Direct Binding of Grb2 Is Required for Efficient Induction of Myeloproliferative Disease in Mice by ETV6/FLT3.

Abstract 2903

Poster Board II-879

FMS-like tyrosine kinase 3 (FLT3), a class III receptor tyrosine kinase, is one of the most frequently mutated genes in hematological malignancies. The most common mutations of FLT3 are internal tandem duplications (ITDs) within the juxtamembrane domain: these mutations occur in 20% to 30% of patients with AML and are closely associated with a poor prognosis. In a small number of patients with myeloproliferative neoplasms (MPNs), FLT3 has been reported to fuse to ETV6 (TEL) and contribute to leukemogenesis, but the leukemogenic mechanism of ETV6/FLT3 remains unclear. We encountered a case of ETV6/FLT3 fusion in a patient with MPN complicated with T-cell lymphoblastic lymphoma. In this case, both myeloid and lymphoma cells shared the same chromosomal translocation, t(12;13)(p13;q12), and allogeneic hematopoietic stem cell transplantation led to complete remission for 3 years. Full-length ETV6/FLT3 fusion cDNA was cloned from the patient's bone marrow cells. Sequence analysis of the PCR product revealed that, in contrast to the finding of previously reported two cases of ETV6/FLT3-positive MPN, ETV6 exon 6 was fused to FLT3 exon 14 and that the fused portion of ETV6 contained 2 potential Grb2-binding sites (Vu et al., Leukemia 2006; Walz et al., Blood 2007a). The ETV6/FLT3 conferred IL-3-independent growth to Ba/F3 and 32Dcl3 cells. Using a dominant negative approach, we showed that both STAT5 and Ras played important roles in ETV6/FLT3-mediated transformation of the hematopoietic cell lines. To investigate the role of the ETV6/FLT3 fusion protein in vivo, we used a murine bone marrow transplant model. Retroviral transduction of the ETV6/FLT3 into primary murine bone marrow cells resulted in a CML-like myeloproliferative disease (MPD) with complete penetrance in the transplanted mice. The disease progressed to cause death at a median of 18 days after transplantation (n = 16). The transplanted mice developed severe leukocytosis (159 × 10³ /μl to 417 × 10³ /μl), splenomegaly, and extensive infiltration of myeloid cells in the bone marrow, spleen, liver, and peripheral blood. ETV6/FLT3-induced MPD was oligoclonal and only 2 of the 9 secondary transplant recipients developed similar MPD when 5 × 10⁶ spleen cells from 3 independent diseased mice were used as donors. We assayed the mutant forms of the ETV6/FLT3 to test their ability to transform hematopoietic cells. Induction of MPD required the oligomerization domain of ETV6 and the tyrosine kinase activity of FLT3. Mice that received the double tyrosine-to-phenylalanine mutant of ETV6/FLT3 at sites 589 and 591 (Y589/591F) in the juxtamembrane domain of FLT3, which are critical for FLT3-ITD-induced MPD, also developed a similar MPD phenotype. Unlike FLT3-ITDs, Y589/591F mutation did not abrogate STAT5 activation in Ba/F3 and 32Dcl3 cells transformed by ETV6/FLT3. A recent study has shown that direct binding of Grb2 to tyrosine 768, 955, and 969 of FLT3 is important for FLT3-ITD-mediated proliferation and survival of hematopoietic cells. Tyrosine 314 in exon 5 of ETV6 has also been reported as the principal Grb2-binding site that contributes to leukemogenesis via oncogenic ETV6 fusion proteins such as ETV6/ABL. Thus, we next investigated the role of Grb2 binding in ETV6/FLT3-mediated leukemogenesis. Using coimmunoprecipitation assays, we demonstrated that Grb2 also binds to the tyrosine 314 and 354 of ETV6 of the ETV6/FLT3, in addition to the tyrosine 768, 955, and 969 of FLT3. Both ETV6/FLT3-Y314/354F and ETV6/FLT3-Y768/955/969F retained their interaction with Grb2 and induced rapidly fatal MPD when they were transduced into primary murine bone marrow cells. On the other hand, the ETV6/FLT3 mutant at all the binding sites of Grb2 (Y314/354/768/955/969F) significantly attenuated MPD development in mice. Simultaneous mutation of these 5 tyrosine residues completely abolished the binding of Grb2 and resulted in a marked decrease in the binding and phosphorylation of Gab2 and impaired activation of STAT5 and Akt in Ba/F3 cells. These results indicate that tyrosine 589 and 591 of FLT3 are dispensable for the ETV6/FLT3-induced MPD phenotype, and suggest that both ETV6 and FLT3 portions contribute to the ETV6/FLT3-mediated leukemogenesis by binding directly to Grb2. Our observations provide deep insights into the oncogenic signaling induced by active FLT3 mutants as well as provide a potential target for therapies.