Expression and purification of a functional human hepatitis B virus polymerase

A critical role of hepatitis B virus polymerase in cirrhosis, hepatocellular carcinoma, and steatosis

Hepatitis B is one of the most common infectious diseases in the world; more than 350 million people are carriers of hepatitis B virus (HBV). Chronic HBV infection (CHB) leads to liver diseases such as cirrhosis, hepatocellular carcinoma (HCC), and steatosis. Despite its seriousness in terms of public health, the pathogenic mechanism of how CHB leads to liver diseases, especially cirrhosis and steatosis, remains unclear. We studied the role of HBV polymerase (HBp) reverse transcriptase (RT) activity in association with the pathogenesis of liver diseases in CHB by developing transgenic mice expressing HBp or the RT domain of HBp. Thorough pathological, serological, and histological analyses of the transgenic mice, as well as mechanistic studies, were conducted. All of the transgenic mice expressing RT in their livers developed early cirrhosis with steatosis by 18 months of age, and 10% developed HCC. The RT activity of HBp stimulates coordinated proapoptotic and proinflammatory responses involving the caspase-9, caspase-3, and caspase-1 pathways that might lead to the development of cirrhosis, HCC, and steatosis. The animal model described here should prove useful for elucidating the molecular events in the CHB-induced liver diseases.

Large-scale production and structural and biophysical characterizations of the human hepatitis B virus polymerase

Hepatitis B virus (HBV) is a major human pathogen that causes serious liver disease and 600,000 deaths annually. Approved therapies for treating chronic HBV infections usually target the multifunctional viral polymerase (hPOL). Unfortunately, these therapies--broad-spectrum antivirals--are not general cures, have side effects, and cause viral resistance. While hPOL remains an attractive therapeutic target, it is notoriously difficult to express and purify in a soluble form at yields appropriate for structural studies. Thus, no empirical structural data exist for hPOL, and this impedes medicinal chemistry and rational lead discovery efforts targeting HBV. Here, we present an efficient strategy to overexpress recombinant hPOL domains in Escherichia coli, purifying them at high yield and solving their known aggregation tendencies. This allowed us to perform the first structural and biophysical characterizations of hPOL domains. Apo-hPOL domains adopt mainly α-helical structures with small amounts of β-sheet structures. Our recombinant material exhibited metal-dependent, reverse transcriptase activity in vitro, with metal binding modulating the hPOL structure. Calcomine orange 2RS, a small molecule that inhibits duck HBV POL activity, also inhibited the in vitro priming activity of recombinant hPOL. Our work paves the way for structural and biophysical characterizations of hPOL and should facilitate high-throughput lead discovery for HBV.

IMPORTANCE

The viral polymerase from human hepatitis B virus (hPOL) is a well-validated therapeutic target. However, recombinant hPOL has a well-deserved reputation for being extremely difficult to express in a soluble, active form in yields appropriate to the structural studies that usually play an important role in drug discovery programs. This has hindered the development of much-needed new antivirals for HBV. However, we have solved this problem and report here procedures for expressing recombinant hPOL domains in Escherichia coli and also methods for purifying them in soluble forms that have activity in vitro. We also present the first structural and biophysical characterizations of hPOL. Our work paves the way for new insights into hPOL structure and function, which should assist the discovery of novel antivirals for HBV.