The genomes of many species on Earth are littered with endogenous retroviruses. These are DNA copies of retroviral genomes from previous infections that are integrated into germ line DNA and passed from parent to offspring. About 8% of the human genome consists of ERVs. The pig genome is no different – it contains PERVs (an acronym made to play with). The genome of an immortalized pig cell line called PK15 contains 62 PERVs. Human cells become infected with porcine retroviruses when they are co-cultured with PK15 cells.
The presence of PERVS is an obvious problem for using pig organs for transplantation into humans – a process called xenotransplantation. The retroviruses produced by pig cells might infect human cells, leading to problems such as immunosuppression and tumor formation. No PERV has ever been shown to be transmitted to a human, but the possibility remains, especially with the transplantation of increasing numbers of pig organs into humans.
The development of CRISPR/Cas9 gene editing technology made it possible to remove PERVs from pigs, potentially easing the fears of xenotransplantation. This technology was first used to remove all 62 copies of PERVS from the PK15 cell line. But having PERV-free pig cells doesn’t help humans in need of pig organs – for that you need pigs.
To make pigs without PERVs, CRISPR/Cas9 was used to remove the PERVs from primary (that is, not immortal) pig cells in culture. Next, the nuclei of these PERV-less cells was used to replace the nucleus of a pig egg cell. After implantation into a female, these cells gave rise to piglets lacking PERVs.
In theory such PERV-less piglets can be used to supply organs for human transplantation, eliminating the worrying about infecting humans with pig retroviruses. But first we have to make sure that the PERV-free pigs, and their organs, are healthy. The more we study ERVs, the more we learn that they supply important functions for the host. For example, the protein syncytin, needed to form the placenta, is a retroviral gene, and the regulatory sequences of interferon genes come from retroviruses. There are likely to be many more examples of essential functions provided by ERVs. It would not be a good idea to have transplanted pig organs fail because they lack an essential PERV!