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Viruses Removed from Pig Organs for Human Transplants

Piglets

(Andrew Martin, Pixabay)

10 August 2017. Researchers in the U.S., Denmark, and China devised a process with genome editing to remove viruses that prevent organs from pigs from being used for human transplants. The process is described by a team from the start-up company eGenesis Inc. in Cambridge, Massachusetts, with colleagues from Harvard University, Aarhus University in Denmark, and several institutions in China, in today’s issue of the journal Science (paid subscription required).

The researchers were led by eGenesis’s co-founder Luhan Yang, a postdoctoral researcher at Harvard Medical School in the lab of geneticist and company co-founder George Church. The company is developing techniques with genome editing to make organs from pigs suitable for human transplantation, and address the continuing problem of shortages in organs needed for transplants. According to the web site OrganDonor.gov, more than 117,000 people in the U.S. are on waiting lists for organ transplants, with another person added to the list every 10 minutes. Despite more than 33,600 organ transplants being performed in 2016, an estimated 22 people die each day waiting for a transplant.

An obstacle to the use of organs from pigs, however, are porcine endogenous retroviruses from pig cells, tissues, and organs. The presence of these retroviruses throughout the pig genome prevents xenotransplantation, or the transfer of pig organs to humans, where they can infect human cells, tissues, and organs. While genome editing is a promising method for removing DNA responsible for these retroviruses, the technique was attempted so far only on individual cell lines, not in live animals.

Yang and colleagues first documented the risks posed by porcine endogenous retroviruses to human cells. In a series of lab tests, the team confirmed the retroviruses can infect human cells, when cultured together. The team then showed the ability for infected human cells to spread the retroviruses to other uninfected human cells. These tests underscored the need to first deactivate porcine endogenous retroviruses in donor animals.

Next, the researchers mapped the genome of connective tissue in pigs and identified 25 regions where porcine endogenous retroviruses could be derived. Using the genome editing technique Crispr — for clustered regularly interspaced short palindromic repeats — and the enzyme Cas9 to perform the edits, the team was able to remove from the pig genome copies of the genes responsible for the retroviruses.

Attempts to clone the connective tissue cells showed the genome editing removed as much as 90 percent of retrovirus genes, but not all. The researchers needed to treat the precursor cells to a cocktail of growth factors and inhibitor proteins during genetic, making it possible to produce pig embryos with inactivated genes that express the retroviruses, indicated by the lack of characteristic virus particles secreted by the modified cells.

The researchers then implanted the embryos in sows determined to be free of the retroviruses, with the resulting piglets also exhibiting no sign of activated genes in the DNA that produce the retroviruses. An analysis of RNA in the pigs’ tissue, the transcription of genetic code that gives instructions for protein in cells, also showed no sign of retrovirus activation. In addition, the researchers were able to produce 37 piglets with deactivated retrovirus genes from 17 sows at the time of the publication, with the oldest piglet still thriving after 4 months.

The eGenesis team says it is monitoring the piglets for long-term effects. “This research represents an important advance in addressing safety concerns about cross-species viral transmission,” says Yang, who serves as the chief scientist at eGenesis, in a company statement. The researchers expect further genome editing may be needed to eliminate potential immune reactions from xenotransplanted organs.

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