High Prevalence of Recombinant Porcine Endogenous Retroviruses (PERV-A/Cs) in Minipigs: A Review on Origin and Presence

Minipigs play an important role in biomedical research and they have also been used as donor animals for preclinical xenotransplantations. Since zoonotic microorganisms including viruses can be transmitted when pig cells, tissues or organs are transplanted, virus safety is an important feature in xenotransplantation. Whereas most porcine viruses can be eliminated from pig herds by different strategies, this is not possible for porcine endogenous retroviruses (PERVs). PERVs are integrated in the genome of pigs and some of them release infectious particles able to infect human cells. Whereas PERV-A and PERV-B are present in all pigs and can infect cells from humans and other species, PERV-C is present in most, but not all pigs and infects only pig cells. Recombinant viruses between PERV-A and PERV-C have been found in some pigs; these recombinants infect human cells and are characterized by high replication rates. PERV-A/C recombinants have been found mainly in minipigs of different origin. The possible reasons of this high prevalence of PERV-A/C in minipigs, including inbreeding and higher numbers and expression of replication-competent PERV-C in these animals, are discussed in this review. Based on these data, it is highly recommended to use only pig donors in clinical xenotransplantation that are negative for PERV-C.

Copy Number and Prevalence of Porcine Endogenous Retroviruses (PERVs) in German Wild Boars

Porcine endogenous retroviruses (PERVs) are integrated in the genome of pigs and are transmitted like cellular genes from parents to the offspring. Whereas PERV-A and PERV-B are present in all pigs, PERV-C was found to be in many, but not all pigs. When PERV-C is present, recombination with PERV-A may happen and the PERV-A/C recombinants are characterized by a high replication rate. Until now, nothing has been known about the copy number of PERVs in wild boars and little is known about the prevalence of the phylogenetically youngest PERV-C in ancient wild boars. Here we investigated for the first time the copy number of PERVs in different populations of wild boars in and around Berlin using droplet digital PCR. Copy numbers between 3 and 69 per genome have been measured. A lower number but a higher variability was found compared to domestic pigs, including minipigs reported earlier (Fiebig et al., Xenotransplantation, 2018). The wild boar populations differed genetically and had been isolated during the existence of the Berlin wall. Despite this, the variations in copy number were larger in a single population compared to the differences between the populations. PERV-C was found in all 92 analyzed animals. Differences in the copy number of PERV in different organs of a single wild boar indicate that PERVs are also active in wild boars, replicating and infecting new cells as has been shown in domestic pigs.

Extensive Mammalian Germline Genome Engineering

Xenotransplantation, specifically the use of porcine organs for human transplantation, has long been sought after as an alternative for patients suffering from organ failure. However, clinical application of this approach has been impeded by two main hurdles: 1) risk of transmission of porcine endogenous retroviruses (PERVs) and 2) molecular incompatibilities between donor pigs and humans which culminate in rejection of the graft. We previously demonstrated that all 25 copies of the PERV elements in the pig genome could be inactivated and live pigs successfully generated. In this study, we improved the scale of porcine germline editing from targeting a single repetitive locus with CRISPR to engineering 13 different genes using multiple genome engineering methods. we engineered the pig genome at 42 alleles using CRISPR-Cas9 and transposon and produced PERVKO·3KO·9TG pigs which carry PERV inactivation, xeno-antigen KO and 9 effective human transgenes. The engineered pigs exhibit normal physiology, fertility, and germline transmission of the edited alleles. In vitro assays demonstrated that these pigs gain significant resistance to human humoral and cell mediated damage, and coagulation dysregulations, similar to that of allotransplantation. Successful creation of PERVKO·3KO·9TG pigs represents a significant step forward towards safe and effective porcine xenotransplantation, which also represents a synthetic biology accomplishment of engineering novel functions in a living organism.One Sentence SummaryExtensive genome engineering is applied to modify pigs for safe and immune compatible organs for human transplantation

Complex evolution of porcine endogenous retroviruses

Background: Porcine endogenous retroviruses (PERVs) are proviruses that can replicate in human cells. However, the evolutionary process leading to the generation of modern PERVs is not well understood.Results: We mined 14 pig genomes and other available 304 mammalian genomes in silico, which led to the documentation of 185 full-length PERVs. Notably, we found two novel ERVs in the lesser Egyptian jerboa ( Jaculus jaculus ) and rock hyrax ( Procavia capensis ) named ERV-Gamma-Jja and ERV-Gamma-Pca, respectively, which were the source of the modern PERVs. Phylogenetic analyses provided evidence for the multiple origins of PERVs involving hosts of rodents, rock hyrax, and pigs. Conclusion: These new findings help us to understand the complex evolution of the modern PERVs.

Genomic insights into evolutionary journey of the porcine endogenous retroviruses

BackgroundXenotransplantation may overcome significant shortage of human allotransplant. Porcine organs are considered favorable for xenotransplantation duo to similar size and function to human organ. However, porcine endogenous retroviruses (PERVs) are potential infectious agents during xenotransplantation as they are able to infect and horizontally transfer among human cells. Furthermore, PERVs can be endogenized in pig genomes and are transmitted genetically in a Mendelian fashion. Here, we depict a complex evolutionary history of modern PERVs.ResultsWein silicomined 142 mammalian genomes and 14 pig genomes. This led to the documentation of 185 PERVs and a new viral cluster. Large-scale genomic alterations were found in most PERVs including many insertion-deletion events and which are suggestive of ancient origins, and pig genomes have been shaped by PERV-mediated genomic rearrangement during evolution. Notably, we found that lesser Egyptian jerboa and rock hyrax harbor ancestral PERV-related elements indicative of ancient cross-species transmission events from none-porcine species to pigs. A comprehensive analysis of these viral “fossils” suggested that recombination among none-porcine endogenous retroviruses led to the origination of PERVs.ConclusionFor the first time, using large scale genomic mining, we decipher a complex evolutionary history for the PERVs. These new findings help us to understand the past of PERVs which pose the potential risk in clinical trials of xenotransplantation and provide novel insights into the origin and evolution of a human-infecting pathogen.