If any swine is fit to be an organ donor for people, then the dozens of pigs snuffling around Qihan Bio’s facility in Hangzhou, China, may be the best candidates so far. The Chinese company and its U.S. collaborators reported today that they have used the genome editor CRISPR to create the most extensively genetically engineered pigs to date—animals whose tissues, the researchers say, finally combine all the features necessary for a safe and successful transplant into humans.
“This is the first prototype,” says Luhan Yang, a geneticist at Qihan Bio. In a preprint published today on BiorXiv, Qihan researchers and collaborators, including Cambridge, Massachusetts-based eGenesis—which Yang co-founded with Harvard geneticist George Church—described the new generation of animals and various tests on their cells; the researchers have already begun transplanting the pigs’ organs into non-human primates, a key step toward human trials.
Qihan and eGenesis aren’t alone in their quest. Several academic and commercial research groups are racing to make up a shortage of life-saving human organs with the comparably-sized hearts, kidneys, and livers of pigs. For these cross-species transplants, also known as xenotransplants, the pig’s genome must be reengineered so that its organs will get along with the new host body. Pigs produce species-specific molecules that set off the human immune system, prompting rejection. Their tissue can also cause abnormal clotting and bleeding when it interacts with human blood. And the pig genome is littered with DNA sequences from viruses that infected the animals long ago and slipped genes into their chromosomes. These sequences, known as porcine endogenous retroviruses (PERVs), have been shown to produce potentially infectious viral particles, though their risk to humans is unclear.
Thanks to CRISPR, researchers have been making increasingly elaborate sets of genetic tweaks to deal with these issues. With researchers at the University of Alabama, Virginia-based company Revivicor has been transplanting baboons with kidneys from pigs that have nine genetic modifications designed to forestall immune rejection and regulate blood coagulation. In 2017, Church, Yang, and their colleagues reported that they had made healthy pigs with genomes stripped of PERV sequences, a tweak the team argues is an important safety measure.
In the new study, the team for the first time combined these PERV “knockouts” with a suite of other changes to prevent immune rejection, for a record-setting 13 modified genes. In pig ear cells, they removed three genes coding for enzymes that help produce molecules on pig cells that provoke an immune response. They also inserted six genes that inhibit various aspects of the human immune response and three more that help regulate blood coagulation.
The researchers then put the DNA-containing nuclei of these edited cells into eggs from pig ovaries collected at a slaughterhouse. These eggs developed into embryos that were implanted into surrogate mothers. Cells from the resulting piglets got another round of edits to remove the PERV sequences, after which their DNA went into another set of egg cells to create a new generation of pigs with all the desired edits. (In future, Yang says, the team will try to make all the modifications in a single generation.
The resulting pigs appeared healthy and fertile with functioning organs, the team reports today. And initial tests of their cells in lab dishes suggest that their organs will be much less prone to immune rejection than those of unmodified pigs: The tendency of the pig cells to bind to certain human antibodies was reduced by 90%, and the modified cells better survived interactions with human immune cells. But a key test is still to come: Yang says her team has begun transplanting organs from the highly edited pigs into baboons and monkeys to gauge their safety and longevity.
The combination of edits described in the new paper is “a technical feat,” says Marilia Cascalho, a transplant immunologist at the University of Michigan in Ann Arbor. “Whether it offers an advantage [over other engineered pig organs]… the jury is out on that,” she says. Questions remain about how many and which genetic changes are really necessary for a human-safe pig organ—particularly when it comes to the PERV sequences. Revivicor has so far opted not to remove PERV sequences from its pigs, and the U.S. Food and Drug Administration (FDA) has not expressly said that removing PERVs will be required for a human trial. But Yang says she and her colleagues got “positive feedback” from the agency about their efforts to eliminate PERV concerns.
Yang says that Qihan plans to remain “laser-focused” on preclinical studies in 2020, but expects to be testing pig organs in humans within 5 years. Many in the field now feel an inevitable momentum around xenotransplantation: “There is so much need for organs,” says Cascalho. “I think it’s going to be a reality.”