The TEL/AML1 fusion associated with t(12;21)(p13;q22) is the most common gene rearrangement in childhood leukemia, occurring in approximately 25% of pediatric acute lymphoblastic leukemia (ALL), and is associated with a favorable prognosis. For example, a cohort of pediatric patients with ALL retrospectively analyzed for the TEL/AML1 fusion treated on Dana-Farber Cancer Institute (DFCI) ALL Consortium protocols between 1980 to 1991 demonstrated a 100% relapse-free survival in TEL/AML1-positive patients with a median of 8.3 years of follow-up. However, two recent studies analyzing pediatric patients with relapsed ALL have reported the same incidence of the TEL/AML1 rearrangement as in patients with newly diagnosed ALL, suggesting that TEL/AML1 positivity is not a favorable prognostic indicator. To clarify this apparent discrepancy, 48 pediatric patients treated on Dana-Farber Cancer Institute (DFCI) protocols with ALL at first or second relapse were tested for TEL/AML1 using reverse transcriptase-polymerase chain reaction (RT-PCR). The TEL/AML1 fusion was identified in only 1 of 32 analyzable relapsed ALL patients, in concordance with our previous reports of improved disease-free survival in TEL/AML1-positive patients. The low frequency of TEL/AML1-positive patients at relapse is significantly different than that reported in other studies. Although there are several potential explanations for the observed differences in TEL/AML1-positive patients at relapse, it is plausible that relapse-free survival in TEL/AML1-positive patients may be changed with different therapeutic approaches. Taken together, these results support the need for prospective analysis of prognosis in TEL/AML1-positive patients.

Abnormalities of the short arm of chromosome 12 including loss of heterozygosity (LOH) and TEL/AML-1 fusion resulting from a t(12;21)(p13;q22) translocation are frequently observed in childhood acute lymphoblastic leukemia (ALL). We investigated 21 DNA samples of childhood ALL which had LOH at 12p13. Rearrangement of TEL was observed in eight cases and another case showed a homozygous deletion of TEL. Two informative samples with TEL rearrangement had a deletion localized to the 5' region of this gene. The deletion in these two cases includes the helix-loop-helix (HLH) domain. This is consistent with the hypothesis that the normal tel can heterodimerize with the TEL/AML-1 gene product and inhibit the transforming capacity of the chimeric protein. Presumably, loss of the HLH of the normal remaining TEL allele abrogates this tumor suppressor-like function. The case with homozygous deletion of TEL is also consistent with this gene having qualities of a tumor suppressor. One unusual case had T-ALL rather than B-lineage ALL and the leukemic cells had rearrangement of TEL, but they did not have an alteration of the remaining TEL allele suggesting that the etiology of this disease may be different. This analysis further emphasizes the importance of loss of the normal TEL allele in childhood precursor B-lineage ALL.

The recurrent (12;21)(p13;q22) translocation fuses the two genes TEL and AML1 that have previously been cloned from translocation breakpoints in myeloid leukemias. Using mainly reverse transcriptase-polymerase chain reaction (RT-PCR), the TEL-AML1 chimeric transcript has been observed in 22-27% of pediatric patients with acute lymphoblastic leukemia (ALL), in particular in the early B-lineage ALL subtype, making it the most common genetic lesion in these patients. The vast majority of acute myeloid leukemias, other ALL subtypes and even adults with early B-lineage ALL were TEL-AML1-negative. We determined whether the TEL-AML1 fusion gene can also be observed in continuous human leukemia cell lines with an early B-lineage phenotype. Twenty-nine such cell lines established from children (n = 13) or adults (n = 13) with early B-lineage ALL and five cell lines derived from chronic myeloid leukemia in blast crisis or B cell non-Hodgkin's lymphoma were investigated for the occurrence of the TEL-AML1 rearrangement by RT-PCR. While all 13 adult early B-lineage ALL cell lines and the five cell lines from other leukemias or lymphomas were negative, 1/13 pediatric cell lines (cell line REH) was found to be positive for TEL-AML1; though neither reciprocal AML1-TEL, nor normal TEL, mRNA was detectable by RT-PCR in this cell line. These findings agreed with the results of conventional cytogenetic and FISH analysis of REH which was found to carry the der(21) partner only of t(12;21)(p13;q22), probably resulting from a complex translocation, t(4;12;21;16)(q32;p13;q22;q24.3). Hybridization with flanking cosmid clones (179A6 and 148B6), covering exons 1 and 8 respectively of TEL, confirmed a rearrangement accompanying the t(12;21), and showed cryptic deletion of the residual allele resulting from an apparently reciprocal t(5;12)(q31;p13). These findings in REH provide a further example of, and possible cytogenetic mechanism for, the paradigm of TEL-AML1 fusion accompanied by deletion of the residual TEL allele. The low rate of early B-lineage ALL cell lines carrying this translocation contrasts clearly with the relative high frequency of TEL-AML1-positive cases in primary material. It is possible that expression of the fusion product hampers the in vitro growth and establishment in culture of such leukemic cells. Nevertheless, the cell line REH represents a powerful tool for the further molecular characterization of this unique breakpoint and can serve as a positive control in routine PCR reactions.

The TEL gene, which is frequently rearranged in human leukemias of both myeloid and lymphoid origin, encodes a member of the Ets family of transcription factors. The TEL gene is widely expressed throughout embryonic development and in the adult. To determine the requirement for the TEL gene product in development we generated TEL knockout mice (TEL-/-) by gene targeting in embryonic stem cells. TEL-/- mice are embryonic lethal and die between E10.5-11.5 with defective yolk sac angiogenesis and intra-embryonic apoptosis of mesenchymal and neural cells. Two-thirds of TEL-deficient yolk sacs at E9.5 lack vitelline vessels, yet possess capillaries, indicative of normal vasculogenesis. Vitelline vessels regress by E10.5 in the remaining TEL-/- yolk sacs. Hematopoiesis at the yolk sac stage, however, appears unaffected in TEL-/- embryos. Our findings demonstrate that TEL is required for maintenance of the developing vascular network in the yolk sac and for survival of selected cell types within the embryo proper.

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.

We have shown previously that loss of heterozygosity at chromosome band 12p13 is among the most frequent genetic abnormalities identified in acute lymphoblastic leukemia (ALL) of childhood. Two known genes map within the critically deleted region of 12p: TEL, the gene encoding a new member of the ETS family of transcription factors, which is rearranged in a variety of hematological malignancies; and KIP1, the gene encoding the cyclin-dependent kinase inhibitor p27. Both genes are, therefore, excellent candidate tumor suppressor genes. In this report, we determined the exon organization of the TEL gene and performed mutational analysis of TEL and KIP1 in 33 childhood ALL patients known to have loss of heterozygosity at this locus. No mutations in either TEL or KIP1 were found; this suggest that neither TEL nor KIP1 is the critical 12p tumor suppressor gene in childhood ALL.

The TEL/PDGF beta R fusion protein is the product of the t(5;12) translocation in patients with chronic myelomonocytic leukemia. The TEL/PDGF beta R is an unusual fusion of a putative transcription factor, TEL, to a receptor tyrosine kinase. The translocation fuses the amino terminus of TEL, containing the helix-loop-helix (HLH) domain, to the transmembrane and cytoplasmic domain of the PDGF beta R. We hypothesized that TEL/PDGF beta R self-association, mediated by the HLH domain of TEL, would lead to constitutive activation of the PDGF beta R tyrosine kinase domain and cellular transformation. Analysis of in vitro-translated TEL/ PDGF beta R confirmed that the protein self-associated and that self-association was abrogated by deletion of 51 aa within the TEL HLH domain. In vivo, TEL/PDGF beta R was detected as a 100-kDa protein that was constitutively phosphorylated on tyrosine and transformed the murine hematopoietic cell line Ba/F3 to interleukin 3 growth factor independence. Transformation of Ba/F3 cells required the HLH domain of TEL and the kinase activity of the PDGF beta R portion of the fusion protein. Immunoblotting demonstrated that TEL/PDGF beta R associated with multiple signaling molecules known to associate with the activated PDGF beta R, including phospholipase C gamma 1, SHP2, and phosphoinositol-3-kinase. TEL/PDGF beta R is a novel transforming protein that self-associates and activates PDGF beta R-dependent signaling pathways. Oligomerization of TEL/PDGF beta R that is dependent on the TEL HLH domain provides further evidence that the HLH domain, highly conserved among ETS family members, is a self-association motif.

TEL is a member of the Ets family of transcription factors which are frequently rearranged in human leukemia. The mechanism of TEL-mediated transformation, however, is unknown. We report the cloning and characterization of a chromosomal translocation associated with acute myeloid leukemia which fuses TEL to the ABL tyrosine kinase. The TEL-ABL fusion confers growth factor-independent growth to the marine hematopoietic cell line Ba/F3 and transforms Rat-1 fibroblasts and primary murine bone marrow cells. TEL-ABL is constitutively tyrosine phosphorylated and localizes to the cytoskeleton. A TEL-ABL mutant containing an ABL kinase-inactivating mutation is not constitutively phosphorylated and is nontransforming but retains cytoskeletal localization. However, constitutive phosphorylation, cytoskeletal localization, and transformation are all dependent upon a highly conserved region of TEL termed the helix-loop-helix (HLH) domain. TEL-ABL formed HLH-dependent homo-oligomers in vitro, a process critical for tyrosine kinase activation. These experiments suggest that oligomerization of TEL-ABL mediated by the TEL HLH domain is required for tyrosine kinase activation, cytoskeletal localization, and transformation. These data also suggest that oligomerization of Ets proteins through the highly conserved HLH domain may represent a previously unrecognized phenomenon.