We have isolated short tandem repeat arrays from the human genome, using a rapid method involving filter hybridization to enrich for tri- or tetranucleotide tandem repeats. About 30% of clones from the enriched library cross-hybridize with probes containing trimeric or tetrameric tandem arrays, facilitating the rapid isolation of large numbers of clones. In an initial analysis of 54 clones, 46 different tandem arrays were identified. Analysis of these tandem repeat loci by PCR showed that 24 were polymorphic in length; substantially higher levels of polymorphism were displayed by the tetrameric repeat loci isolated than by the trimeric repeats. Primary mapping of these loci by linkage analysis showed that they derive from 17 chromosomes, including the X chromosome. We anticipate the use of this strategy for the efficient isolation of tandem repeats from other sources of genomic DNA, including DNA from flow-sorted chromosomes, and from other species.

FDCP‐1 cells are hematopoietic progenitor cells which require interleukin‐3 for survival and proliferation. FDCP‐1 cells stably transfected with the murine erythropoietin receptor cDNA survive and proliferate in the presence of erythropoietin. Erythropoietin induces the activation of the short forms (80 kDa) of STAT5 in the cells. Erythropoietin‐induced activation of STAT5 was strongly reduced in cells expressing mutated variants of the erythropoietin receptors in which tyrosine residues in their intracellular domain have been eliminated. We determined that the erythropoietin receptor tyrosine residues 343 and 401 are independently necessary for STAT5 activation. The amino acid sequences surrounding these two tyrosine residues are very similar. Peptides comprising either phosphorylated Tyr343 or phosphorylated Tyr401, but not their unphosphorylated counterparts, inhibited the STAT5 activation. We propose that these two tyrosine residues of the erythropoietin receptor constitute docking sites for the STAT5 SH2 domain. The growth stimulus mediated by erythropoietin was decreased in cells expressing erythropoietin receptors lacking both Tyr343 and Tyr401. This suggests that STAT5 activation could be involved in the growth control of FDCP‐1 cells.

The TF‐1 cell line has been established from a patient with erythroleukemia. While various cytokines induce TF‐1 cell proliferation, erythropoietin (Epo) only sustains the short‐term growth of these cells and induces their differentiation along the erythroid lineage. A truncated Epo receptor (EpoR) is overexpressed in these cells. The truncation removed the 96 C‐terminal amino acids, including seven tyrosine residues. An additional single mutation at position +3 of Tyr344 led to the replacement of leucine 347 by proline. Stimulation by Epo induced an impaired activation of the STAT5 transcription factor in these cells. The same defect in STAT5 activation was found in the murine FDCP‐1 cell line transfected with a chimeric EpoR containing the abnormal TF‐1 EpoR cytoplasmic domain. Infection of TF‐1 cells with a retrovirus containing a normal murine EpoR was able to restore both Epo‐induced STAT5 activity and cellular proliferation. In contrast, Epo‐induced differentiation was reduced strongly in infected TF‐1ER cells. These results suggest that Epo‐induced differentiation correlates with impaired Epo‐induced STAT5 activation.

The BCR-ABL specific tyrosine kinase inhibitors (TKI) changed the outcome of chronic myeloid leukemia (CML), turning a life-threatening disease into a chronic illness. However, TKI are not yet curative, because most patients retain leukemic stem cells (LSC) and their progenitors in bone marrow and relapse following treatment cessation. At diagnosis, deregulation of the bone morphogenetic protein (BMP) pathway is involved in LSC and progenitor expansion. Here, we report that BMP pathway alterations persist in TKI-resistant patients. In comparison with patients in complete cytogenetic remission, TKI-resistant LSC and progenitors display high levels of BMPR1b expression and alterations of its cellular localization. In vitro treatment of immature chronic phase CML cells with TKI alone, or in combination with interferon-α, results in the preferential survival of BMPR1b cells. We demonstrated persistent and increasing BMP4 production by patients' mesenchymal cells with resistance. Patient follow-up revealed an increase of BMPR1b expression and in BMP4 expression in LSC from TKI-resistant patients in comparison with diagnosis, while remaining unchanged in sensitive patients. Both leukemic and nonleukemic cells exhibit higher BMP4 levels in the bone marrow of TKI-resistant patients. Exposure to BMP2/BMP4 does not alter BCR-ABL transcript expression but is accompanied by the overexpression of TWIST-1, a transcription factor highly expressed in resistant LSC. By modulating BMP4 or BMPR1b expression, we show that these elements are involved in TKI resistance. In summary, we reveal that persistence of BMP alterations and existence of an autocrine loop promote CML-primitive cells' TKI resistance.