Germline-specific RNA helicase DDX4 forms cytoplasmic granules in cancer cells and promotes tumor growth

Cancer cells undergo major epigenetic alterations and transcriptomic changes, including ectopic expression of many tissue- and cell type-specific genes. Here we show that the germline-specific RNA helicase DDX4 (DEAD-box helicase 4) forms germ granule-like cytoplasmic ribonucleoprotein (RNP) granules in various human tumors, but not in cultured cancer cells. These cancerous DDX4-complexes contain RNA-binding proteins and splicing regulators, including many known germ granule components. The deletion of DDX4 in cancer cells induces transcriptomic changes and affects the alternative splicing landscape of a large number of genes involved in cancer growth and invasiveness, leading to compromised capability of DDX4-null cancer cells to form xenograft tumors in immunocompromised mice. Importantly, occurrence of DDX4-granules is associated with poor survival in patients with head and neck squamous cell carcinoma, and higher histological grade of prostate cancer. Taken together, these results show that the germ granule-resembling cancerous DDX4-granules control gene expression and promote malignant and invasive properties of cancer cells. To understand the role of DDX4 in the regulation of cancer-promoting properties of PC3 cells, we carried out RNA-seq of the DDX4-null and WT PC3 cells cultured in monolayers and xenograft tumors.