Fusion of the TEL gene on 12p13 to the AML1 gene on 21q22 in acute lymphoblastic leukemia

The TEL/ETV6 gene is required specifically for hematopoiesis in the bone marrow

The TEL (translocation-Ets-leukemia or ETV6) locus, which encodes an Ets family transcription factor, is frequently rearranged in human leukemias of myeloid or lymphoid origins. By gene targeting in mice, we previously showed that TEL-/- mice are embryonic lethal because of a yolk sac angiogenic defect. TEL also appears essential for the survival of selected neural and mesenchymal populations within the embryo proper. Here, we have generated mouse chimeras with TEL-/- ES cells to examine a possible requirement in adult hematopoiesis. Although not required for the intrinsic proliferation and/or differentiation of adult-type hematopoietic lineages in the yolk sac and fetal liver, TEL function is essential for the establishment of hematopoiesis of all lineages in the bone marrow. This defect is manifest within the first week of postnatal life. Our data pinpoint a critical role for TEL in the normal transition of hematopoietic activity from fetal liver to bone marrow. This might reflect an inability of TEL-/- hematopoietic stem cells or progenitors to migrate or home to the bone marrow or, more likely, the failure of these cells to respond appropriately and/or survive within the bone marrow microenvironment. These data establish TEL as the first transcription factor required specifically for hematopoiesis within the bone marrow, as opposed to other sites of hematopoietic activity during development.

Multiplex Reverse Transcription-Polymerase Chain Reaction for Simultaneous Screening of 29 Translocations and Chromosomal Aberrations in Acute Leukemia

We have developed a multiplex reverse transcription-polymerase chain reaction (RT-PCR) reaction, which enables us to detect 29 translocations/chromosomal aberrations in patients with acute lymphoid leukemia (ALL) and acute myeloid leukemia (AML). Through the construction and optimization of specific primers for each translocation, we have been able to reduce the set-up to 8 parallel multiplex PCR reactions, thus greatly decreasing the amount of work and reagents. We show the value of our set-up in a retrospective analysis on cryopreserved material from 102 AML and 62 ALL patients. The multiplex RT-PCR detected a hybrid mRNA resulting from a structural chromosomal aberration in 45 of 102 (44%) of the AML and in 28 of 62 (45%) of the pediatric ALL cases. Importantly, in 33% of AML and in 47% of the ALL cases with cytogenetic data, submicroscopic chromosomal aberrations or masked translocations were shown that were not detected in the cytogenetic analysis either for structural reasons or because of an insufficient number of metaphases obtained. This multiplex RT-PCR system, which can handle up to 10 patients with a response time of 2 working days, is thus an important tool that complements cytogenetic analysis in the up-front screening of acute leukemia patients and should provide a rapid and efficient characterization of leukemia cells, even in situations with sparse patient material.

Multiplex Reverse Transcription-Polymerase Chain Reaction for Simultaneous Screening of 29 Translocations and Chromosomal Aberrations in Acute Leukemia

We have developed a multiplex reverse transcription-polymerase chain reaction (RT-PCR) reaction, which enables us to detect 29 translocations/chromosomal aberrations in patients with acute lymphoid leukemia (ALL) and acute myeloid leukemia (AML). Through the construction and optimization of specific primers for each translocation, we have been able to reduce the set-up to 8 parallel multiplex PCR reactions, thus greatly decreasing the amount of work and reagents. We show the value of our set-up in a retrospective analysis on cryopreserved material from 102 AML and 62 ALL patients. The multiplex RT-PCR detected a hybrid mRNA resulting from a structural chromosomal aberration in 45 of 102 (44%) of the AML and in 28 of 62 (45%) of the pediatric ALL cases. Importantly, in 33% of AML and in 47% of the ALL cases with cytogenetic data, submicroscopic chromosomal aberrations or masked translocations were shown that were not detected in the cytogenetic analysis either for structural reasons or because of an insufficient number of metaphases obtained. This multiplex RT-PCR system, which can handle up to 10 patients with a response time of 2 working days, is thus an important tool that complements cytogenetic analysis in the up-front screening of acute leukemia patients and should provide a rapid and efficient characterization of leukemia cells, even in situations with sparse patient material.

The ets family member Tel binds to the Fli-1 oncoprotein and inhibits its transcriptional activity

The tel gene, recently shown to be translocated in a spectrum of acute and chronic human leukemias, belongs to the ets family of sequence-specific transcription factors. To determine the role of Tel in normal hematopoietic development, we used the tel gene as the bait in the yeast two-hybrid system to screen a hematopoietic stem cell library. Two partners were identified: Tel binds to itself, and Tel binds to the ets family member Fli-1. In vitro and in vivo assays confirmed these interactions. In transient transfection assays, Fli-1 transactivates megakaryocytic specific promoters, and Tel inhibits this effect of Fli-1. Transactivation studies using deletion mutants of Tel, and the Tel-AML-1 fusion protein, indicate that the helix-loop-helix domain of Tel only partially inhibits transactivation and that complete inhibition requires the full-length Tel molecule, including the DNA binding domain. The Tel and Fli-1 proteins are expressed early in hematopoiesis, and the inability of Tel fusion proteins such as Tel-AML-1 to counteract Fli-1 mediated transactivation may contribute to the malignant phenotype in human leukemias where this fusion protein is present.

Multiple functional domains of AML1: PU.1 and C/EBPalpha synergize with different regions of AML1

Control elements of many genes are regulated by multiple activators working in concert to confer the maximal level of expression, but the mechanism of such synergy is not completely understood. The promoter of the human macrophage colony-stimulating factor (M-CSF) receptor presents an excellent model with which we can study synergistic, tissue-specific activation for two reasons. First, myeloid-specific expression of the M-CSF receptor is regulated transcriptionally by three factors which are crucial for normal hematopoiesis: PU.1, AML1, and C/EBPalpha. Second, these proteins interact in such a way as to demonstrate at least two examples of synergistic activation. We have shown that AML1 and C/EBPalpha activate the M-CSF receptor promoter in a synergistic manner. As we report here, AML1 also synergizes, and interacts physically, with PU. 1. Detailed analysis of the physical and functional interaction of AML1 with PU.1 and C/EBPalpha has revealed that the proteins contact one another through their DNA-binding domains and that AML1 exhibits cooperative DNA binding with C/EBPalpha but not with PU.1. This difference in DNA-binding abilities may explain, in part, the differences observed in synergistic activation. Furthermore, the activation domains of all three factors are required for synergistic activation, and the region of AML1 required for synergy with PU.1 is distinct from that required for synergy with C/EBPalpha. These observations present the possibility that synergistic activation is mediated by secondary proteins contacted through the activation domains of AML1, C/EBPalpha, and PU.1.

Fluorescence in situ hybridization analysis of 12;21 translocation in Japanese childhood acute lymphoblastic leukemia

Fluorescence in situ hybridization (FISH) analysis was applied to detect t(12;21) using two yeast artificial chromosome probes and cosmid probes covering the TEL(ETV6) and the AML1 gene to clarify the incidence of abnormality of t(12;21) in Japanese childhood acute lymphoblastic leukemia (ALL). We detected seven TEL/AML1 fusion positive patients (9.5%), all of whom were diagnosed as B-lineage ALL, among 74 childhood ALL. On the other hand, no TEL/AML1 fusion positive patients were found among 37 adult ALL. The incidence among Japanese seemed to be lower than that among other nations. Of the seven patients with the TEL/AML1 fusion, five exhibited normal karyotype, one was t(8;12)(q11;p13), i(21q) and the remaining one exhibited a near-triploid karyotype in conventional G-banding. The FISH method clearly demonstrated that all patients with the TEL/AML1 fusion had subpopulations of leukemic cells with deletion of the normal TEL allele, which is significant for understanding the progression of leukemia with t(12;21).

10 years of Genomics, chromosome 21, and Down syndrome

During the first 10 years of Genomics (1987-1997), the molecular structure of human chromosome 21 (HC21) has been intensively investigated. Due to its small size and involvement in Down syndrome, it continues to serve as a model in the development of "genomics technologies." Increasingly more detailed genetic, radiation hybrid, physical, and transcription maps, in addition to NotI restriction and chromosomal breakpoint maps, of HC21 have been developed, and approximately 10% of its genes have been cloned. These maps have been vital in the localization of loci for 15 monogenic disorders to HC21, and 10 of these genes have been identified and characterized. The genetic maps have aided in the detailed elucidation of the origin of the supernumerary HC21 in trisomy 21 from investigations of recombination and nondisjunction events. Mouse models of Down syndrome, with partial trisomy 16, the mouse chromosome principally syntenic to HC21, have been created and initially characterized. A substantial number of the above studies related to the molecular mapping, gene cloning, and infrastructure of HC21 were published in Genomics (e.g., approximately 30% of papers describing HC21 maps were published here). The future goals of genomic analysis of HC21 will be the determination of its complete nucleotide sequence and the identification and functional analysis of all of its genes. These advances will help to provide a molecular explanation of the pathophysiology of Down syndrome and aid in the identification of genes for monogenic and polygenic disorders that map on this chromosome. Novel therapeutic interventions for Down syndrome and the monogenic and polygenic disorders that map to HC21 will be designed and tried based on the knowledge of the disease pathogenesis resulting from the genomic analysis.

Haemopoietic transformation by the TEL/ABL oncogene

Rare, novel forms of activated ABL kinase, the result of a fusion between TEL (or ETV6, a member of the ETS transcription factor family), and the non-receptor tyrosine kinase ABL, have been identified. We have analysed the TEL/ABL fusion protein (type A) cloned from an acute lymphoblastic leukaemia patient. In contrast to a second TEL/ABL fusion (type B) identified in two cases of myeloid leukaemia, the portion of TEL contained in the type A TEL/ABL fusion was smaller and did not contain a potential Grb2 binding site. The type A TEL/ABL cDNA we used in this study encoded a 155 kD protein with elevated tyrosine kinase activity and was responsible for the phosphorylation of a number of proteins in vivo. Its expression in factor-dependent murine haemopoietic precursor cells efficiently converted these cells to factor independence for both survival and growth. These cells continued to express high levels of myc mRNA after growth factor depletion. We also demonstrated that type A TEL/ABL self-associated in stably expressing haemopoietic cells. Although the TEL portion of the TEL/ABL fusion protein has no sequence similarity to that of BCR in the BCR/ABL protein, all forms of these fusion proteins contain a structure implicated in oligomerization. Our results support the conclusion that the protein interaction domain of BCR and TEL, but not the Grb2 binding site, are the important functional components in the activation of ABL kinase in haemopoietic discase.

Cytoplasmic sequestration of the polyomavirus enhancer binding protein 2 (PEBP2)/core binding factor alpha (CBFalpha) subunit by the leukemia-related PEBP2/CBFbeta-SMMHC fusion protein inhibits PEBP2/CBF-mediated transactivation

The polyomavirus enhancer binding protein 2 (PEBP2)/core binding factor (CBF) is a transcription factor composed of two subunits, alpha and beta. The gene encoding the beta subunit is disrupted by inv(16), resulting in the formation of a chimeric protein, beta-SMMHC, which is associated with acute myelogenous leukemia. To understand the effect of beta-SMMHC on PEBP2-mediated transactivation, we used a luciferase assay system in which contribution of both the alpha and beta subunits was absolutely required to activate transcription. Using this system, we found that the minimal region of the beta subunit required for transactivation resides between amino acid 1 and 135, which is known to dimerize with the alpha subunit. In contrast, beta-SMMHC, despite having this minimal region for dimerization and transactivation, failed to support transcription with the alpha subunit. Furthermore beta-SMMHC blocked the synergistic transcription achieved by PEBP2 and CCAAT/enhancer binding protein alpha. By using a construct in which the PEBP2 alpha subunit was fused to the glucocorticoid receptor ligand binding domain, we demonstrated that coexpressed beta-SMMHC tightly sequestered the alpha subunit in the cytoplasm and blocked dexamethasone-dependent nuclear translocation of the alpha subunit. Thus, the result suggess that beta-SMMHC inhibits PEBP2-mediated transcription via cytoplasmic sequestration of the alpha subunit. Lastly proliferation of ME-1 cells that harbor inv(16) was blocked by an antisense oligonucleotide complementary to the junction of the chimeric mRNA, suggesting that beta-SMMHC contributes to leukemogenesis by blocking the differentiation of myeloid cells.

The Partner Gene of AML1 in t(16;21) Myeloid Malignancies Is a Novel Member of the MTG8(ETO) Family

The t(16;21)(q24;q22) translocation is a rare but recurrent chromosomal abnormality associated with therapy-related myeloid malignancies and a variant of the t(8;21) translocation in which theAML1 gene on chromosome 21 is rearranged. Here we report the molecular definition of this chromosomal aberration in four patients. We cloned cDNAs from the leukemic cells of a patient carrying t(16;21) by the reverse transcription polymerase chain reaction using anAML1-specific primer. The structural analysis of the cDNAs showed that AML1 was fused to a novel gene named MTG16(Myeloid Translocation Gene on chromosome16) which shows high homology to MTG8(ETO/CDR) and MTGR1. Northern blot analysis usingMTG16 probes mainly detected 4.5 kb and 4.2 kb RNAs, along with several other minor RNAs in various human tissues. As in t(8;21), the t(16;21) breakpoints occurred between the exons 5 and 6 ofAML1, and between the exons 1 and 2 or the exons 3 and 4 ofMTG16. The two genes are fused in-frame, resulting in the characteristic chimeric transcripts of this translocation. Although the reciprocal chimeric product, MTG16-AML1, was also detected in one of the t(16;21) patients, its protein product was predicted to be truncated. Thus, the AML1-MTG16 gene fusion in t(16;21) leukemia results in the production of a protein that is very similar to the AML1-MTG8 chimeric protein.

Interaction and functional cooperation of the leukemia-associated factors AML1 and p300 in myeloid cell differentiation

The AML1 transcription factor and the transcriptional coactivators p300 and CBP are the targets of chromosome translocations associated with acute myeloid leukemia and myelodysplastic syndrome. In the t(8;21) translocation, the AML1 (CBFA2/PEBP2alphaB) gene becomes fused to the MTG8 (ETO) gene. We previously found that the terminal differentiation step leading to mature neutrophils in response to granulocyte colony-stimulating factor (G-CSF) was inhibited by the ectopic expression of the AML1-MTG8 fusion protein in L-G murine myeloid progenitor cells. We show here that overexpression of normal AML1 proteins reverses this inhibition and restores the competence to differentiate. Immunoprecipitation analysis shows that p300 and CREB-binding protein (CBP) interact with AML1. The C-terminal region of AML1 is responsible for the induction of cell differentiation and for the interaction with p300. Overexpression of p300 stimulates AML1-dependent transcription and the induction of cell differentiation. These results suggest that p300 plays critical roles in AML1-dependent transcription during the differentiation of myeloid cells. Thus, AML1 and its associated factors p300 and CBFbeta, all of which are targets of chromosomal rearrangements in human leukemia, function cooperatively in the differentiation of myeloid cells.

The Partner Gene of AML1 in t(16;21) Myeloid Malignancies Is a Novel Member of the MTG8(ETO) Family

The t(16;21)(q24;q22) translocation is a rare but recurrent chromosomal abnormality associated with therapy-related myeloid malignancies and a variant of the t(8;21) translocation in which theAML1 gene on chromosome 21 is rearranged. Here we report the molecular definition of this chromosomal aberration in four patients. We cloned cDNAs from the leukemic cells of a patient carrying t(16;21) by the reverse transcription polymerase chain reaction using anAML1-specific primer. The structural analysis of the cDNAs showed that AML1 was fused to a novel gene named MTG16(Myeloid Translocation Gene on chromosome16) which shows high homology to MTG8(ETO/CDR) and MTGR1. Northern blot analysis usingMTG16 probes mainly detected 4.5 kb and 4.2 kb RNAs, along with several other minor RNAs in various human tissues. As in t(8;21), the t(16;21) breakpoints occurred between the exons 5 and 6 ofAML1, and between the exons 1 and 2 or the exons 3 and 4 ofMTG16. The two genes are fused in-frame, resulting in the characteristic chimeric transcripts of this translocation. Although the reciprocal chimeric product, MTG16-AML1, was also detected in one of the t(16;21) patients, its protein product was predicted to be truncated. Thus, the AML1-MTG16 gene fusion in t(16;21) leukemia results in the production of a protein that is very similar to the AML1-MTG8 chimeric protein.

Expression of a Knocked-In AML1-ETO Leukemia Gene Inhibits the Establishment of Normal Definitive Hematopoiesis and Directly Generates Dysplastic Hematopoietic Progenitors

The t(8;21)-encoded AML1-ETO chimeric product is believed to be causally involved in up to 15% of acute myelogenous leukemias through an as yet unknown mechanism. To directly investigate the role of AML1-ETO in leukemogenesis, we used gene targeting to create anAML1-ETO “knock-in” allele that mimics the t(8;21). Unexpectedly, embryos heterozygous for AML1-ETO(AML1-ETO/+) died around E13.5 from a complete absence of normal fetal liver–derived definitive hematopoiesis and lethal hemorrhages. This phenotype was similar to that seen following homozygous disruption of either AML1 orCBFβ. However, in contrast to AML1- or CBFβ-deficient embryos, fetal livers from AML1-ETO/+ embryos contained dysplastic multilineage hematopoietic progenitors that had an abnormally high self-renewal capacity in vitro. To further document the role of AML1-ETO in these growth abnormalities, we used retroviral transduction to express AML1-ETO in murine adult bone marrow–derived hematopoietic progenitors. AML1-ETO–expressing cells were again found to have an increased self-renewal capacity and could be readily established into immortalized cell lines in vitro. Taken together, these studies suggest that AML1-ETO not only neutralizes the normal biologic activity of AML1 but also directly induces aberrant hematopoietic cell proliferation.

Prognostic significance of TEL/AML1 fusion transcript in childhood B-precursor acute lymphoblastic leukemia

PURPOSE

A retrospective study was conducted to investigate the prognostic significance of TEL/AML1 fusion resulting from a cryptic t(12;21) in Japanese patients with childhood B-precursor acute lymphoblastic leukemia (ALL).

MATERIALS AND METHODS

Leukemic samples from 144 children with newly diagnosed ALL (104 with CD10-positive B-precursor ALL, 11 with CD10-negative B-precursor ALL, 5 with B-ALL, and 24 with T-ALL) were analyzed by reverse-transcription polymerase chain reaction.

RESULTS

The frequency of patients with TEL/AML1 was 16% (23 of 144) and all patients with TEL/AML1 also had CD10-positive B-precursor ALL. TEL/AML1 was not found in any samples from the patients with T-ALL, B-ALL, or CD10-negative B-precursor ALL. Among patients with CD10-positive B-precursor ALL, age, initial white blood cell count, and immunophenotype did not differ with TEL/AML1 positivity, although the patients were predominantly male (p < 0.01). Clinical outcomes of 94 patients treated with recent protocols were analyzed. Five of the 21 (23.8%) patients with TEL/AML1 relapsed and 4 of these relapsed > 24 months after diagnosis. Although the overall 5-year survival rate was better among patients with TEL/AML1 fusion transcript than among those without it (87.3 +/- 8.7% versus 75.9 +/- 5.8%, respectively), the 5-year disease-free survival (DFS) rates of patients with TEL/AML1 fusion transcript and those without it were similar (64.0 +/- 13.5% versus 69.1 +/- 6.3%, respectively). However, for 57 patients treated with the latest intensive protocol, the 4-year DFS rate was much higher for the patients with TEL/AML1 fusion transcript than for those without it (100.0% v.s. 69.6 +/- 8.4%, respectively, p = 0.1472).

CONCLUSIONS

This study confirmed that TEL/AML1 gene fusion is the most common genetic event in pediatric ALL in Japan and is restricted to CD10-positive B-precursor ALL. Moreover, it was associated with an improved survival rate among patients treated with intensive therapy. Therefore, these data suggest that the patients with TEL/AML1 may not necessarily be candidates for less aggressive treatment.

Detection of chromosome over- and underrepresentations in hyperdiploid acute lymphoblastic leukemia by comparative genomic hybridization

Chromosomal analysis of acute lymphoblastic leukemia (ALL) is often difficult because of the suboptimal in vitro growth of the immature lymphoid cell and the poor morphology obtained. In this study, we describe the application of comparative genomic hybridization (CGH) to investigate the genomic abnormalities in 14 patients with ALL, all of whom had cytogenetically identified numerical aberrations or gross chromosomal structural alteration. With the use of CGH, regional or whole chromosome overrepresentation or both were found to be more frequent than underrepresentation (52 gains vs. 6 losses), the most common gains being chromosomes 21 and X. The results of the comparison between CGH and conventional R-banding analysis could be classified into three categories: (1) in three cases, including two with trisomy, CGH and banding analysis gave identical results; (2) in six cases with hyperdiploidy and two cases presenting chromosome structural abnormalities, the results were consistent but with minor discrepancies; (3) in three cases, including two with triploidy and tetraploidy and one with chimeric karyotype together with +22, the data from CGH and cytogenetical analysis were discrepant. CGH could not find the triploidy and tetraploidy. Our results suggest that CGH has certain value in the detection of gains or losses of chromosome materials in hyperdiploid ALL. Nevertheless, the combination of CGH and conventional karyotyping provides more precise information on the genomic imbalance in ALL.

Expression of a Knocked-In AML1-ETO Leukemia Gene Inhibits the Establishment of Normal Definitive Hematopoiesis and Directly Generates Dysplastic Hematopoietic Progenitors

The t(8;21)-encoded AML1-ETO chimeric product is believed to be causally involved in up to 15% of acute myelogenous leukemias through an as yet unknown mechanism. To directly investigate the role of AML1-ETO in leukemogenesis, we used gene targeting to create anAML1-ETO “knock-in” allele that mimics the t(8;21). Unexpectedly, embryos heterozygous for AML1-ETO(AML1-ETO/+) died around E13.5 from a complete absence of normal fetal liver–derived definitive hematopoiesis and lethal hemorrhages. This phenotype was similar to that seen following homozygous disruption of either AML1 orCBFβ. However, in contrast to AML1- or CBFβ-deficient embryos, fetal livers from AML1-ETO/+ embryos contained dysplastic multilineage hematopoietic progenitors that had an abnormally high self-renewal capacity in vitro. To further document the role of AML1-ETO in these growth abnormalities, we used retroviral transduction to express AML1-ETO in murine adult bone marrow–derived hematopoietic progenitors. AML1-ETO–expressing cells were again found to have an increased self-renewal capacity and could be readily established into immortalized cell lines in vitro. Taken together, these studies suggest that AML1-ETO not only neutralizes the normal biologic activity of AML1 but also directly induces aberrant hematopoietic cell proliferation.

Lack of TEL/AML1 fusion in pediatric AML: further evidence for lineage specificity of TEL/AML1

BACKGROUND

The TEL/AML1 fusion associated with t(12;21)(p13;q22) is the most common gene rearrangement in childhood malignancy, occurring in approximately 25% of pediatric acute lymphoblastic leukemia. The TEL/AML1 rearrangement is cryptic at the cytogenetic level but confers a favorable prognosis. The AML1 gene was first identified by virtue of its involvement in adult and pediatric acute myeloid malignancies associated with t(8;21) and t(3;21)(q26;q22.1). We have therefore determined the frequency of the TEL/AML1 fusion in pediatric myeloid leukemias by RT-PCR analysis.

METHODS

Total RNA was isolated from cryopreserved bone marrow samples of 38 pediatric patients with AML. RNA quality was controlled for by amplification of the TEL gene. An RT-PCR assay was then used to test for the presence of the TEL/AML1 fusion.

RESULTS

29 patients had adequate RNA for analysis. Zero out of 29 pediatric AML patients had evidence for the TEL/AML1 fusion by RT-PCR.

CONCLUSIONS

The TEL/AML1 fusion does not occur in children with AML and suggests that the TEL/AML1 rearrangement is restricted in pediatric hematologic malignancy to B lineage ALL.

Intrinsic transcriptional activation-inhibition domains of the polyomavirus enhancer binding protein 2/core binding factor alpha subunit revealed in the presence of the beta subunit

A member of the polyomavirus enhancer binding protein 2/core binding factor (PEBP2/CBF) is composed of PEBP2 alphaB1/AML1 (as the alpha subunit) and a beta subunit. It plays an essential role in definitive hematopoiesis and is frequently involved in the chromosomal abnormalities associated with leukemia. In the present study, we report functionally separable modular structures in PEBP2 alphaB1 for DNA binding and for transcriptional activation. DNA binding through the Runt domain of PEBP2 alphaB1 was hindered by the adjacent carboxy-terminal region, and this inhibition was relieved by interaction with the beta subunit. Utilizing a reporter assay system in which both the alpha and beta subunits are required to achieve strong transactivation, we uncovered the presence of transcriptional activation and inhibitory domains in PEBP2 alphaB1 that were only apparent in the presence of the beta subunit. The inhibitory domain keeps the full transactivation potential of full-length PEBP2 alphaB1 below its maximum potential. Fusion of the transactivation domain of PEBP2 alphaB1 to the yeast GAL4 DNA-binding domain conferred transactivation potential, but further addition of the inhibitory domain diminished the activity. These results suggest that the activity of the alpha subunit as a transcriptional activator is regulated intramolecularly as well as by the beta subunit. PEBP2 alphaB1 and the beta subunit were targeted to the nuclear matrix via signals distinct from the nuclear localization signal. Moreover, the transactivation domain by itself was capable of associating with the nuclear matrix, which implies the existence of a relationship between transactivation and nuclear matrix attachment.

Cloning of a novel human ELF-1-related ETS transcription factor, ELFR, its characterization and chromosomal assignment relative to ELF-1

The ETS gene family encodes a group of proteins that function as transcription factors under physiological conditions and, if aberrantly expressed, can lead to cellular transformation. ETS transcription factors are characterized by a unique conserved DNA binding domain. A subset of these proteins is rearranged with EWS in Ewing tumors (ET). We recently described a spectrum of ETS genes coexpressed with EWS-FLI1 in an ETcell line to define proteins that potentially compete in target site selection. We now report on the cloning and characterization of a novel ETS family member, ELFR, displaying 92% homology to ELF-1 in its DNA binding domain while diverging in the rest of the protein. ELFR expression was found in a very tissue restricted pattern with the highest abundancy in placenta. We also report the chromosomal assignment of ELFR and ELF-1 to Xq26 and 13q13, respectively, by means of fluorescence in-situ hybridization (FISH).

TEL-AML1 Fusion Transcript in Relapsed Childhood Acute Lymphoblastic Leukemia

The cryptic translocation t(12;21)(p13;q22) has been recently recognized as the most common genetic rearrangement in B-lineage childhood acute lymphoblastic leukemia (ALL). The resulting fusion transcript, termed TEL-AML1, has been associated with an excellent prognosis at initial ALL diagnosis. Hence, we postulated that the incidence of TEL-AML1 fusion should be lower in patients with ALL relapse. To address this assumption and to investigate the prognostic significance of TEL-AML1 expression in relapsed childhood ALL, bone marrow samples of 146 children were analyzed by reverse-transcriptase (RT)-polymerase chain reaction (PCR). All children were treated according to Berlin-Frankfurt-Münster (BFM) ALL relapse trial protocols (ALL-REZ BFM 90-96). Their clinical features and outcome were compared with those of 262 patients who could not be tested due to lack of bone marrow samples. Thirty-two of 146 children with relapsed ALL were TEL-AML1–positive. Four of the negative patients had T-lineage and nine Philadelphia chromosome (Ph1)-positive leukemia. Thus, the incidence ofTEL-AML1 in relapsed Ph1-negative, B-cell precursor ALL is 32 of 133 (24%). The 32 TEL-AML1–positive and 101 negative patients differed significantly with respect to duration of last remission (42.5 v 27 months; P = .0001) and age at initial diagnosis (53.5 v 74 months;P = .0269). At a median follow-up time of 21.5 months, children positive for TEL-AML1 had a significantly (P = .0011) higher probability of event-free survival (EFS; 0.79 v 0.33). The predominant majority of patients had been treated for initial ALL according to German multicenter BFM (108 of 133) or Cooperative ALL study group (CoALL) (19 of 133) frontline protocols. The comparison of tested and not-tested (N = 262) patients showed no significant difference.TEL-AML1 positivity predicted a favorable short-term outcome; long-term results are unknown. Screening for TEL-AML1 should become routine at relapse diagnosis and might be used for therapy stratification in future trials.

The AML1/ETO(MTG8) and AML1/Evi-1 Leukemia-Associated Chimeric Oncoproteins Accumulate PEBP2β(CBFβ) in the Nucleus More Efficiently Than Wild-Type AML1

AML1, a gene on chromosome 21 encoding a transcription factor, is disrupted in the (8;21)(q22;q22) and (3;21)(q26;q22) chromosomal translocations associated with myelogenous leukemias; as a result, chimeric proteins AML1/ETO(MTG8) and AML1/Evi-1 are generated, respectively. To clarify the roles of AML1/ETO(MTG8) and AML1/Evi-1 in leukemogenesis, we investigated subcellular localization of these chimeric proteins by immunofluorescence labeling and subcellular fractionation of COS-7 cells that express these chimeric proteins. AML1/ETO(MTG8) and AML1/Evi-1 are nuclear proteins, as is wild-type AML1. Polyomavirus enhancer binding protein (PEBP)2β(core binding factor [CBF]β), a heterodimerizing partner of AML1 that is located mainly in the cytoplasm, was translocated into the nucleus with dependence on the runt domain of AML1/ETO(MTG8) or AML1/Evi-1 when coexpressed with these chimeric proteins. When a comparable amount of wild-type AML1 or the chimeric proteins was coexpressed with PEBP2β(CBFβ), more of the cells expressing the chimeric proteins showed the nuclear accumulation of PEBP2β(CBFβ), as compared with the cells expressing wild-type AML1. We also showed that the chimeric proteins associate with PEBP2β(CBFβ) more effectively than wild-type AML1. These data suggest that the chimeric proteins are able to accumulate PEBP2β(CBFβ) in the nucleus more efficiently than wild-type AML1, probably because of the higher affinities of the chimeric proteins for PEBP2β(CBFβ) than that of wild-type AML1. These effects of the chimeric proteins on the cellular distribution of PEBP2β(CBFβ) possibly cause the dominant negative properties of the chimeric proteins over wild-type AML1 and account for one of the mechanisms through which these chimeric proteins contribute to leukemogenesis.

ETS-core binding factor: a common composite motif in antigen receptor gene enhancers

A tripartite domain of the murine immunoglobulin mu heavy-chain enhancer contains the muA and muB elements that bind ETS proteins and the muE3 element that binds leucine zipper-containing basic helix-loop-helix (bHLH-zip) factors. Analysis of the corresponding region of the human mu enhancer revealed high conservation of the muA and muB motifs but a striking absence of the muE3 element. Instead of bHLH-zip proteins, we found that the human enhancer bound core binding factor (CBF) between the muA and mu elements; CBF binding was shown to be a common feature of both murine and human enhancers. Furthermore, mutant enhancers that bound prototypic bHLH-zip proteins but not CBF did not activate transcription in B cells, and conversely, CBF transactivated the murine enhancer in nonlymphoid cells. Taking these data together with the earlier analysis of T-cell-specific enhancers, we propose that ETS-CBF is a common composite element in the regulation of antigen receptor genes. In addition, these studies identify the first B-cell target of CBF, a protein that has been implicated in the development of childhood pre-B-cell leukemias.

TEL-AML1 Fusion Transcript in Relapsed Childhood Acute Lymphoblastic Leukemia

The cryptic translocation t(12;21)(p13;q22) has been recently recognized as the most common genetic rearrangement in B-lineage childhood acute lymphoblastic leukemia (ALL). The resulting fusion transcript, termed TEL-AML1, has been associated with an excellent prognosis at initial ALL diagnosis. Hence, we postulated that the incidence of TEL-AML1 fusion should be lower in patients with ALL relapse. To address this assumption and to investigate the prognostic significance of TEL-AML1 expression in relapsed childhood ALL, bone marrow samples of 146 children were analyzed by reverse-transcriptase (RT)-polymerase chain reaction (PCR). All children were treated according to Berlin-Frankfurt-Münster (BFM) ALL relapse trial protocols (ALL-REZ BFM 90-96). Their clinical features and outcome were compared with those of 262 patients who could not be tested due to lack of bone marrow samples. Thirty-two of 146 children with relapsed ALL were TEL-AML1–positive. Four of the negative patients had T-lineage and nine Philadelphia chromosome (Ph1)-positive leukemia. Thus, the incidence ofTEL-AML1 in relapsed Ph1-negative, B-cell precursor ALL is 32 of 133 (24%). The 32 TEL-AML1–positive and 101 negative patients differed significantly with respect to duration of last remission (42.5 v 27 months; P = .0001) and age at initial diagnosis (53.5 v 74 months;P = .0269). At a median follow-up time of 21.5 months, children positive for TEL-AML1 had a significantly (P = .0011) higher probability of event-free survival (EFS; 0.79 v 0.33). The predominant majority of patients had been treated for initial ALL according to German multicenter BFM (108 of 133) or Cooperative ALL study group (CoALL) (19 of 133) frontline protocols. The comparison of tested and not-tested (N = 262) patients showed no significant difference.TEL-AML1 positivity predicted a favorable short-term outcome; long-term results are unknown. Screening for TEL-AML1 should become routine at relapse diagnosis and might be used for therapy stratification in future trials.

Correlation of cytogenetic, molecular cytogenetic, and clinical findings in 59 patients with ANLL or MDS and abnormalities of the short arm of chromosome 12

Abnormalities of the short arm of chromosome 12 (12p) are found in about 5% of acute nonlymphocytic leukaemias (ANLL) and myelodysplastic syndromes (MDS). They are described to be characteristic of secondary leukaemias, especially after prior mutagenic exposure, and to be associated with a poor prognosis. In our series of 59 patients with 12p abnormalities and ANLL or MDS, exposure to genotoxic agents was proven only in five patients, but in 13/44 patients ANLL evolved from an MDS. Patients with a small deletion del(12)(p11.2p13) having a mild clinical course were distinguished from those with a large del(12)(p11.2), additional chromosomal anomalies, and a poor clinical course. Among the 31 patients with translocations or dicentric chromosomes involving 12p, a group of eight with t/dic(12;13) was the most frequent and was associated with a poor prognosis. The clinical outcome was adverse in the majority of patients with complex karyotype abnormalities, but in some patients a milder clinical course seems likely. A new, hitherto undescribed, abnormality in an MDS case with a duplication dup(12)(p11.2p13) was the amplification of the signal of the yeast artificial chromosome (YAC) clone 964c10 (D12S736). In 38 cases with deletions or unbalanced translocations/dicentrics one YAC signal was lost. Five patients with balanced translocations demonstrated breakpoints within the YAC, containing the ETV6 (TEL) gene. The breakpoints were telomeric to the YAC 964c10 in seven cases and centromeric in one patient.

The AML1/ETO(MTG8) and AML1/Evi-1 Leukemia-Associated Chimeric Oncoproteins Accumulate PEBP2β(CBFβ) in the Nucleus More Efficiently Than Wild-Type AML1

AML1, a gene on chromosome 21 encoding a transcription factor, is disrupted in the (8;21)(q22;q22) and (3;21)(q26;q22) chromosomal translocations associated with myelogenous leukemias; as a result, chimeric proteins AML1/ETO(MTG8) and AML1/Evi-1 are generated, respectively. To clarify the roles of AML1/ETO(MTG8) and AML1/Evi-1 in leukemogenesis, we investigated subcellular localization of these chimeric proteins by immunofluorescence labeling and subcellular fractionation of COS-7 cells that express these chimeric proteins. AML1/ETO(MTG8) and AML1/Evi-1 are nuclear proteins, as is wild-type AML1. Polyomavirus enhancer binding protein (PEBP)2β(core binding factor [CBF]β), a heterodimerizing partner of AML1 that is located mainly in the cytoplasm, was translocated into the nucleus with dependence on the runt domain of AML1/ETO(MTG8) or AML1/Evi-1 when coexpressed with these chimeric proteins. When a comparable amount of wild-type AML1 or the chimeric proteins was coexpressed with PEBP2β(CBFβ), more of the cells expressing the chimeric proteins showed the nuclear accumulation of PEBP2β(CBFβ), as compared with the cells expressing wild-type AML1. We also showed that the chimeric proteins associate with PEBP2β(CBFβ) more effectively than wild-type AML1. These data suggest that the chimeric proteins are able to accumulate PEBP2β(CBFβ) in the nucleus more efficiently than wild-type AML1, probably because of the higher affinities of the chimeric proteins for PEBP2β(CBFβ) than that of wild-type AML1. These effects of the chimeric proteins on the cellular distribution of PEBP2β(CBFβ) possibly cause the dominant negative properties of the chimeric proteins over wild-type AML1 and account for one of the mechanisms through which these chimeric proteins contribute to leukemogenesis.

The second ETV6 allele is not necessarily deleted in acute leukemias with a ETV6/ABL fusion

The ETV6 (TEL) locus at chromosome band 12p 13 is a major site of translocations in acute leukemia, particularly in childhood acute lymphoblastic leukemia (ALL). In cases with translocations involving ETV6, the normal ETV6 allele is often deleted. In addition, loss of heterozygosity of ETV6 is frequently observed in childhood'ALL. Thus, it has been suggested that ETV6 may have an anti-oncogenic role to play, in addition to its oncogenic role. We have described an unusual case of ALL in which ETV6 is found fused to the ABL gene; ABL is normally activated by fusion to the BCR gene in the 9:22 translocation. We expanded the primary cells from this ETV6/ABL rearranged case of ALL in SCID animals and analyzed them for expression of both ETV6/ABL and the normal ETV6 mRNA. We found that both the rearranged and normal ETV6 mRNAs are expressed in the expanded cell population. Furthermore, sequence analysis of the ETV6 PCR product revealed no point mutations which would influence the amino acid sequence. Thus, deletion of the second ETV6 allele is not necessary for the transformation to leukemia by ETV6/ABL.

Negative regulation of granulocytic differentiation in the myeloid precursor cell line 32Dcl3 by ear-2, a mammalian homolog of Drosophila seven-up, and a chimeric leukemogenic gene, AML1/ETO

The polyomavirus enhancer binding protein 2alphaB (AML1/PEBP2alphaB/Cbfa2) plays a pivotal role in granulocyte colony-stimulating factor (G-CSF)-mediated differentiation of a myeloid progenitor cell line, 32Dc13. In this article, we report the identification of a PEBP2alphaB interacting protein, Ear-2, an orphan member of the nuclear hormone receptor superfamily that directly binds to and can inhibit the function of PEBP2alphaB. Ear-2 is expressed in proliferating 32Dc13 cells in presence of interleukin 3 but is down-regulated during differentiation induced by G-CSF. Interestingly, AML1/ETO(MTG8), a leukemogenic chimeric protein can block the differentiation of 32Dc13 cells, which is accompanied by the sustained expression of ear-2. Overexpression of Ear-2 can prevent G-CSF-induced differentiation, strongly suggesting that ear-2 is a key negative regulator of granulocytic differentiation. Our results indicate that a dynamic balance existing between PEBP2alphaB and Ear-2 appears to determine the choice between growth or differentiation for myeloid cells.

Fluorescence In Situ Hybridization Characterization of New Translocations Involving TEL (ETV6) in a Wide Spectrum of Hematologic Malignancies

The ETV6 (also known as TEL) gene on chromosome 12p13 is the target of a number of translocations associated with various hematologic malignancies. The contribution of ETV6 to leukemogenesis occurs through different mechanisms that involve either its helix-loop-helix dimerization domain or its E26 transformation-specific (ETS) DNA-binding domain. Using fluorescence in situ hybridization we characterized seven newETV6 rearrangements in chronic myeloid leukemia, acute myeloid leukemia, acute lymphoblastic leukemia, and non-Hodgkin's lymphoma. These aberrations, not always discernible at the cytogenetic level, include a t(5;12)(q31;p13), t(6;12;17)(p21;p13;q25), t(7;12)(p15;p13), t(7;12)(p12;p13), t(7;12)(q36;p13), t(12;13)(p13;q12), and a not completely defined t(12;?)(p13;?). Loss or disruption of the secondETV6 allele by a del(12)(p12p13) or by an intragenicETV6 deletion was detected in two cases. In six cases the 12p13 breakpoint occurred in the 5′ end of ETV6, upstream to exons encoding the HLH domain, whereas the remaining case had a breakpoint between the exons coding for the HLH domain and the exons coding for the ETS domain of ETV6. These observations provide further evidence for the multiple contributions of ETV6 in the pathogenesis of a wide range of hematologic malignancies.

The AML1-MTG8 leukemic fusion protein forms a complex with a novel member of the MTG8(ETO/CDR) family, MTGR1

The AML1-CBFbeta transcription factor complex is essential for the definitive hematopoiesis of all lineages and is the most frequent target of chromosomal rearrangements in human leukemia. In the t(8;21) translocation associated with acute myeloid leukemia (AML), the AML1(CBFA2/PEBP2alphaB) gene is juxtaposed to the MTG8(ETO/CDR) gene. We show here that the resultant AML1-MTG8 gene product specifically and strongly interacts with an 85-kDa phosphoprotein. Molecular cloning of cDNA indicated that the AML1-MTG8-binding protein (MTGR1) is highly related to MTG8 and similar to Drosophila Nervy. Comparison of amino acid sequences among MTGR1, MTG8, and Nervy revealed four evolutionarily conserved regions (NHR1 to NHR4). Ectopic expression of AML1-MTG8 in L-G murine myeloid progenitor cells inhibits differentiation to mature neutrophils and induces cell proliferation in response to granulocyte colony-stimulating factor (G-CSF). Analysis with C-terminal deletion mutants of AML1-MTG8 indicated that the region of 51 residues (488 to 538), which contains NHR2, is essential for the induction of G-CSF-dependent cell proliferation. Immunoprecipitation analysis indicates that this region is required for AML1-MTG8 to form a stable complex with MTGR1. Overexpression of MTGR1 stimulates AML1-MTG8 to induce G-CSF-dependent proliferation of L-G cells and to interfere with AML1-dependent transcription. These results suggest that AML1-MTG8 could function as a complex with MTGR1 and that the complex might be important in promoting leukemogenesis.

A novel ETV6-NTRK3 gene fusion in congenital fibrosarcoma

Congenital (or infantile) fibrosarcoma (CFS) is a malignant tumour of fibroblasts that occurs in patients aged two years or younger. CFS is unique among human sarcomas in that it has an excellent prognosis and very low metastatic rate. CFS is histologically identical to adult-type fibrosarcoma (ATFS); however, ATFS is an aggressive malignancy of adults and older children that has a poor prognosis. We report a novel recurrent t(12;15)(p13;q25) rearrangement in CFS that may underlie the distinctive biological properties of this tumour. By cloning the chromosome breakpoints, we show that the rearrangement fuses the ETV6 (also known as TEL) gene from 12p13 with the 15q25 NTRK3 neurotrophin-3 receptor gene (also known as TRKC). Analysis of mRNA revealed the expression of ETV6-NTRK3 chimaeric transcripts in all three CFS tumours analysed. These were not detected in ATFS or infantile fibromatosis (IFB), a histologically similar but benign fibroblastic proliferation occurring in the same age-group as CFS. ETV6-NTRK3 fusion transcripts encode the helix-loop-helix (HLH) protein dimerization domain of ETV6 fused to the protein tyrosine kinase (PTK) domain of NTRK3. Our studies indicate that a chimaeric PTK is expressed in CFS and this may contribute to oncogenesis by dysregulation of NTRK3 signal transduction pathways. Moreover, ETV6-NTRK3 gene fusions provide a potential diagnostic marker for CFS.

Consequences of chromosomal abnormalities in tumor development

This article highlights recent advances in the molecular structure and function of proteins that are activated or created by chromosomal abnormalities and discusses their possible role in tumor development. The molecular characterization of these proteins has revealed that tumor-specific fusion proteins are the consequence of most chromosome translocations associated with leukemias and solid tumors. An emerging common theme is that creation of these proteins disrupts the normal development of tumor-specific target cells by blocking apoptosis. These insights identify these chromosomal translocation-associated genes as potential targets for improved cancer therapies.

Genetics of erythropoiesis: induced mutations in mice and zebrafish

Production of red blood cells (erythropoiesis) in the vertebrate embryo is critical to its survival and subsequent development. As red cells are the first blood cells to appear in embryogenesis, their origin reflects commitment of mesoderm to an hematopoietic fate and provides an avenue by which to examine the development of the hematopoietic system, including the hematopoietic stem cell (HSC). We discuss the genetics of erythropoiesis as studied in two systems: the mouse and zebrafish (Danio rerio). In the mouse, targeted disruption has established several genes as essential at different stages of hematopoiesis or erythroid precursor cell maturation. In the zebrafish, numerous mutants displaying a wide range of phenotypes have been isolated, although the affected genes are unknown. In comparing mouse knockout and zebrafish mutant phenotypes, we propose a pathway for erythropoiesis that emphasizes the apparent similarity of the mutants and the complementary nature of investigation in the two species. We speculate that further genetic studies in mouse and zebrafish will identify the majority of essential genes and define a regulatory network for hematopoiesis in vertebrates.

Overexpression, purification, and biophysical characterization of the heterodimerization domain of the core-binding factor beta subunit

Core-binding factors (CBF) are heteromeric transcription factors essential for several developmental processes, including hematopoiesis. CBFs contain a DNA-binding CBF alpha subunit and a non-DNA binding CBF beta subunit that increases the affinity of CBF alpha for DNA. We have developed a procedure for overexpressing and purifying full-length CBF beta as well as a truncated form containing the N-terminal 141 amino acids using a novel glutaredoxin fusion expression system. Substantial quantities of the CBF beta proteins can be produced in this manner allowing for their biophysical characterization. We show that the full-length and truncated forms of CBF beta bind to a CBF alpha DNA complex with very similar affinities. Sedimentation equilibrium measurements show these proteins to be monomeric. Circular dichroism spectroscopy demonstrates that CBF beta is a mixed alpha/beta protein and NMR spectroscopy shows that the truncated and full-length proteins are structurally similar and suitable for structure determination by NMR spectroscopy.

The t(8;21) fusion product, AML-1-ETO, associates with C/EBP-alpha, inhibits C/EBP-alpha-dependent transcription, and blocks granulocytic differentiation

AML-1B is a hematopoietic transcription factor that is functionally inactivated by multiple chromosomal translocations in human acute myeloblastic and B-cell lymphocytic leukemias. The t(8;21)(q22;q22) translocation replaces the C terminus, including the transactivation domain of AML-1B, with ETO, a nuclear protein of unknown function. We previously showed that AML-1-ETO is a dominant inhibitor of AML-1B-dependent transcriptional activation. Here we demonstrate that AML-1-ETO also inhibits C/EBP-alpha-dependent activation of the myeloid cell-specific, rat defensin NP-3 promoter. AML-1B bound the core enhancer motifs present in the NP-3 promoter and activated transcription approximately sixfold. Similarly, C/EBP-alpha bound NP-3 promoter sequences and activated transcription approximately sixfold. Coexpression of C/EBP-alpha with AML-1B or its family members, AML-2 and murine AML-3, synergistically activated the NP-3 promoter up to 60-fold. The t(8;21) product, AML-1-ETO, repressed AML-1B-dependent activation of NP-3 and completely inhibited C/EBP-alpha-dependent activity as well as the synergistic activation. In contrast, the inv(16) product, which indirectly targets AML family members by fusing their heterodimeric DNA binding partner, CBF-beta, to the myosin heavy chain, inhibited AML-1B but not C/EBP-alpha activation or the synergistic activation. AML-1-ETO and C/EBP-alpha were coimmunoprecipitated and thus physically interact in vivo. Deletion mutants demonstrated that the C terminus of ETO was required for AML-1-ETO-mediated repression of the synergistic activation but not for association with C/EBP-alpha. Finally, overexpression of AML-1-ETO in myeloid progenitor cells prevented granulocyte colony-stimulating factor-induced differentiation. Thus, AML-1-ETO may contribute to leukemogenesis by specifically inhibiting C/EBP-alpha- and AML-1B-dependent activation of myeloid promoters and blocking differentiation.

Molecular genetics of childhood leukemias

PURPOSE

This review summarizes the molecular genetics of childhood leukemias, with emphasis on pathogenesis and clinical applications.

DESIGN

We first describe the most common genetic events that occur in pediatric acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and chronic myeloid leukemia (CML). We then illustrate how these molecular alterations may be used to alter therapy.

RESULTS

In childhood ALL, the TEL-AML1 fusion and hyperdiploidy are both associated with excellent treatment outcomes and therefore identify patients who may be candidates for less intensive therapy. In contrast, MLL gene rearrangements and the BCR-ABL fusion confer a poor prognosis; these patients may be best treated by allogeneic bone marrow transplantation in first remission.

CONCLUSIONS

Although clinical features are important prognostic indicators, genetic alterations of leukemic blasts may be better predictors of outcome for acute leukemia patients. We therefore favor risk-adapted therapy based on classification schemes that incorporate both genetic and clinical features.

Backtracking leukemia to birth: identification of clonotypic gene fusion sequences in neonatal blood spots

Epidemiological evidence has suggested that some pediatric leukemias may be initiated in utero and, for some pairs of identical twins with concordant leukemia, this possibility has been strongly endorsed by molecular studies of clonality. Direct evidence for a prenatal origin can only be derived by prospective or retrospective detection of leukemia-specific molecular abnormalities in fetal or newborn samples. We report a PCR-based method that has been developed to scrutinize neonatal blood spots (Guthrie cards) for the presence of numerically infrequent leukemic cells at birth in individuals who subsequently developed leukemia. We demonstrate that unique or clonotypic MLL-AF4 genomic fusion sequences are present and detectable in neonatal blood spots from individuals who were diagnosed with acute lymphoblastic leukemia at ages 5 months to 2 years and, therefore, have arisen during fetal hematopoiesis in utero. This result provides unequivocal evidence for a prenatal initiation of acute leukemia in young patients. The method should be applicable to other fusion genes in children with common subtypes of leukemia and will be of value in attempts to unravel the natural history and etiology of this major subtype of pediatric cancer.

Detection of the Der (21)t(12;21) chromosome forming the TEL-AML1 fusion gene in childhood acute lymphoblastic leukemia

The t(12;21) (p13;q22) is observed in approximately 20-25% of childhood B-lineage acute lymphoblastic leukemia (ALL) cases in both Asian and Caucasian populations. This translocation results in the fusion of TEL, a recently described ETS-like gene on 12p13, and AML1, which was shown to be involved in the formation of fusion genes with ETO and EVI1 in myeloid leukemias. Fluorescence in situ hybridization (FISH) and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis are useful in detecting this translocation which is not readily identified with routine cytogenetic techniques. The t(12;21) is associated with a distinct subgroup of patients characterized by an age between 1 and 10 years, an early B immunophenotype, and a good prognosis. A high incidence of the deletion of non-translocated TEL is another characteristic of leukemic cells with this translocation. TEL-AML1 hybrid protein thought to be critical in leukemogenesis possesses the HLH domain of TEL fused to almost the entire AML1 protein, although the detailed mechanisms of leukemogenesis remain obscure. RT-PCR combined with FISH analysis of posttreatment samples appears to be useful in detecting early relapse or minimal residual disease and thus, is expected to optimize the treatment strategy for patients with t(12;21).

TEL gene rearrangements in myeloid malignancy

The TEL gene is a recently described, "promiscuous" gene with a role in both myeloid and lymphoid malignancy. It is unusual since there may be more than one mechanism by which its rearrangement through chromosomal translocation is leukemogenic. This article discusses the four potential mechanisms of TEL-mediated transformation. It is conceivable that the TEL gene is the common target for various translocations precisely because of this pleiotropy of pathogenic mechanisms by which TEL gene rearrangements can lead to cell transformation.

12p Abnormalities and the TEL Gene (ETV6) in Childhood Acute Lymphoblastic Leukemia

Although abnormalities involving the short arm of chromosome 12 (12p) are one of the most frequently observed rearrangements in childhood acute lymphoblastic leukemia (ALL), little is known about the frequency of different structural abnormalities and their relationship to the status of the ETV6 (also named TEL) gene in this region. Of 815 children with newly diagnosed ALL, 94 (11.5%) had a total of 104 cytogenetic 12p abnormalities. Loss of genetic material was observed in 67 (64%) of these abnormalities. Cases with 12p alterations had a much lower frequency of hyperdiploidy greater than 50 (7%) than did the ALL population in general, but these cases had a similar distribution of immunophenotype and similar 5-year event-free survival (70% ± 5% SE v 64% ± 2%, P = .64). Rearrangement of the ETV6 gene was identified in 36 (56%) of 64 cases evaluated. The ETV6-CBFA2 (TEL-AML1) fusion transcript was found in 25 (66%) of 38 cases evaluated, and all but one of these showed ETV6 rearrangement. Importantly, ETV6 rearrangement was associated with a favorable prognosis (5-year event-free survival: 89% ± 6% v 60% ± 1%, P < .01). We conclude that most but not all 12p cytogenetic abnormalities in childhood ALL involve ETV6, and that rearrangement of ETV6 is associated with a favorable treatment outcome.

12p Abnormalities and the TEL Gene (ETV6) in Childhood Acute Lymphoblastic Leukemia

Although abnormalities involving the short arm of chromosome 12 (12p) are one of the most frequently observed rearrangements in childhood acute lymphoblastic leukemia (ALL), little is known about the frequency of different structural abnormalities and their relationship to the status of the ETV6 (also named TEL) gene in this region. Of 815 children with newly diagnosed ALL, 94 (11.5%) had a total of 104 cytogenetic 12p abnormalities. Loss of genetic material was observed in 67 (64%) of these abnormalities. Cases with 12p alterations had a much lower frequency of hyperdiploidy greater than 50 (7%) than did the ALL population in general, but these cases had a similar distribution of immunophenotype and similar 5-year event-free survival (70% ± 5% SE v 64% ± 2%, P = .64). Rearrangement of the ETV6 gene was identified in 36 (56%) of 64 cases evaluated. The ETV6-CBFA2 (TEL-AML1) fusion transcript was found in 25 (66%) of 38 cases evaluated, and all but one of these showed ETV6 rearrangement. Importantly, ETV6 rearrangement was associated with a favorable prognosis (5-year event-free survival: 89% ± 6% v 60% ± 1%, P < .01). We conclude that most but not all 12p cytogenetic abnormalities in childhood ALL involve ETV6, and that rearrangement of ETV6 is associated with a favorable treatment outcome.

A domain shared by the Polycomb group proteins Scm and ph mediates heterotypic and homotypic interactions

The Sex comb on midleg (Scm) and polyhomeotic (ph) proteins are members of the Polycomb group (PcG) of transcriptional repressors. PcG proteins maintain differential patterns of homeotic gene expression during development in Drosophila flies. The Scm and ph proteins share a homology domain with 38% identity over a length of 65 amino acids, termed the SPM domain, that is located at their respective C termini. Using the yeast two-hybrid system and in vitro protein-binding assays, we show that the SPM domain mediates direct interaction between Scm and ph. Binding studies with isolated SPM domains from Scm and ph show that the domain is sufficient for these protein interactions. These studies also show that the Scm-ph and Scm-Scm domain interactions are much stronger than the ph-ph domain interaction, indicating that the isolated domain has intrinsic binding specificity determinants. Analysis of site-directed point mutations identifies residues that are important for SPM domain function. These binding properties, predicted alpha-helical secondary structure, and conservation of hydrophobic residues prompt comparisons of the SPM domain to the helix-loop-helix and leucine zipper domains used for homotypic and heterotypic protein interactions in other transcriptional regulators. In addition to in vitro studies, we show colocalization of the Scm and ph proteins at polytene chromosome sites in vivo. We discuss the possible roles of the SPM domain in the assembly or function of molecular complexes of PcG proteins.

Several Cytogenetic Subclones May Be Identified Within Plasma Cells From Patients With Monoclonal Gammopathy of Undetermined Significance, Both at Diagnosis and During the Indolent Course of This Condition

Monoclonal gammopathy of undetermined significance (MGUS) is a frequent condition in patients over 50 years old, that ultimately leads to multiple myeloma (MM) in 20% of patients after 20 to 35 years of follow-up. Little is known about cytogenetic changes associated with this condition. We studied 19 MGUS patients both at diagnosis and after 12 to 35 months of follow-up (mean = 26), using DNA content measurement of bone marrow plasma cells (BMPC), and a new interphase fluorescence in situ hybridization technique (FISH) allowing the simultaneous identification of monotypic BMPC (fluorescent anti light-chain antibodies) and the determination of the number of copies for two different chromosomes within the same PC nucleus (one biotin-labeled probe coupled next to texas red avidin and one FITC-labeled probe). At diagnosis of the MGUS, single interphase FISH showed at least one numeric chromosome change in 13 of 19 patients, after the use of centromeric probes directed against chromosomes no. 3, no. 7, no. 9, and no. 11. At follow-up, abnormalities found at diagnosis in 13 patients were still shown. Moreover, abnormalities occurred in three of the last six patients (trisomy for one to three different chromosomes), although no patient evolved into MM. Dual interphase FISH showed that some BMPC bore numeric changes with both probes tested whereas other BMPC bore abnormality with only one of the probes tested. In patients who showed trisomy for at least three different chromosomes, distribution of numeric changes within BMPC defined significant numbers of up to seven different BMPC clones. All these various clones were shown both at diagnosis and at follow-up. In every patient, these various clones differed only for the number of abnormalities they exhibited, and could be related to each other in a model of gradual acquisition of chromosome changes. Eventually, data reported here show that MGUS patients acquire slowly, gradually, but ineluctably chromosome changes, distributed within several related subclones. However, these changes are not related to transformation into MM: among the various clones coexisting within the same patient, a peculiar change, still to demonstrate, might develop and lead to overt MM.

Abnormalities of Chromosome Band 8p11 in Leukemia: Two Clinical Syndromes Can Be Distinguished on the Basis of MOZ Involvement

Two distinct leukemia syndromes are associated with abnormalities of chromosome band 8p11. First, a myeloproliferative disorder with features characteristic of both chronic myeloid leukemia and non-Hodgkin's lymphoma and second, an acute myeloid leukemia (AML) with French-American-British (FAB) M4/5 morphology and prominent erythrophagocytosis. The two syndromes are exemplified by a t(8; 13)(p11; q12) and a t(8; 16)(p11; p13), respectively, but cytogenetic variants of both have been described. Recently, the t(8; 16) has been cloned and shown to fuse the MOZ gene at 8p11 to the CBP gene at 16p13. We have used fluorescence in situ hybridization (FISH), Southern blotting, and reverse transcriptase-polymerase chain reaction (RT-PCR) to refine the 8p11 breakpoint in three cases with t(8; 13)(p11; q12) and in a single case of AML-M5 with a clinical picture apparently identical to that found in patients with a t(8; 16), but characterized by an inv(8)(p11q13). FISH analysis was performed with several 8p11 CEPH yeast artificial chromosome (YAC) clones. YAC 782H11 was centromeric to the one case with t(8; 13) tested, but was telomeric to the inv(8). YAC 847B12 was telomeric to both the t(8; 13) and the inv(8), whereas YAC 829D12 was centromeric to the t(8; 13), but split by the inv(8). Southern blotting and PCR of YAC 829D12 showed that it contained the MOZ gene. A 900-bp MOZ fragment encompassing the published t(8; 16) breakpoint was amplified by PCR from normal peripheral blood leukocyte cDNA and used to probe Southern blots of patient DNA. A rearrangement was detected in the case with inv(8), but not in any of the three cases with t(8; 13). Southern blotting with a CBP probe and RT-PCR with MOZ and CBP primers suggested that the inv(8) does not result in a cryptic MOZ-CBP fusion. It is likely, therefore, that MOZ is fused to a novel gene at 8q13 in this case. We conclude that the t(8; 13) breakpoint is flanked by YACs 782H11 and 847B12 and is at least 1 Mb telomeric to MOZ. MOZ is involved, however, in a new variant of the t(8; 16).

A chimeric receptor/oncogene that can be regulated by a ligand in vitro and in vivo

The BCR/ABL oncogene encodes an activated tyrosine kinase that causes human chronic myelogenous leukemia. The mechanism of transformation, however, is complex and not well understood. One of the important contributions of BCR to transformation is believed to be dimerization or oligomerization of ABL, thereby activating ABL tyrosine kinase activity. We reasoned that if ABL was dimerized through other mechanisms, activation of the tyrosine kinase activity should also result, and the activated kinase may also be transforming. Erythropoietin is known to activate its receptor by causing dimerization, and therefore a synthetic oncogene was created by linking the extracytoplasmic and transmembrane domains of the EPO receptor with c-ABL. This chimeric receptor was stably expressed in Ba/F3 cells and, in the absence of EPO, had no detectable biological effect on the cells. EPO, however, induced a rapid, dose-dependent activation of ABL tyrosine kinase activity and phosphorylation of several cellular proteins. The major target proteins have been identified, and are very similar to the known substrates of BCR/ABL, including Shc, CBL, CRKL, and several proteins in the cytoskeleton. EPO treatment also resulted in biological effects that were remarkably similar to those of BCR/ABL, including improved viability, altered integrin function, and a weak mitogenic signal. The biological effects were in part dose-dependent, in that low EPO concentrations enhanced viability but did not cause proliferation. At high EPO doses, kinase activation was maximal, and a mitogenic effect was also revealed. In nude mice, Ba/F3 cells expressing this chimeric receptor did not cause detectable disease without administration of pharmacologic doses of EPO. If EPO was given intraperitoneally 5 days a week, however, a dose-dependent lethal leukemia resulted. This ligand-regulatable oncogene mimics some of the biological effects of BCR/ABL, and analysis of ABL mutants in this system will be useful to dissect the signaling pathways that cause CML.

Fusion of TEL, the ETS-Variant Gene 6 (ETV6), to the Receptor-Associated Kinase JAK2 as a Result of t(9; 12) in a Lymphoid and t(9; 15; 12) in a Myeloid Leukemia

Translocations in hematologic disease of myeloid or lymphoid origin with breakpoints at chromosome band 12p13 frequently result in rearrangements of the Ets variant gene 6 (ETV6). As a consequence either the ETS DNA-binding domain or the Helix-Loop-Helix (HLH) oligomerization domain of ETV6 is fused to different partner genes. We show here that a t(9; 12)(p24; p13) in a case of early pre-B acute lymphoid leukemia and a t(9; 15; 12)(p24; q15; p13) in atypical chronic myelogenous leukemia in transformation involve the ETV6 gene at 12p13 and the JAK2 gene at 9p24. In each case different fusion mRNAs were found, with only one resulting in an open reading frame for a chimeric protein consisting of the HLH oligomerization domain of ETV6 and the protein tyrosine kinase (PTK) domain of JAK2. The cloning of the complete human JAK2 coding and genomic sequences and of the genomic junction fragments of the translocations allowed a characterization of the different splice events leading to the various mRNAs. JAK2 plays a central role in non–protein tyrosine kinase receptor signaling pathways, which could explain its involvement in malignancies of different hematologic lineages. Besides hop in Drosophila no member of the JAK family has yet been implicated in tumorigenesis.