Common themes in the pathogenesis of acute myeloid leukemia

Cellular distribution of IDH mutations in AML during morphologic remission

Limited information exists about the cellular distribution of mutations which persist in remission in acute myeloid leukemia (AML) (variably considered pre-leukemic mutations). We hypothesized that mutations detectable in all cell compartments may be less pathogenic than those that are myeloid-restricted. Here, we describe the cellular compartments that have IDH mutations in five patients with IDH-mutated AML in morphologic remission. Unlike pre-leukemic clones harboring the more common DNMT3A, TET2 and ASXL1 (DTA) mutations, we show that IDH mutations are myeloid-restricted. This finding provides an explanation for the reports that IDH mutations carry a higher risk for relapse than DTA mutations. Detailed analysis of one case also shows acquisition of additional mutations in distinct cellular compartments, illustrating subclonal complexity associated with therapeutics.

Understanding a high-risk acute myeloid leukemia by analyzing the interactome of its major driver mutation

The WHO classifies t(6;9)-positive acute myeloid leukemia (AML) as a subgroup of high-risk AML because of its clinical and biological peculiarities, such as young age and therapy resistance. t(6;9) encodes the DEK/NUP214 fusion oncoprotein that targets only a small subpopulation of bone marrow progenitors for leukemic transformation. This distinguishes DEK/NUP214 from other fusion oncoproteins, such as PML/RARα, RUNX1/ETO, or MLL/AF9, which have a broad target population they block differentiation and increase stem cell capacity. A common theme among most leukemogenic fusion proteins is their aberrant localization compared to their wild-type counterparts. Although the actual consequences are widely unknown, it seems to contribute to leukemogenesis most likely by a sequester of interaction partners. Thus, we applied a global approach to studying the consequences of the aberrant localization of t(6;9)-DEK/NUP214 for its interactome. This study aimed to disclose the role of localization of DEK/NUP214 and the related sequester of proteins interacting with DEK/NUP214 for the determination of the biology of t(6;9)-AML. Here we show the complexity of the biological consequences of the expression of DEK/NUP214 by an in-depth bioinformatic analysis of the interactome of DEK/NUP214 and its biologically dead mutants. DEK/NUP214's interactome points to an essential role for aberrant RNA-regulation and aberrant regulation of apoptosis and leukocyte activation as a significant determinant of the phenotype of t(6;9)-AML. Taken together, we provide evidence that the interactome contributes to the aberrant biology of an oncoprotein, providing opportunities for developing novel targeted therapy approaches.

Activation of signaling pathways in models of t(6;9)-acute myeloid leukemia

Patients within the WHO-subgroup of t(6;9)-positive acute myeloid leukemia (AML) differ from other AML subgroups as they are characterised by younger age and a grim prognosis. Leukemic transformation can often be attributed to single chromosomal aberrations encoding oncogenes, in the case of t(6;9)-AML to the fusion protein DEK-CAN (also called DEK-NUP214). As being a rare disease there is the urgent need for models of t(6;9)-AML. The only cell line derived from a t(6;9)-AML patient currently available is FKH1. By using phospho-proteomics on FKH1 cells, we found a strongly activated ABL1 kinase. Further investigation revealed the presence of ETV6-ABL1. This finding renders necessary to determine DEK-CAN- and ETV6-ABL1-related features when using FKH1. This can be done as ETV6-ABL1 activity in FKH1 is responsive to imatinib. Nevertheless, we provided evidence that both SFK and mTOR activation in FKH1 are DEK-CAN-related features as they were activated also in other t(6;9) and DEK-CAN-positive models. The activation of STAT5 previously shown to be strong in t(6;9)-AML and activated by DEK-CAN is regulated in FKH1 by both DEK-CAN and ETV6-ABL1. In conclusion, FKH1 cells still represent a model for t(6;9)-AML and could serve as model for ETV6-ABL1-positive AML if the presence of these leukemia-inducing oncogenes is adequately considered.Taken together, all our results provide clear evidence of novel and specific interdependencies between leukemia-inducing oncogenes and cancer signaling pathways which will influence the design of therapeutic strategies to better address the complexity of cancer signaling.

Artesunate improves venetoclax plus cytarabine AML cell targeting by regulating the Noxa/Bim/Mcl-1/p-Chk1 axis

Venetoclax plus cytarabine therapy is approved for elderly acute myeloid leukemia (AML) patients and needs further improvement. We studied the mechanisms of venetoclax plus cytarabine treatment and searched for a third agent to enhance their effects. Cytarabine induces S phase arrest-mediated DNA damage with activation of DNA replication checkpoint kinase 1 (Chk1) through phosphorylation, while venetoclax induces B cell lymphoma 2 (Bcl-2)-interacting mediator of cell death (Bim)-mediated apoptotic DNA damage. Myeloid cell leukemia-1 (Mcl-1) plays negative roles in both events by sequestering Bim and accelerating Chk1 phosphorylation. Venetoclax releases Bim from Bcl-2 with increased Bim binding to Mcl-1. Artesunate, an antimalaria drug, induces Noxa to replace Bim from Mcl-1 and induces synergistic apoptosis with venetoclax accompanied with Mcl-1 reduction. Silencing Mcl-1 or adding venetoclax/artesunate diminishes the cytarabine resistance pathway p-Chk1. The triple combination exhibits S phase arrest with enhanced DNA damage, improves AML colony formation inhibition, and prolongs survival of two mice xenograft models compared to the venetoclax/cytarabine dual combination. Artesunate serves as a bridge for venetoclax and cytarabine combination by Noxa and Bim-mediated apoptosis and Mcl-1 reduction. We provide a new triple combination for AML treatment by targeting the Noxa/Mcl-1/Bim axis to reverse Mcl-1/p-Chk1 resistance of cytarabine therapy.

Chitosan as possible inhibitory agents and delivery systems in leukemia

Leukemia is a lethal cancer in which white blood cells undergo proliferation and immature white blood cells are seen in the bloodstream. Without diagnosis and management in early stages, this type of cancer can be fatal. Changes in protooncogenic genes and microRNA genes are the most important factors involved in development of leukemia. At present, leukemia risk factors are not accurately identified, but some studies have pointed out factors that predispose to leukemia. Studies show that in the absence of genetic risk factors, leukemia can be prevented by reducing the exposure to risk factors of leukemia, including smoking, exposure to benzene compounds and high-dose radioactive or ionizing radiation. One of the most important treatments for leukemia is chemotherapy which has devastating side effects. Chemotherapy and medications used during treatment do not have a specific effect and destroy healthy cells besides leukemia cells. Despite the suppressing effect of chemotherapy against leukemia, patients undergoing chemotherapy have poor quality of life. So today, researchers are focusing on finding more safe and effective natural compounds and treatments for cancer, especially leukemia. Chitosan is a valuable natural compound that is biocompatible and non-toxic to healthy cells. Anticancer, antibacterial, antifungal and antioxidant effects are examples of chitosan biopolymer properties. The US Food and Drug Administration has approved the use of this compound in medical treatments and the pharmaceutical industry. In this article, we take a look at the latest advances in the use of chitosan in the treatment and improvement of leukemia.

Study of the Chromosomal Abnormalities and Associated Complex Karyotypes in Hematological Cancer in the Population of West Bengal: A Prospective Observational Study

Introduction Chromosomal instability is an important feature of hematological cancer. The pathogenesis is complex and it involves genetic and epigenetic factors. As a genetic factor, chromosomal instability may play a key role in leukemogenesis. Accumulation of genetic alteration is mainly responsible for numerical and structural chromosomal rearrangement or clonal evaluation. But disease progression is often driven by chromosomal translocation, hyper- or hypodiploidy with structural abnormalities, and complex karyotypes.

Objective This research aimed to study the different types of chromosomal abnormalities in clinically suspected hematological cancer patients.

Materials and Methods Cytogenetic analysis was performed based on phytohemaglutinin stimulated peripheral blood lymphocyte cultures and bone marrow culture, without mitogen, of the respective patients of West Bengal from March 2016 to February 2018. All clinically suspected hematological cancer patients referred for karyotyping to the institutional genetics department have been included without any biasness of sex and age. Karyotypes were described according to the International System for Cytogenetic Nomenclature (ISCN 2005).

Results In the present study, 56 clinically suspected hematological cancer cases were observed and 41 cases of chromosomal rearrangement were found which clearly show chromosomal instability as the main driving force for hematological cancer transformation. Presence of variant Philadelphia chromosomes with classical translocation, mosaic complex karyotypes, variable numerical, and structural chromosomal abnormality, along with severe-to-moderate hypo- and hyperdiploidy, and presence of marker chromosomes were the main findings of this study.

Conclusion The result shows that the detection of chromosomal instability was important for preliminary diagnosis, treatment, prognosis, and further management. So the present study provided additional information about chromosomal instability in hematological cancer at Kolkata and adjoining regions.

Identification of predictive genetic signatures of Cytarabine responsiveness using a 3D acute myeloid leukaemia model

This study reports the establishment of a bone marrow mononuclear cell (BMMC) 3D culture model and the application of this model to define sensitivity and resistance biomarkers of acute myeloid leukaemia (AML) patient bone marrow samples in response to Cytarabine (Ara-C) treatment. By mimicking physiological bone marrow microenvironment, the growth conditions were optimized by using frozen BMMCs derived from healthy donors. Healthy BMMCs are capable of differentiating into major hematopoietic lineages and various types of stromal cells in this platform. Cryopreserved BMMC samples from 49 AML patients were characterized for ex vivo growth and sensitivity to Ara-C. RNA sequencing was performed for 3D and 2D cultures to determine differential gene expression patterns. Specific genetic mutations and/or gene expression signatures associated with the ability of the ex vivo expansion and response to Ara-C were elucidated by whole-exome and RNA sequencing. Data analysis identified unique gene expression signatures and novel genetic mutations associated with sensitivity to Ara-C treatment of proliferating AML specimens and can be used as predictive therapeutic biomarkers to determine the optimal treatment regimens. Furthermore, these data demonstrate the translational value of this ex vivo platform which should be widely applicable to evaluate other therapies in AML.

IL-21-mediated expansion of Vγ9Vδ2 T cells is limited by the Tim-3 pathway

Vγ9Vδ2 T cells are the main γδ T subset in the peripheral blood and lymphoid organs. Previous studies have shown that Vγ9Vδ2 T cells could expand in the presence of phosphoantigens and IL-2 and exert antitumor functions. However, their potency was limited because sustained proliferation could not be achieved, possibly due to exhaustion caused by prolonged antigenic stimulation. In this study, we examined the proliferative response of Vγ9Vδ2 T cells to IL-21, a cytokine previously shown to promote NK cell and CD8 T cell cytotoxicity. We found that IL-21 could significantly improve the proliferation of phosphoantigen-stimulated Vγ9Vδ2 T cells in a dose-dependent manner. However, in acute myeloid leukemia (AML) patients, the efficacy of IL-21 was significantly reduced. Vγ9Vδ2 T cells from AML patients exhibited lower expression of IL-21R, and required higher levels of IL-21 for expansion. IL-21-treated Vγ9Vδ2 T cells from AML patients presented lower increase in STAT1 phosphorylation than Vγ9Vδ2 T cells from healthy volunteers. Interestingly, AML Vγ9Vδ2 T cells presented significantly higher Tim-3 expression than healthy Vγ9Vδ2 T cells. IL-21 treatment further induced Tim-3 upregulation. Blocking Tim-3 increased the proliferation and the STAT phosphorylation in Vγ9Vδ2 T cells in response to IL-21. Together, these results demonstrated that IL-21 could significantly expand the Vγ9Vδ2 T cells, but its efficacy was limited since it also increased the expression of checkpoint molecule Tim-3.

Epidermal growth factor signaling in transformed cells

Members of the epidermal growth factor receptor (EGFR/ErbB) family play a critical role in normal cell growth and development. However, many ErbB family members, especially EGFR, are aberrantly expressed or deregulated in tumors and are thought to play crucial roles in cancer development and metastatic progression. In this chapter, we provide an overview of key mechanisms contributing to aberrant EGFR/ErbB signaling in transformed cells, which results in many phenotypic changes associated with the earliest stages of tumor formation, including several hallmarks of epithelial-mesenchymal transition (EMT). These changes often occur through interaction with other major signaling pathways important to tumor progression, causing a multitude of transcriptional changes that ultimately impact cell morphology, proliferation, and adhesion, all of which are crucial for tumor progression. The resulting mesh of signaling networks will need to be taken into account as new regimens are designed for targeting EGFR for therapeutic intervention. As new insights are gained into the molecular mechanisms of cross talk between EGFR signaling and other signaling pathways, including their roles in therapeutic resistance to anti-EGFR therapies, a continual reassessment of clinical therapeutic regimes and strategies will be required. Understanding the consequences and complexity of EGF signaling and how it relates to tumor progression is critical for the development of clinical compounds and establishing clinical protocols for the treatment of cancer.

Epigenetic mechanisms regulating normal and malignant haematopoiesis: new therapeutic targets for clinical medicine

It is now well established that epigenetic phenomena and aberrant gene regulation play a major role in carcinogenesis. These include aberrant gene silencing by imposing inactive histone marks on promoters, aberrant methylation of DNA at CpG islands, and the active repression of promoters by oncoproteins. In addition, many malignant cells also show aberrant gene activation due to constitutively active signalling. The next frontier in cancer research will be to examine how, at the molecular level, small mutations that alter the regulatory phenotype of a cell give rise after a number of cell divisions to the vast deregulation phenomena seen in malignant cells. This review outlines recent insights into how normal cell differentiation in the haematopoietic system is subverted in leukaemia and it introduces the molecular players involved in this process. It also summarises the results of recent clinical trials trying to reverse aberrant epigenetic regulation by employing agents influencing global epigenetic regulators.

AML1/ETO oncoprotein is directed to AML1 binding regions and co-localizes with AML1 and HEB on its targets

A reciprocal translocation involving chromosomes 8 and 21 generates the AML1/ETO oncogenic transcription factor that initiates acute myeloid leukemia by recruiting co-repressor complexes to DNA. AML1/ETO interferes with the function of its wild-type counterpart, AML1, by directly targeting AML1 binding sites. However, transcriptional regulation determined by AML1/ETO probably relies on a more complex network, since the fusion protein has been shown to interact with a number of other transcription factors, in particular E-proteins, and may therefore target other sites on DNA. Genome-wide chromatin immunoprecipitation and expression profiling were exploited to identify AML1/ETO-dependent transcriptional regulation. AML1/ETO was found to co-localize with AML1, demonstrating that the fusion protein follows the binding pattern of the wild-type protein but does not function primarily by displacing it. The DNA binding profile of the E-protein HEB was grossly rearranged upon expression of AML1/ETO, and the fusion protein was found to co-localize with both AML1 and HEB on many of its regulated targets. Furthermore, the level of HEB protein was increased in both primary cells and cell lines expressing AML1/ETO. Our results suggest a major role for the functional interaction of AML1/ETO with AML1 and HEB in transcriptional regulation determined by the fusion protein.

Complementing mutations in core binding factor leukemias: from mouse models to clinical applications

A great proportion of acute myeloid leukemias (AMLs) display cytogenetic abnormalities including chromosomal aberrations and/or submicroscopic mutations. These abnormalities significantly influence the prognosis of the disease. Hence, a thorough genetic work-up is an essential constituent of standard diagnostic procedures. Core binding factor (CBF) leukemias denote AMLs with chromosomal aberrations disrupting one of the CBF transcription factor genes; the most common examples are translocation t(8;21) and inversion inv(16), which result in the generation of the AML1-ETO and CBFbeta-MYH11 fusion proteins, respectively. However, in murine models, these alterations alone do not suffice to generate full-blown leukemia, but rather, complementary events are required. In fact, a substantial proportion of primary CBF leukemias display additional activating mutations, mostly of the receptor tyrosine kinase (RTK) c-KIT. The awareness of the impact and prognostic relevance of these 'second hits' is increasing with a wider range of mutations tested in clinical trials. Furthermore, novel agents targeting RTKs are emanating rapidly and entering therapeutic regimens. Here, we present a concise review on complementing mutations in CBF leukemias including pathophysiology, mouse models, and clinical implications.

Selective anti-leukaemic activity of low-dose histone deacetylase inhibitor ITF2357 on AML1/ETO-positive cells

We analysed the in vitro effects of a new hydroxamate derivative, ITF2357, on AML cells. ITF2357 potently induced histone acetylation. ITF2357 0.1 microM blocked proliferation and induced apoptosis in AML1/ETO-positive Kasumi-1 cells, while AML1/ETO-negative HL60, THP1 and NB4 cell lines were sensitive only to 1 microM ITF2357. Apoptosis was induced by 0.1 microM ITF2357 in AML1/ETO-positive primary blasts and U937-A/E cells induced to express AML1/ETO, but not in U937-A/E cells non-expressing AML1/ETO. In Kasumi-1 cells 0.1 microM ITF2357 induced AML1/ETO degradation through a caspase-dependent mechanism. ITF2357 0.1 microM also determined DNMT1 efflux from, and p300 influx to, the nucleus. Moreover, 0.1 microM ITF2357 determined local H4 acetylation and release of DNMT1, HDAC1 and AML1/ETO, paralleled by recruitment of p300 to the IL-3 gene promoter. ITF2357 treatment, however, did not induce re-expression of IL-3 gene. Accordingly, the methylation level of IL-3 promoter, as well as of several other genes, was unmodified. In conclusion, ITF2357 emerged as an anti-leukaemic agent very potent on AML cells, and on AML1/ETO-positive cells in particular. More relevantly, clearly emerged from our results that ITF2357 could be an ideal agent to treat AML subtypes presenting AML1/ETO fusion protein which determine HDAC involvement in leukaemogenesis.

Butyrates and decitabine cooperate to induce histone acetylation and granulocytic maturation of t(8;21) acute myeloid leukemia blasts

Core histones are proteins organized in octamers, to which DNA is wrapped more or less tightly, depending on their acetylation status. Gene transcription is regulated by a complex series of epigenetic modifications, i.e., histone modification such as methylation and acetylation, events determined by the enzymatic activity of histone methyltransferases, and histone acetyltransferases, respectively, the latter counterbalanced by histone deacetylases (HDAC). Acetylation of histones facilitates destabilization of DNA-nucleosome interaction and renders DNA more accessible to transcription factors. Methylation of different specific lysine residues of histones is differently linked to euchromatin (transcripted DNA) or heterochromatin (silenced DNA). On the other hand, methylation of the promoter regions of some genes by DNA methyltransferases (DNMT) leads to transcriptional silencing and is a common mechanism to regulate gene expression. In normal eukaryotic cells, DNA methylation and histone acetylation are interdependent and maintain equilibrium, allowing temporal expression of genes. In neoplastic cells, this balance is frequently disrupted. In leukemic cells, hypermethylation of CpG islands in the promoter region of genes critical for cell cycle and maturation is frequent, and DNMTs were found to be overexpressed, findings paralleled by evidence of transcriptional repression of downstream genes. Therefore, the combination of HDAC and DNMT inhibitors has been considered to be a possible therapeutic approach to restore normal gene expression in acute myeloid leukemia (AML) and other diseases. Human AML1/ETO Kasumi cells were exposed to the HDAC inhibitor D1 (O-n-butanoil-2,3-O-isopropylidene-alpha-D: -mannofuranoside) and 5-aza-deoxycytidine (decitabine) alone and in combination. Histone acetylation as measured by flow cytometry was increased following treatment with D1 and the combination of D1 and decitabine. Addition of D1 alone or in combination with decitabine also led to inhibition of cell proliferation and induction of apoptosis. Thus, treatment of AML with HDAC inhibitors such as D1 and DNMT inhibitors such as decitabine might have clinical benefit for patients, especially these presenting subtypes of AML, like AML1/ETO, in which the leukemogenic mechanism involves corepressor protein complexes containing HDAC and DNMT.

Structural basis for recognition of SMRT/N-CoR by the MYND domain and its contribution to AML1/ETO's activity

AML1/ETO results from the t(8;21) associated with 12%-15% of acute myeloid leukemia. The AML1/ETO MYND domain mediates interactions with the corepressors SMRT and N-CoR and contributes to AML1/ETO's ability to repress proliferation and differentiation of primary bone marrow cells as well as to enhance their self renewal in vitro. We solved the solution structure of the MYND domain and show it to be structurally homologous to the PHD and RING finger families of proteins. We also determined the solution structure of an MYND-SMRT peptide complex. We demonstrated that a single amino acid substitution that disrupts the interaction between the MYND domain and the SMRT peptide attenuated AML1/ETO's effects on proliferation, differentiation, and gene expression.

The c-Jun-N-terminal-Kinase inhibitor SP600125 enhances the butyrate derivative D1-induced apoptosis via caspase 8 activation in Kasumi 1 t(8;21) acute myeloid leukaemia cells

We recently showed that the histone deacetylase inhibitor D1 induced apoptosis in the t(8;21) Kasumi 1 acute myeloid leukaemia (AML) cell line and activated caspase 9. The present study characterised the effects of the combined administration of D1 with PD98059, SB203580 or SP600125, specific inhibitors of mitogen-activated protein kinase, extracellular signal-regulated kinases 1 and 2 (ERK1/2), p38 or Jun N-terminal kinase (JNK), respectively. Among these inhibitors, SP600125 was the only one to markedly induce apoptosis and decrease cell proliferation. These experiments showed that SP600125 activated caspase 8 and confirmed that D1 activated the intrinsic pathway of apoptosis, as caspase 8 was not affected while Bcl-2 was down-regulated following D1 administration. The combination of the two drugs enhanced caspase-8 activation and induced apoptosis in an additive fashion. JNK was constitutively activated in the Kasumi 1, NB4, HL60 and THP-1 human AML cell lines, as well as in primary blasts from a t(8;21) AML patient. In all these cells, the pro-apoptotic effect of the two drugs alone was increased when they were combined. On this basis, the combined administration of D1 with SP600125 seems to be very promising as a potential anti-leukaemic tool in AML.

Activity of alemtuzumab in patients with CD52-positive acute leukemia

BACKGROUND

Alemtuzumab is a humanized monoclonal antibody directed against the cell surface antigen CD52 and has demonstrated activity in chronic lymphocytic leukemia and other CD52-positive lymphoproliferative disorders. Because CD52 also is expressed on acute leukemic blasts, the authors investigated the safety and efficacy of alemtuzumab in patients with acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL).

METHODS

Fifteen patients with CD52-positive (> or = 20%), recurrent or refractory acute leukemia (9 patients with AML and 6 patients with ALL) received alemtuzumab at a dose of 30 mg intravenously given 3 times a week (dose escalation during Week 1) for a total of 4 to 12 weeks.

RESULTS

The median age of the patients was 39 years (range, 18-71 years). Patients had received a median of 3 prior therapies (range, 1-5 prior therapies). Two patients (13%) achieved a bone marrow complete response and 1 patient achieved a substantial reduction in bone marrow blasts. No complete remissions were observed. Ten patients developed disease progression while on study. Alemtuzumab was myelosuppressive in nearly all patients. Infusion-related toxicities were common, but usually did not exceed Grade 2 (according to the National Cancer Institute Common Toxicity Criteria). Infectious episodes occurred in 13 patients (87%) and included pneumonia (6 patients), bacteremia (11 patients), fungemia (2 patients), and cytomegalovirus reactivation (2 patients).

CONCLUSIONS

Single-agent alemtuzumab was found to have limited activity in recurrent or refractory acute leukemia. An evaluation in patients with a better prognosis, in combination with other agents or as part of consolidation therapy, is warranted.

The molecular pathogenesis of acute myeloid leukemia

The description of the molecular pathogenesis of acute myeloid leukemias (AML) has seen dramatic progress over the last years. Two major types of genetic events have been described that are crucial for leukemic transformation: alterations in myeloid transcription factors governing hematopoietic differentiation and activating mutations of signal transduction intermediates. These processes are highly interdependent, since the molecular events changing the transcriptional control in hematopoietic progenitor cells modify the composition of signal transduction molecules available for growth factor receptors, while the activating mutations in signal transduction molecules induce alterations in the activity and expression of several transcription factors that are crucial for normal myeloid differentiation. The purpose of this article is to review the current literature describing these genetic events, their biological consequences and their clinical implications. As the article will show, the recent description of several critical transforming mutations in AML may soon give rise to more efficient and less toxic molecularly targeted therapies of this deadly disease.

Acute myeloid leukemia: therapeutic impact of epigenetic drugs

Acute myeloid leukemia (AML) is not a single disease but a group of malignancies in which the clonal expansion of various types of hematopoietic precursor cells in the bone marrow leads to perturbation of the delicate balance between self-renewal and differentiation that is characteristic of normal hematopoiesis. An increasing number of genetic aberrations, such as chromosomal translocations that alter the function of transcription regulatory factors, has been identified as the cause of AML and shown to act by deregulating gene programming at both the genetic and epigenetic level. While the genetic aberrations occurring in acute myeloid leukemia are fairly well understood, we have only recently become aware of the epigenetic deregulation associated with leukemia, in particular with myeloid leukemias. The deposition of epigenetic "marks" on chromatin - post-translational modifications of nucleosomal proteins and methylation of particular DNA sequences - is accomplished by enzymes, which are often embedded in multi-subunit "machineries" that have acquired aberrant functionalities during leukemogenesis. These enzymes are targets for so-called "epi-drugs". Indeed, recent results indicate that epi-drugs may constitute an entirely novel type of anti-cancer drugs with unanticipated potential. Proof-of-principle comes from studies with histone deacetylase inhibitors, promising novel anti-cancer drugs. In this review we focus on the epigenetic mechanisms associated with acute myeloid leukemogenesis and discuss the therapeutic potential of epigenetic modulators such as histone deacetylase and DNA methyltransferase inhibitors.

Global approach to the diagnosis of leukemia using gene expression profiling

Accurate diagnosis and classification of leukemias are the bases for the appropriate management of patients. The diagnostic accuracy and efficiency of present methods may be improved by the use of microarrays for gene expression profiling. We analyzed gene expression profiles in 937 bone marrow and peripheral blood samples from 892 patients with all clinically relevant leukemia subtypes and from 45 nonleukemic controls by U133A and U133B GeneChip arrays. For each subgroup, differentially expressed genes were calculated. Class prediction was performed using support vector machines. Prediction accuracy was estimated by 10-fold cross-validation and was assessed for robustness in a 100-fold resampling approach using randomly chosen test sets consisting of one third of the samples. Applying the top 100 genes of each subgroup, an overall prediction accuracy of 95.1% was achieved that was confirmed by resampling (median, 93.8%; 95% confidence interval, 91.4%-95.8%). In particular, acute myeloid leukemia (AML) with t(15;17), AML with t(8;21), AML with inv(16), chronic lymphatic leukemia (CLL), and pro–B-cell acute lymphoblastic leukemia (pro–B-ALL) with t(11q23) were classified with 100% sensitivity and 100% specificity. Accordingly, cluster analysis completely separated all 13 subgroups analyzed. Gene expression profiling can predict all clinically relevant subentities of leukemia with high accuracy.

Acute myeloid leukemia bearing cytoplasmic nucleophosmin (NPMc+ AML) shows a distinct gene expression profile characterized by up-regulation of genes involved in stem-cell maintenance

Approximately one third of acute myeloid leukemias (AMLs) are characterized by aberrant cytoplasmic localization of nucleophosmin (NPMc+ AML), consequent to mutations in the NPM putative nucleolar localization signal. These events are mutually exclusive with the major AML-associated chromosomal rearrangements, and are frequently associated with normal karyotype, FLT3 mutations, and multilineage involvement. We report the gene expression profiles of 78 de novo AMLs (72 with normal karyotype; 6 without major chromosomal abnormalities) that were characterized for the subcellular localization and mutation status of NPM. Unsupervised clustering clearly separated NPMc+ from NPMc– AMLs, regardless of the presence of FLT3 mutations or non–major chromosomal rearrangements, supporting the concept that NPMc+ AML represents a distinct entity. The molecular signature of NPMc+ AML includes up-regulation of several genes putatively involved in the maintenance of a stem-cell phenotype, suggesting that NPMc+ AML may derive from a multipotent hematopoietic progenitor.

New method to detect histone acetylation levels by flow cytometry

BACKGROUND

Reversible histone acetylation affects chromatin structural organization, thus regulating gene expression and other nuclear events. Levels of histone acetylation are tightly modulated in normal cells, and alterations of their regulating mechanisms have been shown to be involved in tumorigenesis.

METHODS

We developed a new flow cytometric technique for detection of histone acetylation, based on a specific monoclonal antibody that recognizes acetylated histone tails. Bivariate analysis for histone acetylation levels and DNA were performed to study modulation of chromatin organization during the cell cycle and after induction of histone hyperacetylation by the histone deacetylase (HDAC) inhibitor trichostatin A (TSA). Histone acetylation and transcription levels were monitored during differentiation induced by retinoic acid alone or in combination with TSA. Blood samples from patients were analyzed with the described protocol to monitor the effects of HDAC inhibitors in vivo and validate the developed protocol for clinical usage.

RESULTS

Flow cytometric detection of acetylation status can successfully detect modifications induced by HDAC inhibitor treatment in vivo as demonstrated by analysis of various blood samples from patients treated with valproic acid. Changes in acetylation levels during the cell cycle demonstrated a reproducible increase in histone acetylation during the replication phase that was subsequently decreased at the G2M entrance, thus paralleling the behavior of DNA replication and transcriptional activity.

CONCLUSIONS

Multiparameter analysis of histone acetylation and expression of molecular markers, DNA ploidy, and/or cell cycle kinetics can provide a quick and statistically reliable tool for the diagnosis and evaluation of treatment efficacy in clinical trials using HDAC inhibitors.

Lozenge directly activates argos and klumpfuss to regulate programmed cell death

We show that reducing the activity of the Drosophila Runx protein Lozenge (Lz) during pupal development causes a decrease in cell death in the eye. We identified Lz-binding sites in introns of argos (aos) and klumpfuss (klu) and demonstrate that these genes are directly activated targets of Lz. Loss of either aos or klu reduces cell death, suggesting that Lz promotes apoptosis at least in part by regulating aos and klu. These results provide novel insights into the control of programmed cell death (PCD) by Lz during Drosophila eye development.

Molecular signature of retinoic acid treatment in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia characterized by a block of differentiation at the promyelocytic stage. APL patients respond to pharmacological concentrations of all-trans retinoic acid (RA) and disease remission correlates with terminal differentiation of leukemic blasts. The PML/RAR oncogenic transcription factor is responsible for both the pathogenesis of APL and for its sensitivity to RA. In order to identify physiological targets of RA therapy, we analysed gene expression profiles of RA-treated APL blasts and found 1056 common target genes. Comparing these results to those obtained in RA-treated U937 cell lines revealed that transcriptional response to RA is largely dependent on the expression of PML/RAR. Several genes involved in the control of differentiation and stem cell renewal are early targets of RA regulation, and may be important effectors of RA response. Modulation of chromatin modifying genes was also observed, suggesting that specific structural changes in local chromatin domains may be required to promote RA-mediated differentiation. Computational analysis of upstream genomic regions in RA target genes revealed nonrandom distribution of transcription factor binding sites, indicating that specific transcriptional regulatory complexes may be involved in determining RA response.

Ceramide reduction and transcriptional up-regulation of glucosylceramide synthase through doxorubicin-activated Sp1 in drug-resistant HL-60/ADR cells

Treatment with doxorubicin (DOX) induced apoptosis with an increase of ceramide content in drug-sensitive HL-60 cells, but not in drug-resistant HL-60/ADR cells. In HL-60/ADR cells (but not in HL-60 cells), the levels of mRNA, protein, and activity in glucosylceramide synthase (GCS), which converts ceramide to glucosylceramide, were up-regulated in response to DOX. Thus, abrogation of apoptosis in HL-60/ADR cells might be involved in ceramide reduction through DOX-induced up-regulation of GCS function. Because we reported that a GC-rich/Sp1 promoter binding region was of importance in the regulation of GCS expression, the role of Sp1 in DOX-induced up-regulation of GCS and apoptosis was investigated. DOX induced Sp1 activation in HL-60/ADR cells, as assessed by Sp1 gel shift and promoter-luciferase reporter assays, whereas transfection of double-stranded oligodeoxynucleotides (ODNs) containing a GC-rich/Sp1 region (Sp1 decoy ODNs) inhibited DOX-induced Sp1 activation. In addition, DOX-increased mRNA and enzyme activity in GCS were inhibited by Sp1 decoy, in conjunction with corresponding elevations of ceramide content. Moreover, DOX-induced apoptotic cell death was significantly increased in Sp1 decoy ODN-transfected HL-60/ADR cells over mock-transfected HL-60/ADR cells. Together, the results suggest that transcriptional up-regulation of GCS through DOX-induced activation of Sp1 is one potential mechanism to regulate ceramide increase and apoptosis in HL-60/ADR cells.

Interpretation with hindsight

In his 1965 paper 'DNA content of tumours: cytophotometric measurements', Sandritter was for the first time able to relate tumour DNA content to the pathology and progression of a small number of tumours. In subsequent publications, these observations were extended to the progression of a much more comprehensive range of tumours. The interpretation of Sandritter's paper below follows the increasing sophistication of methodologies for tumour DNA content through the existing publications and evaluates the conclusions and hypotheses Sandritter proposed in the light of the contemporary account.

PU.1 is a suppressor of myeloid leukemia, inactivated in mice by gene deletion and mutation of its DNA binding domain

In most myeloid leukemias induced in mice by gamma-radiation, one copy of chromosome 2 has suffered a deletion. To search for a potential tumor suppressor gene in that region, we have delineated the deletions in a panel of these tumors. A commonly deleted region of 2 megabase pairs (Mbp) includes the gene encoding the PU.1 transcription factor, a powerful inducer of granulocytic/monocytic differentiation. Significantly, in 87% of these tumors the remaining PU.1 allele exhibited point mutations in the PU.1 DNA binding domain. Surprisingly, 86% of these mutations altered a single CpG, implicating deamination of deoxycytidine, a common mutational mechanism, as the origin of this lesion. The "hot spot" resides in the codon for a contact residue essential for DNA binding by PU.1. In keeping with a tumor suppressor role for PU.1, enforced expression of wild-type PU.1 in the promyelocytic leukemia cells inhibited their clonogenic growth, induced monocytic differentiation, and elicited apoptosis. The mutant PU.1 found in tumors retained only minimal growth suppressive function. The results suggest that PU.1 normally suppresses development of myeloid leukemia by promoting differentiation and that the combination of gene deletion and a point mutation that impairs its ability to bind DNA is particularly leukemogenic.

Epigenomic changes during leukemia cell differentiation: analysis of histone acetylation and cytosine methylation using CpG island microarrays

Dysregulation of epigenetic control is an important participant in carcinogenesis. The PML/RAR alpha translocation in acute promyelocytic leukemia (APL) is an example where the resultant fusion protein recruits histone deacetylase complexes to target genes resulting in their inappropriate transcriptional repression. All-trans-retinoic acid (ATRA) acts as a ligand that relieves this repression and produces an epigenetic transcriptional reprogramming of the cancer cell. CpG island microarrays were used to analyze the DNA methylation and histone acetylation state of the human APL cell line NB4 before and after differentiation with ATRA as well as normal peripheral blood mononuclear cells (PBMC). Over 70 CpG islands within 1 kb of transcription start of a known gene are aberrantly methylated in NB4 cells compared with PBMC; however, no changes in cytosine methylation were detected following ATRA-induced differentiation. With respect to histone H4 acetylation, over 100 single-copy CpG islands within 1 kb of transcription start of a known human gene became hyperacetylated following ATRA-induced differentiation. One CpG island was aberrantly methylated in NB4 cells, but became hyperacetylated and was induced following ATRA treatment and was associated with the HoxA1 gene, suggesting it may be a target gene of ATRA in APL. In addition to single-copy sequences, a selective increase in acetylation was detected in satellite DNA when compared with other high-copy sequences, such as Alu or rDNA. In summary, ATRA stimulates complex epigenomic changes during leukemic cell differentiation, and monitoring these changes may help to identify new targets of epigenetic dysfunction.

Myeloid cells

Granulocytes and monocytes, collectively called myeloid cells, are differentiated descendants from common progenitors derived from hematopoietic stem cells in the bone marrow. Commitment to either lineage of myeloid cells is controlled by distinct transcription factors followed by terminal differentiation in response to specific colony-stimulating factors and release into the circulation. Upon pathogen invasion, myeloid cells are rapidly recruited into local tissues via various chemokine receptors, where they are activated for phagocytosis as well as secretion of inflammatory cytokines, thereby playing major roles in innate immunity. Genetic alterations in myeloid cells may cause an abnormal increase in mature myeloid or blast cells resulting in chronic or acute myelogenous leukemia.

Nucleolar protein NPM interacts with HDM2 and protects tumor suppressor protein p53 from HDM2-mediated degradation

Nucleophosmin (NPM, B23) is an abundant nucleolar phosphoprotein involved in ribosome biogenesis, and interacts with tumor suppressor proteins p53 and Rb. Here we show that NPM is a UV damage response protein that undergoes nucleoplasmic redistribution and regulates p53 and HDM2 levels and their interaction. By utilizing RNAi approaches and analyses of endogenous and ectopically expressed proteins, we demonstrate that NPM binds HDM2 and acts as a negative regulator of p53-HDM2 interaction. Viral stress, enforced by expression of Kaposi's sarcoma virus K cyclin, causes NPM redistribution, K cyclin-NPM association, and p53 stabilization by dissociation of HDM2-p53 complexes. The results demonstrate novel associations of HDM2 and K cyclin with NPM and implicate NPM as a crucial controller of p53 through inhibition of HDM2.

Chimaerism and erythroid marker expression after microinjection of human acute myeloid leukaemia cells into murine blastocysts

It has been suggested that the embryonic microenvironment can control the survival and the transformed phenotype of tumour cells. Here, we addressed the hypothesis that the murine embryonic microenvironment can induce the differentiation of human tumour cells. To examine such interactions, we injected human leukaemic cells into preimplantation murine blastocysts at embryonic day 3.5 of gestation (E3.5). Microinjection of human KG-1 myeloid leukaemia cells and primary human acute myeloid leukaemia (AML) cells led to the generation of chimaeric embryos and adults. We observed that in E12.5 murine embryos, KG-1 cells were preferentially detected in yolk sac and peripheral blood, while primary AML cells mainly seeded the aorta gonad mesonephros region of chimaeric embryos. Analysis of the donor contribution in 15 different adult tissues showed that progeny of primary AML cells seeded to various haematopoietic and nonhaematopoietic tissues. Chimaeric embryos and adults showed no apparent tumour formation. Furthermore, analysis of chimaeric E12.5 embryos revealed that the progeny of human KG-1 cells activated erythroid-specific human globin and glycophorin A expression. In summary, our data indicate that human AML cells activate markers of erythroid differentiation after injection into early murine embryos.

Acute myeloid leukemia fusion proteins deregulate genes involved in stem cell maintenance and DNA repair

Acute myelogenous leukemias (AMLs) are genetically heterogeneous and characterized by chromosomal rearrangements that produce fusion proteins with aberrant transcriptional regulatory activities. Expression of AML fusion proteins in transgenic mice increases the risk of myeloid leukemias, suggesting that they induce a preleukemic state. The underlying molecular and biological mechanisms are, however, unknown. To address this issue, we performed a systematic analysis of fusion protein transcriptional targets. We expressed AML1/ETO, PML/RAR, and PLZF/RAR in U937 hemopoietic precursor cells and measured global gene expression using oligonucleotide chips. We identified 1,555 genes regulated concordantly by at least two fusion proteins that were further validated in patient samples and finally classified according to available functional information. Strikingly, we found that AML fusion proteins induce genes involved in the maintenance of the stem cell phenotype and repress DNA repair genes, mainly of the base excision repair pathway. Functional studies confirmed that ectopic expression of fusion proteins constitutively activates pathways leading to increased stem cell renewal (e.g., the Jagged1/Notch pathway) and provokes accumulation of DNA damage. We propose that expansion of the stem cell compartment and induction of a mutator phenotype are relevant features underlying the leukemic potential of AML-associated fusion proteins.

Diagnostic value of detecting fusion proteins derived from chromosome translocations in acute leukaemia

Clonal chromosomal abnormalities such as balanced translocations are characteristic features of several human leukaemias and have long been detected by conventional cytogenetics on banded metaphases. The advent of molecular biology techniques, advanced karyotyping and immunohistochemistry methods has not only allowed identification of gene involvement at altered chromosome sites and better knowledge of leukaemia pathogenesis, but also contributed important improvements in diagnosis of these heterogeneous diseases. Such novel diagnostic strategies are nowadays being increasingly used to improve leukaemia classification, and in several instances, they help to establish the most appropriate therapeutic strategy in individual patients. Moreover, at least two leukaemia-associated fusion proteins derived from chromosome translocation are specifically targeted by therapeutic approaches which result in significantly increased anti-leukaemia efficacy and reduced toxicity. In this chapter, we highlight the importance of identifying these genetic lesions at diagnosis in acute leukaemia. Further, we discuss briefly the clinical utility of detecting these alterations for prognostic assessment and evaluation of response to treatment.

DNA microarray analysis of stage progression mechanism in myelodysplastic syndrome

Myelodysplastic syndrome (MDS) is a clonal disorder of haematopoietic stem cells. Despite the high incidence of MDS in the elderly, effective treatment of individuals in its advanced stages is problematic. DNA microarray analysis is a potentially informative approach to the development of new treatments for MDS. However, a simple comparison of 'transcriptomes' of bone marrow mononuclear cells among individuals at distinct stages of MDS would result in the identification of genes whose expression differences only reflect differences in the proportion of MDS blasts within bone marrow. Such a 'population shift' effect has now been avoided by purification of haematopoietic stem-like cells that are positive for the cell surface marker AC133 from the bone marrow of healthy volunteers and 30 patients at various stages of MDS. Microarray analysis with the AC133+ cells from these individuals resulted in the identification of sets of genes with expression that was specific to either indolent or advanced stages of MDS. The former group of genes included that for PIASy, which catalyses protein modification with the ubiquitin-like molecule SUMO. Induction of PIASy expression in a mouse myeloid cell line induced apoptosis. A loss of PIASy expression may therefore contribute directly to the growth of MDS blasts and stage progression.

Changes in apoptosis-related pathways in acute myelocytic leukemia

Expression analysis of apoptotic genes was performed for 15 patients with acute myelocytic leukemia (AML) at the time of diagnosis to identify genes and signaling pathways involved in the regulation of cell survival and apoptosis during leukemogenesis. cDNA array analysis revealed 34 genes whose expression was significantly different compared to others. Tumor suppressor genes TP53 and CDKN2A were downregulated and protooncogenes JUN and GRB10 were upregulated. Furthermore, several cellular signaling pathways acting either in cell cycle regulation or in apoptosis were altered. Deregulation was found in pathways that contribute to genomic stability (by downregulation of either TP53 or CSE1L and by upregulation of GADD45A) and regulate cell cycle progression (by downregulation of CDKN2A and upregulation of RBBP4, CDC37, and NEDD5). Alterations at the transcriptional level were identified, namely, upregulation of JUN and E2F5. Abnormalities were observed in the regulation of the caspases through upregulation of CASP8 and by altered expression of BCL2-related pathway. Extrinsic apoptotic signals mediated by IGFs were deregulated and the glutathione detoxification pathway was downregulated. These findings provide insight into the regulation of balance between apoptosis and cell proliferation signals, and suggest that these genes and pathways may have an important role in the pathogenesis of AML.

Comments on the 2001 WHO proposal for the classification of haematopoietic neoplasms

In the preface, the World Health Organization (WHO) classification vows to offer pathologists, oncologists and geneticists worldwide a system of classification for human neoplasms based on histopathological and genetic features. Standardization of nomenclature and agreed-upon criteria for definition of the various types of cancer are felt to be a prerequisite for progress in clinical oncology, multicentre therapy trials and comparative studies in different countries. In fact, the WHO effort represents the first worldwide comprehensive consensus classification of the haematological malignancies. Consensus was reached among a subgroup of investigators, carefully selected for their experience and contributions to existing classifications. In the present climate of daily new discoveries that yield a constant stream of fascinating insights into the biology of leukaemias and lymphomas and, above all, resulting in an explosion of potential therapeutic targets, the WHO system has taken the stand of compiling established classification approaches and providing order to known facts. This furnishes an essential skeleton upon which to build in the future. The WHO committee decided that sorting neoplasms according to prognosis was neither practical nor necessary and could be misleading. While justifiable at the present time, it is important to realize that the classifications of the haematological malignancies are a moving target and that the trend is to move away from currently accepted gold standards, such as morphological evaluations, in favour of genetic characterizations, especially those with therapeutic relevance. The goal of this chapter is to fill in some gaps that, as per the author's opinion, exist in the WHO classification, predominantly, where it concerns the role of immunophenotyping as a complementary discipline for genotyping through its potential to generate surrogate marker profiles for molecular lesions. By introducing some state-of-the-art classification modalities, some of which are still awaiting confirmation, this chapter also aims to spark excitement and provide a glimpse at the future.

Normal and leukemic hematopoiesis: are leukemias a stem cell disorder or a reacquisition of stem cell characteristics?

Leukemia can be viewed as a newly formed, abnormal hematopoietic tissue initiated by a few leukemic stem cells (LSCs) that undergo an aberrant and poorly regulated process of organogenesis analogous to that of normal hematopoietic stem cells. A hallmark of all cancers is the capacity for unlimited self-renewal, which is also a defining characteristic of normal stem cells. Given this shared attribute, it has been proposed that leukemias may be initiated by transforming events that take place in hematopoietic stem cells. Alternatively, leukemias may also arise from more committed progenitors caused by mutations and/or selective expression of genes that enhance their otherwise limited self-renewal capabilities. Identifying the LSCs for each type of leukemia is a current challenge and a critical step in understanding their respective biologies and may provide key insights into more effective treatments. Moreover, LSC identification and purification will provide a powerful diagnostic, prognostic, and therapeutic tool in the clinic.

Megakaryoblastic leukemia 1, a potent transcriptional coactivator for serum response factor (SRF), is required for serum induction of SRF target genes

Megakaryoblastic leukemia 1 (MKL1) is a myocardin-related transcription factor that we found strongly activated serum response element (SRE)-dependent reporter genes through its direct binding to serum response factor (SRF). The c-fos SRE is regulated by mitogen-activated protein kinase phosphorylation of ternary complex factor (TCF) but is also regulated by a RhoA-dependent pathway. The mechanism of this pathway is unclear. Since MKL1 (also known as MAL, BSAC, and MRTF-A) is broadly expressed, we assessed its role in serum induction of c-fos and other SRE-regulated genes with a dominant negative MKL1 mutant (DN-MKL1) and RNA interference (RNAi). We found that DN-MKL1 and RNAi specifically blocked SRE-dependent reporter gene activation by serum and RhoA. Complete inhibition by RNAi required the additional inhibition of the related factor MKL2 (MRTF-B), showing the redundancy of these factors. DN-MKL1 reduced the late stage of serum induction of endogenous c-fos expression, suggesting that the TCF- and RhoA-dependent pathways contribute to temporally distinct phases of c-fos expression. Furthermore, serum induction of two TCF-independent SRE target genes, SRF and vinculin, was nearly completely blocked by DN-MKL1. Finally, the RBM15-MKL1 fusion protein formed by the t(1;22) translocation of acute megakaryoblastic leukemia had a markedly increased ability to activate SRE reporter genes, suggesting that its activation of SRF target genes may contribute to leukemogenesis.

The fusion protein AML1-ETO in acute myeloid leukemia with translocation t(8;21) induces c-jun protein expression via the proximal AP-1 site of the c-jun promoter in an indirect, JNK-dependent manner

Overexpression of proto-oncogene c-jun and constitutive activation of the Jun N-terminal kinase (JNK) signaling pathway have been implicated in the leukemic transformation process. However, c-jun expression and the role of the JNK signaling pathway have not been investigated in primary acute myeloid leukemia (AML) cells with frequently observed balanced rearrangements such as t(8;21). In the present study, we report elevated c-jun mRNA expression in AML patient bone marrow cells with t(8;21), t(15;17) or inv(16), and a high correlation in mRNA expression levels of AML1-ETO and c-jun within t(8;21)-positive AML patient cells. In myeloid U937 cells, c-jun mRNA and protein expression increase upon inducible expression of AML1-ETO. AML1-ETO transactivates the human c-jun promoter through the proximal activator protein (AP-1) site by activating the JNK pathway. Overexpression of JNK-inhibitor JIP-1 and chemical JNK inhibitors reduce the transactivation capacity of AML1-ETO on the c-jun promoter and the proapoptotic function of AML1-ETO in U937 cells. An autocrine mechanism involving granulocyte-colony stimulating factor (G-CSF) and G-CSF receptor (G-CSF-R) might participate in AML1-ETO mediated JNK-signaling, because AML1-ETO induces G-CSF and G-CSF-R expression, and G-CSF-R-neutralizing antibodies reduce AML1-ETO-induced JNK phosphorylation. These data suggest a model in which AML1-ETO induces proto-oncogene c-jun expression via the proximal AP-1 site of the c-jun promoter in a JNK-dependent manner.

Fusion of FIG to the receptor tyrosine kinase ROS in a glioblastoma with an interstitial del(6)(q21q21)

The transmembrane proto-oncogene receptor tyrosine kinase (RTK) ROS is an orphan receptor that is aberrantly expressed in neoplasms of the central nervous system. Here, we report the fusion of its carboxy-terminal kinase domain to the amino-terminal portion of a protein called FIG (Fused in Glioblastoma) in a human glioblastoma multiforme (GBM). By characterizing both FIG and ROS genes in normal and in U118MG GBM cells, we determined that an intra-chromosomal homozygous deletion of 240 kilobases on 6q21 is responsible for the formation of the FIG-ROS locus. The FIG-ROS transcript is encoded by 7 FIG exons and 9 ROS-derived exons. We also demonstrate that the FIG-ROS locus encodes for an in-frame fusion protein with a constitutively active kinase activity, suggesting that FIG-ROS may act as an oncogene. This is the first example of a fusion RTK protein that results from an intra-chromosomal deletion, and it represents the first fusion RTK protein isolated from a human astrocytoma.

IL-6-independent expression of Mcl-1 in human multiple myeloma

Mcl-1 is a critical antiapoptotic survival factor for human multiple myeloma (MM). We examined the importance of IL-6 for Mcl-1 expression in five MM cell lines and in primary MM cells from 14 patients. While culture of MM.1S cells in IL-6 did induce Mcl-1 expression, four other MM cell lines exhibited high levels of Mcl-1 expression that were unaffected by IL-6. Similarly, Mcl-1 expression in 10 of 14 primary MM isolates was found to be IL-6-independent. An analysis of the mechanisms responsible for IL-6-independent Mcl-1 expression was undertaken. ERK1/2 activity did not influence Mcl-1 expression, distinct from Mcl-1 regulation that occurs during myeloid differentiation from hematopoietic progenitor cells. Inhibition of the PI3K pathway led to growth inhibition of 8226 and ANBL-6 cells without reduction of Mcl-1 levels, and high level Mcl-1 expression was maintained in the absence of activated STAT3. Analysis of the transcriptional activity of 5'-regulatory sequences from the human Mcl-1 gene in MM cells demonstrated high levels of IL-6-independent indicator gene activation as predicted. These data demonstrate that the mechanisms regulating Mcl-1 levels in MM cells are heterogeneous, and are often independent from IL-6 signaling pathways.

[Molecular basis of the t(1;22)(p13;q13) specific for human acute megakaryoblastic leukemia]

The t(1;22)(p13;q13) translocation is specifically associated with infant acute megakaryoblastic leukemia (M7). We have recently characterized the two genes involved in this translocation: OTT (One Two Two) and MAL (Megakaryoblastic Acute Leukemia) respectively located on chromosome 1 and 22. The t(1;22) translocation results in the fusion of these genes in all the cases studied to date. We summarize here present knowledge regarding this translocation.

The use of non-tumor data in cancer risk assessment: reflections on butadiene, vinyl chloride, and benzene

The estimation and characterization of a cancer risk is grounded in the observation of tumors in humans and/or experimental animals. Increasingly, however, other kinds of data (non-tumor data) are finding application in cancer risk assessment. Metabolism and kinetics, adduct formation, genetic damage, mode of action, and biomarkers of exposure, susceptibility, and effects are examples. While these and other parameters have been studied for many important chemicals over the past 30-40 years, their use in risk assessments is more recent, and new insights and opportunities are continuing to unfold. To provide some perspective on this field, the ILSI Risk Science Institute asked a select working group to characterize the pertinent non-tumor data available for 1,3-butadiene, benzene, and vinyl chloride and to comment on the utility of these data in characterizing cancer risks. This paper presents the findings of that working group and concludes with 15 simple principles for the use of non-tumor data in cancer risk assessment.

Nuclear receptor superfamily: Principles of signaling

Nuclear receptors (NRs) comprise a family of 49 members that share a common structural organization and act as ligand-inducible transcription factors with major (patho)physiological impact. For some NRs (“orphan receptors”), cognate ligands have not yet been identified or may not exist. The principles of DNA recognition and ligand binding are well understood from both biochemical and crystal structure analyses. The 3D structures of several DNA-binding domains (DBDs),in complexes with a variety of cognate response elements, and multiple ligand-binding domains (LBDs), in the absence (apoLBD)and presence (holoLBD) of agonist, have been established and reveal canonical structural organization. Agonist binding induces a structural transition in the LBD whose most striking feature is the relocation of helix H12, which is required for establishing a coactivator complex, through interaction with members of the p160 family (SRC1, TIF2, AIB1) and/or the TRAP/DRIP complex. The p160-dependent coactivator complex is a multiprotein complex that comprises histone acetyltransferases (HATs), such as CBP,methyltransferases, such as CARM1, and other enzymes (SUMO ligase,etc.). The agonist-dependent recruitment of the HAT complex results in chromatin modification in the environment of the target gene promoters, which is requisite to, or may in some cases be sufficient for, transcription activation. In the absence of ligands, or in the presence of some antagonists, certain NRs are bound to distinct multiprotein complexes through the interaction with corepressors, such as NCoR and SMRT. Corepressor complexes comprise histone deacetylases (HDACs) that have the capacity to condense chromatin over target gene promoters. Ligands have been designed that selectively modulate the interaction between NRs and their coregulators. Both HATs and HDACs can also modify the acetylation status of nonhistone proteins, but the significance in the context of NR signaling is unclear. NRs communicate with other intracellular signaling pathways on a mutual basis, and their functionality may be altered, positively or negatively, by post-translational modification. The majority of NRs act as retinoid X receptor (RXR) heterodimers in which RXR cannot a priori respond autonomously to its cognate ligand to activate target gene transcription. This RXR subordination allows signaling pathway identity for the RXR partner. The corresponding mechanism is understood and reveals cell and NR selectivity, indicating that RXR can, under certain conditions, act autonomously. NRs are regulators of cell life and death,and NR malfunction can be at the basis of both disease and therapy, as is impressively documented in the case of acute promyelocytic leukemia. Recently, several pathways have been uncovered that link NR action with cell proliferation and apoptosis.

Molecular genetics of human leukemias: new insights into therapy

Significant advances have occurred in understanding the molecular pathogenesis of human leukemias. Analysis of patient karyotypes reveals that nonrandom, somatically acquired translocations and inversions occur in most acute myeloid leukemias. Among these, fusion oncogenes have been identified that utilize similar signal transduction pathways and transcriptional activation pathways to mediate their leukemogeneic effect. In chronic myeloid leukemia (CML), both in vitro and in vivo animal studies show that BCR-AB expression leads to clinical manifestations of CML, demonstrating that BCR-AB and its fusion proteins are central mediators of myeloid proliferation and transformation in these malignancies. In other CML syndromes (chronic myelomonocytic leukemia, atypical CML), cloning of chromosomal translocation breakpoints has identified a spectrum of constitutively activated tyrosine kinases. These tyrosine kinase fusions alone apparently are both necessary and sufficient to recapitulate the disease phenotype in the murine model. In contrast, acute myelogenous leukemia (AML) is typified by chromosomal translocations involving transcription factors needed for normal myeloid differentiation. The functional consequence of translocations is loss of function of these transcription factors, resulting in impaired myeloid differentiation. However, these alone are not sufficient to cause acute leukemia; evidence strongly supports the hypothesis that second mutations are required. Data suggest a multistep pathogenesis for AML in which class I mutations, such as activating point mutations in receptor tyrosine kinases (eg, FLT3 and c-KIT), provide a proliferative and/or survival signal to hematopoietic progenitors. Class II mutations are those targeting hematopoietic transcription factors and serving primarily to impair differentiation and subsequent apoptosis. Together, these mutations result in leukemic cells capable of proliferation and survival but not differentiation. The clinical and therapeutic implication is that it may be possible to target both classes of mutations using selected or screened small-molecule inhibitors. Insights gained from molecular genetic analysis of AML provide the basis for a rational, targeted therapeutic approach.

Advancing the treatment of hematologic malignancies through the development of targeted interventions

Significant advances have been made in the development of targeted interventions for hematologic malignancies. Progress has been made in defining the molecular pathogenesis of human leukemias. Data indicate that nonrandom, somatically acquired translocations, inversions, and other abnormalities occur in many acute leukemias. In the treatment of acute promyelocytic leukemia (APL), targeted therapy with all-trans retinoic acid (ATRA) and anthracycline-based chemotherapy leads to dramatic improvements in disease-free survival. Imatinib mesylate, a signal transduction inhibitor that inhibits tyrosine kinase activity, the protein product of the ABL proto-oncogene, has remarkable activity in patients with chronic myeloid leukemia (CML) and Philadelphia chromosome-positive (Ph(+)) acute lymphoblastic leukemia (ALL). Farnesyltransferase inhibitors (FTIs), a promising class of agents that target multiple pathways including Ras proteins, are potential anticancer therapy for a wide range of malignancies, including leukemias and myelodysplastic syndromes (MDS). There also is evidence that recombinant human erythropoietin therapy (r-HuEPO) can benefit patients with chronic lymphocytic leukemia (CLL), multiple myeloma, and lymphomas. This supplement will discuss advances in our understanding of human leukemias, including the use of unconjugated monoclonal antibodies such as Campath-1H (Wellcome, Beckenham, UK, and Ilex Oncology, San Antonio, TX) and rituximab and immunoconjugates such as gemtuzumab ozogamicin and BL-22. Although these novel therapies are beginning to fulfill their promise, continued research efforts are needed to determine the optimal role of targeted therapy in acute and chronic leukemias.

Transcription factor fusions in acute leukemia: variations on a theme

The leukemia-associated fusion proteins share several structural or functional similarities, suggesting that they may impart a leukemic phenotype through common modes of transcriptional dysregulation. The fusion proteins generated by these translocations usually contain a DNA-binding domain, domains responsible for homo- or hetero-dimerization, and domains that interact with proteins involved in chromatin remodeling (e.g., co-repressor molecules or co-activator molecules). It is these shared features that constitute the 'variations on the theme' that underling the aberrant growth and differentiation that is the hallmark of acute leukemia cells.