Heterogeneity of koala retrovirus isolates
Shimode, S, Nakagawa, S, Yoshikawa, R, Shojima, T & Miyazawa, T 2013, FEBS Letters, vol. 558, pp. 41-46.
Koala retrovirus (KoRV) is a gammaretrovirus implicated in immune suppression and leukemia, with three subgroups identified named A, B and J. Phylogenetic analysis demonstrated that KoRV-B and KoRV-J should be considered the same subgroup. A long terminal repeat (LTR) in KoRV-B had four 17 bp tandem repeats (‘DR-1’ for direct repeat 1) and in KoRV-J had three 37 bp tandem repeats (‘DR-2’). The promoter activity in the KoRV-J OJ-4 strain was stronger than in KoRV-A, suggesting more efficient replication.
KoRV-B and KoRV-J were shown to be 91.8% identical, with the difference occurring mainly in the proline rich region (PRR). The PRR has been implicated in fusogenicity differences in distinct murine leukemia virus, suggesting that fusogenicity may vary between KoRV-B and J. The putative receptor binding site VRA had very similar sequences between the two strains, as is consistent with both subgroups using the THTR1 to infect cells. The CETTG motif is interrupted by mutations in the J and A strains, and J substituted a Gln with Arg in the PHQ motif, with both motifs being implicated in fusion activity. The immunosuppressive domain in the transmembrane Env subunit was conserved, as were other important infectivity-linked motifs. A maximum likelihood approach was applied to phylogenetic analysis and revealed that all three subgroups are clustered together. Subgroups B, J and two OJ-4 clones were subclustered together. Amino acid sequences excluding putative recombination sites revealed that the two clone strains appear to have evolved from subgroup A through mutation and recombination while in the koala. LTR sequencing revealed direct repeat sequences and a U3 region that had multiple transcription factors able to bind (according to the computational program MATCH). DR-1 had potential enhancer binding sites for AML1A and C/EP and DR-2 had transcription sites for OCT-1, C/EBP, Pax4 and others. The tandem repeats may therefore be associated with increased viral promoter activity. The luciferase assay was used to compare the promoter activity of subgroups A and J. KoRV-J had significantly higher transcriptional activity than KoRV-A in HEK293 (a potoroo cell line) and PtK2 cells, and both were lower than feline leukaemia virus (FELV) strain FeLV-B in HEK293 cells. The transcription was tested in several human lymphoid/myeloid cell lines, with FeLV-B having higher promoter activity than both, but J having higher activity than A in K-562 cells (human myelogenous lymphoma cell line).
An understanding of the replicative efficiency of the various strains of koala retrovirus in different cell lines improves our knowledge of koala diseases and therefore has implications for their treatment.
Summarised by Laura Wait
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