Virus, helper

Very few self-sufficient viruses have only 60 protein chains in their shells. The satellite viruses do not themselves encode all of the functions required for their replication and are therefore not self-sufficient. The first satellite virus to be discovered, satellite tobacco necrosis virus, which is also one of the smallest known with a diameter of 180 A, has a protein shell of 60 subunits. This virus cannot replicate on its own inside a tobacco cell but needs a helper virus, tobacco necrosis virus, to supply the functions it does not encode. The RNA genome of the satellite virus has only 1120 nucleotides, which code for the viral coat protein of 195 amino acids but no other protein. With this minimal genome the satellite viruses are obligate parasites of the viruses that parasitize cells. 

The gag, pol, and env genes of the retroviral genome, required for retroviral replication and assembly of viral particles, can be replaced with foreign DNA. To assemble viruses that contain the recombinant genetic information, researchers must introduce the DNA into cultured cells that are simultaneously infected with a helper virus that has the genes to produce viral particles but lacks the [Pg.334]

RNA copies of recombinant viruses are produced in cells containing a helper virus and packaged into viral particles. 

Very little is known about viroids except that when they are introduced into a host cell they replicate without the assistance of a helper virus. They are not translated into pioteins and, presumably, their replication must rely entirely upon the enzyme systems of the host. 

Production of vector virions requires transfection of the vector construct in plasmid form together with the introduction on separate plasmids and/or helper viruses of both the AAV genes and the helper virus genes required for a productive AAV infection (Xiao et al., 1998) (Fig. 1.1). Variations have included producing vectors... 

AAV2 and its helper viruses like HSV, AAY2 interacts with the cell surface via heparin sulfate, and AAV internalization is mediated by aV/ 5 integrin, which is also required by adenovirus for uptake. Serotypes other than AAV2 exhibit different tropisms and interact with different cell surface molecules. Some of the cell surface receptors have been identified for AAV3 (HSPG Rabinowitz et al.,... 

Other protocols involve cell lines with integrated rep/cap cassettes (Clark et al., 1995 Gao et al., 1998 Liu et al., 1999 Chadeuf et al., 2000 Mathews et al., 2002 Qiao et al., 2002a,b) infected with adenovirus or, alternatively, a recombinant herpesvirus system has been used to provide both helper virus function and rep/cap (Conway et al., 1997, 1999). In a switch away from using mammalian cell and helper virus production systems, rAAV vectors have been made in insect cells where the AAV genes are expressed under the control of insect promoters and the traditional helper virus gene products are not required (Urabe et al., 2002). Stable producer cell... 

Assay for infectious helper virus contamination of rAAV... 

Matsushita, T. et al. (1998). Adeno-associated virus vectors can be efficiently produced without helper virus. Gene Ther. J, 938-945. 

Donahue RE, Kessler SW, Bodine D, McDonagh K, Dunbar C, Goodman S, Agricola B, Byrne E, Raffeld M, Moen R, Bacher J, Zsebo KM, Nienhuis AW. Helper virus induced T cell lymphoma in nonhuman primates after retroviral mediated gene transfer. J Exp Med 1992 176 1125-35. 

Cumulative experience from 106 monkey-years and 23 human-years of retroviral gene therapy had revealed no side-effects or malignancies. Recently, however, three monkeys used in clinical trials developed T cell lymphomas which were traced to a helper virus-contaminated retroviral vector preparation (Anderson, 1992a). Contamination of defective viral vector stocks by infectious viruses produced in helper cells was a serious problem with the early version of the helper cells. New helper cell lines which greatly reduce the probability of virus production have been developed (Miller, 1990). Eurther, it is also imperative to test the lots of vector virus preparations used in therapy for the presence of other infectious viruses. The probability... 

In the second strategy cells are transfected with plasmids that carry the gene of interest under the control of a vaccinia virus promoter and superinfected with a vaccinia helper virus. In this case, expression occurs in trans by factors provided by the helper virus. The advantage of this... 

Satellite viruses are those that are dependent for their own replication on some (catalytic) activity encoded in another helper virus that coinfects the host cell. The structures of three plant ssRNA satellite viruses represent some of the highest resolutions known and have been comparatively reviewed (Ban et al., 1995). The structures of satellite tobacco mosaic virus (STMV) (Larson et al., 1993a,b), satellite tobacco necrosis virus (STNV) (Jones and LUjas, 1984 Liljas et al., 1982), and satellite panicum mosaic virus (SPMV) (Ban and McPherson, 1995) have T=1 capsids composed of 60 identical copies of unembellished jelly-roll j3 barrels constructed of only 155 to 195 amino acids (Fig. la see Color Insert). What is remarkable is how little the assembly context of these domains is conserved. The same end always points toward the 5-fold axis, but the domains are rotated to different extents around the 5-fold axis. Furthermore, between STNV and the others, there is a 70° rotation of the barrel about its long axis. Contacts across the dimer interface are... 

Parvoviridae is a family of small icosahedral, nonenveloped animal viruses that contain a linear ssDNA genome of approximately 5000 bases with short unique terminal palindromic sequences that fold back on themselves to form hairpin duplexes (van Regenmortel et al, 2000). They are further classified into two subfamilies, Parvcwirinae and Densovirinae, and various genera depending on host specificity, strand specificity of the genomic ssDNA, and on whether helper viruses are required for productive infection (dependoviruses). Members of the Parvoviridae... 

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