AML-060 A Truncated Nucleus Isoform of FGFR1 Drives the Transformation of Leukemia Stem Cells in Leukemogenesis

Abstract

Myeloid and lymphoid malignancies associated with chimeric FGFR1 kinases are the hallmark of stem cell leukemia and lymphoma syndrome (SCLL). In all cases, FGFR1 kinase is constitutively phosphoactivated in the cytoplasm as a result of chromosome translocations. Recently, we demonstrated that these chimeric kinases could be cleaved by granzyme B to generate a truncated derivative, tnFGFR1, which localized exclusively into the nucleus and was not phosphorylated. In this study, we used the mouse bone marrow transduction and transplantation model to demonstrate the leukemic transformation ability of this tnFGFR1 and identified its critical downstream targets, FLT3 and KIT, in mouse and acute myeloid leukemia (AML) patient samples. Stem cell transduction and transplantation in syngeneic mice was used to assess the transforming ability of tnFGFR1 in bone marrow stem cells, and RPPA and RNA-Seq were used to examine the related signaling pathways and regulated target genes. We now show that tnFGFR1 can independently lead to oncogenic transformation of hematopoietic stem cells. These leukemia cells show a mixed immunophenotype with a B-cell B220+Igm- profile in the majority of cells and Kit+ in virtually all cells, suggesting a stem cell disease. tnFGFR1 induces a distinct profile of altered gene expression with significant upregulation of transmembrane signaling receptors including FLT3 and KIT. De novo human AML also expresses tnFGFR1, which correlates with upregulation of FLT3 and KIT as in mouse leukemia cells. ChIP analysis demonstrates tnFGFR1 occupancy at the Flt3 and Kit promoters, suggesting a direct transcriptional regulation. Cells transformed with tnFGFR1 are insensitive to FGFR1 inhibitors but sensitive to the FLT3 inhibitor quizartinib (AC220), suggesting an alternative approach for the treatment of SCLL. This study demonstrates a novel model for the transformation of hematopoietic stem cells by chimeric FGFR1 kinases with the combined effects of direct protein activation by the full-length kinases and transcriptional regulation by tnFGFR1. In human AML, tnFGFR1 activation leads to increased FLT3 and KIT expression, and higher FLT3 and GZMB expression levels are associated with inferior prognosis. These observations provide insights into the relative therapeutic value of targeting FGFR1 and FLT3 in treating AML with this characteristic gene expression profile.