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

Cancer cells undergo major epigenetic alterations and transcriptome changes, including ectopic expression of many tissue- and cell type-specific genes. Here we show that the germline-specific RNA helicase DDX4 forms germ granule-like cytoplasmic ribonucleoprotein (RNP) granules in various human and mouse tumors, but not in cultured cancer cells. The DDX4-compexes in tumors contain RNA-binding proteins and splicing regulators, including many known germ granule components. The deletion of DDX4 in cancer cells induces transcriptome imbalance and affects the alternative splicing landscape of a large number of genes with reported functions in cancer growth and invasiveness, leading to their compromised capability to form xenograft tumors. Importantly, high expression of DDX4-positive granules is associated with poor survival in patients with squamous cell carcinoma of the head and neck, as well as with the histological grade of prostate cancer. Taken together, these results show that the formation of germ granule-resembling DDX4 granules have prognostic value, and they are involved in transcriptome control in cancer cells to promote malignant properties.