from: Retroviruses: Molecular Biology, Genomics and Pathogenesis (Edited by: Reinhard Kurth and Norbert Bannert). Caister Academic Press, U.K. (2010)
Endogenous retroviruses are genetic elements representing the result of retrovirus infections and integration of the proviruses into the germline of vertebrates including humans. Retroviruses use the enzyme reverse transcriptase (RT) to transcribe their RNA genome into cDNA and incorporate it into the cellular genome. Infections of germ cells result in the presence of these viruses in the genome all cells of the organism and transmission of these. sequneces to the offspring, Only some endogenous retroviruses are replication competent and produce infectious particles; most are defective. Although the role of endogenous retroviruses during tumour development and autoimmune diseases is still unclear, sufficient evidence has accumulated indicating that retroviruses play an important role in physiological processes. Endogenous retroviruses are involved in placental differentiation and immunosuppression during pregnancy, and retroviral long term repeats (LTR) regulate the expression of cellular genes. During evolution three main processes took place: First, an accumulation of defective proviral DNA ("junk DNA"), second a development of stronger restriction strategies by the host and third, an utilisation, "enslavement" of retroviral genes and LTRs. Since transspecies transmissions of retroviruses are very common, endogenous retrovirus may be important also for the health of other species. For example, pig cells can release porcine endogenous retroviruses that infect human cells and therefore represent a risk for xenotransplantations involving pig cells or organs read more ...