes that result in the hallmarks of cancer. Furthermore, a novel holistic molecular genetic model of cancer initiation and progression is presented along with a new paradigm for the approach to personalized targeted cancer therapy, clinical monitoring and cancer diagnosis.

Nuclear Pore Complexes and Nucleocytoplasmic Transport: From Structure to Function to Disease

Nucleocytoplasmic transport is an essential cellular activity and occurs via nuclear pore complexes (NPCs) that reside in the double membrane of the nuclear envelope. Significant progress has been made during the past few years in unravelling the ultrastructural organization of NPCs and their constituents, the nucleoporins, by cryo-electron tomography and X-ray crystallography. Mass spectrometry and genomic approaches have provided deeper insight into the specific regulation and fine tuning of individual nuclear transport pathways. Recent research has also focused on the roles nucleoporins play in health and disease, some of which go beyond nucleocytoplasmic transport. Here we review emerging results aimed at understanding NPC architecture and nucleocytoplasmic transport at the atomic level, elucidating the specific function individual nucleoporins play in nuclear trafficking, and finally lighting up the contribution of nucleoporins and nuclear transport receptors in human diseases, such as cancer and certain genetic disorders.

Nucleoporin Gene Fusions and Hematopoietic Malignancies

Nuclear pore complexes (NPCs) are the sole gateways between the nucleus and the cytoplasm of eukaryotic cells and they mediate all macromolecular trafficking between these cellular compartments. Nucleocytoplasmic transport is highly selective and precisely regulated and as such an important aspect of normal cellular function. Defects in this process or in its machinery have been linked to various human diseases, including cancer. Nucleoporins, which are about 30 proteins that built up NPCs, are critical players in nucleocytoplasmic transport and have also been shown to be key players in numerous other cellular processes, such as cell cycle control and gene expression regulation. This review will focus on the three nucleoporins Nup98, Nup214, and Nup358. Common to them is their significance in nucleocytoplasmic transport, their multiple other functions, and being targets for chromosomal translocations that lead to haematopoietic malignancies, in particular acute myeloid leukaemia. The underlying molecular mechanisms of nucleoporin-associated leukaemias are only poorly understood but share some characteristics and are distinguished by their poor prognosis and therapy outcome.

Nucleoporins and nucleocytoplasmic transport in hematologic malignancies

Hematologic malignancies are often associated with chromosomal rearrangements that lead to the expression of chimeric fusion proteins. Rearrangements of the genes encoding two nucleoporins, NUP98 and NUP214, have been implicated in the pathogenesis of several types of hematologic malignancies, particularly acute myeloid leukemia. NUP98 rearrangements result in fusion of an N-terminal portion of NUP98 to one of numerous proteins. These rearrangements often follow treatment with topoisomerase II inhibitors and tend to occur in younger patients. They have been shown to induce leukemia in mice and to enhance proliferation and disrupt differentiation in primary human hematopoietic precursors. NUP214 has only a few fusion partners. DEK-NUP214 is the most common NUP214 fusion in AML; it tends to occur in younger patients and is usually associated with FLT3 internal tandem duplications. The leukemogenic activity of NUP214 fusions is less well characterized. Normal nucleoporins, including NUP98 and NUP214, have important functions in nucleocytoplasmic transport, transcription, and mitosis. These functions and their disruptions by oncogenic nucleoporin fusions are discussed.

Double-strand break repair by interchromosomal recombination: an in vivo repair mechanism utilized by multiple somatic tissues in mammals

Homologous recombination (HR) is essential for accurate genome duplication and maintenance of genome stability. In eukaryotes, chromosomal double strand breaks (DSBs) are central to HR during specialized developmental programs of meiosis and antigen receptor gene rearrangements, and form at unusual DNA structures and stalled replication forks. DSBs also result from exposure to ionizing radiation, reactive oxygen species, some anti-cancer agents, or inhibitors of topoisomerase II. Literature predicts that repair of such breaks normally will occur by non-homologous end-joining (in G1), intrachromosomal HR (all phases), or sister chromatid HR (in S/G²). However, no in vivo model is in place to directly determine the potential for DSB repair in somatic cells of mammals to occur by HR between repeated sequences on heterologs (i.e., interchromosomal HR). To test this, we developed a mouse model with three transgenes—two nonfunctional green fluorescent protein (GFP) transgenes each containing a recognition site for the I-SceI endonuclease, and a tetracycline-inducible I-SceI endonuclease transgene. If interchromosomal HR can be utilized for DSB repair in somatic cells, then I-SceI expression and induction of DSBs within the GFP reporters may result in a functional GFP+ gene. Strikingly, GFP+ recombinant cells were observed in multiple organs with highest numbers in thymus, kidney, and lung. Additionally, bone marrow cultures demonstrated interchromosomal HR within multiple hematopoietic subpopulations including multi-lineage colony forming unit–granulocyte-erythrocyte-monocyte-megakaryocte (CFU-GEMM) colonies. This is a direct demonstration that somatic cells in vivo search genome-wide for homologous sequences suitable for DSB repair, and this type of repair can occur within early developmental populations capable of multi-lineage differentiation.

Novel t(7;10)(p22;p24) along with NPM1 mutation in patient with relapsed acute myeloid leukemia

Chromosomal abnormalities/genetic mutations associated with hematological malignancies alter the structure and function of genes controlling cell proliferation and differentiation through multiple and complex pathways, resulting different clinical outcomes. This is a case study of a lady presented with acute myeloid leukemia (AML M1) at our center who relapsed 10 years after the induction therapy. Cytogenetic and molecular analyses were performed in this case at the time of relapse to find out the chromosomal abnormalities and genetic abnormalities like FMS-like tyrosine kinase (FLT3) and nucleophosmin (NPM1) mutation. The cytogenetic analysis of bone marrow established a novel translocation t(7;10) (p22;q24) in 100% of the cells analyzed. Phytohaemagglutinin (PHA)-stimulated blood culture also revealed the same abnormality. Apart from this, the molecular analysis showed NPM1 exon 12 (hot-spot) mutation in this patient. This was the first report of novel chromosomal translocation in this subset of AML in which a new translocation along with NPM1 mutation was discussed.

Tumours of the lacrimal gland. Epidemiological, clinical and genetic characteristics

Tumours of the lacrimal gland are rare, but the prognosis may be grave. To date, no population-based incidence and distribution data on lacrimal gland tumours exist. In addition, almost nothing is known about the genetic profile of epithelial tumours of the lacrimal gland. We collected specimens and clinical files on all biopsied lacrimal gland lesions in Denmark over a 34-year period and re-evaluated the diagnosis to provide updated population-based incidence rates and epidemiological characteristics. Clinical data regarding symptoms, clinical examinations, treatment and follow-up were collected for patients with adenoid cystic carcinoma (ACC), pleomorphic adenoma (PA), carcinoma ex pleomorphic adenoma (Ca-ex-PA) and mucoepidermoid carcinoma (MEC). Using RT-PCR, FISH, immunohistochemistry, Q-PCR and high-resolution array-based comparative genomic hybridization (arrayCGH) we explored the genetic characteristics including copy number alterations (CNA) in ACC, PA, Ca-ex-PA and MEC. The incidence of biopsied lacrimal gland lesions was 1.3/1,000,000/year, and ~50% were neoplastic lesions. Of these, 55% were malignant tumours with epithelial tumours as the most frequent. The overall incidence was increasing, and this was caused by an increase in biopsied non-neoplastic lesions. We found that 10/14 ACCs either expressed the MYB-NFIB fusion gene and/or had rearrangements of MYB. All ACCs expressed the MYB protein. ACC was characterized by recurrent copy number losses involving 6q, 12q and 17q and gains involving 19q, 8q and 11q. ArrayCGH revealed an apparently normal genomic profile in 11/19 PAs. The remaining 8 PAs had recurrent copy number losses involving 1p, 6q, 8q and 13q and gain involving 9p. PA expressed PLAG1 in all tumours whereas only 2/29 tumours expressed HMGA2. Ca-ex-PA was characterized by recurrent copy number gain involving 22q. PLAG1 was expressed in 3/5 Ca-ex-PA whereas none of these tumours expressed HMGA2. MEC expressed the CRTC1-MAML2, and this fusion was found to be tumour-specific for lacrimal gland MEC. In conclusion, lacrimal gland lesions that require pathological evaluation are rare in the Danish population, and the incidence rate of biopsied benign lesions is increasing. Epithelial tumours of the lacrimal gland are molecularly very similar to their salivary gland counterparts in the expression of the tumour-specific fusion genes and in their genomic imbalances as demonstrated by arrayCGH. MYB-NFIB is a useful biomarker for ACC and MYB, and its downstream target genes may be potential therapeutic targets for these tumours.

Chromosomal aberrations and fusion genes in myeloid malignancies

Since the discovery of the BCR-ABL1 fusion gene in chronic myeloid leukemia, many more fusion genes resulting from chromosomal rearrangements have been identified and characterized. The study of these fusion genes has been extremely important for our understanding of the role of chromosomal rearrangements in leukemogenesis and in oncology in general. In chronic myeloid leukemia, or related myeloproliferative malignancies caused by the expression of oncogenic fusion kinases, tyrosine kinase inhibitors are now successfully used to treat these diseases. In acute myeloid leukemias, the presence of chromosomal rearrangements, oncogenic fusion genes and point mutations in key oncogenic drivers has important prognostic value and determines the choice of therapy. In this review, the authors provide an overview of the important fusion genes present in various myeloid malignancies and their importance for clinical practice.

Statin-dependent activation of protein kinase Cδ in acute promyelocytic leukemia cells and induction of leukemic cell differentiation

Statins are HMG-CoA (3-hydroxy-3-methyl-glutaryl-coenzyme A) reductase inhibitors, which block the conversion of HMG-CoA to mevalonate and have potent cholesterol lowering properties. Beyond their importance in the generation of lipid lowering effects, the regulatory effects of statins on the mevalonate pathway have a significant impact on multiple other cellular functions. There is now extensive evidence that statins have anti-inflammatory and anti-neoplastic properties, but the precise mechanisms by which such responses are generated are not well understood. In the present study we demonstrate that statins engage a member of the protein kinase C (PKC) family of proteins, PKCδ, in acute promyelocytic leukemia (APL) cells. Our study shows that atorvastatin and fluvastatin induce proteolytic activation of PKCδ in the APL NB4 cell line, which expresses the t(15;17) translocation. Such engagement of PKCδ results in induction of its kinase domain and downstream regulation of pathways important for statin-dependent leukemia cell differentiation. Our research shows that the function of PKCδ is essential for statin-induced leukemic cell differentiation, as demonstrated by studies involving selective targeting of PKCδ using siRNAs. We also demonstrate that the potent enhancing effects of statins on all-trans retinoic acid (ATRA)-induced gene expression for CCL3 and CCL4 requires the function of PKCδ, suggesting a mechanism by which statins may promote ATRA-induced antileukemic responses. Altogether, our data establish a novel function for PKCδ as a mediator of statin-induced differentiation of APL cells and antileukemic effects.

The DAC system and associations with acute leukemias and myelodysplastic syndromes

Imbalances of histone acetyltransferase (HAT) and deacetylase activity (DAC) that result in deregulated gene expression are commonly observed in leukemias. These alterations provide the basis for novel therapeutic approaches that target the epigenetic mechanisms implicated in leukemogenesis. As the acetylation status of histones has been linked to transcriptional regulation of genes involved particularly in differentiation and apoptosis, DAC inhibitors (DACi) have attracted considerable attention for treatment of hematologic malignancies. DACi encompass a structurally diverse family of compounds that are being explored as single agents as well as in combination with chemotherapeutic drugs, small molecule inhibitors of signaling pathways and hypomethylating agents. While DACi have shown clear evidence of activity in acute myeloid leukemia, myelodysplastic syndromes and lymphoid malignancies, their precise role in treatment of these different entities remain to be elucidated. Successful development of these compounds as elements of novel targeted treatment strategies for leukemia will require that clinical studies be performed in conjunction with translational research including efforts to identify predictive biomarkers.

Molecular mechanisms of leukemia-associated protein degradation

Chemical biology, using small molecules as probes to study the cellular signaling network, has developed rapidly in recent years. The interaction between chemistry and biology not only provides new insight into the understanding of cellular activities, but also generates new lead compounds for the treatment of diseases. Transcription factors and kinases such as retinoic acid receptor-alpha (RARα), acute myeloid leukemia 1 (AML1), CAAT/enhancer-binding protein α (C/EBPα), c-myc, and c-abl play important roles in the differentiation of hematopoietic stem/progenitor cells. Abnormalities in these proteins may cause the dysregulation of hematopoiesis and even the occurrence of leukemia. Ubiquitin-mediated protein degradation represents a critical mechanism in regulating the cellular levels and functions of these proteins. Thus, targeting protein degradation has been emerging as an important strategy to conquer malignant diseases. In this review, we will summarize the recent advances in the understanding of the roles of protein degradation in leukemia, with an emphasis on the mechanisms revealed by small molecules.

Nup98-homeodomain fusions interact with endogenous Nup98 during interphase and localize to kinetochores and chromosome arms during mitosis

Fusion proteins containing the FG/GLFG repeats of Nup98 joined to a homeodomain are implicated in leukemias. We find expression of these proteins leads to mislocalization of endogenous intranuclear Nup98. During mitosis, fusions concentrate at kinetochores and chromosomes. These findings suggest new possible contributions of Nup98 fusions to leukemogenesis.

Chromosomal translocations involving the Nup98 gene are implicated in leukemias, especially acute myelogenous leukemia. These translocations generate chimeric fusion proteins, all of which have in common the N-terminal half of Nup98, which contains the nucleoporin FG/GLFG repeat motifs. The homeodomain group of Nup98 fusion proteins retain the C-terminus of a homeodomain transcription factor, including the homeobox responsible for DNA binding. Current models for Nup98 leukemogenesis invoke aberrant transcription resulting from recruitment of coregulators by the Nup98 repeat domain. Here we have investigated the behavior of Nup98-homeodomain fusion proteins throughout the cell cycle. At all stages, the fusion proteins exhibit a novel localization distinct from the component proteins or fragments. During interphase, there are dynamic interactions between the Nup98 fusions and endogenous Nup98 that lead to mislocalization of the intranuclear fraction of Nup98, but do not alter the level of Nup98 at the nuclear pore complex. During mitosis, no interaction between the fusion proteins and endogenous Nup98 is observed. However, the fusions are entirely concentrated at kinetochores and on chromosome arms, sites where the APC/C, a target of Nup98 regulation, is also found. Our observations suggest new possibilities for misregulation by which Nup98 translocations may contribute to cellular transformation and leukemogenesis.

Differential methylation pattern of ID4, SFRP1, and SHP1 between acute myeloid leukemia and chronic myeloid leukemia

To gain insight into the differential mechanism of gene promoter hypermethylation in acute and chronic leukemia, we identified the methylation status on one part of 5'CpG rich region of 8 genes, DAB2IP, DLC-1, H-cadherin, ID4, Integrin alpha4, RUNX3, SFRP1, and SHP1 in bone marrows from acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) patients. Also, we compared the methylation status of genes in AML and CML using methylation-specific PCR (MSP). The frequencies of DNA methylation of ID4, SFRP1, and SHP1 were higher in AML patients compared to those in CML patients. In contrast, no statistical difference between AML and CML was detected for other genes such as DLC-1, DAB2IP, H-cadherin, Integrin alpha4, and RUNX3. Taken together, these results suggest that these methylation-controlled genes may have different roles in AML and CML, and thus, may act as a biological marker of AML.

Epigenetic mechanisms in AML - a target for therapy

Epigenetics refers to a stable, mitotically perpetuated regulatory mechanism of gene expression without an alteration of the coding sequence. Epigenetic mechanism include DNA methylation and histone tail modifications. Epigenetic regulation is part of physiologic development and becomes abnormal in neoplasia, where silencing of critical genes by DNA methylation or histone deacetylation can contribute to leukemogenesis as an alternative to deletion or loss-of-function mutation. In acute myelogenous leukemia (AML), aberrant DNA methylation can be observed in multiple functionally relevant genes such as p15, p 73, E-cadherin, ID 4, RARbeta2. Abnormal activities of histone tail-modifying enzymes have also been seen in AML, frequently as a direct result of chromosomal translocations. It is now clear that these epigenetic changes play a significant role in development and progression of AML, and thus constitute important targets of therapy. The aim of targeting epigenetic effector protein or "epigenetic therapy" is to reverse epigenetic silencing and reactive various genes to induce a therapeutic effect such as differentiation, growth arrest, or apoptosis. Recent clinical studies have shown the relative safety and efficacy of such epigenetic therapies.

Chromatin-modifying enzymes as therapeutic targets--Part 1

BACKGROUND

Disease pathogenesis may result from genetic alterations and/or a more diverse group of epigenetic changes. While events such as DNA methylation are well established, there is significant interest in nucleosome remodeling, RNA interference and histone modifications, as mechanisms that underlie epigenetic effects. While genetic mutations are permanent, epigenetic changes can be transitory. The potential to reverse epigenetic changes has led to the development of therapeutic strategies targeting chromatin-modifying enzymes.

OBJECTIVE

To review the roles of chromatin-modifying enzymes in gene regulation and to highlight their potentials as therapeutic targets.

METHODS

This review is based on recently published literature and online resources.

RESULTS/CONCLUSION

This paper focuses on enzymes responsible for histone acetylation, deacetylation, methylation and demethylation, and their potential as targets for epigenetic therapies. A subsequent paper will do the same for enzymes responsible for histone phosphorylation, ubiquitylation, SUMOylation and poly-ADP-ribosylation as well as ATP-dependent nucleosome remodeling.

Multiple ways of C/EBPalpha inhibition in myeloid leukaemia

Transcription factors play a crucial role in myeloid differentiation and lineage determination. Tumour suppressor protein C/EBPalpha is a key regulator of granulocytic differentiation whose functional inactivation has become a pathophysiological signature of myeloid leukaemia. In this review we describe various mechanisms such as antagonistic protein-protein interaction, mutation and posttranslational modifications of C/EBPalpha which lead to its transcriptional inhibition and render C/EBPalpha inactive in its functions.

Emerging treatment strategies for acute myeloid leukemia (AML) in the elderly

Acute myeloid leukemia (AML) is more prevalent in older adults, with an incidence in the United States of 17.6 per 100,000 for those 65 years of age, compared with an incidence of 1.8 per 100,000 for those <65 years of age. While there have been improvements in survival during the last decade for younger patients, prognosis in elderly patients is still poor; approximately 50% achieve complete responses, but many of them relapse. With increasing age, more patients are suboptimal candidates for standard induction chemotherapy due to poor performance status, pre-existing myelodysplasia, unfavorable cytogenetics, treatment-related AML, multidrug resistance protein expression, and CD34 positivity, which are often characteristic of this patient population. In addition, the presence of comorbid conditions make many treatment options less tolerable for elderly patients. Several investigators have described subgroups showing no benefit after intensive treatment approaches in recent years. However, several novel agents have been developed to treat elderly AML patients. These include new chemotherapeutic agents, such as nucleoside analogs, as well as targeted therapies like farnesyltransferase inhibitors, tyrosine kinase inhibitors, epigenetic drugs, and antibodies. On the other hand new insights into the biology of the disease lead to a better understanding of its heterogeneity. Thus, with a variety of novel substances at hand it is increasingly important to introduce a risk-adapted approach for the optimal management of patients. This review will identify subgroups not likely to benefit from intensive chemotherapy and highlight the efficacy and tolerability of new agents in the treatment of AML.

Novel RNA-binding properties of the MTG chromatin regulatory proteins

Background

The myeloid translocation gene (MTG) proteins are non-DNA-binding transcriptional regulators capable of interacting with chromatin modifying proteins. As a consequence of leukemia-associated chromosomal translocations, two of the MTG proteins, MTG8 and MTG16, are fused to the DNA-binding domain of AML1, a transcriptional activator crucial for hematopoiesis. The AML1-MTG fusion proteins, as the wild type MTGs, display four conserved homology regions (NHR1-4) related to the Drosophila nervy protein. Structural protein analyses led us to test the hypothesis that specific MTG domains may mediate RNA binding.

Results

By using an RNA-binding assay based on synthetic RNA homopolymers and a panel of MTG deletion mutants, here we show that all the MTG proteins can bind RNA. The RNA-binding properties can be traced to two regions: the Zinc finger domains in the NHR4, which mediate Zinc-dependent RNA binding, and a novel short basic region (SBR) upstream of the NHR2, which mediates Zinc-independent RNA binding. The two AML1-MTG fusion proteins, retaining both the Zinc fingers domains and the SBR, also display RNA-binding properties.

Conclusion

Evidence has been accumulating that RNA plays a role in transcriptional control. Both wild type MTGs and chimeric AML1-MTG proteins display in vitro RNA-binding properties, thus opening new perspectives on the possible involvement of an RNA component in MTG-mediated chromatin regulation.

Cytogenetic profile of acute lymphocytic leukemia patients: report of a novel translocation t(4;13) (q21 x 3; q35) from an Indian population

Acute lymphocytic leukemia (ALL) is a malignant neoplasm characterized by clonal proliferation, decreased apoptosis and accumulation of immature lymphoid cells in the bone marrow as well as the peripheral blood. The aim of this study was to determine the overall cytogenetic profile of Indian ALL patients along with their frequency and distribution pattern. A total of 75 ALL subjects were included in the study. The major outcome of the work was identification of a novel translocation t(4;13) (q21 x 3;q35) that has not yet been reported. In addition, a few rare chromosomal aberrations such as t(4;16) (p16;q12 x 2) and t(7;10)(q36;q21 x 2) were also detected. Overall, of 75 cases, 67 (89 x 33%) were successfully karyotyped. Normal and abnormal karyotypes were seen in 38 (56 x 7%) and 29 (43 x 3%) cases respectively. Various other abnormalities were hyperdiploidy (20 x 68%), hypodiploidy (10 x 34%), t(8;14) (3 x 44%), t(9;22) (6 x 9%), t(4;16) (3 x 44%), t(7;10) (3 x 44%) and gain of chromosome 8, 13, 16, and 22 was seen in one case each (3 x 44%). Deletions in chromosome 5, 9 and 11 were found to be 3 x 44, 6 x 89 and 6 x 89% respectively, while complex and other aberrations were detected in 3 x 44 and 13 x 8% cases. Finally, we conclude that cytogenetic analysis has an important role in routine genetic diagnostic workup and management of ALL patients.

Roles of the histone acetyltransferase monocytic leukemia zinc finger protein in normal and malignant hematopoiesis

Histone-modified enzymes are involved in various cell functions, including proliferation, differentiation, cell death and carcinogenesis. The protein MOZ (monocytic leukemia zinc finger protein) is a Myst (MOZ, Ybf2 (Sas3), Sas2, Tip60)-type histone acetyltranseferase (HAT) that generates fusion genes, such as MOZ-TIF2, MOZ-CBP and MOZ-p300, in acute myeloid leukemia (AML) by chromosomal translocation. MOZ associates with AML1 (RUNX1), PU.1, and p53, and cooperatively activates target gene transcription. Gene targeting in mice has revealed that MOZ is essential for the generation and maintenance of hematopoietic stem cells (HSC) and for the appropriate development of myeloid, erythroid and B-lineage cell progenitors. In AML, MOZ fusion genes lead to repressed differentiation, hyper-proliferation, and self-renewal of myeloid progenitors through deregulation of MOZ-regulated target gene expression. It is therefore necessary to analyze the roles of MOZ and MOZ fusion genes in normal and malignant hematopoiesis to elucidate the mechanisms underlying and develop therapies for MOZ-related AML.

GATA1-related leukaemias

GATA1 is a prototypical lineage-restricted transcription factor that is central to the correct differentiation, proliferation and apoptosis of erythroid and megakaryocytic cells. Mutations in GATA1 can generate a truncated protein, which contributes to the genesis of transient myeloproliferative disorder (TMD) and acute megakaryoblastic leukaemia (AMKL) in infants with Down syndrome. Similarly, Gata1 knockdown to 5% of its wild-type level causes high incidence of erythroid leukaemia in mice. The GATA1-related leukaemias in both human and mouse could provide important insights into the mechanism of multi-step leukaemogenesis. Efforts are afoot to produce mouse models that are reflective of TMD and AMKL.

Distinct pharmacological properties of second generation HDAC inhibitors with the benzamide or hydroxamate head group

Advanced second generation inhibitors of histone deacetylases (HDAC) are currently used in clinical development. This study aimed at comparing the pharmacological properties of selected second generation HDAC inhibitors with the hydroxamate and benzamide head group, namely SAHA, LAQ824/LBH589, CI994, MS275 and MGCD0103. In biochemical assays using recombinant HDAC1, 3, 6 and 8 isoenzymes, SAHA and LAQ824/LBH589 behave as quite unselective HDAC inhibitors. In contrast, the benzamides CI994, MS275 and MGCD0103 are more selective, potent inhibitors of at least HDAC1 and HDAC3. All HDAC inhibitors induce histone H3 hyperacetylation, correlating with inhibition of proliferation, induction of cell differentiation and apoptosis. A broad cytotoxicity is seen across cell lines from different tumor entities with LAQ824/LBH589 being the most potent agents. The apoptosis inducing activity is evident in arrested and proliferating RKO colon cancer cells with inducible, heterologous p21(waf1) expression, indicative for a cell-cycle independent mode-of-action. Differentiation of MDA-MB468 breast cancer cells is induced by benzamide and hydroxamate analogs. The reversibility of drug action was evaluated by pulse treatment of A549 lung cancer cells. Whereas paclitaxel induced irreversible cell cycle alterations already after 6 hr treatment, HDAC inhibitor action was retarded and irreversible after >16 hr treatment. Interestingly, pulse treatment was equally effective as continous treatment. Finally, the efficacy of LAQ824, SAHA and MS275 in A549 nude mice xenografts was comparable to that of paclitaxel at well tolerated doses. We conclude that despite a different HDAC isoenzyme inhibition profile, hydroxamate and benzamide analogs as studied display similar cellular profiles.

Leukemia, an effective model for chemical biology and target therapy

The rapid rise of chemical biology aimed at studying signaling networks for basic cellular activities using specific, active small molecules as probes has greatly accelerated research on pathological mechanisms and target therapy of diseases. This research is especially important for malignant tumors such as leukemia, a heterogeneous group of hematopoietic malignancies that occurs worldwide. With the use of a chemical approach combined with genetic manipulation, great progress has been achieved over the past few decades on the biological, molecular and cytogenetic aspects of leukemia, and in its diagnosis and therapy. In particular, discoveries of the clinical effectiveness of all-trans retinoic acid and arsenic trioxide in the treatment of acute promyelocytic leukemia and the kinase inhibitors Imatinib and Dasatinib in the treatment of chronic myelogenous leukemia not only make target therapy of leukemia a reality, but also push mechanisms of leukemogenesis and leukemic cell activities forward. This review will outline advances in chemical biology that help our understanding of the molecular mechanisms of cell differentiation and apoptosis induction and target therapy of leukemia.

A homogeneous cellular histone deacetylase assay suitable for compound profiling and robotic screening

Most cellular assays that quantify the efficacy of histone deacetylase (HDAC) inhibitors measure hyperacetylation of core histone proteins H3 and H4. Here we describe a new approach, directly measuring cellular HDAC enzymatic activity using the substrate Boc-K(Ac)-7-amino-4-methylcoumarin (AMC). After penetration into HeLa cervical carcinoma or K562 chronic myeloid leukemia cells, the deacetylated product Boc-K-AMC is formed which, after cell lysis, is cleaved by trypsin, finally releasing the fluorophor AMC. The cellular potency of suberoylanilide hydroxamic acid, LBH589, trichostatin A, and MS275 as well-known HDAC inhibitors was determined using this assay. IC(50) values derived from concentration-effect curves correlated well with EC(50) values derived from a cellomics array scan histone H3 hyperacetylation assay. The cellular HDAC activity assay was adapted to a homogeneous format, fully compatible with robotic screening. Concentration-effect curves generated on a Tecan Genesis Freedom workstation were highly reproducible with a signal-to-noise ratio of 5.7 and a Z' factor of 0.88, indicating a very robust assay. Finally, a HDAC-inhibitor focused library was profiled in a medium-throughput screening campaign. Inhibition of cellular HDAC activity correlated well with cytotoxicity and histone H3 hyperacetylation in HeLa cells and with inhibition of human recombinant HDAC1 in a biochemical assay. Thus, by using Boc-K(Ac)-AMC as a cell-permeable HDAC substrate, the activity of various protein lysine-specific deacetylases including HDAC1-containing complexes is measurable in intact cells in a simple and homogeneous manner.

Acute Myelogeneous Leukemia (M0/M1) with novel chromosomal abnormality of t(14;17) (q32; q11.2)

Acute Myelogeneous Leukemia (AML) is a heterogeneous disease with respect to morphology, immunophenotype, and genetic rearrangements. Multiple recurrent chromosomal aberrations have been identified by conventional cytogenetic analysis. In this study, we report a case whose clinical features were suggestive of AML-M1 subtype with t(14;17) (q32; q11.2) karyotype involving rearrangement of chromosomal segments 17q11.2 and 14q32. This is the first report of novel chromosomal translocation in this subset of AML and has not yet been reported elsewhere. This rearrangement may include certain cancer associated oncogene(s) or genes involved in the differentiation like retionic acid receptor alpha (RARA), which may confer differentiation blockage and/or proliferation advantage.

TEL-AML1 preleukemic activity requires the DNA binding domain of AML1 and the dimerization and corepressor binding domains of TEL

The t(12;21)(p13;q22) translocation generates the TEL-AML1 (TEL, translocation-Ets-leukemia; AML1, acute myeloid leukemia-1) (ETV6-RUNX1) fusion product and is the most common chromosomal abnormality in pediatric leukemia. Our previous studies using a murine fetal liver transplantation model demonstrated that TEL-AML1 promotes the self-renewal of B-cell precursors in vitro and enhances the expansion of hematopoietic stem cells (HSCs) in vivo. This is consistent with the hypothesis that TEL-AML1 induces expansion of a preleukemic clone. Several studies have described domains within TEL-AML1 involved in the transcriptional regulation of specific target genes. However, it is unclear which of these domains is important for the activity of TEL-AML1 in preleukemic hematopoiesis. In order to examine this, we have generated a panel of deletion mutants and expressed them in HSCs. These experiments demonstrate that TEL-AML1 requires multiple domains from both TEL and AML1 to alter hematopoiesis. Furthermore, mutation of a single amino-acid residue within the runt homology domain of AML1, required for DNA binding, was sufficient to abrogate TEL-AML1 activity. These data suggest that TEL-AML1 acts as an aberrant transcription factor to perturb multiple pathways during hematopoiesis.

A distinct epigenetic signature at targets of a leukemia protein

Background

Human myelogenous leukemia characterized by either the non random t(8; 21)(q22; q22) or t(16; 21)(q24; q22) chromosome translocations differ for both their biological and clinical features. Some of these features could be consequent to differential epigenetic transcriptional deregulation at AML1 targets imposed by AML1-MTG8 and AML1-MTG16, the fusion proteins deriving from the two translocations. Preliminary findings showing that these fusion proteins lead to transcriptional downregulation of AML1 targets, marked by repressive chromatin changes, would support this hypothesis. Here we show that combining conventional global gene expression arrays with the power of bioinformatic genomic survey of AML1-consensus sequences is an effective strategy to identify AML1 targets whose transcription is epigenetically downregulated by the leukemia-associated AML1-MTG16 protein.

Results

We interrogated mouse gene expression microarrays with probes generated either from 32D cells infected with a retroviral vector carrying AML1-MTG16 and unable of granulocyte differentiation and proliferation in response to the granulocyte colony stimulating factor (G-CSF), or from 32D cells infected with the cognate empty vector. From the analysis of differential gene expression alone (using as criteria a p value < 0.01 and an absolute fold change > 3), we were unable to conclude which of the 37 genes downregulated by AML1-MTG16 were, or not, direct AML1 targets. However, when we applied a bioinformatic approach to search for AML1-consensus sequences in the 10 Kb around the gene transcription start sites, we closed on 17 potential direct AML1 targets. By focusing on the most significantly downregulated genes, we found that both the AML1-consensus and the transcription start site chromatin regions were significantly marked by aberrant repressive histone tail changes. Further, the promoter of one of these genes, containing a CpG island, was aberrantly methylated.

Conclusion

This study shows that a leukemia-associated fusion protein can impose a distinct epigenetic repressive signature at specific sites in the genome. These findings strengthen the conclusion that leukemia-specific oncoproteins can induce non-random epigenetic changes.

Gene expression regulation and cancer

Gene expression is mostly controlled at the level of the transcription initiation. The transcription control regions of protein-encoding genes include: the core promoter, where RNA polymerase II binds, the proximal and distal promoter, responsible for gene expression regulation, and the enhancers and silencers. Chromatin represents an additional level of regulation of gene expression. The switching between inactive and active chromatin is closely related to the activity of histone-modifying enzymes and chromatin-remodelling complexes. Transcriptional activation of a gene requires the binding of specific transcription factors to regulatory DNA elements, the opening of the chromatin, the binding of Mediator, and the assembly of the preinitiation complex with RNA polymerase and RNA synthesis initiation. Transcription factors ultimately transduce the proliferation signals elicited by growth factors. Moreover, many human oncogenes encode for transcription factors, and some of them are prevalent in particular neoplasias (e.g., MYC, MLL, PML-RARa). Also, some of the most prominent tumor suppressors (e.g. p53) are transcription factors.

Leukemogenic AML1-ETO fusion protein upregulates expression of connexin 43: the role in AML 1-ETO-induced growth arrest in leukemic cells

AML1-ETO, a fusion protein generated by the chromosomal translocation t(8;21), is frequently associated with acute myeloid leukemia (AML). In addition to blocking differentiation, AML1-ETO is also shown to induce growth arrest in AML cells, which is unfavorable for leukemogenesis harboring the t(8;21) translocation. However, its precise mechanism is still unclear. Here we provide the first demonstration that the conditional expression of AML1-ETO by the ecdysone-inducible system dramatically increases the expression of connexin 43 (CX43), together with growth arrest at G1 phase in leukemic U937 cells. We also show that the CX43 induction inhibits the proliferation of U937 cells at G1 phase, while the suppression of CX43 expression by small interfering RNA (siRNA) effectively overcomes the growth-inhibitory effect of AML1 -ETO in leukemic cells. Furthermore, either AML1-ETO or CX43 induction elevates cell-cycle negative regulator P27(kip1) protein by inhibiting its degradation, which is antagonized by siRNA against CX43. Taken together, our data indicate that CX43 plays a role in AML1-ETO-induced growth arrest possibly through the accumulation of P27(kip1) protein. The potential mutation or/and epigenetic alterations of CX43 and its related gene(s) deserve to be explored in AML1-ETO-positive AML patients.

Fusion oncogenes and tumor type specificity--insights from salivary gland tumors

Salivary gland tumors are frequently characterized by recurrent chromosome translocations, which have recently been shown to result in pathogenetically relevant fusion oncogenes. These genes encode novel fusion proteins as well as ectopically expressed normal or truncated proteins, and are found in both benign and malignant salivary gland tumors. The major targets of the translocations are DNA-binding transcription factors (PLAG1 and HMGA2) involved in growth factor signaling and cell cycle regulation, and coactivators of the Notch (MAML2) and cAMP (TORC1) signaling pathways. Identification of these fusion oncogenes has contributed to our knowledge of molecular pathways leading to epithelial tumors in general, and to salivary gland tumors in particular. Interestingly, the fusions in salivary gland tumors do not seem to be as tumor type specific as those in leukemias and sarcomas. Instead, they may function by activating basic transformation pathways that can function in multiple cell types. The downstream gene products of these fusions will be important targets for development of new intracellular therapeutic strategies.

The hematopoietic transcription factor AML1 (RUNX1) is negatively regulated by the cell cycle protein cyclin D3

The family of cyclin D proteins plays a crucial role in the early events of the mammalian cell cycle. Recent studies have revealed the involvement of AML1 transactivation activity in promoting cell cycle progression through the induction of cyclin D proteins. This information in combination with our previous observation that a region in AML1 between amino acids 213 and 289 is important for its function led us to investigate prospective proteins associating with this region. We identified cyclin D3 by a yeast two-hybrid screen and detected AML1 interaction with the cyclin D family by both in vitro pull-down and in vivo coimmunoprecipitation assays. Furthermore, we demonstrate that cyclin D3 negatively regulates the transactivation activity of AML1 in a dose-dependent manner by competing with CBFbeta for AML1 association, leading to a decreased binding affinity of AML1 for its target DNA sequence. AML1 and its fusion protein AML1-ETO have been shown to shorten and prolong the mammalian cell cycle, respectively. In addition, AML1 promotes myeloid cell differentiation. Thus, our observations suggest that the direct association of cyclin D3 with AML1 functions as a putative feedback mechanism to regulate cell cycle progression and differentiation.

AML1-ETO rapidly induces acute myeloblastic leukemia in cooperation with the Wilms tumor gene, WT1

AML1-ETO, a chimeric gene frequently detected in acute myelogenous leukemia (AML), inhibits the differentiation of myeloid progenitors by suppressing genes associated with myeloid differentiation and increases the replating ability of clonogenic myeloid progenitors. However, AML1-ETO alone cannot induce AML and thus additional genetic events are required for the onset of AML. The Wilms tumor gene (WT1), which has been identified as the gene responsible for Wilms tumor, is expressed at high levels in almost all human leukemias. In this study, we have generated transgenic mice (WT1-Tg) that overexpress WT1 in hematopoietic cells to investigate the effects of WT1 on AML1-ETO-associated leukemogenesis. AML1-ETO-transduced bone marrow (BM) cells from WT1-Tg mice exhibited inhibition of myeloid differentiation at more immature stages and higher in vitro colony-forming ability compared with AML1-ETO-transduced BM cells from wild-type mice. Most importantly, all of the mice that received a transplant of AML1-ETO-transduced BM cells from the WT1-Tg mice rapidly developed AML. These results demonstrate that AML1-ETO may exert its leukemogenic function in cooperation with the expression of WT1.

Continuous MLL-ENL expression is necessary to establish a "Hox Code" and maintain immortalization of hematopoietic progenitor cells

The t[(11;19)(p22;q23)] translocation, which gives rise to the MLL-ENL fusion protein, is commonly found in infant acute leukemias of both the myeloid and lymphoid lineage. To investigate the molecular mechanism of immortalization by MLL-ENL we established a Tet-regulatable system of MLL-ENL expression in primary hematopoietic progenitor cells. Immortalized myeloid cell lines were generated, which are dependent on continued MLL-ENL expression for their survival and proliferation. These cells either terminally differentiate or die when MLL-ENL expression is turned off with doxycycline. The expression profile of all 39 murine Hox genes was analyzed in these cells by real-time quantitative PCR. This analysis showed that loss of MLL-ENL was accompanied by a reduction in the expression of multiple Hoxa genes. By comparing these changes with Hox gene expression in cells induced to differentiate with granulocyte colony-stimulating factor, we show for the first time that reduced Hox gene expression is specific to loss of MLL-ENL and is not a consequence of differentiation. Our data also suggest that the Hox cofactor Meis-2 can substitute for Meis-1 function. Thus, MLL-ENL is required to initiate and maintain immortalization of myeloid progenitors and may contribute to leukemogenesis by aberrantly sustaining the expression of a "Hox code" consisting of Hoxa4 to Hoxa11.

Advances in the molecular genetics of acute leukemia

Acute leukemias are characterized by the unrestrained clonal proliferation of hematopoietic precursor cells coupled with aberrant or arrested differentiation. The molecular basis of hematopoiesis and leukemogenesis is still being defined, yet it is increasingly evident that acute leukemias have recurrent molecular features that can be exploited for diagnostic, prognostic, and therapeutic purposes. Modern molecular technologies already influence treatment strategies for these diseases, and it is likely that as such technology matures it will have an increasing impact on all aspects of acute leukemia management. This article reviews recent developments in the molecular classification, prognostication, and treatment of the acute leukemias.

IL-3 Induces Inhibitor of DNA-Binding Protein-1 in Hemopoietic Progenitor Cells and Promotes Myeloid Cell Development

Hemopoiesis depends on the expression and regulation of transcription factors, which control the maturation of specific cell lineages. We found that the helix-loop-helix transcription factor inhibitor of DNA-binding protein 1 (Id1) is not expressed in hemopoietic stem cells (HSC), but is increased in more committed myeloid progenitors. Id1 levels decrease during neutrophil differentiation, but remain high in differentiated macrophages. Id1 is expressed at low levels or is absent in developing lymphoid or erythroid cells. Id1 expression can be induced by IL-3 in HSC during myeloid differentiation, but not by growth factors that promote erythroid and B cell development. HSC were transduced with retroviral vectors that express Id1 and were transplanted in vivo to evaluate their developmental potential. Overexpression of Id1 in HSC promotes myeloid but impairs B and erythroid cell development. Enforced expression of Id1 in committed myeloid progenitor cells inhibits granulocyte but not macrophage differentiation. Therefore, Id1 may be part of the mechanism regulating myeloid vs lymphoid/erythroid cell fates, and macrophage vs neutrophil maturation.