Gammaretrovirus is a genus in the Retroviridae family. Example species are the murine leukemia virus and the feline leukemia virus. They cause various sarcomas, leukemias and immune deficiencies in mammals, reptiles and birds.
Introduction
Many endogenous retroviruses, closely related to exogenous gammaretroviruses, are present in the DNA of mammals (including humans), birds, reptiles and amphibians. Many also share a conserved RNA structural element called a core encapsidation signal.
The avian reticuloendotheliosis viruses are not strictly avian viruses—it appears that reticuloendotheliosis viruses are mammalian viruses that were accidentally introduced into birds in the 1930s during research on malaria.
As a potential vector for gene therapy, gammaretroviruses have some advantages over HIV as a lentiviral vector. Specifically, the gammaretroviral packaging system does not require the incorporation of any sequences overlapping with coding sequences of gag, pol, or accessory genes.
Gammaretroviruses have a wide range of implications for animals. They have been linked with several diseases including cancer, specifically leukemias and lymphomas, various neurological diseases, and some immunodeficiencies in many different species. Gammaretroviruses are similar to other retroviruses and reverse transcribe a positive single strand RNA into double stranded DNA. The double stranded DNA is highly stable and easily integrated into a host genome. A few examples of the virus are Moloney murine leukemia virus, xenotropic MuLB-related virus, feline leukemia virus, and feline sarcoma virus.
Gammaretroviruses are very popular retroviral vectors in laboratory studies. These vectors are crucial for gene therapy and gene transfer. The reason that they are so useful is because their genomes are very simple and easy to use. Retroviruses have the ability to integrate into host cell genomes very well, which allows for the long term expression of their genome. One specific gammaretrovirus that is commonly used as a retroviral vector is the Moloney murine leukemia virus.
A specific gammaretrovirus called xenotropic murine leukemia virus-related virus (XMRV) has been found to infect prostate cancer tissue in laboratories. XMRV is a recombinant virus created in a laboratory accident in the mid-1990s. Although it can infect human tissue, no known disease is associated with the infection and it is unlikely to exist outside laboratories. Alleged discovery of XMRV in blood cells of patients with chronic fatigue syndrome in 2009 caused a controversy and eventual retraction. There were over 50 human cancer cell lines that were claimed to be linked to murine leukemia virus-related virus or murine leukemia virus. There have also been claimed discoveries of murine gammaretroviruses in lung cancer cell lines. While it was unclear what role these viruses have in the cancer development, it was believed that they are most prevalent during the tumor developing stage of the cancer by inhibiting tumor suppressing genes.
Viral classification
Gammaretrovirus is a part of the retroviridae family. Gammaretroviruses are considered zoonotic viruses because they are found in many different mammalian species, such as mice, cats, pigs, primates, cows, and birds. However, bats are the primary reservoir for many gammaretroviruses. Bats can have a prolonged exposure to a variety of pathogens without showing any warning signs, which leads to the debated belief that bats have the ability to develop immunity to viruses that may harm other species. So, it is possible for bats to harbor not only one, but several types of gammaretroviruses. This claim is supported by a sequencing transcriptome profiling technique, and a polymerase chain reaction. Researchers also looked into several different types of bat species to solidify the claim that bats are the main reservoir for gammaretroviruses. The gammaretroviruses can be spread horizontally, animal to animal, or vertically from parent to offspring.
Another gammaretrovirus reservoir was discovered in the genome of the bottlenose dolphin. This gammaretrovirus called Tursiops truncates endogenous retrovirus, was thought to be from extant mammalian endogenous gammaretroviruses. The Tursiops truncates endogenous retrovirus original invasion dates back to approximately 10–19 million years ago, and was identified in killer whale endogenous gammaretrovirus which invaded over 3 million years ago. In 2009, another endogenous gammaretroviruses were detected in a species of killer whale, as well as nine other cetacean genomes. So gammaretrovirus genomes are present in both aquatic and terrestrial mammal species.