Epigenetic mechanisms in hepatitis B virus-associated hepatocellular carcinoma

Abstract

Chronic infection of the liver by the hepatitis B virus (HBV) is associated with increased risk for developing hepatocellular carcinoma (HCC). A multitude of studies have investigated the mechanism of liver cancer pathogenesis due to chronic HBV infection. Chronic inflammation, expression of specific viral proteins such as HBx, the integration site of the viral genome into the host genome, and the viral genotype, are key players contributing to HCC pathogenesis. In addition, the genetic background of the host and exposure to environmental carcinogens are also predisposing parameters in hepatocarcinogenesis. Despite the plethora of studies, the molecular mechanism of HCC pathogenesis remains incompletely understood. In this review, the focus is on epigenetic mechanisms involved in the pathogenesis of HBV-associated HCC. Epigenetic mechanisms are dynamic molecular processes that regulate gene expression without altering the host DNA, acting by modifying the host chromatin structure via covalent post-translational histone modifications, changing the DNA methylation status, expression of non-coding RNAs such as microRNAs and long noncoding RNAs, and altering the spatial, 3-D organization of the chromatin of the virus-infected cell. Herein, studies are described that provide evidence in support of deregulation of epigenetic mechanisms in the HBV-infected/-replicating hepatocyte and their contribution to hepatocyte transformation. In contrast to genetic mutations which are permanent, epigenetic alterations are dynamic and reversible. Accordingly, the identification of essential molecular epigenetic targets involved in HBV-mediated HCC pathogenesis offers the opportunity for the design and development of novel epigenetic therapeutic approaches.

Keywords: DNA methylation; Hepatitis B virus; chromatin/histone modifications; epigenetics; hepatocellular carcinoma; lncRNA; miRNA.

Figure 1.

Deregulation of the epigenetic PRC2 complex by HBV infection. A: Diagram depicts the PRC2 complex, comprised of the core subunits EZH2, EED and SUZ12, in interaction with RNA helicase DDX5 and lncRNA HOTAIR; B: working model of how HBV infection disrupts the transcriptionally silencing PRC2. HBV infection promotes activation of PLK1^[61,62], which phosphorylates SUZ12. E3 ligase Mex3b ubiquitinates SUZ12, leading to SUZ12 proteasomal degradation^[64]. The functional significance of the phosphorylation of DDX5 by PLK1 remains to be determined (Rahman and Andrisani, unpublished results)