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Chapter 14 : Potential Medical Impact of Endogenous Retroviruses
The recent in vitro observation that porcine endogenous retroviruses (PERV) can infect human cells has ignited tremendous fear among investigators in the xenotransplantation field and health officials. This chapter reviews the available experience with other endogenous retroviruses in different animal situations, with the aim of better understanding the potential impact of such organisms in the field of xenotransplantation. All mammals that have been examined so far, including pigs and baboons, have endogenous retrovirus DNA sequences integrated in their genomes. Although transmitted in the germ line, they exhibit a very significant sequence homology to exogenous retroviruses. The true source of endogenous retroviruses remains to be elucidated. One hypothesis is that they are viral remnants of remote infections with exogenous forms of the virus. Alternative explanations include that retroviruses may be derived from retrotransposons or that endogenous retroviruses are the precursors of exogenous retroviral agents. The majority of endogenous retroviruses act as colonizers in the germ line of the host and are noninfectious. The exact functions of the remaining "competent" or partially defective endogenous retroviruses remain enigmatic. It is speculated that, among a number of possible functions, they may enhance the pathogenicity of exogenous retroviruses by the recombination and production of a hybrid virus with altered receptor specificity and species tropism, now capable of infecting a new range of hosts. New animal viruses are being discovered on a regular basis, as detection methods are becoming more sophisticated. Similarly, many more animal endogenous retroviruses are yet to be discovered and fully characterized.