Transspecies transmission of gammaretroviruses and the origin of the gibbon ape leukaemia virus (GaLV) and the koala retrovirus (KoRV)
Denner, J 2016, Viruses, vol. 8, no. 12, 336.
Koala retrovirus (KoRV) is closely related to gibbon ape leukaemia virus (GALV), and it is thought that they both originate from transspecies transmissions from the same host. Bats are discussed as a possible host of a retrovirus transmitted to gibbons and koalas as they could feasibly enter both gibbon habitats in Thailand and koala habitats in Australia and also carry several endogenous and exogenous retroviruses.
Transspecies transmission of retroviruses, including gammaretroviruses, is a common occurrence. KoRV in Australia and GALV in Thailand are closely related genetically, and yet seemingly disparate both geographically and taxonomically. The genetic similarity of the two retroviruses suggests transspecies transmission from a common, but unknown, host. Such an assertion is supported by the pattern of spread of KoRV in koala populations, from the northern tip of the continent southward. It is challenging to identify such a host that could span the geographical distance between Thailand and Australia and that also carries a closely related retrovirus. Previous explorations of this dilemma have described the similarity of retroviruses carried by South East Asian rodents and two subspecies of Melomys burtoni, which occur in Australia and Indonesia, to GaLV and KoRV. The retrovirus carried by Melomys burtoni was found to be 93% similar to GaLV and 83% similar to KoRV. Given their inability to traverse the habitats of both the gibbon and the koala, however, neither of these animals is a feasible candidate for transspecies transmission of KoRV/GaLV. Bats, on the other hand, travel long distances and also carry numerous exogenous and endogenous viruses. Phylogenetically, GaLV and KoRV are related to porcine endogenous retroviruses (PERVs). A retrovirus carried by a particular bat, Rousettus leschenaultia retrovirus (RIRV), is related to PERVs and therefore similar to GaLV and KoRV. Another bat retrovirus, Megaderma lyra retrovirus (MIRV), is also genetically similar to both KoRV and GaLV. GALV has been shown to grow on cells from the respiratory tracts of bats (CCL-88 bat lung fibroblasts), which further suggests that bats could be the source of other mammalian retroviruses. At present, although many unanswered questions remain about the origins of GaLV and KoRV, the bat may be considered as the most likely common transmitter.
Pinpointing the origin of KoRV is useful for understanding the evolution of the retrovirus, its genetic subtypes and its transmission patterns. Knowledge of transspecies transmissions of retroviruses is also currently of interest in the evaluation of potential risks associated with xenotransplantation procedures.
Summarised by Joanna Horsfall
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