DDX17‐regulated alternative splicing that produced an oncogenic isoform of PXN‐AS1 to promote HCC metastasis

Background and Aims The mechanism underlying HCC metastasis remains unclear, many oncogenes are known to regulate this process. However, the role of alternative splicing (AS) in pro‐metastatic HCC is poorly understood. Approach and Results By performing RNA sequencing on nine pairs of primary HCC tissues with extrahepatic metastasis (EHMH) and nine pairs of metastasis‐free HCC (MFH) tissues, we depicted the AS landscape in HCC and found a higher frequency of AS events in EHMH compared with MFH. Moreover, 28 differentially expressed splicing regulators were identified in EHMH compared with MFH. Among these, DEAD‐box RNA helicase 17 (DDX17) was significantly up‐regulated in EHMH and was strongly associated with patient outcome. Functional studies indicated that DDX17 knockout inhibited the degradation of the extracellular matrix, and diminished the invasive ability of HCC cells. A significant reduction in lung metastasis induced by DDX17 deficiency was also demonstrated in a diethylnitrosamine‐induced DDX17HKO mouse model. Mechanistically, high DDX17 induced intron 3 retention of PXN‐AS1 and produced a transcript (termed PXN‐AS1‐IR3). The transcript PXN‐AS1‐IR3 acted as an important promoter of HCC metastasis by inducing MYC transcription activation via recruiting the complex of testis expressed 10 and p300 to the MYC enhancer region, which led to transcriptional activation of several metastasis‐associated downstream genes. Finally, the PXN‐AS1‐IR3 level was significantly higher in serum and HCC tissues with extrahepatic metastasis. Conclusions DDX17 and PXN‐AS1‐IR3 act as important metastatic promoters by modulating MYC signaling, suggesting that DDX17 and PXN‐AS1‐IR3 may be potential prognostic markers for metastatic HCC.