Epigenetic modification and regulation of HBV cccDNA

Abstract

Hepatitis B virus (HBV) infection, which can cause acute or chronic viral hepatitis, hepatic fibrosis, cirrhosis, and carcinoma (HCC), is a major global health problem. After the virus enters a hepatocyte, its genome translocates into the cell nucleus and forms covalently closed circular DNA (cccDNA). HBV cccDNA binds with histones and nonhistone proteins, forming an episomal minichromosome, which underlies the molecular basis of HBV infection persistence and reactivation after antiviral treatment. The term “epigenetics” traditionally refers to heritable traits without changes in DNA sequence, but it also describes general studies of chromatin biology. Increasing evidence suggests that the epigenetic regulation of cccDNA is closely related to viral replication and the outcome of chronic HBV infection. In this review, we mainly discuss the following two mechanisms underlying epigenetic regulation: DNA methylation and histone modification. DNA methylation usually occurs in the CpG dinucleotides; it is mediated by DNA methyltransferases (DNMTs) and is generally associated with transcriptional silencing. There are three predicted CpG islands in the HBV genome. CpG island 1 is rarely methylated, the methylation of CpG island 2 leads to reduced viral 3.5 kb RNA transcription and DNA replication, and the methylation of CpG island 3 is significantly correlated with hepatocarcinogenesis. The exact roles that DNMTs play in HBV DNA methylation have not been completely elucidated. The most studied histone modifications are histone acetylation and methylation. Histone acetylation, which is generally associated with transcriptional activation, is regulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs). Histone methylation, which is associated with both transcriptional activation and repression, is controlled by histone methyltransferases (HMTs) and histone demethylases (HDMs). Some of these enzymes, such as p300/CBP, HDAC1 and PRMT5, have been reported to participate in the epigenetic regulation of HBV cccDNA. Viral proteins are also involved in the epigenetic regulation of HBV cccDNA. HBV core protein (HBc) is a structural protein in the HBV capsid. It has been reported that HBc can be recruited to cccDNA, preferentially binding to CpG island 2. In addition, relative abundances of HBc binding are negatively associated with DNA methylation levels in CpG island 2, which suggests that HBc may be an epigenetic regulatory factor of HBV. HBx is a significant HBV regulatory protein and can also be recruited to cccDNA. In the presence of HBx, transcriptional activation factors such as p300/CBP and GCN5/PCAF are recruited to cccDNA and hyperacetylate the cccDNA-bound histones. This results in an open configuration of the viral minichromosome with higher transcriptional activity. In the absence of HBx, cccDNA-bound HATs are severely impaired, while HDACs and other transcriptional restriction factors are recruited to cccDNA, leading to a tight structure of the minichromosome with lower transcriptional activity. In this review, we summarize the development of epigenetic therapies involving existing and emerging epigenetic drugs and their potential applications in chronic HBV infection and HBV-related HCC. In addition, we describe current techniques and systems for research on cccDNA epigenetic regulation. The discovery that epigenetic regulators play important roles in the persistence of HBV and are potential therapeutic targets has presented a myriad of opportunities to treat or even cure HBV.