Hepatitis B virus (HBV) infection is of global importance with over 2 billion people exposed to the virus during their lifetime and at risk of progressive liver disease, cirrhosis and hepatocellular carcinoma. HBV is a member of the Hepadnaviridae family that replicates via episomal copies of a covalently closed circular DNA (cccDNA) genome. The chromatinization of this small viral genome, with overlapping open reading frames and regulatory elements, suggests an important role for epigenetic pathways to regulate viral transcription. The chromatin‐organising transcriptional insulator protein, CCCTC‐binding factor (CTCF), has been reported to regulate transcription in a diverse range of viruses. We identified two conserved CTCF binding sites in the HBV genome within enhancer I and chromatin immunoprecipitation (ChIP) analysis demonstrated an enrichment of CTCF binding to integrated or episomal copies of the viral genome. siRNA knock‐down of CTCF results in a significant increase in pre‐genomic RNA levels in de novo infected HepG2 cells and those supporting episomal HBV DNA replication. Furthermore, mutation of these sites in HBV DNA minicircles abrogated CTCF binding and increased pre‐genomic RNA levels, providing evidence of a direct role for CTCF in repressing HBV transcription.

中文翻译：

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CCCTC结合因子CTCF抑制乙型肝炎病毒增强子I并调节病毒转录

乙型肝炎病毒 (HBV) 感染具有全球重要性，超过 20 亿人一生中接触过该病毒，并面临进行性肝病、肝硬化和肝细胞癌的风险。HBV 是嗜肝DNA病毒科的一员，通过共价闭合环状DNA (cccDNA) 基因组的游离拷贝进行复制。这种小病毒基因组的染色质化具有重叠的开放阅读框和调控元件，表明表观遗传途径在调控病毒转录方面发挥着重要作用。据报道，染色质组织转录绝缘子蛋白 CCCTC 结合因子 (CTCF) 可调节多种病毒的转录。我们在增强子 I 内鉴定出 HBV 基因组中的两个保守的 CTCF 结合位点，染色质免疫沉淀 (ChIP) 分析证明 CTCF 与病毒基因组的整合或游离拷贝的结合富集。CTCF 的 siRNA 敲低导致从头感染的 HepG2 细胞和支持附加型 HBV DNA 复制的细胞中前基因组 RNA 水平显着增加。此外，HBV DNA 小环中这些位点的突变消除了 CTCF 结合并增加了前基因组 RNA 水平，这为 CTCF 在抑制 HBV 转录中的直接作用提供了证据。