Avian retroviruses viruses

Certain RNA viruses, particularly retroviruses, have also proven capable of inducing cancer. Retroviruses known to induce cancer in animals include Rous sarcoma virus, Kirsten murine sarcoma virus, avian myelocytomatosis virus, as well as various murine leukaemia viruses. Thus far, the only well-characterized human RNA transforming virus is that of human T cell lymphocytotropic virus-1 (HTLV-1), which can induce adult T cell leukaemia/lymphoma (ATL). Identification of antigens uniquely associated with various tumour types, and identification of additional cancer-causing viruses, remain areas of very active research. 

Retroviruses have featured prominently in recent advances in the molecular understanding of cancer. Most retroviruses do not kill their host cells but remain integrated in the cellular DNA, replicating when the cell divides. Some retroviruses, classified as RNA tumor viruses, contain an oncogene that can cause the cell to grow abnormally (see Fig. 12-47). The first retrovirus of this type to be studied was the Rous sarcoma virus (also called avian sarcoma virus Fig. 26-31), named for F. Peyton Rous, who studied chicken tumors now known to be caused by this virus. Since the initial discovery of oncogenes by Flarold Varmus and Michael Bishop, many dozens of such genes have been found in retroviruses. 

After obtaining pure total RNA from eukaryotic cells, the RNA needs to be rewritten into cDNA to serve as a template in PCR, as RNA cannot be amplified by PCR. The task of rewriting is accomplished with reverse transcriptase (RT), a viral enzyme used by retroviruses (whose name stems from harboring this enzyme and the ability to rewrite RNA into cDNA). The group of retroviruses has such members as the AIDS virus, Avian myeloblastosis virus, Murine leukemia virus (Frohmann, 1988 Kawasaki, 1988), and Adenovirus. Commonly employed reverse transcriptases stem from either the avian virus (AMV-RT) or the murine leukemia virus (MMLV, used in the Clontech RT-kit). 

Shimotohno K, Temin HM (1981), Formation of infectious progeny virus after insertion of herpes simplex thymidine kinase gene into DNA of an avian retrovirus, Cell 26 67-77. 

Autographa califomica mononuclear polyhedrosis virus (AcMNPV), 8 Avian leukkosis virus, 343—344 Avian sarcoma/leukosis viral group (ASLV), as retroviruses, 343—344... 

RAV-7, an avian leukosis virus (Carter and Smith, 1984), has an 8.2-kb RNA (Carter et al., 1983a). It is the most common naturally occurring avian retrovirus associated with neoplastic disease condition in domesticated poultry (Fadly, 1997). 

RSV is rather an unusual transforming retrovirus in possessing the complete set of genes for replication, and also an oncogene. Most of the avian transforming viruses lack at least one or part of one of the other genes necessary for replication (see Fig. 11.4). 

Type D (squirrel monkey retrovirus). ALV-related virus (avian leukemia virus, rous sarcoma virus). 

Further investigations of ascorbate inhibition of virus replication have been carried out using retroviruses as models. Bissell et al. (1980), working with the avian retrovirus of chickens, found that while cell-free virus was resistant to ascorbate inactivation upon short-term treatment in vitro, exposure of virus-infected cultures to the vitamin resulted in reduction of virus replication and lowered infec-tivity of newly replicated virus. A subsequent study found that ascorbate interfered with the replication and cell-transforming potential of Rous sarcoma virus by stabilizing the differentiated state of chicken cells (Schwarz, 1991). In a lymphocytic cell line latently infected with human T-cell leukemia virus (HTLV-1), ascorbate was shown to interfere with virus production triggered by chemical inducers added to the culture medium (Blakeslee et al., 1985). 

Acute transforming retroviruses are a unique subset of retroviruses that have acquired the ability to produce cancer in rodents and avian species in a relatively short time period (several weeks). Examples of acute transforming retroviruses include the Rous sarcoma virus (RSV), which produces sarcomas in chickens, and the Harvey murine sarcoma virus (Ha-MSV), which produces sarcomas in rats and mice. Studies utilizing RSV provided the first evidence that cellular genes, now termed oncogenes, exist in vertebrate cells. 

Retroviruses are most familiar to the general reader as the family to which the lentivirus human immunodeficiency virus (HIV) belongs. Other retrovirus groups include the avian sarcoma and leukosis viral group... 

Two avian oncogenes that encode protein kinases are cytoplasmic proteins not associated with membranes rel and mil rel occurs in avian reticuloen-dotheliosis virus (REV-T), which has been described as the most virulent of all retroviruses, causing a rapidly fatal lymphoma with a latent period of one to two weeks and a mortality approaching 100% (Moore Bose, 1989). A modified form of REV has... 

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