Inactivation of a pig retrovirus using CRISPR/Cas9, which will improve success at pig organ transplants in humans

CRISPR/Cas9 is an efficient relatively quick method of gene-editing, and a use of this technique in farmyard animals is shown herein. Previous papers shared by GEITP emails included CRISPR/Cas technology employed in laboratory animals, human clinical experiments, plants of agricultural importance, and even insects.

“Xenotransplantation” describes the process whereby tissue from one species is transplanted into a different species. Xenotransplantation is currently under development to help alleviate the increasing shortage of human tissues and organs for transplantation to treat organ failure. For several reasons –– the size and physiology of the organs, the ease of genetic modification and cloning, and the large number of progeny and short reproduction cycle –– pig is the animal of choice for organ transplant in humans. Three major problems need to be solved, however, before xenotransplantation becomes a clinical reality: immunological rejection, physiological incompatibility, and risk of transmission of porcine microorganisms that are able to induce a disease (this process is called “zoonosis”) in the human recipient.

In the attached paper and editorial, authors describe how to increase safety of xenotransplantation. In addition to the well-known form of immune rejection that can occur after allotransplantation (transplant of non–genetically identical material between the same species), a new form of rejection is observed when using pig cells and organs: hyperacute rejection (HAR); this occurs because of preexisting antibodies in human recipients that recognize sugar molecules on the surface of bacteria as well as pig cells (but not human cells). These antibodies can lead to destruction of the transplant within minutes. In order to prevent immune rejection (including HAR), genetically modified pigs have been generated that lack enzymes responsible for expressing those sugars and that express human proteins that prevent rejection.

Authors previously have demonstrated the feasibility of inactivating porcine endogenous retrovirus (PERV) activity in a pig cell culture line. In the current attached article, authors confirm that PERVs do infect human cells, and they have detected/observed the horizontal gene transfer of PERVs among human cells. Using CRISPR/Cas9, authors inactivated all the PERVs in a porcine primary cell culture line and generated PERV-inactivated pigs via somatic cell nuclear transfer. This study underscores the value of PERV inactivation to prevent cross-species viral transmission. Furthermore, this report demonstrates the successful production of PERV-inactivated animals to address the safety concern in clinical xenotransplantation.

Science 22 Sept 2o17; 357: 1303–1307 plus editorial, pp 1238–1239

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