From the AGA Journals

New HBV model may open door to more effective antivirals

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Long–sought-after breakthrough?

On the heels of the wondrous development of curative antiviral agents for hepatitis C virus (HCV), renewed attention has been directed to efforts to bring about the cure of HBV. However, this task will hinge on successful elimination of covalently closed circular DNA (cccDNA), a highly stable form of viral DNA that is exceedingly difficult to eliminate. Efforts to develop successful curative strategies will in turn rely on development of small animal models that support HBV cccDNA formation and virus production, which has until recently proved elusive. In the past several years, several mouse HBV models supporting cccDNA formation have been constructed using adeno-associated vector (AAV)–mediated transduction of a linearized HBV genome. Both the AAV-HBV linear episome and cccDNA have been consistently replicated and detected in these models. While they recapitulate the key steps of the viral life cycle, these models do not, however, lend themselves to direct assessment of cccDNA, which have traditionally required detection of cccDNA in the liver.

Dr. Raymond T. Chung

Dr. Raymond T. Chung

Xu et al. have now developed a novel mouse model in which generation of HBsAg is directly dependent on generation of cccDNA. This dependence thus yields a simple marker for assessment of cccDNA status and allows monitoring of the therapeutic effects of novel agents targeting cccDNA by simply following HBsAg titers. More studies are required to explore the mechanisms underlying HBV cccDNA formation and elimination, but this work suggests a new way forward to tractably evaluate agents that specifically interrupt cccDNA metabolism, an important step in our systematic march toward HBV cure.

Raymond T. Chung, MD, is a professor of medicine at Harvard Medical School and director of the Hepatology and Liver Center at Massachusetts General Hospital, both in Boston. He has no conflicts to disclose.


 

FROM CELLULAR AND MOLECULAR GASTROENTEROLOGY AND HEPATOLOGY

A new mouse model that better represents chronic infection with hepatitis B virus (HBV) in humans may lead to more effective antiviral therapies for HBV, according to investigators.

During human infection, HBV genomes take the form of covalently closed circular DNA (cccDNA), a structure that has thwarted effective antiviral therapy and, until now, creation of an accurate mouse model, reported lead author Zaichao Xu, PhD, of Wuhan (China) University and colleagues.

“As the viral persistence reservoir plays a central role in HBV infection, HBV cccDNA is the key obstacle for a cure,” the investigators wrote in Cellular and Molecular Gastroenterology and Hepatology.

Although several previous mouse models have approximated this phenomenon with recombinant cccDNA-like molecules (rcccDNA), the present model is the first to achieve genuine cccDNA, which does not naturally occur in mice.

“Although rcccDNA supports persistent viral replication and antigen expression, the nature of rcccDNA may differ from authentic cccDNA, as additional sequences, like LoxP or attR, were inserted into the HBV genome,” the investigators noted.

The new model was created by first constructing an adeno-associated virus vector carrying a replication-deficient HBV1.04-fold genome (AAV-HBV1.04). When injected into mice, the vector led to cccDNA formation via ataxia-telangiectasia and Rad3-related protein (ATR)–mediated DNA damage response, a finding that was confirmed by blocking the same process with ATR inhibitors.

Immediately after injection, mice tested positive for both hepatitis B e antigen (HBeAg) and hepatitis B surface antigen (HBsAg), with peak concentrations after either 4 or 8 weeks depending on dose. HBV DNA was also detected in serum after injection, and 50% of hepatocytes exhibited HBsAg and hepatitis B core protein (HBc) after 1 week. At week 66, HBsAg, HBeAg, and HBc were still detectable in the liver.

“The expression of HBc could only be observed in the liver, but not in other organs or tissues, suggesting that the AAV-HBV1.04 only targeted the mouse liver,” the investigators wrote.

Further experimentation involving known cccDNA-binding proteins supported the similarity between cccDNA in the mouse model and natural infection.

“These results suggested that the chromatinization and transcriptional activation of cccDNA formed in this model dose not differ from wild-type cccDNA formed through infection.”

Next, Dr. Xu and colleagues demonstrated that the infected mice could serve as a reliable model for antiviral research. One week after injection with the vector, mice were treated with entecavir, polyinosinic-polycytidylic acid (poly[I:C]), or phosphate-buffered saline (PBS; control). As anticipated, entecavir suppressed circulating HBV DNA, but not HBsAg, HBeAg, or HBV cccDNA, whereas treatment with poly(I:C) reduced all HBV markers.

“This novel mouse model will provide a unique platform for studying HBV cccDNA and developing novel antivirals to achieve HBV cure,” the investigators concluded.

The study was supported by the National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities, Hubei Province’s Outstanding Medical Academic Leader Program, and others. The investigators reported no conflicts of interest.

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