Porcine endogenous retroviruses: Quantification of the copy number in cell lines, pig breeds, and organs

Background: Porcine endogenous retroviruses (PERVs) may pose a risk of xenotransplantation using porcine cells, tissues, or organs. PERVs are integrated in the genome of all pigs, and some can infect certain human cells. The copy number of PERVs in different pig breeds has been determined by using different methods, with varying results. Methods: To determine the PERV copy number in pig cell lines and in animals, a new method, droplet digital polymerase chain reaction (ddPCR) was used. DNA was isolated from pig cell lines (PK15 and PTK75 cells), from Aachen, Göttingen, and Black minipigs, and from genetically modified and non-modified German landrace pigs. Primers specific for the polymerase gene (pol) were used for the ddPCR. Results: The median copy number of integrated proviruses was found between 46 and 70 copies in three different PK15 cell lines, 49 copies in PTK75 cells, 64 copies in Göttingen minipigs, 69 copies in Aachen minipigs, 117 copies in Black minipigs, and 59 copies in genetically modified pigs generated for xenotransplantation. PERV copy numbers varied between different organs from a single pig, indicating proviral amplification. The study also revealed that different PK15 cell lines from different laboratories which had been used as virus source for infection experiments carry different PERV copies. Furthermore, different copy numbers of cellular reference genes (GAPDH, ACTB) were detected in different cell lines and pigs. Conclusion: The determination of the PERV copy number using ddPCR extended previous data showing differences between the pig breeds and between different organs of a single animal. The determination of PERV copy numbers can be used to select animals less likely to transmit PERVs during xenotransplantation. In addition, this method will be of special value when PERV proviruses are to be inactivated by CRISPR/Cas9.