Abstract 3393: A novel targetable G3BP stress granule assembly factor 1 (G3BP1)-colony stimulating factor 1 receptor (CSF1R) fusion in acute megakaryoblastic leukemia (AMKL)
Authors: Luo Xi , Cheng Chi Keung , Chan Hoi Yun , Kam Yuet Fong , Leung Kam Tong , Li Chi Kong , Ng Margaret Heung-Ling 
Published: April 2023

Abstract
Acute myeloid leukemia (AML) is a heterogeneous disease, and targeted therapy has been shown to improve the outcomes of AML patients. Recently, we identified a novel G3BP stress granule assembly factor 1 (G3BP1)-colony stimulating factor 1 receptor (CSF1R) fusion gene by RNA sequencing in a patient with acute megakaryoblastic leukemia (AMKL). G3BP1-CSF1R (GC) juxtaposed exon 8 of G3BP1 to exon 12 of CSF1R and was predicted to encode a fusion protein containing the disrupted juxtamembrane domain (JMD) and the intact kinase domain of CSF1R. A similar CSF1R rearrangement has been reported in an AMKL cell line, MKPL1, which harbors the RBM6-CSF1R fusion. As the second CSF1R fusion discovered in AMKL, it remains unclear whether GC is a novel oncogenic driver and whether it is involved in dictating megakaryocytic phenotype. The aim of this study was to characterize the GC fusion and to investigate its role in AMKL pathogenesis and its targetability. Immunofluorescence studies showed that GC and G3BP1 were located in the cytoplasm, unlike the wide-type CSF1R which exhibited vesicular localization. Co-IP studies indicated that GC could exist as dimers in the absence of ligand stimulation. Two cytokine-dependent cell lines (TF1 and M07E) transduced with GC lentivirus showed cytokine-independent growth and constitutive activation of CSF1R and its downstream signaling, indicating the transforming ability of GC. Consistently, NSG mice transplanted with GC-expressing TF1 cells developed AML. In vitro drug studies revealed that GC-transformed TF1 (TF1-GC) and MKPL1 cells were sensitive to dasatinib and pexidartinib, as demonstrated by inhibition of kinase activation, reduced cell proliferation, and increased apoptosis. Moreover, in NSG mice transplanted with TF1-GC cells, dasatinib and pexidartinib significantly decreased leukemia burden and prolonged survival. Using flow cytometry, we observed that GC increased cell size, granularity, and megakaryocytic markers (CD41 and CD61) in M07E, TF1, and human cord blood CD34+ stem and progenitor cells, suggesting that GC could promote megakaryocytic skewing. Taken together, we identified and characterized a novel targetable GC fusion in AMKL. These findings may provide insights into the role of CSF1R in AMKL pathogenesis and a rationale for CSF1R-targeted therapy in AML patients with CSF1R activating alterations.
Citation Format: Xi Luo, Chi Keung Cheng, Hoi Yun Chan, Yuet Fong Kam, Kam Tong Leung, Chi Kong Li, Margaret Heung-Ling Ng. A novel targetable G3BP stress granule assembly factor 1 (G3BP1)-colony stimulating factor 1 receptor (CSF1R) fusion in acute megakaryoblastic leukemia (AMKL) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3393.