The Hepatitis B Virus

The Hepatitis B virus (HBV) is a mostly double-stranded DNA virus in the Hepadnaviridae family. One of five hepatitis viruses, HBV causes acute and chronic hepatitis in humans. Despite the current availability of an effective vaccine, almost 1.2 million people worldwide still die each year from HBV related diseases (CDC). An acute HBV infection usually causes only mild symptoms and the majority of infected adults successfully clear the virus and acquire life-long immunity. In acute hepatitis, it takes about 1 to 6 months from the time of infection for the disease to manifest itself. Early symptoms include nausea and vomiting, loss of appetite, fatigue, and muscle and joint aches. Jaundice, together with dark urine and light stools, follows. Only about 1 percent of patients infected with hepatitis B die due to liver damage in this early stage (Poynard, 2002). The risk of becoming chronically infected depends on the age at the time of infection. More than 90 percent of newborns, 50 percent of children,  5 percent of adults infected with HBV develop chronic hepatitis. Those who are unable to produce an effective immune response allow the virus to replicate for long periods in their livers, causing chronic hepatitis HBV infection, cirrhosis of the liver, and hepatocellular carcinoma (HCC) (Yen, 2002). Transmission of the hepatitis B virus is through contact with blood and other bodily fluid. Chronic hepatitis B is treated with a manufactured form of interferon, a protein made naturally by the body to boost the immune system and to regulate other cell functions (CDC).  A vaccine is available to prevent HBV which originally consisted of purified HBV surface antigens (HBsAg) prepared from the serum of carriers and wash chemically treated to kill any contaminating viruses, but the current vaccine is genetically engineered HBsAg produced in yeasts (Yen, 2002).

             Liver Cancer

Genome: The HBV virion genome is circular and approximately 3.2 kb in size and consists of DNA that is mostly double stranded. It has compact organization, with four overlapping reading frames running in one direction and no noncoding regions. The minus strand is unit length and has a protein covalently attached to the 5' end. The other strand, the plus strand, is variable in length, but has less than unit length, and has an RNA oligonulceotide at its 5' end. Thus neither DNA strand is closed and circularity is maintained by cohesive ends (Strauss, 2002). The four overlapping open reading frames (ORFs) in the genome are responsible for the transcription and expression of seven different hepatitis B proteins. The transcription and translation of these proteins is through the used of multiple in-frame start codons. The HBV genome also contains parts that regulate transcription, determine the site of polyadenylation and a specific transcript for encapsidation into the nucleocapsid. The genomic arrangement of the hepatitis B virus family makes it unique among viruses. The unusually packaged may indicate that the method of replication employed by HBV is not of conserved DNA replication (Garces, HBVP).

 
                                                                                                                                                                HBV Genome

Life Cycle: In order to reproduce, the hepatitis B virus, must first attach onto a cell which is capable of supporting its replication. Although hepatocytes are known to be the most effective cell type for replicating HBV, other types of cells in the human body have be found to be able to support replication to a lesser degree. The initial steps following HBV entry are not clearly defined although it is known that the virion initially attaches to a susceptible hepatocyte through recognition of cell surface receptor that has yet to be indified (Garces, HBVP). The DNA is then enters into the nucleus, where it is known to form a convalently closed circular form called CCC DNA. The (-) strand of such CCC DNA is the template for transcription by cellular RNA polymerase II of a longer-than-genome-length RNA called the pregenome and shorter, subgenomic transcripts, all of which serve as mRNAs. The shorter viral mRNAs are translated by ribosomes attached to the cell's endoplasmic reticulum and the proteins that are destined to become HBV surface antigens in the viral envelope are assembled. The pregenome RNA is translated to produce a polymerase protein, P, which then binds to a specific site at the 3' end of its own transcript, where viral DNA synthesis eventually occurs. Occuring at the same time as capsid formation, the RNA-P protein complex is packaged and reverse transcription begins.  At early times after the infection, the DNA is recirculated to the nucleus, where the process is repeated, resulting in the the accumulation of 10 to 30 molecules of CCC DNA and an increase in viral mRNA concentrations (Flint et al., 765).
 

                                                                                                                                                                                                                                                                
                                                            HBV Life Cycle

HBV Particle Types: The hepatitis B virion, also known as the Dane particle, is the one infectious particle found within the body of an infected patient. This virion has a diameter of 42nm and its outer envelope contains a high quantity of hepatitis b surface proteins.  The envelope surrounds the inner nucleocapsid which is made up of 180 hepatitis B core proteins arranged in an icosahedral arrangement. The nucleocapsid also contains at least one hepatitis b ploymerase protein (P) along with the HBV genome.
    In infected people, virions actually compose a small minority of HBV-derived particles. Large numbers of smaller subviral particles are also present, that uusually outnumber the virions by a ratio of 100:1. These two other subviral particles, the hepatitis B filament and the hepatitis B sphere, are often referred to as a group named surface antigen (HBsAg) particles. They are both 22nm in diameter and are totally composed of hepatitis B surface proteins. The sphere contains both middle and small hepatitis surface proteins whereas the filament also includes large hepatitis B surface protein. The absence of the hepatitis B core, polymerase, and genome causes these particles to have a non-infectious nature. High levels of these non-infectious particles can be found during the acute phase of the infection. Since the non-infectious particles present the same sites as the virion, they induce a significant immune response and are thought to be non-advantagous for the virus. However, it is also believed that the presence of high levels of non-infectious particles may allow the infectious viral particles to travel undetected by antibodies through the blood stream (Garces, HBVP).

                                                                                                                                                                                                                                                                              Hepatitis B Virion
 

Hepatitis B Antigens: There are three different types of hepatitis b antigens encoded by the HBV genome.

• Hepatitis B Surface antigen (HBsAg)- There are three different types of hepatitis B surface antigens; small hepatitis B surface antigen (HBsAg or SHBsAg), middle hepatitis B surface antigen (MHBsAg), and large hepatitis B surface Antigen (LHBsAg).  HBsAg is the smallest protein of the hepatitis B surface proteins and has historically been known as the Australia antigen (Au antigen). It is very hydrophobic, containing four-transmembrane spanning regions. This protein is the prime constituent of all hepatitis b particle forms and appears to be manufactured by the virus in high quantities. It also contains a highly antigenic epitope which may be responsible for triggering immune response.  Regardless of the high antigeniticy and prevalence of these particles, the immune system appears basically oblivious to their presence. Reduced production of HBsAg leads to intracellular retention of the virus. MHBsAg contains an additional amino-acid domain and appears to reside extracellularly. Although some believe that the MHBsAg is responsible for HBV attachment, MHBsAg is not required for HBV infectivity and therefore it is more likely that is contributes to viral attachment as a secondary mechanism. LHBsAg is the largest of the HBV surface proteins, containing three domains within the HBV encoding region. HBV is believed to be involved in liver attachment due to its variability among patients. It is also believed to be responsible for mediating viral attachment into host cells, although this has yet to be confirmed experimentally (Yen, 2002)

                                                                                                                                                                                                                                                                              HBsAg

• Hepatitis B Core Antigen (HBcAg)- The only HBV antigen that can not be detected directly by blood test, this antigen can only be isolated by analyzing an infected hepatocyte. A 185 amino acid protein is expressed in the cytoplasm of infected cells, they are highly associated with nucleocapsid assembly (Strauss, 2002).

• Hepatitis B e Antigen (HBeAg)- The e antigen is named due to its "early" appearance during an acute HBV infection. Thought to be located in the core structure of the virus molecule, this antigen can be detected by blood test. If found its usually indicative of complete virus particles in circulation. (Strauss, 2002)