**3. Epigenetic control of HBV transcription: histone modification**

As described above, ultrastructure of HBV cccDNA simulates that of mammalian nucleosome, suggesting the possibility of histone molecules as a main factor for transcriptional control [9]. Indeed, Pollicino et al. demonstrated the feasibility of HBV-associated histone modification as a major transcriptional regulator of HBV [11]. Genome-wise search for posttranslational modification (PTM) of HBV-infected liver cell lines has revealed that active marks of transcription such as H3K4me3, H3K27ac, and H3K122ac are abundant in active chromatin, especially in the core promoter region [19]. Interestingly, however, the repressive marks of transcription, i.e., H3K27me3 and H3K9me2, are depleted in the HBV cccDNA, suggesting that modified histones regulate HBV transcription mainly in favor of active replication. Pol2 enrichment is co-localized at the H3K27me-enriched transcription start site of precore/pregenomic area, and treatment with interferon alpha reduces the active PTMs, also suggesting that active marks of

**Histone change Modifier HBV response and mechanisms References**

levels of H3K27 and H3K122.)

EZH2 on HBV gene expression.)

H3K122ac P300/CBP **Activation** [19, 32, 33]

by MLL3 may facilitate HBV infection in vivo.)

**Repression (**ZHX2 inhibited trimethylation of H3K4. Overexpression of ZHX2 also decreases the acetylation

of HBeAg and ABsAg, indicating a repressive function of

symmetric (H4R3me2s) represses cccDNA transcription.)

HBx **Activation** [20–22, 24]

involving SETDB1-mediated H3K9me3 and HP1 induce silencing of HBV cccDNA transcription through

restricting HBV cccDNA transcription by acting cooperatively with histone methyltransferase.)

function to transcriptional repression via SIRT1-mediated

capacity possibly by modulating the acetylation and deacetylation of cccDNA-bound histones.)

IFNα **Repression** [19, 35, 36]

modulation of chromatin structure.)

SIRT3 **Repression** (SIRT3 is a novel host factor epigenetically

[19, 30]

225

[28, 31]

[28]

[24]

[25]

[34]

[27]

[29]

[26]

Epigenetic Regulation of Hepatitis B Virus Replication http://dx.doi.org/10.5772/intechopen.81711

H3K4me3 MLL3 **Activation** (H3K4me modification of the NTCP promoter

H3K27ac EZH2 **Activation** (Knockdown of EZH2 resulted in upregulation

H4R3me2s PRMT5 **Repression** (PRMT5-mediated histone H4 dimethyl Arg3

H3K9me3 SETDB1 **Repression** (upon HBV infection, cellular mechanisms

H3K27me3 Suz12 **Repression** (downregulation of Suz12 and Znf198 enhances HBV replication.)

H3K79me KDM2B **Repression** (KDM2B as an H3K79 demethylase and link its

etc BCP mutation **Repression** (BCP mutations decrease viral replication

chromatin silencing.)

Zinc finger and homeoboxes (ZHX2)

AcH3/AcH4 H3K4me3

Much is unknown regarding the effects of histone modification on the binding of these transcription factors. Hepatitis X protein (HBx) is the most studied modulator of HBVbound histone [20, 21]. HBx is bound to the cccDNA and enhance transcription by increasing histone acetylation and recruiting cellular coactivators p300, CBP, and PCAF [22], by inhibiting protein arginine methyltransferase 1 and reducing H4 methylation [23]. HBx also increases histone acetylation and H3K4me3 and decreases HP1 binding and

histone modification contribute to the transcriptional activity of HBV.

**Table 1.** Histone modification affecting HBV transcription.


**Table 1.** Histone modification affecting HBV transcription.

It has been known that transcriptional activity of HBV cccDNA varies according to the stage of natural history of chronic hepatitis B (CHB) [3, 4]. Interestingly, many patients with chronic hepatitis B are free from circulating HBV during the natural course despite the presence of HBV cccDNA in the infected nuclei [5]. These findings raise the possibility that replication of HBV is regulated at the transcriptional level. Genetic changes, i.e., DNA mutation, are an attractive explanation for the variable transcriptional activity since the reverse transcriptase activity of HBV is error-prone. However, no universal mutations have been identified associated with transcriptional suppression [6]. Consequently, epigenetic control has been proposed as the mechanism of these variable transcriptional activities in CHB patients [7], and this article covers the current knowledge of epigenetic mechanisms contributing to the

**2. Organization of HBV cccDNA and its transcriptional control**

**3. Epigenetic control of HBV transcription: histone modification**

As described above, ultrastructure of HBV cccDNA simulates that of mammalian nucleosome, suggesting the possibility of histone molecules as a main factor for transcriptional control [9]. Indeed, Pollicino et al. demonstrated the feasibility of HBV-associated histone modification as a major transcriptional regulator of HBV [11]. Genome-wise search for posttranslational modification (PTM) of HBV-infected liver cell lines has revealed that active marks of transcription such as H3K4me3, H3K27ac, and H3K122ac are abundant in active chromatin, especially in the core promoter region [19]. Interestingly, however, the repressive marks of transcription, i.e., H3K27me3 and H3K9me2, are depleted in the HBV cccDNA, suggesting that modified

Hepatitis B virus (HBV) is a partially double-stranded circular DNA virus [8]. The viral DNA goes into nuclei of infected hepatocytes where it is converted to cccDNA [8]. The cccDNA is a viral minichromosome, which takes the form of "beads-on-a-string" conformation of nucleosomal packaging [9], analogous to DNA packaging by mammalian nucleosome. HBV core protein and X protein along with histone H3 and H4 are components of HBV minichromosome [9–11]. A variety of cellular transcription factors bind HBV cccDNA, which in turn control transcriptional activity of HBV promoters: the preC/pregenomic, S1, S2, and X promoters [12]. The core promoter initiates transcription of preC and pregenomic RNA, the template for the viral genome by reverse transcription. Ubiquitous transcription factors such as specificity protein 1 (SP1), nuclear factor kappa B (NF-κβ), activator protein 1 (AP-1), and liver-enriched transcription factors such as hepatocyte nuclear factor 3 (HNF3), CAAT enhancer-binding protein (C/EBP), and several nuclear receptors such as hepatocyte nuclear factor 4 (HNF4), peroxisome proliferator-activated receptors (PPAR) and retinoid X receptors (RXRα), farnesoid acid receptor (FXR), small heterodimer partner (SHP), and testicular orphan receptor 4

transcriptional control of HBV replication.

224 Chromatin and Epigenetics

(TR4) can bind core promoter [13, 14].

histones regulate HBV transcription mainly in favor of active replication. Pol2 enrichment is co-localized at the H3K27me-enriched transcription start site of precore/pregenomic area, and treatment with interferon alpha reduces the active PTMs, also suggesting that active marks of histone modification contribute to the transcriptional activity of HBV.

Much is unknown regarding the effects of histone modification on the binding of these transcription factors. Hepatitis X protein (HBx) is the most studied modulator of HBVbound histone [20, 21]. HBx is bound to the cccDNA and enhance transcription by increasing histone acetylation and recruiting cellular coactivators p300, CBP, and PCAF [22], by inhibiting protein arginine methyltransferase 1 and reducing H4 methylation [23]. HBx also increases histone acetylation and H3K4me3 and decreases HP1 binding and H3K9me3 on the cccDNA [24]. Other host transcription factors, mainly suppressors, that act via epigenetic control of HBV include SIRT3 [25], zinc finger and homeoboxes 2 (ZHX2) [26], KDM2B [27], protein arginine methyltransferase 5 (PRMT5) [28], and SETDB1 [24]. Interestingly, mutations in the basal core promoter are also reported to be associated with histone modification [29]. The effect of histone modification on HBV replication is summarized in **Table 1**.

**5. Therapeutic implications and future perspectives of epigenetics in** 

consequences of epigenetic suppression of HBV replication.

development of HBV cure in the foreseeable future.

\*Address all correspondence to: kimjinw@snu.ac.kr

Perspectives in Medicine. 2015;**5**:a021410

Virology. 2018;**92**:e01391-e01317

NAs, the most commonly used modality of CHB therapy, is costly without definite duration. Interferon induces sustained virologic response with finite duration, but the response rate is suboptimal. The realistic goal of CHB therapy is to render the patients to the clinical situation similar to inactive carrier stage, i.e., normal alanine aminotransferase levels with low or negative serum HBV DNA levels. Since epigenetic silencing may contribute to the suppressive HBV replication status of inactive carrier stage, it would be theoretically feasible and clinically useful to induce epigenetic suppression of HBV replication simulating natural inactive stage of disease. Further studies will be needed to elucidate the mechanisms and long-term

Epigenetic Regulation of Hepatitis B Virus Replication http://dx.doi.org/10.5772/intechopen.81711 227

Epigenetic modification is an important mechanism of host-viral interaction in the transcriptional control of HBV. Current treatment strategy focuses on the inactivation/elimination of HBV cccDNA [17, 52], and knowledge on the epigenetic control is prerequisite for the novel

This work is supported by a National Research Foundation of Korea (NRF) grant to J-W Kim

[1] MacLachlan JH, Cowie BC. Hepatitis B virus epidemiology. Cold Spring Harbor

[2] Li M, Sohn JA, Seeger C. Distribution of hepatitis B virus nuclear DNA. Journal of

that was funded by the Korean Government (2017R1D1A1B03031483).

Seoul National University College of Medicine, Seongnam, South Korea

**chronic hepatitis B**

**6. Conclusions**

**Acknowledgements**

**Author details**

**References**

In Young Moon and Jin-Wook Kim\*
