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The nucleotide sequence of koala (Phascolarctos cinereus) retrovirus: a novel type C endogenous virus related to gibbon ape leukaemia virus

Hanger, JJ, Bromham, LD, McKee, JJ, O’Brien, TM & Robinson, WF 2000, Journal of Virology, vol. 74, no. 9, pp. 4264-4272.

For the first time, the full nucleotide sequence of koala retrovirus (KoRV) is reported and characterised as a novel C-type endogenous retrovirus as a result. Phylogenetic analyses revealed a close genetic relationship between KoRV and gibbon ape leukaemia virus (GALV).

  Using electron microscopy, researchers detected a unique retrovirus with C-type particles in mitogen-stimulated peripheral blood mononuclear cell cultures (PBMCs) in 163 koalas out of 166 tested. In three of the 166 koalas, the C-type retrovirus particles were associated with lymphoma tissue. The polymerase chain reaction (PCR) technique was employed to amplify the KoRV sequence from blood and tissue samples from 17 koalas. As a result, the viral nucleotide sequences originating from genomic DNA and mRNA were compared and found to belong to a single species of retrovirus. A Southern blot analysis, used to detect a specific DNA sequence within a sample, revealed that the genomic DNA from koala tissue was consistent with an endogenous retrovirus. Both full-length and truncated proviruses were present in koala DNA. Truncated proviruses can occur in the genome as a result of recombination excision events, novel superinfection events, or reinsertion events, and suggest that the retrovirus is not stable. These proviruses of different lengths were found in koalas with hematopoietic disease and koalas that appeared healthy. No viruses genetically similar to KoRV have been observed in any other marsupial species. Interestingly, the closest relative of KoRV in phylogenetic analyses was GALV. Given the great taxonomic and geographic divide between koalas and gibbons, the most likely explanation for the similarity between these viruses is a recent cross-species transmission event of a KoRV-like virus to gibbons or koalas from an intermediate host such as a rodent, bat or bird.

  This study has produced formative descriptions of endogenous KoRV that serve as important foundations for further studies. The close phylogenetic relationship between KoRV and GALV, and the potential association between retroviruses and disease in koalas, warrants further investigation.

 

Summarised by Joanna Horsfall

 

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