Abstract 5837: A truncated nucleus-located derivative of FGFR1 contributes to the transformation of leukemia stem cell in leukemogenesis driven by FGFR1 fusion kinases
Authors: Cai Baohuan , Liu Yun , Chong Yating , Zhang Hualei , Mori Stephanie , Matsunaga Atsuko , Fang Xuexiu , Cowell John , Hu Tianxiang 
Published: June 2022

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 as a result of chromosome translocations, which lead to acquisition of dimerization motifs in the chimeric proteins. 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. Stem cell transduction and transplantation in syngeneic mice were used to assess the transforming ability of tnFGFR1 in bone marrow stem cells, and the results showed 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. RPPA and RNA-Seq examination of the related signaling pathways and target genes revealed that tnFGFR1, however, does not activate classic FGFR1 downstream signaling pathways but induces a distinct profile of altered gene expression with significant upregulation of transmembrane signaling receptors including FLT3 and KIT. De novo human AML also expressed 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 treatment of these cells with the Quizartinib (AC220) FLT3 inhibitor, suppresses in vitro growth and development of leukemia in vivo suggesting an alternative approach for treatment of SCLL. This study demonstrates a novel model for transformation of hematopoietic stem cells by chimeric FGFR1 kinases results with the combined effects of direct protein activation by the full-length kinases and gene regulation by the truncated nucleus derivative tnFGFR1, which is associated with GZMB expression levels. Genes significantly upregulated by tnFGFR1 include Flt3 and Kit which promote a leukemia stem cell phenotype. 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.
Citation Format: Baohuan Cai, Yun Liu, Yating Chong, Hualei Zhang, Stephanie Mori, Atsuko Matsunaga, Xuexiu Fang, John Cowell, Tianxiang Hu. A truncated nucleus-located derivative of FGFR1 contributes to the transformation of leukemia stem cell in leukemogenesis driven by FGFR1 fusion kinases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5837.