DDX5 is required for JAK2V617F-induced cell proliferation and tumorigenesis independent of its RNA helicase activity

The constitutively active Janus kinase 2 mutant (JAK2V617F) is a major driver of myeloproliferative neoplasms (MPNs). Our previous studies demonstrated that JAK2V617F upregulates the expression of the RNA helicase DDX5 via activation of the transcription factor, signal transducer and activator of transcription 5 (STAT5). Furthermore, DDX5 facilitates mechanistic target of rapamycin (mTOR) signaling, thereby promoting the proliferation and tumorigenic potential of MPN cells. However, whether the RNA helicase activity of DDX5 is essential for JAK2V617F-driven oncogenesis remained unclear. In this study, we investigated the role of DDX5 in JAK2V617F-induced transformation by knocking down DDX5 in Ba/F3 cells expressing JAK2V617F and the erythropoietin receptor (V617F/EpoR cells), followed by reconstitution with either wild-type DDX5 or the helicase-deficient mutant K144N. DDX5 knockdown suppressed activation of the mTOR pathway, resulting in reduced proliferation and increased apoptosis in V617F/EpoR cells. In contrast, reconstitution with either wild-type DDX5 or the K144N mutant restored mTOR pathway activation, enhanced proliferation, and suppressed apoptosis. Furthermore, transplantation experiments using nude mice showed that DDX5 knockdown inhibited JAK2V617F-mediated tumor formation and hepatosplenomegaly, whereas reconstitution with DDX5 or the K144N mutant reversed these effects. These findings demonstrate that the RNA helicase activity of DDX5 is dispensable for JAK2V617F-induced transformation. These findings suggest that DDX5 promotes oncogenesis through helicase-independent mechanisms and represents a potential therapeutic target in JAK2V617F-driven malignancies.