Reciprocal products of chromosomal translocations in human cancer pathogenesis: key players or innocent bystanders?

`Targeted pharmacologic inhibition of S-phase kinase-associated protein 2 (SKP2) mediated cell cycle regulation in lung and other RB-Related cancers: A brief review of current status and future prospects

Small cell lung cancer (SCLC) often exhibits Rb deficiency, TRβ and p130 deletion, and SKP2 amplification, suggesting TRβ inactivation and SKP2 activation. It is reported that SKP2 targeted therapy is effective in some cancers in vitro and in vivo, but it is not reported for the treatment of SCLC and retinoblastoma. SKP2 is the synthetic lethal gene in SCLC and retinoblastoma, so SKP2 can be used for targeted therapy in SCLC and retinoblastoma. RB1 knockout mice develop several kinds of tumors, but Rb1 and SKP2 double knockout mice are healthy, suggesting that SKP2 targeted therapy may have significant effects on Rb deficient cancers with less side effects, and if successful in SCLC and retinoblastoma in vitro and in animal model, such compounds may be promising for the clinical treatment of SCLC, retinoblastoma, and variety of Rb deficient cancers. Previously our studies showed that retinoblastomas exhibit retinal cone precursor properties and depend on cone-specific thyroid hormone receptor β2 (TRβ2) and SKP2 signaling. In this study, we sought to suppress SCLC and retinoblastoma cell growth by SKP2 inhibitors as a prelude to targeted therapy in vitro and in vivo. We knocked down TRβ2 and SKP2 or over-expressed p27 in SCLC and retinoblastoma cell lines to investigate SKP2 and p27 signaling alterations. The SCLC cell lines H209 as well as retinoblastoma cell lines Y79, WERI, and RB177 were treated with SKP2 inhibitor C1 at different concentrations, following which Western blotting, Immunostaining, and cell cycle kinetics studies were performed to study SKP2 and p27 expression ubiquitination, to determine impact on cell cycle regulation and growth inhibition. TRβ2 knockdown in Y79, RB177 and H209 caused SKP2 downregulation and degradation, p27 up-regulation, and S phase arrest, whereas, SKP2 knockdown or p27 over-expression caused p27 accumulation and G1-S phase arrest. In the cell lines Y79, WERI, RB177, and H209 treatment with C1 caused SKP2 ubiquitination and degradation, p27 de-ubiquitination and accumulation, and cell growth arrest. SKP2 inhibitor C1 significantly suppressed retinoblastoma as well as SCLC cell growth by SKP2 degradation and p27 accumulation. In vivo study also showed inhibition of tumor growth with C1 treatment. Potential limitations of the success of such a therapeutic approach and its translational application in human primary tumors, and alternative approaches to overcome such limitations are briefly discussed for the treatment of retinoblastoma, SCLC and other RB-related cancers.

Enhanced hemato-endothelial specification during human embryonic differentiation through developmental cooperation between AF4-MLL and MLL-AF4 fusions

The t(4;11)(q21;q23) translocation is associated with high-risk infant pro-B-cell acute lymphoblastic leukemia and arises prenatally during embryonic/fetal hematopoiesis. The developmental/pathogenic contribution of the t(4;11)-resulting MLL-AF4 (MA4) and AF4-MLL (A4M) fusions remains unclear; MA4 is always expressed in patients with t(4;11)+ B-cell acute lymphoblastic leukemia, but the reciprocal fusion A4M is expressed in only half of the patients. Because prenatal leukemogenesis manifests as impaired early hematopoietic differentiation, we took advantage of well-established human embryonic stem cell-based hematopoietic differentiation models to study whether the A4M fusion cooperates with MA4 during early human hematopoietic development. Co-expression of A4M and MA4 strongly promoted the emergence of hemato-endothelial precursors, both endothelial- and hemogenic-primed. Double fusion-expressing hemato-endothelial precursors specified into significantly higher numbers of both hematopoietic and endothelial-committed cells, irrespective of the differentiation protocol used and without hijacking survival/proliferation. Functional analysis of differentially expressed genes and differentially enriched H3K79me3 genomic regions by RNA-sequencing and H3K79me3 chromatin immunoprecipitation-sequencing, respectively, confirmed a hematopoietic/endothelial cell differentiation signature in double fusion-expressing hemato-endothelial precursors. Importantly, chromatin immunoprecipitation-sequencing analysis revealed a significant enrichment of H3K79 methylated regions specifically associated with HOX-A cluster genes in double fusion-expressing differentiating hematopoietic cells. Overall, these results establish a functional and molecular cooperation between MA4 and A4M fusions during human hematopoietic development.

The AF4-MLL fusion transiently augments multilineage hematopoietic engraftment but is not sufficient to initiate leukemia in cord blood CD34+ cells

The translocation t(4;11)(q21;q23) is the hallmark genetic abnormality associated with infant pro-B acute lymphoblastic leukemia (B-ALL) and has the highest frequency of rearrangement in Mixed-lineage leukemia (MLL) leukemias. Unlike other MLL translocations, MLL-AF4-induced proB-ALL is exceptionally difficult to model in mice/humans. Previous work has investigated the relevance of the reciprocal translocation fusion protein AF4-MLL for t(4;11) leukemia, finding that AF4-MLL is capable of inducing proB-ALL without requirement for MLL-AF4 when expressed in murine hematopoietic stem/progenitor cells (HSPCs). Therefore, AF4-MLL might represent a key genetic lesion contributing to t(4;11)-driven leukemogenesis. Here, we aimed to establish a humanized mouse model by using AF4-MLL to analyze its transformation potential in human cord blood-derived CD34⁺ HSPCs. We show that AF4-MLL-expressing human CD34⁺ HSPCs provide enhanced long-term hematopoietic reconstitution in primary immunodeficient recipients but are not endowed with subsequent self-renewal ability upon serial transplantation. Importantly, expression of AF4-MLL in primary neonatal CD34⁺ HSPCs failed to render any phenotypic or hematological sign of disease, and was therefore not sufficient to initiate leukemia within a 36-week follow-up. Species-specific (epi)-genetic intrinsic determinants may underlie the different outcome observed when AF4-MLL is expressed in murine or human HSPCs.

Mouse models of MLL leukemia: recapitulating the human disease

Chromosome translocations involving the mixed lineage leukemia (MLL) gene fuse it in frame with multiple partner genes creating novel fusion proteins (MLL-FPs) that cause aggressive acute leukemias in humans. Animal models of human disease are important for the exploration of underlying disease mechanisms as well as for testing novel therapeutic approaches. Patients carrying MLL-FPs have very few cooperating mutations, making MLL-FP driven leukemias ideal for animal modeling. The fact that the MLL-FP is the main driver mutation has allowed for a wide range of different experimental model systems designed to explore different aspects of MLL-FP leukemogenesis. In addition, MLL-FP driven acute myeloid leukemia (AML) in mice is often used as a general model for AML. This review provides an overview of different MLL-FP mouse model systems and discusses how well they have recapitulated aspects of the human disease as well as highlights the biological insights each model has provided into MLL-FP leukemogenesis. Many promising new drugs fail in the early stages of clinical trials. Lessons learned from past and present MLL-FP models may serve as a paradigm for designing more flexible and dynamic preclinical models for these as well as other acute leukemias.

Somatic Engineering of Oncogenic Chromosomal Rearrangements: A Perspective

The ability to engineer specific mutations in mice has proven essential to advancing our understanding of the molecular basis of cancer. Chromosomal rearrangements, a common and clinically relevant class of cancer-causing mutations, have however remained difficult to faithfully recapitulate in vivo The development of genetic tools for in vivo somatic genome editing has recently overcome this limitation and led to the generation of more sophisticated and accurate preclinical models of human cancers. Here, we review the potential applications of these new technologies to the study of tumor biology and discuss their advantages over more conventional strategies, their limitations, and the remaining challenges. Cancer Res; 76(17); 4918-23. ©2016 AACR.

Dynamics of Tumor Heterogeneity Derived from Clonal Karyotypic Evolution

Numerical chromosomal instability is a ubiquitous feature of human neoplasms. Due to experimental limitations, fundamental characteristics of karyotypic changes in cancer are poorly understood. Using an experimentally inspired stochastic model, based on the potency and chromosomal distribution of oncogenes and tumor suppressor genes, we show that cancer cells have evolved to exist within a narrow range of chromosome missegregation rates that optimizes phenotypic heterogeneity and clonal survival. Departure from this range reduces clonal fitness and limits subclonal diversity. Mapping of the aneuploid fitness landscape reveals a highly favorable, commonly observed, near-triploid state onto which evolving diploid- and tetraploid-derived populations spontaneously converge, albeit at a much lower fitness cost for the latter. Finally, by analyzing 1,368 chromosomal translocation events in five human cancers, we find that karyotypic evolution also shapes chromosomal translocation patterns by selecting for more oncogenic derivative chromosomes. Thus, chromosomal instability can generate the heterogeneity required for Darwinian tumor evolution.

Three gangliogliomas: results of GTG-banding, SKY, genome-wide high resolution SNP-array, gene expression and review of the literature

According to the World Health Organization gangliogliomas are classified as well-differentiated and slowly growing neuroepithelial tumors, composed of neoplastic mature ganglion and glial cells. It is the most frequent tumor entity observed in patients with long-term epilepsy. Comprehensive cytogenetic and molecular cytogenetic data including high-resolution genomic profiling (single nucleotide polymorphism (SNP)-array) of gangliogliomas are scarce but necessary for a better oncological understanding of this tumor entity. For a detailed characterization at the single cell and cell population levels, we analyzed genomic alterations of three gangliogliomas using trypsin-Giemsa banding (GTG-banding) and by spectral karyotyping (SKY) in combination with SNP-array and gene expression array experiments. By GTG and SKY, we could confirm frequently detected chromosomal aberrations (losses within chromosomes 10, 13 and 22; gains within chromosomes 5, 7, 8 and 12), and identify so far unknown genetic aberrations like the unbalanced non-reciprocal translocation t(1;18)(q21;q21). Interestingly, we report on the second so far detected ganglioglioma with ring chromosome 1. Analyses of SNP-array data from two of the tumors and respective germline DNA (peripheral blood) identified few small gains and losses and a number of copy-neutral regions with loss of heterozygosity (LOH) in germline and in tumor tissue. In comparison to germline DNA, tumor tissues did not show substantial regions with significant loss or gain or with newly developed LOH. Gene expression analyses of tumor-specific genes revealed similarities in the profile of the analyzed samples regarding different relevant pathways. Taken together, we describe overlapping but also distinct and novel genetic aberrations of three gangliogliomas.

Structural and functional studies of FKHR-PAX3, a reciprocal fusion gene of the t(2;13) chromosomal translocation in alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma (ARMS) is an aggressive pediatric cancer of skeletal muscle. More than 70% of ARMS tumors carry balanced t(2;13) chromosomal translocation that leads to the production of two novel fusion genes, PAX3-FKHR and FKHR-PAX3. While the PAX3-FKHR gene has been intensely studied, the reciprocal FKHR-PAX3 gene has rarely been described. We report here the cloning and functional characterization of the FKHR-PAX3 gene as the first step towards a better understanding of its potential impact on ARMS biology. From RH30 ARMS cells, we detected and isolated three versions of FKHR-PAX3 cDNAs whose C-terminal sequences corresponded to PAX3c, PAX3d, and PAX3e isoforms. Unlike the nuclear-specific localization of PAX3-FKHR, the reciprocal FKHR-PAX3 proteins stayed predominantly in the cytoplasm. FKHR-PAX3 potently inhibited myogenesis in both non-transformed myoblast cells and ARMS cells. We showed that FKHR-PAX3 was not a classic oncogene but could act as a facilitator in oncogenic pathways by stabilizing PAX3-FKHR expression, enhancing cell proliferation, clonogenicity, anchorage-independent growth, and matrix adhesion in vitro, and accelerating the onset of tumor formation in xenograft mouse model in vivo. In addition to these pro-oncogenic behaviors, FKHR-PAX3 also negatively affected cell migration and invasion in vitro and lung metastasis in vivo. Taken together, these functional characteristics suggested that FKHR-PAX3 might have a critical role in the early stage of ARMS development.

Childhood rhabdomyosarcoma: recent advances and prospective views

Rhabdomyosarcoma (RMS) is a morphologically and clinically heterogeneous group of malignant tumors that resemble developing skeletal muscle and is the most common soft-tissue sarcoma in children and adolescents. The most prominent sites involve head and neck structures (~40%), genito-urinary track (~25%), and extremities (~20%). Embryonal (ERMS) and alveolar (ARMS) are the two major RMS subtypes that are distinct in their morphology and genetic make-up. The prognosis for this cancer depends strongly on tumor size, location, staging, and child's age. In general, ERMS has a more favorable outcome, whereas the mortality rate remains high in patients with ARMS, because of its aggressive and metastatic nature. Over the past two decades, researchers have made concerted efforts to delineate genetic and epigenetic changes associated with RMS pathogenesis. These molecular signatures have presented golden opportunities to design targeted therapies for treating this aggressive cancer. This article highlights recent advances in understanding the molecular pathogenesis of RMS, and addresses promising research areas for further exploration.

A pericentric inv(9)(p22q34) of the der(9)t(9;22)(q34;q11.2) is a recurrent secondary anomaly in Ph-positive leukemia

A pericentric inv(9)(p22q34) of the derivative chromosome 9 that resulted from a standard t(9;22)(q34;q11.2) was identified by R-banding karyotypic analysis and fluorescence in situ hybridization (FISH) assays in 4 (0.18%) of 2,200 Philadelphia chromosome (Ph)-positive leukemia patients, including 3 with chronic myeloid leukemia (CML) in chronic phase and 1 with acute myeloid leukemia (AML) in our hospital since 2004. All four patients had two malignant clones: one with only t(9;22)(q34;q11.2) and another with der(9)t(9;22)(q34;q11.2)inv(9)(p22q34) that resulted in the separation of the ABL1/BCR fusion gene. No metaphases with only inv(9)(p22q34) were seen in any of them. FISH also found a deletion of partial sequence of BCR on der(9)t(9;22)(q34;q11.2)inv(9)(p22q34) in 67.5% of bone marrow cells in the AML patient, but did not detect the deletion of the sequence of ASS/9q34 in these four patients. Reverse transcriptase-polymerase chain reaction revealed a b3a2 type of BCR/ABL1 fusion transcript in all of them, proving their disease to be Ph-positive leukemia. On reviewing the literature, only two solitary Ph-positive leukemia patients have been noticed to have the inv(9)(p22q34) anomaly. These two patients, together with our four documented patients, indicate that inv(9)(p22q34) is a novel, rare, but recurrent secondary chromosomal abnormality for Ph-positive leukemia. Despite receiving hydroxyurea therapy (n = 3 patients), combined chemotherapy (n = 2), even imatinib treatment (n = 1), three patients, including one with AML and two with CML (one of whom progressed into the lymphoblastic blast phase), died with survival times of 28 days, 13 months, and 34 months, respectively. Only one patient with CML remained alive for 5.5 months. Their negative outcome implies that inv(9)(p22q34) has an unfavorable impact on prognosis. Presently, no firm conclusions can be drawn from this study. Because the case number reported here is very small, more patients with this anomaly need to be investigated to elucidate its true prognostic significance.

Hematopoiesis and Retinoids: Development and Disease

Retinoids function as activating ligands for a class of nuclear receptors that control gene expression programs for a wide range of tissues and organs during embryogenesis and throughout life. Over the years, three sets of observations have spurred interest in the function of retinoids with respect to development and disease of hematopoietic cells. Since the 1920s, epidemiological studies indicated altered hematopoiesis in vitamin A-deficient (VAD) human populations. More recently, the ability of retinoids to affect various aspects of hematopoietic development has been demonstrated in vitro. Finally, it was discovered that the gene encoding a retinoid receptor is a key target for chromosomal translocations that cause acute promyelocytic leukemia (APL). More recent investigations using targeted gene disruptions, VAD animal models, and mouse models of leukemia have continued to shed light on the function of the retinoid pathway in blood cells. It is now clear that retinoids are required for normal hematopoiesis during both yolk sac and fetal liver stages of hematopoiesis, while the pathway has at least modulatory functions for bone marrow derived progenitors. Studies of normal development and APL have provided complementary insight into the molecular control of blood cell differentiation. Here we review the evidence for retinoid requirements in hematopoiesis and also summarize current ideas regarding how this pathway is subverted in leukemia.

The role of chromosomal alterations in human cancer development

Cancer cells become unstable and compromised because several cancer-predisposing mutations affect genes that are responsible for maintaining the genomic instability. Several factors influence the formation of chromosomal rearrangements and consequently of fusion genes and their role in tumorigenesis. Studies over the past decades have revealed that recurring chromosome rearrangements leading to fusion genes have a biological and clinical impact not only on leukemias and lymphomas, but also on certain epithelial tumors. With the implementation of new and powerful cytogenetic and molecular techniques the identification of fusion genes in solid tumors is being facilitated. Overall, the study of chromosomal translocations have revealed several recurring themes, and reached important insights into the process of malignant transformation. However, the mechanisms behind these translocations remain unclear. A more thorough understanding of the mechanisms that cause translocations will be aided by continuing characterization of translocation breakpoints and by developing in vitro and in vivo model systems that can generate chromosome translocation.

RAR and RXR modulation in cancer and metabolic disease

Retinoic acid receptors (RARs) are ligand-controlled transcription factors that function as heterodimers with retinoid X receptors (RXRs) to regulate cell growth and survival. The success of RAR modulation in the treatment of acute promyelocytic leukaemia (APL) has stimulated considerable interest in the development of RAR and RXR modulators. This has been aided by recent advances in the understanding of the biological role of RARs and RXRs and in the design of selective receptor modulators that might overcome the limitations of current drugs. Here, we discuss the challenges and opportunities for therapeutic strategies based on RXR and RAR modulators, with a focus on cancer and metabolic diseases such as diabetes and obesity.

Cooperative transformation by MEF2D/DAZAP1 and DAZAP1/MEF2D fusion proteins generated by the variant t(1;19) in acute lymphoblastic leukemia

A variant t(1;19)(q23;p13.3) translocation creates reciprocal DAZAP1/MEF2D and MEF2D/DAZAP1 fusion genes that are expressed in acute lymphoblastic leukemia. We used retroviral gene transfer to ectopically express wild-type and chimeric DAZAP1 and MEF2D fusion proteins in NIH 3T3 cells. In soft agar assays, each of the fusion proteins transformed 3T3 cells with a 20-fold increase in colony formation as compared to empty vector or native MEF2D or DAZAP1 proteins. Co-expression of both DAZAP1/MEF2D and MEF2D/DAZAP1 led to a threefold increase in colony formation as compared to either fusion protein alone. Expression of wild-type DAZAP1, MEF2D or DAZAP1/MEF2D allowed 3T3 cells to proliferate under low serum (0.5%) conditions and suppressed apoptosis. In contrast, MEF2D/DAZAP1 expression did not facilitate proliferation in low serum and led to a modest increase in apoptosis. Both MEF2D/DAZAP1 and DAZAP1/MEF2D have oncogenic properties, and co-expression of both fusion proteins is synergistic.

The impact of translocations and gene fusions on cancer causation

Chromosome aberrations, in particular translocations and their corresponding gene fusions, have an important role in the initial steps of tumorigenesis; at present, 358 gene fusions involving 337 different genes have been identified. An increasing number of gene fusions are being recognized as important diagnostic and prognostic parameters in malignant haematological disorders and childhood sarcomas. The biological and clinical impact of gene fusions in the more common solid tumour types has been less appreciated. However, an analysis of available data shows that gene fusions occur in all malignancies, and that they account for 20% of human cancer morbidity. With the advent of new and powerful investigative tools that enable the detection of cytogenetically cryptic rearrangements, this proportion is likely to increase substantially.

Cryptic terminal chromosome rearrangements in colorectal carcinoma cell lines detected by subtelomeric FISH analysis

Epithelial tumour karyotypes are often difficult to study by standard cytogenetic methods because of poor chromosome preparation quality and the high complexity of their genomic rearrangements. Subtelomeric fluorescence in situ hybridisation (FISH) has proved to be a useful method for detecting cryptic constitutional chromosomal rearrangements but little is known about its usefulness for tumour cytogenetic analysis. Using a combination of chromosome banding, multicolour karyotyping and subtelomeric FISH, five colorectal cancer cell lines were characterised. The resulting data were compared to results from previous studies by comparative genomic hybridisation and spectral karyotyping or multicolour FISH. Subtelomeric FISH made it possible to resolve several highly complex chromosome rearrangements, many of which had not been detected or were incompletely characterised by the other methods. In particular, previously undetected terminal imbalances were found in the two cell lines not showing microsatellite instability.

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.

Nonrandom cell-cycle timing of a somatic chromosomal translocation: The t(X;17) of alveolar soft-part sarcoma occurs in G2

The cell-cycle timing of somatic chromosomal translocations in cancer remains poorly understood but may be relevant to their etiology and the mechanism of their formation. Alveolar soft-part sarcoma (ASPS) is a rare malignant soft-tissue tumor of uncertain lineage that provides an opportunity to address this question. The great majority of ASPSs have relatively simple near-diploid karyotypes characterized by an unbalanced der(17)t(X;17)(p11.2;q25), resulting in nonreciprocal fusion of TFE3 with ASPSCR1 (a.k.a. ASPL), with consequent net gain of Xp11.2-->pter and loss of 17q25-->qter. The presence of a normal X along with the der(17)t(X;17) in ASPSs that occur in men has been well described in previous cytogenetic reports and is most readily explained by a translocation in the G2 phase of the cell cycle. To establish whether formation in G2 is a general feature of the t(X;17), we examined polymorphic loci in Xp11.2-->qter in ASPS from 9 women, including 7 with an unbalanced t(X;17). Our analysis showed that all 7 displayed retention of heterozygosity at all informative markers on Xp11.2-->qter, supporting preferential formation of the t(X;17) in the G2 phase of the cell cycle. Given that the two derivative chromosomes of a translocation in G2 would be expected to segregate together half the time, the predominance of an unbalanced der(17)t(X;17) also raises the possibility of a selective advantage in ASPS cells for gain of Xp11.2-->pter or loss of 17q25.3-->qter or retention of an active copy of TFE3.

Opinion: how patients have benefited from mouse models of acute promyelocytic leukaemia

One of the challenges of studying anticancer therapies is that effects observed in cell lines or mouse models are not always good indicators of clinical trial results. The mouse model of acute promyelocytic leukaemia has bucked this trend, as targeted therapies such as retinoic acid and arsenic induce differentiation and clearance of leukaemia cells in both mice and humans. This mouse model has also provided important mechanistic insights into the combinatorial effects of these agents and has promoted combined therapies that have shown recent success in the clinic.

Benzodithiophenes Induce Differentiation and Apoptosis in Human Leukemia Cells

All-trans retinoic acid (ATRA) induces clinical remission in patients with t(15;17) acute promyelocytic leukemia (APL) carrying leukemogenic promyelocytic leukemia-retinoic acid receptor alpha (PML-RARalpha) fusion protein by overcoming PML-RARalpha transcriptional repression and inducing myeloid differentiation. To identify more potent chemical differentiation inducers, a screening assay was developed utilizing an ATRA-insensitive NB4 cell line (NB4-c) in which differentiation could be measured after 48 hours when primed with ATRA followed by other potential inducers. Over 300 cytostatic agents selected from the National Cancer Institute library were screened using this established method. Three compounds, NSC656243, NSC625748, and NSC144168, were identified to amplify ATRA-induced differentiation with acceptable cytotoxicity in NB4-c cells. In the absence of ATRA, these compounds also induced HL-60 and murine erythroleukemia cells to undergo partial differentiation. NSC656243, a benzodithiophene compound, was selected for further studies to examine the underlying mechanism of action. The differentiation effect of NSC656243 was associated with enhanced ATRA-mediated up-regulation of cell cycle regulatory proteins p21waf1 and p27kip1, retinoblastoma dephosphorylation, expression of RIG-E and RIG-G, and myelomonocytic differentiation-specific down-regulation of the myeloperoxidase (MPO) gene. Moreover, at 2- to 3-fold higher concentrations than those used to synergize with ATRA, NSC656243 induced apoptosis in NB4-c cells by reactive oxygen species-mediated pathways. The dual effects of benzodithiophenes (i.e., differentiation and apoptosis induction) support further development of these compounds as therapeutic agents for leukemia.

The versatile mixed lineage leukaemia gene MLL and its many associations in leukaemogenesis

The marked association of abnormalities of chromosome 11 long arm, band q23, with human leukaemia led to the identification of the 11q23 gene called MLL (or HTRX, HRX, TRX1, ALL-1). MLL can become fused with one of a remarkable panoply of genes from other chromosome locations in individual leukaemias, leading to either acute myeloid or lymphoid tumours (hence the name MLL for mixed lineage leukaemia). The unusual finding that a single protein could be involved in both myeloid and lymphoid malignancies and that the truncated protein could do so as a fusion with very disparate partners has prompted studies to define the molecular role of MLL-fusions in leukaemogenesis and to the development of MLL-controlled mouse models of leukaemogenesis. These studies have defined MLL-fusion proteins as regulators of gene expression, controlling such elements as HOX genes, and have indicated a variety of mechanisms by which MLL-fusion proteins contribute to leukaemogenesis.

The role of CREB as a proto-oncogene in hematopoiesis and in acute myeloid leukemia

CREB is a transcription factor that functions in glucose homeostasis, growth factor-dependent cell survival, and memory. In this study, we describe a role of CREB in human cancer. CREB overexpression is associated with increased risk of relapse and decreased event-free survival. CREB levels are elevated in blast cells from patients with acute myeloid leukemia. To understand the role of CREB in leukemogenesis, we studied the biological consequences of CREB overexpression in primary human leukemia cells, leukemia cell lines, and transgenic mice. Our results demonstrate that CREB promotes abnormal proliferation and survival of myeloid cells in vitro and in vivo through upregulation of specific target genes. Thus, we report that CREB is implicated in myeloid cell transformation.

Granulocyte colony-stimulating factor and leukemogenesis

The granulocyte colony-stimulating factor (G-CSF) plays an important role in normal granulopoiesis. Its functions are mediated by specific receptors on the surface of responsive cells and, upon ligand binding, several cytoplasmic tyrosine kinases are activated. The cytoplasmic region proximal to the membrane of the G-CSF receptor (G-CSF-R) transduces proliferative and survival signals, whereas the distal carboxy-terminal region transduces maturation signals and suppresses the receptor's proliferative signals. Mutations in the G-CSF-R gene resulting in truncation of the carboxy-terminal region have been detected in a subset of patients with severe congenital neutropenia who developed acute myelogenous leukemia (AML). In addition, the AML1-ETO fusion protein, expressed in leukemic cells harboring the t(8;21), disrupt the physiological function of transcription factors such as C/EBPalpha and C/EBPepsilon, which in turn deregulate G-CSF-R expression. The resulting high levels of G-CSF-R and G-CSF-dependent cell proliferation may be associated with pathogenesis of AML with t(8;21). Moreover, in vitro and in vivo studies demonstrated that G-CSF may act as a co-stimulus augmenting the response of PML-RARalpha acute promyelocytic leukemia cells to all-trans-retinoic acid treatment. Finally, in the PLZF-RARalpha acute promyelocytic leukemia transgenic model, G-CSF deficiency suppressed leukemia development. Altogether, these data suggest that the G-CSF signaling pathway may play a role in leukemogenesis.

Reciprocal translocation associated with multiple exostoses in seven members of a three generation family and discovered through an infertile male

We report a four generations family with multiple exostoses segregating with a reciprocal translocation t(8;19)(q24.11;q13.13) in 8 members of three generations. FISH investigations detected a breakage of the dosage-sensitive EXT1 gene. Although three members of the family died perinatally from unknown causes and one carrier had four spontaneous abortions, the translocation was discovered only when the cytogenetic analysis was requested in an affected male because of oligozoospermia. In fact, it is well known that infertile males may be carriers of reciprocal or Robertsonian translocations with a higher frequency than the general population. This family stresses the importance of requesting the cytogenetic analysis in all cases in which a dominant disease segregates with repeated miscarriages and/or newborn deaths of unknown cause.

Molecular pathogenesis of acute promyelocytic leukaemia and APL variants

It has been 12 years since the simultaneous discovery of the unique sensitivity of acute promyelocytic leukaemia (APL) to differentiation therapy with all-trans retinoic acid (ATRA) and the discovery that the retinoic acid receptor alpha (RARalpha) gene was rearranged in APL. Nearly 98% of cases of APL are associated with t(15;17) chromosomal translocation and fusion of the PML gene to that encoding RARalpha to yield an abnormal receptor with the capability of de-regulating gene expression in the haematopoietic cell, causing differentiation block and eventually the development of leukaemia. Since this original discovery, four other translocations were described in APL. In each of these the RARalpha gene is fused to different partner genes, all yielding aberrant nuclear receptors. These fusion proteins share in common the ability to repress rather than activate retinoic acid targets, one so strongly that the result is an ATRA-resistant form of the disease. In addition each of the partner proteins is important for normal cell growth and development. In this chapter we explore the biology of the RARalpha, the fusion proteins created in APL and the normal forms of the partner proteins. Through continued study of this disease it is hoped that novel treatments, potentially more applicable to other forms of leukaemia, may arise.

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.

Transgenics at breaking-point

In this issue of Cancer Cell, Forster et al. (2003) have generated mice that recapitulate both the mechanism (sporadic somatic translocation) and the consequences (expression of two translocation fusion genes) leading to an accurate leukemia model.

Polycomb, epigenomes, and control of cell identity

In development, cell identity is maintained by epigenetic functions that prevent changes in cell type-specific transcription programs. Recent insights into gene silencing mechanisms by Polycomb group (PcG) and trithorax group (trxG) proteins reveal that the memory system involves a concerted process of chromatin modification, blocking of RNA polymerase II, and synthesis of noncoding RNA. Remarkably, cell memory is regulated by a balance between repressors and activators that maintains both transcription status and at the same time the possibility of switching to a different state.

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.