More than 114,000 men, women, and children are awaiting organ transplants in the United States. Each day, 22 of them die waiting. To address this shortage, researchers are working hard to grow organs on-demand, using the patient's own cells, to eliminate the need to find a perfectly matched donor.
"The next step is to transplant these cells into a larger animal that will produce an organ that is the right size for a human."
But creating full-size replacement organs in a lab is still decades away. So some scientists are experimenting with the boundaries of nature and life itself: using other mammals to grow human cells. Earlier this year, this line of investigation took a big step forward when scientists announced they had grown sheep embryos that contained human cells.
Dr. Pablo Ross, an associate professor at the University of California, Davis, along with a team of colleagues, introduced human stem cells into the sheep embryos at a very early stage of their development and found that one in every 10,000 cells in the embryo were human. It was an improvement over their prior experiment, using a pig embryo, when they found that one in every 100,000 cells in the pig were human. The resulting chimera, as the embryo is called, is only allowed to develop for 28 days. Leapsmag contributor Caren Chesler recently spoke with Ross about his research. Their interview has been edited and condensed for clarity.
Your goal is to one day grow human organs in animals, for organ transplantation. What does your research entail?
We're transplanting stem cells from a person into an animal embryo, at about day three to five of embryo development.
This concept has already been shown to work between mice and rats. You can grow a mouse pancreas inside a rat, or you can grow a rat pancreas inside a mouse.
For this approach to work for humans, the next step is to transplant these cells into a larger animal that will produce an organ that is the right size for a human. That's why we chose to start some of this preliminary work using pigs and sheep. Adult pigs and adult sheep have organs that are of similar size to an adult human. Pigs and sheep also grow really fast, so they can grow from a single cell at the time of fertilization to human adult size -- about 200 pounds -- in only nine to 10 months. That's better than the average waiting time for an organ transplant.
"You don't want the cells to confer any human characteristics in the animal....Too many cells, that may be a problem, because we do not know what that threshold is."
So how do you get the animal to grow the human organ you want?
First, we need to generate the animal without its own organ. We can generate sheep or pigs that will not grow their own pancreases. Those animals can then be used as hosts for human pancreas generation.
For the approach to work, we need the human stem cells to be able to integrate into the embryo and to contribute to its tissues. What we've been doing with pigs, and more recently, in sheep, is testing different types of stem cells, and introducing them into an early embryo between three to five days of development. We then transfer that embryo to a surrogate female and then harvest the embryos back at day 28 of development, at which point most of the organs are pre-formed.
The human cells will contribute to every organ. But in trying to do that, they will compete with the host organism. Since this is happening inside a pig embryo, which is inside a pig foster mother, the pig cells will win that competition for every organ.
Because you're not putting in enough human cells?
No, because it's a pig environment. Everything is pig. The host, basically, is in control. That's what we see when we do rat mice, or mouse rat: the host always wins the battle.
But we need human cells in the early development -- a few, but not too few -- so that when an organ needs to form, like a pancreas (which develops at around day 25), the pig cells will not respond to that, but if there are human cells in that location, [those human cells] can respond to pancreas formation.
From the work in mice and rats, we know we need some kind of global contribution across multiple tissues -- even a 1% contribution will be sufficient. But if the cells are not there, then they're not going to contribute to that organ. The way we target the specific organ is by removing the competition for that organ.
So if you want it to grow a pancreas, you use an embryo that is not going to grow a pancreas of its own. But you can't control where the other cells go. For instance, you don't want them going to the animal's brain – or its gonads –right?
You don't want the cells to confer any human characteristics in the animal. But even if cells go to the brain, it's not going to confer on the animal human characteristics. A few human cells, even if they're in the brain, won't make it a human brain. Too many cells, that may be a problem, because we do not know what that threshold is.
The objective of our research right now is to look at just 28 days of embryonic development and evaluate what's going on: Are the human cells there? How many? Do they go to the brain? If so, how many? Is this a problem, or is it not a problem? If we find that too many human cells go to the brain, that will probably mean that we wouldn't continue with this approach. At this point, we're not controlling it; we're analyzing it.
"By keeping our research in a very early stage of development, we're not creating a human or a humanoid or anything in between."
What other ethical concerns have arisen?
Conferring human properties to the organism, that is a major concern. I wouldn't like to be involved in that, and so that's what we're trying to assess. By keeping our research in a very early stage of development, we're not creating a human or a humanoid or anything in between.
What specifically sets off the ethical alarms? An animal developing human traits?
Animals developing human characteristics goes beyond what would be considered acceptable. I share that concern. But so far, what we have observed, primarily in rats and mice, is that the host animal dictates development. When you put mouse cells into a rat -- and they're so closely related, sometimes the mouse cells contribute to about 30 percent of the cells in the animal -- the outcome is still a rat. It's the size of a rat. It's the shape of the rat. It has the organ sizes of a rat. Even when the pancreas is fully made out of mouse cells, the pancreas is rat-sized because it grew inside the rat.
This happens even with an organ that is not shared, like a gallbladder, which mice have but rats do not. If you put cells from a mouse into a rat, it never grows a gallbladder. And if you put rat cells into the mouse, the rat cells can end up in the gallbladder even though those rat cells would never have made a gallbladder in a rat.
That means the cell structure is following the directions of the embryo, in terms of how they're going to form and what they're going to make. Based on those observations, if you put human cells into a sheep, we are going to get a sheep with human cells. The organs, the pancreas, in our case, will be the size and shape of the sheep pancreas, but it will be loaded with human cells identical to those of the patient that provided the cells used to generate the stem cells.
But, yeah, if by doing this, the animal acquires the functional or anatomical characteristics associated with a human, it would not be acceptable for me.
So you think these concerns are justified?
Absolutely. They need to be considered. But sometimes by raising these concerns, we prevent technologies from being developed. We need to consider the concerns, but we must evaluate them fully, to determine if they are scientifically justified. Because while we must consider the ethics of doing this, we also need to consider the ethics of not doing it. Every day, 22 people in the US die because they don't receive the organ they need to survive. This shortage is not going to be solved by donations, alone. That's clear. And when people die of old age, their organs are not good anymore.
Since organ transplantation has been so successful, the number of people needing organs has just been growing. The number of organs available has also grown but at a much slower pace. We need to find an alternative, and I think growing the organs in animals is one of those alternatives.
Right now, there's a moratorium on National Institutes of Health funding?
Yes. It's only one agency, but it happens to be the largest biomedical funding source. We have public funding for this work from the California Institute for Regenerative Medicine, and one of my colleagues has funding from the Department of Defense.
"I can say, without NIH funding, it's not going to happen here. It may happen in other places, like China."
Can we put the moratorium in context? How much research in the U.S. is funded by the NIH?
Probably more than 75 percent.
So what kind of impact would lifting that ban have on speeding up possible treatments for those who need a new organ?
Oh, I think it would have a huge impact. The moratorium not only prevents people from seeking funding to advance this area of research, it influences other sources of funding, who think, well, if the NIH isn't doing it, why are we going to do it? It hinders progress.
So with the ban, how long until we can really have organs growing in animals? I've heard five or 10 years.
With or without the ban, I don't think I can give you an accurate estimate.
What we know so far is that human cells don't contribute a lot to the animal embryo. We don't know exactly why. We have a lot of good ideas about things we can test, but we can't move forward right now because we don't have funding -- or we're moving forward but very slowly. We're really just scratching the surface in terms of developing these technologies.
We still need that one major leap in our understanding of how different species interact, and how human cells participate in the development of other species. I cannot predict when we're going to reach that point. I can say, without NIH funding, it's not going to happen here. It may happen in other places, like China, but without NIH funding, it's not going to happen in the U.S.
I think it's important to mention that this is in a very early stage of development and it should not be presented to people who need an organ as something that is possible right now. It's not fair to give false hope to people who are desperate.
So the five to 10 year figure is not realistic.
I think it will take longer than that. If we had a drug right now that we knew could stop heart attacks, it could take five to 10 years just to get it to market. With this, you're talking about a much more complex system. I would say 20 to 25 years. Maybe.
In November 2020, messenger RNA catapulted into the public consciousness when the first COVID-19 vaccines were authorized for emergency use. Around the same time, an equally groundbreaking yet relatively unheralded application of mRNA technology was taking place at a London hospital.
Over the past two decades, there's been increasing interest in harnessing mRNA — molecules present in all of our cells that act like digital tape recorders, copying instructions from DNA in the cell nucleus and carrying them to the protein-making structures — to create a whole new class of therapeutics.
Scientists realized that artificial mRNA, designed in the lab, could be used to instruct our cells to produce certain antibodies, turning our bodies into vaccine-making factories, or to recognize and attack tumors. More recently, researchers recognized that mRNA could also be used to make another groundbreaking technology far more accessible to more patients: gene editing. The gene-editing tool CRISPR has generated plenty of hype for its potential to cure inherited diseases. But delivering CRISPR to the body is complicated and costly.
"Most gene editing involves taking cells out of the patient, treating them and then giving them back, which is an extremely expensive process," explains Drew Weissman, professor of medicine at the University of Pennsylvania, who was involved in developing the mRNA technology behind the COVID-19 vaccines.
But last November, a Massachusetts-based biotech company called Intellia Therapeutics showed it was possible to use mRNA to make the CRISPR system inside the body, eliminating the need to extract cells out of the body and edit them in a lab. Just as mRNA can instruct our cells to produce antibodies against a viral infection, it can also teach them to produce the two molecular components that make up CRISPR — a guide molecule and a cutting protein — to snip out a problem gene.
"The pandemic has really shown that not only are mRNA approaches viable, they could in certain circumstances be vastly superior to more traditional technologies."
In Intellia's London-based clinical trial, the company applied this for the first time in a patient with a rare inherited liver disease known as hereditary transthyretin amyloidosis with polyneuropathy. The disease causes a toxic protein to build up in a person's organs and is typically fatal. In a company press release, Intellia's president and CEO John Leonard swiftly declared that its mRNA-based CRISPR therapy could usher in a "new era of potential genome editing cures."
Weissman predicts that turning CRISPR into an affordable therapy will become the next major frontier for mRNA over the coming decade. His lab is currently working on an mRNA-based CRISPR treatment for sickle cell disease. More than 300,000 babies are born with sickle cell every year, mainly in lower income nations.
"There is a FDA-approved cure, but it involves taking the bone marrow out of the person, and then giving it back which is prohibitively expensive," he says. It also requires a patient to have a matched bone marrow done. "We give an intravenous injection of mRNA lipid nanoparticles that target CRISPR to the bone marrow stem cells in the patient, which is easy, and much less expensive."
Meanwhile, the overwhelming success of the COVID-19 vaccines has focused attention on other ways of using mRNA to bolster the immune system against threats ranging from other infectious diseases to cancer.
The practicality of mRNA vaccines – relatively small quantities are required to induce an antibody response – coupled with their adaptable design, mean companies like Moderna are now targeting pathogens like Zika, chikungunya and cytomegalovirus, or CMV, which previously considered commercially unviable for vaccine developers. This is because outbreaks have been relatively sporadic, and these viruses mainly affect people in low-income nations who can't afford to pay premium prices for a vaccine. But mRNA technology means that jabs could be produced on a flexible basis, when required, at relatively low cost.
Other scientists suggest that mRNA could even provide a means of developing a universal influenza vaccine, a goal that's long been the Holy Grail for vaccinologists around the world.
"The mRNA technology allows you to pick out bits of the virus that you want to induce immunity to," says Michael Mulqueen, vice president of business development at eTheRNA, a Belgium-based biotech that's developing mRNA-based vaccines for malaria and HIV, as well as various forms of cancer. "This means you can get the immune system primed to the bits of the virus that don't vary so much between strains. So you could actually have a single vaccine that protects against a whole raft of different variants of the same virus, offering more universal coverage."
Before mRNA became synonymous with vaccines, its biggest potential was for cancer treatments. BioNTech, the German biotech company that collaborated with Pfizer to develop the first authorized COVID-19 vaccine, was initially founded to utilize mRNA for personalized cancer treatments, and the company remains interested in cancers ranging from melanoma to breast cancer.
One of the major hurdles in treating cancer has been the fact that tumors can look very different from one person to the next. It's why conventional approaches, such as chemotherapy or radiation, don't work for every patient. But weaponizing mRNA against cancer primes the immune cells with the tumor's specific genetic sequence, training the patient's body to attack their own unique type of cancer.
"It means you're able to think about personalizing cancer treatments down to specific subgroups of patients," says Mulqueen. "For example, eTheRNA are developing a renal cell carcinoma treatment which will be targeted at around 20% of these patients, who have specific tumor types. We're hoping to take that to human trials next year, but the challenge is trying to identify the right patients for the treatment at an early stage."
Repairing Damaged mRNA
While hopes are high that mRNA could usher in new cancer treatments and make CRISPR more accessible, a growing number of companies are also exploring an alternative to gene editing, known as RNA editing.
In genetic disorders, the mRNA in certain cells is impaired due to a rogue gene defect, and so the body ceases to produce a particular vital protein. Instead of permanently deleting the problem gene with CRISPR, the idea behind RNA editing is to inject small pieces of synthetic mRNA to repair the existing mRNA. Scientists think this approach will allow normal protein production to resume.
Over the past few years, this approach has gathered momentum, as some researchers have recognized that it holds certain key advantages over CRISPR. Companies from Belgium to Japan are now looking at RNA editing to treat all kinds of disorders, from Huntingdon's disease, to amyotrophic lateral sclerosis, or ALS, and certain types of cancer.
"With RNA editing, you don't need to make any changes to the DNA," explains Daniel de Boer, CEO of Dutch biotech ProQR, which is looking to treat rare genetic disorders that cause blindness. "Changes to the DNA are permanent, so if something goes wrong, that may not be desirable. With RNA editing, it's a temporary change, so we dose patients with our drugs once or twice a year."
Last month, ProQR reported a landmark case study, in which a patient with a rare form of blindness called Leber congenital amaurosis, which affects the retina at the back of the eye, recovered vision after three months of treatment.
"We have seen that this RNA therapy restores vision in people that were completely blind for a year or so," says de Boer. "They were able to see again, to read again. We think there are a large number of other genetic diseases we could go after with this technology. There are thousands of different mutations that can lead to blindness, and we think this technology can target approximately 25% of them."
Ultimately, there's likely to be a role for both RNA editing and CRISPR, depending on the disease. "I think CRISPR is ideally suited for illnesses where you would like to permanently correct a genetic defect," says Joshua Rosenthal of the Marine Biology Laboratory in Chicago. "Whereas RNA editing could be used to treat things like pain, where you might want to reset a neural circuit temporarily over a shorter period of time."
Much of this research has been accelerated by the COVID-19 pandemic, which has played a major role in bringing mRNA to the forefront of people's minds as a therapeutic.
"The pandemic has really shown that not only are mRNA approaches viable, they could in certain circumstances be vastly superior to more traditional technologies," says Mulqueen. "In the future, I would not be surprised if many of the top pharma products are mRNA derived."
"Making Sense of Science" is a monthly podcast that features interviews with leading medical and scientific experts about the latest developments and the big ethical and societal questions they raise. This episode is hosted by science and biotech journalist Emily Mullin, summer editor of the award-winning science outlet Leaps.org.