Identification of a Novel TEL-Lyn Fusion Gene in Chronic Eosinophilic Leukemia with ins (12;8)(p13;q11q21): Effect on Myeloproliferative Transformation and Induction of Myelofibrosis.

Chronic myeloproliferative diseases (CMPD), a group of hematopoietic stem cell disorders often accompanied by myelofibrosis, are associated with several recently identified genetic abnormalities. However, the mechanism responsible for myelofibrosis is still unclear. TEL is an ets family transcription factor located on 12p13 which on translocation is known to form fusion genes with more than 20 partners including protein tyrosine kinases (PTK) and transcription factors. Here, we identified a novel TEL-Lyn fusion gene in chronic eosinophilic leukemia with myelofibrosis, bearing the chromosomal abnormality ins (12;8)(p13;q11q21). The patient was refractory to both imatinib therapy and allogeneic stem cell transplantation and died of blastic transformation. We established that this novel TEL-Lyn fusion gene was expressed by the patient’s peripheral blood mononuclear cells and confirmed that the TEL and Lyn genes were fused in frame at breakpoints at 1010 bp and 638 bp, respectively. This fusion gene contains the TEL PNT domain and the Lyn PTK domain. To test whether the TEL-Lyn fusion product transforms hematopoietic cells, we introduced the gene into murine IL-3 dependent Ba/F3 cells. The 75 kDa TEL-Lyn fusion protein was detected in TEL-Lyn-transfected Ba/F3 cells and found to be constitutively tyrosine-phosphorylated. TEL-Lyn-transfected Ba/F3 cells proliferated in an IL-3-independent manner, which was not blocked by imatinib but could be by dasatinib, which targets Lyn kinase. Next, we isolated CD34-c-Kit+Sca-1+lineage marker- (CD34-KSL) hematopoietic stem cells (HSCs) from C57BL/6 (B6) mouse bone marrow (BM) by FACS sorting and introduced the TEL-Lyn fusion gene using a retroviral vector. HSCs expressing TEL-Lyn formed colonies without the exogenous growth factors SCF, IL-3, EPO and TPO, which was also suppressed by dasatinib, but not imatinib. Finally, we transplanted these transfected HSCs into irradiated hosts using B6-Ly5.2 mice as recipients of TEL-Lyn or control retroviral vector-transfected HSCs from B6-Ly5.1 mice. In the TEL-Lyn group, marked neutrophillia, splenomegaly and BM fibrosis were observed. Six of 10 mice died within 6 weeks after transplantation, while all controls remained healthy over 8 weeks.

Conclusions: Introduction of the TEL-Lyn fusion gene into HSCs results in rapid development of myelofibrosis as well as myeloproliferative transformation. Lyn kinase might be constitutively activated by TEL-induced oligomerization. These data imply for the first time that rearranged or activated Lyn kinase is involved in the pathogenesis of CMPD and myelofibrosis, and provide an ideal model for the latter. Further extensive study on the role of Lyn in CMPD might result in the definition of a novel clinical CMPD entity.