The 5' half of the EWS gene has recently been described to be fused to the 3' regions of genes encoding the DNA-binding domain of several transcriptional regulators, including ATF1, and ERG, in Members of the Ets family form a novel class of sequencespecific DNA-binding proteins which are implicated in developmental processes, in the response of cells to extracellular signals, and in cellular transformation (for reviews, see references 23 and 70). They are characterized by an 85-amino-acid region of similarity, the Ets domain, which is essential for sequence-specific binding to DNA (20,28,30,37,50,64,69,73). DNA sequences bound by several Ets family members have been analyzed in detail and found to include about 10 nucleotides centered over a central GGAA core sequence (11,18,50,67,78). These sequences when multimerized upstream of a minimal promoter or when present in the context of natural viral and cellular promoters/enhancers mediate transcriptional regulation by Ets proteins (8,30,55,56,71,74,79). Consistent with these properties, specific transcriptional activation domains have been mapped in Ets-1, 39,59

The TEL-JAK2 gene fusion, which has been identified in human leukemia, encodes a chimeric protein endowed with constitutive tyrosine kinase activity. TEL-JAK2 transgenic expression in the mouse lymphoid lineage results in fatal and rapid T-cell leukemia/lymphoma. In the present report we show that T-cell leukemic cells from ESR␣-TEL-JAK2 transgenic mice present an aberrant CD8 ؉ differentiation phenotype, as determined by the expression of stage-specific cell surface markers and lineage-specific genes. TEL-JAK2 transforms immature CD4 ؊ CD8 ؊ double-negative thymocytes, as demonstrated by the development of T-cell leukemia with full penetrance in a Rag2-deficient genetic background. This disease is similar to the bona fide TEL-JAK2 disease as assessed by phenotypic and gene profiling analyses. Pre-TCR signaling synergizes with TEL-JAK2 to transform immature thymocytes and initiate leukemogenesis as shown by (1) the delayed leukemia onset in Rag2-, CD3⑀-and pT␣-deficient mice, (2) the occurrence of recurrent chromosomal alterations in pre-TCR-deficient leukemia, and (3) the correction of delayed leukemia onset in Rag2-deficient TEL-JAK2 mice by an H-Y TCR␣␤ transgene that mimics pre-TCR signaling. Although not affecting leukemia incidence and mouse survival, TCR␣␤ expression was shown to facilitate leukemic cell expansion in secondary lymphoid organs. IntroductionIn the thymus, T cells develop from a common CD4 Ϫ CD8 Ϫ double-negative (DN) progenitor into 2 main lineages, ␣␤ and ␥␦, which are defined by the selection of productive rearrangements in the respective T-cell receptor (TCR) loci (for a review, see Aifantis et al 1 ). In mice, DN thymocytes are divided in 4 categories according to CD25 and CD44 expression: DN1 (CD25 Ϫ CD44 ϩ ), DN2 (CD25 ϩ CD44 ϩ ), DN3 (CD25 ϩ CD44 Ϫ ), and DN4 (CD25 Ϫ CD44 Ϫ ). Rag-mediated rearrangement of the TCR␤ locus at the DN3 stage leads to cell surface expression of a functional TCR␤ chain, which assembles with the surrogate pT␣ chain and CD3-signaling proteins to form the pre-TCR complex. Constitutive survival, proliferation, and differentiation signals emanating from the pre-TCR allow T cells to pass through the ␤-selection checkpoint and mature to the DN4, CD4 Ϫ CD8 ϩ immature single-positive (ISP) and CD4 ϩ CD8 ϩ double-positive (DP) stages. The ␤-selected cells rearrange their TCR␣ locus, and only the minority of cells expressing a functional TCR␣␤ complex at the cell surface will either undergo negative selection and die or undergo positive selection and become mature CD4 or CD8 single-positive (SP) thymocytes.Chromosomal rearrangements or point mutations in oncogenes or tumor suppressor genes occur in hematopoietic stem cells (HSCs), uncommitted and committed lymphoid progenitors, or developing thymocytes, thus leading to T-cell leukemia. Chromosomal translocations involving the juxtaposition of protooncogenes to the promoter and enhancer sequences of TCR loci result in deregulated oncogene expression. Chromosomal translocations can also create fusion genes encoding fu...

We previously reported a fusion between TEL and JAK2in a t(9;12)(p24;p13) chromosomal translocation in childhood acute T-cell leukemia. This fusion gene encodes a TEL-JAK2 chimeric protein in which the 336 amino-terminal residues of TEL, including its specific self-association domain, are fused to the kinase domain of JAK2. TEL-JAK2 exhibits constitutive activation of its tyrosine kinase activity which, in turn, confers growth factor–independent proliferation to the interleukin-3–dependent Ba/F3 hematopoietic cell line. To elucidate the properties of TEL-JAK2 in primary cells and to create an animal model for TEL-JAK2–induced leukemia, we generated transgenic mice in which the TEL-JAK2 complementary DNA was placed under the transcriptional control of the EμSR enhancer/promoter. TEL-JAK2 founder mice and their transgenic progeny developed fatal leukemia at 4 to 22 weeks of age. Selective amplification of CD8-positive T cells was observed in blood, lymph nodes, thymus, spleen, and bone marrow. Expression of a tyrosine-phosphorylated TEL-JAK2 protein and activation of STAT1 and STAT5 (signal transducer and activator of transcription) were detected in leukemic tissues. TEL-JAK2 diseased mice also displayed invasion of nonhematopoietic organs, including liver, brain, lung, and kidney, by leukemic T cells. Leukemic organs of founder and transgenic progeny contained a monoclonal/oligoclonal T-cell population as analyzed by the rearrangement of the TCRβ locus. Transplantation of TEL-JAK2 leukemic cells in nude mice confirmed their invasive nature. We conclude that the TEL-JAK2 fusion is an oncogene in vivo and that its expression in lymphoid cells results in the preferential expansion of CD8-positive T cells.