HBx induces hypomethylation of distal intragenic CpG islands required for active expression of developmental regulators.

Epigenetic alterations caused by viral oncoproteins are strong initiation factors for cancer development, but their mechanisms are largely unknown. To identify the epigenetic effects of viral hepatitis B virus X (HBx) that lead to hepatocellular carcinoma (HCC), we profiled the DNA methylomes of normal and HBx transgenic mouse liver. Intriguingly, severe hypomethylation of intragenic CpG islands (CGIs) was observed in HBx liver before the full development of HCC. Normally, these CGIs were highly methylated (mCGIs) by the DNMT3L complex and marked with epigenetic signatures associated with active expression, such as H3K36me3. Hypomethylation of mCGI was caused by the downregulation of Dnmt3L and Dnmt3a due to HBx bound to their promoters, along with HDAC1. These events lead to the downregulation of many developmental regulators that could facilitate tumorigenesis. Here we provide an intriguing epigenetic regulation mediated by mCGI that is required for cell differentiation and describe a previously unidentified epigenetic role for HBx in promoting HCC development.