Functional characterization of ETV6 and ETV6/CBFA2 in the regulation of the MCSFR proximal promoter

Germline ETV6 mutation promotes inflammation and disrupts lymphoid development of early hematopoietic progenitors

Germline mutations in ETV6 are associated with a syndrome of thrombocytopenia and leukemia predisposition, and ETV6 is among the most commonly mutated genes in leukemias, especially childhood B-cell acute lymphoblastic leukemia. However, the mechanisms underlying disease caused by ETV6 dysfunction are poorly understood. To address these gaps in knowledge, using CRISPR/Cas9, we developed a mouse model of the most common recurrent, disease-causing germline mutation in ETV6. We found defects in hematopoiesis related primarily to abnormalities of the multipotent progenitor population 4 (MPP4) subset of hematopoietic progenitor cells and evidence of sterile inflammation. Expression of ETV6 in Ba/F3 cells altered the expression of several cytokines, some of which were also detected at higher levels in the bone marrow of the mice with Etv6 mutation. Among these, interleukin-18 and interleukin-13 abrogated B-cell development of sorted MPP4 cells, but not common lymphoid progenitors, suggesting that inflammation contributes to abnormal hematopoiesis by impairing lymphoid development. These data, along with those from humans, support a model in which ETV6 dysfunction promotes inflammation, which adversely affects thrombopoiesis and promotes leukemogenesis.

Therapeutic targeting of RBPJ, an upstream regulator of ETV6 gene, abrogates ETV6-NTRK3 fusion gene transformations in glioblastoma

Fusion genes are abnormal genes resulting from chromosomal translocation, insertion, deletion, inversion, etc. ETV6, a rather promiscuous partner forms fusions with several other genes, most commonly, the NTRK3 gene. This fusion leads to the formation of a constitutively activated tyrosine kinase which activates the Ras-Raf-MEK and PI3K/AKT/MAPK pathways, leading the cells through cycles of uncontrolled division and ultimately resulting in cancer. Targeted therapies against this ETV6-NTRK3 fusion protein are much needed. Therefore, to find a targeted approach, a transcription factor RBPJ regulating the ETV6 gene was established and since the ETV6-NTRK3 fusion gene is downstream of the ETV6 promoter/enhancer, this fusion protein is also regulated. The regulation of the ETV6 gene via RBPJ was validated by ChIP analysis in human glioblastoma (GBM) cell lines and patient tissue samples. This study was further followed by the identification of an inhibitor, Furamidine, against transcription factor RBPJ. It was found to be binding with the DNA binding domain of RBPJ with antitumorigenic properties and minimal organ toxicity. Hence, a new target RBPJ, regulating the production of ETV6 and ETV6-NTRK3 fusion protein was found along with a potent RBPJ inhibitor Furamidine.

ETV6-RUNX1 and RUNX1 directly regulate RAG1 expression: one more step in the understanding of childhood B-cell acute lymphoblastic leukemia leukemogenesis

ETV6-RUNX1 and RUNX1 directly promote RAG1 expression.

ETV6-RUNX1 and RUNX1 preferentially bind to the −1200 bp enhancer of RAG1 and the −80 bp promoter of RAG1 gene respectively, and compete for these bindings.

ETV6-RUNX1 and RUNX1 induce an excessive RAG recombinase activity.

ETV6-RUNX1 participates directly in two events of the multi-hit ALL leukemogenesis: as an initiating event and as an activator of RAG1 expression.

Diagnosis and treatment of mixed phenotype (T-myeloid/lymphoid) acute leukemia with novel ETV6-FGFR2 rearrangement

Myeloid/lymphoid neoplasms with eosinophilia are driven by aberrant tyrosine kinases in pluripotent cells and display variable phenotypes. FGFR-driven hematolymphoid neoplasms are targetable by TKI inhibitors such as ponatinib; studies of specific FGFR inhibitors are ongoing.

Reduction of RUNX1 transcription factor activity by a CBFA2T3-mimicking peptide: application to B cell precursor acute lymphoblastic leukemia

Background

B Cell Precursor Acute Lymphoblastic Leukemia (BCP-ALL) is the most common pediatric cancer. Identifying key players involved in proliferation of BCP-ALL cells is crucial to propose new therapeutic targets. Runt Related Transcription Factor 1 (RUNX1) and Core-Binding Factor Runt Domain Alpha Subunit 2 Translocated To 3 (CBFA2T3, ETO2, MTG16) are master regulators of hematopoiesis and are implicated in leukemia.

Methods

We worked with BCP-ALL mononuclear bone marrow patients’ cells and BCP-ALL cell lines, and performed Chromatin Immunoprecipitations followed by Sequencing (ChIP-Seq), co-immunoprecipitations (co-IP), proximity ligation assays (PLA), luciferase reporter assays and mouse xenograft models.

Results

We demonstrated that CBFA2T3 transcript levels correlate with RUNX1 expression in the pediatric t(12;21) ETV6-RUNX1 BCP-ALL. By ChIP-Seq in BCP-ALL patients’ cells and cell lines, we found that RUNX1 is recruited on its promoter and on an enhancer of CBFA2T3 located − 2 kb upstream CBFA2T3 promoter and that, subsequently, the transcription factor RUNX1 drives both RUNX1 and CBFA2T3 expression. We demonstrated that, mechanistically, RUNX1 and CBFA2T3 can be part of the same complex allowing CBFA2T3 to strongly potentiate the activity of the transcription factor RUNX1. Finally, we characterized a CBFA2T3-mimicking peptide that inhibits the interaction between RUNX1 and CBFA2T3, abrogating the activity of this transcription complex and reducing BCP-ALL lymphoblast proliferation.

Conclusions

Altogether, our findings reveal a novel and important activation loop between the transcription regulator CBFA2T3 and the transcription factor RUNX1 that promotes BCP-ALL proliferation, supporting the development of an innovative therapeutic approach based on the NHR2 subdomain of CBFA2T3 protein.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13045-021-01051-z.

ETV6-related thrombocytopenia and leukemia predisposition

Germ line mutations in ETV6 are responsible for a familial thrombocytopenia and leukemia predisposition syndrome. Thrombocytopenia is almost completely penetrant and is usually mild. Leukemia is reported in ∼30% of carriers and is most often B-cell acute lymphoblastic leukemia. The mechanisms by which ETV6 dysfunction promotes thrombocytopenia and leukemia remain unclear. Care for individuals with ETV6-related thrombocytopenia and leukemia predisposition includes genetic counseling, treatment or prevention of excessive bleeding and surveillance for the development of hematologic malignancy.

Transcription factor mutations as a cause of familial myeloid neoplasms

The initiation and evolution of myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) are driven by genomic events that disrupt multiple genes controlling hematopoiesis. Human genetic studies have discovered germline mutations in single genes that instigate familial MDS/AML. The best understood of these genes encode transcription factors, such as GATA-2, RUNX1, ETV6, and C/EBPα, which establish and maintain genetic networks governing the genesis and function of blood stem and progenitor cells. Many questions remain unanswered regarding how genes and circuits within these networks function in physiology and disease and whether network integrity is exquisitely sensitive to or efficiently buffered from perturbations. In familial MDS/AML, mutations change the coding sequence of a gene to generate a mutant protein with altered activity or introduce frameshifts or stop codons or disrupt regulatory elements to alter protein expression. Each mutation has the potential to exert quantitatively and qualitatively distinct influences on networks. Consistent with this mechanistic diversity, disease onset is unpredictable and phenotypic variability can be considerable. Efforts to elucidate mechanisms and forge prognostic and therapeutic strategies must therefore contend with a spectrum of patient-specific leukemogenic scenarios. Here we illustrate mechanistic advances in our understanding of familial MDS/AML syndromes caused by germline mutations of hematopoietic transcription factors.

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.

Optimization of proximity ligation assay (PLA) for detection of protein interactions and fusion proteins in non-adherent cells: application to pre-B lymphocytes

BACKGROUND

Genetic abnormalities, including chromosomal translocations, are described for many hematological malignancies. From the clinical perspective, detection of chromosomal abnormalities is relevant not only for diagnostic and treatment purposes but also for prognostic risk assessment. From the translational research perspective, the identification of fusion proteins and protein interactions has allowed crucial breakthroughs in understanding the pathogenesis of malignancies and consequently major achievements in targeted therapy.

METHODS

We describe the optimization of the Proximity Ligation Assay (PLA) to ascertain the presence of fusion proteins, and protein interactions in non-adherent pre-B cells. PLA is an innovative method of protein-protein colocalization detection by molecular biology that combines the advantages of microscopy with the advantages of molecular biology precision, enabling detection of protein proximity theoretically ranging from 0 to 40 nm.

RESULTS

We propose an optimized PLA procedure. We overcome the issue of maintaining non-adherent hematological cells by traditional cytocentrifugation and optimized buffers, by changing incubation times, and modifying washing steps. Further, we provide convincing negative and positive controls, and demonstrate that optimized PLA procedure is sensitive to total protein level. The optimized PLA procedure allows the detection of fusion proteins and protein interactions on non-adherent cells.

CONCLUSION

The optimized PLA procedure described here can be readily applied to various non-adherent hematological cells, from cell lines to patients' cells. The optimized PLA protocol enables detection of fusion proteins and their subcellular expression, and protein interactions in non-adherent cells. Therefore, the optimized PLA protocol provides a new tool that can be adopted in a wide range of applications in the biological field.

Mechanism of ETV6-RUNX1 Leukemia

The t(12;21)(p13;q22) translocation is the most frequently occurring single genetic abnormality in pediatric leukemia. This translocation results in the fusion of the ETV6 and RUNX1 genes. Since its discovery in the 1990s, the function of the ETV6-RUNX1 fusion gene has attracted intense interest. In this chapter, we will summarize current knowledge on the clinical significance of ETV6-RUNX1, the experimental models used to unravel its function in leukemogenesis, the identification of co-operating mutations and the mechanisms responsible for their acquisition, the function of the encoded transcription factor and finally, the future therapeutic approaches available to mitigate the associated disease.

Genome-wide repression of eRNA and target gene loci by the ETV6-RUNX1 fusion in acute leukemia

Approximately 20%–25% of childhood acute lymphoblastic leukemias carry the ETV6-RUNX1 (E/R) fusion gene, a fusion of two central hematopoietic transcription factors, ETV6 (TEL) and RUNX1 (AML1). Despite its prevalence, the exact genomic targets of E/R have remained elusive. We evaluated gene loci and enhancers targeted by E/R genome-wide in precursor B acute leukemia cells using global run-on sequencing (GRO-seq). We show that expression of the E/R fusion leads to widespread repression of RUNX1 motif–containing enhancers at its target gene loci. Moreover, multiple super-enhancers from the CD19⁺/CD20⁺-lineage were repressed, implicating a role in impediment of lineage commitment. In effect, the expression of several genes involved in B cell signaling and adhesion was down-regulated, and the repression depended on the wild-type DNA-binding Runt domain of RUNX1. We also identified a number of E/R-regulated annotated and de novo noncoding genes. The results provide a comprehensive genome-wide mapping between E/R-regulated key regulatory elements and genes in precursor B cell leukemia that disrupt normal B lymphopoiesis.

ETV6/RUNX1 induces reactive oxygen species and drives the accumulation of DNA damage in B cells

The t(12;21)(p13;q22) chromosomal translocation is the most frequent translocation in childhood B cell precursor-acute lymphoblastic leukemia and results in the expression of an ETV6/RUNX1 fusion protein. The frequency of ETV6/RUNX1 fusions in newborns clearly exceeds the leukemia rate revealing that additional events occur in ETV6/RUNX1-positive cells for leukemic transformation. Hitherto, the mechanisms triggering these second hits remain largely elusive. Thus, we generated a novel ETV6/RUNX1 transgenic mouse model where the expression of the fusion protein is restricted to CD19(+) B cells. These animals harbor regular B cell development and lack gross abnormalities. We established stable pro-B cell lines carrying the ETV6/RUNX1 transgene that allowed us to investigate whether ETV6/RUNX1 itself favors the acquisition of second hits. Remarkably, these pro-B cell lines as well as primary bone marrow cells derived from ETV6/RUNX1 transgenic animals display elevated levels of reactive oxygen species (ROS) as tested with ETV6/RUNX1 transgenic dihydroethidium staining. In line, intracellular phospho-histone H2AX flow cytometry and comet assay revealed increased DNA damage indicating that ETV6/RUNX1 expression enhances ROS. On the basis of our data, we propose the following model: the expression of ETV6/RUNX1 creates a preleukemic clone and leads to increased ROS levels. These elevated ROS favor the accumulation of secondary hits by increasing genetic instability and double-strand breaks, thus allowing preleukemic clones to develop into fully transformed leukemic cells.

The impact of TEL-AML1 (ETV6-RUNX1) expression in precursor B cells and implications for leukaemia using three different genome-wide screening methods

The reciprocal translocation t(12;21)(p13;q22), the most common structural genomic alteration in B-cell precursor acute lymphoblastic leukaemia in children, results in a chimeric transcription factor TEL-AML1 (ETV6-RUNX1). We identified directly and indirectly regulated target genes utilizing an inducible TEL-AML1 system derived from the murine pro B-cell line BA/F3 and a monoclonal antibody directed against TEL-AML1. By integration of promoter binding identified with chromatin immunoprecipitation (ChIP)-on-chip, gene expression and protein output through microarray technology and stable labelling of amino acids in cell culture, we identified 217 directly and 118 indirectly regulated targets of the TEL-AML1 fusion protein. Directly, but not indirectly, regulated promoters were enriched in AML1-binding sites. The majority of promoter regions were specific for the fusion protein and not bound by native AML1 or TEL. Comparison with gene expression profiles from TEL-AML1-positive patients identified 56 concordantly misregulated genes with negative effects on proliferation and cellular transport mechanisms and positive effects on cellular migration, and stress responses including immunological responses. In summary, this work for the first time gives a comprehensive insight into how TEL-AML1 expression may directly and indirectly contribute to alter cells to become prone for leukemic transformation.

TEL/AML1-positive patients lacking TEL exon 5 resemble canonical TEL/AML1 cases

BACKGROUND

The TEL/AML1 fusion gene which represents the most frequent genetic abnormality in childhood ALL, usually results from genomic breakpoints in TEL intron 5 and AML1 intron 1 or 2. At the protein level, the helix-loop-helix domain and exon 5-coded central region of TEL are typically fused to almost entire AML1 including DNA-binding domain.

PROCEDURE

We identified two ALL patients with genomic breakpoints within TEL intron 4 resulting in variant TEL/AML1 fusion lacking the TEL exon 5-coded central region. This region was supposed to play an important role in TEL/AML1 function, particularly in TEL/AML1-mediated transcriptional repression of AML1 targets. We aimed at investigating the impact of the loss of this region on disease behavior and TEL/AML1 function. We compared clinical and biological characteristics, treatment response, and outcome of the variant versus classical TEL/AML1 cases, analyzed genome wide gene expression profiles and performed reporter gene assay.

RESULTS

No distinct differences between variant and classical TEL/AML1 cases were observed including gene expression profiling and detailed immunophenotyping. By using reporter gene assay, we showed that the loss of the central region does not influence the TEL/AML1-mediated transcriptional repression.

CONCLUSIONS

The deletion of the central region did not affect the TEL/AML1-specific phenotype; we did not find any relevant differences in clinical and biological features when variant versus classical TEL/AML1-positive cases were compared. Thus, our data does not support hypothesis that the central region of TEL is indispensable for TEL/AML1 driven leukemogenesis.

Revealing the role of TEL/AML1 for leukemic cell survival by RNAi-mediated silencing

Translocation (12;21), the most frequent chromosomal aberration in childhood acute lymphoblastic leukemia, creates TEL/AML1 fusion gene. Resulting hybrid protein was shown to have a role in pre-leukemia establishment. To address its role for leukemic cell survival, we applied RNA interference to silence TEL/AML1 in leukemic cells. We designed and tested 11 different oligonucleotides targeting the TEL/AML1 fusion site. Using most efficient siRNAs, we achieved an average of 74-86% TEL/AML1 protein knockdown in REH and UOC-B6 leukemic cells, respectively. TEL/AML1 silencing neither decreased cell viability, nor induced apoptosis. On the contrary, it resulted in the modest but significant increase in the S phase fraction and in higher proliferation rate. Opposite effects on cell cycle distribution and proliferation were induced by AML1 silencing, thus, supporting our hypothesis that TEL/AML1 may block AML1-mediated promotion of G1/S progression through the cell cycle. In line with the lack of major effect on phenotype, we found no significant changes in clonogenic potential and global gene expression pattern upon TEL/AML1 depletion. Our data suggest that though TEL/AML1 is important for the (pre)leukemic clone development, it may be dispensable for leukemic cell survival and would not be a suitable target for gene-specific therapy.

HDAC3: taking the SMRT-N-CoRrect road to repression

Known histone deacetylases (HDACs) are divided into different classes, and HDAC3 belongs to Class I. Through forming multiprotein complexes with the corepressors SMRT and N-CoR, HDAC3 regulates the transcription of a plethora of genes. A growing list of nonhistone substrates extends the role of HDAC3 beyond transcriptional repression. Here, we review data on the composition, regulation and mechanism of action of the SMRT/N-CoR-HDAC3 complexes and provide several examples of nontranscriptional functions, to illustrate the wide variety of physiological processes affected by this deacetylase. Furthermore, we discuss the implication of HDAC3 in cancer, focusing on leukemia. We conclude with some thoughts about the potential therapeutic efficacies of HDAC3 activity modulation.

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

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

Characteristics of patients with TEL-AML1-positive acute lymphoblastic leukemia with single or multiple fusions

BACKGROUND

The TEL-AML1 fusion in precursor-B ALL is generated by a cryptic 12;21 translocation that is detectable by fluorescence in situ hybridization (FISH). It is generally considered a favorable prognostic indicator. Some TEL-AML1+ ALL patients present at diagnosis with extra copies of the fusion, enumerated by FISH. The aim of the study was to determine whether additional copies of TEL-AML1 have clinical significance.

PROCEDURE

Charts of all TEL-AML1+ ALL patients at the UM and Children's Hospitals and Clinics of Minnesota between 1996 and 2004 were reviewed.

RESULTS

Eight patients (7 males/1 female, mean age 46 months) with two or more TEL-AML1 fusion signals and 24 with single TEL-AML1 fusion signals (18 males/6 females, mean age 52 months) were identified. There was no statistically significant difference in age or gender between the two groups. Patients with double TEL-AML1+ had a higher frequency of myeloid markers CD13 (P = 0.04) or CD33 (P = 0.003) than single TEL-AML1+ patients. Single TEL-AML1+ patients had higher WBC (P = 0.04) than double TEL-AML1+ patients. A trend toward slower therapy response was seen in double TEL-AML1+ patients versus single, (1 of 7 [14%] <5% marrow blasts on Day 7 vs. 13 of 23 [56%], P = 0.09). Double TEL-AML1+ patients had a higher relapse rate (P = 0.09) than single TEL-AML1+ patients.

CONCLUSIONS

Utilizing FISH to distinguish subgroups of TEL-AML1 fusion patients may have important prognostic implications. The presence of an extra fusion may portend poorer prognosis. A larger and longer-term follow-up study will be required to verify the possible clinical significance of the presence of multiple TEL-AML1 fusions.

RUNX1-RUNX1 Homodimerization Modulates RUNX1 Activity and Function

RUNX1 (AML1, CBFalpha2, PEBP2alphaB) is a transcription factor essential for the establishment of the hematopoietic stem cell. It is generally thought that RUNX1 exists as a monomer that regulates hematopoietic differentiation by interacting with tissue-specific factors and its DNA consensus through its N terminus. RUNX1 is frequently altered in human leukemia by gene fusions or point mutations. In general, these alterations do not affect the N terminus of the protein, and it is unclear how they consistently lead to hematopoietic transformation and leukemia. Here we report that RUNX1 homodimerizes through a mechanism involving C terminus-C terminus interaction. This RUNX1-RUNX1 interaction regulates the activity of the protein in reporter gene assays and modulates its ability to induce hematopoietic differentiation of hematopoietic cell lines. The promoters of genes regulated by RUNX1 often contain multiple RUNX1 binding sites. This arrangement suggests that RUNX1 could homodimerize to bring and hold together distant chromatin sites and factors and that if the dimerization region is removed by gene fusions or is altered by point mutations, as observed in leukemia, the ability of RUNX1 to regulate differentiation could be impaired.

Identification of transcripts modulated by ETV6 expression

Deletions at chromosome 12p12-13 are observed in 26-47% of childhood pre-B acute lymphoblastic leukaemia (ALL) cases, suggesting the presence of a tumour suppressor gene (TSG). Accumulating genetic and functional evidence points to ETV6 as being the most probable TSG targeted by the deletions. ETV6 is a ubiquitously expressed transcription factor of the ETS family with very few known targets. To understand its function and to elucidate the impact of its absence in leukaemia, we conducted a study to identify targeted genes. Following the induction of ETV6 expression, global expression was evaluated at different time points. We identified 87 modulated genes, of which 10 (AKR1C1, AKR1C3, IL18, LUM, PHLDA1, PTGER4, PTGS2, SPHK1, TP53 and VEGF) were validated by real-time quantitative reverse transcription-polymerase chain reaction. To assess the significance of the validated candidate genes in leukaemia, their expression patterns were determined, as well as that of ETV6, in pre-B ALL patients. The expression of IL18, LUM, PTGER4, SPHK1 and TP53 was significantly correlated with that of ETV6, further suggesting that ETV6 could regulate the expression of these genes in leukaemia. This work constitutes another step towards the understanding of the functions of ETV6 and the impact of its inactivation in childhood leukaemia.

TEL/ETV6 induces apoptosis in 32D cells through p53-dependent pathways

TEL is an ETS family transcription factor that is critical for maintaining hematopoietic stem cells in adult bone marrow. To investigate the roles of TEL in myeloid proliferation and differentiation, we introduced TEL cDNA into mouse myeloid 32Dcl3 cells. Overexpression of TEL repressed interleukin-3-dependent proliferation through blocking cell cycle progression. Also, the presence of TEL triggered apoptosis through the mitochondrial intrinsic pathway on exposure to granulocyte colony-stimulating factor. We found an increase in p53 protein and its DNA binding in the TEL-overexpressing cells. Forced expression of TEL stimulated transcription via the p53-responsive element and increased the expression of cellular target genes for p53 such as cell cycle regulator p21 and apoptosis inducer Puma. Consistently, induction of apoptosis was delayed by pifithrin-alpha treatment and completely blocked by increased expression of Bcl-2 in the TEL-overexpressing cells. These data collectively suggest that TEL exerts a tumor suppressive function through augmenting the p53 pathway and facilitates normal development of myelopoiesis.

FLT3 is fused to ETV6 in a myeloproliferative disorder with hypereosinophilia and a t(12;13)(p13;q12) translocation

The FMS-like tyrosine kinase 3 (FLT3) gene, belonging to the receptor tyrosine kinase (TK) subclass III family, plays an important role in normal hematopoiesis and is one of the most frequently mutated genes in hematologic malignancies as well as an attractive target for directed inhibition. Activating mutations of this gene, including internal tandem duplication in the juxtamembrane (JM) domain and point mutations in the TK domain, are found in approximately one-third of patients with acute myeloid leukemia and in a smaller subset of patients with acute lymphoblastic leukemia. We report here that FLT3 may contribute to leukemogenesis in a patient with myeloproliferative disorder and a t(12;13)(p13;q12) translocation through generating a fusion gene with the ETS variant gene 6 (ETV6) gene. ETV6 has been reported to fuse to various partner genes, including TK and transcription factors. Both ETV6/FLT3 and reciprocal FLT3/ETV6 transcripts were detected in the patient mRNA by reverse transcriptase-polymerase chain reaction. At the protein level, however, only ETV6/FLT3 products were expressed. Among them, one retains the helix-loop-helix (HLH) oligomerization domain of ETV6 and the JM as well as TK domain of FLT3. FLT3 receptor in leukemic cells might be inappropriately activated through dimerization by HLH domain of ETV6, which consequently interfered with proliferation and differentiation of hematopoietic cells.

MN1-TEL, the product of the t(12;22) in human myeloid leukemia, immortalizes murine myeloid cells and causes myeloid malignancy in mice

MN1-TEL is the product of the recurrent t(12;22)(p12;q11) associated with human myeloid malignancies. MN1-TEL functions as an activated transcription factor, exhibiting weak transforming activity in NIH3T3 fibroblasts that depends on the presence of a functional TEL DNA-binding domain, the N-terminal transactivating sequences of MN1 and C-terminal sequences of MN1. We determined the transforming activity of MN1-TEL in mouse bone marrow (BM) by using retroviral transfer. MN1-TEL-transduced BM showed increased self-renewal capacity of primitive progenitors in vitro, and prolonged in vitro culture of MN1-TEL-expressing BM produced immortalized myeloid, interleukin (IL)-3/stem cell factor-dependent cell lines with a primitive morphology. Transplantation of such cell lines into lethally irradiated mice rescued them from irradiation-induced death and resulted in the contribution of MN1-TEL-expressing cells to all hematopoietic lineages, underscoring the primitive nature of these cells and their capacity to differentiate in vivo. Three months after transplantation, all mice succumbed to promonocytic leukemia. Transplantation of freshly MN1-TEL-transduced BM into lethally irradiated mice also caused acute myeloid leukemia within 3 months of transplantation. We infer that MN1-TEL is a hematopoietic oncogene that stimulates the growth of hematopoietic cells, but depends on secondary mutations to cause leukemia in mice.

Normal and transforming functions of RUNX1: a perspective

Converging studies from many investigators indicate that RUNX1 has a critical role in the correct maintenance of essential cellular functions during embryonic development and after birth. The discovery that this gene is also frequently mutated in human leukemia has increased the interest in the role that RUNX1 plays in both normal and transforming pathways. Here, we provide an overview of the many roles of RUNX1 in hematopoietic self-renewal and differentiation and summarize the information that is currently available on the many mechanisms of RUNX1 deregulation in human leukemia.

Runx1/AML1 in normal and abnormal hematopoiesis

Runx1/AML1 (also known as CBFA2 and PEBP23B) is a Runt family transcription factor critical for normal hematopoiesis. Runx1 forms a heterodimer with CBF3 and binds to the consensus PEBP2 sequence through the Runt domain. Runx1 enhances gene transcription by interacting with transcriptional coactivators such as p300 and CREB-binding protein. However, Runx1 can also suppress gene transcription by interacting with transcriptional corepressors, including mSin3A, TLE (mammalian homolog of Groucho), and histone deacetylases. Runx1 not only is critical for definitive hematopoiesis in the fetus but also is required for normal megakaryocytic maturation and T-lymphocyte and B-lymphocyte development in adult mice. Runx1 has been identified in leukemia-associated chromosomal translocations, including t(8;21) (Runx1-ETO/MTG8), t(16;21) (Runx1-MTG16), t(3;21) (Runx1-Evi1), t(12;21) (TEL-Runx1), and t(X;21) (Runx1-Fog2). The molecular mechanism of leukemogenesis by these fusion proteins is discussed. Various mutant mice expressing these fusion proteins have been created. However, expression of the fusion protein is not sufficient by itself to cause leukemia and likely requires additional events for leukemogenesis. Point mutations in a Runx1 allele cause haploinsufficiency and a biallelic null for Runx1, which are associated with familial platelet disorder with a propensity for acute myeloid leukemia (FPD/AML) and AML-M0, respectively. Thus, the correct protein structure and the precise dosage of Runx1 are essential for the maintenance of normal hematopoiesis.

Cloning and characterization of the novel chimeric gene TEL/PTPRR in acute myelogenous leukemia with inv(12)(p13q13)

We have cloned a novel TEL/protein tyrosine phosphatase receptor-type R (PTPRR) chimeric gene generated by inv(12)(p13q13). PTPRR is the first protein tyrosine phosphatase identified as a fusion partner of TEL. The chimeric gene fused exon 4 of the TEL gene with exon 7 of the PTPRR gene, and produced 10 isoforms through alternative splicing. Two isoforms that were expressed at the highest level in the leukemic cells could have been translated into COOH-terminally truncated TEL protein possessing the helix-loop-helix domain (tTEL) and TEL/PTPRR chimeric protein linking the helix-loop-helix domain of TEL to the catalytic domain of PTPRR. These two mutant proteins exerted a dominant-negative effect over transcriptional repression mediated by wild-type TEL, although they themselves did not show any transcriptional activity. Heterodimerization with wild-type TEL might be an underlying mechanism in this effect. TEL/PTPRR did not exhibit any tyrosine phosphatase activity. Importantly, overexpression of TEL/PTPRR in granulocyte macrophage colony-stimulating factor-dependent UT7/GM cells resulted in their factor-independent proliferation, whereas overexpression of tTEL did not. After cytokine depletion, phosphorylated signal transducers and activators of transcription 3 (STAT3) significantly declined in mock cells, but remained in both tTEL- and TEL/PTPRR-overexpressing cells. Loss of tumor suppressive function of wild-type TEL and maintenance of STAT3-mediated signal could at least partly contribute to the leukemogenesis caused by inv(12)(p13q13).

TEL/AML1 shows dominant-negative effects over TEL as well as AML1

The TEL/AML1 chimeric gene is generated by the t(12;21) translocation in pre-B cell acute lymphoblastic leukemia. TEL/AML1 consists of the helix-loop-helix (HLH) dimerization domain from TEL and almost the entire of AML1, but loses the ETS DNA-binding domain from TEL. Dominant-negative effects of TEL/AML1 over wild-type-AML1 are believed to trigger the development of this type of leukemia. However, it could also be possible that TEL/AML1 affects wild-type-TEL's molecular and tumor suppressive functions through the HLH domain. To test this possibility, we first confirmed that TEL/AML1 associates with wild-type-TEL. TEL/AML1 neither bound to the ETS-binding consensus site nor repressed transcription through it. Regardless, this prevented wild-type-TEL-induced transcriptional repression. Moreover, TEL/AML1 concomitantly inhibited wild-type-TEL-induced growth suppression and wild-type-AML1-mediated transforming activity in NIH3T3 cells. All these data indicate that TEL/AML1 exerts dominant-interfering effects on both AML1 and TEL, and that expression of TEL/AML1 could result in inactivation of TEL's tumor suppressive functions in t(12;21)-carrying leukemia.

Role of the TEL-AML1 fusion gene in the molecular pathogenesis of childhood acute lymphoblastic leukaemia

Balanced chromosomal translocations are frequently associated with haematopoietic neoplasms and often involve genes that encode transcription factors, which play critical roles in normal haematopoiesis. Fusion oncoproteins that arise from chimeric genes generated by such translocations are usually stable and consistent molecular markers for a given disease subtype and contribute to the leukaemogenic processes. The t(12;21)(p13;q22) chromosomal translocation is the most frequent illegitimate gene recombination in paediatric cancer, occurring in approximately 25% of common (c) B-cell precursor acute lymphoblastic leukaemia (cALL) cases. The rearrangement results in the in-frame fusion of the 5' region of the ETS-related gene, TEL (ETV6), to almost the entire AML1 (RUNX1) locus and is associated with favourable prognosis following conventional therapeutic strategies. We discuss here the prenatal origins of the TEL/AML1 translocation as an initiating mutation, the role of TEL-AML1 in cellular transformation and the molecular mechanisms by which the chimeric protein imposes altered patterns of gene expression.

The Runx genes: lineage-specific oncogenes and tumor suppressors

The Runx genes present a challenge to the simple binary classification of cancer genes as oncogenes or tumor suppressors. There is evidence that loss of function of two of the three mammalian Runx genes promotes cancer, but in a highly lineage-restricted manner. In human leukemias, the RUNX1 gene is involved in various chromosomal translocation events that create oncogenic fusion proteins, at least some of which appear to function as dominant-negative inhibitors of the normal gene product. Paradoxically, evidence is mounting that structurally intact Runx genes are also oncogenic when overexpressed. All the three murine genes act as targets for transcriptional activation by retroviral insertional mutagenesis, and the oncogenic potential of Runx2 has been confirmed in transgenic mice. Moreover, the RUNX1 gene is often amplified or overexpressed in cases of acute leukemia. The state of progress in elucidating the oncogenic roles of the Runx genes is the subject of this review, and we draw together recent observations in a tentative model for the effects of Runx deregulation on hematopoietic cell differentiation. We suggest that lineage-specific factors determine the sensitivity to the oncogenic effects of loss or overexpression of Runx factors.

TEL/AML1 Overcomes Drug Resistance Through Transcriptional Repression of Multidrug Resistance-1 Gene Expression

The t(12;21)(p12;q22) chromosomal aberration, which is frequently observed in pediatric precursor B-cell acute lymphoblastic leukemia (ALL), generates the TEL/AML1 chimeric gene and protein. TEL/AML1-positive ALL has a favorable prognosis, and one possible reason is that this subtype of ALL rarely shows drug resistance. AML1/ETO, another AML1-containing chimeric protein, has been shown to transcriptionally repress the activity of the multidrug resistance-1 (MDR-1) gene promoter; thus, we examined whether TEL/AML1 also represses MDR-1 gene expression, possibly preventing the emergence of multidrug resistance. In this study, we show that the TEL/AML1 protein binds to the consensus AML1 binding site in the MDR-1 promoter and transcriptionally represses its activity. Following transient transfection of TEL/AML1 protein into Adriamycin-resistant K562/Adr cells, we also demonstrate that TEL/AML1 can down-regulate the expression of P-glycoprotein, a product of the MDR-1 gene, and restore the chemosensitivity to the cells. Furthermore, we report that MDR-1 mRNA levels in leukemic cells obtained from TEL/AML1-positive ALL patients are lower than those from TEL/AML1-negative ALL patients. Thus, TEL/AML1 protein acts as a transcriptional repressor of MDR-1 gene expression, and although TEL/AML1 has been implicated in leukemogenesis, its effects on the MDR-1 gene may contribute to the excellent prognosis of TEL/AML1-positive ALL with current therapy.

Association of the T allele of an intronic single nucleotide polymorphism in the colony stimulating factor 1 receptor with Crohn's disease: a case-control study

BACKGROUND: Polymorphisms in several genes (NOD2, MDR1, SLC22A4) have been associated with susceptibility to Crohn's disease. Identification of the remaining Crohn's susceptibility genes is essential for the development of disease-specific targets for immunotherapy. Using gene expression analysis, we identified a differentially expressed gene on 5q33, the colony stimulating factor 1 receptor (CSF1R) gene, and hypothesized that it is a Crohn's susceptibility gene. The CSF1R gene is involved in monocyte to macrophage differentiation and in innate immunity. METHODS: Patients provided informed consent prior to entry into the study as approved by the Institutional Review Board at LSU Health Sciences Center. We performed forward and reverse sequencing of genomic DNA from 111 unrelated patients with Crohn's disease and 108 controls. We also stained paraffin-embedded, ileal and colonic tissue sections from patients with Crohn's disease and controls with a polyclonal antibody raised against the human CSF1R protein. RESULTS: A single nucleotide polymorphism (A2033T) near a Runx1 binding site in the eleventh intron of the colony stimulating factor 1 receptor was identified. The T allele of this single nucleotide polymorphism occurred in 27% of patients with Crohn's disease but in only 13% of controls (X2 = 6.74, p < 0.01, odds ratio (O.R.) = 2.49, 1.23 < O.R. < 5.01). Using immunohistochemistry, positive staining with a polyclonal antibody to CSF1R was observed in the superficial epithelium of ileal and colonic tissue sections. CONCLUSIONS: We conclude that the colony stimulating factor receptor 1 gene may be a susceptibility gene for Crohn's disease.

Myeloid ELF1-like Factor Is a Potent Activator of Interleukin-8 Expression in Hematopoietic Cells

Myeloid ELF1-like factor (MEF), also known as ELF4, is a member of the ETS family of transcription factors which is expressed in hematopoietic cells. MEF-deficient mice have defects in natural killer cell and natural killer T cell development, suggesting a role for MEF in regulating innate immunity. MEF also functions in myeloid cells, where it can transactivate target genes. To identify MEF target genes in a "myeloid" environment, we created an inducible expression system and used oligonucleotide microarrays to examine the transcript profile of HEL cells after induction of MEF expression. Sixteen genes were reproducibly turned on or off more than 2-fold, 8 h after induction of MEF expression, and we examined one of the genes, interleukin-8 (IL-8), in greater detail. IL-8 is a CXC chemokine involved in neutrophil chemoattraction, angiogenesis, and stem cell mobilization. It is expressed by several tumor types, and its expression is regulated primarily transcriptionally. The IL-8 promoter contains three ETS binding sites, and we identified the specific site that binds MEF and is required for MEF responsiveness. MEF, but not the closely related ETS factors PEA3, ETS1, ETS2, ELF1, or PU.1, strongly activates the IL-8 promoter. MEF overexpression is sufficient to induce IL-8 protein expression, and reduction in MEF expression (using RNA interference) results in decreased IL-8 levels. These data demonstrates that MEF is an important regulator of IL-8 expression.

The emerging role of the myeloid Elf-1 like transcription factorin hematopoiesis

MEF (myeloid Elf-1 like factor) is a member of the ETS family of transcription factors (TF) with transcriptional activating properties. ETS proteins have been implicated in widely divergent physiological and pathological processes (such as development and oncogenesis). MEF is expressed in non-hematopoietic and hematopoietic (lymphoid and myeloid) tissues, and after generating MEF-deficient mice by homologous recombination, we have studied its role in lymphopoiesis (Immunity 17 (2002), 437). MEF plays a critical role in NK and NK-T cell development and the constitutive expression of perforin by NK cells. MEF interacts with other TFs such as AML1 (Runx1) and with the cyclin A/cdk2 kinase complex. In this review, we discuss the biology of MEF in the context of the other members of this family of transcriptional regulators.

SUV39H1 interacts with AML1 and abrogates AML1 transactivity. AML1 is methylated in vivo

Acute myeloid leukemia 1 (AML1) belongs to a family of DNA-binding proteins highly conserved through evolution. AML1 regulates the expression of several hematopoietic genes and is essential for murine fetal liver hematopoiesis. We report here that the histone methyltransferase SUV39H1, a mammalian ortholog of the Drosophila melanogaster SU(VAR) 3-9, forms complex with AML1. SUV39H1 methylates lysine 9 of the histone protein H3 leading to the formation of the high-affinity binding site on chromatin for proteins of the heterochromatin protein 1 family (HP1). The interaction of AML1 with SUV39H1 requires the N-terminus of AML1 where the Runt domain is located. Binding of AML1 to SUV39H1 abrogates the transactivating and DNA-binding properties of AML1 and dissociates the net-like nuclear structure of AML1. It has been reported that AML1 is capable of interaction with histone acetyl transferases (CBP, p300, and MOZ) and with component of the histone deacetylase complex (Sin3), and that the interaction with these coregulators affects the strength of AML1 in promoter regulation. Our data suggest that other enzymes are also involved in gene regulation by AML1 activity by modulating the affinity of AML1 for DNA.

The human L(3)MBT polycomb group protein is a transcriptional repressor and interacts physically and functionally with TEL (ETV6)

H-L(3)MBT, the human homolog of the Drosophila lethal(3)malignant brain tumor protein, is a member of the polycomb group (PcG) of proteins, which function as transcriptional regulators in large protein complexes. Homozygous mutations in the l(3)mbt gene cause brain tumors in Drosophila, identifying l(3)mbt as a tumor suppressor gene. The h-l(3)mbt gene maps to chromosome 20q12, within a common deleted region associated with myeloid hematopoietic malignancies. H-L(3)MBT contains three repeats of 100 residues called MBT repeats, whose function is unknown, and a C-terminal alpha-helical structure, the SPM (SCM, PH, MBT domain, which is structurally similar to the SAM (sterile alpha motif) protein-protein interaction domain, found in several ETS transcription factors, including TEL (translocation Ets leukemia). We report that H-L(3)MBT is a transcriptional repressor and that its activity is largely dependent on the presence of a region containing the three MBT repeats. H-L(3)MBT acts as a histone deacetylase-independent transcriptional repressor, based on its lack of sensitivity to trichostatin A. We found that H-L(3)MBT binds in vivo to TEL, and we have mapped the region of interaction to their respective SPM/SAM domains. We show that the ability of TEL to repress TEL-responsive promoters is enhanced by the presence of H-L(3)MBT, an effect dependent on the H-L(3)MBT and the TEL interacting domains. These experiments suggest that histone deacetylase-independent transcriptional repression by TEL depends on the recruitment of PcG proteins. We speculate that the interaction of TEL with H-L(3)MBT can direct a PcG complex to genes repressed by TEL, stabilizing their repressed state.

AML1 interconnected pathways of leukemogenesis

The AML1 transcription factor, identified by the cloning of the translocation t(8;21) breakpoint, is one of the most frequent targets for chromosomal translocations in leukemia. Furthermore, polysomies and point mutations can also alter AML1 function. AML1, also called CBF alpha 2, PEBP alpha 2 or RUNX1, is thus implicated in a great number of acute leukemias via a variety of pathogenic mechanisms and seems to act either as an oncogene or a tumor suppressor gene. Characterization of AML1 knockout mice has shown that AML1 is necessary for normal development of all hematopoietic lineages and alterations in the overal functional level of AML1 can have a profound effect on hematopoiesis. Numerous studies have shown that AML1 plays a vital role in the regulation of expression of many genes involved in hematopoietic cell development, and the impairment of AML1 function disregulates the pathways leading to cellular proliferation and differentiation. However, heterozygous AML1 mutations alone may not be sufficient for the development of leukemia. A cumulative process of mutagenesis involving additional genetic events in functionally related molecules, may be necessary for the development of leukemia and may determine the leukemic phenotype. We review the known AML1 target genes, AML1 interacting proteins, AML1 gene alterations and their effects on AML1 function, and mutations in AML1-related genes associated with leukemia. We discuss the interconnections between all these genes in cell signaling pathways and their importance for future therapeutic developments.

Differential expression of TCL1 during pre-B-cell acute lymphoblastic leukemia progression

Nonrandom, recurring chromosomal translocations are critical events in the pathogenesis of leukemia. The recently identified TEL/AML1 (CBFA2/EVT6) fusion gene occurs as a result of the t(12;21)(p13;q22) in approximately 25% of children with diagnosed pre-B-cell acute lymphoblastic leukemia (PBC-ALL). To identify changes in gene expression patterns that occur during PBC-ALL disease progression, we used cDNA microarrays to compare expressed sequences from the AT-1 and AT-2 cell lines. These cell lines, from the same patient, were established from two distinct stages of PBC-ALL disease progression, namely, first and second relapse. Analysis of both cell lines with spectral karyotying (SKY) revealed an insertion of chromosome 8 into chromosome 5 and a previously undetected translocation in AT-2 involving chromosomes 1 and 17. Hybridization of cDNA microarrays identified the TCL1 transcript as being overexpressed in the AT-2 cell line relative to AT-1. Northern blot analysis showed an eightfold increase of the TCL1 transcript in AT-2 over AT-1 cells. Western blot analysis showed that the TCL1 protein was expressed more than 50-fold higher in AT-2 than AT-1 cells. TCL1 expression was correlated with TEL expression by reintroducing TEL into AT-2 cells and demonstrating that those cells expressing TEL at high levels showed a decreased expression of endogenous TCL1.

Transcription factor fusions in acute leukemia: variations on a theme

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

Polymerization of the SAM domain of TEL in leukemogenesis and transcriptional repression

TEL is a transcriptional repressor that is a frequent target of chromosomal translocations in a large number of hematalogical malignancies. These rearrangements fuse a potent oligomerization module, the SAM domain of TEL, to a variety of tyrosine kinases or transcriptional regulatory proteins. The self-associating property of TEL-SAM is essential for cell transformation in many, if not all of these diseases. Here we show that the TEL-SAM domain forms a helical, head-to-tail polymeric structure held together by strong intermolecular contacts, providing the first clear demonstration that SAM domains can polymerize. Our results also suggest a mechanism by which SAM domains could mediate the spreading of transcriptional repression complexes along the chromosome.

Proteins of the ETS family with transcriptional repressor activity

ETS proteins form one of the largest families of signal-dependent transcriptional regulators, mediating cellular proliferation, differentiation and tumorigenesis. Most of the known ETS proteins have been shown to activate transcription. However, four ETS proteins (YAN, ERF, NET and TEL) can act as transcriptional repressors. In three cases (ERF, NET and TEL) distinct repression domains have been identified and there are indications that NET and TEL may mediate transcription via Histone Deacetylase recruitment. All four proteins appear to be regulated by MAPKs, though for YAN and ERF this regulation seems to be restricted to ERKs. YAN, ERF and TEL have been implicated in cellular proliferation although there are indications suggesting a possible involvement of YAN and TEL in differentiation as well. Other ETS-domain proteins have been shown to repress transcription in a context specific manner, and there are suggestions that the ETS DNA-binding domain may act as a transcriptional repressor. Transcriptional repression by ETS domain proteins adds an other level in the orchestrated regulation by this diverse family of transcription factors that often recognize similar if not identical binding sites on DNA and are believed to regulate critical genes in a variety of biological processes. Definitive assessment of the importance of this novel regulatory level will require the identification of ETS proteins target genes and the further analysis of transcriptional control and biological function of these proteins in defined pathways.

Regulation of Ets function by protein - protein interactions

Ets proteins are a family of transcription factors that share an 85 amino acid conserved DNA binding domain, the ETS domain. Over 25 mammalian Ets family members control important biological processes, including cellular proliferation, differentiation, lymphocyte development and activation, transformation and apoptosis by recognizing the GGA core motif in the promoter or enhancer of their target genes. Protein - protein interactions regulates DNA binding, subcellular localization, target gene selection and transcriptional activity of Ets proteins. Combinatorial control is a characteristic property of Ets family members, involving interaction between Ets and other key transcriptional factors such as AP-1, NFkappaB and Pax family members. Specific domains of Ets proteins interact with many protein motifs such as bHLH, bZipper and Paired domain. Such interactions coordinate cellular processes in response to diverse signals including cytokines, growth factors, antigen and cellular stresses.

Ets target genes: past, present and future

Ets is a family of transcription factors present in species ranging from sponges to human. All family members contain an approximately 85 amino acid DNA binding domain, designated the Ets domain. Ets proteins bind to specific purine-rich DNA sequences with a core motif of GGAA/T, and transcriptionally regulate a number of viral and cellular genes. Thus, Ets proteins are an important family of transcription factors that control the expression of genes that are critical for several biological processes, including cellular proliferation, differentiation, development, transformation, and apoptosis. Here, we tabulate genes that are regulated by Ets factors and describe past, present and future strategies for the identification and validation of Ets target genes. Through definition of authentic target genes, we will begin to understand the mechanisms by which Ets factors control normal and abnormal cellular processes.

Alterations of the AML1 transcription factor in human leukemia

The identification of clonal chromosomal translocations in human leukemias provided one of the first insights into the underlying pathogenesis of this clinically heterogeneous disease. Over the last decade a large number of these chromosomal rearrangements have been molecularly cloned and the involved genes identified. A surprising finding that has emerged from this work is that many of these chromosomal alterations target the genes encoding the AML1/CBFbeta transcription factor complex, a critical regulator of normal hematopoiesis. In this review, we summarize our present understanding of the mechanisms through which alterations of AML1/CBFbeta contribute to leukemogenesis.

TEL, a putative tumor suppressor, modulates cell growth and cell morphology of ras-transformed cells while repressing the transcription of stromelysin-1

TEL is a member of the ETS family of transcription factors that interacts with the mSin3 and SMRT corepressors to regulate transcription. TEL is biallelically disrupted in acute leukemia, and loss of heterozygosity at the TEL locus has been observed in various cancers. Here we show that expression of TEL in Ras-transformed NIH 3T3 cells inhibits cell growth in soft agar and in normal cultures. Unexpectedly, cells expressing both Ras and TEL grew as aggregates. To begin to explain the morphology of Ras-plus TEL-expressing cells, we demonstrated that the endogenous matrix metalloproteinase stromelysin-1 was repressed by TEL. TEL bound sequences in the stromelysin-1 promoter and repressed the promoter in transient-expression assays, suggesting that it is a direct target for TEL-mediated regulation. Mutants of TEL that removed a binding site for the mSin3A corepressor but retained the ETS domain failed to repress stromelysin-1. When BB-94, a matrix metalloproteinase inhibitor, was added to the culture medium of Ras-expressing cells, it caused a cell aggregation phenotype similar to that caused by TEL expression. In addition, TEL inhibited the invasiveness of Ras-transformed cells in vitro and in vivo. Our results suggest that TEL acts as a tumor suppressor, in part, by transcriptional repression of stromelysin-1.

Lineage specificity of CBFA2 fusion transcripts

The CBFA2 gene on chromosome band 21q22 is one of the most commonly translocated genes in leukemia. As with other translocations, those involving CBFA2 are associated with specific disease phenotypes. Only one of the different translocations involving CBFA2, the t(12;21), has been associated with a non-myeloid lineage. Several different CBFA2 fusion transcripts were expressed in the myeloid 32Dcl3 cell line, and show that unlike the myeloid specific fusion transcripts, the lymphoid specific ETV6/CBFA2 transcript is not compatible with myeloid cell differentiation. It is shown that myeloid cells expressing the ETV6/CBFA2 transcript undergo apoptosis in response to a G-CSF differentiation signal. The molecular differences in the cells we studied are characterized using Western blot analysis to show that t(12;21) expressing cells fail to express the G-CSF receptor.

The leukemia-associated gene TEL encodes a transcription repressor which associates with SMRT and mSin3A

The E-26 transforming specific (ETS)-related gene TEL, also known as ETV6, encodes a strong transcription repressor that is rearranged in several recurring chromosomal rearrangements associated with leukemia and congenital fibrosarcoma. The TEL protein contains two functional domains that have been partially characterized: a helix-loop-helix (HLH) domain (also known as a pointed domain) at the N-terminus, which physically interacts with itself, with the SUMO-conjugating enzyme UBC9, and with FLI1; and, at the C-terminus, an ETS domain with DNA-binding properties. Little is known about the function of the central region of TEL. The HLH domain and the central region of TEL are consistently maintained in the t(12;21), which is the most frequent chromosomal translocation involving TEL. In this study, we found that the HLH domain and the central region of TEL mediate transcription repression by two distinct mechanisms. The central region involves the recruitment of a repression complex, including SMRT and mSin3A. The HLH domain represses gene transcription through a mechanism that is independent of known corepressors. Thus, TEL belongs to a growing number of transcription factors rearranged by chromosomal translocations that are associated with the corepressor complexes.