Chimeric homeobox gene E2A-PBX1 induces proliferation, apoptosis, and malignant lymphomas in transgenic mice

Comprehensive summary: the role of PBX1 in development and cancers

PBX1 is a transcription factor that can promote the occurrence of various tumors and play a reg-ulatory role in tumor growth, metastasis, invasion, and drug resistance. Furthermore, a variant generated by fusion of E2A and PBX1, E2A-PBX1, has been found in 25% of patients with childhood acute lymphoblastic leukemia. Thus, PBX1 is a potential therapeutic target for many cancers. Here, we describe the structure of PBX1 and E2A-PBX1 as well as the molecular mecha-nisms whereby these proteins promote tumorigenesis to provide future research directions for developing new treatments. We show that PBX1 and E2A-PBX1 induce the development of highly malignant and difficult-to-treat solid and blood tumors. The development of specific drugs against their targets may be a good therapeutic strategy for PBX1-related cancers. Furthermore, we strongly recommend E2A-PBX1 as one of the genes for prenatal screening to reduce the incidence of childhood hematological malignancies.

PBX1: a key character of the hallmarks of cancer

Pre-B-cell leukemia homeobox transcription factor 1 (PBX1) was first identified as part of a fusion protein resulting from the chromosomal translocation t(1;19) in pre-B cell acute lymphoblastic leukemias. Since then, PBX1 has been associated with important developmental programs, and its expression dysregulation has been related to multifactorial disorders, including cancer. As PBX1 overexpression in many cancers is correlated to poor prognosis, we sought to understand how this transcription factor contributes to carcinogenesis, and to organize PBX1's roles in the hallmarks of cancer. There is enough evidence to associate PBX1 with at least five hallmarks: sustaining proliferative signaling, activating invasion and metastasis, inducing angiogenesis, resisting cell death, and deregulating cellular energetics. The lack of studies investigating a possible role for PBX1 on the remaining hallmarks made it impossible to defend or refute its contribution on them. However, the functions of some of the PBX1's transcription targets indicate a potential engagement of PBX1 in the avoidance of immune destruction and in the tumor-promoting inflammation hallmarks. Interestingly, PBX1 might be a player in tumor suppression by activating the transcription of some DNA damage response genes. This is the first review organizing PBX1 roles into the hallmarks of cancer. Thus, we encourage future studies to uncover the PBX1's underlying mechanisms to promote carcinogenesis, for it is a promising diagnostic and prognostic biomarker, as well as a potential target in cancer treatment.

Oncogenic heterogeneous nuclear ribonucleoprotein D-like modulates the growth and imatinib response of human chronic myeloid leukemia CD34+ cells via pre-B-cell leukemia homeobox 1

Chronic myeloid leukemia (CML) originates from normal hematopoietic stem cells acquiring BCR-ABL fusion gene, specific BCR-ABL inhibitors (e.g., imatinib mesylate, IM) have greatly improved patient management. However, some patients are still suffering from relapse and drug resistance, which urges better understanding of the growth/survival mechanisms of CML stem/progenitor cells. In the present study, the role and its underlying mechanism of heterogeneous nuclear ribonucleoprotein D-like (HNRPDL) in CML cells were investigated. Firstly, overexpression of HNRPDL promoted the growth of murine BaF3 cells in vitro and induced leukemia in vivo, which was enhanced by co-expression of BCR-ABL. Conversely, HNRPDL silencing inhibited colony-forming cell (CFC) production of CML CD34⁺ cells and attenuated BCR-ABL induced leukemia. In addition, HNRPDL modulated imatinib response of K562 cells and HNRPDL silencing sensitized CML CD34⁺ cells to imatinib treatment. Mechanistically, we found the stability of pre-B-cell leukemia homeobox 1 (PBX1) mRNA was sustained by HNRPDL through its binding to a specific motif (ACUAGC) in 3'-untranslated region (3'-UTR) of PBX1. The expression of PBX1 was significantly higher in CML CD34⁺ cells than that in control cells and PBX silencing inhibited the growth of CML cells and sensitized them to imatinib treatment. In contrast, overexpression of PBX1 elevated the CFC production of normal hematopoietic CD34⁺ cells and "rescued" HNRPDL silencing induced growth inhibition and imatinib sensitization. Taken together, our data have demonstrated that HNRPDL transforms hematopoietic cells and a novel HNRPDL/PBX1 axis plays an important role in human CML CD34⁺ cells.

Comparative analysis of lncRNA and mRNA expression profiles between periodontal ligament stem cells and gingival mesenchymal stem cells

Oral tissue-derived mesenchymal stem cells, such as periodontal ligament stem cells (PDLSCs) and gingival mesenchymal stem cells (GMSCs), possess different biological characteristics, but the molecular mechanism remains unclear, which restricts their application in tissue engineering. Long noncoding RNAs (lncRNAs) are known to be significant regulators of gene expression, but our knowledge about their roles in the regulation of stem cell biological properties is still limited. This study compared the lncRNA and mRNA expression profiles between PDLSCs and GMSCs through microarray analysis, and applied bioinformatics methods to analyze and predict the function and connection of differentially expressed genes, aiming to screen potential key regulators of diverse biological characteristics in PDLSCs and GMSCs. Microarray analysis showed that 2162 lncRNAs and 1347 mRNAs were significantly differentially expressed between PDLSCs and GMSCs. Gene ontology (GO) analysis and pathway analysis indicated that these differentially expressed genes were involved in diverse biological processes and signaling pathways. The gene signal network and pathway relation network predicted some potentially important regulators. The coding-noncoding gene coexpression network (CNC network) revealed many potential lncRNA-mRNA connection pairs that participated in the regulation of biological behaviors. These results stressed the roles of lncRNAs in controlling stem cell biological behaviors and provided guides for molecular mechanistic study of different biological characteristics in PDLSCs and GMSCs.

Paradoxical role of Id proteins in regulating tumorigenic potential of lymphoid cells

A family of transcription factors known as Id proteins, or inhibitor of DNA binding and differentiation, is capable of regulating cell proliferation, survival and differentiation, and is often upregulated in multiple types of tumors. Due to their ability to promote self-renewal, Id proteins have been considered as oncogenes, and potential therapeutic targets in cancer models. On the contrary, certain Id proteins are reported to act as tumor suppressors in the development of Burkitt's lymphoma in humans, and hepatosplenic and innate-like T cell lymphomas in mice. The contexts and mechanisms by which Id proteins can serve in such contradictory roles to determine tumor outcomes are still not well understood. In this review, we explore the roles of Id proteins in lymphocyte development and tumorigenesis, particularly with respect to inhibition of their canonical DNA binding partners known as E proteins. Transcriptional regulation by E proteins, and their antagonism by Id proteins, act as gatekeepers to ensure appropriate lymphocyte development at key checkpoints. We re-examine the derailment of these regulatory mechanisms in lymphocytes that facilitate tumor development. These mechanistic insights can allow better appreciation of the context-dependent roles of Id proteins in cancers and improve considerations for therapy.

PBX1 promotes the cell proliferation via JAK2/STAT3 signaling in clear cell renal carcinoma

PBX1 was abnormally overexpressed and its intracellular localization was found to be frequently amplified in many types of cancer, including renal clear cell carcinoma. PBX1 displays oncogenic activity in several different types of cells, but little is known about how signaling pathways are altered, and the function of PBX1 in renal clear cell carcinoma has not been well investigated. In this study, we demonstrate that the expression of PBX1 was significantly upregulated in 30 pairs of human tissues compared to adjacent normal tissues and the overall survival rate of PBX1-high group was significantly worse than that of PBX1-low group. Furthermore, JAK2 expression is significantly correlated to PBX1 expression in human clinical specimen and PBX1 knockdown inhibits STAT3 phosphorylation and reduced transcription of STAT3 target genes Cyclin D1. More interestingly, PBX1 knockdown inhibits ccRCC cell viability, proliferation and cell cycle progression in vivo and in vitro. Thus, our results demonstrate that PBX1 plays an oncogenic role in ccRCC via JAK2/STAT3 pathway and indicate its potential application for the treatment of human ccRCC in future.

The Molecular Mechanism Underlying Pro-apoptotic Role of Hemocytes Specific Transcriptional Factor Lhx9 in Crassostrea hongkongensis

Hemocytes are the central organ of immune defense against pathogens by means of inflammation, phagocytosis, and encapsulation in mollusks. The well-functioning of the host immune system relies on the hemocytes’ task exertion and frequent renewal, but the underlying renewal mechanism remains elusive at the gene level. Here, we identified one transcription factor, LIM homeobox 9, in Crassostrea hongkongensis (ChLhx9) that could be involved in hemocyte apoptosis or renewal. ChLhx9 contains a homeodomain and two LIM domains. The expression profile of ChLhx9 showed that it was specific and had high expression in hemocytes, and it significantly increased under the bacterial challenge. RNA interference of ChLhx9 dramatically decreased the apoptosis rate of hemocytes when compared with a control group, which strongly implies its pro-apoptotic role in hemocytes. Furthermore, the genomic responses to the knockdown of ChLhx9 were examined through RNA-seq, which showed that multiple pathways associated with cell apoptosis, including the apoptosis pathway, hippo signal pathway and p53 signaling pathway, were significantly down-regulated. Meanwhile, seven of the key apoptotic genes were confirmed to be upregulated by ChLhx9, among which ChASPP1 (apoptosis stimulating protein of p53) was confirmed to induce hemocyte apoptosis strongly, which demonstrates that ChASPP1 was a downstream target mediated by ChLhx9 that caused apoptosis. In conclusion, tissue-specific transcription factor ChLhx9 induces hemocyte apoptosis through activating apoptotic genes or pathways, which could contribute to hemocyte renewal and immune defense in oysters.

Nonhistone targets of KAT2A and KAT2B implicated in cancer biology 1

Lysine acetylation is a critical post-translation modification that can impact a protein's localization, stability, and function. Originally thought to only occur on histones, we now know thousands of nonhistone proteins are also acetylated. In conjunction with many other proteins, lysine acetyltransferases (KATs) are incorporated into large protein complexes that carry out these modifications. In this review we focus on the contribution of two KATs, KAT2A and KAT2B, and their potential roles in the development and progression of cancer. Systems biology demands that we take a broad look at protein function rather than focusing on individual pathways or targets. As such, in this review we examine KAT2A/2B-directed nonhistone protein acetylations in cancer in the context of the 10 "Hallmarks of Cancer", as defined by Hanahan and Weinberg. By focusing on specific examples of KAT2A/2B-directed acetylations with well-defined mechanisms or strong links to a cancer phenotype, we aim to reinforce the complex role that these enzymes play in cancer biology.

Molecular processes involved in B cell acute lymphoblastic leukaemia

B cell leukaemia is one of the most frequent malignancies in the paediatric population, but also affects a significant proportion of adults in developed countries. The majority of infant and paediatric cases initiate the process of leukaemogenesis during foetal development (in utero) through the formation of a chromosomal translocation or the acquisition/deletion of genetic material (hyperdiploidy or hypodiploidy, respectively). This first genetic insult is the major determinant for the prognosis and therapeutic outcome of patients. B cell leukaemia in adults displays similar molecular features as its paediatric counterpart. However, since this disease is highly represented in the infant and paediatric population, this review will focus on this demographic group and summarise the biological, clinical and epidemiological knowledge on B cell acute lymphoblastic leukaemia of four well characterised subtypes: t(4;11) MLL-AF4, t(12;21) ETV6-RUNX1, t(1;19) E2A-PBX1 and t(9;22) BCR-ABL1.

Identification of cooperative genes for E2A-PBX1 to develop acute lymphoblastic leukemia

E2A-PBX1 is a chimeric gene product detected in t(1;19)-bearing acute lymphoblastic leukemia (ALL) with B-cell lineage. To investigate the leukemogenic process, we generated conditional knock-in (cKI) mice for E2A-PBX1, in which E2A-PBX1 is inducibly expressed under the control of the endogenous E2A promoter. Despite the induced expression of E2A-PBX1, no hematopoietic disease was observed, strongly suggesting that additional genetic alterations are required to develop leukemia. To address this possibility, retroviral insertional mutagenesis was used. Virus infection efficiently induced T-cell, B-cell, and biphenotypic ALL in E2A-PBX1 cKI mice. Inverse PCR identified eight retroviral common integration sites, in which enhanced expression was observed in the Gfi1, Mycn, and Pim1 genes. In addition, it is of note that viral integration and overexpression of the Zfp521 gene was detected in one tumor with B-cell lineage; we previously identified Zfp521 as a cooperative gene with E2A-HLF, another E2A-involving fusion gene with B-lineage ALL. The cooperative oncogenicity of E2A-PBX1 with overexpressed Zfp521 in B-cell tumorigenesis was indicated by the finding that E2A-PBX1 cKI, Zfp521 transgenic compound mice developed B-lineage ALL. Moreover, upregulation of ZNF521, the human counterpart of Zfp521, was found in several human leukemic cell lines bearing t(1;19). These results indicate that E2A-PBX1 cooperates with additional gene alterations to develop ALL. Among them, enhanced expression of ZNF521 may play a clinically relevant role in E2A fusion genes to develop B-lineage ALL.

Conditional Expression of E2A-HLF Induces B-Cell Precursor Death and Myeloproliferative-Like Disease in Knock-In Mice

Chromosomal translocations are driver mutations of human cancers, particularly leukemias. They define disease subtypes and are used as prognostic markers, for minimal residual disease monitoring and therapeutic targets. Due to their low incidence, several translocations and their biological consequences remain poorly characterized. To address this, we engineered mouse strains that conditionally express E2A-HLF, a fusion oncogene from the translocation t(17;19) associated with 1% of pediatric B-cell precursor ALL. Conditional oncogene activation and expression were directed to the B-cell compartment by the Cre driver promoters CD19 or Mb1 (Igα, CD79a), or to the hematopoietic stem cell compartment by the Mx1 promoter. E2A-HLF expression in B-cell progenitors induced hyposplenia and lymphopenia, whereas expression in hematopoietic stem/progenitor cells was embryonic lethal. Increased cell death was detected in E2A-HLF expressing cells, suggesting the need for cooperating genetic events that suppress cell death for B-cell oncogenic transformation. E2A-HLF/Mb1.Cre aged mice developed a fatal myeloproliferative-like disorder with low frequency characterized by leukocytosis, anemia, hepatosplenomegaly and organ-infiltration by mature myelocytes. In conclusion, we have developed conditional E2A-HLF knock-in mice, which provide an experimental platform to study cooperating genetic events and further elucidate translational biology in cross-species comparative studies.

Comparative genomics reveals multistep pathogenesis of E2A-PBX1 acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is the most common childhood cancer; however, its genetic diversity limits investigation into the molecular pathogenesis of disease and development of therapeutic strategies. Here, we engineered mice that conditionally express the E2A-PBX1 fusion oncogene, which results from chromosomal translocation t(1;19) and is present in 5% to 7% of pediatric ALL cases. The incidence of leukemia in these mice varied from 5% to 50%, dependent on the Cre-driving promoter (Cd19, Mb1, or Mx1) used to induce E2A-PBX1 expression. Two distinct but highly similar subtypes of B cell precursor ALLs that differed by their pre-B cell receptor (pre-BCR) status were induced and displayed maturation arrest at the pro-B/large pre-B II stages of differentiation, similar to human E2A-PBX1 ALL. Somatic activation of E2A-PBX1 in B cell progenitors enhanced self-renewal and led to acquisition of multiple secondary genomic aberrations, including prominent spontaneous loss of Pax5. In preleukemic mice, conditional Pax5 deletion cooperated with E2A-PBX1 to expand progenitor B cell subpopulations, increasing penetrance and shortening leukemia latency. Recurrent secondary activating mutations were detected in key signaling pathways, most notably JAK/STAT, that leukemia cells require for proliferation. These data support conditional E2A-PBX1 mice as a model of human ALL and suggest targeting pre-BCR signaling and JAK kinases as potential therapeutic strategies.

A robust in vivo model for B cell precursor acute lymphoblastic leukemia

B cell precursor acute lymphoblastic leukemia (BCP ALL) is the most common malignancy in children. While treatments have improved remarkably over the past four decades, resistant disease and late effects that result from cytotoxic chemotherapy remain serious problems for individuals with BCP ALL. Improved genetic tools have led to the discovery of numerous somatic mutations associated with BCP ALL, leading to a framework for the genetic classification of BCP ALL. In this issue of the JCI, Duque-Afonso et al. develop an accurate in vivo model for BCP ALL that recapitulates the key features of human disease, including acquired mutations in genes encoding PAX5 and components of the JAK/STAT pathway. The authors further show, as proof of principle, that this model can be used to evaluate the efficacy of drugs designed to target specific acquired mutations in patients with BCP ALL.

Genetically engineered mouse models of human B-cell precursor leukemias

B-cell precursor acute lymphoblastic leukemias (pB-ALLs) are the most frequent type of malignancies of the childhood, and also affect an important proportion of adult patients. In spite of their apparent homogeneity, pB-ALL comprises a group of diseases very different both clinically and pathologically, and with very diverse outcomes as a consequence of their biology, and underlying molecular alterations. Their understanding (as a prerequisite for their cure) will require a sustained multidisciplinary effort from professionals coming from many different fields. Among all the available tools for pB-ALL research, the use of animal models stands, as of today, as the most powerful approach, not only for the understanding of the origin and evolution of the disease, but also for the development of new therapies. In this review we go over the most relevant (historically, technically or biologically) genetically engineered mouse models (GEMMs) of human pB-ALLs that have been generated over the last 20 years. Our final aim is to outline the most relevant guidelines that should be followed to generate an “ideal” animal model that could become a standard for the study of human pB-ALL leukemia, and which could be shared among research groups and drug development companies in order to unify criteria for studies like drug testing, analysis of the influence of environmental risk factors, or studying the role of both low-penetrance mutations and cancer susceptibility alterations.

Retrovirus-Mediated Expression of E2A-PBX1 Blocks Lymphoid Fate but Permits Retention of Myeloid Potential in Early Hematopoietic Progenitors

The oncogenic transcription factor E2A-PBX1 is expressed consequent to chromosomal translocation 1;19 and is an important oncogenic driver in cases of pre-B-cell acute lymphoblastic leukemia (ALL). Elucidating the mechanism by which E2A-PBX1 induces lymphoid leukemia would be expedited by the availability of a tractable experimental model in which enforced expression of E2A-PBX1 in hematopoietic progenitors induces pre-B-cell ALL. However, hematopoietic reconstitution of irradiated mice with bone marrow infected with E2A-PBX1-expressing retroviruses consistently gives rise to myeloid, not lymphoid, leukemia. Here, we elucidate the hematopoietic consequences of forced E2A-PBX1 expression in primary murine hematopoietic progenitors. We show that introducing E2A-PBX1 into multipotent progenitors permits the retention of myeloid potential but imposes a dense barrier to lymphoid development prior to the common lymphoid progenitor stage, thus helping to explain the eventual development of myeloid, and not lymphoid, leukemia in transplanted mice. Our findings also indicate that E2A-PBX1 enforces the aberrant, persistent expression of some genes that would normally have been down-regulated in the subsequent course of hematopoietic maturation. We show that enforced expression of one such gene, Hoxa9, a proto-oncogene associated with myeloid leukemia, partially reproduces the phenotype produced by E2A-PBX1 itself. Existing evidence suggests that the 1;19 translocation event takes place in committed B-lymphoid progenitors. However, we find that retrovirus-enforced expression of E2A-PBX1 in committed pro-B-cells results in cell cycle arrest and apoptosis. Our findings indicate that the neoplastic phenotype induced by E2A-PBX1 is determined by the developmental stage of the cell into which the oncoprotein is introduced.

Genetic Modeling of PIM Proteins in Cancer: Proviral Tagging and Cooperation with Oncogenes, Tumor Suppressor Genes, and Carcinogens

The PIM proteins, which were initially discovered as proviral insertion sites in Moloney-murine leukemia virus infection, are a family of highly homologous serine/threonine kinases that have been reported to be overexpressed in hematological malignancies and solid tumors. The PIM proteins have also been associated with metastasis and overall treatment responses and implicated in the regulation of apoptosis, metabolism, the cell cycle, and homing and migration, which makes these proteins interesting targets for anti-cancer drug discovery. The use of retroviral insertional mutagenesis and refined approaches such as complementation tagging has allowed the identification of myc, pim, and a third group of genes (including bmi1 and gfi1) as complementing genes in lymphomagenesis. Moreover, mouse modeling of human cancer has provided an understanding of the molecular pathways that are involved in tumor initiation and progression at the physiological level. In particular, genetically modified mice have allowed researchers to further elucidate the role of each of the Pim isoforms in various tumor types. PIM kinases have been identified as weak oncogenes because experimental overexpression in lymphoid tissue, prostate, and liver induces tumors at a relatively low incidence and with a long latency. However, very strong synergistic tumorigenicity between Pim1/2 and c-Myc and other oncogenes has been observed in lymphoid tissues. Mouse models have also been used to study whether the inhibition of specific PIM isoforms is required to prevent carcinogen-induced sarcomas, indicating that the absence of Pim2 and Pim3 greatly reduces sarcoma growth and bone invasion; the extent of this effect is similar to that observed in the absence of all three isoforms. This review will summarize some of the animal models that have been used to understand the isoform-specific contribution of PIM kinases to tumorigenesis.

Murine models of acute leukemia: important tools in current pediatric leukemia research

Leukemia remains the most common diagnosis in pediatric oncology and, despite dramatic progress in upfront therapy, is also the most common cause of cancer-related death in children. Much of the initial improvement in outcomes for acute lymphoblastic leukemia (ALL) was due to identification of cytotoxic agents that are active against leukemia followed by the recognition that combination of these cytotoxic agents and prolonged therapy are essential for cure. Recent data demonstrating lack of progress in patients for whom standard chemotherapy fails suggests that the ability to improve outcome for these children will not be dramatically impacted through more intensive or newer cytotoxic agents. Thus, much of the recent research focus has been in the area of improving our understanding of the genetics and the biology of leukemia. Although in vitro studies remain critical, given the complexity of a living system and the increasing recognition of the contribution of leukemia extrinsic factors such as the bone marrow microenvironment, in vivo models have provided important insights. The murine systems that are used can be broadly categorized into syngeneic models in which a murine leukemia can be studied in immunologically intact hosts and xenograft models where human leukemias are studied in highly immunocompromised murine hosts. Both of these systems have limitations such that neither can be used exclusively to study all aspects of leukemia biology and therapeutics for humans. This review will describe the various ALL model systems that have been developed as well as discuss the advantages and disadvantages inherent to these systems that make each particularly suitable for specific types of studies.

Integrated genetic and epigenetic analysis of childhood acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is the commonest childhood malignancy and is characterized by recurring structural genetic alterations. Previous studies of DNA methylation suggest epigenetic alterations may also be important, but an integrated genome-wide analysis of genetic and epigenetic alterations in ALL has not been performed. We analyzed 137 B-lineage and 30 T-lineage childhood ALL cases using microarray analysis of DNA copy number alterations and gene expression, and genome-wide cytosine methylation profiling using the HpaII tiny fragment enrichment by ligation-mediated PCR (HELP) assay. We found that the different genetic subtypes of ALL are characterized by distinct DNA methylation signatures that exhibit significant correlation with gene expression profiles. We also identified an epigenetic signature common to all cases, with correlation to gene expression in 65% of these genes, suggesting that a core set of epigenetically deregulated genes is central to the initiation or maintenance of lymphoid transformation. Finally, we identified aberrant methylation in multiple genes also targeted by recurring DNA copy number alterations in ALL, suggesting that these genes are inactivated far more frequently than suggested by structural genomic analyses alone. Together, these results demonstrate subtype- and disease-specific alterations in cytosine methylation in ALL that influence transcriptional activity, and are likely to exert a key role in leukemogenesis.

Detection of E2A-PBX1 fusion transcripts in human non-small-cell lung cancer

Background

E2A-PBX1 fusion gene caused by t(1;19)(q23;p13), has been well characterized in acute lymphoid leukemia (ALL). There is no report on E2A-PBX1 fusion transcripts in non-small-cell lung cancer (NSCLC).

Methods

We used polymerase chain reaction (PCR) to detect E2A-PBX1 fusion transcripts in human NSCLC tissue specimens and cell lines. We analyzed correlation of E2A-PBX1 fusion transcripts with clinical outcomes in 76 patients with adenocarcinoma in situ (AIS) and other subgroups. We compared mutation status of k-ras, p53 and EGFR in 22 patients with E2A-PBX1 fusion transcripts.

Results

We detected E2A-PBX1 transcripts in 23 of 184 (12.5%) NSCLC tissue specimens and 3 of 13 (23.1%) NSCLC cell lines. Presence of E2A-PBX1 fusion transcripts correlated with smoking status in female patients (P = 0.048), AIS histology (P = 0.006) and tumor size (P = 0.026). The overall survival was associated with gender among AIS patients (P = 0.0378) and AIS patients without E2A-PBX1 fusion transcripts (P = 0.0345), but not among AIS patients with E2A-PBX1 fusion transcripts (P = 0.6401). The overall survival was also associated with status of E2A-PBX1 fusion transcripts among AIS stage IA patients (P = 0.0363) and AIS stage IA female patients (P = 0.0174). In addition, among the 22 patients with E2A-PBX1 fusion transcripts, 12 (54.5%) patients including all four non-smokers, showed no common mutations in k-ras, p53 and EGFR.

Conclusions

E2A-PBX1 fusion gene caused by t(1;19)(q23;p13) may be a common genetic change in AIS and a survival determinant for female AIS patients at early stage.

Transgenic studies on homeobox genes in nervous system development: spina bifida in Isl1 transgenic mice

To develop in vivo assays for homeobox gene function in neural development, we generated transgenic mice in which the expression of a homeobox gene is altered only within the nervous system, in neurons or neuronal precursor cells. Transgenic expression of Hoxc8 did not result in gross abnormalities, while a Hoxd4 transgene caused death shortly after birth. In neural progenitor cells, the motorneuron-specific homeodomain transcription factor Isl1 induced early developmental defects, including absence of anterior neural structures, profound defects in the neuroepithelium and defective neural tube closure. A fraction of Isl1 transgenic mice exhibited spina bifida. Isl1 transgene expression was also associated with decreased proliferation and increased Pbx1 expression in the ventral neural tube. Our results suggest a function for some homeobox genes in development of the nervous system, and that cell-type- and region-specific transgenic models will be useful to identify the cellular and molecular targets of homeobox transcription factors in nervous system development.

Crosstalk between ROR1 and the Pre-B cell receptor promotes survival of t(1;19) acute lymphoblastic leukemia

We report that t(1;19) ALL cells universally exhibit expression of and dependence on the cell surface receptor ROR1. We further identify t(1;19) ALL cell sensitivity to the kinase inhibitor dasatinib due to its inhibition of the pre-B cell receptor (pre-BCR) signaling complex. These phenotypes are a consequence of developmental arrest at an intermediate/late stage of B-lineage maturation. Additionally, inhibition of pre-BCR signaling induces further ROR1 upregulation, and we identify distinct ROR1 and pre-BCR downstream signaling pathways that are modulated in a counterbalancing manner-both leading to AKT phosphorylation. Consistent with this, AKT phosphorylation is transiently eliminated after dasatinib treatment, but is partially restored following dasatinib potentiation of ROR1 expression. Consequently, ROR1 silencing accentuates dasatinib killing of t(1;19) ALL cells.

Motif-guided sparse decomposition of gene expression data for regulatory module identification

Background

Genes work coordinately as gene modules or gene networks. Various computational approaches have been proposed to find gene modules based on gene expression data; for example, gene clustering is a popular method for grouping genes with similar gene expression patterns. However, traditional gene clustering often yields unsatisfactory results for regulatory module identification because the resulting gene clusters are co-expressed but not necessarily co-regulated.

Results

We propose a novel approach, motif-guided sparse decomposition (mSD), to identify gene regulatory modules by integrating gene expression data and DNA sequence motif information. The mSD approach is implemented as a two-step algorithm comprising estimates of (1) transcription factor activity and (2) the strength of the predicted gene regulation event(s). Specifically, a motif-guided clustering method is first developed to estimate the transcription factor activity of a gene module; sparse component analysis is then applied to estimate the regulation strength, and so predict the target genes of the transcription factors. The mSD approach was first tested for its improved performance in finding regulatory modules using simulated and real yeast data, revealing functionally distinct gene modules enriched with biologically validated transcription factors. We then demonstrated the efficacy of the mSD approach on breast cancer cell line data and uncovered several important gene regulatory modules related to endocrine therapy of breast cancer.

Conclusion

We have developed a new integrated strategy, namely motif-guided sparse decomposition (mSD) of gene expression data, for regulatory module identification. The mSD method features a novel motif-guided clustering method for transcription factor activity estimation by finding a balance between co-regulation and co-expression. The mSD method further utilizes a sparse decomposition method for regulation strength estimation. The experimental results show that such a motif-guided strategy can provide context-specific regulatory modules in both yeast and breast cancer studies.

Pre-B cell receptor signaling in acute lymphoblastic leukemia

B cell lineage ALL represents by far the most frequent malignancy in children and is also common in adults. Despite significant advances over the past four decades, cytotoxic treatment strategies have recently reached a plateau with cure rates at 80 percent for children and 55 percent for adults. Relapse after cytotoxic drug treatment, initial drug-resistance and dose-limiting toxicity are among the most frequent complications of current therapy approaches. For this reason, pathway-specific treatment strategies in addition to cytotoxic drug treatment seem promising to further improve therapy options for ALL patients. In a recent study on 111 cases of pre-B cell-derived human ALL, we found that ALL cells carrying a BCR-ABL1-gene rearrangement lack expression of a functional pre-B cell receptor in virtually all cases. In a proof-of-principle experiment, we studied pre-B cell receptor function during progressive leukemic transformation of pre-B cells in BCR-ABL1-transgenic mice: Interestingly, signaling from the pre-B cell receptor and the oncogenic BCR-ABL1 kinase are mutually exclusive and only "crippled" pre-B cells that fail to express a functional pre-B cell receptor are permissive to transformation by BCR-ABL1.

Prognostic significance of pre B cell leukemia transcription factor 2 (PBX2) expression in non-small cell lung carcinoma

Previous studies on the mammary carcinoma cell line have shown that the pre B cell leukemia transcription factor 1 (PBX1) was a transcription factor for valosin-containing protein (VCP), which is involved in invasion and metastasis of cancers. The roles of PBX1 and PBX2, a highly homologous transcription factor to PBX1, for expression of VCP were examined in the cell lines from non-small cell lung cancer (NSCLC). The effects of PBX1 and PBX2 on VCP expression were examined with siRNA in A549 and PC14 NSCLC cell lines. Expression levels of PBX2 and VCP were immunohistochemically examined and compared with each other in 206 NSCLC cases. Subsequently, significance of PBX expression in clinical behavior of NSCLC patients was evaluated. Expression levels of VCP mRNA significantly decreased when PBX2 but not PBX1 expression was knocked down in NSCLC cell lines. Immunohistochemically, staining intensity of PBX2 was correlated with that of VCP in clinical samples. Then correlation of PBX2 expression and clinical behavior of NSCLC patients was evaluated. Univariate analysis revealed high expression levels of PBX2 and VCP to be poor prognosticators for overall and disease-free survival. Multivariate analysis revealed that high expression of VCP but not PBX2 to be an independent prognostic factor. PBX2 is a transcription factor for VCP in NSCLC. Because high levels of PBX2 expression correlated with prognosis of NSCLC, PBX2 could be a target molecule for treatment of NSCLC.

A retroviral mutagenesis screen reveals strong cooperation between Bcl11a overexpression and loss of the Nf1 tumor suppressor gene

NF1 inactivation occurs in specific human cancers, including juvenile myelomonocytic leukemia, an aggressive myeloproliferative disorder of childhood. However, evidence suggests that Nf1 loss alone does not cause leukemia. We therefore hypothesized that inactivation of the Nf1 tumor suppressor gene requires cooperating mutations to cause acute leukemia. To search for candidate genes that cooperate with Nf1 deficiency in leukemogenesis, we performed a forward genetic screen using retroviral insertion mutagenesis in Nf1 mutant mice. We identified 43 common proviral insertion sites that contain candidate genes involved in leukemogenesis. One of these genes, Bcl11a, confers a growth advantage in cultured Nf1 mutant hematopoietic cells and causes early onset of leukemia of either myeloid or lymphoid lineage in mice when expressed in Nf1-deficient bone marrow. Bcl11a-expressing cells display compromised p21(Cip1) induction, suggesting that Bcl11a's oncogenic effects are mediated, in part, through suppression of p21(Cip1). Importantly, Bcl11a is expressed in human chronic myelomonocytic leukemia and juvenile myelomonocytic leukemia samples. A subset of AML patients, who had poor outcomes, of 16 clusters, displayed high levels of BCL11A in leukemic cells. These findings suggest that deregulated Bcl11a cooperates with Nf1 in leukemogenesis, and a therapeutic strategy targeting the BCL11A pathway may prove beneficial in the treatment of leukemia.

Expanding the specificity of DNA targeting by harnessing cooperative assembly

The precise control of developmental and regulatory processes in the cell requires accurate recognition of specific DNA sites. For genomes as large as that of humans, single-molecule-DNA binders have difficulties accurately and specifically recognizing the intended targets. Natural transcription factors overcome these difficulties by forming non-covalent complexes on the DNA with other transcription factors. These cooperative complexes overcome the difficulties of single-molecule transcription factors, allowing specific, combinatorial control of a range of transcriptional targets. Artificial transcription factors have been designed to take advantage of this technique of cooperative assembly, facilitating future studies in whole genome targeting. In contrast to a simple model of component independence, cooperative complexes as a whole often display slightly altered DNA specificity from what would be expected from the analysis of their separate components. The true sequence specificity of cooperative complexes, and thus their presumed in vivo targets, have to be experimentally probed. A number of techniques, such as the cognate site identity array, now allow for rapid, high-throughput determination of the specificity of cooperative complexes.

E2A-ZNF384 and NOL1-E2A fusion created by a cryptic t(12;19)(p13.3; p13.3) in acute leukemia

A 5-year-old boy who initially presented with ALL and relapsed 4 months later with AML was found to have an add(19) in the leukemia cells. FISH revealed that the add(19) was really a cryptic t(l2;l9)(p13.3;p13.3) interrupting E2A (TCF3). Nucleotide sequences of cloned genomic fragments with the E2A rearrangements revealed that the der(12) contained E2A joined to an intron of the NOLI (p120) gene. Reverse transcriptase (RT)-PCR of patient lymphoblast RNA showed expression of in-frame fusion cDNAs consisting of most of NOL1 fused to the 3' portion of E2A that encoded part of the second transcriptional activation domain and the DNA binding and protein dimerization motifs. The reciprocal der(19) E2A genomic rearrangements included 5' regions of E2A joined to an intron of the ZNF384 (NMP4, CIZ) gene, located approximately 450 kb centromeric to NOL1 on chromosome 12. RT-PCR showed expression of in-frame E2A-ZNF384 fusion cDNAs. To our knowledge, this is the second report of a chromosome translocation in leukemia resulting in two different gene fusions. This is the first report of expression of E2A fusion protein that includes the DNA binding and protein dimerization domains due to a more proximal break in E2A compared to those described previously.

Interleukin-10 expression in macrophages during phagocytosis of apoptotic cells is mediated by homeodomain proteins Pbx1 and Prep-1

Production of interleukin (IL)-10, a major immunoregulatory cytokine, by phagocytes during clearance of apoptotic cells is critical to ensuring cellular homeostasis and suppression of autoimmunity. Little is known about the regulatory mechanisms in this fundamental process. We report that IL-10 production stimulated by apoptotic cells was regulated at the point of transcription in a manner dependent on p38 mitogen-activated protein kinase, partially on the scavenger receptor CD36, and required cell-cell contact but not phagocytosis. By using a reporter assay, we mapped the apoptotic-cell-response element (ACRE) in the human IL10 promoter and provide biochemical and physiological evidence that ACRE mediates the transcriptional activation of IL10 by pre-B cell leukemia transcription factor-1b and another Hox cofactor Pbx-regulating protein 1 in response to apoptotic cells. This study establishes a role of two developmentally critical factors (Pbx1 and Prep-1) in the regulation of homeostasis in the immune system.

Mechanisms of transcription factor deregulation in lymphoid cell transformation

The most frequent targets of genetic alterations in human lymphoid leukemias are transcription factor genes with essential functions in blood cell development. TAL1, LYL1, HOX11 and other transcription factors essential for normal hematopoiesis are often misexpressed in the thymus in T-cell acute lymphoblastic leukemia (T-ALL), leading to differentiation arrest and cell transformation. Recent advances in the ability to assess DNA copy number have led to the discovery that the MYB transcription factor oncogene is tandemly duplicated in T-ALL. The NOTCH1 gene, which is essential for key embryonic cell-fate decisions in multicellular organisms, was found to be activated by mutation in a large percentage of T-ALL patients. The gene encoding the FBW7 protein ubiquitin ligase, which regulates the turnover of the intracellular form of NOTCH (ICN), is also mutated in T-ALL, resulting in stabilization of the ICN and activation of the NOTCH signaling pathway. In mature B-lineage ALL and Burkitt lymphoma, the MYC transcription factor oncogene is overexpressed due to translocation into the IG locus. PAX5, a transcription factor essential for B-lineage commitment, is inactivated in 32% of cases of B-progenitor ALL. Translocations resulting in oncogenic fusion transcription factors also occur frequently in this form of ALL. The most frequent transcription factor chimeric fusion, TEL-AML1, is an initiating event in B-progenitor ALL that acts by repressing transcription. Therefore, deregulated transcription and its consequent effects on key developmental pathways play a major role in the molecular pathogenesis of lymphoid malignancy. Once the full complement of cooperating mutations in transformed B- and T-progenitor cells is known, and the deregulated downstream pathways have been elucidated, it will be possible to identify vulnerable components and to target them with small-molecule inhibitors.

Loss of bHLH transcription factor E2A activity in primary effusion lymphoma confers resistance to apoptosis

Similar to classical Hodgkin lymphoma (HL) tumour cells, primary effusion lymphoma (PEL) originates from mature B cells but displays a non-B cell phenotype, the mechanisms and consequences of which are not yet understood. This study showed that PEL lacked DNA binding activity of the B cell-determining transcription factors E2A, EBF and Pax5. PEL overexpressed the E2A antagonists ABF-1 and Id2, which have been described to block the B-cell differentiation program in classical HL. However, in contrast to HL cells, B lineage-inappropriate genes were not similarly upregulated in PEL, and reconstitution of B cell-specific E2A homodimer activity in PEL induced apoptosis. These data demonstrate that lineage infidelity in PEL is not as pronounced as in HL, and that the loss of the B cell-specific transcription factor E2A in PEL is implicated in apoptosis protection.

Pre-B-cell leukemia transcription factor 1 regulates expression of valosin-containing protein, a gene involved in cancer growth

Valosin-containing protein (VCP) is involved in a wide variety of cellular functions. Our previous studies showed that the enhanced expression of VCP in cancer cells correlated with invasion and metastasis of cancers. Here, the regulatory mechanism for VCP transcription was investigated. Luciferase reporter constructs containing serially deleted 5'-flanking region of the VCP gene were transfected into MCF7 mammary carcinoma cell line, in which VCP was abundantly expressed. The deletion and mutation at the two binding motifs for pre-B-cell leukemia transcription factor 1 (PBX1) reduced the luciferase activity, indicating that these two PBX1 motifs mediated the transactivation of the VCP gene. Chromatin immunoprecipitation assay showed the binding of PBX-1 to the 5'-flanking region of the VCP gene. The knockdown of PBX1 by siRNA decreased the expression level of VCP. VCP is reported to maintain cell viability after the treatment of tumor necrosis factor-alpha. The viability of tumor necrosis factor-alpha-treated cells was significantly reduced in PBX1 knockdown MCF7. These findings indicate that PBX1 plays a crucial role in VCP expression and function and that the PBX-VCP pathway might be important for cell survival under cytokine stress.

ABCB1 over-expression and drug-efflux in acute lymphoblastic leukemia cell lines with t(17;19) and E2A-HLF expression

BACKGROUND

The t(17;19)(q21;p13), which occurs in a small subset of acute lymphoblastic leukemias (ALLs) and is associated with a dismal prognosis, creates a chimeric E2A-HLF transcription factor with transforming properties.

PROCEDURE

We used representational difference analysis to identify candidate E2A-HLF target genes. Transient transfection assays and an inducible expression model system were then used to evaluate the ability of E2A-HLF to modulate target gene expression.

RESULTS

We identified ABCB1 (MDR1, P-glycoprotein) as a gene differentially expressed in ALL cell lines with and without E2A-HLF expression and demonstrated that t(17;19)+ ALL cell lines expressed high levels of ABCB1 protein and had a drug efflux-positive phenotype. Although ABCB1 transcription is regulated by C/EBPbeta via interaction with a DNA response element that shares significant homology with the optimal E2A-HLF binding site, E2A-HLF did not directly activate transcription of reporter genes under control of ABCB1 promoter elements in transient transfection assays. However, ABCB1 expression was induced in a DNA-binding independent manner by E2A-HLF, E2A-PBX1, and truncated E2A polypeptides consisting of those portions of E2A present in leukemic fusion proteins.

CONCLUSIONS

E2A-HLF-mediated over-expression of ABCB1 may play a critical role in defining the clinical phenotype of ALLs with a t(17;19), suggesting pharmacologic modulation of ABCB1 activity as a rational therapeutic strategy for this chemotherapy resistant subtype of ALL.

Critical role for a single leucine residue in leukemia induction by E2A-PBX1

In roughly 5% of cases of acute lymphoblastic leukemia, a chromosomal translocation leads to expression of the oncogenic protein E2A-PBX1. The N-terminal portion of E2A-PBX1, encoded by the E2A gene, is identical in sequence to the corresponding portion of the E proteins E12/E47 and includes transcriptional activation domains. The C terminus consists of most of the HOX interacting transcription factor PBX1, including its DNA-binding homeodomain. Structure-function correlative experiments have suggested that oncogenesis by E2A-PBX1 requires an activation domain, called AD1, at the extreme N terminus. We recently demonstrated that a potentially helical portion of AD1 interacts directly with the transcriptional coactivator protein cyclic AMP response element-binding protein (CBP) and that this interaction is essential in the immortalization of primary bone marrow cells in tissue culture. Here we show that a conserved LXXLL motif within AD1 is required in the interaction between E2A-PBX1 and the KIX domain of CBP. We show by circular dichroism spectroscopy that the LXXLL-containing portion of AD1 undergoes a helical transition upon interacting with the KIX domain and that amino acid substitutions that prevent helix formation prevent both the KIX interaction and cell immortalization by E2A-PBX1. Perhaps most strikingly, substitution of a single, conserved leucine residue (L20) within the LXXLL motif impairs leukemia induction in mice after transplantation with E2A-PBX1-expressing bone marrow. The KIX domain of CBP mediates well-characterized interactions with several transcription factors of relevance to leukemia induction. Circumstantial evidence suggests that the side chain of L20 might interact with a deep hydrophobic pocket in the KIX domain. Therefore, our results serve to identify a potential new drug target.

Fusion oncogenes in tumor development

Currently, all identified fusion oncogenes are found in rare tumor forms, and most of them only in specific tumor types. Some fusion oncogenes are frequent in healthy individuals suggesting that they rarely induce tumor growth. Multiple double-strand breaks that cluster in time and space increases the risk for formation of fusion oncogenes genes. The normal cell type specific spatial distribution of chromatin and genes in interphase nuclei may affect the risk for fusion of specific genes. Transcriptional orientation, splicing of reading frames, size and sequences of breakpoint introns are other risk factors. The biological activity of fusion oncoproteins is the most important factor for penetrance. The effects in specific target cells may explain the tumor type specificity of most fusion oncogenes.

Cloning and functional characterization of MEF2D/DAZAP1 and DAZAP1/MEF2D fusion proteins created by a variant t(1;19)(q23;p13.3) in acute lymphoblastic leukemia

We analyzed the TS-2 acute lymphoblastic leukemia (ALL) cell line that contains a t(1;19)(q23;p13.3) but lacks E2A-PBX1 fusion typically present in leukemias with this translocation. We found that the t(1;19) in TS-2 fuses the 19p13 gene DAZAP1 (Deleted in Azoospermia-Associated Protein 1) to the 1q23 gene MEF2D (Myocyte Enhancer Factor 2D), leading to expression of reciprocal in-frame DAZAP1/MEF2D and MEF2D/DAZAP1 transcripts. MEF2D is a member of the MEF2 family of DNA binding proteins that activate transcription of genes involved in control of muscle cell differentiation, and signaling pathways that mediate response to mitogenic signals and survival of neurons and T-lymphocytes. DAZAP1 is a novel RNA binding protein expressed most abundantly in the testis. We demonstrate that MEF2D/DAZAP1 binds avidly and specifically to DNA in a manner indistinguishable from that of native MEF2D and is a substantially more potent transcriptional activator than MEF2D. We also show that DAZAP1/MEF2D is a sequence-specific RNA-binding protein. MEF2D has been identified as a candidate oncogene in murine retroviral insertional mutagenesis studies. Our data implicate MEF2D in human cancer and suggest that MEF2D/DAZAP1 and/or DAZAP1/MEF2D contribute to leukemogenesis by altering signaling pathways normally regulated by wild-type MEF2D and DAZAP1.

High incidence of proviral integrations in the Hoxa locus in a new model of E2a-PBX1-induced B-cell leukemia

Relevant mouse models of E2a-PBX1-induced pre-B cell leukemia are still elusive. We now report the generation of a pre-B leukemia model using E2a-PBX1 transgenic mice, which lack mature and precursor T-cells as a result of engineered loss of CD3epsilon expression (CD3epsilon(-/-)). Using insertional mutagenesis and inverse-PCR, we show that B-cell leukemia development in the E2a-PBX1 x CD3epsilon(-/-) compound transgenic animals is significantly accelerated when compared to control littermates, and document several known and novel integrations in these tumors. Of all common integration sites, a small region of 19 kb in the Hoxa gene locus, mostly between Hoxa6 and Hoxa10, represented 18% of all integrations in the E2a-PBX1 B-cell leukemia and was targeted in 86% of these leukemias compared to 17% in control tumors. Q-PCR assessment of expression levels for most Hoxa cluster genes in these tumors revealed an unprecedented impact of the proviral integrations on Hoxa gene expression, with tumors having one to seven different Hoxa genes overexpressed at levels up to 6600-fold above control values. Together our studies set the stage for modeling E2a-PBX1-induced B-cell leukemia and shed new light on the complexity pertaining to Hox gene regulation. In addition, our results show that the Hoxa gene cluster is preferentially targeted in E2a-PBX1-induced tumors, thus suggesting functional collaboration between these oncogenes in pre-B-cell tumors.

Meis1-mediated apoptosis is caspase dependent and can be suppressed by coexpression of HoxA9 in murine and human cell lines

Coexpression of the homeodomain protein Meis1 and either HoxA7 or HoxA9 is characteristic of many acute myelogenous leukemias. Although Meis1 can be overexpressed in bone marrow long-term repopulating cells, it is incapable of mediating their transformation. Although overexpressing HoxA9 alone transforms murine bone marrow cells, concurrent Meis1 overexpression greatly accelerates oncogenesis. Meis1-HoxA9 cooperation suppresses several myeloid differentiation pathways. We now report that Meis1 overexpression strongly induces apoptosis in a variety of cell types in vitro through a caspase-dependent process. Meis1 requires a functional homeodomain and Pbx-interaction motif to induce apoptosis. Coexpressing HoxA9 with Meis1 suppresses this apoptosis and provides protection from several apoptosis inducers. Pbx1, another Meis1 cofactor, also induces apoptosis; however, coexpressing HoxA9 is incapable of rescuing Pbx-mediated apoptosis. This resistance to apoptotic stimuli, coupled with the previously reported ability to suppress multiple myeloid differentiation pathways, would provide a strong selective advantage to Meis1-HoxA9 coexpressing cells in vivo, leading to leukemogenesis.

The 5′-upstream region of human programmed cell death 5 gene contains a highly active TATA-less promoter that is up-regulated by etoposide

The PDCD5 (programmed cell death 5), a novel apoptosis related gene, is functionally associated with cell apoptosis, exhibits a ubiquitous expression pattern and is up-regulated in some types of tumor cells undergoing apoptosis. To study the transcriptional regulation of the PDCD5 gene, we have cloned 1.1 kb of its 5'-upstream region. The DNA sequencing analysis revealed a major transcriptional start site at 72 base pairs in front of the ATG translational start codon. The upstream of the transcriptional start site lacks a canonical TATA box and CAAT box. Transient transfection and luciferase assay demonstrate that this region presents extremely strong promoter activity. The 5'-deleted sequences fused to a luciferase reporter gene demonstrated that the -555/-383 region from the transcription start site is crucial for transcriptional regulation, and the luciferase reporter gene's expression significantly increased in the early stage of cell apoptosis induced by etoposide. These results imply that the PDCD5 gene may be a target gene under the control of some important apoptosis-related transcriptional factors during the cell apoptosis.

E2a/Pbx1 induces the rapid proliferation of stem cell factor-dependent murine pro-T cells that cause acute T-lymphoid or myeloid leukemias in mice

Oncoprotein E2a/Pbx1 is produced by the t(1;19) chromosomal translocation of human pre-B acute lymphoblastic leukemia. E2a/Pbx1 blocks differentiation of primary myeloid progenitors but, paradoxically, induces apoptosis in established pre-B-cell lines, and no transforming function of E2a/Pbx1 has been reported in cultured lymphoid progenitors. Here, we demonstrate that E2a/Pbx1 induces immortal proliferation of stem cell factor (SCF)-dependent pro-T thymocytes by a mechanism dependent upon both its transactivation and DNA-binding functions. E2a-Pbx1 cooperated with cytokines or activated signaling oncoproteins to induce cell division, as inactivation of conditional E2a/Pbx1 in either factor-dependent pro-T cells or pro-T cells made factor independent by expression of Bcr/Abl resulted in pro-T-cell quiescence, while reactivation of E2a/Pbx1 restored cell division. Infusion of E2a/Pbx1 pro-T cells in mice caused T lymphoblastic leukemia and, unexpectedly, acute myeloid leukemia. The acute lymphoblastic leukemia did not evidence further maturation, suggesting that E2a/Pbx1 establishes an early block in pro-T-cell development that cannot be overcome by marrow or thymic microenvironments. In an E2a/Pbx1 pro-T thymocyte clone that induced only pro-T acute lymphoblastic leukemia, coexpression of Bcr/Abl expanded its leukemic phenotype to include acute myeloid leukemia, suggesting that unique functions of cooperating signaling oncoproteins can influence the lymphoid versus myeloid character of E2a/Pbx1 leukemia and may cooperate with E2a/Pbx1 to dictate the pre-B-cell phenotype of human leukemia containing t(1;19).

Transgenic Mice as an In Vivo Model of Lymphomagenesis

This review covers multiple data obtained on genetically modified mice that help to elucidate various intricate molecular mechanisms of lymphomagenesis in humans. We are in a "golden age" of mouse genetics. The mouse is by far the most accessible mammalian system physiologically similar to humans. Transgenic mouse models have illuminated how different genes contribute to human lymphomagenesis. Multiple experiments with transgenic mice have not only confirmed the data obtained for human lymphomas but also gave additional evidence for the role of some genes and cooperative participation of their products in the development of human lymphomas. Genes and gene networks detected on transgenic mice can successfully serve as molecular targets for tumor therapy. This review demonstrates the extraordinary possibilities of transgenic technology, which is presently one of the readily available, efficient, and accurate tools to solve the problem of cancer.

Leukemic transformation of hematopoietic progenitors by MLL-GAS7 in the absence of Hoxa7 or Hoxa9

Differential expression of Hox genes is associated with normal hematopoiesis, whereas inappropriate maintenance of Hox gene expression, particularly Hoxa7 and Hoxa9, is a feature of leukemias harboring mixed-lineage leukemia (MLL) mutations. To understand the pathogenic roles of Hox genes in MLL leukemias, we assessed the impact of Hoxa7 or Hoxa9 nullizygosity on hematopoietic progenitor compartments and their susceptibility to MLL-induced leukemias. Selective reductions in the absolute numbers of committed progenitors, but not of hematopoietic stem cells, distinguished Hoxa7- and Hoxa9-deficient mice. Megakaryocytic/erythroid progenitor (MEP) reductions in Hoxa7(-/-) mice correlated with reticulocytosis and thrombocytopenia without anemia. Conversely, Hoxa9(-/-) mice displayed marked lymphopenia and substantial reductions of common lymphoid progenitors (CLPs) and lymphoid precursors, in addition to significant reductions of common myeloid progenitors (CMPs) and granulocyte/monocyte progenitors (GMPs). In retroviral transduction/transplantation assays, Hoxa7- and Hoxa9-deficient progenitors remained susceptible to transformation by MLL-GAS7, which activates MLL through a dimerization-dependent mechanism. However, Hoxa7(-/-) or Hoxa9(-/-) progenitors were less efficient in generating transformed blast colony-forming units (CFUs) in vitro and induced leukemias with longer disease latencies, reduced penetrance, and less mature phenotypes. Thus, Hoxa7 and Hoxa9 contribute to hematopoietic progenitor homeostasis but are not necessary for MLL-GAS7-mediated leukemogenesis, yet they appear to affect disease latency, penetrance, and phenotypes consistent with their critical roles as downstream targets of MLL fusion proteins.

Standardization and quality control studies of ‘real-time’ quantitative reverse transcriptase polymerase chain reaction of fusion gene transcripts for residual disease detection in leukemia – A Europe Against Cancer Program

Detection of minimal residual disease (MRD) has proven to provide independent prognostic information for treatment stratification in several types of leukemias such as childhood acute lymphoblastic leukemia (ALL), chronic myeloid leukemia (CML) and acute promyelocytic leukemia. This report focuses on the accurate quantitative measurement of fusion gene (FG) transcripts as can be applied in 35-45% of ALL and acute myeloid leukemia, and in more than 90% of CML. A total of 26 European university laboratories from 10 countries have collaborated to establish a standardized protocol for TaqMan-based real-time quantitative PCR (RQ-PCR) analysis of the main leukemia-associated FGs within the Europe Against Cancer (EAC) program. Four phases were scheduled: (1) training, (2) optimization, (3) sensitivity testing and (4) patient sample testing. During our program, three quality control rounds on a large series of coded RNA samples were performed including a balanced randomized assay, which enabled final validation of the EAC primer and probe sets. The expression level of the nine major FG transcripts in a large series of stored diagnostic leukemia samples (n=278) was evaluated. After normalization, no statistically significant difference in expression level was observed between bone marrow and peripheral blood on paired samples at diagnosis. However, RQ-PCR revealed marked differences in FG expression between transcripts in leukemic samples at diagnosis that could account for differential assay sensitivity. The development of standardized protocols for RQ-PCR analysis of FG transcripts provides a milestone for molecular determination of MRD levels. This is likely to prove invaluable to the management of patients entered into multicenter therapeutic trials.