Interview on genome-editing and xenotransplantation

I was pleased to be invited to reflect on the future of xenotransplantation and human genome editing recently in an interview on ABC’s The World with Beverley O’Connor. The interview was broadcast following news of the unfortunate passing of Richard “Rick” Slayman — the first human to have received a genome-edited pig kidney. Mr Slayman passed less than two months following the xenotransplantation, although the General Massachusetts Hospital has stated that there is no indication that the transplant was the cause of death.

As I note in the interview, it is likely that the pig from which the kidney was sourced was a cloned pig. While this has not been confirmed (to my knowledge) by eGenesis, there are reports that the firm uses cloned pigs.

Moreover, the protocol by which pigs are prepared for this process is confirmed in the academic literature as involving somatic cell nuclear transfer (SCNT). SCNT is a kind of cloning that makes use of cultured fibroblasts — skin cells that excrete proteins like collagen — that are transferred into the enucleated nucleus of endogenous oocytes (eggs) . Willard Eyestone and colleagues describes the process in this way:

A major step forward in the generation of pigs as organ donors was the advent of somatic cell nuclear transfer (SCNT). In the pig, cultured fibroblasts were used as nuclear donors to replace the endogenous nuclei of porcine oocytes. Upon fusion with an enucleated oocyte, fibroblast nuclei were reprogrammed to totipotency by factors in oocyte cytoplasm. The newly reconstructed oocyte then developed into a new individual with the genetic constitution of the donor nucleus. SCNT technology opened the door for genetic modification of cultured somatic cells, which could be used to generate pigs bearing those modifications.

https://link.springer.com/chapter/10.1007/978-3-030-49127-7_6

In essence, pig oocytes or eggs are enucleated — their nucleuses removed — and then they are fused with the somatic (adult) nuclei of the fibroblast cells. The fused or engineered egg undergoes ‘reprogramming’ as a result of this process. This means that the genes within the engineered egg cells are expressed differently once the fusion occurs; indeed, the so-called ‘fate’ of the cell is ‘switched.’ This means that ‘potency’ (or differentiation pathway) of the egg cell is changed. By ‘differentiation pathway,’ I mean the cells’ ability to differentiate into other cell types.

Cell biologists speak of several different cell potencies. Stem cells can express different degrees of potency, and may be classified as totipotent, pluripotent, multipotent, oligopotent, and unipotent cells:

  • totipotent stem cells can differentiate into all adult somatic cell types, as well as tissues of the placental and fetal membranes; eg, the zygote (until the 16-cell stage)
  • pluripotent stem cells are capable of differentiation into all adult somatic cells in all three germ layers: the endoderm, mesoderm and ectoderm. They have two defining features: the ability to form teratomas when injected in immune-deficient mice and the ability to form chimera or contribute to the germline of a mouse if injected into the blastocyst. An example is an embryonic stem cell or a somatic cell reprogrammed into the pluripotent state using somatic cell nuclear transfer (SCNT).
  • multipotent stem cells are capable of differentiating into multiple but limited cell types, usually within one germ layer: eg, hematopoietic stem cells can differentiate into lymphoid, myeloid, erythroid and megakaryocyte precursors; mesenchymal stem cells, which are often used for attempt to regenerate tissue and other cells, can differentiate into osteogenic, chondrogenic, and adipogenic cells.

As the pig oocytes undergo reprogramming through the fusion process, they become pluripotent cells. This means they can give rise to another new form of life (eg, a pig may be ‘cloned’ from this engineered donor cell). This is how the pig was likely to have been created in respect of this process; and the pig kidney would have been harvested from the pig that was made through this process.

I suspect the 69 gene edits that were made to the pig, which included pig gene knockouts, human gene knock-ins, and pig endogenous retrovirus (PERV) gene knockouts, was done at the pre-fertilisation stage — that is, that were applied to the engineered pig oocyte.

In any event, it will be important to understand how well these treatments last, especially given that they will continue to be offered. A second recipient of a xenotransplanted pig kidney, a woman from New Jersey, was given her xenotransplanted organ at the New York University Langone Health around 24 April 2023. The organ, however, also included the pig’s thymus glad, according to reports.

Around the same time as the organ xenotransplant, the NYU surgical team also transplanted a mechanical heart pump into this patient. It is noted in reports that the patient was given these treatments under an FDA emergency authorisation; that would mean that it was likely authorised by an Investigational New Drug licence under pt 312 of Title 21 of the Code of Federal Regulations.

Salient details about this pig kidney from the media release include the following points:

  • The genome-edited pig kidney was sourced from biotech firm United Therapeutics Corporation
  • It was an investigational xenokidney that ‘matched’ the donee (presumably this is something like a HLA match?)
  • Although chronic kidney failure ordinarily rules out patients from receiving a mechanical heart pump, the potential for this patient to live without a need for kidney dialysis (provided the xenotransplant succeeds) meant that the heart pump could be given to this patient
  • The pig kidney was engineered to “knock out” the gene responsible for producing the sugar known as alpha-gal
  • NYU Langone studies (although it is not clear what kinds of studies — presumably non-human primate studies?) demonstrated that removing alpha-gal was sufficient to prevent an antibody reaction that causes hyperacute rejection
  • The donor pig’s thymus gland, which is said to “educate” the immune system, was included: it was surgically placed under the covering of the kidney to reduce the likelihood of rejection
  • The xenokidney and the thymus tissue combined are called a UThymoKidney
  • The gene edits, pig breeding, and production of the investigational UThymoKidney used in this procedure were performed by United Therapeutics Corporation. No other unapproved devices or medications were used in the procedure.

Chimeric monkey sheds light on what can be done with embryonic stem cells

I was quoted in this article today regarding a newly published study in Cell that demonstrates, in the author’s words, that ‘mammalian pluripotent stem cells possess preimplantation embryonic cell-like (naive) pluripotency.’ As the summary notes, this discovery about embryonic stem cells can now be said to have been demonstrated experimentally through the generation of a chimeric animal — a monkey whose embryonic development has been ‘complemented’ by homologous embryonic stem cells derived from another ‘donated’ line of cells. The monkey, in short, has developed from a blastocyst that is a compound of two embryonic stem cell lines.

Unsurprisingly, news stories have been focusing on one of the eye-grabbing aspects of this experiment: that the monkey in question has fluorescent green fingers and eyes. Unfortunately, the monkey died after only 10 days — which is still the longest period of time for which such a chimeric organism has lived before.

The reason that the monkey has these features is because the researchers used green fluorescent protein (GFP) to ‘label’ the embryonic stem cells (ESCs) that were incorporated into the host embryo at the blastocyst stage. And so what one is looking at when one sees the monkey with green fingers and eyes (visible even to the naked eye) is visual evidence that the embryonic stem cells have survived the process of being ‘complemented’ into the blastocyst of the host monkey and have spread throughout its body. In other words, the cells have been incorporated into the monkey’s cellular DNA; the monkey has both its ‘natural’ DNA and a ‘foreign’ line of DNA. Indeed, as the images indicate, there is a proliferation of these complemented ESCs throughout the monkey’s organs, including plenty in the brain and ileum (small intestine).

As the ‘Highlights’ section of the article points out, when these embryonic stem cells (ESCs) in the body of the monkey were ‘characterised’ (assessed), it was revealed that they remained in a so-called pluripotent state. In other words, the ESCs seem to have been able to differentiate into the different kinds of cellular categories: glial (brain) cells, heart cells (myocytes), lung cells (epithelial cells), and so on. Indeed, they continue to be in this pluripotent state, even as they maintain a ‘functional’ presence in the monkey’s body.

The news.com.au story quotes me as follows:

Sydney University lecturer in health law Dr Christopher Rudge told new.com.au the medical experiment had been on the cards for a long time.

“This is another step along the journey,” he said.

“The advancement here is that scientists have never been able to show such a prolific survival / proliferation of donated (or ‘complemented’) embryonic cells through a single organism.

“You’ve got more of these donated or secondary cells throughout the organism in a mammal.”

But he cautioned whether it would lead to anything substantive.

“Regenerative medicine has been hyped since the late 1990s,” he said. “Unfortunately it has not borne fruit.”

See https://www.news.com.au/technology/science/stunning-monkey-born-with-glowing-eyes-and-fingers/news-story/27d84700628476da1579968e76cbda5d

Obviously this scientific study demonstrates that certain new techniques can be adopted to expand the capacity of scientists to create chimeras. Scientists have long had the capacity to infuse mouse and rat blastocysts with pluripotent stem cells to generate live chimeric animals that feature this high proliferation of homologous cells. What is new here is that this capacity now extends to non-human primates — a species of animal much closer, in evolutionary terms, to humans.

It is arguably another step along the way in discovering how stem cells, including pluripotent embryonic stem cells, can be used as technologies of biological inquiry (for diagnosis, and to study developmental mechanisms) and, ultimately, to biological treatments. Of course, there is still so much more to learn.

Whether an experiment of this nature would be approved in Australia is an interesting question. If nothing else, this finding indicates that discoveries in stem cell medicine are continuing apace. Of course, given that this involved the effective fusing of two monkey embryos (or embryonic cell lines), the more serious bioethical questions regarding human-monkey chimeras, which have been posed before, do not arise in this instance.