From Bench to BedsideNew insight on hepatitis B virus persistence from the study of intrahepatic viral cccDNA
Introduction
Chronic hepatitis B virus (HBV) infection remains a major health problem affecting approximately 400 million people worldwide. Although HBV replication is only mildly cytopathic, cellular immune responses directed against the virus can produce substantial liver damage and result in chronic hepatitis, cirrhosis, and hepatocellular carcinoma [1], [2]. Chronic hepatitis B (CH-B) encompasses a broad spectrum of disease and can be divided into several phases of natural history which are distinguishable by serological and virological markers. One goal of antiviral therapy is to accelerate patients with chronic active disease into later natural history phases wherein viral replication abates and the incidence of serious liver disease is significantly reduced. With the knowledge gained from the molecular biology of HBV infection, it was shown that HBV covalently closed circular (ccc) DNA plays a key role in viral persistence, viral reactivation after treatment withdrawal, and drug resistance.
Section snippets
What is cccDNA?
Chronic infection is believed to be maintained by a replicative form of HBV DNA termed covalently closed circular (ccc) DNA (Fig. 1). Formation of cccDNA from incoming virions implies the trafficking of nucleocapsids to the nucleus, the entry of relaxed circular DNA in the nucleus, the completion of plus strand DNA synthesis, the removal of the polymerase primer for minus strand DNA synthesis and of the RNA primer for plus strand DNA synthesis, the ligation of both DNA strand extremities, and
Is cccDNA stable?
The importance of cccDNA in HBV persistence at the level of a single cell has been demonstrated in tissue culture experiments. Using nucleoside analogs to block HBV DNA synthesis and recycling of cccDNA by the trafficking of nucleocapsids to the nucleus, it was possible to analyze the half-life of cccDNA in non-dividing cells. Depending on experimental conditions, i.e. using primary duck or woodchuck hepatocytes, or in hepatoma cell lines, a range of results were obtained. Some studies showed
Detection of intrahepatic cccDNA
Despite the crucial role of cccDNA during persistent infection and the importance of understanding clearance mechanisms, few data have been collected from patients [26], [27]. Indeed, our current understanding of cccDNA has been obtained primarily through studies of the woodchuck and duck hepatitis B virus models [8], [9], [28], [29]. Historical obstacles to the study of HBV cccDNA have been (1) the requirement for liver biopsies, which are difficult to collect, especially from patients in
Perspectives for antiviral therapy
The detection of cccDNA in the liver of infected patients may represent an important marker to monitor antiviral therapy. As assays for its quantification are not used routinely, the quantification of cccDNA decline during antiviral therapy may represent an important treatment endpoint in the setting of clinical trials using new antiviral drugs or new combinations. Information gained from such studies may help to provide recommendation for the duration of antiviral therapy and new insight in
Acknowledgements
This work was supported by funding from the European Community (HepBvar project contract QLRT2001-00977; and viRgil network contract LSHM-CT-2004-503359).
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