Angelika Schnieke: making pigs more human
Angelika Schnieke, image courtesy of Angelika Schnieke
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We've heard how scientists have manipulated 10 genes in a pig to produce a pig heart that could be transplanted into a human without the human immune system rejecting it. But how exactly do you produce such a pig? And are we at risk of transferring more than just the heart from the pig into humans? Could we be setting ourselves up for a new pig-human zoonotic virus?
Dr Kat Arney raised some of these concerns with Professor Angelika Schnieke, the chair of livestock biotechnology at the Technical University of Munich.
Kat: So we have these molecules encoded by genes on the surface of the pig cells that mean that the organs will be rejected. But also I know that a lot of our genomes are made up of things like viruses and stuff like that. So is, is that a problem as well?
Angelika: It's a theoretical problem. So we ourselves have endogenous retroviruses, the pigs have endogenous retroviruses. And there is a theoretical risk that if you now have a pig organ which moves into the human we might have then, in the end, maybe a recombinant virus which is sort of part human part pig and maybe then start completely new diseases.
Kat: That sounds bad. How big of risk is it?
Angelika: It sounds bad, but so far is there's no evidence that it actually occurs after the organ transplantation.
So it has only been achieved experimentally that you try to reactivate the virus, as most of the viruses are not intact viruses, they parts of viruses. And when these parts would be expressed, some might be still functional, but generally not.
Kat: And how many viruses are we talking about in the pig genome that are potentially risky?
Because I know that some companies, some researchers, have just basically been trying to get rid of them, because even if it's a theoretical risk, it's still a risk. I don't want some weird mutated pig disease going around.
Angelika: Well there are two things you can do. So, A, we can find animals which do not express any functional virus which we are worried about, and there are animals which just don't express it right from the beginning. So they should be safe. And should then we have, depending a bit on the pig breed, between 25 - 60, and sometimes more, copies of mostly non-intact retroviruses.
And like you mentioned before, there was a Chinese group, they had found and then switched about 25 copies of it inactivated all of the cells, they haven't deleted it. They have inactivated those.
Kat: So let's dig a bit more into the process by which you would genetically modify a pig or another animal to make their organs suitable. So how does that process start?
Angelika: So you first have to define what are the important genes. So once you know which genes, and then maybe also the PERVs, you can inactivate those genes. Now we have CRISPR-Cas, and so we can use CRISPR-Cas and can actually inactivate like all 25 retroviruses, were inactivated at the same time. Other groups have done four different genes. Others what you have to do is there are some genes where you do not have a complementarity between the pig and the human. And then you can go ahead and add the human genes to the pigs.
Kat: So we are removing the retroviruses. We are removing or inactivating genes that might be a problem. And then adding in genes that will make this tissue more compatible with humans.
Angelika: Exactly.
Kat: That that's a lot, at what stage would you do this? Then how would you, like, how do you get this into a pig and into an organ? So at what stage are you modifying these genes?
Angelika: We still use a lot of nuclear transfer to do this. So we do all the modifications in the cells of a pig and then we use the cells and make a pig out of it.
Kat: Hang on. Hang on. We take the cells, we make a pig out of it. How does that work?
Angelika: We do something called somatic cell nuclear transfer. You can take any type of cells, usually it is either fetal cells, kidney cells, you keep them in culture, you modify them. Once you have identified the cells which has exactly all the modifications you want, you then take an oozyte which is not fertilised so it can't really develop, you even then remove the genetic material from that oozyte, add the new genetic material from the cell, implant it into a pig and hopefully you get an animal born.
Kat: Fingers crossed, this is the kind of cloning technology that Dolly the sheep and, and all the other cloned animals used.
Angelika: Absolutely, yes. So knockouts, you can still do without a problem sometimes directly in the embryo, but if you do some more complicated or multiple knockouts and you want to prove right away that animal you generate is really genetically modified then you do it with somatic cell nuclear transfer.
Kat: So what are the limitations of this technology? Because we've seen a pig heart recently been transplanted into a human, and we're seeing advances in this kind of, a lot of interest in the area of xenotransplantation. I mean, is this technology really like, yep, they're good to go? Or are there areas where we could still really improve this?
Angelika: Well we still have technical limitations, not so much on the genetic engineering side because that has really improved over the last few years, but we don't know all the important genes we need to modify for particular organs.
And so we need the experiments which we do. We then often do single cell sequencing to see what is still going wrong. And once we know that, we can adjust this. The heart is a relatively simple organ, this was done first, people have had some success also with kidneys. And then you probably also heard people even trying to grow human organs in animals.
Kat: Oh yes. So how does that work? Presumably, it's sort of the other way round of the process?
Angelika: Ideally what people had hoped we can make chimeric animals, you might know about that. That we can take cells from closely related animals, like the rat, and put that into an early embryo of a mouse and then you get an animal which is born, which is half mouse and half rat.
So people had took that idea and thought well could that also work if we go for larger animals, which could then provide an organ. So when you take the embryo from a pig add human cells to it. And would you then get a chimera?
Kat: This doesn't sound good.
Angelika: I would agree. And you have to add one more trick. You have to actually also delete some genes so that particular organ can't grow. So that the animal would have a pancreas and then the new cells which come in should then produce purely this pancreas.
Kat: So you're effectively making sure that even though it's this hybrid animal, that it's only growing human pancreatic cells, right?
Angelika: Yep
Kat: This still doesn't seem good to me.
Angelika: Well, it still doesn't work.
Kat: That's a good start.
Angelika: I think that there is a difference in the species, and of course there are a lot of ethical problems.
Kat: Yeah. That's mainly the ones I'm worried about.
Angelika: Yes
