Phylogeny, recombination and expression of porcine endogenous retrovirus γ2 nucleotide sequences

Endogenous retroviral sequences in the pig genome represent a potential infectious risk in xenotransplantation. Porcine endogenous retrovirus (PERV) γ sequences described to date have been classified into several families. The known infectious, human-tropic PERVs have been assigned to the PERV γ1 subfamilies A, B and C. High copy numbers and full-length clones have also been observed for an additional family, designated PERV γ2. The aim of this study was to examine the PERV γ2 family by analysis of retroviral pro/pol gene sequences. The proviral load was observed to be similar among various pig breeds. Although clones harbouring an open reading frame in the examined region were found, analysis of published large PERV γ2 clones revealed multiple deleterious mutations in each of the retroviral genes. Various recombination events between γ2 genomes were revealed. In contrast to PERV γ1, phylogenetic analyses did not distinguish defined subfamilies, but indicated the independent evolution of the proviruses after a single event of retroviral amplification. Expression analysis showed large PERV γ2 transcripts and variable transcription in several tissues. Analysis of the two published γ2 env gene sequences observed the partial lack of the receptor-binding domain. Overall, this study indicated the low infectious potential for PERV γ2.