The Kinase Inhibitor TKI258 Is Active Against the Novel CUX1-FGFR1 Fusion Detected In a Patient with T-Lymphoblastic Leukemia/Lymphoma and t(7;8)(q22;p11)

Abstract 2715

The 8p11 myeloproliferative syndrome (EMS) is an aggressive, atypical stem cell disorder associated with chromosome translocations that constitutively activate FGFR1 by fusion to diverse partner genes. Here, we describe a case with a clinical and hematological diagnosis of T-lymphoblastic leukemia/lymphoma and with a t(7;8)(q22;p11) on cytogenetic analysis. We identified the fusion partner involved and characterized this translocation functionally in vitro using the interleukin 3 (IL3) dependent Ba/F3 cell line.

The translocation was analyzed in more detail by FISH using FGFR1 flanking probes. We could confirm the 8p11 breakpoint and 7q as the partner chromosome. Using 5'-RACE CUX1 (7q22) was identified as the fusion partner of FGFR1 in this patient with T-lymphoblastic leukemia/lymphoma. CUX1 is a homeobox family DNA binding protein not previously described as a fusion partner in hematological malignancies. To evaluate the transforming potential of this novel fusion, the CUX1-FGFR1 fusion was cloned and used to transform Ba/F3 cells. CUX1-FGFR1 expressing Ba/F3 cells displayed IL3 independent proliferation thus demonstrating the oncogenic character of this fusion protein. Western blotting of the transformed Ba/F3 cells showed activation of FGFR1 as well as its downstream target STAT5. Treatment of the CUX1-FGFR1 expressing Ba/F3 cells with the kinase inhibitors PKC412 and TKI258 significantly inhibited cell growth with an IC₅₀ of 483 and 489 nM respectively. With western blotting a direct effect of both inhibitors on FGFR1 kinase activity as well as on different downstream effectors was proven. Furthermore using an annexinV/propidium iodide-based apoptosis assay, we could show that PKC412 and TKI258 both induced apoptosis followed by cell death in inhibitor treated CUX1-FGFR1 transformed Ba/F3 cells. The antiproliferative effect of the inhibitors could be rescued by addition of IL3 for the TKI258 treated but not for PKC412 treated CUX1-FGFR1 expressing cells. This observation indicates a selective action of TKI258 on FGFR1 signaling at the concentrations used. In contrast, for PKC412 non-specific cytotoxicity is also contributing to the antiproliferative effect. In summary, we identified a novel CUX1-FGFR1 fusion in a case with EMS and a novel t(7;8)(q22;p11), and demonstrated the oncogenic potential of CUX1-FGFR1 in the Ba/F3 cell line. This new fusion partner CUX1 contains a potential coiled coil domain that can explain the observed constitutive FGFR1 activation, as has been elaborately demonstrated for other oncogenic kinase fusions. The in vitro data presented here using the inhibitor TKI258 support the use of this compound for the treatment of the novel CUX1-FGFR1 fusion as well as other constitutively active FGFR1 fusion proteins.