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| __NOTOC__
| | #REDIRECT[[Hepatitis B virus]] |
| {{Hepatitis B}}
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| {{CMG}}; {{AE}} {{JS}}
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| ==Overview==
| | [[Category:Emergency mdicine]] |
| The [[hepatitis B virus]] is an [[hepadnavirus]] with a DNA [[genome]]. The viral particle consists of an outer [[lipid]] envelope and an [[icosahedron|icosahedral]] [[nucleocapsid]] core composed of [[protein]]. The nucleocapsid encloses the viral DNA and a DNA [[polymerase]] that has [[reverse transcriptase]] activity. It shows [[tropism]] for [[hepatocytes]] and humans are its only [[natural reservoir]].
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| ==Taxonomy==
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| [[Viruses]]; Retro-transcribing viruses; [[Hepadnaviridae]]; Orthohepadnavirus
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| ==Biology==
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| {| style="float: right;"
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| | [[File:Hepatitis B virus.png|200px|thumb|none|Hepatitis B virions <SMALL>Courtesy: ''[http://www.who.int/en/ World Health Organization]''<ref>{{Cite web | title = http://www.who.int/en/ | url = http://www.who.int/en/}}</ref></SMALL>]]
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| | [[File:HBV Genome.png|200px|thumb|none|The genome organisation of HBV. The genes overlap. <SMALL>Courtesy: ''[Wikimedia Commons]''<ref>{{Cite web | title = http://commons.wikimedia.org/wiki/File:HBV_Genome.svg | url = http://commons.wikimedia.org/wiki/File:HBV_Genome.svg}}</ref></SMALL>]]
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| The [[hepatitis B virus]], an [[hepadnavirus]], is a 42 nm partially double stranded [[DNA]] virus, composed of a 27 nm nucleocapsid core (HBcAg), surrounded by an outer [[lipoprotein]] coat (also called envelope) containing the surface antigen ([[HBsAg]]).
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| [[Hepatocytes]] infected in vivo by [[hepadnaviruses]] produce an excess of noninfectious viral [[lipoprotein]] particles.
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| ===Genome===
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| *[[HBV]] virion [[DNA]] is a relaxed circular, partially duplex molecule of 3.2 kb, whose circularity is maintained by 5' cohesive ends.<ref name=WHO>{{cite web | title = Hepatitis B | url = http://www.who.int/csr/disease/hepatitis/HepatitisB_whocdscsrlyo2002_2.pdf }}</ref>
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| *A virion-associated [[polymerase]] can repair gaps and generate a fully duplex [[genome]]. Negative strand [[DNA]] is the template for the synthesis of the viral [[mRNA]] transcripts. HBV DNA has a very compact coding organization with four partially overlapping open reading frames (ORFs) that are translated into seven known proteins.<ref name=WHO>{{cite web | title = Hepatitis B | url = http://www.who.int/csr/disease/hepatitis/HepatitisB_whocdscsrlyo2002_2.pdf }}</ref>
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| *Noncoding regions are not present.<ref name=WHO>{{cite web | title = Hepatitis B | url = http://www.who.int/csr/disease/hepatitis/HepatitisB_whocdscsrlyo2002_2.pdf }}</ref>
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| *Four separate viral [[promoters]] have been identified, driving expression of:<ref name=WHO>{{cite web | title = Hepatitis B | url = http://www.who.int/csr/disease/hepatitis/HepatitisB_whocdscsrlyo2002_2.pdf }}</ref>
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| ::*Genomic, P, and pre-C and C RNAs
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| ::*L protein mRNA
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| ::*M and S protein mRNAs
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| ::*X protein mRNA
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| They are referred to as the [[genomic]], pre-S1, S, and X [[promoters]], respectively.
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| * Two major classes of transcripts exist:
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| :* The subgenomic [[RNA]]s function exclusively as messenger [[RNA]]s ([[mRNA]]s) for translation of envelope and X [[proteins]].
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| :* The [[genomic]] [[RNA]]s are bifunctional, serving as both the templates for viral [[DNA]] synthesis and as messages for ORF pre-C, C, and P translation.<ref name=WHO>{{cite web | title = Hepatitis B | url = http://www.who.int/csr/disease/hepatitis/HepatitisB_whocdscsrlyo2002_2.pdf }}</ref><ref>{{cite book | last = Plotkin | first = Stanley | title = Vaccines | publisher = W.B. Saunders Co | location = Philadelphia | year = 1999 | isbn = 0721674437 }}</ref>
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| * ORF P encodes the viral [[polymerase]] and the terminal [[protein]] found on minus strand DNA. ORF C encodes the structural protein of the nucleocapsid and the HBeAg, and ORF S/pre-S encodes the viral surface glycoproteins. The product of ORF X is a poorly understood regulatory protein that enhances the expression of heterologous and homologous cellular genes in trans.
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| * The three envelope [[glycoproteins]] are not distributed uniformly among the various [[HBV]] particle types. Subviral 22 nm particles are composed predominantly of ''S proteins'', with variable amounts of ''M proteins'' and few or no ''L proteins''. Virus particles are enriched for L proteins. L proteins carry the receptor recognition domain, which allows efficient binding to cell surface receptors.
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| * HBcAg is the most conserved [[polypeptide]] among the mammalian [[hepadnaviruses]] with 68% homology between [[HBV]] and GSHV and 92% between GSHV and WHV. Furthermore, it plays important roles in the encapsidation of the viral pregenomic [[RNA]]. The [[polymerase]] protein is quite immunogenic during both acute and chronic infection.<ref name=WHO>{{cite web | title = Hepatitis B | url = http://www.who.int/csr/disease/hepatitis/HepatitisB_whocdscsrlyo2002_2.pdf }}</ref>
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| ===Serotypes and Genotypes ===
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| The [[virus]] is divided into:
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| * Four major [[serotype]]s (adr, adw, ayr, ayw) based on antigenic [[epitope]]s presented on its envelope [[proteins]]
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| * Ten [[genotype]]s (A-J) according to overall [[nucleotide]] sequence variation of the [[genome]]
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| The genotypes have a distinct geographical distribution and are used in tracing the evolution and transmission of the virus.
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| Differences between [[genotype]]s affect:<ref name="pmid15752827">{{cite journal |author=Kramvis A, Kew M, François G |title=Hepatitis B virus genotypes |journal=[[Vaccine]] |volume=23 |issue=19 |pages=2409–23 |year=2005 |month=March |pmid=15752827 |doi=10.1016/j.vaccine.2004.10.045 |url=http://linkinghub.elsevier.com/retrieve/pii/S0264-410X(04)00849-7 |accessdate=2012-02-08}}</ref><ref name="pmid8666521">{{cite journal |author=Magnius LO, Norder H |title=Subtypes, genotypes and molecular epidemiology of the hepatitis B virus as reflected by sequence variability of the S-gene |journal=[[Intervirology]] |volume=38 |issue=1-2 |pages=24–34 |year=1995 |pmid=8666521 |doi= |url= |accessdate=2012-02-08}}</ref>
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| * Disease severity
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| * Course of the disease
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| * Likelihood of [[complications]]
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| * Response to treatment and possibly [[vaccination]]
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| ==Structure==
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| [[Hepatitis B virus]] ([[HBV]]) is a member of the [[Hepadnaviridae|Hepadnavirus family]].<ref name=Baron>{{Cite book |author=Zuckerman AJ |chapter=Hepatitis Viruses |title=Baron's Medical Microbiology |editor=Baron S, ''et al'' |edition=4th |publisher=University of Texas Medical Branch |year=1996|url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.3738|isbn=0-9631172-1-1 }}</ref>
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| The viral particle ([[virion]]) consists of an outer [[lipid]] envelope and an [[icosahedron|icosahedral]] [[nucleocapsid]] core composed of [[protein]]. The nucleocapsid encloses the viral DNA and a DNA [[polymerase]] that has [[reverse transcriptase]] activity.<ref name="pmid15192795">{{cite journal |author=Locarnini S |title=Molecular virology of hepatitis B virus |journal=[[Seminars in Liver Disease]] |volume=24 Suppl 1 |issue= |pages=3–10 |year=2004 |pmid=15192795 |doi=10.1055/s-2004-828672 |url=http://www.thieme-connect.com/DOI/DOI?10.1055/s-2004-828672 |accessdate=2012-02-08}}</ref>
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| The outer envelope contains embedded proteins that are involved in viral binding of, and entry into, susceptible cells. The virus is one of the smallest enveloped animal viruses, with a virion diameter of 42nm, but pleomorphic forms exist, including filamentous and spherical bodies lacking a core. These particles are not infectious and are composed of the lipid and protein that forms part of the surface of the virion, which is called the surface antigen ([[HBsAg]]), and is produced in excess during the life cycle of the virus.<ref name="pmid3014045">{{cite journal |author=Howard CR |title=The biology of hepadnaviruses |journal=[[The Journal of General Virology]] |volume=67 ( Pt 7) |issue= |pages=1215–35 |year=1986 |month=July |pmid=3014045 |doi= |url=http://vir.sgmjournals.org/cgi/pmidlookup?view=long&pmid=3014045 |accessdate=2012-02-08}}</ref>
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| The protein of the virion coat is termed "surface antigen" or HBsAg. It is sometimes extended as a tubular tail on one side of the virus particle. The surfaceantigen is generally produced in vast excess, and is found in the blood of infected individuals in the form of filamentous and spherical particles. Filamentous particles are identical to the virion "tails" - they vary in length and have a mean diameter of about 22nm. They sometimes display regular, non-helical transverse striations.<ref name=WHO>{{cite web | title = Hepatitis B | url = http://www.who.int/csr/disease/hepatitis/HepatitisB_whocdscsrlyo2002_2.pdf }}</ref>
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| ==Tropism==
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| [[Hepatitis B virus]] shows tropism for [[hepatocytes]].
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| ==Natural Reservoir==
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| The [[natural reservoir]] for [[hepatitis B virus]] is man. Closely related [[hepadnaviruses]] have been found in woodchucks and ducks, but they are not [[infectious]] for humans.<ref name=WHO1>{{cite web | title = Hepatitis B | url = http://www.who.int/csr/disease/hepatitis/whocdscsrlyo20022/en/index3.html }}</ref>
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| == References ==
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| {{Reflist|2}}
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| {{STD/STI}}
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| [[Category:Hepatitis|B]]
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| [[Category:Viruses]]
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| [[Category:Infectious disease]] | | [[Category:Infectious disease]] |
| | [[Category:Hepatology]] |
| [[Category:Gastroenterology]] | | [[Category:Gastroenterology]] |
| [[Category:Mature chapter]]
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| [[Category:Disease]]
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| {{WH}}
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| {{WS}}
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