Dependovirus
Dependovirus ( synonym: adeno- associated virus group) is a genus of Parvoviridae, a family of DNA viruses. Known representative of dependoviruses is the adeno-associated virus ( AAV). The replication of dependoviruses is dependent on a coinfection with a helper virus, from where the name originates. When helper viruses adenovirus, or herpes viruses are used.
Properties
Virion
The virions of dependoviruses have a non-enveloped icosahedral capsid of about 22 nm in diameter. The capsid is composed of the three capsid proteins VP1 -3 consists of 60 proteins with Triangulationszahl of one. Each capsid proteins VP1 has five, five proteins VP2 and VP3 proteins 50. This reflects the viral DNA is single-stranded.
Genome
The genome is single-stranded DNA of 4.7 kilobases ( Kb ) in length with a different polarity, and has two open reading frames. At the 3 ' end is the cap gene for the capsid proteins, which proteins VP1 or VP2 and VP3 results from alternative splicing. The second gene encodes rep for the replication- related proteins. Alternative splicing caused either Rep78, Rep68, Rep52 or Rep40, wherein the numbering is done according to the electrophoretic mobility. Six mRNA are formed from the genome (4.2 Kb, 3.9 Kb, 3.6 Kb, 3.3 Kb, 2.6 Kb and 2.3 Kb in length), all carrying a poly -A tail.
Which at the ends of the genome of the inverted repeat of about 145 bases form a T-shaped secondary structure due to the palindromic DNA sequence of the first 125 bases. The inverted repeat is used both as the starting point of replication as well as for the insertion of the viral DNA into the host DNA on chromosome 19, the complementary regions of inverted repeats having a free hydroxyl group at the 3'- end of replication. The 3 'end is used as a primer for synthesis of the leading beach, then double transitional forms are formed.
Dependoviruses are replication deficient themselves, that they need a helper virus. The proteins necessary for replication of the adenoviruses have been identified and are used for the preparation of dependoviralen vectors. Furthermore, a replication of the dependoviruses by UV radiation, cycloheximide, aphidicolin, topoisomerase inhibitors, hydroxyurea and various chemical carcinogens can be triggered.
The insertion of the viral DNA is preferably carried on chromosome 19 in troponin genes. The genes contained are therefore increasingly read in muscle cells. The insertion and the genetic repression produced the viral latency and persistent infection. Dependoviruses therefore be used as viral vectors in gene therapy, where a long-term expression of the DNA of the recombinant protein is desired in a vector with no production of viral proteins. In addition, the genomes of certain AAV can also be present as an episome.
Proteome
The proteome of dependoviruses includes, occurring in the three virion structural proteins VP1 -3 and the non-structural proteins and helicases Rep78, Rep68, Rep52 and Rep40. Rep78 is a helicase and a repressor protein on rep cohesive element ( RBE ) in the p5 promoter, whereby the production of Rep78 and Rep68 in the absence of a helper virus is largely ceased and instead the insertion takes place. Rep52 is a repressor of the p19 promoter which controls the gene expression of Rep52 and Rep40.
Replication cycle
After adsorption of the virions to the cell membrane invagination occurs by endocytosis. During maturation of the endosome to the lysosome, the penetration of the endosomal membrane and the release of the genome takes place in the cytosol. The viral genome is imported into the nucleus, after which the gene expression of the rep gene is carried out and the replication of the viral genome is introduced by a double stranded DNA intermediate. Through gene expression of the cap genes, the components of the virion to be completed. After the binding of single-stranded DNA is positive or negative polarity of the capsid proteins, thereby assemble the virions. Dependoviruses leave the cell after its destruction by the lytic helper viruses.
System
For genus Dependovirus include:
- Adeno- associated virus -1
- Adeno- associated virus -2
- Adeno- associated virus -3
- Adeno- associated virus -4
- Adeno- associated virus -5
- Avian adeno- associated virus
- Bovine adeno- associated virus
- Canine adeno- associated virus
- Duck parvovirus
- Equine adeno- associated virus
- Goose parvovirus
- Ovine adeno- associated virus
Host range
Dependoviruses occur in different vertebrates. They are primarily dependent upon the presence of a helper virus. There is no associated with dependoviruses infectious diseases, where antibodies are produced during an infection. More than 90 % of adults are seropositive for AAV, ie they are or were infected with AAV.
Gene Therapy
Various dependoviruses integrate their viral DNA into the genome of their host, preferably in chromosome 19, Thus dependoviruses such as the AAV used in the gene therapy viral vector.
As viral vectors used dependoviruses have a relatively small DNA - packaging capacity. The adeno-associated viral vectors have a maximum length of eight DNA kilobases ( single-stranded ), which can still be packaged into virions. When using self - complementary DNA, the maximum length of a transgene is lowered to five kilobases in length about the virus titer is reduced. As with all viral vectors occur in the context of the innate and adaptive immunity vector antibodies against the viral proteins (in this case against the capsid proteins ). Therefore, one can use a serotype per patient only once to prevent premature degradation of the vector or excessive immune reactions, so that continuously new serotypes are being developed. However, insertions were found by AAV vectors in transcriptionally active regions of the genome outside the Troponingene, which may contribute to the formation of tumors.