Inappropriate activation of Abl family kinases plays a crucial role in di erent human leukaemias. In addition to the well known oncoproteins p190Bcr-Abl and p210Bcr-Abl, Tel-Abl, a novel fusion protein resulting from a di erent chromosomal translocation, has recently been described. In this study, the kinase speci®cities of the Bcr-Abl and Tel-Abl proteins were compared to the physiological Abl family kinases c-Abl and Arg (abl related gene). Using short peptides which correspond to the target epitopes in known substrate proteins of Abl family kinases, we found a higher catalytic promiscuity of Bcr-Abl and Tel-Abl. Similar to Bcr-Abl, Tel-Abl was found in complexes with the adapter protein CRKL. In addition, c-Crk II and CRKL are tyrosine phosphorylated and complexed with numerous other tyrosine phosphorylated proteins in Tel-Abl expressing Ba/F3 cells. GTPase analysis with a Ras´GTP-speci®c precipitation assay showed constitutive elevation of GTPloaded Ras in cells expressing the leukaemic Abl proteins. The mitogenic MAPK/Erk kinases as well as Akt/PKB, a kinase implicated to negatively regulate apoptosis, were also constitutively activated by both BcrAbl and Tel-Abl. The results indicate that the leukaemic Abl-fusion proteins have catalytic speci®cities di erent from the normal kinases c-Abl and Arg and that Tel-Abl is capable to activate at least some pathways which are also upregulated by Bcr-Abl.

An amino-terminally truncated ␤ C receptor (␤ C -R) subunit of the interleukin-3 (IL3)/granulocyte-macrophage colony-stimulating factor/IL5 receptor complex mediates factor-independent and tumorigenic growth in two spontaneous mutants of a promyelocytic cell line. The constitutive activation of the JAK2 protein kinase in these mutants confirms that signaling occurs through the truncated receptor protein. Noteworthily, in addition to a 10-kb deletion in the ␤ C -R subunit gene encoding the truncated receptor, several secondary and independent mutations that result in the deletion or functional inactivation of the allelic ␤ C -R subunit and the closely related ␤ IL3 -R subunit genes were observed in both mutants, suggesting that such mutations are necessary for the full oncogenic penetrance of the truncated ␤ C -R subunit. Reversion of these mutations by the expression of the wild-type ␤ C -R in the two mutants resulted in a fivefold decrease in cloning efficiency of the mutants in the absence of IL3, confirming a functional interaction between the wild-type and truncated proteins. Furthermore, expression of the truncated ␤ C -R subunit in factor-dependent myeloid cells did not immediately render the cells autonomous but increased the spontaneous frequency to factor-independent growth by 4 orders of magnitude. Implications for both leukemogenic progression and receptor-subunit interaction and signaling are discussed.Adult hematopoiesis, in which the constant replenishment of short-lived mature blood cells from a pool of multipotential cells is tightly regulated yet able to respond rapidly to environmental changes, is controlled by at least two interacting systems. The maintenance of the stem cell pool and the production of lineage-committed precursors are thought to be controlled by the interaction of specialized cells of the bone marrow with hematopoietic precursors. This interaction is presumably regulated by the secretion of short-range regulatory factors or direct cell-cell interactions. In contrast, the proliferation of the committed precursors and their more differentiated progeny, as well as the induction of cell maturation, is primarily regulated by the interaction of a relatively large group of hematopoietic growth factors or cytokines that are released by auxiliary cells in hematopoietic tissues (e.g., T lymphocytes, endothelial cells, fibroblasts, and mononuclear phagocytes) and act at relatively large distances (2, 31).These cytokines interact with the hematopoietic target cell through receptor molecules present on the cell surface in varying numbers. The receptor molecules transduce the binding of their cognate cytokines into cytoplasmic signals that eventually trigger a cascade of intracellular responses (11,24). Although the receptor molecules of a few of these cytokines belong to the classical family of receptor tyrosine kinases (53), the majority make up a relatively new superfamily of receptors (3). This family is characterized by conserved amino acid sequence motifs in their extracellular domain...

The t(12;21)(p13;q22) chromosomal translocation is the most frequent illegitimate gene recombination in a pediatric cancer and occurs in approximately 25% of common acute lymphoblastic leukemia (cALL) cases. This rearrangement results in the in frame fusion of the 5′-region of the ETS-related gene, TEL(ETV6), to almost the entire acute myeloid leukemia 1 (AML1) (also called CBFA2 orPEBP2AB1) locus and expression of the TEL-AML1 chimeric protein. Although AML1 stimulates transcription, TEL-AML1 functions as a repressor of some AML1 target genes. In contrast to the wild type AML1 protein, both TEL and TEL-AML1 interact with N-CoR, a component of the nuclear receptor corepressor complex with histone deacetylase activity. The interaction between TEL and N-CoR requires the central region of TEL, which is retained in TEL-AML1, and TEL lacking this domain is impaired in transcriptional repression. Taken together, our results suggest that TEL-AML1 may contribute to leukemogenesis by recruiting N-CoR to AML1 target genes and thus imposing an altered pattern of their expression.

Summary. 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 disease.