text: For humans, porcine endogenous retroviruses (PERVs) pose a concern when using porcine tissues and organs in xenotransplantion, the transplanting of living cells, tissues, and organs from an organism of one species to an organism of different species. Although pigs are generally the most suitable donors to treat human organ diseases due to practical, financial, safety, and ethical reasons, PERVs previously could not be removed from pigs, due to their viral ability to integrate into the host genome and to be passed into offspring, until the year 2017, when one lab, using CRISPR-Cas9, removed all 62 retroviruses from the pig genome. The consequences of cross-species transmission remain unexplored and have dangerous potential.
  Researchers have indicated that infection of human tissues by PERVs is very possible, especially in immunosuppressed individuals. An immunosuppressed condition could potentially permit a more rapid and tenacious replication of viral DNA, and would later have less difficulty adapting to human-to-human transmission. Although known infectious pathogens present in the donor organ/tissue can be eliminated by breeding pathogen-free herds, unknown retroviruses can be present in the donor. These retroviruses are often latent and asymptomatic in the donor, but can become active in the recipient. Some examples of endogenous viruses that can infect and multiply in human cells are from baboons (BaEV), cats (RD114), and mice.
  There are three different classes of PERVs, PERV-A, PERV-B, and PERV-C. PERV-A and PERV-B are polytropic and can infect human cells in vitro, while PERV-C is ecotropic and does not replicate on human cells. The major differences between the classes is in the receptor binding domain of the env protein and the long terminal repeats (LTRs) that influence the replication of each class. PERV-A and PERV-B display LTRs that have repeats in the U3 region. However, PERV-A and PERV-C show repeatless LTRs. Researchers found that PERVs in culture actively adapted to the repeat structure of their LTR in order to match the best replication performance a host cell could perform. At the end of their study, researchers concluded that repeatless PERV LTR evolved from the repeat-harboring LTR. This was likely to have occurred from insertional mutation and was proven through use of data on LTR and env/Env. It is thought that the generation of repeatless LTRs could be reflective of an adaptation process of the virus, changing from an exogenous to an endogenous lifestyle.
  A clinical trial study performed in 1999 sampled 160 patients who were treated with different living pig tissues and observed no evidence of a persistent PERV infection in 97% of the patients for whom a sufficient amount of DNA was available to PCR for amplification of PERV sequences. This study stated that retrospective studies are limited to find the true incidence of infection or associated clinical symptoms, however. It suggested using closely monitored prospective trials, which would provide a more complete and detailed evaluation of the possible cross-species PERV transmission and a comparison of the PERV.