Organ Transplant: Humanized Pig Hearts | nd-aktuell.de

Solving the shortage of human donor organs using organs from animals is an old idea. The developments in genome editing technologies have given the research field a new impetus. Around ten years after the discovery of the Crispr-Cas9 genetic scissors, clinical trials on patients are being carried out again.

In January last year, international media attention focused on the American David Bennett, who had the world’s first transgenic pig heart implanted at the University of Maryland Medical Center in the USA. Before the procedure, the 57-year-old had been kept alive for six weeks on a heart-lung machine because of a life-threatening cardiac arrhythmia. He was just as out of the question for a normal heart transplant as he was for an artificial heart. The US Food and Drug Administration (FDA) approved the high-risk experiment through its Compassionate Access policy, which allows unapproved treatments to be used as last-ditch treatments. Two months after the procedure, Bennett died of sudden heart failure, even though the transplant had proven fully functional for weeks, according to his doctors.

Initially the message from the clinic was that there was no rejection. A study published three months later pointed to several overlapping factors that led to the heart failure: in addition to the patient’s poor health and weakened immune system, antibody-mediated rejection. In addition, the reactivation of a pig virus in the transplant “possibly triggered a harmful inflammatory reaction.” Undeterred by this setback, a pig’s heart was transplanted in September in Maryland to the second patient, 58-year-old Lawrence Faucette, who had end-stage heart disease. Initially, the patient made significant progress, the hospital said. Faucette died on October 30th after his heart showed the first signs of rejection days earlier.

Immune rejection as the first hurdle

Even if the result of this experiment is still pending, historically the approach of xenotransplantation, that is, the transplantation of an animal organ into a human, has proven to be difficult on several levels. Until the middle of the 20th century, xenotransplantations were unthinkable – due to the limited state of medicine. There had already been unsuccessful attempts to transplant chimpanzee organs into humans in the 1960s. The first high-profile experiment in which the patient initially survived was carried out on “Baby Fae”, a few days old baby with an inoperable congenital heart defect. In 1984, surgeon Leonard Bailey transplanted a baboon heart into the child at Loma Linda Medical Center in California, but the child’s immune system rejected the organ and the child died after 20 days. Since then, ever better immunosuppressants have been developed and the understanding of the immunobiology of transplant rejection has expanded.

The first major hurdle in the case of a transplant is the acute rejection reaction, triggered by foreign markers on the animal cells, which manifests itself in destruction of the transplant and blood clotting. Researchers hope to be able to keep the rejection reaction as low as possible through genetic engineering “humanization” of the donor animals. To do this, certain human genes are inserted into the animal cells. Another danger is disease transmission. This is to be countered by keeping the animals germ-free, and since Crispr-Cas9 also through the genetic engineering inactivation of certain viruses that lie dormant in the animals’ genome and can be reactivated.

Foreign germs could be dangerous, and not just for the patients affected. The experiences from the corona pandemic have increased concerns about “xenozoonosis”; after all, it is estimated that three out of four newly occurring human diseases come from animals. Transplant patients in particular, with their weakened immune systems, could represent the ideal breeding ground for new infectious diseases. Last but not least, the different organ sizes are also problematic: organs from domestic pigs continue to grow too quickly after transplants. This can be inhibited by removing the genes for growth hormone receptors, but it introduces new biological problems. And finally, the organ must of course be functional, i.e. it must fit not only anatomically but also physiologically, for example into the hormonal signaling pathways of the recipient.

Race of biotech companies

Pig organs are the focus of research because they already have a high degree of similarity to those of humans, which should be further increased through genetic engineering interventions. This approach is also being researched in Germany. The company XTransplant was founded in Munich in 2020 to “create a world in which there is no shortage of donor hearts.” The scientists involved are developing genetically “humanized,” patented pigs as organ donors in collaboration with the Ludwig Maximilian University of Munich.

The transgenic pig heart used by Bennett and Faucette with the brand name “UHeart™” was prepared by the company Revivicor. The University of Maryland Medical Center previously received $15.7 million to test transplanting the hearts into baboons. According to one publication, they remained functional in the monkeys for up to three years.

Similar successes are also reported in a recently published study in which kidneys from transgenic pigs were transplanted into monkeys and some of them were kept alive for over two years. The experiment was carried out by the biotech startup eGenesis, which was co-founded by the controversial geneticist George Church, who still owes the world the promised resurrection of the mammoth. Unlike Revivicor, the scientists involved used a miniature pig breed for the appropriate organ size and compared the effect of various genetic changes on the rejection reaction and function of the organ after transplantation in long-tailed macaques.

A total of up to 69 genetic changes were carried out in the pigs – more than ever before for a xenotransplantation. The monkeys with organs that had all the genetic modifications lived the longest – in one case, 768 days, before the animal was euthanized due to edema and kidney failure. The researchers discovered cysts of unknown origin in his pig kidney. According to Joachim Denner, an expert in xenotransplantation at the Institute of Virology at the Free University of Berlin, it must first be clarified why some animals survived for a much shorter time; but “in principle, the kidneys from the genetically modified animals described (…) could be used for a clinical study with immunosuppression adapted to humans.”

Patients unable to consent

The company eGenesis is not only aiming for xenotransplantation of kidneys, in another study it is also testing the transplantation of the hearts of its transgenic pigs into twelve baboon babies. The study is intended to lay the foundation for use on humans. As early as next year, eGenesis hopes to transplant pig hearts into human babies with severe heart defects to keep them alive until a human donor organ can be used. However, the first baboons undergoing surgery only survived a few days, due to surgical complications rather than rejection, according to the company.

The American ethicist Syd Johnson pointed out that babies – unlike the two previous subjects in the experiments with the Revivicor hearts – cannot give informed consent. Parents are “desperately looking for anything that could save their child’s life,” said Johnson. The hearts could actually be a chance for children whose chances of survival are otherwise very poor. However, a failure of the risky experiment would mean a prolongation of their suffering through additional operations and intensive care treatments, which those affected do not understand and cannot refuse due to their young age. Ultimately, however, clinical studies are needed because due to the differences between species, results from animal experiments can only provide limited information about the functionality of the organs.

Xenotransplantations have also been tested several times on brain-dead people. In August, doctors at NYU Langone Hospital in New York reported that the genetically modified pig kidney used in a brain-dead patient had been functioning for 32 days.

Complex consideration

In Germany alone there are currently around 8,500 people on the waiting list for a donor organ, most of them waiting for a kidney. Last year, 743 people died on the waiting list. In contrast, there will only be 869 organ donors nationwide in 2022. The need for alternative sources of organs is therefore high. Although xenotransplantation is not the only research approach, attempts to grow organs from human stem cells in the laboratory have so far shown only modest success.

In addition to concerns about patients unable to consent, there are also concerns about the welfare of animals kept under severe restrictions. It is not for nothing that eGenesis keeps the location of its research facility with around 400 cloned pigs secret for fear of animal rights protests. The more pigs are genetically “humanized,” the more ethical complexity arises. One branch of research, for example, is concerned with the creation of chimeras, i.e. hybrid creatures from species that are supposed to have lower rejection reactions. These research goals awaken dystopian thoughts about the creation of pig people who vegetate in organ farms until the “harvest”. It is questionable whether we as a society really want to promote research in this direction – at the same time, there currently seems to be no alternative.

Dr. Isabelle Bartram is a molecular biologist and employee at the Gene-ethical Network eV