Written by Dylan Roussouw MBChB IV
Figure 1
January 7, 2022, marked the first pig-to-human heart transplant. David Bennett (57) had been on cardiac support for two months because of his terminal heart disease. He was illegible for both a mechanical heart pump and human heart transplantation owing to an irregular heartbeat and poor compliance with previous transplants. This qualified him for the experimental procedure of receiving a genetically modified pig heart (Figure 2). Surgeons from the University of Maryland Medical Centre performed the surgery as Bennett’s other options had been exhausted. This was a unique opportunity to observe the long-term impact of xenotransplantation on a surviving patient because the US Food and Drugs Administration (FDA) has not yet allowed clinical trial of porcine organs in humans.1
Figure 2: Surgeons at the University of Maryland Medical Centre with the genetically modified pig heart.
History
Xenotransplantation is the term for the transplantation of organs or tissues from a non-human donor to a human recipient. The demand for organs for transplantation severely exceeds the supply, so xenotransplantation provides a possible solution to this shortage.2 However, xenotransplantation is not without its challenges. Two major problems the field of xenotransplantation faces are organ rejection and zoonotic infections. As with human organs, there is potential for the transplanted organ to be rejected, which is a process mediated by the immune system. Both hyperacute rejection (HAR) and acute humoral xenograft rejection (AHXR) are forms of rejection seen in xenotransplantation. HAR is characterised by diffuse interstitial haemorrhage, oedema, and small vessel thrombosis occurring within minutes to hours of transplantation, while AHXR develops over days to weeks.2
As for zoonotic infections, there are several diseases being researched about their potential to cause harm in transplant recipients. Of concern are porcine endogenous retroviruses (PERVs).2 PERVs exist as part of the genome of many pigs. Organs infected with PERVs cannot be safely transplanted into humans. In specialised facilities pigs are selectively bred to exclude zoonotic infections – including PERVs – from their genetic makeup. A drove of pigs must be considered clinical grade before their organs can be used in transplantation.1 As such, in a drove of clinical grade pigs there must be genome-wide inactivation of PERV related genes.3
The problems associated with xenotransplantation of porcine organs cannot be solely fixed by selective breeding, as such, genetic modification of their genome is required to produce a clinical grade organ. Revivicor, a company situated in Blacksburg Virginia, is currently the only company that has suitable facilities to produce clinical grade pigs and is doing research into genetic modifications that limit rejection in humans and other primates.1 The development of CRISPR-Cas9 genome editing has been integral in creating porcine organs that are less likely to be attacked by the human immune system.1 Through CRISPR pig genes have been silenced or “knocked-out” while human genes have been incorporated into the pigs’ genome1 . In Bennett’s case the transplanted heart had three porcine genes silenced, as they normally induce an immune response, while an additional six human genes were incorporated into the organ to aid with acceptance of the organ post-transplant.1 In-total the transplanted heart had ten genetic modifications applied to it.4
Human versus Animal Models
The results of Bennett’s transplantation will help shape the future of xenotransplantation.1,4 There are no ongoing clinical trials in humans for xenotransplantation.4 One patient cannot provide sufficient safety and effectiveness data for xenotransplantation to be implemented into daily clinical practice. But Bennett’s progress can be used as a case study for proof of concept going forward.4
Most data we have on the effects of xenotransplantation comes from pig-to-primate studies.1,5 In most cases monkeys that have received a pig heart have not survived long term, hence why clinical trials in humans have been deemed too dangerous.5 In three test groups of baboons that receive a pig heart transplant survival was 1 day, 3 days, and 30 days across the three groups(Figure 1a). Längin et al6 have done multiple pig-to-baboon transplants and documented several issues that stand as obstacles to successful xenotransplantation. Problematic laboratory findings included increased troponin levels (Figure 3b), which is associated with cardiac damage, as well as increased bilirubin levels (Figure 3c) and an increased liver mass (Figure 3d) which together indicate terminal liver disease. On autopsy there was evident cardiac hypertrophy (Figure 3e) with a thickened myocardium and decreased left ventricular cavity (Figure 3f, 3g). These indicate that the heart had to work too hard to pump blood, which will lead to heart failure.
Figure 3: Survival, laboratory parameters, and complications of pig-to-baboon heart transplants.6
From their research Längin et al6 produced genetically modified hearts that, once implanted, allowed survival of baboons for more than six months. However, this could only be achieved in baboons whose hearts were fully functional before the transplant. In baboons that required lifesaving transplants the greatest survival achieved was 57 days.6 Problems noted during and after the transplantation included immune responses, excessive blood clotting of the heart’s vessels, damage to the heart muscle through ischaemia, hypertrophy of the heart muscle, and high blood pressure.5 Genetic modification was utilised to minimise the chance of an immune response as well as clotting of the heart’s vessels. To prevent damage owing to ischaemia the hearts were intermittently perfused with an oxygenated blood-based protective solution (figure 4). The baboons would be placed on further immunosuppression, and be given treatment to lower blood pressure, prevent blood clotting, and inhibit cell proliferation to prevent cardiac hypertrophy, also indicated in figure 4.
Figure 4: Methodology to improve pig-to-primate heart transplantation.5
Animal models may provide a large pool of data to analyse, but they are not without their limitations. Studies with baboons will never be able to accurately replicate the clinical outcome expected in human recipients. This is because different animals produce different antibodies. Work done on making an organ suitable for baboon transplantation does not translate into making an organ suitable for human transplantation. Furthermore, human physiology greatly differs from baboon physiology. Animal models cannot predict if a sick human will respond to a modified porcine organ similar to how healthy baboons respond.1
A Look to the Future
Despite decades of experience in genetically modifying pig organs, xenotransplantation is still a growing field. Almost all studies have been in animal models, and there are no defined criteria for when clinical trials involving humans will begin. David Bennett’s case marks an integral milestone in the progression of xenotransplantation. It is bound to provide vital insight into the advantages, shortcomings, and unknowns of the field in relation to human models. At time of writing Bennett has survived 43 days with his porcine heart.
Use the following link to read the full article through the UP Library: First pig-to-human heart transplant: what can scientists learn?
Bibliography
- Reardon S. First pig-to-human heart transplant: what can scientists learn? [Internet]. Nature.com. 2022 [cited 2022Feb12]. Available from: https://www-nature-com.uplib.idm.oclc.org/articles/d41586-022-00111-9
- Yang Y-G, Sykes M. Xenotransplantation: current status and a perspective on the future [Internet]. Proquest.com. Nature Reviews. 2007 [cited 2022Feb12]. Available from: https://www.proquest.com/docview/224156473?accountid=14717
- Yang L, Güell M, Niu D, George H, Lesha E, Grishin D, et al. Genome-wide inactivation of porcine endogenous retroviruses (PERVs) [Internet]. JSTOR. Science. 2015 [cited 2022Feb12]. Available from: https://www-jstor-org.uplib.idm.oclc.org/stable/24740941?seq=1#metadata_info_tab_contents
- Burki T. Pig-heart transplantation surgeons look to the next steps [Internet]. ClinicalKey.com. The Lancet; 2022 [cited 2022Feb12]. Available from: https://www-clinicalkey-com.uplib.idm.oclc.org/#!/content/playContent/1-s2.0-S0140673622000976?returnurl=null&referrer=null
- Knosalla C. Success for pig-to-baboon heart transplants [Internet]. Nature.com. 2018 [cited 2022Feb12]. Available from: https://www-nature-com.uplib.idm.oclc.org/articles/d41586-018-07419-5
- Längin M, Mayr T, Reichart B, Michel S, Buccholz S, Guethoff S, et al. Consistent success in life-supporting porcine cardiac xenotransplantation [Internet]. Nature.com. 2018 [cited 2022Feb12]. Available from: https://www-nature-com.uplib.idm.oclc.org/articles/s41586-018-0765-z
Figure references
Figure 1: Ryan P. Illustration of CRISPR gene editing [Internet]. Time.com. 2015 [cited 2022Feb12]. Available from: https://time.com/5159889/why-pig-organs-could-be-the-future-of-transplants/
Figure2: Reardon S. The heart used in the transplant came from a pig with several genetic modifications, including some to knock out genes that trigger the human immune system. [Internet]. Nature.com. University of Maryland School of Medicine; 2022 [cited 2022Feb12]. Available from: https://www-nature-com.uplib.idm.oclc.org/articles/d41586-022-00111-9
Figure 3: Längin M, Mayr T, Reichart B, Michel S, Buccholz S, Guethoff S, et al. Survival, laboratory parameters, necropsy and histology after orthotopic xenotransplantation [Internet]. Nature.com. 2018 [cited 2022Feb12]. Available from: https://www-nature-com.uplib.idm.oclc.org/articles/s41586-018-0765-z
Figure 4: Knosalla C. Improving pig-to-primate heart transplants [Internet]. Nature.com. 2018 [cited 2022Feb12]. Available from: https://www-nature-com.uplib.idm.oclc.org/articles/d41586-018-07419-5
