Addressing Compatibility Issues

Pig organs are not inherently compatible with human anatomy. To enhance this compatibility, scientists at a Chinese biotech firm, co-founded by Harvard geneticist George Church, PhD, are employing CRISPR to implement 42 modifications across 13 genes in pigs. This initiative, spearheaded by Qihan Biotech, has been ongoing since its establishment in 2017, with the latest prototype referred to as "pig 3.0" being the most modified to date.

The genetic alterations aim to mitigate immune rejection, prevent blood clotting, and combat infection in those receiving transplants. The objective is to deceive the human immune system into perceiving the transplanted pig organ as a human one. The company recently published its findings in the journal Nature Biomedical Engineering on September 21.

“This represents a remarkable advancement,” stated Jay Fishman, MD, associate director of the transplant program at Massachusetts General Hospital, who was not part of the research. “We face an organ shortage for transplantation. Now we possess the capabilities to modify animal organs, potentially making them functional.”

The Potential of Pigs in Transplantation

Due to their similar organ size and functionality to humans, pigs have long been viewed as a viable source for human organ transplants. However, two significant challenges exist: rapid immune rejection and the risk of infection.

Pigs possess surface molecules known as antigens that appear foreign to the human immune system, causing any transplanted pig organ to elicit a strong immune response and rapid rejection. While pig heart valves have been successfully used in human transplants for years, the tissue is chemically preserved, rendering the cells non-viable.

In a significant breakthrough, doctors successfully transplanted genetically modified pig skin onto a human burn victim last year, overcoming the immune response challenge. These pigs were bred specifically to avoid skin rejection. Although the pig skin grafts are temporary, they represent a critical step toward human clinical trials involving entire pig organs.

Another challenge for pig-to-human organ transplants is the presence of porcine endogenous retroviruses (PERVs) in pigs' DNA. The implications of these viruses for human health during transplantation remain uncertain.

In 2017, Church and his colleagues, including Luhan Yang, PhD, who previously worked in his lab, announced the creation of the first CRISPR-engineered pig prototype, which was free of PERVs. The subsequent version, pig 2.0, included modifications to the immune system, while pig 3.0 encompasses all previous edits.

“They are quite healthy,” Yang, now CEO of Qihan Biotech, shared in an interview. “Thus far, their physiology, fertility, and reproductive capabilities appear normal.” Blood tests have indicated that the livers, hearts, and kidneys of the modified pigs are functioning well.

Qihan Biotech, headquartered in Hangzhou, has produced around 2,000 of these 3.0 pigs in China, with some allowed to breed, passing on their genetic modifications to offspring.

The next crucial step is to determine if these alterations genuinely enhance the suitability of pig organs for human transplantation. “We must take a leap of faith that these organs will function in humans,” Fishman remarked. “However, this remains unproven.”

Before human application, these genetically modified organs must first be tested in monkeys, our closest relatives. Yang noted that Qihan Biotech has performed numerous pig-to-monkey organ transplants, primarily focusing on kidneys due to high demand, while also exploring multi-organ transplants to assess other pig organs' viability. Recent modifications have led to reduced immune reactions in monkeys compared to earlier pig prototypes, though Yang refrained from disclosing the duration of survival for transplanted monkeys.

In 2016, researchers at the U.S. National Institutes of Health achieved a milestone by sustaining a pig heart in a baboon for over two years, setting a new record.

Yang and Church also co-founded eGenesis, Qihan Biotech's partner in Boston, which has produced over 100 edited pigs, as reported by MIT Technology Review. However, research involving monkeys is costly and controversial in the United States, while Chinese regulations are more accommodating for studies involving nonhuman primates.

Fishman expressed optimism that the pigs' apparent health and fertility, along with their normal-sized litters, indicate that the genetic modifications did not adversely impact the animals. Nonetheless, CRISPR is not infallible; it is known to produce “off-target” edits, which are unintended modifications elsewhere in the genome. While some of these changes are likely benign, others could potentially have health implications.

When Yang and her team utilized genome sequencing to investigate off-target effects in the edited pigs, they identified several unintended insertions and deletions. These alterations occurred in “noncoding” regions, which were once considered “junk DNA” but are now understood to play significant roles.

Muhammad Mohiuddin, MD, director of the cardiac xenotransplantation program at the University of Maryland School of Medicine, cautioned that extensive genetic modifications could lead to abnormalities within the animals' organs. He also noted that varying genetic edits might be necessary for different organs. “We must be cautious to limit alterations to only those genes essential to mitigate unknown risks,” he advised.