ETV6::FRK gene expression analysis in Ba/F3 cells by whole transcriptome sequencing

ETV6::FRK is a rare kinase-related fusion gene which was identified only in AML but not in ALL. Herein, we firstly identified ETV6::FRK fusion gene in a patient with pediatric B-ALL. Because FRK is Src family tyrosine kinase, we performed functional analysis of ETV6::FRK to establish molecular targeting therapy. Our case with B-ALL was refractory to conventional chemotherapy and received allogeneic bone marrow transplantation following administration of blinatumomab. Cytogenetic analysis demonstrated 46,XY,t(6;12)(q21;p13) and target capture mRNA sequencing revealed ETV6::FRK. Ba/F3 cells expressing ETV6::FRK generated by retroviral transduction (Ba/F3-ETV6::FRK) and analyzed for IL-3 independent growth. Gene expression analysis using whole transcriptome sequencing and gene set enrichment analysis (GSEA) was performed for comprehensive analysis of gene expression profile related to ETV6::FRK. It was also analyzed whether dasatinib, which is Src-kinase inhibitor, suppressed the growth of Ba/F3-ETV6::FRK in vitro and in vivo. Ba/F3-ETV6::FRK proliferated without IL-3, suggesting ETV6::FRK had proliferation activity. Western blot revealed that constitutive phosphorylation of tyrosine residue of ETV6::FRK and STAT3/STAT5, suggesting constitutive activation of FRK-STAT3/STAT5 pathway. GSEA of oncogenic gene sets (C6) from the GSEA Molecular Signatures Database revealed that, compared with control cells, Ba/F3-ETV6::FRK cells were enriched for up-regulation of SNF5 target genes and down-regulation of RB target genes involved in cell cycle regulation. In vitro killing assay showed that dasatinib killed efficiently Ba/F3-ETV6::FRK. Dasatinib also suppressed the growth of Ba/F3-ETV6::FRK in vivo and extended the survival time of the xenografted NSG mice. These findings suggest that activation of FRK-STAT3/STAT5 pathway contributes aberrant growth promotion of Ba/F3-ETV6::FRK. Our study demonstrated that dasatinib might be effective for the patient with B-ALL harboring ETV6::FRK. Ba/F3 cells were transduced with ETV6::FRK fusion protein by the Retro-X Tet-On Advanced Inducible Expression System (Takara Bio), and Ba/F3 cells expressing ETV6::FRK under doxycycline (DOX) dependent manner were established. To reveal which pathways were affected by ETV6::FRK, Ba/F3 cells expressing ETV6::FRK induced by DOX (Ba/F3-ETV6::FRK) or non-DOX-induced Ba/F3 cells were cultured in triplicate, and RNA was extracted from Ba/F3-ETV6::FRK or non-DOX-induced Ba/F3 48 hours later.