Hepatitis-C-Virus

The hepatitis C virus is an enveloped, single-stranded RNA virus with positive polarity (ss ( ) RNA), and is the causative agent of hepatitis C. It is the only previously known RNA virus (without a retrovirus to be ) can cause a chronic infectious disease. At diagnosis, diseases and prevention, see Hepatitis C.

Although it was known since the first reports in 1974 about an infectious agent as the causative agent of the then so-called " non-A, non-B hepatitis ," the virus for a long time was not identified. Only after the first application of the cloning of genomic fragments from the serum of artificially with HCV infected chimpanzee came in 1989 and 1990, the discovery of the sequencing of the then hepatitis C virus called pathogen. It is also the first virus whose genome sequence and viral proteins subject to a comprehensive patent protection, which greatly hampered the development of new test methods. Patent holder is, after the acquisition of Chiron Corporation in 2006, the pharmaceutical company Novartis.

Hepatitis C virus ( HCV) belongs together with the hepatitis B virus ( HBV), Epstein- Barr virus ( EBV), human papillomavirus (HPV), human T- lymphotropic virus 1 (HTLV -1) and the Human herpes virus 8 (HHV -8, Kaposi 's sarcoma herpesvirus, KSHV ) to a group of human viruses that are responsible for 10 to 15 percent of all cancers worldwide.

Molecular Biology

System

By sequence comparisons and homology to other viral proteins, the HCV has been classified in the family Flaviviridae, which initially was not clear assignment to the genus flavivirus or pestivirus. Phylogenetically the HCV appears to be more closely related to the otherwise only animal pathogenic pestiviruses, this place hydrophobicity of the proteins as well as the genomic organization close. It appeared taxonomically useful HCV as the only representative to group into a new genus hepacivirus. Very closely related to HCV, but not assigned to any genus, the GB viruses ( GB virus A and B in New World monkeys and tamarins ) especially the in humans occurring, but non-pathogenic GB virus C.

Variability and subtypes

The variability of HCV is also compared to other single-stranded RNA viruses relatively high. To be found at the genomic level when comparing different isolates up to 40% deviations. Therefore, the HCV is divided (1-6) in six genotypes, with a single genotype must have agreed, at least 72 % similarity at the amino acid level. These genotypes are further subdivided into 30 subtypes ( 1a, 1b, 3a, etc. ), the homology is at the amino acid level 73-86 % within the subtypes.

Due to the inaccuracy of reading of the viral RNA polymerase with a mutation rate of up per nucleotide and replication, develop in an infected host other deviations from the original sequence is referred to as quasispecies. Rapid formation of the quasi-species has been associated with the ability of the HCV in combination, to cause chronic infection. The constantly emerging variants soft (similar to HIV infection) from the access of the immune system; this mechanism is also called immune evasion.

Structure

The single-stranded RNA genome (negatively charged as an acid ) is located in the virion at the core protein (basic, positively charged) before attached. The core protein is most likely not a classic capsid with a fixed symmetry (eg icosahedral ). Free capsids could so far not be displayed or stored together credible. The core protein is anchored in the inside of the viral envelope and a transmembrane helix in the complex of the two envelope proteins of E1/E2 ( envelope Engl. Shell ) attached. The transmembrane portion of the HCV core protein corresponds to the most likely ( large ), M (matrix) protein in virus of the genus Flavivirus (e.g. yellow fever virus, FSME - virus). This M- protein is cleaved from the core protein and clothes the viral envelope from the inside out; the HCV apparently omitted, this cleavage of an M- protein.

HCV is extremely difficult to represent in the electron microscope, does not have a publication to show a final Proven picture. Some pictures show particles having anticipated size of about 50 nm, which are visible after certain cleaning processes (see above). This difficult visualization probably due to a structural instability and sensitivity to normal preparation methods of HCV is a reason for its very late discovery despite intensive search.

In the genome of the HCV there is a large open reading frames ( open reading frame, ORF) of the a single polyprotein is read 3008-3037 amino acids in length. This protein is cleaved by cellular proteases ( signalase ) the ER membrane and viral proteases already during the translation into the structural proteins (Core, E1, and E2 ) and non-structural proteins ( NS2 through NS5 ). This means that in addition to the existing viral genome, no further messenger RNA is transcribed. The table below lists these proteins with their (partly still unclear ) function, the molecular weight and the amino acid position in the polyprotein on:

Oligomerization and lipid binding, activation and inhibition of cellular genes ( leucine zipper motif )

Hypervariable region ( HVR1 ), antibody -binding region

The open reading frame of about 9.5 kb in length HCV genome is noncoding regions of two (NCR, Eng. Non-coding region) flanked, which play a regulatory role during virus replication. Before the start codon ( nucleotide position defined as 1) is located approximately 340 bases long 5'- NCR, which shows the greatest sequence match and a complex folding of the RNA strand in the HCV isolates. At the end of the genome to find the 250-300 base long 3'- NCR. This consists essentially of a polymeric uracil ( polyU ) or adenine ( polyA ) and a highly conserved nucleotide sequence of 98 bases in length, referred to as X -tail.

Replication

The detection of the liver cell ( hepatocyte) as the target cell and entry into the cell to be forwarded with high probability of one or more receptors. The receptor for HCV is not clearly identified; some evidence suggests that binding to the beta- lipoprotein receptor, CD81 receptor, HDL receptor ( SRB1 ). Inhibition of CD81 and SRB1 the inclusion of E1/E2 could be blocked in hepatocytes, but none of these receptors alone is sufficient to infect a cell. In addition, none of these receptors is liver-specific.

In analogy to other members of the Flaviviridae, it is believed that the HCV by itself abschnürendes vesicles of the cell membrane ( endosome ) enters the cell, where the fusion of the endosomal membrane with the viral envelope and thus the release of the core protein - RNA complex in the cytosol comes. The readable as mRNA HCV genome first passes directly to the ribosomes of the rough ER, where a first synthesis of viral proteins occurs. This first step is needed because the virus first needs a few molecules of the viral RNA - dependent RNA polymerase of NS5B, and this enzyme is not present in the cell.

To increase the amount of available viral mRNA follows the further step of multiplication of the viral RNA. These synthesized the NS5B polymerase a strand ( negative strand ) of the viral ( ) RNA as a template ( template) for the subsequent synthesis of additional plus strands. As an initiation signal for the NS5B polymerase are believed to act specifically folded portions in the two noncoding regions of the viral RNA. There are indications that close both ends of the RNA genome by binding to cellular proteins to form a ring and thus a replication complex is formed in terms of a rolling circle mechanism. Replicating HCV in cell culture striking membrane structures were found in the electron microscope, which are regarded as the HCV replication complex. Known as " membranous web" ( membranous network) designated structures are induced by the hydrophobic membrane protein NS4B. Very similar specific membrane changes can be found as the so-called Viroplasma at all ( ) ssRNA viruses, have been investigated in this regard.

After sufficient synthesis of viral mRNA now begins the translation of viral proteins, mainly of the structural proteins. For initiation of translation at the ribosomes use eukaryotic mRNAs usually a modification of the 5'-end, the so-called cap. However, HCV has by the folding of the 5'- NCR a special structure for cap-independent initiation, internal ribosomal entry site called (English Internal ribosomal entry site, IRES). Wherein the HCV-RNA requires no cellular factors to bind to the ribosomes, and for the start of protein synthesis. The IRES is normally only found in some picornaviruses such as poliovirus.

Even during the translation, the structural proteins of cellular proteases ( signalase ) are cut from the resulting Polyproteinstrang; the envelope proteins E1 and E2 to enter the lumen of the ER, are deposited in the membrane and to be glycosylated. The core protein is deposited from the outside in the ER membrane and binds, due to its charging characteristics viral mRNA. It now comes to the packaging of the core RNA complex and abscission ( budding, Eng. Budding ) of the HCV particle in the lumen of the ER. The new virions leave now by cellular secretion via the Golgi apparatus to the cell.

Hosts

Man is the only natural host of HCV. In the study of HCV in the 1980s and the attempts to prepare a vaccine from 1990 apes were (mostly chimpanzees) infected artificially. These are just as susceptible to infection, but chronic infection is rare. This then predominantly conducted in the U.S. trials are prohibited in Europe for ethical reasons.

Despite intensive experiments since 2004 is a virus -producing cell culture system for HCV is available that should greatly accelerate the study of the replication of the virus in the cell and the development of new antiviral agents.

Epidemiology

The prevalence ratio ( number of patients in relation to the number of examinees ) is 0.031 world, in Europe and the United States less than 0.02. In Japan, Mongolia and Egypt it is at 0,181, in the latter caused by errors in treatment against schistosomiasis (contaminated needles ). Worldwide, the WHO goes from 170 million chronic carriers of the virus. The number of hepatitis C patients is much lower in some countries.

The distribution of genotypes and subtypes also follows geographical patterns: in Europe and America are predominantly found the otherwise occurring worldwide subtypes 1a, 1b and 3a (mainly in drug addicts ); in Asia, the subtype 1b is the dominant, in Africa, the genotype 4, genotype 5 in South Africa and Hong Kong and Vietnam, the genotype 6 genotypes 2 and 3 are located worldwide, but to a much lesser extent.

The period of the phylogenetic separation of the genotypes could be estimated by means of the sequence variability and the calculated mutation frequency to about 500 years. It therefore seems likely that the diversification of genotypes was associated with the advent of the global voyages around the year 1500.

Transmission

The virus is transmitted parenterally, that is, primarily by blood and blood products; sexual transmission is rare. Thus, the following risks are defined: dialysis (especially before 1991 ), intravenous drug abuse ( intravenous drug users, IVDU ), tattooing and piercing. About 30% of patients in the transmission is unknown.

Swell

• H.-J. Thiel et al. Genus hepacivirus. In: CM Fauquet, MA Mayo et al.: Eighth Report of the International Committee on Taxonomy of Viruses. London, San Diego, pp. 993-998 2005 ISBN 0-12-249951-4
• David M. Knipe, Peter M. Howley ( ed. -in- chief): Fields' Virology. 5th edition, 2 volumes Philadelphia 2007, pp. 1113-1126 and 1253-1291 ISBN 0-7817-6060-7